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    Add as FriendWeb Programming Spring 2004

    by: Rogers

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    1 : 1 CSC 551: Web ProgrammingSpring 2004 Internet & World Wide Web Protocols network layers TCP/IP domain name system, IP addresses, routing protocols HTTP GET/POST, headers, caching, cookies
    2 : 2 OSI 7-layer model in the 70's, computer networks were ad hoc, vendor-specific Open Systems Interconnection model developed by the ISO in 1984 provides an abstract model of networking divides the tasks involved in moving info between networked computers into 7 task groups each task group is assigned a layer Each layer is reasonably self-contained, so can be implemented independently changes/updates to a layer need not effect other layers
    3 : 3 Protocol layers Application layer describes how applications will communicate e.g., HTTP, FTP, Telnet, SMTP Presentation layer describes the form of data being transferred & ensures that it will be readable by receiver e.g., floating point formats, data compression, encryption Session layer describes the organization of large data sequences & manages communication session e.g., coordinates requests/responses Transport layer describes the quality and nature of data delivery e.g., how retransmissions are used to ensure delivery Network layer describes how a series of exchanges over various data links can deliver data across a network e.g., addressing and routing Data Link layer describes the logical organization of data bits transmitted on a particular medium e.g., frame sequencing, error notification Physical layer: describes the physical & electrical properties of the communications media e.g., voltage levels, data rates, max distances
    4 : 4 Layer protocols across the network, processes at the same level can (seemingly) communicate e.g., Web server & browser run at the application level, communicate via HTTP in reality, actual communication takes place at the physical layer upper layers can only communicate with those above and below at the source, as data is passed down the layers: the protocol for each layer adds control information to the data at the destination, as data is passed up the layers: the protocol for each layer strips and analyzes the control information for that layer
    5 : 5 Internet protocol suite Network layer: Internet Protocol (IP) provides generalized packet network interface handles routing through the Internet connectionless and unreliable Transport layer: Transmission Control Protocol (TCP) provides a virtual circuit over which two processes can communicate supplies logic to give reliable, connection-oriented session FTP (file transfer) and HTTP are built on top of TCP
    6 : 6 Internet Protocol (IP) the IP protocol adds packet routing info (20 bytes) Time-to-live (TTL): indicates number of hops packet is allowed to take before being discarded Source address: IP address of host sending the packet Destination address: IP address of host to receive the packet Options: options such as sender-specified routing or security
    7 : 7 IP addresses IP addresses are 32 bits long 10010011 10000110 00000010 00010100 ? written as a dotted sequence divided into 5 classes class A: start with 0, then 7-bit code 224 = 16,777,216 hosts in subnetwork class B: start with 10, then 14-bit code 216 = 65,536 hosts in subnetwork class C: start with 110, then 21-bit code 28 = 256 hosts in subnetwork class D: start with 1110 used for multicasting class E: start with 11110 reserved for future use IPv6 extends address size to 128 bits extensions support authentication, data integrity, confidentiality
    8 : 8 Domain name system rarely do applications deal directly with IP addresses a hierarchical system of domain names can be used instead top level domains: edu, com, gov, org, net, … commonly: hostname.subdomain.domain (possibly many subdomains) e.g., a domain name server (DNS) is a machine that keeps a table of names and corresponding IP addresses there are 13 root servers in the world (mirrored) when an application specifies a host name, go to local domain name server and try lookup if not stored there, then local DNS requests address from a root server root server determines appropriate name server & forwards request
    9 : 9 Routing protocols routers (or gateways) are special purpose machines on the Internet that determine the path for packets from source to destination when a router receives a packet, inspects the destination address looks up that address in a routing table based on the contents of the table, forwards the packet to another router (or to its final destination if possible) Routing Information Protocol (RIP) describes how routers exchange routing table information uses hop-count as the metric of a path's cost Open Shortest Path First Protocol (OSPF) more robust, scalable protocol than RIP doesn't exchange entire tables, only updates changed links Internet Control Message Protocol (ICMP) adjunct to IP, notifies sender (or other router) of abnormal events e.g., unreachable host, net congestion
    10 : 10 Transmission Control Protocol (TCP) the TCP protocol adds info for providing a virtual circuit, including message formatting, circuit management, flow control, error correction Source & destination ports a port is analogous to a mailbox Sequence number: identifies its place in sequence (byte # in overall message) Acknowledgement number: specifies the next byte # in sequence, if destination does not receive it in X amount of time, will notify sender Control flags: used to set up connection (3-way handshake: request, ack, ack), mark as urgent, terminate connection, …
    11 : 11 User Datagram Protocol (UDP) UDP protocol is a simple, connectionless alternative to TCP used in many Internet applications that require only simple query/response e.g., time Source & destination ports same as in TCP Length: number of bytes in the packet Checksum: rudimentary error detection
    12 : 12 World Wide Web the Web is the world’s largest client/server system communication occurs via message passing within browser, select URL of desired page browser requests page from server server responds with message containing type of page (HTML, gif, pdf, zip, …) page contents browser uses type info to correctly display page if page contains other items (images, applets, …), browser must request each separately
    13 : 13 Hypertext Transfer Protocol (HTTP): application-level protocol for distributed, collaborative, hypermedia information systems generic, stateless, object-oriented can be used for many tasks, such as name servers & distributed object management systems underlying language of the Web HTTP HTTP/1.0 allowed only connectionless message passing each request/response required a new connection to download a page with images required multiple connections can overload the server, require lots of overhead HTTP/1.1 provides persistent connection by default once client & server connect, remains open until told to close it (or timeout) reduces number of connections, saves overhead client can send multiple requests without waiting for responses e.g., can request all images in a page at once
    14 : 14 GET request most URL’s have the form: protocol://serverName URI e.g., to retrieve a document via HTTP from the server, issue a GET request GET URI HTTP/1.1 Host: serverName Web server only knows the contents of the GET request message automatically generated by browser when you select a URL could also come from a link checker, a search engine robot, … can come directly from a telnet connection using port 80
    15 : 15 GET example bluejay> telnet 80 Trying... Connected to Escape character is '^]'. GET /~davereed/index.html HTTP/1.1 Host: HTTP/1.1 200 OK Date: Thu, 22 Jan 2004 18:35:24 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 Last-Modified: Tue, 13 Jan 2004 17:38:00 GMT ETag: "155005-1a4-40042cf8" Accept-Ranges: bytes Content-Length: 420 Content-Type: text/html Dave Reed's Home Page server response has assorted header information, followed by the page
    16 : 16 Response header fields the first line of the server’s response contains a status code 200 OK request was processed successfully 301 Moved permanently document has been moved 304 Not modified if cached version is up-to-date 400 Bad request syntax error in client’s request 403 Forbidden client is not allowed access (e.g., protected) 404 Not found file could not be found 500 Internal server error server failed 503 Service unavailable server is overloaded
    17 : 17 Other response header fields in addition to the status code, the server’s response may include Date response time (in GMT) Server identification info on the server Last-modified time document was last changed (in GMT) Content-length size of document, in bytes Content-type file format (e.g., html, gif, pdf) Expires prevents browser from caching beyond date
    18 : 18 File not found bluejay> telnet 80 Trying... Connected to Escape character is '^]'. GET /~davereed/foo.html HTTP/1.1 Host: HTTP/1.1 404 Not Found Date: Thu, 22 Jan 2004 18:37:29 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 Content-Type: text/html Requested Page Not Found!

