Re: Do piggybacked ACKs work?

[Vlad Yasevich <vladislav.yasevich@hp.com>]

Doug Graham wrote:
> Hello,
> 
> I have a little test that simply has a client send 32 bytes of data to
> server, which then replies back with the same data.  What doesn't look
> right to me is that I never see piggybacked ACKs.  I see the client
> send 32 bytes, then the server reply in a packet containing a single
> DATA chunk of 32 bytes, and then 200ms later both SACKs are sent.

Piggybacked ACKs will not be sent every packet.  Looks like you are hitting
the typical single packet request-response senario.

I'd recommend you read RFC 4960 section 6.2.

Now, it's possible to do some piggy-backing when bulk data is flowing in
both directions, but that would different then you proposal below.

-vlad

> 
> Looking at the code in net/sctp/output.c I see this:
> 
>         /* If sending DATA and haven't aleady bundled a SACK, try to
>          * bundle one in to the packet.
>          */
>         if (sctp_chunk_is_data(chunk) && !pkt->has_sack &&
>             !pkt->has_cookie_echo) {
>                 if (asoc->a_rwnd > asoc->rwnd)
>                     <append the SACK chunk to the DATA>
> 
> The test that is failing is the "a_rwnd > rwnd" test, and I don't
> understand that test at all.  As far as I know, a_rwnd is the last value
> of rwnd that was advertised to the peer, and rwnd is the current receive
> window.  What's happening is that after the association has settled,
> a_rwnd and rwnd are equal.  The client then sends 32 bytes to the peer
> SCTP on the server.  The server SCTP receives these 32 bytes and decreases
> its rwnd.  Then the userspace server process reads the 32 bytes from the
> socket, thus freeing up the buffer space, causing SCTP to increase rwnd back
> to where it was initially.  When the server process echoes the data,
> SCTP constructs a DATA chunk and then runs the above code to see whether
> it needs to append a SACK.  But since rwnd is no longer less than a_rwnd,
> no SACK is sent.
> 
> I think this logic is wrong.  I don't think the decision as to whether
> or not to piggyback a SACK on a DATA packet should have anything to do
> with the receive window.  If there is unacknowledged data, then the
> SACK should be sent, regardless of the state of the receive window.
> Am I right about this?
> 
> Thanks,
> Doug.
>