[IPse]c (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) for encrypting packet flows, and the rarely used Authentication Header (AH) which provides authentication and message integrity guarantees for such flows, but does not offer confidentiality. 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, 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:

Field meanings:

Next Header 

Identifies the protocol of the transferred data.

Payload Length 

Size of AH package.

RESERVED 

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 package.

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:

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 bits.

Next Header 

Identifies the protocol of the transferred data.

Authentication Data 

Contains the data used to authenticate the package.

Implementations

The FreeS/WAN project has developed an open source implementation of IPsec for GNU/Linux. An IPsec implementation based on the KAME project is included in NetBSD and FreeBSD, as well as the 2.6 Linux kernel. Thus, the KAME code will be widely available as GNU/Linux distributions change over to 2.6; in part because of this, development of the Free S/WAN project was discontinued in March 2004. Openswan and Strongswan are continuations of FreeS/WAN.

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

• NRL IPsec , one of the original
• OpenBSD, with its own code derived from NRL IPsec
• Mac OS X, which includes the Kame IPsec code
• Cisco IOS
• Microsoft Windows Win2K and WinXP
• SSH Sentinel (now part of SafeNet ) provides toolkits
• Solaris

See also

• Information security

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

• Securing Data in Transit with IPSec
• The IPsec Working Group at IETF.
• Free S/WAN project homepage.
• Openswan project homepage.
• The VPN Consortium.
• A long thread on the ipsec@lists.tislabs.com about whether uppercasing the S or not. The RFCs indicate that it is spelled "IPsec".