RFC 2828 (Internet Security Glossary) defines OAKLEY as a key establishment protocol (proposed for IPsec but superseded by IKE) based on the Diffie-Hellman algorithm and designed to be a compatible component of ISAKMP.

ISAKMP is an Internet IPsec protocol to negotiate, establish, modify, and delete security associations, and to exchange key generation and authentication data, independent of the details of any specific key generation technique, key establishment protocol, encryption algorithm, or authentication mechanism.

SKIP is a key distribution protocol that uses hybrid encryption to convey session keys that are used to encrypt data in IP packets.

ISAKMP provides a framework for authentication and key exchange but does not define them. ISAKMP is designed to be key exchange independant; that is, it is designed to support many different key exchanges.

Oakley and SKEME each define a method to establish an authenticated key exchange. This includes payloads construction, the information payloads carry, the order in which they are processed and how they are used.

Oakley describes a series of key exchanges-- called modes and details the services provided by each (e.g. perfect forward secrecy for keys, identity protection, and authentication).

SKEME describes a versatile key exchange technique which provides anonymity, repudiability, and quick key refreshment.

RFC 2049 describes the IKE protocol using part of Oakley and part of SKEME in conjunction with ISAKMP to obtain authenticated keying material for use with ISAKMP, and for other security associations such as AH and ESP for the IETF IPsec DOI.

While Oakley defines "modes", ISAKMP defines "phases". The relationship between the two is very straightforward and IKE presents different exchanges as modes which operate in one of two phases.

Phase 1 is where the two ISAKMP peers establish a secure, authenticated channel with which to communicate. This is called the ISAKMP Security Association (SA). "Main Mode" and "Aggressive Mode" each accomplish a phase 1 exchange. "Main Mode" and "Aggressive Mode" MUST ONLY be used in phase 1.

Phase 2 is where Security Associations are negotiated on behalf of services such as IPsec or any other service which needs key material and/or parameter negotiation. "Quick Mode" accomplishes a phase 2 exchange. "Quick Mode" MUST ONLY be used in phase 2.

References:

CISSP: Certified Information Systems Security Professional Study Guide By James Michael Stewart, Ed Tittel, Mike Chappl, page 397

RFC 2049 at: http://www.ietf.org/rfc/rfc2409

SHIREY, Robert W., RFC2828: Internet Security Glossary, may 2000.

The All-in-one CISSP Exam Guide, 3rd Edition, by Shon Harris, page 674

The CISSP and CAP Prep Guide, Platinum Edition, by Krutz and Vines