CS402: Computer Communications and Networks

The original TCP data transfer mechanisms were defined in RFC 793. Based on the experience of using TCP on the growing global Internet, this part of the TCP specification has been updated and improved several times, always while preserving the backward compatibility with older TCP implementations. In this section, we review the main data transfer mechanisms used by TCP.

TCP is a window-based transport protocol that provides a bi-directional byte stream service. This has several implications on the fields of the TCP header and the mechanisms used by TCP. The three fields of the TCP header are:

• sequence number. TCP uses a 32 bits sequence number. The sequence number placed in the header of a TCP segment containing data is the sequence number of the first byte of the payload of the TCP segment.
• acknowledgment number. TCP uses cumulative positive acknowledgments. Each TCP segment contains the sequence number of the next byte that the sender of the acknowledgment expects to receive from the remote host. In theory, the acknowledgment number is only valid if the ACK flag of the TCP header is set. In practice, almost all 15 TCP segments have their ACK flag set.
• window. a TCP receiver uses this 16 bits field to indicate the current size of its receive window expressed in bytes.

Note: The Transmission Control Block

For each established TCP connection, a TCP implementation must maintain a Transmission Control Block (TCB). A TCB contains all the information required to send and receive segments on this connection RFC 793. This includes 16:

• the local IP address
• the remote IP address
• the local TCP port number
• the remote TCP port number
• the current state of the TCP FSM
• the maximum segment size (MSS)
• snd.nxt: the sequence number of the next byte in the byte stream (the first byte of a new data segment that you send uses this sequence number)
• snd.una: the earliest sequence number that has been sent but has not yet been acknowledged
• snd.wnd: the current size of the sending window (in bytes)
• rcv.nxt: the sequence number of the next byte that is expected to be received from the remote host
• rcv.wnd: the current size of the receive window advertised by the remote host
• sending buffer: a buffer used to store all unacknowledged data
• receiving buffer: a buffer to store all data received from the remote host that has not yet been delivered to the user. Data may be stored in the receiving buffer because either it was not received in sequence or because the user is too slow to process it

The original TCP specification can be categorized as a transport protocol that provides a byte stream service and uses go-back-n.

To send new data on an established connection, a TCP entity performs the following operations on the corresponding- ing TCB. It first checks that the sending buffer does not contain more data than the receive window advertised by the remote host (rcv.wnd). If the window is not full, up to MSS bytes of data are placed in the payload of a TCP segment. The sequence number of this segment is the sequence number of the first byte of the payload. It is set to the first available sequence number: snd.nxt and snd.nxt is incremented by the length of the payload of the TCP segment. The acknowledgment number of this segment is set to the current value of rcv.nxt and the window field of the TCP segment is computed based on the current occupancy of the receiving buffer. The data is kept in the sending buffer in case it needs to be retransmitted later.

When a TCP segment with the ACK flag set is received, the following operations are performed. rcv.wnd is set to the value of the window field of the received segment. The acknowledgment number is compared to snd.una. The newly acknowledged data is remove from the sending buffer and snd.una is updated. If the TCP segment contained data, the sequence number is compared to rcv.nxt. If they are equal, the segment was received in sequence and the data can be delivered to the user and rcv.nxt is updated. The contents of the receiving buffer is checked to see whether other data already present in this buffer can be delivered in sequence to the user. If so, rcv.nxt is updated again. Otherwise, the segment’s payload is placed in the receiving buffer.

Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
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