2- Application Level Protocols HTTP 1.0/1.1/2
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1 2- Application Level Protocols HTTP 1.0/1.1/2
2 HTTP, (HyperText Transfer Protocol) Basis for fetching Web pages request Network CSE 461 University of Washington 2
3 Sir Tim Berners-Lee (1955 ) Inventor of the Web Dominant Internet app since mid 90s He now directs the W3C Developed Web at CERN in 89 Browser, server and first HTTP Popularized via Mosaic ( 93), Netscape First WWW conference in 94 Source: By Paul Clarke, CC-BY-2.0, via Wikimedia CSE 461 University of Washington 3
4 Web Context Page as a set of related HTTP transactions HTTP request HTTP response CSE 461 University of Washington 4
5 Web Protocol Context HTTP is a request/response protocol for fetching Web resources Runs on TCP, typically port 80 HTTPS typically on port 443 Part of browser/server app browser HTTP TCP IP request response server HTTP TCP IP
6 Static vs Dynamic Web pages Static web page is a file contents, e.g., image Dynamic web page is the result of program execution Javascript on client, PHP on server, or both CSE 461 University of Washington 6
7 HTTP Protocol Originally a simple protocol, with many options added over time HTTP transports typed data TCP transports bytes HTTP is a request-response protocol Client sends request message, server sends response message HTTP messages have a header and a payload section Header is encoded as text HTTP is carried over a TCP CSE 461 University of Washington 7
8 Fetching a Web Page From a Browser Browser starts with the page URL: Protocol Browser steps: Server Page on server Resolve the server name to an IP address (DNS) Set up a TCP connection to the server Send HTTP request for the page Wait for and then read HTTP response (Assuming no errors) Process response data and render page Clean up any idle TCP connections CSE 461 University of Washington 8
9 HTTP Message Format Special command line\r\n Tag: value\r\n Tag: value\r\n... Tag: value\r\n \r\n <payload> Header is encoded as text Header is a sequence of lines Each line ends with \r\n Header ends with \r\n\r\n Payload length is given by either: Content-length tag in header Payload is encoded in a format that uses a sentinel (special value that marks the end)
10 Try It Yourself: View HTTP Request $ nc l 8080 Opens a TCP socket on port 8080 and waits for an incoming connection Point a browser running on the same machine to The output of the nc window is the HTTP request sent by the browser
11 Example HTTP Request $ nc -l 8080 GET /first/second/third.html HTTP/1.1 Host: localhost:8080 Connection: keep-alive User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/ (KHTML, like Gecko) Chrome/ Safari/ Upgrade-Insecure-Requests: 1 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/a png,*/*;q=0.8 DNT: 1 Accept-Encoding: gzip, deflate, br Accept-Language: en-us,en;q=0.8 What the browser sent
12 Try It Yourself: HTTP Response request response $ nc 80 GET / HTTP/1.0 Host: HTTP/ OK Date: Tue, 10 Oct :58:14 GMT Server: Apache/ (Unix) mod_ssl/ OpenSSL/1.0.1e-fips PHP/ mod_pubcookie/3.3.4a mod_uwa/3.2.1 Last-Modified: Mon, 09 Oct :45:12 GMT ETag: "180cd3-c459-55b241ae94a00" Accept-Ranges: bytes Content-Length: Vary: Accept-Encoding,User-Agent Connection: close Content-Type: text/html <!DOCTYPE html><html class="no-js"><head><meta content="ie=edge" http-equiv="x-ua- Compatible"/><title> UW Homepage </title><meta charset="utf-8"/><meta content="university of Washington" name="d... <50265 bytes of data in all>
13 Try It Yourself: HTTP Response 2 $ nc uw.edu 80 GET / HTTP/1.0 Host: uw.edu HTTP/ Moved Permanently Date: Tue, 10 Oct :04:20 GMT Server: Apache/ (Unix) mod_ssl/ OpenSSL/1.0.1e-fips PHP/ mod_pubcookie/3.3.4a mod_uwa/3.2.1 Location: Vary: Accept-Encoding Content-Length: 385 Connection: close Content-Type: text/html; charset=iso <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>301 Moved Permanently</title> </head><body> <h1>moved Permanently</h1> <p>the document has moved <a href=" <hr> <address>apache/ (Unix) mod_ssl/ OpenSSL/1.0.1e-fips PHP/ mod_pubcookie/3.3.4a mod_uwa/3.2.1 Server at uw.edu Port 80</address> </body></html>
14 HTTP Protocol Commands used in the request Fetch page Upload data Method Description GET Read a Web page HEAD Read a Web page's header POST Append to a Web page PUT Store a Web page DELETE Remove the Web page TRACE Echo the incoming request CONNECT Connect through a proxy OPTIONS Query options for a page Basically defunct
15 HTTP Protocol Result codes returned with the response Code Meaning Examples Yes! 1xx Information 100 = server agrees to handle client's request 2xx Success 200 = request succeeded; 204 = no content present 3xx Redirection 301 = page moved; 304 = cached page still valid 4xx Client error 403 = forbidden page; 404 = page not found 5xx Server error 500 = internal server error; 503 = try again later CSE 461 University of Washington 15
16 PERFORMANCE
17 Performance Measure: PLT (Page Load Time) PLT is the key measure of web performance From click until user sees page PLT depends on many factors Structure of page/content HTTP (and TCP!) protocol Network RTT and bandwidth CSE 461 University of Washington 17
18 v.
