EDA344 - Review Questions
Chapter 4 R1a What is the name of a network-layer packet? (Recall that both routers and link-layer switches are called packet switches.)
A network-layer packet is a datagram.
Chapter 4 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"
Chapter 1 R23 What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?
- Application layer (Network applications and their respective protocols) - Transport layer (Transport application-level messages between application endpoints) - Network layer (Routes a datagram though a series of routers between the source and destination) - Link layer (Delivery of frame from node to node) - Physical layer (Delivery of individual bits from node to node)
Chapter 1 R22 List five tasks that a layer can perform. Is it possible that one (or more) of these tasks could be performed by two (or more) layers?
- Error Control - Flow Control - Segmentation & Reassembly - Multiplexing - Connection Setup Example: Error control is often provided at more than one layer
Chapter 6 R3 What are the differences between the following types of wireless channel impairments: 1. Path loss 2. Multipath propagation 3. Interference from other sources?
- Path loss is due to the attenuation of the electromagnetic signal when it travels through matter. - Multipath propagation results in blurring of the received signal at the receiver and occurs when portions of the electromagnetic wave reflect off objects and ground, taking paths of different lengths between a sender and receiver. - Interference from other sources occurs when the other source is also transmitting in the same frequency range as the wireless network.
Chapter 1 R16. Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?
- Process delay - Queuing delay - Transmission delay - Propagation delay All delays are fixed, except for the queueing delay,s, which are variable.
Chapter 2 R1 List five nonpropriary Internet applications and the application-layer protocols that they use.
- The web: HTTP - File transfer: FTP - Remote Login: Telnet - e-mail: SMTP - BitTorrent file sharing: BitTorrent protocol
Chapter 1 R18. How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5 · 10^8 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length L to propa- gate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?
10msec; d/s; no; no
Chapter 4 R16 Suppose an application generates chunks of 40 bytes of data every 20 msec, and each chunk gets encapsulated in a TCP segment and then an IP datagram. What percentage of each datagram will be overhead, and what percentage will be application data?
50% overhead
Chapter 4 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 desti- nation host will travel over how many interfaces? How many forwarding tables will be indexed to move the datagram from the source to the destination?
8 interfaces; 3 forwarding tables.
Chapter 1 R12. What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM have over FDM in a circuit-switched network?
A circuit-switched network can guarantee a certain amount of end-to-end bandwidth for the duration of a call. Most packet-switched networks today (including the Internet) cannot make any end-to-end guarantees for bandwidth. FDM requires sophisticated analog hardware to shift signal into appropriate frequency bands.
Chapter 6 R17 What is the difference between a permanent address and a care-of address? Who assigns a care-of address?
A permanent address for a mobile node is its IP address when it is at its home network. A care-of-address is the one its gets when it is visiting a foreign network. The COA is assigned by the foreign agent (which can be the edge router in the foreign network or the mobile node itself).
Chapter 2 R10 What is meant by a handshaking protocol?
A protocol uses handshaking if the two communicating entities first exchange control packets before sending data to each other. SMTP uses handshaking at the application layer whereas HTTP does not.
Chapter 4 R1b 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 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.
Chapter 4 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).
Chapter 4 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.
Chapter 3 R11 Suppose that the roundtrip delay between sender and receiver is constant and known to the sender. Would a timer still be necessary in protocol rdt 3.0, assuming that packets can be lost? Explain.
A timer would still be necessary in the protocol rdt 3.0. If the round trip time is known then the only advantage will be that, the sender knows for sure that either the packet or the ACK (or NACK) for the packet has been lost, as compared to the real scenario, where the ACK (or NACK) might still be on the way to the sender, after the timer expires. However, to detect the loss, for each packet, a timer of constant duration will still be necessary at the sender.
Chapter 6 R5 Describe the role of the beacon frames in 802.11.
APs transmit beacon frames. An AP's beacon frames will be transmitted over one of the 11 channels. The beacon frames permit nearby wireless stations to discover and identify the AP.
Chapter 5 R2 If all the links in the Internet were to provide reliable delivery service, would the TCP reliable delivery service be redundant? Why or why not?
Although each link guarantees that an IP datagram sent over the link will be received at the other end of the link without errors, it is not guaranteed that IP datagrams will arrive at the ultimate destination in the proper order. With IP, datagrams in the same TCP connection can take different routes in the network, and therefore arrive out of order. TCP is still needed to provide the receiving end of the application the byte stream in the correct order. Also, IP can lose packets due to routing loops or equipment failures.
