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Network Working Group S. Kent
Request for Comments: 4302 BBN Technologies
Obsoletes: 2402 December 2005
Category: Standards Track
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 (2005).
Abstract
This document describes an updated version of the IP Authentication
Header (AH), which is designed to provide authentication services in
IPv4 and IPv6. This document obsoletes RFC 2402 (November 1998).
Table of Contents
1. Introduction ....................................................3
2. Authentication Header Format ....................................4
2.1. Next Header ................................................5
2.2. Payload Length .............................................5
2.3. Reserved ...................................................6
2.4. Security Parameters Index (SPI) ............................6
2.5. Sequence Number ............................................8
2.5.1. Extended (64-bit) Sequence Number ...................8
2.6. Integrity Check Value (ICV) ................................9
3. Authentication Header Processing ................................9
3.1. Authentication Header Location .............................9
3.1.1. Transport Mode ......................................9
3.1.2. Tunnel Mode ........................................11
3.2. Integrity Algorithms ......................................11
3.3. Outbound Packet Processing ................................11
3.3.1. Security Association Lookup ........................12
3.3.2. Sequence Number Generation .........................12
3.3.3. Integrity Check Value Calculation ..................13
3.3.3.1. Handling Mutable Fields ...................13
3.3.3.2. Padding and Extended Sequence Numbers .....16
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RFC 4302 IP Authentication Header December 2005
3.3.4. Fragmentation ......................................17
3.4. Inbound Packet Processing .................................18
3.4.1. Reassembly .........................................18
3.4.2. Security Association Lookup ........................18
3.4.3. Sequence Number Verification .......................19
3.4.4. Integrity Check Value Verification .................20
4. Auditing .......................................................21
5. Conformance Requirements .......................................21
6. Security Considerations ........................................22
7. Differences from RFC 2402 ......................................22
8. Acknowledgements ...............................................22
9. References .....................................................22
9.1. Normative References ......................................22
9.2. Informative References ....................................23
Appendix A: Mutability of IP Options/Extension Headers ............25
A1. IPv4 Options ...............................................25
A2. IPv6 Extension Headers .....................................26
Appendix B: Extended (64-bit) Sequence Numbers ....................28
B1. Overview ...................................................28
B2. Anti-Replay Window .........................................28
B2.1. Managing and Using the Anti-Replay Window ............29
B2.2. Determining the Higher-Order Bits (Seqh) of the
Sequence Number ......................................30
B2.3. Pseudo-Code Example ..................................31
B3. Handling Loss of Synchronization due to Significant
Packet Loss ................................................32
B3.1. Triggering Re-synchronization ........................33
B3.2. Re-synchronization Process ...........................33
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RFC 4302 IP Authentication Header December 2005
1. Introduction
This document assumes that the reader is familiar with the terms and
concepts described in the "Security Architecture for the Internet
Protocol" [Ken-Arch], hereafter referred to as the Security
Architecture document. In particular, the reader should be familiar
with the definitions of security services offered by the
Encapsulating Security Payload (ESP) [Ken-ESP] and the IP
Authentication Header (AH), the concept of Security Associations, the
ways in which ESP can be used in conjunction with the Authentication
Header (AH), and the different key management options available for
ESP and AH.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in RFC 2119 [Bra97].
The IP Authentication Header (AH) is used to provide connectionless
integrity and data origin authentication for IP datagrams (hereafter
referred to as just "integrity") and to provide protection against
replays. This latter, optional service may be selected, by the
receiver, when a Security Association (SA) is established. (The
protocol default requires the sender to increment the sequence number
used for anti-replay, but the service is effective only if the
receiver checks the sequence number.) However, to make use of the
Extended Sequence Number feature in an interoperable fashion, AH does
impose a requirement on SA management protocols to be able to
negotiate this new feature (see Section 2.5.1 below).
AH provides authentication for as much of the IP header as possible,
as well as for next 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 piecemeal. (See
Appendix A.)
AH may be applied alone, in combination with the IP Encapsulating
Security Payload (ESP) [Ken-ESP], or in a nested fashion (see
Security Architecture document [Ken-Arch]). 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 anti-replay and similar
integrity services, and it also provides a confidentiality
(encryption) service. The primary difference between the integrity
provided by ESP and AH is the extent of the coverage. Specifically,
ESP does not protect any IP header fields unless those fields are
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RFC 4302 IP Authentication Header December 2005
encapsulated by ESP (e.g., via use of tunnel mode). For more details
on how to use AH and ESP in various network environments, see the
Security Architecture document [Ken-Arch].
Section 7 provides a brief review of the differences between this
document and RFC 2402 [RFC2402].
2. Authentication Header Format
The protocol header (IPv4, IPv6, or IPv6 Extension) immediately
preceding the AH header SHALL contain the value 51 in its Protocol
(IPv4) or Next Header (IPv6, Extension) fields [DH98]. Figure 1
illustrates the format for AH.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Integrity Check Value-ICV (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. AH Format
The following table refers to the fields that comprise AH,
(illustrated in Figure 1), plus other fields included in the
integrity computation, and illustrates which fields are covered by
the ICV and what is transmitted.
