Week14 lec1

Chapter 4Network Layer

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July

2007.

Hierarchical Routing

Scale: With millions of destinations:

Can’t store all destinations in routing tables

Exchange of Routing updates would swamp links!

Administrative Autonomy

Internet = network of networks

Each network administrator may want to control routing in its own network

All routers run the same routing Algorithm?

… not true in practice

Hierarchical Routing Routers are organized

into “Autonomous Systems” (ASs). Routers in same

administrative domain Routers in same AS

run same routing protocol The routing algorithm

within an autonomous system is called an “intra-AS” routing protocol

Routers in different AS can run different intra-AS routing protocol

Gateway Routers One or more of the

routers in an AS will have the added task of forwarding packets to destinations outside the AS.

1d

1c

AS11a 1b

3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

Interconnected ASs

1b, 1c, 2a and 3a are Gateway Routers

Inter-AS tasks Suppose router in AS1

receives datagram destined outside of AS1: Router should forward

packet to gateway router, but which one?

AS1 must:1. Learn which destinations are

reachable through AS2, which through AS3

2. Propagate this reachability information to all routers in AS1. Configure

Forwarding tableJob of inter-AS routing protocol!

3b

1d

3a1c

2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Example: Setting forwarding table in router 1d

Suppose AS1 learns (via inter-AS protocol) that network x reachable via AS3 (gateway 1c) but not via AS2.

Inter-AS protocol propagates reachability info to all internal routers.

Router 1d determines from intra-AS routing info that its interface I is on the least cost path to 1c. installs forwarding table entry (x,I)

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

x…

Example: Choosing among multiple ASs Now suppose AS1 learns from inter-AS protocol

that a network x is reachable from AS3 and from AS2.

To configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x.

Hot Potato Routing: Send packet towards closest of two routers.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3cx… …

Learn from inter-AS protocol that network

x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routing:

Choose the gateway

that has the smallest least

cost

Determine the interface I that leads to least-cost gateway.

Enter (x,I) in forwarding table

Example: Choosing among multiple ASs

Steps in adding an outside AS destination in a routers forwarding Table

Routing Protocols

Path Vector Routing

Path Vector RoutingLoop Prevention

If a router receives a message, checks to see if its autonomous system is in the path list to the destination.

If it is, looping is involved and the message is ignored.

Policy RoutingIf one of the path in the path vector

is against the policy it can ignore that path and does not update its routing table.

Inter-AS Routing in Internet: BGP BGP (Border Gateway Protocol)

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate reachability information to all AS-

internal routers.3. Determine “good” routes to subnets based

on reachability information and policy. Allows subnet to advertise its existence

to rest of Internet: “I am here”

BGP Basics Pairs of routers (BGP peers) exchange routing info over

TCP connections using port 179. For each TCP connection, the two routers at the end of

the connection are called BGP Peers The TCP connection along with all the BGP messages

sent over the connection is called a BGP Session Internal (IBGP) neighbours

A pair of BGP speakers within the same AS External (EBGP) neighbours

Two BGP speakers from two different AS

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

BGP Basics BGP allows each AS to learn which

destinations are reachable via its neighboring ASs.

Destinations are not hosts but instead are CIDRized prefixes. E.g. AS1 has four subnets attached to it

It will aggregate the prefixes of these four and will advertise the single prefix

AS1 and AS2 send reachability information trough their gateway routers 1b and 2a.

When a gateway router receives e-BGP learned prefixes, the gateway router uses iBGP sessions to distribute the prefixes to other routers in the AS.

Distributing Reachability info Using eBGP session between 3a and 1c, AS3

sends prefix reachability info to AS1. 1c can then use iBGP do distribute new

prefix info to all routers in AS1 1b can then re-advertise new reachability

info to AS2 over 1b-to-2a eBGP session When router learns of new prefix, it creates

entry for prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

Path Attributes & BGP Routes

In BGP an AS is recognized by its globally unique Autonomous System Number (ASN).Assigned by ICANN Regional

Registries.Advertised prefix includes BGP

attributes. prefix + attributes = “route”

Two important attributes:AS-PATH and NEXT-HOP

Path Attributes & BGP Routes

AS-PATHContains ASs through which prefix

advertisement has passed.When a prefix is passed into an AS, the

As adds its ASN to the AS-PATH attribute.E.g suppose that the prefix

138.16.64.0/24 is first advertised from AS2 to AS1.

If AS1 then advertises the prefix to AS3, AS-PATH would be AS2 AS1.

Detect and prevent looping advertisements

Routing Loop Prevention

AS1

AS2

AS3

AS4

Net1

[Net1, (AS4)]

[ Net1, (AS3,AS4) ]

[ Net1, (AS1,AS2,AS3,AS4)]

[ Net1, (AS2,AS3,AS4)]

AS3 won’t forward this further