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Versions: 00 01 02 03 04 05 rfc2406                                     
Network Working Group                             Stephen Kent, BBN Corp
Internet Draft                           Randall Atkinson, @Home Network
draft-ietf-ipsec-esp-v2-00.txt                              21 July 1997

                IP Encapsulating Security Payload (ESP)

Status of This Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its working groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of 6 months.
   Internet Drafts may be updated, replaced, or obsoleted by other
   documents at any time. It is not appropriate to use Internet Drafts
   as reference material or to cite them other than as "work in

   This particular Internet Draft is a product of the IETF's IPsec
   working group. It is intended that a future version of this draft be
   submitted to the IPng Area Directors and the IESG for possible
   publication as a standards-track protocol.

Kent, Atkinson                                                  [Page 1]

Internet Draft              IP Encapsulating                21 July 1997
                         Security Payload (ESP)

Table of Contents

   1. Introduction......................................................3
   2. Encapsulating Security Payload Packet Format......................4
      2.1  Security Parameters Index....................................5
      2.2  Sequence Number .............................................5
      2.3  Payload Data.................................................5
      2.4  Padding (for Encryption).....................................6
      2.5  Pad Length...................................................7
      2.6  Next Header..................................................7
      2.7  Authentication Data..........................................7
   3. Encapsulating Security Protocol Processing........................7
      3.1  ESP Header Location..........................................7
      3.2  Outbound Packet Processing..................................10
         3.2.1  Security Association Lookup............................10
         3.2.2  Sequence Number Generation.............................10
         3.2.3  Packet Encryption......................................10
   Scope of Encryption................................10
   Encryption Algorithms..............................11
         3.2.4  Integrity Check Value Calculation......................11
    Scope of Authentication Protection................11
    Authentication Padding............................11
    Authentication Algorithms.........................12
         3.2.5  Fragmentation..........................................12
      3.3  Inbound Packet Processing...................................12
         3.3.1  Pre-ESP Processing Overview............................12
         3.3.2  Security Association Lookup............................12
         3.3.3  Sequence Number Verification...........................13
         3.3.4  Integrity Check Value Verification.....................14
         3.3.5  Packet Decryption......................................15
   4. Auditing.........................................................15
   5. Conformance Requirements.........................................16
   6. Security Considerations..........................................16
   7. Differences from RFC 1827........................................16
   Author Information..................................................19

Kent, Atkinson                                                  [Page 2]

Internet Draft              IP Encapsulating                21 July 1997
                         Security Payload (ESP)

1.  Introduction

   The Encapsulating Security Payload (ESP) header is designed to
   provide a mix of security services in IPv4 and IPv6.  ESP may be
   applied alone, in combination with the IP Authentication Header (AH)
   [KA97b], or in a nested fashion, e.g., through the use of tunnel mode
   (see "Security Architecture for the Internet Protocol" [KA97a],
   hereafter referred to as the Security Architecture document).
   Security services can be provided between a pair of communicating
   hosts, between a pair of communicating security gateways, or between
   a security gateway and a host.  For more details on how to use ESP
   and AH in various network environments, see the Security Architecture
   document [KA97a].

   The ESP header is inserted after the IP header and before the upper
   layer protocol header (transport mode) or before  an encapsulated IP
   header (tunnel mode).  These modes are described in more detail

   ESP is used to provide confidentiality, data origin authentication,
   connectionless integrity, an anti-replay service (a form of partial
   sequence integrity), and limited traffic flow confidentiality.  The
   set of services provided depends on options selected at the time of
   Security Association establishment and on the placement of the
   implementation.  Confidentiality may be selected independent of all
   other services.  However, use of confidentiality without
   integrity/authentication (either in ESP or separately in AH) may
   subject traffic to certain forms of active attacks that could
   undermine the confidentiality service (see [Bel96].  Data origin
   authentication and connectionless integrity are joint services
   (hereafter referred to jointly as "authentication) and are offered as
   an option in conjunction with confidentiality.  The anti-replay
   service may be selected only if data origin authentication is
   selected, and its election is solely at the discretion of the
   receiver.  Traffic flow confidentiality requires selection of tunnel
   mode, and is most effective if implemented at a security gateway,
   where traffic aggregation may be able to mask true source-destination

   It is assumed that the reader is familiar with the terms and concepts
   described in the Security Architecture document.  In particular, the
   reader should be familiar with the definitions of security services
   offered by ESP and AH, the concept of Security Associations, the ways
   in which ESP can be used in conjunction with the Authentication
   Header (AH), and the different key management options available for
   ESP and AH.  (With regard to the last topic, the current key
   management options required for both AH and ESP are manual keying and

Kent, Atkinson                                                  [Page 3]

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                         Security Payload (ESP)

   automated keying via Oakley/ISAKMP.)

