1. Hypertext Transfer Protocol (HTTP)
Paul Amer
CISC 856: TCP/IP and Upper Layer Protocols
Fall 2003

2. HTTP Background Created by Tim Berners-Lee at CERN
– physicists, not computer scientists
– to share data from physics experiments
– because ftp was “too heavy”
Standardized and much expanded by IETF
Aerial View of CERN

3. Basic HTTP Protocol HTTP HTTP TCP TCP IP IP Link Link Physical
Goal: transfer data
Stateless
Request-Response Protocol

4. Terminology: URL vs URN ?
URN : Uniform Resource Name
e.g., a book’s ISBN
URN does not tell where to find a resource
URL : Uniform Resource Locator
e.g.,
URL tells where a resource is

5. HTTP Version HTTP/0.9 HTTP/1.0 HTTP/1.1
HTTP/<major>.<minor>
HTTP/0.9
HTTP/1.0
HTTP/1.1

6. Overview of a browser HTTP request TCP connection HTTP response
URL
User-Agent (browser/client)
Origin Server
DNS Server
DNS query
DNS response
HTTP request
TCP connection
HTTP response
optional TCP connections

7. HTTP/0.9 – simple ftp -- Client says “GET /index.html”
Client/Server, request/response:
-- Client says “GET /index.html”
-- Server returns file named index.html
Implementable in ~40 lines of C
No support for
-- variant kinds of text (languages)
-- variant kinds of pictures (gif, jpeg, png)
-- object versioning and caching
-- error codes
Simple, But Enough to Start a Revolution

8. Request Message 3 request methods: A-PDU GET, HEAD, POST request line
headers
blank line
body
3 request methods:
GET, HEAD, POST
GET /pub/index.html HTTP/1.0
Date: Wed, 20 Mar :00:02 GMT
Pragma: no-cache
From:
User-Agent: Mozilla/4.03
GET: Retrieves resource indicated by URI
HEAD: Retrieves ONLY metadata indicated by URI
POST: Pushes the resource in place of the URI (or creates a new one) or inputs the entity (data part) to the program(ex: CGI script) pointed by the URI

9. Request Methods GET HEAD POST PATCH LINK UNLINK PUT DELETE OPTION
TRACE
CONNECT
Methods present in HTTP/1.0 & HTTP/1.1
New Methods added in HTTP/1.1

10. Response Message status line headers blank line body HTTP/1.1 200 OK
Date: Tue, 08 Oct :31:35 GMT
Server: Apache/ tomcat/1.0
Last-Modified: 7Oct :40:01 GMT
ETag: "20f-6c4b-3da21b51"
Accept-Ranges: bytes
Content-Length: 27723
Keep-Alive: timeout=5, max=300
Connection: Keep-Alive
Content-Type: text/html
Why a human readeable phrase?
To make extensibility easier. You can add new status codes without waiting for a protocol standard

11. Status Codes Classes: 200 OK 201 Created 202 Accepted 204 No Content
301 Moved Permanently
302 Moved Temporarily
304 Not Modified
400 Bad Request
401 Unauthorized
403 Forbidden
404 Not Found
500 Internal Server Error
501 Not Implemented
502 Bad Gateway
503 Service Unavailable
Classes:
1xx: Informational - not used, reserved for future
2xx: Success - action was successfully received, understood, and accepted
3xx: Redirection - further action needed to complete request
4xx: Client Error - request contains bad syntax or cannot be fulfilled
5xx: Server Error - server failed to fulfill an apparently valid request
404, 403, 200, 304

12. Headers Request Line A Blank Line Body Entity Headers Request Headers
General Headers
Status Line
A Blank Line
Body
Entity Headers
Response Headers
General Headers

13. Headers (cont’d) General Headers Request Headers
Date
Pragma
Cache Control
Connection
Trailer
Transfer-Encoding
Upgrade
Via
Warning
Authorization
From
If-Modified-Since Referer
User-Agent
Accept
Accept-Charset
Accept-Encoding
Language
Expect
Host
If-Match
If-None-Match
If-Range
If-Unmodified-Since
Max-Forwards
Proxy-Authorization
Range TE
Headers present in HTTP/1.0 & HTTP/1.1
New Headers added in HTTP/1.1

14. Headers (cont’d) Entity Headers Response Headers
Allow
Content-Encoding
Content-Length
Content-Type
Expires
Last-Modified
extension-header
Content-Language
Content-Location
Content-MD5
Content-Range
Location
Server
WWW-Authenticate
Accept-Ranges
Age
ETag
Proxy-Authenticate
Retry-After
Vary
Headers present in HTTP/1.0 & HTTP/1.1
New Headers added in HTTP/1.1

15. Performance Issues

16. FTP vs. HTTP

17. FTP vs. HTTP (cont’d)

18. HTTP/1.0 Nonpersistent Connections
Client
Server
SYN
SYN-ACK
3-way handshake
ACK
GET URI HTTP/1.0
URL
ACK OK
ACK
ACK
DATA
web page
web page transferred
client parses HTML web page
ACK
Nonpersistent Connection (short-lived connection)
-- use a new TCP connection for each request;
-- seldom get past the “slow-start” region.
-- fail to maximize their use of the available bandwidth
FIN
connection close
ACK
FIN
ACK

