Transcript Protocols and Layering

Lecture 2 Overview
Multiaccess vs. Point-to-point
• Multi-access means shared medium
– many end-systems share the same physical
communication resources (wire, frequency, ...)
– There must be some arbitration mechanism.
• Point-to-point
– only 2 systems involved
– no doubt about where data came from !
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The Internet
PC
server
wireless
laptop
cellular
handheld
• millions of connected
computing devices:
hosts = end systems
– running network apps
• communication links
access
points
wired
links
router
– fiber, copper, radio,
satellite
Mobile network
Global ISP
Home network
Regional ISP
Institutional network
• routers:
– forward packets
(chunks of data)
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A Network of Networks
• roughly hierarchical
– Tier-1 ISPs provide national,
international coverage
– Tier-2 ISPs provide
regional coverage
– Tier-3 and lower levels
provide local coverage
• any tier may sell to
business and residential
customers
• any ISP may have a
link to any other ISP
– not strictly hierarchical
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Internet Design Goals
• primary goal: interoperability among existing networks
– a network of networks
– obey administrative boundaries
• secondary goals
–
–
–
–
–
fault tolerance
multiple transport protocols
support a variety of networks
distributed management
cost effective, low effort for host attachment, accountability
• first three were more important, so remaining four did not
receive as much attention
• no mention of security
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Internet Design Principles
• minimal assumptions about services network
should support
– ability to send packets
– no reliability or security
• end-to-end principle
– keep the core of the network as simple as
possible,
– put complex functionality at the edges
– exception: significant performance improvement
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Layering
• Divide a task into pieces and then solve each
piece independently (or nearly so)
• Establishing a well defined interface between
layers makes porting easier
• Major Advantages:
Code Reuse
Extensibility
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The Internet Hourglass
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The Internet at each Hop
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Lecture 3
Protocols and Layering
Lecture 1
Internet
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger
What’s a protocol?
human protocols:
• “what’s the time?”
• “I have a question”
• introductions
network protocols:
• machines rather than
humans
• all communication activity
in Internet governed by
protocols
… specific msgs sent
… specific actions taken when msgs received, or other events
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What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
request
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
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Protocol
• An agreed upon convention for communication.
– both endpoints need to understand the protocol.
• Protocols must be formally defined and unambiguous!
• Protocols define
– format,
– order of msgs sent and received among network entities,
– actions taken on msg transmission, receipt
• We will study lots of existing protocols and perhaps develop a
few of our own.
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Programs & Processes
• A program is an executable file.
• A process or task is an instance of a program
that is being executed.
• A single program can generate multiple
processes.
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Client - Server
• A server is a process - not a machine !
• A server waits for a request from a client.
• A client is a process that sends a request to an
existing server and (usually) waits for a reply.
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Client - Server Examples
• Server returns the time-of-day.
• Server returns a document.
• Server prints a file for client.
• Server does a disk read or write.
• Server records a transaction.
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Servers
• Servers are generally more complex (more
interesting).
• Basic types of servers:
Iterative - server handles one client at a time.
Concurrent - server handles many clients at a
time.
• We will study the differences later.
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Layering
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OSI Reference Model
The International Standards Organization (ISO) proposal
for the standardization of various protocols used in
computer networks is called the Open Systems
Interconnection Reference Model.
Although the OSI model is a just a model (not a
specification), it is generally regarded as the most
complete model.
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OSI 7 Layer Model:
7
6
5
4
3
2
1
Application
Presentation
Session
Transport
Network
Data-Link
Physical
High level protocols
TCP/IP Model
Low level protocols
Protocol Layers
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Simplified Network Model
Application
Application
Interface Protocols
Transport
Transport
Peer-to-peer Protocols
Network
Network
Data Link
Data Link
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The Physical Layer
• Responsibility:
– transmission of raw bits over a communication
channel
• Issues:
– mechanical and electrical interfaces
– time per bit
– distances
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The Data Link Layer
• Responsibility:
– provide an error-free communication link
• Issues:
– framing (dividing data into chunks)
• header & trailer bits
– addressing
10110110101
Protocol Layers
01100010011
10110000001
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The Data Link Layer
• Data Link Control sublayer
• Medium Access Control sublayer
– needed by mutiaccess networks.
• MAC provides Data Link Control with “virtual
wires” on multiaccess networks.
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The Network Layer
• Responsibilities:
– path selection between end-systems (routing).
– flow control.
– fragmentation & reassembly
– translation between different network types.
• Issues:
– packet headers
– virtual circuits
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The Transport Layer
• Responsibilities:
– provides virtual end-to-end links between peer
processes.
– end-to-end flow control
• Issues:
– headers
– error detection
– reliable communication
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The Session Layer
• Responsibilities:
– establishes, manages, and terminates sessions
between applications.
– service location lookup
• Many protocol suites do not include a session
layer.
– Not in TCP/IP model
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The Presentation Layer
• Responsibilities:
– data encryption
– data compression
– data conversion
• Many protocol suites do not include a
Presentation Layer.
– Not in TCP/IP model
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The Application Layer
• Responsibilities:
– anything not provided by any of the other layers
– TCP/IP model
• Session and Presentation Layer functions
• Issues:
– application level protocols
– appropriate selection of “type of service”
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Layering & Headers
• Each layer needs to add some control information to
the data in order to do it’s job.
• This information is typically prepended to the data
before being given to the lower layer.
• Once the lower layers deliver the data and control
information - the peer layer uses the control
information.
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Headers
DATA
Application
H
DATA
Transport
H H
DATA
Network
H H H
DATA
Data Link
Application
Transport
Network
Data Link
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What are the headers?
Physical:
– no header - just a bunch of bits
Data Link:
– address of the receiving endpoints
– address of the sending endpoint
– length of the data
– checksum
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What are the headers?
• Network:
–
–
–
–
–
–
–
–
–
–
Protocol
Protocol version
type of service
packet identifier
time to live
source network address
destination network address
length of the data
fragment number
header checksum
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Layers Summary
• Data-Link: communication between machines
on the same network.
• Network: communication between machines
on possibly different networks.
• Transport: communication between processes
(running on machines on possibly different
networks).
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