Graphics Programs Reference
In-Depth Information
The two major protocols at this layer are the Transmission Control
Protocol (TCP) and User Datagram Protocol (UDP). TCP is the most
commonly used protocol for services on the Internet: telnet, HTTP (web
traffic), SMTP (email traffic), and FTP (file transfers) all use TCP. One of
the reasons for TCP's popularity is that it provides a transparent, yet reliable
and bidirectional, connection between two IP addresses. Stream sockets use
TCP/IP connections. A bidirectional connection with TCP is similar to using
a telephone—after dialing a number, a connection is made through which
both parties can communicate. Reliability simply means that TCP will ensure
that all the data will reach its destination in the proper order. If the packets
of a connection get jumbled up and arrive out of order, TCP will make sure
they're put back in order before handing the data up to the next layer. If
some packets in the middle of a connection are lost, the destination will hold
on to the packets it has while the source retransmits the missing packets.
All of this functionality is made possible by a set of flags, called
TCP flags
,
and by tracking values called
sequence numbers
. The TCP flags are as follows:
TCP flag
Meaning
Purpose
URG
Urgent
Identifies important data
ACK
Acknowledgment
Acknowledges a packet; it is turned on for the majority of the
connection
PSH
Push
Tells the receiver to push the data through instead of buffering it
RST
Reset
Resets a connection
SYN
Synchronize
Synchronizes sequence numbers at the beginning of a connection
FIN
Finish
Gracefully closes a connection when both sides say goodbye
These flags are stored in the TCP header along with the source and
destination ports. The TCP header is specified in RFC 793.
From RFC 793
[Page 14]
September 1981
Transmission Control Protocol
3. FUNCTIONAL SPECIFICATION
3.1. Header Format
TCP segments are sent as internet datagrams. The Internet Protocol
header carries several information fields, including the source and
destination host addresses [2]. A TCP header follows the internet
header, supplying information specific to the TCP protocol. This
division allows for the existence of host level protocols other than
TCP.
TCP Header Format
