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New draft -- IPSEC AH
Network Working Group Stephen Kent, BBN Corp
Internet Draft Randall Atkinson, @Home Network
draft-ietf-ipsec-auth-05.txt July 21 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
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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 IPsec
Working Group. 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 21 July, 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......................................................5
2.4 Security Parameters Index (SPI)...............................5
2.5 Sequence Number...............................................5
2.6 Authentication Data ..........................................5
3. Authentication Header Processing..................................6
3.1 Authentication Header Location...............................6
3.2 Outbound Packet Processing...................................8
3.2.1 Security Association Lookup.............................8
3.2.2 Sequence Number Generation..............................8
3.2.3 Integrity Check Value Calculation.......................9
3.2.3.1 Handling Mutable Fields............................9
3.2.3.1.1 ICV Computation for IPv4......................9
3.2.3.1.1.1 Base Header Fields........................9
3.2.3.1.1.2 Options..................................10
3.2.3.1.2 ICV Computation for IPv6.....................10
3.2.3.1.2.1 Base Header Fields.......................10
3.2.3.1.2.2 Extension Headers -- Options.............11
3.2.3.1.2.3 Extension Headers -- non-Options.........11
3.2.3.2 Padding...........................................11
3.2.3.2.1 Authentication Data Padding..................11
3.2.3.2.2 Implicit Packet Padding......................12
3.2.3.3 Authentication Algorithms.........................12
3.2.4 Fragmentation..........................................12
3.3 Inbound Packet Processing...................................13
3.3.1 Reassembly.............................................13
3.3.2 Security Association Lookup............................13
3.3.3 Sequence Number Verification...........................13
3.3.4 Integrity Check Value Verification.....................14
4. Auditing.........................................................15
5. Conformance Requirements.........................................15
6. Security Considerations..........................................16
7. Differences from RFC 1826........................................16
Acknowledgements....................................................17
Appendix A -- Mutability of IP Options/Extension Headers............18
1. IPv4 Options..................................................18
2. IPv6 Extension Headers........................................19
References..........................................................21
Disclaimer..........................................................22
Author Information..................................................22
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Internet Draft IP Authentication Header 21 July, 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, by
the receiver, 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 "Security Architecture for the Internet
Protocol" [KA97a], hereafter referred to as the Security Architecture
document). Security services can be provided between a pair of
communicating hosts, between a pair of communicating security
gateways, or between a security gateway and a host. ESP may be used
to provide the same security services, and it also provides a
confidentiality (encryption) service. The primary difference between
the authentication provided by ESP and AH is the extent of the
coverage. Specifically, ESP does not protect any IP header fields
unless those fields are encapsulated by ESP (tunnel mode). For more
details on how to use AH and ESP in various network environments, see
the Security Architecture document [KA97a].
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 AH and ESP, the concept of Security Associations, the ways
in which AH can be used in conjunction with ESP, and the different
key management options available for AH and ESP. (With regard to the
last topic, the current key management options required for both AH
and ESP are manual keying and automated keying via Oakley/ISAKMP.)
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2. Authentication Header Format
The protocol header (IPv4, IPv6, or Extension) immediately preceding the
AH header will contain the value 51 in its Protocol (IPv4) or Next
Header (IPv6, Extension) field [STD-2].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Payload Len | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number Field |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Authentication Data (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following subsections define the fields that comprise the AH
format. All the fields described here are mandatory, i.e., they are
always present in the AH format and are included in the ICV
computation.
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).
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 (as defined in the
original AH spec) of AH is not counted. (Since the Sequence Number
field is always present, the fixed portion of AH is now three 32-bit
words. However, the "minus 2" length adjustment has been retained
for backwards compatibility.) In the "standard" case of a 96-bit
authentication value plus the 3 32-bit word fixed portion, this
length field will be "4". A "null" authentication algorithm may be
used only for debugging purposes. Its use would result in a "1"
value for this field, as there would be no corresponding
Authentication Data field.
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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.)
