ESP (Encapsulating Security Payload) is a cryptographic process that provides origin confidentiality, integrity, and origin authentication for packets. ESP encrypts the payload of an IP packet with a symmetric key, and adds a header and a trailer to the packet. The header contains a security parameter index (SPI) and a sequence number, which are used to identify the security association (SA) and prevent replay attacks. The trailer contains padding and a next header field, which are used to align the packet and indicate the type of the original payload. ESP also adds an authentication data field at the end of the packet, which contains a message authentication code (MAC) that is computed over the entire ESP packet (except for the authentication data field itself) using a secret key and a hash function. The MAC provides data integrity and origin authentication for the packet. ESP can operate in two modes: tunnel mode and transport mode. In tunnel mode, ESP encapsulates the entire original IP packet, including the IP header, and adds a new IP header. This mode provides protection for the entire packet, but adds more overhead. In transport mode, ESP only encapsulates the payload of the original IP packet, and leaves the IP header intact. This mode provides protection only for the payload, but preserves the original IP header information. ESP is one of the two main protocols of IPsec, along with AH (Authentication Header). AH only provides data integrity and origin authentication, but not confidentiality. AH adds a header to the IP packet, which contains a MAC that is computed over the immutable fields of the IP header and the entire payload. AH does not encrypt the payload, and therefore does not protect it from eavesdropping. AH can also operate in tunnel mode or transport mode, but it is incompatible with NAT devices, which modify the IP header fields. IKE (Internet Key Exchange) is a protocol that is used to establish and manage SAs for IPsec. IKE negotiates the security parameters, such as the encryption and authentication algorithms, the keys, and the SPIs, for the IPsec protocols. IKE also performs mutual authentication between the IPsec peers, and establishes a secure channel for exchanging keying material. IKE has two versions: IKEv1 and IKEv2. IKEv1 consists of two phases: phase 1 and phase 2. In phase 1, IKEv1 establishes an IKE SA, which is a secure channel for phase 2. In phase 2, IKEv1 negotiates one or more IPsec SAs, which are used to protect the IPsec traffic. IKEv2 simplifies the IKE protocol by combining the two phases of IKEv1 into a single exchange. IKEv2 also supports more features, such as NAT traversal, EAP authentication, and MOBIKE. References :=

Implementing and Operating Cisco Security Core Technologies (SCOR) v1.0, Module 3: VPN Technologies, Lesson 3.1: Site-to-Site VPNs, Topic 3.1.1: IPsec VPNs

IPsec - Wikipedia

AH and ESP protocols - IBM

How TLS provides identification, authentication, confidentiality, and integrity - IBM