Independent Submission J. Touch
Request for Comments: 9187 Independent Consultant
Category: Informational January 2022
ISSN: 2070-1721
Sequence Number Extension for Windowed Protocols
Abstract
Sliding window protocols use finite sequence numbers to determine
segment placement and order. These sequence number spaces wrap
around and are reused during the operation of such protocols. This
document describes a way to extend the size of these sequence numbers
at the endpoints to avoid the impact of that wrap and reuse without
transmitting additional information in the packet header. The
resulting extended sequence numbers can be used at the endpoints in
encryption and authentication algorithms to ensure input bit patterns
do not repeat over the lifetime of a connection.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9187.
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Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Background
3. Related Discussion
4. Using SNE in Protocol Design
5. Example Code
6. Validation Suite
7. Security Considerations
8. IANA Considerations
9. Informative References
Acknowledgments
Author's Address
1. Introduction
Protocols use sequence numbers to maintain ordering and, in sliding
window systems, to control the amount of outstanding unacknowledged
information. These sequence numbers are finite and thus commonly
wrap around during long connections, reusing past values.
It can be useful for protocols to keep track of this wrap around in a
separate counter, such that the sequence number and counter together
form an equivalent number space that need not wrap. This technique
was introduced as "Sequence Number Extension" in the TCP
Authentication Option (TCP-AO) [RFC5925]. The example provided there
was intended to introduce the concept, but the pseudocode provided is
not complete.
This document presents the formal requirements for Sequence Number
Extension (SNE), a code example, and a check sequence that can be
used to validate this and alternate implementations. Sequence
numbers are used in a variety of protocols to support loss detection,
reordering, flow control, and congestion control. Limitations in the
size of a sequence number protocol field can limit the ways in which
these capabilities can be supported.
Under certain conditions, it is possible for both endpoints of a
protocol to keep track of sequence number rollover and effectively
extend the sequence number space without requiring modification of
the sequence number field used within protocol messages. These
conditions assume that the received sequence numbers never vary by
more than half the size of the space of the field used in messages,
i.e., they never hop forward or backward by more than half that
space. This constraint is typical in sliding window protocols, such
as TCP. However, although both ends can track rollover
unambiguously, doing so can be surprisingly complex. This document
provides examples and test cases to simplify that process.
This document is intended for protocol designers who seek to use
larger sequence numbers at the endpoints without needing to extend
the sequence number field used in messages, such as for
authentication protocols, e.g., TCP-AO [RFC5925]. Use of extended
sequence numbers should be part of a protocol specification so that
both endpoints can ensure they comply with the requirements needed to
enable their use in both locations.
The remainder of this document describes how SNE can be supported and
provides the pseudocode to demonstrate how received messages can
unambiguously determine the appropriate extension value, as long as
the reordering is constrained. Section 2 provides background on the
concept. Section 3 discusses currently known uses of SNE. Section 4
discusses how SNE is used in protocol design and how it differs from
in-band use of sequence numbers. Section 5 provides a framework for
testing SNE implementations, including example code for the SNE
function, and Section 6 provides a sequence that can be used by that
code for validation. Section 7 concludes with a discussion of
security issues.
2. Background
Protocols use sequence numbers to maintain message order. The
transmitter typically increments them either once per message or by
the length of the message. The receiver uses them to reorder
messages and detect gaps due to inferred loss.
Sequence numbers are represented within those messages (e.g., in the
headers) as values of a finite, unsigned number space. This space is
typically represented in a fixed-length bit string, whose values
range from 0..(2^N)-1, inclusive.
The use of finite representations has repercussions on the use of
these values at both the transmitter and receiver. Without
additional constraints, when the number space "wraps around", it
would be impossible for the receiver to distinguish between the uses
of the same value.
As a consequence, additional constraints are required. Transmitters
are typically required to limit reuse until they can assume that
receivers would successfully differentiate the two uses of the same
value. The receiver always interprets values it sees based on the
assumption that successive values never differ by just under half the
number space. A receiver cannot detect an error in that sequence,
but it will incorrectly interpret numbers if reordering violates this
constraint.
