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RFC 6479

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IPsec Anti-Replay Algorithm without Bit Shifting


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Independent Submission                                          X. Zhang
Request for Comments: 6479                                       T. Tsou
Category: Informational                           Futurewei Technologies
ISSN: 2070-1721                                             January 2012

            IPsec Anti-Replay Algorithm without Bit Shifting


   This document presents an alternate method to do the anti-replay
   checks and updates for IP Authentication Header (AH) and
   Encapsulating Security Protocol (ESP).  The method defined in this
   document obviates the need for bit shifting and it reduces the number
   of times an anti-replay window is adjusted.

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 a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 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.

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Table of Contents

   1. Introduction ....................................................2
   2. Description of New Anti-Replay Algorithm ........................3
   3. Example of New Anti-Replay Algorithm ............................5
   4. Security Considerations .........................................9
   5. Acknowledgements ................................................9
   6. Normative References ............................................9

1.  Introduction

   "IP Authentication Header" [RFC4302] and "IP Encapsulating Security
   Payload (ESP)" [RFC4303] define an anti-replay service that employs a
   sliding window mechanism.  The mechanism, when enabled by a receiver,
   uses an anti-replay window of size W.  This window limits how far out
   of order a packet can be, relative to the packet with the highest
   sequence number that has been authenticated so far.  The window can
   be represented by a range [WB, WT], where WB=WT-W+1.  The whole
   anti-replay window can be thought of as a string of bits.  The value
   of each bit indicates whether or not a packet with that sequence
   number has been received and authenticated, so that the replay packet
   can be detected and rejected.  If the packet is received, the
   receiver gets the sequence number S in the packet.  If S is inside
   window (S<=WT and S>=WB), then the receiver checks the corresponding
   bit (location is S-WB) in the window to see if this S has already
   been seen.  If S<WB, the packet is dropped.  If S>WT and is
   validated, the window is advanced by (S-WT) bits.  The new window
   becomes [WB+S-WT, S].  The new bits in this new window are set to
   indicate that no packets with those sequence numbers have been
   received.  The typical implementation (for example, the integrity
   algorithm [RFC4302]) is done by shifting (S-WT) bits.  In normal
   cases, the packets arrive in order, which results in continuous
   updates and bit-shifting operations.

   [RFC4302] and [RFC4303] define minimum window sizes of 32 and 64.
   But no requirement is established for minimum or recommended window
   sizes beyond 64 packets.  The window size needs to be based on
   reasonable expectations for packet re-ordering.  For a high-end,
   multi-core network processor with multiple crypto cores, a window
   size bigger than 64 or 128 is needed due to the varied IPsec
   processing latency caused by different cores.  In such a case, the
   window sliding is tremendously costly even with hardware acceleration
   to do the bit shifting.  This document describes an alternate method
   to avoid bit shifting.  It only discusses the anti-replay processing
   at the receiving side.  The processing is always safe and has no
   interoperability effects.  Even with a window size bigger than the
   usual 32- or 64-bit window, no interoperability issues are caused.

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   Any node employing practices that potentially cause reordering beyond
   the usual 32- or 64-bit window may lead to interoperability or
   performance problems, however.  For instance, if either the sending
   node or routers along the path cause significant re-ordering, this
   can lead to inability of the receiving IPsec endpoint to process the
   packets, as many current implementations do not support the
   extensions defined in this memo.  Similarly, such reordering can
   cause significant problems for transport and upper-layer protocols,
   and is generally best avoided.

2.  Description of the New Anti-Replay Algorithm

   Here we present an easy way to update the window index only, while
   also reducing the number window updates.  The basic idea is
   illustrated in the following figures.  Suppose that we configure the
   window size W, which consists of M-1 blocks, where M is a power of
   two (2).  Each block contains N bits, where N is also a power of two
   (2).  It can be a byte (8 bit) or word (32 bit), or multiple words.
   The supported sliding window size is (M-1)*N.  However, it covers up
   M blocks (four blocks as shown in Figure 1).  All these M blocks are
   circulated and become a ring of blocks, each with N bits.  In this
   way, the supported sliding window (M-1 blocks) is always a subset
   window of the actual window when the window slides.

   Initially, the actual window is defined by a low- and high-end index
   [WB, WT], as illustrated in Figure 1.

             ^                         ^
             |                         |
             WB                        WT

      Figure 1: The sliding window [WB, WT] in which WT is the last
      validated sequence number, and the supported window size W is
      WT-WB+1.  (x=don't care bit, c=check bit)

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   If we receive a packet with the sequence number (S) greater than WT,
   we slide the window.  But we only change the window index by adding
   the difference (S-WT) to both WT and WB (WB is automatically changed
   as the window size is fixed).  So, S becomes the largest sequence
   number of the received packets.  Figure 2 shows the case that the
   packet with sequence number S=WT+1 is received.

           ^                         ^
           |                         |
           WB                        WT

      Figure 2: The sliding window [WB, WT] after S=WT+1

   If S is in a different block from where WT is, we have to initialize
   all bit values in the blocks to 0 without bit shifting.  If S passes
   several blocks, we have to initialize several blocks instead of only
   one block.  Figure 3 shows that the sequence number already passed
   the block boundary.  Immediately after the update, all the check bits
   should be 0 in the block where WT resides.

       ^         ^
       |         |
       WT        WB

      Figure 3: The sliding window [WB, WT] after S passes the boundary

   After the update, the new window still covers the configured window.
   This means the configured sub-window also slides, conforming to the
   sliding window protocol.  The actual effect is somewhat like
   shifting the block.  In this way, the bit shifting is deemed

   It is also easier and much faster to check the window with the
   sequence number because the sequence number check does not
   depend on the lowest index WB.  Instead, it only depends on the
   sequence number of the received packet.  If we receive a sequence
   number S, the bit location is the lowest several bits of the
   sequence number, which only depends on the block size (N).  The
   block index is several bits before the location bits, which only
   depends on the window size (M).

