NETWORK Exam 2 Study
Ethernet
A broadcast link-layer technology, a popular and widely deployer CSMA protocol.
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. 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?
No, it is impossible to forward both packets through the switch at the same time. When a shared bus topology is used only one packet is allowed through at a time.
managed device
a piece of network equipment (including it software) that resides on a managed network; might be a host, router, switch, middlebox, modem, thermometer, or other network connected devices
Subnet
a portion of a larger network; a subnet does not contain a router; its boundaries are defined by the router and host interfaces
network management agent
a process running in the managed device that communicates with the managing server taking local actions at the managed device under the command and control of the managing server
FIFO
a queuing discipline that all packets depart in the order in which they arrive
Communication layer
communication between the SDN controller and controlled network devices
Service abstraction layer
where internal applications communicate with each other
Medium Access Control [MAC]
- For point-to-point links that have a single sender at one end of the link and a single receiver at the other end of the link the sender can send a frame whenever the link is idle. - When multiple nodes share a single broadcast link, the MAC protocol serves to coordinate the frame transmissions of the many nodes.
Suppose the information content of a packet is the bit pattern 1110 0110 1001 1101 and an even parity scheme is being used. What would the value of the field containing the parity bits be for the case of a two-dimensional parity scheme? Your answer should be such that a minimum-length checksum field is used.
1 1 1 0 1 1 0 1 1 1 1 0 0 1 0 1 1 0 1 1 0 0 0 1 1
Time division multiplexing [TDM]
A Channel Partitioning Protocol technique to partition a broadcast channel's bandwidth among all nodes, that divides time into time frames, and each time frame - into time slots.
Dynamic Host Configuration Protocol [DHCP]
A client-server protocol. Allows a host to obtain (be allocated) an IP address automatically. A network administrator can configure it so that a given host receives the same IP address each time it connects to the network, or a host may be assigned a temporary IP address that will be different each time the host connects to the network. It also allows a host to learn additional information, such as its subnet mask, the address of its first-hop router (default gateway), and the address of its local DNS server. Also is referred as Plug-and-play protocol.
Switching via an interconnection network
A crossbar switch is an interconnection network consisting of 2n buses that connect n input ports to n output ports. An example of this would be what is used to interconnect processors in a multiprocessor computer.
Multiprotocol Label Switching (MPLS)
A different way of switching packets that can be used on a dedicated line
RIP (Routing Information Protocol)
A distance-vector protocol that operates in a manner very close to ideal DV protocol . Each link has a cost of 1. Costs are from source router to a destination subnet. Hop = number of subnets traversed along the shortest path. - Max cost of a path limited by 15. - Routing updates are exchanged every 30 seconds between neighbors using RIP response messages (RIP advertisements) Deployed in lower-tier ISPs.
Autonomous system
A group of routers that are under the same administrative control (same ISP / same network), running same routing algorithm, and have information about each other.
Carrier Sense Multiple Access (CSMA)
A method used by Ethernet networks to allow shared usage of a baseband network and avoid collisions.; under collision detection this helps stop transmitting to prevent collision with someone else; carrier sensing determines whether someone is listening before speaking
Carrier Sense Multiple Access [CSMA]
A node listens to the cannel before transmitting. If a frame from another node is currently being transmitted into the channel, a node then waits until it detects no transmissions for a short amount of time and then begins transmission.
Head-of-line blocking
A phenomenon when a queued packet in an input queue must wait for transfer through the fabric (even though its output port is free) because it is blocked by another packet at the head of the line.
Prefix
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.
IPv6
A successor to the IPv4 protocol that introduced: expanded addressing capabilities, a streamlined 40-byte header, and flow labeling and priority.
Address Resolution Protocol [ARP]
A translator between network-layer addresses and link-layer addresses (IP addresses into MAC addresses)
Network address translation [NAT]
A way to map a network to a single IP address. -=or=- Remapping one IP address space into another by modifying network address information in IP datagram packet headers while they are in transit across a router/switch.
Cyclic redundancy check (CRC)
A.k.a. polynomial codes. For a given piece of data, D, the sender will choose r additional bits, R, and append them to D such that the resulting d + r bit pattern (interpreted as a binary number) is exactly divisible by G (i.e., has no remainder) using modulo-2 arithmetic. The receiver divides the d + r received bits by G. If the remainder is nonzero, the receiver knows that an error has occurred; otherwise the data is accepted as being correct.