The constraint requires that "forward" values advance the values by
less than half the sequence number space, ensuring that receivers
never experience a series of values that violate that rule.
We define a sequence space as follows:
* An unsigned integer within the range of 0..(2^N)-1, i.e., for N
bits.
* An operation that increments values in that space by K, where K <
2^(N-1), i.e., less than half the range. This operation is used
exclusively by the transmitter.
* An operation that compares two values in that space to determine
their order, e.g., where X < Y implies that X comes before Y.
We assume that both sides begin with the same initial value, which
can be anywhere in the space. That value is either assumed (e.g., 0)
before the protocol begins or coordinated before other messages are
exchanged (as with TCP Initial Sequence Numbers (ISNs) [RFC0793]).
It is assumed that the receiver always receives values that are
always within (2^N)-1, but the successive received values never jump
forward or backward by more than 2^(N-1)-1, i.e., just under half the
range.
No other operations are supported. The transmitter is not permitted
to "backup", such that values are always used in "increment" order.
The receiver cannot experience loss or gaps larger than 2^(N-1)-1,
which is typically enforced either by assumption or by explicit
endpoint coordination.
An SNE is a separate number space that can be combined with the
sequence number to create a larger number space that need not wrap
around during a connection.
On the transmit side, SNE is trivially accomplished by incrementing a
local counter once each time the sequence number increment "wraps"
around or by keeping a larger local sequence number whose least-
significant part is the message sequence number and most-significant
part can be considered the SNE. The transmitter typically does not
need to maintain an SNE except when used in local computations, such
as for Message Authentication Codes (MACs) in TCP-AO [RFC5925].
The goal of this document is to demonstrate that SNE can be
accomplished on the receiver side without transmitting additional
information in messages. It defines the stateful function
compute_sne() as follows:
SNE = compute_sne(seqno)
The compute_sne() function accepts the sequence number seen in a
received message and computes the corresponding SNE. The function
includes persistent local state that tracks the largest currently
received SNE and seqno combination. The concatenation of SNE and
seqno emulates the equivalent larger sequence number space that can
avoid wrap around.
Note that the function defined here is capable of receiving any
series of seqno values and computing their correct corresponding SNE,
as long as the series never "jumps" more than half the number space
"backward" from the largest value seen "forward".
3. Related Discussion
The DNS uses sequence numbers to determine when a Start of Authority
(SOA) serial number is more recent than a previous one, even
considering sequence space wrap [RFC1034][RFC1035]. The use of
wrapped sequence numbers for sliding windows in network protocols was
first described as a sequence number space [IEN74].
A more recent discussion describes this as "serial number arithmetic"
and defines a comparison operator it claimed was missing in IEN-74
[RFC1982]. That document defines two operations: addition
(presumably shifting the window forward) and comparison (defining the
order of two values). Addition is defined in that document as
limited to values within the range of 0..windowsize/2-1. Comparison
is defined in that document by a set of equations therein, but that
document does not provide a way for a receiver to compute the correct
equivalent SNE, especially including the potential for sequence
number reordering, as is demonstrated in this document.
4. Using SNE in Protocol Design
As noted in the introduction, message sequence numbers enable
reordering, loss detection, flow control, and congestion control.
They are also used to differentiate otherwise potentially identical
messages that might repeat as part of a sequence or stream.
The size of the sequence number field used within transferred
messages defines the ability of a protocol to tolerate reordering and
gaps, notably limited to half the space of that field. For example,
a field of 8 bits can reorder and detect losses of smaller than 2^7,
i.e., 127 messages. When used for these purposes -- reordering, loss
detection, flow control, and congestion control -- the size of the
field defines the limits of those capabilities.
Sequence numbers are also used to differentiate messages; when used
this way, they can be problematic if they repeat for otherwise
identical messages. Protocols using sequence numbers tolerate that
repetition because they are aware of the rollover of these sequence
number spaces at both endpoints. In some cases, it can be useful to
track this rollover and use the rollover count as an extension to the
sequence number, e.g., to differentiate authentication MACs. This
SNE is never transmitted in messages; the existing rules of sequence
numbers ensure both ends can keep track unambiguously -- both for new
messages and reordered messages.
