Network Working Group S. Kent
Request for Comments: 2406 BBN Corp
Obsoletes: 1827 R. Atkinson
Category: Standards Track @Home Network
November 1998

IP 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 (1998). All Rights Reserved.

Table of Contents

1. IntrodUCtion..................................................2
2. Encapsulating Security Payload Packet Format..................3
2.1 Security Parameters Index................................4
2.2 Sequence Number .........................................4
2.3 Payload Data.............................................5
2.4 Padding (for Encryption).................................5
2.5 Pad Length...............................................7
2.6 Next Header..............................................7
2.7 Authentication Data......................................7
3. Encapsulating Security Protocol Processing....................7
3.1 ESP Header Location......................................7
3.2 Algorithms..............................................10
3.2.1 Encryption Algorithms..............................10
3.2.2 Authentication Algorithms..........................10
3.3 Outbound Packet Processing..............................10
3.3.1 Security Association Lookup........................11
3.3.2 Packet Encryption..................................11
3.3.3 Sequence Number Generation.........................12
3.3.4 Integrity Check Value Calculation..................12
3.3.5 Fragmentation......................................13
3.4 Inbound Packet Processing...............................13
3.4.1 Reassembly.........................................13
3.4.2 Security Association Lookup........................13
3.4.3 Sequence Number Verification.......................14
3.4.4 Integrity Check Value Verification.................15

3.4.5 Packet Decryption..................................16
4. Auditing.....................................................17
5. Conformance Requirements.....................................18
6. Security Considerations......................................18
7. Differences from RFC1827....................................18
Acknowledgements................................................19
References......................................................19
Disclaimer......................................................20
Author Information..............................................21
Full Copyright Statement........................................22

1. Introduction

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

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

ESP is used to provide confidentiality, data origin authentication,
connectionless integrity, an anti-replay service (a form of partial
sequence integrity), and limited traffic flow confidentiality. The
set of services provided depends on options selected at the time of
Security Association establishment and on the placement of the
implementation. Confidentiality may be selected independent of all
other services. However, use of confidentiality without
integrity/authentication (either in ESP or separately in AH) may
subject traffic to certain forms of active attacks that could
undermine the confidentiality service (see [Bel96]). Data origin
authentication and connectionless integrity are joint services
(hereafter referred to jointly as "authentication) and are offered as
an option in conjunction with (optional) confidentiality. The anti-
replay service may be selected only if data origin authentication is
selected, and its election is solely at the discretion of the
receiver. (Although the default calls for the sender to increment
the Sequence Number used for anti-replay, the service is effective
only if the receiver checks the Sequence Number.) Traffic flow

confidentiality requires selection of tunnel mode, and is most
effective if implemented at a security gateway, where traffic
aggregation may be able to mask true source-destination patterns.
Note that although both confidentiality and authentication are
optional, at least one of them MUST be selected.

It is assumed that the reader is familiar with the terms and concepts
described in the Security Architecture document. In particular, the
reader should be familiar with the definitions of security services
offered by ESP and AH, the concept of Security Associations, the ways
in which ESP can be used in conjunction with the Authentication
Header (AH), and the different key management options available for
ESP and AH. (With regard to the last topic, the current key
management options required for both AH and ESP are manual keying and
automated keying via IKE [HC98].)

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 in RFC2119 [Bra97].

2. Encapsulating Security Payload Packet Format

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

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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

* If included in the Payload field, cryptographic
synchronization data, e.g., an Initialization Vector (IV, see

Section 2.3), usually is not encrypted per se, although it
often is referred to as being part of the ciphertext.

The following subsections define the fields in the header format.
"Optional" means that the field is omitted if the option is not
selected, i.e., it is present in neither the packet as transmitted
nor as formatted for computation of an Integrity Check Value (ICV,
see Section 2.7). Whether or not an option is selected is defined as
part of Security Association (SA) establishment. Thus the format of
ESP packets for a given SA is fixed, for the duration of the SA. In
contrast, "mandatory" fields are always present in the ESP packet
format, for all SAs.

2.1 Security Parameters Index

The SPI is an arbitrary 32-bit value that, in combination with the
destination IP address and security protocol (ESP), uniquely
identifies the Security Association for this datagram. The set of
SPI values in the range 1 through 255 are reserved by the Internet
Assigned Numbers Authority (IANA) for future use; a reserved SPI
value will not normally be assigned by IANA unless the use of the
assigned SPI value is specified in an RFC. It is ordinarily selected
by the destination system upon establishment of an SA (see the
Security Architecture document for more details). The SPI field is
mandatory.

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 MAY use the zero SPI value to mean "No
Security Association Exists" 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 monotonically increasing
counter value (sequence number). It is mandatory and is always
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, i.e., the sender MUST always
transmit this field, but the receiver need not act upon it (see the
discussion of Sequence Number Verification in the "Inbound Packet
Processing" section 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; see Section 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.