What What
# of Requ'd Integ is
bytes [1] Covers Xmtd
------ ------ ------ ------
IP Header variable M [2] plain
Next Header 1 M Y plain
Payload Len 1 M Y plain
RESERVED 2 M Y plain
SPI 4 M Y plain
Seq# (low-order 32 bits) 4 M Y plain
ICV variable M Y[3] plain
IP datagram [4] variable M Y plain
Seq# (high-order 32 bits) 4 if ESN Y not xmtd
ICV Padding variable if need Y not xmtd
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[1] - M = mandatory
[2] - See Section 3.3.3, "Integrity Check Value Calculation", for
details of which IP header fields are covered.
[3] - Zeroed before ICV calculation (resulting ICV placed here
after calculation)
[4] - If tunnel mode -> IP datagram
If transport mode -> next header and data
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).
Note: All of the cryptographic algorithms used in IPsec expect their
input in canonical network byte order (see Appendix of RFC 791
[RFC791]) and generate their output in canonical network byte order.
IP packets are also transmitted in network byte order.
AH does not contain a version number, therefore if there are concerns
about backward compatibility, they MUST be addressed by using a
signaling mechanism between the two IPsec peers to ensure compatible
versions of AH, e.g., IKE [IKEv2] or an out-of-band configuration
mechanism.
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 on the
web page of Internet Assigned Numbers Authority (IANA). For example,
a value of 4 indicates IPv4, a value of 41 indicates IPv6, and a
value of 6 indicates TCP.
2.2. Payload Length
This 8-bit field specifies the length of AH in 32-bit words (4-byte
units), minus "2". Thus, for example, if an integrity algorithm
yields a 96-bit authentication value, this length field will be "4"
(3 32-bit word fixed fields plus 3 32-bit words for the ICV, minus
2). For IPv6, the total length of the header must be a multiple of
8-octet units. (Note that although IPv6 [DH98] characterizes AH as
an extension header, its length is measured in 32-bit words, not the
64-bit words used by other IPv6 extension headers.) See Section 2.6,
"Integrity Check Value (ICV)", for comments on padding of this field,
and Section 3.3.3.2.1, "ICV Padding".
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2.3. Reserved
This 16-bit field is reserved for future use. It MUST be set to
"zero" by the sender, and it SHOULD be ignored by the recipient.
(Note that the value is included in the ICV calculation, but is
otherwise ignored by the recipient.)
2.4. Security Parameters Index (SPI)
The SPI is an arbitrary 32-bit value that is used by a receiver to
identify the SA to which an incoming packet is bound. For a unicast
SA, the SPI can be used by itself to specify an SA, or it may be used
in conjunction with the IPsec protocol type (in this case AH).
Because for unicast SAs the SPI value is generated by the receiver,
whether the value is sufficient to identify an SA by itself or
whether it must be used in conjunction with the IPsec protocol value
is a local matter. The SPI field is mandatory, and this mechanism
for mapping inbound traffic to unicast SAs described above MUST be
supported by all AH implementations.
If an IPsec implementation supports multicast, then it MUST support
multicast SAs using the algorithm below for mapping inbound IPsec
datagrams to SAs. Implementations that support only unicast traffic
need not implement this de-multiplexing algorithm.
In many secure multicast architectures, e.g., [RFC3740], a central
Group Controller/Key Server unilaterally assigns the group security
association's SPI. This SPI assignment is not negotiated or
coordinated with the key management (e.g., IKE) subsystems that
reside in the individual end systems that comprise the group.
Consequently, it is possible that a group security association and a
unicast security association can simultaneously use the same SPI. A
multicast-capable IPsec implementation MUST correctly de-multiplex
inbound traffic even in the context of SPI collisions.
Each entry in the Security Association Database (SAD) [Ken-Arch] must
indicate whether the SA lookup makes use of the destination, or
destination and source, IP addresses, in addition to the SPI. For
multicast SAs, the protocol field is not employed for SA lookups.
For each inbound, IPsec-protected packet, an implementation must
conduct its search of the SAD such that it finds the entry that
matches the "longest" SA identifier. In this context, if two or more
SAD entries match based on the SPI value, then the entry that also
matches based on destination, or destination and source, address
comparison (as indicated in the SAD entry) is the "longest" match.
This implies a logical ordering of the SAD search as follows:
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RFC 4302 IP Authentication Header December 2005
1. Search the SAD for a match on {SPI, destination
address, source address}. If an SAD entry
matches, then process the inbound AH packet with that
matching SAD entry. Otherwise, proceed to step 2.
2. Search the SAD for a match on {SPI, destination
address}. If an SAD entry matches, then process
the inbound AH packet with that matching SAD
entry. Otherwise, proceed to step 3.
3. Search the SAD for a match on only {SPI} if the receiver
has chosen to maintain a single SPI space for AH and ESP,
or on {SPI, protocol} otherwise. If an SAD
entry matches, then process the inbound AH packet with
that matching SAD entry. Otherwise, discard the packet
and log an auditable event.