2.  Encapsulating Security Payload Packet Format

   The protocol header (IPv4, IPv6, or Extension) immediately preceding the
   ESP header will contain the value 50 in its Protocol (IPv4) or Next
   Header (IPv6, Extension) field [STD-2].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ----
   |               Security Parameters Index (SPI)                 | ^
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Auth.
   |                      Sequence Number                          | |Coverage
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | -----
   |                    Payload Data* (variable)                   | |   ^
   ~                                                               ~ |   |
   |                                                               | |   |
   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Confid.
   |               |     Padding (0-255 bytes)                     | |Coverage*
   +-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   |
   |                               |  Pad Length   | Next Header   | v   v
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -------
   |                 Authentication Data (variable)                |
   ~                                                               ~
   |                                                               |

        * If included in the Payload field, cryptographic synchronization
          data, e.g., an IV, usually is not encrypted per se, although it
          often is referred to as being part of the ciphertext.

   The following subsections define the fields in the header format.
   "Optional" means that the field is omitted if the option is not
   selected, i.e., it is present in neither the packet as transmitted
   nor as formatted for computation of an ICV.  Whether or not an option
   is selected is defined as part of Security Association (SA)
   establishment.  Thus the format of ESP packets for a given SA is
   fixed, for the duration of the SA.  In contrast, "mandatory" fields
   are always present in the ESP packet format, for all SAs.

Kent, Atkinson                                                  [Page 4]

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                         Security Payload (ESP)

2.1  Security Parameters Index

   The SPI is an arbitrary 32-bit value that uniquely identifies the
   Security Association for this datagram, relative to the destination
   IP address contained in the IP header (with which this security
   header is associated) and relative to the security protocol employed.
   The set of SPI values in the range 1 through 255 are reserved by the
   Internet Assigned Numbers Authority (IANA) for future use; a reserved
   SPI value will not normally be assigned by IANA unless the use of the
   assigned SPI value is specified in an RFC.  It is ordinarily selected
   by the destination system upon establishment of an SA (see the
   Security Architecture document for more details).  (A zero value may
   be used within an ESP implementation for local debugging purposes,
   but no ESP packets should be transmitted with a zero SPI value.)  The
   SPI field is mandatory.

2.2  Sequence Number

   This unsigned 32-bit field contains a monotonically increasing
   counter value (sequence number).  The sender's counter and the
   receiver's counter are initialized to 0 when an SA is established.
   (The first packet sent using a given SA will have a Sequence Number
   of 1; see Section 3.2.2 for more details on how the Sequence Number
   is generated.) The transmitted Sequence Number must never be allowed
   to cycle.  Thus, the sender's counter and the receiver's counter MUST
   be reset (by establishing a new SA and thus a new key) prior to the
   transmission of 2^32nd packet on an SA.

   The Sequence Number is mandatory.  It is always included in an ESP
   packet, to ensure alignment of the Payload field on an 8-byte
   boundary (in support of IPv6).  Even if authentication is not
   selected as a security service for the SA, or if ESP is employed in
   an IPv4 environment, this field MUST be present.

   Processing of the Sequence Number field is at the discretion of the
   receiver, i.e., the sender MUST always transmit this field, but the
   receiver need not act upon it (see the discussion of Sequence Number
   Verification in the "Inbound Processing" section below).

2.3  Payload Data

   Payload Data is a variable-length field containing data described by
   the Next Header field. The Payload Data field is mandatory and is an
   integral number of bytes in length.  If the algorithm used to encrypt
   the payload requires cryptographic synchronization data, e.g., an
   Initialization Vector (IV), then this data MAY be carried explicitly

Kent, Atkinson                                                  [Page 5]

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                         Security Payload (ESP)

   in the Payload field.  Any encryption algorithm that requires such
   explicit, per-packet synchronization data MUST indicate the length,
   any structure for such data, and the location of this data as part of
   an RFC specifying how the algorithm is used with ESP.  If such
   synchronization data is implicit, the algorithm for deriving the data
   MUST be part of the RFC.

2.4  Padding (for Encryption)

   Several factors require or motivate use of the Padding field.

