Computer Networking
R26. Fill in the blank: RIP advertisements typically announce the number of hops to various destinations. BGP updates, on the other hand, announce the __________ to the various destinations.
"sequence of ASs on the routes"
Fragment
(1) Refers to the condition of a disk in which files are divided into pieces scattered around the disk. Fragmentation occurs naturally when you use a disk frequently, creating, deleting, and modifying files. At some point, the operating system needs to store parts of a file in noncontiguous clusters. This is entirely invisible to users, but it can slow down the speed at which data is accessed because the disk drivemust search through different parts of the disk to put together a single file. In DOS 6.0 and later systems, you can defragment a disk with the DEFRAG command. You can also buy software utilities, called disk optimizers or defragmenters,that defragment a disk. (2) Fragmentation can also refer to RAM that has small, unused holes scattered throughout it. This is called external fragmentation. With modern operating systems that use a paging scheme, a more common type of RAM fragmentation is internal fragmentation.This occurs when memory is allocated in frames and the frame size is larger than the amount of memory requested.
Subnet
(SUBNETwork) A logical division of a local area network, which is created to improve performance and provide security. To enhance performance, subnets limit the number of nodes that compete for available bandwidth. Instead of one network handling all the traffic, the network is divided into groups of clients and servers that interact with each other most of the time. For security, the subnet divisions can be based on servers that have restricted applications. Routers are bridges are used to traverse network segments. In an IP network, the subnet is identified by a subnet mask (see subnet mask).
Gateway Routers
(n.) (1) A node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISPthat connects the user to the internet. In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway. (2) A computer system located on earth that switches data signals and voice signals between satellites and terrestrialnetworks.
Tunneling
(n.) A technology that enables one network to send its data via another network's connections. Tunneling works by encapsulating a network protocol within packets carried by the second network. For example, Microsoft's PPTP technology enables organizations to use the Internet to transmit data across a VPN. It does this by embedding its own network protocol within the TCP/IP packets carried by the Internet. Tunneling is also called encapsulation.
Routing Information Protocol (RIP)
(n.) Abbreviated as RIP, an interior gateway protocol defined by RFC 1058 that specifies how routers exchange routing table information. With RIP, routers periodically exchange entire tables. Because this is inefficient, RIP is gradually being replaced by a newer protocol called Open Shortest Path First (OSPF).
Routing
(n.) In internetworking, the process of moving a packet of data from source to destination. Routing is usually performed by a dedicated device called a router. Routing is a key feature of the Internet because it enables messages to pass from one computer to another and eventually reach the target machine. Each intermediary computer performs routing by passing along the message to the next computer. Part of this process involves analyzing a routing tableto determine the best path. Routing is often confused with bridging, which performs a similar function. The principal difference between the two is that bridging occurs at a lower level and is therefore more of a hardware function whereas routing occurs at a higher level where the software component is more important. And because routing occurs at a higher level, it can perform more complex analysis to determine the optimal path for the packet.
Interface
(n.)A boundary across which two independent systems meet and act on or communicate with each other. In computer technology, there are several types of interfaces. user interface - the keyboard, mouse, menus of a computer system. The user interface allows the user to communicate with the operating system. Also see GUI. software interface - the languages and codes that the applications use to communicate with each other and with the hardware. hardware interface - the wires, plugs and sockets that hardware devices use to communicate with each other. (v.) To connect with or interact with by means of an interface.
Routers
(row´ter) (n.) A router is a device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers are located at gateways, the places where two or more networks connect. Routers use headers and forwarding tables to determine the best path for forwarding the packets, and they use protocols such as ICMP to communicate with each other and configure the best route between any two hosts. Very little filtering of data is done through routers.
P44. Consider the seven-node network (with nodes labeled t to z) in Problem P26. Show the minimal-cost tree rooted at z that includes (as end hosts) nodes u, v, w, and y. Informally argue why your tree is a minimal-cost tree.
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P20. Suppose datagrams are limited to 1,500 bytes (including header) between source Host A and destination Host B. Assuming a 20-byte IP header, how many datagrams would be required to send an MP3 consisting of 5 million bytes? Explain how you computed your answer.
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P25. Repeat Problem P24 for paths from x to z, z to u, and z to w.
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P26. Consider the following network. With the indicated link costs, use Dijkstra's shortest-path algorithm to compute the shortest path from x to all network nodes. Show how the algorithm works by computing a table similar to
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P42. In Figure 4.42, suppose that there is another stub network V that is a customer of ISP A. Suppose that B and C have a peering relationship, and Ais a customer of both B and C. Suppose that Awould like to have the traffic destined to Wto come from B only, and the traffic destined to V from either B or C. How should A advertise its routes to B and C? What AS routes does C receive?
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P43. Suppose ASs X and Z are not directly connected but instead are connected by AS Y. Further suppose that X has a peering agreement with Y, and that Y has a peering agreement with Z. Finally, suppose that Z wants to transit all of Y's traffic but does not want to transit X's traffic. Does BGP allow Z to implement this policy?
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Broadcast Sequence Number
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Import policy
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Multicast Group
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Neighbor
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P1. In this question, we consider some of the pros and cons of virtual-circuit and datagram networks. a. Suppose that routers were subjected to conditions that might cause them to fail fairly often. Would this argue in favor of a VC or datagram architecture? Why? b. Suppose that a source node and a destination require that a fixed amount of capacity always be available at all routers on the path between the source and destination node, for the exclusive use of traffic flowing between this source and destination node. Would this argue in favor of a VC or datagram architecture? Why? c. Suppose that the links and routers in the network never fail and that routing paths used between all source/destination pairs remains constant. In this scenario, does a VC or datagram architecture have more control traffic overhead? Why?
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P11. Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has the following forwarding table: Prefix Match Interface 00 0 010 1 011 2 10 2 11 3 For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range.
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P12. Consider a datagram network using 8-bit host addresses. Suppose a router uses longest prefix matching and has the following forwarding table: Prefix Match Interface 1 0 10 1 111 2 otherwise 3 For each of the four interfaces, give the associated range of destination host addresses and the number of addresses in the range.
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P13. Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3. Suppose all of the interfaces in each of these three subnets are required to have the prefix 223.1.17/24. Also suppose that Subnet 1 is required to support at least 60 interfaces, Subnet 2 is to support at least 90 interfaces, and Subnet 3 is to support at least 12 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.
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P14. In Section 4.2.2 an example forwarding table (using longest prefix matching) is given. Rewrite this forwarding table using the a.b.c.d/x notation instead of the binary string notation.
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P15. In Problem P10 you are asked to provide a forwarding table (using longest prefix matching). Rewrite this forwarding table using the a.b.c.d/x notation instead of the binary string notation.
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P16. Consider a subnet with prefix 128.119.40.128/26. Give an example of one IP address (of form xxx.xxx.xxx.xxx) that can be assigned to this network. Suppose an ISP owns the block of addresses of the form 128.119.40.64/26. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. What are the prefixes (of form a.b.c.d/x) for the four subnets?
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P17. Consider the topology shown in Figure 4.17. Denote the three subnets with hosts (starting clockwise at 12:00) as Networks A, B, and C. Denote the subnets without hosts as Networks D, E, and F. a. Assign network addresses to each of these six subnets, with the following constraints: All addresses must be allocated from 214.97.254/23; Subnet A should have enough addresses to support 250 interfaces; Subnet B should have enough addresses to support 120 interfaces; and Subnet C should have enough addresses to support 120 interfaces. Of course, subnets D, E and F should each be able to support two interfaces. For each subnet, the assignment should take the form a.b.c.d/x or a.b.c.d/x - e.f.g.h/y. b. Using your answer to part (a), provide the forwarding tables (using longest prefix matching) for each of the three routers.
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P18. Use the whois service at the American Registry for Internet Numbers (http://www.arin.net/whois) to determine the IP address blocks for three universities. Can the whois services be used to determine with certainty the geographical location of a specific IP address? Use www.maxmind.com to determine the locations of the Web servers at each of these universities.
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P19. Consider sending a 2400-byte datagram into a link that has an MTU of 700 bytes. Suppose the original datagram is stamped with the identification number 422. How many fragments are generated? What are the values in the various fields in the IP datagram(s) generated related to fragmentation?
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P2. Consider a virtual-circuit network. Suppose the VC number is an 8-bit field. a. What is the maximum number of virtual circuits that can be carried over a link? b. Suppose a central node determines paths and VC numbers at connection setup. Suppose the same VC number is used on each link along the VC's path. Describe how the central node might determine the VC number at connection setup. Is it possible that there are fewer VCs in progress than the maximum as determined in part (a) yet there is no common free VC number? c. Suppose that different VC numbers are permitted in each link along a VC's path. During connection setup, after an end-to-end path is determined, describe how the links can choose their VC numbers and configure their forwarding tables in a decentralized manner, without reliance on a central node.
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P27. Consider the network shown in Problem P26. Using Dijkstra's algorithm, and showing your work using a table similar to Table 4.3, do the following: a. Compute the shortest path from t to all network nodes. b. Compute the shortest path from u to all network nodes. c. Compute the shortest path from v to all network nodes. d. Compute the shortest path from w to all network nodes. e. Compute the shortest path from y to all network nodes. f. Compute the shortest path from z to all network nodes.
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P28. Consider the network shown below, and assume that each node initially knows the costs to each of its neighbors. Consider the distance-vector algorithm and show the distance table entries at node z.
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P29. Consider a general topology (that is, not the specific network shown above) and a synchronous version of the distance-vector algorithm. Suppose that at each iteration, a node exchanges its distance vectors with its neighbors and receives their distance vectors. Assuming that the algorithm begins with each node knowing only the costs to its immediate neighbors, what is the maximum number of iterations required before the distributed algorithm converges? Justify your answer.
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P3. A bare-bones forwarding table in a VC network has four columns. What is the meaning of the values in each of these columns? A bare-bones forwarding table in a datagram network has two columns. What is the meaning of the values in each of these columns?
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P30. Consider the network fragment shown below. x has only two attached neighbors, w and y. w has a minimum-cost path to destination u (not shown) of 5, and y has a minimum-cost path to u of 6. The complete paths from w and y to u (and between w and y) are not shown. All link costs in the network have strictly positive integer values. a. Give x's distance vector for destinations w, y, and u. b. Give a link-cost change for either c(x,w) or c(x,y) such that x will inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm. c. Give a link-cost change for either c(x,w) or c(x,y) such that x will not inform its neighbors of a new minimum-cost path to u as a result of executing the distance-vector algorithm.
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P31. Consider the three-node topology shown in Figure 4.30. Rather than having the link costs shown in Figure 4.30, the link costs are c(x,y) = 3, c(y,z) = 6, c(z,x) = 4. Compute the distance tables after the initialization step and after each iteration of a synchronous version of the distance-vector algorithm (as we did in our earlier discussion of Figure 4.30).
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P32. Consider the count-to-infinity problem in the distance vector routing. Will the count-to-infinity problem occur if we decrease the cost of a link? Why? How about if we connect two nodes which do not have a link?
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P33. Argue that for the distance-vector algorithm in Figure 4.30, each value in the distance vector D(x) is non-increasing and will eventually stabilize in a finite number of steps.
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P34. Consider Figure 4.31. Suppose there is another router w, connected to router y and z. The costs of all links are given as follows: c(x,y) = 4, c(x,z) = 50, c(y,w) = 1, c(z,w) = 1, c(y,z) = 3. Suppose that poisoned reverse is used in the distance-vector routing algorithm. a. When the distance vector routing is stabilized, router w, y, and z inform their distances to x to each other. What distance values do they tell each other? b. Now suppose that the link cost between x and y increases to 60. Will there be a count-to-infinity problem even if poisoned reverse is used? Why or why not? If there is a count-to-infinity problem, then how many iterations are needed for the distance-vector routing to reach a stable state again? Justify your answer. c. How do you modify c(y,z) such that there is no count-to-infinity problem at all if c(y,x) changes from 4 to 60?
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P35. Describe how loops in paths can be detected in BGP.
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P36. Will a BGP router always choose the loop-free route with the shortest ASpath length? Justify your answer.
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P37. Consider the network shown below. Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Initially suppose there is no physical link between AS2 and AS4. a. Router 3c learns about prefix x from which routing protocol: OSPF, RIP, eBGP, or iBGP? b. Router 3a learns about x from which routing protocol? c. Router 1c learns about x from which routing protocol? d. Router 1d learns about x from which routing protocol?
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P38. Referring to the previous problem, once router 1d learns about x it will put an entry (x, I) in its forwarding table. a. Will I be equal to I1 or I2 for this entry? Explain why in one sentence. b. Now suppose that there is a physical link between AS2 and AS4, shown by the dotted line. Suppose router 1d learns that x is accessible via AS2 as well as via AS3. Will I be set to I1 or I2? Explain why in one sentence. c. Now suppose there is another AS, called AS5, which lies on the path between AS2 and AS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as via AS3 AS4. Will I be set to I1 or I2? Explain why in one sentence.