In practice, an implementation MAY choose any method to accelerate
this search, although its externally visible behavior MUST be
functionally equivalent to having searched the SAD in the above
order. For example, a software-based implementation could index into
a hash table by the SPI. The SAD entries in each hash table bucket's
linked list are kept sorted to have those SAD entries with the
longest SA identifiers first in that linked list. Those SAD entries
having the shortest SA identifiers are sorted so that they are the
last entries in the linked list. A hardware-based implementation may
be able to effect the longest match search intrinsically, using
commonly available Ternary Content-Addressable Memory (TCAM)
features.
The indication of whether source and destination address matching is
required to map inbound IPsec traffic to SAs MUST be set either as a
side effect of manual SA configuration or via negotiation using an SA
management protocol, e.g., IKE or Group Domain of Interpretation
(GDOI) [RFC3547]. Typically, Source-Specific Multicast (SSM) [HC03]
groups use a 3-tuple SA identifier composed of an SPI, a destination
multicast address, and source address. An Any-Source Multicast group
SA requires only an SPI and a destination multicast address as an
identifier.
The set of SPI values in the range 1 through 255 is reserved by the
Internet Assigned Numbers Authority (IANA) for future use; a reserved
SPI value will not normally be assigned by IANA unless the use of the
assigned SPI value is specified in an RFC. The SPI value of zero (0)
is reserved for local, implementation-specific use and MUST NOT be
sent on the wire. (For example, a key management implementation
might use the zero SPI value to mean "No Security Association Exists"
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RFC 4302 IP Authentication Header December 2005
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 counter value that increases by
one for each packet sent, i.e., a per-SA packet sequence number. For
a unicast SA or a single-sender multicast SA, the sender MUST
increment this field for every transmitted packet. Sharing an SA
among multiple senders is permitted, though generally not
recommended. AH provides no means of synchronizing packet counters
among multiple senders or meaningfully managing a receiver packet
counter and window in the context of multiple senders. Thus, for a
multi-sender SA, the anti-reply features of AH are not available (see
Sections 3.3.2 and 3.4.3).
The field is mandatory and MUST always be present even if the
receiver does not elect to enable the anti-replay service for a
specific SA. Processing of the Sequence Number field is at the
discretion of the receiver, but all AH implementations MUST be
capable of performing the processing described in Section 3.3.2,
"Sequence Number Generation", and Section 3.4.3, "Sequence Number
Verification". Thus, the sender MUST always transmit this field, but
the receiver need not act upon it.
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.
2.5.1. Extended (64-bit) Sequence Number
To support high-speed IPsec implementations, a new option for
sequence numbers SHOULD be offered, as an extension to the current,
32-bit sequence number field. Use of an Extended Sequence Number
(ESN) MUST be negotiated by an SA management protocol. Note that in
IKEv2, this negotiation is implicit; the default is ESN unless 32-bit
sequence numbers are explicitly negotiated. (The ESN feature is
applicable to multicast as well as unicast SAs.)
The ESN facility allows use of a 64-bit sequence number for an SA.
(See Appendix B, "Extended (64-bit) Sequence Numbers", for details.)
Only the low-order 32 bits of the sequence number are transmitted in
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the AH header of each packet, thus minimizing packet overhead. The
high-order 32 bits are maintained as part of the sequence number
counter by both transmitter and receiver and are included in the
computation of the ICV, but are not transmitted.
2.6. Integrity Check Value (ICV)
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 (IPv4 or IPv6) in length. The details of ICV processing
are described in Section 3.3.3, "Integrity Check Value Calculation",
and Section 3.4.4, "Integrity Check Value Verification". This field
may include explicit padding, if required 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 and MUST
insert only enough padding to satisfy the IPv4/IPv6 alignment
requirements. Details of how to compute the required padding length
are provided below in Section 3.3.3.2, "Padding". The integrity
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
AH may be employed in two ways: transport mode or tunnel mode. (See
the Security Architecture document for a description of when each
should be used.)
3.1.1. Transport Mode
In transport mode, AH is inserted after the IP header and before a
next 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 next layer protocol. (Note that
the term "transport" mode should not be misconstrued as restricting
its use to TCP and UDP.) The following diagram illustrates AH
transport mode positioning for a typical IPv4 packet, on a "before
and after" basis.
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RFC 4302 IP Authentication Header December 2005
BEFORE APPLYING AH
----------------------------
IPv4 |orig IP hdr | | |
|(any options)| TCP | Data |
----------------------------
AFTER APPLYING AH
-------------------------------------------------------
IPv4 |original IP hdr (any options) | AH | TCP | Data |
-------------------------------------------------------
|<- mutable field processing ->|<- immutable fields ->|
|<----- 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
before or after or both before and 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 |
------------------------------------------------------------
|<--- mutable field processing -->|<-- immutable fields -->|
|<---- 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.
Note that in transport mode, for "bump-in-the-stack" or "bump-in-
the-wire" implementations, as defined in the Security Architecture
document, inbound and outbound IP fragments may require an IPsec
implementation to perform extra IP reassembly/fragmentation in order
to both conform to this specification and provide transparent IPsec
support. Special care is required to perform such operations within
these implementations when multiple interfaces are in use.