19 Page Load Time Impact From How One Second Could Cost Amazon $1.6 Billion In Sales, March 15,
20 HTTP 1.0 (1996) HTTP/1.0 uses one TCP connection to fetch one web resource Made HTTP very easy to build But gave fairly poor PLT Framing? Length? Sentinel? CSE 461 University of Washington 20
21 HTTP 1.0 Many reasons why PLT is larger than necessary Sequential request/responses, even when to different servers Multiple TCP connection setups to the same server CSE 461 University of Washington 21
22 Parallel Connections One simple way to reduce PLT Browser runs multiple (8, say) HTTP instances in parallel Server is unchanged; already handled concurrent requests for many clients How does this help? Single HTTP wasn t using network much So parallel connections aren t slowed much Pulls in completion time of last fetch CSE 461 University of Washington 22
23 HTTP 1.1 (1997) Persistent Connections Parallel connections compete with each other for network resources 1 parallel client 8 sequential clients? Exacerbates network bursts, and loss Persistent connection alternative Make 1 TCP connection to 1 server Use it for multiple HTTP requests CSE 461 University of Washington 23
24 Persistent Connections One request per connection Sequential requests per connection Pipelined requests per connection CSE 461 University of Washington 24
25 Persistent Connections: Framing How are requests and responses framed? Enforce use of content-length header field? What if content is dynamically generated? If not that, then what?
26 Persistent Connections Widely used as part of HTTP/1.1 Supports optional pipelining (?) PLT benefits depending on page structure, but easy on network How can we reduce PLT even more? CSE 461 University of Washington 26
27 HTTP 2 (2015) HTTP 2 preserves the semantics of HTTP 1.0 / 1.1 Client still says GET and server still responds OK However, the requests are encoded differently (compressed) transferred differently (streams and frames) IETF RFC 7540, May 2015 Successor to Google s SPDY protocol
28 HTTP 2 browser HTTP 1.1 TCP IP request server HTTP 1.1 response HTTP 2 HTTP 2 TCP IP This is the idea of how HTTP 2 fits in. A particular implementation might well combine HTTP 1.1 and HTTP 2
29 HTTP 2 Main Features Allows real pipelining of requests on persistent connections We have to name each request explicitly so that we can match responses to requests Why can t we use ordering? Headers have gotten big compress headers Servers can supply data that wasn t requested server push Clients can advertise priorities among their requests Note: real pipelining allows the server to apply its own idea of priority, since it doesn t have to reply in order
30 HTTP 2 Streams and Frames A connection is a TCP connection between client and server long lived, just like HTTP 1.1 A stream is an ordered, bidirectional flow of information between client and server There is one connection between a client and server There is (roughly) one stream per HTTP request
31 HTTP 2 Streams & Frames
32 Streams Each stream has a unique ID Successive stream IDs must be increasing When run out of stream IDs, have to create a new connection Race condition if both ends try to create stream IDs Solution: client uses odd numbers, server uses evens A stream is created by sending a frame with a new stream ID
33 Frame Types
34 Simples encoding of an HTTP request Send a HEADER frame followed by zero or more CONTINUATION frames Set END_HEADERS flag on last one Send DATA frames for request data, if needed Set END_STREAM flag on last Response is the same, in reverse
35 Frame Header Length: length of payload header is always 9 bytes Type: frame type Flags: depends on type R: reserved; must be unset when sending and ignored when receiving Stream ID: 0x0 is reserved for frames associated with the connection (not an individual stream)
36 HEADER frame Padding is for security obfuscate lengths Stream dependency make this stream a child of named stream If server can t make progress on parent, assign resources proportional to weights to children Header block fragment take the HTTP 1.1 header and compress it, then send it in chunks (if necessary) Frame header flags: END_HEADERS and END_STREAM
37 DATA Frame
38 PRIORITY Frame E: exclusive bit inserts this stream as only child of parent stream, moving existing children to be children of this stream
