Download A Perspective on Software Communication Overhead

Network Performance Effects
on HTTP/1.1, CSS1, and PNG
Henrik Frystyk Nielsen : W3C
James Gettys : DEC
Anselm Baird-Smith: W3C
Luis Rivera
2/24/2000
Outline
• Goals
• HTTP 1.0 vs HTTP 1.1
» Specification
» Performance
• Further improvements
» Compression
» Web content: CSS, PNB and MNG
• Conclusions
• This was two years ago, was about today?
Luis Rivera
2/24/2000
Paper Goals
• Main Goal
» Present key performance differences between HTTP/1.1 and
HTTP/1.0
» Stress the importance of changes in Web Content
• Secondary Goals
» Expose the performance advantages of a well implemented
HTTP/1.1 protocol
» Show advantages of CSS, PNG and MNG
• Did I miss something?
Luis Rivera
2/24/2000
Problem?
• HTTP/1.0
» Not designed for multiple transmissions in the same request
» Makes use of multiple TCP connections
» Small multiple transmissions are affected by TCP slow start
mechanism
» Patch: “Keep-Alive” extensions don’t work with multiple
proxies
» Low support for compression
• However it is still heavily used !!!
Luis Rivera
2/24/2000
Solution?
• HTTP/1.1
» “Persistent connections”
» Data compression
» Ranges facilities
– Partial retrieval of objects
» Cache validation
– “poor man’s multiplexing”
• Design Goals
» Increase network use efficiency
» Allow reliable use of caching
» Improve performance perceived by end user!!!
Luis Rivera
2/24/2000
Implementation headaches
• Testbed
» LAN, WAN and PPP
» Jigsaw and Apache servers (SPARC platform)
» Libwww robot, Netscape and Explorer (Pentium and Alpha
platforms)
» Microscape!
• Initial testing
» Pipelining: Achieves better efficiency!
» Persistent connections: Decreases TCP packets
» BUT: Multiple connections outperforms both !!!
• What went wrong?
Luis Rivera
2/24/2000
Pipelining
• Maintains several outstanding requests on the same
connection
• Tuned to produce as few small headers as possible
• Requests are buffered
» Improves network utilization
» Need to be flushed (size threshold, timer)
» Flushing can take advantage of the application knowledge!
• Interfere with Nagle’s algorithm
• Can you really rely on the application knowledge?
Luis Rivera
2/24/2000
Performance Tuned Version
• Tuned HTTP/1.1 outperforms HTTP/1.0
• Buffering
» Reduces the number of TCP packets required
» Increases packet size
• Pipelining
» Needs to be complemented with buffering
• Bigger and fewer packets increase server
performance!!!
• Look at performance tables !!!
Luis Rivera
2/24/2000
Compression
• Data, images are already compressed
• Could help to overcome TCP slow start effects
• Increases the chances to generate a new batch of
request
• Decreases elapsed time (using zip compression)
• HTML tags should be lower cased for best
compression
• This is good, but what is the cost of compressing web
pages?
Luis Rivera
2/24/2000
Web Content
• Fact
» HTML was not born to be used the way it is being used!
» However, it has been proved flexible
• Changes represent
» Potential web traffic reduction
» Web page development could be easied
• Style Sheets for web page designers: CSS
» Better control of page representation and layout
» Saves requests by replacing images in the Unicode char set
• PNG and MNG for images and animation
Luis Rivera
2/24/2000
Conclusions
• HTTP/1.1+pipelining outperforms HTTP/1.0
• Pipelining can significantly improve performance
» Must be used with buffering and other tricks to be effective
» Specially useful revalidating cached items
• HTTP/1.1 new techniques saves bandwidth and uses
the network more efficiently
• Such changes could be improved further by changes
in web content
» For style sheets, images, animations, more?
Luis Rivera
2/24/2000
Discussion
• What about interactions between HTTP/1.1 and a
hierarchy of proxy caches
• Performance improvements are nice but,
» Will developers really care if they are achieved though
heavy modification of existing technologies?
» Are those performance improvements really significant to
justify the change?
• Changes to web content make sense in an ever
changing web environment !!!
Luis Rivera
2/24/2000