Network SystemsSpring 2002

CSCI 4273/5273

Professor Rick HanUniversity of Colorado at Boulder

rhan@cs.colorado.edu

Prof. Rick Han, University of Colorado at Boulder

Network Systems

• Goals: – Learn how the Internet works under the

covers!– Learn general “computer networking”

concepts and principles– Obtain practical experience in network

programming– Have fun in the process

• Information for CSCI 4273/5273:– http://www.cs.colorado.edu/~rhan, then– follow the Network Systems link at the top

Prof. Rick Han, University of Colorado at Boulder

Announcements

• Textbooks: • Colorado book store had 19 as of 1/7/2002• CU book store’s order of 40 should arrive

this/next week

• Waiting List• Students who have not satisfied the OS

prerequisite should drop• Admit students from wait list• Fire regulations limit class size

• Questions so far?• Next, a preview and Chapter 1

Prof. Rick Han, University of Colorado at Boulder

Historical Perspective on Computer Networking

• 1957: USSR launches Sputnik. In response, US forms Advanced Research Projects Agency (ARPA) with Department of Defense (DoD)

• 1969: ARPANET commissioned by DoD for research into networking.• Kleinrock, 4 nodes, packet-

switched, 50 kbps links• “Do you see the L?”

• “Yes, we can see the L,”• “Do you see the O?”

• Yes, we see the O”• “Then we typed the G, and the system crashed”

Prof. Rick Han, University of Colorado at Boulder

Historical Perspective on Computer Networking (2)

• 1971: Logical map of the ARPANET

• 1972: ARPA renamed to DARPA

Prof. Rick Han, University of Colorado at Boulder

Historical Perspective on Computer Networking (3)

• 1971: email invented, “@” sign chosen• 1973: ARPANET becomes global with addition of

nodes in England and Norway.• 1976: UUCP (Unix-to-Unix copy) developed at

AT&T Bell Labs and distributed with Unix.• 1981: IBM PC introduced• 1982: TCP and IP defined as a protocol suite• 1984: > 1,000 hosts, DNS introduced, Cisco

founded• 1989-90: > 100,000 hosts, ARPANET

decommissioned• 1991: 727,000 unique hosts on Internet, CERN’s

Berners-Lee invents Web (HTML, HTTP, URL)• 1993: Mosaic developed by Andreessen at NCSA• 2001: 175,000,000 unique hosts on Internet

Prof. Rick Han, University of Colorado at Boulder

Bottom-Up Approach to Computer Networking

• Approach taken by the textbook• Two hosts want to communicate with each

other• First, how do I send a “bit” from host A to

host B?

Host A Host B

“1” or “0” ?

Prof. Rick Han, University of Colorado at Boulder

Layer 1: The Physical Layer

• Solution: Host A encodes the bit into an analog signal. Host B decodes the analog signal into a received bit.

• Physical Layer, also called Layer 1, ensures that host B can decode a digital bit from an analog representation of the digital bit sent by host A• Modem has advanced DSP to achieve 56 kbps

Host A Host B

“1” or “0” over wire

Prof. Rick Han, University of Colorado at Boulder

Layer 2: The Data Link Layer

• Next Problem: How do I send a message from Host A to Host B?

• Data Link Layer, also called Layer 2, ensures that host B can decode a digital message from a stream of bits sent by host A

Host A Host B

1011000…?

Prof. Rick Han, University of Colorado at Boulder

The Data Link Layer (cont.)

