Internet Protocol: Routing AlgorithmsInternet Protocol: Routing Algorithms

Srinidhi Varadarajan

RoutingRouting

Graph abstraction for routing algorithms:graph nodes are routersgraph edges are physical links– link cost: delay, $ cost,

or congestion level

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– other def’s possible

Routing Algorithm classificationRouting 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 neighborsiterative process of computation, exchange of info with neighbors“distance vector” algorithms

Static or dynamic?Static:

routes change slowly over time

Dynamic:routes change more quickly– periodic update– in response to link

cost changes

A LinkA Link--State Routing AlgorithmState Routing Algorithm

Dijkstra’s algorithmnet topology, link costs known to all nodes– accomplished via “link

state broadcast” – all nodes have same

infocomputes least cost paths from one node (‘source”) to all other nodes– gives routing table for

that nodeiterative: after k iterations, know least cost path to k dest.’s

Notation:c(i,j): link cost from node i to j. cost infinite if not direct neighborsD(v): current value of cost of path from source to dest. Vp(v): predecessor node along path from source to v, that is next vN: set of nodes whose least cost path definitively known

Dijsktra’sDijsktra’s AlgorithmAlgorithm1 Initialization:2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 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 N

Dijkstra’s Dijkstra’s algorithm: examplealgorithm: exampleStep

012345

start NA

ADADE

ADEBADEBC

ADEBCF

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)infinity

2,D

D(F),p(F)infinityinfinity

4,E4,E4,E

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Dijkstra’s Dijkstra’s algorithm, discussionalgorithm, discussionAlgorithm computational complexity: n nodes

each iteration: need to check all nodes, w, not in Nn*(n+1)/2 comparisons: O(n2)more efficient implementations possible: – O(nlogn): Use a heap (sorted) to maintain interim table

Oscillations possible:e.g., link cost = amount of carried traffic

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001+e 1

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initially … recomputerouting

… recompute … recompute

Link State: Reliable FloodingLink State: Reliable FloodingLink State routers exchange information using Link State Packets (LSP).LSP contains– 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

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 TTL=0

Distance Vector Routing AlgorithmDistance Vector Routing Algorithmiterative:

continues until no nodes exchange info.self-terminating: no “signal” to stop

asynchronous:nodes need notexchange info/iterate in lock step!

distributed:each node communicates onlywith directly-attached neighbors

Distance Table data structureeach node has its ownrow for each possible destinationcolumn for each directly-attached neighbor to node

Distance Table: exampleDistance Table: example

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E cost to destination via

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D (C,D)E

c(E,D) + min {D (C,w)}Dw=

= 2+2 = 4

D (A,D)E

c(E,D) + min {D (A,w)}Dw=

= 2+3 = 5

D (A,B)E

c(E,B) + min {D (A,w)}Bw=

= 8+6 = 14

loop!

loop!

Distance table Distance table givesgives routing tablerouting table

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Outgoing link to use, cost

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Distance table Routing table

Distance Vector Routing: overviewDistance Vector Routing: overviewIterative, asynchronous:

each local iteration caused by: local link cost change message from neighbor: its least cost path change from neighbor

Distributed:each node notifies neighbors only when its least cost path to any destination changes– neighbors then notify

their neighbors if necessary

wait for (change in local link cost of msg from neighbor)

recompute distance table

if least cost path to any dest has changed, notifyneighbors

Each node:

Distance Vector: link cost changesDistance 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)

X Z14

50

Y1

algorithmterminates

“goodnews travelsfast”

Distance Vector: link cost changesDistance Vector: link cost changesLink cost changes:

good news travels fast bad news travels slow -“count to infinity” problem!

X Z14

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Y60

algorithmcontinues

on!

Distance Vector: poisoned reverseDistance Vector: poisoned reverse

If Z routes through Y to get to X :Z tells Y its (Z’s) distance to X is infinite (so Y won’t route to X via Z)Does not work on larger loops

X Z14

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Y60

algorithmterminates

Comparison of LS and DV algorithmsComparison of LS and DV algorithms

Message complexityLS: with n nodes, with an average of l links/node, each node sends O(nl). Total messages O(n2l)DV: exchange between neighbors only– 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 tableDV:

– DV node can advertise incorrect path cost

– each node’s table used by others

• error propagate thru network

Hierarchical RoutingHierarchical Routing

scale: with 50 million destinations:can’t store all dest’s in routing tables!routing table exchange would swamp links!

administrative autonomyinternet = network of networkseach network admin may want to control routing in its own network

Our routing study thus far -idealization all routers identicalnetwork “flat”

… not true in practice

Hierarchical RoutingHierarchical Routing

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

special routers in ASrun intra-AS routing protocol with all other routers in ASalso responsible for routing to destinations outside AS– run inter-AS routing

protocol with other gateway routers

gateway routers

Why different IntraWhy different Intra-- and Interand Inter--AS routing ?AS routing ?

