# libcsp: a small network stack for cubesats that speak like TCP/IP

> Cubesat Space Protocol is a C implementation of a compact network layer for embedded systems, giving spacecraft subsystems a multi-master bus with a TCP-like socket API in a few kilobytes of RAM, ported to FreeRTOS, Zephyr and Linux. It suits a small satellite team that wants a standard bus between subsystems, and it is not a general purpose network stack for a ground station.

**libcsp/libcsp** — Cubesat Space Protocol - A small network-layer delivery protocol designed for Cubesats

- Repository: https://github.com/libcsp/libcsp
- Website: http://libcsp.github.io/libcsp/
- Stars: 692 · Forks: 353
- Language: C
- License: MIT
- Published: 2026-10-21 · Updated: 2026-10-21 · Language: en
- Canonical page: https://hysenlabs.com/projects/libcsp-libcsp

## The same TCP/IP features without the IP header

Cubesat Space Protocol is a small protocol stack written in C, designed to ease communication between distributed embedded systems in smaller networks such as cubesats. The design follows the TCP/IP model and includes a transport protocol, a routing protocol and several MAC-layer interfaces. The core of the library includes a router, a connection-oriented socket API and message and connection pools.

The central idea is stated plainly in the project description: the protocol uses a very lightweight header that carries both transport and network-layer information, and the implementation is designed for embedded systems with very limited CPU and memory resources. It is written in GNU C and is currently ported to run on FreeRTOS, Zephyr and Linux as a POSIX system.

The framing is deliberately TCP-like rather than TCP-like enough. The feature list names connection-oriented operation with references to RFC 908 and RFC 1151, and connection-less operation described as similar to UDP. Both modes are there, and a caller picks per message rather than per connection.

The alternative would have been to run IP on the spacecraft, which is what the project is arguing against. The stated benefit is giving cubesat subsystem developers the same features of a TCP/IP stack without the overhead of the IP header. The small footprint and the simple implementation are what allow a small 8-bit system to be fully connected on the network.

The point of the design is that all subsystems can provide their services at the same network level with no master node required. The project lays out the service-oriented advantages that follow from dropping the traditional master and slave topology: a standardised network protocol where all subsystems can talk to each other, loose coupling because services maintain a relationship that minimises dependencies, service abstraction so that services hide their logic from the outside, service reusability because logic is divided into services with the intention of promoting reuse, service autonomy because services control the logic they encapsulate, service redundancy that is easy to add to the bus, and the reduction of a single point of failure since the complexity moves from one master node to several well-defined services on the network.

## A router with quality of service, and a zero-copy buffer system

The feature list is short, and every entry is a design decision rather than a capability boast.

The socket API is thread safe, which matters because a spacecraft subsystem can have several tasks writing to the bus. The router task carries Quality of Services, so the implementation can prioritise traffic on a link where bandwidth is scarce. There is a loopback interface, broadcast traffic and promiscuous mode, which is the set of things you need when one node has to see everything on the bus or when you are debugging on a bench.

There are ICMP-like requests, specifically ping and buffer status. Buffer status is the interesting one for an embedded stack: it lets a node ask another node how much room is left in its receive buffers, which is how a sender avoids filling a buffer on a slower subsystem and losing data.

The zero-copy buffer and queue system is the other load-bearing piece. Instead of copying a payload into a freshly allocated buffer for every send, the system hands ownership of an existing buffer to the queue and hands it back on receipt. On a system with tens of kilobytes of RAM, avoiding a copy per message is the difference between a bus that fits and one that does not.

The network interface system is modular, which is how a single stack runs over several physical layers. The MAC layer interfaces are separate from the transport and routing layers, and the repository layout shows how that separates: there are example sources for a CAN to UDP bridge, a serial frame protocol bridge and a ZeroMQ proxy, plus a POSIX helper and an architecture-specific file. The examples also include a plain client, a server, a combined server and client, and Python binding examples for both a client and a server, which is how the project demonstrates that the same protocol can be spoken from a ground station script and from flight code.

## Configuration at compile time, and the build systems in the tree

The implementation is written with simplicity in mind, but the compile time configuration lets it carry some fairly advanced features. That phrase does the work: the features are not runtime options, they are choices made when the library is built, which is what keeps the footprint small.

The repository shows three build systems, which is a decision rather than an accident. There is a CMakeLists.txt at the root, a meson.build with a meson_options.txt, and the waf build system with a wscript. There is also csp_autoconfig.h.in, a generated configuration header, and a zephyr/ directory for the Zephyr integration, which is how a library with a portable core also plugs into a vendor RTOS build system.

The supporting directories tell the rest of the story. include/ holds the public headers, src/ the implementation, doc/ the documentation, examples/ the sample programs, samples/ additional samples, unittests/ the unit tests, utils/ the utilities, contrib/ the contributed material, and there is a .clang-format, a .gitlint, a .linelint.yml and a .codespell-ignore, which are the housekeeping files a project with a contribution process accumulates.

The documentation is the part to look at first. The doc folder is compiled to HTML and hosted at libcsp.github.io/libcsp/, and contributing is gated on reviewing the guidelines in doc/contributing.md before opening a pull request. The project description gives the layering in one line, with a router, a connection-oriented socket API and message and connection pools at the core, so a reader can decide quickly whether this is the kind of stack they are looking for.