2.4 Security Parameters Index (SPI)
The SPI is an arbitrary 32-bit value that uniquely identifies the
Security Association for this datagram, relative to the destination
IP address contained in the IP header with which this security header
is associated, and relative to the security protocol employed. The
set of SPI values in the range 1 through 255 are reserved by the
Internet Assigned Numbers Authority (IANA) for future use; a reserved
SPI value will not normally be assigned by IANA unless the use of the
assigned SPI value is specified in an RFC. It is ordinarily selected
by the destination system upon establishment of an SA (see the
Security Architecture document for more details). (A zero value may
be used for local debugging purposes, but no AH packets should be
transmitted with a zero SPI value.)
2.5 Sequence Number
This unsigned 32-bit field contains a monotonically increasing
counter value (sequence number). The sender's counter and the
receiver's counter are initialized to 0 when an SA is established.
(The first packet sent using a given SA will have a Sequence Number
of 1; see Section 3.2.2 for more details on how the Sequence Number
is generated.) The transmitted Sequence Number must never be allowed
to cycle. Thus the sender's counter and the receiver's counter MUST
be reset (by establishing a new SA and thus a new key) prior to the
transmission of 2^32nd packet on an SA.
This field is always present, even if the receiver does not elect to
enable the anti-replay service for a specific SA, in order to ensure
8-byte alignment for the IPv6 environment, when the default integrity
algorithms are employed.
Processing of the Sequence Number field is at the discretion of the
receiver, i.e., the sender MUST always transmit this field, but the
receiver need not act upon it (see the discussion of Sequence Number
Verification in the "Inbound Processing" section below).
2.6 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
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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 below.
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
provides protection for upper layer protocols, in addition to
selected IP header fields. (In this mode, note that for "bump-in-
the-stack" or "bump-in-the-wire" implementations, as defined in the
Security Architecture document, inbound and outbound IP fragments may
require an IPsec implementation to perform extra IP
reassembly/fragmentation in order to both conform to this
specification and provide transparent IPsec support. Special care is
required to perform such operations within these implementations when
multiple interfaces are in use.)
In transport mode, AH is inserted after the IP header and before an
upper layer protocol, e.g., TCP, UDP, ICMP, etc. or before any other
IPsec headers that have already been inserted, e.g., ESP. 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
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Internet Draft IP Authentication Header 21 July, 1997
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.
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
(or in so-called "bump-in-the-stack" or "bump-in-the-wire"
implementations, as defined in the Security Architecture document).
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.
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------------------------------------------------
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.
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 Security Architecture document.
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 Security Architecture document.
3.2.2 Sequence Number Generation
The sender's counter is initialized to 0 when an SA is established.
The transmitter increments the Sequence Number for this SA, checks to
ensure that the counter has not cycled, and inserts the new value
into the Sequence Number Field. Thus the first packet sent using a
given SA will have a Sequence Number of 1. A transmitter MUST not
send a packet on an SA if doing so would cause the sequence number to
cycle. An attempt to transmit a packet that would result in sequence
number overflow is an auditable event. (Note that this approach to
Sequence Number management does not require use of modular
arithmetic.)
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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 may be 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. Also, the zero-fill approach ensures that the length of
the fields that are so handled cannot be changed during transit, even
though their contents are not explicitly covered by the ICV.
As a new extension header or IPv4 option is created, it will be
defined in its own RFC and SHOULD include (in the Security
Considerations section) directions for how it should be handled when
calculating the AH ICV. If the IPSEC implementation encounters an
extension header that it does not recognize, it MUST zero the whole
header except for the Next Header and Hdr Ext Len fields. The length
of the extension header MUST be computed by 8 * Hdr Ext Len value +
8. If the IPSEC implementation encounters an IPv4 option that it
does not recognize, it should zero the whole option, using the second
byte of the option as the length. (IPv6 options contain a flag
indicating mutability, which determines appropriate processing for
such options.)
