TCP Cram Sheet

• Sequence Numbers. Each byte has a sequence number, beginning with a randomly chosen Initial Sequence Number. For simplicity, we often assume the ISN is 0.
• Acknowledgement. Each side sends acknowledgements, known as ACK's, in response to data recieved. The ACK contains the ISN of the next byte it expects to receive.
• Segments. Since IP is packetized, TCP sends its stream of bytes in packets which are called Segments.
• Sliding Window. The sender can send multiple segments, up to the byte with sequence number W plus the last Sequence Number acknowledged, where W is the window size. As new acknowledgements come in all bytes up to by not including the ISN of the ACK have been confirmed received, so the window slides to allow more segments to be sent.
• Window Advertisements. The window is announced by the receiver in the header of each TCP packet, along with the ACK. Every packet has an ACK number and a Window.
• Flow control. Sender cannot send beyond window. If the window size is zero, the sender must wait. an unsolicited ACK might follow eventually with a non-zero window to reopen the window.
• Piggy-back Ack: each TCP header contains, as well as Flags, an ACK and a Sequence Number. Packets can simultaneously acknowledge data sent towards the software with the ACK field and send data using the sequence number field. In fact, both these fields must always be valid.
• Reliability. Unacknowledged bytes will be resent when sender's ACK timer expires. Although only missing segments need be resent, in the case of multiple outstanding segments, sender knows only the lowest Sequence Numbered segment which must be resent. Other segment may be OK, but until the ACK comes in, sender can't be sure.
• RTT. The timer for the ACK is sent by time Round Trip Time of a packet, which is estimated dynamically.
• Karn's Algorithm. The RTT is estimated by measuring only packets which have an unambiguous round trip time, that is, not for retransimitted packets. In addition, the timeout value, which is a multiple of the RTT, is grown geometrically (backoff strategy) for which retransmitted packet. A successful packet resets the backoff.
• Variance. The timeout to RTT ratio is proportional variance.
• Multiplicative Decrease. Sender congestion avoidance technique. Inside the window, keep a congestion window, which halves for each lost packet.
• Slow-Start. Sender congestion avoidance technique. Open the congestion window by adding one segment (MTU) for each successful packet. Also do this for a new connection, starting with a congestion window of 1.
• Silly Window Syndrome. In a filled window, sender sends one addtional byte for each byte receiver consumes. Small packets waste bandwidth, and there is no down side to waiting.
• Receive Side Silly Window Avoidance. Do not open window unless you can advertise substantial space. Delay Acknowledgements until you can open the window substantially, if possible to wait this long.
• Nagle Algorithm. Send Side Silly Window Avoidance. To clump data, a self-clocking algorithm consisting of: if awaiting an acknowledgement, clump data until either the send buffer full or acknowledgement arrives.

Last Modified: Fri Mar 10 12:23:58 EST 2006