Packet size and TSO

Discussion in 'Linux Networking' started by paul, Nov 15, 2012.

  1. paul

    paul Guest

    I have two servers whose ethernet interfaces have an MTU of 9000. The
    switch supports jumbo frames, and I can successfully exchange full-sized
    packets (9000 bytes IP MTU, plus ethernet headers).

    If I run tcpdump during an iSCSI data exchange, I see packets with a
    reported length of more than 9000 (up to 20000 and more). I checked
    whether TSO was enabled, and indeed it was. So to make some tests, I
    disabled it with ethtool, expecting then tcpdump to see packet lengths up to
    9000 bytes. However, after disabling TSO, the maximum packet length I
    see with tcpdump is 1500.

    The physical card is an Intel Corporation 82576 Gigabit (kernel driver
    igb). Is what I see normal?

    paul, Nov 15, 2012
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  2. paul

    Rick Jones Guest

    It depends on what you see in the TCP MAXSEG option at connection
    establishment time.

    rick jones
    Rick Jones, Nov 19, 2012
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  3. paul

    paul Guest

    Well, I see 1460, but that only changes the question slightly, which now
    is: why do I see 1460 and not 8960?
    paul, Nov 19, 2012
  4. What does ifconfig output for its MTU?
    Johann Klammer, Nov 19, 2012
  5. paul

    Rick Jones Guest

    That suggests the side sending that SYNchronize segment does not
    actually believe the MTU of its first-hop link is 9000 bytes. Given
    the way the MSS gets computed, it would seem to believe the first-hop
    is a 1500 byte MTU.

    Or, perhaps, there is something "in the middle" re-writing TCP
    Rick Jones, Nov 19, 2012
  6. paul

    paul Guest

    It outputs 9000. I can indeed do a ping -s 8000 or so and see packets
    pass unfragmented.
    paul, Nov 20, 2012
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