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Final Notes - I II Delay(P.34 a Processing Delay i Time...

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I. Delay (P.34) a. Processing Delay i. Time required to examine a packet’s header and determine where to direct the packet. ii. Typically microseconds or less. b. Queuing Delay i. Time it takes before packet is transmitted from router onto the link. ii. Microseconds or milliseconds, depending on traffic. iii. Example of varying queuing delay 1. First packet arrives at 0, second packet arrives at L/R, and third packet arrives at 2L/R. So the Nth packet will arrive at (N-1)L/R. 2. Total delay given by the sum of all the individual delays. Total delay = (0+1+2+…+ n-1) (L/R) 3. Can rewrite the summation as (n)(n-1) / 2 4. Total Delay = (n-1) L / 2R iv. c. Transmission Delay i. Also called store-and-forward delay ii. Amount of time to transmit all of the packet’s bits onto the link. iii. Microseconds to milliseconds iv. Calculated by Length of packet (in bits) / transmission Rate d. Propagation Delay i. Time it takes to propagate (travel) from to the router after it has been pushed onto the link. ii. Propagation speed is the speed of light or a little less, depends on the physical material of the link. iii. The entire delay is milliseconds. iv. Calculated by Distance between routers A and B / Speed of link II. HTTP (HyperText Transfer Protocol) (P.96) a. Uses TCP as underlying transport protocol. (Provides reliable data transfer) b. Stateless protocol. (Does not maintain information about the client). c. Non-persistent connection sends each request/response over separate TCP connections. d. Persistent connection sends each request/response over the same TCP connection. i. Default format for HTTP ii. Eventually closed after not used for a certain amount of time. e. HTTP Message Format i. Request Message 1. GET /somedir/page.html HTTP/1.1 2. Host: www.someschool.edu 3. Connection: close 4. User-agent: Mozilla/4.0
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5. Accept-language: fr a. 1 is request line, has 3 fields: method, URL, HTTP version i. Methods are GET, POST, HEAD, PUT, DELETE b. 2 specifies the host on which the object resides c. 3 specifies a non-persistent connection. Client wants the server to close the connection after sending object. d. 4 specifies which browser is being used. (Can send different versions of same object to different browsers) e. 5 specifies that the user prefers French, if available else default ii. Response Message 1. HTTP/1.1 200 OK 2. Connection: close 3. Date: Thu, 06 Aug 1998 12:00:15 GMT 4. Server: Apache/1.3.0 (Unix) 5. Last-Modified: Mon, 6 May 1998 … 6. Content-Length: 6821 7. Content-Type: text/html 8. 9. (data data data…) a. 1 is status line, has 3 fields: protocol version, status code, status message i. 200 = OK, 301 = Moved Permanently, 400 = Bad Request, 404 = Not Found, 505 = HTTP Version Not Supported b. 2 tells client that server will disconnect TCP connection c. 3 specifies time and date the response was sent by the server. d. 4 specifies the server and version used to send the response.
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