Subject: Re: [PATCHv2] sctp: Enforce retransmission limit during shutdown

From: Vladislav Yasevich <[email-masked]>
Date: Wed, 06 Jul 2011 10:31:56 -0400

On 07/06/2011 10:18 AM, Thomas Graf wrote:
> On Wed, Jul 06, 2011 at 09:42:42AM -0400, Vladislav Yasevich wrote:
>> On a related note, were you going to re-submit the receiver patch as well?
> 
> Yes
> 
>> On 07/04/2011 09:50 AM, Thomas Graf wrote:
>>> +			 * retransmission limit. Stop that timer as soon
>>> +			 * as the receiver acknowledged any data.
>>> +			 */
>>> +			t = &asoc->timers[SCTP_EVENT_TIMEOUT_T5_SHUTDOWN_GUARD];
>>> +			if (asoc->state == SCTP_STATE_SHUTDOWN_PENDING &&
>>> +			    timer_pending(t) && del_timer(t))
>>> +				sctp_association_put(asoc);
>>> +
>>
>> I believe 'state' and 'timers' are in different cache lines, so might be able to optimize it
>> a little by checking the state prior to referencing timers array.
> 
> gcc should do that but I'm fine with changing it.
> 
>>> +			 *
>>> +			 * Allow the association to timeout if SHUTDOWN is
>>> +			 * pending in case the receiver stays in zero window
>>> +			 * mode forever.
>>>  			 */
>>>  			if (!q->asoc->peer.rwnd &&
>>>  			    !list_empty(&tlist) &&
>>> -			    (sack_ctsn+2 == q->asoc->next_tsn)) {
>>> +			    (sack_ctsn+2 == q->asoc->next_tsn) &&
>>> +			    !(q->asoc->state >= SCTP_STATE_SHUTDOWN_PENDING)) {
>>
>> Would a test for (q->asoc->state != SCTP_STATE_SHUTDOWN_PENDING) be clearer?  We only
>> care about the PENDING state here.
> 
> I think SHUTDOWN_RECEIVED should also be included. We continue to transmit and
> process SACKs after receiving a SHUTDOWN.

I am not sure about SHUTDOWN_RECEIVED.  If we received shutdown, then we are not in
a 0 window situation.  Additionally, the sender of the SHUTDOWN started the GUARD timer
and will abort after it expires.  So there is no special handling on our part.

-vlad

> 
>>> +	 * Although RFC2960 and RFC4460 specify that the overall error
>>> +	 * count must be cleared when a HEARTBEAT ACK is received this
>>> +	 * behaviour may prevent the maximum retransmission count from
>>> +	 * being reached while in SHUTDOWN. If the peer keeps its window
>>> +	 * closed not acknowledging any outstanding TSN we may rely on
>>> +	 * reaching the max_retrans limit via the T3-rtx timer to close
>>> +	 * the association which will never happen if the error count is
>>> +	 * reset every heartbeat interval.
>>> +	 */
>>> +	if (!(t->asoc->state >= SCTP_STATE_SHUTDOWN_PENDING))
>>> +		t->asoc->overall_error_count = 0;
>>
>> Same here.  We only care about the PENDING state. Also, please fix the comment to reflect
>> the code.
> 
> Agreed.
> 
>>> +		if (asoc->state == SCTP_STATE_SHUTDOWN_PENDING) {
>>> +			/*
>>> +			 * We are here likely because the receiver had its rwnd
>>> +			 * closed for a while and we have not been able to
>>> +			 * transmit the locally queued data within the maximum
>>> +			 * retransmission attempts limit.  Start the T5
>>> +			 * shutdown guard timer to give the receiver one last
>>> +			 * chance and some additional time to recover before
>>> +			 * aborting.
>>> +			 */
>>> +			sctp_add_cmd_sf(commands, SCTP_CMD_TIMER_RESTART,
>>> +				SCTP_TO(SCTP_EVENT_TIMEOUT_T5_SHUTDOWN_GUARD));
>>
>> This is bug.  You don't want to restart the timer every time you hit a T3-timeout.  Remember, since you fall
>> through here, you do another retransmission and schedule another timeout.  So next time the timeout happens,
>> you'll restart the SHUTDOWN_GUARD, which is not what you want.
>>
>> We want to start it once if it isn't pending, and leave it running without restart if it is already pending.
> 
> Doh, absolutely. The timer_pending() check got lost between testing and submission.
>