• Eric Dumazet's avatar
    net: tcp: GRO should be ECN friendly · a9e050f4
    Eric Dumazet authored
    While doing TCP ECN tests, I discovered GRO was reordering packets if it
    receives one packet with CE set, while previous packets in same NAPI run
    have ECT(0) for the same flow :
    
    09:25:25.857620 IP (tos 0x2,ECT(0), ttl 64, id 27893, offset 0, flags
    [DF], proto TCP (6), length 4396)
        172.30.42.19.54550 > 172.30.42.13.44139: Flags [.], seq
    233801:238145, ack 1, win 115, options [nop,nop,TS val 3397779 ecr
    1990627], length 4344
    
    09:25:25.857626 IP (tos 0x3,CE, ttl 64, id 27892, offset 0, flags [DF],
    proto TCP (6), length 1500)
        172.30.42.19.54550 > 172.30.42.13.44139: Flags [.], seq
    232353:233801, ack 1, win 115, options [nop,nop,TS val 3397779 ecr
    1990627], length 1448
    
    09:25:25.857638 IP (tos 0x0, ttl 64, id 34581, offset 0, flags [DF],
    proto TCP (6), length 64)
        172.30.42.13.44139 > 172.30.42.19.54550: Flags [.], cksum 0xac8f
    (incorrect -> 0xca69), ack 232353, win 1271, options [nop,nop,TS val
    1990627 ecr 3397779,nop,nop,sack 1 {233801:238145}], length 0
    
    We have two problems here :
    
    1) GRO reorders packets
    
      If NIC gave packet1, then packet2, which happen to be from "different
    flows"  GRO feeds stack with packet2, then packet1. I have yet to
    understand how to solve this problem.
    
    2) GRO is not ECN friendly
    
    Delivering packets out of order makes TCP stack not as fast as it could
    be.
    
    In this patch I suggest we make the tos test not part of the 'same_flow'
    determination, but part of the 'should flush' logic
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Cc: Herbert Xu <herbert@gondor.apana.org.au>
    Acked-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    a9e050f4
af_inet.c 43 KB