Ethereal-users: Re: [Ethereal-users] Are unchanging sequence numbers normal in TCP Acks?

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From: Guy Harris <gharris@xxxxxxxxx>
Date: Wed, 16 Jun 2004 20:45:33 -0700
On Tue, Jun 15, 2004 at 03:59:53PM -0700, David Wuertele wrote:
> Notice that everything from the client (169.254.242.55) has the same
> Sequence Number.  Is that normal when the packet has nothing in it but
> a TCP Ack?

Yes - in fact, it's what I'd *expect* for a packet with nothing but an
ACK.  To quote RFC 793:

    The TCP must recover from data that is damaged, lost, duplicated, or
    delivered out of order by the internet communication system.  This
    is achieved by assigning a sequence number to each octet
    transmitted, and requiring a positive acknowledgment (ACK) from the
    receiving TCP.

An ACK-only packet transmits no octets, so it doesn't cause the sequence
number to change.