2.3 Payload Data

Payload Data is a variable-length field containing data described by
the Next Header field. The Payload Data field is mandatory and is an
integral number of bytes in length. If the algorithm used to encrypt
the payload requires cryptographic synchronization data, e.g., an
Initialization Vector (IV), then this data MAY be carried eXPlicitly
in the Payload field. Any encryption algorithm that requires such
explicit, per-packet synchronization data MUST indicate the length,
any structure for such data, and the location of this data as part of
an RFCspecifying how the algorithm is used with ESP. If such
synchronization data is implicit, the algorithm for deriving the data
MUST be part of the RFC.

Note that with regard to ensuring the alignment of the (real)
ciphertext in the presence of an IV:

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.
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)

Several 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, Pad
Length and Next Header fields, as well as the Padding) 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 figure above, to ensure that the Authentication Data
field (if present) is aligned on a 4-byte boundary.

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

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

a. For the purpose of ensuring that the bits to be encrypted
are a multiple of the algorithm's blocksize (first bullet
above), the padding computation applies to the Payload
Data exclusive of the IV, the Pad Length, and Next Header
fields.

b. For the purposes of ensuring that the Authentication Data
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 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.)

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

receiver MUST inform senders of how the receiver will handle the
Padding field.

2.5 Pad Length

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

2.6 Next Header

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

2.7 Authentication Data

The Authentication Data is a variable-length field containing an
Integrity Check Value (ICV) computed over the ESP packet minus the
Authentication Data. The length of the field is specified by the
authentication function selected. The Authentication Data field is
optional, and is included only if the authentication service has been
selected for the SA in question. The authentication algorithm
specification MUST specify the length of the ICV and the comparison
rules and processing steps for validation.

3. Encapsulating Security Protocol Processing

3.1 ESP Header Location

Like AH, ESP may be employed in two ways: transport mode or tunnel
mode. The former mode is applicable only to host implementations and
provides protection for upper layer protocols, but not the IP header.
(In this mode, note that for "bump-in-the-stack" or "bump-in-the-
wire" implementations, as defined in the Security Architecture
document, inbound and outbound IP fragments may require an IPsec
implementation to perform extra IP reassembly/fragmentation in order
to both conform to this specification and provide transparent IPsec
support. Special care is required to perform such operations within
these implementations when multiple interfaces are in use.)

In transport mode, ESP is inserted after the IP header and before an
upper layer protocol, e.g., TCP, UDP, ICMP, etc. or before any other
IPsec headers that have already been inserted. In the context of

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

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

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

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

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 hdrrouting,fragment.ESPopt*TCPDataTrailerAuth
---------------------------------------------------------
<---- encrypted ---->
<---- authenticated ---->

* = if present, could be before ESP, after ESP, or both

ESP and AH headers can be combined in a variety of modes. The IPsec
Architecture document describes the combinations of security
associations that must be supported.

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

-----------------------------------------------------------
IPv4 new IP hdr* orig IP hdr* ESP ESP
(any options) ESP (any options) TCPDataTrailerAuth
-----------------------------------------------------------
<--------- encrypted ---------->
<----------- authenticated ---------->

------------------------------------------------------------
IPv6 new* new ext orig*orig ext ESP ESP
IP hdr hdrs* ESPIP hdr hdrs * TCPDataTrailerAuth
------------------------------------------------------------
<--------- encrypted ----------->
<---------- authenticated ---------->

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

3.2 Algorithms

The mandatory-to-implement algorithms are described in Section 5,
"Conformance Requirements". Other algorithms MAY be supported. Note
that although both confidentiality and authentication are optional,
at least one of these services MUST be selected hence both algorithms
MUST NOT be simultaneously NULL.

3.2.1 Encryption Algorithms

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

3.2.2 Authentication Algorithms

The authentication algorithm employed for the ICV computation is
specified by the SA. For point-to-point communication, suitable
authentication algorithms include keyed Message Authentication Codes
(MACs) based on symmetric encryption algorithms (e.g., DES) or on
one-way hash functions (e.g., MD5 or SHA-1). For multicast
communication, one-way hash algorithms combined with asymmetric
signature algorithms are appropriate, though performance and space
considerations currently preclude use of such algorithms. Note that
since authentication is optional, this algorithm may be "NULL".

3.3 Outbound Packet Processing

In transport mode, the sender encapsulates the upper layer protocol
information in the ESP header/trailer, and retains the specified IP
header (and any IP extension headers in the IPv6 context). In tunnel
mode, the outer and inner IP header/extensions can be inter-related
in a variety of ways. The construction of the outer IP
header/extensions during the encapsulation process is described in
the Security Architecture document. If there is more than one IPsec
header/extension required by security policy, the order of the
application of the security headers MUST be defined by security
policy.