19. HTTP/1.0 Nonpersistent (cont’d)
SYN
SYN-ACK
ACK
Client
Server
GET URI HTTP/1.0 OK
FIN
image 1 transfer
connect close
3-Way Handshake
URL
DATA
web page
Client parses HTML web page
HTML file gets parsed GET Image 1 will be issued
SYN
SYN-ACK
ACK
GET IMAGE1 HTTP/1.0 OK
FIN
image 2 transfer
connect close
3-Way Handshake
URL
DATA
image1

20. Nonpersistent with pipelining (a.k.a. Parallel Connections Hack)
SYN
SYN-ACK
ACK
Client
Server
GET URI HTTP/1.0 OK
FIN
DATA Transfer
CONN. Close
3-Way Handshake
URL
DATA
Client initiates new TCP connections for each embedded object after parsing the HTML file
SYN
SYN-ACK
ACK
Client
Server
GET URI HTTP/1.0 OK
FIN
DATA Transfer
CONN. Close
3-Way Handshake
URL
DATA
SYN
SYN-ACK
ACK
Client
Server
GET URI HTTP/1.0 OK
FIN
DATA Transfer
CONN. Close
3-Way Handshake
URL
DATA
Increases network congestion, burstiness
Is not fair for well-behaved connections

21. Nonpersistent with pipelining (a. k. a
Nonpersistent with pipelining (a.k.a. Parallel Connections Hack) (cont’d)
RTTs 00 01
port 1114 02 03 04 05 06
1118
1115
1116
1117 07 08 09
1120 10
1121
1119 11 12

22. HTTP Delay Estimation Non Persistent Non Persistent with Pipelining
assume web page with 2 images
Non Persistent
Non Persistent with Pipelining
Client
Server
Client
Server
Time Delay in RTTs = 4
Delay Due to Connection Request/Handshake
Delay Due to HTML Page Request
Time Delay in RTTs = 6
Delay Due to Object Request

23. Potential HTTP 1.0 Inefficiencies
v1.0 fetches single URL per TCP connection
Mean size of responses only a few thousand bytes  inefficient use of available network bandwidth
Server resources wasted
User perceived latency is high

24. HTTP/1.1 Default: Persistent Connections
SYN
SYN-ACK
ACK
Client
Server
3-Way Handshake
GET URI HTTP/1.0
OK-ACK
ACK
DATA
DATA Transfer
URL
GET image 3
GET URI HTTP/1.0
OK-ACK
ACK
DATA
DATA Transfer
URL
Client parses HTML GET image 1
Conn. timeout
GET URI HTTP/1.0
OK-ACK
ACK
DATA
DATA Transfer
URL
FIN
CONN. Close
ACK
GET image 2
GET URI HTTP/1.0
OK-ACK
ACK
DATA
DATA Transfer
URL
Either client or server can close the connection

25. Why Persistent Connections?
CPU time is saved in routers and hosts
Memory used for TCP PCB can be saved in hosts.
Reduced Network Congestion
Reduced perceived latency on subsequent requests
HTTP can evolve more gracefully

26. Persistent with pipelining
SYN
SYN-ACK
ACK
Client
Server
3-Way Handshake
GET Image 1
OK-ACK
URL Image 1
GET Image 2
GET URI HTTP/1.0
OK-ACK
ACK
DATA
DATA Transfer
URL
OK-ACK
URL Image 2
GET Image 3
OK-ACK
URL Image 3
ACK
DATA Transfer
Image 1
DATA
Client parses HTML GET image 1
ACK
DATA
Image 2
DATA Transfer

27. Persistent with pipelining (cont’d)
ACK
DATA Transfer
Image 3
DATA
Conn. timeout
FIN
CONN. Close
ACK
Reduces user perceived latency even more than persistent connections
Encouraged, but not default

28. HTTP Delay Estimation (cont’d)
Persistent w/o pipelining
Persistent with pipelining
Client
Server
Client
Server
Time Delay in RTTs = 3
Time Delay in RTTs = 4
Delay Due to Connection Request/Handshake
Delay Due to HTML Page Request
Delay Due to Object Request

29. Questions ? References: -- Jeffrey C. Mogul (DEC)
-- John Heidemann (USC)
-- Balachander Krishnamurthy (AT&T)
-- James F. Kurose/Keith W. Ross
-- W. Richard Stevens
-- Behrouz A. Forouzan
-- Ali Yilmaz Kumcu (Univ of Delaware)

30. Why HTTP/1.1 ? v1.0 fetches single URL per TCP connection
Mean size of responses only a few thousand bytes
TCP Congestion control not used due to short transfers
Server resources wasted
User perceived latency is high
Naïve caching
Goal: Making HTTP a good Internet citizen, while improving performance for both clients and servers

31. What is different in HTTP/1.1 ?
Over 40 header fields as compared to 16 in v1.0
Host header
Reliable caching (semantically transparent)
Use of Age header for cache control
If-Modified-Since, If-Unmodified-Since
Range requests (Range, If-Range, Content-Range)
A cache behaves in a “semantically transparent” manner, with respect to a particular response, when its use affects neither the requesting client nor the origin server, except to improve performance. When a cache is semantically transparent, the client receives exactly the same response that it would have received had its request been handled directly by the origin server.

