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Re: smarter seqno handling?

From: Chris Kemp
Subject: Re: smarter seqno handling?
Date: Fri, 6 Jun 2003 02:46:53 +0100

On 2003.06.05 21:03 Osma Suominen wrote:
> Hi all,
> continuing my crusade to find weaknesses in gnokii and writing reports
> about them in the hope to inspire someone ;)
> This time I noticed that handling of sequence numbers in gnokii is
> currently suboptimal (IMHO). The problem is when messages have to be
> resent because there was no ack or no reply message. This is done in the
> sm_block*() family of functions.
> When a message times out, it is automatically resent. However, when it
> is sent again, the new message gets a new sequence number. I think this
> is bad. One example where things can go wrong: if the send times out
> because no ack was received from the phone, but the problem was that the
> ack was corrupted, the phone actually gets two different copies of the
> same message and may react twice. This can cause all sorts of problems,
> like a storm of messages from the phone.
> I've seen this a few times when testing the 3110: I do something with
> command line gnokii, and then try it once again, and the first thing
> that happens is that spurious messages appear from the phone. They were
> requested by a duplicate message in the previous session, and the phone
> keeps resending them until they are acked by gnokii - which only happens
> on the second run, and then it can confuse gnokii.
> I did some tests and noticed that when I leave a message from the phone
> unacked, it resends the exact same frame i.e. keeps the sequence number.
> I think the phone also ignores a frame if it has the same seqno as the
> previous frame, but I haven't tested this.
> I think there should be a way for the statemachine to tell the link
> driver that the message is a resend and thus the seqno should be kept
> the same. The link drivers may ignore it if that seems more correct.
> On a related note, the link drivers probably should discard duplicate
> frames i.e. those having the same seqno as the previous message. I don't
> think any link drivers currently do this. I'll hack fbus-3110.c to do
> this soon.

All seems sensible to me.  Would be worth doing the test to check whether
phones generally ignore a second message with recently used sequence number


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