go-gtp: GTP in Go

Package gtp provides simple and painless handling of GTP (GPRS Tunneling Protocol) in the Go programming language.

Project Status

This project is still EXPERIMENTAL.
Any part of the implementations (including exported APIs) may be changed before released as v1.0.0.

Features

Flexible enough to control everything in the GTP protocol, making it suitable for developing mobile core network nodes or testing tools for them.
Provides many helpers that are kind to developers, such as session, bearer, and TEID management.
Makes it easy to handle multiple connections with fixed IP and Port with UDP (or other net.PacketConn).
Currently works only on Linux and macOS since netlink support is introduced. However, the plan is to make it work on Windows in the future.

Getting Started

Prerequisites

go-gtp supports Go Modules. Run go mod tidy in your project's directory to collect the required packages automatically.

go mod tidy

This project follows the Release Policy of Go.

Running examples

End-to-end

We have a set of tools called GW Tester at examples/gw-tester. See the document for how it works and how to run it. It is also used for the integration test of this project. Workflow setting may help you understand it as well.

Individual node

The examples work as it is by go build and executing commands in the following way.

Note for macOS users: before running any go service, make sure to execute ./mac_local_host_enabler.sh you will find at examples/utils.

Open four terminals on a machine and start capturing on the loopback interface.
Start P-GW on terminal #1 and #2

// on terminal #1
./pgw

// on terminal #2
./pgw -s5c 127.0.0.53 -s5u 127.0.0.5

Start S-GW on terminal #3

// on terminal #3
./sgw

Start MME on terminal #4

// on terminal #4
./mme

You will see the nodes exchanging Create Session and Modify Bearer on C-Plane, and ICMP Echo on U-Plane afterwards.

The "mme" is not an MME per se. In addition to S11 interface, it also mocks UEs and an eNB to establish sessions and send packets on U-Plane.

Developing with go-gtp

This section briefly describes how to develop your own GTP node with go-gtp. For the detailed usage of a specific version, see README.md under each version's directory.

Version	Details
GTPv0	README.md
GTPv1	README.md
GTPv2	README.md

Establishing a connection between nodes

Each version has net.PacketConn-like APIs and GTP-specific ones, which are often version-specific. The basic idea behind the current implementation is;

Dial or ListenAndServe to create a connection (Conn) between nodes.
Register handlers to the Conn for specific messages with AddHandler, allowing users to handle the messages coming from the remote endpoint as flexible as possible, with less pain.
CreateXXX to create session or PDP context with arbitrary IEs given. Session/PDP context is structured, and they also have some helpers like AddTEID to handle known TEID properly.

Handling messages and IEs

Messages

All the messages implement the same interface: message.Message, and have their own structs named <MessageName>, which can be created by New<MessageName> with given ie.IEs. message.Message can be sent on top of Conn with SendMessageTo, or can be serialized into []byte with Marshal.

To parse the message from []byte, use message.Parse. The parsed message will be one of the structs that implement message.Message, and you can type-assert it to the corresponding struct to access the fields which are a set of ie.IEs.

IEs

All the IEs are of the same type: ie.IE (not an interface). An IE can be created either with New<IEName>, with ie.New, or with ie.New<TypeIE>. The latter two are useful when you want to create an IE with an unsupported type or our constructor does not work well for you.

To parse the IE from []byte, use ie.Parse (note that message.Parse parses all the IEs on a message - you don't need to call ie.Parse when you're handling IEs on a message). The value of the parsed ie.IE can be retrieved with <IEName>, ValueAs<IEType>. Some of the complicated IEs have their own struct named <IEName>Fields to get the values by accessing the fields.

For grouped IEs, accesing the ChildIEs field and iterating over the list of IEs contained is the most efficient way in most cases. Though there are the methods to get the specific IE value from the list (e.g., BearerFlags can be called upon BearerContext IE), they are not recommended since they always parse the whole list of IEs again.

Supported Features

Note that "supported" means that the package provides helpers that make it easier to handle. In other words, even if a message/IE is not marked as "Yes", you can make it work with some additional effort.

Your contribution is welcome to implement helpers for all the types, of course!

Version	Messages	IEs	Networking (state machine)	Details
GTPv0	~35%	~80%	not implemented yet	gtpv0/README
GTPv1	~25%	~30%	v1-U: functional v1-C: not implemented	gtpv1/README
GTPv2	~40.0%	~45%	functional	gtpv2/README
GTP' (Prime)	N/A	N/A	N/A	not planned

You may also be interested in the sibling project go-pfcp which is a PFCP implementation in Go.

Contributing

With the design goal of being flexible and easy to use, go-gtp is still in the early stage of development. Any contribution is welcome! Please feel free to open an issue or a pull request.

Please don't forget to run tests once you are done with your changes. Additionally, running the fuzz test is recommended to make sure that the implementation is robust enough.

go test ./...
go test -fuzz .

Note for macOS users: the first time you run any test, make sure to execute ./mac_local_host_enabler.sh you will find at examples/utils. You will have to run the script again after each reboot.

Authors

Yoshiyuki Kurauchi and contributors.

LICENSE

MIT

wmnsk/go-gtp