Packet Size greater than MTU set

Discussion in 'Cisco' started by Akhtar, Nov 17, 2006.

  1. Akhtar

    Akhtar Guest

    Dear all,

    If i need to transport packets of size ( say 1524 bytes ) greater than
    MTU set ( 1500 bytes ), & DF bit of all the packets is set to 1, how
    could i transport these packets...

    1. Changing the MTU size on Physical Interface or Logical interface (
    tunnel ).
    2. or else

    Please suggest...


    Cheers,

    Akhtar
     
    Akhtar, Nov 17, 2006
    #1
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  2. * Akhtar wrote:
    > If i need to transport packets of size ( say 1524 bytes ) greater than
    > MTU set ( 1500 bytes ), & DF bit of all the packets is set to 1, how
    > could i transport these packets...


    Which kind of transport medium do you use?
    - Ethernet - Ethernet with crossover cable?
    - Ethernet via switch infrastructure?
    - IP connection via Int(er|ra)net?
    - Anything else?

    > 1. Changing the MTU size on Physical Interface or Logical interface (
    > tunnel ).


    If you set up a tunnel the command "ip mtu 1524" command on both sides is
    sufficient.

    > 2. or else


    Please describe your layer 1 and layer 2 setup.
     
    Lutz Donnerhacke, Nov 17, 2006
    #2
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  3. Akhtar

    Rainer Temme Guest

    Akhtar wrote:
    > If i need to transport packets of size ( say 1524 bytes ) greater than
    > MTU set ( 1500 bytes ), & DF bit of all the packets is set to 1, how
    > could i transport these packets...
    >
    > 1. Changing the MTU size on Physical Interface or Logical interface (
    > tunnel ).
    > 2. or else


    .... or else ... clear the DF-bit ...

    Usually DF-bit is set because the sender of the packet wants to
    get informed about the fact that the MTU is (somewhere on the
    routing path) smaller than the packet he sent. An icmp (type3 code4)
    would be generated for this. If the sender gets this icmp, he would
    resent the data in the packet, but would repacketize into smaller
    packets. (This is part of TCPs path-mtu-detection).

    However, there are situations, when the original sender cannot be
    determined anymore ... or cannot be reached from where the bottleneck
    (regarding mtu-size) is. In such a case, it might be a good idea,
    to clear the DF bit ... by doing that, you allow your router to
    break the packet into two ... and route them (rather than discard
    the packet).

    Rainer
     
    Rainer Temme, Nov 17, 2006
    #3
  4. In article <>,
    Akhtar <> wrote:

    >If i need to transport packets of size ( say 1524 bytes ) greater than
    >MTU set ( 1500 bytes ), & DF bit of all the packets is set to 1, how
    >could i transport these packets...


    >1. Changing the MTU size on Physical Interface or Logical interface (
    >tunnel ).
    >2. or else


    You would have to use an encapsulating protocol that handled the
    fragmentation itself, with the decapsulation routine
    reassembling the packets before submitting them to the layer that
    believes it needs the larger packet size.

    But normally this is not a problem because normally you send
    a stream, not a packet.

    This is especially true for TCP: you might happen to have 1524 bytes to
    send, but TCP would look at the MTU, see that only 1460 bytes would fit,
    and would prepare a packet with those bytes, leaving the remaining
    64 bytes in the transmit buffers, to remain there until more data
    is ready to send, or a PUSH or URG packet is formed, or a timer
    goes off and the packet is sent without being full. TCP does not
    deal in packets, and makes absolutely no promise that packet boundaries
    will be preserved.

    In order to send a "packet" with a larger MTU, you would need to
    be building RAW packets, or else be attempting to use send() or
    sendto() or sendmsg() -- and for the send*() family, if you
    attempt to send something too large, the send*() will error out.
     
    Walter Roberson, Nov 17, 2006
    #4
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