Lecture 10 Overview

Lecture 10 Overview

Border Gateway Protocol(BGP)• De facto standard for Internet inter-AS routing• allows subnet to advertise its existence to rest

of Internet: “I am here”• BGP provides each AS a means to:– Obtain subnet reachability information from

neighboring ASs– Propagate reachability information to all AS-internal

routers– Determine “good” routes to subnets based on

reachability information and policyCPE 401/601 Lecture 10 : Border Gateway Protocol 2

BGP basics• Pairs of routers (BGP peers) exchange routing info

over semi-permanent TCP connections: BGP sessions– BGP sessions need not correspond to physical links

• when AS2 advertises a prefix to AS1:– AS2 promises it will forward datagrams towards that prefix– AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2a

AS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

CPE 401/601 Lecture 10 : Border Gateway Protocol 3

Distributing reachability info

3b

1d

3a

1c2a

AS3

AS1

AS21a

2c

2b

1b

3ceBGP session

iBGP session

• 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


Path attributes & BGP routes• advertised prefix includes BGP attributes – prefix + attributes = “route”

• two important attributes– AS-PATH: contains ASs through which prefix

advertisement has passed: e.g, AS 67, AS 17 – NEXT-HOP: indicates specific internal-AS router to

next-hop AS• may be multiple links from current AS to next-hop-AS

• when gateway router receives advertisement, uses import policy to accept/decline


BGP route selection

• router may learn about more than 1 route to some prefix– Router must select route

• elimination rules– local preference value attribute: policy decision– shortest AS-PATH – closest NEXT-HOP router: hot potato routing– additional criteria


BGP messages

• BGP messages exchanged using TCP

• 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 ACKs OPEN request– NOTIFICATION: reports errors in previous msg;

also used to close connectionCPE 401/601 Lecture 10 : Border Gateway Protocol 7

BGP routing policy

• A,B,C are provider networks• X,W,Y are customer (of provider networks)• X is dual-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

A

B

C

W X

Y

legend:

customer network:

provider network


BGP routing policy (2)

• A advertises path AW to B• B advertises path BAW to X • Should B advertise path BAW to C?

– 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!

A

B

C

W X

Y

legend:

customer network:

provider network


Why different Intra- and Inter-AS routing ?

• Policy: – Inter-AS: admin wants control over how its traffic

routed, who routes through its net– Intra-AS: single admin, no need for policy decisions

• Scale:– hierarchical routing saves table size, reduced update

traffic

• Performance: – Intra-AS: can focus on performance– Inter-AS: policy may dominate over performance


Lecture 11

Router Architectures

CPE 401 / 601

Computer Network Systems

slides are modified from Dave Hollingerslides are modified from Dave Hollinger

Router Architecture Overview• Two key router functions: – run routing algorithms/protocol (RIP, OSPF, BGP)– forwarding datagrams from incoming to outgoing link

CPE 401/601 Lecture 11 : Router Architectures 12

Input Port Functions

• Goal: complete input port processing at ‘line speed’

• Decentralized switching: – given datagram dest., lookup output port using

forwarding table in input port memory


Input Port Functions

Physical layer:bit-level reception

Data link layer:e.g., Ethernet

Network layer:Routing & forwarding


Input Port Queuing• Queuing when datagrams arrive faster than

forwarding rate into switch fabric• queuing delay and loss due to input buffer overflow!• Head-of-the-Line (HOL) blocking: queued datagram

at front of queue prevents others in queue from moving forward


Three types of switching fabrics


Output Ports

• Buffering required when datagrams arrive from fabric faster than the transmission rate

• Scheduling discipline chooses among queued datagrams for transmission


Output port queueing

• Queuing (delay) and loss due to output port buffer overflow!


How much buffering?

• RFC 3439 rule of thumb: – buffering equal to typical RTT (say 250 msec)

times link capacity C• e.g., C = 10 Gps link:

– 2.5 Gbit buffer

• Recent recommendation: – with N flows, buffering equal to

RTT C.N


Bridges

Bridge Software

• A bridge connects networks and forwards frames from one network to another.

21

BRIDGE

A B

C D

E F

G H

PORTSPORTS

CPE 401/601 Lecture 11 : Bridges

Selective Forwarding

• If A sends a frame to E– the frame must be forwarded by the bridge

• If A sends a frame to B– there is no reason to forward the frame

22

BRIDGE

A B

C D

E F

G H


Bridge Database

• The bridge needs a database that contains information about which hosts are on which network.

• The realistic options are:– The system administrator can create and maintain

the database. – The bridge can acquire the database on the fly.

23

Hard to addnew computers

Some loss of efficiency


Learning the host mapping

• The bridge forwards packets for which it does not know which network the destination is on

• Every time the bridge forwards a packet it can record the network on which the sender is located

• Each host mapping expires when it is unused for a “long” period of time.

24CPE 401/601 Lecture 11 : Bridges

“Learning” Bridge

• A host can be moved to another network.

• New hosts can be added at any time.

• Requires no setup information from humans.


What is the problem ?

26

Bridge 1 10

Bridge 2 10

A

C

B

D

F

E


Possible Solutions

• Forget about smart bridges

• Tell customers to avoid topologies that include loops– design bridges so that they detect loops and

scream at the customer

• Design bridges that prune the network topology into something with no loops.


Spanning Tree Creation

• The bridges must communicate!– They send configuration bridge protocol data units

(BPDUs)– Multicast: special data link address

• Each bridge has a unique ID

• Use an algorithm to construct a spanning tree based on local messages


Spanning Tree

29

B

BB

B

B

B

B

BB

B

B

B

Prune

B bridge networkCPE 401/601 Lecture 11 : Bridges

Documents

Lecture 10 Overview