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Routing Protocols

RIP and OSPF

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Introduction - IP Protocol

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Classful IP Addresses IP address

IPv4: 32-bit address dotted-quad or dotted decimal ex. 130.221.203.154 decimal = 82.DD.CB.9A hex = 1000 0010 . 1101 1101 . 1100 1011 . 1001 1010 binary only 232 (4,294,967,296) IPv4 addresses available

2 parts: netid & hostid

“Classful” addressing Class A Class B Class C Class D - Mulitcast Class E - Reserved for future use

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Classful IP Addresses - Class A

starts with binary 0 27 - 2 (126) Class A networks

2 reserved Class A networks00000000 ( 0.0.0.0 ) : default route01111111 ( 127.0.0.0 ) : loopback

224 - 2 (16,777,214) hosts per Class A network all 0’s : ‘this network’ all 1’s : ‘broadcast’

00 1 2 3 4 8 16 24 31

netid hostid

7 bits 24 bits

Lowest network address : 1.0.0.0Highest network address: 126.0.0.0

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Classful IP Addresses - Class B

Start with 10 Second Octet also included in network address 214 = 16,384 class B addresses

1 00 1 2 3 4 8 15 16 24 31

netid hostid

14 bits 16 bits

Lowest address : 128.0.0.0Highest address: 191.255.0.0

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Classful IP Addresses - Class C Start with 110 Second and third octet also part of network

address 221 = 2,097,152 addresses

1 1 00 1 2 3 7 15 23 24 31

netid hostid

21 bits 8 bits

Lowest address : 192.0.0.0Highest address: 223.255.255.0

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Classful IP Addresses - Class D Start with 1110 IP Multicasting

1 1 1 00 1 2 3 7 15 23 24 31

28 bits

Lowest address : 224.0.0.0Highest address: 239.255.255.255

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Classful IP Addresses - Class E Start with 1111 Experimental

1 1 1 10 1 2 3 7 15 23 24 31

28 bits

Lowest address : 240.0.0.0Highest address: 255.255.255.254

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Classless Interdomain Routing (CIDR) pronounced “cider” RFC 1518 & 1519 addresses two scaling problems on the Internet

growth of backbone routing tables potential for the 32-bit IP address space to be exhausted

Current IP address inefficiency exists because of the address Class requirements (i.e., A, B, C, etc.) a network with 2 hosts needs a Class C network address space

(2/254 = 0.79%) for 256 hosts ->Class B (256/65,534 = 0.39%) Class B exhaustion is more severe - so give out multiple Class

C’s If one AS has 16 Class C’s, each backbone router would need 16

routing table entries for that one AS

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CIDR CIDR helps to aggregate routes

hand out contiguous blocks of Class C addresses ex: 192.4.16 - 192.4.31

16 Class C’s They all start with 1100 0000 - 0000 0100 - 0001 . . . looks like a 20-bit network number - something between a

Class B and a Class C each block must contain a number of Class C networks that

is a power of 2

need a routing protocol that can deal with these “classless” addresses (i.e., a non-standard network number) BGP version 4 is able to do this Network numbers are represented by (length, value) pairs

example above would have length = 20 similar to the (mask, value) for subnets

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IP Routing In a packet-switched network, routing relates to

the process of choosing a link to send the packets over. Router: the computer that makes this choice.

an internet is composed of multiple physical networks interconnected by computers (or network devices) called routers

forwarding: take a packet, look at its destination address, consult a table, send packet to its destination based on that table

routing: process which builds the forwarding tables

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IP Routing For routing to scale, a hierarchical routing

infrastructure is used (Internet) Autonomous System (AS)

a group of routers exchanging info with a common routing protocol

a set of routers and networks managed by a single organization

connected (except during failures): a path exists between any two pair of nodes

Interior Gateway Protocols (IGPs) - within an AS Exterior Gateway Protocols (EGPs) - between ASs

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IP Routing

AS #1 AS #2

IGP1 IGP2BGP

Interior Gateway Protocol (IGP)Exterior Gateway Protocol (EGP)

