1. Routing & IP Routing Protocols

2. Routing: Problem Definition
You are a router in a packet switched network and you receive a packet destined to some remote node
E.g., router A below receives a packet destined to node F
Question: How does A know where to send this packet?
Does A send it to B? C? or D?
Answer: Recall from our earlier discussion in IP that router A consults a forwarding table to make this decision
Routing Problem: How does router A build this forwarding table?
Built by a routing algorithm (protocol): The job of the routing algorithm is to determine the next hop router for ALL destinations in the network A E D C B F 2 1 3 5
Dest Next Hop Cost
B B
D D
C D
E D
F D
Forwarding Table in A

3. End-to-End Path Determination: Routing principles
Routing protocol A E D C B F 2 1 3 5
Goal: determine “good” path
(sequence of routers) thru
network from source to dest.
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links
link cost: delay, $ cost, or congestion level (amount of traffic carried on the link)
“good” path:
typically means minimum cost path

4. Forwarding vs Routing Distinction between “Forwarding” and “Routing”
Forwarding consists of taking a packet, looking at its destination address, consulting the forwarding table, and sending the packet in a direction determined by the table
Very easy once the forwarding table has been built
Routing is the process by which the forwarding table is built
Need a routing protocol to dynamically build and maintain the table
Dest Next Hop Cost
B B
D D
C D
E D
F D
Forwarding table in A A E D C B F 2 1 3 5

5. Routing Algorithm classification
Global or decentralized information?
Global:
all routers have complete topology, link cost info
“link state” algorithms
Decentralized:
router knows physically-connected neighbors, link costs to neighbors
iterative process of computation, exchange of info with neighbors
“distance vector” algorithms

6. Link-State Algorithms: General Idea
Have each router build the complete topology of the network
Once the complete topology is built, have each router run an algorithm to compute the shortest path from itself to all other routers (nodes) in the network
2 Issues
How does a router build the complete topology of the network?
How does a router compute the shortest path to all other nodes in the network using this topology information?
Dijkstra’s Shortest Path Algorithm A E D C B F 2 1 3 5

7. Building the Network Topology
At the heart of a link state algorithm is the discovery of a node’s links’ states
Each node is assumed to capable of finding out the state of the link to its neighbors (up or down) and the cost of each link
Each node creates an update packet, also called a link-state packet (LSP) and periodically sends this information to all of its neighbors
Node’s neighbors send the packet to their neighbors and so on until the LSP is received by all nodes in the network
LSP Contents
ID of the node -- A
List of neighbors and the cost to each neighbor – (B, 2), (C, 5), (D, 1)
A sequence number
A time-to-live (TTL) A E D C B F 2 1 3 5

8. Reliable Flooding
Reliable flooding is the process of making sure that all nodes get a copy of LSP from all other nodes in the network
When a node X receives a copy of an LSP that originated at some other node Y, it checks to see if it has already stored a copy of an LSP from Y.
If not, it stores the LSP.
If it already has a copy, it compares the SeqNos; if the new LSP has a larger SeqNo, it is assumed to be more recent, and the last LSP is stored replacing the old one. Otherwise the new LSP is discarded A E D C B F 2 1 3 5
The new LSP is then forwarded on all neighbors of X except the neighbor from which the LSP was just received

9. Computing the Shortest Path: Dijkstra’s Algorithm
Notation:
Cost(i,j): link cost from node i to j.
Cost(A, B) = 2
Cost(A, C) = 5
Cost(A, F) = infinity
Distance(v): current value of cost of path from source to destination V
Distance(D) = 1
Pred(v): predecessor node of v along path from source to v
Pred(D) = A
N: set of nodes whose least cost path definitively known A E D C B F 2 1 3 5

10. Dijsktra’s Algorithm – where A is the source node
1 Initialization:
2 N = {A}
3 for all nodes v
if v adjacent to A
then Distance (v) = cost(A,v)
else Distance (v) = infinity 7
8 Loop
9 find w not in N such that Distance(w) is a minimum
10 add w to N
11 update Distance(v) for all v adjacent to w and not in N:
Distance(v) = min( Distance(v), Distance(w) + cost(w,v) )
13 /* new distance to v is either old distance to v or known
shortest path distance to w plus cost from w to v */
15 until all nodes in N

11. Dijkstra’s algorithm: example
Step 1 2 3 4 5 N A AD
ADE
ADEB
ADEBC
ADEBCF
D(B),p(B)
2,A
D(C),p(C)
5,A
4,D
3,E
D(D),p(D)
1,A
D(E),p(E)
infinity
2,D
D(F),p(F)
infinity
4,E 5 3 B C 5 2 A 2 1 F 3 1 2 D E 1
Algorithm complexity: n nodes
each iteration: need to check all nodes, w, not in N
n*(n+1)/2 comparisons: O(n2) – O(nlogn) algorithm possible

12. Distance Vector Algos: General Idea
We do NOT need to know the complete topology of the network to build the forwarding table!
If a node X simply tells its neighbor Y the cost of reaching another node Z via itself (X), then
neighbor Y can compute the cost of reaching Z via X by simply adding the cost of reaching X from Y and the cost of reaching Z from X, which was advertised by X
Example
If D tells A that it can reach E with a cost of 1, then A knows it can reach E via D with a cost of 1+1 = 2
If D tells A that it can reach F with a cost of 3, then A knows it can reach F via D with a cost of 1+3 = 4 A E D C B F 2 1 3 5