3.2.3.1.1 ICV Computation for IPv4
3.2.3.1.1.1 Base Header Fields
The IPv4 base header fields are classified as follows:
Immutable
Version
Internet Header Length
Total Length
Identification
Protocol
Source Address
Destination Address (without loose or strict source routing)
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Mutable but predictable
Destination Address (with loose or strict source routing)
Mutable (zeroed prior to ICV calculation)
Type of Service (TOS)
Flags
Fragment Offset
Time to Live (TTL)
Header Checksum
TOS -- This field is excluded 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.
Flags -- This field is excluded since an intermediate router might
set the DF bit, even if the source did not select it.
Fragment Offset -- Since AH is applied only to non-fragmented IP
packets, the Offset Field must always be zero, and thus it is
excluded (even though it is predictable).
TTL -- This is changed en-route as a normal course of processing by
routers, and thus its value at the receiver is not predictable
by the sender.
Header Checksum -- This will change if any of these other fields
changes, and thus its value upon reception cannot be predicted
by the sender.
3.2.3.1.1.2 Options
For IPv4 (unlike IPv6), there is no mechanism for tagging options as
mutable in transit. Hence the IPv4 options are explicitly listed in
Appendix A and classified as immutable, mutable but predictable, or
mutable. For IPv4, the entire option is viewed as a unit; so even
though the 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.
3.2.3.1.2 ICV Computation for IPv6
3.2.3.1.2.1 Base Header Fields
The IPv6 base header fields are classified as follows:
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Immutable
Version
Payload Length
Next Header
Source Address
Destination Address (without Routing Extension Header)
Mutable but predictable
Destination Address (with Routing Extension Header)
Mutable (zeroed prior to ICV calculation)
Priority
Flow Label
Hop Limit
3.2.3.1.2.2 Extension Headers -- Options
The IPv6 extension headers (that are options) are explicitly listed
in Appendix A and classified as immutable, mutable but predictable,
or mutable.
IPv6 options in the Hop-by-Hop and Destination Extension Headers
contain a bit that indicates whether the option might change
(unpredictably) 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 options for which the bit indicates immutability are included in
the ICV calculation. See the IPv6 specification [DH95] for more
information.
3.2.3.1.2.3 Extension Headers -- non-Options
The IPv6 extension headers (that are not options) are explicitly
listed in Appendix A and classified as immutable, mutable but
predictable, or mutable.
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 two factors:
- the length of the ICV
- the IP protocol version (v4 or v6)
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For example, if a default, 96-bit truncated (see Section 3.2.3.3)
HMAC algorithm is selected no padding is required for either IPv4 nor
for IPv6. However, if a different length ICV is generated, due to
use of a different algorithm, then padding may be required for the
IPv6 environment. 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 padding 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 appropriate, though performance and space
considerations currently preclude use of such algorithms. As of this
writing, the mandatory-to-implement authentication algorithms are
based on the former class, i.e., HMAC [KBC97] with SHA-1 [SHA] or
HMAC with MD5 [Riv92]. The output of the HMAC computation is
truncated to the leftmost 96 bits. Other algorithms, possibly with
different ICV lengths, MAY be supported.
3.2.4 Fragmentation
If required, IP fragmentation occurs after AH processing within an
IPsec implementation. Thus, transport mode AH is applied only to
whole IP datagrams (not to IP fragments). An IP packet to which AH
has been applied may itself be fragmented by routers en route, and
such fragments must be reassembled prior to AH processing at a
receiver. In tunnel mode, AH is applied to an IP packet, the payload
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of which may be a fragmented IP packet. For example, a security
gateway or a "bump-in-the-stack" or "bump-in-the-wire" IPsec
implementation (see the Security Architecture document for details)
may apply tunnel mode AH to such fragments.
3.3 Inbound Packet Processing
3.3.1 Reassembly
If required, reassembly is performed prior to AH processing. If a
packet offered to AH 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, Source Address, Destination Address, and (in IPv6) the
Flow ID.
