Network Systems Spring 2002 CSCI 4273/5273

1. Network SystemsSpring 2002CSCI 4273/5273 Professor Rick Han University of Colorado at Boulder rhan@cs.colorado.edu

2. 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

3. 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

4. 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

5. 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

6. 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

7. “1” or “0” ? Host A Host B 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? Prof. Rick Han, University of Colorado at Boulder

8. “1” or “0” over wire Host A Host B 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 Prof. Rick Han, University of Colorado at Boulder

9. 1011000…? Host A Host B 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 Prof. Rick Han, University of Colorado at Boulder

10. 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 • … 1011000 Host A Host B Prof. Rick Han, University of Colorado at Boulder

11. 1011000 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 Prof. Rick Han, University of Colorado at Boulder

12. 1011000 1011000 1011000 How Physical and Data-Link Layers Interact… Host A Host B 1011000 1011000 Layer 2 Layer 2 Layer 1 Layer 1 Prof. Rick Han, University of Colorado at Boulder

13. 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 1011000 Defining a Service Host A 1011000 Layer 2 Layer 1 Prof. Rick Han, University of Colorado at Boulder

14. 1011000 Communication Across a Network of Hosts Host A Host B Host C • 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 Prof. Rick Han, University of Colorado at Boulder

15. Layer 3: The Network Layer • The Network layer is responsible for routing a message across an interconnected mesh of hosts Host C Host A Host B Host F Host E Host D • Network often represented as a “cloud” • Routers have specialized hardware & OS’s, but initially were just computer hosts Prof. Rick Han, University of Colorado at Boulder

16. Network B Network C Network E Network D Internet: Communication Across a Network of Networks • The Internet is a network of heterogeneous networks Host A Host F • The Internet Protocol, or IP, is an example of a Network Layer protocol • IP routers at entry/exit to cloud (& possibly within cloud) Prof. Rick Han, University of Colorado at Boulder

17. 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

18. Host A Router Host C Multiplexed Time slots Host B 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. Prof. Rick Han, University of Colorado at Boulder

19. Host A Host F Network B Network C Network E Network D Layer 4: The Transport Layer • 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

20. 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

21. 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

22. Application Layer OSI Stack Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer The Layered Network Stack Internet Stack Application Layer Transport Layer (TCP/UDP) Network Layer (IP) Data Link Layer Prof. Rick Han, University of Colorado at Boulder Physical Layer