Advanced Computer Networks

Transcription

1 istance Vector dvanced omputer Networks Internal routing - distance vector protocols Prof. ndrzej uda duda@imag.fr ontents Principles of internal routing istance vector (ellman-ford) principles case of link failures count to infinity split horizon RIP RIP v IGRP Routing Routing protocol 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 good path: 5 typically means minimum cost path other def s possible F Routing algorithms Problem find the best route to a destination What does it mean the best? metric to measure how a route is good hops link capacity performance measures: link load, delay cost Graph optimization - Shortest Path find the shortest path in a graph shortest in the sense of a metric 3 4 Main algorithms Metrics istance vector (ellman-ford) routers only know their local state link metric and neighbor estimates internal routing protocols (RIP, IGRP) Link state knowledge of the global state metrics of all links global optimization (Shortest Path First - ijkstra) internal routing protocols (OSPF, PNNI (TM)) Path vector knowledge of the global state path: sequence of S with attributes global optimization and policy routing external routing protocols (GP) 5 Static - do not depend on the network state number of hops link capacity and static delay cost ynamic - depend on the network state link load current delay 6

2 istance Vector Traffic matrix Traffic Link model queueing system M/M/ exponentially distributed service and interarrival times λ µ T = µ "! 7 8 elay Parameters Mb/s and.5 Mb/s links mean packet length /µ - 5 Kbytes (4 ) bits transmission time on Mb/s link: 4 ms transmission time on.5 Mb/s link: 8 ms λ pq/s Mb/s µ pq/s T ms ms ms ms 9 Flooding Simple and robust routing no need for routing tables each packet duplicated on each outgoing link packet duplication duplicated packets destroyed at destination robust - tolerates link or router failures optimal in some sense the first packet has found the shortest path to the destination cannot be compared to the shortest path calculated by Link State - no packet duplication Problem increased load due to packet duplication Used in OSPF to distribute link state information and in ad hoc routing protocols (OV, OLSR) istance vector ynamic routing based on distributed estimation of the distance to the destination uses the distributed algorithm by ellman-ford (dynamic programming) each router receives aggregated information from its neighbors estimates the local cost to its neighbors computes the best routes no global network states istance number of hops delay ellman-ford algorithm ellman-ford algorithm node i knows cost c(i,k) to its immediate neighbours (+ for most values of k) distance (i,n) is given by: (i,n) = min k (c(i,k) + (k,n)) in the worst case, convergence after N- iterations istributed ellman-ford algorithm initially: (i,n) = if i directly connected to n and (i,n) = + otherwise node i receives from neighbour k latest values of (k,n) for all n (distance vector) node i computes the best estimates (i,n) = min k (c(i,k) + (k,n))

3 istance Vector ellman-ford algorithm xample of ellman-ford i c(i,) c(i,k) k (,n) (k,n) n G H I H K Table of J G H H 7 9 H I 4 I J K 4 6 K J: 8 6 c(i,m) m (m,n) I J K 3 computation of G : 8+8=6, 3+=4, 6+=8, 6+3=37 choice of 8, H 4 istance vector example Simple network routers connected by links destinations = subnetworks connected to routers symmetric links cost = number of hops Initialization local local l local local local 5 6 istance vector announcement istance vector announcement local from : from : local l l local local l from : local l l from : local local l3 local local l3 from : local l4 l

4 istance Vector istance vector announcement Final local l l3 from : local l3 local l l from : local l4 l4 local 9 local l l3 l l local l3 l3 local l l4 l l local l4 l4 local Link failure Link failure local l l3 l l local l l4 l local l l3 l l local l l4 l from :,, local l3 l3 local l4 l4 local local l3 from :,, local l4 l4 local Final state after failure qual link costs - link failures local l3 3 l3 l3 3 l3 local l4 3 l4 l4 local l3 3 l3 l3 3 l3 local l4 3 l4 l4 local l3 local l4 local 3 3 local l3 l3 local l4 local 3 4 4

5 istance Vector ounting to infinity Split horizon local l3 3 l3 l3 3 l3 from :, 3 l3 Loop between and xchange of routes, costs increase by each cycle onvergence to a stable state = large number e.g. RIP: = 6 Minimize the effects of bouncing and counting to infinity Rule if routes packets to X via, it does not announce this route to local l3 l3 4 l3 4 l xample of split horizon Split horizon local l3 3 l3 l3 3 l3 local l3 l3 local l4 3 l4 l4 local l4 local 3 7 local l3 3 l3 l3 3 l3 from : local l3 l3 l3 Split horizon cuts the process of counting to infinity 8 Split horizon Split horizon may fail local l3 l3 l3 l3 from :,, local l3 l3 l3 Split horizon cuts the process of counting to infinity 9 from : l4 local l4 l4 l4 local l4 local 3 3 5

6 istance Vector Split horizon may fail Split horizon may fail local 4 l4 3 from : 3 local 4 l4 3 l4 local l4 local 3 from : 3 l4 from : 4 3 local l4 5 l4 l4 4 local 3 3 RIP v Message format istance vector protocol Metric - hops Network span limited to 5 = 6 Split horizon estination network identified by IP address no prefix/subnet information - derived from address class ncapsulated as UP packets, port 5 Largely implemented (routed on Unix) roadcast every 3 seconds or when update detected Route not announced during 3 minutes cost becomes 33 3 command version address family May be repeated 5 times IP address metric ommand RQUST - (sent at boot to initialize) RSPONS - (broadcast each 3 sec) 34 Missing netmask RIP v (RF 453) (55...) F... (55...) packet to... Subnetworks take into account IR prefixes and netmasks uthentication Multicast mapped to M on LN only, no need for IGMP and can forward to... Packet to... can go through F or if sent to, it goes through and If link - broken, no route to destination

7 istance Vector Message format 3 command version unused address family IP address netmask next router metric ommand, version unchanged One address family - authentication Next router route tag used at the border of different routing domains (e.g. RIP and OSPF) Route tag nnouncing netmasks for external routes (used by GP) ( ) F... ( ) can forward to... Packet to... can go through F packet to... Routing domains Simple authentication domain (OSPF) 3 command version unused xffff authentication type = password on 6 bytes F domain (RIP) ifferent routing domains e.g. routers under different administrations that run different routing protocols (RIP, OSPF) If wants to send a packet to F, it goes through and When announcing F, adds as next router 39 onfiguration of gated (/etc/gated.conf) rip yes { interface all version multicast authentication simple "qptszwmz" } 4 M5 authentication M5 authentication 3 command version unused xffff authentication type = 3 packet length key Id auth. length increasing sequence no. route info xffff x seal Seal M5 digest on the message using a shared secret sequence number avoids replay attacks onfiguration of gated (/etc/gated.conf) rip yes { interface all version multicast authentication md5 "qptszwmz" } 4 4 7

8 istance Vector onclusion Main distance vector protocols Largely deployed (Unix S routed) Simplicity Slow convergence Not suited for large and complex networks Link State protocols should be used instead