The OSI Reference Model

• not strictly followed by any system
• useful as a conceptual model
• its terminology is still useful

The OSI model prescribes seven layers:

1. Application
2. Presentation
3. Session
4. Transport
5. Network
6. Data link
7. Physical

We'll talk briefly about each one.

Physical layer

deals with bits

• How to transmit a sequence of bits along some physical medium (e.g., wire, fiber, microwave)
• A different standard is needed for each type of media.
• For example, are 0's and 1's represented by differences in voltages, frequencies, or what?
• How long (in time) is each bit?

Data link layer

deals with frames

• Give some structure to a bit stream
• Some error checking may be done here
• Synchronization between sender and receiver may be done here

To do this, divide the bit stream into frames of say, 1000 bytes each
If there is a problem (an error or lost frame) the damage is limited, and we can resynchronize

What is involved?

• Coding schemes (with redundancy) so errors can be detected
• Acknowledgements from receiver to sender so that the sender knows if a frame arrived correctly
• Flow control -- so a slow receiver can tell a fast sender to slow down
• Access control for broadcast networks

These issues involved in both point-to-point and broadcast networks

Network layer

deals with packets

• The biggest issue here is routing
• A packet may have to go through many intermediate nodes before arriving at its destination
• "store and forward"

Transport layer

deals with datagrams, streams

The objective is to provide a more transparent level of service, for use by applications

example  Socket is an abstraction which forms the interface from transport layer to applications.  Sockets may support TCP (stream) or UDP (datagram) service.

A stream socket allows an application to write a sequence of bytes to the socket, which can be read by an application on the other side.

Issues

• connection-oriented vs. connectionless
• In a connection-oriented service, once a connection is made, each communication is separate; it can be used
• reliable vs. unreliable
• reliable ==> error-checking
• data is guaranteed to arrive in correct order without errors or duplication
• examples
• file transfer, telnet, email, ecommerce (reliable)
• streaming video, video conferencing

Session layer

deals with sessions

A session between processes on different machines
May provide basic transport + enhanced services

example  When a session is terminated, all processes during the session are terminated.
example  Establish checkpoints, so that in case of a crash, an operation can be resumed where it left off instead of starting over

Presentation layer

example  Data formatting for basic data types, data structures

Application layer

Application-specific protocols, such as

• telnet
• ftp
• smtp
• http
• dns

The TCP/IP Reference Model

The basis for the Internet
Evolved from ARPANET -- Advanced Research Projects Agency Network

Goal:  A way for various Department of Defense installations to communicate, which could survive a calamity like a nuclear war.

Critical design ideas:

• Packet switching
• Host computers + subnet structure

==> Provide alternative routes between hosts through the subnet.  Rerouting possible if some points were damaged





Subnet consists of IMPs (Interface Message Processors) connected by leased telephone lines.

1 host may be connected to several IMPs (for safety if an IMP goes down)
1 IMP may be connected to several hosts (for efficiency)

Only three layers are defined:  Internet, transport, and application





OSI was created to form model for the design of new networks; TCP/IP arose to describe the existing TCP/IP networks.

Internet layer

• packet-switched
• connectionless -- each packet is handled independently
• IP (Internet protocol) defines the packet format

The job of the internet layer is to deliver packets to the correct address.
Major concern:  routing

• IPv4 uses 32-bit IP addresses
• IPv6 uses 128-bit IP addresses

Transport layer

Goal:  allow a "conversation" between peer processes at each end (source and destination)

TCP -- Transmission Control Protocol

• reliable, connection-oriented
• interface to TCP is a byte stream
• TCP breaks the stream into chunks and sends each one through the internet layer, which takes it and constructs an IP packet

UDP -- User Datagram Protocol

• unreliable, connectionless
• each datagram corresponds to a single IP packet
• used when prompt delivery is more important than accurate delivery
• games
• video conferencing
• streaming audio/video

Application layer

All higher level protocols

ftp
telnet
dns -- Domain Name Service
http -- the web

OSI vs. TCP/IP

TCP/IP is intended as a heterogeneous internetworking protocol, so it assumes that lower levels are handled by a local network (i.e., somebody else's problem)

The OSI model provides a good way of abstractly viewing network protocols, their structure and functions.

• service
• interface
• protocol

TCP/IP describes what is really in use.

Tanenbaum suggests using a hybrid model, consisting of 5 layers:

Application
Transport
Network
Data link -- LAN protocols
Physical -- communication media

A word about middleware

Middleware is software lying between network software and application software, used to support applications.  Some examples:

COM -- Component object model (Microsoft)
CORBA -- Common Object Request Broker Architecture
RPC -- Remote Procedure Call
RMI -- Remote Method Invocation (Java)

Other network technologies

TCP/IP is not the only important network technology.  Many others exist.  How did network services evolve?

The early days of networking saw many vendor-specific technologies, such as

• IBM Token Ring
• Novell Netware
• AppleTalk
• Banyan vines
• Ethernet
• FDDI
• ATM

Good ones became industry standards.

ATM

One example of an alternative high-speed network is ATM (Asynchronous Transfer Mode)

• Fast cell-switching (cell = packet)
• Connection-oriented
• 155 Mbps - 622 Mbps
• Developed by a consortium of telephone companies and computer vendors
• Objective:  Support new services like video on demand

ATM Reference Model

3-d model

• 3 planes:
• User plane - user data
• Control plane - connection control
• Management plane
• 3 layers
• AAL - ATM Adaptation Layer
• CS - Convergence sublayer
• SAR - Segmentation and reassembly
• ATM - handles
• Flow control
• Cell header operation/extraction
• cell multiplex/demultiplex
• virtual circuit management
• Physical - two sublayers
• TC - transmission convergence
• PM - physical medium

Ethernet

• Most widely used LAN technology
• Conceptual origin -- ALOHANET
• Hawaii, early 1970s
• Objective:  connect users on remote islands to main computer in Honolulu
• Broadcast transmission using radio waves
• Ethernet originated at Xerox PARC in the 1970s
• Metcalfe and Boggs, "Ethernet:  Distributed Packet Switching for Local Computer Networks," CACM, July, 1976.
• Bus topology:  a single coaxial cable with lines dropped off to individual machines