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Network Working Group                                            S. KentRequest for Comments: 4303                              BBN TechnologiesObsoletes:2406                                            December 2005Category: Standards TrackIP Encapsulating Security Payload (ESP)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   This document describes an updated version of the Encapsulating   Security Payload (ESP) protocol, which is designed to provide a mix   of security services in IPv4 and IPv6.  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.  This document   obsoletesRFC 2406 (November 1998).Table of Contents1. Introduction ....................................................32. Encapsulating Security Payload Packet Format ....................52.1. Security Parameters Index (SPI) ...........................102.2. Sequence Number ...........................................122.2.1. Extended (64-bit) Sequence Number ..................122.3. Payload Data ..............................................132.4. Padding (for Encryption) ..................................142.5. Pad Length ................................................152.6. Next Header ...............................................162.7. Traffic Flow Confidentiality (TFC) Padding ................172.8. Integrity Check Value (ICV) ...............................173. Encapsulating Security Protocol Processing .....................183.1. ESP Header Location .......................................183.1.1. Transport Mode Processing ..........................183.1.2. Tunnel Mode Processing .............................19Kent                        Standards Track                     [Page 1]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20053.2. Algorithms ................................................203.2.1. Encryption Algorithms ..............................213.2.2. Integrity Algorithms ...............................213.2.3. Combined Mode Algorithms ...........................223.3. Outbound Packet Processing ................................223.3.1. Security Association Lookup ........................22           3.3.2. Packet Encryption and Integrity Check Value                  (ICV) Calculation ..................................22                  3.3.2.1. Separate Confidentiality and                           Integrity Algorithms ......................23                  3.3.2.2. Combined Confidentiality and                           Integrity Algorithms ......................243.3.3. Sequence Number Generation .........................253.3.4. Fragmentation ......................................263.4. Inbound Packet Processing .................................273.4.1. Reassembly .........................................273.4.2. Security Association Lookup ........................273.4.3. Sequence Number Verification .......................283.4.4. Integrity Check Value Verification .................30                  3.4.4.1. Separate Confidentiality and                           Integrity Algorithms ......................30                  3.4.4.2. Combined Confidentiality and                           Integrity Algorithms ......................324. Auditing .......................................................335. Conformance Requirements .......................................346. Security Considerations ........................................347. Differences fromRFC 2406 ......................................348. Backward-Compatibility Considerations ..........................359. Acknowledgements ...............................................3610. References ....................................................3610.1. Normative References .....................................3610.2. Informative References ...................................37Appendix A: Extended (64-bit) Sequence Numbers ....................38      A1. Overview ...................................................38      A2. Anti-Replay Window .........................................38          A2.1. Managing and Using the Anti-Replay Window ............39          A2.2. Determining the Higher-Order Bits (Seqh) of the                Sequence Number ......................................40          A2.3. Pseudo-Code Example ..................................41      A3. Handling Loss of Synchronization due to Significant          Packet Loss ................................................42          A3.1. Triggering Re-synchronization ........................43          A3.2. Re-synchronization Process ...........................43Kent                        Standards Track                     [Page 2]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20051.  Introduction   This document assumes that the reader is familiar with the terms and   concepts described in the "Security Architecture for the Internet   Protocol" [Ken-Arch], hereafter referred to as the Security   Architecture document.  In particular, the reader should be familiar   with the definitions of security services offered by the   Encapsulating Security Payload (ESP) and the IP Authentication Header   (AH), the concept of Security Associations, the ways in which ESP can   be used in conjunction with AH, and the different key management   options available for ESP and AH.   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this   document, are to be interpreted as described inRFC 2119 [Bra97].   The Encapsulating Security Payload (ESP) header is designed to   provide a mix of security services in IPv4 and IPv6 [DH98].  ESP may   be applied alone, in combination with AH [Ken-AH], or in a nested   fashion (see the Security Architecture document [Ken-Arch]).   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 [Ken-Arch].   The ESP header is inserted after the IP header and before the next   layer protocol header (transport mode) or before an encapsulated IP   header (tunnel mode).  These modes are described in more detail   below.   ESP can be 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 (SA) establishment and   on the location of the implementation in a network topology.   Using encryption-only for confidentiality is allowed by ESP.   However, it should be noted that in general, this will provide   defense only against passive attackers.  Using encryption without a   strong integrity mechanism on top of it (either in ESP or separately   via AH) may render the confidentiality service insecure against some   forms of active attacks [Bel96,Kra01].  Moreover, an underlying   integrity service, such as AH, applied before encryption does not   necessarily protect the encryption-only confidentiality against   active attackers [Kra01].  ESP allows encryption-only SAs because   this may offer considerably better performance and still provideKent                        Standards Track                     [Page 3]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   adequate security, e.g., when higher-layer authentication/integrity   protection is offered independently.  However, this standard does not   require ESP implementations to offer an encryption-only service.   Data origin authentication and connectionless integrity are joint   services, hereafter referred to jointly as "integrity".  (This term   is employed because, on a per-packet basis, the computation being   performed provides connectionless integrity directly; data origin   authentication is provided indirectly as a result of binding the key   used to verify the integrity to the identity of the IPsec peer.   Typically, this binding is effected through the use of a shared,   symmetric key.)  Integrity-only ESP MUST be offered as a service   selection option, e.g., it must be negotiable in SA management   protocols and MUST be configurable via management interfaces.   Integrity-only ESP is an attractive alternative to AH in many   contexts, e.g., because it is faster to process and more amenable to   pipelining in many implementations.   Although confidentiality and integrity can be offered independently,   ESP typically will employ both services, i.e., packets will be   protected with regard to confidentiality and integrity.  Thus, there   are three possible ESP security service combinations involving these   services:            - confidentiality-only (MAY be supported)            - integrity only (MUST be supported)            - confidentiality and integrity (MUST be supported)   The anti-replay service may be selected for an SA only if the   integrity service is selected for that SA.  The selection of this   service is solely at the discretion of the receiver and thus need not   be negotiated.  However, to make use of the Extended Sequence Number   feature in an interoperable fashion, ESP does impose a requirement on   SA management protocols to be able to negotiate this feature (seeSection 2.2.1 below).   The traffic flow confidentiality (TFC) service generally is effective   only if ESP is employed in a fashion that conceals the ultimate   source and destination addresses of correspondents, e.g., in tunnel   mode between security gateways, and only if sufficient traffic flows   between IPsec peers (either naturally or as a result of generation of   masking traffic) to conceal the characteristics of specific,   individual subscriber traffic flows.  (ESP may be employed as part of   a higher-layer TFC system, e.g., Onion Routing [Syverson], but such   systems are outside the scope of this standard.)  New TFC features   present in ESP facilitate efficient generation and discarding of   dummy traffic and better padding of real traffic, in a backward-   compatible fashion.Kent                        Standards Track                     [Page 4]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005Section 7 provides a brief review of the differences between this   document andRFC 2406.2.  Encapsulating Security Payload Packet Format   The (outer) protocol header (IPv4, IPv6, or Extension) that   immediately precedes the ESP header SHALL contain the value 50 in its   Protocol (IPv4) or Next Header (IPv6, Extension) field (see IANA web   page athttp://www.iana.org/assignments/protocol-numbers).  Figure 1   illustrates the top-level format of an ESP packet.  The packet begins   with two 4-byte fields (Security Parameters Index (SPI) and Sequence   Number).  Following these fields is the Payload Data, which has   substructure that depends on the choice of encryption algorithm and   mode, and on the use of TFC padding, which is examined in more detail   later.  Following the Payload Data are Padding and Pad Length fields,   and the Next Header field.  The optional Integrity Check Value (ICV)   field completes the packet. 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)                 | ^Int.+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-|                      Sequence Number                          | |ered+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----|                    Payload Data* (variable)                   | |   ^~                                                               ~ |   ||                                                               | |Conf.+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-|               |     Padding (0-255 bytes)                     | |ered*+-+-+-+-+-+-+-+-+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |   ||                               |  Pad Length   | Next Header   | v   v+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ------|         Integrity Check Value-ICV   (variable)                |~                                                               ~|                                                               |+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+            Figure 1.  Top-Level Format of an ESP Packet    * If included in the Payload field, cryptographic synchronization      data, e.g., an Initialization Vector (IV, seeSection 2.3),      usually is not encrypted per se, although it often is referred      to as being part of the ciphertext.Kent                        Standards Track                     [Page 5]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   The (transmitted) ESP trailer consists of the Padding, Pad Length,   and Next Header fields.  