BGP AS Number Exhaustion

Geoff Huston

Research activitysupported by APNIC

The Problem

• The 16 bit AS number field in BGP has 64,510 available values to use in the Internet’s public routing space

• Some 30,000 AS numbers have already been assigned by the RIRs

• This BGP protocol field will be exhausted at some point in the future

The Solution

• Use a 32 bit field for this value– draft-ietf-idr-as4bytes-06.txt describes how

– This is proposed for publication as an experimental RFC

The Issue

• At some point we will need to start testing various transition plans and vendor implementations, set up a new AS number registry, and commence deployment of these extended length protocol objects in BGP

When?• Before we run completely out of 16 bit AS

numbers!• Need to allow a lead time for testing,

deployment of 4-byte AS BGP implementations and development of appropriate transition arrangements– Allow 2 – 3 years to undertake this smoothly

• So we’d like to know when we have 3 years to go before we run out of AS numbers

When?

• A number of views can be used to make forward projections:– The growth of the number of announced AS’s in

the BGP routing table– The rate at which AS number blocks are passed

from IANA to the RIRs– The rate at which RIRs undertake assignments

of As’s to LIRs and end users

The BGP Routing TableAnnounced AS’s

BGP Table - AS Count

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BGP Table

The BGP Routing TableGrowth Projections

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pre-02 Exp TrendExponential TrendLinear TrendBGP Data

Feb 2013 Dec 2027Oct 2005

IANA AS block AllocationsIANA AS Allocations

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IANA AS Allocations

From the IANA AS number Registry

IANA AS Allocation Projection

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IANA AllocationsExp ProjectionLinear Projection

May 2007 Nov 2010

RIR Assignments

RIR Assignments

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RIR Assignments

From the RIR stats reports

RIR Projection

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Exp ProjectionLinear ProjectionRIR Assignments

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Combining these views

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IANAIANA EXPIANA LinearRIRRIR ExpRIR LinearBGPBGP exp 96-01BGP exp 02BGP Linear 02

Combined View + differences

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IANARIR AssignedBGP AnnouncedNOT assignedNOT Announced

Observations

• RIRs operate with an allocation buffer of around 5,000 numbers

• 10,000 AS numbers (40% of the assigned AS numbers) are not announced in the BGP table.– Is this the result of old AS assignments falling

into disuse? – Or recent AS assignments being hoarded?– This pool creates uncertainty in 2 byte AS

number pool exhaustion predictions

Announced and Unannounced ASs

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Unannounced : Announced AS’s

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Ratio Unann:Ann

Trend:unannounced : announced ratio

Ratio Unann:Ann

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Ratio Unann:Ann

Announced / Unannounced Distribution by Date

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Normalized Announced / Unannounced

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Distribution by AS Number Range

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UnassignedNOT AnnouncedAnnounced

Observations

• Low AS number ranges have the highest unannounced / announced ratios– Reclamation of unused AS numbers in the low

number ranges is likely to be a useful exercise

• Recent assignments show a 45% announcement utilization ratio for AS numbers– LIR staging point factors– Inadequate incentives to return if no

immediate requirement for deployment

Forecast 1 – AS Reclamation in effect

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IANARIRBGP ProjectionBGPUnassignedUnrouted

Forecast2 – No significant reclamation

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IANARIRBGP ProjectionBGPUnassignedUnrouted

Current Forecast• The available AS number pool will exhaust in the

timeframe of 2009-2011 if current AS use trends continue

2009• no significant reclamation in old AS number space

• No coordinated effort to increase utilization density of AS numbers

2011• reclamation and increased deployment efficiency