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Routing & IP Routing Protocols
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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
ED
CB
F
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Dest. Next Hop Cost B B 2 D D 1 C D 3 E D 2 F D 4
Forwarding Table in A
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End-to-End Path Determination: Routing principles
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)
Goal: determine “good” path
(sequence of routers) thru network from source to
dest.
Routing protocol
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• “good” path:– typically means
minimum cost path
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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
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Dest. Next Hop Cost
B B 2 D D 1 C D 3 E D 2 F D 4
Forwarding table in A
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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
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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
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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
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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
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5LSP 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)
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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
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5• The new LSP is then forwarded on all neighbors of X except the neighbor from which the LSP was just received
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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
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Dijsktra’s Algorithm – where A is the source node
1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then Distance (v) = cost(A,v) 6 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: 12 Distance(v) = min( Distance(v), Distance(w) + cost(w,v) ) 13 /* new distance to v is either old distance to v or known 14 shortest path distance to w plus cost from w to v */ 15 until all nodes in N
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Dijkstra’s algorithm: example
Step012345
NAADADEADEBADEBCADEBCF
D(B),p(B)2,A2,A2,A
D(C),p(C)5,A4,D3,E3,E
D(D),p(D)1,A
D(E),p(E)infinity2,D
D(F),p(F)infinityinfinity4,E4,E4,E
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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
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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
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5Example
• 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
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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 toY via Z as next hop
c(X,Z) + D (Z)Y
=
=
D (Y) X
min {D (Y,w)}X
w=
X Y ZC(X,Y)
D (Z)Y
T
C(X,T)
D (Z)T
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Distance Table StructureDistance 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:
A
E D
CB7
8
1
2
1
2
D ()
A
B
C
D
A
1
7
6
4
B
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D
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Ecost to destination via
dest
inat
ion
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Distance Table gives Forwarding Table
D ()
A
B
C
D
A
1
7
6
4
B
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8
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D
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Ecost to destination via
dest
inat
ion
A
B
C
D
A,1
D,5
D,4
D,4
Outgoing link to use, cost
dest
inat
ion
Distance Table Forwarding (Routing) Table
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Distance Vector Routing: overviewEach 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
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Initialization
1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infinity /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v)
5 for all destinations, y 6 send D (y) to each neighbor /* Cost of reaching Y via X */
XX
X
At all nodes, X:
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Distance Vector Algorithm: Example
X Z12
7
Y
This columnshows theInit. results
InitInfo sent
We need to adjustvalues
if necessary
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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) 11 /* shortest path from V to some Y has changed */ 12 /* V has sent a new value for its DV(Y) */ 13 /* call this received new value is "newval" */ 14 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 17 send new value of D (Y) to all neighbors 18 19 forever
X
X
X
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X Z12
7
Y
D (Y,Z)X
c(X,Z) + D (Y)=
= 7+1 = 8
Z
D (Z,Y)X
c(X,Y) + D (Z)=
= 2+1 = 3
Y
Distance Vector Algorithm: Exampleinfo sent
Initial values
Adjusted new value
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Distance Vector Algorithm: Example
X Z12
7
Y
This columnShows theInit. results
InitInfo sent
Adjusted new
values
NewInfo sent
Adjustvalues
if necessar
y
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Hierarchical RoutingOur 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– http://www.ilkertemir.com/backbone-map/
• Turkey’s past and current Internet backbone map
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Hierarchical Routing
a
b
b
aaC
A
Bd
A.a
A.c
C.bB.a
cb
c
• 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
gateway routers• 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
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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)
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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!
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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
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RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B 7
x -- 1…. …. ....
w x y
z
A
C
D B
Routing table in D
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RIP: Example
Destination Network Next Router Num. of hops to dest. w A 2
y B 2 z B A 7 5
x -- 1…. …. ....Routing table in D
w x y
z
A
C
D B
Dest Next hops w - - x - - z C 4 …. … ...
Advertisementfrom A to D
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RIP Table processing• RIP routing tables managed by application-level
process called route-d (daemon)• advertisements sent in UDP packets, periodically
repeated
physical
link
network forwarding (IP) table
Transprt (UDP)
routed
physical
link
network (IP)
Transprt (UDP)
routed
forwardingtable
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Inter-AS routing in the Internet: BGP
Figure 4.5.2-new2: BGP use for inter-domain routing
AS2 (OSPF
intra-AS routing)
AS1 (RI P intra-AS
routing) BGP
AS3 (OSPF intra-AS
routing)
BGP
R1 R2
R3
R4
R5
• 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
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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
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