Re: Re: [Idr] why has 4096 bytes limit on BGP messages size?

On Thu, Jun 14, 2007 at 06:58:06PM +0800, Fenggen Jia wrote:
> Then why not other limit,how is 4096 bytes derived, thanks.

It wouldn't shock me if it had to do with the MTU of a FDDI link but
that's complete speculation.

Aside from the 4k limit, here's some things to keep in mind when writing
protocols.  Mostly IMO:
- Messages encoded in some sort of TLV format probably need a full PDU
  kept in memory at the same time.  This is because a sane
  implementation will validate the PDU contents against TLV constraints
  (either implied by the TLV or by the protocol) before doing *anything*
  to it.
- Having a fixed sized buffer means you can allocate it *once* and be
  done with it.  
- You're going to have one of these buffers for each of your peering
  sessions.  You don't want it to be large.
- You want the PDU to be "big enough" to be able to transport your
  information usefully.  If you're regularly exceeding a 4K message in
  BGP, you're probably not passing around Internet reachability.

-- Jeff

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