32. What is different in HTTP/1.1 ? (cont’d)
Persistent Connections
Default behavior for HTTP/1.1
Server can indicate connection will be closed by:
Connection: close
Request/responses can be pipelined
Major performance gain for users, and major goodness to the network
Chunked Encoding
Hop-by-Hop behavior
New Methods
Trace, Put, Delete, Options, Upgrade
Trace, Used in concert with Via: and Max-Forwards: for debugging
Some entities do not have known length (e.g. those generated by scripts)
Chunked Encoding allows transmission of entities where the length is not known in advance
New header
Upgrade:
Supports protocol switching on open connection
Content-Transfer-Encoding: Hop-by-hop compression of entities

33. HTTP on top of TCP Different Connection Types
HTTP/1.0 style connections (serial)
Extended HTTP/1.0 style (by Netscape)
open 4 parallel connections for embedded objects
Persistent Connections
HTTP/1.1 default
Persistent with pipelining

34. Design Issues for P-HTTP
Effects on Reliability
Interaction with current proxy-servers
Connection Lifetimes
Server Resource Utilization
Server Congestion control
Network Resources
Users Perceived Performance (UPP)

35. Quantifying TCP connection overhead
Throughput (bits/second)
Connection Length (bytes)
Figure 3-2: Throughput vs. connection length, RTT = 70 msec

36. Experimental Results Network Latency (seconds)
(NP HTTP/1.0)
Network Latency (seconds)
Number of in lined images
Figure 6-1: Latencies for a remote server, image size = 2544 bytes

37. Experimental Results (contd..)
(NP HTTP/1.0)
Network Latency (seconds)
Number of in lined images
Figure 6-2: Latencies for a remote server, image size = bytes

38. Persistent Connection and Pipelining
Differences between HTTP/1.0 and HTTP/1.1
Persistent Connection and Pipelining
Support for Semantically Transparent Caching
Range Requests
Chunked Encoding
Expect/Continue
Host Header

39. Effects of changes in HTTP Protocol
HTTP 1.1 Feature
Implication
Persistent Connection
Lowers Number of Connection Setups
Pipelining
Shortens Inter arrival of requests
Expires
Lowers number of validations
Entity Tags
Lowers Frequency of Validations
Max-Age, Max-Stale etc
Changes Frequency of Validations
Range Request
Lowers bytes transferred
Chunked Encoding
Lowers User perceived Latency
Expect/Continue
Lowers error response/bandwidth
Host Header
Reduces Proliferation of IP addresses

40. What’s Wrong with HTTP/1.1 ?
HTTP’s Existing Data Type Model
Resources
Entities and Entity Tags
Problems with Current Model
How to specify HTTP Caching
Consistent Handling of Partial Results
Categorization of Headers

41. Food For Thought !!! Network What’s the Bottleneck CPU
Hard Disk/Memory

42. Client
Server
SYN
SYN-ACK
3-Way Handshake
ACK
GET URI HTTP/1.0 OK
URL
ACK
ACK
ACK
DATA
DATA
DATA Transfer
ACK
Client parses HTML
FIN
CONN. Close
ACK

43. TCP-PDUs and round-trip times for HTTP/1.0
Client
Server
0 RTT
SYN
Client opens TCP connection
SYN-ACK
1 RTT
ACK
Client sends HTTP GET request for /index.html
DAT
Server reads from Disk
ACK
DAT
2 RTT
FIN
ACK
Client parses /index.html
ACK
FIN
Client Opens TCP Connection
SYN
ACK
3 RTT
SYN-ACK
ACK
Client Sends HTTP Request for image
DAT
Server Reads from Disk
ACK
DAT
4 RTT
Image Begins to Arrive

44. TCP-PDUs for HTTP/1.1 (persistent connections)
Client
Server
0 RTT
SYN
Client opens TCP connection
SYN-ACK
1 RTT
ACK
Client Sends HTTP GET request for /index.html
DAT
Server Reads from Disk
ACK
DAT
2 RTT
ACK
Client Parses HTML
DAT
Client Sends HTTP Request for image 1-n
ACK
Server Reads from Disk
DAT
3 RTT
Image Begins to Arrive

45. TCP-PDUs for HTTP/1.1 (persistent connections with pipelining)
Client
Server
0 RTT
SYN
Client opens TCP connection
SYN-ACK
1 RTT
ACK
Client Sends HTTP GET request for /index.html
DAT
Server Reads from Disk
ACK
DAT
2 RTT
ACK
Client Parses HTML
GET Image-1
Client Sends HTTP Request for image 1-n
GET Image-2
Server Reads from Disk
GET Image-3
ACK
3 RTT
DAT
Image Begins to Arrive