Transcript cs622present1

DHTTP: An Efficient and CacheFriendly Transfer
Protocol for the Web
By Michael Rabinovich and Hua Wang
Presented by Jerry Usery
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Overview
Introduction
• DHTTP protocol
• Implications of DHTTP
• Server design
• Performance analysis
• Future work
• Summary
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Introduction
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Two issues addressed
1) Violation of end-to-end principle by interception
caches (Web proxies)
Impersonation of origin servers
• Web interactions may be disrupted
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2) Performance implications of client-initiated
TCP (Transmission Control Protocol) as transport
protocol
HTTP generally conceived as file transfer protocol
• TCP connection overhead, persistence & pipelining
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Introduction
Interception Cache
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Introduction
Performance Implications
TCP connection overhead, persistence &
pipelining penalties
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Persistent connections
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Degradation of throughput and increasing connections
Pipelined transmissions
Server must maintain connections, send responses in
order
• Head-of-line delays can occur with slow responses
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Introduction
Main Ideas of DHTTP
Two main ideas of DHTTP (Dual-Transport
HTTP protocol)
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1) Split Web traffic between UDP (User Datagram
Protocol) and TCP
Client typically sends requests via UDP
• Server sends response via UDP or TCP
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Response size
• Network conditions
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2) Server establishes TCP connection to client
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Introduction
UDP vs TCP
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UDP use for short responses
Reduced number of open server connections
• Reduced number of TCP connection setups
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DHTTP benefits
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Improves client latency
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Increases server capacity
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Servers manage fewer open connections
Remaining TCP connections reserved for larger objects
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Fewer Web interactions wait for connections
Improved utilization of TCP connections
Ordering constraints of pipelining doesn’t exist
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Introduction
DHTTP Server-established TCP connections
TCP connection client/server roles reversed
• Some firewall implications (existing
countermeasures suffice)
• Benefits
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Retains end-to-end Internet principle
True interception cache IP address use
• Allows arbitrary deployment of interception caches
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Server-initiated TCP reduces message round trips
(even with initial UDP request)
• Message round trips decreases with UDP
• Bottleneck process removed (server-accepted TCP
connections)
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DHTTP Protocol
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Web clients and servers listen on two ports
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One UDP, one TCP
Servers use well-known ports (or URL-specified)
• Clients use ephemeral ports (short-lived, per download)
• Two channels exist between client and server
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UDP used for requests below 1460 bytes
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Most HTTP requests met
Server opens TCP connection or reuses open
one for larger messages
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If TCP request sent by client, server may use it
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DHTTP Protocol
Message Exchange
(a) HTTP (b) DHTTP over UDP (c) DHTTP over TCP
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DHTTP Protocol
Message Format Description
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Response may arrive to client out of order
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Client assigns request ID, then matches it
Client advises server of listening port numbers
Channel’s source port number included in IP header
already
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UDP request contains client’s TCP port number
• TCP request contains client’s UDP port number
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Flag field used for duplicate (resend) requests
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DHTTP Protocol
Message Format
(a) – Request (b) – Response
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DHTTP Protocol
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Reliability
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DHTTP stipulates that a client may resend UDP requests
Lessens overhead
• Fighting denial-of-service attacks beyond paper’s scope
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Nonidempotent (e-commerce, etc) requests
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Delegated to TCP channel
Congestion control
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DHTTP responds to any resent client requests via TCP
Aids packet loss issues
• In congested Internet experiments, only 6% of responses
sent via UDP
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DHTTP Protocol
Channel Selection/Algorithm
Response size
• Network condition
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Server maintains “fresh requests” and “resent requests” counters
• Loss threshold parameter “L” (currently 1%)
• Size threshold parameter “S” (1460 bytes, default)
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Algorithm
1) Choose TCP for all large responses, i.e., whose size exceeds S, as well
as for all resent requests.
