IP-Based Multihoming: Implementation, Challenges, and Alternatives, Papers of Computer Science

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Multihoming and Source-Controlled Routing

CS 7260

Nick Feamster

February 1, 2006

Today’s Topic

IP-Based Multihoming

• What is it? – What problem is it solving? (Why multihome?) – How is it implemented today (in IP)? – Traffic Engineering – How many upstream ISPs are enough?

Problems with IP-based multihoming

• Inbound route control – Routing table growth

Another approach:

host-based multihoming

Why Multihome?

Redundancy

Availability

Performance

Cost

Interdomain traffic engineering:

the process by

which a multihomed network configures its

network to achieve these goals

Redundancy

Maintain connectivity in the face of:

• Physical connectivity problems (fiber cut, device

failures, etc.)

• Failures in upstream ISP

Multihoming in IP Networks Today

Stub AS:

no transit service for other ASes

• No need to use BGP

Multi-homed stub AS:

has connectivity to multiple

immediate upstream ISPs

• Need BGP – No need for a public AS number – No need for IP prefix allocation

Multi-homed transit AS:

connectivity to multiple ASes

and

transit service

• Need BGP, public AS number, IP prefix allocation

Multihomed Stub: One Link

Upstream

ISP

Multiple linksbetween samepair of routers.

Default routes to “border”

“Stub”

ISP

Downstream ISP’s routers configure default(“static”) routes pointing to border router.

Upstream ISP advertises reachability

Multihomed Stub: Multiple ISPs

“Stub”

ISP

Upstream

ISP 1

Upstream

ISP 2

Many possibilities

• Load sharing – Primary-backup – Selective use of different ISPs

Requires BGP, public AS number, etc.

Multihomed Transit Network

Transit

ISP

ISP 1

ISP 2

ISP 3

BGP everywhere

Incoming and outcoming traffic

Challenge:

balancing load on intradomain

and

egress

links, given an offered traffic load

Outbound Traffic Control

Easier to control than inbound traffic

Destination-based routing:

sender determines where

the packets go

Control over

next-hop AS only

• Cannot control selection of the entire path

Provider 1

Provider 2

Control with localpreference

Outbound Traffic: Load Balancing

Control routes to provider

per-prefix

• Assign local preference across destination prefixes – Change the local preference assignments over time

Useful inputs to load balancing

• End-to-end path performance data – Outbound traffic statistics per destination prefix

Challenge:

Getting from traffic volumes to

groups of prefixes that should be assigned toeach link

Premise of “intelligent route control” preoducts.

Managing Scale

Destination prefixes

• More than 90,000 destination prefixes
  • Don’t want to have per-prefix routing policies
    • Small fraction of prefixes contribute most of the traffic
      • Focus on the small number of heavy hitters
        • Define routing policies for selected prefixes

Routing choices

• About 27,000 unique “routing choices”
  • Help in reducing the scale of the problem
    • Small fraction of “routing choices” contribute most traffic
      • Focus on the very small number of “routing choices”
        • Define routing policies on common attributes

Achieving Predictability

Route prediction with static analysis

• Helpful to know effects

before

deployment

• Static analysis can help

Topology

BGP policy

configuration

eBGP routes

BGP routing

model

Offered

traffic

Flow of traffic through the network

Inter-AS Negotiation

“Hot Potato”

routing

Coordination aidspredictability

• Negotiate where to send– Inbound and outbound– Mutual benefits

How to implement?

• What info to exchange?– Protecting privacy?– How to prioritize choices?– How to prevent cheating?

Destination 2

multiplepeeringpoints

Destination 1

Provider B Provider A

Outbound: Multihoming Goals

Redundancy

• Dynamic routing will failover to backup link

Performance

• Select provider with best performance per prefix – Requires active probing

Cost

• Select provider per prefix over time to minimize the

total financial cost