The constraints required to use SNE have already been presented as
background in Section 2. The transmitter must never send messages
out of sequence beyond half the range of the sequence number field
used in messages. A receiver uses this assumption to interpret
whether received numbers are part of pre-wrap sequences or post-wrap
sequences. Note that a receiver cannot enforce or detect if the
transmitter has violated these assumptions on its own; it relies on
explicit coordination to ensure this property is maintained, such as
the exchange of acknowledgements.
SNEs are intended for use when it is helpful for both ends to
unambiguously determine whether the sequence number in a message has
wrapped and whether a received message is pre-wrap or post-wrap for
each such wrap. This can be used by both endpoints to ensure all
messages of arbitrarily long sequences can be differentiated, e.g.,
ensuring unique MACs.
SNE does not extend the actual sequence space of a protocol or (thus)
its tolerance to reordering or gaps. It also cannot improve its
dynamic range for flow control or congestion control, although there
are other somewhat related methods that can, such as window scaling
[RFC7323] (which increases range at the expense of granularity).
SNE is not needed if messages are already unique over the entirety of
a transfer sequence, e.g., either because the sequence number field
used in its messages never wrap around or because other fields
provide that disambiguation, such as timestamps.
5. Example Code
The following C code is provided as a verified example of SNE from 16
to 32 bits. The code includes both the framework used for validation
and the compute_sne() function, the latter of which can be used
operationally.
A correct test will indicate "OK" for each test. An incorrect test
will indicate "ERROR" where applicable.
<CODE BEGINS> file "compute_sne.c"
#include <stdio.h>
#include <sys/param.h>
#define distance(x,y) (((x)<(y))?((y)-(x)):((x)-(y)))
#define SNEDEBUG 1
// This is the core code, stand-alone, to compute SNE from seqno
// >> replace this function with your own code to test alternates
unsigned long compute_sne(unsigned long seqno) {
// INPUT: 32-bit unsigned sequence number (low bits)
// OUTPUT: 32-bit unsigned SNE (high bits)
// variables used in this code example to compute SNE:
static unsigned long
RCV_SNE = 0; // high-watermark SNE
static int
RCV_SNE_FLAG = 1; // set during first half rollover
// (prevents re-rollover)
static unsigned long
RCV_PREV_SEQ = 0; // high-watermark SEQ
unsigned long
holdSNE; // temp copy of output
holdSNE = RCV_SNE; // use current SNE to start
if (distance(seqno,RCV_PREV_SEQ) < 0x80000000) {
// both in same SNE range?
if ((seqno >= 0x80000000) && (RCV_PREV_SEQ < 0x80000000)) {
// jumps fwd over N/2?
RCV_SNE_FLAG = 0; // reset wrap increment flag
}
RCV_PREV_SEQ = MAX(seqno,RCV_PREV_SEQ);
// move prev forward if needed
} else {
// both in diff SNE ranges
if (seqno < 0x80000000) {
// jumps forward over zero?
