More on Internet Routing

• A large portion of this lecture material comes from BGP tutorial given by Philip Smith from Cisco (ftp://ftp-eng.cisco.com/pfs/seminars/APRICOT2004-BGP00.pdf)

More on Internet Routing

• What does a router do?– Find path (routing)– Forward packet from one interface to another

interface (forwarding)• Routing versus forwarding

– Routing: building maps and giving directions– Forwarding: moving packet based on the

directions.

• IP Routing:– Path derived from information received from a

routing protocol.– Several alternative paths may exist

• Best next hop stored in forwarding table.– Decisions are updated periodically or as

topology changes (event driven)– Decisions are based on

• Topology, policies and metrics (hop count, filtering, delay, bandwidth)

• IP route lookup (part of forwarding):– Based on destination address– “longest match” routing

• More specific prefix preferred over less specific prefix.

• Example: packet with destination of 10.1.1.1/32 is sent to the router announcing 10.1/16 rather than the router announcing 10/8

• Explicit versus default routing:– Default:

• Simple, cheap• Low granularity

– Explicit• High overhead, complex, high cost, high granularity

– Hybrid• Minimize overhead• Provide useful granularity

• Autonomous System (AS)– Collective of networks with same routing

policy– Single routing protocol– Usually under single ownership and

administrative control– Identified by the autonomous system number

(ASN), which is a 16-bit number.

• Border Gateway Protocol (BGP)– Described in RFC 1771– Routing protocol used to exchange routing

information between networks– The AS is BGP’s fundamental operating unit

• Used to identify networks with common routing polocy.

– BGP-4 is the de facto standard in the Internet

• BGP terms:– Neighbours: AS’s which directly exchange

routing information– Announce: send route information to the

neighbor– Accept: receive and use the information sent by

a neighbour– Originate: insert routing information into

external announcement– Peers: routers that can directly exchange

routing and policy information

Routing flow and packet flow:

• BGP basic:– Runs over TCP – port 179– Path vector protocol (distance vector algorithm

+ explicit AS path)– Two types of updates

• (Route) announcement and withdraw– BGP update attributes

• ASPATH and some other information

An example

NLRI=128.186.0.0/16ASPATH=[0]

128.186.0.0/16

NLRI=128.186.0.0/16ASPATH=[10]

NLRI=128.186.0.0/16ASPATH=[10]

NLRI=128.186.0.0/16ASPATH=[210]

NLRI=128.186.0.0/16ASPATH=[610]

NLRI=128.186.0.0/16ASPATH=[610]

NLRI=128.186.0.0/16ASPATH=[210]

NLRI=128.186.0.0/16ASPATH=[7610]

NLRI=128.186.0.0/16ASPATH=[4210]

NLRI=128.186.0.0/16ASPATH=[3210]

[3210]*[4210][7610]

NLRI=128.186.0.0/16ASPATH=[53210]

• BGP general operation– Learn multiple paths via internal and external

BGP speakers– Picks the best path and installs in the

forwarding table– Best path is sent to external BGP neighbours– Policies applies by influencing the best path

selection

• Policy based routing protocol• BGP route selection:

– Prefer routes with highest local preference– Prefer routers with shortest AS path– Prefer routers with lowest MED value– Prefer EBGP routes over IBGP routes– Prefer routes via nearest IGP neighbor– Tie breaking: lowest router ID

Policy disputes

• Global routing system never converge

Security: control plane security

• problems– Anyone can claim ownership of NLRI– Anyone can claim en route to NLRI