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RFC2402 - IP Authentication Header

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

Network Working Group S. Kent

Request for Comments: 2402 BBN Corp

Obsoletes: 1826 R. Atkinson

Category: Standards Track @Home Network

November 1998

IP Authentication Header

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Table of Contents

1. IntrodUCtion......................................................2

2. Authentication Header Format......................................3

2.1 Next Header...................................................4

2.2 Payload Length................................................4

2.3 Reserved......................................................4

2.4 Security Parameters Index (SPI)...............................4

2.5 Sequence Number...............................................5

2.6 Authentication Data ..........................................5

3. Authentication Header Processing..................................5

3.1 Authentication Header Location...............................5

3.2 Authentication Algorithms....................................7

3.3 Outbound Packet Processing...................................8

3.3.1 Security Association Lookup.............................8

3.3.2 Sequence Number Generation..............................8

3.3.3 Integrity Check Value Calculation.......................9

3.3.3.1 Handling Mutable Fields............................9

3.3.3.1.1 ICV Computation for IPv4.....................10

3.3.3.1.1.1 Base Header Fields.......................10

3.3.3.1.1.2 Options..................................11

3.3.3.1.2 ICV Computation for IPv6.....................11

3.3.3.1.2.1 Base Header Fields.......................11

3.3.3.1.2.2 Extension Headers Containing Options.....11

3.3.3.1.2.3 Extension Headers Not Containing Options.11

3.3.3.2 Padding...........................................12

3.3.3.2.1 Authentication Data Padding..................12

3.3.3.2.2 Implicit Packet Padding......................12

3.3.4 Fragmentation..........................................12

3.4 Inbound Packet Processing...................................13

3.4.1 Reassembly.............................................13

3.4.2 Security Association Lookup............................13

3.4.3 Sequence Number Verification...........................13

3.4.4 Integrity Check Value Verification.....................15

4. Auditing.........................................................15

5. Conformance Requirements.........................................16

6. Security Considerations..........................................16

7. Differences from RFC1826........................................16

Acknowledgements....................................................17

Appendix A -- Mutability of IP Options/Extension Headers............18

A1. IPv4 Options.................................................18

A2. IPv6 Extension Headers.......................................19

References..........................................................20

Disclaimer..........................................................21

Author Information..................................................22

Full Copyright Statement............................................22

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. (Although

the default calls for the sender to increment the Sequence Number

used for anti-replay, the service is effective only if the receiver

checks the Sequence Number.) 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 sender. 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 IKE [HC98].)

The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,

SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this

document, are to be interpreted as described in RFC2119 [Bra97].

2. 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 Integrity

Check Value (ICV) computation (see Sections 2.6 and 3.3.3).

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] RFCfrom 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". (All IPv6 extension headers, as per RFC1883,

encode the "Hdr Ext Len" field by first suBTracting 1 (64-bit word)

from the header length (measured in 64-bit words). AH is an IPv6

extension header. However, since its length is measured in 32-bit

words, the "Payload Length" is calculated by subtracting 2 (32 bit

words).) 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 for

IPv4 or a "2" for IPv6, as there would be no corresponding

Authentication Data field (see Section 3.3.3.2.1 on "Authentication

Data Padding").

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, in combination with the

destination IP address and security protocol (AH), uniquely

identifies the Security Association for this datagram. The set of

SPI values in the range 1 through 255 are reserved by the Internet

Assigned Numbers Authority (IANA) for future use; a reserved SPI

value will not normally be assigned by IANA unless the use of the

assigned SPI value is specified in an RFC. It is ordinarily selected

by the destination system upon establishment of an SA (see the

Security Architecture document for more details).

The SPI value of zero (0) is reserved for local, implementation-

specific use and MUST NOT be sent on the wire. For example, a key

management implementation MAY use the zero SPI value to mean "No

Security Association Exists" during the period when the IPsec

implementation has requested that its key management entity establish

a new SA, but the SA has not yet been established.

