little-dude / smoltcp

a smol tcp/ip stack

smoltcp

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 1.20 and later.

Features

smoltcp is missing many widely deployed features, usually 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.
• Unicast and broadcast packets are supported, multicast packets are not supported.
• ARP packets (including gratuitous requests and replies) are supported.
• ARP rate limiting and cache expiration is not supported.
• 802.3 frames and 802.1Q are not supported.
• Jumbo frames are not supported.

IP layer

The only supported internetworking protocol is IPv4.

• IPv4 header checksum is generated and validated.
• IPv4 time-to-live value is configurable per socket, set to 64 by default.
• IPv4 default gateway is supported.
• IPv4 fragmentation is not supported.
• IPv4 options are not supported and are silently ignored.
• IPv4 routes (other than the default one) are not supported.

ICMP layer

The ICMPv4 protocol is supported, and ICMP sockets are available.

• ICMPv4 header checksum is supported.
• ICMPv4 echo replies are generated in response to echo requests.
• ICMP sockets can listen to ICMPv4 Port Unreachable messages, or any ICMPv4 messages with a given IPv4 identifier field.
• ICMPv4 protocol unreachable messages are not passed to higher layers when received.
• ICMPv4 parameter problem messages are not generated.

UDP layer

The UDP protocol is supported over IPv4, and UDP sockets are available.

• Header checksum is always generated and validated.
• In response to a packet arriving at a port without a listening socket, an ICMP destination unreachable message is generated.

TCP layer

The TCP protocol is supported over IPv4, and server and client TCP sockets are available.

• Header checksum is generated and validated.
• Maximum segment size is negotiated.
• Multiple packets are transmitted without waiting for an acknowledgement.
• Reassembly of out-of-order segments is supported, with no more than 4 gaps in sequence space.
• Keep-alive packets may be sent at a configurable interval.
• Retransmission timeout starts at a fixed interval of 100 ms and doubles every time.
• Time-wait timeout has a fixed interval of 10 s.
• User timeout has a configurable interval.
• Window scaling is not supported, and the maximum buffer size is 65536.
• Selective acknowledgements are not implemented.
• Delayed acknowledgements are not implemented.
• Silly window syndrome avoidance is not implemented.
• Nagle's algorithm is not implemented.
• Congestion control is not implemented.
• Timestamping is not supported.
• Urgent pointer is ignored.

Installation

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

[dependencies]
smoltcp = "0.4"

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

[dependencies]
smoltcp = { version = "0.4", default-features = false, features = ["log"] }

Feature std

The std feature enables use of objects and slices owned by the networking stack through a dependency on std::boxed::Box and std::vec::Vec.

This feature is enabled by default.

Feature alloc

The alloc feature enables use of objects owned by the networking stack through a dependency on collections from the alloc crate. This only works on nightly rustc.

This feature is disabled by default.

Feature log

The log feature enables logging of events within the networking stack through the log crate. Normal events (e.g. buffer level or TCP state changes) are emitted with the TRACE log level. Exceptional events (e.g. malformed packets) are emitted with the DEBUG 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.

Features phy-raw_socket and phy-tap_interface

Enable smoltcp::phy::RawSocket and smoltcp::phy::TapInterface, respectively.

These features are enabled by default.

Features socket-raw, socket-udp, and socket-tcp

Enable smoltcp::socket::RawSocket, smoltcp::socket::UdpSocket, and smoltcp::socket::TcpSocket, respectively.

These features are enabled by default.

Hosted usage examples

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 192.168.69.100/24 dev tap0

It's possible to let smoltcp access Internet by enabling routing for the tap interface:

sudo iptables -t nat -A POSTROUTING -s 192.168.69.0/24 -j MASQUERADE
sudo sysctl net.ipv4.ip_forward=1

Fault injection

In order to demonstrate the response of smoltcp to adverse network conditions, all examples implement fault injection, available through command-line options:

• The --drop-chance option randomly drops packets, with given probability in percents.
• The --corrupt-chance option randomly mutates one octet in a packet, with given probability in percents.
• The --size-limit option drops packets larger than specified size.
• The --tx-rate-limit and --rx-rate-limit options set the amount of tokens for a token bucket rate limiter, in packets per bucket.
• The --shaping-interval option sets the refill interval of a token bucket rate limiter, in milliseconds.

