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README.md
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 1.91 and later.
smoltcp achieves ~Gbps of throughput when tested against
the Linux TCP stack in loopback mode.
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
There are 3 supported mediums.
Ethernet
Regular Ethernet II frames are supported.
Unicast, broadcast and multicast packets are supported.
ARP packets (including gratuitous requests and replies) are supported.
ARP requests are sent at a rate not exceeding one per second.
Cached ARP entries expire after one minute.
802.3 frames and 802.1Q are not supported.
Jumbo frames are not supported.
IP
Unicast, broadcast and multicast packets are supported.
IEEE 802.15.4
Only support for data frames.
IP layer
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.
Routing outgoing IPv4 packets is supported, through a default gateway or a CIDR route table.
IPv4 fragmentation and reassembly is supported.
IPv4 options are not supported and are silently ignored.
IPv6
IPv6 hop-limit value is configurable per socket, set to 64 by default.
Routing outgoing IPv6 packets is supported, through a default gateway or a CIDR route table.
IPv6 hop-by-hop header is supported.
ICMPv6 parameter problem message is generated in response to an unrecognized IPv6 next header.
ICMPv6 parameter problem message is not generated in response to an unknown IPv6
hop-by-hop option.
Fragmentation is supported, as defined in RFC4944.
UDP header compression/decompression is supported.
Extension header compression/decompression is supported.
Uncompressed IPv6 Extension Headers are not supported.
IP multicast
IGMP
The IGMPv1 and IGMPv2 protocols are supported, and IPv4 multicast is available.
Membership reports are sent in response to membership queries at
equal intervals equal to the maximum response time divided by the
number of groups to be reported.
ICMP layer
ICMPv4
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 by default.
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.
ICMPv6
The ICMPv6 protocol is supported, and ICMP sockets are available.
ICMPv6 header checksum is supported.
ICMPv6 echo replies are generated in response to echo requests by default.
ICMPv6 protocol unreachable messages are not passed to higher layers when received.
NDISC
Neighbor Advertisement messages are generated in response to Neighbor Solicitations.
Router Advertisement messages are read, but not generated.
Router Solicitation messages are generated, but not read.
Redirected Header messages are not generated or read.
UDP layer
The UDP protocol is supported over IPv4 and IPv6, 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 IPv6, and server and client TCP sockets are available.
Header checksum is generated and validated.
Maximum segment size is negotiated.
Window scaling is negotiated.
Multiple packets are transmitted without waiting for an acknowledgement.
Reassembly of out-of-order segments is supported, with no more than 4 or 32 gaps in sequence space.
Keep-alive packets may be sent at a configurable interval.
Retransmission timeout starts at at an estimate of RTT, and doubles every time.
Time-wait timeout has a fixed interval of 10 s.
User timeout has a configurable interval.
Delayed acknowledgements are supported, with configurable delay.
Nagle's algorithm is implemented.
Selective acknowledgements are not implemented.
Silly window syndrome avoidance is not implemented.
Congestion control is optional, CUBIC and Reno are implemented.
Timestamping is not supported.
Urgent pointer is ignored.
Probing Zero Windows is implemented.
Packetization Layer Path MTU Discovery PLPMTU is not implemented.
Installation
To use the smoltcp library in your project, add the following to Cargo.toml:
[dependencies]
smoltcp = "0.10.0"
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 = "0.10.0", default-features = false, features = ["log"] }
Feature flags
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 defmt
The defmt feature enables logging of events with the defmt crate.
This feature is disabled by default, and cannot be used at the same time as log.
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-tuntap_interface
Enable smoltcp::phy::RawSocket and smoltcp::phy::TunTapInterface, respectively.
These features are enabled by default.
Features socket-raw, socket-udp, socket-tcp, socket-icmp, socket-dhcpv4, socket-dns
Enable the corresponding socket type.
These features are enabled by default.
Features proto-ipv4, proto-ipv6 and proto-sixlowpan
smoltcp has some configuration settings that are set at compile time, affecting sizes
and counts of buffers.
They can be set in two ways:
Via Cargo features: enable a feature like <name>-<value>. name must be in lowercase and
use dashes instead of underscores. For example. iface-max-addr-count-3. Only a selection of values
is available, check Cargo.toml for the list.
Via environment variables at build time: set the variable named SMOLTCP_<value>. For example
SMOLTCP_IFACE_MAX_ADDR_COUNT=3 cargo build. You can also set them in the [env] section of .cargo/config.toml.
Any value can be set, unlike with Cargo features.
Environment variables take precedence over Cargo features. If two Cargo features are enabled for the same setting
with different values, compilation fails.
IFACE_MAX_ADDR_COUNT
Max amount of IP addresses that can be assigned to one interface (counting both IPv4 and IPv6 addresses). Default: 2.
IFACE_MAX_MULTICAST_GROUP_COUNT
Max amount of multicast groups that can be joined by one interface. Default: 4.
IFACE_MAX_SIXLOWPAN_ADDRESS_CONTEXT_COUNT
Max amount of 6LoWPAN address contexts that can be assigned to one interface. Default: 4.
IFACE_NEIGHBOR_CACHE_COUNT
Amount of "IP address -> hardware address" entries the neighbor cache (also known as the "ARP cache" or the "ARP table") holds. Default: 4.
