Let me try to cover all types of diameter nodes -
Diameter Client - A Diameter Client is a device at the edge of the network that performs access control.
Diameter Server - A Diameter Server is one that handles authentication, authorization, and accounting requests for a particular realm.
Relay Agent - A Relay Agent routes Diameter messages based on information found in the messages. This routing decision is performed using a list of supported realms and known peers. Relay agents are largely transparent. A Relay Agent may modify Diameter messages only by inserting and/or removing routing information but may not modify any other portion of a message.
Proxy Agent - A Proxy agent also routes Diameter messages. However, a proxy agent may modify messages to implement policy decisions, such as controlling resource usage, providing admission control, and provisioning.
Redirect Agent - A redirect agent also provides a routing function, generally acting as a centralized source of Realm to Server address mappings for members of a roaming consortium. Unlike the other agents that relay requests, a redirect agent returns a special type of answer message to the peer that sent the request. This answer message contains routing information that allows the peer to resend the request directly to the correct destination server. A redirect agent does not relay requests.
Translation Agent - A Translation Agent translates between two protocols, such as RADIUS and Diameter. In this case, the translation agent supports a RADIUS to Diameter migration, allowing server conversions to Diameter.
Edge Agent - A Diameter Edge Agent provides the extra security communications service providers (CSPs) require to connect securely with third-party partners when needed and share resources, such as in roaming and billing use cases. It does this by enabling Diameter signaling messages to flow smoothly among networks while also adequately disguising the topology of both networks. Its normalization engine ensures that only supported AVPs (attribute-value pairs) and content enter the network. It can also protect Diameter servers and the entire network from overload conditions brought on by DoS attacks or other types of traffic overload.