    Requested Page Not Found!

    The URL you requested was not found on this server. (Error 404)

    . . . if file not found, response includes 404 status code and generic error page

    19 : 19 Directories as URI’s bluejay> telnet 80 Trying... Connected to Escape character is '^]'. GET /~davereed/ HTTP/1.1 Host: if a directory is specified, will look for a file named index.html HTTP/1.1 200 OK Date: Thu, 22 Jan 2004 18:41:17 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 Last-Modified: Tue, 13 Jan 2004 17:38:00 GMT ETag: "155005-1a4-40042cf8" Accept-Ranges: bytes Content-Length: 420 Content-Type: text/html Dave Reed's Home Page
    20 : 20 Redirection bluejay> telnet 80 Trying... Connected to Escape character is '^]'. GET /~davereed HTTP/1.1 Host: HTTP/1.1 301 Moved Permanently Date: Thu, 22 Jan 2004 18:42:53 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 Location: Transfer-Encoding: chunked Content-Type: text/html; charset=iso-8859-1 166 301 Moved Permanently

    Moved Permanently

    The document has moved here.

    Apache/1.3.27 Server at Port 80
    0 since URI is missing / at end, browser must do 2 requests
    21 : 21 Request header fields the client can specify additional information in the request User-Agent specifies the browser version Referer tells server where the user came from useful for logging and customer tracking From contains email address of user generally not used for privacy reasons Authorization can send username & password used with documents that require authorization If-Modified-Since only send document if newer than specified date used for caching
    22 : 22 Conditional GET bluejay> telnet 80 Trying... Connected to Escape character is '^]'. GET /~davereed/ HTTP/1.1 Host: If-Modified-Since: Wed, 21 Jan 2004 14:00:00 GMT HTTP/1.1 304 Not Modified Date: Thu, 22 Jan 2004 18:45:25 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 ETag: "155005-1a4-40042cf8" since the document has not been modified since the specified date, the page is not sent by the server (status code 304)
    23 : 23 Other request methods HEAD similar to GET, but requests header information only useful for checking to see if a document exists, how recent POST similar to GET, but encodes inputs differently useful for submitting form contents to a CGI program PUT upload a document to the server new in HTTP/1.1 DELETE delete a document from the server new in HTTP/1.1
    24 : 24 HEAD example bluejay> telnet 80 Trying... Connected to Escape character is '^]'. HEAD /~davereed/index.html HTTP/1.1 Host: HTTP/1.1 200 OK Date: Thu, 22 Jan 2004 18:47:16 GMT Server: Apache/1.3.27 (Unix) PHP/4.1.2 Last-Modified: Tue, 13 Jan 2004 17:38:00 GMT ETag: "155005-1a4-40042cf8" Accept-Ranges: bytes Content-Length: 420 Content-Type: text/html server does not send the page, only the header information
    25 : 25 Caching browsers cache pages to save downloading maintain temporary storage (cache) for recent pages when a page is requested, check to see if already in cache if not in the cache, issue GET request when response message arrives, display page and store in cache (along with header info) if already stored in the cache, send GET request with If-Modified-Since header set to the data of the cached page when response message arrives, if status code 200, then display and store in cache if status code 304, then display cached version instead
    26 : 26 Cookies Netscape’s solution: cookies a cookie is a collection of information about the user server can download a cookie to the client’s machine using the “Set-cookie” header in a response Set-cookie: CUSTOMER=Dave_Reed; PATH=/; EXPIRES=Thursday, 29-Jan-04 12:00:00 when user returns to URL on the specified path, the browser returns the cookie data as part of its request Cookie: CUSTOMER=Dave_Reed HTTP message passing is transaction-based, stateless many e-commerce apps require persistent memory of customer interactions e.g., remembers your name, credit card, past purchases, interests

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