TCP/IP Architecture and the TCP/IP Model
(Page 3 of 3)
This layer corresponds to the network layer in the OSI Reference Model (and for that reason is sometimes called the network layer even in TCP/IP model discussions). It is responsible for typical layer three jobs, such as logical device addressing, data packaging, manipulation and delivery, and last but not least, routing. At this layer we find the Internet Protocol (IP), arguably the heart of TCP/IP, as well as support protocols such as ICMP and the routing protocols (RIP, OSFP, BGP, etc.) The new version of IP, called IP version 6, will be used for the Internet of the future and is of course also at this layer.
This primary job of this layer is to facilitate end-to-end communication over an internetwork. It is in charge of allowing logical connections to be made between devices to allow data to be sent either unreliably (with no guarantee that it gets there) or reliably (where the protocol keeps track of the data sent and received to make sure it arrives, and re-sends it if necessary). It is also here that identification of the specific source and destination application process is accomplished
The formal name of this layer is often shortened to just the transport layer; the key TCP/IP protocols at this layer are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The TCP/IP transport layer corresponds to the layer of the same name in the OSI model (layer four) but includes certain elements that are arguably part of the OSI session layer. For example, TCP establishes a connection that can persist for a long period of time, which some people say makes a TCP connection more like a session.
This is the highest layer in the TCP/IP model. It is a rather broad layer, encompassing layers five through seven in the OSI model. While this seems to represent a loss of detail compared to the OSI model, I think this is probably a good thing! The TCP/IP model better reflects the blurry nature of the divisions between the functions of the higher layers in the OSI model, which in practical terms often seem rather arbitrary. It really is hard to separate some protocols in terms of which of layers five, six or seven they encompass. (I didn't even bother to try in this Guide which is why the higher-level protocols are all in the same chapter, while layers one through four have their protocols listed separately.)
Numerous protocols reside at the application layer. These include application protocols such as HTTP, FTP and SMTP for providing end-user services, as well as administrative protocols like SNMP, DHCP and DNS.
In the topic that follows I provide a brief look at each of the TCP/IP protocols covered in detail in this Guide and more detail on where they all fit into the TCP/IP architecture. There I will also cover a couple of protocols that don't really fit into the TCP/IP layer model at all.
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