Distance Vector Algorithm and Routing Protocols
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Questions and Answers

What is the purpose of the 'Longest Prefix Matching Rule'?

  • To map a range of IP addresses to one output interface
  • To allow hosts to obtain IP addresses dynamically
  • To fragment large IP datagrams into smaller ones
  • To select the entry in the forwarding table that has the longest match with the destination address (correct)

    • What happens when a large IP datagram is sent over a link layer with a smaller MTU?

  • The datagram is routed to a different link layer
  • The datagram is discarded
  • The datagram is sent as is, without fragmentation
  • The datagram is fragmented into smaller IP datagrams (correct)

    • What is the unit of measurement for the fragment offset field in an IPv4 datagram?

  • Bits
  • Octets
  • Bytes
  • Multiples of 8 bytes (correct)

    • What is the purpose of a subnet in IPv4?

    <p>To divide a network into smaller groups of devices</p> Signup and view all the answers

    What is the notation '/24' in an IP address?

    <p>Subnet mask</p> Signup and view all the answers

    What is CIDR?

    <p>A classless subnetting method</p> Signup and view all the answers

    What is the destination IP address of a DHCP discovery message?

    <p>255.255.255.255</p> Signup and view all the answers

    Why are all DHCP messages broadcast?

    <p>Because the client initially doesn't know the IP address of the DHCP server</p> Signup and view all the answers

    What is the purpose of the fragment offset field in an IPv4 datagram?

    <p>To determine the size of the fragments (except the last)</p> Signup and view all the answers

    What happens to a large IP datagram that is sent over a link layer with a small MTU?

    <p>It is fragmented into smaller IP datagrams</p> Signup and view all the answers

    Study Notes

    Distance Vector Algorithm

    • A distance vector contains estimates of path costs from node x to y
    • Each node x maintains information for each direct neighbor: cost c(x,v) from x to v
    • Node x's distance vector: Dx = [Dx(y): y in N]

    Internet Approach to Scalable Routing

    • Routers are aggregated into regions known as Autonomous Systems (AS)
    • Intra-AS routing (Interior Gateway Protocols): within the same AS
    • Inter-AS routing: among different ASes (e.g. ISPs)
    • Border Gateway Protocol (BGP) is used for inter-AS routing

    Border Gateway Protocol (BGP)

    • Provides each AS a means to exchange routes with other ASes (eBGP) and distribute reachability information to routers within the AS (iBGP)
    • Determines "good" routes to other networks based on:
      • Local preference value attribute
      • Shortest AS-PATH
      • Closest NEXT-HOP router
      • Additional criteria

    Subnetting

    • A single prefix is used to advertise multiple subnets (address aggregation or supernetting)
    • ISP gets IP address from ICANN
    • Subnet gets IP address from ISP

    Network Address Translation (NAT)

    • NAT router behaves to the outside world as a single device having a public IP address
    • NAT router hides details of the home network
    • NAT router has a private IP address for the home network

    IPv4 Checksum

    • Header Checksum field is used to detect bit errors in received IP datagrams
    • At the sender, the checksum is calculated by summing the 2-byte numbers in the header using 1's complement arithmetic

    From IPv4 to IPv6

    • IPv4 address depletion: 32-bit address allows only 2^32 unique IP addresses
    • IPv6: 128-bit address
    • Tunneling: IPv6 datagram is carried as payload in IPv4 datagram among IPv4 routers

    Network Layer - Control Plane

    • Routing algorithms determine "good" paths from sending host to receiving host through network of routers
    • Global (Link-State) algorithms: OSPF
    • Decentralized (Distance Vector) algorithms

    Graph Abstraction of the Network

    • Graph: G = (N, E) where N is the set of routers and E is the set of links

    Dijkstra's Algorithm

    • Used for routing algorithms

    Distance Vector Algorithm

    • Each node maintains distance vector Dx = [Dx(y): y in N]

    IPv4 - Fragmentation and Reassembly

    • Data link layer frames carry IP datagrams
    • Maximum Transmission Unit (MTU): maximum amount of data that a link layer frame can carry
    • Large IP datagram is fragmented into smaller IP datagrams to fit into the link layer frame MTU
    • Fragment offset field is 13 bits and determines the size of the fragments (except the last)

    Subnetting

    • Subnet: devices that have the same subnet part in their IP addresses
    • CIDR notation is also called subnet mask
    • Subnet portion (Prefix) can be of arbitrary length (classless)

    How to Get an IP Address?

    • Hosts get IP address using DHCP (Dynamic Host Configuration Protocol)
    • DHCP process:
      1. Discovery: src=0.0.0.0 , dest = 255.255.255.255
      2. Offer: broadcast to 255.255.255.255
      3. Request: broadcast
      4. ACK: broadcast
    • All DHCP messages are broadcast because client initially doesn't know DHCP server's IP

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    Description

    This quiz covers the basics of distance vector algorithm, including its application in routing protocols such as OSPF and BGP, used in internet routing and autonomous systems.

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