2.5 Sequence Number

This unsigned 32-bit field contains a monotonically increasing

counter value (sequence number). It is mandatory and is always

present even if the receiver does not elect to enable the anti-replay

service for a specific SA. Processing of the Sequence Number field

is at the discretion of the receiver, i.e., the sender MUST always

transmit this field, but the receiver need not act upon it (see the

discussion of Sequence Number Verification in the "Inbound Packet

Processing" section below).

The sender's counter and the receiver's counter are initialized to 0

when an SA is established. (The first packet sent using a given SA

will have a Sequence Number of 1; see Section 3.3.2 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.

2.6 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 3.3.2 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. The

authentication algorithm specification MUST specify the length of the

ICV and the comparison rules and processing steps for validation.

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. 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.

BEFORE APPLYING AH

---------------------------------------

IPv6 ext hdrs

orig IP hdr if present TCP Data

---------------------------------------

AFTER APPLYING AH

------------------------------------------------------------

IPv6 hop-by-hop, dest*, dest

orig IP hdr routing, fragment. AH opt* TCP Data

------------------------------------------------------------

<---- authenticated except for mutable fields ----------->

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

ESP and AH headers can be combined in a variety of modes. The IPsec

Architecture document describes the combinations of security

associations that must be supported.

Tunnel mode 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 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 -->

in the new IP hdr

--------------------------------------------------------------

IPv6 ext hdrs* ext hdrs*

new IP hdr*if present AH orig IP hdr*if presentTCPData

--------------------------------------------------------------

<-- authenticated except for mutable fields in new IP hdr ->

* = construction of outer IP hdr/extensions and modification

of inner IP hdr/extensions is discussed below.

3.2 Authentication Algorithms

The authentication algorithm employed for the ICV computation is

specified by the SA. For point-to-point communication, suitable

authentication algorithms include keyed Message Authentication Codes

(MACs) based on symmetric encryption algorithms (e.g., DES) or on

one-way hash functions (e.g., MD5 or SHA-1). For multicast

communication, one-way hash algorithms combined with asymmetric

signature algorithms are appropriate, though performance and space

considerations currently preclude use of such algorithms. The

mandatory-to-implement authentication algorithms are described in

Section 5 "Conformance Requirements". Other algorithms MAY be

supported.

3.3 Outbound Packet Processing

In transport mode, the sender 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.

If there is more than one IPsec header/extension required, the order

of the application of the security headers MUST be defined by

security policy. For simplicity of processing, each IPsec header

SHOULD ignore the existence (i.e., not zero the contents or try to

predict the contents) of IPsec headers to be applied later. (While a

native IP or bump-in-the-stack implementation could predict the

contents of later IPsec headers that it applies itself, it won't be

possible for it to predict any IPsec headers added by a bump-in-the-

wire implementation between the host and the network.)

3.3.1 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.

3.3.2 Sequence Number Generation

The sender's counter is initialized to 0 when an SA is established.

The sender increments the Sequence Number for this SA and inserts the

new value into the Sequence Number Field. Thus the first packet sent

using a given SA will have a Sequence Number of 1.

If anti-replay is enabled (the default), the sender checks to ensure

that the counter has not cycled before inserting the new value in the

Sequence Number field. In other words, the sender MUST NOT send a

packet on an SA if doing so would cause the Sequence Number to cycle.

An attempt to transmit a packet that would result in Sequence Number

overflow is an auditable event. (Note that this approach to Sequence

Number management does not require use of modular arithmetic.)

The sender assumes anti-replay is enabled as a default, unless

otherwise notified by the receiver (see 3.4.3). Thus, if the counter

has cycled, the sender will set up a new SA and key (unless the SA

was configured with manual key management).

If anti-replay is disabled, the sender does not need to monitor or

reset the counter, e.g., in the case of manual key management (see

Section 5.) However, the sender still increments the counter and when

it reaches the maximum value, the counter rolls over back to zero.

