smoltcp 0.2.1

A TCP/IP stack designed for bare-metal, real-time systems without a heap.


smoltcp is a standalone, event-driven TCP/IP stack that is designed for bare-metal, real-time systems. Its design goals are simplicity and robustness. Its design anti-goals include complicated compile-time computations, such as macro or type tricks, even at cost of performance degradation.

smoltcp does not need heap allocation at all, is extensively documented, and compiles on stable Rust 1.15 and later.


smoltcp is missing many widely deployed features, whether by design or simply because no one implemented them yet. To set expectations right, both implemented and omitted features are listed.

Media layer

The only supported medium is Ethernet.

  • Regular Ethernet II frames are supported.
  • ARP packets (including gratuitous requests and replies) are supported.
  • 802.3 and 802.1Q are not supported.
  • Jumbo frames are not supported.

IP layer

The only supported internetworking protocol is IPv4.

  • IPv4 header checksum is supported.
  • IPv4 fragmentation is not supported.
  • IPv4 options are not supported.
  • ICMPv4 header checksum is supported.
  • ICMPv4 echo requests and replies are supported.
  • ICMPv4 destination unreachable message is supported.
  • ICMPv4 parameter problem message is not supported.

UDP layer

The UDP protocol is supported over IPv4.

  • UDP header checksum is supported.
  • UDP sockets are supported.

TCP layer

The TCP protocol is supported over IPv4.

  • TCP header checksum is supported.
  • Multiple packets will be transmitted without waiting for an acknowledgement.
  • Lost packets will be retransmitted with exponential backoff, starting at a fixed delay of 100 ms.
  • TCP urgent pointer is not supported; any urgent octets will be received alongside data octets.
  • Reassembly of out-of-order segments is not supported.
  • TCP options are not supported, in particular:
    • Maximum segment size is hardcoded at the default value, 536.
    • Window scaling is not supported, and the maximum buffer size is 65536.
  • Keepalive is not supported.


To use the smoltcp library in your project, add the following to Cargo.toml:

smoltcp = "0.1"

The default configuration assumes a hosted environment, for ease of evaluation. You probably want to disable default features and configure them one by one:

smoltcp = { version = ..., default-features = false, features = [...] }

Feature use_std

The use_std feature enables use of objects and slices owned by the networking stack through a dependency on std::boxed::Box and std::vec::Vec. It also enables smoltcp::phy::RawSocket and smoltcp::phy::TapInterface, if the platform supports it.

This feature is enabled by default.

Feature use_alloc

The use_alloc feature enables use of objects owned by the networking stack through a dependency on alloc::boxed::Box. This only works on nightly rustc.

Feature use_collections

The use_collections feature enables use of slices owned by the networking stack through a dependency on collections::vec::Vec. This only works on nightly rustc.

Feature use_log

The use_log feature enables logging of events within the networking stack through the log crate. The events are emitted with the TRACE log level.

This feature is enabled by default.

Feature verbose

The verbose feature enables logging of events where the logging itself may incur very high overhead. For example, emitting a log line every time an application reads or writes as little as 1 octet from a socket is likely to overwhelm the application logic unless a BufReader or BufWriter is used, which are of course not available on heap-less systems.

This feature is disabled by default.

Usage example

smoltcp, being a freestanding networking stack, needs to be able to transmit and receive raw frames. For testing purposes, we will use a regular OS, and run smoltcp in a userspace process. Only Linux is supported (right now).

On *nix OSes, transmiting and receiving raw frames normally requires superuser privileges, but on Linux it is possible to create a persistent tap interface that can be manipulated by a specific user:

sudo ip tuntap add name tap0 mode tap user $USER
sudo ip link set tap0 up
sudo ip addr add dev tap0


examples/ is a tiny clone of the tcpdump utility.

Unlike the rest of the examples, it uses raw sockets, and so it can be used on regular interfaces, e.g. eth0 or wlan0, as well as the tap0 interface we've created above.

Read its source code, then run it as:

cargo build --example tcpdump
sudo ./target/debug/tcpdump eth0


examples/ emulates a network host that can serve requests.

The host is assigned the hardware address 02-00-00-00-00-01 and IPv4 address

Read its source code, then run it as:

cargo run --example server -- tap0

It responds to:

  • pings (ping;
  • UDP packets on port 6969 (socat stdio udp4-connect: <<<"abcdefg"), where it will respond "yo dawg" to any incoming packet;
  • TCP packets on port 6969 (socat stdio tcp4-connect:, where it will respond "yo dawg" to any incoming connection and immediately close it;
  • TCP packets on port 6970 (socat stdio tcp4-connect: <<<"abcdefg"), where it will respond with reversed chunks of the input indefinitely.

The buffers are only 64 bytes long, for convenience of testing resource exhaustion conditions.

Fault injection is available through the --drop-chance and --corrupt-chance options, with values in percents. A good starting value is 15%.


smoltcp is distributed under the terms of 0-clause BSD license.

See LICENSE-0BSD for details.