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ultimately, to Internet Standard status.

       Experimental specifications: These are a result of research or development efforts. They’re not intended to be standards, but the information they contain may be of use to the Internet community.

       Informational specifications: These simply provide general information for the Internet community.

       Historic specifications: These RFCs have been superseded by a more recent RFC and are thus considered obsolete.

       Best Current Practice (BCP): RFCs are documents that summarize the consensus of the Internet community’s opinion on the best way to perform an operation or procedure. BCPs are guidelines, not standards.

TABLE 2-1 Maturity Levels for Internet Standards Track RFCs xxx

Maturity Level	Description
Proposed Standard	Generally stable, have resolved known design choices, are believed to be well understood, have received significant community review, and appear to enjoy enough community interest to be considered valuable.
Draft Standard	Well understood and known to be quite stable. At least two interoperable implementations must exist, developed independently from separate code bases. The specification is believed to be mature and useful.
Internet Standard	Have been fully accepted by the Internet community as highly mature and useful standards.

Table 2-2 summarizes the RFCs that apply to the key Internet standards described in this book.

TABLE 2-2 RFCs for Key Internet Standards

RFC	Date	Description
768	August 1980	User Datagram Protocol (UDP)
791	September 1981	Internet Protocol (IP)
792	September 1981	Internet Control Message Protocol (ICMP)
793	September 1981	Transmission Control Protocol (TCP)
826	November 1982	Ethernet Address Resolution Protocol (ARP)
950	August 1985	Internet Standard Subnetting Procedure
959	October 1985	File Transfer Protocol (FTP)
1034	November 1987	Domain Names — Concepts and Facilities (DNS)
1035	November 1987	Domain Names — Implementation and Specification (DNS)
1939	May 1996	Post Office Protocol Version 3 (POP3)
2131	March 1997	Dynamic Host Configuration Protocol (DHCP)
3376	November 1997	Internet Group Management Protocol (IGMP) (Updates RFC 2236 and 1112)
7230 through 7235	June 2014	Hypertext Transfer Protocol – HTTP/1.1
5321	October 2008	Simple Mail Transfer Protocol (SMTP)

My favorite RFC is 1149, an experimental specification for the “Transmission of IP datagrams on avian carriers.” The specification calls for IP datagrams to be written in hexadecimal on scrolls of paper and secured to “avian carriers” with duct tape. (Not surprisingly, it’s dated April 1, 1990. Similar RFCs are frequently submitted on April 1.)

The TCP/IP Protocol Framework

Like the seven-layer OSI Reference Model, TCP/IP protocols are based on a layered framework. TCP/IP has four layers, as shown in Figure 2-1. These layers are described in the following sections.

      FIGURE 2-1: The four layers of the TCP/IP framework.

      Network interface layer

      The lowest level of the TCP/IP architecture is the network interface layer. It corresponds to the OSI physical and data link layers. You can use many different TCP/IP protocols at the network interface layer, including Ethernet and token ring for LANs and protocols such as X.25, Frame Relay, and ATM for wide area networks (WANs).

      The network interface layer is assumed to be unreliable.

      Network layer

      The network layer is where data is addressed, packaged, and routed among networks. Several important Internet protocols operate at the network layer:

       Internet Protocol (IP): A routable protocol that uses IP addresses to deliver packets to network devices. IP is an intentionally unreliable protocol, so it doesn’t guarantee delivery of information.

       Address Resolution Protocol (ARP): Resolves IP addresses to hardware Media Access Control (MAC) addresses, which uniquely identify hardware devices.

       Internet Control Message Protocol (ICMP): Sends and receives diagnostic messages. ICMP is the basis of the ubiquitous ping command.

       Internet Group Management Protocol (IGMP): Used to multicast messages to multiple IP addresses at once.

      Transport layer

      The transport layer is where sessions are established and data packets are exchanged between hosts. Two core protocols are found at this layer:

       Transmission Control Protocol (TCP): Provides reliable connection-oriented transmission between two hosts. TCP establishes a session between hosts, and then ensures delivery of packets between the hosts.

       User Datagram Protocol (UDP): Provides connectionless, unreliable,

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