Section 6: Upper Layers of the OSI Model CSIS 479R Fall 1999 “Network +” George D. Hickman, CNI, CNE

Objectives  Identify the purpose of the OSI Session Layer  Identify the characteristics of the three dialog control methods  Identify the elements of session administration

Objectives (con’t)  Identify the purpose of the OSI Presentation layer  Identify the characteristics of the translation methods that can be implemented at the Presentation layer

Objectives (con’t)  Identify the purpose of the OSI Application layer  Identify the characteristics of the two service advertisement methods used at the Application layer  Identify the characteristics of the service use methods used at the Application layer

Session layer  “The Session layer facilitates communications between service requesters and providers”

Processes and Methods--Session  Dialog Control –Simplex  one way –Half-Duplex   two way, one at a time –Full-Duplex  two way, concurrently  Session Administration –Connection establishment –Data transfer –Connection release

Connection Establishment  All subtasks needed for entities to recognize each other and agree to communicate –Verify login names and passwords –Establish connection ID numbers –Agreeing on which services are required, and for how long –Which entity begins the conversation –Coordinate acknowledgment numbering and retransmission procedures

Data Transfer  Maintains the connection, passes messages between entities –Transfer of data –Acknowledgement of data receipt (or NAK) –Resumption of interrupted communications

Connection Release  Ends communications session  Ended by agreement or loss of connection –If not ended by agreement, determined by lack of ACK/NAK

OSI Presentation Layer  Transforms data into a mutually agreed upon format (transfer syntax)  Computers and Network Applications can understand  Can also compress/expand, encrypt/decrypt data

Processes and Methods  Translation –Bit order –Byte order –Character code –File syntax  Encryption (Section 10) –Secret key –Public/private key

Translation methods  Bit Order –7, 8, 16, or 32 bits per “piece of data” –Order or reverse order  Byte Order –8 or 16 bits 2 bytes (16 bit) called a word –Intel processors are little-endian Reverse-byte ordering Least significant bit appears first –Motorola processors are big-endian Most significant bit appears first –Least significant bit is lowest binary value

Translation methods (con’t)  Character code –ASCII –EBCDIC –Shift-JIS  File syntax –PCs use a single entry file table (file name) –Macs use a double entry File name Application that created the file

OSI Application layer  NOT WordPerfect  “All processes and functions specific to each network service”  Lower 6 layers support network services, the Application layer provides the protocols to perform the functions or services

Processes and Methods  Network Services –Covered in section 1  Service Advertisement –Active –Passive  Service Use –OS call interception –Remote operation –Collaborative

Service Advertisement Methods  Active Service Advertisement –Periodic messages sent out to announce availability  Passive Service Advertisement –Servers register availability with a directory –Clients query directory for service location

Service Use Methods  OS Call Interception –Local OS is unaware a network exists  Remote operation –Client is aware of network, but server is unaware of client –Requests all appear internal  Collaborative computing –Network aware (Peer-to-peer networks) –Sharing processing abilities to accomplish a single task