IPsec Working Group S. Kent
Internet Draft BBN Technologies
Draft-ietf-ipsec-esp-v3-04.txt March 2003
Expires September 2003
IP Encapsulating Security Payload (ESP)
Status of This Memo
This document is an Internet Draft and is subject to all provisions
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Copyright (C) The Internet Society (2003). All Rights Reserved.
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
is based upon RFC 2406 (November 1998). Section 7 provides a brief
review of the differences between this document and RFC 2406.
Comments should be sent to Stephen Kent (kent@bbn.com).
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Table of Contents
1. Introduction...................................................4
2. Encapsulating Security Payload Packet Format...................6
2.1 Security Parameters Index...............................10
2.2 Sequence Number.........................................12
2.2.1 Extended (64-bit) Sequence Number..................12
2.3 Payload Data............................................13
2.4 Padding (for Encryption)................................14
2.5 Pad Length..............................................15
2.6 Next Header.............................................15
2.7 Traffic Flow Confidentiality (TFC) Padding..............15
2.8 Integrity Check Value (ICV).............................16
3. Encapsulating Security Protocol Processing....................16
3.1 ESP Header Location.....................................16
3.1.1 Transport Mode Processing..........................17
3.1.2 Tunnel Mode Processing.............................18
3.2 Algorithms..............................................19
3.2.1 Encryption Algorithms..............................19
3.2.2 Integrity Algorithms...............................20
3.2.3 Combined Mode Algorithms...........................20
3.3 Outbound Packet Processing..............................21
3.3.1 Security Association Lookup........................21
3.3.2 Packet Encryption and Integrity Check Value (ICV)
Calculation........................................21
3.3.2.1 Separate Confidentiality and Integrity
Algorithms....................................21
3.3.2.2 Combined Confidentiality and Integrity
Algorithms....................................23
3.3.3 Sequence Number Generation.........................23
3.3.4 Fragmentation......................................24
3.4 Inbound Packet Processing...............................25
3.4.1 Reassembly.........................................25
3.4.2 Security Association Lookup........................25
3.4.3 Sequence Number Verification.......................26
3.4.4 Integrity Check Value Verification.................28
3.4.4.1 Separate Confidentiality and Integrity
Algorithms....................................28
3.4.4.2 Combined Confidentiality and Integrity
Algorithms....................................30
4. Auditing......................................................31
5. Conformance Requirements......................................32
6. Security Considerations.......................................32
7. Differences from RFC 2406.....................................33
Acknowledgements.................................................33
References.......................................................34
Disclaimer.......................................................34
Author Information...............................................35
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Appendix -- Extended Sequence Number.............................36
Full Copyright Statement.........................................42
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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)
[KA98b], or in a nested fashion, (see "Security Architecture for the
Internet Protocol" [KA98a], 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 [KA98a].
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 provide
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, but an asymmetric cryptographic algorithm also may be
employed, e.g., to sign a hash.) Integrity-only ESP MUST be offered
as a service selection option, e.g., it must be negotiable in SA
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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,
most ESP use 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 (see
Section 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 backwards
compatible fashion.
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.
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 RFC 2119 [Bra97].
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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 at http://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 (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) | ^Integ.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
| Sequence Number | |erage
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ----
| Payload Data* (variable) | | ^
~ ~ | |
| | |Conf.
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Cov-
| | Padding (0-255 bytes) | |erage*
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
| | 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, see Section 2.3),
usually is not encrypted per se, although it often is referred
to as being part of the ciphertext.
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.
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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.,
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, while others can provide integrity for some additional
data, data that is not encrypted for transmission. Since 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) is omitted. 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: This
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 (see Section 2.2.1).
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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 (see
Section 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 is omitted (see Section 3.3.2.2 below). Since
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.
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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] See section 2.2.1
[6] mandatory if a separate integrity algorithm is used
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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 omitted when this mode is employed
[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] See section 2.2.1
[5] The algorithm choices determines whether these are
transmitted, but in either case, the result is invisible
to ESP
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 Integrity Check Value (ICV,
see Section 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 in RFC 791) and
generate their output in canonical network byte order. IP packets
are also transmitted in network byte order.
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. For a unicast
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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).
