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Evaluation of the Proximity between Web Clients and their Local DNS Servers. Z. Morley Mao UC Berkeley ([email protected]) C. Cranor, M. Rabinovich, O. Spatscheck, and J. Wang AT&T Labs-Research F. Douglis IBM Research. Origin servers. Clients. Clients. Motivation.

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Evaluation of the proximity between web clients and their local dns servers

Evaluation of the Proximity between Web Clients and their Local DNS Servers

Z. Morley Mao

UC Berkeley ([email protected])

C. Cranor, M. Rabinovich, O. Spatscheck, and J. Wang

AT&T Labs-Research

F. Douglis

IBM Research


Motivation

Origin servers Local DNS Servers

Clients

Clients

Motivation

  • Content Distribution Networks (CDNs)

    • Attempt to deliver content from servers close to users

Internet

Cache server

Cache server

Cache server


Dns based server selection

www.service.com? Local DNS Servers

Server IP address

www.service.com?

www.service.com?

Server IP address

ns.service.com

Client.myisp.net

DNS based server selection

  • Originator problem

    • Assumes that clients are close to their local DNS servers

Authoritative DNS server

ns.service.com

Local DNS Server

ns.myisp.net

A.GTLD-SERVERS.NET

Verify the assumption that clients are close to

their local DNS servers


Measurement setup

www.att.com Local DNS Servers

Measurement setup

  • Three components

    • 1x1 pixel embedded transparent GIF image

      • <img src=http://xxx.rd.example.com/tr.gif height=1 width=1>

    • A specialized authoritative DNS server

      • Allows hostnames to be wild-carded

    • An HTTP redirector

      • Always responds with “302 Moved Temporarily”

      • Redirect to a URL with client IP address embedded

1x1 transparent GIF


Embedded image request sequence

1. HTTP GET request for the image Local DNS Servers

2. HTTP redirect to

IP10-0-0-1.cs.example.com

Client

[10.0.0.1]

Redirector for

xxx.rd.example.com

7. HTTP GET request for the image

8. HTTP response

6. Reply: content server

IP address

3. Request to resolve

IP10-0-0-1.cs.example.com

Content server for the image

4. Request to resolve IP10-0-0-1.cs.example.com

5. Reply: IP address of content server

Name server for

*.cs.example.com

Local DNS server

Embedded image request sequence


Measurement data
Measurement Data Local DNS Servers


Measurement statistics
Measurement statistics Local DNS Servers


Proximity metrics
Proximity metrics: Local DNS Servers

  • AS clustering

  • Network clustering

  • Traceroute divergence

  • Roundtrip time correlation


As clustering
AS clustering Local DNS Servers

  • Autonomous System (AS)

    • A single administrative entity with unified routing policy

  • Observes if client and LDNS belong to the same AS


Network clustering
Network clustering Local DNS Servers

  • [Krishnamurthy,Wang sigcomm00]

  • Based on BGP routing information using the longest prefix match

  • Each prefix identifies a network cluster

  • Observes if client and LDNS belong to the same network cluster


Traceroute divergence

client Local DNS Servers

Local DNS server

Traceroute divergence

Probe machine

a

  • [Shaikh et al. infocom00]

  • Use the last point of

  • divergence

  • Traceroute divergence:

  • Max(3,4)=4

b

1

1

2

2

3

3

4


Roundtrip time correlation
Roundtrip time correlation Local DNS Servers

  • Correlation between message roundtrip times from a probe site to the client and its LDNS server

  • The probe site represents a potential cache server location

  • A crude metric, highly dependent on the probe site


Aggregate statistics of as network clustering
Aggregate statistics of AS/network clustering Local DNS Servers

  • More than 13,000 ASes

    • Close to 75% total ASes

  • 440,000 unique prefixes

    • Close to 25% of all possible network clusters

       We have a representative data set


Proximity analysis as network clustering
Proximity analysis: Local DNS ServersAS, network clustering

  • AS clustering: coarse-grained

  • Network clustering: fine-grained

  • Most clients not in the same routing entity as their LDNS

  • Clients with LDNS in the same cluster slightly more active


Proximity analysis traceroute divergence
Proximity analysis: Local DNS ServersTraceroute divergence

  • Probe sites:

    • NJ(UUNET), NJ(AT&T), Berkeley(Calren), Columbus(Calren)

    • Sampled from top half of busy network clusters

  • Median divergence: 4

  • Mean divergence: 5.8-6.2

  • Ratio of common to disjoint path length

    • 72%-80% pairs traced have common path at least as long as disjoint path


Improved local dns configuration
Improved local DNS configuration Local DNS Servers

  • For client-LDNS associations not in the same cluster, do we know a LDNS in the client’s cluster?

Client IPs

HTTP requests


Impact on commercial cdns

Data set Local DNS Servers

Client-LDNS associations

LDNS-CDN associations

Available CDN servers

Verifiable clients:

w/ responsive

LDNS

Misdirected clients:

directed to a cache

not in client’s cluster

Clients with LDNS

not in same cluster

Impact on commercial CDNs

Client w/ CDN server

in cluster


Impact on commercial cdns as clustering
Impact on commercial CDNs Local DNS ServersAS clustering


Impact on commercial cdns network clustering

Less than 10% of all clients Local DNS Servers

Impact on commercial CDNsNetwork clustering


Conclusion
Conclusion Local DNS Servers

  • Novel technique for finding client and local DNS associations

    • Fast, non-intrusive, and accurate

  • DNS based server selection works well for coarse-grained load-balancing

    • 64% associations in the same AS

    • 16% associations in the same network cluster

  • Server selection can be inaccurate if server density is high


Related work
Related work Local DNS Servers

  • Measurement methodology

    • IBM (Shaikh et al.)

      • Time correlation of DNS and HTTP requests from DNS and Web server logs

    • Univ of Boston (Bestavros et al.)

      • Assigning multiple IP addresses to a Web server

    • Differences from our work:

      • Our methodology: efficient, accurate, nonintrusive

    • Web bugs

  • Proximity metrics

    • Cisco’s Boomerang protocol: uses latency from cache servers to the LDNS


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