1. CPE 401 / 601 Computer Network Systems
Lecture 1 Introduction
Lecture 1 Internet
CPE 401 / 601
Computer Network Systems
slides are modified from Dave Hollinger and Daniel Zappala

2. Network “ ... communication system for connecting end-systems”
End-systems a.k.a. “hosts”
PCs, workstations
dedicated computers
network components

3. 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 !

4. Internetwork Net A Net B
Connection of 2 or more distinct (possibly dissimilar) networks.
Requires some kind of network device to facilitate the connection.
Net A
Net B

5. 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

6. The Internet
Internet
Internet Mapping Project, Bill Cheswick

7. 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

8. Internet

9. Many Different Internet Service Providers
Each network is independent
Interoperability requires using Internet standards: IP, TCP
the Internet is global and must run these standards
your private intranet can do whatever you want it to do

10. 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

11. 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

12. 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

13. The Internet Hourglass

14. The Internet at each Hop

15. Interface and Peer-to-peer Protocols
Interface protocols describe communication between layers on the same endpoint.
Peer-to-peer protocols describe communication between peers at the same layer.
Process
Transport
Network
Data Link
Interface Protocols
Peer-to-peer Protocols

16. What’s a protocol? a human protocol and a computer network protocol:
… specific msgs sent
… specific actions taken when msgs received, or other events Hi
TCP connection
request Hi
TCP connection
response
Got the
time?
Get
2:00
<file>
time
Q: Other human protocols?

17. 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.

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

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

20. 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.

21. 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

22. 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

23. 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”

24. 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.