Routing Protocols 1 ProtocolsLayer name DNSApplication TCP, UDPTransport IPInternet (Network ) WiFi, Ethernet Link (Physical)

Terminology 2 An IP address followed by a slash(/) and a number (i.e /8 ) indicates a block of addresses named subnet.  This includes all the addresses starting with the same 8 bits as the given address.

Flat network 3 Flat network based on the idea that all the nodes know each other. scale: with 50 million destinations: can’t store all destinations in routing tables! routing table exchange would swamp links

Hierarchical Routing 4 internet = network of networks Each administrative autonomy network admin may want to control routing in its own network Internet consists of Autonomous Systems interconnected with each other!

Internet AS Hierarchy Inter-AS border (exterior gateway routers) Intra-AS (interior gateway routers) 5

Static Routing Typically used in hosts  To configure it, enter subnet mask and the router (gateway) IP address.  Perfect for cases with few connections, doesn’t change much  E.g. host with a single router connecting to the rest of the Internet H1H2H3 R1 Internet For H1 Next Hop: IP:

Dynamic Routing Most routers use dynamic routing  Automatically build the routing tables  There are two major approaches  Distance Vector Algorithms  Link State Algorithms 7

Autonomous System Contiguous set of networks under one administrative authority Common routing protocol For example, The Hebrew university of Jerusalem A connected network  There is at least one route between any pair of nodes 8

Intra-AS Routing Named Interior Routing Protocol (IRP) or Interior Gateway Protocol (IGP)  Passes routing information between routers within AS  Can use routing metric, e.g. hop count or administrative cost 9

Inter-AS Routing Named Exterior Routing Protocol (ERP) or Exterior Gateway Protocol (EGP)  Passes routing information between routers across AS  Routing algorithms and tables may differ between different AS  Finds a path, but can’t find an optimal path since it can’t compare routing metrics via multiple AS 10

Application of IRP and ERP 11

Path Sub-optimality in Inter-AS Routing 12 Lecture #13: hop red path vs. 2 hop green path start end

Intra-AS Routing Also known as Interior Router Protocols (IRP) or Interior Gateway Protocols (IGP) Most common:  RIP: Routing Information Protocol  OSPF: Open Shortest Path First  IGRP: Interior Gateway Routing Protocol (Cisco proprietary) 13

Routing Information Protocol (RIP) Distance vector algorithm  Each node contains a vector with the distances of all the nodes in the AS. Included in BSD-UNIX Distribution in 1982  routed Distance metric: # of hops (max = 15 hops) Distance vectors: exchanged every 30 sec via Response Message (update) 14

Routing Information Protocol (RIP) 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 15

RIP: Link Failure and Recovery If no advertisement heard after 180 sec  neighbor/link declared dead  routes via neighbor invalidated  new responses sent to neighbors  neighbors in turn send out new responses (if tables changed)  link failure info quickly propagates to entire net 16

RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated 17

RIP Table example (continued) Router: giroflee.eurocom.fr via: netstat -rn Router only knows routes to attached LANs Default router used to “go up” Destination Gateway Flags Ref Use Interface UH lo U 2 13 fa U le U 2 25 qaa U 3 0 le0 default UG

RIP Problems (1) Massive traffic burst every 30 seconds  Because the networks grew in size Very slow coverage time  Failure takes at least 3 minutes to be observed  If we have 15 routers in line-like order, the last router will be updated with a change of the first touter only after 14*30 seconds 19

RIP Problems (1) Slow recovery from loops. If C clopases, A tries to route via B, and B via A. the loop will persist until the metrics for C reach infinity In most current networking environments, RIP is not the preferred choice for routing 20

OSPF (Open Shortest Path First) Available since 1998 for IPv4 (for IPV6 since 2008). Uses Link State algorithm  Link State packet dissemination  Topology map at each node  Route computation using Dijkstra’s algorithm OSPF update carries one entry per neighbor router Updates disseminated to entire AS (via flooding) Conceived as a successor to RIP 21

Link state protocols A B C D E F G 1 2 H I 3 router 1 A, B, C, G, H router 3 H, I router 2 D, E, F, G, I 22

OSPF “advanced” features (not in RIP) Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used Multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different Type Of Service (e.g., satellite link cost set “low” for best effort; high for real time) Hierarchical OSPF in large domains.  Reduces the routing traffic. 23

Hierarchical OSPF 24

Distance vector algorithm vs Link state algorithm Distance vector algorithm pros  Minimal configuration required.  Low CPU/memory overhead. Link state algorithm pros  Less updates  Loops are essentially impossible in a link-state network.  Fast convergence time. 25

IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP Several cost metrics (delay, bandwidth, reliability, load etc) Uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation 26

Inter-AS routing (Exterior Route Protocols) 27

Internet inter-AS routing: BGP BGP (Border Gateway Protocol): the de facto standard  Version 4 the current standard Path 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 28

Internet inter-AS routing: BGP Suppose: router X send its path to peer router W W may or may not select path offered by X  cost, policy (don’t route via competitors AS), loop prevention reasons, many other metrics E.g. X advertises path to Z: XY 1 Y 2 Y 3 Z  If W selects path advertised by X, then: Path (W,Z) = WXY 1 Y 2 Y 3 Z 29

Internet inter-AS 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;  NOTIFICATION: reports errors in previous msg; also used to close connection 30

Why different Interior/Exterior routing ? Scale: hierarchical routing saves table size, reduced update traffic, hierarchical scheme allows different interior routing protocols Policy: Intra-AS / Interior: single admin, so no policy decisions needed Inter-AS / Exterior: admin wants control over how its traffic routed, who routes through its net. Performance: Intra-AS / Interior: can focus on performance, customization Inter-AS / Exterior: policy may dominate over performance 31

Policy with BGP BGP provides capability for enforcing various policies Policies are not part of BGP: they are provided to BGP as configuration information BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement (Updates) to other AS’s 32

Examples of BGP Policies A AS refuses to act as transit  Limit path advertisement A AS can become transit for some AS’s  Only advertise paths to some AS’s An AS can favor or disfavor certain AS’s for traffic transit from itself 33