Odd tftp problem

Discussion in 'Cisco' started by Tarvos{k}, Sep 30, 2005.

  1. Tarvos{k}

    Tarvos{k} Guest

    Greetings all,

    The other day during my CCNA class we were working on transfering the IOS to
    a switch by tftp. During the process we got some odd characters on the
    hyperterminal display during the transfer. Instead of getting all !'s we
    had several (proabably 5 spaced out on each line) O's (letter o not zero).
    Many of us have looked for some explaination but no one seems to be able to
    find one. The image came through just fine, but we are all curious as to
    what this might mean, including the instructor who has never seen this in
    the many years he had worked on any of Cisco's products. The switch is a
    2950 and the image was being moved from a local tftp server in the same
    room.

    Thanks for any help

    Tarvos
    Tarvos{k}, Sep 30, 2005
    #1
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  2. Tarvos{k}

    Peter Guest

    Hi Tarvos,

    > The other day during my CCNA class we were working on transfering the IOS to
    > a switch by tftp. During the process we got some odd characters on the
    > hyperterminal display during the transfer. Instead of getting all !'s we
    > had several (proabably 5 spaced out on each line) O's (letter o not zero).


    I think you may find that the O indicates a error in that block and
    the block was retransmitted. This is most likely if you are using a
    HUB or Half-Duplex segment that has other traffic on it at the time
    the transfer is running, and the sending end detected a collision (IE
    an OUTPUT error).

    Cheers.............pk.

    --
    Peter from Auckland.
    Peter, Sep 30, 2005
    #2
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  3. Tarvos{k} wrote:

    > hyperterminal display during the transfer. Instead of getting all !'s we
    > had several (proabably 5 spaced out on each line) O's (letter o not zero).
    > Many of us have looked for some explaination but no one seems to be able to
    > find one.


    That's an 'O'out of order packet. It means that the router received
    (say) packet 5 before it got packet 4. Shouldn't really happen on a LAN,
    though...

    copy Command Character Descriptions

    Character Description
    ! For net transfers an exclamation point indicates that the copy
    process is taking place. Each exclamation point indicates the successful
    transfer of ten packets (512 bytes each).

    .. For net transfers a period indicates the copy process timed out. Many
    periods in a row typically mean that the copy process may fail.

    O For net transfers an uppercase O indicates a packet was received out
    of order and the copy process may fail.

    e For flash erasures, a lowercase e indicates a device is being erased.

    E An uppercase E indicates an error. The copy process may fail.

    V A series of uppercase Vs indicates the progress during the
    verification of the image checksum.
    M.C. van den Bovenkamp, Sep 30, 2005
    #3
  4. Tarvos{k}

    Tarvos{k} Guest

    I appreciate the answers that have been sent, but the question I have about
    this is that if tftp uses udp then sequence shouldn't matter. Or is there
    some other "order/sequence" that I hadn't heard about involved?

    Tarvos{k}
    Tarvos{k}, Sep 30, 2005
    #4
  5. Tarvos{k}

    Merv Guest

    If you take a look at the TFTP RFC 1350 you will see that TFTP packets
    contain a block number within each packet.
    Merv, Sep 30, 2005
    #5
  6. Tarvos{k}

    Merv Guest

    If you take a look at the TFTP RFC 1350 you will see that TFTP packets
    contain a block number within each packet.
    Merv, Sep 30, 2005
    #6
  7. Tarvos{k}

    John Agosta Guest

    The TFTP session can sometimes fail. To help determine why a TFTP session
    failed, TFTP generates an "E" character if it receives an erroneous packet,
    and an "O" character if it receives an out-of-sequence packet. A period (.)
    indicates a timeout. The transfer session may still succeed even if TFTP
    generates these characters, but the output is useful for diagnosing the
    transfer failure

    -ja




    "Tarvos{k}" <> wrote in message
    news:JQ3%e.11128$...
    > Greetings all,
    >
    > The other day during my CCNA class we were working on transfering the IOS
    > to a switch by tftp. During the process we got some odd characters on the
    > hyperterminal display during the transfer. Instead of getting all !'s we
    > had several (proabably 5 spaced out on each line) O's (letter o not zero).
    > Many of us have looked for some explaination but no one seems to be able
    > to find one. The image came through just fine, but we are all curious as
    > to what this might mean, including the instructor who has never seen this
    > in the many years he had worked on any of Cisco's products. The switch is
    > a 2950 and the image was being moved from a local tftp server in the same
    > room.
    >
    > Thanks for any help
    >
    > Tarvos
    >
    John Agosta, Oct 1, 2005
    #7
  8. Tarvos{k}

    Guest

    > The other day during my CCNA class we were working on transfering the IOS to
    > a switch by tftp. During the process we got some odd characters on the
    > hyperterminal display during the transfer. Instead of getting all !'s we
    > had several (proabably 5 spaced out on each line) O's (letter o not zero).
    > Many of us have looked for some explaination but no one seems to be able to
    > find one. The image came through just fine, but we are all curious as to
    > what this might mean, including the instructor who has never seen this in
    > the many years he had worked on any of Cisco's products.


    As I understand it TFTP was designed to use as
    little code as possible and is therefore basiclly quite crude.
    It uses:
    - 512 byte blocks
    - each block is numbered (with a 16 bit unsigned - 0-65535)
    - the sender sends the (n + 1)th block when it receives the
    acknowledgement for the n th block
    - there must be some way of detecting the last block (read the RFC)

    Given these constraints (particularly the third one) out of order
    blocks
    should not occur, ever.

    So what could be causing them?

    Duplicated packets within the network could cause them.

    e.g. Block n gets duplicated

    The receiver is expecting block n,
    it gets the first one and acknowledges it
    it is then expecting block n+1 but it gets block n again
    and flags it as out of order.

    e.g Ack n gets duplicated.

    the receiver gets block n and sends the Ack.
    On receiving the ack the sender send block n+1.
    The receiver receives block n+1 and sends Ack n+1
    The duplicate Ack n then appears at the Sender
    The Sender sends block n+1 (again).

    Note that in these cases the duplicated packet stream continues
    to the end of the file.

    tftp is limited to 32M Byte files 512 * 65535

    I have heard that some implementations incorrectly use signed integers
    internally for the block counter and are therefore limited to 16M
    files.
    512 * 32767.

    To investigate this use a packet sniffer to see what is going on.
    You can probably very easily run one on the tftp server.

    On Windows;
    winpcap and ethereal (and windump) are worth getting to know.

    On unix:
    pcap, ethereal, tcpdump.

    Linux, for example sems to come with tcpdump built in.

    I seem to recall that some tftp inplementations were
    flawed and caused duplicate packets. This is catastrophic
    since every duplicate packet creates an additional stream that
    goes on to the end of the file.

    I like tftpd32.exe (I am on windows). I don't like the tftpd that cisco

    had on their web site a few years ago.
    , Oct 3, 2005
    #8
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