Re: "RFC 2461bis" issue: MTU handling

[Fred Templin <ftemplin@iprg.nokia.com>]

Itojun,

I disagree with your statement. Two examples where the per-neighbor MTU
negotiation would be very useful:

1) Peer-to-peer wireless (e.g. IEEE 802.11):
   On certain wireless media, receiver A can sense QoS metrics (e.g. SNR)
   for the packets it receives from senders B and C. If the SNR for B 
is strong,
   A may wish to inform B that a larger MTU (e.g. 1500 bytes) is 
acceptable.
   Conversely, if the SNR for C is weak, A may wish to inform C that a 
smaller
   MTU (e.g. 1300 bytes) is desired.

 2) Constrained nodes on large links:
   On large-MTU media (e.g., HiPPI, Gig-E, etc.), nodes with small 
receive buffers
   (e.g. boot ROMs, embedded devices) may wish to receive smaller 
packets than
   the MTU for the link. For example, a constrained node on a link with 
MTU of
   64KB may wish to inform a neighbor that an MTU of 10KB is desired.

Fred
ftemplin@iprg.nokia.com

Jun-ichiro itojun Hagino wrote:

> > I'm not sure this should go into a replacement specification for RFC 
> > 2461, but I'll bring it up anyway:
> >
> > Currently, routers can advertise an MTU for a link. That's nice. But 
> > what we really need is a way for hosts to find out the MTU each 
> > individual neighbor can handle. 100 Mbps and slower ethernet interfaces 
> > can typically handle only the standard 1500 byte ethernet MTU, while 
> > gigabit ethernet interfaces usually support a much larger MTU.
> >
> > However, in most cases hosts with different MTUs are present on the 
> > same subnet, so simply advertising a larger MTU wouldn't solve this. 
> > (Not that this would work anyway as hosts are instructed to only listen 
> > to MTU advertisements where the MTU is between 1280 and 1500 (for 
> > ethernet).)
> >
> > But if hosts can tell each other the MTU they support, each set of two 
> > hosts is always able to use the largest possible MTU between them. 
> > (This would also require a new link MTU option that conveys the maximum 
> > MTU the lower layer equipment supports. Switches have their own MTU and 
> > even some hubs start doing strange things when a larger than expected 
> > MTU is used.)
> >    
> 	the assumption made in RFC2461 is that the link MTU is constant
> 	over the link, i guess.  i don't think it is necessary to make
> 	MTU negotiable between peers, it would complicate too many things.
>
> itojun
>
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