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Date: Mon, 09 Jun 2014 15:13:32 +0200 From: Per Hurtig <per.hurtig@....se> To: Nandita Dukkipati <nanditad@...gle.com> CC: Eric Dumazet <eric.dumazet@...il.com>, Netdev <netdev@...r.kernel.org>, Anna Brunström <anna.brunstrom@....se>, mohammad.rajiullah@....se, Neal Cardwell <ncardwell@...gle.com>, Sergei Shtylyov <sergei.shtylyov@...entembedded.com> Subject: Re: [PATCH] tcp: fixing TLP's FIN recovery See inline, On 2014-06-09 09:02, Nandita Dukkipati wrote: > On Sun, Jun 8, 2014 at 12:41 AM, Per Hurtig <per.hurtig@....se> wrote: >> >> >> On sön 8 jun 2014 04:58:25, Eric Dumazet wrote: >>> >>> On Sat, 2014-06-07 at 16:34 +0200, Per Hurtig wrote: >>>> >>>> Fix to a problem observed when losing a FIN segment that does not >>>> contain data. In such situations, TLP is unable to recover from >>>> *any* tail loss and instead adds at least PTO ms to the >>>> retransmission process, i.e., RTO = RTO + PTO. >>>> >>>> Signed-off-by: Per Hurtig <per.hurtig@....se> >>>> --- >>>> net/ipv4/tcp_output.c | 6 ++++-- >>>> 1 file changed, 4 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c >>>> index d463c35..6573765 100644 >>>> --- a/net/ipv4/tcp_output.c >>>> +++ b/net/ipv4/tcp_output.c >>>> @@ -2130,8 +2130,10 @@ void tcp_send_loss_probe(struct sock *sk) >>>> if (WARN_ON(!skb || !tcp_skb_pcount(skb))) >>>> goto rearm_timer; >>>> >>>> - /* Probe with zero data doesn't trigger fast recovery. */ >>>> - if (skb->len > 0) >>>> + /* Probe with zero data doesn't trigger fast recovery, if FIN >>>> + * flag is not set. >>>> + */ >>>> + if ((skb->len > 0) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)) >>>> err = __tcp_retransmit_skb(sk, skb); >>>> >>>> /* Record snd_nxt for loss detection. */ >>> >>> >>> >>> You know, I believe the test was exactly to avoid sending data less FIN >>> packets. >>> >>> If you write : >>> >>> if (A || !A) >>> >>> Better remove the condition, completely ;) >>> After looking more closely, I see that we only enter this part when the FIN flag is set on an otherwise empty segment. I guess I was distracted by the comment that suggested a more general scenario than the actual one, a bit confusing ;) >> Obviously, but I don't think that FINs are the only segments >> who are targeted by this condition (or targeted at all given >> the implications of this statement). Furthermore, the comment above >> the if statement would probably have mentioned FINs explicity >> and not zero sized segments in general if this were the case. >> >> >> >>> >>> Nandita, why FIN packet wont trigger fast retransnmits ? >>> >> >> They do, that's the whole thing with this patch. >> >> >>> It sounds like if the timer is the issue you want to fix, you might >>> simply rearm a timer with RTO-PTO instead of RTO ? >>> >>> >> No I want to enable TLP for tail loss where an empty FIN is involved, >> this does not work now. > > I understand the tail loss case you want to solve - essentially when a > tail loss occurs that involves data segments as well as that of an > empty FIN. However, have you verified that re-sending an empty FIN > triggers fast recovery? I would be surprised if it did, because I > think the sender needs to receive a SACK of at least 1-byte of data > before sender can trigger FACK based fast recovery. > Yes, it needs a SACK that covers one "sequence number", which a FIN does. I don't see why it shouldn't generate a SACK? See below for some packet dumps. > If you have verified that a pure FIN does indeed trigger recovery, can > you tell me what part of the code makes that happen? > Scenario: Eleven segments are sent back-to-back (ten data and one empty FIN), the last three segments (the FIN + two others) are dropped. Other relevant info: RTT of 20ms. Transfer time of the entire flow (from the receiver's point of view): TCP w TLP: 324ms TCP w modified TLP: 122ms Detailed TLP behavior: The entire transfer including retransmissions takes approx 324ms. The retransmissions are conducted in frames 23 and 25. Sender-side packet trace: 1 0.000000 10.0.1.1 -> 10.0.2.1 