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 Security Architecture document.) The SA dictates
whether the Sequence Number field will be checked, specifies the
algorithm(s) employed for ICV computation, and indicates 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; this is
an auditable event. The audit log entry for this event SHOULD
include the SPI value, date/time, Source Address, Destination
Address, and (in IPv6) the Flow ID.
3.3.3 Sequence Number Verification
All AH implementations MUST support the anti-replay service, though
its use may be enabled or disabled on a per-SA basis. (Note that
there are no provisions for managing transmitted Sequence Number
values among multiple senders directing traffic to a single,
multicast SA. Thus the anti-replay service SHOULD NOT be used in a
multi-sender multicast environment that employs a single, multicast
SA.) If an SA establishment protocol such as Oakley/ISAKMP is
employed, then the receiver SHOULD notify the transmitter, during SA
establishment, if the receiver will provide anti-replay protection
and SHOULD inform the transmitter of the window size.
If the receiver has enabled the anti-replay service for this SA, the
receiver packet counter for the SA MUST be initialized to zero when
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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 AH check applied to a
packet after it has been matched to an SA, to speed rejection of
duplicate packets.
Duplicates are rejected through the use of a sliding receive window.
(How the window is implemented is a local matter, but the following
text describes the functionality that the implementation must
exhibit.) A MINIMUM window size of 32 MUST be supported; but a
window size of 64 is preferred and SHOULD be employed as the default.
A window size of 64 or larger MAY be chosen by the receiver. If a
larger window size is chosen, it MUST be a multiple of 32. If any
window size other than the default of 64 is employed by the receiver,
it MUST be reported to the transmitter during SA negotiation.
The "right" edge of the window represents the highest, validated
Sequence Number value received on this SA. Packets that contain
Sequence Numbers lower than the "left" edge of the window are
rejected. Packets falling within the window are checked against a
list of received packets within the window. An efficient means for
performing this check, based on the use of a bit mask, is described
in the Security Architecture document.
If the received packet falls within the window and is new, or if the
packet is to the right of the window, then the receiver proceeds to
ICV verification. If the ICV validation fails, the receiver MUST
discard the received IP datagram as invalid; this is an auditable
event. The audit log entry for this event SHOULD include the SPI
value, date/time, Source Address, Destination Address, the Sequence
Number, and (in IPv6) the Flow ID. The receive window is updated
only if the ICV verification succeeds.
DISCUSSION:
Note that if the packet is either inside the window and new, or is
outside the window on the "right" side, the receiver MUST
authenticate the packet before updating the Sequence Number window
data.
3.3.4 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.
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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 audit log entry SHOULD include the SPI value, date/time,
Source Address, Destination Address, and (in IPv6) the Flow ID.
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 saved value. (For the mandatory-to-
implement authentication algorithms, HMAC [KBC97] with SHA-1 [SHA]
or HMAC with MD5 [Riv92], the output of the HMAC computation is
truncated to the leftmost 96 bits. Other algorithms may have
different ICV lengths.) (If a digital signature and one-way hash
are used for the ICV computation, the matching process is more
complex and will be described in the algorithm specification.)
4. Auditing
Not all systems that implement AH will implement auditing. However,
if AH is incorporated into a system that supports auditing, then the
AH implementation MUST also support auditing and MUST allow a system
administrator to enable or disable auditing for AH. For the most
part, the granularity of auditing is a local matter. However,
several auditable events are identified in this specification and for
each of these events a minimum set of information that SHOULD be
included in an audit log is defined. Additional information also MAY
be included in the audit log for each of these events, and additional
events, not explicitly called out in this specification, also MAY
result in audit log entries. There is no requirement for the
receiver to transmit any message to the purported transmitter in
response to the detection of an auditable event, because of the
potential to induce denial of service via such action.
