Description of TCP and UDP Packet Headers

Posted on 10:45 AM by Bharathvn

Interviewers love to throw this question in to see how well you know the fields in the TCP and UDP headers. RFC 791 defines the standard TCP header. RFC 768 defines the standard UDP header. The TCP header has a minimum size of 20 bytes and a maximum of 60 bytes, and it uses the IP protocol number of 6. The UDP header has a static size of 8 bytes and uses the IP protocol number of 17.

Following is a list of the fields in the TCP header (shown in Figure 2-7):

❑ Source Port (16 bits): Defines the source port of the connection.
❑ Destination Port (16 bits): Defines the destination port of the connection.
❑ Sequence Number (32 bits): Identifies the byte count relative to the initial sequence number established in the first SYN packet during the three-way handshake.
❑ Acknowledgment Number (32 bits): Contains the last sequence number of the current transmission as well as those received correctly by the host.
❑ Data Offset (4 bits): Identifies the number of 32-bit words in the header portion of the packet.
❑ Reserved (6 bits): As the name implies, this field is not currently used and is reserved for future use.
❑ Flags (6 bits): These are a series of 1-bit fields that are controls for the connection, as follows:

Urgent (URG); Acknowledgment (ACK); Push (PSH); Reset (RST); Synchronize (SYN); and
Finish (FIN).

❑ Window (16 bits): This field defines how many packets can be sent before an acknowledgment is received. This is critical to the performance of a connection, and the window size is adjusted accordingly.
❑ Checksum (16 bits): This field is a simple ones-complement function of a pseudo-header, the TCP header, and data that is computed by the client and recomputed by the server for integrity. If these do not match, a packet retransmission occurs. The pseudo header is the source and destination IP addresses and the TCP header.
❑ Urgent Pointer (16 bits): This field is the byte location of any urgent data that signals to the destination host to process immediately as opposed to buffering.
❑ Options and Padding (32 bits): These fields are used to pad the length of the TCP header so that it results in a multiple of 32 bits.
❑ Data: This is the data passed from the application layer (of the TCP/IP model).


As stated earlier in the chapter, UDP is used by applications that are not concerned with reliability or packet loss. These applications just need to transmit quickly and with as little overhead as possible. The UDP header has four 16-bit fields (see Figure 2-8), making the total overhead 8 bytes. Similarly to TCP, the Source Port and Destination Port fields define the ports to be used. The Length field is the length of the entire packet (header and data). As is the TCP checksum, the UDP checksum field is a onescomplement of a pseudo header, the UDP header and the data. One important note is that the UDP checksum is optional and can be set to 0.