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P39. Consider the following network. ISP B provides national backbone service to regional ISP A. ISP C provides national backbone service to regional ISP D. Each ISP consists of one AS. B and C peer with each other in two places using BGP. Consider traffic going from A to D. B would prefer to hand that traffic over to C on the West Coast (so that C would have to absorb the cost of carrying the traffic cross-country), while C would prefer to get the traffic via its East Coast peering point with B (so that B would have carried the traffic across the country). What BGP mechanism might C use, so that B would hand over A-to-D traffic at its East Coast peering point? To answer this question, you will need to dig into the BGP specification.
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P40. In Figure 4.42, consider the path information that reaches stub networks W, X, and Y. Based on the information available at Wand X, what are their respective views of the network topology? Justify your answer. The topology view at Y is shown below.
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P41. Consider Figure 4.42. B would never forward traffic destined to Y via X based on BGP routing. But there are some very popular applications for which data packets go to X first and then flow to Y. Identify one such application, and describe how data packets follow a path not given by BGP routing.
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P45. Consider the two basic approaches identified for achieving broadcast, unicast emulation and network-layer (i.e., router-assisted) broadcast, and suppose spanning-tree broadcast is used to achive network-layer broadcast. Consider a single sender and 32 receivers. Suppose the sender is connected to the receivers by a binary tree of routers. What is the cost of sending a broadcast packet, in the cases of unicast emulation and network-layer broadcast, for this topology? Here, each time a packet (or copy of a packet) is sent over a single link, it incurs a unit of cost. What topology for interconnecting the sender, receivers, and routers will bring the cost of unicast emulation and true network-layer broadcast as far apart as possible? You can choose as many routers as you'd like.
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P46. Consider the operation of the reverse path forwarding (RPF) algorithm in Figure 4.44. Using the same topology, find a set of paths from all nodes to the source node A (and indicate these paths in a graph using thicker-shaded lines as in Figure 4.44) such that if these paths were the least-cost paths, then node B would receive a copy of A's broadcast message from nodes A, C, and D under RPF.
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P47. Consider the topology shown in Figure 4.44. Suppose that all links have unit cost and that node E is the broadcast source. Using arrows like those shown in Figure 4.44 indicate links over which packets will be forwarded using RPF, and links over which packets will not be forwarded, given that node E is the source.
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P48. Repeat Problem P47 using the graph from Problem P26. Assume that z is the broadcast source, and that the link costs are as shown in Problem P26.
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P49. Consider the topology shown in Figure 4.46, and suppose that each link has unit cost. Suppose node C is chosen as the center in a center-based multicast routing algorithm. Assuming that each attached router uses its least-cost path to node C to send join messages to C, draw the resulting center-based routing tree. Is the resulting tree a minimum-cost tree? Justify your answer.
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P5. Consider a VC network with a 2-bit field for the VC number. Suppose that link C, and link D. Suppose that each of these links is currently carrying two other virtual circuits, and the VC numbers of these other VCs are as follows: Link A Link B Link C Link D 00 01 10 11 01 10 11 00 In answering the following questions, keep in mind that each of the existing VCs may only be traversing one of the four links. a. If each VC is required to use the same VC number on all links along its path, what VC number could be assigned to the new VC? b. If each VC is permitted to have different VC numbers in the different links along its path (so that forwarding tables must perform VC number translation), how many different combinations of four VC numbers (one for each of the four links) could be used?
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P50. Repeat Problem P49, using the graph from Problem P26. Assume that the center node is v.
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P51. In Section 4.5.1 we studied Dijkstra's link-state routing algorithm for computing the unicast paths that are individually the least-cost paths from the source to all destinations. The union of these paths might be thought of as forming a least-unicast-cost path tree (or a shortest unicast path tree, if all link costs are identical). By constructing a counterexample, show that the least-cost path tree is not always the same as a minimum spanning tree.
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P52. Consider a network in which all nodes are connected to three other nodes. In a single time step, a node can receive all transmitted broadcast packets from its neighbors, duplicate the packets, and send them to all of its neighbors (except to the node that sent a given packet). At the next time step, neighboring nodes can receive, duplicate, and forward these packets, and so on. Suppose that uncontrolled flooding is used to provide broadcast in such a network. At time step t, how many copies of the broadcast packet will be transmitted, assuming that during time step 1, a single broadcast packet is transmitted by the source node to its three neighbors.
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P53. We saw in Section 4.7 that there is no network-layer protocol that can be used to identify the hosts participating in a multicast group. Given this, how can multicast applications learn the identities of the hosts that are participating in a multicast group?
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P54. Design (give a pseudocode description of) an application-level protocol that maintains the host addresses of all hosts participating in a multicast group. Specifically identify the network service (unicast or multicast) that is used by your protocol, and indicate whether your protocol is sending messages in band or out-of-band (with respect to the application data flow among the multicast group participants) and why.
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P55. What is the size of the multicast address space? Suppose now that two multicast groups randomly choose a multicast address. What is the probability that they choose the same address? Suppose now that 1,000 multicast groups are ongoing at the same time and choose their multicast group addresses at random. What is the probability that they interfere with each other?
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P6. In the text we have used the term connection-oriented service to describe a transport-layer service and connection service for a network-layer service. Why the subtle shades in terminology?
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P9. Consider the switch shown below. Suppose that all datagrams have the same fixed length, that the switch operates in a slotted, synchronous manner, and that in one time slot a datagram can be transferred from an input port to an output port. The switch fabric is a crossbar so that at most one datagram can be transferred to a given output port in a time slot, but different output ports can receive datagrams from different input ports in a single time slot. What is the minimal number of time slots needed to transfer the packets shown from input ports to their output ports, assuming any input queue scheduling order you want (i.e., it need not have HOL blocking)? What is the largest number of slots needed, assuming the worst-case scheduling order you can devise, assuming that a non-empty input queue is never idle?
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Prefix
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Prefixes
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R5. Describe some hypothetical services that the network layer can provide to a single packet. Do the same for a flow of packets. Are any of can provide to a single packet. Do the same for a flow of packets. Are any of your hypothetical service provided by the Internet's network layer? Are any provided by ATM's CBR service model? Are any provided by ATM's ABR service model?
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Center-based Approach
...Creating a center based spanning-tree A central node is defined. Then all nodes sends a message at the same time towards the center node until they arrive at ether the center node or a node that's already a part of the tree.
Link-layer Switches
.A network switch (sometimes known as a switching hub) is a computer networking device that is used to connect devices together on a computer network, by using a form of packet switching to forward data to the destination device. A network switch is considered more advanced than a hub because a switch will only forward a message to one or multiple devices that need to receive it, rather than broadcasting the same message out of each of its ports. A network switch is a multi-port network bridge that processes and forwards data at the data link layer (layer 2) of the OSI model. Switches can also incorporate routing in addition to bridging; these switches are commonly known as layer-3 or multilayer switches.[2] Switches exist for various types of networks including Fibre Channel, Asynchronous Transfer Mode, InfiniBand, Ethernet and others. The first Ethernet switch was introduced by Kalpana in 1990.
R13. What is the 32-bit binary equivalent of the IP address 223.1.3.27?
11011111 00000001 00000011 00011100
R33. For each of the three general approaches we studied for broadcast communication (uncontrolled flooding, controlled flooding, and spanning-tree broadcast), are the following statement true or false? You may assume that no packets are lost due to buffer overflow and all packets are delivered on a link in the order in which they were sent. a. A node may receive multiple copies of the same packet. b. A node may forward multiple copies of a packet over the same outgoing link.
32. a) uncontrolled flooding: T; controlled flooding: T; spanning-tree: F b) uncontrolled flooding: T; controlled flooding: F; spanning-tree: F
R15. Suppose there are three routers between a source host and a destination host. Ignoring fragmentation, an IP datagram sent from the source host to the destination host will travel over how many interfaces? How many forwarding tables will be indexed to move the datagram from the source to the destination?
50% overhead
R14. Visit a host that uses DHCP to obtain its IP address, network mask, default router, and IP address of its local DNS server. List these values.
8 interfaces; 3 forwarding tables
First-hop router
A First Hop Redundancy Protocol (FHRP) is a computer networking protocol which is designed to protect the default gateway used on a subnetwork by allowing two or more routers to provide backup for that address; in the event of failure of the/an active router, the backup router will take over the address, usually within a few seconds. In practice, such protocols can also be used to protect other services operating on a single IP address, not just routers.
RIP response message
A RIPv1 Response Message A RIP response message consists of a RIP header, followed by one or more RIP route entries (up to a maximum of 25). Each route entry consists of a 20 byte record that specifies a reachable network along with its metric: RIPv1 Response Message Format Some Things to Note The Command field specifies if this is a RIP request (0x01) or response (0x02). The Version field specifies the RIP version of this message. As you may have guessed, for RIPv1 this has a value of 0x01. The Address Family Identifier (AFI) identifies the type of network address specified - for RIPv1 this always has a value of 0x0002 which corresponds to IPv4 (look for the #define AF_INET in socket.h on a nearby Unix system). All "unused" fields must be zeroed. These unused fields exist due to historical and alignment reasons, however are recycled in RIPv2. All fields are stored in network byte order or big endian form, meaning that there is no need to transpose bytes. Off the Wire The following is a RIPv1 response message captured "off the wire" from a nearby network, minus the link-level header - the IP and UDP encapsulation however remain intact: 45c0 0034 0000 0000 0211 a9fd 0a01 03fc ffff ffff 0208 0208 0020 0000 0201 0000 0002 0000 0a01 0100 0000 0000 0000 0000 0000 0001
Rendezvous Point or core
A Rendezvous Point (RP) is used as a temporary way to connect a would-be multicast receiver to an existing shared multicast tree passing through the rendezvous point. When volume of traffic crosses a threshold, the receiver is joined to a source-specific tree, and the feed through the RP is dropped. You can think of this as obtaining copies of something through a friend who already subscribes, and when it proves useful or interesting, it's worth the bother to become a direct subscriber. So: the scalable way to do multicast is PIM Sparse Mode (PIM-SM), and PIM-SM requires that you have at least one RP.
Backbone
A backbone network or network backbone is a part of computer network infrastructure that interconnects various pieces of network, providing a path for the exchange of information between different LANs or subnetworks.[1] A backbone can tie together diverse networks in the same building, in different buildings in a campus environment, or over wide areas. Normally, the backbone's capacity is greater than the networks connected to it.[2] A Diagram of a Typical Nation-wide network backbone. A large corporation that has many locations may have a backbone network that ties all of the locations together, for example, if a server cluster needs to be accessed by different departments of a company that are located at different geographical locations. The pieces of the network connections (for example: ethernet, wireless) that bring these departments together is often mentioned as network backbone. Network congestion is often taken into consideration while designing backbones.[3][4] One example of a backbone network is the Internet backbone.[5]
Virtual circuits (VCs)
A connection between two devices that acts as though it's a direct connection even though it may physically be circuitous. The term is used most frequently to describe connections between two hosts in a packet-switching network. In this case, the two hosts can communicate as though they have a dedicated connection even though the packets might actually travel very different routes before arriving at their destination. An X.25 connection is an example of a virtual circuit. Virtual circuits can be either permanent (called PVCs) or temporary
Dual-stack
A dual stack network is a network in which all of the nodes are both IPv4 and IPv6 enabled. This is especially important at the router, as the router is typically the first node on a given network to receive traffic from outside of the network. Many experts believe that network infrastructure will shift from IPv4 to IPv6 in order to provide more adress space and serve growing global connecitivity. Dual stack networks are one of the many IPv4 to IPv6 migration strategies that have been presented in recent years.
Forwarding Table
A forwarding information base (FIB), also known as a forwarding table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should forward a packet.
Packet Scheduler
A function at the network protocol level that allocates bandwidth to competing online connections. Residing in the transmitting machine, it determines how many packets are handed to each connection (each flow) at a given time. A packet scheduler makes its determinations by observing the packet flows from the applications or by request from a quality of service (QoS) protocol such as RSVP or Diffserv. See QoS.
Subnet Mask
A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.
Routing Loop
A network problem in which packets continue to be routed in an endless circle. It is caused by a router or line failure, and the notification of the downed link has not yet reached all the other routers. It can also occur over time due to normal growth or when networks are merged together. See route poisoning.
R1. Let's review some of the terminology used in this textbook. Recall that the name of a transport-layer packet is segment and that the name of a link-layer packet is frame. What is the name of a network-layer packet? Recall that both routers and link-layer switches are called packet switches. What is the fundamental difference between a router and link-layer switch? Recall that we use the term routers for both datagram networks and VC networks.