3.3.3 Integrity Check Value Calculation

The AH ICV is computed over:

o IP header fields that are either immutable in transit or

that are predictable in value upon arrival at the endpoint

for the AH SA

o the AH header (Next Header, Payload Len, Reserved, SPI,

Sequence Number, and the Authentication Data (which is set

to zero for this computation), and explicit padding bytes

(if any))

o the upper level protocol data, which is assumed to be

immutable in transit

3.3.3.1 Handling Mutable Fields

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 is also 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 RFCand SHOULD include (in the Security

Considerations section) directions for how it should be handled when

calculating the AH ICV. If the IP (v4 or v6) implementation

encounters an extension header that it does not recognize, it will

discard the packet and send an ICMP message. IPsec will never see

the packet. 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 (in

Destination extension headers or Hop by Hop extension header) contain

a flag indicating mutability, which determines appropriate processing

for such options.

3.3.3.1.1 ICV Computation for IPv4

3.3.3.1.1.1 Base Header Fields

The IPv4 base header fields are classified as follows:

Immutable

Version

Internet Header Length

Total Length

Identification

Protocol (This should be the value for AH.)

Source Address

Destination Address (without loose or strict source routing)

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.

3.3.3.1.1.2 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.

3.3.3.1.2 ICV Computation for IPv6

3.3.3.1.2.1 Base Header Fields

The IPv6 base header fields are classified as follows:

Immutable

Version

Payload Length

Next Header (This should be the value for AH.)

Source Address

Destination Address (without Routing Extension Header)

Mutable but predictable

Destination Address (with Routing Extension Header)

Mutable (zeroed prior to ICV calculation)

Class

Flow Label

Hop Limit

3.3.3.1.2.2 Extension Headers Containing 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.

3.3.3.1.2.3 Extension Headers Not Containing Options

The IPv6 extension headers that do not contain options are explicitly

listed in Appendix A and classified as immutable, mutable but

predictable, or mutable.

3.3.3.2 Padding

3.3.3.2.1 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)

For example, if the output of the selected algorithm is 96-bits, no

padding is required for either IPv4 or for IPv6. However, if a

different length ICV is generated, due to use of a different

algorithm, then padding may be required depending on the length and

IP protocol version. 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.

3.3.3.2.2 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. Note that MD5 and SHA-1 are viewed as having a 1-byte

blocksize because of their internal padding conventions.

3.3.4 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

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.

3.4 Inbound Packet Processing

If there is more than one IPsec header/extension present, the

processing for each one ignores (does not zero, does not use) any

IPsec headers applied subsequent to the header being processed.

3.4.1 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.

NOTE: For packet reassembly, the current IPv4 spec does NOT require

either the zero'ing of the OFFSET field or the clearing of the MORE

FRAGMENTS flag. In order for a reassembled packet to be processed by

IPsec (as opposed to discarded as an apparent fragment), the IP code

must do these two things after it reassembles a packet.

3.4.2 Security Association Lookup

Upon receipt of a packet containing an IP Authentication Header, the

receiver determines the appropriate (unidirectional) SA, based on the

destination IP address, security protocol (AH), and the SPI. (This

process is described in more detail in the Security Architecture

document.) The SA indicates whether the Sequence Number field will

be checked, 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.

3.4.3 Sequence Number Verification

All AH implementations MUST support the anti-replay service, though

its use may be enabled or disabled by the receiver 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 SA

(irrespective of whether the destination address is unicast,

broadcast, or multicast). Thus the anti-replay service SHOULD NOT be

used in a multi-sender environment that employs a single SA.)

If the receiver does not enable anti-replay for an SA, no inbound

checks are performed on the Sequence Number. However, from the

perspective of the sender, the default is to assume that anti-replay

is enabled at the receiver. To avoid having the sender do

unnecessary sequence number monitoring and SA setup (see section

3.3.2), if an SA establishment protocol such as IKE is employed, the

receiver SHOULD notify the sender, during SA establishment, if the

receiver will not provide anti-replay protection.

If the receiver has enabled the anti-replay service for this SA, the

receiver 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 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.

Another window size (larger than the MINIMUM) MAY be chosen by the

receiver. (The receiver does NOT notify the sender of the window

size.)

The "right" edge of the window represents the highest, validated

Sequence Number value received on this SA. Packets that contain

Sequence Numbers lower than the "left" edge of the window are

rejected. Packets falling within the window are checked against a

list of received packets within the window. An efficient means for

performing this check, based on the use of a bit mask, is described

in the Security Architecture document.

