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new AH spec (big message)
Network Working Group Stephen Kent, BBN Corp
Internet Draft Randall Atkinson, @Home Network
draft-ietf-ipsec-auth-04.txt 26 March 1997
IP Authentication Header
Status of This Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of 6 months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time. It is not appropriate to use Internet Drafts
as reference material or to cite them other than as a "working draft"
or "work in progress". Please check the I-D abstract listing
contained in each Internet Draft directory to learn the current
status of this or any other Internet Draft.
This particular Internet Draft is a product of the IETF's IPng and
IPsec Working Groups. It is intended that a future version of this
draft will be submitted for consideration as a standards-track
document. Distribution of this document is unlimited.
Kent, Atkinson [Page 1]
Internet Draft IP Authentication Header 26 March 1997
Table of Contents
1. Introduction......................................................3
2. Authentication Header Format......................................4
2.1 Next Header...................................................4
2.2 Payload Length................................................4
2.3 Reserved......................................................4
2.4 Security Parameters Index (SPI)...............................5
2.5 Sequence Number...............................................5
2.6 Authentication Data ..........................................5
3. Authentication Header Processing..................................5
3.1 Authentication Header Location...............................5
3.2 Outbound Packet Processing...................................8
3.2.1 Security Association Lookup.............................8
3.2.2 Sequence Number Field...................................8
3.2.3 Integrity Check Value Calculation.......................8
3.2.3.1 Handling Mutable Fields............................8
3.2.3.1.1 ICV Computation for IPv4......................9
3.2.3.1.2 ICV Computation for IPv6......................9
3.2.3.2 Padding...........................................10
3.2.3.2.1 Authentication Data Padding..................10
3.2.3.2.2 Implicit Packet Padding......................10
3.2.3.3 Authentication Algorithms.........................10
3.2.4 Fragmentation..........................................11
3.3 Inbound Packet Processing...................................11
3.3.1 Reassembly.............................................11
3.3.2 Security Association Lookup............................11
3.3.3 Sequence Number Verification...........................11
3.3.4 Integrity Check Value Verification.....................12
4. Conformance Requirements.........................................13
5. Security Considerations..........................................13
Acknowledgements....................................................13
References..........................................................14
Disclaimer..........................................................15
Author Information..................................................15
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Internet Draft IP Authentication Header 26 March 1997
1. Introduction
The IP Authentication Header (AH) is used to provide connectionless
integrity and data origin authentication for IP datagrams (hereafter
referred to as just "authentication"), and to provide protection
against replays. This latter, optional service may be selected when
a Security Association is established. AH provides authentication
for as much of the IP header as possible, as well as for upper level
protocol data. However, some IP header fields may change in transit
and the value of these fields, when the packet arrives at the
receiver, may not be predictable by the transmitter. The values of
such fields cannot be protected by AH. Thus the protection provided
to the IP header by AH is somewhat piecemeal.
AH may be applied alone, in combination with the IP Encapsulating
Security Payload (ESP) [KA97b], or in a nested fashion through the
use of tunnel mode (see below). 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. ESP may be used to provide the same security services, and it
also provides an optional confidentiality (encryption) service. The
primary difference between ESP and AH, when used for authentication,
is the extent of the coverage. Specifically, ESP does not protect
any IP header fields unless those fields are encapsulated by ESP.
For more details on how to use AH and ESP in various network
environments, see "Security Architecture for the Internet Protocol"
[KA97a].
It is assumed that the reader is familiar with the terms and concepts
described in the document "Security Architecture for the Internet
Protocol" [KA97a]. In particular, the reader should be familiar with
the definitions of security services offered by AH (and by ESP), the
concept of Security Associations, the different key management
options available for AH (and ESP), and the ways in which AH can be
used in conjunction with ESP.
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2. Authentication Header Format
+---------------+---------------+---------------+---------------+
| Next Header(8)| Payload Len(8)| RESERVED (16) |
+---------------+---------------+---------------+---------------+
| Security Parameters Index (32) |
+---------------+---------------+---------------+---------------+
| Sequence Number Field (32) |
+---------------+---------------+---------------+---------------+
| |
+ Authentication Data (variable number of 32-bit words) |
| |
+---------------+---------------+---------------+---------------+
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
The following subsections define the fields that comprise the AH
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 the Integrity Check
Value (ICV). Whether or not an option is selected is defined as part
of the Security Association. In contrast, "mandatory" fields are
always present in the AH format.
