Communications Protocol
Engineering
中華大學資工系
嚴力行
1
Protocol Hierarchies
• Most networks are organized as a series of
layers or levels.
– Each layer offers certain services to the higher
layers
• Protocol
– an agreement between the communicating
parties on how communication is to proceed.
– Layer n protocol
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Peers and Interfaces
• Peers
– The entities comprising the corresponding
layers on different machines
• Interface
– between each pair of adjacent layers
– defines which primitive operations and services
the lower layer offers to the upper one.
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Entities and Peer Entities
• An entity
– an active element in a layer
– can be a software entity (a process), a hardware
entity (an I/O chip), or both (an I/O chip with
its driver).
• Peer entities
– entities in the same layer on different machines
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Protocol Stacks
• A protocol stack.
– A list of protocols used by a certain system, one
protocol per layer
• Network architecture
– A set of layers and protocols
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Layers at An Interface
SAP = Service Access Point
IDU = Interface Data Unit
SDU = Service Data Unit
PDU = Protocol Data Unit
ICI = Interface Control Information
IDU
Layer N+1
ICI
SDU
SAP
Interface
Layer N
ICI
SDU
SDU
N-PDU
Layer N entities
exchange N-PDUs
in their layer N
protocol
Header
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Service Primitives
• Request
Layer N+1
Layer N+1
1
3
– layer n+1 to layer n
• Indication
– layer n to layer n+1
– signal an event
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Layer N
2
Layer N
• Response
– layer n+1 to layer n
– respond to the previous
indication
• Confirm
– layer n to layer n+1
– confirm the previous request
1. CONNECT.request
2. CONNECT.indication
3. CONNECT.response
4. CONNECT.confirm
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Reference Models
• The OSI Reference Model
– based on a proposal developed by the
International Standards Organization (ISO)
– a first step toward international standardization
of the protocols used in the various layers.
• The TCP/IP Reference Model
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The OSI Reference Model
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Application
6
Presentation
5
Session
4
Transport
Application Protocol
Presentation Protocol
Application
APDU
Presentation PPDU
Session Protocol
Transport Protocol
Session
SPDU
Transport
TPDU
Network
Packet
Frame
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Network
Internal Subnet Protocol
Network
Network
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Data Link
Data Link
Data Link
Data Link
1
Physical
Physical
Physical
Physical
Layer
Host A
Router
Router
Host B
Bit
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The Physical Layer
• Functions
– Concerned with transmission of unstructured bit stream over
physical medium
– Deals with the mechanical, electrical, functional, and procedural
characteristics to access the physical medium
• Examples
–
–
–
–
–
baud rate
simplex/half duplex/full duplex
parallel/serial
error detection (odd/even parity)
synchronization/asynchronization
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The Data Link Layer
• Main task
– to take a raw transmission facility and transform it into
a line that appears free of transmission errors to the
network layer
• The sender
– breaks the input data up into data frames
– transmits the frames sequentially
– processes the acknowledgement frames sent back by
the receiver
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Issues of the Data Link Layer
• error handling: solve the problem caused by
damaged, lost and duplicate frames
• flow control: keep a fast transmitter from
drowning a slower receiver in data
• piggybacking: attach data with ack. information
• medium access control (MAC)
– a sublayer that controls access to the shared channel in
broadcast network
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The Network Layer
• determine how packets are routed from source to
destination (routing)
• congestion control
• accounting function
• to allow heterogeneous networks to be interconnected
– address conversion
– packet conversion
• In broadcast networks
– the network layer is often thin or even nonexistent
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The Transport Layer
• Functions
– Provides reliable, transparent transfer of data between
end-points
– Provides end-to-end error recovery and flow control
• Tasks
–
–
–
–
–
error control
flow control
multiplexing
fragmentation/reassembly
quality of service
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The Session Layer
• Functions
– Provides the control structure for communication
between applications
– Establishes, manages, and terminates connections
(sessions) between cooperating applications
• Examples
– dialogue control (simplex/half-duplex/full-duplex)
– synchronization (checkpoint)
– token management
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The Presentation Layer
• Functions
– Provides independence to the application processes
from differences in data representation (syntax)
• Examples
– data encoding (character string, integer, floating-point
number)
– data conversion (Big-5/IBM 5550/ET/Telecom,
ASCII/EBCDIC)
– data encryption
– data compression/expansion
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The Application Layer
• Functions
– Provides access to the OSI environment for users and
also provides distributed information services
• Examples
– virtual terminal
• deals with different terminal types, screen layouts,
escape sequences for inserting and deleting text, etc.
– file transfer: naming, line feed, ...
– electronic mail
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The TCP/IP Reference Model
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6
5
4
3
2
1
Application
Application
Presentation
Not present
in the model
Session
Transport
Transport
Network
Internet
Data Link
Physical
Host-tonetwork
OSI
TCP/IP
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Protocol Stack Performance
• Depends on structure and environment
• Structure
– partitioning of functionality between user and kernel
– separation of layer processing (interface)
• Environment (cost of O.S. functions)
–
–
–
–
–
data copy cost
interrupt overhead
context switch time
latency in accessing memory
cache effects
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Partitioning Strategies
• How much to put in user space, and how much in
kernel space? tradeoff between
• software engineering
• customizability
• security
• performance
• Monolithic in kernel space
• Monolithic in user space
• Per-process in user space
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Monolithic in Kernel
Application
Application
User
Kernel
Session
• Place all but the
application layer in
kernel
• Great security
Transport
• Potentially the best
performance
Network
Device
Driver
• expensive
implementation cost
Device
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Monolithic in User Space
Session
Application
• Secure
Application
• reasonable protocol
implementation cost
Transport
Network
Device
Driver
User
Kernel
Proxy
Device
• Poor performance
– every read or write
accesses requires 2
context switches
A trusted user-level
server process
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Per-process in User Space
• Implement stack
as a library linked
to an application
• high performance
• low cost
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Interface Strategies
• Single-context
– protocol layers share a single uninterrupted
thread of execution
• Tasks
– write each layer as a task
• Upcalls
– associated a thread per packet
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Single context
A whole
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Tasks
Task: a procedure called
from task scheduler with
a pointer to a data buffer
containing a packet
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Upcalls
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Implement a Single Layer
Entry points to a
protocol layer
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