1. Lecture 2 Overview

2. 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 !
Internet

3. The Internet
millions of connected computing devices: hosts = end systems
running network apps
communication links
fiber, copper, radio, satellite
routers:
forward packets (chunks of data)
Home network
Institutional network
Mobile network
Global ISP
Regional ISP PC
server
wireless
laptop
cellular
handheld
wired
links
access
points
router
Internet

4. 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
Internet

5. 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
Internet

6. 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
Internet

7. 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
Internet

8. The Internet Hourglass

9. The Internet at each Hop

10. CPE 401 / 601 Computer Network Systems
Lecture 3 Protocols and Layering
Lecture 1 Internet
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger

11. What’s a protocol? human protocols: network protocols:
“what’s the time?”
“I have a question”
introductions
… specific msgs sent
… specific actions taken when msgs received, or other events
network protocols:
machines rather than humans
all communication activity in Internet governed by protocols
Internet

12. What’s a protocol? a human protocol and a computer network protocol:Hi
TCP connection
request Hi
TCP connection
response
Got the
time?
Get
2:00
<file>
time
Q: Other human protocols?
Internet

13. 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.
Internet

14. 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.
Internet

15. 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.
Internet

16. 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.
Internet

17. 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.
Internet

18. Layering
Internet

19. 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.
Protocol Layers

20. OSI 7 Layer Model:
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data-Link 1 Physical
High level protocols
TCP/IP Model
Low level protocols
Protocol Layers

21. Simplified Network Model
Application
Application
Interface Protocols
Transport
Transport
Peer-to-peer Protocols
Network
Network
Data Link
Data Link
Protocol Layers

22. The Physical Layer Responsibility: Issues:
transmission of raw bits over a communication channel
Issues:
mechanical and electrical interfaces
time per bit
distances
Protocol Layers

23. The Data Link Layer Responsibility: Issues:
provide an error-free communication link
Issues:
framing (dividing data into chunks)
header & trailer bits
addressing
Protocol Layers

24. 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.
Protocol Layers

25. The Network Layer Responsibilities: Issues:
path selection between end-systems (routing).
flow control.
fragmentation & reassembly
translation between different network types.
Issues:
packet headers
virtual circuits
Protocol Layers

26. The Transport Layer Responsibilities: Issues:
provides virtual end-to-end links between peer processes.
end-to-end flow control
Issues:
headers
error detection
reliable communication
Protocol Layers

27. 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
Protocol Layers

28. The Presentation Layer
Responsibilities:
data encryption
data compression
data conversion
Many protocol suites do not include a Presentation Layer.
Not in TCP/IP model
Protocol Layers

29. The Application Layer Responsibilities: Issues:
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”
Protocol Layers

30. 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.
Protocol Layers

31. Headers DATA Application Application Transport H DATA Transport
Network H H
DATA
Network
Data Link H H H
DATA
Data Link
Protocol Layers

32. What are the headers? Physical: Data Link:
no header - just a bunch of bits
Data Link:
address of the receiving endpoints
address of the sending endpoint
length of the data
checksum
Protocol Layers

33. 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
Protocol Layers

34. 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).
Protocol Layers