Additional, implicit ESP trailer data (which   is not transmitted) is included in the integrity computation, as   described below.   If the integrity service is selected, the integrity computation   encompasses the SPI, Sequence Number, Payload Data, and the ESP   trailer (explicit and implicit).   If the confidentiality service is selected, the ciphertext consists   of the Payload Data (except for any cryptographic synchronization   data that may be included) and the (explicit) ESP trailer.   As noted above, the Payload Data may have substructure.  An   encryption algorithm that requires an explicit Initialization Vector   (IV), e.g., Cipher Block Chaining (CBC) mode, often prefixes the   Payload Data to be protected with that value.  Some algorithm modes   combine encryption and integrity into a single operation; this   document refers to such algorithm modes as "combined mode   algorithms".  Accommodation of combined mode algorithms requires that   the algorithm explicitly describe the payload substructure used to   convey the integrity data.   Some combined mode algorithms provide integrity only for data that is   encrypted, whereas others can provide integrity for some additional   data that is not encrypted for transmission.  Because the SPI and   Sequence Number fields require integrity as part of the integrity   service, and they are not encrypted, it is necessary to ensure that   they are afforded integrity whenever the service is selected,   regardless of the style of combined algorithm mode employed.   When any combined mode algorithm is employed, the algorithm itself is   expected to return both decrypted plaintext and a pass/fail   indication for the integrity check.  For combined mode algorithms,   the ICV that would normally appear at the end of the ESP packet (when   integrity is selected) may be omitted.  When the ICV is omitted and   integrity is selected, it is the responsibility of the combined mode   algorithm to encode within the Payload Data an ICV-equivalent means   of verifying the integrity of the packet.   If a combined mode algorithm offers integrity only to data that is   encrypted, it will be necessary to replicate the SPI and Sequence   Number as part of the Payload Data.   Finally, a new provision is made to insert padding for traffic flow   confidentiality after the Payload Data and before the ESP trailer.   Figure 2 illustrates this substructure for Payload Data.  (Note: ThisKent                        Standards Track                     [Page 6]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   diagram shows bits-on-the-wire.  So even if extended sequence numbers   are being used, only 32 bits of the Sequence Number will be   transmitted (seeSection 2.2.1).)     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)                 |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                      Sequence Number                          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---   |                    IV (optional)                              | ^ p   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | a   |                    Rest of Payload Data  (variable)           | | y   ~                                                               ~ | l   |                                                               | | o   +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | a   |               |         TFC Padding * (optional, variable)    | v d   +-+-+-+-+-+-+-+-+         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+---   |                         |        Padding (0-255 bytes)        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                               |  Pad Length   | Next Header   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |         Integrity Check Value-ICV   (variable)                |   ~                                                               ~   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               Figure 2. Substructure of Payload Data         * If tunnel mode is being used, then the IPsec implementation           can add Traffic Flow Confidentiality (TFC) padding (seeSection 2.4)  after the Payload Data and before the Padding           (0-255 bytes) field.   If a combined algorithm mode is employed, the explicit ICV shown in   Figures 1 and 2 may be omitted (seeSection 3.3.2.2 below).  Because   algorithms and modes are fixed when an SA is established, the   detailed format of ESP packets for a given SA (including the Payload   Data substructure) is fixed, for all traffic on the SA.   The tables below refer to the fields in the preceding figures and   illustrate how several categories of algorithmic options, each with a   different processing model, affect the fields noted above.  The   processing details are described in later sections.Kent                        Standards Track                     [Page 7]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005          Table 1. Separate Encryption and Integrity Algorithms                                            What    What    What                          # of     Requ'd  Encrypt Integ    is                          bytes      [1]   Covers  Covers  Xmtd                          ------   ------  ------  ------  ------   SPI                       4        M              Y     plain   Seq# (low-order bits)     4        M              Y     plain       p                                                                ------ a   IV                     variable    O              Y     plain     | y   IP datagram [2]        variable  M or D    Y      Y     cipher[3] |-l   TFC padding [4]        variable    O       Y      Y     cipher[3] | o                                                                ------ a   Padding                 0-255      M       Y      Y     cipher[3]   d   Pad Length                1        M       Y      Y     cipher[3]   Next Header               1        M       Y      Y     cipher[3]   Seq# (high-order bits)    4     if ESN [5]        Y     not xmtd   ICV Padding            variable if need           Y     not xmtd   ICV                    variable   M [6]                 plain           [1] M = mandatory; O = optional; D = dummy           [2] If tunnel mode -> IP datagram               If transport mode -> next header and data           [3] ciphertext if encryption has been selected           [4] Can be used only if payload specifies its "real" length           [5] Seesection 2.2.1           [6] mandatory if a separate integrity algorithm is usedKent                        Standards Track                     [Page 8]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005                  Table 2. Combined Mode Algorithms                                             What    What    What                            # of     Requ'd  Encrypt Integ    is                            bytes      [1]   Covers  Covers  Xmtd                            ------   ------  ------  ------  ------    SPI                        4        M                    plain    Seq# (low-order bits)      4        M                    plain    p                                                                  --- a    IV                      variable    O              Y     plain  | y    IP datagram [2]         variable  M or D    Y      Y     cipher |-l    TFC padding [3]         variable    O       Y      Y     cipher | o                                                                  --- a    Padding                  0-255      M       Y      Y     cipher   d    Pad Length                 1        M       Y      Y     cipher    Next Header                1        M       Y      Y     cipher    Seq# (high-order bits)     4     if ESN [4]        Y     [5]    ICV Padding             variable if need           Y     [5]    ICV                     variable    O [6]                plain            [1] M = mandatory; O = optional; D = dummy            [2] If tunnel mode -> IP datagram                If transport mode -> next header and data            [3] Can be used only if payload specifies its "real" length            [4] SeeSection 2.2.1            [5] The algorithm choices determines whether these are                transmitted, but in either case, the result is invisible                to ESP            [6] The algorithm spec determines whether this field is                present   The following subsections describe 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 (seeSection 2.7).   Whether or not an option is selected is determined 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.   Note: All of the cryptographic algorithms used in IPsec expect their   input in canonical network byte order (see Appendix ofRFC 791   [Pos81]) and generate their output in canonical network byte order.   IP packets are also transmitted in network byte order.Kent                        Standards Track                     [Page 9]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   ESP does not contain a version number, therefore if there are   concerns about backward compatibility, they MUST be addressed by   using a signaling mechanism between the two IPsec peers to ensure   compatible versions of ESP (e.g., Internet Key Exchange (IKEv2)   [Kau05]) or an out-of-band configuration mechanism.2.1.  Security Parameters Index (SPI)   The SPI is an arbitrary 32-bit value that is used by a receiver to   identify the SA to which an incoming packet is bound.  The SPI field   is mandatory.   For a unicast SA, the SPI can be used by itself to specify an SA, or   it may be used in conjunction with the IPsec protocol type (in this   case ESP).  Because the SPI value is generated by the receiver for a   unicast SA, whether the value is sufficient to identify an SA by   itself or whether it must be used in conjunction with the IPsec   protocol value is a local matter.  This mechanism for mapping inbound   traffic to unicast SAs MUST be supported by all ESP implementations.   If an IPsec implementation supports multicast, then it MUST support   multicast SAs using the algorithm below for mapping inbound IPsec   datagrams to SAs.  Implementations that support only unicast traffic   need not implement this de-multiplexing algorithm.   In many secure multicast architectures (e.g., [RFC3740]), a central   Group Controller/Key Server unilaterally assigns the group security   association's SPI.  This SPI assignment is not negotiated or   coordinated with the key management (e.g., IKE) subsystems that   reside in the individual end systems that comprise the group.   Consequently, it is possible that a group security association and a   unicast security association can simultaneously use the same SPI.  A   multicast-capable IPsec implementation MUST correctly de-multiplex   inbound traffic even in the context of SPI collisions.   Each entry in the Security Association Database (SAD) [Ken-Arch] must   indicate whether the SA lookup makes use of the destination, or   destination and source, IP addresses, in addition to the SPI.  For   multicast SAs, the protocol field is not employed for SA lookups.   For each inbound, IPsec-protected packet, an implementation must   conduct its search of the SAD such that it finds the entry that   matches the "longest" SA identifier.  In this context, if two or more   SAD entries match based on the SPI value, then the entry that also   matches based on destination, or destination and source, address   comparison (as indicated in the SAD entry) is the "longest" match.   This implies a logical ordering of the SAD search as follows:Kent                        Standards Track                    [Page 10]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005         1. Search the SAD for a match on {SPI, destination address,            source address}.  If an SAD entry matches, then process the            inbound ESP packet with that matching SAD entry.  Otherwise,            proceed to step 2.         2. Search the SAD for a match on {SPI, destination address}.            If the SAD entry matches, then process the inbound ESP            packet with that matching SAD entry.  Otherwise, proceed to            step 3.         3. Search the SAD for a match on only {SPI} if the receiver has            chosen to maintain a single SPI space for AH and ESP, or on            {SPI, protocol} otherwise.  