Intra-domain%20Routing - PowerPoint PPT Presentation

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Intra-domain%20Routing

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Title: Intra-domain%20Routing


1
Intra-domain Routing
  • Outline
  • Introduction to Routing
  • Distance Vector Algorithm

2
Overview
  • Forwarding vs Routing
  • forwarding to select an output port based on
    destination address and routing table
  • routing process by which routing table is built
  • Network as a Graph
  • Assume single admin. authority
  • Assume nodes are routers
  • Assume links have cost
  • Problem Find lowest cost path (sum of links)
    between two nodes

3
Routing Protocol Issues
  • It may be simple to calculate least cost path if
    graph is static but
  • Links and routers go down
  • Links and routers are added
  • Traffic can cause links to overload
  • How are costs calculated?
  • Algorithm must be distributed in order to scale
  • Rich area for research due to distributed,
    dynamic nature of the problem
  • Different routers can have different routes at
    same time

4
Distance Vector (Bellman-Ford,RIP)
  • Find SP for node such that paths contain at most
    1 hop, then 2
  • Each node maintains a set of triples
  • (Destination, Cost, NextHop)
  • Assume each node knows cost of directly connected
    neighbors
  • Exchange updates directly connected neighbors
  • periodically ( on the order of several seconds)
    even if there are no changes
  • whenever its table changes (called triggered
    update)
  • Each update is a list of pairs
  • (Destination, Cost)
  • Update local table if receive a better route
  • smaller cost
  • came from next-hop
  • Refresh existing routes delete if they time out

5
Example
5
  • Destination Cost NextHop
  • B 1 B
  • C 3 D
  • D 1 D
  • E 2 D
  • F 4 D
  • G 1 G

3
B
C
1
5
2
1
F
A
1
2
1
1
E
D
G
6
Routing Loops
  • Example 1
  • E detects that link to C has failed
  • E sets distance to C to infinity and sends update
    to D
  • D sets distance to C to infinity since it uses D
    to reach E
  • D receives periodic update from B with 2-hop path
    to C
  • D sets distance to C to 5 and sends update to E
  • E decides it can reach C in 3 hops via B
  • Example 2
  • link from A to G fails
  • A advertises distance of infinity to G
  • B and D advertise a distance of 2 to G
  • B decides it can reach G in 3 hops advertises
    this to A
  • A decides it can read G in 4 hops advertises
    this to D
  • D decides that it can reach G in 5 hops

7
Loop-Breaking Heuristics
  • Set infinity to 16
  • Assume this is maximum number of hops in network
  • Split horizon
  • Dont send routes learned from a neighbor back to
    a neighbor
  • Split horizon with poison reverse
  • Send route back to neighbor with negative
    information ie. Infinite
  • What are the problems?

8
Link State (Dijkstras algorithm,OSPF)
  • Find SP from a given node by sending path data to
    all nodes and developing paths in order of
    increasing length
  • Strategy
  • send to all nodes (not just neighbors)
    information about directly connected links (not
    entire routing table)
  • Link State Packet (LSP)
  • id of the node that created the LSP
  • cost of the link to each directly connected
    neighbor
  • sequence number (SEQNO)
  • time-to-live (TTL) for this packet

9
Link State (cont)
  • Reliable flooding
  • store most recent LSP from each node
  • forward LSP to all nodes but one that sent it
  • generate new LSP periodically
  • increment SEQNO
  • start SEQNO at 0 when reboot
  • decrement TTL of each stored LSP
  • discard when TTL0
  • This is not easy!
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