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?

    Thanks.
     
    paul, Nov 15, 2012
    #1
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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
    #2
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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
    #3
  4. What does ifconfig output for its MTU?
     
    Johann Klammer, Nov 19, 2012
    #4
  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
    headers.
     
    Rick Jones, Nov 19, 2012
    #5
  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
    #6
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