If an SAD entry matches, then            process the inbound ESP packet with that matching SAD entry.            Otherwise, discard the packet and log an auditable event.   In practice, an implementation MAY choose any method to accelerate   this search, although its externally visible behavior MUST be   functionally equivalent to having searched the SAD in the above   order.  For example, a software-based implementation could index into   a hash table by the SPI.  The SAD entries in each hash table bucket's   linked list are kept sorted to have those SAD entries with the   longest SA identifiers first in that linked list.  Those SAD entries   having the shortest SA identifiers are sorted so that they are the   last entries in the linked list.  A hardware-based implementation may   be able to effect the longest match search intrinsically, using   commonly available Ternary Content-Addressable Memory (TCAM)   features.   The indication of whether source and destination address matching is   required to map inbound IPsec traffic to SAs MUST be set either as a   side effect of manual SA configuration or via negotiation using an SA   management protocol, e.g., IKE or Group Domain of Interpretation   (GDOI) [RFC3547].  Typically, Source-Specific Multicast (SSM) [HC03]   groups use a 3-tuple SA identifier composed of an SPI, a destination   multicast address, and source address.  An Any-Source Multicast group   SA requires only an SPI and a destination multicast address as an   identifier.   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.  The SPI value of zero (0)   is reserved for local, implementation-specific use and MUST NOT be   sent on the wire.  (For example, a key management implementation   might use the zero SPI value to mean "No Security Association Exists"Kent                        Standards Track                    [Page 11]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   during the period when the IPsec implementation has requested that   its key management entity establish a new SA, but the SA has not yet   been established.)2.2.  Sequence Number   This unsigned 32-bit field contains a counter value that increases by   one for each packet sent, i.e., a per-SA packet sequence number.  For   a unicast SA or a single-sender multicast SA, the sender MUST   increment this field for every transmitted packet.  Sharing an SA   among multiple senders is permitted, though generally not   recommended.  ESP provides no means of synchronizing packet counters   among multiple senders or meaningfully managing a receiver packet   counter and window in the context of multiple senders.  Thus, for a   multi-sender SA, the anti-replay features of ESP are not available   (see Sections3.3.3 and3.4.3.)   The field is mandatory and MUST always be present even if the   receiver does not elect to enable the anti-replay service for a   specific SA.  Processing of the Sequence Number field is at the   discretion of the receiver, but all ESP implementations MUST be   capable of performing the processing described in Sections3.3.3 and   3.4.3. Thus, 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 Packet Processing" section (3.4.3)   below).   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; seeSection 3.3.3 for more details   on how the sequence number is generated.)  If anti-replay is enabled   (the default), 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 the 2^32nd packet on an SA.2.2.1.  Extended (64-bit) Sequence Number   To support high-speed IPsec implementations, Extended Sequence   Numbers (ESNs) SHOULD be implemented, as an extension to the current,   32-bit sequence number field.  Use of an ESN MUST be negotiated by an   SA management protocol.  Note that in IKEv2, this negotiation is   implicit; the default is ESN unless 32-bit sequence numbers are   explicitly negotiated.  (The ESN feature is applicable to multicast   as well as unicast SAs.)Kent                        Standards Track                    [Page 12]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   The ESN facility allows use of a 64-bit sequence number for an SA.   (SeeAppendix A, "Extended (64-bit) Sequence Numbers", for details.)   Only the low-order 32 bits of the sequence number are transmitted in   the plaintext ESP header of each packet, thus minimizing packet   overhead.  The high-order 32 bits are maintained as part of the   sequence number counter by both transmitter and receiver and are   included in the computation of the ICV (if the integrity service is   selected).  If a separate integrity algorithm is employed, the high   order bits are included in the implicit ESP trailer, but are not   transmitted, analogous to integrity algorithm padding bits.  If a   combined mode algorithm is employed, the algorithm choice determines   whether the high-order ESN bits are transmitted or are included   implicitly in the computation.  SeeSection 3.3.2.2 for processing   details.2.3.  Payload Data   Payload Data is a variable-length field containing data (from the   original IP packet) 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 is carried explicitly in the Payload field, but it is not called   out as a separate field in ESP, i.e., the transmission of an explicit   IV is invisible to ESP.  (See Figure 2.)  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.  (Typically, the IV immediately precedes the ciphertext.  See   Figure 2.)  If such synchronization data is implicit, the algorithm   for deriving the data MUST be part of the algorithm definition RFC.   (If included in the Payload field, cryptographic synchronization   data, e.g., an Initialization Vector (IV), usually is not encrypted   per se (see Tables 1 and 2), although it sometimes is referred to as   being part of the ciphertext.)   Note that the beginning of the next layer protocol header MUST be   aligned relative to the beginning of the ESP header as follows.  For   IPv4, this alignment is a multiple of 4 bytes.  For IPv6, the   alignment is a multiple of 8 bytes.   With regard to ensuring the alignment of the (real) ciphertext in the   presence of an IV, note the following:         o For some IV-based modes of operation, the receiver treats           the IV as the start of the ciphertext, feeding it into the           algorithm directly.  In these modes, alignment of the start           of the (real) ciphertext is not an issue at the receiver.Kent                        Standards Track                    [Page 13]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005         o In some cases, the receiver reads the IV in separately from           the ciphertext.  In these cases, the algorithm specification           MUST address how alignment of the (real) ciphertext is to be           achieved.2.4.  Padding (for Encryption)   Two primary factors require or motivate use of the Padding field.      o 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,        Padding, Pad Length, and Next Header fields) to the size        required by the algorithm.      o 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 figures above, to ensure that the ICV field (if        present) is aligned on a 4-byte boundary.   Padding beyond that required for the algorithm or alignment reasons   cited above could be used to conceal the actual length of the   payload, in support of TFC.  However, the Padding field described is   too limited to be effective for TFC and thus should not be used for   that purpose.  Instead, the separate mechanism described below (seeSection 2.7) should be used when TFC is required.   The sender MAY add 0 to 255 bytes of padding.  Inclusion of the   Padding field in an ESP packet is optional, subject to the   requirements noted above, but all implementations MUST support   generation and consumption of padding.      o For the purpose of ensuring that the bits to be encrypted        are a multiple of the algorithm's block size (first bullet        above), the padding computation applies to the Payload Data        exclusive of any IV, but including the ESP trailer        fields.  If a combined algorithm mode requires transmission        of the SPI and Sequence Number to effect integrity, e.g.,        replication of the SPI and Sequence Number in the Payload        Data, then the replicated versions of these data items, and        any associated, ICV-equivalent data, are included in the        computation of the pad length.  (If the ESN option isKent                        Standards Track                    [Page 14]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005        selected, the high-order 32 bits of the ESN also would enter        into the computation, if the combined mode algorithm        requires their transmission for integrity.)      o For the purposes of ensuring that the ICV is aligned on a        4-byte boundary (second bullet above), the padding        computation applies to the Payload Data inclusive of the IV,        the Pad Length, and Next Header fields.  If a combined mode        algorithm is used, any replicated data and ICV-equivalent        data are included in the Payload Data covered by the padding        computation.   If Padding bytes are needed but the encryption algorithm does not   specify the padding contents, then the following default processing   MUST be used.  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.)   If an encryption or combined mode algorithm imposes constraints on   the values of the bytes used for padding, they MUST be specified by   the RFC defining how the algorithm is employed with ESP.  If the   algorithm requires checking of the values of the bytes used for   padding, this too MUST be specified in that RFC.2.5.  Pad Length   The Pad Length field indicates the number of pad bytes immediately   preceding it in the Padding field.  The range of valid values is 0 to   255, where a value of zero indicates that no Padding bytes are   present.  As noted above, this does not include any TFC padding   bytes.  The Pad Length field is mandatory.Kent                        Standards Track                    [Page 15]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20052.6.  Next Header   The Next Header is a mandatory, 8-bit field that identifies the type   of data contained in the Payload Data field, e.g., an IPv4 or IPv6   packet, or a next layer header and data.  The value of this field is   chosen from the set of IP Protocol Numbers defined on the web page of   the IANA, e.g., a value of 4 indicates IPv4, a value of 41 indicates   IPv6, and a value of 6 indicates TCP.   To facilitate the rapid generation and discarding of the padding   traffic in support of traffic flow confidentiality (seeSection 2.4),   the protocol value 59 (which means "no next header") MUST be used to   designate a "dummy" packet.  A transmitter MUST be capable of   generating dummy packets marked with this value in the next protocol   field, and a receiver MUST be prepared to discard such packets,   without indicating an error.  All other ESP header and trailer fields   (SPI, Sequence Number, Padding, Pad Length, Next Header, and ICV)   MUST be present in dummy packets, but the plaintext portion of the   payload, other than this Next Header field, need not be well-formed,   e.g., the rest of the Payload Data may consist of only random bytes.   Dummy packets are discarded without prejudice.   Implementations SHOULD provide local management controls to enable   the use of this capability on a per-SA basis.  The controls should   allow the user to specify if this feature is to be used and also   provide parametric controls; for example, the controls might allow an   administrator to generate random-length or fixed-length dummy   packets.   DISCUSSION: Dummy packets can be inserted at random intervals to mask   the absence of actual traffic.  One can also "shape" the actual   traffic to match some distribution to which dummy traffic is added as   dictated by the distribution parameters.  As with the packet length   padding facility for Traffic Flow Security (TFS), the most secure   approach would be to generate dummy packets at whatever rate is   needed to maintain a constant rate on an SA.  If packets are all the   same size, then the SA presents the appearance of a constant bit rate   data stream, analogous to what a link crypto would offer at layer 1   or 2.  