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3.1.2. Tunnel Mode
In tunnel mode, the "inner" IP header carries the ultimate (IP)
source and destination addresses, while an "outer" IP header contains
the addresses of the IPsec "peers," e.g., addresses of security
gateways. Mixed inner and outer IP versions are allowed, i.e., IPv6
over IPv4 and IPv4 over IPv6. 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 | | | orig IP hdr* | | |
|new IP header * (any options) | AH | (any options) |TCP| Data |
----------------------------------------------------------------
|<- mutable field processing ->|<------ immutable fields ----->|
|<- 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 present|TCP|Data|
--------------------------------------------------------------
|<--- mutable field -->|<--------- immutable fields -------->|
| processing |
|<-- authenticated except for mutable fields in new IP hdr ->|
* = if present, construction of outer IP hdr/extensions and
modification of inner IP hdr/extensions is discussed in
the Security Architecture document.
3.2. Integrity Algorithms
The integrity algorithm employed for the ICV computation is specified
by the SA. For point-to-point communication, suitable integrity
algorithms include keyed Message Authentication Codes (MACs) based on
symmetric encryption algorithms (e.g., AES [AES]) or on one-way hash
functions (e.g., MD5, SHA-1, SHA-256, etc.). For multicast
communication, a variety of cryptographic strategies for providing
integrity have been developed and research continues in this area.
3.3. Outbound Packet Processing
In transport mode, the sender inserts the AH header after the IP
header and before a next layer protocol header, as described above.
In tunnel mode, the outer and inner IP header/extensions can be
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RFC 4302 IP Authentication Header December 2005
interrelated in a variety of ways. The construction of the outer IP
header/extensions during the encapsulation process is described in
the Security Architecture document.
3.3.1. Security Association Lookup
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 (or ESN) counter for this
SA and inserts the low-order 32 bits of the value into the Sequence
Number field. Thus, the first packet sent using a given SA will
contain a sequence number of 1.
If anti-replay is enabled (the default), the sender checks to ensure
that the counter has not cycled before inserting the new value in the
Sequence Number field. In other words, the sender MUST NOT send a
packet on an SA if doing so would cause the sequence number to cycle.
An attempt to transmit a packet that would result in sequence number
overflow is an auditable event. The audit log entry for this event
SHOULD include the SPI value, current date/time, Source Address,
Destination Address, and (in IPv6) the cleartext Flow ID.
The sender assumes anti-replay is enabled as a default, unless
otherwise notified by the receiver (see Section 3.4.3) or if the SA
was configured using manual key management. Thus, typical behavior
of an AH implementation calls for the sender to establish a new SA
when the Sequence Number (or ESN) cycles, or in anticipation of this
value cycling.
If anti-replay is disabled (as noted above), the sender does not need
to monitor or reset the counter, e.g., in the case of manual key
management (see Section 5). However, the sender still increments the
counter and when it reaches the maximum value, the counter rolls over
back to zero. (This behavior is recommended for multi-sender,
multicast SAs, unless anti-replay mechanisms outside the scope of
this standard are negotiated between the sender and receiver.)
If ESN (see Appendix B) is selected, only the low-order 32 bits of
the sequence number are transmitted in the Sequence Number field,
although both sender and receiver maintain full 64-bit ESN counters.
However, the high-order 32 bits are included in the ICV calculation.
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RFC 4302 IP Authentication Header December 2005
Note: If a receiver chooses not to enable anti-replay for an SA, then
the receiver SHOULD NOT negotiate ESN in an SA management protocol.
Use of ESN creates a need for the receiver to manage the anti-replay
window (in order to determine the correct value for the high-order
bits of the ESN, which are employed in the ICV computation), which is
generally contrary to the notion of disabling anti-replay for an SA.
3.3.3. Integrity Check Value Calculation
The AH ICV is computed over:
o IP or extension header fields before the AH header 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 (low-order 32 bits), and the ICV (which is set
to zero for this computation), and explicit padding bytes (if
any))
o everything after AH is assumed to be immutable in transit
o the high-order bits of the ESN (if employed), and any implicit
padding required by the integrity algorithm
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 Integrity Check Value 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 RFC and 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 the Hop-by-Hop Extension Header)
contain a flag indicating mutability, which determines appropriate
processing for such options.
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RFC 4302 IP Authentication Header December 2005
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)
Differentiated Services Code Point (DSCP)
(6 bits, see RFC 2474 [NBBB98])
Explicit Congestion Notification (ECN)
(2 bits, see RFC 3168 [RFB01])
Flags
Fragment Offset
Time to Live (TTL)
Header Checksum
DSCP - Routers may rewrite the DS field as needed to provide a
desired local or end-to-end service, thus its value upon reception
cannot be predicted by the sender.
ECN - This will change if a router along the route experiences
congestion, and thus its value upon reception cannot be predicted by
the sender.
Flags - This field is excluded because 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.
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RFC 4302 IP Authentication Header December 2005
Header Checksum - This will change if any of these other fields
change, 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
Source Address
Destination Address (without Routing Extension Header)
Mutable but predictable
Destination Address (with Routing Extension Header)
Mutable (zeroed prior to ICV calculation)
DSCP (6 bits, see RFC2474 [NBBB98])
ECN (2 bits, see RFC3168 [RFB01])
Flow Label (*)
Hop Limit
(*) The flow label described in AHv1 was mutable, and in
RFC 2460 [DH98] was potentially mutable. To retain
compatibility with existing AH implementations, the
flow label is not included in the ICV in AHv2.
