1. Protocol Layering
Chapter 11

2. Protocol Layering Why is software broken into modules?
Why is program translation done in steps with distinct functions and output?
Why must those functions expect a precise format of data?

3. Conceptual Protocol Software Layers
Sender
Receiver
Layer n
Layer n
...
...
Layer 2
Layer 2
Layer 1
Layer 1
Network

4. Conceptual Protocol Software Layers
Sending a message from an application program on one machine to an application program on another machine means:
Transferring a message down through successive layers on the sender’s machine
Forwarding the message across the network
Transferring the message up through successive layers on the receiver’s machine

5. Conceptual Protocol Software Layers
Each layer makes decisions about the correctness of the message and chooses an appropriate action based on the message type or destination address
Keep the message or forward it to another machine?
Which application program should receive the message?
A more realistic picture is in Figure 11.2b, which shows that IP software may deal with multiple high level protocols or multiple network interfaces

6. ISO Model of Layering ISO’s 7 layer model
X.25 had a suite of protocols based on ISO and was widely used
The network operates much like the telephone system
Packet switches have intelligence to route packets
Hosts attach directly to the packet switches by a serial communication line

7. ISO Model of Layering ISO Layers
Physical Layer - specifies physical interconnection including specifics on voltage and current
Data Link Layer - defines the format of frames and frame boundaries, error detection, acknowledgements
Network Layer - destination addressing and routing
Transport Layer - end to end reliability, checks that machines in the middle did not fail in transmission
Session Layer - describe how protocol software can be organized to provide functionality for applications

8. ISO Model of Layering
Presentation Layer - provides functions needed by application programs, such as text compression, graphics conversion into streams
Application Layer - electronic mail, file transfer, etc.

9. TCP/IP Model of Layering
Conceptual Layer Objects Passed
Between Layers
Application
Messages or Streams
Transport
Transport Protocol Packets
Internet
IP Datagrams
Network Interface
( aka Data Link )
Network-Specific Frames
Hardware

10. TCP/IP Model of Layering
TCP/IP Layers
Network Interface - accepts IP datagrams and transmits them over a network (device driver or specific data link protocol)
Internet - routing and delivery of datagrams
Transport - provides communication from one application program on one machine to another (end-to-end communication), adds codes to header indicating the application involved
Application - individual messages or continuous stream of bytes

11. Differences between ISO and Internet Layering
The ISO model handles errors at all layers, checksums, acknowledgments (including timeout and retransmission) are used
TCP/IP assumes that reliability is an end-to-end problem and handles it in the transport layer
the internet is built to handle expected load, but data may be lost along the way
this assumption makes the software simpler to understand and implement
detection and recovery of datagram loss is handled by source and destination hosts

12. Differences between ISO and Internet Layering
Networks using X.25 assume that the network vendor provides a transport service
the network controls network access, monitors traffic for billing, handles routing, flow control, acknowledgements, etc.
the network is complex
TCP/IP expects the hosts to implement those types of functions
routing (they choose a router), handling ICMP traffic
the network is simple, intelligence is in the hosts

13. The Protocol Layering Principle
Layered protocols are designed so that layer n at the destination receives exactly the same object sent by layer n at the source
This allows the protocol designer to focus on one layer at a time
See Figures 11.6 and 11.7
In the higher layers, the layering principle applies across end-to-end transfers
At the lowest layer, it applies to transfer from one machine to a directly connected machine

14. Point-to-Point Connections
Recall the ARPANET design in which each host was connected to one of 22 ports on a PSN
The set of such point-to-point connections can function like a single network or like a set of networks
Acting like a single network
use a single IP address for and provide new functionality to provide for intranetwork routing
Acting like a set of networks

15. Boundaries in TCP/IP Model
Boundary between the network interface layer and the Internet layer
above the boundary only IP addresses are used
below the boundary only hardware addresses are used
Boundary between the application layer and the transport layer
the application layer is considered outside the operating system
the transport layer is considered part of the operating system

16. The Disadvantage of Layering
Software can be inefficient
the transport layer may not know what the Internet layer is doing, so instead of choosing the largest possible packet size from the beginning, it will not know what that size is
The strict layering scheme can be relaxed
information like route selection and network MTU can be made known

17. Multiplexing and Demultiplexing
The source sends a message and includes extra bits indicating message type, originating program and protocols used
Eventually, all messages are placed into frames and multiplexed, or combined into a stream
The destination network interface decodes, or demultiplexes, the messages as seen in Figure 11.10
Both occur at almost every protocol layer

18. Summary
Protocols
are the standards that specify how data is represented when transferred from one machine to another
specify how the transfer occurs, how errors are detected, how acknowledgements are passed
Communication problems are segregated into subproblems that can be solved independently
Witha layered approach, each subproblem is handled by a separate layer

19. For Next Time
Read Chapter 12