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   We do not specify how many redundancy bits are needed, except that
   it should be a power of two (2) for computation efficiency.  If the
   microprocessor is 32 bits, 32 might be a better choice while 64
   might be better for 64-bit microprocessor.  For a microprocessor
   with cache support, one cache line is also a good choice.  It also
   depends on the size of the sliding window.  If we have N
   redundancy bits (for example, 32 bits in the above description),
   we only need 1/N times update of blocks, comparing to the
   bit-shifting algorithm in [RFC4302].

   The cost of this method is extra byte(s) being used as a redundant
   window.  The cost will be minimal if the window size is big enough.
   Actually, the extra redundant bits are not completely wasted.  We
   could reuse the unused bits in the block where index WB resides,
   i.e., the supported window size could be (M-1)*N, plus the unused
   bits in the last block.

3.  Example of the New Anti-Replay Algorithm

   Here is the example code to implement the algorithm of anti-replay
   checks and updates, which is described in the previous sections.


 * Copyright (c) 2012 IETF Trust and the persons identified as
 * authors of the code. All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, is permitted pursuant to, and subject to the license
 * terms contained in, the Simplified BSD License set forth in Section
 * 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents
 * (

 * In this algorithm, the hidden window size must be a power of two,
 * for example, 1024 bits.  The redundant bits must also be a power of
 * two, for example 32 bits.  Thus, the supported anti-replay window
 * size is the hidden window size minus the redundant bits.  It is 992
 * in this example.  The size of the integer depends on microprocessor
 * architecture.  In this example, we assume that the software runs on
 * a 32-bit microprocessor.  So the size of the integer is 32.  In order
 * to convert the bitmap into an array of integers, the total number of
 * integers is the hidden window size divided by the size of the
 * integer.

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 * struct ipsec_sa contains the window and window related parameters,
 * such as the window size and the last acknowledged sequence number.
 * all the value of macro can be changed, but must follow the rule
 * defined in the algorithm.

#define SIZE_OF_INTEGER       32 /** 32-bit microprocessor */
#define BITMAP_LEN            (1024/ SIZE_OF_INTEGER)
                                /** in terms of the 32-bit integer */

ipsec_check_replay_window (struct ipsec_sa *ipsa,
                           uint32_t sequence_number)
    int bit_location;
    int index;

     * replay shut off
    if (ipsa->replaywin_size == 0) {
        return 1;

     * first == 0 or wrapped
    if (sequence_number == 0) {
        return 0;

     * first check if the sequence number is in the range
    if (sequence_number>ipsa->replaywin_lastseq) {
        return 1;  /** larger is always good */

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     * The packet is too old and out of the window
    if ((sequence_number + ipsa->replaywin_size) <
        ipsa->replaywin_lastseq) {
          return 0;

     * The sequence is inside the sliding window
     * now check the bit in the bitmap
     * bit location only depends on the sequence number
    bit_location = sequence_number&BITMAP_LOC_MASK;
    index = (sequence_number>>REDUNDANT_BIT_SHIFTS)&BITMAP_INDEX_MASK;

     * this packet has already been received
    if (ipsa->replaywin_bitmap[index]&(1<<bit_location)) {
        return 0;

    return 1;

ipsec_update_replay_window (struct ipsec_sa *ipsa,
                            uint32_t sequence_number)
    int bit_location;
    int index, index_cur, id;
    int diff;

    if (ipsa->replaywin_size == 0) {  /** replay shut off */
        return 1;

    if (sequence_number == 0) {
        return 0;     /** first == 0 or wrapped */

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     * the packet is too old, no need to update
    if ((ipsa->replaywin_size + sequence_number) <
        ipsa->replaywin_lastseq) {
           return 0;

     * now update the bit
    index = (sequence_number>>REDUNDANT_BIT_SHIFTS);

 * first check if the sequence number is in the range
if (sequence_number>ipsa->replaywin_lastseq) {
    index_cur = ipsa->replaywin_lastseq>>REDUNDANT_BIT_SHIFTS;
    diff = index - index_cur;
    if (diff > BITMAP_LEN) {  /* something unusual in this case */
        diff = BITMAP_LEN;

    for (id = 0; id < diff; ++id) {
            = 0;

    ipsa->replaywin_lastseq = sequence_number;

    index &= BITMAP_INDEX_MASK;
    bit_location = sequence_number&BITMAP_LOC_MASK;

    /* this packet has already been received */
    if (ipsa->replaywin_bitmap[index]&(1<<bit_location)) {
    return 0;

    ipsa->replaywin_bitmap[index] |= (1<<bit_location);

    return 1;


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4.  Security Considerations

    This document does not change [RFC4302] or [RFC4303].  It provides
    an alternate method for anti-replay.

5.  Acknowledgements

    The idea in this document came from the software design on one
    high-performance multi-core network processor.  The new network
    processor core integrates a dozen of crypto core in a distributed
    way, which makes hardware anti-replay service impossible.

6.  Normative References

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302, December

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
              4303, December 2005.

Authors' Addresses

   Xiangyang Zhang
   Futurewei Technologies
   2330 Central Expressway
   Santa Clara, California  95051

   Phone: +1-408-330-4545

   Tina Tsou (Ting Zou)
   Futurewei Technologies
   2330 Central Expressway
   Santa Clara, California  95051

   Phone: +1-408-859-4996