AS number related metrics

11 December 1997, hwb/kc -- caida

some problems are just too complicated for rational logical solutions.
they admit of insights, not answers.

jerome bert weisner, A Scientist's Advice II, New Yorker 26 jan 1963

Autonomous system (AS) numbers in BGP-based routing tables are, with some degree of risk, indicative of how networks and ISPs are interconnected. Those BGP tables offer an AS-based path from the table-generating BGP peer to a target network/mask pair, i.e., a destination network. Furthermore in some cases it may be useful to consider via which AS a specific network was learned, though the degree of risk is even higher here, as the routing table may indicate that the information is incomplete.

While it would be useful to map AS numbers to ISP, many ISPs hoard many AS numbers, and there is no data base that one could ask simple questions such as "tell me all the AS numbers belonging to a specific ISP." This is further complicated by current and pending mergers and restructuring of the ISP landscape. As an example, one of the most successful collector of ISPs appears to be Worldcom. Trying out the hard way hop many AS numbers belong or will belong to Worldcom, one has to ask and look around. A possible answer may be:

mfs
- 6066 6324 6374
wiltel
- 6248
uunet
- 284 701 702 703 704 705 813 814 815 816 817
mci
- 180 1743 1964 2757 2758 2759 2760 2761 2888 2889 2890 2891 2892 3369 3370 3371 3372 3373 3374 3375 3376 3377 3378 3561 3617 3618 3619 3620 3621 3622 3623 3624 3625 3626 4281 4282 4283 4284 4285 4286 4287 4288 4289 4290 4291 4292 4293 4294 4295 4296 4297 4298
ans
- 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1934
compuserve
- 4183
gridnet
- 6113
pipex
- 1849 4571

In other words, abstracting AS numbers to ISPs is not a straightforward undertaking. But then, neither is abstracting multiple network prefixes to AS numbers, as there is no one-to-one relationship, and also networks are at times connected via multiple ISPs.

Hence, while there are no really good mappings that would accommodate this in a straightforward and "absolutely correct" sense, when allowing for a margin of error, such abstractions may still yield valuable insight into the structure of the Internet system. Insight that would be hard to get otherwise. Therefore BGP tables from Internet routers or measurement systems (e.g., the one described in http://www.antc.uoregon.edu/route-views) can be valuable instruments.

Example questions for network metrics to be asked include:

what percentage of all ASes are related to a specific ISP
what percentage of all used (BGP-known) ASes are related to a specific ISP
what percentage of ASes in the BGP routing system have adjacencies related to a specific ISP
what percentage of all AS routing announcements are related to a specific ISP, perhaps based on BGP traffic traces
what percentage of all total address space (via network prefixes) are announced for from a specific ISP
what percentage of all allocated address space (via network prefixes) are announced for from a specific ISP
what percentage of all announced address space (via network prefixes) are announced for from a specific ISP
what percentage of all traffic does a specific ISP route to, perhaps based on packet traces
what percentage of all Internet transit-related revenues is a specific ISP responsible for

given the aforementioned Worldcom example, the answer to question (2.) would be for the 10 December 1997 to be 143 out of 3154 known AS numbers. However, in answer to question (1.), the InterNIC's 26-Aug-97 asn.txt file (minus the reserved space) reports 7383 assigned AS numbers, out of the 2^16 or 65536 total, i.e., many more AS numbers are assigned than actually known to the global interconnected BGP system.

The following graph describes the number of BGP adjacencies based on the BGP information, and considers the subset that neighbors the above mentioned Worldcom related AS numbers, with the lower graph describing the Worldcom fraction.

Going a step further, one can assess what address space is being advertised, what address space is injected to the BGP system via specific clusters of AS numbers (ignoring for the moment whether the information was learned from an IGP or derived from incomplete information), and what address space is no more than one AS hop away from a specific AS (again, ignoring for the moment whether the information was learned from an IGP or derived from incomplete information). This is illustrated in the following graph.