Subject: RE: A question about RFC2960.

From: [email-masked]
Date: Fri, 5 Sep 2003 18:20:06 +0300

Ay, Brian, you always so lovely and positive...

   B) At any given time, the sender MUST NOT transmit new data to a
      given transport address if it has cwnd or more bytes of data
      outstanding to that transport address.

	I don't see here anything that says that you cannot exceed the MTU. It says "don't send more if you have already cwnd or more bytes", not that you are not able to slop over the MTU... Obviously, if there is a possibility of having "cwnd *or more* bytes", that's because you are able to slop over the MTU...

	You probably understood it in the right way from the beginning, but as people happened to ask about this, that's why the IG tries to leave it clearer...

	Whatever...

	Iván Arias Rodríguez

> -----Original Message-----
> From: ext Brian F. G. Bidulock [mailto:[email-masked]]
> Sent: Friday, 05 September, 2003 18:10
> To: Arias-Rodriguez Ivan (NRC/Helsinki)
> Cc: Wei Fusheng (NRC/Beijing); [email-masked]
> Subject: Re: A question about RFC2960.
> 
> 
> Ivan.Arias-Rodriguez,
> 
> On Fri, 05 Sep 2003, [email-masked] wrote:
> 
> > 	Otherwise, why explaining that you have to send one 
> datagram? I think
> > 	that if the idea was simply "send as much as you can" 
> you wouldn't
> > 	explicitly state that you have to send one datagram after the T3
> > 	expiration... that would only cause confusion...
> 
> No more confusion that neglecting to mention in the document that
> one can slop over the MTU.  Thus the embarassment of IG 2.7.
> 
> --brian
> 
> -- 
> Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
> [email-masked]    ¦ world; the unreasonable one persists in  ¦
> http://www.openss7.org/ ¦ trying  to adapt the  world  to himself. ¦
>                         ¦ Therefore  all  progress  depends on the ¦
>                         ¦ unreasonable man. -- George Bernard Shaw ¦
>

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 5 Sep 2003 09:33:01 -0600

Ivan.Arias-Rodriguez,

On Fri, 05 Sep 2003, [email-masked] wrote:

> 	I don't see here anything that says that you cannot exceed the MTU. It
> 	says "don't send more if you have already cwnd or more bytes", not
> 	that you are not able to slop over the MTU... Obviously, if there is a
> 	possibility of having "cwnd *or more* bytes", that's because you are
> 	able to slop over the MTU...

You can have "cwnd or more" without slopping over MTU:  e.g. MTU falls from
PMTU discovery and you still have data outstanding...

So the above does not necessitate any slopping at all.

> 
> 	You probably understood it in the right way from the beginning, but as
> 	people happened to ask about this, that's why the IG tries to leave it
> 	clearer...

An embarrasing correction indeed.

--brian

---

From: Qiaobing Xie <[email-masked]>
Date: Wed, 17 Sep 2003 13:16:09 -0500

Hi, guys,

I just read through this thread. Firstly let me say that I think both
views are correct and reasonable and there is no need to argue over
this. Here is why:

The spirit of that particular aspect of the design is very simple: when
the sender senses (by seeing a recent T3 expiration) that a path may be
congested, it will retrans only a single packet to that path. 

Why not more than one? because the likelihood of losing another packet
on that path is now high (learn by experience principle) and if the
sender foolishly retrans, say, 10 tiny packets to the path, even it is
within the cwnd limit and unlikely to do harm to the network, if all the
10 packets subsequently get lost, the sender will have a hell of time to
clean up the mess created by itself. So, for singly-homed association,
not to retrans >1 packet after T3 exp is just a smart thing for the
sender to do.

Now things become a little fuzzy with a multi-homed association, because
there is no guarantee for path diversity as we all well understand. If
you are optimistic and assume multi-homed likely means multi-path, then
you probably will agree with Brian and send another packet to an
alternate addr. If you are more conservative minded and not assume
multi-path, then you probably will agree with Ivan and Wei, and will
only still retrans a single packet.

regards,
-Qiaobing