CIS 553: Networked Systems

HTTP Performance

April 6, 2020

Agenda

n Router-assisted congestion controln HTTP

n Protocoln Fetching web pages

n HTTP Optimizations

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From HTTP to Web Pages

n Most Web pages have multiple objectsn e.g., HTML file and a bunch of embedded images

n How do you retrieve those objects (naively)?n One item at a time

n New TCP connection per (small) object!

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Non-persistent connections

n Default in HTTP/1.0n 2RTT for each object in the HTML file!

n One more 2RTT for the HTML file itself

n Inefficient due to repeated work

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Persistent connections

n Maintain TCP connection across multiple requestsn Including transfers subsequent to current pagen Client or server can tear down connection

n Advantagesn Avoid overhead of connection set-up and tear-downn Allow underlying layers (e.g., TCP) to learn about RTT and

bandwidth characteristics

n Default in HTTP/1.1

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Pipelined requests & responses

n Batch requests and responses to reduce the number of packets

n Multiple requests can be contained in one TCP segment

Client Server

Request 1Request 2Request 3

Transfer 1

Transfer 2

Transfer 3

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Concurrent requests and responses

n Use multiple connections in parallel

n Does not necessarily maintain order of responses

R1R2 R3

T1

T2 T3

Client

Server

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Concurrent Connections (Part 2)

n Initial congestion window & slow start limits BWn Use more connections!

n Per-connection RWNDs are sometimes set lown Use more connections!

n Congestion control limits how much we can sendn Use more connections!

n Adds complexityn Goes against original intent of TCP

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Scorecard: Getting n small objects

n Time dominated by latency

n One-at-a-time: ~2n RTTn m concurrent: ~2[n/m] RTTn Persistent: n Pipelined: n Pipelined/Persistent:

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Scorecard: Getting n small objects

n Time dominated by latency

n One-at-a-time: ~2n RTTn m concurrent: ~2[n/m] RTTn Persistent: ~ (n+1)RTTn Pipelined: ~2 RTTn Pipelined/Persistent: ~2 RTT first time, RTT later

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Scorecard: Getting n large objects each of size F

n Time dominated by bandwidth

n One-at-a-time: ~ nF/Bn m concurrent: ~ [n/m] F/B

n Assuming shared with large population of users and each TCP connection gets the same bandwidth

n Pipelined and/or persistent: ~ nF/Bn The only thing that helps is getting more bandwidth

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Agenda

n Router-assisted congestion controln HTTP

n Protocoln Fetching web pages

n HTTP Optimizations

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Every Millisecond Counts

500ms delay causes 1.2% decrease in Bing revenue [Souders 2009]

400ms delay causes 0.74% decrease in Google searches [Brutlag 2009]

100ms delay causes 1% decrease in Amazon revenue [Linden 2013]

Many Web services companies spend considerable effort reducing Web response time.

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Background - Critical Path

Critical Path: the longest chain of dependent browser tasks

Fetch Delay: Network DelayRender Delay: Computational Delay

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Background - Page Load Time (PLT)

Order matters!

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How can we reduce PLT?

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Agenda

n Router-assisted congestion controln HTTP

n Protocoln Fetching web pages

n HTTP Optimizationsn CDNsn HTTP/2

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Proxies

n Computer that acts a broker between client and servern Speaks to server on client’s behalf

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HTTP Proxies

n Accept requests from multiple clients

n Takes request and reissues it to server

n Takes response and forwards to client

client

proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

originserver

originserver

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Types of Proxies

n Explicitn Requires configuring browser

n Implicitn Service provider deploys an “on path” proxyn … that intercepts and handles Web requests

n Also, forward and reverse

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Forward Proxy

n Proxy on behalf of the client

n Usually under administrativecontrol of client-side AS

proxyserver

HTTP request

HTTP request

HTTP response

HTTP response

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client

client

Reverse Proxy

n Cache on behalf of the server

n Generally implicit proxyserver HTTP request

HTTP response

originserver

originserver

HTTP requestHTTP response

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Examples of Proxies

n Forward:n Privacy – Hide your true IPn Firewalls – limit access to the Internet, secure the proxyn ISP caching – reduce traffic exiting the AS

n Reverse:n CDNs – reduce traffic exiting the ASn Load balancers

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Before CDNs

n Sending content from the source to 4 users takes 4 x 3 = 12 “network hops” in the example

source

client

. . .

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client

After CDNs

n Sending content via replicas takes only 4 + 2 = 6 “network hops”

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source

client

. . .

client

replica

Popularity of Content

n Zipf’s Law: few popular items, many unpopular ones; both matter

Zipf popularity(kth item is 1/k)

Rank Source: Wikipedia

George Zipf (1902-1950)

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Content Distribution Networks (CDN)

n Caching and replication as a servicen Large-scale distributed storage infrastructure

(usually) administered by one entityn e.g., Akamai has servers in 20,000+ locations

n Combination of caching and replicationn Pull: Direct result of clients’requests (caching)n Push: Expectation of high access rate (replication)

n Can do some processing to handle dynamic webpage content

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CDNs (e.g., Akamai)

Resulting in traffic of:30 terabits / sec3+ trillion hits / day Responsible for 15-30% of all web traffic

The Akamai EdgePlatform:

240,000+Servers

2200+POPs

130Countries

1600+Networks

800+ Cities

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Server Selection Policies

n Live servern For availability

n Lowest loadn To balance load across the

servers

n Closestn Nearest geographically, or in

round-trip time

n Best performancen Throughput, latency, …

n Cheapest bandwidth, electricity, …

origin server in North America

CDN distribution node

CDN serverin S. America CDN server

in Europe

CDN serverin Asia

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Mechanism: HTTP Redirection

n Advantagesn Fine-grain controln Selection based on client

IP address

n Disadvantagesn Extra round-trips for TCP

connection to servern Overhead on the server

GET

Redirect

GET

OK

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Mechanism: Anycast Routing

n Advantagesn No extra round tripsn Route to nearby server

n Disadvantagesn Does not consider

network or server loadn Different packets may go

to different serversn Used only for simple

request-response apps

1.2.3.0/24

1.2.3.0/24

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Mechanism: DNSn Advantages

n Avoid TCP set-up delayn DNS caching reduces

overheadn Relatively fine control

n Disadvantagen Based on IP address of

local DNS servern “Hidden load” effectn DNS TTL limits adaptation

DNSquery

local DNS server

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1.2.3.4

1.2.3.5