Re: [tcpm] Francesca Palombini's No Objection on draft-ietf-tcpm-rfc793bis-25: (with COMMENT)

[Francesca Palombini <[e-mail]>]

Ben,

Thank you for your reply. I did read “occupy sequence number space” as “the peer needs to increase the sequence number before sending”, which is why I was puzzled by the ACK getting sent with SEQ += 1 (might be worth to clarify).

However I am still missing something: when I look at Figure 7 I see something similar to what happens in Figure 14, but this time with SYN:

   2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...

   3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT

   4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED

   5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...

I am looking at lines 2 and 5. Peer A sends the SYN with SEQ 100, and then sends the SYN and ACK with the same SEQ 100. Making the parallel with what you say below, shouldn’t then in this case the SYN message occupy sequence number space, and the next message sent (line 5.) getting SEQ=101?

Francesca

From: Benjamin Kaduk <[e-mail]>
Date: Thursday, 30 September 2021 at 22:12
To: Francesca Palombini <[e-mail]>
Cc: The IESG <[e-mail]>, [e-mail] <[e-mail]>, [e-mail] <[e-mail]>, [e-mail] <[e-mail]>, [e-mail] <[e-mail]>
Subject: Re: Francesca Palombini's No Objection on draft-ietf-tcpm-rfc793bis-25: (with COMMENT)
On Wed, Sep 22, 2021 at 02:19:17PM -0700, Francesca Palombini via Datatracker wrote:
> 4. -----
>
> FP: This is surely me missing something but, in section 3.5 I see:
>
>    4.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK>       --> ESTABLISHED
>
>    5.  ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED
>
> which is followed by:
>
>    Note that the sequence number of the segment in line 5 is the same as
>    in line 4 because the ACK does not occupy sequence number space (if
>    it did, we would wind up ACKing ACKs!).
>
> However, later on, in Figure 13:
>
>    2.  (Close)                                              (Close)
>        FIN-WAIT-1  --> <SEQ=100><ACK=300><CTL=FIN,ACK>  ... FIN-WAIT-1
>                    <-- <SEQ=300><ACK=100><CTL=FIN,ACK>  <--
>                    ... <SEQ=100><ACK=300><CTL=FIN,ACK>  -->
>
>    3.  CLOSING     --> <SEQ=101><ACK=301><CTL=ACK>      ... CLOSING
>                    <-- <SEQ=301><ACK=101><CTL=ACK>      <--
>                    ... <SEQ=101><ACK=301><CTL=ACK>      -->
>
> I am confused why in this case, in line 3, ACK does in fact occupy sequence
> number space. What am I missing?

It is the FIN that occupies sequence number space, not the ACK.

Both steps 2 and 3 show messages being sent "in parallel", i.e., the "..."
in the respective column shows that the event is not currently occuring at
that endpoint.

So in (2), left sends SEQ=100,FIN, and right sends SEQ=300,FIN; since those
sequence numbers are thus committed, in (3) left has to send SEQ=101 and
right has to send SEQ=301.

-Ben