Chapter 5 R11 Why is an ARP query sent within a broadcast frame? Why is an ARP response sent within a frame with a specific destination MAC address?
An ARP query is sent in a broadcast frame because the querying host does not which adapter address corresponds to the IP address in question. For the response, the sending node knows the adapter address to which the response should be sent, so there is no need to send a broadcast frame (which would have to be processed by all the other nodes on the LAN).
Chapter 3 R14f Suppose that the last SampleRTT in a TCP connection is equal to 1 sec. The current value of TimeoutInterval for the connection will neces- sarily be ≥ 1 sec.
False
Chapter 3 R4 Describe why an application developer might choose to run an application over UDP rather than TCP.
An application developer may not want its application to use TCP's congestion control, which can throttle the application's sending rate at times of congestion. Often, designers of IP telephony and IP videoconference applications choose to run their applications over UDP because they want to avoid TCP's congestion control. Also, some applications do not need the reliable data transfer provided by TCP.
Chapter 6 R20 What is the role of the anchor MSC in GSM networks?
Anchor MSC is the MSC visited by the mobile when a call first begins; anchor MSC thus remains unchanged during the call. Throughout the call's duration and regardless of the number of inter-MSC transfers performed by the mobile, the call is routed from the home MSC to the anchor MSC, and then from the anchor MSC to the visited MSC where the mobile is currently located.
Chapter 6 R12 What are the differences between a master device in a Bluetooth network and a base station in an 802.11 network?
Any ordinary Bluetooth node can be a master node whereas access points in 802.11 networks are special devices (normal wireless devices like laptops cannot be used as access points).
Chapter 1 R20.3 Why is packet switching in the Internet analagous to driving from one city to another and asking directions along the way?
Asking which road to take is analogous to a packet asking which outgoing link it should be forwarded on, given the packet's destination address.
Chapter 4 R10 Describe how packet loss can occur at output ports. Can this loss be prevented by increasing the switch fabric speed?
Assuming input and output line speeds are the same, packet loss can still occur if the rate at which packets arrive to a single output port exceeds the line speed. If this rate mismatch persists, the queues will get larger and larger and eventually overflow the output port buffers, causing packet loss. Note that increasing switch fabric speed cannot prevent this problem from occurring.
Chapter 3 R14g Suppose Host A sends one segment with sequence number 38 and 4bytes of data over a TCP connection to Host B. In this same segment the acknowledgment number is necessarily 42.
False
Chapter 4 R34 When a host joins a multicast group, must it change its IP address to that of the multicast group it is joining?
False
Chapter 1 R11. Suppose there is exactly one packet switch between a sending host and a receiving host. The transmission rates between the sending host and the switch and between the switch and the receiving host are R1 and R2, respec- tively. Assuming that the switch uses store-and-forward packet switching, what is the total end-to-end delay to send a packet of length L? (Ignore queuing, propagation delay, and processing delay.)
At time t0 the sending host begins to transmit. At time t1 = L/R1, the sending host completes transmission and the entire packet is received at the router (no propagation delay). Because the router has the entire packet at time t1, it can begin to transmit the packet to the receiving host at time t1. At time t2 = t1 + L/R2, the router completes transmission and the entire packet is received at the receiving host (again, no propagation delay). Thus, the end-to-end delay is L/R1 + L/R2.
Chapter 5 R10 Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters). If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the MAC address of B, will C's adapter process these frames? If so, will C's adapter pass the IP datagrams in these frames to the network layer C? How would your answers change if A sends frames with the MAC broadcast address?
C's adapter will process the frames, but the adapter will not pass the datagrams up the protocol stack. If the LAN broadcast address is used, then C's adapter will both process the frames and pass the datagrams up the protocol stack.
Chapter 6 R6 True or false: Before an 802.11 station transmits a data frame, it must first send an RTS frame and receive a corresponding CTS frame.
False
Chapter 6 R8 True or false: Ethernet and 802.11 use the same frame structure.