2.1 Next Header
The Next Header is an 8-bit field that identifies the type of the
next payload after the Authentication Header. The value of this
field is chosen from the set of IP Protocol Numbers defined in the
most recent "Assigned Numbers" [STD-2] RFC from the Internet Assigned
Numbers Authority (IANA). The Next Header field is mandatory.
2.2 Payload Length
This 8-bit field specifies the length of AH, in 32-bit words (4-byte
units), minus "2," i.e., the fixed portion of AH is not counted. The
minimum value is 0, which is used only in the degenerate case of a
"null" authentication algorithm. The Payload Length field is
mandatory.
*** Do we want to retain a null authentication algorithm as part of the
*** spec at this point? What purpose does it serve?
2.3 Reserved
This 16-bit field is reserved for future use. It MUST be set to
"zero." (Note that the value is included in the Authentication Data
calculation, but is otherwise ignored by the recipient.) The
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Internet Draft IP Authentication Header 26 March 1997
Reserved field is mandatory.
2.4 Security Parameters Index (SPI)
The SPI is an arbitrary 32-bit value identifying the Security
Association for this datagram (relative to the destination IP address
contained in the IP header with which this security header is
associated). 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. A
value of zero indicates that no Security Association exists. The SPI
field is mandatory. It is ordinarily selected by the destination
system upon establishment of an SA (see "Security Architecture for
the Internet Protocol" [KA97a] for more details).
*** Under what circumstances will a zero SPI be employed? Is this
*** still relevant or is it vestigial?
2.5 Sequence Number
This unsigned 32-bit field contains a monotonically increasing
counter value (sequence number). The counter is initialized to 1
when an SA is established. The sequence number must never be allowed
to cycle; thus, it MUST be reset (by establishing a new SA and thus a
new key) prior to the transmission of 2^32-1 packets on an SA. The
Sequence Number field is optional. It is included only if the anti-
replay service (a form of loose sequence integrity) is selected as a
security service for the SA.
2.6 Authentication Data
This is a variable-length field that contains the Integrity Check
Value (ICV) for this packet. The field must be an integral multiple
of 32 bits in length. The details of the ICV computation are
described in Section 3.2.3 below. This field may include explicit
padding. This padding is included to ensure that the length of the
AH header is an integral multiple of 32 bits (IPv4) or 64 bits
(IPv6). All implementations MUST support such padding. Details of
how to compute the required padding length are provided in Section
3.2.3.2.1 below. The Authentication Data field is mandatory.
3. Authentication Header Processing
3.1 Authentication Header Location
Like ESP, AH may be employed in two ways: transport mode or tunnel
mode. The former mode is applicable only to host implementations and
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Internet Draft IP Authentication Header 26 March 1997
provides protection for upper layer protocols, in addition to
selected IP header fields. In this mode, AH is inserted after the IP
header and before an upper layer protocol, e.g., TCP, UDP, ICMP, etc.
In the context of IPv4, this calls for placing AH 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 AH transport mode
positioning for a typical IPv4 packet, on a "before and after" basis.
BEFORE APPLYING AH
----------------------------
IPv4 |orig IP hdr | | |
|(any options)| TCP | Data |
----------------------------
AFTER APPLYING AH
---------------------------------
IPv4 |orig IP hdr | | | |
|(any options)| AH | TCP | Data |
---------------------------------
|<------ authenticated ------->|
except for mutable fields
In the IPv6 context, AH 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 AH header depending on the semantics
desired. The following diagram illustrates AH transport mode
positioning for a typical IPv6 packet.
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BEFORE APPLYING AH
---------------------------------------
IPv6 | | ext hdrs | | |
| orig IP hdr |if present| TCP | Data |
---------------------------------------
AFTER APPLYING AH
------------------------------------------------------------
IPv6 | |hxh,rtg,frag| dest | | dest | | |
|orig IP hdr |if present**| opt* | AH | opt* | TCP | Data |
------------------------------------------------------------
|<-------------------- authenticated --------------------->|
except for mutable fields
* = if present, could be before AH, after AH, or both
** = hop by hop, routing, fragmentation headers
Tunnel mode AH may be employed in either hosts or security gateways.
When AH 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, AH
protects the entire inner IP packet, including the entire inner IP
header. The position of AH in tunnel mode, relative to the outer IP
header, is the same as for AH in transport mode. The following
diagram illustrates AH tunnel mode positioning for typical IPv4 and
IPv6 packets.
