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

Hi Jeff,

I had a few doubts.

> - 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.
Are you saying keeping/ allocating a PDU bigger than 4k bytes is a problem?

> - Having a fixed sized buffer means you can allocate it *once* and be
>   done with it.
If I remember correctly what glibc does, it does not make a brk system
call everytime, it itself keeps a pool of memory, and does not give it
back to the OS. I think allocating a few buffers of different sizes
for such a design would help.

> - You're going to have one of these buffers for each of your peering
>   sessions.  You don't want it to be large.
Its again a memory management issue of an implementation right. We
could allocate a central pool and extract memory from there.

Thanks,
Vishwas

On 6/14/07, Jeffrey Haas <jhaas@pfrc.org> wrote:
> 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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