Computer Networks with Internet TechnologyWilliam Stallings

Chapter 11Interior Routing Protocols

Autonomous Systems (AS)• Group of routers exchanging information

via common routing protocol• Set of routers and networks managed by

single organization• Connected

—Except in time of failure

Interior Routing Protocol (IRP)• Passes routing information between routers

within AS• Does not need to be implemented outside AS

—Allows IRP to be tailored

• May be different algorithms and routing information in different connected AS

• Need minimum information from other connected AS—At least one router in each AS must talk—Use Exterior Routing Protocol (ERP)

Exterior Routing Protocol (ERP)

• Pass less information than IRP• Router in first system determines route to

target AS• Routers in target AS then co-operate to

deliver datagram• ERP does not deal with details within

target AS

Figure 11.4 Application of Exterior and Interior Routing Protocols

Approaches to Routing – Distance-vector • Each node (router or host) exchange information

with neighboring nodes—Neighbors are both directly connected to same network

• First generation routing algorithm for ARPANET• Node maintains vector of link costs for each directly

attached network and distance and next-hop vectors for each destination

• Used by Routing Information Protocol (RIP) • Requires transmission of lots of information by

each router—Distance vector to all neighbors—Contains estimated path cost to all networks in

configuration—Changes take long time to propagate

Approaches to Routing – Link-state• Designed to overcome drawbacks of distance-vector• When router initialized, it determines link cost on each

interface• Advertises set of link costs to all other routers in topology

— Not just neighboring routers• From then on, monitor link costs

— If significant change, router advertises new set of link costs• Each router can construct topology of entire configuration

— Can calculate shortest path to each destination network• Router constructs routing table, listing first hop to each

destination• Router does not use distributed routing algorithm

— Use any routing algorithm to determine shortest paths— In practice, Dijkstra's algorithm

• Open shortest path first (OSPF) protocol uses link-state routing.

• Also second generation routing algorithm for ARPANET

Exterior Router Protocols –Path-vector

• Provide information about which networks can be reached by a given router and ASs crossed to get there—Does not include distance or cost estimate

• Each block of information lists all ASs visited on this route—Enables router to perform policy routing

• e.g. avoid path to avoid transiting particular AS

—e.g. link speed, capacity, tendency to become congested, and overall quality of operation, security

—e.g. minimizing number of transit ASs

11.2 Least Cost Algorithms• Least-cost criterion

—If minimize number of hops, link value 1—Link value may be inversely proportional to capacity,

proportional to current load, or some combination—May differ in different two directions—E.g. if cost equaled length of queue

• Cost of path between two nodes as sum of costs of links traversed

• For each pair of nodes, find least cost path • Two common algorithmsTwo common algorithms

—Dijkstra's algorithm Dijkstra's algorithm —Bellman-Ford algorithmBellman-Ford algorithm

11.3Distance Vector Routing: RIP• Each node exchange information with

neighbors—Directly connected by same network

• Each node maintains three vectors—Link cost—Distance vector—Next hop vector

• Every 30 seconds, exchange distance vector with neighbors

• Use this to update distance and next hop vector

Figure 11.1 A Configuration of Routers and Networks

(changed to 1)

(changed to 1)

Figure 11.8 Distance Vector Algorithm Applied to Figure 11.1

RIP Details – Incremental Update• Updates do not arrive from neighbors

within small time window• RIP packets use UDP• Tables updated after receipt of individual

distance vector—Add any new destination network—Replace existing routes with small delay ones—If update from router R, update all routes

using R as next hop

RIP Details –Topology Change• If no updates received from a router

within 180 seconds, mark route invalid—Invalid timer: 180 sec—Assumes router crash or network connection

unstable—Set distance value to infinity

• Actually 16

Counting to Infinity Problem (1)• Slow convergence may cause:• All links are assumed cost 1• B has distance to network 5 as 2, next hop D• A & C have distance 3 and next hop B

Counting to Infinity Problem (2)• Suppose router D fails:

— B determines network 5 no longer reachable via D• Sets distance to 4 based on report from A or C

— At next update, B tells A and C this— A and C receive this and increment their network 5 distance to 5

• 4 from B plus 1 to reach B

— B receives distance count 5 and assumes network 5 is 6 away— Repeat until reach infinity (16)— Takes 8 to 16 minutes to resolve

×

Split Horizon

• Counting to infinity problem caused by misunderstanding between B and A, and B and C—Each thinks it can reach network 5 via the other

• Split Horizon rule says do not send information about a route back in the direction it came from—Router sending information is nearer destination than

you

—That is, A should not tell B “the distance to network 5”.

