Network Working Group C. Partridge
Request for Comments: 1151 BBN Systems and Technologies
Updates: RFC 908 R. Hinden
BBN Communications Corp.
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.
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.
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
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
"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
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
Security issues are not addressed in this memo.
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