Messages

Message Format

A Diameter message consists of a Diameter header and a variable number of Attribute-Value Pairs (AVPs) for encapsulating information that is relevant to the Diameter message.

Diameter Message Format

If the Request (R) bit is set, the message is a request. If this bit is cleared, the message is an answer.

If the Proxiable (P) bit is set, the message may be proxied, relayed, or redirected. If this bit is cleared, the message must be processed locally.

If the Error (E) bit is set, the message contains a protocol error and is referred to as an answer error message.

If the Potentially Retransmitted Message (T) is set, the message indicates a link failover to help management of duplicate requests.

Maximum Diameter Message Size

The Ribbon DSC supports the following maximum Diameter message sizes across various transports:

• TCP: 60,000
• SCTP: ~32,700
• DTLS/SCTP: 16,000

Message Processing: Request and Answer

The Diameter protocol requires that answers be returned to the host that originated the routing request. The Ribbon DSC answers on the same connection as the request. The answer is forwarded to the proper DSC Process only if the Adjacent Diameter Node is up. In the rare case that the DSC Process no longer has connectivity to the incoming ADN, the answer message is discarded.

Message Processing: Queuing

The following image displays the message processing path of a Diameter request or answer for a single operation through a DSC Node within a DSC Process with optional TLS security.

DSC Diameter Message Processing Queue

DSC Answer Modifications

DSC Answer Modification allows the modification of the AVPs for a correlated answer message before it is routed back to the original incoming ADN Connection, ideally using the full power of the DSC modification and routing tables. The answer message will always return to its original incoming ADN Connection regardless of which rule terminates the chain of routing tables (that is, any Result Table, dsc_default rule or one of several forms of dsc_error* rules).

Ingress Filtering

Ingress filtering ensures that the Ribbon DSC filters Diameter messages from unknown realms and rejects illegitimate messages. If a message fails ingress filtering, a log is generated with message details and the message is returned as an ERROR_UNABLE_TO_DELIVER. Also, a dynamic measurement (statistic) is generated to indicate that a message was discarded due to ingress filtering.

The DSC uses the Realm Routing Table to determine which requests are allowed to pass through the system. The cost attribute of a realm routing record has no impact on this feature.

Topology Mapping

One to one public topology mapping and one to N public topology mapping are supported for the Ribbon DSC. These topology mapping scenarios apply to both request and answer Diameter messages.

Configuring a one to one Topology Mapping record allows you to replace the DID Host Name that is internal for a protected DSC Node with an Alias name.

For Mobility Management Entity/Home Subscriber Server (MME/HSS) S6a/S6d and MME/Equipment Identity Register (EIR) S13/S13 message flows, one private to N public obfuscation is supported. In this topology mapping scenario, the Public DID Host Name is a combination of a base value and a numeric suffix. The base value can be shared among multiple rows (records) in the Topology Mapping Table, but the suffix must be unique.