TCP 74 36713 > search-agent [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 TSval=1171292524 TSecr=0 WS=128 2 0.000020 10.0.2.1 -> 10.0.1.1 TCP 74 search-agent > 36713 [SYN, ACK] Seq=0 Ack=1 Win=28960 Len=0 MSS=1460 SACK_PERM=1 TSval=1171296150 TSecr=1171292524 WS=128 3 0.019818 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=1 Win=29312 Len=0 TSval=1171292529 TSecr=1171296150 4 0.019854 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=1 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 5 0.019864 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=1449 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 6 0.019868 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=2897 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 7 0.019871 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=4345 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 8 0.019875 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=5793 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 9 0.019878 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=7241 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 10 0.019881 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=8689 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 11 0.019922 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=10137 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 12 0.019929 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 36713 [PSH, ACK] Seq=11585 Ack=1 Win=29056 Len=1448 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 13 0.019930 10.0.2.1 -> 10.0.1.1 TCP 1513 search-agent > 36713 [PSH, ACK] Seq=13033 Ack=1 Win=29056 Len=1447 TSval=1171296155 TSecr=1171292529[Packet size limited during capture] 14 0.019971 10.0.2.1 -> 10.0.1.1 TCP 66 search-agent > 36713 [FIN, ACK] Seq=14480 Ack=1 Win=29056 Len=0 TSval=1171296155 TSecr=1171292529 15 0.039635 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=1449 Win=32128 Len=0 TSval=1171292534 TSecr=1171296155 16 0.039643 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=2897 Win=35072 Len=0 TSval=1171292534 TSecr=1171296155 17 0.039646 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=4345 Win=37888 Len=0 TSval=1171292534 TSecr=1171296155 18 0.039650 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=5793 Win=40832 Len=0 TSval=1171292534 TSecr=1171296155 19 0.039653 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=7241 Win=43520 Len=0 TSval=1171292534 TSecr=1171296155 20 0.039655 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=8689 Win=43520 Len=0 TSval=1171292534 TSecr=1171296155 21 0.039657 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=10137 Win=43520 Len=0 TSval=1171292534 TSecr=1171296155 22 0.039660 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=11585 Win=43520 Len=0 TSval=1171292534 TSecr=1171296155 23 0.283780 10.0.2.1 -> 10.0.1.1 TCP 1514 [TCP Retransmission] search-agent > 36713 [PSH, ACK] Seq=11585 Ack=1 Win=29056 Len=1448 TSval=1171296221 TSecr=1171292534[Packet size limited during capture] 24 0.303267 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [ACK] Seq=1 Ack=13033 Win=43520 Len=0 TSval=1171292600 TSecr=1171296221 25 0.303276 10.0.2.1 -> 10.0.1.1 TCP 1513 [TCP Retransmission] search-agent > 36713 [FIN, PSH, ACK] Seq=13033 Ack=1 Win=29056 Len=1447 TSval=1171296225 TSecr=1171292600[Packet size limited during capture] 26 0.324085 10.0.1.1 -> 10.0.2.1 TCP 66 36713 > search-agent [FIN, ACK] Seq=1 Ack=14481 Win=43520 Len=0 TSval=1171292605 TSecr=1171296225 27 0.324093 10.0.2.1 -> 10.0.1.1 TCP 66 search-agent > 36713 [ACK] Seq=14481 Ack=2 Win=29056 Len=0 TSval=1171296231 TSecr=1171292605 ---- Modified TLP behavior: The entire transfer including retransmissions takes approx 122ms. The TLP probe is sent in frame 23 below, and you can see in frame 24 below that a SACK covering one sequence number is returned from the receiver and used to trigger retransmissions of the other lost segments. Sender-side packet trace: 1 0.000000 10.0.1.1 -> 10.0.2.1 TCP 74 37730 > search-agent [SYN] Seq=0 Win=29200 Len=0 