On 16 Sep 2019, at 20:37, Job Snijders <j...@instituut.net> wrote: > Limiting the AS_PATH length - from an IETF RFC publication process in context > of providing operational guidance, probably shouldn’t be “limit the path > length to avoid vendor bugs”.
> Instead, the guidance perhaps should be “please report and fix bugs”, right? > :-) In theory, yes. In practice, how does that work? Suppose I prepend 250 times, and a few hops away, routers start to explode because they hit 255 or 256 ASes in the path? How do these people debug the issue? Once they know the problem, how do they install a temporary fix as they wait for a real fix? Also, it's unlikely that any given bug will ever be completely eradicated from the internet even if fixed software is available. The real time global nature of BGP routing makes all of this much more complex. And what if I make it 675 ASes instead and watch sparks fly as a few hops away routers hit the 4096-byte BGP message size? Or I make it 700 ASes with only a 16-bit AS path or a truncated 32-bit AS path, so the first 32-bit router that tries to create a 700-hop 32-bit AS path exceeds 4096 bytes? Now all of these problems can easily be avoided by a 120 or so AS limit. Currently, the longest path that I see at Routeviews is 45, with 40 prepends. (That same network also advertises about a /13 worth of space as 2200 individual /24s for good measure.) 120 won't be hitting any overflow bugs, but it's still rather excessive at three times the longest non-prepended path. Also, 40 prepends is not going to give you anything that 39 prepends, or 30 or 20, for that matter, won't accomplish. I've seen max path length limits of 40 or even as low as 20 mentioned. But before we decide _where_ to draw the line, we first have to decide if we think this particular line should be drawn at all. It looks to me like AS path length filtering is sufficiently common that trying to come up with a coordinated limit as a best practice would be worthwhile. (BTW, as far as I'm concerned we deprecate the use of AS_SETs and multiple AS_PATHs in order to remove some unnecessary complexity from BGP.) _______________________________________________ GROW mailing list GROW@ietf.org https://www.ietf.org/mailman/listinfo/grow
