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Why the Sequence number in an ACK packet is incremented?

Big IP orphaned connectionsHelp! Cisco VOIP phone are flooding my PC with network trafficTCP Window Size going to 0 / Wget stops downloading?syn/ack sequence number confusionIs my TCP connections sabotaged by my country's government?TCP Sequence & Acknowledgment numbersNFS stuck in ack loopserver stops sending SYN ACK after several normal connectionsHTTP 502 response generated by a proxy after it tries to send data upstream to a partially closed connection (reset packet)Can a TCP packet don't reach the 7th OSI layer

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2

1

I was wondering if there is any particular reason to increment the ACK sequence number instead of acknowledging the received sequence number. Why the RFC designed it this way?

Actual:
[SYN] SEQ=100
[SYN, ACK] Seq=300 Ack=101
[ACK] Seq=101 Ack=301

Why not:
[SYN] Seq=100
[SYN/ACK] Seq=300 Ack=100
[ACK] Seq=101 Ack=300

Naturally it would make sense to acknowledge the sequence number you just received instead of the sequence number you received + 1?

networking tcp

share|improve this question

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

add a comment | 

2

1

I was wondering if there is any particular reason to increment the ACK sequence number instead of acknowledging the received sequence number. Why the RFC designed it this way?

Actual:
[SYN] SEQ=100
[SYN, ACK] Seq=300 Ack=101
[ACK] Seq=101 Ack=301

Why not:
[SYN] Seq=100
[SYN/ACK] Seq=300 Ack=100
[ACK] Seq=101 Ack=300

Naturally it would make sense to acknowledge the sequence number you just received instead of the sequence number you received + 1?

networking tcp

share|improve this question

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

add a comment | 

I was wondering if there is any particular reason to increment the ACK sequence number instead of acknowledging the received sequence number. Why the RFC designed it this way?

Actual:
[SYN] SEQ=100
[SYN, ACK] Seq=300 Ack=101
[ACK] Seq=101 Ack=301

Why not:
[SYN] Seq=100
[SYN/ACK] Seq=300 Ack=100
[ACK] Seq=101 Ack=300

Naturally it would make sense to acknowledge the sequence number you just received instead of the sequence number you received + 1?

networking tcp

share|improve this question

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

Actual:
[SYN] SEQ=100
[SYN, ACK] Seq=300 Ack=101
[ACK] Seq=101 Ack=301

Why not:
[SYN] Seq=100
[SYN/ACK] Seq=300 Ack=100
[ACK] Seq=101 Ack=300

share|improve this question

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

share|improve this question

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

asked Mar 27 '18 at 9:13

L.S.L.S.

1132

1 Answer
1

active

oldest

votes

0

In TCP, the protocol keeps track of what has been sent by using a Sequence number. Effectively it's a counter of everything that was sent+1.

More details on https://tools.ietf.org/html/rfc793#section-3.3

ACK is incremented by 1 because the packet is carrying a SYN, it's not empty.
SYNs contribute to incrementing the SEG.LEN, as explained in the rfc:

SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)

Had the packet been empty and without SYN/FIN, the counter would have not been incremented.

This is also contemplated in the rfc where it states that the next sequence number to be sent must be greater or equal than the one indicated in the ACK:

A new acknowledgment (called an "acceptable ack"), is one for which the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

where SND.UNA is the oldest unacknowledged sequence number and SND.NXT is the next sequence number to be sent.

By incrementing the seq. number the packet is basically asking the other party "I expect you to send me the first byte of data now"

share|improve this answer

edited Mar 27 '18 at 16:49

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Yes. But why is it +1? The RFC states: "The acknowledgment mechanism[...]indicates that all octets up to but not including X have been received." Why not including X? This is the source of the need to ack the last received octet +1.
  
  – L.S.
  Mar 27 '18 at 12:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  added some clarification. Bottom line: the rfc says SEG.LEN must count amount of data in the payload, SYN and FIN.
  
  – Luca Gibelli
  Mar 27 '18 at 14:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ok thanks, i have read some more about the topic and came to the conclusion that the ACK is stating "I expect you to send me <this> Segment next." instead of "I received the segments up to <this>". I think both ways would be possible but this is how it was done. Maybe there is some indication that it had programmatic reasons to do it, maybe the implementation is easier if the sending host just has to start streaming from the ACK number he received in case of a retransmission. However your answer was useful to clarify, thus i marked it as valid. Thanks.
  
  – L.S.
  Mar 28 '18 at 10:36
  

  
  
  
  

add a comment | 

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0

In TCP, the protocol keeps track of what has been sent by using a Sequence number. Effectively it's a counter of everything that was sent+1.

