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

[Curtis Villamizar <[e-mail]>]

Hi Tony,

Some further clarification inline, not for you since you know this,
but for anyone that might still be confusing BGP message size and TCP
buffer size (aka TCP window size).

In message <[e-mail]>
Tony Li writes:
>  
> On Jun 17, 2007, at 6:12 PM, Fenggen Jia wrote:
>  
> > 	I think my inital question is why the protocol has 4K limit on  
> > messages sizes,that is different from the implementation,an  
> > implementation may chose to use large read or write buffer(>4K) to  
> > handle multiple updates one time,still my question is if message  
> > size limit is a good pratice in protocol design?
>  
> Hmmm, ok, I guess we haven't been explicit enough.  I'll try again:

We've been clear to the careful reader.  I mentioned that the TCP
window has nothing to do with the BGP message size.

That was a different person confused about this.

> 1) First, an implementation should NOT be using a message size that  
> is different than the specified 4k message size limit.  If an  
> implementation sends messages more than 4k, then other  
> implementations will not be able to parse them.  If an implementation  
> cannot receive 4k messages, then it will also not be able to  
> interoperate.
>  
> 1a) Having a fixed size is good because it makes the protocol  
> implementations easy.  There is no point to having complexity in an  
> implementation if it provides no benefit.  Large messages don't  
> provide a wonderful benefit, as they need to be large enough to carry  
> the path attributes and associated prefixes.  For this purpose 4k is  
> probably adequate to date.

Right.

> 1b) Historically, 4k was considered a bit wasteful.  Of course, it  
> was wonderfully simple compared to EGP which used fragmented  
> packets.  Care to parse a 16k jumbo-gram?  Care to debug that?  Trust  
> me, it's not fun.

Actually EGP was worse than that.  EGP required that you put everythin
in one dategram packet which eventually exceeded the IP 64KB limit.
Fortunately only one network was using EGP when that happenned (NASA
still had Proteon routers that ran EGP and not BGP so they had to take
partial routing and use default - they were even worse off when CIDR
came along but finally retired the Proteons).  Then there was the AGS
FDDI interface that couldn't handle 5 consecutive IP packets or
fragments without dropping the 5th one.  Not amusing when EGP packets
required 5 fragments (somewhere beyond 20KB in size).  Those were
interesting times.  Its boring at times now that things mostly work.  :-)

> 2) The 4k message size is *completely independent* of the TCP window  
> size.  An implementation is perfectly free to compose any number of  
> messages, each of which is within the 4k limit.  The implementation  
> can then cram any number of messages into its TCP socket, up to the  
> buffering limits of that TCP.

I think that is the point people are missing.

> 2a) Thus, the message size is *NOT* performance limiting, except when  
> an implementation could actually overfill a message.  Folks  
> maintaining current implementations might chime in here as to whether  
> or not they see this.

Yes BGP updates are filled and spill into a second update.  No it is
not performance limiting.  But you knew that.

Performance is determined by the TCP buffer size set.  TCP buffer size
in BSD is set with a setsockopt as I had mentioned before.  TCP buffer
size has nothing to do with BGP message size.

BGP message size has little to do with performance unless as I had
mentioned parsing the attributes such as AS Path more than once ever
became an issue.  If anything it is less an issue over time.

> So, in summary, yes, a 4k message size limit is a fine situation *for  
> BGP*, for the way that it behaves and the job that it does.  This  
> does *NOT* necessarily generalize to other protocols, (e.g. OSPF)  
> where 4k exceeds the most common MTUs.  In those cases, you'd end up  
> with fragmentation, and that's bad.

OSPF does not use TCP therefore its message size is relevant to
performance and cannot exceed the MTU.

BGP uses TCP therefore its message size is not relevant to performance
except for internal parsing issues and the MTU is irrelevant to the
BGP message size.

> Regards,
> Tony

Curtis

ps - For those new to IDR (and some of the posters on this thread seem
to be) this sort of mini-lecture on how TCP works and how BGP uses it
is somewhat of a rerun.  The topic or something similar comes up on
this mailing list every few years.

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