------------------------------------------------
IPv4 | new IP hdr* | | orig IP hdr* | | |
|(any options)| AH | (any options) |TCP | Data |
------------------------------------------------
|<---------------- authenticated ------------->|
except for mutable fields
--------------------------------------------------------------
IPv6 | | ext hdrs*| | | ext hdrs*| | |
|new IP hdr*|if present| AH |orig IP hdr*|if present|TCP|Data|
--------------------------------------------------------------
|<---------------------- authenticated --------------------->|
except for mutable fields
* = construction of outer IP hdr/extensions and
modification of inner IP hdr/extensions is
discussed below.
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3.2 Outbound Packet Processing
In transport mode, the transmitter inserts the AH header after the IP
header and before an upper layer protocol header, as described above.
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 document, "Security Architecture for the Internet Protocol".
3.2.1 Security Association Lookup
AH is applied to an outbound packet only after an IPsec
implementation determines that the packet is associated with an SA
that calls for AH processing. The process of determining what, if
any, IPsec processing is applied to outbound traffic is described in
the document, "Security Architecture for the Internet Protocol".
3.2.2 Sequence Number Field
If the anti-replay service has been selected for this SA, the
transmitter increments the sequence number for this SA, checks to
ensure that the counter has not cycled, and inserts the new value
into the Sequence Number Field. A transmitter MUST not send a packet
on an SA if doing so would cause the sequence number to cycle.
3.2.3 Integrity Check Value Calculation
3.2.3.1 Handling Mutable Fields
The AH ICV is computed over IP header fields that are either
immutable in transit or that are predictable in value upon arrival at
the endpoint for the AH SA. The ICV also encompasses the upper level
protocol data, which is assumed to be immutable in transit. If a
field is modified during transit, the value of the field is set to
zero for purposes of the ICV computation. If a field is mutable, but
its value at the (IPsec) receiver is predictable, then that value is
inserted into the field for purposes of the ICV calculation. The
Authentication Data field also is set to zero in preparation for this
computation. (Note that by replacing each field's value with zero,
rather than omitting the field, alignment is preserved for the ICV
calculation.)
DISCUSSION:
For IPv4 (unlike IPv6), there is no mechanism for tagging options
as mutable in transit. Hence the IPv4 options are explicitly
listed here and classified as either mutable or immutable. For
IPv4, the entire option is viewed as a unit; so even though the
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type and length fields within most options are immutable in
transit, if an option is classified as mutable, the entire option
is zeroed for ICV computation purposes. The mutable IPv4 header
fields also are enumerated below. The ICV calculation is
restricted to the immutable options and (base) header fields.
3.2.3.1.1 ICV Computation for IPv4
The IPv4 base header fields "Time to Live", "Header Checksum",
"Offset", "Flags", and "Type of Service" are zeroed prior to the
computation of the ICV. (The TOS field is included here because some
routers are known to change the value of this field, even though the
IP specification does not consider TOS to be a mutable header field.)
*** What about OFFSET and FLAGS. Since reassembly takes place before
*** AH processing why are these fields omitted from the ICV
*** computation?
The following IPv4 options are mutable: record route, timestamp,
loose source routing, and strict source routing. These options are
treated as zero-filled for purposes of the ICV computation. The IP
Security Options, BSO and ESO (RFC-1038, RFC-1108) and the CIPSO
(option number 134) option are included in the ICV calculation and
are not zeroed.
3.2.3.1.2 ICV Computation for IPv6
In IPv6, the "Hop Limit" field in the IPv6 base header is zeroed
prior to performing the ICV calculation. IPv6 options contain a bit
that indicates whether the option might change during transit. For
any option for which contents may change en-route, the entire "Option
Data" field must be treated as zero-valued octets when computing or
verifying the ICV. The Option Type and Opt Data Len are included in
the ICV calculation. All other options are also included in the ICV
calculation. See the IPv6 specification [DH95] for more information.
Note that the IPv6 Routing Header "Type 0" will rearrange the address
fields within the packet during transit from source to destination.
However, the contents of the packet as it will appear at the receiver
are known to the sender and to all intermediate hops. Hence, the
IPv6 Routing Header "Type 0" is included in the Authentication Data
calculation as an immutable option. The transmitter must order the
field so that it appears as it will at the receiver, prior to
performing the ICV computation.