## Where libcsp is the wrong choice

The first limitation is the one the design implies but does not state: CSP is a different network layer, not an implementation of IP. A subsystem that expects to talk IP, to sit behind a router, or to be reachable from a conventional network is not going to work with it. The project is explicit that the point is to avoid the IP header, and that decision cuts both ways. For a cubesat internal bus this is exactly right. For anything that has to interoperate with existing IP equipment, it is disqualifying.

The second is the port list. Three operating systems are named, FreeRTOS, Zephyr and Linux as POSIX. A flight computer running something else, or an unusual bare-metal target, needs a port that does not exist yet, and writing a port means writing the OS abstraction layer yourself.

The third is the age of the specification. The connection-oriented operation is based on RFC 908 and RFC 1151, which are decades-old documents for a protocol that was designed for embedded systems before those RFCs were relevant. That is not a criticism, since a stable specification is exactly what you want in space, but it does mean the protocol will not gain new transport features the way TCP does.

The fourth is the trust model. CSP gives every subsystem full network membership, and the multi-master design means there is no master to enforce policy. Promiscuous mode and broadcast are there on purpose. A subsystem that should not be able to command a payload is not separated from one that should, at the protocol level, and access control has to come from somewhere else.

The fifth is the release rhythm against the commit activity. The tags are v2.0 in April 2024 and v2.1 in October 2025, while the last push was on 2026-10-17, so the working tree has moved well past the last published release. There is a CHANGELOG file in the tree, which is the place to read what actually changed, and it is a fair assumption that a flight team will end up building from a commit rather than from the tag.

## The bus argument, and who it is for

The best way to judge libcsp is to ask the question the project asks. On many cubesats, the subsystems are wired to a central node, and that node is a single point of failure and a single bottleneck for every service. CSP replaces that with a flat network where the attitude control system, the telemetry unit and the payload computer each listen for the services they need and answer on the same bus.

For that shape of satellite, the advantages are not incremental. A subsystem can be added or replaced without renegotiating a central contract, redundancy is a matter of running two services on the bus, and the failure of one node does not take the rest of the spacecraft with it. The small footprint means an 8-bit microcontroller can join the same network as a Linux companion computer, which is the kind of mixed-processor spacecraft that are now common.

The cost is that CSP gives you a small network layer and nothing else. There is no TLS, no encryption, no authentication, no rate limiting, and no quality-of-service guarantee end to end, only a router task that carries the traffic. For a spacecraft bus where the physical link is short, the radio link is controlled and the threat model is different from a terrestrial network, that is a defensible position. It is also a position that has to be argued for rather than assumed.

So libcsp is for a team that has decided the spacecraft bus should be a service-oriented multi-master network and wants an implementation of that idea rather than a general purpose stack. It is a well-scoped piece of engineering with a clear architecture, a documented configuration model, a porting layer and unit tests, and it is MIT licensed with the source available. For anyone building the wrong shape of system, it will look like an interesting toy that solves a problem you do not have.

## Conclusion

libcsp fits a satellite team that wants every subsystem on one network without a master node, since the multi-master design, the service oriented argument and the small memory footprint are exactly what a cubesat needs and a general purpose IP stack cannot deliver inside an 8-bit budget. It is the wrong choice if you need IP, since CSP is a separate network layer rather than an IP implementation, and it is the wrong choice if you want a maintained general purpose networking library. Before adopting it, read the doc folder compiled at libcsp.github.io/libcsp/, check that your flight operating system is one of the three ports listed, and confirm the version you take, since the last push was on 2026-10-17 with v2.1 published on 2025-10-11.

## FAQ

### What is Cubesat Space Protocol?

It is a small network-layer delivery protocol written in C for distributed embedded systems, following the TCP/IP model with a transport protocol, a routing protocol and several MAC-layer interfaces. It uses a lightweight header carrying both transport and network information, avoiding the overhead of an IP header.

### Which operating systems does libcsp run on?

The implementation is written in GNU C and is currently ported to FreeRTOS, Zephyr and Linux as a POSIX system. The repository also carries a zephyr/ directory for the Zephyr integration.

### Does libcsp support both connection-oriented and connection-less communication?

Yes. Connection-oriented operation is based on RFC 908 and RFC 1151, and connection-less operation is described as similar to UDP. There are also ICMP-like requests for ping and buffer status.

### What makes libcsp suitable for a small satellite?

The project describes a very small footprint in code and memory, a zero-copy buffer and queue system, and a router task with Quality of Services. Together these allow a small 8-bit system to be fully connected on the network, and the multi-master design removes the single point of failure of a master/slave topology.

### What licence is libcsp released under?

MIT, with the source code available under that licence and the licence text in the LICENSE file at the root of the repository.

## Sources

- [libcsp/libcsp on GitHub](https://github.com/libcsp/libcsp)
- [License: MIT](https://github.com/libcsp/libcsp/blob/develop/LICENSE)
- [Project website](http://libcsp.github.io/libcsp/)
- [README](https://github.com/libcsp/libcsp/blob/develop/README.md)
- [Releases](https://github.com/libcsp/libcsp/releases)

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Hysen Labs editorial analysis, written from the project's own repository and release notes. Cite the canonical page: https://hysenlabs.com/projects/libcsp-libcsp