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IPsec (IP security) is a suite of protocols for securing Internet Protocol (IP) communications by authenticating and/or encrypting each IP packet in a data stream. IPsec also includes protocols for cryptographic key establishment.



IPsec protocols operate at the network layer, layer 3 of the OSI model. Other Internet security protocols in widespread use, such as SSL, TLS and SSH, operate from the transport layer up (OSI layers 4 - 7). This makes IPsec more flexible, as it can be used for protecting layer 4 protocols, including both TCP and UDP, the most commonly used transport layer protocols. IPSec has an advantage over SSL and other methods that operate at higher layers. For an application to use IPsec no code change in the applications is required whereas to use SSL and other higher level protocols, applications must undergo code changes.

Security architecture

IPsec is implemented by a set of cryptographic protocols for (1) securing packet flows, (2) mutual authentication and (3) establishing cryptographic parameters.

The IP security architecture uses the concept of a security association as the basis for building security functions into IP. A security association is simply the bundle of algorithms and parameters (such as keys) that is being used to encrypt and authenticate a particular flow in one direction. Therefore, in normal bi-directional traffic, the flows are secured by a pair of security associations. The actual choice of encryption and authentication algorithms (from a defined list) is left to the IPsec administrator.

In order to decide what protection is to be provided for an outgoing packet, IPsec uses the security parameter index (SPI), an index to the security association database (SADB), along with the destination address in a packet header, which together uniquely identify a security association for that packet. A similar procedure is performed for an incoming packet, where IPsec gathers decryption and verification keys from the security association database.

For multicast, a security association is provided for the group, and is duplicated across all authorized receivers of the group. There may be more than one security association for a group, using different SPIs, thereby allowing multiple levels and sets of security within a group. Indeed, each sender can have multiple security associations, allowing authentication, since a receiver can only know that someone knowing the keys sent the data. Note that the relevant standard does not describe how the association is chosen and duplicated across the group; it is assumed that a responsible party will have made the choice.

Current status as a standard

IPsec is a mandatory part of IPv6, and is optional for use with IPv4. While the standard is designed to be indifferent to IP versions, current widespread deployment and experience concerns IPv4 implementations.

IPsec protocols were originally defined by RFCs 1825–1829, published in 1995. In 1998, these documents were obsoleted by RFCs 2401–2412, which are not compatible with 1825–1829, although they are conceptually identical. In December 2005, third-generation documents, RFCs 4301–4309, were produced. They are largely a superset of 2401–2412, but provide a second Internet Key Exchange standard. These third-generation documents standardized the abbreviation of IPsec to uppercase “IP” and lowercase “sec”.

It is unusual to see any product that offers RFC1825–1829 support. “ESP” generally refers to 2406, while ESPbis refers to 4303.

Design intent

IPsec was intended to provide either transport mode (end-to-end) security of packet traffic in which the end-point computers do the security processing, or tunnel mode (portal-to-portal) communications security in which security of packet traffic is provided to several machines (even to whole LANs) by a single node.

IPsec can be used to create Virtual Private Networks (VPN) in either mode, and this is the dominant use. Note, however, that the security implications are quite different between the two operational modes.

End-to-end communication security on an Internet-wide scale has been slower to develop than many had expected. Part of the reason is that no universal, or universally trusted, Public Key Infrastructure (PKI) has emerged (DNSSEC was originally envisioned for this); another part is that many users understand neither their needs nor the available options well enough to promote inclusion in vendors' products.

Since the Internet Protocol does not inherently provide any security capabilities, IPsec was introduced to provide security services such as the following:

  1. Encrypting traffic (so it cannot be read by parties other than those for whom it is intended)
  2. Integrity validation (ensuring traffic has not been modified along its path)
  3. Authenticating the peers (ensuring that traffic is from a trusted party)
  4. Anti-replay (protecting against replay of the secure session).


There are two modes of IPsec operation: transport mode and tunnel mode.

Transport mode

In transport mode, only the payload (the data you transfer) of the IP packet is encrypted and/or authenticated. The routing is intact, since the IP header is neither modified nor encrypted; however, when the authentication header is used, the IP addresses cannot be translated, as this will invalidate the hash value. The transport and application layers are always secured by hash, so they cannot be modified in any way (for example by translating the port numbers). Transport mode is used for host-to-host communications.

A means to encapsulate IPsec messages for NAT traversal has been defined by RFC documents describing the NAT-T mechanism.

