TCP ACK 1

[jamesjlee50]

Device “A” sends a 500b file to “B” with a sequence number of 4512. What ACK does “B” return to “A” to indicate positive reception? What ACK does it send to indicate a negative reception?

 

[rwullems]

ACK = SEQ + LEN

LEN = TCP payload length field found in the TCP header. So if this will be 500, the ACK will be 5012.
The ACK number sent by B will be the next SEQ nummer that A will sent, so the next packet from A will have SEQ = 5012, if the lenght again is 500, the ACK from B will be 5512

If B does not recieve the package, it will not know it was ever sent. A timer wil run out on A and it will retransmit the packet using the same SEQ number.

Regards,
Robert



Robert Wullems
Network Specialist
SCP/SCE/SCM/CNX/MCP/MCSA/Network+
***************************************
If you can Sniff it, you can solve it!
***************************************

 

[jamesjlee50]

What ACK does "B" send to "A" to indicate negative reception in case of data corruption ("B" received the corrupted file)?

 

[rwullems]

That depents. It will ack until the last correct received packet. You also have a windows size in TCP that comes into play if you look at things like this. B does not have to ACK every packet. Simple example:

- B says to A: I have a window size of 100 bytes.
- A sents packets of 10 bytes
- A can sent 10 packets x 10 bytes = 100 bytes before B has to sent an ACK.
- Packet 7 gets lost
- B wil ack until 6
- A has to sent 7 to 10 agian.

This is a very simple example, it is a bit more complicated, but this is the basics.
There is also an option in TCP called SACK (Selective ACK). Litle bit to complecated to discuss in a few words.
Regards,
Robert



Robert Wullems
Network Specialist
SCP/SCE/SCM/CNX/MCP/MCSA/Network+/CNA
***************************************
If you can Sniff it, you can solve it!
***************************************

 

[Parcival21]

If B gets a corrupted packet (wrong checksum or something similar...) B woin't send an Ack packet for this packet. As R.Wullems said B will send an ack, that says that it received packets until ... .I think I remember that there's also a possibility that B can send an Ack in which it says I received packets 1,2,3,4,5,7,9 not talking about the lost packages. That has the opportunity that A will not send all packages starting from 6 again. It will only need to send 6 and 8 again. But I'm not quite sure about this option.
I't might even be the selective Ack option,that r.wullems is talking about(pls tell me).

bye,
busche

 

[rwullems]

Hi Percival21,

That is indeed the SACK option. But not many of todays TCP/IP implementations are using it. You will see it rearly.

There are currently three RFC's concerning SACK, RFC3517, RFC2883 and RFC2018. If you need some reading on this go to

http://www.rfc-editor.org

Regards,
Robert


Robert Wullems
Network Specialist
SCP/SCE/SCM/CNX/MCP/MCSA/Network+/CNA
***************************************
If you can Sniff it, you can solve it!
***************************************

 

[Parcival21]

Thank's for the information,
bye,
busche