The Internet Link protocol or IL is a connection-based transport-layer protocol designed at Bell Labs originally as part of the Plan 9 operating system and is used to carry 9P. It is assigned the Internet Protocol number of 40. It is similar to TCP but much simpler.
Its main features are:
Reliable datagram service
In-sequence delivery
Internetworking using IP
Low complexity, high performance
Adaptive timeouts
As of the Fourth Edition of Plan 9, 2003, IL is deprecated in favor of TCP/IP because it doesn't handle long-distance connections well.[1]
structIPIL{bytevihl;/* Version and header length */bytetos;/* Type of service */bytelength[2];/* packet length */byteid[2];/* Identification */bytefrag[2];/* Fragment information */bytettl;/* Time to live */byteproto;/* Protocol */bytecksum[2];/* Header checksum */bytesrc[4];/* Ip source */bytedst[4];/* Ip destination */byteilsum[2];/* Checksum including header */byteillen[2];/* Packet length */byteiltype;/* Packet type */byteilspec;/* Special */byteilsrc[2];/* Src port */byteildst[2];/* Dst port */byteilid[4];/* Sequence id */byteilack[4];/* Acked sequence */};
^"Plan 9 — Fourth Edition Release Notes". Lucent Technologies. June 2003. Retrieved August 3, 2018. We are phasing out the IL protocol since it doesn't handle long-distance connections well (and long-distance networks don't handle it well, either). IL is still used by fs(4) but TCP has become the standard protocol for all other services.
