Internet Protocol Security (IPsec) 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: an application doesn't need to be designed to use IPsec, whereas the ability to use SSL or another higher-layer protocol must be incorporated into the design of an application.

IPsec is a framework of open standards that provides data confidentiality, data integrity, and data authentication between participating peers. IPsec provides these security services at the IP layer; it uses IKE to handle negotiation of protocols and algorithms based on local policy and to generate the encryption and authentication keys to be used by IPsec. IPsec can be used to protect one or more data flows between a pair of hosts, between a pair of security gateways, or between a security gateway and a host.

The official term of "IPsec" as defined by the IETF is often wrongly written as "IPSec".

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 implementation is a mandatory part of IPv6 but is not an integral part of IPv4. However, because of the slow uptake of IPv6, IPsec is most commonly used to secure IPv4 traffic. IPsec protocols were originally defined in RFCs by RFC 1825 & RFC 1829, published in 1995. In 1998, these documents were obsoleted by RFC 2401 & RFC 2412 (with which they were not compatible) although they were conceptually identical. In addition, a mutual authentication and key exchange protocol Internet Key Exchange (IKE), was defined to create and manage security associations. In December 2005, these RFCs were themselves obsoleted by RFC 4301 & RFC 4309, which are largely a superset of the previous editions, and a second version of the Internet Key Exchange standard, IKEv2, was defined. These third-generation documents standardized the abbreviation of IPsec to uppercase “IP” and lowercase “sec”. It is unusual to see any product that offers support for RFCs 1825 & 1829. “ESP” generally refers to RFC 2406, while ESPbis refers to RFC 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 (VPNs) 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

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, e.g. for VPNs) 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, 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 4835 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 : 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.. 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. Padding : 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


External links


  • RFC 2367: PF_KEY Interface
  • RFC 2401: Security Architecture for the Internet Protocol (IPsec overview)
  • 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 2367: PF_KEY Interface
  • RFC 2405: The ESP DES-CBC Cipher Algorithm With Explicit IV
  • RFC 2409: The Internet Key Exchange
  • 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: Security Architecture for the Internet Protocol
  • RFC 4302: IP Authentication Header
  • RFC 4303: IP Encapsulating Security Payload
  • RFC 4304: Extended Sequence Number (ESN) Addendum to IPsec Domain of Interpretation (DOI) for Internet Security Association and Key Management Protocol (ISAKMP)
  • RFC 4306: 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: 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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