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DIAMETER is a planned replacement of RADIUS.
DIAMETER is a AAA protocol for the applications such as net work access and IP mobility.
Intended to work in both local & Roaming AAA situations.
DIAMETER is just twice the predecessor protocol RADIUS.
Uses TCP or SCTP and not UDP.
It uses transport level security ( IPSEC or TLS ).
32 bit identifier in stead of 8 bit.
Stateless as well as stateful modes supported.
Supports application layer acknowledgement, define failover.
Better roaming support.
Easy to extend, new commands and attributes can be defined.
Now you have basic understanding of RADIUS & DIAMETER. To go in deeep detail for both the protocols you need to go through various RFCs and other resources mentioned in the next chapter.