Notes - Reference Models

The following are notes from Computer Networks written by Tanenbaum 5th edition.

• layered networks in the abstract
• TCP/IP reference model

The OSI Reference Model

• OSI modle minus physical medium is based on proposal developed by ISO (International Standards Organization)
• OSI model
• seven layers
• a layer should be created where a different abstraction is needed
• each layer should perform a well defined function
• function should be chosen to define international standards
• layer boundaries chosen to minimize information flow across interfaces
• number of layers large enough that distinct functions don't need to be thrown together in same layer, small enough that architecture isn't unwieldy
• Physical Layer
• transmit of raw bits over communication channel
• 1,0 bit communication
• electrical signals, how connection established, how connections torn down
• pins network connector has
• mechanical electrical and timing interfaces
• Data Link Layer
• raw transmission into a line free of undetected transmission errors
• mask real errors so network layer doesn't see them
• data frames
• send back acknowledgement frame
• special sublayer of data link layer
• MAC (Medium Access Control) sublayer
• Network Layer
• controls operation of subnet
• determine how packets are routed from source to destination
• routes based on tables, updatable to reflect machines
• handling congestion
• quality of service
• delay
• transmit time
• jitter
• broadcast networks, routing is simple so this layer is thin for that system
• Transport Layer
• Accept data from above it
• split into smaller units
• determines type of service to provide to session layer
• error free point to point channel is most popular
• type of service determined when connection established
• end to end layer, carries data from source to destination
• Session Layer
• establish sessions
• dialog control keep track of whose turn it is to transmit
• token management prevent parties from attempting same operatio simultaneously
• synchronization
• Presentation layer
• concerned with syntax and semantics of information transmitted
• manages abstract data structures
• allows for higher level data structures to be defined
• Application Layer
• HTTP(hyper text transfer protocol)
• variety of protocols for users
• e-mail, file transfer, network news

The TCP/IP Reference Model

• TCP/IP used in ARPANET
• ARPANET was a research network sponsored by the department of defense
• used to connect multiple networks seamlessly
• survive loss of subnet hardware
• survive as long as source/destination functioning
• The Link Layer
• packet switching network
• serial lines
• interface between hosts and transmission links
• Internet Layer
• corresponds roughly to OSI Network layer
• permit hosts to inject packets into any network and travel independently to destination
• may arrive out of order
• snail mail equivalent
• defines official packet format and protocol called IP(Internet Protocol)
• ICMP(Internet Control Message Protocol)
• helps function
• Transport Layer
• layer above the internet layer in TCP/IP
• allow peer entities on source and destination to carry out a conversation
• two end to end transport protocols have been defined here
• TCP
• Transmission Control Protocol
• connection oriented
• byte stream delivered without error to other machine
• reassembles received messages into output stream
• UDP
• User Datagram Protocol
• unreliable connectionless protocol
• one shot client server request reply queries
• used for prompt delivery over accuracy
• speech or video
• The Application Layer
• does not have session or presentation layers
• applications includes the above
• all higher level protocols
• TELNET
• virtual terminal
• FTP
• File transfer protocol
• SMTP
• simple mail protocol

The Model Used in This Book

• OSI strength is the model itself used for discussion
• TCP/IP is used as the protocols
• hybrid scheme
• Application
• contains programs
• Transport
• transmit
• Network
• how to combine multiple links into networks
• how to find paths
• Link
• how to send finite length messages
• Physical
• how to transmit bits

A Comparison of the OSI and TCP/IP Reference Models

• Both are based on the concept of a stack of independent protocols
• functionality roughly similar
• OSI
• differentiation is explicit
• Services
• performs services for layer above it
• defines semantics
• Interfaces
• tells process how to access
• Protocols
• used in a layer and is the layer's own business
• fits with object oriented style programming
• devised before protocols invented, so extremely general
• convergence sublayers have to be grafted on to account for network differences
• supports connection oriented/connectionless
• TCP/IP
• does not distinguish
• have been modeled to make it similar to OSI
• protocols came first model defined afterwards, easy fit
• not useful in describing non TCP/IP networks
• supports connectionless in network, both in transport layer

A Critique of the OSI Model and Protocols

• Bad Timing
• time of standardization is crucial
• in between research and investment
• TCP/IP already in use by universities by the time OSI appeared
• universities/companies did not want to support
• Bad Technology
• model and protocols are flawed
• very complex
• functions repeated
• Bad Implementation
• complexity means implementations large unwieldy and slow
• OSI associated with poor quality
• TCP/IP was good and free from Berkeley, large user communities/improvements
• Bad Politics
• TCP/IP was seen as part of UNIX
• OSI seen as government control

A Critique of the TCP/IP Reference Model

• Models don't distinguish specifications/implementations
• not much of a guide for design
• not general
• link layer is not really a layer, just an interface
• does not distinguish physical and data layers
• Many protocols ad hoc and distributed free
• TELNET designed for 10 character per second mechanical teletype terminal no gui or mice

Back to ToC