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Multi-path Interdomain ROuting by Xu and Rexford. Alan Dunn Topics in Network Protocol Design March 5, 2010. Outline. Motivation Protocol Description Evaluation. Motivation. Examine “large scale” ( interdomain ) routing Involves parties with varying security and performance needs

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Multi path interdomain routing by xu and rexford

Multi-path InterdomainROutingby Xu and Rexford

Alan Dunn

Topics in Network Protocol Design

March 5, 2010


  • Motivation

  • Protocol Description

  • Evaluation


  • Examine “large scale” (interdomain) routing

  • Involves parties with varying security and performance needs

  • Claim: Current interdomain routing is too inflexible for current needs

Reminder bgp border gateway protocol
Reminder: BGP – Border Gateway Protocol

  • Standard Autonomous System (AS) level routing protocol

  • “BGP speakers” for AS advertise complete AS paths for aggregate routes as represented by IP prefixes

  • BGP routes combine with Interior Gateway Protocol (IGP) routing schemes (RIP, OSPF) to form full routes across Internet

Bgp example
BGP Example

AS 1239


next-hop = 3

AS_PATH = <7018,209,6922,18>

AS 7018

(AT&T) -> 1 -> 1


??? -> 2 -> 2

next-hop = 1

AS_PATH = <209,6922,18>

AS 6543

next-hop = 2

AS_PATH = <6543,270,123,18>



AS 209

next-hop =

AS_PATH = <18>

AS 18

(UT Austin)

Bgp strengths weaknesses
BGP Strengths/Weaknesses

  • AS level management allows tractability (16-bit AS space)

  • Routing metric is coarse

    • Number of ASes: not necessarily related to path performance or length

  • Inflexible

    • ASes export one path per prefix

    • No recourse for downstream nodes for undesirable paths

Problem description
Problem Description

  • Allow for greater flexibility in path selection

  • Concerns:

    • Incrementality – hard to modify deployed system

    • Manageability – can’t keep track of too much data

    • Authority – allow for AS autonomy

Potential solutions
Potential Solutions

  • Source routing: Let hosts or edge routers pick route

  • Very flexible

  • Problems:

    • Ends have to know which route to take

    • Intermediate domains lose control

Source routing example
Source Routing Example

Feedback Based Routing (Zhu et. al. 02)

  • Two types of routers: Access routers and Transit routers

    • One access router per end organization

    • Transit routers in non-stub ASes

  • Transit routers only forward packets based upon contained tokens and propagate (flood) link existence information

  • Access routers are responsible for route computation and monitoring (select best route based upon RTT)

Potential solutions cont d
Potential Solutions (cont’d)

  • Overlay routing: Impose virtual topology over the Internet

  • Choice in routing by picking intermediate nodes from the overlay which in turn forward toward the ultimate destination

  • Problems:

    • Encapsulation overhead

    • Little path control: dependent on underlying paths that exist between intermediate nodes (runs on top of existing Internet protocols)

Miro protocol
MIRO Protocol

  • Main idea: Add route negotiation to BGP

    • Party that desires change in routing contacts other ASes

    • ASes may reveal other stored routes that were not previously exported

  • Exact form of querying – types of policy that routers will be able to understand - left unspecified

Example of miro negotiation
Example of MIRO Negotiation

A queries B: Any route to F avoiding E?

AS A wants to select a route to F that avoids AS E

2. B responds to A: BCF

3. A accepts

Miro protocol cont d
MIRO Protocol (cont’d)

How does this address our goals?

  • Incrementality: Nodes that don’t know about MIRO simply ignore it and continue with BGP

  • Manageability: Routing still on AS level, only new state is that of added tunnels, which should be small compared to BGP routing

  • Authority: Intermediate ASes get to choose which routes to reveal - additionally could set conditions for revealed routes (eg: charge for traffic)

Miro protocol additional details
MIRO Protocol – Additional Details

  • Negotiation Details

    • No special multilateral negotiation

    • Querying can be done by proxy – AS that receives a request can in turn query other nodes

    • Not necessarily clear who to query for particular request

  • Conceptual state for established routes

    • When path established by negotiation, tunnel identifier created at AS that originated new route

