The Internet protocols discussed above are defined by the Internet Engineering Task Force, or IETF (under the aegis of the Internet Architecture Board, or IAB, in turn under the aegis of the Internet Society, ISOC). The IETF publishes “Request For Comment” or RFC documents that contain all the formal Internet standards; these are available at http://www.ietf.org/rfc.html (note that, by the time a document appears here, the actual comment-requesting period is generally long since closed). The five-layer model is closely associated with the IETF, though is not an official standard.
RFC standards sometimes allow modest flexibility. With this in mind, RFC 2119declares official understandings for the words MUST and SHOULD. A feature labeled with MUST is “an absolute requirement for the specification”, while the term SHOULD is used when
there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.
The original ARPANET network was developed by the US government’s Defense Advanced Research Projects Agency, or DARPA; it went online in 1969. The National Science Foundation began NSFNet in 1986; this largely replaced ARPANET. In 1991, operation of the NSFNet backbone was turned over to ANSNet, a private corporation. The ISOC was founded in 1992 as the NSF continued to retreat from the networking business.
- Hallmarks of the IETF design approach were David Clark’s declaration
We reject: kings, presidents and voting.We believe in: rough consensus and running code.
- and RFC Editor Jon Postel ’s Robustness Principle
Be liberal in what you accept, and conservative in what you send.
Postel’s aphorism has come in for criticism in recent years, especially with regard to cryptographic protocols, for which lax enforcement can lead to security vulnerabilities. To be fair, however, Postel wrote this in an era when protocol specifications sometimes failed to fully spell out the rules in every possible situation; today’s cryptographic protocols are generally much more complete. One way to read Postel’s rule is that protocol implementations should be as strict as necessary, but no stricter.
There is a persistent – though false – notion that the distributed-routing architecture of IP was due to a US Department of Defense mandate that the original ARPAnet be able to survive a nuclear attack. In fact, the developers of IP seemed unconcerned with this. However, Paul Baran did write, in his 1962 paper outlining the concept of packet switching, that
If [the number of stations] is made sufficiently large, it can be shown that highly survivable system structures can be built – even in the thermonuclear era.
In 1977 the International Organization for Standardization, or ISO, founded the Open Systems Interconnection project, or OSI, a process for creation of new network standards. OSI represented an attempt at the creation of networking standards independent of any individual government.
The OSI project is today perhaps best known for its seven-layer networking model: between Transport and Application were inserted the Session and Presentation layers. The Session layer was to handle “sessions” between applications (including the graceful closing of Transport-layer connections, something included in TCP, and the re-establishment of “broken” Transport-layer connections, which TCP could sorely use), and the Presentation layer was to handle things like defining universal data formats (eg for binary numeric data, or for non-ASCII character sets), and eventually came to include compression and encryption as well.
Data presentation and session management are important concepts, but in many cases it has not proved necessary, or even particularly useful, to make them into true layers, in the sense that a layer communicates directly only with the layers adjacent to it. What often happens is that the Application layer manages its own Transport connections, and is responsible for reading and writing data directly from and to the Transport layer. The application then uses conventional libraries for Presentation actions such as encryption, compression and format translation, and for Session actions such as handling broken Transport connections and multiplexing streams of data over a single Transport connection. Version 2 of the HTTP protocol, for example, contains a subprotocol for managing multiple streams; this is generally regarded as part of the Application layer.
However, the SSL/TLS transport-encryption service, 22.10.2 TLS, can be viewed as an example of a true Presentation layer. Applications generally read and write data directly to the SSL/TLS endpoint, which in turn mostly encapsulates the underlying TCP connection. The encapsulation is incomplete, though, in that SSL/TLS applications generally are responsible for creating their own Transport-layer (TCP) connections; see 22.10.3 A TLS Programming Example and the note at the end of 126.96.36.199 TLSserver.
OSI has its own version of IP and TCP. The IP equivalent is CLNP, the ConnectionLess Network Protocol, although OSI also defines a connection-oriented protocol CMNS. The TCP equivalent is TP4; OSI also defines TP0 through TP3 but those are for connection-oriented networks.
It seems clear that the primary reasons the OSI protocols failed in the marketplace were their ponderous bureaucracy for protocol management, their principle that protocols be completed before implementation began, and their insistence on rigid adherence to the specifications to the point of non-interoperability; indeed, Postel’s aphorism above may have been intended as a response to this last point. In contrast, the IETF had (and still has) a “two working implementations” rule for a protocol to become a “Draft Standard”. From RFC 2026:
A specification from which at least two independent and interoperable implementations from different code bases have been developed, and for which sufficient successful operational experience has been obtained, may be elevated to the “Draft Standard” level. [emphasis added]
This rule has often facilitated the discovery of protocol design weaknesses early enough that the problems could be fixed. The OSI approach is a striking failure for the “waterfall” design model, when competing with the IETF’s cyclic “prototyping” model. However, it is worth noting that the IETF has similarly been unable to keep up with rapid changes in html, particularly at the browser end; the OSI mistakes were mostly evident only in retrospect.
Trying to fit protocols into specific layers is often both futile and irrelevant. By one perspective, the Real-Time Protocol RTP lives at the Transport layer, but just above the UDP layer; others have put RTP into the Application layer. Parts of the RTP protocol resemble the Session and Presentation layers. A key component of the IP protocol is the set of various router-update protocols; some of these freely use higher-level layers. Similarly, tunneling might be considered to be a Link-layer protocol, but tunnels are often created and maintained at the Application layer.
A sometimes-more-successful approach to understanding “layers” is to view them instead as parts of a protocol graph. Thus, in the following diagram we have two protocol sublayers within the transport layer (UDP and RTP), and one protocol (ARP) not easily assigned to a layer.