AUTONOMOUS SYSTEM NUMBERING PLAN



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International Civil Aviation Organization

WORKING PAPER |ACP-SWGN1-13/WP-1304

13 Apr 2007

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AERONAUTICAL COMMUNICATIONS PANEL (ACP)

13th MEETING OF SUB-WORKING GROUP N-1

Montreal, Canada 16-20 April 2007

|Agenda Item : |4.5. (AS Numbering Plan) |

AUTONOMOUS SYSTEM NUMBERING PLAN

(Presented by Eivan Cerasi)

|SUMMARY |

|The working paper proposes a numbering plan for ATN IPS autonomous systems. |

1. INTRODUCTION

The ATN IPS mandates BGP for dynamic routing between ATN Administrative Domains.

Previously Eurocontrol submitted WP1005 in order to highlight a number of areas in which SGN1 should develop material in order to ensure a coherent BGP deployment within a de-centralised environment. These areas are:

• A unique Autonomous System (AS) numbering scheme;

• The IP address space used for the interconnection of BGP peers should come from ranges belonging to Administrative Domains (bilateral agreement) or should be globally reserved to avoid any source of conflict;

• BGP router IDs do not need global uniqueness but it is strongly suggested to develop a unique numbering scheme to avoid problems in the future (draft-dupont-durand-idr-ipv6-bgp-routerid-01.txt can be used as a guideline);

• Timer consistency between BGP routers;

• Consistency of route prefix advertisements and aggregation;

This working paper addresses the first topic, namely the development of a numbering plan for autonomous systems.

2. BGP AS Numbers

The AS number space is a finite resource. An AS number is currently defined as a 16-bit integer which implies 65536 possible values.

To limit the consumption of AS numbers, the Internet Assigned Numbers Authority (IANA) reserved AS number range for private use (i.e. not to be advertised on the global Internet). The range is defined as values 64512 to 65535.

Despite this private AS number range, IANA studies predicted that all public 16-bit AS numbers would be assigned by 2010. As a result, IANA has expanded the AS number registry from its original 16-bit range (AS numbers 0 through 65535) to a 32-bit range (AS numbers 0 through 4,294,967,295).

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As of January 1st, 2007 the internet authorities can now assign AS numbers on the basis of 32-bit values. A three year transition period is foreseen after which 16-bit AS numbers will no longer be assigned.

BGP routers need to be modified (BGP OPEN and UPDATE messages) to support the 32-bit AS numbers and provide backwards compatibility with 16-bit environments.

Backwards compatibility implies that when passing a routing update from the 32- to 16-bit environment, the BGP router converts all the values of the AS numbers in the AS PATH. When the AS number is between 0 and 65535 the 16 leading zero bits are dropped. When the AS number is greater than 65535, it is converted to the AS transition value of 23456.

Although all AS numbers greater than 65535 are converted to 23456, the 32-bit AS number is not lost as it is stored in the NEW_AS_PATH attribute. This allows the new 32-bit environments to convert from a 16-bit to 32-bit environment.

3. IMPLICATIONs

In view of the planning of the transition period, it would make sense that the ATN IPS builds a numbering scheme on the basis of 32 bits rather than the “old” 16-bit.

Nevertheless, the private AS number range has not been extended within the 32 bit environment. In this new environment, former AS numbers in the range 0 to 65535 are to be simply prefixed with 16 zeros.

As a result, three ranges would be available for the ATN IPS:

• Assignments from the “old” public range 0 to 64511

• Assignments from the private range 64512 to 65535

• Assignments from the new public range: 65536 to 4,294,967,295

Regional Internet Registries (RIRs) have published their policies for requesting and assigning new 32 bit AS numbers. The RIR covering Europe, RIPE, states that AS numbers are to be requested through Local Internet Registries (LIRs) and are only allocated for networks that are multi-homed to the public internet. Other regional entities apply a similar policy.

It is considered that the ATN IPS will not be directly connected to the internet in terms of layer 3 IP routing exchanges. It is understood that the ATN IPS may make use of the internet as a sub-network to provide interconnectivity between autonomous systems e.g. tunnels. This means that requests for public AS number values for the ATN IPS are likely to be rejected. As a result it is proposed that the ATN IPS establishes an AS numbering plan based on the used of the private range.

4. Proposal

As part of its role as Local Internet Registry (LIR), EUROCONTROL has already planned use of the AS numbers within the private range.

Indeed assignments within the range of 64608 to 65236 have been foreseen but very few have been assigned. This means that two ranges remain free for use: 64512 through 64607 and 65237 through 65535.

Without any change this leaves 396 assignments for non-European regions.

The following table attempts to estimate the number of required AS number values for non-European regions based on the number of contracted States. For the purpose of this working paper it is assumed that assignments could be co-ordinated by the ICAO regional offices.

|Regional Office that could |Estimated number of Autonomous |Range |

|administer assignments |Systems | |

|Asia and Pacific |36 |64512 – 64579 |

|South America |13 |64580 – 64607 |

|Europe and North Atlantic |56 |64608 – 65236 |

|Middle East |19 |65237 –65279 |

|Western and Central Africa |24 |65280 - 65329 |

|North American, Central American |40 |65330 - 65419 |

|and Caribbean | | |

|Eastern and Southern Africa |25 |65420 – 65479 |

|Reserved |- |65479 - 65535 |

Furthermore, EUROCONTROL could revise its planned assignments in order to reduce its range to 64672 - 64800. However in view of the above table, this action does not seem necessary.

5. Recommendations

It is recommended that the ATN IPS autonomous systems shall use private AS numbers.

It is recommended to consider that the proposed AS numbering plan for the ATN IPS.

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