10.3 The Diameter side

[!IMPORTANT] SIP carries the call; Diameter carries what the call depends on — subscriber identity, authorisation, media QoS, charging. The CSCF can register and authorise nobody without it, so Kamailio runs a full Diameter peer alongside its SIP stack.

Why Diameter

SIP is a session protocol; it doesn't carry subscription, authentication, or charging data. 3GPP put that on Diameter (RFC 6733) — an AAA protocol with a request/answer command model, structured AVPs (Attribute-Value Pairs), and persistent peer connections. Each Diameter link in IMS is a reference point from 10.1: Cx to the HSS, Rx to the PCRF, Ro/Rf to charging.

cdp — the C Diameter Peer

Kamailio's Diameter stack is the cdp module (C Diameter Peer), with cdp_avp providing typed helpers for reading and building AVPs. cdp is loaded once and shared by all the IMS modules that need Diameter — ims_icscf, ims_registrar_scscf, ims_auth, ims_qos, ims_charging all call into it rather than each opening their own connection.

Key properties:

• Persistent peers. cdp maintains long-lived connections to each Diameter peer (HSS, PCRF, OCS, or a DRA in front of them), with its own connection state machine: it does the Capabilities-Exchange (CER/CEA) handshake on connect and Device-Watchdog (DWR/DWA) keepalives to detect dead peers.
• Config in XML. Unlike most Kamailio config, cdp is configured by an external XML file (realm, peer list, transport, AVP dictionaries) referenced from the cfg. This is OpenIMSCore heritage.
• Async by nature. A Diameter round-trip happens inside SIP processing — the S-CSCF can't answer the REGISTER until the MAA comes back. The IMS modules use Kamailio's async/transaction-suspend machinery (see 8.2 Async transactions) so a worker isn't blocked on the wire waiting for the HSS.

What Kamailio speaks, and on which module

Interface	Kamailio side	Peer	What it does
Cx	ims_icscf, ims_registrar_scscf, ims_auth	HSS	Auth, S-CSCF selection, profile/iFC download
Rx	ims_qos	PCRF / PCF	Authorise media, request a dedicated bearer with QoS
Ro (online) / Rf (offline)	ims_charging	OCS / CHF	Credit control: reserve units, debit, release

Command codes you'll meet

Cx (HSS):

Code	Command	Trigger
300	UAR/UAA — User-Authorization	I-CSCF on REGISTER
303	MAR/MAA — Multimedia-Auth	S-CSCF fetching the auth vector
301	SAR/SAA — Server-Assignment	S-CSCF registering itself + pulling iFC
302	LIR/LIA — Location-Info	I-CSCF on a terminating call
304	RTR/RTA — Registration-Termination	HSS pushing a deregistration to the S-CSCF
305	PPR/PPA — Push-Profile	HSS pushing a profile update

Rx (PCRF), Ro (OCS):

Code	Command	Trigger
265	AAR/AAA — AA-Request	P-CSCF authorising media for a session
275	STR/STA — Session-Termination	P-CSCF tearing the media authorisation down
274	ASR/ASA — Abort-Session	PCRF aborting a session
272	CCR/CCA — Credit-Control	S-CSCF reserving/debiting charging units

[!NOTE] When the network multi-homes its HSS, the I/S-CSCF first asks an SLF (Subscription Locator Function) which HSS holds a given user — the SLF answers with a Diameter Redirect (the Dx reference point). In practice this job is increasingly handed to a DRA (Diameter Routing Agent) that proxies Cx to the right HSS; the CSCF just points cdp at the DRA. More on the DRA in 10.4.

What kills you

• Peer flapping. If the HSS/DRA connection drops, cdp tears it down and retries; during that window every REGISTER fails (no MAA). A peer silent on DWR watchdogs will drop registrations — alert on CER/DWR state.
• AVP dictionary mismatch. cdp only understands the AVPs in its loaded dictionaries. A vendor-specific AVP the dictionary lacks is dropped or rejects the message, surfacing as a SIP-side error far from the cause.
• Blocking the worker. A synchronous Diameter call (async wiring wrong) parks a SIP worker for the whole HSS timeout; at registration storms this cascades. The suspend/continue machinery exists to prevent it.

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