Since, for unicast SAs, the SPI value is generated by the receiver,
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. The SPI field is mandatory and the mechanism for
mapping inbound traffic to unicast SAs described above MUST be
supported by all ESP implementations.
IPsec implementations SHOULD be prepared to support both unicast and
multicast SAs using the following algorithm for mapping inbound IPsec
datagrams to SAs. Each entry in the Security Association Database
(SAD) [KA97a] must indicate whether the SA lookup makes use of the
source and destination IP addresses, in addition to the SPI (and,
optionally, the protocol field). Nominally, this indication can be
represented by two bits, one associated with the source IP address
and the other for the destination IP address. A "1" value for each
bit indicates the need to match against the corresponding address
field as part of the SA lookup process. Thus, for example, unicast
SAs would have both bits set to zero, since neither the source nor
destination IP address is used for SA matching. (Only the SPI, and,
optionally, the protocol field are employed.) For multicast methods
that rely only on the destination address to specify a multicast
group, only the second bit would be set to "1," implying the need to
use the destination address plus the SPI (and, optionally the
protocol) to determine the appropriate SA. For multicast methods
(e.g., SSM [HC03]) that also require the source address to identify a
multicast group, both bits would be set to "1." (There is no current
requirement to support SA mapping based on the source address but not
the destination address, thus one of the possible four values is not
meaningful.) The indication 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.
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"
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.)
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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 deprecated, since there is no general means
of synchronizing packet counters among the senders or meaningfully
managing a receiver packet counter and window in the context of
multiple senders.
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 Sequence Number processing described in
Sections 3.3.3 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; 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.
2.2.1 Extended (64-bit) Sequence Number
To support high-speed IPsec implementations, a new option for
sequence numbers SHOULD be offered, as an extension to the current,
32-bit sequence number field. Use of an Extended Sequence Number
(ESN) MUST be negotiated by an SA management protocol. (The ESN
feature is applicable to multicast as well as unicast SAs.)
The ESN facility allows use of a 64-bit sequence number for an SA.
(See Appendix on "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
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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. See Section 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.
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.
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2.4 Padding (for Encryption)
Three 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 (see
Section 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 blocksize (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 is
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,
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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 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.
2.6 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 (see 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.
2.7 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.
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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,
e.g., if tunnel mode is employed. This 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.
TFC padding takes advantage of an intrinsic feature of IP, i.e.,
other data may be present in a buffer delivered to an IP interface,
beyond the packet length indicated by the IP total length field.
Thus, in tunnel mode, a compliant IP stack at a receiver should
ignore this padding. In this sense, 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 in section 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.
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 a separate (not
combined mode) integrity algorithm is employed. The length of the
field is 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 Processing
3.1 ESP Header Location
ESP may be employed in two ways: transport mode or tunnel mode.
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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, since 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.
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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. 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.
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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 are described in Section 5,
"Conformance Requirements." Other algorithms 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.
3.2.1 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
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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, this could significantly
extend the scope of a cryptographic module evaluation.) 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) MAY be
an optional service (e.g., integrity-only ESP), this algorithm MAY be
"NULL" [KA98a]
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 since 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.
3.2.3 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
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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 inter-related in a variety of ways. The
construction of the outer IP header/extensions during the
encapsulation process is described in the Security Architecture
document.
3.3.1 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.
3.3.2.1 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.
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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 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 since 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, since encryption is performed first.) If
the ESN option is enabled for the SA, it 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.
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 padding
octets MUST have a value of zero. 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. Note
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that MD5 and SHA-1 are viewed as having a 1-byte block size because
of their internal padding conventions.
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 of
the combined mode algorithm employed and thus not
specified in this standard.
- The (explicit) ICV field is NOT part of the ESP packet
format when a combined mode algorithm is employed,
although an analogous field usually will 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) for this SA and
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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 (see 3.4.3) or if the SA was
configured using manual key management. 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 anti-replay is disabled (as noted above), 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. (This behavior is recommended for multi-sender,
multicast SAs, unless anti-replay mechanisms outside 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.
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.
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NOTE: For transport mode -- As mentioned at the end of Section 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 PMTU
discovery. In any case, an ESP implementation MUST 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 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 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 in Section 2.1. For multicast SAs,
the destination address is also employed in the lookup (in addition
to the SPI and, optionally the protocol), and the sender address also
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may be employed, as described in Section 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, whether the (explicit) ICV field should be present (and
if so, its size), and it 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.