MSS=1460 SACK_PERM=1 TSval=1194757582 TSecr=0 WS=128 2 0.000021 10.0.2.1 -> 10.0.1.1 TCP 74 search-agent > 37730 [SYN, ACK] Seq=0 Ack=1 Win=28960 Len=0 MSS=1460 SACK_PERM=1 TSval=222654 TSecr=1194757582 WS=128 3 0.020765 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=1 Win=29312 Len=0 TSval=1194757587 TSecr=222654 4 0.020800 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=1 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 5 0.020810 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=1449 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 6 0.020814 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=2897 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 7 0.020818 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=4345 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 8 0.020821 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=5793 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 9 0.020824 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=7241 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 10 0.020827 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=8689 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 11 0.020870 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=10137 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 12 0.020877 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=11585 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 13 0.020879 10.0.2.1 -> 10.0.1.1 TCP 1514 search-agent > 37730 [PSH, ACK] Seq=13033 Ack=1 Win=29056 Len=1448 TSval=222659 TSecr=1194757587[Packet size limited during capture] 14 0.020918 10.0.2.1 -> 10.0.1.1 TCP 66 search-agent > 37730 [FIN, ACK] Seq=14481 Ack=1 Win=29056 Len=0 TSval=222659 TSecr=1194757587 15 0.040583 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=1449 Win=32128 Len=0 TSval=1194757593 TSecr=222659 16 0.040591 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=2897 Win=35072 Len=0 TSval=1194757593 TSecr=222659 17 0.040594 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=4345 Win=37888 Len=0 TSval=1194757593 TSecr=222659 18 0.040597 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=5793 Win=40832 Len=0 TSval=1194757593 TSecr=222659 19 0.040599 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=7241 Win=43520 Len=0 TSval=1194757593 TSecr=222659 20 0.040601 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=8689 Win=43520 Len=0 TSval=1194757593 TSecr=222659 21 0.040604 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=10137 Win=43520 Len=0 TSval=1194757593 TSecr=222659 22 0.040606 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=11585 Win=43520 Len=0 TSval=1194757593 TSecr=222659 23 0.078751 10.0.2.1 -> 10.0.1.1 TCP 66 [TCP Retransmission] search-agent > 37730 [FIN, ACK] Seq=14481 Ack=1 Win=29056 Len=0 TSval=222674 TSecr=1194757593 24 0.098093 10.0.1.1 -> 10.0.2.1 TCP 78 [TCP Dup ACK 22#1] 37730 > search-agent [ACK] Seq=1 Ack=11585 Win=43520 Len=0 TSval=1194757607 TSecr=222659 SLE=14481 SRE=14482 25 0.102752 10.0.2.1 -> 10.0.1.1 TCP 1514 [TCP Retransmission] search-agent > 37730 [PSH, ACK] Seq=11585 Ack=1 Win=29056 Len=1448 TSval=222680 TSecr=1194757607[Packet size limited during capture] 26 0.102757 10.0.2.1 -> 10.0.1.1 TCP 1514 [TCP Retransmission] search-agent > 37730 [PSH, ACK] Seq=13033 Ack=1 Win=29056 Len=1448 TSval=222680 TSecr=1194757607[Packet size limited during capture] 27 0.121854 10.0.1.1 -> 10.0.2.1 TCP 78 37730 > search-agent [ACK] Seq=1 Ack=13033 Win=43520 Len=0 TSval=1194757613 TSecr=222680 SLE=14481 SRE=14482 28 0.121862 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [ACK] Seq=1 Ack=14482 Win=43520 Len=0 TSval=1194757613 TSecr=222680 29 0.121868 10.0.1.1 -> 10.0.2.1 TCP 66 37730 > search-agent [FIN, ACK] Seq=1 Ack=14482 Win=43520 Len=0 TSval=1194757613 TSecr=222680 30 0.121875 10.0.2.1 -> 10.0.1.1 TCP 66 search-agent > 37730 [ACK] Seq=14482 Ack=2 Win=29056 Len=0 TSval=222684 TSecr=1194757613 ---- Thanks, Per > Nandita > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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