TCP Flags

TCP flags are control bit values set on TCP packets which indicate a purpose or action.

Note

Some areas of TNSR use TCP flags by name and others use them by their bit value. Consult the documentation for each relevant area, and the command reference, to determine proper usage for each area.

This list contains TCP flag names, their corresponding bit value, and a brief description of their purpose.

fin (1)

Indicates there is no more data from the sender, which starts the process of closing a connection.

syn (2)

Synchronize sequence numbers. Indicates a new connection attempt.

rst (4)

Reset the connection.

This flag is set when replying to a request to open a connection on a port which has no listening daemon. Can also be set by peers or by firewall software to turn away undesirable connections.

psh (8)

Indicates that the peer should push or flush data, including data in this packet, by passing the data up to the application.

ack (16)

Indicates ACKnowledgment of data. These are replies to let the sender know data was received OK. This is typically found on all packets from a client after the initial syn.

urg (32)

Indicates that the urgent field is significant, and this packet should be sent before data that is not urgent.

ece (64)

Explicit Congestion Notification (ECN) echo flag. The meaning of this flag depends on the value of syn.

If syn is set, the ece flag indicates that the peer is capable of ECN.

If syn is unset, the ece flag indicates that the peer received a packet with the ECN bit set and is confirming the notification.

cwr (128)

Congestion window reduced. Set in response to packets with ece set, indicating that the peer is performing congestion control.