CS 204: BGP Jiasi Chen Lectures: MWF 12:10-1pm in WCH 139 - - PowerPoint PPT Presentation

CS 204: BGP

Jiasi Chen Lectures: MWF 12:10-1pm in WCH 139 http://www.cs.ucr.edu/~jiasi/teaching/cs204_spring16/

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Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

2

Autonomous Systems

• Autonomous system (AS)
• Unit of routing policy
• ~50k ASes in use
• E.g., UCR has AS#6061, AT&T has AS#144, Princeton has AS#88

3

AS6061 3b 3c 3a AS144 1c 1a 1d 1b

AS88

2a 2c 2b

• ther

networks

• ther

networks

Review of Routing

• Inter-AS routing
• BGP
• Intra-AS routing
• RIP
• OSPF

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Link-state? Distance vector?

AS6061 3b 3c 3a AS144 1c 1a 1d 1b

AS88

2a 2c 2b

• ther

networks

• ther

networks

Why different Intra-, Inter-AS routing ?

policy:

• inter-AS: admin wants control over how its traffic

routed, who routes through its net.

• intra-AS: single admin, so no policy decisions needed

scale:

• hierarchical routing saves table size, reduced update

traffic performance:

• intra-AS: can focus on performance
• inter-AS: policy may dominate over performance

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3b 1d 3a 1c 2a AS3 AS1

AS2

1a 2c 2b 1b

Intra-AS Routing algorithm Inter-AS Routing algorithm

Forwarding table

3c

Interconnected ASes

vforwarding table configured by both intra- and inter-AS routing algorithm § intra-AS sets entries for internal dests § inter-AS & intra-AS sets entries for external dests

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Example: setting forwarding table in router 1d

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

• ther

networks

• ther

networks

x

a b c

Send packet on interface a, b, or c? Determine b is on least cost path, install forwarding table entry (x,b)

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Example: choosing among multiple ASes

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

• ther

networks

• ther

networks

x Which path to choose? This is also the job of the inter-AS protocol

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learn from inter-AS protocol that subnet x is reachable via multiple gateways use routing info from intra-AS protocol to determine costs of least-cost paths to each

• f the gateways

hot potato routing: choose the gateway that has the smallest least cost determine from forwarding table the interface that leads to least-cost gateway. Enter that interface in forwarding table

Example: choosing among multiple ASes

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

• ther

networks

• ther

networks

x

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Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

10

Internet inter-AS routing: BGP

• BGP (Border Gateway Protocol):thede facto

inter-domain routing protocol

• “glue that holds the Internet together”
• BGP provides each AS a means to:
• eBGP:obtain subnet reachability information from

neighboring ASs.

• iBGP:propagate reachability information to all AS-

internal routers.

• determine “good” routes to other networks based on

reachability information and policy.

• allows subnet to advertise its existence to rest of

Internet: “I am here”

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BGP basics

• when AS3 advertises a prefix to AS1:
• AS3 promises it will forward datagrams towards that prefix
• AS3 can aggregate prefixes in its advertisement

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

• ther

networks

• ther

networks

v BGP session: two BGP routers (“peers”) exchange BGP

messages:

§ advertising paths to different destination network prefixes (“path vector” protocol) § exchanged over semi-permanent TCP connections

BGP message

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Path attributes and BGP routes

• advertised prefix includes BGP attributes
• prefix + attributes = “route”
• two important attributes:
• AS-PATH: contains ASs through which prefix advertisement

has passed

• NEXT-HOP: indicates specific internal-AS router to next-hop
• AS. (may be multiple links from current AS to next-hop-AS)

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BGP basics: distributing path information

AS3

AS2

3b 3a AS1 1c 1a 1d 1b 2a 2c 2b

• ther

networks

• ther

networks eBGP session iBGP session

3c

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BGP route selection

vrouter may learn about more than 1 route to destination AS, selects route based on:

• 1. local preference value attribute: policy decision
• 2. shortest AS-PATH
• 3. closest NEXT-HOP router: hot potato routing
• 4. additional criteria
• gateway router receiving route advertisement uses

import policy to accept/decline

• e.g., never route through AS x
• policy-based routing

15

BGP messages

• BGP messages exchanged between peers over TCP connection
• 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 connection

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Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

17

1

2 3

Dest IP

routing algorithms local forwarding table prefix

• utput port

138.16.64/22 124.12/16 212/8 ………….. 3 2 4 …

How does entry get in forwarding table?

entry

Assume prefix is in another AS.

