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    ipv6: RTAX_FEATURE_ALLFRAG causes inefficient TCP segment sizing · 67469601
    Eric Dumazet 提交于
    Quoting Tore Anderson from :
    https://bugzilla.kernel.org/show_bug.cgi?id=42572
    
    When RTAX_FEATURE_ALLFRAG is set on a route, the effective TCP segment
    size does not take into account the size of the IPv6 Fragmentation
    header that needs to be included in outbound packets, causing every
    transmitted TCP segment to be fragmented across two IPv6 packets, the
    latter of which will only contain 8 bytes of actual payload.
    
    RTAX_FEATURE_ALLFRAG is typically set on a route in response to
    receving a ICMPv6 Packet Too Big message indicating a Path MTU of less
    than 1280 bytes. 1280 bytes is the minimum IPv6 MTU, however ICMPv6
    PTBs with MTU < 1280 are still valid, in particular when an IPv6
    packet is sent to an IPv4 destination through a stateless translator.
    Any ICMPv4 Need To Fragment packets originated from the IPv4 part of
    the path will be translated to ICMPv6 PTB which may then indicate an
    MTU of less than 1280.
    
    The Linux kernel refuses to reduce the effective MTU to anything below
    1280 bytes, instead it sets it to exactly 1280 bytes, and
    RTAX_FEATURE_ALLFRAG is also set. However, the TCP segment size appears
    to be set to 1240 bytes (1280 Path MTU - 40 bytes of IPv6 header),
    instead of 1232 (additionally taking into account the 8 bytes required
    by the IPv6 Fragmentation extension header).
    
    This in turn results in rather inefficient transmission, as every
    transmitted TCP segment now is split in two fragments containing
    1232+8 bytes of payload.
    
    After this patch, all the outgoing packets that includes a
    Fragmentation header all are "atomic" or "non-fragmented" fragments,
    i.e., they both have Offset=0 and More Fragments=0.
    
    With help from David S. Miller
    Reported-by: NTore Anderson <tore@fud.no>
    Signed-off-by: NEric Dumazet <edumazet@google.com>
    Cc: Maciej Żenczykowski <maze@google.com>
    Cc: Tom Herbert <therbert@google.com>
    Tested-by: NTore Anderson <tore@fud.no>
    Signed-off-by: NDavid S. Miller <davem@davemloft.net>
    67469601
inet_connection_sock.h 10.7 KB