Do not fragment

Discussion in 'Cisco' started by KAL, Mar 8, 2006.

  1. KAL

    KAL Guest

    What happens if a receiver gets a packet with DF set to 1 but can not
    process it? What will it notify the sender?


    Also can somebody tell me the size of a UDP frame?

    Thanks.
     
    KAL, Mar 8, 2006
    #1
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  2. KAL

    Dan C Guest

    On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

    > What happens if a receiver gets a packet with DF set to 1 but can not
    > process it? What will it notify the sender?


    > Also can somebody tell me the size of a UDP frame?


    Do your own homework.
    Ask your teacher.

    --
    If you're not on the edge, you're taking up too much space.
    Linux Registered User #327951
     
    Dan C, Mar 8, 2006
    #2
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  3. KAL

    noEMA Guest

    On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

    > What happens if a receiver gets a packet with DF set to 1 but can not
    > process it? What will it notify the sender?


    - First : fragmentation :
    Not all networking technology have the same frame size.
    Case in point Ethernet and Token Ring.
    The MSS (Maximum Segment Size) of an Ethernet network is 1500 Bytes.
    The MSS of a Token Ring network is 4500.
    The MSS of some other networking technology is only 476 bytes.

    So a router transferring a Token Ring frame to an Ethernet network need to
    cut that packet in roughly 3 parts. This is why you need to do
    fragmentation. Now, since transferring a TCP packet onto that frame
    require some headers the number are actually smaller.

    Routers will fragment packets too big for the next step on the network
    path. Only the destination host will (except for IDS/IPSes) have to
    reassemble all the part to make it whole and ordered again. In the case
    where the network is not very efficient and loose packets badly, you will
    miss parts and your communication will have to manage timeouts and ask for
    retries.

    All that take time and computer resources. In some cases, it's better if
    there is never any fragmentation.

    As per in our example, if the 4500 bytes Token ring packet goes to a
    router with the DF flag set, then the router will send back to the
    originating host an ICMP type 3 with code 4 (Destination Unreachable ) and
    (Frag needed and DF set.)




    > Also can somebody tell me the size of a UDP frame?


    The maximum size of a TCP segment or UDP datagram is 65565 bytes
    minus the header size of the IP packet header and the header of the
    TCP/UDP part.

    Since TCP is more complex than UDP its header take more space.


    All of that is is clearly written in the specifications of the protocols
    in documents called RFCs. All RFCs can be found at :

    http://www.rfc-editor.org

    RFC 791 id for IP
    RFC 768 is for UDP
    and
    RFC 793 is for TCP.


    Well I hope it help.



    >
    > Thanks.
     
    noEMA, Mar 8, 2006
    #3
  4. KAL

    noEMA Guest

    On Tue, 07 Mar 2006 16:46:25 -0800, KAL wrote:

    > What happens if a receiver gets a packet with DF set to 1 but can not
    > process it? What will it notify the sender?
    >
    >
    > Also can somebody tell me the size of a UDP frame?
    >
    > Thanks.



    By the way if I may correct the vocabulary for
    the name of a header and a data payload at OSI layers

    At layer 1 (physical) it's called a bit stream... or serial comm.
    At layer 2 (data) it's called a "frame" (Ethernet)
    At layer 3 (network) it's called a "packet" (IP)
    At layer 4 (transport) it's a "segment"(TCP) or a "datagram"(UDP)
    Want me to go further ?

    And at layer 8 (human!) it's called correct vocabulary...
     
    noEMA, Mar 8, 2006
    #4
  5. KAL

    Rick Jones Guest

    In comp.os.linux.networking KAL <> wrote:
    > What happens if a receiver gets a packet with DF set to 1 but can
    > not process it? What will it notify the sender?


    What exactly do you mean by "process it" and by receiver do you mean
    the actual end destination, or an intermediate router?

    Details about IP and ICMP processing can be found in their respective
    RFC's which are archived at www.ietf.org.

    > Also can somebody tell me the size of a UDP frame?


    Approximately the size of New Jersey, or the length of a piece of
    string. :) The UDP RFC(s) are also at www.ietf.org if your question
    was meant to be "the maximum size of a UDP datagram"

    stricly speaking (at least as strictly as I can recall)

    Application "packets" are called messages
    TCP "packets" are called segments
    UDP "packets" are called datagrams
    IP "packets" are called datagrams
    Ethernet "packets" are called frames

    Sooo we can have for example an application-level message carried in a
    UDP datagram that is carried in some number of IP datagram fragments,
    each of which are carried in separate Ethernet frames. If that is
    then set over an ATM network those frames are further bend, folded,
    spindled and mutilated into cells.

    rick jones
    --
    portable adj, code that compiles under more than one compiler
    these opinions are mine, all mine; HP might not want them anyway... :)
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
     
    Rick Jones, Mar 8, 2006
    #5
  6. KAL

    Rick Jones Guest

    In comp.os.linux.networking noEMA <> wrote:
    > By the way if I may correct the vocabulary for
    > the name of a header and a data payload at OSI layers


    > At layer 1 (physical) it's called a bit stream... or serial comm.
    > At layer 2 (data) it's called a "frame" (Ethernet)
    > At layer 3 (network) it's called a "packet" (IP)


    I've always understood that IP packets were called datagrams.

    > At layer 4 (transport) it's a "segment"(TCP) or a "datagram"(UDP)
    > Want me to go further ?


    > And at layer 8 (human!) it's called correct vocabulary...


    We were all new at this once. Just that as we get old at it it
    becomes harder to remember what it was like to be new. To layer is
    human, to tolerate divine :) :) :)

    Oh, and layer 8 is actually the Financial layer:

    https://secure.isc.org/index.pl?/store/t-shirt/

    followed by layer 9, the Political layer. :)

    rick jones
    --
    denial, anger, bargaining, depression, acceptance, rebirth...
    where do you want to be today?
    these opinions are mine, all mine; HP might not want them anyway... :)
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...
     
    Rick Jones, Mar 8, 2006
    #6
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