You should be aware of the fact, that when talking about networking you will hear the word "protocol" all the
time. This is because protocols are sets of standards that define all operations within a network. They define how various operations are to be performed. They may even define how devices outside the network can interact with the network. Protocols define everything from basic networking data structures, to higher level application programs. They define various services and utility programs. Protocols operate at many layers of the network models described below. There are protocols considered to be transport protocols such as TCP and UDP. Other protocols work at the network layer of the OSI network model shown below, and some protocols work at several of the network layers.
RFCs
Protocols are outlined in Request for Comments (RFCs). At the end of this document is a list of protocols and associated RFC numbers.Protocols. Although RFCs define protocols not all RFCs define protocols but may define other requirements for the internet such as RFC 1543 which provides information about the preparation of RFCs. The following RFCs are very central to the TCP/IP protocol.
l RFC 1122 - Defines host requirements of the TCP/IP suite of protocols covering the link, network (IP),
and transport (TCP, UDP) layers. l RFC 1123 - The companion RFC to 1122 covering requirements for internet hosts at the application layer l RFC 1812 - Defines requirements for internet gateways which are IPv4 routers Network Models
There are several network models which you may hear about but the one you will hear about most is the ISO
network model described below. You should realize, however that there are others such as:
l The internet layered protocol
l The TCP/IP 4 layered protocol
l The Microsoft networking protocol
If you don't like any of these models, feel free to invent your own along with your own networking scheme of
course, and add it to the list above. You can call it "The MyName Protocol". Ever wonder why networking can be so complex and confusing? Welcome to the world of free enterprise!
The ISO Network Model Standard
The International Standards Organization (ISO) has defined a standard called the Open Systems Interconnection (OSI) reference model. This is a seven layer architecture listed below. Each layer is considered to be responsible for a different part of the communications. This concept was developed to accommodate changes in technology.
The layers are arranged here from the lower levels starting with the physical (hardware) to the higher levels.
1. Physical Layer - The actual hardware.
2. Data Link Layer - Data transfer method (802x ethernet). Puts data in frames and ensures error free
transmission. Also controls the timing of the network transmission. Adds frame type, address, and error
control information. IEEE divided this layer into the two following sublayers.
1. Logical Link control (LLC) - Maintains the Link between two computers by establishing Service
Access Points (SAPs) which are a series of interface points. IEEE 802.2.
2. Media Access Control (MAC) - Used to coordinate the sending of data between computers. The
802.3, 4, 5, and 12 standards apply to this layer. If you hear someone talking about the MAC
address of a network card, they are referring to the hardware address of the card.
3. Network Layer - IP network protocol. Routes messages using the best path available.
4. Transport Layer - TCP, UDP. Ensures properly sequenced and error free transmission.
5. Session Layer - The user's interface to the network. Determines when the session is begun or opened, how
long it is used, and when it is closed. Controls the transmission of data during the session. Supports
security and name lookup enabling computers to locate each other.
6. Presentation Layer - ASCII or EBCDEC data syntax. Makes the type of data transparent to the layers
around it. Used to translate date to computer specific format such as byte ordering. It may include
compression. It prepares the data, either for the network or the application depending on the direction it is
going.
7. Application Layer - Provides services software applications need. Provides the ability for user applications
to interact with the network. Many protocol stacks overlap the borders of the seven layer model by operating at multiple layers of the model. File Transport Protocol (FTP) and telnet both work at the application, presentation, and the session layers.
