RFC 2406 (rfc2406) - Page 2 of 22

IP Encapsulating Security Payload (ESP)

RFC 2406           IP Encapsulating Security Payload       November 1998


         3.4.5  Packet Decryption..................................16
   4. Auditing.....................................................17
   5. Conformance Requirements.....................................18
   6. Security Considerations......................................18
   7. Differences from RFC 1827....................................18
   Acknowledgements................................................19
   References......................................................19
   Disclaimer......................................................20
   Author Information..............................................21
   Full Copyright Statement........................................22

1.  Introduction

   The Encapsulating Security Payload (ESP) header is designed to
   provide a mix of security services in IPv4 and IPv6.  ESP may be
   applied alone, in combination with the IP Authentication Header (AH)
   [KA97b], or in a nested fashion, e.g., 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.  For more details on how to use ESP
   and AH in various network environments, see the Security Architecture
   document [KA97a].

   The ESP header is inserted after the IP header and before the upper
   layer protocol header (transport mode) or before  an encapsulated IP
   header (tunnel mode).  These modes are described in more detail
   below.

   ESP is used to provide confidentiality, data origin authentication,
   connectionless integrity, an anti-replay service (a form of partial
   sequence integrity), and limited traffic flow confidentiality.  The
   set of services provided depends on options selected at the time of
   Security Association establishment and on the placement of the
   implementation.  Confidentiality may be selected independent of all
   other services.  However, use of confidentiality without
   integrity/authentication (either in ESP or separately in AH) may
   subject traffic to certain forms of active attacks that could
   undermine the confidentiality service (see [Bel96]).  Data origin
   authentication and connectionless integrity are joint services
   (hereafter referred to jointly as "authentication) and are offered as
   an option in conjunction with (optional) confidentiality.  The anti-
   replay service may be selected only if data origin authentication is
   selected, and its election is solely at the discretion of the
   receiver.  (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.)  Traffic flow



Kent & Atkinson             Standards Track