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IP Routing routing: graph theory problem

nodes : hosts, switches, routers, or networks initial case: consider all nodes as routers

edges of graph: network links (assume undirected)

cost is associated with each edge relates to desirability of sending traffic over particular link routing problem: find the lowest-cost path between any

two nodes cost = sum of costs of all of the edges that form the path

CB

A

DE F4

9

3 6

1 21

node (router)

edge (link)

cost

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IP routing for a simple network

calculate all shortest paths and store in a table problems with this static approach:

does not handle node or link failures does not consider addition of new nodes or links assumes fixed edge costs (may want to adjust cost upward

for increased loading)

to deal with the static routing problems, routing protocols are used between nodes to discover the lowest cost paths and are distributed

centralization inhibits scalability distributed algorithms may cause synchronization

problems dynamic

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IP intradomain routing Look at two main classes of IGPs

distance vector (RIP) link state (OSPF)

assume all edge costs are known

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Routing Information Protocol (RIP) built on a Distance-Vector algorithm

also called Bellman-Ford algorithm, after the inventors each node constructs a one-dimensional array (a vector)

of the “distances” (costs) to all of the other nodes and distributes the vector to its immediate neighbors

assumes that each node knows the cost of the links to its immediate (directly connected) neighbors

a link outage is assigned an infinite cost assume each link cost = 1, so now least-cost path is the

fewest number of router hops One of the most widely-used routing protocols RFC 1058

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RIP example

Info Stored Distance to reach node at node A B C D E F G

A 0 1 1 1 1 B 1 0 1 C 1 1 0 1 D 1 0 1 E 1 0 F 1 0 1 G 1 1 0

initial distances/costs stored at each node

Destination Cost NextHop

B 1 BC 1 CD -E 1 EF 1 FG -

initial routing table at node A

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RIP example, cont.

Info Stored Distance to reach node at node A B C D E F G

A 0 1 1 2 1 1 2B 1 0 1 2 2 2 3C 1 1 0 1 2 2 2D 2 2 1 0 3 2 1 E 1 2 2 3 0 2 3 F 1 2 2 2 2 0 1 G 2 3 2 1 3 1 0

final distances/costs stored at each node

Destination Cost NextHop

B 1 BC 1 CD 2 CE 1 EF 1 FG 2 F

final routing table at node A

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RIP example, cont in the absence of any topology changes, only a few

exchanges are required between neighbors before each node has completed its routing table

convergence: the process of getting consistent routing information to all of the nodes

there is no one node in the network that has all of the information in the complete routing table

each node only has knowledge of its own routing table each node has a consistent view of the network in the

absence of any centralized authority routing updates

periodic (seconds to minutes) triggered

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RIP, example RIP packets advertise costs to reach networks

(rather than routers/ nodes)

RIP packet format

command: ‘1’ (request), ‘2’ (reply)version: ‘1’ (or ‘2’ for RIPv2)address family: ‘2’ (IP)address: IP addressdistance: cost metric - hop count

up to 25 routes per RIP messagewell-known RIP port: UDP 520

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RIP RIP messages carried in UDP datagrams RIP version 2 (RFC 1388)

RIP-2 pass additional information

routing domain, route tag, subnet mask interoperable with RIP

cisco’s proprietary distance-vector Interior Gateway Routing Protocol (IGRP)

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Open Shortest Path First (OSPF) another intradomain or interior gateway protocol (IGP) Link-state ‘Open’ : non-proprietary (IETF) vs. proprietary EIGRP (cisco)

RFC 1247 each node is assumed to be capable of finding the state of

the link to its neighbors (up or down) and the cost of each link

assume reliable dissemination of link-state info

reliable flooding (all of node’s L-S info to all attached nodes) update packet (link-state packet [LSP])

calculation of routes from the sum of all the accumulated link-state knowledge

Dijkstra’s shortest-path algorithm

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OSPF Uses IP directly (does not use UDP or TCP)

has it’s own value (protocol ID) in the IP header can calculate a separate set of routes for each IP type-of-

service there can be multiple routing table entries for any

destination, one for each TOS each interface is assigned a dimensionless cost

can be throughput, RTT, reliability, etc. separate cost for each TOS

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OSPF when more than one equal-cost routes exist to a

destination, OSPF distributes traffic equally among routes (load balancing)

supports subnets: an associated subnet mask with each advertised route allows a single IP address of any class to be broken into

multiple subnets of various sizes (variable-length subnets)

simple authentication scheme (cleartext password, similar to RIP-2) can be used

replaces RIP

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Exterior Gateway Protocols (EGPs) interdomain routing protocols used between routers of different AS’s historically, the predominant EGP was a protocol

called EGP (confusing) the newer EGP is the Border Gateway Protocol

(BGP) version 3 (RFC 1267) RFC 1268 (use of BGP in the Internet) version 4 (RFC 1654)

message types (RFC 1771) updates sent using TCP