Bus
An input port transfers a packet directly to the output port, without intervention by the routing processor.
Code division multiple access [CDMA]
Assigns a different code to each node. Each node then uses its unique code to encode the data bits it sends. If the codes are chosen carefully, different nodes can transmit simultaneously and yet have their respective receivers correctly receive a sender's encoded data bits in spite of interfering transmissions by other nodes. CDMA has been used in military systems for some time.
Switching fabric
Connects the router's input ports to its output ports. It is completely contained within the router - a network inside a network router.
What is the "count to infinity" problem in distance vector routing?
Counting to infinity is just another name for a routing loop. In distance vector routing, routing loops usually occur when an interface goes down, or when two routers send updates to each other at the same time.
OSPF (Open Shortest Path First)
Deployed in upper-level ISPs. It is a link-state protocol, that uses flooding of link-state information and a Dijkstra's least-cost path algorithm. Constructs a complete topological map of the entire AS, and runs Dijkstra's to determine the shortest path tree.
How does generalized forwarding differ from destination-based forwarding?
Destination-based routing is the typical, most common type of routing. For this, each message that we send contains the address of the destination and the forwarding decision process makes its forwarding decision solemnly based on this address (and independent of the original sender). So, when constructing a routes, one thing we can do is root a spanning tree at the destination. This creates a path from all possible sources to that destination. This is called a "sink tree". Source-based routing is the reverse of destination-based routing. The decision process is based on the source address. So, when sending a message, we only include the address of the sender (and not of the destination). The decision on where to forward the message is based on this source address. Source-based routing is only really useful for doing some kind of broadcast / multicast (e.g. consider a delivery tree when building a multicast group).
Compare and contrast link-state and distance-vector routing algorithms
Distance vectors and link-state routing are both methods that choose the routing method for forwarding packets between networks. However, while distance vectors choose the shortest path from one point to the next, link-state routing track the status and connection type of each link and produces a calculated metric based on these and other factors.
Control plane
Functions executed on the routing processor. (Executing the routing protocols, responding to attached links that go up or down, management functions) Implemented in SOFTWARE.
Collision detection
If a node detects that another node is transmitting an interfering frame, it stops transmitting and waits a random amount of time before repeating the sense-and-transmit-when-idle cycle.
Describe how packet loss can occur at output ports. Can this loss be prevented by increasing the switch fabric speed?
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.
What is meant by an area in a OSPF autonomous system? Why was the concept of an area introduced?
In OSPF, a single autonomous system (AS) can be divided into smaller groups called areas. This reduces the number of link-state advertisements (LSAs) and other OSPF overhead traffic sent on the network, and it reduces the size of the topology database that each router must maintain. The routing devices that participate in OSPF routing perform one or more functions based on their location in the network.
Dijkstra's algorithm
Is a link-state routing algorithm, where for a given source node algorithm finds the shortest path between that node and every other.
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. 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?
It is possible to forward both packets through the switch at the same time as long as both have different destinations. That's the whole idea about crossbars.
Distance vector (DV) algorithm
Iterative, asynchronous, and distributed algorithm. - Each node receives information from one or more of its directly attached neighbors, calculates and distributes the information back to its neighbors. - Continues until no more information is exchanged between neighbors - Does not require all of the nodes to operate in lockstep with one another.
Network adapter
Link-layer controller (chip) that implements link-layer services (framing, link access, error detection) and is also called Network Interface Card (NIC).
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. 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?
No, it is not possible to forward two packets at the same time through the router to one output port. This is because although the crossbar allows multiple packets through, they cannot be at the same place at the same time
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.
ALOHA
One of the most commonly used Random Access Protocol.
Routing loop
Problem associated with Distance vector protocols, when a data packet is continually routed through the same routers over and over.
Do routers have IP addresses? If so, how many?
Routers do have IP addresses. Each router has two IP addresses.
How does BGP 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.
Suppose you want to implement a new routing protocol in the SDN control plane. At which layer would you implement that protocol? Explain.
Routing would occur in the management layer. The management layer is the one that interacts with SNMP, SSH, Telnet, and TFTP.
MAC address
Same as LAN address and Physical address. It is 6 bytes long, expressed in HEX. No adapters have same address. Is managed by IEEE.
What are the purposes of the SNMP GetRequest and SetRequest messages?
SetRequest, value is applied to the specified OID of the SNMP agent. GetRequest, value is a Null that acts as a placeholder for the return data.
