TCP Segment Structure Flashcards
What is the sequence number?
All bytes in a TCP connection are numbered, beginning at a randomly chosen initial sequence number (ISN). The SYN packets consume one sequence number, so actual data will begin at ISN+1. The sequence number is the byte number of the first byte of data in the TCP packet sent (also called a TCP segment).
What is the acknowledgement number?
The acknowledgement number is the sequence number of the next byte the receiver expects to receive after the sequence number.
Where are the sequence and acknowledgement number fields used?
The 32-bit sequence number field and the 32-bit acknowledgment number field are used by the TCP sender and receiver in implementing a reliable data transfer service, as discussed below. (counting
by bytes of data (not segments!))
What is the receive window used for?
The 16-bit receive window field is used for flow control. We will see shortly that it is used to indicate the number of bytes that a receiver is willing to accept. (# bytes rcvr willing to accept)
What is the header length field used for?
The 4-bit header length field specifies the length of the TCP header in 32-bit words. The TCP header can be of variable length due to the TCP options field. (Typically, the options field is empty, so that the length of the typical TCP header is 20 bytes.)
What is the options field for?
The optional and variable-length options field is used when a sender and receiver negotiate the maximum segment size (MSS) or as a window scaling factor for use in high-speed networks. A time-
stamping option is also defined. See RFC 854 and RFC 1323 for additional details.
What is the flag field for?
-contains 6 bits
- ACK bit is used to indicate that the value carried in the acknowledgment field is valid(segment contains an acknowledgment for a segment that
has been successfully received)
- RST, SYN, FIN bits are used for connection and setup
- CWR, ECE bits are used in congestion control
- PSH bit indicates that the receiver should pass the data to the upper layer immediately
- URG bit is used to indicate that there is data in this segment that the
sending-side upper-layer entity has marked as “urgent.” The location of the last byte of this urgent data is indicated by the 16-bit urgent data pointer field.
