Re: [rohc] Path MTU discovery when having varying MTU

On 19 mei 2009, at 20:51, Joe Touch wrote:

>> Assuming that we can predict if and how well a certain flow will
>> compress, we could also simply drop a) and send back a too big with  
>> the
>> size that will compress to what fits inside the MTU and just send b)
>> through the tunnel.

> Your solution tells the endpoint that all packets have to be smaller,
> even when subsequent packets with the large MTU would have headers  
> that
> compressed enough to make space for the ROHC header.

No, what I mean is that if you get a packet that is 1500 bytes, your  
IPsec is 50 and ROHC for this packet at this time is +1 but we know  
that subsequent packets will be -30, then we return an MTU of 1500 -  
50 - -30 = 1480. Next packet is 1480, we do ROHC and it's 1450 and  
then IPsec = 1500 and we're in business.

Interesting question about what we do with packets that conform to the  
compressable MTU. In this case, 1480. Too big is unnecessary because  
the source is already sending small enough, but we'd have to drop (the  
data) anyway. With TCP, fast retransmit will catch this. Are there any  
corner cases we should worry about?