IPsec, an abbreviation of IP security, is a standard for securing Internet Protocol (IP) communications by encrypting and authenticating all IP packets. IPsec provides security at the network layer.

IPsec is a protocol suite (i.e., a set of interdependent protocols) consisting of (1) protocols for securing packet flows and (2) key exchange protocols used for setting up those secure flows. Of the former, there are two: Encapsulating Security Payload (ESP) provides authentication, data confidentiality and message integrity; Authentication Header (AH) provides authentication and message integrity, but does not offer confidentiality (which is why it is not used as pervasively as ESP). Currently only one key exchange protocol is defined, the IKE protocol.


Current status as a standard

IPsec is an obligatory part of IPv6, the new IETF Internet standard for Internet Protocol (IP) packet traffic, and is optional for use with IPv4. As a result, IPsec is expected to become more widely deployed as IPv6 becomes more popular. IPsec protocols are defined by RFCs 2401-2412. As of 2004, work is progressing to release updated replacement documents.

Design intent

IPsec was intended to provide either (1) portal-to-portal communications security in which security of packet traffic is provided to several machines (even to whole LANs) by a single node, or (2) end-to-end security of packet traffic in which the end-point computers do the security processing. It can be used to construct 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 has emerged (DNSSEC was originally envisioned for this), part is that many users understand neither their needs nor the available options well enough to force inclusion in vendors' products (which would lead to widespread adoption), and part is probably due to degradation (or anticipated degradation) of Net responsivity due to bandwidth loss from such things as spam.

IPsec vs. other Internet security protocols

IPsec protocols operate at layer 3 of the OSI model, which makes them suitable for protecting both TCP and UDP-based protocols when used alone. This means that, compared with transport layer and above protocols such as SSL (OSI Layer 6), which cannot protect UDP level traffic, the IPsec protocols must cope with reliability and fragmentation issues, adding their complexity and processing overhead. SSL/TLS, in contrast, rely on a higher level layer TCP (OSI Layer 4) to manage reliability and fragmentation.

Technical details

Authentication Header

Authentication Header (AH) is intended to guarantee the integrity and authenticity of the transferred packets. Further, it protects against replay attacks. AH tries to protect all fields of an IP datagram. Only fields changeable during transfer of an IP packet are excluded.

An AH packet diagram:

0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
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 in combination with IP address.
Sequence Number 
A monotonically increasing number, used to prevent replay attacks.
Authentication Data 
Contains the data necessary to authenticate the packet.

Encapsulated Security Payload (ESP)

The Encapsulating Security Payload (ESP) protocol provides origin authenticity, integrity, and confidentiality of a packet. Unlike the AH header, the IP packet header is not accounted for.

An ESP packet diagram:

0 1 2 3
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Security Parameters Index (SPI)
Sequence Number

Payload * (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 bits.
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 GNU/Linux. It consists of a kernel IPsec stack (KLIPS), as well as a key management daemon (pluto). 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, as well as GNU/Linux. Its key management daemon is called racoon.

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.

Therefore, contrary to popular belief, the Linux IPsec stack did not originate from the KAME project. As it supports the standard PFKEY protocol and the native XFRM interface for key management, the Linux IPsec stack can be used in conjunction with either pluto from Openswan or racoon from the KAME project.

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

See also

Overview of IPsec Related RFCs

RFC 2401
Security Architecture for the Internet Protocol
RFC 2402
Authentication Header
RFC 2406
Encapsulating Security Payload
RFC 2407
IPsec Domain of Interpretation for ISAKMP (IPsec DoI)
RFC 2408
Internet Security Association and Key Management Protocol (ISAKMP)
RFC 2409
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

External links

es:IPsec nl:IPSec ja:IPsec pl:IPsec fi:IPsec zh:安全IP


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