    • Querying AS uses that identifier to send along the chosen AS route

Miro state establishment
MIRO State Establishment

4. Confirmed, your tunnel ID is 7

3. A accepts route BCF

Miro implementation considerations
MIRO – Implementation Considerations

  • Implementation of tunnel concept

    • Option 1: IP address per link

    • Option 2: Single link for all tunnels + additional tunnel identifier

Tunneladdr: 3

Linkaddr: 4

Linkaddr: 3





Linkaddr: 5

Miro implementation considerations cont d
MIRO – Implementation Considerations (cont’d)

  • IP address per link

    • Easier to implement

    • Reveals extra topology information to upstream

    • Could break from changes in internal AS topology

  • Single address and separate tunnel identifier

    • Reveals no extra topology information

    • Requires packet rewriting

Evaluation what to evaluate
Evaluation – What to Evaluate

  • Goal: Demonstrate enough flexibility to implement some potentially desired policies

  • Policies chosen: Avoiding an AS, load balancing

Evaluation difficulties
Evaluation – Difficulties

  • Ideally, would test on Internet

    • Unfortunately, impossible for protocol that changes core routing

  • Instead used RouteViews

    • As part of AS 6447, conducts eBGP pairings with a number of network backbones

    • Allowed to do this because it does not pass on any information learned

    • Can use this data to see which BGP routes an AS knows and further which routes it actually propagated

Evaluation difficulties cont d
Evaluation – Difficulties (cont’d)

  • AS generally not willing to export all routes

  • Economic reasons at play: AS relationships

  • AS relationships and their details are often not public information

  • Four main roles (Gao 01):

    • Provider

    • Consumer

    • Siblings

    • Peers

As relationships
AS Relationships

  • Guiding principle for AS relationships: Don’t carry traffic unless it benefits your AS

  • Producer: Provides service to Consumer, wants Consumer to know routes through it

  • Consumer: Receives routes from Producers, does not want to transit traffic between them (would get charged twice!)





= Provider to Consumer

As relationships cont d
AS Relationships (cont’d)

  • Peers: Willing to cooperate to service each others customers, but not willing to transit for other peers

  • Siblings: Full cooperation – willing to transit for other siblings









= Provider to Consumer

= Siblings

= Peers

As relationships cont d1
AS Relationships (cont’d)

  • Can infer AS relationships from BGP route data

  • Idea: Allowed exports have a certain pattern – moved “uphill” to providers and then redistributed “downhill” to consumers





  • AS_PATH = <D,…,C,B,…,A>


  • MIRO evaluated under three conditions depending on willingness of ASes to export routes:

    • Strict: ASes only export routes with same local preference

    • Respect export policy: ASes willing to export routes obeying previously described relationships

    • Most flexible ASes willing to export all routes (mostly hypothetical scenario)

Evaluation avoiding an as
Evaluation – Avoiding an AS

  • For each AS, examine ability to avoid every non-neighbor AS

  • Why is this not 100%? Although any AS path can be chosen with source routing, sometimes there is no path that can avoid an AS

    • Probably due to distribution of number of neighbors per node

Evaluation cost of avoiding an as
Evaluation – Cost of Avoiding an AS

AS#/tuple: Number of ASes that were contacted in order to establish a new pair

(= communication cost)

Path#/tuple: Number of candidate paths that were considered (= server load)

Note: Unclear from their work whether these include unsuccessful attempts, which could be costly

Evaluation incrementality
Evaluation - Incrementality

  • Observation: Only a small number of ASes in the BGP topology are particularly well connected

  • Deploying MIRO at these nodes may have a greater effect

Evaluation incrementality cont d
Evaluation – Incrementality (cont’d)

  • Deploying at 0.2 percent most connected nodes will yield 50% success rate

Evaluation load balancing
Evaluation – Load Balancing

  • AS wants to rebalance incoming traffic among its incoming links

  • Can do this by asking upstream node to advertise alternate routes

  • Upstream nodes that lie on route to destination for many sources: “power nodes”

D: Can you use routes to me that don’t end in AD?






Evaluation load balancing cont d
Evaluation – Load Balancing (cont’d)

Important assumption: Equal traffic per link