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

Group,

	If one decided to pre-allocate fix buffer sizes,
	 then..
	What is wrong with a pre-allocating a buffer size 
	 of say 128bytes and assume that it will cover 90+%
	 of your needs and on the other 10% of the time,
	 allocate a buffer larger on demand??

	I believe this is legacy..

	However, if one likes to look at memory allocators
	then the "Slab Allocator" would be one I would suggest
	to look at first.

	Mitchell Erblich
	----------------

	

Vishwas Manral wrote:
> 
> 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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> >
> 
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