The standard model for networking protocols & distributed applications is the International Standard Organization’s Open System Interconnect
(ISO/OSI) model. It defines seven network layers.
Short for Open System Interconnection, an ISO standard for worldwide communications that defines a
networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station,
proceeding to the bottom layer, over the channel to the next station & back up the hierarchy.
At one time, most vendors agreed to support OSI in one form
or another, but OSI was too loosely defined & proprietary standards were too entrenched. Except for the OSI-compliant X.400 & X.500 e-mail & directory
standards, which are widely used, what was once thought to become the universal communications standard now serves as the teaching model for all other
protocols.
Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the
next station & back up the hierarchy.
Layer 1 - Physical
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet,
unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience & cost of installation & maintenance. Converters
from one media to another operate at this level.
Layer 2 - Data
LinkData Link layer defines the format of data on the network. A network data frame,
aka packet, includes checksum, source & destination address, & data. The largest packet that can be sent through a data link layer defines the Maximum
Transmission Unit (MTU). The data link layer handles the physical & logical connections to the packet’s destination, using a network interface. A host connected
to an Ethernet would have an Ethernet interface to handle connections to the outside world, & a loopback interface to send packets to itself.
Ethernet
addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six
colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is completely unique & is associated with a particular Ethernet device. Hosts with multiple network
interfaces should use the same MAC address on each. The data link layer’s protocolspecific header specifies the MAC address of the packet’s source &
destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.
Layer 3 - Network
NFS uses Internetwork
Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large
datagrams, larger than MTU, into smaller packets & host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol
identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 & 255, e.g., 129.79.16.40. The leading
1-3 bytes of the IP identify the network & the remaining bytes identifies the host on that network. The network portion of the IP is assigned by InterNIC Registration
Services, under the contract to the National Science Foundation, & the host portion of the IP is assigned by the local network administrators. For large sites, the
first two bytes represents the network portion of the IP, & the third & fourth bytes identify the subnet & host respectively.
Even though IP packets are
addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used
to map the IP address to it hardware address.
Layer 4 - Transport
Transport layer subdivides user-buffer into network-buffer sized datagrams &
enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) & User Datagram Protocol (UDP), sits at the transport layer.
Reliability & speed are the primary difference between these two protocols. There is more. TCP establishes connections between two hosts on the network through ’sockets’
which are determined by the IP address & port number. TCP keeps track of the packet delivery order & the packets that must be resent. Maintaining this
information for each connection makes TCP a stateful protocol. UDP on the other hand gives a low overhead transmission service, but with less error checking.
NFS is built on top of UDP because of its speed & statelessness. Statelessness simplifies the crash recovery.
Layer 5 - Session
The session
protocol defines the format of the data sent over the connections. There is more. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on
either TCP or UDP. Login sessions uses TCP whereas NFS & broadcast use UDP.
Layer 6 - Presentation
External Data Representation (XDR) sits
at the presentation level. It converts local representation of data to its canonical form & vice versa. The canonical uses a standard byte ordering & structure
packing convention, independent of the host.
Layer 7 - Application
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are
examples of network applications.
Bob Bello,
is the owner of http://www.CiscoKits.com At his site you’ll find over 200 various articles dedicated to
with CCNA & CCNP home lab kits.