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
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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 [DH98] 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 and Extended Sequence Numbers
3.3.3.2.1. ICV Padding
As mentioned in Section 2.6, the ICV field may include explicit
padding if required 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 IPv4 or 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 ICV calculation,
counted as part of the Payload Length, and transmitted at the end of
the ICV field to enable the receiver to perform the ICV calculation.
Inclusion of padding in excess of the minimum amount required to
satisfy IPv4/IPv6 alignment requirements is prohibited.
3.3.3.2.2. Implicit Packet Padding and ESN
If the ESN option is elected for an SA, then the high-order 32 bits
of the ESN must be included in the ICV computation. For purposes of
ICV computation, these bits are appended (implicitly) immediately
after the end of the payload, and before any implicit packet padding.
For some integrity 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 and the 32
high-order bits of the ESN, if enabled) 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
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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. The document that
defines an integrity algorithm MUST be consulted to determine if
implicit padding is required as described above. If the document
does not specify an answer to this, then the default is to assume
that implicit padding is required (as needed to match the packet
length to the algorithm's blocksize.) If padding bytes are needed
but the algorithm does not specify the padding contents, then the
padding octets MUST have a value of zero.
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 IPv4 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. (This does not apply to IPv6, where there is no router-
initiated fragmentation.) 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.
NOTE: For transport mode -- As mentioned at the end of Section 3.1.1,
bump-in-the-stack and bump-in-the-wire implementations may have to
first reassemble a packet fragmented by the local IP layer, then
apply IPsec, and then fragment the resulting packet.
NOTE: For IPv6 -- For bump-in-the-stack and bump-in-the-wire
implementations, it will be necessary to examine all the extension
headers to determine if there is a fragmentation header and hence
that the packet needs reassembling prior to IPsec processing.
Fragmentation, whether performed by an IPsec implementation or by
routers along the path between IPsec peers, significantly reduces
performance. Moreover, the requirement for an AH receiver to accept
fragments for reassembly creates denial of service vulnerabilities.
Thus, an AH implementation MAY choose to not support fragmentation
and may mark transmitted packets with the DF bit, to facilitate Path
MTU (PMTU) discovery. In any case, an AH implementation MUST support
generation of ICMP PMTU messages (or equivalent internal signaling
for native host implementations) to minimize the likelihood of
fragmentation. Details of the support required for MTU management
are contained in the Security Architecture document.
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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 nonzero 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 zeroing of the OFFSET field or the clearing of the MORE
FRAGMENTS flag. In order for a reassembled packet to be processed by
IPsec (as opposed to discarded as an apparent fragment), the IP code
must do these two things after it reassembles a packet.
3.4.2. Security Association Lookup
Upon receipt of a packet containing an IP Authentication Header, the
receiver determines the appropriate (unidirectional) SA via lookup in
the SAD. For a unicast SA, this determination is based on the SPI or
the SPI plus protocol field, as described in Section 2.4. If an
implementation supports multicast traffic, the destination address is
also employed in the lookup (in addition to the SPI), and the sender
address also may be employed, as described in Section 2.4. (This
process is described in more detail in the Security Architecture
document.) The SAD entry for the SA also indicates whether the
Sequence Number field will be checked and whether 32- or 64-bit
sequence numbers are employed for the SA. The SAD entry for the SA
also specifies the algorithm(s) employed for ICV computation, and
indicates the key required to validate the ICV.
If no valid Security Association exists for this packet the receiver
MUST discard the packet; this is an auditable event. The audit log
entry for this event SHOULD include the SPI value, date/time, Source
Address, Destination Address, and (in IPv6) the Flow ID.
(Note that SA management traffic, such as IKE packets, does not need
to be processed based on SPI, i.e., one can de-multiplex this traffic
separately based on Next Protocol and Port fields, for example.)
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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.
Anti-replay is applicable to unicast as well as multicast SAs.
However, this standard specifies no mechanisms for providing anti-
replay for a multi-sender SA (unicast or multicast). In the absence
of negotiation (or manual configuration) of an anti-replay mechanism
for such an SA, it is recommended that sender and receiver checking
of the Sequence Number for the SA be disabled (via negotiation or
manual configuration), as noted below.
If the receiver does not enable anti-replay for an SA, no inbound
checks are performed on the Sequence Number. However, from the
perspective of the sender, the default is to assume that anti-replay
is enabled at the receiver. To avoid having the sender do
unnecessary sequence number monitoring and SA setup (see Section
3.3.2, "Sequence Number Generation"), if an SA establishment protocol
such as IKE is employed, the receiver SHOULD notify the sender,
during SA establishment, if the receiver will not provide anti-replay
protection.
If the receiver has enabled the anti-replay service for this SA, the
receive packet counter for the SA MUST be initialized to zero when
the SA is established. For each received packet, the receiver MUST
verify that the packet contains a Sequence Number that does not
duplicate the Sequence Number of any other packets received during
the life of this SA. This SHOULD be the first 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.