False
Chapter 1 R27 Describe how a botnet can be created, and how it can be used for a DDoS attack
Creation of a botnet requires an attacker to find vulnerability in some application or system (e.g. exploiting the buffer overflow vulnerability that might exist in an application). After finding the vulnerability, the attacker needs to scan for hosts that are vulnerable. The target is basically to compromise a series of systems by exploiting that particular vulnerability. Any system that is part of the botnet can automatically scan its environment and propagate by exploiting the vulnerability. An important property of such botnets is that the originator of the botnet can remotely control and issue commands to all the nodes in the botnet. Hence, it becomes possible for the attacker to issue a command to all the nodes, that target a single node (for example, all nodes in the botnet might be commanded by the attacker to send a TCP SYN message to the target, which might result in a TCP SYN flood attack at the target).
Chapter 1 R24.1 What is an Application-layer message?
Data which an application wants to send and passed onto the transport layer
Chapter 4 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?
Datagram-based network layer: forwarding; routing. Additional function of VC-based network layer: call setup.
Chapter 6 R9 Describe how the RTS threshold works.
Each wireless station can set an RTS threshold such that the RTS/CTS sequence is used only when the data frame to be transmitted is longer than the threshold. This ensures that RTS/CTS mechanism is used only for large frames.
Chapter 1 R24.4 What is a link-layer frame?
Encapsulates network-layer datagram with a link-layer header
Chapter 1 R24.3 What is a network-layer datagram?
Encapsulates transport-layer segment with a network-layer header
Chapter 1 R20.1 Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file.
End system A breaks the large file into chunks. It adds headers to each chunk, thereby generating multiple packets from the file.
Chapter 3 R14a True or false: Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments on data.
False
Chapter 3 R14b b. The size of the TCP rwnd never changes throughout the duration of the connection.
False
Chapter 3 R14d Suppose Host A is sending a large file to Host B over a TCP connection. If the sequence number for a segment of this connection is m, then the sequence number for the subsequent segment will necessarily be m + 1.
False
Chapter 3 R18 True or false? Consider congestion control in TCP. When the timer expires at the sender, the value of ssthresh is set to one half of its previous value.
False, it is set to half of the current value of the congestion window.
Chapter 3 R8 Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses persistent connections, and is currently receiving requests from two different Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If they are being passed through different sockets, do both of the sockets have port 80? Discuss and explain.
For each persistent connection, the Web server creates a separate "connection socket". Each connection socket is identified with a four-tuple: (source IP address, source port number, destination IP address, destination port number). When host C receives and IP datagram, it examines these four fields in the datagram/segment to determine to which socket it should pass the payload of the TCP segment. Thus, the requests from A and B pass through different sockets. The identifier for both of these sockets has 80 for the destination port; however, the identifiers for these sockets have different values for source IP addresses. Unlike UDP, when the transport layer passes a TCP segment's payload to the application process, it does not specify the source IP address, as this is implicitly specified by the socket identifier.
Chapter 4 R3 What is the difference between routing and forwarding?
Forwarding is about moving a packet from a router's input port to the appropriate output port. Routing is about determining the end-to-routes between sources and destinations.
Chapter 5 R3 What are some of the possible services that a link-layer protocol can offer to the network layer? Which of these link-layer services have corresponding services in IP? In TCP?
Framing: there is also framing in IP and TCP; link access; reliable delivery: there is also reliable delivery in TCP; flow control: there is also flow control in TCP; error detection: there is also error detection in IP and TCP; error correction; full duplex: TCP is also full duplex.
Chapter 1 R24.2 What is a Transport-layer segment?
Generated by the transport layer and encapsulates application-layer message with transport layer header
Chapter 4 R11 What is HOL blocking? Does it occur in input ports or output ports?
HOL blocking: Sometimes the a packet that is first in line at an input port queue must wait because there is no available buffer space at the output port to which it wants to be forwarded. When this occurs, all the packets behind the first packet are blocked, even if their output queues have room to accommodate them. HOL blocking occurs at the input port.
Chapter 1 R25.3 Which layers does a host process?
Hosts process all five layers.
Chapter 4 R35 What are the roles played by the 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.
Chapter 4 R19 Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in common?
IPv6 has a fixed length header, which does not include most of the options an IPv4 header can include. Even though the IPv6 header contains two 128 bit addresses (source and destination IP address) the whole header has a fixed length of 40 bytes only. Several of the fields are similar in spirit. Traffic class, payload length, next header and hop limit in IPv6 are respectively similar to type of service, datagram length, upper-layer protocol and time to live in IPv4.
