Network Working Group                                      C. Partridge
Request for Comments: 1151                 BBN Systems and Technologies
Updates: RFC 908                                              R. Hinden
                                               BBN Communications Corp.
                                                             April 1990


             Version 2 of the Reliable Data Protocol (RDP)

Status of this Memo

   This RFC suggests several updates to the specification of the
   Reliable Data Protocol (RDP) in RFC-908 based on experience with the
   protocol.  This revised version of the protocol is experimental.

   Distribution of this memo is unlimited.

Introduction

   Experiments in 1986 and 1987 turned up some ambiguities and problems
   with the RDP specification.  At the time, it was hoped that the
   authors might find the time to revise the entire RDP specification to
   fix these problems, however given the limited demand for RDP
   implementations, the authors were never able to justify the time
   involved in revising the spec.  This document lists the changes that
   we believe are appropriate to make to RDP version 1.

   Readers are expected to be familiar with RFC-908.

Changes To The Protocol Header

   There are three changes to the protocol header: the checksum
   algorithm has been changed, the port size increased, and the version
   number incremented.  The new header format is shown in Figure 1.

   The major discovery during the testing of the protocol is that cost
   of computing the the RDP checksum proved surprisingly variable; its
   performance was more heavily affected by the host's data
   representation than anticipated.  Optimized checksum implementations
   on two comparable hardware bases gave performance that differed by a
   factor of five.  Since the speed of the checksum is a key factor in
   the performance of the protocol itself, this variation caused a
   noticeable difference in throughput.

   The wide variation in performance on comparable machines was felt to
   be undesirable, so the checksum has been changed.  RDP now uses the
   16-bit TCP checksum, which is specified on page 16 of RFC-793.




Partridge & Hinden                                              [Page 1]

RFC 1151                    RDP - Version 2                   April 1990


   The 8-bit port size is probably too small to support a large range of
   applications.  Accordingly, the port size is now 16-bits.  Port
   numbers less than 1024 are reserved for well-defined applications.
   Allocable ports begin at port number 1024.

   Finally, because the checksum and port size have been changed, the
   version number has been increased to 2.


                   0             0 0   1         1
                   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                  +-+-+-+-+-+-+---+---------------+
                  |S|A|E|R|N| |Ver|    Header     |
                0 |Y|C|A|S|U|0|No.|    Length     |
                  |N|K|K|T|L| |   |               |
                  +-+-+-+-+-+-+---+---------------+
                1 |         Source Port           |
                  +---------------+---------------+
                2 |       Destination Port        |
                  +---------------+---------------+
                3 |          Data  Length         |
                  +---------------+---------------+
                4 |                               |
                  +---    Sequence Number      ---+
                5 |                               |
                  +---------------+---------------+
                6 |                               |
                  +--- Acknowledgement Number  ---+
                7 |                               |
                  +---------------+---------------+
                8 |           Checksum            |
                  +---------------+---------------+
                9 |     Variable Header Area      |
                  .                               .
                  .                               .
                  |                               |
                  +---------------+---------------+

                         RDP Header Format
                             Figure 1

Minor Errors and Ambiguities

   Some ambiguities and minor errors have been found in RFC-908.  They
   are corrected in this section.

   The value of the state variable, SND.UNA is treated inconsistently in
   the pseudo-code on pages 21-29.  On page 12, SND.UNA is defined as



Partridge & Hinden                                              [Page 2]

RFC 1151                    RDP - Version 2                   April 1990


   "the sequence number of the oldest unacknowledged segment", and on
   page 21 it is appropriately set to the initial sequence number when
   the connection is opened.  But on page 28, when an acknowledgement is
   received, SND.UNA is set to SEG.ACK, the sequence number being
   acknowledged, instead of SEG.ACK+1.  A similar inconsistency occurs
   on page 26.  The proper fix is to always set SND.UNA to SEG.ACK+1.

   The pseudo-code on page 25 for the SYN-SENT state is incorrect.  The
   first few lines cause all packets with the ACK bit set to be
   discarded, but later lines test the ACK bit.  The test for the ACK
   bit should be placed after all the other tests.  Also note that if
   the ACK bit is set, a RST segment is sent to reset the remote peer,
   but the local peer is left half-open.  There is a similar problem in
   the SYN-RCVD state.  The local peer should deallocate the connection
   record and close.

   On page 24, the pseudo-code indicates that if non-data packets are
   received in the CLOSED state, a RST segment with SEG.ACK set to
   RCV.CUR should be sent.  RCV.CUR is not defined in the CLOSED state.
   SEG.ACK should be set to SEG.SEG.

   There is some inconsistency about how to handle a RST packet in the
   CLOSE-WAIT state.  On page 24, the pseudo-code shows that a RST
   should cause the connection state to become CLOSED.  Text on page 13
   and the state diagram on page 10 suggest the connection state should
   stay in CLOSE-WAIT.  The implementation should stay in CLOSE-WAIT.

   On page 29, the pseudo-code for the OPEN state suggests that if a
   data packet is received in sequence, the acknowledgement packet
   should not contain EACKs.  This is misleading.  Implementations may
   include EACKs in the acknowledgement.

   On page 18, it is possible to interpret the right edge as being
   either inside or outside the window.  This results in a one segment
   difference in the window size.  The proper interpretation is that the
   right edge is outside the window.  In other words, the right edge is
   the first sequence number that cannot be sent or received and the
   total window size is 2*X, where X is the maximum number of
   outstanding segments.

   One final problem is that RDP's flow control scheme does not allow
   the receiver to close the sender's window.  As a result, if the
   receiver acknowledges segments when they are received the sender can
   easily send more data than the receiver is prepared to buffer.  A
   solution to this problem (suggested by members of the End-2-End
   Research Group) is to only acknowledge a segment after it has been
   delivered to the application.  This scheme, however, has not be
   tested.



Partridge & Hinden                                              [Page 3]

RFC 1151                    RDP - Version 2                   April 1990


Security Considerations

   Security issues are not addressed in this memo.

Authors' Addresses

   Craig Partridge
   Bolt Beranek and Newman Inc.
   50 Moulton Street
   Cambridge, MA 02138

   Phone: (617) 873-2459

   EMail: craig@BBN.COM


   Robert Hinden
   Bolt Beranek and Newman Inc.
   50 Moulton Street
   Cambridge, MA 02238

   Phone: (617) 873-3757

   Email: HINDEN@BBN.COM



























Partridge & Hinden                                              [Page 4]