Subject: Re: [Sigtran] Repost - More M2PA 7 questions]

From: [email-masked]
Date: Fri, 7 Mar 2003 12:48:36 +0530

Tom,

Comments below...

Regards
Anshoo.
> Andrew, Brian, and others,
>
> IF M2PA delays acking a data message, and then gets a Retrieve BSNT,
> there would be a problem with returning the number of the last message
> for which the ack was sent.
>
> For example, say messages up to number 35 have been received and
> delivered to MTP3 (where they may or may not be queued). But the acks
> have been sent out up to number 34. For some reason, M2PA is delaying
> the ack of number 35. Now if M2PA receives a Retrieve BSNT from MTP3. If
> M2PA returns the number of the last message acked (34), the other end
> would retransmit beginning with 35 and there could be a duplication. And
> duplication is very bad.

Why not send return the FSN on the last message passed on to MTP3 instead ?
If we are explicitly acknowledging all received packets, half of the packets
sent on the network would be acknowledgements. Instead, we can wait for a
particular time duration to see if the acknowledgement can be piggybacked on an
outgoing MSU. In case no MSU been sent by the Local M2PA before timeout, an
explicit ack should be sent to the Peer. Whenever a Retrieve BSNT from MTP3 is
received, the sequence number returned is the FSN value of the last Message
given to MTP3 and not the last Message acknowledged to Peer M2PA.
The advantage here is that in the scenario where both peers are exchanging MSUs
at a high rate, no explicit acknowledgement would be sent on the network.

>
> But IMHO the right way to deal with this is to send the ack right away.
>
> tomG.

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 7 Mar 2003 01:18:52 -0700

anssharma,

[email-masked] wrote:                                                     (Fri, 07 Mar 2003 12:48:36)
> at a high rate, no explicit acknowledgement would be sent on the network.

Many MSUs are quite short comparted to the MTU of the connection.  Rather than
delay acknowledgements, I acknowledge immediately and let SCTP running a
modified Nagle/Minshall algorithm determine optimal bundling.  This addresses
both bundling of acks and bundling of many MSUs into a single SCTP packet.
M2PA cannot be expected to perform the bundling operation itself: bundling of
packets is something for which SCTP (or other transport) is responsible.
M2PA lacks access to the information necessary to make a good decision on
bundling.

So, I agree with Tom as below: "the right way to deal with this is to send the
ack right away."

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/