Border Gateway Protocol (BGP)
Standard inter-AS routing protocol in today's Internet. Provides each AS means to: 1. Obtain subnet reachability information from neighboring ASs. 2. Propagate (spread) the reachability information to all routers internal to the AS. 3. Determine "good" routes to subnets based on the reachability information and on AS policy. Allows each subnet to advertise its existence to the rest of the Internet.
Describe the main role of the communication layer, the network-wide state-management layer, and the network control application layer in an SDN controller.
The Management Plane handles functions such device management, firmware updates, SNMP and external configuration via the CLI. The Data Plane refers to packet and frame forwarding through the device. The Control Plane is routing protocols such as BGP & OSPF and switching protocols such as STP & TRILL. The control plane will use the routing table to build the forwarding table used by data plane. The forwarding table is delivered to the data plane by the management plane as part of the device operating system. Thus when an Ethernet frame arrives on the switch interface, the data plane then forwards it to output port.
How does BGP use the NEXT-HOP attribute?
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.
Switching via bus
The input ports transfer a packet directly to the output port over a shared bus, without intervention by the routing processor. Although the routing processor is not involved in the bus transfer, because the bus is shared, only one packet at a time can be transferred over the bus
Subnet
The network interconnecting the host interfaces and a router/switch interface. -=or=- A logical visible subdivision of an IP network.
Parity bit
The simplest form of error detection. In an even parity scheme the sender includes one additional bit and chooses its value such that the total number of 1s in datalength+1 bits is even. In an odd parity scheme, the total number of 1s is odd.
Switching via memory
The simplest, earliest routers were traditional computers, with switching between input and output ports being done under direct control of the CPU (routing processor)
Three types of switching fabrics
The three types of switching are switching via memory, switching via bus, and switching via an interconnection network.
Describe the design principle of Traceroute software
Traceroute is a utility that records the route (the specific gateway computers at each hop) through the Internet between your computer and a specified destination computer. It also calculates and displays the amount of time each hop took. Traceroute is a handy tool both for understanding where problems are in the Internet network and for getting a detailed sense of the Internet itself
True or False. When an OSPF route sends its link state information, it is sent only to those nodes directly attached neighbors. Explain.
True, when an OSPF route sends its link state information, it is sent only to those nodes directly attached neighbors.
Send Packet
Type of northbound message contains a specific packet to be sent from a specific port; packer is contain in its payload
Flow removed
Type of southbound message that informs the controller that the flow table has been removed a requested by the modify stater message from the controller
ICMP
Used by hosts and routers to communicate network-layer information to each other. The most typical use for it is error reporting.
BGP route
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
Network Control Application Layer
allows to read/write network state and flow tables within the state management layer
managing server
an application, typically with a human in the loop, tuning in a centralized network management station in the network operations center; locus of activity for network management; it controls the processing, analysis, collection, and display of network management information
Simple Network Management Protocol
application layer protocol used to convey network management control and information messages between a managing server and an agent executing on behalf of the managing server
Generalized-based forwarding
form of packet forwarding that is a key component of a modern network data plane; including data plane in software-defined networks (SDNs); if a packet not deemed worthy, it can be blocked and not allowed to pass through
Border Gateway Protocol (BGP)
glue that holds the internet together
Frequency Division Multiple Access
limited collision, limited timeslot
data plane
local per-router function that determines how datagram arriving on router input port is forwarded to router output port
Management Information Base (MIB)
set of network objects that is managed using SNMP; shows different statistics about hosts, routers, DNS servers, etc
network wide state management layer
state of network links, switches, services; ex: configuring flow tables in all switches to achieve the desired end-end forwarding; ex: distributed databases
Classes InterDomain Routing (CIDR)
subnet portion of address of arbitrary length; address format: a.b.c.d/x where x is number of bits
Destination-based forwarding
traditional forwarding where packets maybe forwarded to output ports based on their destination IP addresses; helps packer determine where to go next to reach path of final destination
Address Resolution Protocol
translator between link layer addresses and the network layer addresses
Configuration message
type of northbound message that allows controller to query and set a switch's configuration parameters in order to determine roles
Port Status
type of southbound messaged used by the switch to inform the controller that the status of a port has been changed
Internet Control Message Protocol (ICMP)
used in level 3 network layer; used by hosts and routers to communicate network layer information with each other; most typical use is for error reporting