A network-layer packet is a datagram. A router forwards a packet based on the packet's IP (layer 3) address. A link-layer switch forwards a packet based on the packet's MAC (layer 2) address.
Subnet
A portion of a network that shares a common address component. On TCP/IP networks, subnets are defined as all devices whose IP addresses have the same prefix. For example, all devices with IP addresses that start with 100.100.100. would be part of the same subnet. Dividing a network into subnets is useful for both security and performance reasons. IP networks are divided using a subnet mask.
Autonomous system number (ASN)
A public AS has a globally unique number, an ASN, associated with it. This number is used both in the exchange of exterior routing information (between neighboring ASes) and as an identifier of the AS itself. There are two types of ASNs: •Public ASNs •Private ASNs
Connection Reversal
A reverse connection is usually used to bypass firewall restrictions on open ports. A firewall usually blocks open ports, but does not block outgoing traffic. In a normal forward connection, a client connects to a server through the server's open port, but in the case of a reverse connection, the client opens the port that the server connects to. The most common way a reverse connection is used is to bypass firewall and router security restrictions. For example, a backdoor running on a computer behind a firewall that blocks incoming connections can easily open an outbound connection to a remote host on the Internet. Once the connection is established, the remote host can send commands to the backdoor. Remote administration tools (RAT) that use a reverse connection usually send SYN packets to the client's IP address. The client listens for these SYN packets and accepts the desired connections. If a computer is sending SYN packets or is connected to an client's computer, the connections can be discovered by using the netstat command or a common port listener like "Active Ports". If the Internet connection is closed down and an application still tries to connect to remote hosts it may be infected with malware. Keyloggers and other malicious programs are harder to detect once installed, because they connect only once per session. Note that SYN packets by themselves are not necessarily a cause for alarm, as they are a standard part of all TCP connections. There are legitimate uses for using reverse connections, for example to allow hosts behind a NAT firewall to be administered remotely. These hosts do not normally have public IP addresses, and so must either have ports forwarded at the firewall, or open reverse connections to a central administration server.
Broadcast Storm
A state in which a message that has been broadcast across a network results in even more responses, and each response results in still more responses in a snowball effect. A severe broadcast storm can block all other network traffic, resulting in a network meltdown. Broadcast storms can usually be prevented by carefully configuring a network to block illegal broadcast messages.
Stub Network
A stub network has only one default path to non-local hosts and no outside network knowledge. Non-local stub network traffic uses a single logical path when traveling in and out of the network. Stub networks are essentially local area networks (LAN) that either do not connect to the outside and relay data packets internally or are dead-end LANs that know of only one network exit. Stub networks may have multiple connections but use one path to single points of destination.
R29. Define and contrast the following terms: subnet, prefix, and BGP route.
A subnet is a portion of a larger network; a subnet does not contain a router; its boundaries are defined by the router and host interfaces. A prefix is the network portion of a CDIRized address; it is written in the form a.b.c.d/x ; A prefix covers one or more subnets. When a router advertises a prefix across a BGP session, it includes with the prefix a number of BGP attributes. In BGP jargon, a prefix along with its attributes is a BGP route (or simply a route).
R31. Describe how a network administrator of an upper-tier ISP can implement policy when configuring BGP.
A tier-1 ISP B may not to carry transit traffic between two other tier-1 ISPs, say A and C, with which B has peering agreements. To implement this policy, ISP B would not advertise to A routes that pass through C; and would not advertise to C routes that pass through A.
Spanning Tree
Abbreviated STP, a link management protocol that is part of the IEEE 802.1 standard for media access control bridges. Using the spanning tree algorithm, STP provides path redundancy while preventing undesirable loops in a network that are created by multiple active paths between stations. Loops occur when there are alternate routes between hosts. To establish path redundancy, STP creates a tree that spans all of the switches in an extended network, forcing redundant paths into a standby, or blocked, state. STP allows only one active path at a time between any two network devices (this prevents the loops) but establishes the redundant links as a backup if the initial link should fail. If STP costs change, or if one network segment in the STP becomes unreachable, the spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating the standby path. Without spanning tree in place, it is possible that both connections may be simultaneously live, which could result in an endless loop of traffic on the LAN.
Source-Specific Multicast (SSM)
Abbreviated as SSM, a strategic service management is a solution comprised of software, services and knowledge that assist companies in efficiently delivering service commitments. SSM solutions offer comprehensive management, scheduling and planning of service parts and service technicians, the related pricing of such resources, and the real-time, closed-loop management of exceptions that jeopardize a company's capability to meet customer commitments.
Address Aggregation (route aggregation or route summarization).
Address aggregation On the accompanying visual, one can see a small example of the benefits of address aggregation and gain an appreciation for the process that saves so much room in router memory. The example centers upon a network access point (NAP) or large ISP—called NAP2 in the graphic. The has Internet Assigned Numbers Authority (IANA) has seen fit to grant this NAP a large block of 65536 Class C IP networks (all those between and including 199.0.0.0 and 199.255.255.0). Using classless interdomain routing (CIDR) notation, this block of addresses is represented by 199.0.0.0/8, with the "/8" signifying the number of bits in the IP network mask for the CIDR block. This so-called "slash" notation is becoming more common as CIDR makes its way into common parlance, although the block could also be expressed with the mask 255.0.0.0. Address Aggregation Enlarge Address Aggregation Note that the /8 designation overrides any notion of the address's "natural" or "default" mask, which in this case would be 255.255.255.0 or /24. The CIDR designation of the block means that all the other NAPs need to keep track of only one routing table entry for all 64K networks! Their routers need to follow the rule, "If it starts with 199, send it to NAP2 and let that provider figure out what to do with it." NAP2 has up to 255 ISP customers; two are represented by ISP1 and ISP2, each of which is assigned a block of 255 Class C addresses. Although that means there are 65,536 possible networks for routers in NAP2 to track, NAP2 has also used address aggregation to assign its own addresses in blocks to its customers, giving 199.1.0.0/16 to ISP1, 199.2.0.0/16 to ISP2, and so on. Thus all the possible routes that NAP2's routers must maintain can be expressed in 255 routing table entries. Following the example given above, if NAP2 receives a packet destined for 199.2.3.4, it need look only at the first two octets to know that the packet should be forwarded to ISP2. Finally, it is left up to the individual ISPs to determine actual destinations for packets, since they are the only ones connected to the destination networks themselves. Address aggregation saves much memory in routers higher up in the tree, but the actual location of the destinations must be recorded somewhere. What CIDR really does is reduce the amount of redundancy in the routers, without adversely affecting (indeed, perhaps speeding up) the process of finding the best route.
Path
Also referred to as a transmission channel, the path between two nodes of a network that a data communication follows. The term can refer to the physical cabling that connects the nodes on a network, the signal that is communicated over the pathway or a subchannel in a carrier frequency.
Tunnel
An IP tunnel is an Internet Protocol (IP) network communications channel between two networks. It is used to transport another network protocol by encapsulation of its packets. IP tunnels are often used for connecting two disjoint IP networks that don't have a native routing path to each other, via an underlying routable protocol across an intermediate transport network. In conjunction with the IPsec protocol they may be used to create a virtual private network between two or more private networks across a public network such as the Internet. Another prominent use is to connect islands of IPv6 installations across the IPv4 Internet. IP tunnelling encapsulation In IP tunnelling, every IP packet, including addressing information of its source and destination IP networks, is encapsulated within another packet format native to the transit network. At the borders between the source network and the transit network, as well as the transit network and the destination network, gateways are used that establish the end-points of the IP tunnel across the transit network. Thus, the IP tunnel endpoints become native IP routers that establish a standard IP route between the source and destination networks. Packets traversing these end-points from the transit network are stripped from their transit frame format headers and trailers used in the tunnelling protocol and thus converted into native IP format and injected into the IP stack of the tunnel endpoints. In addition, any other protocol encapsulations used during transit, such as IPsec or Transport Layer Security, are removed. IP in IP, sometimes called ipencap, is an example of IP encapsulation within IP and is described in RFC 2003. Other variants of the IP-in-IP variety are IPv6-in-IPv4 (6in4) and IPv4-in-IPv6 (4in6). IP tunneling often bypasses simple firewall rules transparently since the specific nature and addressing of the original datagrams are hidden. Content-control software is usually required to block IP tunnels.
Area Border Routers
An area border router (ABR) is a kind of router that is located near the border between one or more Open Shortest Path First (OSPF) areas. It is used to establish a connection between backbone networks and the OSPF areas. It is a member of both the main backbone network and the specific areas to which it connects, so it stores and maintains separate routing information or routing tables regarding the backbone and the topologies of the area to which it is connected.
Intra-autonomous System Routing Protocol
An intra-AS routing protocol is used to configure and maintain the routing tables within an autonomous system (AS). Once the routing tables are configured, datagrams are routed within the AS as described in the previous section. Inter-AS routing protocols are also known as interior gateway protocols. Historically, three routing protocols have been used extensively for routing within an autonomous system in the Internet: RIP (the Routing Information Protocol), and OSPF (Open Shortest Path First), and IGRP (Cisco's propriety Interior Gateway Routing Protocol).
Connection Setup
Connection Setup automatically configures your mobile network connection to work with your service provider.
Head-of-the-line (HOL) Blocking
Head-of-line blocking (HOL blocking) in computer networking is a performance-limiting phenomenon that occurs when a line of packets is held-up by the first packet, for example in input buffered network switches, out-of-order delivery, and multiple requests in HTTP pipelining.
Inter-AS Routing Protocol
Border Gateway Protocol (BGP) is one of the major routing protocols of the Internet. It is an "external gateway" protocol, meaning that it handles the routing of data between autonomous systems (AS), typically internet service providers (ISP).
Multi-homed Stub Network
As businesses rely more and more on the Internet, having multiple points of connection to the Internet is fast becoming an integral part of their network strategy. Multiple connections, known as multi-homing, reduces the chance of a potentially catastrophic shutdown if one of the connections should fail. In addition to maintaining a reliable connection, multi-homing allows a company to perform load-balancing by lowering the number of computers connecting to the Internet through any single connection. Distributing the load through multiple connections optimizes the performance and can significantly decrease wait times. Multi-homed networks are often connected to several different ISPs (Internet Service Providers). Each ISP assigns an IP address (or range of IP addresses) to the company. Routers use BGP (Border Gateway Protocol), a part of the TCP/IP protocol suite, to route between networks using different protocols. In a multi-homed network, the router utilizes IBGP (Internal Border Gateway Protocol) on the stub domain side, and EBGP (External Border Gateway Protocol) to communicate with other routers. Multi-homing really makes a difference if one of the connections to an ISP fails. As soon as the router assigned to connect to that ISP determines that the connection is down, it will reroute all data through one of the other routers. NAT can be used to facilitate scalable routing for multi-homed, multi-provider connectivity. For more on multi-homing, see Cisco: Enabling Enterprise Multihoming. For lots more information on NAT and related topics, check out the links on the next page.
BGP Attributes
BGP Attributes Routes learned via BGP have associated properties that are used to determine the best route to a destination when multiple paths exist to a particular destination. These properties are referred to as BGP attributes, and an understanding of how BGP attributes influence route selection is required for the design of robust networks. This section describes the attributes that BGP uses in the route selection process: ## Weight ## Local preference ## Multi-exit discriminator ## Origin ## AS_path ## Next hop ## Community
Global Routing Algorithm
Based on how routers gather information about the structure of a network and their analysis of information to specify the best route, we have two major routing algorithms: global routing algorithms and decentralized routing algorithms. In decentralized routing algorithms, each router has information about the routers it is directly connected to -- it doesn't know about every router in the network. These algorithms are also known as DV (distance vector) algorithms. In global routing algorithms, every router has complete information about all other routers in the network and the traffic status of the network. These algorithms are also known as LS (link state) algorithms. We'll discuss LS algorithms in the next section.
Border Gateway Protocol (BGP)
Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information between autonomous systems (AS) on the Internet.
Broadcast Routing
Broadcast Routing Broadcasting: sending a packet to all N receivers . routing updates in LS routing . service/request advertisement in application layer (e.g., Novell) Broadcast algorithm 1: N point-to-point sends . send packet to every destination, point-to-point . wasteful of bandwidth . requires knowledge of all destinations Broadcast algorithm 2: flooding . when node receives a broadcast packet, send it out on every link . node may receive many copies of broadcast packet, hence must be able to detect duplicates
Circuit-Switched Routing Algorithms
Circuit switching (CS) The communication between a source and destination has two phases: circuit establishment phase and message transmission phase. A physical path from the source to the destination is reserved prior to the transmission of data by injecting a routing probe, which contains destination address and some control information. It usually needs more that one phit. Let p be the size of the probe in phits. The probe progresses towards the destination node, reserving physical links as it is transmitted through intermediate routers (see Figure 5(a)). The path is set up after the probe has reached the destination. Then an acknowledgment flit is sent back (see Figure 5(b)). Upon reception of the acknowledgment, the sender transmits the whole message at the full bandwidth of the path. The path froms a HW circuit that is reserved for the whole time of the message transmission (see Figure 5(c)). The circuit is released either by destination node or by the last bits of the message.
