1 Network Architecture Section 1.3

2 Network Architecture Challenge –Fill the gap between hardware capabilities and application expectations, and to do so while delivering “good” performance Hardware and expectations are moving targets

3 How do network designers cope with complexity? Networking basic concepts: –Layering –Protocols –Standards

4 Abstraction through layering Decompose the problem of building a network into manageable components Network components are organized in layers –Each layer provides some functionality –Each layer relies on services from layer below and provide services to layer above Link hardware Host-to-host connectivity Application-to-application channels Distributed Applications ( e.g. WWW )

5 Layering: continued Interface between layers defines interaction –Hides implementation details –Layers can change without disturbing other layers  modify a layer independently of other layers –Allows alternative abstractions within the same layer Application programs Hardware Host-to-host connectivity Request/reply channelMessage stream channel

6 How do network designers cope with complexity? Networking basic concepts: Layering –Protocols –Standards

7 Example Protocol for A Calendar Service Making an appointment with your instructor Specifying the messages that go back and forth –And an understanding of what each party will do Can I meet with you for 1.5 hours starting at 4:30pm on February 28, 2008? I can’t. Yes! Can I meet with you for 1.5 hours starting at 3:00pm on February 28, 2008? Can I meet with you for 1.5 hours starting at 1:30pm on February 28, 2008?

8 This is Getting Tedious You: When are you free to meet for 1.5 hours during the next two weeks? Instructor: 10:30am on Feb 27 and 1:15pm on Feb 28. You: Book me for 1.5 hours at 10:30am on Feb 27. Instructor : Yes.

9 Well, Not Quite Enough Student #1: When can you meet for 1.5 hours during the next two weeks? Instructor : 10:30am on Feb 27 and 1:15pm on Feb 28. Student #2: When can you meet for 1.5 hours during the next two weeks? Instructor: 10:30am on Feb 27 and 1:15pm on Feb 28. Student #1: Book me for 1.5 hours at 10:30am on Feb 27. Instructor: Yes. Student #2: Book me for 1.5 hours at 10:30am on Feb 27. Instructor: Uh… well… I can no longer can meet then. I’m free at 1:15pm on Feb 28. Student #2: Book me for 1.5 hours at 1:15pm on Feb 28. Instructor: Yes.

10 Specifying the Details How to identify yourself? –Name? National number? How to represent dates and time? –Time, day, month, year? In what time zone? What granularities of times to use? –Any possible start time and meeting duration? –Multiples of five minutes? How to represent the messages? –Strings? Record with name, start time, and duration? What do you do if you don’t get a response? –Ask again? Reply again?

11 Example: Hyper Text Transfer Protocol, HTTP GET /ecourse/netcourse/netindex.html/ HTTP/1.1 Host: User-Agent: Mozilla/4.03 CRLF HTTP/ OK Date: Mon, 25 Feb :09:03 GMT Server: Netscape-Enterprise/3.5.1 Last-Modified: Mon, 25 Feb :12:23 GMT Content-Length: 21 CRLF Site under construction Request Response

12 Example: IP Packet 4-bit Version 4-bit Header Length 8-bit Type of Service (TOS) 16-bit Total Length (Bytes) 16-bit Identification 3-bit Flags 13-bit Fragment Offset 8-bit Time to Live (TTL) 8-bit Protocol 16-bit Header Checksum 32-bit Source IP Address 32-bit Destination IP Address Options (if any) Payload 20-byteheader

13 Network Protocols Definition –A protocol is an abstract object that makes up a layer of a network system –A protocol provides a communication service that higher- layer objects use to exchange messages  Service interface To objects on the same computer that want to use its services  Peer interface To its counterpart on a different machine Peers communicate using the services of lower-level protocols

14 Interfaces Host 1Host 2 Service interface Higher- level protocol (TCP) Peer-to-peer interface Lower-level Protocol (IP) Peer-to-peer interface

15 Terminology Term “protocol” is overloaded –specification of peer-to-peer interface –module that implements this interface

16 Layering Concepts: Encapsulation Application program Request/ Reply Host-to-Host DATA RRP HDRDATA Application program Request/ Reply Host-to-Host DATA RRP HDRDATA HHP HDR RRP HDRDATA

17 Layering Concepts (continued) Encapsulation – Higher layer protocols create messages and send them via the lower layer protocols – These messages are treated as data by the lower-level protocol – A layer protocol adds its own control information in the form of headers or trailers Multiplexing and Demultiplexing – Multiplexing: Messages from different higher layer protocols (e.g. TCP & UDP) could use the same lower layer protocol (e.g. IP) – Demultiplexing: Use protocol keys in the header to determine correct upper-layer protocol

18 OSI Architecture Open Systems Interconnect (OSI) Architecture –International Standards Organization (ISO) –International Telecommunications Union (ITU, formerly CCITT) –“X dot” series: X.25, X.400, X.500 –Primarily a reference model

19 OSI Protocol Stack Application Presentation Physical Transport Session Data Link Network Application: Application specific protocols Presentation: Format of exchanged data Session: Name space for connection mgmt Transport: Process-to-process channel Network: Host-to-host packet delivery Data Link: Framing of data bits Physical: Transmission of raw bits

20 OSI Protocol Stack (continued) Application Presentation Physical Transport Session Data Link Network Physical Data Link Network Application Presentation Physical Transport Session Data Link Network Host User- Level Host OS Kernel Router

21 Internet Architecture Internet Architecture (TCP/IP) –Developed with ARPANET and NSFNET –Internet Engineering Task Force (IETF)  Culture: implement, then standardize  OSI culture: standardize, then implement –Popular with release of UC Berkeley Software Distribution (BSD) Unix; i.e., free software –Standard suggestions debated publicly through “requests for comments” (RFC’s)

22 Internet Architecture – Hourglass Design FTP TCP ModemATMFDDIEthernet IP UDP TFTPNVHTTP

23 TCP/IP Suite: End Hosts vs. Routers HTTP TCP IP Ethernet interface HTTP TCP IP Ethernet interface IP Ethernet interface Ethernet interface SONET interface SONET interface host router1 router2 HTTP message TCP segment IP packet

24 Internet Architecture Features: – No strict layering – Hourglass shape – IP is the focal point Application Network IP UDPTCP

25 Protocol Acronyms (T)FTP - (Trivial) File Transfer Protocol HTTP - HyperText Transport Protocol NV - Network Video SMTP – Simple Mail Transfer Protocol NTP - Network Time Protocol TCP - Transmission Control Protocol UDP - User Datagram Protocol IP - Internet Protocol FDDI -Fiber Distributed Data Interface ATM -Asynchronous Transfer Mode

26 Summary Goal –Understanding of computer network functionality, with experience building and using computer networks Steps –Identify what functions we expect from a network –Define a layered architecture –Implement network protocols and application programs