Network Layer4-1 Chapter 4 roadmap 4.1 Introduction and Network Service Models 4.2 Routing Principles 4.3 Hierarchical Routing 4.4 The Internet (IP) Protocol 4.5 Routing in the Internet m Intra-AS routing: RIP and OSPF m Inter-AS routing: BGP 4.6 What’s Inside a Router?

Network Layer4-2 Routing in the Internet r The Global Internet consists of Autonomous Systems (AS) interconnected with each other: m Stub AS: small corporation: one connection to other AS’s m Multihomed AS: large corporation (no transit): multiple connections to other AS’s m Transit AS: provider, hooking many AS’s together r Two-level routing: m Intra-AS: administrator responsible for choice of routing algorithm within network m Inter-AS: unique standard for inter-AS routing: BGP

Network Layer4-3 Internet AS Hierarchy Intra-AS border (exterior gateway) routers Inter-AS interior (gateway) routers

Network Layer4-4 Intra-AS Routing r Also known as Interior Gateway Protocols (IGP) r Most common Intra-AS routing protocols: m RIP: Routing Information Protocol m OSPF: Open Shortest Path First m IGRP: Interior Gateway Routing Protocol (Cisco proprietary) m IS-IS: Intermediate System to Intermediate System

Network Layer4-5 RIP ( Routing Information Protocol) r Distance vector algorithm r Included in BSD-UNIX Distribution in 1982 r Distance metric: # of hops (max = 15 hops) m Can you guess why? r Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) r Each advertisement: list of up to 25 destination nets within AS

Network Layer4-6 RIP: Example Destination Network Next Router Num. of hops to dest. wA2 yB2 zB7 x--1 ….…..... w xy z A C D B Routing table in D

Network Layer4-7 RIP: Example Destination Network Next Router Num. of hops to dest. wA2 yB2 zB A7 5 x--1 ….…..... Routing table in D w xy z A C D B Dest Next hops w - - x - - z C 4 …. …... Advertisement from A to D

Network Layer4-8 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead m routes via neighbor invalidated m new advertisements sent to neighbors m neighbors in turn send out new advertisements (if tables changed) m link failure info quickly propagates to entire net m poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)

Network Layer4-9 RIP Table processing r RIP routing tables managed by application-level process called route-d (daemon) r advertisements sent in UDP packets, periodically repeated physical link network forwarding (IP) table Transprt (UDP) routed physical link network (IP) Transprt (UDP) routed forwarding table

Network Layer4-10 RIP Table example (continued) Router: giroflee.eurocom.fr r Three attached class C networks (LANs) r Router only knows routes to attached LANs r Default router used to “go up” r Route multicast address: r Loopback interface (for debugging) Destination Gateway Flags Ref Use Interface UH lo U 2 13 fa U le U 2 25 qaa U 3 0 le0 default UG

Network Layer4-11 Weaknesses of RIP r INFINITY defined as 15, thus RIP cannot be used in networks where routes are more than 15 hops r Difficulty in supporting multiple metrics (default metric: # of hops) m the potential range for such metrics as bandwidth, throughput, delay, and reliability can be large m thus the value for INFINITY should be large; but this can result in slow convergence of RIP due to count-to- infinity problem

Network Layer4-12 OSPF (Open Shortest Path First) r “open”: publicly available r Uses Link State algorithm m LS packet dissemination m Topology map at each node m Route computation using Dijkstra’s algorithm r OSPF advertisement carries one entry per neighbor router r Advertisements disseminated to entire AS (via flooding) m Carried in OSPF messages directly over IP (rather than TCP or UDP

Network Layer4-13 OSPF “advanced” features (not in RIP) r Security: all OSPF messages authenticated (to prevent malicious intrusion) r Multiple same-cost paths allowed (only one path in RIP) r For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time) r Integrated uni- and multicast support: m Multicast OSPF (MOSPF) uses same topology data base as OSPF r Hierarchical OSPF in large domains.

Network Layer4-14 Hierarchical OSPF

Network Layer4-15 Hierarchical OSPF r Two-level hierarchy: local area, backbone. m Link-state advertisements only in area m each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. r Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. r Backbone routers: run OSPF routing limited to backbone. r Boundary routers: connect to other AS’s.

Network Layer4-16 Inter-AS routing in the Internet: BGP

Network Layer4-17 Internet inter-AS routing: BGP r BGP (Border Gateway Protocol): the de facto standard r Path Vector protocol: m similar to Distance Vector protocol m each Border Gateway broadcast to neighbors (peers) entire path (i.e., sequence of AS’s) to destination m BGP routes to networks (ASs), not individual hosts m E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z

Network Layer4-18 Internet inter-AS routing: BGP Suppose: gateway X send its path to peer gateway W r W may or may not select path offered by X m cost, policy (don’t route via competitors AS), loop prevention reasons. r If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) r Note: X can control incoming traffic by controlling it route advertisements to peers: m e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z

Network Layer4-19 BGP: controlling who routes to you r A,B,C are provider networks r X,W,Y are customer (of provider networks) r X is dual-homed: attached to two networks m X does not want to route from B via X to C m.. so X will not advertise to B a route to C

Network Layer4-20 BGP: controlling who routes to you r A advertises to B the path AW r B advertises to X the path BAW r Should B advertise to C the path BAW? m No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers m B wants to force C to route to w via A m B wants to route only to/from its customers!

Network Layer4-21 BGP operation Q: What does a BGP router do? r Receiving and filtering route advertisements from directly attached neighbor(s). r Route selection. m To route to destination X, which path )of several advertised) will be taken? r Sending route advertisements to neighbors.

Network Layer4-22 BGP messages r BGP messages exchanged using TCP. r BGP messages: m OPEN: opens TCP connection to peer and authenticates sender m UPDATE: advertises new path (or withdraws old) m KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request m NOTIFICATION: reports errors in previous msg; also used to close connection

Network Layer4-23 Why different Intra- and Inter-AS routing ? Policy: r Inter-AS: admin wants control over how its traffic routed, who routes through its net. r Intra-AS: single admin, so no policy decisions needed Scale: r hierarchical routing saves table size, reduced update traffic Performance: r Intra-AS: can focus on performance r Inter-AS: policy may dominate over performance

Network Layer4-24 BGP fragility r BGP can contribute to lot of the routing instability in the internet r Interactions between IGPs & EGPs poorly understood (OSPF timeouts etc.) r Incorrect information being fed from an IGP to an EGP (and vice versa) can result in catastrophic meltdown r BGP route flaps and associated dampening introduces more complexity than before r A routing protocol that does not work in the simple case?!