*** Do we want to make any recommendation for what an AH implementation
*** should do if it encounters an unfamiliar IPv6 extension header,
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Internet Draft IP Authentication Header 26 March 1997
*** e.g., Routing Header "Type 1" (aka Nimrod)?
3.2.3.2 Padding
3.2.3.2.1 Authentication Data Padding
As mentioned in section 2.6, the Authentication Data field explicitly
includes padding to ensure that the AH header is a multiple of 32
bits (IPv4) or 64 bits (IPv6). If padding is required, its length is
determined by three factors:
- the presence or absence of the Sequence Number field
- the length of the ICV
- the IP protocol context (v4 or v6)
For example, if the Sequence Number field is present and a default,
96-bit truncated HMAC algorithm is selected, no padding is required
for either IPv4 nor IPv6. In contrast, if the anti-replay service is
not selected, and a default 96-bit truncated HMAC algorithm is
selected, no padding is required for IPv4, but 4 bytes of padding are
required for IPv6. The content of the padding field is arbitrarily
selected by the sender. (The padding is arbitrary, but need not be
random to achieve security.) These bytes are included in the
Authentication Data calculation, counted as part of the Payload
Length, and transmitted at the end of the Authentication Data field
to enable the receiver to perform the ICV calculation.
3.2.3.2.2 Implicit Packet Padding
For some authentication algorithms, the byte string over which the
ICV computation is performed must be a multiple of a blocksize
specified by the algorithm. If the IP packet length (including AH)
does not match the blocksize requirements for the algorithm, implicit
padding MUST be appended to the end of the packet, 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.
3.2.3.3 Authentication Algorithms
The authentication algorithm employed for the ICV computation is
specified by the SA. For point-to-point communication, suitable
authentication algorithms include keyed Message Authentication Codes
(MACs) based on symmetric encryption algorithms (e.g., DES) or on
one-way hash functions (e.g., MD5 or SHA-1). For multicast
communication, one-way hash algorithms combined with asymmetric
signature algorithms are suitable. As of this writing, the
mandatory-to-implement authentication algorithms are based on the
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Internet Draft IP Authentication Header 26 March 1997
former class, i.e., HMAC [KBC97] with SHA-1 [SHA] or HMAC with MD5
[Riv92]. The output of the HMAC computation is truncated to (the
leftmost) 96 bits. Other algorithms, possibly with different ICV
lengths, MAY be supported.
3.2.4 Fragmentation
If required, IP fragmentation occurs after AH processing within an
IPsec implementation. However, an IP packet to which AH has been
applied may itself be fragmented by routers en route, including
security gateways that may apply AH or ESP (tunnel mode) to the
already-protected packet or fragments.
3.3 Inbound Packet Processing
3.3.1 Reassembly
If required, reassembly is performed prior to AH processing.
3.3.2 Security Association Lookup
Upon receipt of a packet containing an IP Authentication Header, the
receiver determines the appropriate (unidirectional) SA, based on the
destination IP address and the SPI. (This process is described in
more detail in the document, "Security Architecture for the Internet
Protocol".) The SA will indicate whether the Sequence Number field
should be present, will specify the algorithm(s) employed for ICV
computation, and will indicate the key(s) required to validate the
ICV.
If no valid Security Association exists for this session (e.g., the
receiver has no key), the receiver MUST discard the packet and the
failure MUST be recorded in an audit log. The log entry SHOULD
include the SPI value, date/time, Source Address, Destination
Address, and (in IPv6) the Flow ID. The log entry MAY also include
other identifying data. There is no requirement for the receiver to
transmit any message to the purported transmitter in response to
receipt of such packets (because of the potential to induce denial of
service via such actions).
3.3.3 Sequence Number Verification
If the anti-replay service has been selected for this SA, 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 AH check
applied to a packet after it has been matched to an SA, to speed
rejection of duplicate packets.
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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 default window size is 32 and all AH implementations
MUST support this window size. A larger window size MAY be
established during SA negotiation. If a larger window size is
negotiated it MUST be a multiple of 32.
The "right" edge of the window represents the highest, validated
Sequence Number value received on this SA. Packets that contain
Sequence Number values 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 [KA97a].
If the received packet falls within the window, then the receiver
proceeds to ICV verification. If the ICV validation fails, the
receiver MUST discard the received IP datagram as invalid and MUST
record the authentication failure in an audit log. If such a failure
occurs, the log entry MUST include the SPI value, date/time received,
Sending Address, Destination Address, and (in IPv6) Flow ID. The log
data MAY also include other information about the failed packet. The
window is updated only if the ICV verification succeeds.
DISCUSSION:
Note that if the packet is either inside the window and new, or
outside the window on the "right" side, the receiver MUST
authenticate the Sequence Number field before updating the bit
mask (either turning on a bit or updating the "right" side of the
window).
