Internet Routing (COS Internet Routing (COS 598A)598A)

Today: Topology SizeToday: Topology Size

Jennifer RexfordJennifer Rexford

http://www.cs.princeton.edu/~jrex/teaching/http://www.cs.princeton.edu/~jrex/teaching/spring2005spring2005

Tuesdays/Thursdays 11:00am-12:20pmTuesdays/Thursdays 11:00am-12:20pm

Outline

• BGP distribution inside an AS– Full-mesh iBGP – Route reflectors– Routing anomalies caused by route

reflectors– Pros and cons of proposed solutions

• IGP topology– OSPF areas– Summarization– Multiple ASes

Routers Running eBGP, iBGP, and IGP

destination

AS A AS B

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4

1 2Legend

eBGP sessioniBGP session

IGP link

r1 r2 r3 r4

r1r2

r4

Route r1 has closest egress point

Roles of eBGP, iBGP, and IGP

• eBGP: External BGP– Learn routes from neighboring ASes– Advertise routes to neighboring ASes

• iBGP: Internal BGP– Disseminate BGP information within the AS

• IGP: Interior Gateway Protocol– Compute shortest paths between routers in

AS– Identify closest egress point in BGP path

selection

Full Mesh iBGP Configuration

• Internal BGP session– Forward best BGP route to a neighbor– Do not send from one iBGP neighbor to another

• Full-mesh configuration– iBGP session between each pair of routers– Ensures complete visibility of BGP routes

Why Do Point-to-Point Internal BGP?

• Reusing the BGP protocol– iBGP is really just BGP– … except you don’t add an AS to the AS

path– … or export routes between iBGP neighbors

• No need to create a second protocol– Another protocol would add complexity

• And, full-mesh is workable for many networks– Well, until they get too big…

Scalability Limits of Full Mesh on the Routers

• Number of iBGP sessions– TCP connection to every other router

• Bandwidth for update messages– Every BGP update sent to every other router

• Storage for the BGP routing table– Storing many BGP routes per destination

prefix

• Configuration changes when adding a router– Configuring iBGP session on every other

router

Route Reflectors

• Relax the iBGP propagation rule– Allow sending updates between iBGP neighbors

• Route reflector– Receives iBGP updates from neighbors– Send a single BGP route to the clients

• Very much like provider, peer, and customer– To client: send all BGP routes– To peer route reflector: send client-learned

routes– To route reflector: send all client-learned routes

Example: Single Route Reflector

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1 2

r1 r2 r3 r4

r1 r2

r4

r2

Router only learns about r2

The Problem About Route Reflectors

• Advantage: scalability– Fewer iBGP sessions– Lower bandwidth for update messages– Smaller BGP routing tables– Lower configuration overhead

• Disadvantage: changes the answers– Clients only learn a subset of the BGP routes– Does not result is same choices as a full

mesh– ... especially if RR sees different IGP

distances

Routing Anomaly: Forwarding Loop

r1 r2

1

1

1r1r2

Picks r2 Picks r1

Packet deflected toward other egress point, causing a loop

Routing Anomaly: Protocol Oscillation

5 5 5

r1 r2 r3

1 1 11 2 3

RR1 prefers r2 over r1RR2 prefers r3 over r2RR3 prefers r1 over r3

Avoiding Routing Anomalies

• Reduce impact of route reflectors– Ensure route reflector is close to its clients– … so the RR makes consistent decisions

• Sufficient conditions for ensuring consistency– RR preferring routes through clients over “peers”– BGP messages should traverse same path as data

• Forces a high degree of replication– Many route reflectors in the network– E.g., a route reflector per PoP for correctness– E.g. have a second RR per PoP for reliability

http://www.acm.org/sigs/sigcomm/sigcomm2002/papers/ibgp.pdf

Possible Solution: Disseminating More Routes

• Make route reflectors more verbose– Send all BGP routes to clients, not just best route– Send all equally-good BGP routes (up to IGP cost)

• Advantages– Client routers have improved visibility– Make the same decisions as in a full mesh

• Disadvantages– Higher overhead for sending and storing routes– Requires protocol changes to send multiple routes– Not backwards compatible with legacy routers

http://www.acm.org/sigs/sigcomm/sigcomm2002/papers/bgposci.pdf

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1 2

r1 r2 r3 r4

r1 r2

r4

r1, r2, r4

Possible Solution: Customized Dissemination

• Make route reflector more intelligent– Send customized BGP route to each client– Tell each client what he would pick himself

• Advantages– Make the same decisions as in a full mesh– Remain compatible with legacy routers

• Disadvantages– Intelligent RR must make decisions per

client– … and select closest egress from each

viewpointhttp://www.rnp.br/ietf/internet-drafts/draft-bonaventure-bgp-route-reflectors-00.txthttp://www.cs.princeton.edu/~jrex/papers/rcp-nsdi.pdf

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1 2

r1 r2 r3 r4

r1 r2

r4

r1

Possible Solution: Multicast/Flooding

• Replace point-to-point distribution– Apply a multicast protocol to distribute

messages– Or, flood the BGP messages to all routers

• Advantages– Complete distribution without route reflectors– Avoids configuration overhead of a full mesh

