How many packets are needed to establish IKEv1?

The Internet Key Exchange version 1 (IKEv1) protocol establishes a Security Association (SA) in what is known as Phase 1. This phase has two primary modes: Main Mode and Aggressive Mode. Main Mode is the more secure and commonly used method, as it protects the identities of the communicating peers. It involves a three-step, two-way exchange, resulting in a total of six packets. The first two packets negotiate cryptographic policies, the next two perform a Diffie-Hellman key exchange, and the final two encrypted packets authenticate the peers. While Aggressive Mode is faster, using only three packets, Main Mode's six-packet exchange is the standard for a complete and secure IKEv1 establishment.

A. Main mode requires six packets for its three-step exchange. Aggressive mode, not Main mode, uses three packets.
B. Eight packets is not a standard number for establishing an IKEv1 Phase 1 SA in either Main or Aggressive mode.
C. Five packets is not a standard number for establishing an IKEv1 Phase 1 SA in either Main or Aggressive mode. ---

1. Check Point Official Documentation: Check Point R80.40 VPN Administration Guide, Page 21, "IKE (Phase 1)". The document states: "Main Mode - This is a three-step exchange of a total of six messages. The identities of the two gateways are encrypted and therefore hidden from any eavesdroppers."

2. Internet Engineering Task Force (IETF) Standard: RFC 2409: The Internet Key Exchange (IKE), Section 5.1, "Main Mode". This foundational document details the six-message exchange between the Initiator and Responder to establish the IKE SA. The messages are listed as: (1) SA, (2) SA, (3) KE, Ni, (4) KE, Nr, (5) IDi, AUTH, and (6) IDr, AUTH.

3. University Courseware: Purdue University, CS 555: Introduction to Cryptography, Lecture 21: IPsec and IKE. The lecture notes explicitly diagram the IKE Phase 1 Main Mode, showing the six distinct messages exchanged between the initiator and responder to set up the secure channel.