Classful Addressing
Classful addressing, formally adopted as part of the Internet Protocol (IP) in RFC 791, was the Internet's first major addressing scheme. The IP address was 32 bits in size, just as today, but was managed considerably differently. There were three address classes to chose from: A, B, or C, corresponding to 8-bit, 16-bit, or 24-bit prefixes. No other prefix lengths were allowed, and there was no concept of nesting a group of 24-bit prefixes, for example, within a 16-bit prefix. An address was slotted into one of three address classes based on its high-order bits. Addresses beginning with 0 were considered class A; addresses beginning 10 were class B; addresses beginning 110 class C. Two other classes were also defined, class D addresses beginning 1110 and class E addresses beginning 1111, though neither of these two address classes were normally used. For humans, the easiest way to distinguish between different address classes is to use the first decimal number in the IP address: First octet Address Class 0-127 Class A 128-191 Class B 192-223 Class C 224-239 Class D 240-255 Class E For example, 128.8.74.1 is a Class B address because the first octet, 128, lies in the 128-191 range. Likewise, 10.10.191.1 is a Class A address (because the first octet is 10) and 208.130.29.33 is a Class C (because the first octet is 208). If this seems at all confusing, convert these addresses into binary and verify for yourselves that the initial bits correspond to the pattern shown in the diagram. Upon installing a new Internet connection, the network engineer would request a Class A, B, or C network, depending on the expected size of the installed network. For example, the U.S. Department of Defense, a very large network, was assigned a Class A; the University of Maryland, a typical mid-sized network, was assigned a Class B network; and a small consulting firm I once worked for was assigned a Class C network. The Internet Assigned Numbers Authority (IANA) oversaw all classful network assignments. Only the network bits were assigned by IANA. For example, a request for a Class C network might have been met by assigning 192.17.34.0. As a Class C, the first three bytes were fixed by IANA, and the last byte was assigned by the local network administrator. No attempt was made to assign the addresses in a hierarchical fashion. The first Class B assigned was 128.1.0.0, the next was 128.2.0.0, and so on. Routers processed packets according to their classful network. For example, consider a packet addressed to 130.17.44.2. First, the address is determined to be a Class B (its two high bits are 10), then split to determine its membership in the 130.17.0.0 classful network. The routing table would have an entry for each classful network, in this case 130.17.0.0, which would determine how the packet should be delivered.
Anycast Address
Communication that takes place over a network between a single sender and the nearest of a group of receivers. Anycast is used in IPv6 as a method of updating routing tables. One host initiates an update of a router table for a group of hosts, sending the data to the nearest host. That host then sends the message on to its nearest router until all the routing tables in that group are updated.
Datagram Network
Datagram Networks In Datagram networks, also called Connectionless Networks, the communication between the two sites is on a one-off basis; the packet contains the full addressing information needed to transmit it. If a site sends two datagrams in quick succession to the same address they could arrive in reverse order for the intervening nodes are allowed to change the exact path through the network depending on traffic flows and to bypass faulty nodes. An analogy is the postal service; each packet is a separate letter.
Datagram networks
Datagram Networks Two basic approaches to packet switching are common: The most common is datagram switching (also known as a "best-effort network" or a network supporting the connection-less network service).This is what is used in the network layer of the Internet. Datagram Packet Networks Datagram transmission uses a different scheme to determine the route through the network of links. Using datagram transmission, each packet is treated as a separate entity and contains a header with the full information about the intended recipient. The intermediate nodes examine the header of a packet and select an appropriate link to an intermediate node which is nearer the destination. In this system, the packets do not follow a pre-established route, and the intermediate nodes (usually known as "routers") do not require prior knowledge of the routes that will be used. A datagram network is analogous to sending a message as a series of postcards through the postal system. Each card is independently sent to the final destination (using the postal system). To receive the whole message, the receiver must collect all the postcards and sort them into the original order. Not all postcards need be delivered by the postal system, and not all take the same length of time to arrive. In a datagram network delivery is not guaranteed (although they are usually reliably sent). Enhancements, if required, to the basic service (e.g. reliable delivery) must be provided by the end systems (i.e. user's computers) using additional software. The most common datagram network is the Internet which uses the IP network protocol. Applications which do not require more than a best effort service can be supported by direct use of packets in a datagram network (using the User Datagram Protocol (UDP) transport protocol). Such applications include Internet Video, Voice Communication, messages notifying a user that she/he has received new email, etc. Most Internet applications need additional functions to provide reliable communication (such as end-to-end error and sequence control). Examples include sending email, browsing a web site, or sending a file using the file transfer protocol (ftp). This reliability ensures all the data is received in the correct order with no duplication or omissions. It is provided by additional layers of software algorithms implemented in the End Systems (A,D). Two examples of this are the Transmission Control Protocol (TCP), and the Trivial File Transfer Protocol (TFTP) which uses UDP. One merit of the datagram approach is that not all packets need to follow the same path (route) through the network (although frequently packets do follow the same route). This removes the need to set-up and tear-down the path, reducing the processing overhead, and a need for Intermediate Systems to execute an additional protocol. Packets may also be routed around busy parts of the network when alternate paths exist. This is useful when a particular intermediate system becomes busy or overloaded with excessive volumes of packets to send. It can also provide a high degree of fault tolerance, when an individual intermediate system or communication circuit fails. As long as a route exists through the network between two end systems, they are able to communicate. Only if there is no possible way to send the packets, will the packets be discarded and not delivered. The fate (success/failure) of an application therefore depends only on existance of an actual path between the two End Systems (ESs). This is known as "fate sharing" - since the application shares the "fate" of the network. There is another type of network known as a virtual circuit network. This has some advantages in particular scenarios. Although it is not covered as a part of the course, there is a web page comparing the two approaches.
Decentralized Routing Algorithm
Decentralized routing algorithms - No node has complete information about the costs of all links - A node initially knows only its direct links - Iterative process: calculate & exchange info with neighbors • Eventually calculate the least-cost path to a destination
P10. Consider a datagram network using 32-bit host addresses. Suppose a router has four links, numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows: Destination Address Range Link Interface 11100000 00000000 00000000 00000000 through 0 11100000 00111111 11111111 11111111 11100000 01000000 00000000 00000000 through 1 11100000 01000000 11111111 11111111 11100000 01000001 00000000 00000000 through 2 11100001 01111111 11111111 11111111 otherwise 3 a. Provide a forwarding table that has five entries, uses longest prefix matching, and forwards packets to the correct link interfaces. b. Describe how your forwarding table determines the appropriate link interface for datagrams with destination addresses: 11001000 10010001 01010001 01010101 11100001 01000000 11000011 00111100 11100001 10000000 00010001 01110111
Destination Address Range Link Interface 00000000 through 0 00111111 01000000 through 1 01111111 10000000 through 2 10111111 11000000 through 3 11111111 number of addresses in each range =
Shortest Path
Dijkstra's algorithm (note* there are different kinds of dijkstra's implementation) and growth graph algorithm
Dotted-Decimal Notation
Dot-decimal notation is a presentation format for numerical data. It consists of a string of decimal numbers, each pair separated by a full stop (dot). A common use of dot-decimal notation is in information technology where it is a method of writing numbers in octet-grouped base-10 (decimal) numbers separated by dots (full stops). In computer networking, Internet Protocol Version 4 addresses are commonly written using the quad-dotted notation of four decimal integers, ranging from 0 to 255 each.
DHCP Simplifies Network Administration
Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task. This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. Many ISPs use dynamic IP addressing for Internet subscribers.
Dynamic Routing Algorithms
Dynamic routing algorithms - Change routing paths as network traffic loads or topology change
temporary IP address
Establishing Temporary IP Connectivity Between the Management Host and Array Controllers In order to assign IP addresses to the controllers, you must establish temporary IP connectivity between the management host and Ethernet port 1 of each controller. There are two methods by which to do that, depending on the method by which the management host and controller's Ethernet ports are physically connected to the Ethernet, and the availability of an Ethernet interface on the management host. The two methods of establishing temporary IP connectivity are as follows: Assigning a temporary IP address to a management host Ethernet interface in the same subnet as the default IP addresses of the controller's Ethernet ports (for example, IP address 192.168.128.100). Use this method if the following conditions are true: You have an available Ethernet interface on the management host or you can temporarily reassign the IP address of an Ethernet interface on the management host. Ethernet port 1 of each controller can be directly connected to an Ethernet interface on the management host by an Ethernet crossover cable, or Ethernet port 1 of each controller and an Ethernet interface of the management host are connected to the same Ethernet hub. For information on changing the IP address of an Ethernet interface on the management host, see Configuring the IP Address of the Management Host. Creating a temporary virtual subnet on the management host. Use this method if there is not an available Ethernet interface on the management host or if Ethernet port 1 of each controller is connected to a subnet on the local area network (LAN) that is not the subnet of the management host. For information on creating a temporary virtual subnet on the management host, see Creating a Temporary Virtual Subnet on a Management Host.
Best-Effort Service
Evolution of the Internet Service Model Traditionally, the Best Effort Internet has provided the worst possible service: packets are forwarded by routers solely on the basis that there is any known route, irrespective of traffic conditions along that route. Routers that are overloaded discard packets, typically dropping packets at the tail of the queue of those awaiting to depart along their way. Other types of digital networks have been built, most notably, for wide public access, the digital telephone network, with user access based on the narrow band Integrated Services Digital Network architecture. This is in fact, the Fixed Effort ISDN, which gives you a constant data rate from source to sink, irrespective of whether you have something ready to send at any moment or not (or whether you have something that needs to be sent at the offered rates!). More recently, we have seen the evolution of both of these network architectures towards more flexible support for multiple service categories. Multiservice IP and Broadband ISDN, provided by ATM are both being redesigned from the ground up to cater for actual perceived multimedia application requirements. To this end, the notion of Traffic Classes, each of which have a range of parameters (usually known as Quality of Service parameters, even though they are quantitative) have been designed. In the ITU and ATM Forum, these are called bearer service classes, and in the IP Integrated Services Internet work, they are Flow Classes.
Prefix
Extended-Network-Prefix For enhancing subnetting Extended-Network-Prefix is introduced, where to the default subnet mask (class A - 255.0.0.0, Class B - 255.255.0.0, Class C - 255.255.255.0) some more 1bits is used further contiguous 1bits and the length of network address increase. This idea is to divide the standard classful host-number field into two parts - the subnet-number and the host number on that subnet. It is three level hierarchy instead of two level hierarchy. To calculate the number of subnets or hosts, use the formula (2^n-2) where n = number of bits in either field, and 2^n represents 2 raised to the nth power. Multiplying the number of subnets by the number of hosts available per subnet gives you the total number of nodes available for your class and subnet mask. For example read the post Subnet Design.
R34. When a host joins a multicast group, must it change its IP address to that of the multicast group it is joining?
False
Flooding
Flooding is a Denial of Service (DoS) attack that is designed to bring a network or service down by flooding it with large amounts of traffic. Flood attacks occur when a network or service becomes so weighed down with packets initiating incomplete connection requests that it can no longer process genuine connection requests. By flooding a server or host with connections that cannot be completed, the flood attack eventually fills the host��s memory buffer. Once this buffer is full no further connections can be made, and the result is a Denial of Service.
Forwarding Function
Forward is an option found in an e-mail client, which allows you to forward a received e-mail to another recipient. The e-mail will be sent with the body of the e-mail showing as "quoted text". Recipients of a forwarded e-mail can usually tell the e-mail is a forward. Some clients will place the letters "FWD" in front of the Subject field.
Router Forwarding Plane
Forwarding data planes typically come either centralized or distributed. This means the forwaring engine is either centrally located across the ethernet fabric/crossbar or pushed all the way to the edge. The more performance required the more that distributed forwarding is pushed to the edge. How we design networks is packet forwarding engine centralized decentralized
R3. What is the difference between routing and forwarding?
Forwarding is about moving a packet from a router's input link to the appropriate output link. Routing is about determining the end-to-routes between sources and destinations.