However, this is unlikely to be practical in many contexts,   e.g., when there are multiple SAs active, because it would imply   reducing the allowed bandwidth for a site, based on the number of   SAs, and that would undermine the benefits of packet switching.   Implementations SHOULD provide controls to enable local   administrators to manage the generation of dummy packets for TFC   purposes.Kent                        Standards Track                    [Page 16]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20052.7.  Traffic Flow Confidentiality (TFC) Padding   As noted above, the Padding field is limited to 255 bytes in length.   This generally will not be adequate to hide traffic characteristics   relative to traffic flow confidentiality requirements.  An optional   field, within the payload data, is provided specifically to address   the TFC requirement.   An IPsec implementation SHOULD be capable of padding traffic by   adding bytes after the end of the Payload Data, prior to the   beginning of the Padding field.  However, this padding (hereafter   referred to as TFC padding) can be added only if the Payload Data   field contains a specification of the length of the IP datagram.   This is always true in tunnel mode, and may be true in transport mode   depending on whether the next layer protocol (e.g., IP, UDP, ICMP)   contains explicit length information.  This length information will   enable the receiver to discard the TFC padding, because the true   length of the Payload Data will be known.  (ESP trailer fields are   located by counting back from the end of the ESP packet.)   Accordingly, if TFC padding is added, the field containing the   specification of the length of the IP datagram MUST NOT be modified   to reflect this padding.  No requirements for the value of this   padding are established by this standard.   In principle, existing IPsec implementations could have made use of   this capability previously, in a transparent fashion.  However,   because receivers may not have been prepared to deal with this   padding, the SA management protocol MUST negotiate this service prior   to a transmitter employing it, to ensure backward compatibility.   Combined with the convention described inSection 2.6 above, about   the use of protocol ID 59, an ESP implementation is capable of   generating dummy and real packets that exhibit much greater length   variability, in support of TFC.   Implementations SHOULD provide local management controls to enable   the use of this capability on a per-SA basis.  The controls should   allow the user to specify if this feature is to be used and also   provide parametric controls for the feature.2.8.  Integrity Check Value (ICV)   The Integrity Check Value is a variable-length field computed over   the ESP header, Payload, and ESP trailer fields.  Implicit ESP   trailer fields (integrity padding and high-order ESN bits, if   applicable) are included in the ICV computation.  The ICV field is   optional.  It is present only if the integrity service is selected   and is provided by either a separate integrity algorithm or a   combined mode algorithm that uses an ICV.  The length of the field isKent                        Standards Track                    [Page 17]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   specified by the integrity algorithm selected and associated with the   SA.  The integrity algorithm specification MUST specify the length of   the ICV and the comparison rules and processing steps for validation.3.  Encapsulating Security Protocol Processing3.1.  ESP Header Location   ESP may be employed in two ways: transport mode or tunnel mode.3.1.1.  Transport Mode Processing   In transport mode, ESP is inserted after the IP header and before a   next layer protocol, e.g., TCP, UDP, ICMP, etc.  In the context of   IPv4, this translates to placing ESP after the IP header (and any   options that it contains), but before the next layer protocol.  (If   AH is also applied to a packet, it is applied to the ESP header,   Payload, ESP trailer, and ICV, if present.)  (Note that the term   "transport" mode should not be misconstrued as restricting its use to   TCP and UDP.)  The following diagram illustrates ESP transport mode   positioning for a typical IPv4 packet, on a "before and after" basis.   (This and subsequent diagrams in this section show the ICV field, the   presence of which is a function of the security services and the   algorithm/mode selected.)                  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 | ICV|             -------------------------------------------------                                 |<---- encryption ---->|                           |<-------- integrity ------->|   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.  Destination options extension header(s) could appear   before, after, or both before and after the ESP header depending on   the semantics desired.  However, because ESP protects only fields   after the ESP header, it generally will 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                        Standards Track                    [Page 18]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005                      BEFORE APPLYING ESP             ---------------------------------------       IPv6  |             | ext hdrs |     |      |             | orig IP hdr |if present| TCP | Data |             ---------------------------------------                      AFTER APPLYING ESP             ---------------------------------------------------------       IPv6  | orig |hop-by-hop,dest*,|   |dest|   |    | ESP   | ESP|             |IP hdr|routing,fragment.|ESP|opt*|TCP|Data|Trailer| ICV|             ---------------------------------------------------------                                          |<--- encryption ---->|                                      |<------ integrity ------>|                 * = if present, could be before ESP, after ESP, or both   Note that in transport mode, for "bump-in-the-stack" or "bump-in-   the-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.3.1.2.  Tunnel Mode Processing   In tunnel mode, the "inner" IP header carries the ultimate (IP)   source and destination addresses, while an "outer" IP header contains   the addresses of the IPsec "peers", e.g., addresses of security   gateways.  Mixed inner and outer IP versions are allowed, i.e., IPv6   over IPv4 and IPv4 over IPv6.  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.Kent                        Standards Track                    [Page 19]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005                 BEFORE APPLYING ESP            ----------------------------      IPv4  |orig IP hdr  |     |      |            |(any options)| TCP | Data |            ----------------------------                 AFTER APPLYING ESP            -----------------------------------------------------------      IPv4  | new IP hdr* |     | orig IP hdr*  |   |    | ESP   | ESP|            |(any options)| ESP | (any options) |TCP|Data|Trailer| ICV|            -----------------------------------------------------------                                |<--------- encryption --------->|                          |<------------- integrity ------------>|                      BEFORE APPLYING ESP            ---------------------------------------      IPv6  |             | ext hdrs |     |      |            | orig IP hdr |if present| TCP | Data |            ---------------------------------------                     AFTER APPLYING ESP            ------------------------------------------------------------      IPv6  | new* |new ext |   | orig*|orig ext |   |    | ESP   | ESP|            |IP hdr| hdrs*  |ESP|IP hdr| hdrs *  |TCP|Data|Trailer| ICV|            ------------------------------------------------------------                                |<--------- encryption ---------->|                            |<------------ integrity ------------>|            * = if present, construction of outer IP hdr/extensions and                modification of inner IP hdr/extensions is discussed in                the Security Architecture document.3.2.  Algorithms   The mandatory-to-implement algorithms for use with ESP are described   in a separate RFC, to facilitate updating the algorithm requirements   independently from the protocol per se.  Additional algorithms,   beyond those mandated for ESP, MAY be supported.  Note that although   both confidentiality and integrity are optional, at least one of   these services MUST be selected, hence both algorithms MUST NOT be   simultaneously NULL.Kent                        Standards Track                    [Page 20]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20053.2.1.  Encryption Algorithms   The encryption algorithm employed to protect an ESP packet is   specified by the SA via which the packet is transmitted/received.   Because IP packets may arrive out of order, and not all packets may   arrive (packet loss), 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 plaintext portions of the (outer IP or ESP) packet   header.  (Note that if plaintext header information is used to derive   an IV, that information may become security critical and thus the   protection boundary associated with the encryption process may grow.   For example, if one were to use the ESP Sequence Number to derive an   IV, the Sequence Number generation logic (hardware or software) would   have to be evaluated as part of the encryption algorithm   implementation.  In the case of FIPS 140-2 [NIST01], this could   significantly extend the scope of a cryptographic module evaluation.)   Because ESP makes provision for padding of the plaintext, encryption   algorithms employed with ESP may exhibit either block or stream mode   characteristics.  Note that because encryption (confidentiality) MAY   be an optional service (e.g., integrity-only ESP), this algorithm MAY   be "NULL" [Ken-Arch].   To allow an ESP implementation to compute the encryption padding   required by a block mode encryption algorithm, and to determine the   MTU impact of the algorithm, the RFC for each encryption algorithm   used with ESP must specify the padding modulus for the algorithm.3.2.2.  Integrity Algorithms   The integrity algorithm employed for the ICV computation is specified   by the SA via which the packet is transmitted/received.  As was the   case for encryption algorithms, any integrity algorithm employed with   ESP must make provisions to permit processing of packets that arrive   out of order and to accommodate packet loss.  The same admonition   noted above applies to use of any plaintext data to facilitate   receiver synchronization of integrity algorithms.  Note that because   the integrity service MAY be optional, this algorithm may be "NULL".   To allow an ESP implementation to compute any implicit integrity   algorithm padding required, the RFC for each algorithm used with ESP   must specify the padding modulus for the algorithm.Kent                        Standards Track                    [Page 21]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20053.2.3.  Combined Mode Algorithms   If a combined mode algorithm is employed, both confidentiality and   integrity services are provided.  As was the case for encryption   algorithms, a combined mode algorithm must make provisions for per-   packet cryptographic synchronization, to permit decryption of packets   that arrive out of order and to accommodate packet loss.  The means   by which a combined mode algorithm provides integrity for the   payload, and for the SPI and (Extended) Sequence Number fields, may   vary for different algorithm choices.  In order to provide a uniform,   algorithm-independent approach to invocation of combined mode   algorithms, no payload substructure is defined.  For example, the SPI   and Sequence Number fields might be replicated within the ciphertext   envelope and an ICV may be appended to the ESP trailer.  None of   these details should be observable externally.   To allow an ESP implementation to determine the MTU impact of a   combined mode algorithm, the RFC for each algorithm used with ESP   must specify a (simple) formula that yields encrypted payload size,   as a function of the plaintext payload and sequence number sizes.3.3.  