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High-level overview

• 1. Router becomes aware of prefix
• 2. Router determines output port for prefix
• 3. Router enters prefix-port in forwarding table

How does entry get in forwarding table?

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Router becomes aware of prefix

AS3

AS2

3b 3c AS1 1c 1a 1d 2a 2c 2b

• ther

networks

BGP message v BGP message contains “routes” v “route” is a prefix and attributes: AS-PATH, NEXT-

HOP,…

v Example v Prefix: 138.16.64/22 v AS-PATH: AS3 AS15 … v NEXT-HOP: 201.44.13.125

138.16.64/22

3a 1b

AS15

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Router may receive multiple routes

v Router may receive multiple routes for same prefix v Has to select one route

AS3

AS2

3b 3c AS1 1c 1a 1d 2a 2c 2b

• ther

networks AS131

3a 1b

138.16.64/22

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• Router selects route based on shortest AS-PATH

Select best BGP route to prefix

v Example: v AS2 AS17 to 138.16.64/22 v AS3 AS131 AS201 to 138.16.64/22 v What if there is a tie?

We’ll come back to that!

select

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Find best intra-route to BGP route

• Use selected route’s NEXT-HOP attribute
• Route’s NEXT-HOP attribute is the IP address of the router

interface that begins the AS PATH.

• Example:

v AS-PATH: AS2 AS17 …; NEXT-HOP: 111.99.86.55

• Router uses OSPF to find shortest path from 1c to

111.99.86.55

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

AS17 AS15

111.99.86.55

2a 3a

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Router identifies port for route

vIdentifies port along the OSPF shortest path vAdds prefix-port entry to its forwarding table:

§ (dest IP , port 4)

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

AS17 AS131

router port

1 2 3 4

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Hot Potato Routing

vSuppose there two or more best inter-routes. vThen choose route with closest NEXT-HOP

§ Use OSPF to determine which gateway is closest § Q: From 1c, chose AS3 AS131 or AS2 AS17? § A: route AS3 AS131 since it is closer

AS3

AS2

3b 3c 3a AS1 1c 1a 1d 1b 2a 2c 2b

AS17 AS131

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Summary

• 1. Router becomes aware of prefix

§ via BGP route advertisements from other routers

• 2. Determine router output port for prefix

§ Use BGP route selection to find best inter-AS route § Use OSPF to find best intra-AS route leading to best inter-AS route § Router identifies router port for that best route

• 3. Enter prefix-port entry in forwarding table

How does entry get in forwarding table?

26

Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

27

Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

28

BGP routing policy

v A,B,C are provider networks v X,W,Y are customer (of provider networks) v 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

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BGP routing policy (2)

v A advertises path AW to B v B advertises path BAW to X v 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

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Overview

• Inter-AS routing
• BGP
• Forwarding example
• AS hierarchy
• Paper discussion

31

Stable BGP Routing

• Trying to get to destination A
• Routes listed in order of preference

32

A B C

(B C A) (B A) (C B A) (C A)

Stable BGP routing

• Suppose we start off with the second choice options…

33

A B C

(B C A) (B A) (C B A) (C A) (B C A) (B A) (C B A) (C A) (B C A) (B A) (C B A) (C A)

time

Paper Discussion

• What are implicit and explicit policies?
• What are the underlying assumptions?

34

Sources

• Computer Networking: A Top-Down Approach, Kurose & Ross
• Lixin Gao and Jennifer Rexford, “Stable Internet Routing Without

Global Coordination,” IEEE Trans. Networking, 2001.

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