IFACE_MAX_ROUTE_COUNT
Max amount of routes that can be added to one interface. Includes the default route. Includes both IPv4 and IPv6. Default: 2.
IFACE_MAX_PREFIX_COUNT
Max amount of IPv6 prefixes that can be added to one interface via SLAAC.
Should be lower or equal to IFACE_MAX_ADDR_COUNT.
FRAGMENTATION_BUFFER_SIZE
Size of the buffer used for fragmenting outgoing packets larger than the MTU. Packets larger than this setting will be dropped instead of fragmented. Default: 1500.
ASSEMBLER_MAX_SEGMENT_COUNT
Maximum number of non-contiguous segments the assembler can hold. Used for both packet reassembly and TCP stream reassembly. Default: 4.
REASSEMBLY_BUFFER_SIZE
Size of the buffer used for reassembling (de-fragmenting) incoming packets. If the reassembled packet is larger than this setting, it will be dropped instead of reassembled. Default: 1500.
REASSEMBLY_BUFFER_COUNT
Number of reassembly buffers, i.e how many different incoming packets can be reassembled at the same time. Default: 1.
DNS_MAX_RESULT_COUNT
Maximum amount of address results for a given DNS query that will be kept. For example, if this is set to 2 and the queried name has 4 A records, only the first 2 will be returned. Default: 1.
DNS_MAX_SERVER_COUNT
Maximum amount of DNS servers that can be configured in one DNS socket. Default: 1.
DNS_MAX_NAME_SIZE
Maximum length of DNS names that can be queried. Default: 255.
IPV6_HBH_MAX_OPTIONS
The maximum amount of parsed options the IPv6 Hop-by-Hop header can hold. Default: 4.
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, transmitting 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
sudo ip -6 addr add fe80::100/64 dev tap0
sudo ip -6 addr add fdaa::100/64 dev tap0
sudo ip -6 route add fe80::/64 dev tap0
sudo ip -6 route add fdaa::/64 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
sudo ip6tables -t nat -A POSTROUTING -s fdaa::/64 -j MASQUERADE
sudo sysctl -w net.ipv6.conf.all.forwarding=1
# Some distros have a default policy of DROP. This allows the traffic.
sudo iptables -A FORWARD -i tap0 -s 192.168.69.0/24 -j ACCEPT
sudo iptables -A FORWARD -o tap0 -d 192.168.69.0/24 -j ACCEPT
Bridged connection
Instead of the routed connection above, you may also set up a bridged (switched)
connection. This will make smoltcp speak directly to your LAN, with real ARP, etc.
It is needed to run the DHCP example.
NOTE: In this case, the examples' IP configuration must match your LAN's!
NOTE: this ONLY works with actual wired Ethernet connections. It
will NOT work on a WiFi connection.
# Replace with your wired Ethernet interface name
ETH=enp0s20f0u1u1
sudo modprobe bridge
sudo modprobe br_netfilter
sudo sysctl -w net.bridge.bridge-nf-call-arptables=0
sudo sysctl -w net.bridge.bridge-nf-call-ip6tables=0
sudo sysctl -w net.bridge.bridge-nf-call-iptables=0
sudo ip tuntap add name tap0 mode tap user $USER
sudo brctl addbr br0
sudo brctl addif br0 tap0
sudo brctl addif br0 $ETH
sudo ip link set tap0 up
sudo ip link set $ETH up
sudo ip link set br0 up
# This connects your host system to the internet, so you can use it
# at the same time you run the examples.
sudo dhcpcd br0
To tear down:
sudo killall dhcpcd
sudo ip link set br0 down
sudo brctl delbr br0
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.
cargo run --example httpclient -- --tap tap0 ADDRESS URL
For example:
cargo run --example httpclient -- --tap tap0 93.184.216.34 http://example.org/
or:
cargo run --example httpclient -- --tap tap0 2606:2800:220:1:248:1893:25c8:1946 http://example.org/
It connects to the given address (not a hostname) and URL, and prints any returned response data.
The TCP socket buffers are limited to 1024 bytes to make packet traces more interesting.
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.
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, 192.168.69.100. It cannot reach itself because
packets entering a tap interface do not loop back.
examples/server.rs
examples/server.rs emulates a network host that can respond to basic requests.
The host is assigned the hardware address 02-00-00-00-00-01 and IPv4 address 192.168.69.1.
UDP packets on port 6969 (socat stdio udp4-connect:192.168.69.1:6969 <<<"abcdefg"),
where it will respond with reversed chunks of the input indefinitely;
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 basic requests.
The host is assigned the hardware address 02-00-00-00-00-02 and IPv4 address 192.168.69.2.
cargo run --example client -- --tap 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/benchmark.rs
examples/benchmark.rs implements a simple throughput benchmark.
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 alloc feature to run, as well as log, proto-ipv4 and socket-tcp.
cargo run --example loopback --no-default-features --features="log proto-ipv4 socket-tcp alloc"
... or with std (in this case the features don't have to be explicitly listed):
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.
examples/loopback_benchmark.rs
examples/loopback_benchmark.rs is another simple throughput benchmark.