Tunnel mode

In tunnel mode, the entire IP packet (data plus the message headers) is encrypted and/or authenticated. It must then be encapsulated into a new IP packet for routing to work. Tunnel mode is used for network-to-network communications (secure tunnels between routers) or host-to-network and host-to-host communications over the Internet.

Technical details

Two protocols have been developed to provide packet-level security for both IPv4 and IPv6:

  • The IP Authentication Header provides integrity and authentication and non-repudiation, if the appropriate choice of cryptographic algorithms is made.
  • The IP Encapsulating Security Payload provides confidentiality, along with optional (but strongly recommended) authentication and integrity protection.

Cryptographic algorithms defined for use with IPsec include HMAC-SHA1 for integrity protection, and TripleDES-CBC and AES-CBC for confidentiality. Refer to RFC 4305 for details.

Authentication header (AH)

The AH is intended to guarantee connectionless integrity and data origin authentication of IP datagrams. Further, it can optionally protect against replay attacks by using the sliding window technique and discarding old packets. 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. In IPv4, mutable (and therefore unauthenticated) IP header fields include TOS, Flags, Fragment Offset, TTL and Header Checksum. AH operates directly on top of IP, using IP protocol number 51. An AH packet diagram:

0 - 7 bit 8 - 15 bit 16 - 23 bit 24 - 31 bit
Next header Payload length RESERVED
Security parameters index (SPI)
Sequence number

Authentication data (variable)

Field meanings:

Next header 
Identifies the protocol of the transferred data.
Payload length 
Size of AH packet.
Reserved for future use (all zero until then).
Security parameters index (SPI) 
Identifies the security parameters, which, in combination with the IP address, then identify the security association implemented with this packet.
Sequence number 
A monotonically increasing number, used to prevent replay attacks.
Authentication data 
Contains the integrity check value (ICV) necessary to authenticate the packet; it may contain padding.

Encapsulating Security Payload (ESP)

The ESP protocol provides origin authenticity, integrity, and confidentiality protection of a packet. ESP also supports encryption-only and authentication-only configurations, but using encryption without authentication is strongly discouraged because it is insecure.<ref> {{{author}}}, Proceedings of the Sixth Usenix Unix Security Symposium, Problem Areas for the IP Security Protocols </ref> <ref> K.G. Paterson and A. Yau, Eurocrypt 2006, Lecture Notes in Computer Science Vol. 4004, Cryptography in theory and practice: The case of encryption in IPsec </ref> <ref> J.P. Degabriele and K.G. Paterson, IEEE Symposium on Security and Privacy, IEEE Computer Society, Attacking the IPsec Standards in Encryption-only Configurations </ref>. Unlike AH , the IP packet header is not protected by ESP. (Although in tunnel mode ESP, protection is afforded to the whole inner IP packet, including the inner header; the outer header remains unprotected.) ESP operates directly on top of IP, using IP protocol number 50.

An ESP packet diagram:

0 - 7 bit 8 - 15 bit 16 - 23 bit 24 - 31 bit
Security parameters index (SPI)
Sequence number

Payload data (variable)

  Padding (0-255 bytes)  
    Pad Length Next Header

Authentication Data (variable)

Field meanings:

Security parameters index (SPI) 
Identifies the security parameters in combination with IP address.
Sequence number 
A monotonically increasing number, used to prevent replay attacks.
Payload data 
The data to be transferred.
Used with some block ciphers to pad the data to the full length of a block.
Pad length 
Size of padding in bytes.
Next header 
Identifies the protocol of the transferred data.
Authentication data 
Contains the data used to authenticate the packet.


IPsec support is usually implemented in the kernel with key management and ISAKMP/IKE negotiation carried out from user-space. Existing IPsec implementations tend to include both of these functionalities. However, as there is a standard interface for key management, it is possible to control one kernel IPsec stack using key management tools from a different implementation.

Because of this, there is confusion as to the origins of the IPsec implementation that is in the Linux kernel. The FreeS/WAN project made the first complete and open source implementation of IPsec for Linux. It consists of a kernel IPsec stack (KLIPS), as well as a key management daemon (pluto) and many shell scripts. The FreeS/WAN project was disbanded in March 2004. Openswan and strongSwan are continuations of FreeS/WAN. The KAME project also implemented complete IPsec support for NetBSD, FreeBSD. Its key management daemon is called racoon. OpenBSD made its own ISAKMP/IKE daemon, simply named isakmpd (which was also ported to other systems, including Linux).

However, none of these kernel IPsec stacks were integrated into the Linux kernel. Alexey Kuznetsov and David S. Miller wrote a kernel IPsec implementation from scratch for the Linux kernel around the end of 2002. This stack was subsequently released as part of Linux 2.6, and is referred to variously as "native" or "NETKEY".

