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Protocol Layering
Sender
Receiver
Layer N
Layer N
...
...
Layer 2
Layer 1
Layer 2
Layer 1
Network
Motivation for Protocol Layering - The
Programming Language Analogy
• Computer programming:
– Program -> Compiler -> Assembly Code ->
Assembler -> Machine Code -> Hardware
• Internet programming:
– Application -> Transport Protocol ->
Connectionless Datagram Delivery Protocol ->
Physical Network
The Internet Protocol Suite
• Internetworking does not use a single protocol to
transmit data
• Imagine how hard it would be to write one that:
– Ran on all underlying network technologies
– Transmitted and routed all traffic on the internet
– Handled: hardware failure, network congestion, delays,
data loss, data corruption, data duplication, sequencing
errors
• Internetworking uses a set of cooperating
protocols (each with its own job)
Benefits of Protocol Layering
• Complexity hidden from application-level
programmer
• Physical network details hidden in low-level
protocols
• Each layer of software can be written,
tested, and modified independently
Protocol Layering
Sender
Receiver
Layer N
Layer N
Layer 2
Layer 2
Layer 1
Layer 1
...
...
Network
• Each layer provides services to the layer above
and utilizes services from the layer below
Protocol Layering
• The path of a datagram:
Send
Recv
Layer N
...
Layer N
...
Network (IP)
Network (IP)
Data Link Layer
Data Link Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
Physical Layer
Physical Layer
Network 1
Network 2
Network 3
Functionality of the Layers
• ISO 7-Layer Reference Model:
7
Application Layer
6
Presentation Layer
5
Session Layer
4
Transport Layer
3
Network Layer
2
Data Link Layer
1
Physical Layer
ISO 7-Layer Reference Model
• Physical Layer (ISO Layer 1)
– Service: transmission of a raw bit stream over a
communication channel ensuring a reliable delivery of
0’s and 1’s
– Functions: conversion of bits into electrical or optical
signals
– Examples: X.21, RS-232-C
ISO 7-Layer Reference Model
• Data Link Layer (ISO Layer 2)
– Service: reliable transfer of frames over a link
– Functions: synchronization, error control, flow control
– Examples: HDLC, CCITT, LAP-D
• Network Layer (ISO Layer 3)
– Service: provide switching and routing functions to
transfer data between hosts
– Functions: routing, addressing, switching, congestion
control
– Examples: IP, X.25, CLNP
ISO 7-Layer Reference Model
• Transport Layer (ISO Layer 4)
– Service: control delivery of messages between hosts
– Functions: connection management, error control, flow
control, multiplexing
– Examples: TCP, UDP, ISO TP0 - TP4
• Session Layer (ISO Layer 5)
– Service: support communication between cooperating
application programs
– Functions: session management, synchronization,
recovery
– Examples: ISO session protocol, RPC
ISO 7-Layer Reference Model
• Presentation Layer (ISO Layer 6)
– Service: handle compatibility issues
– Functions: virtual device support, syntax conversion,
cryptography
– Examples: ASN.1, ISO presentation protocol
• Application Layer (ISO Layer 7)
– Service: provide network access to application
programs
– Functions: (application specific)
– Examples: Telnet, FTP, E-mail, WWW
ISO 7-Layer Reference Model
• Designed by committee - International
Organization for Standardization (ISO)
• Reference model, not an implementation
• Implemented by a set of protocols known as
X.25
TCP/IP Internet Layering Model
• Functionality of the layers
Conceptual Layer
Application Layer
Transport Layer
Internet Layer
Network Interface Layer
Hardware
Objects Passed
Between Layers
Messages or Streams
Transport Protocol Packets
IP Datagrams
Network-Specific Frames
TCP/IP Internet Layering Model
• Network Interface Layer (TCP/IP Layer 1)
– Service: accepts IP datagrams and transmits them over
a specific network
– Functions: conversion of datagrams to physical frames
– Examples: device drivers
• Internet Layer (TCP/IP Layer 2)
– Service: connectionless datagram delivery
– Functions: inter-host communication, routing
– Examples: IP
TCP/IP Internet Layering Model
• Transport Layer (TCP/IP Layer 3)
– Service: provide reliable end-to-end communication
– Functions: flow control, sequencing, acknowledgments
– Examples: TCP
• Application Layer (TCP/IP Layer 4)
– Service: (application specific)
– Functions: (application specific)
– Examples: Telnet, FTP, E-mail, WWW
Internet Model vs. ISO Model
• Similarities
• Differences
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.”
Application
Transport
Internet
Network Interface
Identical message
Identical packet
Identical datagram
Identical frame
Network
Application
Transport
Internet
Network Interface
Boundaries in the TCP/IP Model
Conceptual Layer
Application Layer
Transport Layer
Internet Layer
Network Interface Layer
Hardware
Boundary
Software outside the operating system
Software inside the operating system
Only IP addresses used
Physical addresses used
Protocol Layering
• Advantages
• Disadvantages