Protocol(TCP/IP, HTTP) 송준화 조경민 2001/03/13

Network Computing Lab.2 Layering of TCP/IP-based protocols

2001/03/13Network Computing Lab.3 TCP Characteristic TCP is connection-oriented 3-way handshake is used for connection setup/teardown TCP provides a stream-of-bytes service. TCP is reliable Acknowledgements indicate delivery of data. Checksums are used to detect corrupted data. Sequence numbers detect missing, or mis-sequenced data. Corrupted data is retransmitted after a timeout. Mis-sequenced data is re-sequenced. (Window-based) Flow control prevents over-run of receiver. TCP uses congestion control to share network capacity among users.

2001/03/13Network Computing Lab.4 TCP is connection-oriented

2001/03/13Network Computing Lab.5 Connection Establishment/Close telnet address port Active open telnet> quit  equivalent to “ close ” Makes the system to send “ FIN ” for “ active close ” Time Wait State (2MSL state) On “ active close ” side MSL: maximum segment lifetime Two effects: lost ACK Port reuse

2001/03/13Network Computing Lab.6 TCP segment header 20 bytes

2001/03/13Network Computing Lab.7 (some) TCP Header Fields URG: urgent bit One end tells the other that “ urgent data ” of some form has been placed into the normal stream of data Works with 16 bit “ urgent pointer ” field ACK: PSH: RST: SYN: FIN:

2001/03/13Network Computing Lab.8 IP characteristic Connectionless mis-sequencing Unreliable may drop packets Best Effort

2001/03/13Network Computing Lab.9 IP : Internet Protocol

2001/03/13Network Computing Lab.10 The IP Datagram 20 bytes

2001/03/13Network Computing Lab.11 (some) IP Header Fields TOS: 3 bit precedence (ignored) 4 bit TOS : (minimize delay, maximize throughput, maximize reliability, minimize monetary cost) Only one bit can be “ on ” Usually call bits “ off ” ID: unique id for each datagram from a sender TTL: upper bound for # of hops

2001/03/13Network Computing Lab.12 HTTP Stateless protocol Not include the concept of a session or interaction beyond delivery of the requested document. Do not need to keep track of who the clients are or what requests were serviced in the past.

2001/03/13Network Computing Lab.13 HTTP 1.0 vs. HTTP 1.1(1) HTTP 1.0 For each additional request, a new TCP connection must be established. The system incurs again in a connection setup overhead. HTTP 1.1 Persistent connection Pipeline of requests

2001/03/13Network Computing Lab.14 HTTP 1.0 vs. HTTP 1.1(2) HTTP 1.1 Persistent connection Avoids many round trip delays Improving performance Reducing the number of packets exchanged Pipeline of requests Allows multiple requests to be sent without waiting for a response.

2001/03/13Network Computing Lab.15 HTTP interaction HTTP 1.0HTTP 1.1

2001/03/13Network Computing Lab.16 Anatomy of an HTTP transaction

2001/03/13Network Computing Lab.17 (Network) System Overview The main issue is how we can improve network system performance at each component.

2001/03/13Network Computing Lab.18 Where are the delays?(1) Browser The end user clicks on a hyperlink and requests a document. In the case of a hit, returns the requested document in the local cache. In the case of a miss mapping the server hostname to an IP address. The clients opens a TCP connection to the server. The clients sends an HTTP request to the server. The browser formats and displays the document and renders the associated images.

2001/03/13Network Computing Lab.19 Where are the delays?(2) Network The network imposes delays to deliver information from the client to server and back from the server to client.

2001/03/13Network Computing Lab.20 Where are the delays?(3) Server A request arrives from the client. The server parses the request, according to the HTTP protocol. The server executes the method requested. The server reads the contents of the file from disk or from its main memory cache and writes it to the network port. Close the connection Service time and waiting time at the various components of the server, such as processor, disk, and network

2001/03/13Network Computing Lab.21 QoS(Quality of Service)(1) Customers want to have good services available at any time. Customers do not care about site failures, traffic jams, network bandwidth, or other indicators of system activities. Besides contents and aesthetics, online users want performance and security.

2001/03/13Network Computing Lab.22 QoS(Quality of Service)(2) Users perceive Web services through performance metrics such as: Response time Availability The percentage of time a site is available during an observation period. Reliability Measuring the occurrence of failures during the processing or services. Predictability Cost

2001/03/13Network Computing Lab.23 QoS(Quality of Service)(3) Compounding the need for high reliability and availability is the dynamic nature of the Web, which internal and external customers rely on for up-to- date business, professional, and personal information.

2001/03/13Network Computing Lab.24 QoS(Quality of Service)(4) Unpredictable For example, in a large trade show company, 250,000 hits a week during normal weeks A million hits a day at the peak of its largest Fall show. Companies must measure traffic, performance, and usage patterns in order to monitor the quality of service of their Web sites. Management has to decide if additional capacity is needed to stay ahead of customer demands.