Web Server Design Week 7 Old Dominion University Department of Computer Science CS 495/595 Spring 2010 Martin Klein 2/24/10

Encodings gzip –extension:.gz –(sometimes seen as x-gzip) compress –extension:.Z –(sometimes seen as x-compress) deflate –extension:.zip identity –no encoding at all chunked –breaks the body into a series of server-chosen “chunks” –optimization for dynamically produced content

Content Encoding vs. Transfer Encoding Client Origin Server GET /bottle1 HTTP/1.1 HTTP/ OK images from: Content-Encoding Transfer-Encoding

Content-Encodings “Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type.” –section 14.11

Content-Encoding Example (Correct) AIHT:~/Desktop/cs595-s06 mln$ telnet 80 Trying Connected to xenon.cs.odu.edu. Escape character is '^]'. HEAD /~mln/pubs/bollenj_adaptive.ps.gz HTTP/1.1 Host: Connection: close HTTP/ OK Date: Mon, 20 Feb :30:25 GMT Server: Apache/ (Unix) ApacheJServ/1.1.2 PHP/4.3.4 Last-Modified: Thu, 25 Jul :58:58 GMT ETag: "1c16-139ea-3d402e52" Accept-Ranges: bytes Content-Length: Connection: close Content-Type: application/postscript Content-Encoding: x-gzip Connection closed by foreign host.

Content-Encoding Example (Incorrect) AIHT:~/Desktop mln$ telnet 80 Trying Connected to xenon.cs.odu.edu. Escape character is '^]'. HEAD /~mln/pubs/bollenj_adaptive.ps.gz HTTP/1.1 Host: Connection: close HTTP/ OK Date: Mon, 26 Feb :06:25 GMT Server: Apache/2.2.0 Last-Modified: Thu, 25 Jul :58:58 GMT ETag: "1c16-139ea-92cab880" Accept-Ranges: bytes Content-Length: Connection: close Content-Type: application/x-gzip Connection closed by foreign host. Wrong, Wrong, Wrong!!!!!!!

Transfer Encodings “Transfer-Encoding is a property of the message, not of the entity, and thus MAY be added or removed by any application along the request/response chain.” –section 4.3

TE Request Header & Transfer-Encoding Response Header Client specifies preferences for transfer encoding in the “TE:” header –section Server marks the encoding used with the “Transfer-Encoding” header –section Both headers use the same encoding values available with “Content-Encoding”, plus the special “chunked” encoding and the “Trailers” keyword

TE & Transfer-Encoding dhcp :~/Desktop/cs595-s06 mln$ telnet 80 Trying Connected to xenon.cs.odu.edu. Escape character is '^]'. GET / HTTP/1.1 TE: gzip;q=1.0, Trailers Host: HTTP/ OK Date: Mon, 27 Feb :52:33 GMT Server: Apache/ (Unix) ApacheJServ/1.1.2 PHP/4.3.4 Transfer-Encoding: chunked Content-Type: text/html 5f6 [more html deleted]

Time / Space Tradeoff Hard to find examples of compression used in transfer encoding – – –idea: for very heavy volume web servers, answering the request quickly is more important than preserving bandwidth Complexity of management seems to be the limiting factor

Chunked Encoding “The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an OPTIONAL trailer containing entity-header fields. This allows dynamically produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.” –sections 3.6.1,

Chunked Encoding Example AIHT:~/Desktop/cs595-s06 mln$ telnet 80 | tee 1-6.out Trying Connected to xenon.cs.odu.edu. Escape character is '^]'. GET /~mln HTTP/1.1 Connection: close Host: Connection closed by foreign host. HTTP/ Moved Permanently Date: Mon, 09 Jan :32:24 GMT Server: Apache/ (Unix) ApacheJServ/1.1.2 PHP/4.3.4 Location: Connection: close Transfer-Encoding: chunked Content-Type: text/html; charset=iso e 301 Moved Permanently Moved Permanently The document has moved here. Apache/ Server at Port 80 0

Chunked Encoding Example 2 AIHT:~/Desktop/cs595-s06 mln$ telnet 80 Trying Connected to xenon.cs.odu.edu. Escape character is '^]'. GET / HTTP/1.1 Host: HTTP/ OK Date: Tue, 21 Feb :54:31 GMT Server: Apache/ (Unix) ApacheJServ/1.1.2 PHP/4.3.4 Transfer-Encoding: chunked Content-Type: text/html 5f6 Department Of Computer Science <LINK title=style href="files/style.css" type=text/css rel=stylesheet> [lots of html deleted] [demo this example to see the various “chunks”] 0

Trailer Response Header The “Trailer” response header lets the client know that additional headers will appear at the end of the chunked response –section –headers can be reconstructed by downstream servers –headers that can never be trailers: Transfer-Encoding Content-Length Trailer

Trailer Example HTTP/ OK Date: Mon, 22 Mar :15:03 GMT Content-Type: text/html Content-Length: 129 Expires: Sat, 27 Mar :12:00 GMT The file you requested is 3,400 bytes long and was last modified: Sat, 20 Mar :12:00 GMT. HTTP/ OK Date: Mon, 22 Mar :15:03 GMT Content-Type: text/html Transfer-Encoding: chunked Trailer: Expires 29 The file you requested is 5 3, bytes long and was last modified: 1d Sat, 20 Mar :12:00 GMT Expires: Sat, 27 Mar :12:00 GMT Examples from: “Expires:” response header covered in section 14.21

Two More Request Headers to Process

User-Agent Request Header User-Agent: –section –“This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. User agents SHOULD include this field with requests.” –examples: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 User-Agent: CS 495/595 Spring 2006 Automated Checker (A3)

Referer Request Header Referer: –section –“The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained (the "referrer", although the header field is misspelled.) The Referer request-header allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard.” –examples: Referer: Referer: /opening-page.php