             If an encryption algorithm is employed that requires the
             plaintext to be a multiple of some number of bytes, e.g., the
             block size of a block cipher, the Padding field is used to fill
             the plaintext (consisting of the Payload Data, Pad Length and
             Next Header fields, as well as the Padding) to the size required
             by the algorithm.

             Padding also may be required, irrespective of encryption
             algorithm requirements, to ensure that the resulting ciphertext
             terminates on a 4-byte boundary. Specifically, the Pad Length
             and Next Header fields must be right aligned within a 4-byte
             word, as illustrated in the ESP packet format figure above.

             Padding beyond that required for the algorithm or alignment
             reasons cited above, may be used to conceal the actual length of
             the payload, in support of (partial) traffic flow
             confidentiality.  However, inclusion of such additional padding
             has adverse bandwidth implications and thus its use should be
             undertaken with care.

   The transmitter MAY add 0-255 bytes of padding.  Inclusion of the
   Padding field in an ESP packet is optional, but all implementations
   MUST support generation and consumption of padding.

   As a default, the Padding bytes are initialized with a series of
   (unsigned, 1-byte) integer values.  The first padding byte appended
   to the plaintext is numbered 1, with subsequent padding bytes making
   up a monotonically increasing sequence: 1, 2, 3, ...  When this
   padding scheme is employed, the receiver SHOULD inspect the Padding
   field.  (This scheme was selected because of its relative simplicity,
   ease of implementation in hardware, and because it offers limited
   protection against certain forms of "cut and paste" attacks in the
   absence of other integrity measures, if the receiver checks the
   padding values upon decryption.)

Kent, Atkinson                                                  [Page 6]

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                         Security Payload (ESP)

   Any encryption algorithm that requires Padding other than the default
   described above, MUST define the Padding contents (e.g., zeros or
   random data) and any required receiver processing of these Padding
   bytes in an RFC specifying how the algorithm is used with ESP.  In
   such circumstances, the content of the Padding field will be
   determined by the encryption algorithm and mode selected and defined
   in the corresponding algorithm RFC.  The relevant algorithm RFC MAY
   specify that a receiver MUST inspect the Padding field or that a
   receiver MUST inform senders of how the receiver will handle the
   Padding field.

2.5  Pad Length

   The Pad Length field indicates the number of pad bytes immediately
   preceding it.  The range of valid values is 0-255, where a value of
   zero indicates that no Padding bytes are present.  The Pad Length
   field is mandatory.

2.6  Next Header

   The Next Header is an 8-bit field that identifies the type of data
   contained in the Payload Data field, e.g., an extension header in
   IPv6 or an upper layer protocol identifier.  The value of this field
   is chosen from the set of IP Protocol Numbers defined in the most
   recent "Assigned Numbers" [STD-2] RFC from the Internet Assigned
   Numbers Authority (IANA).  The Next Header field is mandatory.

2.7  Authentication Data

   The Authentication Data is a variable-length field containing an
   Integrity Check Value (ICV) computed over the ESP packet minus the
   Authentication Data.  The length of the field depends upon the
   authentication function selected.  The mandatory-to-implement
   authentication algorithms, HMAC with MD5 or SHA-1, both yield 96-bit
   ICV's because of the truncation convention (see Section
   adopted for use in IPsec.  The Authentication Data field is optional,
   and is included only if the authentication service has been selected
   for the SA in question.

3.  Encapsulating Security Protocol Processing

   3.1 ESP Header Location

   Like AH, ESP may be employed in two ways: transport mode or tunnel
   mode.  The former mode is applicable only to host implementations and
   provides protection for upper layer protocols, but not the IP header.
   (In this mode, note that for "bump-in-the-stack" or "bump-in-the-

Kent, Atkinson                                                  [Page 7]

Internet Draft              IP Encapsulating                21 July 1997
                         Security Payload (ESP)

   wire" implementations, as defined in the Security Architecture
   document, inbound and outbound IP fragments may require an IPsec
   implementation to perform extra IP reassembly/fragmentation in order
   to both conform to this specification and provide transparent IPsec
   support.  Special care is required to perform such operations within
   these implementations when multiple interfaces are in use.)

   In transport mode, ESP is inserted after the IP header and before an
   upper layer protocol, e.g., TCP, UDP, ICMP, etc.  or before any other
   IPsec headers that have already been inserted, e.g., AH.  In the
   context of IPv4, this translates to placing ESP after the IP header
   (and any options that it contains), but before the upper layer
   protocol.  (Note that the term "transport" mode should not be
   misconstrued as restricting its use to TCP and UDP. For example, an
   ICMP message MAY be sent using either "transport" mode or "tunnel"
   mode.)  The following diagram illustrates ESP transport mode
   positioning for a typical IPv4 packet, on a "before and after" basis.
   (The "ESP trailer" encompasses any Padding, plus the Pad Length, and
   Next Header fields.)

