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Internet Protocol Security (IPsec) is a protocol suite for securing Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session. IPsec also includes protocols for establishing mutual authenticationbetween agents at the beginning of the session and negotiation of cryptographic keys to be used during the session.

IPsec is an end-to-end security scheme operating in the Internet Layer of the Internet Protocol Suite. It can be used in protecting data flows between a pair of hosts (host-to-host), between a pair of security gateways (network-to-network), or between a security gateway and a host (network-to-host).[1]

Some other Internet security systems in widespread use, such as Secure Sockets Layer (SSL), Transport Layer Security (TLS) and Secure Shell (SSH), operate in the upper layers of the TCP/IP model. Hence, IPsec protects any application traffic across an IP network. Applications do not need to be specifically designed to use IPsec. The use of TLS/SSL, on the other hand, must be designed into an application to protect the application protocols.

IPsec is a successor of the ISO standard Network Layer Security Protocol (NLSP). NLSP was based on the SP3 protocol that was published by NIST, but designed by the Secure Data Network System project of the National Security Agency (NSA).

IPsec is officially specified by the Internet Engineering Task Force (IETF) in a series of Request for Comment documents addressing various components and extensions. It specifies the spelling of the protocol name to be IPsec.[2]

Internet Protocol Suite
Application Layer


Transport Layer


Internet Layer

IP (IPv4IPv6· ICMP · ICMPv6 · IGMP ·IPsec ·

Link Layer
ARP/InARP · NDP · OSPF ·Tunnels (L2TP· PPP · Media Access Control (EthernetDSLISDNFDDI·(more)
v · d · e



[edit]Security architecture

The IPsec suite is an open standard. IPsec uses the following protocols to perform various functions:[3][4]

[edit]Authentication Header

Authentication Header (AH) is a member of the IPsec protocol suite. AH guarantees connectionless integrity and data origin authentication of IP packets. Further, it can optionally protect against replay attacks by using the sliding window technique and discarding old packets (see below).

  • In IPv4, the 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). Mutable (and therefore unauthenticated) IP header fields are DSCP/TOSECN, Flags, Fragment Offset,TTL and Header Checksum.[6]
  • In IPv6, the AH protects the AH itself, the Destination Options extension header after the AH, and the IP payload. It also protects the fixed IPv6 header and all extension headers before the AH, except for the mutable fields: DSCPECN, Flow Label, and Hop Limit.[6]

AH operates directly on top of IP, using IP protocol number 51.[12]

The following AH packet diagram shows how an AH packet is constructed and interpreted:[5][6]

Authentication Header format
00Next HeaderPayload LenReserved
432Security Parameters Index (SPI)
864Sequence Number
C96Integrity Check Value (ICV)
Next Header (8 bits) 
Type of the next header, indicating what upper-layer protocol was protected. The value is taken from the list of IP protocol numbers.
Payload Len (8 bits) 
The length of this Authentication Header in 4-octet units, minus 2 (a value of 0 means 8 octets, 1 means 12 octets, etcetera). Although the size is measured in 4-octet units, the length of this header needs to be a multiple of 8 octets if carried in an IPv6 packet. This restriction does not apply to an Authentication Header carried in an IPv4 packet.
Reserved (16 bits) 
Reserved for future use (all zeroes until then).
Security Parameters Index (32 bits) 
Arbitrary value which is used (together with the source IP address) to identify the security association of the sending party.
Sequence Number (32 bits) 
monotonic strictly increasing sequence number (incremented by 1 for every packet sent) to prevent replay attacks. When replay detection is enabled, sequence numbers are never reused because a new security association must be renegotiated before an attempt to increment the sequence number beyond its maximum value.[6]
Integrity Check Value (multiple of 32 bits) 
Variable length check value. It may contain padding to align the field to an 8-octet boundary for IPv6, or a 4-octet boundary for IPv4.

