The Network Layer: Routing in the Internet. The Network Layer: Routing & Addressing Outline

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1 CPSC 852 Internetworking The Network Lyer: Routing in the Internet Mihele Weigle Deprtment of Computer Siene Clemson University 1 The Network Lyer: Routing & Addressing Outline! Network lyer funtions! Router rhiteture! IP Internet Protool» Addressing! Routing lgorithms» Lest ost pth omputtion lgorithms! Hierrhil routing» Conneting networks of networks! Routing on the Internet» Intr-domin routing» Inter-domin routing pplition trnsport network link physil pplition trnsport network link physil 2

2 The Network Lyer: Routing & Addressing Hierrhil routing! The theory of routing: reltively simple lgorithms with mngele shortomings! Critil ssumptions:» All routers re identil» The network is flt! The relity: Routing is dominted y issues of sle» The Internet hs 100 million hosts! " Cn t store ll host destintions in routing tles! " Routing tle exhnge would swmp links!» We must route to networks, not hosts! Routing lso dominted y issues of dministrtive utonomy» The Internet is network of networks eh network owner my wnt to ontrol routing in its own network 3 Hierrhil Routing Gtewy routers! Aggregte routers into regions, utonomous systems (AS)! All routers inside sme AS run sme routing protool mong themselves» Intr-AS routing protool» Routers in different AS n run different intr- AS routing protool Gtewy routers! Speil routers in AS! Run intr-as routing protool with ll other routers inside AS! Responsile for routing to destintions outside AS» Also run inter-as routing protool with gtewy routers in djent AS 4

3 Hierrhil Routing Intr-AS & Inter-AS Routing C C. d A A. A. B. B! Gtewys:» Perform inter-as routing mong themselves» Perform intr-as routing with other routers in their AS Inter-AS nd intr-as routing in gtewy A. Intr-AS Routing Algorithm Routing Tle Inter-AS Routing Algorithm network lyer link lyer physil lyer to/from A. DL PHY DL PHY to/from A.d DL PHY to/from B. 5 Hierrhil Routing Intr-AS & Inter-AS Routing Host h1 C C. A. A. d A Intr-AS routing within AS A Inter-AS routing etween A nd B B. B Host h2 Intr-AS routing within AS B! We ll exmine speifi inter-as nd intr-as Internet routing protools shortly 6

4 Routing on the Internet The Internet AS hierrhy Inter-AS order (exterior gtewy) routers C C. d A A. A. B. B Intr-AS (interior gtewy) routers 7 The Internet AS Hierrhy Intr-AS routing C C. d A A. A. B. B! Also known s Interior Gtewy Protools (IGPs)! Most ommon IGPs:» RIP: Routing Informtion Protool» OSPF: Open Shortest Pth First» IGRP: Interior Gtewy Routing Protool (Ciso proprietry) 8

5 Intr-AS routing RIP! Distne vetor-like lgorithm» Commonly used in the erly Internet! Distne metri: Numer of hops» Mximum vlue = 15 hops (! = 16)! Routing tles (inluding osts) re exhnged with djent nodes every 30 seonds vi Response Messge (lso lled route dvertisement )» Reeiver determines if djent node hs found new minimum ost to destintion! Eh dvertisement onsists of the minimum ost route for up to 25 destintion networks 9 RIP Link filure nd reovery! If no dvertisement herd fter 180 seonds, djent node/link delred filed» Routes vi tht djent node invlidted» New dvertisements sent to other djent nodes» Advertisement reeivers in turn send out new dvertisements (if their tles hnged)» Link filure informtion quikly propgtes to entire net» Poisoned reverse used to prevent ping-pong loops " infinity = 16 10

6 Intr-AS routing IGRP! CISCO proprietry; suessor of RIP (mid 80s)! Distne Vetor-like similr to RIP» Severl ost metris: dely, ndwidth, reliility, lod, et.» Uses TCP to exhnge routing updtes 11 Intr-AS routing OSPF! Open : pulily ville! Uses the Link Stte minimum ost pth omputtion lgorithm» LS updte flooding» Topology mp t eh node» Route omputtion using Dijkstr s lgorithm! OSPF dvertisement rries one entry per djent node! Advertisements disseminted to n entire AS (vi flooding) 12

7 OSPF Advned fetures (not in RIP)! Seurity: ll OSPF messges uthentited to prevent mliious intrusion» TCP onnetions used in flooding! Multiple sme-ost pths llowed (only one pth in RIP)! For eh link, multiple ost metris for different network-lyer servies» (e.g., stellite link ost set low for est effort; high for rel time)! Hierrhil OSPF used in lrge networks 13 OSPF Why hierrhil OSPF? 14

8 OSPF Hierrhil OSPF To other AS 15 OSPF Hierrhil OSPF! Two-level hierrhy: lol re, kone» Link-stte dvertisements only in re» Eh re node hs detiled re topology; only knows shortest pth to networks in other res! Are Border Routers: summrize distnes to nets in own re nd dvertise to other Are Border routers! Bkone Routers: run OSPF routing limited to kone! Boundry Routers: onnet to other AS 16

9 The Internet AS Hierrhy Inter-AS Routing C C. d A A. A. B. B! Border Gtewy Protool (BGP) is the de fto stndrd! Pth Vetor protool:» Similr to Distne Vetor protool» Eh Border Gtewy dvertises to djent nodes (peers) the entire pth (i.e., sequene of AS numers) to destintion» e.g., Gtewy X my send its pth to destintion Z: pth (X,Z) = X, Y 1, Y 2, Y 3,, Z 17 Internet Inter-AS Routing BGP! Suppose gtewy X sends pth to peer gtewy W! W my or my not selet the pth dvertised y X» Cost, poliy ( don t route vi ompetitor X s network ), or loop prevention resons! If W selets the pth dvertised y X to Z, then: pth (W,Z) = W + pth (X,Z)! Note tht X n ontrol its inoming trffi y ontrolling its route dvertisements to djent order gtewys:» If X does not wnt to route trffi to Z, then X will not dvertise ny routes to Z 18

10 Internet Inter-AS Routing BGP! BGP messges exhnged using TCP! BGP messges:» OPEN: opens TCP onnetion to peer nd uthentites sender» UPDATE: dvertises new pth (or withdrws old)» KEEPALIVE: keeps onnetion live in sene of UPDATES; lso ACKs OPEN request» NOTIFICATION: reports errors in previous messge; lso used to lose onnetion 19 The Internet AS Hierrhy Why different intr- nd inter-as routing?! Poliy:» Inter-AS: dministrtion wnts ontrol over how its trffi routed nd who routes through its network» Intr-AS: single dministrtion, so no poliy deisions needed! Sle:» Hierrhil routing sves tle size, redued updte trffi! Performne:» Intr-AS: n fous on performne» Inter-AS: poliy my dominte over performne 20