Is it necessary for an RST packet to have an acknowledgement number?

Question

I am studying about TCP reset attack. I know that some RST packets have an acknowledgement number (with the ACK bit set), and some RST packets do not have an acknowledgement number (the acknowledgement number is set to 0, and the ACK bit is not set).

Now if someone were to send a forged RST packet, is there a situation where it is necessary for the RST packet to have an acknowledgement number, or will an RST packet without the acknowledgement number always work?

Accepted Answer

Is it necessary for an RST packet to have an acknowledgement number?

No. RFC 793, section 3.1 "Header format", 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.

so if a connection isn’t established - for example, if a packet comes in for a connection that was closed - a packet sent out doesn’t have to have an acknowledgment number and thus doesn’t have to have ACK set.