Chapter 4 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.
Chapter 4 R9 Describe how packet loss can occur at input ports. Describe how packet loss at input ports can be eliminated (without using infinite buffers).
If the rate at which packets arrive to the fabric exceeds switching fabric rate, then packets will need to queue at the input ports. If this rate mismatch persists, the queues will get larger and larger and eventually overflow the input port buffers, causing packet loss. Packet loss 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.
Chapter 4 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 trees for a given multicast group.
Chapter 6 R1 What does it mean for a wireless network to be operating in "infrastructure mode?" If the network is not in infrastructure mode, what mode of operation is it in, and what is the difference between that mode of operation and infra- structure mode?
In infrastructure mode of operation, each wireless host is connected to the larger network via a base station (access point). If not operating in infrastructure mode, a network operates in ad-hoc mode. In ad-hoc mode, wireless hosts have no infrastructure with which to connect. In the absence of such infrastructure, the hosts themselves must provide for services such as routing, address assignment, DNS-like name translation, and more.
Chapter 5 R7 Describe polling and token-passing protocols using the analogy of cocktail party interactions.
In polling, a discussion leader allows only one participant to talk at a time, with each participant getting a chance to talk in a round-robin fashion. For token ring, there isn't a discussion leader, but there is wine glass that the participants take turns holding. A participant is only allowed to talk if the participant is holding the wine glass.
Chapter 6 R11 Section 6.3.4 discusses 802.11 mobility, in which a wireless station moves from one BSS to another within the same subnet. When the APs are interconnected with a switch, an AP may need to send a frame with a spoofed MAC address to get the switch to forward the frame properly. Why?
Initially the switch has an entry in its forwarding table which associates the wireless station with the earlier AP. When the wireless station associates with the new AP, the new AP creates a frame with the wireless station's MAC address and broadcasts the frame. The frame is received by the switch. This forces the switch to update its forwarding table, so that frames destined to the wireless station are sent via the new AP.
Chapter 6 R16 If a node has a wireless connection to the Internet, does that node have to be mobile? Explain. Suppose that a user with a laptop walks around her house with her laptop, and always accesses the Internet through the same access point. Is this user mobile from a network standpoint? Explain.
No. A node can remain connected to the same access point throughout its connection to the Internet (hence, not be mobile). A mobile node is the one that changes its point of attachment into the network over time. Since the user is always accessing the Internet through the same access point, she is not mobile.
Chapter 4 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.
Chapter 4 R6 List some applications that would benefit from ATM's CBR service model.
Interactive live multimedia applications, such as IP telephony and video conference, could benefit from ATM CBR's service, which maintains timing.
Chapter 2 R21 In BitTorrent, suppose Alice provides chunks to Bob throughout a 30-second interval. Will Bob necessarily return the favor and provide chunks to Alice in this same interval? Why or why not?
It is not necessary that Bob will also provide chunks to Alice. Alice has to be in the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur even if Alice provides chunks to Bob throughout a 30-second interval.
Chapter 1 R25.2 Which layers does a link-layer switch process?
Link layer switches process link and physical layers (Layers 1 through 2)
Chapter 4 R21 Compare and contrast link-state and distance-vector routing algorithms.
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.
Chapter 5 R9 How big is the MAC address space? The IPv4 address space? The IPv6 address space?
MAC: 48 bit IPv4: 32 bit IPv6: 128 bit
Chapter 2 R24 Consider a DHT with a mesh overlay topology (that is, every peer tracks all peers in the system). What are the advantages and disadvantages of such a design? What are the advantages and disadvantages of a circular DHT (with no shortcuts)?
Mesh DHT: The advantage is in order to a route a message to the peer (with ID) that is closest to the key, only one hop is required; the disadvantage is that each peer must track all other peers in the DHT. Circular DHT: the advantage is that each peer needs to track only a few other peers; the disadvantage is that O(N) hops are needed to route a message to the peer that is closest to the key.
Chapter 4 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
Chapter 2 R2 What's the difference between network architecture and application architecture?
Network architecture refers to the organization mof the communication process into layers (e.g. the five-layer Internet architecture) Application architecture, on the other hand, is designed by an application developer and dictates the boraod structure of the application (e.g. client-server or P2P)
Chapter 6 R10 Suppose the IEEE 802.11 RTS and CTS frames were as long as the standard DATA and ACK frames. Would there be any advantage to using the CTS and RTS frames? Why or why not?