The "right" edge of the window represents the highest, validated
Sequence Number value received on this SA. Packets that contain
sequence numbers lower than the "left" edge of the window are
rejected. Packets falling within the window are checked against a
list of received packets within the window.
If the ESN option is selected for an SA, only the low-order 32 bits
of the sequence number are explicitly transmitted, but the receiver
employs the full sequence number computed using the high-order 32
bits for the indicated SA (from his local counter) when checking the
received Sequence Number against the receive window. In constructing
the full sequence number, if the low-order 32 bits carried in the
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packet are lower in value than the low-order 32 bits of the
receiver's sequence number counter, the receiver assumes that the
high-order 32 bits have been incremented, moving to a new sequence
number subspace. (This algorithm accommodates gaps in reception for
a single SA as large as 2**32-1 packets. If a larger gap occurs,
additional, heuristic checks for re-synchronization of the receiver's
sequence number counter MAY be employed, as described in Appendix B.)
If the received packet falls within the window and is not a
duplicate, 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.
A MINIMUM window size of 32 packets 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 receive window size should be increased for higher-speed
environments, irrespective of assurance issues. Values for minimum
and recommended receive window sizes for very high-speed (e.g.,
multi-gigabit/second) devices are not specified by this standard.
3.4.4. Integrity Check Value Verification
The receiver computes the ICV over the appropriate fields of the
packet, using the specified integrity algorithm, and verifies that it
is the same as the ICV included in the ICV field of the packet.
Details of the computation are provided below.
If the computed and received ICVs 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.
Implementation Note:
Implementations can use any set of steps that results in the same
result as the following set of steps. Begin by saving the ICV
value and replacing it (but not any ICV field padding) with zero.
Zero all other fields that may have been modified during transit.
(See Section 3.3.3.1, "Handling Mutable Fields", for a discussion
of which fields are zeroed before performing the ICV calculation.)
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If the ESN option is elected for this SA, append the high-order 32
bits of the ESN after the end of the packet. Check the overall
length of the packet (as described above), and if it requires
implicit padding based on the requirements of the integrity
algorithm, append zero-filled bytes to the end of the packet
(after the ESN if present) 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 for unicast traffic, and MUST comply with all
requirements of the Security Architecture document [Ken-Arch].
Additionally, if an implementation claims to support multicast
traffic, it MUST comply with the additional requirements specified
for support of such traffic. 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.
The mandatory-to-implement algorithms for use with AH are described
in a separate RFC [Eas04], to facilitate updating the algorithm
requirements independently from the protocol per se. Additional
algorithms, beyond those mandated for AH, MAY be supported.
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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 2402
This document differs from RFC 2402 [RFC2402] in the following ways.
o SPI -- modified to specify a uniform algorithm for SAD lookup
for unicast and multicast SAs, covering a wider range of
multicast technologies. For unicast, the SPI may be used
alone to select an SA, or may be combined with the protocol,
at the option of the receiver. For multicast SAs, the SPI is
combined with the destination address, and optionally the
source address, to select an SA.
o Extended Sequence Number -- added a new option for a 64-bit
sequence number for very high-speed communications. Clarified
sender and receiver processing requirements for multicast SAs
and multi-sender SAs.
o Moved references to mandatory algorithms to a separate
document [Eas04].
8. Acknowledgements
The author would like to acknowledge the contributions of Ran
Atkinson, who played a critical role in initial IPsec activities, and
who authored the first series of IPsec standards: RFCs 1825-1827.
Karen Seo deserves special thanks for providing help in the editing
of this and the previous version of this specification. The author
also would like to thank the members of the IPsec and MSEC working
groups who have contributed to the development of this protocol
specification.
9. References
9.1. Normative References
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Level", BCP 14, RFC 2119, March 1997.
[DH98] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
Kent Standards Track [Page 22]
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[Eas04] 3rd Eastlake, D., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4305, December 2005.
[Ken-Arch] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC1108] Kent, S., "U.S. Department of Defense Security Options for
the Internet Protocol", RFC 1108, November 1991.
9.2. Informative References
[AES] Advanced Encryption Standard (AES), Federal Information
Processing Standard 197, National Institutes of Standards
and Technology, November 26, 2001.
[HC03] Holbrook, H. and B. Cain, "Source Specific Multicast for
IP", Work in Progress, November 3, 2002.
[IKEv2] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, December 2005.
[Ken-ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[NBBB98] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December
1998.
[RFB01] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1385] Wang, Z., "EIP: The Extended Internet Protocol", RFC 1385,
November 1992.
Kent Standards Track [Page 23]
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[RFC1393] Malkin, G., "Traceroute Using an IP Option", RFC 1393,
January 1993.
[RFC1770] Graff, C., "IPv4 Option for Sender Directed Multi-
Destination Delivery", RFC 1770, March 1995.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, February
1997.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, March 2004.
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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 RFC 1700, "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
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, DH98 (IPv6)]
0 0 10 Experimental Measurement
1 2 13 Experimental Flow Control
1 0 14 Experimental Access Ctl
0 0 15 ???
