Strategic placement of Diameter Routing Agents (DRAs) in the network helps Communication Service Providers (CSPs) control Diameter signaling as they grow their networks.
Diameter signaling creates challenges
The Diameter protocol is an extension of, and replacement for, the Remote Authentication Dial-In User Service (RADIUS) protocol for Authentication, Authorization and Accounting (AAA).
Diameter is used as the basis for communications across many control elements in 3GPP LTE deployments, including the Policy and Charging Rules Function (PCRF), Home Subscriber Server (HSS) and Online Charging System (OCS). As a result, it is emerging as the control protocol standard across mobile networks.
As mobile traffic continues to increase, so does the volume of Diameter signaling traffic in the network. Diameter signaling is used by many network control elements to exchange information about end-user device tracking, session tracking, session management, data usage, entitlements and other details.
With more smartphones, mobile applications and sophisticated services in use, more signaling information must be exchanged. Some of the major signaling sources highlighted in a recent report from telecom and datacom market research firm, Infonetics Research, include:
- Smartphones that spur signaling every time the device is reactivated
- Advanced, personalized and application-specific services that require more frequent exchanges of policy information
- The growing number of 4G Long Term Evolution (LTE) network elements that must communicate with each other
Together, these factors are creating extremely high volumes of Diameter signaling traffic in networks. High signaling volumes can degrade network performance and end-user quality of experience. It can also increase network operations and maintenance costs and lead to network outages.
In its report, Infonetics noted that CSPs including Verizon Wireless, NTT DoCoMo, Telenor and TeliaSonera have all experienced significant signaling storms in their LTE networks. In some cases these storms led to network outages. The signaling storms were caused by increasing network usage and by architectures that allow Diameter signaling to be deployed on a peer-to-peer (P2P) basis with mesh topologies.
DRAs help control Diameter signaling
A special Diameter node called an “agent” is defined within the IETF Diameter specification (RFC 3588) and is used to provide Diameter “routing” capabilities. A node that offers this Diameter “agent” capability is also known as a Diameter Routing Agent (DRA). A DRA offers 4 basic functions: relay, proxy, redirect and translation. Collectively these functions represent the foundation of the use cases that a DRA can offer.
To bring Diameter signaling under control, CSPs need to deploy DRAs in the network. Adding DRAs to the network helps CSPs:
- Improve the scalability and performance of the Diameter network
- Ensure network stability during sudden floods of signaling messages
- Maintain high availability in cases where alternate Diameter servers must be used
- Support different versions or implementations of the same Diameter applications
- Ensure interworking with other network protocols such as RADIUS
- Ensure secure communications at peering points with other networks while enforcing specific roaming agreements
- Quickly and easily add Diameter clients and servers to networks without extensive operational effort or network re-design
DRAs deliver these benefits by allowing CSPs to:
- Simplify the Diameter mesh
- Mediate between control elements
- Precisely define the control behavior
- Create a secure roaming infrastructure
Simplify the Diameter Topology
The first step in networks of any size is to simplify the Diameter mesh by adding DRAs to proxy multiple Diameters server clusters in specific locations based on geography and function.
Every Diameter element in the network connects to another Diameter element in a P2P relationship. As more Diameter elements are added to the network, a complicated mesh structure is quickly created. Inserting DRAs into the network allows CSPs to collapse parts of this complicated mesh into a hierarchical hub-and-spoke topology. Figure 1 shows a simplified example of how that can be achieved. This particular example uses an LTE network but the logic would apply to any Diameter-based topology.
The DRA in Action
With DRAs inserted between control elements and a simplified control topology, CSPs can add more advanced capabilities, such as defining the Diameter control behavior, mediating between Diameter control elements, as well as offering robust control resiliency capabilities:
- Defining the control behavior allows CSPs to use the relay and proxy functionality within the DRA to define the exact route that Diameter messages take within the network and the servers that each client uses. Routing decisions can be based on both header and payload information. The DRA can do this by offering simple operator-configured routing rules that select specific Diameter servers or server partitions based on initial session establishment information (IMSI, MSISDN, Country Code, APN, etc). It then binds this selection for all subsequent Diameter packets within a given session (i.e. session binding). This session binding can then be used for other related applications that are related to this session such as a Voice-over-IP (VoIP) call that is being requested through the P-CSCF. In addition, with access to this session binding information, related sessions can be correlated to the same binding (i.e. family plan). This binding can be propagated to other Diameter Routing agents in the network or can be accessed through a centralized session binding repository.