Minimum Spanning Tree
Given a connected, undirected graph, a spanning tree of that graph is a subgraph that is a tree and connects all the vertices together. A single graph can have many different spanning trees. We can also assign a weight to each edge, which is a number representing how unfavorable it is, and use this to assign a weight to a spanning tree by computing the sum of the weights of the edges in that spanning tree. A minimum spanning tree (MST) or minimum weight spanning tree is then a spanning tree with weight less than or equal to the weight of every other spanning tree. More generally, any undirected graph (not necessarily connected) has a minimum spanning forest, which is a union of minimum spanning trees for its connected components. One example would be a telecommunications company laying cable to a new neighborhood. If it is constrained to bury the cable only along certain paths, then there would be a graph representing which points are connected by those paths. Some of those paths might be more expensive, because they are longer, or require the cable to be buried deeper; these paths would be represented by edges with larger weights. A spanning tree for that graph would be a subset of those paths that has no cycles but still connects to every house. There might be several spanning trees possible. A minimum spanning tree would be one with the lowest total cost.
Graph
Graph G = (N, E) N = set of nodes (routers) E = set of edges (links) Each edge = pair of nodes in N Node y is a neighbor of node x if (x, y) ∈ E
R11. What is HOL blocking? Does it occur in input ports or output port?
HOL blocking - a queued packet in an input queue must wait for transfer through the fabric because it is blocked by another packet at the head of the line. It occurs at the input port.
Hot-Potato Routing
Hot-potato routing is the normal behavior of most settlement-free peering agreements. Hot-potato routing has the effect that the network receiving the data bears the cost of carrying it between cities. When the traffic ratio (the ratio of traffic flowing in one direction to the traffic flowing in the other direction between peers) is reasonably even, this is considered fair, because the networks will share evenly in carrying traffic exchanged by their customers between cities. The marginal cost of carrying traffic between cities depends on how the network has purchased those links; some networks own dark fiber, which can be upgraded by merely replacing the equipment on each end of the fiber, and possibly the amplifiers along the path between cities. In other cases, the network has an agreement with a telco that allows for a specific amount of bandwidth, and upgrading involves paying more money to the telco.
R35. What are the roles played by IGMP protocol and a wide-area multicast routing protocol?
IGMP is a protocol run only between the host and its first-hop multicast router. IGMP allows a host to specify (to the first-hop multicast router) the multicast group it wants to join. It is then up to the multicast router to work with other multicast routers (i.e., run a multicast routing protocol) to ensure that the data for the host-joined multicast group is routed to the appropriate last-hop router and from there to the host.
R28. Why are policy considerations as important for intra-AS protocols, such as OSPF and RIP, as they are for an inter-AS routing protocol like BGP?
ISP C can use the BGP Multi-Exit Descriptor to suggest to ISP B that the preferred route to ISP D is through the east coast peering point. For example, the east coast BGP router in ISP C can advertise a route to D with an MED value of 5. The west coast router in ISP C can advertise a route to D with an MED value of 10. Since a lower value is preferred, ISP B knows that ISP C wants to receive traffic on the east coast. In practice, a router can ignore the MED value, and so ISP B can still use hot potato routing to pass traffic to ISP C destined to ISP D via the west coast peering point.
Active Queue Management (AQM)
In Internet routers, active queue management (AQM) is the arbitrary reorder or drop of network packets inside the transmit buffer of a network interface controller. The task is performed by the network scheduler.
Virtual-circuit (VC) networks
In a Virtual Circuit network, also called a Connection Network, the sender first attempts to create a "virtual circuit" to the receiver. If successful the communication is assigned a virtual circuit number in each of the participating nodes and then each packet's address is just this virtual circuit data. Unlike a real circuit switch exchange there is no resource, other than some space reserved in circuit tables, that is reserved for the duration of the communication. Note: the terms``sender'' and ``receiver'' apply only to the process of establishing the virtual circuit; once set up, data can flow in either direction. An analogy is a phone network. Dialing the number sets up the call establishing a circuit through all intervening phone exchanges. After that all data passes over this circuit.
R36. What is the difference between a group-shared tree and a source-based tree in the context of multicast routing?
In a group-shared tree, all senders send their multicast traffic using the same routing tree. With source-based tree, the multicast datagrams from a given source are routed over s specific routing tree constructed for that source; thus each source may have a different source-based tree and a router may have to keep track of several source-based tress for a given multicast group.
Connection State Information
In a virtual circuit network, information exchange occurs in a dedicated path between the source and the destination. Each active path is given some Virtual Circuit number on the fly. The routers in this path need to keep information about this path and to which interface the data, coming from a particular interface, has to be redirected. This information is termed as connection state information. Whenever a new connection is established, this 'connection state information' has to be updated in router's table, and whenever the connection completes exchanging data this 'connection state information' has to be discarded. In short, connection state information is the information required/maintained by a router to redirect data, to proper interface, in an active connection.
Distance-Vector (DV)
In computer communication theory relating to packet-switched networks, a distance-vector routing protocol is one of the two major classes of routing protocols, the other major class being the link-state protocol. Distance-vector routing protocols use the Bellman-Ford algorithm, Ford-Fulkerson algorithm, or DUAL FSM (in the case of Cisco Systems's protocols) to calculate paths. A distance-vector routing protocol requires that a router informs its neighbors of topology changes periodically. Compared to link-state protocols, which require a router to inform all the nodes in a network of topology changes, distance-vector routing protocols have less computational complexity and message overhead.[citation needed] The term distance vector refers to the fact that the protocol manipulates vectors (arrays) of distances to other nodes in the network. The vector distance algorithm was the original ARPANET routing algorithm and was also used in the internet under the name of RIP (Routing Information Protocol). Examples of distance-vector routing protocols include RIPv1 and RIPv2 and IGRP.
Routing Table
In computer networking a routing table, or routing information base (RIB), is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes.
Routing Tables
In computer networking a routing table, or routing information base (RIB), is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. The routing table contains information about the topology of the network immediately around it. The construction of routing tables is the primary goal of routing protocols. Static routes are entries made in a routing table by non-automatic means and which are fixed rather than being the result of some network topology "discovery" procedure.
NAT Translation Table
In computer networking, network address translation (NAT) provides a method[1] of modifying network address information in Internet Protocol (IP) datagram packet headers while they are in transit across a traffic routing device for the purpose of remapping one IP address space into another. The term NAT44 is sometimes used to more specifically indicate mapping between two IPv4 addresses; this is the typical case while IPv4 carries the majority of traffic on the Internet. NAT64 refers to the mapping of an IPv4 address to an IPv6 address, or vice versa.
Source Router
In computer networking, source routing allows a sender of a packet to partially or completely specify the route the packet takes through the network. In contrast, in non-source routing protocols, routers in the network determine the path based on the packet's destination. Source routing allows easier troubleshooting, improved traceroute, and enables a node to discover all the possible routes to a host. It does not allow a source to directly manage network performance by forcing packets to travel over one path to prevent congestion on another. In the Internet Protocol, two header options are available which are rarely used: "strict source and record route" (SSRR) and "loose source and record route" (LSRR). Because of security concerns, packets marked LSRR are frequently blocked on the Internet. If not blocked, LSRR can allow an attacker to spoof its address but still successfully receive response packets.[1] Policy-based routing can also be used to route packets using their source addresses. Software Defined Networking can also be enhanced when source routing is used in the forwarding plane. Studies have shown significant improvements in convergence times as a result of the reduced state that must be distributed by the controller into the network.
Soft State
In computer science, soft state is state which is useful for efficiency, but not essential, as it can be regenerated or replaced if needed. The term is often used in network protocol engineering. It is a term that is used for 'information that times out (goes away) unless refreshed. While in general less efficient than well-designed "hard state" protocols when tuned for a particular network regime, soft state protocols behave much better than hard state protocols in an unpredictable network environment such as the Internet.[1]
Flow
In desktop publishing, to insert a body of text into a document such that it wraps (or flows) around any objects on the page.
Forwarding
In home networking, port forwarding, also called port mapping or punch-through, enables you to create a permanent translation entry that maps a protocol port on your gateway machine to an IP address and protocol port on your private LAN. It's a transparent process, meaning network clients cannot see that port forwarding is being done. This process enables you to run a public Internet service on a machine that is otherwise hidden from the Internet by your gateway. Port forwarding may also be used to aggregate traffic from an application that uses several ports for transactions and consolidate it into one port for reporting the total traffic identified with that application.
DHCP request message
In response to the DHCP offer, the client replies with a DHCP request, broadcast to the server, requesting the offered address. A client can receive DHCP offers from multiple servers, but it will accept only one DHCP offer. Based on required server identification option in the request and broadcast messaging, servers are informed whose offer the client has accepted.[5]:Section 3.1, Item 3 When other DHCP servers receive this message, they withdraw any offers that they might have made to the client and return the offered address to the pool of available addresses.
Router Control Plane
In routing, the control plane is the part of the router architecture that is concerned with drawing the network map, or the information in a (possibly augmented) routing table that defines what to do with incoming packets. Control plane functions, such as participating in routing protocols, run in the architectural control element.[1] In most cases, the routing table contains a list of destination addresses and the outgoing interface(s) associated with them. Control plane logic also can define certain packets to be discarded, as well as preferential treatment of certain packets for which a high quality of service is defined by such mechanisms as differentiated services. Depending on the specific router implementation, there may be a separate forwarding information base that is populated (i.e., loaded) by the control plane, but used by the forwarding plane to look up packets, at very high speed, and decide how to handle them.
Destination Router
In telecommunications, destination routing is a process that defines a sequential pathway that messages must pass through to reach a target destination.
IS-IS
Intermediate System-Intermediate System is an International Organization for Standardization (ISO) hierarchical routing protocol where IS routers exchange routing information based on a single metric to determine network topology. IS-IS is often used in IP networks.
R21. Compare and contrast link state and distance-vector routing algorithms.
Key: 1.Link-state algorithms is a global routing algorithm, and it requires each node to know the cost of each link in the network. Also, whenever a link cost changes, new link cost must be sent to all nodes. Distance-vector algorithm is a decentralized routing algorithm, and it requires message exchanges between directly connected Neighbors at each iteration. When link costs change ,the DV algorithm will propagate the results of the changed link only if the new link cost result in a changed least-cost path for one of the nodes attached to the link. 2.LS is an O(n^2)algorithm requiring O(nE)messages ,and that it potentially suffers from oscillations. The DV algorithm can converge slowly and have routing loops .DV also suffer from the count-to-infinity problem. 3. When a router fails, under LS, a router could broadcast an incorrect cost for one of its attached links; under DV, a node can advertise incorrect least-cost paths to any/all destinations.
Link-State (LS) Algorithms
LS Algorithms In LS algorithms, every router has to follow these steps: Identify the routers that are physically connected to them and get their IP addresses When a router starts working, it first sends a "HELLO" packet over network. Each router that receives this packet replies with a message that contains its IP address. Measure the delay time (or any other important parameters of the network, such as average traffic) for neighbor routers In order to do that, routers send echo packets over the network. Every router that receives these packets replies with an echo reply packet. By dividing round trip time by 2, routers can count the delay time. (Round trip time is a measure of the current delay on a network, found by timing a packet bounced off some remote host.) Note that this time includes both transmission and processing times -- the time it takes the packets to reach the destination and the time it takes the receiver to process it and reply. Broadcast its information over the network for other routers and receive the other routers' information In this step, all routers share their knowledge and broadcast their information to each other. In this way, every router can know the structure and status of the network. Using an appropriate algorithm, identify the best route between two nodes of the network In this step, routers choose the best route to every node. They do this using an algorithm, such as the Dijkstra shortest path algorithm. In this algorithm, a router, based on information that has been collected from other routers, builds a graph of the network. This graph shows the location of routers in the network and their links to each other. Every link is labeled with a number called the weight or cost. This number is a function of delay time, average traffic, and sometimes simply the number of hops between nodes. For example, if there are two links between a node and a destination, the router chooses the link with the lowest weight
Least-Cost Path
Least-cost path analysis If the shortest path between any two points is a straight line, then the least-cost path is the path of least resistance. Least-cost path analyses use the cost weighted distance and direction surfaces for an area to determine a cost-effective route between a source and a destination. For example, you can use least-cost path analysis to find the cheapest route for building a pipeline or the quickest way to a set of observation points. In a least-cost path analysis, the eight neighbors of a cell are evaluated and the path moves to the cell with the smallest accumulated value. The process repeats itself until the source and destination are connected. The completed path represents the smallest sum of cell values between the two points. The least-cost path can travel through cells in both orthogonal and diagonal directions. Any combination of sources and destinations can be part of a least-cost path analysis. For example, you can find the least-cost path from one source to many destinations, or from many sources to a single destination.
Link-State Broadcast
Link State Update packets are OSPF packet type 4. These packets implement the flooding of link state advertisements. Each Link State Update packet carries a collection of link state advertisements one hop further from its origin. Several link-state advertisement may be included in a single packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded advertisements are acknowledged in Link State Acknowledgment packets. If retransmission of certain advertisements is necessary, the retransmitted advertisements are always carried by unicast Link State Update packets.
