Re: [lwip-users] lwip 1.4.1 send a sporadic RST ACK

[Joel Cunningham]

I've updated the remaining comments to mention the same priority on git master

Another thing you could do is have your application track number of active connections, make sure enough TCP pcbs are configured and then close the listener pcb once the maximum is reach.

Joel

On 10/24/2017 10:38 AM, Mattia Settin wrote:

Yes

The main problem was related that the tcp_kill_prio() kills the active pcb with a priority equals to the pcb passaed in input.

For my specific web server application, this lead to send a (sporadic) RST ACK to the client, when the tcp_alloc fails to allocate new connection.

br

On Tue, Oct 24, 2017 at 4:34 PM, Axel Lin <address@hidden> wrote:

2017-10-24 16:37 GMT+08:00 Axel Lin <address@hidden>:
> 2017-10-24 1:13 GMT+08:00 Mattia Settin <address@hidden>:
>> Dear all
>> After a week of debugging I finally find the issue. The problem is related to priority in pcb.
>> Taking a deep look I saw that the priority in the stack 1.4.1 is change, in particular:
>> Tcp_alloc return e pcb with a prior equal to the prior passed in input.
>> Tcp_listen_with_backlog return e pcb with a prior equal to the pcb.prior passed in input.
>> This two changes lead to a sporadic RST ACK call from the kill_prio routine.
>> Finally I fix this issue removing the equal condition in the kill_prio function.
>
> The comment on the code seems confusion for "the same priority case":
> (I check the latest git tree)
>
> Th comment on the caller:
>           /* Try killing active connections with lower priority than
> the new one. */
>           LWIP_DEBUGF(TCP_DEBUG, ("tcp_alloc: killing connection with
> prio lower than %d\n", prio));
>           tcp_kill_prio(prio);
>
> But the comment on tcp_kill_prio() function:
> /**
>  * Kills the oldest active connection that has the same or lower priority than
>  * 'prio'.

The comment for tcp_kill_prio() is updated by:
commit 6a4c30fe5d65 fixed bug #31723 (tcp_kill_prio() kills pcbs with
the same prio) by updating its documentation only

And take a look at bug #31723: tcp_kill_prio() kills pcbs with the same prio

According to its documentation, tcp_kill_prio() kills "the oldest
active connection that has lower priority than prio". However, the
current code also kills connections with a priority equal to 'prio'.
We should either change the documentation or the implementation.
The downside of the current implementation is that every call to
tcp_new() can lead to aborting an old (active) connection.
This has been in there since revision 1.1 of tcp.c, so changing it
might involve changing applications relying on this (like I have one
:).

I'm wondering if it should change the implementation rather than the
documentation.

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--

Mattia Settin

System engineer

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