Alsoran

Alsoran is a Rust implementation of the gNodeB Centralized Unit (gNB-CU) of the 5G Radio Access Network (RAN).

The gNodeB is the component that manages the radio access of 5G User Equipment (UEs) and connects them to a 5G Core.

This project is currently a proof of concept and not yet a fully functional gNB-CU.

Quickstart

cargo build
./target/debug/gnb-cu --mcc 111 --mnc 11 --amf-ip 5.5.5.5

Current support

• UE registration demo against free5GC.
• PDU session setup and release (TS 23.502, figures 4.3.2.2.1-1 and 4.3.4.2-1).
• Relay of userplane packets between GNB-DU and UPF.
• Separate CU-CP and CU-UP communicating using E1.
• Scale-out of GNB-CU-CP workers using multiple TNLAs.
• ASN.1 libraries for NGAP, E1AP, F1AP and RRC.
• Rust ASN.1 autogenerator (written in Python).
• UE state in Redis datastore.
• Triangular redirection
• SCTP connection management stack

Generally only the success cases are covered, and there are a lot of 'To Dos'.

Procedure support

CU-CP

• NG Setup
• RAN Configuration Update
• F1 Setup
• E1 Setup
• Initial Access
• Uplink NAS
• Downlink NAS
• Initial Context Setup
• Pdu Session Resource Setup
• Pdu Session Resource Release
• AMF Status Indication
• GNB CU Configuration Update
• GNB DU Configuration Update
• GNB CU CP Configuration Update.

CU-UP

• E1 Setup
• Bearer Context Setup
• Bearer Context Modification
• Bearer Context Release
• GNB CU CP Configuration Update
• Downlink user data
• Uplink user data

What's different about Alsoran?

It's written in Rust and it has a "scale-out single hop" design.

"Scale-out" means that it has multiple interchangeable stateless worker processes. A request can be processed by any worker and no worker is a single point of failure. A Coordinator process coordinates the interface management exchanges of the workers when the topology changes. The motivation is scalability and fault tolerance.

"Single hop" means that, in the mainline case, a message is processed by a single worker (rather than chained through multiple microservices or load balancers). Each Alsoran CU-CP worker has its own SCTP connection to the AMF, the DU and the CU-UP. The motivation is speed and system simplicity.

Rust is an attractive choice of language for new O-RAN development. The main barrier to entry is the ASN.1 and SCTP based protocols. This project attempts to prove that this barrier is surmountable.

Building and running integration tests

The build relies on lld to reduce linker memory needs. You will either need to install LLD (sudo apt install lld or similar), or edit .cargo/config to remove the target.x86_64-unknown-linux-gnu config, which reverts to plain cc linking.

cargo test runs the integration test suite, minus the live Redis test.

To run the live Redis test, cargo test live_redis -- --ignored. For this to pass, you need to have redis-server in your path. Get Redis here: https://redis.io/docs/getting-started/.

A quick tour

The following test shows the Alsoran CU-CP and CU-UP carrying out UE registration, session establishment and userplane forwarding.

RUST_LOG=info cargo test successful_pdu_session_setup --test pdu_session -- --nocapture

This test shows two workers starting up, and the Coordinator instructing the workers how to initialize their NGAP, E1AP and F1AP interfaces.

RUST_LOG=info cargo test two_workers_base --test two_workers -- --nocapture

You can packet capture during these tests by running the following in parallel.

sudo tcpdump -w alsoran.pcap -i lo port 38472 or port 38412 or port 38462 or port 38462 or port 2152

...then Ctrl-C at the end of the test and open alsoran.pcap in Wireshark.

To run the live registration against free5GC takes a bit more setup - see the demo instructions.

Finally you might want to browse the design notes in documentation/design, which give an idea of the design thinking that has gone into Alsoran so far.

Contributing

If you would like to contribute, start by creating a Github issue or discusion to propose the change you want to make.

The backlog shows the main items being worked on and also tracks areas of tech debt.

The instructions for regenerating the two OpenAPI interfaces are in OpenAPI generation.

3GPP and O-RAN specifications

Alsoran protocol handling and workflow logic is based on the following specifications.

• 3GPP TS23.501 - System architecture for the 5G System
• 3GPP TS23.502 - Procedures for the 5G System
• 3GPP TS29.281 - General Packet Radio System (GPRS) Tunnelling Protocol User Plane (GTPv1-U)
• 3GPP TS37.324 - Service Data Adaptation Protocol (SDAP) specification
• 3GPP TS38.300 - NR and NG-RAN Overall Description
• 3GPP TS38.323 - Packet Data Convergence Protocol (PDCP) Specification
• 3GPP TS38.331 - Radio Resource Control (RRC) protocol specification
• 3GPP TS38.401 - NG-RAN; Architecture description
• 3GPP TS38.412 - NG signalling transport
• 3GPP TS38.413 - NG Application Protocol (NGAP)
• 3GPP TS38.414 - NG data transport
• 3GPP TS38.415 - PDU Session User Plane Protocol
• 3GPP TS38.462 - E1 signalling transport
• 3GPP TS38.463 - E1 Application Protocol (E1AP)
• 3GPP TS38.472 - F1 signalling transport
• 3GPP TS38.473 - F1 Application Protocol (F1AP)
• 3GPP TS38.474 - F1 data transport
• O-RAN.WG5.C.1 - NR C-plane profile