Download Chapter 11

Protocol Layering
Chapter 11
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?
Conceptual Protocol Software
Layers
Sender
Receiver
Layer n
...
Layer n
...
Layer 2
Layer 1
Layer 2
Layer 1
Network
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
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
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
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
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.
TCP/IP Model of Layering
Conceptual Layer
Application
Objects Passed
Between Layers
Messages or Streams
Transport
Transport Protocol Packets
Internet
IP Datagrams
Network Interface
( aka Data Link )
Hardware
Network-Specific Frames
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-toend communication), adds codes to header indicating
the application involved
– Application - individual messages or continuous
stream of bytes
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
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
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
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
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
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
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
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
For Next Time
• Read Chapter 12