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

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. Accordingly, the sender:

1. encapsulates (into the ESP Payload field):
- for transport mode -- just the original upper layer
protocol information.
- for tunnel mode -- the entire original IP datagram.
2. adds any necessary padding.
3. encrypts the result (Payload Data, Padding, Pad Length, and
Next Header) using the key, encryption algorithm, algorithm
mode indicated by the SA and cryptographic synchronization
data (if any).
- 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 synchronication data, e.g.,
an IV, is indicated, it is constructed and input to
the encryption algorithm as per the algorithm
specification.

The exact steps for constructing the outer IP header depend on the
mode (transport or tunnel) and are described in the Security
Architecture document.

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

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 for this SA and inserts the
new value into the Sequence Number field. Thus the first packet sent
using a given SA will have a Sequence Number of 1.

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. (Note that this approach to Sequence
Number management does not require use of modular arithmetic.)

The sender assumes anti-replay is enabled as a default, unless
otherwise notified by the receiver (see 3.4.3). Thus, if the counter
has cycled, the sender will set up a new SA and key (unless the SA
was configured with manual key management).

If anti-replay is disabled, the sender does not need to monitor or
reset the counter, e.g., in the case of manual key management (see
Section 5). However, the sender still increments the counter and
when it reaches the maximum value, the counter rolls over back to
zero.

3.3.4 Integrity Check Value Calculation

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

For some authentication algorithms, the byte string over which the
ICV computation is performed must be a multiple of a blocksize
specified by the algorithm. If the length of this byte string does
not match the blocksize requirements for the algorithm, implicit
padding MUST be appended to the end of the ESP packet, (after the
Next Header field) prior to ICV computation. The padding octets MUST
have a value of zero. The blocksize (and hence the length of the
padding) is specified by the algorithm specification. This padding
is not transmitted with the packet. Note that MD5 and SHA-1 are
viewed as having a 1-byte blocksize because of their internal padding
conventions.

3.3.5 Fragmentation

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

NOTE: For transport mode -- As mentioned at the beginning of Section
3.1, 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 walk through all the
extension headers to determine if there is a fragmentation header and
hence that the packet needs reassembling prior to IPsec processing.

3.4 Inbound Packet Processing

3.4.1 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 zero'ing 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 (reassembled) packet containing an ESP Header, the
receiver determines the appropriate (unidirectional) SA, based on the
destination IP address, security protocol (ESP), and the SPI. (This
process is described in more detail in the Security Architecture
document.) The SA indicates whether the Sequence Number field will

be checked, whether the Authentication Data field should be present,
and it will specify the algorithms and keys to be employed for
decryption and ICV computations (if applicable).

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

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 authentication service
also is enabled for the SA, since otherwise the Sequence Number field
has not been integrity protected. (Note that there are no provisions
for managing transmitted Sequence Number values among multiple
senders directing traffic to a single SA (irrespective of whether the
destination address is unicast, broadcast, or multicast). Thus the
anti-replay service SHOULD NOT be used in a multi-sender environment
that employs a single SA.)

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 (see section
3.3.3), if an SA establishment protocol such as IKE 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.

Duplicates are rejected through the use of a sliding receive window.
(How the window is implemented is a local matter, but the following
text describes the functionality that the implementation must
exhibit.) A MINIMUM window size of 32 MUST be supported; but a
window size of 64 is preferred and SHOULD be employed as the default.

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

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

DISCUSSION:

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

3.4.4 Integrity Check Value Verification

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

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

DISCUSSION:

Begin by removing and saving the ICV value (Authentication Data
field). Next check the overall length of the ESP packet minus the
Authentication Data. If implicit padding is required, based on

the blocksize of the authentication algorithm, append zero-filled
bytes to the end of the ESP packet directly after the Next Header
field. Perform the ICV computation and compare the result with
the saved value, using the comparison rules defined by the
algorithm specification. (For example, if a digital signature and
one-way hash are used for the ICV computation, the matching
process is more complex.)

3.4.5 Packet Decryption

As in section 3.3.2, "Packet Encryption", we speak here 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.
Accordingly, the receiver:

1. 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.
- 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. processes any padding as specified in the encryption
algorithm specification. If the default padding scheme (see
Section 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.
3. reconstructs the original IP datagram from:
- for transport mode -- original IP header plus the
original upper layer protocol information in the ESP
Payload field
- for tunnel mode -- tunnel IP header + 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.

If authentication has been selected, verification and decryption MAY
be performed serially or in parallel. If performed serially, then
ICV verification SHOULD be performed first. If performed in
parallel, verification 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 bogus
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 authentication,
care must be taken to avoid possible race conditions with regard to
packet Access and reconstruction of the decrypted packet.