                 BEFORE APPLYING ESP
      IPv4  |orig IP hdr  |     |      |
            |(any options)| TCP | Data |

                 AFTER APPLYING ESP
      IPv4  |orig IP hdr  | ESP |     |      |   ESP   | ESP|
            |(any options)| Hdr | TCP | Data | Trailer |Auth|
                                |<----- encrypted ---->|
                          |<------ authenticated ----->|

   In the IPv6 context, ESP is viewed as an end-to-end payload, and thus
   should appear after hop-by-hop, routing, and fragmentation extension
   headers.  The destination options extension header(s) could appear
   either before or after the ESP header depending on the semantics
   desired.  However, since ESP protects only fields after the ESP
   header, it generally may be desirable to place the destination
   options header(s) after the ESP header.  The following diagram
   illustrates ESP transport mode positioning for a typical IPv6 packet.

Kent, Atkinson                                                  [Page 8]

Internet Draft              IP Encapsulating                21 July 1997
                         Security Payload (ESP)

                     BEFORE APPLYING ESP
      IPv6  |             | ext hdrs |     |      |
            | orig IP hdr |if present| TCP | Data |

                     AFTER APPLYING ESP
      IPv6  | orig |hxh,rtg,frag|dest|ESP|dest|   |    | ESP   | ESP|
            |IP hdr|if present**|opt*|Hdr|opt*|TCP|Data|Trailer|Auth|
                                         |<---- encrypted ---->|
                                     |<---- authenticated ---->|

                * = if present, could be before ESP, after ESP, or both
               ** = hop by hop, routing, fragmentation headers

   Tunnel mode ESP may be employed in either hosts or security gateways.
   When ESP is implemented in a security gateway (to protect subscriber
   transit traffic), tunnel mode must be used.  In tunnel mode, the
   "inner" IP header carries the ultimate source and destination
   addresses, while an "outer" IP header may contain distinct IP
   addresses, e.g., addresses of security gateways.  In tunnel mode, ESP
   protects the entire inner IP packet, including the entire inner IP
   header. The position of ESP in tunnel mode, relative to the outer IP
   header, is the same as for ESP in transport mode.  The following
   diagram illustrates ESP tunnel mode positioning for typical IPv4 and
   IPv6 packets.

      IPv4  | new IP hdr* |     | orig IP hdr*  |   |    | ESP   | ESP|
            |(any options)| ESP | (any options) |TCP|Data|Trailer|Auth|
                                |<--------- encrypted ---------->|
                          |<----------- authenticated ---------->|

      IPv6  | new* | ext hdrs*|   | orig*| ext hdrs*|   |    | ESP   | ESP|
            |IP hdr|if present|ESP|IP hdr|if present|TCP|Data|Trailer|Auth|
                                  |<---------- encrypted ----------->|
                              |<----------- authenticated ---------->|

               * = construction of outer IP hdr/extensions and modification
                      of inner IP hdr/extensions is discussed below.

Kent, Atkinson                                                  [Page 9]

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                         Security Payload (ESP)

3.2  Outbound Packet Processing

   In transport mode, the transmitter encapsulates the upper layer
   protocol information in the ESP header/trailer, and retains the
   specified IP header (and any IP extension headers in the IPv6
   context).  In tunnel mode, the outer and inner IP header/extensions
   can be inter-related in a variety of ways.  The construction of the
   outer IP header/extensions during the encapsulation process is
   described in the Security Architecture document.

3.2.1  Security Association Lookup

   ESP is applied to an outbound packet only after an IPsec
   implementation determines that the packet is associated with an SA
   that calls for ESP processing.  The process of determining what, if
   any, IPsec processing is applied to outbound traffic is described in
   the Security Architecture document.

