Goals of Routing Protocols ❒ Find the “optimal route” 10: ❒ Rapid Convergence Inter and intra AS, RIP, OSPF, ❒ Robustness BGP, Router Architecture ❒ Configurable to respond to changes in many variables (changes in bandwidth, delay, Last Modified: queue size, policy, etc.) 3/24/2003 2:39:16 PM ❒ Ease of configuration 4: Network Layer 4: Network Layer 4a-1 4a-2 Real Internet Routing? Recall CIDR We already talked about how routing based on hierarchical allocation of IP address space can allows efficient advertisement ❒ CIDR? of routing information: ❒ Dynamic routing protocols running between Organization 0 every router? 200.23.16.0/23 Organization 1 “Send me anything 200.23.18.0/23 with addresses beginning Organization 2 . 200.23.16.0/20” 200.23.20.0/23 . Fly-By-Night-ISP . . . Internet . Organization 7 200.23.30.0/23 “Send me anything ISPs-R-Us with addresses beginning 199.31.0.0/16” 4: Network Layer 4: Network Layer 4a-3 4a-4 CIDR? Dynamic Routing? Dynamic Routing Protocols? Our study of dynamic routing protocols thus far = ❒ CIDR by itself is a nice idea but.. idealized graph problem ❍ Hard to maintain ❒ all routers identical ❍ Work around existing IP address space ❒ network “flat” allocations … not true in practice ❍ What about redundant paths? ❒ Dynamic routing protocols? scale: with 50 million destinations: ❍ They maintain/update themselves ❒ can’t store all destinations in routing tables! ❍ Allow for redundant paths ❒ routing table exchange would swamp links! ❍ But could every router in the Internet be a ❒ Neither link state nor distance vector could node in the graph? handle the whole Internet! 4: Network Layer 4: Network Layer 4a-5 4a-6
Hierarchical Routing Routing in the Internet gateway routers ❒ Administrative Autonomy Routers in same AS run routing protocol ❍ Internet = network of networks ❒ special routers in AS ❍ Each network controls routing in its own network chosen by ❒ run intra-AS routing ❍ Global routing system to route between Autonomous Systems administrators of that protocol with all other (AS) routers in AS domain ❒ Two-level routing: ❒ also responsible for ❍ “intra-AS” routing routing to destinations ❍ Intra-AS: administrator is responsible for choice protocol outside AS ❍ routers in different AS ❍ Inter-AS: unique standard ❍ run inter-AS routing can run different intra- protocol with other AS routing protocol gateway routers 4: Network Layer 4: Network Layer 4a-7 4a-8 Internet AS Hierarchy Intra-AS and Inter-AS routing C.b Gateways: Intra-AS border (exterior gateway) routers B.a •perform inter-AS A.a routing amongst b c A.c themselves a a C b •perform intra-AS a B routers with other d c routers in their b A AS network layer inter-AS, intra-AS link layer routing in physical layer gateway A.c Inter-AS interior (gateway) routers 4: Network Layer 4: Network Layer 4a-10 4a-9 Intra vs Inter AS Routing Intra-AS and Inter-AS routing protcols Inter-AS ❒ For Intra AS routing protocols: many choices; For routing C.b between B.a Inter AS routing protocols: standard A and B A.a Host ❍ Why does this make sense? b c h2 A.c ❒ Intra AS routing protocols focus on performance a a C b a B optimization; Inter AS routing protocols focus on Host d administrative issues Intra-AS routing c h1 b A within AS B ❍ Why does this make sense? Intra-AS routing ❒ Choice in Intra-AS within AS A ❍ Intra-AS often static routing based on CIDR, can also be ❒ Single datagram is often routed over many hops dynamic (usually RIP or OSPF) via routes established by several intra-AS ❒ Standard Inter-AS BGP is dynamic routing protocols and an inter-AS routing protocol 4: Network Layer 4a-11 4: Network Layer 4a-12