A good starting value for --drop-chance and --corrupt-chance is 15%. A good starting value for --?x-rate-limit is 4 and --shaping-interval is 50 ms.

Note that packets dropped by the fault injector still get traced; the rx: randomly dropping a packet message indicates that the packet above it got dropped, and the tx: randomly dropping a packet message indicates that the packet below it was.

Packet dumps

All examples provide a --pcap option that writes a libpcap file containing a view of every packet as it is seen by smoltcp.

examples/tcpdump.rs

examples/tcpdump.rs 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/server.rs

examples/server.rs emulates a network host that can respond to requests.

The host is assigned the hardware address 02-00-00-00-00-01 and IPv4 address 192.168.69.1.

Read its source code, then run it as:

cargo run --example server -- tap0

It responds to:

• pings (ping 192.168.69.1);
• UDP packets on port 6969 (socat stdio udp4-connect:192.168.69.1:6969 <<<"abcdefg"), where it will respond "hello" to any incoming packet;
• TCP connections on port 6969 (socat stdio tcp4-connect:192.168.69.1:6969), where it will respond "hello" to any incoming connection and immediately close it;
• TCP connections on port 6970 (socat stdio tcp4-connect:192.168.69.1:6970 <<<"abcdefg"), where it will respond with reversed chunks of the input indefinitely.
• TCP connections on port 6971 (socat stdio tcp4-connect:192.168.69.1:6971 </dev/urandom), which will sink data. Also, keep-alive packets (every 1 s) and a user timeout (at 2 s) are enabled on this port; try to trigger them using fault injection.
• TCP connections on port 6972 (socat stdio tcp4-connect:192.168.69.1:6972 >/dev/null), which will source data.

Except for the socket on port 6971. the buffers are only 64 bytes long, for convenience of testing resource exhaustion conditions.

examples/client.rs

examples/client.rs emulates a network host that can initiate requests.

The host is assigned the hardware address 02-00-00-00-00-02 and IPv4 address 192.168.69.2.

Read its source code, then run it as:

cargo run --example client -- tap0 ADDRESS PORT

It connects to the given address (not a hostname) and port (e.g. socat stdio tcp4-listen:1234), and will respond with reversed chunks of the input indefinitely.

examples/ping.rs

examples/ping.rs implements a minimal version of the ping utility using raw sockets.

The host is assigned the hardware address 02-00-00-00-00-02 and IPv4 address 192.168.69.1.

Read its source code, then run it as:

cargo run --example ping -- tap0 ADDRESS

It sends a series of 4 ICMP ECHO_REQUEST packets to the given address at one second intervals and prints out a status line on each valid ECHO_RESPONSE received.

The first ECHO_REQUEST packet is expected to be lost since arp_cache is empty after startup; the ECHO_REQUEST packet is dropped and an ARP request is sent instead.

Currently, netmasks are not implemented, and so the only address this example can reach is the other endpoint of the tap interface, 192.168.1.100. It cannot reach itself because packets entering a tap interface do not loop back.

Bare-metal usage examples

Examples that use no services from the host OS are necessarily less illustrative than examples that do. Because of this, only one such example is provided.

examples/loopback.rs

examples/loopback.rs sets up smoltcp to talk with itself via a loopback interface. Although it does not require std, this example still requires the collections feature to run.

Read its source code, then run it without std:

cargo run --example loopback --no-default-features --features collections

... or with std:

cargo run --example loopback -- --pcap loopback.pcap

It opens a server and a client TCP socket, and transfers a chunk of data. You can examine the packet exchange by opening loopback.pcap in Wireshark.

If the std feature is enabled, it will print logs and packet dumps, and fault injection is possible; otherwise, nothing at all will be displayed and no options are accepted.

License

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

See LICENSE-0BSD for details.