• Daniel Borkmann's avatar
    net: sctp: fix suboptimal edge-case on non-active active/retrans path selection · aa4a83ee
    Daniel Borkmann authored
    In SCTP, selection of active (T.ACT) and retransmission (T.RET)
    transports is being done whenever transport control operations
    (UP, DOWN, PF, ...) are engaged through sctp_assoc_control_transport().
    
    Commits 4c47af4d ("net: sctp: rework multihoming retransmission
    path selection to rfc4960") and a7288c4d ("net: sctp: improve
    sctp_select_active_and_retran_path selection") have both improved
    it towards a more fine-grained and optimal path selection.
    
    Currently, the selection algorithm for T.ACT and T.RET is as follows:
    
    1) Elect the two most recently used ACTIVE transports T1, T2 for
       T.ACT, T.RET, where T.ACT<-T1 and T1 is most recently used
    2) In case primary path T.PRI not in {T1, T2} but ACTIVE, set
       T.ACT<-T.PRI and T.RET<-T1
    3) If only T1 is ACTIVE from the set, set T.ACT<-T1 and T.RET<-T1
    4) If none is ACTIVE, set T.ACT<-best(T.PRI, T.RET, T3) where
       T3 is the most recently used (if avail) in PF, set T.RET<-T.PRI
    
    Prior to above commits, 4) was simply a camp on T.ACT<-T.PRI and
    T.RET<-T.PRI, ignoring possible paths in PF. Camping on T.PRI is
    still slightly suboptimal as it can lead to the following scenario:
    
    Setup:
            <A>                                <B>
        T1: p1p1 (10.0.10.10) <==>  .'`)  <==> p1p1 (10.0.10.12)  <= T.PRI
        T2: p1p2 (10.0.10.20) <==> (_ . ) <==> p1p2 (10.0.10.22)
    
        net.sctp.rto_min = 1000
        net.sctp.path_max_retrans = 2
        net.sctp.pf_retrans = 0
        net.sctp.hb_interval = 1000
    
    T.PRI is permanently down, T2 is put briefly into PF state (e.g. due to
    link flapping). Here, the first time transmission is sent over PF path
    T2 as it's the only non-INACTIVE path, but the retransmitted data-chunks
    are sent over the INACTIVE path T1 (T.PRI), which is not good.
    
    After the patch, it's choosing better transports in both cases by
    modifying step 4):
    
    4) If none is ACTIVE, set T.ACT_new<-best(T.ACT_old, T3) where T3 is
       the most recently used (if avail) in PF, set T.RET<-T.ACT_new
    
    This will still select a best possible path in PF if available (which
    can also include T.PRI/T.RET), and set both T.ACT/T.RET to it.
    
    In case sctp_assoc_control_transport() *just* put T.ACT_old into INACTIVE
    as it transitioned from ACTIVE->PF->INACTIVE and stays in INACTIVE just
    for a very short while before going back ACTIVE, it will guarantee that
    this path will be reselected for T.ACT/T.RET since T3 (PF) is not
    available.
    
    Previously, this was not possible, as we would only select between T.PRI
    and T.RET, and a possible T3 would be NULL due to the fact that we have
    just transitioned T3 in sctp_assoc_control_transport() from PF->INACTIVE
    and would select a suboptimal path when T.PRI/T.RET have worse properties.
    
    In the case that T.ACT_old permanently went to INACTIVE during this
    transition and there's no PF path available, plus T.PRI and T.RET are
    INACTIVE as well, we would now camp on T.ACT_old, but if everything is
    being INACTIVE there's really not much we can do except hoping for a
    successful HB to bring one of the transports back up again and, thus
    cause a new selection through sctp_assoc_control_transport().
    
    Now both tests work fine:
    
    Case 1:
    
     1. T1 S(ACTIVE) T.ACT
        T2 S(ACTIVE) T.RET
    
     2. T1 S(ACTIVE) T.ACT, T.RET
        T2 S(PF)
    
     3. T1 S(ACTIVE) T.ACT, T.RET
        T2 S(INACTIVE)
    
     5. T1 S(PF) T.ACT, T.RET
        T2 S(INACTIVE)
    
    [ 5.1 T1 S(INACTIVE) T.ACT, T.RET
          T2 S(INACTIVE) ]
    
     6. T1 S(ACTIVE) T.ACT, T.RET
        T2 S(INACTIVE)
    
     7. T1 S(ACTIVE) T.ACT
        T2 S(ACTIVE) T.RET
    
    Case 2:
    
     1. T1 S(ACTIVE) T.ACT
        T2 S(ACTIVE) T.RET
    
     2. T1 S(PF)
        T2 S(ACTIVE) T.ACT, T.RET
    
     3. T1 S(INACTIVE)
        T2 S(ACTIVE) T.ACT, T.RET
    
     5. T1 S(INACTIVE)
        T2 S(PF) T.ACT, T.RET
    
    [ 5.1 T1 S(INACTIVE)
          T2 S(INACTIVE) T.ACT, T.RET ]
    
     6. T1 S(INACTIVE)
        T2 S(ACTIVE) T.ACT, T.RET
    
     7. T1 S(ACTIVE) T.ACT
        T2 S(ACTIVE) T.RET
    Signed-off-by: default avatarDaniel Borkmann <dborkman@redhat.com>
    Acked-by: default avatarNeil Horman <nhorman@tuxdriver.com>
    Acked-by: default avatarVlad Yasevich <vyasevich@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    aa4a83ee
associola.c 48.1 KB