Editing IPsec (section)

===Authentication Header===
[[File:Ipsec-ah.svg|thumb|Usage of IPsec Authentication Header format in Tunnel and Transport modes]]
The Security Authentication Header (AH) was developed at the [[US Naval Research Laboratory]] in the early 1990s and is derived in part from previous IETF standards' work for authentication of the [[Simple Network Management Protocol]] (SNMP) version 2. Authentication Header (AH) is a member of the IPsec protocol suite. AH ensures connectionless [[Data integrity|integrity]] by using a [[hash function]] and a secret shared key in the AH algorithm. AH also guarantees the data origin by [[authenticating]] IP [[Packet (information technology)|packet]]s. Optionally a sequence number can protect the IPsec packet's contents against [[replay attack]]s,<ref>{{Cite book|title= Carrier-Scale IP Networks: Designing and Operating Internet Networks|author =Peter Willis |publisher= IET|year=2001 |isbn= 9780852969823|page=270}}</ref>{{Ref RFC|4949}} using the [[sliding window]] technique and discarding old packets.

* In [[IPv4]], AH prevents option-insertion attacks. In [[IPv6]], AH protects both against header insertion attacks and option insertion attacks.
* In [[IPv4]], the AH protects the IP payload and all header fields of an [[IP datagram]] except for mutable fields (i.e. those that might be altered in transit), and also IP options such as the IP Security Option.{{Ref RFC|1108}} Mutable (and therefore unauthenticated) IPv4 header fields are [[Differentiated services code point|DSCP]]/[[Type of service|ToS]], [[Explicit Congestion Notification|ECN]], Flags, [[IP fragmentation|Fragment]] [[Offset (computer science)|Offset]], [[Time to live|TTL]] and [[IPv4 header checksum|Header Checksum]].{{Ref RFC|4302}}
* In [[IPv6]], the AH protects most of the IPv6 base header, AH itself, non-mutable extension headers after the AH, and the IP payload. Protection for the IPv6 header excludes the mutable fields: [[Differentiated services code point|DSCP]], [[Explicit Congestion Notification|ECN]], Flow Label, and Hop Limit.{{Ref RFC|4302}}
AH operates directly on top of IP, using [[List of IP protocol numbers|IP protocol number {{Mono|51}}]].<ref name="iana">{{cite web|url=https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xml |title=Protocol Numbers |date=2010-05-27 |archive-url=https://web.archive.org/web/20100529122930/https://www.iana.org/assignments/protocol-numbers/protocol-numbers.xml |archive-date=2010-05-29 |work=IANA |url-status=dead }}</ref>

The following AH packet diagram shows how an AH packet is constructed and interpreted:{{Ref RFC|4302}}
{{APHD|start|title=Authentication Header format}}
{{APHD|0|bits1=8|field1=Next Header|bits2=8|field2=Payload Len|bits3=16|field3=Reserved}}
{{APHD|4|bits1=32|field1=Security Parameters Index}}
{{APHD|8|bits1=32|field1=Sequence Number}}
{{APHD|999|hoctets=12|hbits=96|bits1=64|field1=Integrity Check Value}}
{{APHD|end}}
;{{APHD|def|name=Next Header|length=8 bits|text=Type of the next header, indicating what upper-layer protocol was protected. The value is taken from the [[list of IP protocol numbers]].}}
;{{APHD|def|name=Payload Len|length=8 bits|text=The length of this ''Authentication Header'' in 4-octet units, minus 2. For example, an AH value of 4 equals 3×(32-bit fixed-length AH fields) + 3×(32-bit ICV fields) − 2 and thus an AH value of 4 means 24 octets. Although the size is measured in 4-octet units, the length of this header needs to be a multiple of 8 octets if carried in an IPv6 packet. This restriction does not apply to an ''Authentication Header'' carried in an IPv4 packet.}}
;{{APHD|def|name=Reserved|length=16 bits|text=Reserved for future use (all zeroes until then).}}
;{{APHD|def|name=Security Parameters Index|length=32 bits|text=Arbitrary value which is used (together with the destination IP address) to identify the [[security association]] of the receiving party.}}
;{{APHD|def|name=<span id="sequence number">Sequence Number</span>|length=32 bits|text=A [[monotonic]] strictly increasing sequence number (incremented by 1 for every packet sent) to prevent [[replay attack]]s. When replay detection is enabled, sequence numbers are never reused, because a new security association must be renegotiated before an attempt to increment the sequence number beyond its maximum value.{{Ref RFC|4302}}}}
;{{APHD|def|name=Integrity Check Value|length=multiple of 32 bits|text=Variable length check value.  It may contain padding to align the field to an 8-octet boundary for [[IPv6]], or a 4-octet boundary for [[IPv4]].}}