39 RST_STREAM Frame Ends a stream Why is this useful? Also have END_STREAM flag bit...
40 GOAWAY Frame Closes connection Provides largest id of any stream that the server may have acted on Why?
41 PUSH_PROMISE Frame Allows server to send something not yet asked for E.g., a style sheet or a javascript program or an embedded image Acts like a HEADERS frame Can have CONTINUATIONs following for more header
42 PING Frame Is other end still there? Responds with PING with ACK flag bit set Measure latency to other end PING frames have highest priority...
43 WINDOW_UPDATE Frame TCP does flow control on entire connection but need flow control on a per stream basis as well
44 Getting There From Here HTTP 2 is supposed to be an optimized transport of HTTP requests Needs to be backward compatible with HTTP 1/1.1 Main problem: How to tell if client and server can both speak HTTP 2? Client could try HTTP 2 and then revert to 1.1 Client could start with HTTP 1.1 then upgrade to 2
45 Dynamically Upgrading to HTTP 2 Client: GET / HTTP/1.1 Host: server.example.com Connection: Upgrade, HTTP2-Settings Upgrade: h2c HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>
46 Server Refuses Upgrade Server may simply not recognize the upgrade request if it isn t HTTP 2 capable HTTP/ OK Content-Length: 243 Content-Type: text/html...
47 Server Wants to Upgrade HTTP/ Switching Protocols Connection: Upgrade Upgrade: h2c [ HTTP/2 connection...
48 HTTP 2 Wrap-up
49 Web Caching Users often revisit web pages Big win from reusing local copy! This is caching Local copies Cache Network Server Key question: When is it OK to reuse local copy? CSE 461 University of Washington 49
50 Web Caching (2) Locally determine copy is still valid Based on expiry information such as Expires header from server Or use a heuristic to guess (cacheable, freshly valid, not modified recently) Content is then available right away Cache Network Server CSE 461 University of Washington 50
51 Web Caching (3) Revalidate copy with remote server Based on timestamp of copy such as Last-Modified header from server Or based on content of copy such as Etag server header Content is available after 1 RTT Cache Network Server CSE 461 University of Washington 51
52 Web Caching (4) Putting the pieces together: CSE 461 University of Washington 52
53 Web Proxies Place intermediary between pool of clients and external web servers Benefits for clients include caching and security checking Organizational access policies too! Proxy caching Clients benefit from larger, shared cache Benefits limited by secure / dynamic content, as well as long tail CSE 461 University of Washington 53
54 Web Proxies Clients contact proxy; proxy contacts server Cache Near client Far from client CSE 461 University of Washington 54
55 Content Delivery Networks As the web took off in the 90s, traffic volumes grew and grew. This: Idea: 1. Concentrated load on popular servers 2. Led to congested networks and need to provision more bandwidth 3. Gave a poor user experience Place popular content near clients Helps with all three issues above CSE 461 University of Washington 55
56 Before CDNs Sending content from the source to 4 users takes 4 x 3 = 12 network hops in the example User... Server User CSE 461 University of Washington 56
57 After CDNs Sending content via replicas takes only = 6 network hops User Server Replica... User CSE 461 University of Washington 57
58 After CDNs Benefits assuming popular content: Reduces server, network load Improves user experience (PLT) User Source Replica... User CSE 461 University of Washington 58
59 Popularity of Content Zipf s Law: few popular items, many unpopular ones; both matter George Zipf ( ) Zipf popularity (kth item is ~1/k) Rank Source: Wikipedia CSE 461 University of Washington 59
60 How to place content near clients? Use browser and proxy caches Helps, but limited to one client or clients in one organization Want to place replicas across the Internet for use by all nearby clients Done by clever use of DNS CSE 461 University of Washington 60
61 Content Delivery Network CSE 461 University of Washington 61
62 Content Delivery Network (2) DNS gives different answers to clients Tell each client the nearest replica (map client IP) CSE 461 University of Washington 62
63 Business Model Clever model pioneered by Akamai Placing site replica at an ISP is win-win Improves site experience and reduces ISP bandwidth usage Consum er site ISP Replica User... User CSE 461 University of Washington 63
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