Chapter 9Application Layer, HTTP

Professor Rick HanUniversity of Colorado at Boulder

rhan@cs.colorado.edu

Prof. Rick Han, University of Colorado at Boulder

Announcements

• Read Sections 9.1 - 9.2, Skip 9.3• HW #4 due April 16• Programming Assignment #3 soon…• Midterm: hand back April 4

• Next, Application Layer

Prof. Rick Han, University of Colorado at Boulder

Recap of Previous Lecture• Domain Name Service

• Translate/resolve a name to an IP address• www.cs.colorado.edu => 128.9.17.42

• Hierarchical name space• Hierarchical name servers

• Root name servers – about a dozen• Then, .edu, .com, .gov, .mil, .org, .net, …• Local name server• Authoritative name server – gives back final IP

address• Recursive vs. iterative queries• Caching

Prof. Rick Han, University of Colorado at Boulder

HyperText Transfer Protocol (HTTP)

• Basis for Web• Application-layer protocol built on top of

TCP• Request-Response type of protocol

• Request: e.g. “GET URL HTTP_version_#”• Response from server• Requests and responses are encoded in text• Stateless: after request and response, no

further state maintained• Cookies maintain session state outside of

HTTP

Prof. Rick Han, University of Colorado at Boulder

HTTP Request• Request headers

• Authorization – authentication info• Acceptable document types/encodings• From – user email• If-Modified-Since return page only if mod after

date

• Referrer – what caused this page to be requested• User-Agent – client software

• Blank line• Body

Prof. Rick Han, University of Colorado at Boulder

HTTP Request Example: GET

GET / HTTP/1.1Accept: */*Accept-Language: en-usAccept-Encoding: gzip, deflateUser-Agent: Mozilla/4.0 (compatible; MSIE

5.5; Windows NT 5.0)Host: www.seshan.orgConnection: Keep-Alive

Prof. Rick Han, University of Colorado at Boulder

HTTP Response• Headers

– Location – for redirection– Server – server software– WWW-Authenticate – request for authentication– Allow – list of methods supported (get, head, etc)– Content-Encoding – E.g x-gzip– Content-Length # bytes in content– Content-Type MIME type– Expires when contents become stale– Last-Modified time contents last mod by servr

• Blank-line• Body

Prof. Rick Han, University of Colorado at Boulder

HTTP Response ExampleHTTP/1.1 200 OKDate: Tue, 27 Mar 2001 03:49:38 GMTServer: Apache/1.3.14 (Unix) (Red-Hat/Linux)

mod_ssl/2.7.1 OpenSSL/0.9.5a DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24

Last-Modified: Mon, 29 Jan 2001 17:54:18 GMTETag: "7a11f-10ed-3a75ae4a"Accept-Ranges: bytesContent-Length: 4333Keep-Alive: timeout=15, max=100Connection: Keep-AliveContent-Type: text/html MIME Type…..

Prof. Rick Han, University of Colorado at Boulder

HTTP 0.9/1.0

• One HTTP 1.0 request/response per TCP connection– Simple to implement

• Disadvantages– Multiple connection setups three-way

handshake each time• Several extra round trips added to transfer• Netscape browser opens up to 4 parallel HTTP 1.0

connections

– Multiple slow starts

Prof. Rick Han, University of Colorado at Boulder

HTTP 1.0 Interaction With TCP

Client

ServerSYN

SYN

SYN

SYN

ACK

ACK

ACK

ACK

ACK

DAT

DAT

DAT

DAT

FIN

ACK

0 RTT

1 RTT

2 RTT

3 RTT

4 RTT

Server reads from disk

FIN

Server reads from disk

Client opens TCP connection

Client sends HTTP request for HTML

Client parses HTMLClient opens TCP connection

Client sends HTTP request for image

Image begins to arrive

Courtesy: Srini Seshan

Prof. Rick Han, University of Colorado at Boulder

More HTTP 1.0 & TCP Interaction Problems

• Lots of extra connections– Increases server state/processing

• Server also forced to keep TIME_WAIT connection state for dead TCP connections– Tends to be an order of magnitude

greater than # of active connections, why?

Prof. Rick Han, University of Colorado at Boulder

HTTP 1.1 Persistent Connection Solution

• Multiplex multiple requests onto one open TCP connection (& multiple responses in reverse direction)– Serialize transfers client makes next

request only after previous response– Reduce slow start latency– Reduce amount of TCP state at both endpoints– Reduce overhead

• HTTP 1.1 adds complexity because multiple requests (and responses) have to be multiplexed and demultiplexed

Prof. Rick Han, University of Colorado at Boulder

HTTP 1.1 Persistent Connection Example

Client

Server

ACK

ACK

DAT

DAT

ACK

0 RTT

1 RTT

2 RTT

Server reads from diskClient sends HTTP request for HTML

Client parses HTMLClient sends HTTP request for image

Image begins to arrive

DAT

Server reads from disk

DAT

Courtesy: Srini Seshan

Prof. Rick Han, University of Colorado at Boulder

Web Caching Proxies

• Place a Web caching proxy in the network between Web client and Web server– Reduces client response time

• HTTP GET only goes as far as intermediate cache, rather than all the way to server

– Reduces network bandwidth usage• HTTP GET doesn’t travel over wide area from

caching proxy to server

– Reduces server load• HTTP GET never reaches server

HTTP CachingProxy

HTTP Web

Server

HTTP Client/

Browser

Prof. Rick Han, University of Colorado at Boulder

Web Proxies• Used for Caching

– Improved response time, etc. from previous slide– Provides a centralized coordination point to share

cached information across all of a company’s client hosts

• Also used for security– Proxy for a company can be the only host that

can access Internet– Administrators makes sure that it is secure

• Used for protocol translation– Translate HTTP 1.0 to/from HTTP 1.1. Enables old

HTTP 1.0 clients to connect to HTTP 1.1 servers, and benefit from HTTP 1.1 performance boosts

Prof. Rick Han, University of Colorado at Boulder

Designing Caching Proxies• How much can/should be cached?

– How large a cache is necessary?– On disk vs. in memory typically on disk– What are the cache hit rates?

• If user behavior is uncorrelated, have to cache a lot of data to improve response time, resulting in small cache hit rate

• If user behavior is correlated, i.e. everyone visits only a few Web sites, then cache less data and still improve response time (high cache hit rate)

Prof. Rick Han, University of Colorado at Boulder

Designing Caching Proxies (2)

• What can be cached?– Cache first-time unknown

documents/objects– Non-cacheable documents

• CGI-scripts• Personalized documents (cookies, etc)• Encrypted data (SSL)

– Document should no longer be cached if updated/expired before reuse

Prof. Rick Han, University of Colorado at Boulder

Designing Caching Proxies (3)• Performance:

– How many TCP connections can the proxy handle?– How to efficiently index into database/cache?

• Early caches used file system to find file• Metadata now kept in memory on most caches

• Prefetching – combine with caching to reduce response time– Proxy parses a Web page and prefetch its

hyperlinked objects before the client asks for them• Example: when a proxy fetches a Web page on behalf of a

client, the proxy will parse and cache the Web page returned by the server, and then prefetch all links before client requests them

– Not widely used due to poor hit rates?