Data Communication and Networks Lecture 11 Internet Routing Algorithms and Protocols December 5, 2002 Joseph Conron Computer Science Department New York University

Some Perspective on Routing ….. When we wish to take a long trip by car, we consult a road map. The road map shows the possible routes to our destination. It might show us the shortest distance, but, it can’t always tell us what we really want to know: –What is the fastest route! –Why is this not always obvious? Question: What’s the difference between you and an IP Packet?

Packets are Dumb, Students are Smart! We adapt to traffic conditions as we go. Packets depend on routers to choose how they get their destination. Routers have maps just like we do. These are called routing tables. What we want to know is: –How to these tables get constructed/updated? –How are routes chosen using these tables?

Static Vs. Dynamic Routing Routes are static if they do not change. –Route table is loaded once at startup and all changes are manual –Computers at the network edge use static routing. Routes are dynamic if the routing table information can change over time (without human intervention. –Internet routers use dynamic routing.

Routing Table Example

Dynamic Routing and Routers To insure that routers know how to reach all possible destinations, routers exchange information using a routing protocol. But, we cannot expect every router to know about every other router. –Too much Internet traffic would be generated. –Tables would be huge (10 6 routers) –Algorithms to choose “best” path would never terminate. How to handle this?

Autonomous Systems (AS) Routers are divided into groups known as an autonomous systems (AS). ASs communicate using an Exterior Routing Protocol (Intra-AS Routing) Routers within an AS communicate using an Interior Routing Protocol (Inter-AS Routing)

Interior and Exterior Routing

Why different Intra and Inter-AS routing ? Policy: Inter-AS: admin wants control over how its traffic routed, 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 performance Inter-AS: policy may dominate over performance

Routing Algorithms Graph abstraction for routing algorithms: graph nodes are routers graph 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 A E D CB F “good” path: –typically means minimum cost path –other def’s possible

Routing Algorithm classification Static or dynamic? Static: –routes change slowly over time Dynamic: –routes change more quickly periodic update in response to link cost changes

Routing Algorithm classification Global or decentralized? 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

A Link-State Routing Algorithm Dijkstra’s algorithm net topology, link costs known to all nodes –accomplished via “link state broadcast” –all nodes have same info computes least cost paths from one node (‘source”) to all other nodes –gives routing table for that node Iterative –after k iterations, know least cost path to k dest.’s

A Link-State Routing Algorithm Notation: 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 dest. V p(v): predecessor node along path from source to v, that is next v N: set of nodes whose least cost path definitively known

Dijsktra’s Algorithm 1 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 Loop 9 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 algorithm: example Step start 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 A E D CB F

Dijkstra’s algorithm, discussion Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n**2) more efficient implementations possible: O(nlogn) Oscillations possible: e.g., link cost = amount of carried traffic A D C B 1 1+e e 0 e A D C B 2+e e 1 A D C B 0 2+e 1+e A D C B 2+e 0 e 0 1+e 1 initially … recompute routing … recompute

Distance Vector Routing Algorithm iterative: continues until no nodes exchange info. self-terminating: no “signal” to stop asynchronous: nodes need not exchange info/iterate in lock step! distributed: each node communicates only with directly-attached neighbors Distance Table data structure each node has its own row for each possible destination column for each directly-attached neighbor to node example: in node X, for dest. Y via neighbor Z: D (Y,Z) X distance from X to Y, via Z as next hop c(X,Z) + min {D (Y,w)} Z w = =

Distance Table: example A E D CB D () A B C D A1764A1764 B D5542D5542 E cost to destination via destination D (C,D) E c(E,D) + min {D (C,w)} D w = = 2+2 = 4 D (A,D) E c(E,D) + min {D (A,w)} D w = = 2+3 = 5 D (A,B) E c(E,B) + min {D (A,w)} B w = = 8+6 = 14 loop!

Distance table gives routing table D () A B C D A1764A1764 B D5542D5542 E cost to destination via destination ABCD ABCD A,1 D,5 D,4 D,2 Outgoing link to use, cost destination Distance table Routing table

Distance Vector Routing: overview Iterative, 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, notify neighbors Each node:

Distance Vector Algorithm: 1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infty /* 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 At all nodes, X:

Distance Vector Algorithm (cont.): 8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 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 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 DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 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 forever w X X X X X w w

Distance Vector Algorithm: example X Z Y

X Z Y D (Y,Z) X c(X,Z) + min {D (Y,w)} w = = 7+1 = 8 Z D (Z,Y) X c(X,Y) + min {D (Z,w)} w = = 2+1 = 3 Y

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) X Z Y 1 algorithm terminates “good news travels fast”

Distance Vector: link cost changes Link cost changes: good news travels fast bad news travels slow - “count to infinity” problem! X Z Y 60 algorithm continues on!

Distance 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) will this completely solve count to infinity problem? X Z Y 60 algorithm terminates

Comparison of LS and DV algorithms Message complexity LS: with n nodes, E links, O(nE) msgs sent each DV: exchange between neighbors only –convergence time varies Speed of Convergence LS: O(n**2) algorithm requires O(nE) msgs –may have oscillations DV: convergence time varies –may be routing loops –count-to-infinity problem

Comparison of LS and DV algorithms 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 node’s table used by others error propagates thru network

Interior and Exterior Routing

Interior Router Protocol (IRP) Passes routing information between routers within AS May be more than one AS in internet Routing algorithms and tables may differ between different AS Routers need some info about networks outside their AS Use exterior router protocol (ERP) IRP needs detailed model ERP supports summary information on reachability

Interior Routing Also known as Interior Gateway Protocols (IGP) Passes routing information between routers within AS Most common IGPs: –RIP: Routing Information Protocol –OSPF: Open Shortest Path First Routing algorithms and tables may differ between different AS

RIP ( Routing Information Protocol) Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) –Can you guess why? Distance vectors: exchanged every 30 sec via Response Message (also called advertisement) Each advertisement: route to up to 25 destination nets

OSPF (Open Shortest Path First) “open”: publicly available Uses Link State algorithm –LS packet dissemination –Topology map at each node –Route computation using Dijkstra’s algorithm OSPF advertisement carries one entry per neighbor router Advertisements disseminated to entire AS (via flooding)

Sample AS

Directed Graph of AS

Exterior Routing

Border Gateway Protocol (BGP) For use with TCP/IP internets Preferred EGP of the Internet Procedures –Neighbor acquisition –Neighbor reachability –Network reachability

Internet Exterior 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

BGP Procedure Open TCP connection Send Open message –Includes proposed hold time Receiver selects minimum of its hold time and that sent –Max time between Keep alive and/or update messages

BGP Message Types Keep Alive –To tell other routers that this router is still here Update –Info about single routes through internet –List of routes being withdrawn –Includes path info Origin (IGP or EGP) AS_Path (list of AS traversed) Next_hop (IP address of border router) Multi_Exit_Disc (Info about routers internal to AS) Local_pref (Inform other routers within AS) Atomic_Aggregate, Aggregator (Uses address tree structure to reduce amount of info needed)

Uses of AS_Path and Next_Hop AS_Path –Enables routing policy Avoid a particular AS Security Performance Quality Number of AS crossed Next_Hop –Only a few routers implement BGP Responsible for informing outside routers of routes to other networks in AS

BGP Routing Information Exchange Within AS, router builds topology picture using IGP Router issues Update message to other routers outside AS using BGP These routers exchange info with other routers in other AS Routers must then decide best routes