I'm trying to find out on which OSI layer the ISAKMP protocol resides on? It isn't listed under the wiki's list of network protocols, and the internet says it's either on the transport, network or the application layer.

I did find out where it resides on the tcp model, but still, I need to know for the OSI model.

  • Umm... you do know that the OSI model isn't actually USED on the internet today, right? That's why you see it on the TCP model (which is used nearly universally), and find nothing regarding the OSI model - which is, basically, irrelevant.
    – AviD
    Nov 25, 2014 at 11:33

2 Answers 2


According to this and Wikipedia, the OSI Session Layer is responsible for setting up any kind of conversation/dialogue.

As ISAKMP runs on top of UDP (port 500) and it sets up a secure and authenticated channel for communication, i would say that it is part of OSI Session Layer.


ISAKMP isn't a protocol as much as a framework for key exchanges (I know it has protocol in the name). Implementations of the framework include the Internet Key Exchange (IKE) and Kerberized Internet Negotiation of Keys (KINK).

If you read the ISAKMP RFC (RFC2408) it has a nice diagram for where ISAKMP sits in the network stack.

RFC2408 Section 2.2 ISAKMP Placement

 +------------+        +--------+                +--------------+
 !     DOI    !        !        !                !  Application !
 ! Definition ! <----> ! ISAKMP !                !    Process   !
 +------------+    --> !        !                !--------------!
+--------------+   !   +--------+                ! Appl Protocol!
! Key Exchange !   !     ^  ^                    +--------------+
!  Definition  !<--      !  !                           ^
+--------------+         !  !                           !
                         !  !                           !
        !----------------!  !                           !
        v                   !                           !
    +-------+               v                           v
    !  API  !        +---------------------------------------------+
    +-------+        !                Socket Layer                 !
        !            !---------------------------------------------!
        v            !        Transport Protocol (TCP / UDP)       !
 +----------+        !---------------------------------------------!
 ! Security ! <----> !                     IP                      !
 ! Protocol !        !---------------------------------------------!
 +----------+        !             Link Layer Protocol             !

                 Figure 1:  ISAKMP Relationships

This is a little bit ambiguous because it wants to protect the IP layer, but it must also perform negotiations between two IPs prior to encapsulation. However, in section 2.5.1 it states the following:

ISAKMP can be implemented over any transport protocol or over IP itself. Implementations MUST include send and receive capability for ISAKMP using the User Datagram Protocol (UDP) on port 500. UDP Port 500 has been assigned to ISAKMP by the Internet Assigned Numbers Authority (IANA). Implementations MAY additionally support ISAKMP over other transport protocols or over IP itself.

ISAKMP can be implemented at either Layer 3 (IP) or Layer 4 (Transport). Implementations must have support for transport layers, and optionally support an IP implementation.

IKE is one of the most common protocols used to negotiate IPSec keys. However both KINK (RFC4430 Section 9) and IKE use UDP packets to perform their exchange of keys between two peers.

While ISAKMP can occur at either IP or Transport it's most commonly implemented over Transport which would place it at Layer 5 (Session), but it could sit at Layer 4 (Transport) if it's implemented over Layer 2.

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