3.2.2  Sequence Number Generation

   As noted in Section 2.2, the Sequence Number field is always included
   in ESP packets, even if the anti-replay service, or the
   authentication service, have not been enabled for the SA.  The
   sender's counter is initialized to 0 when an SA is established.  The
   transmitter increments the Sequence Number for this SA, checks to
   ensure that the counter has not cycled, and inserts the new value
   into the Sequence Number field.  Thus the first packet sent using a
   given SA will have a Sequence Number of 1.  A transmitter MUST NOT
   send a packet on an SA if doing so would cause the Sequence Number to
   cycle.  An attempt to transmit a packet that would result in sequence
   number overflow is an auditable event.  (Note that this approach to
   Sequence Number management does not require use of modular

3.2.3  Packet Encryption Scope of Encryption

   In transport mode, the transmitter encapsulates the original upper
   layer protocol information into the ESP payload field, adds any
   necessary padding, and encrypts the result (Payload Data, Padding,
   Pad Length, and Next Header) using the key, encryption algorithm, and
   algorithm mode indicated by the SA.  In tunnel mode, the transmitter
   encapsulates and encrypts the entire original IP datagram (plus the
   Padding, Pad Length, and Next Header).

   If authentication is selected, encryption is performed first, before

Kent, Atkinson                                                 [Page 10]

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   the authentication, and the encryption does not encompass the
   Authentication Data field.  This order of processing facilitates
   rapid detection and rejection of replayed or bogus packets by the
   receiver, prior to decrypting the packet, hence potentially reducing
   the impact of denial of service attacks.  It also allows for the
   possibility of parallel processing of packets at the receiver, i.e.,
   decryption can take place in parallel with authentication.  Note that
   since the Authentication Data is not protected by encryption, a keyed
   authentication algorithm must be employed to compute the ICV. Encryption Algorithms

The encryption algorithm employed is specified by the SA.  ESP is
designed for use with symmetric encryption algorithms.  Because IP
packets may arrive out of order, each packet must carry any data
required to allow the receiver to establish cryptographic
synchronization for decryption.  This data may be carried explicitly in
the payload field, e.g., as an IV (as described above), or the data may
be derived from the packet header.  Since ESP makes provision for
padding of the plaintext, encryption algorithms employed with ESP may
exhibit either block or stream mode characteristics.

At the time of writing, one mandatory-to-implement encryption algorithm
and mode has been defined for ESP.  It is based on the Data Encryption
Standard (DES) [NIST77] in Cipher Block Chaining Mode [NIST80].  Details
of use of this mode are contained in [MS97].

3.2.4  Integrity Check Value Calculation  Scope of Authentication Protection

   If authentication is selected for the SA, the transmitter computes
   the ICV over the ESP packet minus the Authentication Data.  Thus the
   SPI, Sequence Number, Payload Data, Padding (if present), Pad Length,
   and Next Header are all encompassed by the ICV computation.  Note
   that the last 4 fields will be in ciphertext form, since encryption
   is performed prior to authentication.  Authentication Padding

   For some authentication algorithms, the byte string over which the
   ICV computation is performed must be a multiple of a blocksize
   specified by the algorithm.  If the length of this byte string does
   not match the blocksize requirements for the algorithm, implicit
   padding MUST be appended to the end of the ESP packet, prior to ICV
   computation.  The padding octets MUST have a value of zero.  The

Kent, Atkinson                                                 [Page 11]

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   blocksize (and hence the length of the padding) is specified by the
   algorithm specification.  This padding is not transmitted with the
   packet.  Authentication Algorithms

   The authentication algorithm employed for the ICV computation is
   specified by the SA.  For point-to-point communication, suitable
   authentication algorithms include keyed Message Authentication Codes
   (MACs) based on symmetric encryption algorithms (e.g., DES) or on
   one-way hash functions (e.g., MD5 or SHA-1).  For multicast
   communication, one-way hash algorithms combined with asymmetric
   signature algorithms are suitable.  As of this writing, the
   mandatory-to-implement authentication algorithms are based on the
   former class, i.e., HMAC [KBC97] with SHA-1 [SHA] or HMAC with MD5
   [Riv92].  The output of the HMAC computation is truncated to the
   leftmost 96 bits.  Other algorithms, possibly with different ICV
   lengths, MAY be supported.

3.2.5  Fragmentation

   If necessary, fragmentation is performed after ESP processing within
   an IPsec implementation.  Thus, transport mode ESP is applied only to
   whole IP datagrams (not to IP fragments).  An IP packet to which ESP
   has been applied may itself be fragmented by routers en route, and
   such fragments must be reassembled prior to ESP processing at a
   receiver.  In tunnel mode, ESP is applied to an IP packet, the
   payload of which may be a fragmented IP packet.  For example, a
   security gateway or a "bump-in-the-stack" or "bump-in-the-wire" IPsec
   implementation (as defined in the Security Architecture document) may
   apply tunnel mode ESP to such fragments.

