<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080><BR></FONT>

</TD>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Optional: ULP timeout, timeout action, precedence, security, options

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ACTIVE-OPEN-WITH-DATA

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Source port, destination socket, data, data length, push flag, urgent flag

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080><BR></FONT>

</TD>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Optional: ULP timeout, timeout action, precedence, security

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ALLOCATE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, data length

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CLOSE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>FULL-PASSIVE-OPEN

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local port, destination socket

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080><BR></FONT>

</TD>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Optional: ULP timeout, timeout action, precedence, security, options

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>RECEIVE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, buffer address, byte count, push flag, urgent flag

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SEND

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, buffer address, data length, push flag, urgent flag

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080><BR></FONT>

</TD>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Optional: ULP timeout, timeout action

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>STATUS

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>UNSPECIFIED-PASSIVE-OPEN

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local port

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080><BR></FONT>

</TD>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Optional: ULP timeout, timeout action, precedence, security, options

<BR>

</FONT>

<TR>

<TD COLSPAN=2 ALIGN=center  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P ALIGN=CENTER>

<CENTER><B>TCP to ULP Service Request Primitives</B></CENTER>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CLOSING

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>DELIVER

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, buffer address, data length, urgent flag

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ERROR

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, error description

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>OPEN-FAILURE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>OPEN-ID

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, remote socket, destination address

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>OPEN-SUCCESS

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>STATUS RESPONSE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, source port, source address, remote socket, connection state, receive window, send window, amount waiting ACK, amount waiting receipt, urgent mode, precedence, security, timeout, timeout action

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>TERMINATE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name, description</FONT>

</TABLE></CENTER><BR>

<BR>

<A ID="E68E39" NAME="E68E39"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>Passive and Active Ports</B></FONT></CENTER></H3>

<BR>

<P>TCP enables two methods to establish a connection: active and passive. An active connection establishment happens when TCP issues a request for the connection, based on an instruction from an upper-level protocol that provides the socket number. A passive approach takes place when the upper-level protocol instructs TCP to wait for the arrival of connection requests from a remote system (usually from an active open instruction). When TCP receives the request, it assigns a port number. This enables a connection to proceed rapidly, without waiting for the active process.

<BR>

<P>There are two passive open primitives. A specified passive open creates a connection when the precedence level and security level are acceptable. An unspecified passive open opens the port to any request. The latter is used by servers that are waiting for clients of an unknown type to connect to them.

<BR>

<P>TCP has strict rules about the use of passive and active connection processes. Usually a passive open is performed on one machine, while an active open is performed on the other, with specific information about the socket number, precedence (priority), and security levels.

<BR>

<P>Although most TCP connections are established by an active request to a passive port, it is possible to open a connection without a passive port waiting. In this case, the TCP that sends a request for a connection includes both the local socket number and the remote socket number. If the receiving TCP is configured to enable the request (based on the precedence and security settings, as well as application-based criteria), the connection can be opened. This process is looked at again in the section titled &quot;TCP and Connections.&quot;

<BR>

<BR>

<A ID="E68E40" NAME="E68E40"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>TCP Timers</B></FONT></CENTER></H3>

<BR>

<P>TCP uses several timers to ensure that excessive delays are not encountered during communications. Several of these timers are elegant, handling problems that are not immediately obvious at first analysis. The timers used by TCP are examined in the following sections, which reveal their roles in ensuring that data is properly sent from one connection to another.

<BR>

<BR>

<A ID="E69E64" NAME="E69E64"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>The Retransmission Timer</B></FONT></CENTER></H4>

<BR>

<P>The retransmission timer manages retransmission timeouts (RTOs), which occur when a preset interval between the sending of a datagram and the returning acknowledgment is exceeded. The value of the timeout tends to vary, depending on the network type, to compensate for speed differences. If the timer expires, the datagram is retransmitted with an adjusted RTO, which is usually increased exponentially to a maximum preset limit. If the maximum limit is exceeded, connection failure is assumed, and error messages are passed back to the upper-layer application.

<BR>

<P>Values for the timeout are determined by measuring the average time that data takes to be transmitted to another machine and the acknowledgment received back, which is called the round-trip time, or RTT. From experiments, these RTTs are averaged by a formula that develops an expected value, called the smoothed round-trip time, or SRTT. This value is then increased to account for unforeseen delays.

<BR>

<BR>

<A ID="E69E65" NAME="E69E65"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>The Quiet Timer</B></FONT></CENTER></H4>

<BR>

<P>After a TCP connection is closed, it is possible for datagrams that are still making their way through the network to attempt to access the closed port. The quiet timer is intended to prevent the just-closed port from reopening again quickly and receiving these last datagrams.

<BR>

<P>The quiet timer is usually set to twice the maximum segment lifetime (the same value as the Time to Live field in an IP header), ensuring that all segments still heading for the port have been discarded. Typically, this can result in a port being unavailable for up to 30 seconds, prompting error messages when other applications attempt to access the port during this interval.

<BR>

<BR>

<A ID="E69E66" NAME="E69E66"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>The Persistence Timer</B></FONT></CENTER></H4>

<BR>

<P>The persistence timer handles a fairly rare occurrence. It is conceivable that a receive window might have a value of 0, causing the sending machine to pause transmission. The message to restart sending might be lost, causing an infinite delay. The persistence timer waits a preset time and then sends a one-byte segment at predetermined intervals to ensure that the receiving machine is still clogged.

<BR>

<P>The receiving machine resends the zero window-size message after receiving one of these status segments, if it is still backlogged. If the window is open, a message giving the new value is returned, and communications are resumed.

<BR>

<BR>

<A ID="E69E67" NAME="E69E67"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>The Keep-Alive Timer and the Idle Timer</B></FONT></CENTER></H4>

<BR>

<P>Both the keep-alive timer and the idle timer were added to the TCP specifications after their original definition. The keep-alive timer sends an empty packet at regular intervals to ensure that the connection to the other machine is still active. If no response has been received after sending the message by the time the idle timer has expired, the connection is assumed to be broken.

<BR>

<P>The keep-alive timer value is usually set by an application, with values ranging from 5 to 45 seconds. The idle timer is usually set to 360 seconds.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>TCP uses adaptive timer algorithms to accommodate delays. The timers adjust themselves to the delays experienced over a connection, altering the timer values to reflect inherent problems.</NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<BR>

<A ID="E68E41" NAME="E68E41"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>Transmission Control Blocks and Flow Control</B></FONT></CENTER></H3>

<BR>

<P>TCP has to keep track of a lot of information about each connection. It does this through a Transmission Control Block (TCB), which contains information about the local and remote socket numbers, the send and receive buffers, security and priority values, and the current segment in the queue. The TCB also manages send and receive sequence numbers.

<BR>

<P>The TCB uses several variables to keep track of the send and receive status and to control the flow of information. These variables are shown in Table 4.3.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 4.3. TCP send and receive variables.</B></FONT></CENTER>

<BR>



<CENTER><TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 CELLPADDING=3 >

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Variable Name</I></B>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Description</I></B>

</FONT>

<TR>

<TD COLSPAN=2 ALIGN=center  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P ALIGN=CENTER>

<CENTER><B><I>Send Variables</I></B></CENTER>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SND.UNA

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Send Unacknowledged

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SND.NXT

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Send Next

<BR>

</FONT>

<TR>