Module 4: Routing Part 2

Transcription

1 Module 4: Routing Part 2 SMD123 Computer Communications Johan Nykvist Department of Computer Science and Electrical Engineering Luleå University of Technology Lecture Objectives Routing in the Internet Intra-Domain Routing RIP, OSPF Inter-Domain Routing BGP

2 Intra-Domain Routing Handle routing within Autonomous Systems Interior-Gateway Protocols (IGP) Most common IGP protocols: Distance-Vector Protocols RIP: Routing Information Protocol IGRP: Interior Gateway Routing Protocol (Cisco proprietary) EIGRP: Enhanced IGRP, uses diffusion computations to avoid the count-to-infinity problem (Cisco proprietary) Link-State Protocols OSPF: Open Shortest Path First Protocol IS-IS: Intermediate System-to-Intermediate System (ISO) Routing Information Protocol (RIP) Uses the Distance-Vector Algorithm Hop-count as routing metric One hop == one subnet traversal Infinity == 16 z u A C B D v y w x A s Routing table: destination hops u 1 v 2 w 2 x 3 y 3 z 2

3 Routing Information Protocol (RIP) Each router keeps a routing table with the router s distances to subnets in the AS (Distance Vector) The distance vectors are exchanged between neighbors with response messages Also known as advertisements Advertisements are sent every 30 seconds (no need to store neighbors routing tables) Contain subnet addresses and distances to the subnets (max. 25 addresses per message) Routing Information Protocol (RIP) w x y A D B z C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x Routing table in D

4 Routing Information Protocol (RIP) Dest Next hops w - - x - - z C Advertisement from A to D w x y A D B z C Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B A 7 5 x Routing table in D Routing Information Protocol (RIP) Timeout after 180 seconds 1. Neighbor is considered to be no longer reachable 2. Routes via neighbor are removed from the routing table 3. Advertisements are sent to the other neighbors RIP applies Poison Reverse to avoid ping-pong loops Still suffers from loops that are longer than 3 hops (Infinity is therefore set to 16)

5 Routing Information Protocol (RIP) Advertisements are exchanged with UDP on port 520 In UNIX, the RIP protocol is executed by an application-level process called route-d (daemon) routed routed Transport (UDP) Transport (UDP) network (IP) forwarding table forwarding table network (IP) link link physical physical Routing Information Protocol (RIP) Was one of the earliest IGPs Included in BSD UNIX in 1982 Three versions exist: RIP version 1 (RFC 1058) No support for subnets No router authentication RIP version 2 (RFC 2453) Backward-compatible with RIPv1 Subnet support Router authentication support RIP Next Generation (RFC 2080) Support for IPv6

6 Open Shortest Path First (OSPF) Open : publicly available (version 2, RFC 2328) Uses Link State algorithm Link-State packets are disseminated regularly and at topology changes Each router has a complete topology map Route computation is done using Dijkstra s algorithm (each router regards itself as the source router) Advertisements disseminated to entire AS via flooding Carried in OSPF messages directly over IP (rather than TCP or UDP) OSPF advanced features (not in RIP) Security: all OSPF messages authenticated (to prevent malicious intrusion) Multiple same-cost paths allowed (only one path in RIP) For each link, multiple cost metrics for different TOS (e.g., satellite link cost set low for best effort; high for real time) Hierarchical OSPF in large domains

7 Hierarchical OSPF Two-level hierarchy: local areas and backbone area Link-state advertisements kept within each area each node has detailed area topology: only knows direction (shortest path) to nets in other areas Area border routers: summarize distances to nets in own area, advertise to other area border routers Backbone routers: run OSPF routing limited to backbone Boundary routers: connect to other AS s Lecture Objectives Routing in the Internet Intra-Domain Routing RIP, OSPF Inter-Domain Routing BGP

8 Inter-Domain Routing Handle routing *between* Autonomous Systems Exterior-Gateway Protocols (EGP) Do not care about routing within ASes The standard EGP used in Internet today: BGP AS3 AS2 AS1 Border Gateway Protocol (BGP) Border Gateway Protocol version 4 (BGP) RFC 1771 BGP provides each AS a means to 1. Obtain network reachability information from neighboring ASes 2. Propagate received information to routers within the AS 3. Determine good routes to distant networks based on received information and routing policies 4. Advertise its network prefixes to neighboring ASes Each AS is assigned an unique AS Number (ASN) by ICANN Uses a path-vector routing algorithm Similar to the Distance-Vector algorithm (distributed computation) The AS-path to a destination is included in the routing information Routing decisions made on metrics and local routing policies

9 BGP Basics Pairs of routers (BGP peers) exchange routing information over semi-permanent TCP connections: BGP sessions (Note that BGP sessions do not correspond to physical links) External BGP (ebgp) sessions between ASes, Internal BGP (ibgp) sessions within ASes When AS2 advertises a prefix to AS1, AS2 is promising it will forward any datagrams destined to that prefix towards the prefix 3c 3a 3b AS3 1a AS1 1c 1d 1b 2a 2c 2b AS2 ebgp session ibgp session Distributing reachability information With an ebgp session between 3a and 1c, AS3 sends prefix reachability info to AS1 1c can then use ibgp do distribute this new prefix reach info to all routers in AS1 1b can then re-advertise the new reach info to AS2 over the 1b-to-2a ebgp session 3c 3a 3b AS3 1a AS1 1c 1d 1b 2a 2c 2b AS2 ebgp session ibgp session

10 BGP Routes and Path Attributes When advertising a prefix, the advertisement includes BGP attributes. prefix + attributes = route Two important attributes: AS-PATH: contains the ASNs of the ASs through which the advert for the prefix passed, e.g., [ ] NEXT-HOP: Indicates the specific internal-as router nexthop AS. (There may be multiple links from current AS to next-hop-as) When a gateway router receives a route advertisement, it applies import policies to accept / decline the route BGP Route Selection Router may learn about more than 1 route to some prefix. Router must select route Per-AS routing policies are applied to implement business agreements Elimination rules for selecting the best route: 1. Local preference value attribute: policy decision 2. Shortest AS-PATH 3. Closest NEXT-HOP router (hot potato routing) 4.

11 BGP Messages BGP messages exchanged using TCP, port 179. 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; also used to ACK OPEN request NOTIFICATION: reports errors in previous message also used to close connection BGP Routing Policy W A B C X legend: provider network customer network: Y Figure 4.5-BGPnew: a simple BGP scenario A,B,C are provider networks (called Transit ASes) X,W,Y are customers of provider networks (called Stub ASes) X is multi-homed: attached to two networks X does not want to route from B via X to C.. so X will not advertise to B a route to C (local routing policies applied)

12 BGP Routing Policy W A B C X legend: provider network customer network: Y Figure 4.5-BGPnew: a simple BGP scenario A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW? No way! B gets no revenue for routing CBAW since neither W nor C are B s customers B wants to force C to route to w via A B wants to route only to / from its customers! Inter-Domain Routing Network prefix growth Collected at AS 1221 (Telstra)

13 Inter-Domain Routing ASN growth Collected at AS 1221 (Telstra) Inter-Domain Routing Avg AS Path Length Collected at AS 1221 (Telstra)