3.3  Inbound Packet Processing

3.3.1  Pre-ESP Processing Overview

   If required, reassembly is performed prior to ESP processing.

3.3.2  Security Association Lookup

   Upon receipt of a (reassembled) packet containing an ESP Header, the
   receiver determines the appropriate (unidirectional) SA, based on the
   destination IP address and the SPI.  (This process is described in
   more detail in the Security Architecture document.)  The SA indicates
   whether the Authentication Data field should be present, and it will
   specify the algorithms and keys to be employed for decryption and ICV
   computations (if applicable).

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   If no valid Security Association exists for this session (for
   example, the receiver has no key), the receiver MUST discard the
   packet; this is an auditable event.  The audit log entry for this
   event SHOULD include the SPI value, date/time, Source Address,
   Destination Address, and (in IPv6) the cleartext Flow ID.

3.3.3  Sequence Number Verification

   All ESP implementations MUST support the anti-replay service, though
   its use may be enabled or disabled on a per-SA basis.  This service
   MUST NOT be enabled unless the authentication service also is enabled
   for the SA, since otherwise the Sequence Number field has not been
   integrity protected.  (Note that there are no provisions for managing
   transmitted Sequence Number values among multiple senders directing
   traffic to a single, multicast SA.  Thus the anti-replay service
   SHOULD NOT be used in a multi-sender multicast environment that
   employs a single, multicast SA.)  If an SA establishment protocol
   such as Oakley/ISAKMP is employed, then the receiver SHOULD notify
   the transmitter, during SA establishment, if the receiver will
   provide anti-replay protection and SHOULD inform the transmitter of
   the window size.

   If the receiver enables the anti-replay service for this SA, the
   receive packet counter for the SA MUST be initialized to zero when
   the SA is established.  For each received packet, the receiver MUST
   verify that the packet contains a Sequence Number that does not
   duplicate the Sequence Number of any other packets received during
   the life of this SA.  This SHOULD be the first ESP check applied to a
   packet after it has been matched to an SA, to speed rejection of
   duplicate packets.

   Duplicates are rejected through the use of a sliding receive window.
   (How the window is implemented is a local matter, but the following
   text describes the functionality that the implementation must
   exhibit.)  A MINIMUM window size of 32 MUST be supported; but a
   window size of 64 is preferred and SHOULD be employed as the default.
   A window size of 64 or larger MAY be chosen by the receiver.  If a
   larger window size is chosen, it MUST be a multiple of 32.  If any
   window size other than the default of 64 is employed by the receiver,
   it MUST be reported to the transmitter during SA negotiation.

   The "right" edge of the window represents the highest, validated
   Sequence Number value received on this SA.  Packets that contain
   Sequence Numbers lower than the "left" edge of the window are
   rejected.  Packets falling within the window are checked against a
   list of received packets within the window.  An efficient means for
   performing this check, based on the use of a bit mask, is described

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   in the Security Architecture document.

   If the received packet falls within the window and is new, or if the
   packet is to the right of the window, then the receiver proceeds to
   ICV verification.  If the ICV validation fails, the receiver MUST
   discard the received IP datagram as invalid; this is an auditable
   event.  The audit log entry for this event SHOULD include the SPI
   value, date/time, Source Address, Destination Address, the Sequence
   Number, and (in IPv6) the Flow ID.  The receive window is updated
   only if the ICV verification succeeds.


      Note that if the packet is either inside the window and new, or is
      outside the window on the "right" side, the receiver MUST
      authenticate the packet before updating the Sequence Number window

3.3.4  Integrity Check Value Verification

   If authentication has been selected, the receiver computes the ICV
   over the ESP packet minus the Authentication Data using the specified
   authentication algorithm and verifies that it is the same as the ICV
   included in the Authentication Data field of the packet.  Details of
   the computation are provided below.

   If the computed and received ICV's match, then the datagram is valid,
   and it is accepted.  If the test fails, then the receiver MUST
   discard the received IP datagram as invalid; this is an auditable
   event.  The log data SHOULD include the SPI value, date/time
   received, Source Address, Destination Address, and (in IPv6) the
   cleartext Flow ID.


