lwIP is a small independent implementation of the TCP/IP protocol suite targeted at embedded systems.

The focus of the lwIP TCP/IP implementation is to reduce resource usage while still having a full scale TCP. This makes lwIP suitable for use in embedded systems with tens of kilobytes of free RAM and room for around 40 kilobytes of code ROM.

Main features include:
- Protocols: IP, IPv6, ICMP, ND, MLD, UDP, TCP, IGMP, ARP, PPPoS, PPPoE, 6LowPAN (via IEEE 802.15.4, BLE or ZEP; since v2.1.0)
- DHCP client, stateless DHCPv6 (since v2.1.0), DNS client (incl. mDNS hostname resolver), AutoIP/APIPA (Zeroconf), SNMP agent (v1, v2c, v3 (since v2.1.0), private MIB support & MIB compiler)
- APIs: specialized APIs for enhanced performance & zero copy, optional Berkeley-alike socket API
- Extended features: IP forwarding over multiple network interfaces
- Extended TCP features: congestion control, RTT estimation and fast recovery/fast retransmit, sending SACKs (since v2.1.0), "altcp": nearly transparent TLS for any tcp pcb (since v2.1.0)
- Addon applications: HTTP server (HTTPS via altcp), HTTP(S) client (since v2.1.0), SNTP client, SMTP client (SMTPS via altcp), ping, NetBIOS nameserver, mDNS responder, MQTT client (TLS support since v2.1.0), TFTP server, iPerf2 counterpart