Policy:Inter-AS: admin wants control over how its traffic is routed and who routes through its net. Intra-AS: single admin, so no policy decisions needed

Scale:hierarchical routing saves table size, reduced update traffic

Performance:Intra-AS: can focus on performanceInter-AS: policy may dominate over performance

IntraIntra--AS and InterAS and Inter--AS routingAS routingGateways:

•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layerlink layer

physical layer

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IntraIntra--AS and InterAS and Inter--AS routingAS routing

Host h2

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Intra-AS routingwithin AS A

Inter-ASrouting

between A and B

Intra-AS routingwithin AS B

Routing in the InternetRouting in the Internet

The Global Internet consists of Autonomous Systems (AS) interconnected with each other:– Stub AS: small corporation– Multihomed AS: large corporation (no transit)– Transit AS: provider

Two-level routing: – Intra-AS: administrator is responsible for choice– Inter-AS: unique standard

Internet AS HierarchyInternet AS HierarchyIntra-AS border (exterior gateway) routers

Inter-AS interior (gateway) routers

IntraIntra--AS RoutingAS Routing

Also known as Interior Gateway Protocols (IGP)Most common IGPs:

– RIP: Routing Information Protocol

– OSPF: Open Shortest Path First

– IGRP: Interior Gateway Routing Protocol (Cisco proprietary.)

RIP ( Routing Information Protocol)RIP ( Routing Information Protocol)

Distance vector algorithmIncluded in BSD-UNIX Distribution in 1982Distance metric: # of hops (max = 15 hops)– Can you guess why?

Distance vectors: exchanged every 30 sec via Response Message (also called advertisement)Each advertisement: routes for up to 25 destination nets

RIP (Routing Information Protocol)RIP (Routing Information Protocol)

Destination Network Next Router Num. of hops to dest.w A 2y B 2z B 7x -- 1…. …. ....

w x y

z

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Routing table in D

RIP: Link Failure and Recovery RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declared dead– routes via neighbor invalidated– new advertisements sent to neighbors– neighbors in turn send out new

advertisements (if tables changed)– link failure info quickly propagates to

entire net– poison reverse used to prevent ping-pong

loops (infinite distance = 16 hops)

RIP Table processingRIP Table processing

RIP routing tables managed by application-level process called routed (daemon)advertisements sent in UDP packets, periodically repeated

RIP Table example (continued)RIP Table example (continued)Router: giroflee.eurocom.fr

Three attached class C networks (LANs)Router only knows routes to attached LANsDefault router used to “go up”Route multicast address: 224.0.0.0Loopback interface (for debugging)

Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ ---------127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454

OSPF (Open Shortest Path First)OSPF (Open Shortest Path First)

“open”: publicly availableUses Link State algorithm – LS packet dissemination– Topology map at each node– Route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor routerAdvertisements disseminated to entire AS (via flooding)

OSPF “advanced” features (not in RIP)OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections usedMultiple same-cost paths allowed (only one path in RIP)For each link, multiple cost metrics for different TOS (eg, satellite link cost set “low” for best effort; high for real time)Integrated uni- and multicast support: – Multicast OSPF (MOSPF) uses same topology

data base as OSPFHierarchical OSPF in large domains.

Hierarchical OSPFHierarchical OSPF

Hierarchical OSPFHierarchical OSPFTwo-level hierarchy: local area, backbone.– Link-state advertisements only in area – each nodes has detailed area topology;

only know direction (shortest path) to nets in other areas.

Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.Backbone routers: run OSPF routing limited to backbone.Boundary routers: connect to other ASs.

IGRP (Interior Gateway Routing Protocol)IGRP (Interior Gateway Routing Protocol)

CISCO proprietary; successor of RIP (mid 80s)Distance Vector, like RIP– Hold time– Split Horizon– Poison Reverse

several cost metrics (delay, bandwidth, reliability, load etc)uses TCP to exchange routing updatesEIGRP (Garcia-Luna): Loop-free routing via Distributed Updating Algorithm. (DUAL) based on diffused computation– Uses a mix of link-state and distance vector

InterInter--AS routingAS routing

Internet interInternet inter--AS routing: BGPAS routing: BGP

BGP (Border Gateway Protocol): the de facto standardPath Vector protocol:– similar to Distance Vector protocol– each Border Gateway broadcast to neighbors

(peers) entire path (I.e, sequence of ASs) to destination

– E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z

Internet interInternet inter--AS routing: BGPAS routing: BGP

Suppose: gateway X send its path to peer gateway WW may or may not select path offered by X– cost, policy (don’t route via competitors

AS), loop prevention reasons.If W selects path advertised by X, then:

Path (W,Z) = w, Path (X,Z)Note: X can control incoming traffic by controlling it route advertisements to peers:– e.g., don’t want to route traffic to Z -> don’t

advertise any routes to Z

Internet interInternet inter--AS routing: BGPAS routing: BGP

BGP messages exchanged using TCP.BGP messages:– OPEN: opens TCP connection to peer and

authenticates sender– UPDATE: advertises new path (or

withdraws old)– KEEPALIVE keeps connection alive in

absence of UPDATES; also ACKs OPEN request

– NOTIFICATION: reports errors in previous msg; also used to close connection

Other Routing TechniquesOther Routing Techniques

Hot-Potato Routing a.k.a Deflection Routing– Use the first available link irrespective of

whether it leads to the destination or not.Cut Through routing– Non-store and forward: Routes before entire

packet is received at the router.– Outgoing link is reserved. What happens if a

fast links succeeds a slow link?

ReadingReading

Recommended– End-To-End Routing Behavior in the Internet,

V. Paxson, SIGCOMM 1996. ftp://ftp.ee.lbl.gov/papers/routing.SIGCOMM.ps.Z

– Persistent Route Oscillations in Inter-Domain Routing, K. Varadhan, R. Govindan, D. Estrin, ftp://ftp.isi.edu/ra/Publications/bgp_osc.ps.gz