Outbound Packet Processing   In transport mode, the sender encapsulates the next layer protocol   information between the ESP header and the ESP trailer fields, 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 interrelated in a variety of ways.  The   construction of the outer IP header/extensions during the   encapsulation process is described in the Security Architecture   document.3.3.1.  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.3.3.2.  Packet Encryption and Integrity Check Value (ICV) Calculation   In this section, we speak in terms of encryption always being applied   because of the formatting implications.  This is done with the   understanding that "no confidentiality" is offered by using the NULL   encryption algorithm (RFC 2410).  There are several algorithmic   options.Kent                        Standards Track                    [Page 22]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 20053.3.2.1.  Separate Confidentiality and Integrity Algorithms   If separate confidentiality and integrity algorithms are employed,   the Sender proceeds as follows:         1. Encapsulate (into the ESP Payload field):                 - for transport mode -- just the original next layer                   protocol information.                 - for tunnel mode -- the entire original IP datagram.         2. Add any necessary padding -- Optional TFC padding and            (encryption) Padding         3. Encrypt the result using the key, encryption algorithm,            and algorithm mode specified for the SA and using any            required cryptographic synchronization data.                 - If explicit cryptographic synchronization data,                   e.g., an IV, is indicated, it is input to the                   encryption algorithm per the algorithm specification                   and placed in the Payload field.                 - If implicit cryptographic synchronization data is                   employed, it is constructed and input to the                   encryption algorithm as per the algorithm                   specification.                 - If integrity is selected, encryption is performed                   first, before the integrity algorithm is applied, and                   the encryption does not encompass the ICV 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                   (DoS) attacks.  It also allows for the possibility of                   parallel processing of packets at the receiver, i.e.,                   decryption can take place in parallel with integrity                   checking.  Note that because the ICV is not protected                   by encryption, a keyed integrity algorithm must be                   employed to compute the ICV.         4. Compute the ICV over the ESP packet minus the ICV field.            Thus, the ICV computation encompasses the SPI, Sequence            Number, Payload Data, Padding (if present), Pad Length, and            Next Header.  (Note that the last 4 fields will be in            ciphertext form, because encryption is performed first.)  If            the ESN option is enabled for the SA, the high-order 32            bits of the sequence number are appended after the Next            Header field for purposes of this computation, but are not            transmitted.Kent                        Standards Track                    [Page 23]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   For some integrity algorithms, the byte string over which the ICV   computation is performed must be a multiple of a block size specified   by the algorithm.  If the length of ESP packet (as described above)   does not match the block size requirements for the algorithm,   implicit padding MUST be appended to the end of the ESP packet.   (This padding is added after the Next Header field, or after the   high-order 32 bits of the sequence number, if ESN is selected.)  The   block size (and hence the length of the padding) is specified by the   integrity algorithm specification.  This padding is not transmitted   with the packet.  The document that defines an integrity algorithm   MUST be consulted to determine if implicit padding is required as   described above.  If the document does not specify an answer to this   question, then the default is to assume that implicit padding is   required (as needed to match the packet length to the algorithm's   block size.)  If padding bytes are needed but the algorithm does not   specify the padding contents, then the padding octets MUST have a   value of zero.3.3.2.2.  Combined Confidentiality and Integrity Algorithms   If a combined confidentiality/integrity algorithm is employed, the   Sender proceeds as follows:         1. Encapsulate into the ESP Payload Data field:                 - for transport mode -- just the original next layer                   protocol information.                 - for tunnel mode -- the entire original IP datagram.         2. Add any necessary padding -- includes optional TFC padding            and (encryption) Padding.         3. Encrypt and integrity protect the result using the key            and combined mode algorithm specified for the SA and using            any required cryptographic synchronization data.                 - If explicit cryptographic synchronization data,                   e.g., an IV, is indicated, it is input to the                   combined mode algorithm per the algorithm                   specification and placed in the Payload field.                 - If implicit cryptographic synchronization data is                   employed, it is constructed and input to the                   encryption algorithm as per the algorithm                   specification.                 - The Sequence Number (or Extended Sequence Number, as                   appropriate) and the SPI are inputs to the                   algorithm, as they must be included in the integrity                   check computation.  The means by which these values                   are included in this computation are a function ofKent                        Standards Track                    [Page 24]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005                   the combined mode algorithm employed and thus not                   specified in this standard.                 - The (explicit) ICV field MAY be a part of the ESP                   packet format when a combined mode algorithm is                   employed.  If one is not used, an analogous field                   usually will be a part of the ciphertext payload.                   The location of any integrity fields, and the means                   by which the Sequence Number and SPI are included in                   the integrity computation, MUST be defined in an RFC                   that defines the use of the combined mode algorithm                   with ESP.3.3.3.  Sequence Number Generation   The sender's counter is initialized to 0 when an SA is established.   The sender increments the sequence number (or ESN) counter for this   SA and inserts the low-order 32 bits of the value into the Sequence   Number field.  Thus, the first packet sent using a given SA will   contain a sequence number of 1.   If anti-replay is enabled (the default), the sender checks to ensure   that the counter has not cycled before inserting the new value in the   Sequence Number field.  In other words, the sender 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.  The audit log entry for this event   SHOULD include the SPI value, current date/time, Source Address,   Destination Address, and (in IPv6) the cleartext Flow ID.   The sender assumes anti-replay is enabled as a default, unless   otherwise notified by the receiver (seeSection 3.4.3).  Thus,   typical behavior of an ESP implementation calls for the sender to   establish a new SA when the Sequence Number (or ESN) cycles, or in   anticipation of this value cycling.   If the key used to compute an ICV is manually distributed, a   compliant implementation SHOULD NOT provide anti-replay service.  If   a user chooses to employ anti-replay in conjunction with SAs that are   manually keyed, the sequence number counter at the sender MUST be   correctly maintained across local reboots, etc., until the key is   replaced.  (SeeSection 5.)   If anti-replay is disabled (as noted above), the sender does not need   to monitor or reset the counter.  However, the sender still   increments the counter and when it reaches the maximum value, the   counter rolls over back to zero.  (This behavior is recommended for   multi-sender, multicast SAs, unless anti-replay mechanisms outsideKent                        Standards Track                    [Page 25]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   the scope of this standard are negotiated between the sender and   receiver.)   If ESN (see Appendix) is selected, only the low-order 32 bits of the   sequence number are transmitted in the Sequence Number field,   although both sender and receiver maintain full 64-bit ESN counters.   The high order 32 bits are included in the integrity check in an   algorithm/mode-specific fashion, e.g., the high-order 32 bits may be   appended after the Next Header field when a separate integrity   algorithm is employed.   Note: If a receiver chooses to not enable anti-replay for an SA, then   the receiver SHOULD NOT negotiate ESN in an SA management protocol.   Use of ESN creates a need for the receiver to manage the anti-replay   window (in order to determine the correct value for the high-order   bits of the ESN, which are employed in the ICV computation), which is   generally contrary to the notion of disabling anti-replay for an SA.3.3.4.  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, which may   be a fragment of an IP datagram.  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.   NOTE: For transport mode -- As mentioned at the end ofSection 3.1.1,   bump-in-the-stack and bump-in-the-wire implementations may have to   first reassemble a packet fragmented by the local IP layer, then   apply IPsec, and then fragment the resulting packet.   NOTE: For IPv6 -- For bump-in-the-stack and bump-in-the-wire   implementations, it will be necessary to examine all the extension   headers to determine if there is a fragmentation header and hence   that the packet needs reassembling prior to IPsec processing.   Fragmentation, whether performed by an IPsec implementation or by   routers along the path between IPsec peers, significantly reduces   performance.  Moreover, the requirement for an ESP receiver to accept   fragments for reassembly creates denial of service vulnerabilities.   Thus, an ESP implementation MAY choose to not support fragmentation   and may mark transmitted packets with the DF bit, to facilitate Path   MTU (PMTU) discovery.  In any case, an ESP implementation MUSTKent                        Standards Track                    [Page 26]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   support generation of ICMP PMTU messages (or equivalent internal   signaling for native host implementations) to minimize the likelihood   of fragmentation.  Details of the support required for MTU management   are contained in the Security Architecture document.3.4.  Inbound Packet Processing3.4.1.  Reassembly   If required, reassembly is performed prior to ESP processing.  If a   packet offered to ESP for processing appears to be an IP fragment,   i.e., the OFFSET field is non-zero or the MORE FRAGMENTS flag is set,   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 received, Source Address, Destination Address, Sequence   Number, and (in IPv6) the Flow ID.   NOTE: For packet reassembly, the current IPv4 spec does NOT require   either the zeroing of the OFFSET field or the clearing of the MORE   FRAGMENTS flag.  In order for a reassembled packet to be processed by   IPsec (as opposed to discarded as an apparent fragment), the IP code   must do these two things after it reassembles a packet.3.4.2.  Security Association Lookup   Upon receipt of a packet containing an ESP Header, the receiver   determines the appropriate (unidirectional) SA via lookup in the SAD.   For a unicast SA, this determination is based on the SPI or the SPI   plus protocol field, as described inSection 2.1.  If an   implementation supports multicast traffic, the destination address is   also employed in the lookup (in addition to the SPI), and the sender   address also may be employed, as described inSection 2.1.  (This   process is described in more detail in the Security Architecture   document.)  The SAD entry for the SA also indicates whether the   Sequence Number field will be checked, whether 32- or 64-bit sequence   numbers are employed for the SA, and whether the (explicit) ICV field   should be present (and if so, its size).  Also, the SAD entry will   specify the algorithms and keys to be employed for decryption and ICV   computation (if applicable).   If no valid Security Association exists for this packet, 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   received, Source Address, Destination Address, Sequence Number, and   (in IPv6) the cleartext Flow ID.Kent                        Standards Track                    [Page 27]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   (Note that SA management traffic, such as IKE packets, does not need   to be processed based on SPI, i.e., one can demultiplex this traffic   separately based on Next Protocol and Port fields, for example.)3.4.3.  Sequence Number Verification   All ESP implementations MUST support the anti-replay service, though   its use may be enabled or disabled by the receiver on a per-SA basis.   This service MUST NOT be enabled unless the ESP integrity service   also is enabled for the SA, because otherwise the Sequence Number   field has not been integrity protected.  Anti-replay is applicable to   unicast as well as multicast SAs.  However, this standard specifies   no mechanisms for providing anti-replay for a multi-sender SA   (unicast or multicast).  In the absence of negotiation (or manual   configuration) of an anti-replay mechanism for such an SA, it is   recommended that sender and receiver checking of the sequence number   for the SA be disabled (via negotiation or manual configuration), as   noted below.   If the receiver does not enable anti-replay for an SA, no inbound   checks are performed on the Sequence Number.  However, from the   perspective of the sender, the default is to assume that anti-replay   is enabled at the receiver.  To avoid having the sender do   unnecessary sequence number monitoring and SA setup (seesection3.3.3), if an SA establishment protocol is employed, the receiver   SHOULD notify the sender, during SA establishment, if the receiver   will not provide anti-replay protection.   If the receiver has enabled 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.   ESP permits two-stage verification of packet sequence numbers.  This   capability is important whenever an ESP implementation (typically the   cryptographic module portion thereof) is not capable of performing   decryption and/or integrity checking at the same rate as the   interface(s) to unprotected networks.  If the implementation is   capable of such "line rate" operation, then it is not necessary to   perform the preliminary verification stage described below.   The preliminary Sequence Number check is effected utilizing the   Sequence Number value in the ESP Header and is performed prior to   integrity checking and decryption.  If this preliminary check fails,Kent                        Standards Track                    [Page 28]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   the packet is discarded, thus avoiding the need for any cryptographic   operations by the receiver.  If the preliminary check is successful,   the receiver cannot yet modify its local counter, because the   integrity of the Sequence Number has not been verified at this point.   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.   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.  If the ESN option is   selected for an SA, only the low-order 32 bits of the sequence number   are explicitly transmitted, but the receiver employs the full   sequence number computed using the high-order 32 bits for the   indicated SA (from his local counter) when checking the received   Sequence Number against the receive window.  In constructing the full   sequence number, if the low-order 32 bits carried in the packet are   lower in value than the low-order 32 bits of the receiver's sequence   number, the receiver assumes that the high-order 32 bits have been   incremented, moving to a new sequence number subspace.  (This   algorithm accommodates gaps in reception for a single SA as large as   2**32-1 packets.  If a larger gap occurs, additional, heuristic   checks for re-synchronization of the receiver sequence number counter   MAY be employed, as described in the Appendix.)   If the received packet falls within the window and is not a   duplicate, or if the packet is to the right of the window, and if a   separate integrity algorithm is employed, then the receiver proceeds   to integrity verification.  If a combined mode algorithm is employed,   the integrity check is performed along with decryption.  In either   case, if the integrity check 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 received, Source Address, Destination Address, the Sequence   Number, and (in IPv6) the Flow ID.  The receive window is updated   only if the integrity verification succeeds.  (If a combined mode   algorithm is being used, then the integrity protected Sequence Number   must also match the Sequence Number used for anti-replay protection.)   A minimum window size of 32 packets MUST be supported when 32-bit   sequence numbers are employed; a window size of 64 is preferred and   SHOULD be employed as the default.  Another window size (larger than   the minimum) MAY be chosen by the receiver.  (The receiver does NOT   notify the sender of the window size.)  The receive window sizeKent                        Standards Track                    [Page 29]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   should be increased for higher-speed environments, irrespective of   assurance issues.  Values for minimum and recommended receive window   sizes for very high-speed (e.g., multi-gigabit/second) devices are   not specified by this standard.3.4.4.  Integrity Check Value Verification   As with outbound processing, there are several options for inbound   processing, based on features of the algorithms employed.3.4.4.1.  Separate Confidentiality and Integrity Algorithms   If separate confidentiality and integrity algorithms are employed   processing proceeds as follows:         1. If integrity has been selected, the receiver computes the            ICV over the ESP packet minus the ICV, using the specified            integrity algorithm and verifies that it is the same as the            ICV carried in the packet.  Details of the computation are            provided below.            If the computed and received ICVs 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, the Sequence Number, and (for IPv6) the cleartext            Flow ID.            Implementation Note:            Implementations can use any set of steps that results in the            same result as the following set of steps.  Begin by            removing and saving the ICV field.  Next check the overall            length of the ESP packet minus the ICV field.  If implicit            padding is required, based on the block size of the            integrity algorithm, append zero-filled bytes to the end of            the ESP packet directly after the Next Header field, or            after the high-order 32 bits of the sequence number if ESN            is selected.  Perform the ICV computation and compare the            result with the saved value, using the comparison rules            defined by the algorithm specification.         2. The receiver decrypts the ESP Payload Data, Padding, Pad            Length, and Next Header using the key, encryption algorithm,            algorithm mode, and cryptographic synchronization data (if            any), indicated by the SA.  As inSection 3.3.2, we speak            here in terms of encryption always being applied because ofKent                        Standards Track                    [Page 30]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005            the formatting implications.  This is done with the            understanding that "no confidentiality" is offered by using            the NULL encryption algorithm (RFC 2410).                 - If explicit cryptographic synchronization data, e.g.,                   an IV, is indicated, it is taken from the Payload                   field and input to the decryption algorithm as per                   the algorithm specification.                 - If implicit cryptographic synchronization data is                   indicated, a local version of the IV is constructed                   and input to the decryption algorithm as per the                   algorithm specification.         3. The receiver processes any Padding as specified in the            encryption algorithm specification.  If the default padding            scheme (seeSection 2.4) has been employed, the receiver            SHOULD inspect the Padding field before removing the padding            prior to passing the decrypted data to the next layer.         4. The receiver checks the Next Header field.  If the value is            "59" (no next header), the (dummy) packet is discarded            without further processing.         5. The receiver reconstructs the original IP datagram from:                 - for transport mode -- outer IP header plus the                   original next layer protocol information in the ESP                   Payload field                 - for tunnel mode -- the entire IP datagram in the ESP                   Payload field.            The exact steps for reconstructing the original datagram            depend on the mode (transport or tunnel) 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.  This            processing "discards" any (optional) TFC padding that has            been added for traffic flow confidentiality.  (If present,            this will have been inserted after the IP datagram (or            transport-layer frame) and before the Padding field (seeSection 2.4).)   If integrity checking and encryption are performed in parallel,   integrity checking MUST be completed before the decrypted packet is   passed on for further processing.  This order of processing   facilitates rapid detection and rejection of replayed or bogusKent                        Standards Track                    [Page 31]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   packets by the receiver, prior to decrypting the packet, hence   potentially reducing the impact of denial of service attacks.   Note: If the receiver performs decryption in parallel with integrity   checking, care must be taken to avoid possible race conditions with   regard to packet access and extraction of the decrypted packet.3.4.4.2.  Combined Confidentiality and Integrity Algorithms   If a combined confidentiality and integrity algorithm is employed,   then the receiver proceeds as follows:         1. Decrypts and integrity checks the ESP Payload Data, Padding,            Pad Length, and Next Header, using the key, algorithm,            algorithm mode, and cryptographic synchronization data (if            any), indicated by the SA.  The SPI from the ESP header, and            the (receiver) packet counter value (adjusted as required            from the processing described inSection 3.4.3) are inputs            to this algorithm, as they are required for the integrity            check.                 - If explicit cryptographic synchronization data, e.g.,                   an IV, is indicated, it is taken from the Payload                   field and input to the decryption algorithm as per                   the algorithm specification.                 - If implicit cryptographic synchronization data, e.g.,                   an IV, is indicated, a local version of the IV is                   constructed and input to the decryption algorithm as                   per the algorithm specification.         2. If the integrity check performed by the combined mode            algorithm fails, 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, the Sequence Number,            and (in IPv6) the cleartext Flow ID.         3. Process any Padding as specified in the encryption algorithm            specification, if the algorithm has not already done so.         4. The receiver checks the Next Header field.  