5. Conformance Requirements
Implementations that claim conformance or compliance with this
specification MUST fully implement the AH syntax and processing
described here and MUST comply with all requirements of the Security
Architecture document. If the key used to compute an ICV is manually
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distributed, correct provision of the anti-replay service would
require correct maintenance of the counter state at the transmitter,
until the key is replaced, and there likely would be no automated
recovery provision if counter overflow were imminent. Thus a
compliant implementation SHOULD NOT provide this service in
conjunction with SAs that are manually keyed. A compliant AH
implementation MUST support the following mandatory-to-implement
algorithms (specified in [KBC97]):
- HMAC with MD5
- HMAC with SHA-1
6. Security Considerations
Security is central to the design of this protocol, and these
security considerations permeate the specification. Additional
security-relevant aspects of using the IPsec protocol are discussed
in the Security Architecture document.
7. Differences from RFC 1826
This specification of AH differs from RFC 1826 [ATK95] in several
important respects, but the fundamental features of AH remain intact.
One goal of the revision of RFC 1826 was to provide a complete
framework for AH, with ancillary RFCs required only for algorithm
specification. For example, the anti-replay service is now an
integral, mandatory part of AH, not a feature of a transform defined
in another RFC. Carriage of a sequence number to support this
service is now required at all times, to meet IPv6 alignment
requirements (even when anti-replay is not enabled for an SA). The
default algorithms required for interoperability have been changed to
HMAC with MD5 or SHA-1 (vs. keyed MD5), for security reasons. The
list of IPv4 header fields excluded from the ICV computation has been
expanded to include the OFFSET and FLAGS fields.
Another motivation for revision was to provide additional detail and
clarification of subtle points. This specification provides
rationale for exclusion of selected IPv4 header fields from AH
coverage and provides examples on positioning of AH in both the IPv4
and v6 contexts. Auditing requirements have been clarified in this
version of the specification. Tunnel mode AH was mentioned only in
passing in RFC 1826, but now is a mandatory feature of AH.
Discussion of interactions with key management and with security
labels have been moved to the Security Architecture document.
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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 Karen Seo for providing
extensive help in the review, editing, background research, and
coordination for this version of the specification. The authors
would also like to thank the members of the IPsec and IPng working
groups, with special mention of the efforts of (in alphabetic order):
Steve Bellovin, Steve Deering, Francis Dupont, Phil Karn, Frank
Kastenholz, Perry Metzger, David Mihelcic, Hilarie Orman, William
Simpson, and Nina Yuan.
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Appendix A -- Mutability of IP Options/Extension Headers
1. IPv4 Options
This table shows how the IPv4 options are classified with regard to
"mutability". Where two references are provided, the second one
supercedes the first. This table is based in part on information
provided in RFC1700, "ASSIGNED NUMBERS", (October 1994).
Opt.
Copy Class # Name Reference
---- ----- --- ------------------------- ---------
IMMUTABLE -- included in ICV calculation
0 0 0 End of Options List [RFC791]
0 0 1 No Operation [RFC791]
1 0 2 Security [RFC1108(historic but in
use)]
1 0 5 Extended Security [RFC1108(historic but in
use)]
1 0 6 Commercial Security [expired I-D, now US MIL STD]
1 0 20 Router Alert [RFC2113]
1 0 21 Sender Directed Multi- [RFC1770]
Destination Delivery
MUTABLE -- zeroed
1 0 3 Loose Source Route [RFC791]
0 2 4 Time Stamp [RFC791]
0 0 7 Record Route [RFC791]
1 0 9 Strict Source Route [RFC791]
0 2 18 Traceroute [RFC1393]
EXPERIMENTAL, SUPERCEDED -- zeroed
1 0 8 Stream ID [RFC791, RFC1122 (Host Req)]
0 0 11 MTU Probe [RFC1063, RFC1191 (PMTU)]
0 0 12 MTU Reply [RFC1063, RFC1191 (PMTU)]
1 0 17 Extended Internet Protocol [RFC1385, RFC1883 (IPv6)]
0 0 10 Experimental Measurement [ZSu]
1 2 13 Experimental Flow Control [Finn]
1 0 14 Experimental Access Ctl [Estrin]
0 0 15 ??? [VerSteeg]
1 0 16 IMI Traffic Descriptor [Lee]
1 0 19 Address Extension [Ullmann IPv7]
NOTE: Use of the Router Alert option is potentially incompatible with
use of IPSEC. Although the option is immutable, its use implies that
each router along a packet's path will "process" the packet and
consequently might change the packet. This would happen on a hop by
hop basis as the packet goes from router to router. Prior to being
processed by the application to which the option contents are
directed, e.g., RSVP/IGMP, the packet should encounter AH processing.
