RFC 791

RFC 791 is the introduction of Internet Protocol, or IP, as it is known today. The publication of this document occurred in September 1981. It is an amalgamation of six prior RFCs, into a formalized standard that has been in use for almost the past 30 years. The document provides an overview of the protocol and then drills down into the specifications of how Internet Protocol works. The following is a brief review of this document.
Internet Protocol is the addressing scheme that is used to move packets between connected systems. The packets, referred to as datagrams in this document, are moved from one node to another by stripping off and adding local network headers as needed. These are then passed to the next node in the network. This is continued until the packet reaches its destination.
The most prevalent subject of this RFC is the header of the packet. The header that is created within this protocol, provide routing support throughout local networks and across the internet. The basis of this header is the four octet address. These are 32-bit sections of the header that are used to list out the source address of the packet, and the destination module that the packet is being routed to.
There are 11 other field specifications of the header. The first is the version field, which is a four bit representation of the protocol header version. The next field is the internet header length, or IHL, which is a four bit representation of the length of the header. Following that is the type of service field. This is an eight bit section of the header that indicates the packet priority. The next field is the representation of the total length. This is a 16 bit field that measures the number of octets in the packet. On a side note, the maximum standard size of a packet is 576 octets, unless the receiver node has been prepared to accept larger packets. The next field is the Identification field. This is also a 16 bit field that directs the destination node in reassembling the fragment of the packet. The next field is the Flags field, which is a three bit control field. Following the flag field is the Fragment Offset field. This is the location of the fragment in a series of packets. Next is the eight bit time to live field. This is the maximum amount of time a packet can have to reach its destination. The next field is the Protocol field, or the representation of the protocol that the packet is being sent with. Currently the most popular is TCP. The last field before the addresses is the Header Checksum. This is a 16 bit field that is recalculated and verified at each intermediate node.
The packet header also has an Options field. This field is located after the source and destination addresses. It can be of variable length or non-existent. The rest of the RFC is comprised of usage procedures and options. These are organized as examples in the Appendices. Overall, this is nice reference material for network architecture.

Postel, J.,(September 1981). "INTERNET PROTOCOL." Retrieved August 19, 2009, from http://www.ietf.org/rfc/rfc0791.txt?number=791