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.

Header

struct IPIL { byte vihl; /* Version and header length */ byte tos; /* Type of service */ byte length; /* packet length */ byte id; /* Identification */ byte frag; /* Fragment information */ byte ttl; /* Time to live */ byte proto; /* Protocol */ byte cksum; /* Header checksum */ byte src; /* Ip source */ byte dst; /* Ip destination */ byte ilsum; /* Checksum including header */ byte illen; /* Packet length */ byte iltype; /* Packet type */ byte ilspec; /* Special */ byte ilsrc; /* Src port */ byte ildst; /* Dst port */ byte ilid; /* Sequence id */ byte ilack; /* Acked sequence */

See also Fast Local Internet Protocol

References

Further reading —The original paper describing IL