2) If the ratio of resent request counter to fresh request counter exceeds L,
enter a “high-loss” mode, else enter a “low-loss” mode.
3) In the low-loss mode, choose UDP for all small responses, i.e., those
below the size threshold S.
4) In the high-loss mode, choose TCP for the 1-L fraction of small
responses and UDP for the remaining L small responses.
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DHTTP Implications
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DHTTP and Interception Caches
DHTTP interception caches intercept UDP
requests – TCP requests pass through
• Client is aware it speaks with cache
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Retains end-to-end principle (even with caching)
DHTTP uses UDP channel for short requests
Reduces TCP setup costs, connections,
response time
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DHTTP allows client or server to unilaterally
close TCP connection
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Ensures no in-transit data exists
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DHTTP Implications
Server Design Description
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Master process
Accepts incoming requests
• Three threads
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ReadRequest thread
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Reads incoming UDP request, copies into global buffer
PipeRequest thread
Pipes global buffer requests to worker processes
• Global buffer moves requests ASAP so UDP port buffer
will not fill up, requests don’t get dropped
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Maintenance thread checks worker process status
every second
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If too few idle worker processes, it forks new ones
• If too many, it kills some
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DHTTP Implications
Server Design Description
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Worker processes
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Execute individual requests, respond to clients
Reads pipe requests, generates response, chooses
UDP or TCP, sends response
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If TCP connection chosen and one exists to client, it
reuses it
• One TCP connection per client, possibly many clients
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Includes Timeout thread
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Closes TCP connections idle > timeout period
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DHTTP Implications
Server Design
Modified Apache 1.3.6 Web server
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Performance Analysis
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Three-pronged performance study
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Trace-driven simulation
Measured number/utilization of TCP connections
experienced by server (HTTP and DHTTP)
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Benchmarked Apache HTTP/DHTTP servers with
clients on same LAN
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Compared peak performance and scalability
Tested both servers in WAN environment with
congested Internet connection
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Performance Analysis
Simulation
Used access log from AT&T low-end hosting
services
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Three month duration
• Contained over 100 million accesses
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Average response size of 13 K
Two threshold values used
4 K, optimistic value for many small Web responses
• 1460 bytes, conservative value, one Ethernet MTU
(maximum transfer unit)
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Performance Analysis
Simulation
Number of TCP connections at a server with three connections per client
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Performance Analysis
Simulation
Connection utilization with three connections per client
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Performance Analysis
Simulation
Number of TCP connections at a server with one connection per client
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Performance Analysis
Simulation
Connection utilization with one connection per client
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Performance Analysis
Prototyped Testing Results
Apache performance (bottleneck at server)
(a) Throughput with three connections per client
(b) Latency with three connections per client
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Performance Analysis
Prototyped Testing Results
Apache performance (bottleneck at server)
(c) Throughput with one connection per client
(d) Latency with one connection per client
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Performance Analysis
Prototyped Testing Results
DHTTP server performance (bottleneck at server)
(a) Throughput with three connections per client
(b) Latency with three connections per client
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Performance Analysis
Prototyped Testing Results
DHTTP server performance (bottleneck at server)
(c) Throughput with one connection per client
(d) Latency with one connection per client
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Performance Analysis
Performance Comparison
Comparison of Apache and DHTTP servers
(a) Throughput (b) Latency
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Performance Analysis
Performance Comparison
Apache and DHTTP server performance under network congestion
(a) Throughput (b) Latency
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Performance Analysis
Performance Comparison
Effectiveness of congestion detection in DHTTP server
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Future Work
Dividing response among several UDP
packets
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Likely allows higher size thresholds
Building native support for nonidempotent
requests
• Investigation of finer algorithms that track
network conditions at the client and/or subnet
• Dynamic policies for size threshold selection
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High-loss vs low-loss environments
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Summary
Introduction
• DHTTP protocol
• Implications of DHTTP
• Server design
• Performance analysis
• Future work
• Summary
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