Re: [Idr] why has 4096 bytes limit on BGP messages size?
Tony Li <tli@cisco.com> Mon, 18 June 2007 03:35 UTC
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From: Tony Li <tli@cisco.com>
Subject: Re: [Idr] why has 4096 bytes limit on BGP messages size?
Date: Sun, 17 Jun 2007 20:34:56 -0700
To: Fenggen Jia <fgjia@mail.zjgsu.edu.cn>
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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: 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. 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. 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. 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. 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. Regards, Tony _______________________________________________ Idr mailing list Idr@ietf.org https://www1.ietf.org/mailman/listinfo/idr
- [Idr] why has 4096 bytes limit on BGP messages si… Fenggen Jia
- Re: [Idr] why has 4096 bytes limit on BGP message… Tony Li
- Re: Re: [Idr] why has 4096 bytes limit on BGP mes… Fenggen Jia
- Re: Re: [Idr] why has 4096 bytes limit on BGP mes… Jeffrey Haas
- Re: [Idr] why has 4096 bytes limit on BGP message… Enke Chen
- Re: [Idr] why has 4096 bytes limit on BGP message… Pekka Savola
- Re: Re: [Idr] why has 4096 bytes limit on BGP mes… Vishwas Manral
- Re: [Idr] why has 4096 bytes limit on BGP message… Erblichs
- Re: [Idr] why has 4096 bytes limit on BGP message… Danny McPherson
- Re: [Idr] why has 4096 bytes limit on BGP message… Danny McPherson
- Re: [Idr] why has 4096 bytes limit on BGP message… Erblichs
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- Re: [Idr] why has 4096 bytes limit on BGP message… Pekka Savola
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- RE: [Idr] why has 4096 bytes limit on BGP message… Susan Hares
- RE: [Idr] why has 4096 bytes limit on BGP message… Susan Hares
- Re: Re: Re: [Idr] why has 4096 bytes limit on BGP… Fenggen Jia
- Re: [Idr] why has 4096 bytes limit on BGP message… Tony Li
- Re: [Idr] why has 4096 bytes limit on BGP message… Curtis Villamizar
- Re: [Idr] why has 4096 bytes limit on BGP message… Vishwas Manral
- RE: [Idr] why has 4096 bytes limit on BGP message… Bhatia, Manav (Manav)