Note that there are several ways in which the decryption can "fail":

a. The selected SA may not be correct -- The SA may be
mis-selected due to tampering with the SPI, destination
address, or IPsec protocol type fields. Such errors, if they
map the packet to another extant SA, will be
indistinguishable from a corrupted packet, (case c).
Tampering with the SPI can be detected by use of
authentication. However, an SA mismatch might still occur
due to tampering with the IP Destination Address or the IPsec
protocol type field.

b. The pad length or pad values could be erroneous -- Bad pad
lengths or pad values can be detected irrespective of the use
of authentication.

c. The encrypted ESP packet could be corrupted -- This can be
detected if authentication is selected for the SA.,

In case (a) or (c), the erroneous result of the decryption operation
(an invalid IP datagram or transport-layer frame) will not
necessarily be detected by IPsec, and is the responsibility of later
protocol processing.

4. Auditing

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

events, not explicitly called out in this specification, also MAY
result in audit log entries. There is no requirement for the
receiver to transmit any message to the 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 and MUST comply with all requirements of the Security
Architecture document. If the key used to compute an ICV is manually
distributed, correct provision of the anti-replay service would
require correct maintenance of the counter state at the sender, until
the key is replaced, and there likely would be no automated recovery
provision if counter overflow were imminent. Thus a compliant
implementation SHOULD NOT provide this service in conjunction with
SAs that are manually keyed. A compliant ESP implementation MUST
support the following mandatory-to-implement algorithms:

- DES in CBC mode [MD97]
- HMAC with MD5 [MG97a]
- HMAC with SHA-1 [MG97b]
- NULL Authentication algorithm
- NULL Encryption algorithm

Since ESP encryption and authentication are optional, support for the
2 "NULL" algorithms is required to maintain consistency with the way
these services are negotiated. NOTE that while authentication and
encryption can each be "NULL", 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 from RFC1827

This document differs from RFC1827 [ATK95] in several significant
ways. The major difference is that, this document attempts to
specify a complete framework and context for ESP, whereas RFC1827
provided a "shell" that was completed through the definition of
transforms. The combinatorial growth of transforms motivated the
reformulation of the ESP specification as a more complete document,
with options for security services that may be offered in the context
of ESP. Thus, fields previously defined in transform documents are

now part of this base ESP specification. For example, the fields
necessary to support authentication (and anti-replay) are now defined
here, even though the provision of this service is an option. The
fields used to support padding for encryption, and for next protocol
identification, are now defined here as well. Packet processing
consistent with the definition of these fields also is included in
the document.

Acknowledgements

Many of the concepts embodied in this specification were derived from
or influenced by the US Government's SP3 security protocol, ISO/IEC's
NLSP, or from the proposed swIPe security protocol. [SDNS89, ISO92,
IB93].

For over 3 years, this document has evolved through multiple versions
and iterations. During this time, many people have contributed
significant ideas and energy to the process and the documents
themselves. The authors would like to thank Karen Seo for providing
extensive help in the review, editing, background research, and
coordination for this version of the specification. The authors
would also like to thank the members of the IPsec and IPng working
groups, with special mention of the efforts of (in alphabetic order):
Steve Bellovin, Steve Deering, Phil Karn, Perry Metzger, David
Mihelcic, Hilarie Orman, Norman Shulman, William Simpson and Nina
Yuan.

References

[ATK95] Atkinson, R., "IP Encapsulating Security Payload (ESP)",
RFC1827, August 1995.

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

[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC2119, March 1997.

[HC98] Harkins, D., and D. Carrel, "The Internet Key Exchange
(IKE)", RFC2409, November 1998.

[IB93] John Ioannidis & Matt Blaze, "Architecture and
Implementation of Network-layer Security Under Unix",
Proceedings of the USENIX Security Symposium, Santa Clara,
CA, October 1993.

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

[KA97a] Kent, S., and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.

[KA97b] Kent, S., and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.

[MD97] Madson, C., and N. Doraswamy, "The ESP DES-CBC Cipher
Algorithm With Explicit IV", RFC2405, November 1998.

[MG97a] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH", RFC2403, November 1998.

[MG97b] Madson, C., and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC2404, November 1998.

[STD-2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994. See also:
http://www.iana.org/numbers.Html

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

Disclaimer

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

Author Information

Stephen Kent
BBN Corporation
70 Fawcett Street
Cambridge, MA 02140
USA

Phone: +1 (617) 873-3988
EMail: kent@bbn.com

Randall Atkinson
@Home Network
425 Broadway,
Redwood City, CA 94063
USA

Phone: +1 (415) 569-5000
EMail: rja@corp.home.net

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