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Version 0.0.0 @ 03:55/08.07.2000
Decomposition of a TCP Packet
The above is an example of an TCP Header. Rows 1 through 5 are commonly used, and when options are included in the header, then rows 1 through 6 or more depending upon how many 32-bit option words are used by special options. The payload contained is the encapsulated data from the upper layer. For example, a Synthesis request from a client to a server to create a connection and session.
Description of fields listed in above TCP header starting from left to right, top to bottom.
- Source Port [SP] (16 bits): When a connection is attempted, or being conducted, this specifies what port the local machine is waiting to listen for responses from the destination machine.
- Destination Port [DP] (16 bits): When a user desires to connect up to a service on a remote machine, the Application Layer program specifies what port initial connections should use. When not as part of an initial connection, this specifies what port number is going to be used for the remote machine as a packet is being sent out to its destination.
- Sequence Number [SN] (32 bits): In a sliding window protocol like TCP, the sequence number allows both TCP stacks to know what packets have been received and which ones have not. Say for instance I get mail messages 1,2,3,5,6,7,8,9, and 10 from you when I know you are sending 10 messages. If you numbered each of your messages, I can look through and see that I do not have message number 4, and I can tell you to send me another copy of that. The sequence number works very much like this, as well as to allow for a little security so that other users cannot easily break into the middle of your connection and continue where you left off.
Predictable sequence numbers for certain implementations of TCP/IP stacks has led to some security problems with attacks geared to anything from blind Denial of Service, to taking over connections in use by other users that have authenticated themselves. This discussion is beyond the scope of this paper, but plenty of other talented coders and researchers have published a great deal of work on this topic.
Once there is a communication session over TCP set up between two machines, the sequence number increases are directly proportional to the number of data bytes that are transmitted.
- Acknowledgment Number [TL] (32 bits): This works by acknowledging the sequence number as sent by the remote host. The local host's Acknowledgement Number is a reference to the remote machine's Sequence number, and the local machine's sequence number is related to the remote machine's acknowledgement number.
- Header Length [HL] (4 bits): Just as the TCP Header Length, this one also is a measure of the length of the header in 32-bit sized words.
- Reserved (6 bits): This is reserved but I was not able to find out the specific purposes for its reservation. It could be a to aid in the identification of a packet when other things are mangled, but this is just theory.
- Urgent flag [URG] (1 bit): This specifies that the Urgent point included in this packet is valid.
- Acknowledgement flag [ACK] (1 bit): This specifies that the portion of the header that has the acknowledgement number is is valid.
- Push flag [PSH] (1 bit): This tells the TCP/IP stack that this should be pushed up to the Application Layer program that needs, or requires it as soon as time allows.
- Reset flag [RST] (1 bit): This is used to reset the connection.
- Synthesis flag [SYN] (1 bit): This is to Synchronize sequence numbers with acknowledgement numbers for both hosts, sometimes referred to synthesis of a connection. The former reference is more common, while the latter is more rare.
- Finish Flag [FIN] (1 bit): This is to specify that a connection is finished according to the side that sent the packet with the FIN flag set.
- Window size [WS] (16 bits): This specifies how many bytes may be received on the receiving side before being halted from sliding any further and receiving any more bytes as a result of a packet at the beginning of the sliding window not having been acknowledged or received.
- TCP Checksum [TCPCS] (16 bits):This is a checksum that covers the header and data portion of a TCP packet to allow the receiving host to verify the integrity of an incoming TCP packet.
- Urgent Pointer [UP] (16 bits): this allows for a section of data as specified by the Urgent pointer to be passed up by the receiving host quickly.
- Options (Variable bits if used): Seldom ever used.
- Data (Variable bits): As you might expect, this is the payload, or data portion of an TCP packet. The payload may be any number of application layer protocols. The most common are HTTP, Telnet, SSH, FTP, but other popular protocols also use TCP.
This ends the brief preview of an TCP packet.
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