RCV_PREV_SEQ = seqno; // update prev
if (RCV_SNE_FLAG == 0) {
// first jump over zero? (wrap)
RCV_SNE_FLAG = 1;
// set flag so we increment once
RCV_SNE = RCV_SNE + 1;
// increment window
holdSNE = RCV_SNE;
// use updated SNE value
}
} else {
// jump backward over zero
holdSNE = RCV_SNE - 1;
// use pre-rollover SNE value
}
}
#ifdef SNEDEBUG
fprintf(stderr,"state RCV_SNE_FLAG = %1d\n",
RCV_SNE_FLAG);
fprintf(stderr,"state RCV_SNE = %08lx\n", RCV_SNE);
fprintf(stderr,"state RCV_PREV_SEQ = %08lx\n", RCV_PREV_SEQ);
#endif
return holdSNE;
}
int main() {
// variables used as input and output:
unsigned long SEG_SEQ; // input - received SEQ
unsigned long SNE; // output - SNE corresponding
// to received SEQ
// variables used to validate the computed SNE:
unsigned long SEG_HIGH; // input - xmitter side SNE
// -> SNE should match this value
unsigned long long BIG_PREV; // prev 64-bit total seqno
unsigned long long BIG_THIS = 0; // current 64-bit total seqno
// -> THIS, PREV should never jump
// more than half the SEQ space
char *prompt = "Input hex numbers only (0x is optional)\n\n")
"\tHex input\n"
"\t(2 hex numbers separated by whitespace,"
"each with 8 or fewer digits)";
fprintf(stderr,"%s\n",prompt);
while (scanf("%lx %lx",&SEG_HIGH,&SEG_SEQ) == 2) {
BIG_PREV = BIG_THIS;
BIG_THIS = (((unsigned long long)SEG_HIGH) << 32)
| ((unsigned long long)SEG_SEQ);
// given SEG_SEQ, compute SNE
SNE = compute_sne(SEG_SEQ);
fprintf(stderr," SEG_SEQ = %08lx\n", SEG_SEQ);
fprintf(stderr," SNE = %08lx\n", SNE);
fprintf(stderr," SEG_HIGH = %08lx %s\n",SEG_HIGH,
(SEG_HIGH == SNE)? " - OK" : " - ERROR !!!!!!!");
fprintf(stderr,"\t\tthe jump was %16llx %s %s\n",
distance(BIG_PREV,BIG_THIS),
((BIG_PREV < BIG_THIS)?"+":"-"),
(((distance(BIG_PREV,BIG_THIS)) > 0x7FFFFFFF)
? "ILLEGAL JUMP" : "."));
fprintf(stderr,"\n");
fprintf(stderr,"\n");
fprintf(stderr,"%s\n",prompt);
}
}
<CODE ENDS>
6. Validation Suite
The following numbers are used to validate SNE variants and are shown
in the order they legitimately could be received. Each line
represents a single 64-bit number, represented as two hexadecimal
32-bit numbers with a space between. The numbers are formatted for
use in the example code provided in Section 5.
A correctly operating extended sequence number system can receive the
least-significant half (the right side) and compute the correct most-
significant half (the left side) correctly. It specifically tests
both forward and backward jumps in received values that represent
legitimate reordering.
00000000 00000000
00000000 30000000
00000000 90000000
00000000 70000000
00000000 a0000000
00000001 00000001
00000000 e0000000
00000001 00000000
00000001 7fffffff
00000001 00000000
00000001 50000000
00000001 80000000
00000001 00000001
00000001 40000000
00000001 90000000
00000001 b0000000
00000002 0fffffff
00000002 20000000
00000002 90000000
00000002 70000000
00000002 A0000000
00000003 00004000
00000002 D0000000
00000003 20000000
00000003 90000000
00000003 70000000
00000003 A0000000
00000004 00004000
00000003 D0000000
7. Security Considerations
Sequence numbers and their extensions can be useful in a variety of
security contexts. Because the extension part (most-significant
half) is determined by the previously exchanged sequence values
(least-significant half), the extension should not be considered as
adding entropy for the purposes of message authentication or
encryption.
8. IANA Considerations
This document has no IANA actions.
9. Informative References
[IEN74] Plummmer, W., "Sequence Number Arithmetic", IEN-74,
September 1978.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
DOI 10.17487/RFC1982, August 1996,
<https://www.rfc-editor.org/info/rfc1982>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC7323] Borman, D., Braden, B., Jacobson, V., and R.
Scheffenegger, Ed., "TCP Extensions for High Performance",
RFC 7323, DOI 10.17487/RFC7323, September 2014,
<https://www.rfc-editor.org/info/rfc7323>.
Acknowledgments
The need for this document was first noted by Juhamatti Kuusisaari in
April 2020 during discussions of the pseudocode in RFC 5925.
Author's Address
Joe Touch
Manhattan Beach, CA 90266
United States of America
Phone: +1 (310) 560-0334
Email: touch@strayalpha.com