- The aforementioned functionality can be extended to offer load balancing of Diameter messages in a geographic partition or server cluster. By load balancing across multiple servers in a given server partition CPU efficiencies can be gained as well as providing inherent resiliency benefits.
- Mediation between control elements helps CSPs ensure that P2P communications work across the many different versions and variants of Diameter interfaces and vendor-specific implementations that are found in most networks. It also ensures smooth interoperation with non-Diameter elements, such as RADIUS and SS7. In multivendor networks with increasing numbers of control elements, mediation is essential to avoid interoperability challenges. Mediation capabilities use custom data dictionaries and attribute value pair (AVP) manipulations. This capability effectively turns the DRA into a Diameter translator, dramatically reducing operational costs as new Diameter elements are deployed and new SW loads are installed.
- By providing a simplified DRA topology the operator will enjoy a more resilient control infrastructure ensuring higher availability. The DRA function will be able to route around link failures thus augmenting link-level resiliency. In addition DRAs within a given geo-location can be mirrored in another geo-location offering a form of geo-redundancy.
Adding DRAs to small networks
A small-scale network that does not have a large geographic span is a good candidate for a co-located DRA function on an existing PCRF (Figure 2). This DRA will act as the Diameter messaging hub for all Diameter messages within the packet core. This approach is both cost-effective and operationally efficient, and can be easily scaled by simply adding routing blades on the existing server or chassis as Diameter traffic increases.
Adding DRAs to medium-sized networks
As networks grow in size, creating a dedicated, centralized DRA function within a given geography or packet core cluster often makes sense (Figure 3). With this approach, multiple elements can use the centralized DRA with no further topological design considerations. In this case, each centralized regional DRA communicates with centralized national IMS resources and with centralized billing and charging resources.
Once the network reaches this scale, it is essential that the DRA include functionality that allows CSPs to mediate between Diameter control elements and to define the control behavior for efficient operation, scalability, and performance. As part of defining the control server partitioning, load balancing becomes a very valuable operator tool to help efficiently scale the network.
Adding DRAs to large networks
As networks grow to a larger scale, they will typically include multiple packet cores and cover a larger geographic area. At this point, each packet core should have its own DRA function (Figure 4).
As in medium-sized networks, the ability to mediate between control elements and to define the control behavior are very important.
Because large-scale networks typically cover a large geographic area, it’s also very important that the DRA supports session binding to ensure that related applications and devices are served from like elements. This streamlined routing of Diameter messages simplifies operations and improves performance.
Using DRAs to secure roaming infrastructures
Most CSPs have roaming agreements with other CSPs. To ensure a secure roaming infrastructure, the DRA should include Diameter Edge Agent (DEA) functionality.
DEA functionality allows CSPs to configure policy rules that securely exchange Diameter signaling information across disparate and partner networks in a roaming infrastructure. The DEA should:
- Act as a single point of entry for roaming traffic
- Maintain and enforce white lists and black lists of roaming partners
- Control complex roaming agreements
- Hide the network topology from roaming partners
- Provide overload protection mechanisms
- Remove key message content and filter out unknown and unwanted data
DRAs that are used to secure the roaming infrastructure are located at the edge of the operator’s network (Figure 5). DRAs can also be used by Mobile Virtual Network Operators (MVNOs) that maintain OCSs that need to communicate with the home packet core network.
Choosing the right DRA solution
It’s still early days for DRA solutions. Most commercial DRA solutions available today were developed by vendors with a history of working with Diameter and the Signaling System 7 (SS7) protocol.
To effectively use Diameter Routing, operators must have a solution in place that is suitably easy to use, flexible, and scalable to accommodate a demanding and dynamic environment where networks are growing rapidly and requirements are changing all the time.
A powerful rules engine is required that allows operators the ability to:
- Make it very easy to quickly create and modify routing policies, with no need for coding, patching, professional services or downtime
- Offer Diameter routing use cases that can be tailored for specific scenarios
- Scale to accommodate large numbers of new Diameter elements — a critical capability in LTE networks
- Provide very high performance that does not deteriorate with complexity or scale; low latency is key metric to consider when evaluating performance levels
- Adapt to the ever-changing protocol environment and be able to mediate across different Diameter implementations and interface versions
- Be developed by a vendor with extensive experience in Diameter and the network elements that use the protocol
For more information about Diameter signaling control in multivendor networks and its relationship to policy control, please visit the Alcatel-Lucent 5780 Dynamic Services Controller page.
To contact the authors or request additional information, please send an e-mail to firstname.lastname@example.org.