• Alexander Duyck's avatar
    tcp: Set ECT0 bit in tos/tclass for synack when BPF needs ECN · 407c85c7
    Alexander Duyck authored
    When a BPF program is used to select between a type of TCP congestion
    control algorithm that uses either ECN or not there is a case where the
    synack for the frame was coming up without the ECT0 bit set. A bit of
    research found that this was due to the final socket being configured to
    dctcp while the listener socket was staying in cubic.
    
    To reproduce it all that is needed is to monitor TCP traffic while running
    the sample bpf program "samples/bpf/tcp_cong_kern.c". What is observed,
    assuming tcp_dctcp module is loaded or compiled in and the traffic matches
    the rules in the sample file, is that for all frames with the exception of
    the synack the ECT0 bit is set.
    
    To address that it is necessary to make one additional call to
    tcp_bpf_ca_needs_ecn using the request socket and then use the output of
    that to set the ECT0 bit for the tos/tclass of the packet.
    
    Fixes: 91b5b21c ("bpf: Add support for changing congestion control")
    Signed-off-by: default avatarAlexander Duyck <alexanderduyck@fb.com>
    Link: https://lore.kernel.org/r/160593039663.2604.1374502006916871573.stgit@localhost.localdomainSigned-off-by: default avatarJakub Kicinski <kuba@kernel.org>
    407c85c7
tcp_ipv6.c 57 KB