Diameter protocol was created on 1998 to be a successor of RADIUS protocol, the main goal was to create a protocol more secure and implemented on signalling network.

It can use TCP or SCTP at transport layer and as TLS and DTLS for transport layer security and it has a fail over mechanism using internal watchdogs.

diameter protocol stack — Diameter Protocol Stack

It has another mechanism to discover peers and configure them in order to maintained the sessions bindings.

Diameter communicate as a client and a server

Diameter Protocol RFCs

First RFC 3588
Additional Updates RFC 5729, 5719, 6408
The RFC 6733 made all other obsolete.

RFC 6733 Specifies

Protocol Format
Transport
Peer Connection
User Sessions
Accounting

Types of Nodes

Clients
Servers
Agents

Node

Any device using the Diameter protocol stack.

Client

A Diameter client is any device who trigger a request to a server requesting information about the user and its level of access on the local or roaming network (AAA).

Types of Diameter Clients

MME
PCEF

Server

Handles AAA requests

PCRF
HSS

Diameter Agents

Relay

Route messages without change them

Proxy

Route a message and can change it

Redirect

Just simple redirect the traffic

Translation

Converts into another protocol

DEA Diameter Edge Agent

Located at border network

DRA Diameter Routing Agent

Core network providing routing capabilities

IWF Node

Convert into another protocol

DSC Diameter Signaling Controller

Groups DEA / DRA / IWF into one single device.

Diameter Applications

Diameter Messages command codes

Diameter Interfaces

https://tools.ietf.org/html/rfc6733

Diameter Protocol : Easy understand the basic functionality 2020