3.3.4 Integrity Check Value Verification
The receiver computes the ICV over the appropriate fields of the
packet, 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 and MUST record the
authentication failure in an audit log. The log data MUST include the
SPI value, date/time received, Source Address, Destination Address,
and (in IPv6) the Flow ID. The log data also MAY include other
information about the failed packet.
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DISCUSSION:
Begin by saving the ICV value and replacing it (but not any
Authentication Data padding) with zero. Zero all other fields
that may have been modified during transit. (See section 3.2.3.1
for a discussion of which fields are zeroed before performing the
ICV calculation.) Check the overall length of the packet, and if
it requires implicit padding based on the requirements of the
authentication algorithm, append zero-filled bytes to the end of
the packet as required. Now perform the ICV computation and
compare the result with the received value. (If a digital
signature and one-way hash are used for the ICV computation, the
matching process is more complex and will be described in the
algorithm specification.)
4. Conformance Requirements
Implementations that claim conformance or compliance with this
specification MUST fully implement the AH syntax and processing
described here and MUST comply with all requirements of the "Security
Architecture for the Internet Protocol." Note that support for
manual key distribution is required, but its use is inconsistent with
the anti-replay service, and thus a compliant implementation must not
negotiate this service in conjunction with SAs that are manually
keyed. A compliant AH implementation MUST support the following
mandatory-to-implement algorithms (specified in [KBC97]):
- HMAC with MD5
- HMAC with SHA-1
5. Security Considerations
Security is central to the design of this protocol, and this security
considerations permeate the specification. Additional security-
relevant aspects of using IPsec protocol are discussed in the
document, "Security Architecture for the Internet Protocol".
Acknowledgements
For over 2 years, this document has evolved through multiple versions
and iterations. During this time, many people have contributed
significant ideas and energy to the process and the documents
themselves. The authors would like to thank the members of the IPsec
and IPng working groups, with special mention of the efforts of (in
alphabetic order): Steve Bellovin, Steve Deering, Francis Dupont,
Phil Karn, Frank Kastenholz, Perry Metzger, David Mihelcic, Hilarie
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Internet Draft IP Authentication Header 26 March 1997
Orman, and William Simpson. In addition, Charlie Lynn, Karen Seo,
and Nina Yuan provided extensive help in the review and editing of
this version of the specification.
References
[BCCH94] R. Braden, D. Clark, S. Crocker, & C.Huitema, "Report of IAB
Workshop on Security in the Internet Architecture", RFC-
1636, 9 June 1994, pp. 21-34.
[Bel89] Steven M. Bellovin, "Security Problems in the TCP/IP
Protocol Suite", ACM Computer Communications Review, Vol.
19, No. 2, March 1989.
[CER95] Computer Emergency Response Team (CERT), "IP Spoofing
Attacks and Hijacked Terminal Connections", CA-95:01,
January 1995. Available via anonymous ftp from
info.cert.org in /pub/cert_advisories.
[DH95] Steve Deering & Bob Hinden, "Internet Protocol version 6
(IPv6) Specification", RFC-1883, December 1995.
[GM93] James Galvin & Keith McCloghrie, Security Protocols for
version 2 of the Simple Network Management Protocol
(SNMPv2), RFC-1446, April 1993.
[KBC97] Hugo Krawczyk, Mihir Bellare, and Ran Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC-2104,
February 1997.
[Ken91] Steve Kent, "US DoD Security Options for the Internet
Protocol", RFC-1108, November 1991.
[KA96a] Steve Kent, Randall Atkinson, "Security Architecture for
the Internet Protocol", Internet Draft, ?? 1997.
[KA96b] Steve Kent, Randall Atkinson, "IP Encapsulating Security
Payload (ESP)", Internet Draft, March 1997.
[KA96c] Steve Kent, Randall Atkinson, "IP Authentication Header",
Internet Draft, March 1997.
[Riv92] Ronald Rivest, MD5 Digest Algorithm, RFC-1321, April 1992.
[SHA] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995
[STD-1] J. Postel, "Internet Official Protocol Standards", STD-1,
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Internet Draft IP Authentication Header 26 March 1997
March 1996.
[STD-2] J. Reynolds & J. Postel, "Assigned Numbers", STD-2, 20
October 1994.
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
Telephone: +1 (617) 873-3988
Randall Atkinson <rja@inet.org>
@Home Network
385 Ravendale Drive
Mountain View, CA 94043
USA
Kent, Atkinson [Page 15]
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