(Note that SA management traffic, e.g., IKE packets, does not need to
be processed based on SPI, i.e., one can demultiplex this traffic
separately, e.g., based on Next Protocol and Port fields.)
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, since 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 (see section 3.3.3), 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
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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,
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 it's local counter, since 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 resynchronization 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
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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 size
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 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 (for IPv6)
the cleartext Flow ID.
DISCUSSION:
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 blocksize of the
integrity algorithm, append zero-filled bytes to the end of
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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.
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 in
section 3.3.2, 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 (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 (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.
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
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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 (see
section 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 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 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. Decryps and integrity check 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
in Section 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.
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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)
padding 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 (see section 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,
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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 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 additional packet processing
requirements levied by the Security Architecture document [KA98a].
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 anti-replay service in conjunction with SAs that
are manually keyed. A compliant ESP implementation MUST support the
following mandatory-to-implement algorithms:
- AES (with 128-bit keys) in CBC mode
- HMAC with MD5 [MG98a]
- HMAC with SHA-1 [MG98b]
- NULL Encryption algorithm (RFC 2410)
Since use of encryption in ESP is optional, support for the "NULL"
encryption algorithm 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 while 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-
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relevant aspects of using the IPsec protocol are discussed in the
Security Architecture document.
7. Differences from RFC 2406
This document differs from RFC 2406 in a number of significant ways.
o Confidentiality-only service -- now a MAY, not a MUST.
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; for multicast, the SPI is
combined with destination address, and optionally the
source address, to select an SA. If the receiver (unicast)
or the multicast controller (multicast) opted to use the
security protocol (ESP) in selecting the SPI, then the
security protocol is also used in the lookup.
o 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 Inbound and Outbound packet processing -- there are now two
paths -- (1) separate confidentiality and integrity
algorithms, (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.
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.
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References
[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, RFC 2119, March 1997.
[HC98] Harkins, D., and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[HC03] Holbrook, H., and Cain, B., "Source Specific Multicast for
IP", Internet Draft, draft-ietf-ssm-arch-01.txt, November
3, 2002.
[KA98a] Kent, S., and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[KA98b] Kent, S., and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[Kra01] Krawczyk, H., "The Order of Encryption and Authentication
for Protecting Communications (Or: How Secure Is SSL?)",
CRYPTO' 2001.
[MD98] Madson, C., and N. Doraswamy, "The ESP DES-CBC Cipher
Algorithm With Explicit IV", RFC 2405, November 1998.
[MG98a] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
ESP and AH", RFC 2403, November 1998.
[MG98b] Madson, C., and R. Glenn, "The Use of HMAC-SHA-1-96 within
ESP and AH", RFC 2404, November 1998.
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.
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Author Information
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3988
EMail: kent@bbn.com
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Appendix -- Extended (64-bit) Sequence Numbers
A1. 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 Seq
When performing the anti-replay check, or when determining which high
order bits to use to authenticate an incoming packet, there are two
cases:
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+ 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 zero's 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 B
A2.1. 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.
When a packet with a 64-bit sequence number (Seq) greater than T is
received and validated,
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+ 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 (see Section 2.2. 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 (see Section 2.2. below for
determination of Seqh).
A2.2. Determining the Higher Order Bits (Seqh) of the Sequence Number
Since 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 3 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 = Th
A2.3. 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.
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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
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 packet
Else
If (pass integrity check)
Tl = Seql (shift bits)
Set the bit corresponding to Seql
Pass packet on
Else reject packet
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A3. 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 2.2. 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 seems unlikely to occur
if any significant fraction of the traffic on the SA in question is
TCP, because 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.
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 resynchronization process.
+ limit the recovery mechanism to the receiver -- since 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
backwards compatibility.
A3.1. Triggering Resynchronization
For each SA, the receiver records the number of consecutive packets
that fail authentication. This count is used to trigger the
resynchronization 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
resynchronization 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.
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A3.2. Resynchronization 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 end point of the payload, then
compute different candidate ICV's 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 backwards compatibility. Also, because
resynchronization 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.
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