No, there wouldn't be any advantage. Suppose there are two stations that want to transmit at the same time, and they both use RTS/CTS. If the RTS frame is as long as a DATA frames, the channel would be wasted for as long as it would have been wasted for two colliding DATA frames. Thus, the RTS/CTS exchange is only useful when the RTS/CTS frames are significantly smaller than the DATA frames.
Chapter 2 R4 For a P2P file-sharing application, do you agree with the statement "There is no notion of client and server sides of a communication session"? Why or why not?
No. In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server.
Chapter 2 R15 Why is it said that FTP sends the control information "out-of-band"?
Out-of-band control passes control data on a separate connection from main data
Chapter 4 R27 Why are different inter-AS and intra-AS protocols used in the Internet?
Policy: Among ASs, policy issues dominate. It may well be important that traffic originating in a given AS not be able to pass through another specific AS. Similarly, a given AS may want to control what transit traffic it carries between other ASs. Within an AS, everything is nominally under the same administrative control and thus policy issues a much less important role in choosing routes with in AS. Scale: The ability of a routing algorithm and its data structures to scale to handle routing to/among large numbers of networks is a critical issue in inter-AS routing. Within an AS, scalability is less of a concern. For one thing, if a single administrative domain becomes too large, it is always possible to divide it into two ASs and perform inter-AS routing between the two new ASs. Performance: Because inter-AS routing is so policy oriented, the quality (for example, performance) of the routes used is often of secondary concern (that is, a longer or more costly route that satisfies certain policy criteria may well be taken over a route that is shorter but does not meet that criteria). Indeed, we saw that among ASs, there is not even the notion of cost (other than AS hop count) associated with routes. Within a single AS, however, such policy concerns are of less importance, allowing routing to focus more on the level of performance realized on a route.
Chapter 3 R17 Suppose two TCP connections are present over some bottleneck link of rate R bps. Both connections have a huge file to send (in the same direction over the bottleneck link). The transmissions of the files start at the same time. What transmission rate would TCP like to give to each of the connections?
R/2
Chapter 2 R22 Consider a new peer Alice that joins BitTorrent without possessing any chunks. Without any chunks, she cannot become a top-four uploader for any of the other peers, since she has nothing to upload. How then will Alice get her first chunk?
Recall that in BitTorrent, a peer picks a random peer and optimistically unchokes the peer for a short period of time. Therefore, Alice will eventually be optimistically unchoked by one of her neighbours, during which time she will receive chunks from that neighbour.
Chapter 4 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. An interconnection network can forward packets in parallel as long as all the packets are being forwarded to different output ports.
Chapter 4 R22 Discuss how a hierarchical organization of the Internet has made it possible to scale to millions of users.
Routers are organized into autonomous systems (ASs). Within an AS, all routers run the same intra-AS routing protocol. The problem of scale is solved since an router in an AS need only know about routers within its AS and the subnets that attach to the AS. To route across ASes, the inter-AS protocol is based on the AS graph and does not take individual routers into account.
Chapter 1 R25.1 Which layers in the Internet protocol stack does a router process?
Routers process network, link and physical layers (layers 1 through 3) (This is a white lie as modern routers sometimes act as firewalls or caching components, and process Transport layer as well)
Chapter 4 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.
Chapter 2 R9 Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do?
SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application.
Chapter 3 R9 In our rdt protocols, why did we need to introduce sequence numbers?
Sequence numbers are required for a receiver to find out whether an arriving packet contains new data or is a retransmission.
Chapter 3 R5 Why is it that voice and video traffic is often sent over TCP rather than UDP in today's Internet? (Hint: The answer we are looking for has nothing to do with TCP's congestion-control mechanism.)
Since most firewalls are configured to block UDP traffic, using TCP for video and voice traffic lets the traffic though the firewalls.
Chapter 3 R3 Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number x and destination port number y. What are the source and destination port num- bers for the segments traveling from Host B to Host A?
Source port number y and destination port number x.
Chapter 4 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 to UDP or to something else?
The 8-bit protocol field in the IP datagram contains information about which transport layer protocol the destination host should pass the segment to.
Chapter 2 R5 What information is used by a process running on one host to identify a process running another host?