      Begin by removing and saving the ICV value (Authentication Data
      field).  Next check the overall length of the ESP packet minus the
      Authentication Data.  If implicit padding is required, based on
      the blocksize of the authentication algorithm, append zero-filled
      bytes to the end of the ESP packet directly after the Next Header
      field.  Perform the ICV computation and compare the result with
      the saved value.  (For the mandatory-to-implement authentication
      algorithms, HMAC [KBC97] with SHA-1 [SHA] or HMAC with MD5
      [Riv92], the output of the HMAC computation is truncated to the
      leftmost 96 bits.  Other algorithms may have different ICV
      lengths.) (If a digital signature and one-way hash are used for
      the ICV computation, the matching process is more complex and will

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      be described in the algorithm specification.)

3.3.5  Packet Decryption

   The receiver decrypts the ESP Payload Data, Padding, Pad Length, and
   Next Header using the session key that has been established for this
   traffic.  If an explicit IV is present in the Payload Field, it is
   input to the decryption algorithm as per the algorithm specification.
   If an implicit IV is employed, a local version of the IV is
   constructed and input to the decryption algorithm as per the
   algorithm specification.  (Decryption may take place in parallel with
   authentication, but care must be taken to avoid possible race
   conditions with regard to packet access and reconstruction of the
   decrypted packet.)

   After decryption, the original IP datagram is reconstructed and
   processed per the normal IP protocol specification.  The exact steps
   for reconstructing the original datagram depend on the mode (tunnel
   vs transport) and are described in the Security Architecture
   document.  At a minimum, in an IPv6 context, the receiver SHOULD
   ensure that the decrypted data is 8-byte aligned, to facilitate
   processing by the protocol identified in the Next Header field.

   Note that there are two ways in which the decryption can "fail".  The
   selected SA may not be correct or the encrypted ESP packet could be
   corrupted.  (The latter case would be detected if authentication is
   selected for the SA, as would tampering with the SPI.  However, an SA
   mismatch might still occur due to tampering with the IP Destination
   Address.)  In either case, the erroneous result of the decryption
   operation (an invalid IP datagram or transport-layer frame) will not
   necessarily be detected by IPsec, and is the responsibility of later
   protocol processing.

4. Auditing

   Not all systems that implement ESP will implement auditing.  However,
   if ESP is incorporated into a system that supports auditing, then the
   ESP implementation MUST also support auditing and MUST allow a system
   administrator to enable or disable auditing for ESP.  For the most
   part, the granularity of auditing is a local matter.  However,
   several auditable events are identified in this specification and for
   each of these events a minimum set of information that SHOULD be
   included in an audit log is defined.  Additional information also MAY
   be included in the audit log for each of these events, and additional
   events, not explicitly called out in this specification, also MAY

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   result in audit log entries.  There is no requirement for the
   receiver to transmit any message to the purported transmitter in
   response to the detection of an auditable event, because of the
   potential to induce denial of service via such action.

5.  Conformance Requirements

   Implementations that claim conformance or compliance with this
   specification MUST implement the ESP syntax and processing described
   here and MUST comply with all requirements of the Security
   Architecture document.  If the key used to compute an ICV is manually
   distributed, correct provision of the anti-replay service would
   require correct maintenance of the counter state at the transmitter,
   until the key is replaced, and there likely would be no automated
   recovery provision if counter overflow were imminent.  Thus a
   compliant implementation SHOULD NOT provide this service in
   conjunction with SAs that are manually keyed.  A compliant ESP
   implementation MUST support the following mandatory-to-implement
   algorithms (specified in [KBC97] and in [MS97].

             - DES in CBC mode
             - HMAC with MD5
             - HMAC with SHA-1

6.  Security Considerations

   Security is central to the design of this protocol, and this security
   considerations permeate the specification.  Additional security-
   relevant aspects of using IPsec protocol are discussed in the
   Security Architecture document.

7. Differences from RFC 1827

   This document differs from RFC 1827 [ATK95] in several significant
   ways.  The major difference is that, this document attempts to
   specify a complete framework and context for ESP, whereas RFC 1827
   provided a "shell" that was completed through the definition of
   transforms.  The combinatorial growth of transforms motivated the
   reformulation of the ESP specification as a more complete document,
   with options for security services that may be offered in the context
   of ESP.  Thus, fields previously defined in transform documents are
   now part of this base ESP specification.  For example, the fields
   necessary to support authentication (and anti-replay) are now defined
   here, even though the provision of this service is an option.  The

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   fields used to support padding for encryption, and for next protocol
   identification, are now defined here as well.  Packet processing
   consistent with the definition of these fields also is included in
   the document.


