Re: Death by extension header (was:RE: New Version Notification for draft-li-6man-hbh-fwd-hdr-00.txt)

[Toerless Eckert <[e-mail]>]

On Mon, Jul 13, 2020 at 11:53:08AM -0700, Tom Herbert wrote:
> Wrt byte length, the most interesting limit with regards to routers is
> when the aggregate length of the header chain exceeds the size of the
> parsing buffer in the device. As I understand it common lengths of
> parsing buffers are 128, 256, or 512 bytes. That might suggest the
> default minimum should be 128 bytes or maybe 256 bytes. That is, we
> would expect that routers should be able to process IP header,
> extension headers, and first four bytes of transport header (e.g. to
> get ports for ECMP) fit in the parsing buffer in the first N bytes of
> the packet. We can narrow this down if the standard is followed such
> that routers only process the IPv6 and HBH headers, intermediate
> destinations in a routing header only process headers through the
> routing header, and end hosts process all options.

Indeed, the parsing depth is a key criteria. There is also the
question of how high the cost of rewrites in the packet header is.
That is much less clear to me. Programming languages like P4 make
it look as if you could arbitrarily reshuffle headers, but the
compiled code might expose extremely different performance output
based on the amount of rewrites.

For example, when we do PE-P-PE designs, we typically create a new
additional encap to minimize lookup depth for P nodes. And the overhead
of this is particularily big for IPv6. Leander headers or reordering
header fields might be a better cost effective option going forward.
Hard to say. Would be great to collect researched data points.

Cheers
   Toerless

> Tom
> 
> 
> > Thanks,
> > --
> > Fernando Gont
> > SI6 Networks
> > e-mail: [e-mail]
> > PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492
> >
> >
> >
> >
> 
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