• A Data Link Layer Protocol implements:• Delimiting/framing of a message• Fragmenting of a long message• Retransmission of a lost message• …

Host A Host B

1011000

Prof. Rick Han, University of Colorado at Boulder

Defining a Protocol

• A protocol is an agreement between two parties or endpoints as to how information is to be transmitted

• A protocol implements this agreement via:• A Header• How each endpoint responds to control

info in the header (& external input)

Host A Host B

1011000

Prof. Rick Han, University of Colorado at Boulder

How Physical and Data-Link Layers Interact…

Layer 1 Layer 1

Layer 2 Layer 2

Host A Host B1011000 1011000

1011000

1011000

1011000

Prof. Rick Han, University of Colorado at Boulder

Defining a Service

Layer 1

Layer 2

Host A1011000

1011000

• A Service Interface separates two layers • Acts as standard API

• Lower layer promises to support API primitives

• Isolates Physical Layer’s implementation from DL Layer Protocol

• e.g. mail envelope• e.g. socket API

Prof. Rick Han, University of Colorado at Boulder

Communication Across a Network of Hosts

Host CHost A Host B

• Suppose there is a link connecting A to B, and B to C

• How does Host B know that a message from Host A should be forwarded to Host C?

• Therefore, the intermediate Host B needs:• Address information• Topology information

1011000

Prof. Rick Han, University of Colorado at Boulder

Layer 3: The Network Layer

• The Network layer is responsible for routing a message across an interconnected mesh of hosts

Host A

• Network often represented as a “cloud”• Routers have specialized hardware & OS’s,

but initially were just computer hosts

Host C

Host D

Host B

Host E

Host F

Prof. Rick Han, University of Colorado at Boulder

Internet: Communication Across a Network of

Networks• The Internet is a network of heterogeneous networks

Host A

• The Internet Protocol, or IP, is an example of a Network Layer protocol

• IP routers at entry/exit to cloud (& possibly within cloud)

Host F

Network B

Network C

Network ENetwork

D

Prof. Rick Han, University of Colorado at Boulder

Design Considerations for the Internet

• Correctness – can IP route a packet correctly from source to destination?

• Reliability – if a router crashes/joins, can the Internet adapt to changes in topology?

• Scalability – can IP routing scale to support millions of nodes?

• Performance – can the packet quickly be routed to its destination?

• Efficiency – how much overhead does the routing take?

• …

Prof. Rick Han, University of Colorado at Boulder

Design Philosophies for the Internet

• Early (and winning) design philosophy: keep the network simple• Smart clients + “dumb” network = Internet• In contrast, telephone network: dumb clients,

smart network – hard to introduce new services

• A second design philosophy: share the wealth• Bandwidth is statistically multiplexed, or

shared• In contrast, telephone network sets up a

dedicated circuit that is not shared.

Host A

Host B

Router Host CMultiplexedTime slots

Prof. Rick Han, University of Colorado at Boulder

Layer 4: The Transport Layer

Host A

Host F

Network B

Network C

Network ENetwork

D

• The Transport Layer offers end-to-end delivery of packets across a network• Packets may be lost due to stat mux

congestion or router failure

Prof. Rick Han, University of Colorado at Boulder

Internet Transport Layers: TCP and UDP

• The Transmission Control Protocol, or TCP, is a reliable Transport Layer protocol above IP• Reliable delivery• In-order or stream delivery

• The User Datagram Protocol, or UDP is an unreliable protocol above IP• Unreliable packet delivery• Out-of-order packet delivery

• Why would you ever want unreliable out-of-order delivery?

Prof. Rick Han, University of Colorado at Boulder

Layer 5: The Internet Application Layer

• Given reliable in-order delivery by TCP, the Internet Application Layer builds application-level protocols above TCP like:• HTTP – Hypertext Transfer Protocol• SMTP – Simple Mail Transfer Protocol• FTP – File Transfer Protocol• NNTP – Network News Transfer Protocol

• The Open Systems Interconnection (OSI) model expands its Application Layer into 3 sub-layers: Application, Presentation, and Session

Prof. Rick Han, University of Colorado at Boulder

The Layered Network Stack

Physical Layer

Application Layer

Transport Layer (TCP/UDP)

Network Layer (IP)

Data Link Layer

Internet Stack

Physical Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

OSI StackPresentation Layer

Application Layer