The IP address of the destination host and the port number of the socket in the destination process
Chapter 2 R11 Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?
The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.
Chapter 6 R19 What are the purposes of the HLR and VLR in GSM networks? What ele- ments of mobile IP are similar to the HLR and VLR?
The home network in GSM maintains a database called the home location register (HLR), which contains the permanent cell phone number and subscriber profile information about each of its subscribers. The HLR also contains information about the current locations of these subscribers. The visited network maintains a database known as the visitor location register (VLR) that contains an entry for each mobile user that is currently in the portion of the network served by the VLR. VLR entries thus come and go as mobile users enter and leave the network. The edge router in home network in mobile IP is similar to the HLR in GSM and the edge router in foreign network is similar to the VLR in GSM.
Chapter 2 R16 Suppose Alice, with a Web-based e-mail account (such as Hotmail or gmail), sends a message to Bob, who accesses his mail from his mail server using POP3. Discuss how the message gets from Alice's host to Bob's host. Be sure to list the series of application-layer protocols that are used to move the mes- sage between the two hosts.
The message is first sent from Alice's host to her mail server over HTTP. Alice's mail server then sends the message to Bob's mail server over SMTP. Bob then transfers the message from his mail server to his host over POP3.
Chapter 2 R23 What is an overlay network? Does it include routers? What are the edges in the overlay network?
The overlay network in a P2P file sharing system consists of the nodes participating in the file sharing system and the logical links between the nodes. There is a logical link (an "edge" in graph theory terms) from node A to node B if there is a semi- permanent TCP connection between A and B. An overlay network does not include routers.
Chapter 1 R20.2 When a packets arrive to a packet switch, what infromation in the packet does the switch use to determine the link onto which the packet is forwarded?
The packets switch uses the destination IP address in the packet to determine the outgoing link.
Chapter 2 R3 For a communications session between a pair of processes, which process is the client and which is the server?
The process which initiates the communication is the client; the process that waits to be contacted is the server
Chapter 5 R1 Consider the transportation analogy in Section 5.1.1. If the passenger is analagous to a datagram, what is analogous to the link layer frame?
The transportation mode, e.g., car, bus, train, car.
Chapter 3 R10 In our rdt protocols, why did we need to introduce timers?
To handle losses in the channel. If the ACK for a transmitted packet is not received within the duration of the timer for the packet, the packet (or its ACK or NACK) is assumed to have been lost. Hence, the packet is retransmitted.
Chapter 3 R14c Suppose Host A is sending Host B a large file over a TCP connection. The number of acknowledged bytes that A sends cannot exceed the size of the receive buffer.
True
Chapter 3 R14e The TCP segment has a field in its header for rwnd.
True
Chapter 4 R18 Suppose you purchase a wireless router and connect it to your cable modem. Also suppose that your ISP dynamically assigns 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 wirelessly connect to your wireless router. How are IP addresses assigned to the five PCs? Does the wireless router use NAT? Why or why not?
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.
Chapter 1 R26 What's the difference between a virus and a worm?
Virus - Requires some form of human interaction to spread. Worms - No user replication needed. Worm in infected host scans IP addresses and port numbers, looking for vulnerable processes to infect.
Chapter 2 R13 Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?
Web caching can bring the desired content "closer" to the user, possibly to the same LAN to which the user's host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links.
Chapter 5 R8 Why would the token-ring protocol be inefficient if a LAN had a very large perimeter?
When a node transmits a frame, the node has to wait for the frame to propagate around the entire ring before the node can release the token. Thus, if L/R is small as compared to tprop, then the protocol will be inefficient.
Chapter 2 R12 Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies.
When the user first visits the site, the server creates a unique identification number, creates an entry in its back-end database, and returns this identification number as a cookie number. This cookie number is stored on the user's host and is managed by the browser. During each subsequent visit (and purchase), the browser sends the cookie number back to the site. Thus the site knows when this user (more precisely, this browser) is visiting the site.
Chapter 4 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.
Chapter 2 R18 From a user's perspective, what is the difference between the download-and-delete mode and the download-and-keep mode in POP3?
With download and delete, after a user retrieves its messages from a POP server, the messages are deleted. This poses a problem for the nomadic user, who may want to access the messages from many different machines (office PC, home PC, etc.). In the download and keep configuration, messages are not deleted after the user retrieves the messages. This can also be inconvenient, as each time the user retrieves the stored messages from a new machine, all of non-deleted messages will be transferred to the new machine (including very old messages).
