In TCP/IP communication, we know that when a sender sends a packet containing some data, the receiver has to return an ACK packet. Does this ACK packet has to carry the same payload that the receiver got or it is just turning on ACK flag in the header and the next serial number?
TCP Acknowledgment Payload – Understanding TCP Acknowledgment
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Related Solutions
If the packet doesn't contain the IP of the sender, how does the receiver know from where it got the packet?
TCP Syn-segments are always sent uncompressed. After each host has received a SYN-segment, they know the other side's IP address. They can then compress the following segments' headers, because the IP address is remembered and re-entered in the header by the decompressor.
...and according to which packet headers that it (receiver) stored it "rebuilds" the packet?
According to the headers of the packets that were tranmitted uncompressed, at least the first two (SYN) segments of the connection, one in each direction.
1. step
The client begins the three-way handshake by sending a segment with the synchronize sequence number (SYN) control flag set, which indicates an initial value in the sequence number field. This value is known as the initial sequence number (ISN). The ISN is randomly chosen and is increased by one for each byte of data sent from the client to the server.
ACK : not set | SYN : set
2. step
The server must acknowledge the receipt of the SYN segment, so it sends a segment back with the ACK flag set. The client recognizes this as an acknowledgement that the server received the SYN from the client.
The value of the ACK number field is the ISN + 1.
The server also must initiate a response to the client. To achieve that the server uses the SYN flag in the same way that the client did. (So it is one segment)
ACK : set | SYN : set
3. step
The client responds with a segment containing ACK that is the response to the SYN sent by the server. There is no user data in this segment. The ACK field contains one more than the ISN received from the server.
After both sessions are established between client and server, all segments exchanged will have the ACK flag set.
ACK : set | SYN : not set
(Note: the whole conversation between the client and the server is actually two one-way sessions)
Best Answer
Yup.
Repeating the payload would just lead to unnecessary duplicating of data that has to be carried across the network. The Acknowledgement number itself is sufficient to tell the initial sender exactly what bits of data have been received.