Cell Relay Archive

[fahad@cs.pitt.edu (Fahad A Hoymany)]

Ernst Lopes Cardozo (e.lopes.cardozo@aranea.nl) wrote:
: Hmm, this not not what I understood so far. Thought it is more like this:
: Maximum Segment Size (MSS) is indicated by both parties during TCP 
: session setup (in the initial frames that have the SYN flag up). MSS 
: says: "this is the size of my front door, don't send anything larger or I 
: won't be able to receive it. The indicated MSS is based on the 
: size of internal buffers, etc.. MSS can very well be larger than the MTU 
: (Maximum Transfer Unit) of the underlying network - IP takes care of 
: fragmentation and reassembly. (MSS *can* be larger than MTU, but it 
: wouldn't neccesarily be efficient of course). It is funny that it is 
: called 'MSS negociation', because - my understanding - the MSS in one 
: direction has nothing to do with the MSS in the other direction. It is 
: not so much an negociation as a simple message: keep whatever you send 
: down to this size or you have a problem. If host A has MSS=1k and host 
: B's MSS=512, there is no reason for B to limit it's transmission to 512 


  If B is able to support more than 512 then its MSS is more than 512
  and it won't report less than that.  The MSS is always the smaller of the
  two. In your analogy, if B's front door cannot allow more than 512 bytes
  then B is neither able to send nor receive more than 512 bytes.

Fahad


: (unless it wants to do so for it's own internal reasons).

: I can immagine that some implementations try to optimize things by taking 
: (path) MTU into account when advertising their MSS - but that is at best 
: a guessing game: path MTU varies over time.

: Please correct me if I'm wrong.

: Ernst Lopes Cardozo