Why the Control Plane Needs an Intermediary
A mobile core is rarely built from a single vendor’s equipment. One PGW came with the core, a second was bought from another vendor for a new service, and a third is left over from a partner or an old 3G network. Each of these nodes has to be integrated into the network separately, with its own configuration and testing.
GTPCProxy eliminates that work every time a new PGW is added. The proxy is inserted inline on the S5 interface in the home network, on S8 in roaming, and on Gn/Gp in 2G/3G networks. The operator points the relevant APNs to the proxy address in DNS, and from then on the proxy selects the subscriber’s PGW based on its own rules.
To the SGW, the proxy looks like a PGW; to the PGW, it looks like an SGW. The proxy uses only standard TS 29.274 (GTPv2-C) and TS 29.060 (GTPv1-C) messages, so the configuration of other core nodes stays the same. User-plane GTP-U traffic does not pass through the proxy and flows directly between the SGW and the PGW.
Every Session to the Right PGW
When a subscriber attaches to the network, the proxy decides which PGW to send them to. The operator defines rules for this. A rule can take into account the subscriber’s SIM card number (IMSI prefixes and ranges), phone number (MSISDN prefixes), access point (APN), serving network (PLMN), radio access type and location.
Each rule points to a pool, that is, a group of gateways. The pool defines what share of the load each gateway receives, which gateways are on standby, and how a specific gateway is chosen for a subscriber. The proxy monitors the health of every PGW on its own. It regularly polls the gateways with keepalive requests and checks whether sessions can be created through them.
This setup gives the operator four benefits:
- Multivendor gateways in one network without integration projects. PGWs from different vendors work in the same pool, while the SGW and SGSN address a single endpoint: the proxy.
- Subscribers don’t notice when a PGW fails. If a gateway is overloaded, the proxy sends the subscriber to another one. If a gateway fails, its subscribers reconnect to healthy ones. Once the gateway recovers, load is shifted back to it gradually.
- Policies without reconfiguring the PGW. The proxy can modify a subscriber’s session parameters, such as the access point, charging characteristics, PDN type or RAT type. The change can be made before gateway selection, in which case it affects which PGW the proxy picks, or after it. In addition, at the operator’s request, the proxy can enable subscriber location reporting from the network’s control nodes.
- Rule changes without restarting the proxy. Rules can be updated from a file, from a Redis database or through the web interface, with no proxy restart. The proxy validates new rules before applying them, and if something goes wrong, you can roll back to the previous version.
| What the rule considers | Session parameter |
| SIM card number (prefix or range) | IMSI |
| Phone number (prefix) | MSISDN |
| Access point | APN |
| Network the subscriber connects through | PLMN |
| Radio network type | RAT Type |
| User location information | ULI |
Moving Between 2G/3G and LTE Without Dropping the Session
Operators are in no hurry to retire 2G/3G, and these networks will keep running alongside LTE for a long time. That’s why the proxy supports both versions of GTP, the protocol nodes use to set up a subscriber’s connection. LTE uses the newer version, and 2G/3G networks use the older one.
| Network | Protocol | Home network interface | Roaming interface | Between which nodes | Standard |
| LTE | GTPv2-C | S5 | S8 | SGW and PGW | TS 29.274 |
| 2G/3G | GTPv1-C | Gn | Gp | SGSN and PGW acting as GGSN | TS 29.060 |
Version switching during subscriber mobility is described in TS 23.401, Annex D.
A subscriber can move from 2G/3G to LTE or back while keeping their session and IP address. The proxy finds the subscriber’s session and switches the protocol version on both sides as the 3GPP standard prescribes. If a gateway responds that it doesn’t support the newer version, the proxy falls back to the older one with it.
Another capability is translating messages from one protocol version to the other. This lets a modern PGW that only understands the newer version serve a 2G/3G subscriber. Previously, this required a separate GGSN, the predecessor of the PGW in 2G/3G networks. The same applies to inbound roamers coming from other 2G/3G networks.
Translation is enabled in the pool settings and kicks in as a fallback if the primary gateway is unavailable. This way the operator can decommission legacy GGSNs even while the 2G/3G network itself is still running.
Roaming and Regional Pools
In roaming, the proxy works the same way as in the home network. Other operators’ subscribers go through the same pools and rules, and their session parameters can be modified in the same way.
Location-based routing lets operators split PGWs by region and set up dedicated pools for mass-event areas and for M2M: meters, sensors and other equipment that runs without human involvement.
If a subscriber moves into an area served by a different pool, the proxy can close their session and ask the device to reconnect. The device then connects to the gateway for its region. If a voice call is in progress at that moment, the proxy waits for it to end and only then closes the session.
Installation and Operation
GTPCProxy ships as two RPM packages for VEOS 8.x and EL8: the daemon and the web interface. Upgrades and rollbacks are done with the standard dnf upgrade and dnf downgrade commands.
Session data is stored in an external Redis instance, so subscriber sessions are not dropped when the proxy restarts. For monitoring, there are Prometheus metrics and Grafana dashboards, and for management, a web interface with administrator and observer roles.
Conclusion
GTPCProxy lets operators evolve the core step by step. You can add PGWs from the vendor of your choice, decommission legacy nodes and keep 2G/3G and LTE under a single policy without touching the MME, SGW or SGSN.
The easiest way to verify this on your own nodes is in a lab. VAS Experts offers a lab validation package with configuration, test scenarios and a report template. Contact us, and we’ll agree on a maintenance window and the testbed setup.
Glossary
| Term | In plain words |
| PGW | The gateway through which a subscriber reaches the internet and other networks |
| GGSN | The equivalent gateway in 2G/3G networks, the predecessor of the PGW |
| SGW | The LTE gateway that carries traffic from base stations to the PGW |
| MME | The LTE control node. It registers the subscriber and tracks their movement |
| SGSN | The 2G/3G network node that does the job of both the MME and the SGW |
| APN | Access point name. It tells the network which service to connect the subscriber to |
| IMSI | The SIM card number in the operator’s network |
| MSISDN | The subscriber’s phone number |
| PLMN | A specific operator’s network |
| GTP | The protocol network nodes use to set up a subscriber’s connection. GTPv1 is used in 2G/3G, GTPv2 in LTE |
| S5, S8, Gn, Gp | Interfaces between nodes. S5 and Gn work within the home network, S8 and Gp in roaming |
| TAC, LAC, ECI, CGI, SAI, RAI | Area and cell identifiers the network uses to determine where the subscriber is |
| UE | The subscriber’s device |
| M2M | Devices that exchange data without human involvement |