Therefore, contrary to popular belief, the Linux IPsec stack did not originate from the KAME project. As it supports the standard PF_KEY protocol (RFC 2367) and the native XFRM interface for key management, the Linux IPsec stack can be used in conjunction with either pluto from Openswan/strongSwan, isakmpd from OpenBSD project, racoon from the KAME project or without any ISAKMP/IKE daemon (using manual keying).

The new architectures of network processors, including multi-core processors with integrated encryption engines, change the way the IPsec stacks are designed. A dedicated Fast Path is used in order to offload the processing of the IPsec processing (SA, SP lookups, encryption, etc.). These Fast Path stacks must be co-integrated on dedicated cores with Linux or RTOS running on other cores. These OS are the control plane that runs ISAKMP/IKE of the Fast Path IPsec stack.

There are a number of implementations of IPsec and ISAKMP/IKE protocols. These include:

See also


List of IPsec-related RFCs

RFC 2367
PF_KEY Interface
RFC 2401 (obsoleted by RFC 4301)
Security Architecture for the Internet Protocol
RFC 2402 (obsoleted by RFC 4302 and RFC 4305)
Authentication Header
RFC 2403
The Use of HMAC-MD5-96 within ESP and AH
RFC 2404
The Use of HMAC-SHA-1-96 within ESP and AH
RFC 2405
The ESP DES-CBC Cipher Algorithm With Explicit IV
RFC 2406 (obsoleted by RFC 4303 and RFC 4305)
Encapsulating Security Payload
RFC 2407 (obsoleted by RFC 4306)
IPsec Domain of Interpretation for ISAKMP (IPsec DoI)
RFC 2408 (obsoleted by RFC 4306)
Internet Security Association and Key Management Protocol (ISAKMP)
RFC 2409 (obsoleted by RFC 4306)
Internet Key Exchange (IKE)
RFC 2410
The NULL Encryption Algorithm and Its Use With IPsec
RFC 2411
IP Security Document Roadmap
RFC 2412
The OAKLEY Key Determination Protocol
RFC 2451
The ESP CBC-Mode Cipher Algorithms
RFC 2857
The Use of HMAC-RIPEMD-160-96 within ESP and AH
RFC 3526
More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)
RFC 3706
A Traffic-Based Method of Detecting Dead Internet Key Exchange (IKE) Peers
RFC 3715
IPsec-Network Address Translation (NAT) Compatibility Requirements
RFC 3947
Negotiation of NAT-Traversal in the IKE
RFC 3948
UDP Encapsulation of IPsec ESP Packets
RFC 4106
The Use of Galois/Counter Mode (GCM) in IPsec Encapsulating Security Payload (ESP)
RFC 4301 (obsoletes RFC 2401)
Security Architecture for the Internet Protocol
RFC 4302 (obsoletes RFC 2402)
IP Authentication Header
RFC 4303 (obsoletes RFC 2406)
IP Encapsulating Security Payload (ESP)
RFC 4304
Extended Sequence Number (ESN) Addendum to IPsec Domain of Interpretation (DOI) for Internet Security Association and Key Management Protocol (ISAKMP)
RFC 4305 (obsoleted by RFC 4835)
Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)
RFC 4306 (obsoletes RFC 2407, RFC 2408, and RFC 2409)
Internet Key Exchange (IKEv2) Protocol
RFC 4307
Cryptographic Algorithms for Use in the Internet Key Exchange Version 2 (IKEv2)
RFC 4308
Cryptographic Suites for IPsec
RFC 4309
Using Advanced Encryption Standard (AES) CCM Mode with IPsec Encapsulating Security Payload (ESP)
RFC 4478
Repeated Authentication in Internet Key Exchange (IKEv2) Protocol
RFC 4543
The Use of Galois Message Authentication Code (GMAC) in IPsec ESP and AH
RFC 4555
IKEv2 Mobility and Multihoming Protocol (MOBIKE)
RFC 4621
Design of the IKEv2 Mobility and Multihoming (MOBIKE) Protocol
RFC 4718
IKEv2 Clarifications and Implementation Guidelines
RFC 4806
Online Certificate Status Protocol (OCSP) Extensions to IKEv2
RFC 4809
Requirements for an IPsec Certificate Management Profile
RFC 4835 (obsoletes RFC 4305)
Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH)
RFC 4945
The Internet IP Security PKI Profile of IKEv1/ISAKMP, IKEv2, and PKIX


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External links

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