R20. It has been said that when IPv6 tunnels through IPv4 router, IPv6 treats the IPv4 tunnels as link layer protocols. Do you agree with this statement? Why or why not?
Link state algorithms: Computes the least-cost path between source and destination using complete, global knowledge about the network. Distance-vector routing: The calculation of the least-cost path is carried out in an iterative, distributed manner. A node only knows the neighbor to which it should forward a packet in order to reach given destination along the least-cost path, and the cost of that path from itself to the destination
Load-Insensitive
Load-insensitive algorithms - Ignore current or recent levels of congestion
Load-Sensitive Algorithms
Load-sensitive algorithms - Link costs vary to reflect the current level of congestion
Longest prefix matching rule
Longest prefix match (also called Maximum prefix length match) refers to an algorithm used by routers in Internet Protocol (IP) networking to select an entry from a routing table .[1] Because each entry in a routing table may specify a network, one destination address may match more than one routing table entry. The most specific of the matching table entries — the one with the highest subnet mask — is called the longest prefix match. It is called this because it is also the entry where the largest number of leading address bits of the destination address match those in the table entry. For example, consider this IPv4 routing table (CIDR notation is used): 192.168.20.16/28 192.168.0.0/16 When the address 192.168.20.19 needs to be looked up, both entries in the routing table "match". That is, both entries contain the looked up address. In this case, the longest prefix of the candidate routes is 192.168.20.16/28, since its subnet mask (/28) is higher than the other entry's mask (/16), making the route more specific. Routing tables often contain a default route, which has the shortest possible prefix match, to fall back on in case matches with all other entries fail.
Signaling Message
MTP3 processes all incoming MSUs to determine whether they should be sent to one of the MTP3 users or routed to another destination. The term "MTP3 user" refers to any user of MTP3 services, as indicated by the Service Indicator in the SIO. This includes messages generated by MTP3 itself, such as SNM, or those that are passed down from the User Parts at level 4 of the SS7 protocol, like ISUP and SCCP. The term "MTP User Part" is also used, but more specifically refers to the User Parts at level 4. When a node generates an MSU, MTP3 is responsible for determining how to route the message toward its destination using the DPC in the Routing Label and the Network Indicator in the SIO. Figure 7-8 shows how MTP3 message processing can be divided into three discrete functions: discrimination, distribution, and routing.
Multicast Routing Problem
Multicast Routing GOAL: deliver packet from one sender to many (but not all) other hosts . deliver to M hosts in N-host network (M<N) . option 1: sender establishes M point-to-point connections . option 2: sender sends one packet, which is duplicated and forwarded, as needed by routers: u router A duplicates packet u router B selectively forwards
R32. What is an important difference between implementing the broadcast abstraction via multiple unicasts, and a single network - (router-) supported broadcast?
N-way unicast has a number of drawbacks, including: • Efficiency: multiple copies of the same packet are sent over the same link for potentially many links; source must generate multiple copies of same packet • Addressing: the source must discover the address of all the recipients
R23. Is it necessary that every autonomous system use the same intra-AS routing algorithm? Why or why not?
No. Each AS has administrative autonomy for routing within an AS.
R24. Consider Figure 4.37. Starting with the original table in D, suppose that D receives from A the following advertisement: Destination Subnet Next Router Number of Hops to Destination z c 10 w __ 1 x __ 1 . . . . . . . . . Will the table in A change? If so how?
No. The advertisement tells D that it can get to z in 11 hops by way of A. However, D can already get to z by way of B in 7 hops. Therefore, there is no need to modify the entry for z in the table. If, on the other hand, the advertisement said that A were only 4 hops away from z by way of C, then D would indeed modify its forwarding table.
R10. Describe how packet loss can occur at input ports. Describe how packet loss at input ports can be eliminated (without using infinite buffers).
Packet loss can occur if the queue size at the output port grows large because of slow outgoing line-speed.
R9. Describe how packet loss can occur at input ports. Describe how packet loss at input ports can be eliminated (without using infinite buffers).
Packet loss occurs if queue size at the input port grows large because of slow switching fabric speed and thus exhausting router's buffer space. It can be eliminated if the switching fabric speed is at least n times as fast as the input line speed, where n is the number of input ports.
NAT Traversal
Passing through network address translation (NAT) to reach a user. NAT hides private IP addresses from the public Internet; however, voice over IP (VoIP) and videoconferencing calls that originate from outside the network must locate the user's IP address. See STUN, UPnP and NAT.
Protocol-Independent Multicast (PIM) Routing Protocol
Protocol-independent multicast (PIM) is a set of four specifications that define modes of Internet multicasting to allow one-to-many and many-to-many transmission of information... (Continued) Protocol-independent multicast (PIM) is a set of four specifications that define modes of Internet multicasting to allow one-to-many and many-to-many transmission of information. The four modes are: •sparse mode (SM) •dense mode (DM) •source-specific multicast (SSM) •bidirectional. The most common mode in PIM is the sparse mode. It is used for transmission of data to nodes in multiple Internet domains, where it is expected that only a small proportion of the potential nodes will actually subscribe. Dense mode, in contrast to sparse mode, is used when it is expected that a large proportion of the potential nodes will subscribe to the multicast. In source-specific multicast, paths (also called trees) originate (or are rooted) at a single, defined source, whereas bidirectional PIM is not source-specific. The term "protocol independent" means that PIM can function by making use of routing information supplied by a variety of communications protocols. In information technology, a protocol is a defined set of rules that end points in a circuit or network employ to facilitate communication.
RIP advertisements
RIP advertisements. ❒ Distance vectors: exchanged among neighbors every 30 sec via Response. Message (also called advertisement). ❒ Each advertisement: ....
Random Early Detection (RED)
Random early detection (RED), also known as random early discard or random early drop is an queueing discipline for a network scheduler suited for congestion avoidance.[1] In the conventional tail drop algorithm, a router or other network component buffers as many packets as it can, and simply drops the ones it cannot buffer. If buffers are constantly full, the network is congested. Tail drop distributes buffer space unfairly among traffic flows. Tail drop can also lead to TCP global synchronization as all TCP connections "hold back" simultaneously, and then step forward simultaneously. Networks become under-utilized and flooded by turns. RED addresses these issues.
Realm
Realm-Specific IP is an Experimental IETF framework and protocol intended as an alternative to NAT in which the end-to-end integrity of packets is maintained. RSIP lets a host borrow one or more IP addresses (and UDP/TCP port) from one or more RSIP gateways, by leasing (usually public) IP addresses and ports to RSIP hosts located in other (usually private) addressing realms. The RSIP client requests registration with an RSIP gateway. The gateway in turn delivers either a unique IP address or a shared IP address and a unique set of TCP/UDP ports and associates the RSIP host address to this address. The RSIP host uses this address to send packets to destinations in the other realm. The tunnelled packets between RSIP host and gateway contain both addresses, and the RSIP gateway strips off the host address header and sends the packet to the destination. RSIP can also be used to relay traffic between several different privately addressed networks by leasing several different addresses to reach different destination networks. RSIP should be useful for NAT traversal as an IETF standard alternative to Universal Plug and Play (UPnP). As of November 2004, the protocol was in the experimental stage and was not yet in widespread use. IETF activities: RFC 3103 Realm Specific IP: Protocol Specification
plug-and-play protocol
Refers to the ability of a computer system to automatically configure expansion boards and other devices. You should be able to plug in a device and play with it, without worrying about setting DIP switches, jumpers, and other configuration elements. Since the introduction of the NuBus, the Apple Macintosh has been a plug-and-play computer.
Reverse Path Forwarding (RPF)
Reverse path forwarding (RPF) is a technique used in modern routers for the purposes of ensuring loop-free forwarding of multicast packets in multicast routing and to help prevent IP address spoofing in unicast routing.
R22. Discuss how a hierarchical organization of the Internet has made it possible to scale to millions of users.
Routers are aggregated into autonomous systems (ASs). Within an AS, all routers run the same intra-AS routing protocol. Special gateway routers in the various ASs run the inter-autonomous system routing protocol that determines the routing paths among the ASs. The problem of scale is solved since an intra-AS router need only know about routers within its AS and the gateway router(s) in its AS.
Routing Algorithms
Routers use routing algorithms to find the best route to a destination. When we say "best route," we consider parameters like the number of hops (the trip a packet takes from one router or intermediate point to another in the network), time delay and communication cost of packet transmission. Based on how routers gather information about the structure of a network and their analysis of information to specify the best route, we have two major routing algorithms: global routing algorithms and decentralized routing algorithms. In decentralized routing algorithms, each router has information about the routers it is directly connected to -- it doesn't know about every router in the network. These algorithms are also known as DV (distance vector) algorithms. In global routing algorithms, every router has complete information about all other routers in the network and the traffic status of the network. These algorithms are also known as LS (link state) algorithms. We'll discuss LS algorithms in the next
R30. How does BGP use the NEXT-HOP attribute? How does it use the AS-PATH attribute?
Routers use the AS-PATH attribute to detect and prevent looping advertisements; they also use it in choosing among multiple paths to the same prefix. The NEXT-HOP attribute indicates the IP address of the first router along an advertised path (outside of the AS receiving the advertisement) to a given prefix. When configuring its forwarding table, a router uses the NEXT-HOP attribute.
R27. Why are different inter-AS and intra-AS protocols used in the Internet?
See "Principles in Practice" on page 384
R18. Suppose you purchase a wireless router and connect it to your cable modem. Also suppose that your ISP dynamically assign your connected device (that is, your wireless router) one IP address. Also suppose that you have five PCs at home that use 802.11 to wireless connect to your wireless router. How are IP addresses assigned the five PCs? Does the wireless router use NAT? Why or why not?
See Section 4.4.4
Classless Inter-domain Routing (CIDR)
Short for Classless Inter-Domain Routing, an IP addressing scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP network prefix.For example: 172.200.0.0/16 The IP network prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP network prefix of /12,for example, can be used to address 1,048,576 former Class C addresses.
Distance-Vector Multicast Routing Protocol (DVMRP)
Short for Distance Vector Multicast Routing Protocol, an interior gateway protocol based on RIP that supports connectionless multicast data transmission to a group of hosts over a network. DVMRP tunnels multicast transmission within unicast packets that are reassembled into multicast data when they arrive at their destination
Dynamic Host Configuration Protocol (DHCP)
Short for Dynamic Host Configuration Protocol, a protocol for assigning dynamic IP addresses to devices on a network. With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device's IP address can even change while it is still connected. DHCP also supports a mix of static and dynamic IP addresses.
Network address translation (NAT)
Short for Network Address Translation, an Internet standard that enables a local-area network (LAN) to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. A NAT boxlocated where the LAN meets the Internet makes all necessary IP address translations. NAT serves three main purposes: •Provides a type of firewallby hiding internal IP addresses •Enables a company to use more internal IP addresses. Since they're used internally only, there's no possibility of conflict with IP addresses used by other companies and organizations. •Allows a company to combine multiple ISDN connections into a single Internet connection. Also see dynamic NAT and static NAT.
Open Shortest Path First (OSPF)
Short for Open Shortest Path First, an interior gateway routing protocol developed for IP networks based on the shortest path first or link-state algorithm. Routers use link-state algorithms to send routing information to all nodes in an internetwork by calculating the shortest path to each node based on a topography of the Internet constructed by each node. Each router sends that portion of the routing table (keeps track of routes to particular network destinations) that describes the state of its own links, and it also sends the complete routing structure (topography). The advantage of shortest path first algorithms is that they results in smaller more frequent updates everywhere. They converge quickly, thus preventing such problems as routing loops and Count-to-Infinity (when routers continuously increment the hop count to a particular network). This makes for a stable network. The disadvantage of shortest path first algorithms is that they require a lot of CPU power and memory. In the end, the advantages out weigh the disadvantages. OSPF Version 2 is defined in RFC1583. It is rapidly replacing RIP on the Internet.
Quality-of-Service Guarantees
Short for Quality of Service, a networking term that specifies a guaranteed throughput level. One of the biggest advantages of ATM over competing technologies such as Frame Relay and Fast Ethernet, is that it supports QoS levels. This allows ATM providers to guarantee to their customers that end-to-end latency will not exceed a specified level.
Available bit rate (ABR) ATM network service
Short for available bit rate, or Class C quality of service, an ATM bandwidth-allocation service that adjusts the amount of bandwidth based on the amount of traffic in the network. ABR service provides a guaranteed minimum bandwidth capacity but allows data to be bursted at higher capacities when the network is free.
Constant bit rate (CBR) ATM network service
Short for constant bit rate, or Class A quality of service, an ATM bandwidth-allocation service that requires the user to determine a fixed bandwidth requirement at the time the connection is set up so that the data can be sent in a steady stream. CBR service is often used when transmitting fixed-rate uncompressed video.