• Disadvantages– Requires an additional, new protocol– Not backwards-compatible with legacy routers– Large BGP routing tables, like in a full mesh

http://www.nanog.org/mtg-0302/ppt/van.pdf

Possible Solution: Tunnel Between Edge Routers

r1 r2

1

1

1r1r2tunneltunnel

• Tunneling through the core– Ingress router selects ingress point– Other routers blindly forward to the egress

• Advantages– No risk of forwarding loops– No BGP running on interior routers

• Disadvantages– Overhead of tunneling protocol/technology– Still has a risk of protocol oscillations

State-of-the-Art of BGP Distribution in an AS

• When full-mesh doesn’t scale– Hierarchical route-reflector configuration

• One or two route reflectors per PoP

– Some networks use “confederations” (mini ASes)• Recent ideas

– Sufficient conditions to avoid anomalies– Enhanced RRs sending multiple or custom

routes– Flooding/multicast of BGP updates– Tunneling to avoid packet deflections

• Open questions– Are the sufficient conditions too restrictive?– Good comparison of the various approaches

IGP Topology

Interior Gateway Protocols (IGPs)

• Protocol overhead depends on the topology– Bandwidth: flooding of link state advertisements – Memory: storing the link-state database– Processing: computing the shortest paths

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Improving the Scaling

• Dijkstra’s shortest-path algorithm– Simplest version: O(N2), where N is # of nodes– Better algorithms: O(L*log(N)), where L is #

links– Incremental algorithms: great for small

changes

• Timers to pace operations– Minimum time between LSAs for the same link– Minimum time between path computations

• More resources on the routers– Routers with more CPU and memory

Introducing Hierarchy: OSPF Areas

• Divide network into regions– Backbone (area 0) and non-backbone areas– Each area has its own link-state database– Advertise only path distances at area boundaries

Area 0

Area 1 Area 2

Area 3 Area 4

areaborderrouter

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1

13

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5

3

To area 0

cost 3

cost 8

Summarization at Area Boundaries

• Areas only help so much– Advertising path costs to reach each component– Single link failure may change multiple path costs

• Summarization: LSA for multiple components– LSA for an IP prefix containing the addresses– LSA carries cost for the maximum path cost

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1

13

1

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3

To area 0

cost 8

Assigning OSPF Areas

• Group related routers– E.g., in a Point-of-Presence– Assign to single OSPF area– Put inter-PoP links in area 0

• Enable summarization– Select an address block for

the equipment in the area– Assign IP addresses in the

block to router CPUs and interfaces

Intra-PoP

Other networks

Inter-PoP

Pros and Cons of Summarization

• Advantages: scalability– Reduce the size of the link-state database

• One entry per summary prefix

– Isolate the rest of the network from changes• Only advertise when max path cost changes

• Disadvantages– Complexity

• Extra configuration details for areas & summarization

• Requires tight coupling with IP address assignment

– Inefficiency• Summarization hides details that affect path

selection• Data packets may traverse a less-attractive path

Dividing into Multiple ASes

• Divide the network into regions– Separate instance of IGP per region– Interdomain routing between regions– Loss of visibility into differences within region

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10020 20 20 20 20 20

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North America Europe Asia

Multi-AS Networks, Not Just for Scalability

• Administrative reasons– Separate networks per geographic region– Mergers/acquisitions that combine networks

• Why not merge to single AS?– Using different intradomain protocols– Managed by different people– Fear of encountering scalability problems– Fear of losing the benefits of isolation

• Why merge to a single AS?– Simpler configuration– More efficient routing– Avoid having separate AS hop in BGP AS paths

Which Approach is Better?

• Ideal: flat IGP network– Single AS– Single IGP instance, no areas

• Hierarchical IGP– Single AS– Single IGP instance, using areas &

summarization

• Multiple ASes– Multiple ASes– Separate IGP instances

• Some other approach???

Comparison Metrics

• Scalability– Protocol overhead

• Storing and flooding link-state advertisements• Overhead of Dijkstra shortest-path computation

– Effects of topology changes• Number of advertisements after a change• Likelihood a change must be propagated

• Efficiency– Stretch: comparing path lengths

• In ideal flat intradomain routing• In alternative scheme

– How much longer do the paths get?

Interesting Research Questions

• Routing protocols that achieve small stretch– Theory work on algorithms to minimize stretch– Protocol work on hierarchy and aggregation– Any new distributed protocols with low stretch?– Avoid sharp boundaries between areas/ASes?

• Identifying good places to hide information– Given a network graph with link weights– Decide where to put area and AS boundaries– … with the goal of minimizing stretch– … within some max size of each area or AS

Conclusion

• Networks are getting bigger– Growth of a network topology– Merger/acquisition of other networks

• Techniques for scaling the routing design– BGP route reflection– OSPF areas– Multiple BGP ASes

• Relatively open research area– Rich theoretical tradition on compact routing– Common operational practices for protocol

scaling– Not much work has been done in between

Next Time: Router Configuration

• Two papers– “Automated provisioning of BGP customers”

(just sections 1-3)– “Detecting BGP faults with static analysis”

• Review only of second paper– Summary– Why accept– Why reject– Future work

• Optional– Short survey on BGP routing policies for ISPs– NANOG video covering material in second

paper