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ISD and AS numbering

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Stephen Shirley edited this page Sep 5, 2019 · 12 revisions

This page outlines the numbering scheme for SCION ISD and AS numbers. An ISD-AS number is 64-bits, with the top 16 bits indicating the ISD, and the bottom 48 bits indicating the AS. The text representation uses a --separator between the ISD and AS numbers. E.g. 4-ff00:1:f.

ISD numbers

Formatting

ISD numbers are formatted as decimal.

Allocations

ISD Description
0 The wildcard ISD.
1 - 15 Reserved for documentation and sample code [0].
16 - 63 Private Use [1]. They can be used for testing and private deployments.
64 - 4094 Public ISDs. They should be allocated in ascending order, without gaps and "vanity" numbers.
4095 (212-1) Reserved.
4096 - 65535 Reserved.

AS numbers

AS numbering is globally unique, and uses a super-set of the existing BGP AS numbering scheme [2].

Formatting

The default formatting for AS numbers is very similar to IPv6 (summary, RFC 5952). It uses a 16-bit :-separated lower-case hex encoding with leading 0's ommitted: 0:0:0 to ffff:ffff:ffff.

N.b. that the :: zero-compression feature of IPv6 is not legal. It has very limited use in a 48-bit address space, and would just add an extra complication for very little gain.

As : isn't a legal filename character on both windows and mac platforms, a file/directory name that contains an AS number should use _ as a separator instead. E.g. a dir for 1-11:bc:1c could be ISD1-AS11_bc_1c.

In order to provide easy comparison with BGP AS numbers, any AS number in the BGP AS range (0 - 232-1) is represented as decimal. While it is legal to write a BGP AS number using the SCION AS syntax, programs should always use the decimal representation for display. E.g. if a program receives 0:1:f it would display it as 65551.

Allocations

AS Size Description
0 1 The wildcard AS.
1 - 4294967295 (~0:0:0/16) ~4.3 bil 32-bit BGP AS numbers [2], formatted as decimal. If a BGP AS deploys SCION, it has the same AS number for both BGP and SCION.
1:0:0 1 Reserved.
2:0:0/16 ~4.3 bil Public SCION-only ASes (i.e. ASes that are created for SCION, and aren’t existing BGP ASes). They should be allocated in ascending order, without gaps and "vanity" numbers.
ff00:0:0/32 65535 Reserved for documentation and test/sample code [0].
ffaa:0:0/24 ~16.8 mil Reserved for Private Use [1]. They can be used for testing/private deployments.
ffff:ffff:ffff 1 Reserved.

The rest of the space is currently unallocated.

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