Packet acknowledgement question - TCP-IP

This is a discussion on Packet acknowledgement question - TCP-IP ; Hi, I am doing some tcpdump-ing and have one question: Host A sends Packet 1 to Host B with 1448 Bytes Data: SeqNr = a677 1af7 Host A sends Packet 2 to Host B with 1448 Bytes Data. SeqNr = ...

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  1. Packet acknowledgement question

    Hi,
    I am doing some tcpdump-ing and have one question:

    Host A sends Packet 1 to Host B with 1448 Bytes Data: SeqNr = a677 1af7

    Host A sends Packet 2 to Host B with 1448 Bytes Data. SeqNr = a677 209f
    Host B sends ACK to Host A with ACKNr: a677 2647.

    I suppose that the ACK Paket is for Paket 2 because a677 209f + 5a8
    (Data) = a677 2647, am I right? And since I am not getting an ACK for
    Paket 2 my question is: Does the ACK Paket acknowledge both Pakets, 1
    and 2, or just the second one??

    Thank you.


  2. Re: Packet acknowledgement question

    In article <1150195726.199497.56630@f6g2000cwb.googlegroups.co m>,
    "inf009" wrote:

    > Hi,
    > I am doing some tcpdump-ing and have one question:
    >
    > Host A sends Packet 1 to Host B with 1448 Bytes Data: SeqNr = a677 1af7
    >
    > Host A sends Packet 2 to Host B with 1448 Bytes Data. SeqNr = a677 209f
    > Host B sends ACK to Host A with ACKNr: a677 2647.
    >
    > I suppose that the ACK Paket is for Paket 2 because a677 209f + 5a8
    > (Data) = a677 2647, am I right? And since I am not getting an ACK for
    > Paket 2 my question is: Does the ACK Paket acknowledge both Pakets, 1


    I assume you mean "not getting an ACK for Packet 1"

    > and 2, or just the second one??


    You don't acknowledge packets, you acknowledge sequences. ACK = X means
    "I have received everything up to X-1, and expect X next." So it
    acknowledges both segments. It's normal to send an ACK for every other
    segment received.

    There's also a "selective acknowledgement" option that can be used to
    acknowledge out-of-order sequences. If A and B were using this option,
    B could send a packet with ACK = a677 1af7 and SACK = {a677 209f, a677
    2646} to acknowledge only packet 2.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

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