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I understand seq/ack is used to provide a reliable connection service. But it seems that during TCP data transmission the ACK bit is ALWAYS set, as demonstrated in the following picture.
Suppose A has just acked a packet from B, and now A wants to send something to B. What is the point of A setting the ACK bit and ACK number, which are already known to B?
A related question: wouldn't it be more clear to always separate ACK control segment from data segment?
Pic from internet:
acks themselves are not acknowledged - so your notion that the peer already knows the information in the seemingly redundant ack is not necessarily true.. and its free to encode it this way, so doing so helps prevent retrasmissions from lost acks at no cost.
The ACK bit means that the Acknowledgement number in this packet is valid. In the SYN packet there is no SEQ# from the other side yet so there is nothing to ACK. There is a zero in the ACKnowledgement field, but it is just a placeholder (some decoders show it some don't, but you can see it in the hex). Every packet after the first packet will have the ACK bit set. Every packet has a SEQ and ACK number, so there is always a valid ACK, even if the ACK was the same as the last ACK.
As RFC 793 says:
Acknowledgment Number. 32 bits.
If the ACK control bit is set this field contains the value of the next sequence number the sender of the segment is expecting to receive. Once a connection is established this is always sent.
As others have said, instead of leaving the ACK field empty, it is always populated. It doesn't "cost" anything and it always makes sure that the other end knows what byte is expected next.
The ACK bit is continuously used as part of the process of transferring data.
Since the purpose of TCP is to be a reliable protocol, it has to have some way to acknowledge that all pieces of data were correctly received. The ACK associated with the Acknowledgment number is used to do so.
From RFC793
Reliability:
The TCP must recover from data that is damaged, lost, duplicated, or delivered out of order by the internet communication system. This is achieved by assigning a sequence number to each octet transmitted, and requiring a positive acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout interval, the data is retransmitted. At the receiver, the sequence numbers are used to correctly order segments that may be received out of order and to eliminate duplicates. Damage is handled by adding a checksum to each segment transmitted, checking it at the receiver, and discarding damaged segments. As long as the TCPs continue to function properly and the internet system does not become completely partitioned, no transmission errors will affect the correct delivery of data. TCP recovers from internet communication system errors.Flow Control:
TCP provides a means for the receiver to govern the amount of data sent by the sender. This is achieved by returning a "window" with every ACK indicating a range of acceptable sequence numbers beyond the last segment successfully received. The window indicates an allowed number of octets that the sender may transmit before receiving further permission.
