Multi path interdomain routing by xu and rexford
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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


Outline

Outline

  • Motivation

  • Protocol Description

  • Evaluation


Motivation

Motivation

  • 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

(Sprint)

128.83.0.0/16

next-hop = 3

AS_PATH = <7018,209,6922,18>

AS 7018

(AT&T)

128.83.0.0/16 -> 1

128.83.0.0/16 -> 1

3

???

128.83.0.0/16 -> 2

128.83.0.0/16 -> 2

128.83.0.0/16

next-hop = 1

AS_PATH = <209,6922,18>

AS 6543

128.83.0.0/16

next-hop = 2

AS_PATH = <6543,270,123,18>

1

2

AS 209

128.83.0.0/16

next-hop = 128.83.4.5

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

2

2

1

1

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!)

A

B

C

AS_PATH = <C,A>

= 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

AS_PATH = <B,C>

AS_PATH = <D,B,C>

A

B

C

D

E

F

= 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

A

B

C

D

  • AS_PATH = <D,…,C,B,…,A>


Evaluation

Evaluation

  • 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?

A

D

C

B

Congested


Evaluation load balancing cont d

Evaluation – Load Balancing (cont’d)

Important assumption: Equal traffic per link


Extra slides

Extra Slides


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