Chapter 4 R7 Discuss why each input port in a high-speed router stores a shadow copy of the forwarding table.
With the shadow copy, the forwarding lookup is made locally, at each input port, without invoking the centralized routing processor. Such a decentralized approach avoids creating a lookup processing bottleneck at a single point within the router.
Chapter 2 R19 Is it possible for an organisation's Web server and mail server to have exactly the same alias for a hostname (for example, foo.com)? What would be the type for the RR that contains the hostname of the mail server?
Yes, an organization's mail server and Web server can have the same alias for a host name. The MX record is used to map the mail server's host name to its IP address.
Chapter 4 R20 It has been said that when IPv6 tunnels through IPv4 routers, IPv6 treats the IPv4 tunnels as link-layer protocols. Do you agree with this statement? Why or why not?
Yes, because the entire IPv6 datagram (including header fields) is encapsulated in an IPv4 datagram.
Chapter 3 R7 Suppose a process in Host C has a UDP socket with port number 6789. Sup- pose both Host A and Host B each send a UDP segment to Host C with desti- nation port number 6789. Will both of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts?
Yes, both segments will be directed to the same socket. For each received segment, at the socket interface, the operating system will provide the process with the IP addresses to determine the origins of the individual segments.
Chapter 4 R4 Do the routers in both datagram networks and virtual-circuit networks 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.
Chapter 3 R6 Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how?
Yes. The application developer can put reliable data transfer into the application layer protocol. This would require a significant amount of work and debugging, however.
Chapter 4 R12 Do routers have IP addresses? If so, how many?
Yes. They have one address for each interface.
Chapter 2 R6 Suppose you wanted to do a transaction from a remote client ot a server as fast as possible. Would you use UDP or TCP? Why?
You would use UDP. With UDP, the transaction can be completed in one roundtrip time (RTT) the client sends the transaction request into a UDP socket, and the server seds the reply back to the client's UDP socket. With TCP, a minimum of two RTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply.
Chapter 3 R15 Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The first segment has sequence number 90; the second has sequence number 110. a. How much data is in the first segment? b. Suppose that the first segment is lost but the second segment arrives at B. In the acknowledgment that Host B sends to Host A, what will be the acknowledgment number?
a) 20 bytes b) Ack number = 90
Chapter 2 R25 List at least four different applications that are naturally suitable for P2P architectures. (Hint: File distribution and instant messaging are two.)
a) File Distribution b) Instant Messaging c) Video Streaming d) Distributed Computing
Chapter 6 R4 As a mobile node gets farther and farther away from a base station, what are two actions that a base station could take to ensure that the loss probability of a transmitted frame does not increase?
a) Increasing the transmission power b) Reducing the transmission rate
Chapter 6 R21 What are three approaches that can be taken to avoid having a single wireless link degrade the performance of an end-to-end transport-layer TCP connection?
a) Local recovery b) TCP sender awareness of wireless links c) Split-connection approaches
Chapter 2 R8 List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service
a) Reliable data transfer - TCP provides a reliable byte-stream between client and server but UDP does not b) A guarantee that a certain value for throughput will be maintained - Neither (Does circuit switching guarantee this?) c) A guarantee that data will be delivered within a specified amount of time - Neither d) Confidentiality (via encryption) - Neither
Chapter 6 R2 What are the four types of wireless networks identified in our taxonomy in Section 6.1?
a) Single hop, infrastructure-based b) Single hop, infrastructure-less c) Multi-hop, infrastructure-based d) Multi-hop, infrastructure-less
Chapter 4 R33b For each of the three general approaches we studied for broadcast communi- cation (uncontrolled flooding, controlled flooding, and spanning-tree broadcast), are the following statements 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 node may forward multiple copies of a packet over the same outgoing link.
uncontrolled flooding: T; controlled flooding: F; spanning-tree: F
Chapter 4 R33a For each of the three general approaches we studied for broadcast communication (uncontrolled flooding, controlled flooding, and spanning-tree broadcast), are the following statements 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. True or false: A node may receive multiple copies of the same packet.
uncontrolled flooding: T; controlled flooding: T; spanning-tree: F
