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

[David Borman <[e-mail]>]

> On Jan 6, 2022, at 10:26 PM, Wesley Eddy <[e-mail]> wrote:
> 
> Sorry for the very slow response, but I think all questions are answered below.  Thanks for your comments and patience.
> 
> 
> On 9/22/2021 5:19 PM, Francesca Palombini via Datatracker wrote:
>> Francesca Palombini has entered the following ballot position for
>> draft-ietf-tcpm-rfc793bis-25: No Objection
...
> 
>> 
>> 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?
> 
> In line 3 if the sequence number was 100, that would be a retransmission of the FIN, rather than just an ACK that covers the other side's FIN (300).  There's no data in the segment, so the sequence number is just meaningful for validation.

To be clear, it is the FIN that occupies one byte in the sequence space, not the ACK.  In line 3, each side is ACKing the other side's FIN; they do not include the FIN bit so the sequence number is advanced beyond the FIN.

			-David Borman