   Many of the concepts embodied in this specification were derived from
   or influenced by the US Government's SP3 security protocol, ISO/IEC's
   NLSP, or from the proposed swIPe security protocol.  [SDNS89, ISO92

   For over 2 years, this document has evolved through multiple versions
   and iterations.  During this time, many people have contributed
   significant ideas and energy to the process and the documents
   themselves.  The authors would like to thank Karen Seo for providing
   extensive help in the review, editing, background research, and
   coordination for this version of the specification.  The authors
   would also like to thank the members of the IPSEC and IPng working
   groups, with special mention of the efforts of (in alphabetic order):
   Steve Bellovin, Steve Deering, Phil Karn, Perry Metzger, David
   Mihelcic, Hilarie Orman, William Simpson and Nina Yuan.


   [ATK95]   R. Atkinson, "IP Encapsulating Security Payload (ESP)", RFC
             1827, August 1997.

   [Bel89]   Steven M. Bellovin, "Security Problems in the TCP/IP
             Protocol Suite", ACM Computer Communications Review, Vol.
             19, No. 2, March 1989.

   [Bel96]   Steven M. Bellovin, "Problem Areas for the IP Security
             Protocols", Proceedings of the Sixth Usenix Unix Security
             Symposium, July, 1996.

   [CERT95]  Computer Emergency Response Team (CERT), "IP Spoofing
             Attacks and Hijacked Terminal Connections", CA-95:01,
             January 1995.  Available via anonymous ftp from

   [DH95]    Steve Deering & Robert Hinden, Internet Protocol Version 6
             (Ipv6)  Specification, RFC 1883, December 1995.

   [IB93]    John Ioannidis & Matt Blaze, "Architecture and

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             Implementation of Network-layer Security Under Unix",
             Proceedings of the USENIX Security Symposium, Santa Clara,
             CA, October 1993.

   [ISO92]   ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
             DIS 11577, International Standards Organisation, Geneva,
             Switzerland, 29 November 1992.

   [KA97a]   Steve Kent, Randall Atkinson, "Security Architecture for
             the Internet Protocol", Internet Draft, ?? 1997.

   [KA97b]   Steve Kent, Randall Atkinson, "IP Authentication Header",
             Internet Draft, ?? 1997.

   [KBC97]   Hugo Krawczyk, Mihir Bellare, and Ran Canetti, "HMAC:
             Keyed-Hashing for Message Authentication", RFC-2104,
             February 1997.

   [Ken91]   Steve Kent, "US DoD Security Options for the Internet
             Protocol (IPSO)", RFC-1108, November 1991.

   [MS97]    Perry Metzger & W.A. Simpson, "The ESP DES-CBC Transform",
             RFC-xxxx, August 1997.

   [NIST77]  US National Bureau of Standards, "Data Encryption
             Standard", Federal Information Processing Standard (FIPS)
             Publication 46, January 1977.

   [NIST80]  US National Bureau of Standards, "DES Modes of Operation"
             Federal Information Processing Standard (FIPS) Publication
             81, December 1980.

   [NIST81]  US National Bureau of Standards, "Guidelines for
             Implementing and Using the Data Encryption Standard",
             Federal Information Processing Standard (FIPS) Publication
             74, April 1981.

   [NIST88]  US National Bureau of Standards, "Data Encryption
             Standard", Federal Information Processing Standard (FIPS)
             Publication 46-1, January 1988.

   [Riv92]   Ronald Rivest, "The MD5 Message Digest Algorithm," RFC-
             1321, April 1992.

   [SHA]     NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995

   [STD-2]   J. Reynolds and J. Postel, "Assigned Numbers", STD-2, 20

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             October 1994.

   [Sch94]   Bruce Schneier, Applied Cryptography, John Wiley & Sons,
             New York, NY, 1994. ISBN 0-471-59756-2

   [SDNS89]  SDNS Secure Data Network System, Security Protocol 3, SP3,
             Document SDN.301, Revision 1.5, 15 May 1989, as published
             in NIST Publication NIST-IR-90-4250, February 1990.


   The views and specification here are those of the authors and are not
   necessarily those of their employers.  The authors and their
   employers specifically disclaim responsibility for any problems
   arising from correct or incorrect implementation or use of this

Author Information

   Stephen Kent
   BBN Corporation
   70 Fawcett Street
   Cambridge, MA  02140
   E-mail: kent@bbn.com
   Telephone: +1 (617) 873-3988

   Randall Atkinson
   @Home Network
   385 Ravendale Drive
   Mountain View, CA 94043
   E-mail: rja@inet.org

Kent, Atkinson                                                 [Page 19]