However, AH processing would require that each router along the path
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is a member of a multicast-SA defined by the SPI. This might pose
problems for packets that are not strictly source routed, and it
requires multicast support techniques not currently available.
NOTE: Addition or removal of any security labels (BSO, ESO, CIPSO) by
systems along a packet's path conflicts with the classification of these
IP Options as immutable and is incompatible with the use of IPSEC.
2. IPv6 Extension Headers
This table shows how the IPv6 Extension Headers are classified with
regard to "mutability".
Option/Extension Name Reference
----------------------------------- ---------
MUTABLE BUT PREDICTABLE -- included in ICV calculation
Routing (Type 0) [RFC1883]
BIT INDICATES IF OPTION IS MUTABLE (CHANGES UNPREDICTABLY DURING
TRANSIT)
Hop by Hop options [RFC1883]
Destination options [RFC1883]
NOT APPLICABLE
Fragmentation [RFC1883]
Options -- IPv6 options in the Hop-by-Hop and Destination Extension
Headers contain a bit that indicates whether the option might
change (unpredictably) 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 options for which the bit
indicates immutability are included in the ICV calculation. See
the IPv6 specification [DH95] for more information.
Routing (Type 0) -- 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 mutable but
predictable. The transmitter must order the field so that it
appears as it will at the receiver, prior to performing the ICV
computation.
Fragmentation -- Fragmentation occurs after outbound IPSEC processing
(section 3.2.4) and reassembly occurs before inbound IPSEC
processing (section 3.3.1). So the Fragmentation Extension
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Header, if it exists, is not seen by IPSEC.
Note that on the receive side, the IP implementation could leave
a Fragmentation Extension Header in place when it does
re-assembly. If this happens, then when AH receives the packet,
before doing ICV processing, AH MUST "remove" (or skip over)
this header and change the previous header's "Next Header" field
to be the "Next Header" field in the Fragmentation Extension
Header.
Note that on the send side, the IP implementation could give the
IPSEC code a packet with a Fragmentation Extension Header with
Offset of 0 (first fragment) and a More Fragments Flag of 0
(last fragment). If this happens, then before doing ICV
processing, AH MUST first "remove" (or skip over) this header
and change the previous header's "Next Header" field to be the
"Next Header" field in the Fragmentation Extension Header.
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References
[ATK95] R. Atkinson, "The IP Authentication Header," RFC 1826,
August 1995.
[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.
[KA97a] Steve Kent, Randall Atkinson, "Security Architecture for
the Internet Protocol", Internet Draft, ?? 1997.
[KA97b] Steve Kent, Randall Atkinson, "IP Encapsulating Security
Payload (ESP)", Internet Draft, ?? 1997.
[KA97c] Steve Kent, Randall Atkinson, "IP Authentication Header",
Internet Draft, ?? 1997.
[KBC97] Hugo Krawczyk, Mihir Bellare, and Ran Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC-2104,
February 1997.
[Ken91] Steve Kent, "US DoD Security Options for the Internet
Protocol", RFC-1108, November 1991.
[KA97a] Steve Kent, Randall Atkinson, "Security Architecture for
the Internet Protocol", Internet Draft, ?? 1997.
[Riv92] Ronald Rivest, "The MD5 Message Digest Algorithm," RFC-
1321, April 1992.
[SHA] NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995
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[STD-1] J. Postel, "Internet Official Protocol Standards", STD-1,
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
E-mail: kent@bbn.com
Telephone: +1 (617) 873-3988
Randall Atkinson
@Home Network
385 Ravendale Drive
Mountain View, CA 94043
USA
E-mail: rja@inet.org
Kent, Atkinson [Page 22]