Interior Gateway Protocols
Short for interior gateway protocol, IGP is a generic term for a routing protocol that is used to exchange routing information among routers in an autonomous network, such as an enterprise LAN. IGPs typically support confined geographical areas. RIP and OSPF are two examples of an IGP. Compare with EGP.
R6. List some applications that would benefit from ATM's CBR service model.
Single packet: guaranteed delivery; guaranteed delivery with guaranteed maximum delay. Flow of packets: in-order packet delivery; guaranteed minimal bandwidth; guaranteed maximum jitter. None of these services is provided by the Internet's network layer. ATM's CBR service provides both guaranteed delivery and timing. ABR does not provide any of these services.
P7. Suppose two packets arrive to two different input ports of a router at exactly the same time. Also suppose there are no other packets anywhere in the router. a. Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a shared bus? b. Suppose the two packets are to be forwarded to two different output ports. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a crossbar? c. Suppose the two packets are to be forwarded to the same output port. Is it possible to forward the two packets through the switch fabric at the same time when the fabric uses a crossbar?
Solution: a) No, you can only transmit one packet at a time over a shared bus. b)Yes, as long as the two packets use different input busses and different output busses,they can be forwarded in parallel. c)No, in this case the two packets would have to be sent over the same output bus at thesame time, which is not possible. 123412342341
Static Routing Algorithms
Static routing algorithms - Routes change very slowly over time
Subnetting
Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is: 10010110.11010111.00010001.00001001 The Class B network part is: 10010110.11010111 and the host address is 00010001.00001001 If this network is divided into 14 subnets, however, then the first 4 bits of the host address (0001) are reserved for identifying the subnet. The subnet mask is the network address plus the bits reserved for identifying the subnetwork -- by convention, the bits for the network address are all set to 1, though it would also work if the bits were set exactly as in the network address. In this case, therefore, the subnet mask would be 11111111.11111111.11110000.00000000. It's called a mask because it can be used to identify the subnet to which an IP address belongs by performing a bitwise AND operation on the mask and the IP address. The result is the subnetwork address: Subnet Mask 255.255.240.000 11111111.11111111.11110000.00000000 IP Address 150.215.017.009 10010110.11010111.00010001.00001001 Subnet Address 150.215.016.000 10010110.11010111.00010000.00000000 The subnet address, therefore, is 150.215.016.000. Subnet Calculator An IP subnet mask calculator is used to automatically calculate subnets. The calculator allows you to input an IP address and choose the Subnet Mask, Network class and other variables to calculate subnet network mask. Results of the calculation will provide the hexadecimal IP address, the wildcard mask, subnet ID, broadcast address and the subnet address range for the resulting subnet network.
R8. Three types of switching fabrics are discussed in Section 4.3. List and briefly describe each type. Which, if any, can send multiple packets across the fabric in parallel.
Switching via memory; switching via a bus; switching via an interconnection network
Drop-tail
Tail Drop, or Drop Tail, is a very simple queue management algorithm used by Internet routers, e.g. in the network schedulers, and network switches to decide when to drop packets. In contrast to the more complex algorithms like RED and WRED, in Tail Drop the traffic is not differentiated. Each packet is treated identically. With tail drop, when the queue is filled to its maximum capacity, the newly arriving packets are dropped until the queue has enough room to accept incoming traffic. The name arises from the effect of the policy on incoming datagrams. Once a queue has been filled, the router begins discarding all additional datagrams, thus dropping the tail of the sequence of datagrams. The loss of datagrams causes the TCP sender to enter slow-start, which reduces throughput in that TCP session until the sender begins to receive acknowledgements again and increases its congestion window. A more severe problem occurs when datagrams from multiple TCP connections are dropped, causing global synchronization; i.e. all of the involved TCP senders enter slow-start. This happens because, instead of discarding many segments from one connection, the router would tend to discard one segment from each connection.
R16. Suppose an application generates chunks of 40 bytes of data every 20 msec, and each chunk get encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram will be overhead, and what percentage will application data?
The 8-bit protocol field in the IP datagram contains information about which transport layer protocol the destination host should pass the segment to.
DHCP discover message
The client broadcasts messages on the network subnet using the destination address 255.255.255.255 or the specific subnet broadcast address. A DHCP client may also request its last-known IP address. If the client remains connected to the same network, the server may grant the request. Otherwise, it depends whether the server is set up as authoritative or not. An authoritative server denies the request, causing the client to issue a new request. A non-authoritative server simply ignores the request, leading to an implementation-dependent timeout for the client to expire the request and ask for a new IP address.
Address Lease Time
The client sends a request to the DHCP server to release the DHCP information and the client deactivates its IP address. As client devices usually do not know when users may unplug them from the network, the protocol does not mandate the sending of DHCP Release.
Packet Loss
The discarding of data packets in a network when a device (switch, router, etc.) is overloaded and cannot accept any incoming data at a given moment. High-level transport protocols such as TCP/IP ensure that all the data sent in a transmission is received properly at the other end. See packet switching.
Default Router
The router used to forward all traffic that is not addressed to a station within the local network or local subnet. Its primary purpose in most SOHO applications (homes and small businesses) is to direct Internet traffic from the local network to the cable or DSL modem, which connects to the Internet service provider (ISP).
R2. What are the two most important network-layer functions in a datagram network? What are the three most important network-layer functions in a virtual-circuit network?
The two functions of datagram-based network layer are path determination and switching. The additional function of a vc-based network is call setup.
P23. In this problem we'll explore the impact of NATs on P2P applications. Suppose a peer with username Arnold discovers through querying that a peer with username Bernard has a file it wants to download. Also suppose that Bernard and Arnold are both behind a NAT. Try to devise a technique that will allow Arnold to establish a TCP connection with Bernard without application-specific NAT configuration. If you have difficulty devising such a technique, discuss why.
There is a super-peer (say Cindy) which is not behind any NAT. Arnold and Bernard connect to the super-peer first, and send it the IP address they think they have; the server notes both that address and the address it sees in the UDP header. The server then sends both addresses to the other peers. At this point, everyone knows everyone else's address(es). To open up peer-to-peer connections, Arnold and Bernard send a UDP packet to the new peer, and the new peer sends a UDP packet to each of Arnold and Bernard. Since nobody knows at first whether they are behind the same NAT, the first packet is always sent to both the public and the private address. This causes everyone's NAT to open up a bidirectional hole for the UDP traffic to go through. Once the first reply comes back from each peer, the sender knows which return address to use, and can stop sending to both addresses. This technique is also called NAT P2P hole punching.
Pruning
These protocols are more correctly known as reverse-path multicast algorithms. When a sender first starts sending, traffic is flooded out through the network. A router may receive the traffic along multiple paths on different interfaces, in which case it rejects any packet that arrives on any other interface other than the one it would use to send a unicast packet back to the source. It then sends a copy of each packet out of each interface other than the one back to the source. In this way each link in the whole network is traversed at most once in each direction, and the data is received by all routers in the network. So far, this describes reverse-path broadcast. Many parts of the network will be receiving traffic although there are no receivers there. These routers know they have no receivers (otherwise IGMP would have told them) and they can then send prune messages back towards the source to stop unnecessary traffic flowing. Thus the delivery tree is pruned back to the minimal tree that reaches all the receivers. The final distribution tree is what would be formed by the union of shortest paths from each receiver to the sender, and so this type of distribution tree is known as a shortest-path tree 3.5. Two commonly used multicast routing protocols fall in the class - DVMRP (the Distance Vector Multicast Routing Protocol) and Dense-mode PIM (Protocol Independent Multicast). The primary difference between these is that DVMRP computes its own routing table to determine the best path back to the source, whereas DM-PIM uses that of the underlying unicast routing hence the term "Protocol Independent". It should be fairly obvious that sending traffic everywhere and getting people to tell you what they don't want is not a terribly scalable mechanism. Sites get traffic they don't want (albeit very briefly), and routers not on the delivery tree need to store prune state. For example, if a group has one member in the UK and two in France, routers in Australia still get some of the packets and need to hold prune state to prevent more packets arriving! However, for groups where most places actually do have receivers (receivers are ``densely'' distributed), this sort of protocol works well, and so although these protocols are poor choices for a global scheme, they might be appropriate within some organisations.
multicast
To transmit a single message to a select group of recipients. A simple example of multicasting is sending an e-mail message to a mailing list. Teleconferencing and videoconferencing also use multicasting, but require more robust protocols and networks. Standards are being developed to support multicasting over a TCP/IP network such as the Internet. These standards, IP Multicast and Mbone, will allow users to easily join multicast groups. Note that multicasting refers to sending a message to a select group whereas broadcasting refers to sending a message to everyone connected to a network. The terms multicast and narrowcast are often used interchangeably, although narrowcast usually refers to the business model whereas multicast refers to the actual technology used to transmit the data.
Route
To understand what a route is, we have to explain what routing is, why we need it and how routers do routing. Routing is the process of moving a packet of data from one network to another network based on the destination IP address. The Internet uses routing to move data from your computer, across several networks, to reach a final destination, like a website. Specialized computer devices that perform this routing function are referred to as routers. Routers use the information contained in a route to make decisions about which network interface to forward a packet through in order to reach the destination address in the packet. Routers maintain a list of routes which is often referred to as a routing table. Routers look up routes in the routing table to figure out how to move data from one network to another network. Routes are simply the signposts that tell a router which network interface to forward a packet through in order to reach the packet's intended destination. - See more at: http://www.inetdaemon.com/tutorials/internet/ip/routing/route.shtml#sthash.exNcGSZ9.dpuf
R17. Suppose Host A sends Host B a TCP segment encapsulated in an IP datagram. When Host B receives the datagram, how does the network layer in Host B know it should pass the segment (that is, the payload of the datagram) to TCP rather than a UDP or to something else?
Typically the wireless router includes a DHCP server. DHCP is used to assign IP addresses to the 5 PCs and to the router interface. Yes, the wireless router also uses NAT as it obtains only one IP address from the ISP.
external BGP (eBGP) session
Understanding External BGP Peering Sessions To establish point-to-point connections between peer autonomous systems (ASs), you configure a BGP session on each interface of a point-to-point link. Generally, such sessions are made at network exit points with neighboring hosts outside the AS. Figure 1 shows an example of a BGP peering session. BGP Peering Session In Figure 1, Router A is a gateway router for AS 3, and Router B is a gateway router for AS 10. For traffic internal to either AS, an interior gateway protocol (IGP) is used (OSPF, for instance). To route traffic between peer ASs, a BGP session is used. You arrange BGP routing devices into groups of peers. Different peer groups can have different group types, AS numbers, and route reflector cluster identifiers. To define a BGP group that recognizes only the specified BGP systems as peers, statically configure all the system's peers by including one or more neighbor statements. The peer neighbor's address can be either an IPv6 or IPv4 address. As the number of external BGP (EBGP) groups increases, the ability to support a large number of BGP sessions might become a scaling issue. The preferred way to configure a large number of BGP neighbors is to configure a few groups consisting of multiple neighbors per group. Supporting fewer EBGP groups generally scales better than supporting a large number of EBGP groups. This becomes more evident in the case of hundreds of EBGP groups when compared with a few EBGP groups with multiple peers in each group. After the BGP peers are established, BGP routes are not automatically advertised by the BGP peers. At each BGP-enabled device, policy configuration is required to export the local, static, or IGP-learned routes into the BGP RIB and then advertise them as BGP routes to the other peers. BGP's advertisement policy, by default, does not advertise any non-BGP routes (such as local routes) to peers.
BGP session
Understanding External BGP Peering Sessions To establish point-to-point connections between peer autonomous systems (ASs), you configure a BGP session on each interface of a point-to-point link. Generally, such sessions are made at network exit points with neighboring hosts outside the AS. Figure 1 shows an example of a BGP peering session. Figure 1: BGP Peering Session BGP Peering Session In Figure 1, Router A is a gateway router for AS 3, and Router B is a gateway router for AS 10. For traffic internal to either AS, an interior gateway protocol (IGP) is used (OSPF, for instance). To route traffic between peer ASs, a BGP session is used. You arrange BGP routing devices into groups of peers. Different peer groups must have different group types, AS numbers, or route reflector cluster identifiers. To define a BGP group that recognizes only the specified BGP systems as peers, statically configure all the system's peers by including one or more neighbor statements. The peer neighbor's address can be either an IPv6 or IPv4 address. As the number of external BGP (EBGP) groups increases, the ability to support a large number of BGP sessions might become a scaling issue. The preferred way to configure a large number of BGP neighbors is to configure a few groups consisting of multiple neighbors per group. Supporting fewer EBGP groups generally scales better than supporting a large number of EBGP groups. This becomes more evident in the case of hundreds of EBGP groups when compared with a few EBGP groups with multiple peers in each group. After the BGP peers are established, BGP routes are not automatically advertised by the BGP peers. At each BGP-enabled device, policy configuration is required to export the local, static, or IGP-learned routes into the BGP RIB and then advertise them as BGP routes to the other peers. BGP's advertisement policy, by default, does not advertise any non-BGP routes (such as local routes) to peers.