If the value is            "59" (no next header), the (dummy) packet is discarded            without further processing.         5. Extract the original IP datagram (tunnel mode) or            transport-layer frame (transport mode) from the ESP Payload            Data field.  This implicitly discards any (optional) paddingKent                        Standards Track                    [Page 32]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005            that has been added for traffic flow confidentiality.  (If            present, the TFC padding will have been inserted after the            IP payload and before the Padding field (seeSection 2.4).)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.         - No valid Security Association exists for a session.  The           audit log entry for this event SHOULD include the SPI value,           date/time received, Source Address, Destination Address,           Sequence Number, and (for IPv6) the cleartext Flow ID.         - A packet offered to ESP for processing appears to be an IP           fragment, i.e., the OFFSET field is non-zero or the MORE           FRAGMENTS flag is set.  The audit log entry for this event           SHOULD include the SPI value, date/time received, Source           Address, Destination Address, Sequence Number, and (in IPv6)           the Flow ID.         - Attempt to transmit a packet that would result in Sequence           Number overflow.  The audit log entry for this event SHOULD           include the SPI value, current date/time, Source Address,           Destination Address, Sequence Number, and (for IPv6) the           cleartext Flow ID.         - The received packet fails the anti-replay checks.  The audit           log entry for this event SHOULD include the SPI value,           date/time received, Source Address, Destination Address, the           Sequence Number, and (in IPv6) the Flow ID.         - The integrity check fails.  The audit log entry for this           event SHOULD include the SPI value, date/time received,           Source Address, Destination Address, the Sequence Number, and           (for IPv6) the Flow ID.   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 result in audit log entries.  There is   no requirement for the receiver to transmit any message to theKent                        Standards Track                    [Page 33]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   purported sender 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 for unicast traffic, and MUST comply with all additional packet   processing requirements levied by the Security Architecture document   [Ken-Arch].  Additionally, if an implementation claims to support   multicast traffic, it MUST comply with the additional requirements   specified for support of such traffic.  If the key used to compute an   ICV is manually distributed, correct provision of the anti-replay   service requires correct maintenance of the counter state at the   sender (across local reboots, etc.), 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 anti-replay service in conjunction with SAs that are manually   keyed.   The mandatory-to-implement algorithms for use with ESP are described   in a separate document [Eas04], to facilitate updating the algorithm   requirements independently from the protocol per se.  Additional   algorithms, beyond those mandated for ESP, MAY be supported.   Because use of encryption in ESP is optional, support for the "NULL"   encryption algorithm also is required to maintain consistency with   the way ESP services are negotiated.  Support for the   confidentiality-only service version of ESP is optional.  If an   implementation offers this service, it MUST also support the   negotiation of the "NULL" integrity algorithm.  NOTE that although   integrity and encryption may each be "NULL" under the circumstances   noted above, they MUST NOT both be "NULL".6.  Security Considerations   Security is central to the design of this protocol, and thus security   considerations permeate the specification.  Additional security-   relevant aspects of using the IPsec protocol are discussed in the   Security Architecture document.7.  Differences fromRFC 2406   This document differs fromRFC 2406 in a number of significant ways.        o Confidentiality-only service -- now a MAY, not a MUST.Kent                        Standards Track                    [Page 34]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005        o SPI -- modified to specify a uniform algorithm for SAD lookup          for unicast and multicast SAs, covering a wider range of          multicast technologies.  For unicast, the SPI may be used          alone to select an SA, or may be combined with the protocol,          at the option of the receiver.  For multicast SAs, the SPI is          combined with the destination address, and optionally the          source address, to select an SA.        o Extended Sequence Number -- added a new option for a 64-bit          sequence number for very high-speed communications.  Clarified          sender and receiver processing requirements for multicast SAs          and multi-sender SAs.        o Payload data -- broadened model to accommodate combined mode          algorithms.        o Padding for improved traffic flow confidentiality -- added          requirement to be able to add bytes after the end of the IP          Payload, prior to the beginning of the Padding field.        o Next Header -- added requirement to be able to generate and          discard dummy padding packets (Next Header = 59)        o ICV -- broadened model to accommodate combined mode          algorithms.        o Algorithms -- Added combined confidentiality mode algorithms.        o Moved references to mandatory algorithms to a separate          document.        o Inbound and Outbound packet processing -- there are now two          paths: (1) separate confidentiality and integrity          algorithms and (2) combined confidentiality mode          algorithms.  Because of the addition of combined mode          algorithms, the encryption/decryption and integrity sections          have been combined for both inbound and outbound packet          processing.8.  Backward-Compatibility Considerations   There is no version number in ESP and no mechanism enabling IPsec   peers to discover or negotiate which version of ESP each is using or   should use.  This section discusses consequent backward-compatibility   issues.   First, if none of the new features available in ESP v3 are employed,   then the format of an ESP packet is identical in ESP v2 and v3.  If a   combined mode encryption algorithm is employed, a feature supported   only in ESP v3, then the resulting packet format may differ from the   ESP v2 spec.  However, a peer who implements only ESP v2 would never   negotiate such an algorithm, as they are defined for use only in the   ESP v3 context.Kent                        Standards Track                    [Page 35]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   Extended Sequence Number (ESN) negotiation is supported by IKE v2 and   has been addressed for IKE v1 by the ESN Addendum to the IKE v1   Domain of Interpretation (DOI).   In the new ESP (v3), we make two provisions to better support traffic   flow confidentiality (TFC):        - arbitrary padding after the end of an IP packet        - a discard convention using Next Header = 59   The first feature is one that should not cause problems for a   receiver, since the IP total length field indicates where the IP   packet ends.  Thus, any TFC padding bytes after the end of the packet   should be removed at some point during IP packet processing, after   ESP processing, even if the IPsec software does not remove such   padding.  Thus, this is an ESP v3 feature that a sender can employ   irrespective of whether a receiver implements ESP v2 or ESP v3.   The second feature allows a sender to send a payload that is an   arbitrary string of bytes that do not necessarily constitute a well-   formed IP packet, inside of a tunnel, for TFC purposes.  It is an   open question as to what an ESP v2 receiver will do when the Next   Header field in an ESP packet contains the value "59".  It might   discard the packet when it finds an ill-formed IP header, and log   this event, but it certainly ought not to crash, because such   behavior would constitute a DoS vulnerability relative to traffic   received from authenticated peers.  Thus this feature is an   optimization that an ESP v3 sender can make use of irrespective of   whether a receiver implements ESP v2 or ESP v3.9.  Acknowledgements   The author would like to acknowledge the contributions of Ran   Atkinson, who played a critical role in initial IPsec activities, and   who authored the first series of IPsec standards: RFCs 1825-1827.   Karen Seo deserves special thanks for providing help in the editing   of this and the previous version of this specification.  The author   also would like to thank the members of the IPSEC and MSEC working   groups who have contributed to the development of this protocol   specification.10.  References10.1.  Normative References   [Bra97]    Bradner, S., "Key words for use in RFCs to Indicate              Requirement Level",BCP 14,RFC 2119, March 1997.Kent                        Standards Track                    [Page 36]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   [DH98]     Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, December 1998.   [Eas04]    3rd Eastlake, D., "Cryptographic Algorithm Implementation              Requirements for Encapsulating Security Payload (ESP) and              Authentication Header (AH)",RFC 4305, December 2005.   [Ken-Arch] Kent, S. and K. Seo, "Security Architecture for the              Internet Protocol",RFC 4301, December 2005.   [Pos81]    Postel, J., "Internet Protocol", STD 5,RFC 791, September              1981.10.2.  Informative References   [Bel96]    Steven M. Bellovin, "Problem Areas for the IP Security              Protocols", Proceedings of the Sixth Usenix Unix Security              Symposium, July, 1996.   [HC03]     Holbrook, H. and B. Cain, "Source-Specific Multicast for              IP", Work in Progress, November 3, 2002.   [Kau05]    Kaufman, C., Ed., "The Internet Key Exchange (IKEv2)              Protocol",RFC 4306, December 2005.   [Ken-AH]   Kent, S., "IP Authentication Header",RFC 4302, December              2005.   [Kra01]    Krawczyk, H., "The Order of Encryption and Authentication              for Protecting Communications (Or: How Secure Is SSL?)",              CRYPTO' 2001.   [NIST01]   Federal Information Processing Standards Publication 140-2              (FIPS PUB 140-2), "Security Requirements for Cryptographic              Modules", Information Technology Laboratory, National              Institute of Standards and Technology, May 25, 2001.   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The              Group Domain of Interpretation",RFC 3547, July 2003.   [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security              Architecture",RFC 3740, March 2004.   [Syverson] P. Syverson, D. Goldschlag, and M. Reed, "Anonymous              Connections and Onion Routing", Proceedings of the              Symposium on Security and Privacy, Oakland, CA, May 1997,              pages 44-54.Kent                        Standards Track                    [Page 37]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005Appendix A: Extended (64-bit) Sequence NumbersA1.  Overview   This appendix describes an extended sequence number (ESN) scheme for   use with IPsec (ESP and AH) that employs a 64-bit sequence number,   but in which only the low-order 32 bits are transmitted as part of   each packet.  It covers both the window scheme used to detect   replayed packets and the determination of the high-order bits of the   sequence number that are used both for replay rejection and for   computation of the ICV.  It also discusses a mechanism for handling   loss of synchronization relative to the (not transmitted) high-order   bits.A2.  Anti-Replay Window   The receiver will maintain an anti-replay window of size W.  This   window will limit how far out of order a packet can be, relative to   the packet with the highest sequence number that has been   authenticated so far.  (No requirement is established for minimum or   recommended sizes for this window, beyond the 32- and 64-packet   values already established for 32-bit sequence number windows.   However, it is suggested that an implementer scale these values   consistent with the interface speed supported by an implementation   that makes use of the ESN option.  Also, the algorithm described   below assumes that the window is no greater than 2^31 packets in   width.)  All 2^32 sequence numbers associated with any fixed value   for the high-order 32 bits (Seqh) will hereafter be called a sequence   number subspace.  The following table lists pertinent variables and   their definitions.        Var.   Size        Name  (bits)            Meaning        ----  ------  ---------------------------        W       32    Size of window        T       64    Highest sequence number authenticated so far,                      upper bound of window          Tl      32    Lower 32 bits of T          Th      32    Upper 32 bits of T        B       64    Lower bound of window          Bl      32    Lower 32 bits of B          Bh      32    Upper 32 bits of B        Seq     64    Sequence Number of received packet          Seql    32    Lower 32 bits of Seq          Seqh    32    Upper 32 bits of SeqKent                        Standards Track                    [Page 38]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   When performing the anti-replay check, or when determining which   high-order bits to use to authenticate an incoming packet, there are   two cases:     + Case A: Tl >= (W - 1). In this case, the window is within one                              sequence number subspace.  (See Figure 1)     + Case B: Tl < (W - 1).  In this case, the window spans two                              sequence number subspaces.  (See Figure 2)   In the figures below, the bottom line ("----") shows two consecutive   sequence number subspaces, with zeros indicating the beginning of   each subspace.  The two shorter lines above it show the higher-order   bits that apply.  The "====" represents the window.  The "****"   represents future sequence numbers, i.e., those beyond the current   highest sequence number authenticated (ThTl).        Th+1                         *********        Th               =======*****              --0--------+-----+-----0--------+-----------0--                         Bl    Tl            Bl                                        (Bl+2^32) mod 2^32                            Figure 1 -- Case A        Th                           ====**************        Th-1                      ===              --0-----------------+--0--+--------------+--0--                                  Bl    Tl            Bl                                                 (Bl+2^32) mod 2^32                            Figure 2 -- Case BA2.1.  Managing and Using the Anti-Replay Window   The anti-replay window can be thought of as a string of bits where   `W' defines the length of the string.  W = T - B + 1 and cannot   exceed 2^32 - 1 in value.  The bottom-most bit corresponds to B and   the top-most bit corresponds to T, and each sequence number from Bl   through Tl is represented by a corresponding bit.  The value of the   bit indicates whether or not a packet with that sequence number has   been received and authenticated, so that replays can be detected and   rejected.Kent                        Standards Track                    [Page 39]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005   When a packet with a 64-bit sequence number (Seq) greater than T is   received and validated,      + B is increased by (Seq - T)      + (Seq - T) bits are dropped from the low end of the window      + (Seq - T) bits are added to the high end of the window      + The top bit is set to indicate that a packet with that sequence        number has been received and authenticated      + The new bits between T and the top bit are set to indicate that        no packets with those sequence numbers have been received yet.      + T is set to the new sequence number   In checking for replayed packets,      + Under Case A: If Seql >= Bl (where Bl = Tl - W + 1) AND Seql <=        Tl, then check the corresponding bit in the window to see if        this Seql has already been seen.  If yes, reject the packet.  If        no, perform integrity check (seeAppendix A2.2. below for        determination of Seqh).      + Under Case B: If Seql >= Bl (where Bl = Tl - W + 1) OR Seql <=        Tl, then check the corresponding bit in the window to see if        this Seql has already been seen.  If yes, reject the packet.  If        no, perform integrity check (seeAppendix A2.2. below for        determination of Seqh).A2.2.  Determining the Higher-Order Bits (Seqh) of the Sequence Number   Because only `Seql' will be transmitted with the packet, the receiver   must deduce and track the sequence number subspace into which each   packet falls, i.e., determine the value of Seqh.  The following   equations define how to select Seqh under "normal" conditions; see   Section A3 for a discussion of how to recover from extreme packet   loss.      + Under Case A (Figure 1):        If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th        If Seql <  Bl (where Bl = Tl - W + 1), then Seqh = Th + 1      + Under Case B (Figure 2):        If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th - 1        If Seql <  Bl (where Bl = Tl - W + 1), then Seqh = ThKent                        Standards Track                    [Page 40]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005A2.3.  Pseudo-Code Example   The following pseudo-code illustrates the above algorithms for anti-   replay and integrity checks.  The values for `Seql', `Tl', `Th' and   `W' are 32-bit unsigned integers.  Arithmetic is mod 2^32.        If (Tl >= W - 1)                            Case A            If (Seql >= Tl - W + 1)                Seqh = Th                If (Seql <= Tl)                    If (pass replay check)                        If (pass integrity check)                            Set bit corresponding to Seql                            Pass the packet on                        Else reject packet                    Else reject packet                Else                    If (pass integrity check)                        Tl = Seql (shift bits)                        Set bit corresponding to Seql                        Pass the packet on                    Else reject packet            Else                Seqh = Th + 1                If (pass integrity check)                    Tl = Seql (shift bits)                    Th = Th + 1                    Set bit corresponding to Seql                    Pass the packet on                Else reject packet        Else                                    Case B            If (Seql >= Tl - W + 1)                Seqh = Th - 1                If (pass replay check)                    If (pass integrity check)                        Set the bit corresponding to Seql                        Pass packet on                    Else reject packet                Else reject packet            Else                Seqh = Th                If (Seql <= Tl)                    If (pass replay check)                        If (pass integrity check)                            Set the bit corresponding to Seql                            Pass packet on                        Else reject packet                    Else reject packetKent                        Standards Track                    [Page 41]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005                Else                    If (pass integrity check)                        Tl = Seql (shift bits)                        Set the bit corresponding to Seql                        Pass packet on                    Else reject packetA3.  Handling Loss of Synchronization due to Significant Packet Loss   If there is an undetected packet loss of 2^32 or more consecutive   packets on a single SA, then the transmitter and receiver will lose   synchronization of the high-order bits, i.e., the equations in   Section A2.2. will fail to yield the correct value.  Unless this   problem is detected and addressed, subsequent packets on this SA will   fail authentication checks and be discarded.  The following procedure   SHOULD be implemented by any IPsec (ESP or AH) implementation that   supports the ESN option.   Note that this sort of extended traffic loss is likely to be detected   at higher layers in most cases, before IPsec would have to invoke the   sort of re-synchronization mechanism described in A3.1 and A3.2. If   any significant fraction of the traffic on the SA in question is TCP,   the source would fail to receive ACKs and would stop sending long   before 2^32 packets had been lost.  Also, for any bi-directional   application, even ones operating above UDP, such an extended outage   would likely result in triggering some form of timeout.  However, a   unidirectional application, operating over UDP, might lack feedback   that would cause automatic detection of a loss of this magnitude,   hence the motivation to develop a recovery method for this case.   Note that the above observations apply to SAs between security   gateways, or between hosts, or between host and security gateways.   The solution we've chosen was selected to:     + minimize the impact on normal traffic processing     + avoid creating an opportunity for a new denial of service attack       such as might occur by allowing an attacker to force diversion of       resources to a re-synchronization process     + limit the recovery mechanism to the receiver -- because anti-       replay is a service only for the receiver, and the transmitter       generally is not aware of whether the receiver is using sequence       numbers in support of this optional service, it is preferable for       recovery mechanisms to be local to the receiver.  This also       allows for backward compatibility.Kent                        Standards Track                    [Page 42]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005A3.1.  Triggering Re-synchronization   For each SA, the receiver records the number of consecutive packets   that fail authentication.  This count is used to trigger the re-   synchronization process, which should be performed in the background   or using a separate processor.  Receipt of a valid packet on the SA   resets the counter to zero.  The value used to trigger the re-   synchronization process is a local parameter.  There is no   requirement to support distinct trigger values for different SAs,   although an implementer may choose to do so.A3.2.  Re-synchronization Process   When the above trigger point is reached, a "bad" packet is selected   for which authentication is retried using successively larger values   for the upper half of the sequence number (Seqh).  These values are   generated by incrementing by one for each retry.  The number of   retries should be limited, in case this is a packet from the "past"   or a bogus packet.  The limit value is a local parameter.  (Because   the Seqh value is implicitly placed after the ESP (or AH) payload, it   may be possible to optimize this procedure by executing the integrity   algorithm over the packet up to the endpoint of the payload, then   compute different candidate ICVs by varying the value of Seqh.)   Successful authentication of a packet via this procedure resets the   consecutive failure count and sets the value of T to that of the   received packet.   This solution requires support only on the part of the receiver,   thereby allowing for backward compatibility.  Also, because re-   synchronization efforts would either occur in the background or   utilize an additional processor, this solution does not impact   traffic processing and a denial of service attack cannot divert   resources away from traffic processing.Author's Address   Stephen Kent   BBN Technologies   10 Moulton Street   Cambridge, MA  02138   USA   Phone: +1 (617) 873-3988   EMail: kent@bbn.comKent                        Standards Track                    [Page 43]

RFC 4303        IP Encapsulating Security Payload (ESP)    December 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Kent                        Standards Track                    [Page 44]