—Erroneous route now eliminated within time out period (180 seconds)

Poisoned Reverse• Send updates with hop count of 16 to

neighbors for route learned from those neighbors—If two routers have routes pointing at each

other advertising reverse route with metric 16 breaks loop immediately

—B tells A and C “distance to network 5 is 16”

Figure 11.9 RIP Packet Format (v1)

Command: 1: request, 2: response

Address Family identifier: IP, IPX, …

• Over UDP• Multicast: 224.0.0.9

RIP v2

Route Tag: 0 or AS#

RIP Packet Format Notes• Command: 1=request 2=reply

—Updates are replies whether asked for or not—Initializing node broadcasts request—Requests are replied to immediately

• Version: 1 or 2• Address family: 2 for IP• IP address: non-zero network portion, zero host

portion—Identifies particular network

• Metric—Path distance from this router to network—Typically 1, so metric is hop count

RIP Limitations• Destinations with metric more than 15 are

unreachable—If larger metric allowed, convergence becomes

lengthy

• Simple metric leads to sub-optimal routing tables—Packets sent over slower links

• Accept RIP updates from any device—Misconfigured device can disrupt entire

configuration

11.4Link-State Protocol: OSPF• RIP limited in large internets• Open Shortest Path First (OSPF)• OSPF preferred interior routing protocol

for TCP/IP based internets• Link state routing used• Directly over IP

Link State Routing• When initialized, router determines link cost on

each interface• Router advertises these costs to all other routers

in topology• Router monitors its costs

—When changes occurs, costs are re-advertised

• Each router constructs topology and calculates shortest path to each destination network

• Not distributed version of routing algorithm• Can use any algorithm

—Dijkstra

Flooding• Packet sent by source router to every neighbor• Incoming packet resent to all outgoing links except

source link• Duplicate packets already transmitted are discarded

—Prevent incessant retransmission

• All possible routes tried so packet will get through if route exists—Highly robust

• At least one packet follows minimum delay route—Reach all routers quickly

• All nodes connected to source are visited—All routers get information to build routing table

• High traffic load

Figure 11.10 Flooding Example

OSPF Overview• Router maintains descriptions of state of

local links• Transmits updated state information to all

routers it knows about• Router receiving update must

acknowledge—Lots of traffic generated

• Each router maintains database—Directed graph

Router Database Graph• Vertices

—Router—Network

• Transit• Stub

• Edges—Connecting two routers—Connecting router to network

• Built using link state information from other routers

Figure 11.11 Sample Autonomous System

Figure 11.12 Directed Graph of Autonomous System of Figure 19.7

Link Costs• Cost of each hop in each direction is called

routing metric• OSPF provides flexible metric scheme

based on type of service (TOS)—Normal (TOS 0)—Minimize monetary cost (TOS 2)—Maximize reliability (TOS 4)—Maximize throughput (TOS 8)—Minimize delay (TOS 16)

• Each router generates 5 spanning trees (and 5 routing tables)

Figure 11.13 The SPF Tree for Router R6

Areas• Make large internets more manageable• Configure as backbone and multiple areas• Area – Collection of contiguous networks

and hosts plus routers connected to any included network

• Backbone – contiguous collection of networks not contained in any area, their attached routers and routers belonging to multiple areas

Operation of Areas• Each area runs a separate copy of the link

state algorithm—Topological database and graph of just that

area—Link state information broadcast to other

routers in area—Reduces traffic—Intra-area routing relies solely on local link

state information

Inter-Area Routing• Path consists of three legs

—Within source area• Intra-area

—Through backbone• Has properties of an area• Uses link state routing algorithm for inter-area

routing

—Within destination area• Intra-area

Figure 11.14OSPF Packet Header

* directly over IP

Packet Format Notes• Version number: 2 is current• Type: one of 5, see next slide• Packet length: in octets including header• Router id: this packet’s source, 32 bit• Area id: Area to which source router

belongs• Authentication type: null, simple password

or encryption• Authentication data: used by

authentication procedure

OSPF Packet Types

• Hello: used in neighbor discovery• Database description: Defines set of link

state information present in each router’s database

• Link state request• Link state update• Link state acknowledgement