Understanding Layered Protocols & Message-Oriented Communication in Networking, Slides of Advanced Operating Systems

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CS 620 Advanced Operating

Systems

Lecture 7 – Communication

Layered Protocols

• As we saw previously, network software is often

structured as a layered protocol suite. We will now examine these protocols in somewhat more detail.

• Protocol : An agreement between communicating

parties on how communication is to proceed.

• Error correction codes.
• Blocksize.
• Ack/Nak.

Layered Protocols

• Layer X of the sender acts as if it is directly communicating with layer X of the receiver but in fact it is communicating with layer X-1 of the sender.
• Similarly layer X of the sender acts as a virtual layer X+1 of the receiver to layer X+1 of the sender.
• A famous example is the ISO OSI ( International Standards Organization Open Systems Interconnection Reference Model ).

Layered Protocols

Layered Protocols

• So the network layer adds a header (in front of the transport layer's header) and sends to the other network layer (really its own data link layer that adds a header in front of the network layer's and a trailer).
• So headers get added as you go down the sender's layers (often called the Protocol Stack or Protocol Suite ).
• They get used (and stripped off) as the message goes up the receiver's stack.

Layered Protocols

Layered Protocols

• Frames contain error detection (and correction) bits.
• This is what the pair of data link layers do when viewed as an extension of the physical.
• But when being used, the sending DL layer gets a packet from the network layer and breaks it into frames and adds the error detection bits.

Data Link Layer

• Discussion between a receiver and a sender in the data link layer.

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Layered Protocols

• Transport layer : make reliable and ordered (but not always). - Break incoming message into packets and send to corresponding transport layer (really send to ...). They are sequence numbered. - Header contains info as to which packets have been sent and received. - These sequence numbers are for the end to end message.

Layered Protocols

• I.e. if grail.cba.csuohio.edu sends message to www.microsoft.com the transport layer breaks message into packets and numbers the packets. - These packets may take different routes. - On any one hop the data link layer keeps the frames ordered.
• If you use connection-oriented network layer there is little for transport layer to do.
• If you use IP for network layer, there is a lot to do.
• If use connection-oriented TCP for transport layer of client-server system, slower than need be - Can use transactional TCP

Layered Protocols

• Session Layer : dialog and synchronization.
  • Dialog control
  • Synchronization facilities
• Presentation layer : Describes "meaning" of fields.
  • Record definition
• Application layer : For specific applications (e.g. mail, news, ftp). - Middleware logically resides in the application layer, but contains functionality that is quite general - Authentication - Authorization - Multicast, etc. - This leads to a slightly modified reference model

Middleware Protocols

• An adapted reference model for networked communication.

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Remote Procedure Call (RPC)

• read is a user mode program.
• read manipulates registers and then does a trap to the kernel.
• After the trap, the kernel manipulates registers and then does a C-language routine and lots of work gets done (drivers, disks, etc).
• After the I/O, the process get unblocked, the kernel read manipulates registers, and returns. The user mode read manipulates registers and returns to the original caller.
• Let’s do something similar with request reply:

Remote Procedure Call (RPC)

• User (client) does a subroutine call to getchar (or read ). - Client knows nothing about messages.
• We link in a user mode program called the client stub (analogous to the user mode read above). - This takes the parameters to read and converts them to a message ( marshalls the arguments). - Sends a message to machine containing the server directed to a server stub. - Does a blocking receive (of the reply message).