13. Distance Vector Routing Algorithm
Each node tells its neighbors the cost of reaching every other node via itself
A node computes its cost of reaching a destination via each of its neighbors and picks the best one
distributed:
each node communicates only with directly-attached neighbors
iterative:
continues until no nodes exchange new information
self-terminating: no “signal” to stop
D (Y,Z) X
distance from X to
Y via Z as next hop
c(X,Z) + D (Z) Y =
D (Y)
min {D (Y,w)} w
D (Z) Y
C(X,Y) X Y Z
C(X,T) T
D (Z) T

14. Distance Table Structure
Distance Table Data Structure
each node has its own distance table
row for each possible destination
column for each directly-attached neighbor to node
example: in node X, for dest. Y via neighbor Z:
D () A B C D 1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination A E D C B 7 8 1 2

15. Distance Table gives Forwarding Table1 7 6 4 14 8 9 11 5 2 E
cost to destination via
destination
Outgoing link
to use, cost A B C D
A,1
D,5
D,4
destination
Distance Table
Forwarding (Routing) Table

16. Distance Vector Routing: overview
Each node:
wait for (msg from neighbor)
recompute distance table
if least cost path to any dest has changed, notify neighbors
Initialize the Distance Table Structure

17. Initialization At all nodes, X: 1 Initialization:
2 for all adjacent nodes v:
D (*,v) = infinity /* the * operator means "for all rows" */
D (v,v) = c(X,v)
5 for all destinations, y
send D (y) to each neighbor /* Cost of reaching Y via X */ X X X

18. Distance Vector Algorithm: Example
We need to adjust
values
if necessary X Z 1 2 7 Y
This column
shows the
Init. results
Init
Info
sent

19. Distance Vector Algorithm (cont.):
8 loop
9 wait (until I receive update from neighbor V)
10 if (update received from neighbor V wrt destination Y)
/* shortest path from V to some Y has changed */
/* V has sent a new value for its DV(Y) */
/* call this received new value is "newval" */
for the single destination Y: D (Y,V) = c(X,V) + newval 15
16 if we have a new D (Y) for any destination Y
send new value of D (Y) to all neighbors 18
19 forever X X X

20. Distance Vector Algorithm: Example
info
sent
Adjusted new value
Initial values X Z 1 2 7 Y
D (Z,Y) X
c(X,Y) + D (Z) =
2+1 = 3 Y
D (Y,Z) X
c(X,Z) + D (Y) =
7+1 = 8 Z

21. Distance Vector Algorithm: Example
Adjust
values
if necessary
This column
Shows the
Init. results
Init
Info
sent
Adjusted new values
New
Info
sent X Z 1 2 7 Y

22. Hierarchical Routing 1. scale: with 200 million destinations:
Our routing study thus far assumed
all routers to be “identical” and the network to be “flat”
2 Problems exist with such a model:
1. scale: with 200 million destinations:
can’t store all destinations in routing tables!
routing table exchange would swamp links!
2. administrative autonomy
Can’t assume that a network with the scale of Internet will be administered by a single authority
Solution?
Internet is NOT flat, but is a network of networks
each network admin controls routing in its own network – next
Turkey’s past and current Internet backbone map

23. Hierarchical Routing gateway routers C.b B.a A.a b c A.c a a C b a B d
special routers in AS
run intra-AS routing protocol with all other routers in AS
also responsible for routing to destinations outside AS
run inter-AS routing protocol with other gateway routers
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol
“intra-AS” routing protocol
routers in different AS can run different intra-AS routing protocol

24. Intra-AS Routing Also known as Interior Gateway Protocols (IGP)
Most common Intra-AS routing protocols:
OSPF: Open Shortest Path First
RIP: Routing Information Protocol
IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

25. OSPF (Open Shortest Path First)
“open”: publicly available
Uses Link State algorithm
LS advertisement dissemination to entire AS via flooding
Topology map at each node
Route computation using Dijkstra’s algorithm
Carried in OSPF messages directly over IP
OSPF has its own network layer protocol number like IP!

26. RIP (Routing Information Protocol)
Distance vector algorithm
Included in BSD-UNIX Distribution in 1982
Distance metric: # of hops (max = 15 hops)
16 is the infinity to eliminate routing loops
Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)
Each advertisement: list of up to 25 destination nets within AS
If more destinations, send multiple advertisements

27. RIP: Example z w x y A D B C y B 2 z B 7 x -- 1
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B 7 x
…. …
Routing table in D

28. RIP: Example w x y z A C D B y B 2 z B A 7 5 x -- 1 Advertisement
Dest Next hops w x
z C 4
…. …
Advertisement
from A to D w x y z A C D B
Destination Network Next Router Num. of hops to dest.
w A 2
y B 2
z B A 7 5 x
…. …
Routing table in D

29. RIP Table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
routed
routed
Transprt
(UDP)
Transprt
(UDP)
network forwarding
(IP) table
network
(IP)
forwarding
table
link
link
physical
physical

30. Inter-AS routing in the Internet: BGP
Each BGP router communicates only with its neighbors
R1 with R2, R3 with R4
Global info about routes to destination networks propagates in an AS-by-AS manner via the exchange of BGP messages

31. Why different Intra- and Inter-AS routing ?
Policy:
Intra-AS: single admin, so no policy decisions needed
Inter-AS: admin wants control over how its traffic routed, who routes through its net.
Performance:
Intra-AS: can focus on performance
Inter-AS: policy may dominate over performance