[edit]Encapsulating Security Payload

Encapsulating Security Payload (ESP) is a member of the IPsec protocol suite. In IPsec it provides origin authenticityintegrity, and confidentiality protection of packets. ESP also supports encryption-only and authentication-only configurations, but using encryption without authentication is strongly discouraged because it is unsecure.[13][14][15] Unlike Authentication Header (AH), ESP in transport mode does not provide integrity and authentication for the entire IP packet. However, in Tunnel Mode, where the entire original IP packet is encapsulated with a new packet header added, ESP protection is afforded to the whole inner IP packet (including the inner header) while the outer header remains unprotected. ESP operates directly on top of IP, using IP protocol number 50.[12]

The following ESP packet diagram shows how an ESP packet is constructed and interpreted:[1][16]

Encapsulating Security Payload format
00Security Parameters Index (SPI)
432Sequence Number
864Payload data
 Padding (0-255 octets) 
 Pad LengthNext Header
Integrity Check Value (ICV)
Security Parameters Index (32 bits) 
Arbitrary value which is used (together with the source IP address) to identify the security association of the sending party.
Sequence Number (32 bits) 
monotonically increasing sequence number (incremented by 1 for every packet sent) to protect against replay attacks. There is a separate counter kept for every security association.
Payload data (variable) 
The protected contents of the original IP packet, including any data used to protect the contents (e.g. an Initialisation Vector for the cryptographic algorithm). The type of content that was protected is indicated by the Next Header field.
Padding (0-255 octets) 
Padding for encryption, to extend the payload data to a size that fits the encryption's cypher block size, and to align the next field.
Pad Length (8 bits) 
Size of the padding in octets.
Next Header (8 bits) 
Type of the next header. The value is taken from the list of IP protocol numbers.
Integrity Check Value (multiple of 32 bits) 
Variable length check value. It may contain padding to align the field to an 8-octet boundary for IPv6, or a 4-octet boundary for IPv4.

[edit]Security association

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.

Security associations are established using the Internet Security Association and Key Management Protocol (ISAKMP). ISAKMP is implemented by manual configuration with pre-shared secrets, Internet Key Exchange (IKE and IKEv2), Kerberized Internet Negotiation of Keys (KINK), and the use of IPSECKEY DNS records.[11][17][18]

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.

[edit]Modes of operation

IPsec can be implemented in a host-to-host transport mode, as well as in a network tunnel mode.

[edit]Transport mode

In transport mode, only the payload (the data you transfer) of the IP packet is usually 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.

[edit]Tunnel mode

In tunnel mode, the entire IP packet is encrypted and/or authenticated. It is then encapsulated into a new IP packet with a new IP header. Tunnel mode is used to create virtual private networks for network-to-network communications (e.g. between routers to link sites), host-to-network communications (e.g. remote user access), and host-to-host communications (e.g. private chat).

Tunnel mode supports NAT traversal.

[edit]Cryptographic algorithms

Cryptographic algorithms defined for use with IPsec include:

  • HMAC-SHA1 for integrity protection and authenticity.
  • TripleDES-CBC for confidentiality
  • AES-CBC for confidentiality.

Refer to RFC 4835 for details.

[edit]Software implementations

IPsec support is usually implemented in the kernel with key management and ISAKMP/IKE negotiation carried out from user-space. Existing IPsec implementations often include both.

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

[edit]Standards status

IPsec was developed in conjunction with IPv6 and must be available in all standards-compliant implementations of IPv6,[31]. It is optional for IPv4 implementations. However, because of the slow deployment of IPv6, IPsec is most commonly used to secure IPv4 traffic. IPsec protocols were originally defined in RFC 1825 and RFC 1829, published in 1995. In 1998, these documents were superseded by RFC 2401 and RFC 2412 with incompatible aspects, 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, new standards were defined in RFC 4301 and RFC 4309 which are largely a superset of the previous editions with a second version of the Internet Key Exchange standard IKEv2. 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 and 1829. “ESP” generally refers to RFC 2406, while ESPbis refers to RFC 4303.

Since mid-2008, an IPsec Maintenance and Extensions working group is active at the IETF.[32][33]