1 0 16 IMI Traffic Descriptor
1 0 19 Address Extension
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
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being processed by the application to which the option contents are
directed (e.g., Resource Reservation Protocol (RSVP)/Internet Group
Management Protocol (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 security labels (e.g., Basic Security
Option (BSO), Extended Security Option (ESO), or Commercial Internet
Protocol Security Option (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 that 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) [DH98]
BIT INDICATES IF OPTION IS MUTABLE (CHANGES UNPREDICTABLY DURING
TRANSIT)
Hop-by-Hop options [DH98]
Destination options [DH98]
NOT APPLICABLE
Fragmentation [DH98]
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
[DH98] for more information.
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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 Integrity Check Value 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.
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Appendix B: Extended (64-bit) Sequence Numbers
B1. Overview
This appendix describes an Extended Sequence Number (ESN) scheme for
use with IPsec (ESP and AH) that employs a 64-bit sequence number,
but in which only the low-order 32 bits are transmitted as part of
each packet. It covers both the window scheme used to detect
replayed packets and the determination of the high-order bits of the
sequence number that are used both for replay rejection and for
computation of the ICV. It also discusses a mechanism for handling
loss of synchronization relative to the (not transmitted) high-order
bits.
B2. Anti-Replay Window
The receiver will maintain an anti-replay window of size W. This
window will limit how far out of order a packet can be, relative to
the packet with the highest sequence number that has been
authenticated so far. (No requirement is established for minimum or
recommended sizes for this window, beyond the 32- and 64-packet
values already established for 32-bit sequence number windows.
However, it is suggested that an implementer scale these values
consistent with the interface speed supported by an implementation
that makes use of the ESN option. Also, the algorithm described
below assumes that the window is no greater than 2^31 packets in
width.) All 2^32 sequence numbers associated with any fixed value
for the high-order 32 bits (Seqh) will hereafter be called a sequence
number subspace. The following table lists pertinent variables and
their definitions.
Var. Size
Name (bits) Meaning
---- ------ ---------------------------
W 32 Size of window
T 64 Highest sequence number authenticated so far,
upper bound of window
Tl 32 Lower 32 bits of T
Th 32 Upper 32 bits of T
B 64 Lower bound of window
Bl 32 Lower 32 bits of B
Bh 32 Upper 32 bits of B
Seq 64 Sequence Number of received packet
Seql 32 Lower 32 bits of Seq
Seqh 32 Upper 32 bits of Seq
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When performing the anti-replay check, or when determining which
high-order bits to use to authenticate an incoming packet, there are
two cases:
+ Case A: Tl >= (W - 1). In this case, the window is within one
sequence number subspace. (See Figure 1)
+ Case B: Tl < (W - 1). In this case, the window spans two
sequence number subspaces. (See Figure 2)
In the figures below, the bottom line ("----") shows two consecutive
sequence number subspaces, with zeros indicating the beginning of
each subspace. The two shorter lines above it show the higher-order
bits that apply. The "====" represents the window. The "****"
represents future sequence numbers, i.e., those beyond the current
highest sequence number authenticated (ThTl).
Th+1 *********
Th =======*****
--0--------+-----+-----0--------+-----------0--
Bl Tl Bl
(Bl+2^32) mod 2^32
Figure 1 -- Case A
Th ====**************
Th-1 ===
--0-----------------+--0--+--------------+--0--
Bl Tl Bl
(Bl+2^32) mod 2^32
Figure 2 -- Case B
B2.1. Managing and Using the Anti-Replay Window
The anti-replay window can be thought of as a string of bits where
`W' defines the length of the string. W = T - B + 1 and cannot
exceed 2^32 - 1 in value. The bottom-most bit corresponds to B and
the top-most bit corresponds to T, and each sequence number from Bl
through Tl is represented by a corresponding bit. The value of the
bit indicates whether or not a packet with that sequence number has
been received and authenticated, so that replays can be detected and
rejected.
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When a packet with a 64-bit sequence number (Seq) greater than T is
received and validated,
+ B is increased by (Seq - T)
+ (Seq - T) bits are dropped from the low end of the window
+ (Seq - T) bits are added to the high end of the window
+ The top bit is set to indicate that a packet with that sequence
number has been received and authenticated
+ The new bits between T and the top bit are set to indicate that
no packets with those sequence numbers have been received yet.
+ T is set to the new sequence number
In checking for replayed packets,
+ Under Case A: If Seql >= Bl (where Bl = Tl - W + 1) AND
Seql <= Tl, then check the corresponding bit in the window to
see if this Seql has already been seen. If yes, reject the
packet. If no, perform integrity check (see Appendix B2.2
below for determination of SeqH).
+ Under Case B: If Seql >= Bl (where Bl = Tl - W + 1) OR
Seql <= Tl, then check the corresponding bit in the window to
see if this Seql has already been seen. If yes, reject the
packet. If no, perform integrity check (see Appendix B2.2
below for determination of Seqh).
B2.2. Determining the Higher-Order Bits (Seqh) of the Sequence Number
Because only `Seql' will be transmitted with the packet, the receiver
must deduce and track the sequence number subspace into which each
packet falls, i.e., determine the value of Seqh. The following
equations define how to select Seqh under "normal" conditions; see
Appendix B3 for a discussion of how to recover from extreme packet
loss.
