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Sequence Number Extension for Windowed Protocols

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9187.
Author Dr. Joseph D. Touch
Last updated 2022-02-07 (Latest revision 2021-11-24)
RFC stream Independent Submission
Intended RFC status Informational
IETF conflict review conflict-review-touch-sne
Stream ISE state Published RFC
Consensus boilerplate Unknown
Document shepherd Eliot Lear
Shepherd write-up Show Last changed 2021-11-24
IESG IESG state Became RFC 9187 (Informational)
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ISE Stream                                                     J. Touch
Internet Draft                                   Independent consultant
Intended status: Informational                        November 24, 2021
Expires: May 2022

             Sequence Number Extension for Windowed Protocols


   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.

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79. This document may not be modified,
   and derivative works of it may not be created, except to format it
   for publication as an RFC or to translate it into languages other
   than English.

   Internet-Drafts are working documents of the Internet Engineering
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   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on May 24, 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document. Code Components extracted from this
   document must include Simplified BSD License text as described in
   Section 4.e of the Trust Legal Provisions and are provided without
   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................2
   2. Background.....................................................4
   3. Related Discussion.............................................5
   4. Using SNE in Protocol Design...................................6
   5. Example Code...................................................7
   6. Validation Suite..............................................10
   7. Security Considerations.......................................11
   8. IANA Considerations...........................................11
   9. References....................................................11
      9.1. Normative References.....................................11
      9.2. Informative References...................................11
   10. Acknowledgments..............................................12

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 TCP-AO

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   [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 surprizingly 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 sequence number
   extension 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

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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 and ensuring that receivers
   never experience a series of values that violate that rule.

   We define a sequence space as follows:

   o  An unsigned integer range from 0..(2^N)-1, i.e., for N bits

   o  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.

   o  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, i.e., ISNs
   [RFC793]). The receiver is assumed to always receive values that are

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   always within (2^N)-1 but that 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.

   A sequence number extension (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 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)

   Compute_sne() 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, 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].

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   A more recent discussion describes this as "serial number
   arithmetic" and defines a comparison operator it claimed was missing
   in IEN74 [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 value 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 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. E.g., 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 sequence number extension (SNE) is never transmitted in
   messages; the existing rules of sequence number ensure both ends can
   keep track unambiguously - both for new messages and reordered

   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.

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   SNE 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 message authentication codes (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

   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 wraparound or because other fields
   provide that disambiguation, such as timestamps.

5. Example Code

   The following C code is provided as a verified example of sequence
   number extension 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.

   #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:

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       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",

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       fprintf(stderr,"state      RCV_SNE = %08lx\n", RCV_SNE);
       fprintf(stderr,"state RCV_PREV_SEQ = %08lx\n", RCV_PREV_SEQ);
       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 = "Hex input (2 groups of 8 hex chars with a
   space): ";

       fprintf(stderr,"Input hex numbers only (0x is optional)\n\n");


       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",

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                   ((BIG_PREV < BIG_THIS)?"+":"-"),
                   (((distance(BIG_PREV,BIG_THIS)) > 0x7FFFFFFF)
                    ? "ILLEGAL JUMP" : "."));



6. Validation Suite

   The following numbers are used to validate sequence number extension
   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

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   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

8. IANA Considerations

   This document contains no IANA issues. This section should be
   removed upon publication as an RFC.

9. References

9.1. Normative References

9.2. Informative References

   [IEN74]   Plummmer, W., "Sequence Number Arithmetic," IEN 74, Sept.

   [RFC793]  Postel, J., "Transmission Control Protocol," RFC 793,
             September 1981.

   [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities,"
             RFC 1034, Nov. 1987.

   [RFC1035] Mockapetris, P., "Domain Names - Implementation and
             Specification," Nov. 1987.

   [RFC1982] Elz, R., Bush, R., "Serial Number Arithmetic," RFC 1982,
             Aug. 1996.

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   [RFC5925] Touch, J., A. Mankin, R. Bonica, "The TCP Authentication
             Option," RFC 5925, June 2010.

   [RFC7323] Borman, D., D. Braden, V. Jacobson, R. Scheffenegger, Ed.,
             "TCP Extensions for High Performance" RFC 7323, Sep. 2014.

10. Acknowledgments

   The need for this document was first noted by Juhamatti Kuusisaari
   in April 2020 during discussions of the pseudocode in RFC 5925.

   This document was prepared using

Authors' Addresses

   Joe Touch
   Manhattan Beach, CA 90266 USA

   Phone: +1 (310) 560-0334

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