If the received packet falls within the window and is new, or if the

packet is to the right of the window, then the receiver proceeds to

ICV verification. If the ICV validation fails, the receiver MUST

discard the received IP datagram as invalid; this is an auditable

event. The audit log entry for this event SHOULD include the SPI

value, date/time, Source Address, Destination Address, the Sequence

Number, and (in IPv6) the Flow ID. The receive window is updated

only if the ICV verification succeeds.

DISCUSSION:

Note that if the packet is either inside the window and new, or is

outside the window on the "right" side, the receiver MUST

authenticate the packet before updating the Sequence Number window

data.

3.4.4 Integrity Check Value Verification

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; this is an auditable

event. The audit log entry SHOULD include the SPI value, date/time

received, 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 3.3.3.1

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. Perform the ICV computation and compare

the result with the saved value, using the comparison rules

defined by the algorithm specification. (For example, if a

digital signature and one-way hash are used for the ICV

computation, the matching process is more complex.)

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 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 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

distributed, correct provision of the anti-replay service would

require correct maintenance of the counter state at the sender, until

the key is replaced, and there likely would be no automated recovery

provision if counter overflow were imminent. Thus a compliant

implementation SHOULD NOT provide this service in conjunction with

SAs that are manually keyed. A compliant AH implementation MUST

support the following mandatory-to-implement algorithms:

- HMAC with MD5 [MG97a]

- HMAC with SHA-1 [MG97b]

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 RFC1826

This specification of AH differs from RFC1826 [ATK95] in several

important respects, but the fundamental features of AH remain intact.

One goal of the revision of RFC1826 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. 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 RFC1826, 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.

Acknowledgements

For over 3 years, this document has evolved through multiple versions

and iterations. During this time, many people have contributed

significant ideas and energy to the process and the documents

themselves. The authors would like to thank Karen Seo for providing

extensive help in the review, editing, background research, and

coordination for this version of the specification. The authors

would also like to thank the members of the IPsec and IPng working

groups, with special mention of the efforts of (in alphabetic order):

Steve Bellovin, Steve Deering, Francis Dupont, Phil Karn, Frank

Kastenholz, Perry Metzger, David Mihelcic, Hilarie Orman, Norman

Shulman, William Simpson, and Nina Yuan.

Appendix A -- Mutability of IP Options/Extension Headers

A1. 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 Proto [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

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.

NOTE: End of Options List options SHOULD be repeated as necessary to

ensure that the IP header ends on a 4 byte boundary in order to

ensure that there are no unspecified bytes which could be used for a

covert channel.

A2. 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 sender 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 3.3) and reassembly occurs before

inbound IPsec processing (section 3.4). So the

Fragmentation Extension 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.

References

[ATK95] Atkinson, R., "The IP Authentication Header", RFC1826,

August 1995.

[Bra97] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Level", BCP 14, RFC2119, March 1997.

[DH95] Deering, S., and B. Hinden, "Internet Protocol version 6

(IPv6) Specification", RFC1883, December 1995.

[HC98] Harkins, D., and D. Carrel, "The Internet Key Exchange

(IKE)", RFC2409, November 1998.

[KA97a] Kent, S., and R. Atkinson, "Security Architecture for the

Internet Protocol", RFC2401, November 1998.

[KA97b] Kent, S., and R. Atkinson, "IP Encapsulating Security

Payload (ESP)", RFC2406, November 1998.

[MG97a] Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within

ESP and AH", RFC2403, November 1998.

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

ESP and AH", RFC2404, November 1998.

[STD-2] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC

1700, October 1994. See also:

http://www.iana.org/numbers.Html

Disclaimer

The views and specification here are those of the authors and are not

necessarily those of their employers. The authors and their

employers specifically disclaim responsibility for any problems

arising from correct or incorrect implementation or use of this

specification.

Author Information

Stephen Kent

BBN Corporation

70 Fawcett Street

Cambridge, MA 02140

USA

Phone: +1 (617) 873-3988

EMail: kent@bbn.com

Randall Atkinson

@Home Network

425 Broadway,

Redwood City, CA 94063

USA

Phone: +1 (415) 569-5000

EMail: rja@corp.home.net

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

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