Internet Routing (COS 598A) Today: Topology Size

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Internet Routing (COS 598A)Today Topology Size

• Jennifer Rexford
• http//www.cs.princeton.edu/jrex/teaching/spring2
  005
• Tuesdays/Thursdays 1100am-1220pm

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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

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Routers Running eBGP, iBGP, and IGP
destination
AS A
AS B
Legend eBGP session iBGP session IGP link
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1
2
4
Route r1 has closest egress point
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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

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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

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Why Do Point-to-Point Internal BGP?

• Reusing the BGP protocol
• iBGP is really just BGP
• except you dont 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

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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

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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

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Example Single Route Reflector
1
2
2
4
r2
Router only learns about r2
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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

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Routing Anomaly Forwarding Loop
r1
r2
1
1
r1
r2
1
Picks r2
Picks r1
Packet deflected toward other egress point,
causing a loop
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Routing Anomaly Protocol Oscillation
1
2
3
1
1
1
5
5
5
RR1 prefers r2 over r1 RR2 prefers r3 over r2 RR3
prefers r1 over r3
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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
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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

1
2
2
4
r1, r2, r4
http//www.acm.org/sigs/sigcomm/sigcomm2002/papers
/bgposci.pdf
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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 viewpoint

1
2
2
4
r1
http//www.rnp.br/ietf/internet-drafts/draft-bonav
enture-bgp-route-reflectors-00.txt http//www.cs.p
rinceton.edu/jrex/papers/rcp-nsdi.pdf
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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
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Possible Solution Tunnel Between Edge Routers

• 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

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

• When full-mesh doesnt 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

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IGP Topology
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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

2
1
3
1
3
2
1
5
4
3
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Improving the Scaling

• Dijkstras shortest-path algorithm
• Simplest version O(N2), where N is of nodes
• Better algorithms O(Llog(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

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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

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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

2
1
3
1
To area 0
3
2
1
5
cost 8
4
3
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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

Inter-PoP
Intra-PoP
Other networks
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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

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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

50
50
100
100
50
100
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20
20
20
20
20
100
100
100
50
50
50
North America
Europe
Asia
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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

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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???

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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?

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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

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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

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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