EE 122: Intradomain routing

1
EE 122 Intra-domain routing

• Ion Stoica
• September 30, 2002

( this presentation is based on the on-line
slides of J. Kurose K. Rose)
2
Internet Routing

• Internet organized as a two level hierarchy
• First level autonomous systems (ASs)
• AS region of network under a single
  administrative domain
• ASs run an intra-domain routing protocols
• Distance Vector, e.g., RIP
• Link State, e.g., OSPF
• Between ASs runs inter-domain routing protocols,
  e.g., Border Gateway Routing (BGP)
• De facto standard today, BGP-4

3
Example
Interior router
BGP router
AS-1
AS-3
AS-2
4
Intra-domain Routing Protocols

• Based on unreliable datagram delivery
• Distance vector
• Routing Information Protocol (RIP), based on
  Bellman-Ford
• Each neighbor periodically exchange reachability
  information to its neighbors
• Minimal communication overhead, but it takes long
  to converge, i.e., in proportion to the maximum
  path length
• Link state
• Open Shortest Path First Protocol (OSPF), based
  on Dijkstra
• Each network periodically floods immediate
  reachability information to other routers
• Fast convergence, but high communication and
  computation overhead

5
Routing

• Goal determine a good path through the network
  from source to destination
• Good means usually the shortest path
• Network modeled as a graph
• Routers ? nodes
• Link ?edges
• Edge cost delay, congestion level,

6
A Link State Routing Algorithm

• Dijkstras algorithm
• Net topology, link costs known to all nodes
• Accomplished via link state broadcast
• All nodes have same info
• Compute least cost paths from one node (source)
  to all other nodes
• Iterative after k iterations, know least cost
  path to k closest destinations

• Notations
• c(i,j) link cost from node i to j cost infinite
  if not direct neighbors
• D(v) current value of cost of path from source
  to destination v
• p(v) predecessor node along path from source to
  v, that is next to v
• S set of nodes whose least cost path
  definitively known

7
Dijsktras Algorithm
1 Initialization 2 S A 3 for all
nodes v 4 if v adjacent to A 5 then
D(v) c(A,v) 6 else D(v) 7 8
Loop 9 find w not in S such that D(w) is a
minimum 10 add w to S 11 update D(v)
for all v adjacent to w and not in S 12
D(v) min( D(v), D(w) c(w,v) ) 13 //
new cost to v is either old cost to v or known
14 // shortest path cost to w plus cost
from w to v 15 until all nodes in S
8
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A
D(E),p(E)
start S A
D(F),p(F)
5
3
5
2
2
1
3
1
2
1
9
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D
D(E),p(E) 2,D
start S A AD
D(F),p(F)
5
3
5
2
2
1
3
1
2
1
10
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D 3,E
D(E),p(E) 2,D
start S A AD ADE
D(F),p(F) 4,E
5
3
5
2
2
1
3
1
2
1
11
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D 3,E
D(E),p(E) 2,D
start S A AD ADE ADEB
D(F),p(F) 4,E
5
3
5
2
2
1
3
1
2
1
12
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D 3,E
D(E),p(E) 2,D
start S A AD ADE ADEB ADEBC
D(F),p(F) 4,E
5
3
5
2
2
1
3
1
2
1
13
Example Dijkstras Algorithm
D(B),p(B) 2,A
D(D),p(D) 1,A
Step 0 1 2 3 4 5
D(C),p(C) 5,A 4,D 3,E
D(E),p(E) 2,D
start S A AD ADE ADEB ADEBC ADEBCF
D(F),p(F) 4,E
5
3
5
2
2
1
3
1
2
1
14
Dijkstras Algorithm Discussion

• Algorithm complexity n nodes
• Each iteration need to check all nodes, w, not
  in S
• n(n1)/2 comparisons O(n2)
• More efficient implementations possible
  O(nlog(n))
• Oscillation possible
• E.g., link cost amount of carried traffic

1
1e
0
0
e
0
1
1
e
initially
15
Distance Vector Routing Algorithm

• Iterative continues until no nodes exchange info
• Asynchronous nodes need not exchange
  info/iterate in lock step!
• Distributed each node communicates only with
  directly-attached neighbors
• Routing (distance) table data structure each
  router maintains
• Row for each possible destination
• Column for each directly-attached neighbor to
  node
• Entry in row Y and column Z of node X ? distance
  from X to Y, via Z as next hop

16
Example Distance (Routing) Table
1
6
2
8
1
E
2
loop!
loop!
17
Routing Table ? Forwarding Table
Outgoing link to use, cost
A B C D
A,1 D,5 D,4 D,2
destination
Forwarding table
Distance (routing) table
18
Distance Vector Routing Overview
Each node

• Each local iteration caused by
• Local link cost change
• Message from neighbor its least cost path change
  from neighbor
• Each node notifies neighbors only when its least
  cost path to any destination changes
• Neighbors then notify their neighbors if
  necessary

19
Distance Vector Algorithm
At all nodes, X
1 Initialization 2 for all adjacent nodes v
3 D (,v) / the operator
means "for all rows" / 4 D (v,v) c(X,v)
5 for all destinations, y 6 send min D
(y,w) to each neighbor / w over all X's
neighbors /
X
X
X
w
20
Distance Vector Algorithm (contd)
8 loop 9 wait (until I see a link cost
change to neighbor V 10 or until I
receive update from neighbor V) 11 12 if
(c(X,V) changes by d) 13 / change cost to
all dest's via neighbor v by d / 14 /
note d could be positive or negative / 15
for all destinations y D (y,V) D (y,V) d
16 17 else if (update received from V wrt
destination Y) 18 / shortest path from V to
some Y has changed / 19 / V has sent a
new value for its min D (Y,w) / 20 /
call this received new value is "newval" /
21 for the single destination y D (Y,V)
c(X,V) newval 22 23 if we have a new min
D (Y,w) for any destination Y 24 send new
value of min D (Y,w) to all neighbors 25 26
forever
X
X
V
w
X
X
w
X
w
21
Example Distance Vector Algorithm
22
Example Distance Vector Algorithm
23
Example Distance Vector Algorithm
24
Example Distance Vector Algorithm
25
Distance Vector Link Cost Changes

• Link cost changes
• Node detects local link cost change
• Updates distance table (line 15)
• If cost change in least cost path, notify
  neighbors (lines 23,24)

algorithm terminates
good news travels fast
26
Distance Vector Link Cost Changes

• Link cost changes
• Good news travels fast
• Bad news travels slow - count to infinity
  problem!

algorithm continues on!
27
Distance Vector Poisoned Reverse

• If Z routes through Y to get to X
• Z tells Y its (Zs) distance to X is infinite (so
  Y wont route to X via Z)
• Will this completely solve count to infinity
  problem?

algorithm terminates
28
Link State vs. Distance Vector

• Per node message complexity
• LS O(ne) messages n number of nodes e
  number of edges
• DV O(d) messages where d is nodes degree
• Complexity
• LS O(n2) with O(ne) messages
• DV convergence time varies
• may be routing loops
• count-to-infinity problem

• Robustness what happens if router malfunctions?
• LS
• node can advertise incorrect link cost
• each node computes only its own table
• DV
• DV node can advertise incorrect path cost
• each nodes table used by others error propagate
  through network