+ Under Case A (Figure 1):
If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th
If Seql < Bl (where Bl = Tl - W + 1), then Seqh = Th + 1
+ Under Case B (Figure 2):
If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th - 1
If Seql < Bl (where Bl = Tl - W + 1), then Seqh = Th
Kent Standards Track [Page 30]
RFC 4302 IP Authentication Header December 2005
B2.3. Pseudo-Code Example
The following pseudo-code illustrates the above algorithms for anti-
replay and integrity checks. The values for `Seql', `Tl', `Th', and
`W' are 32-bit unsigned integers. Arithmetic is mod 2^32.
If (Tl >= W - 1) Case A
If (Seql >= Tl - W + 1)
Seqh = Th
If (Seql <= Tl)
If (pass replay check)
If (pass integrity check)
Set bit corresponding to Seql
Pass the packet on
Else reject packet
Else reject packet
Else
If (pass integrity check)
Tl = Seql (shift bits)
Set bit corresponding to Seql
Pass the packet on
Else reject packet
Else
Seqh = Th + 1
If (pass integrity check)
Tl = Seql (shift bits)
Th = Th + 1
Set bit corresponding to Seql
Pass the packet on
Else reject packet
Else Case B
If (Seql >= Tl - W + 1)
Seqh = Th - 1
If (pass replay check)
If (pass integrity check)
Set the bit corresponding to Seql
Pass packet on
Else reject packet
Else reject packet
Else
Seqh = Th
If (Seql <= Tl)
If (pass replay check)
If (pass integrity check)
Set the bit corresponding to Seql
Pass packet on
Else reject packet
Else reject packet
Kent Standards Track [Page 31]
RFC 4302 IP Authentication Header December 2005
Else
If (pass integrity check)
Tl = Seql (shift bits)
Set the bit corresponding to Seql
Pass packet on
Else reject packet
B3. Handling Loss of Synchronization due to Significant Packet Loss
If there is an undetected packet loss of 2^32 or more consecutive
packets on a single SA, then the transmitter and receiver will lose
synchronization of the high-order bits, i.e., the equations in
Appendix B2.2. will fail to yield the correct value. Unless this
problem is detected and addressed, subsequent packets on this SA will
fail authentication checks and be discarded. The following procedure
SHOULD be implemented by any IPsec (ESP or AH) implementation that
supports the ESN option.
Note that this sort of extended traffic loss seems unlikely to occur
if any significant fraction of the traffic on the SA in question is
TCP, because the source would fail to receive ACKs and would stop
sending long before 2^32 packets had been lost. Also, for any bi-
directional application, even ones operating above UDP, such an
extended outage would likely result in triggering some form of
timeout. However, a unidirectional application, operating over UDP,
might lack feedback that would cause automatic detection of a loss of
this magnitude, hence the motivation to develop a recovery method for
this case.
The solution we've chosen was selected to:
+ minimize the impact on normal traffic processing.
+ avoid creating an opportunity for a new denial of service attack
such as might occur by allowing an attacker to force diversion of
resources to a re-synchronization process.
+ limit the recovery mechanism to the receiver because anti-replay
is a service only for the receiver, and the transmitter generally
is not aware of whether the receiver is using sequence numbers in
support of this optional service. It is preferable for recovery
mechanisms to be local to the receiver. This also allows for
backward compatibility.
Kent Standards Track [Page 32]
RFC 4302 IP Authentication Header December 2005
B3.1. Triggering Re-synchronization
For each SA, the receiver records the number of consecutive packets
that fail authentication. This count is used to trigger the re-
synchronization process, which should be performed in the background
or using a separate processor. Receipt of a valid packet on the SA
resets the counter to zero. The value used to trigger the re-
synchronization process is a local parameter. There is no
requirement to support distinct trigger values for different SAs,
although an implementer may choose to do so.
B3.2. Re-synchronization Process
When the above trigger point is reached, a "bad" packet is selected
for which authentication is retried using successively larger values
for the upper half of the sequence number (Seqh). These values are
generated by incrementing by one for each retry. The number of
retries should be limited, in case this is a packet from the "past"
or a bogus packet. The limit value is a local parameter. (Because
the Seqh value is implicitly placed after the AH (or ESP) payload, it
may be possible to optimize this procedure by executing the integrity
algorithm over the packet up to the endpoint of the payload, then
compute different candidate ICVs by varying the value of Seqh.)
Successful authentication of a packet via this procedure resets the
consecutive failure count and sets the value of T to that of the
received packet.
This solution requires support only on the part of the receiver,
thereby allowing for backward compatibility. Also, because re-
synchronization efforts would either occur in the background or
utilize an additional processor, this solution does not impact
traffic processing and a denial of service attack cannot divert
resources away from traffic processing.
Author's Address
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge, MA 02138
USA
Phone: +1 (617) 873-3988
EMail: kent@bbn.com
Kent Standards Track [Page 33]
RFC 4302 IP Authentication Header December 2005
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Kent Standards Track [Page 34]