Intra-AS Routing RIP ( Routing Information Protocol) ❒ Also known as Interior Gateway Protocols (IGP) ❒ Distance vector algorithm ❒ Most common IGPs: ❒ Included in BSD-UNIX Distribution in 1982 ❒ Single Distance metric: # of hops (max = 15 hops) ❍ RIP: Routing Information Protocol ❍ Can you guess why? ❍ Count to infinity less painful if infinity = 16 ☺ ❍ OSPF: Open Shortest Path First ❍ But limits RIP to networks with a diameter of 15 hops ❍ IGRP: Interior Gateway Routing Protocol (Cisco proprietary) ❒ Distance vectors: exchanged every 30 sec via ❍ Can also be static (via CIDR) but that is not Response Message (also called advertisement ) called an IGP ❒ Each advertisement: route to up to 25 destination nets 4: Network Layer 4a-13 4: Network Layer 4a-14 RIP: Link Failure and Recovery RIP Table processing If no advertisement heard after 180 sec --> ❒ RIP routing tables managed by a pplication-level neighbor/link declared dead process called route-d (daemon) ❍ routes via neighbor invalidated ❒ advertisements sent in UDP packets, periodically ❍ new advertisements sent to neighbors repeated ❍ neighbors in turn send out new advertisements (if ❒ Periodically inform kernel of routing table to use tables changed) ❍ link failure info quickly propagates to entire net ❍ poison reverse used to prevent small loops ❍ infinite distance = 16 hops to make make problem with larger loops less painful 4: Network Layer 4a-15 4: Network Layer 4a-16 RIP Table example: netstat -rn OSPF (Open Shortest Path First) ❒ “open”: publicly available ❒ Uses Link State algorithm Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- ❍ LS packet dissemination 127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 ❍ Topology map at each node 193.55.114. 193.55.114.6 U 3 58503 le0 ❍ Route computation using Dijkstra’s algorithm 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454 ❒ OSPF advertisement carries one entry per neighbor ❒ Three attached class C networks (LANs) router (i.e. cost to each neighbor) ❒ Router only knows routes to attached LANs ❒ Advertisements disseminated to entire AS (via ❒ Default router used to “go up” flooding) ❒ Route multicast address: 224.0.0.0 ❒ Loopback interface (for debugging) 4: Network Layer 4a-17 4: Network Layer 4a-18
OSPF “advanced” features (not in RIP) Hierarchical OSPF: Mini Internet ❒ Many have nothing to do with link-state vs distance Within each area, border router responsible for routing outside vector!! the area Exactly one area ❒ Security: all OSPF messages authenticated (to is backbone area prevent malicious intrusion); TCP connections used ❒ Multiple same-cost paths can be used at once (single path need not be chosen as in RIP) ❒ For each link, multiple cost metrics for different TOS (eg, high BW, high delay satellite link cost may set “low” for best effort; high for real time) ❒ Integrated uni- and multicast support: ❍ Multicast OSPF (MOSPF) uses same topology data base as OSPF ❒ Hierarchical OSPF in large domains Backbone area contains all area border routers and possibly others ❍ Full broadcast in each sub domain only 4: Network Layer 4a-19 4: Network Layer 4a-20 Hierarchical OSPF IGRP (Interior Gateway Routing Protocol) ❒ Two-level hierarchy: local area, backbone. ❒ CISCO proprietary; successor of RIP (mid 80s) ❍ Link-state advertisements only in area ❒ Distance Vector, like RIP but with advanced ❍ each nodes has detailed area topology; only know features like OSPF direction (shortest path) to nets in other areas. ❒ several cost metrics (delay, bandwidth, reliability, ❒ Area border routers: “summarize” distances to nets load etc); administer decides which cost metrics in own area, advertise to other Area Border routers. to use ❒ Backbone routers: run OSPF routing limited to ❒ uses TCP to exchange routing updates backbone. ❒ Loop-free routing via Distributed Updating Alg. ❒ Boundary routers: connect to other ASs. (DUAL) based on diffused computation 4: Network Layer 4a-21 4: Network Layer 4a-22 Autonomous systems Now on to Inter-AS routing ❒ The Global Internet consists of Autonomous Systems (AS) interconnected with each other: ❍ Stub AS : small corporation ❍ Multihomed AS : large corporation (no transit traffic) ❍ Transit AS : provider (carries transit traffic) ❒ Major goal of Inter-AS routing protocol is to reduce transit traffic 4: Network Layer 4a-23 4: Network Layer 4a-24
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