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Network Working Group                                      C. PartridgeRequest for Comments: 1151                 BBN Systems and TechnologiesUpdates: RFC908                                              R. Hinden                                               BBN Communications Corp.                                                             April 1990Version 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) inRFC-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 withRFC-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 ofRFC-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 1Minor Errors and Ambiguities   Some ambiguities and minor errors have been found inRFC-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 asPartridge & 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 1990Security 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.COMPartridge & Hinden                                              [Page 4]