Internal BGP (iBGP) session
Understanding Internal BGP Peering Sessions When two BGP-enabled devices are in the same autonomous system (AS), the BGP session is called an internal BGP session, or IBGP session. BGP uses the same message types on IBGP and external BGP (EBGP) sessions, but the rules for when to send each message and how to interpret each message differ slightly. For this reason, some people refer to IBGP and EBGP as two separate protocols. Figure 1: Internal and External BGP Internal and External BGP In Figure 1, Device Jackson, Device Memphis, and Device Biloxi have IBGP peer sessions with each other. Likewise, Device Miami and Device Atlanta have IBGP peer sessions between each other. The purpose of IBGP is to provide a means by which EBGP route advertisements can be forwarded throughout the network. In theory, to accomplish this task you could redistribute all of your EBGP routes into an interior gateway protocol (IGP), such as OSPF or IS-IS. This, however, is not recommended in a production environment because of the large number of EBGP routes in the Internet and because of the way that IGPs operate. In short, with that many routes the IGP churns or crashes. Generally, the loopback interface (lo0) is used to establish connections between IBGP peers. The loopback interface is always up as long as the device is operating. If there is a route to the loopback address, the IBGP peering session stays up. If a physical interface address is used instead and that interface goes up and down, the IBGP peering session also goes up and down. Thus the loopback interface provides fault tolerance in case the physical interface or the link goes down, if the device has link redundancy. While IBGP neighbors do not need to be directly connected, they do need to be fully meshed. In this case, fully meshed means that each device is logically connected to every other device through neighbor peer relationships. The neighbor statement creates the mesh. Because of the full mesh requirement of IBGP, you must configure individual peering sessions between all IBGP devices in the AS. The full mesh need not be physical links. Rather, the configuration on each routing device must create a full mesh of peer sessions (using multiple neighbor statements). Note: The requirement for a full mesh is waived if you configure a confederation or route reflection. To understand the full-mesh requirement, consider that an IBGP-learned route cannot be readvertised to another IBGP peer. The reason for preventing the readvertisement of IBGP routes and requiring the full mesh is to avoid routing loops within an AS. The AS path attribute is the means by which BGP routing devices avoid loops. The path information is examined for the local AS number only when the route is received from an EBGP peer. Because the attribute is only modified across AS boundaries, this system works well. However, the fact that the attribute is only modified across AS boundaries presents an issue inside the AS. For example, suppose that routing devices A, B, and C are all in the same AS. Device A receives a route from an EBGP peer and sends the route to Device B, which installs it as the active route. The route is then sent to Device C, which installs it locally and sends it back to Device A. If Device A installs the route, a loop is formed within the AS. The routing devices are not able to detect the loop because the AS path attribute is not modified during these advertisements. Therefore, the BGP protocol designers decided that the only assurance of never forming a routing loop was to prevent an IBGP peer from advertising an IBGP-learned route within the AS. For route reachability, the IBGP peers are fully meshed. IBGP supports multihop connections, so IBGP neighbors can be located anywhere within the AS and often do not share a link. A recursive route lookup resolves the loopback peering address to an IP forwarding next hop. The lookup service is provided by static routes or an IGP such as OSPF, or BGP routes.
DHCP offer message
When a DHCP server receives a DHCPDISCOVER message from a client, which is an IP address lease request, the server reserves an IP address for the client and makes a lease offer by sending a DHCPOFFER message to the client. This message contains the client's MAC address, the IP address that the server is offering, the subnet mask, the lease duration, and the IP address of the DHCP server making the offer. The server determines the configuration based on the client's hardware address as specified in the CHADDR (client hardware address) field. Here the server, 192.168.1.1, specifies the client's IP address in the YIADDR (your IP address) field.
N-way-unicast
When a source node sends a packet or frame destined to exactly one machine / node, then it is termed as Unicast communication. This is basically point to point communication. Unicasting can be done at the physical layer (point to point links) or at the data link layer or at the network layer. Most of the standard application layer protocols are from a source node to a single destination node. Examples include HTTP, FTP, SMTP etc.
Multicast Routing
When a source node sends a packet or frame to a group address, destined only to a set of nodes that are part of a specific group, then it is termed as multicast communication. The group is termed as a multicast group. Multicasting is a form of point to multi-point communication, where communication is from a source node to multiple end nodes, but not to all end nodes of a network. It is communication from a source node confined to all the members of a specific group. Multicasting can be done at both data link and network layers. Multicasting is used for real-time applications like Voice over IP, audio/video conferencing, video or audio broadcasting to specific groups, Video On Demand (VOD) etc.
DHCP ACK message
When the DHCP server receives the DHCPREQUEST message from the client, the configuration process enters its final phase. The acknowledgement phase involves sending a DHCPACK packet to the client. This packet includes the lease duration and any other configuration information that the client might have requested. At this point, the IP configuration process is completed. The protocol expects the DHCP client to configure its network interface with the negotiated parameters. After the client obtains an IP address, the client may use the Address Resolution Protocol (ARP) to prevent address conflicts caused by overlapping address pools of DHCP servers.
Network Service Model
When the transport layer at a sending host transmits a packet into the network (i.e., passes it down to the network layer at the sending host), can the transport layer count on the network layer to deliver the packet to the destination? When multiple packets are sent, will they be delivered to the transport layer in the receiving host in the order in which they were sent? Will the amount of time between the sending of two sequential packet transmissions be the same as the amount of time between their reception? Will the network provide any feedback about congestion in the network? What is the abstract view (properties) of the channel connecting the transport layer in the two hosts? The answers to these questions and others are determined by the service model provided by the network layer. The network service model defines the characteristics of end-to-end transport of data between one "edge" of the network and the other, i.e., between sending and receiving end systems.
R25. Compare and contrast the advertisements used by RIP and OSPF.
With OSPF, a router periodically broadcasts routing information to all other routers in the AS, not just to its neighboring routers. This routing information sent by a router has one entry for each of the router's neighbors; the entry gives the distance from the router to the neighbor. A RIP advertisement sent by a router contains information about all the networks in the AS, although this information is only sent to its neighboring routers.
R7. Discuss why each input port in a high-speed router stores shadow copy of the forwarding table.
With the shadow copy, the forwarding decision is made locally, at each input port, without invoking the centralized routing processor. Such decentralized forwarding avoids creating a forwarding processing bottleneck at a single point within the router.
Autonomous System (ASs)
Within the Internet, an autonomous system (AS) is a collection of connected Internet Protocol (IP) routing prefixes under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet.[1] Originally the definition required control by a single entity, typically an Internet service provider or a very large organization with independent connections to multiple networks, that adhere to a single and clearly defined routing policy, as originally defined in RFC 1771.[2] The newer definition in RFC 1930 came into use because multiple organizations can run BGP using private AS numbers to an ISP that connects all those organizations to the Internet. Even though there may be multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have an officially registered autonomous system number (ASN). A unique ASN is allocated to each AS for use in BGP routing. AS numbers are important because the ASN uniquely identifies each network on the Internet. Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65536 assignments. RFC 4893 introduced 32-bit AS numbers, which Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs) have begun to allocate, although this proposed standard has now been replaced by RFC 6793. These numbers are written preferably as simple integers (in a notation sometimes referred to as "asplain") ranging from 0 to 4,294,967,295, or in the form called "asdot" which looks like x.y, where x and y are 16-bit numbers. Numbers of the form 0.y are exactly the old 16-bit AS numbers. The accepted textual representation of autonomous system numbers is defined in RFC 5396 as "asplain".[3] The special 16-bit ASN 23456 ("AS_TRANS"[4]) was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ("new BGP speakers") send BGP messages to routers with older BGP software ("old BGP speakers") which do not understand the new 32-bit ASNs.[5] The first and last ASNs of the original 16-bit integers, namely 0 and 65535, and the last ASN of the 32-bit numbers, namely 4,294,967,295 are reserved and should not be used by operators. ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use by RFC 6996, meaning they can be used internally but should not be announced to the global Internet. All other ASNs are subject to assignment by IANA. The number of unique autonomous networks in the routing system of the Internet exceeded 5000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, and 47,000 in mid-2014. [6]
R19. Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in common?
Yes, because the entire IPv6 datagram (including header fields) is encapsulated in an an IPv4 datagram
R4. Do the routers in both datagram networks and virtual-circuit network use forwarding tables? If so, describe the forwarding tables for both classes of networks.
Yes, both use forwarding tables. For descriptions of the tables, see Section 4.2.
R12. Do router have IP addresses? If so, how many?
Yes. They have one address for each interface.
BGP peers
You can group BGP neighbors who share the same outbound policies together in what is called a BGP peer group. Instead of configuring each neighbor with the same policy individually, a peer group allows you to group the policies which can be applied to individual peers thus making efficient update calculation along with simplified configuration.
P4. Consider the network below. a. Suppose that this network is a datagram network. Show the forwarding table in router A, such that all traffic destined to host H3 is forwarded through interface 3. b. Suppose that this network is a datagram network. Can you write down a forwarding table in router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while all traffic from H2 destined to host H3 is forwarded through interface 4? (Hint: this is a trick question.) c. Now suppose that this network is a virtual circuit network and that there is one ongoing call between H1 and H3, and another ongoing call between H2 and H3. Write down a forwarding table in router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while all traffic from H2 destined to host H3 is forwarded through interface 4. d. Assuming the same scenario as (c), write down the forwarding tables in nodes B, C, and D.
a) For Router A, data destined to host H3 is forwarded through interface 3.Distention address Link interfaceH3 #3 b) No, because, for datagram networks, forwarding rule is only based only on destinationaddress (not the source address). c) One possible configuration for Router A is:Incoming interface Incoming VC# Outgoing Interface Outgoing VC#1 12 3 222 63 4 18Note, that the two flows could actually have the same VC numbers. d) One possible configuration is: for Router BIncoming interface Incoming VC# Outgoing Interface Outgoing VC#1 22 2 24 For Router CIncoming interface Incoming VC# Outgoing Interface Outgoing VC#1 18 2 50For Router DIncoming interface Incoming VC# Outgoing Interface Outgoing VC#1 24 3 702 50 3 76
P21. Consider the network setup in Figure 4.22. Suppose that the ISP instead assigns the router the address 24.34.112.235 and that the network address of the home network is 192.168.1/24. a. Assign addresses to all interfaces in the home network. b. Suppose each host has two ongoing TCP connections, all to port 80 at host 128.119.40.86. Provide the six corresponding entries in the NAT translation table.
a) Home addresses - 192.168.0.1, 192.168.0.2, 192.168.0.3 with the router interface being 192.168.0.4 b) NAT Translation Table WAN Side LAN Side 128.119.40.86, 80 192.168.0.1, 3345 128.119.40.86, 80 192.168.0.1, 3355 128.119.40.86, 80 192.168.0.2, 3365 128.119.40.86, 80 192.168.0.2, 3375 128.119.40.86, 80 192.168.0.3, 3245 128.119.40.86, 80 192.168.0.3, 3945
P22. Suppose you are interested in detecting the number of hosts behind a NAT. You observe that the IP layer stamps an identification number sequentially on each IP packet. The identification number of the first IP packet generated by a host is a random number, and the identification numbers of the subsequent IP packets are sequentially assigned. Assume all IP packets generated by hosts behind the NAT are sent to the outside world. a. Based on this observation, and assuming you can sniff all packets sent by the NAT to the outside, can you outline a simple technique that detects the number of unique hosts behind a NAT? Justify your answer. b. If the identification numbers are not sequentially assigned but randomly assigned, would your technique work? Justify your answer.
a) NAT will not have an entry for a connection initiated from the WAN side, hence will drop incoming packets from Arnold. b) Bernard can know the IP address of Arnold through Cindy. Then, the p2p application can initiate a connection through NAT to Arnold and upload the file.
Sequence-Number-Controlled Flooding
controlled flooding: sequence number controlled •sequence number controlled flooding • each sender can put its address and a sequence number in every packet • each router keeps track of senders and sequence numbers, and only forwards packets it has never seen before • nodes may receive a packet more than once, but will only forward each packet once • OSPF uses 32-bit sequence numbers to control the flooding of Link-State Advertisements (LSAs) • the sequence number also helps routers ignore earlier LSAs that might be delivered out of sequence • as an alternative, can remember the packet itself, or a checksum for the packet • Gnutella uses this algorithm, with 16-bit IDs and 16-bit descriptors, and TCP connections as "links"
Address Indirection
noun, Computers. 1. the address in a storage location that contains the actual machine address of a data item or of other information, as the next instruction, or that contains another indirect address