3.7 Data Communication

Once the connection is established data is communicated by the exchange of segments. Because segments may be lost due to errors (checksum test failure), or network congestion, TCP uses retransmission (after a timeout) to ensure delivery of every segment. Duplicate segments may arrive due to network or TCP retransmission. As discussed in the section on sequence numbers the TCP performs certain tests on the sequence and acknowledgment numbers in the segments to verify their acceptability.

The sender of data keeps track of the next sequence number to use in the variable SND.NXT. The receiver of data keeps track of the next sequence number to expect in the variable RCV.NXT. The sender of data keeps track of the oldest unacknowledged sequence number in the variable SND.UNA. If the data flow is momentarily idle and all data sent has been acknowledged then the three variables will be equal.

When the sender creates a segment and transmits it the sender advances SND.NXT. When the receiver accepts a segment it advances RCV.NXT and sends an acknowledgment. When the data sender receives an acknowledgment it advances SND.UNA. The extent to which the values of these variables differ is a measure of the delay in the communication. The amount by which the variables are advanced is the length of the data in the segment. Note that once in the ESTABLISHED state all segments must carry current acknowledgment information.

The CLOSE user call implies a push function, as does the FIN control flag in an incoming segment.

Retransmission Timeout

Because of the variability of the networks that compose an internetwork system and the wide range of uses of TCP connections the retransmission timeout must be dynamically determined. One procedure for determining a retransmission time out is given here as an illustration.

    An Example Retransmission Timeout Procedure

      Measure the elapsed time between sending a data octet with a
      particular sequence number and receiving an acknowledgment that
      covers that sequence number (segments sent do not have to match
      segments received).  This measured elapsed time is the Round Trip
      Time (RTT).  Next compute a Smoothed Round Trip Time (SRTT) as:

        SRTT = ( ALPHA * SRTT ) + ((1-ALPHA) * RTT)

      and based on this, compute the retransmission timeout (RTO) as:

        RTO = min[UBOUND,max[LBOUND,(BETA*SRTT)]]

      where UBOUND is an upper bound on the timeout (e.g., 1 minute),
      LBOUND is a lower bound on the timeout (e.g., 1 second), ALPHA is
      a smoothing factor (e.g., .8 to .9), and BETA is a delay variance
      factor (e.g., 1.3 to 2.0).

The Communication of Urgent Information

The objective of the TCP urgent mechanism is to allow the sending user to stimulate the receiving user to accept some urgent data and to permit the receiving TCP to indicate to the receiving user when all the currently known urgent data has been received by the user.

This mechanism permits a point in the data stream to be designated as the end of urgent information. Whenever this point is in advance of the receive sequence number (RCV.NXT) at the receiving TCP, that TCP must tell the user to go into "urgent mode"; when the receive sequence number catches up to the urgent pointer, the TCP must tell user to go into "normal mode". If the urgent pointer is updated while the user is in "urgent mode", the update will be invisible to the user.

The method employs a urgent field which is carried in all segments transmitted. The URG control flag indicates that the urgent field is meaningful and must be added to the segment sequence number to yield the urgent pointer. The absence of this flag indicates that there is no urgent data outstanding.

To send an urgent indication the user must also send at least one data octet. If the sending user also indicates a push, timely delivery of the urgent information to the destination process is enhanced.

Managing the Window

The window sent in each segment indicates the range of sequence numbers the sender of the window (the data receiver) is currently prepared to accept. There is an assumption that this is related to the currently available data buffer space available for this connection.

Indicating a large window encourages transmissions. If more data arrives than can be accepted, it will be discarded. This will result in excessive retransmissions, adding unnecessarily to the load on the network and the TCPs. Indicating a small window may restrict the transmission of data to the point of introducing a round trip delay between each new segment transmitted.

The mechanisms provided allow a TCP to advertise a large window and to subsequently advertise a much smaller window without having accepted that much data. This, so called "shrinking the window," is strongly discouraged. The robustness principle dictates that TCPs will not shrink the window themselves, but will be prepared for such behavior on the part of other TCPs.

The sending TCP must be prepared to accept from the user and send at least one octet of new data even if the send window is zero. The sending TCP must regularly retransmit to the receiving TCP even when the window is zero. Two minutes is recommended for the retransmission interval when the window is zero. This retransmission is essential to guarantee that when either TCP has a zero window the re-opening of the window will be reliably reported to the other.

When the receiving TCP has a zero window and a segment arrives it must still send an acknowledgment showing its next expected sequence number and current window (zero).

The sending TCP packages the data to be transmitted into segments which fit the current window, and may repackage segments on the retransmission queue. Such repackaging is not required, but may be helpful.

In a connection with a one-way data flow, the window information will be carried in acknowledgment segments that all have the same sequence number so there will be no way to reorder them if they arrive out of order. This is not a serious problem, but it will allow the window information to be on occasion temporarily based on old reports from the data receiver. A refinement to avoid this problem is to act on the window information from segments that carry the highest acknowledgment number (that is segments with acknowledgment number equal or greater than the highest previously received).

The window management procedure has significant influence on the communication performance. The following comments are suggestions to implementers.

    Window Management Suggestions

      Allocating a very small window causes data to be transmitted in
      many small segments when better performance is achieved using
      fewer large segments.

      One suggestion for avoiding small windows is for the receiver to
      defer updating a window until the additional allocation is at
      least X percent of the maximum allocation possible for the
      connection (where X might be 20 to 40).

      Another suggestion is for the sender to avoid sending small
      segments by waiting until the window is large enough before
      sending data.  If the the user signals a push function then the
      data must be sent even if it is a small segment.

      Note that the acknowledgments should not be delayed or unnecessary
      retransmissions will result.  One strategy would be to send an
      acknowledgment when a small segment arrives (with out updating the
      window information), and then to send another acknowledgment with
      new window information when the window is larger.

      The segment sent to probe a zero window may also begin a break up
      of transmitted data into smaller and smaller segments.  If a
      segment containing a single data octet sent to probe a zero window
      is accepted, it consumes one octet of the window now available.
      If the sending TCP simply sends as much as it can whenever the
      window is non zero, the transmitted data will be broken into
      alternating big and small segments.  As time goes on, occasional
      pauses in the receiver making window allocation available will
      result in breaking the big segments into a small and not quite so
      big pair. And after a while the data transmission will be in
      mostly small segments.

      The suggestion here is that the TCP implementations need to
      actively attempt to combine small window allocations into larger
      windows, since the mechanisms for managing the window tend to lead
      to many small windows in the simplest minded implementations.