1. Top Three Layers Session Layer Presentation Layer Application Layer

2. Session Layer Design Marks the division between “upper layers and lower layers” –lower layers:= end to end communications –upper layers:= user-oriented services Invented by ISO Can support transport layer QoS Not explicitly used by TCP/IP “Generally connection-oriented”

3. Mission of the Session Layer Provides a way for the session users to establish connections, called sessions, and transfer data over the sessions in an orderly way. Examples: –remote logins –file transfers –Remote Procedure Calls

4. Session Layer & Transport Layer Usually a transport connection is required to establish a session Session Layer provides Control. It says what to do, not necessarily how to do it. Can provide redundancy and multiplexing of transport layer resources Administrative control point

5. Data Exchange Same three phases as transport layer –establishment –use –release In many cases the session layer simply passes the primitive requests to the transport layer. orderly (graceful) release vs. abrupt release –abrupt release is analogous to hanging up the phone –orderly release uses a full handshake, (not provided in transport layer)

6. Dialog Management In principle, all OSI connections are full duplex Some upper layer software is structured to be half-duplex This is to simplify the software and has nothing to do with hardware limitations The session layer supports data token schemes to support half-duplex transmissions

7. Synchronization Transport layer provides error recovery only from communication errors Synchronization in the transport layer provides for returning the session entries back to the last known correct state. Example: Session established and large file transmitted; receiving end has no storage and directly prints the incoming data. The printer dies and data is lost. Session users could split the text into pages and insert a synchronization point between each page.

8. Activity Management Users split the message stream into activities delimiters are added in the session to separate activities First File Sent Start Stop Start Stop Second File Sent Session

9. Remote Procedure Calls Based on the connectionless model Generally outside the OSI model Can be implemented in the application layer Multiple Flavors –{Sun} ONC RPC –{OSF} DCE RPC

10. “RPC School of Thought” Clients sending messages are viewed as procedure calls with the reply being the return from the procedure Contrast this to the more “conventional” school which views I/O with primitives such as X-DATA.request and X- DATA.return as I/O and interrupts respectively.

11. RPC Invocation ClientClient StubNetwork{Portmapper}Server Stub Server 1 2 3 4 5 7 6

12. RPC Semantics and Orphans Operations –exactly once –at most once –at least once Handling crashes: –1. Hang forever, waiting for reply that never comes –2. Time out and raise an exception –3. Time out and retransmit Orphans –extermination –expiration –reincarnation (kill & restart) –gentle reincarnation (selectively kill and restart)

13. Session Layer Conclusions Not used in ARPANET or USENET Some debate on including it in the ISO model Closest Internet example of an OSI (not ISO) stack: NFS XDR RPC UDP IP whatever