More details on https://tools.ietf.org/html/rfc793#section-3.3

ACK is incremented by 1 because the packet is carrying a SYN, it's not empty.
SYNs contribute to incrementing the SEG.LEN, as explained in the rfc:

SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)

Had the packet been empty and without SYN/FIN, the counter would have not been incremented.

This is also contemplated in the rfc where it states that the next sequence number to be sent must be greater or equal than the one indicated in the ACK:

A new acknowledgment (called an "acceptable ack"), is one for which the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

where SND.UNA is the oldest unacknowledged sequence number and SND.NXT is the next sequence number to be sent.

By incrementing the seq. number the packet is basically asking the other party "I expect you to send me the first byte of data now"

share|improve this answer

edited Mar 27 '18 at 16:49

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Yes. But why is it +1? The RFC states: "The acknowledgment mechanism[...]indicates that all octets up to but not including X have been received." Why not including X? This is the source of the need to ack the last received octet +1.
  
  – L.S.
  Mar 27 '18 at 12:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  added some clarification. Bottom line: the rfc says SEG.LEN must count amount of data in the payload, SYN and FIN.
  
  – Luca Gibelli
  Mar 27 '18 at 14:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ok thanks, i have read some more about the topic and came to the conclusion that the ACK is stating "I expect you to send me <this> Segment next." instead of "I received the segments up to <this>". I think both ways would be possible but this is how it was done. Maybe there is some indication that it had programmatic reasons to do it, maybe the implementation is easier if the sending host just has to start streaming from the ACK number he received in case of a retransmission. However your answer was useful to clarify, thus i marked it as valid. Thanks.
  
  – L.S.
  Mar 28 '18 at 10:36
  

  
  
  
  

add a comment | 

0

In TCP, the protocol keeps track of what has been sent by using a Sequence number. Effectively it's a counter of everything that was sent+1.

More details on https://tools.ietf.org/html/rfc793#section-3.3

ACK is incremented by 1 because the packet is carrying a SYN, it's not empty.
SYNs contribute to incrementing the SEG.LEN, as explained in the rfc:

SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)

Had the packet been empty and without SYN/FIN, the counter would have not been incremented.

This is also contemplated in the rfc where it states that the next sequence number to be sent must be greater or equal than the one indicated in the ACK:

A new acknowledgment (called an "acceptable ack"), is one for which the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

where SND.UNA is the oldest unacknowledged sequence number and SND.NXT is the next sequence number to be sent.

By incrementing the seq. number the packet is basically asking the other party "I expect you to send me the first byte of data now"

share|improve this answer

edited Mar 27 '18 at 16:49

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Yes. But why is it +1? The RFC states: "The acknowledgment mechanism[...]indicates that all octets up to but not including X have been received." Why not including X? This is the source of the need to ack the last received octet +1.
  
  – L.S.
  Mar 27 '18 at 12:47
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  added some clarification. Bottom line: the rfc says SEG.LEN must count amount of data in the payload, SYN and FIN.
  
  – Luca Gibelli
  Mar 27 '18 at 14:31
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Ok thanks, i have read some more about the topic and came to the conclusion that the ACK is stating "I expect you to send me <this> Segment next." instead of "I received the segments up to <this>". I think both ways would be possible but this is how it was done. Maybe there is some indication that it had programmatic reasons to do it, maybe the implementation is easier if the sending host just has to start streaming from the ACK number he received in case of a retransmission. However your answer was useful to clarify, thus i marked it as valid. Thanks.
  
  – L.S.
  Mar 28 '18 at 10:36
  

  
  
  
  

add a comment | 

In TCP, the protocol keeps track of what has been sent by using a Sequence number. Effectively it's a counter of everything that was sent+1.

More details on https://tools.ietf.org/html/rfc793#section-3.3

ACK is incremented by 1 because the packet is carrying a SYN, it's not empty.
SYNs contribute to incrementing the SEG.LEN, as explained in the rfc:

SEG.LEN = the number of octets occupied by the data in the segment
(counting SYN and FIN)

Had the packet been empty and without SYN/FIN, the counter would have not been incremented.

This is also contemplated in the rfc where it states that the next sequence number to be sent must be greater or equal than the one indicated in the ACK:

A new acknowledgment (called an "acceptable ack"), is one for which the inequality below holds:

SND.UNA < SEG.ACK =< SND.NXT

where SND.UNA is the oldest unacknowledged sequence number and SND.NXT is the next sequence number to be sent.

By incrementing the seq. number the packet is basically asking the other party "I expect you to send me the first byte of data now"

share|improve this answer

edited Mar 27 '18 at 16:49

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

share|improve this answer

edited Mar 27 '18 at 16:49

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424

answered Mar 27 '18 at 11:47

Luca GibelliLuca Gibelli

2,11611424