oasis: anycast for any service michael j. freedman karthik lakshminarayanan david mazières in nsdi...

Oasis: Anycast for Any Service

Michael J. FreedmanKarthik Lakshminarayanan

David Mazières

in NSDI 2006

Presented by:Sailesh Kumar

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Overview

Why Anycast?» Availability, Load balancing, Performance

How to Implement Anycast?» Replica selection problem

Oasis» Replica Selection for many Services (amortized cost)» Use Geographic Coordinates» Fast Response Time» Accurate Results

Architecture of Oasis» How it works?» Design choices

Evaluation Discussion

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Anycast/Replica Selection

Data/Service replication is very common in Internet» CDNs (e.g. Akamai, Coral)» Distributed web mirrors, » DNS servers» Distributed (DHT based) storage/file systems

What is thestate today?

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Anycast is the Solution

Anycast aids in selecting the replica which is» Near the client» Less loaded» IP Anycast does not provide comprehensive selection policies

?

mycdn

Animation taken from Freedman NSDI’06 slide

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Traditional Approach

Round-robin and DNS based techniques» spread load but do nothing for network locality

Use On-demand probing» probe client from multiple vantage points» Usually accurate» Significant probing required, can incur long latencies» Many probing to a client can trigger intrusion-detection alarms

Such probing is needed for every service» Access IBM (Akamai will probe you)» Access Google video (you are again probed)» Access a high volume website (you again need to be probed)

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Introducing Oasis

A single Anycast Solution for many services» Amortize costs of probing

Perform probing off-line (probe the entire Internet)» Faster response

Use geographic coordinates» Geographic coordinates of IP prefixes are relatively stable» Better than virtual coordinates» Results may be more accurate

Provide flexible Anycast policies» Balance between

– Bandwidth– Accuracy– Latency

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Oasis Architecture

A two-tier architecture» A reliable core of hosts» Replicas belonging to different services» Replicas run Oasis specific code to

– Report their liveness and load information– assist core nodes in network measurements

There are two primary ways clients access Oasis» DNS redirection» HTTP redirection

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DNS Redirection

Client issues DNS request for a service using Oasis OASIS will redirect the client to the nearest replica

mycdn

OASIS core

Client Resolver

12

Animation taken from Freedman NSDI’06 slide

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HTTP Redirection

Client issues HTTP request for the service Service invokes Oasis code and contacts Oasis core With Oasis info, client is redirected to better replica

Animation taken from Freedman NSDI’06 slide

123

OASIS core

mycdn

Client

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How Oasis resolves Anycast requests?

Core node maps client IP address to network bucket» Network bucket consists of IP address prefixes

IP prefix is mapped to a location, say x (coordinates)» Geographic coordinates of IP prefixes are stable

Replica whose location is closest to x is returned

Optionally, livenessand load of replicascan also determinereplica selection

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proximityIP prefix (y) ( Latx, Lngx, RTT distance )

location accuracy

Mapping IP prefix to geographic coords?

First step is to map IP addresses a.b.c.d to the nearest IP prefix say a.b.c.0/24

For the prefix, measurement is done by the replica» Replicas form an overlay» They traceroute to the IP prefix and registers the RTT» If replica x has minimum RTT to prefix y, then the prefix y’s

geographic coordinates becomes the coords. of replica x

Such address aggregation reduces the amount of probing

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Two-pronged approach» Find closest replica proxy with less probing» Use closest replica’s geo-coords + error RTT as location

18.168.0.23

18.0.0.0/8

“Probe 18.0.0.0/8”

Animation taken from Freedman NSDI’06 slide

How to find replica nearest to prefix?

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Two-pronged approach» Find closest replica proxy with less probing» Use closest replica’s geo-coords + error RTT as location

18.168.0.23

[ Meridian 05 ]

Animation taken from Freedman NSDI’06 slide

How to find replica nearest to prefix?

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18.168.0.23

[ Meridian 05 ] Two-pronged approach

» Find closest replica proxy with less probing» Use closest replica’s geo-coords + error RTT as location

Probing of prefixes are done very infrequently (once a week)

:18.0.0.0/8 , 6.0 ms(42N,71W)

Animation taken from Freedman NSDI’06 slide

How to find replica nearest to prefix?

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How Oasis maintains its database?

For every service, there is a rendezvous core node (via consistent hashing, H(serv))

Whenever replicas join, leave, change state, they notify its rendezvous node, also notify abt. liveness

Rendezvous gossip liveness to other core nodes Every replica also sends keepalives to its nearest core

Oasis node Nodes in the core keeps their data structures weakly

consistent via gossiping» Uses incarnation numbers to indicate the latest info» Uses consistent hashing to distribute load across nodes

DNS is one of the services provided by Oasis» Thus, first step in resolving any Anycast request is to provide

the nearest core node to the client

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Operation of Oasis?

A client queries OASIS for hostname coralcdn.nyuld.net:» The client queries the DNS root servers, finds OASIS core node I» Core lookup: Node I finds other core nodes near the client:

– I maps client’s IP address to IP prefix, and then prefix to location coords.– I queries the rendezvous node for service dns, H(dns). Call this node SI.– SI responds with the best-suited OASIS node for the specified coordinates.– I returns this set of DNS replicas to the client. Let this set include node J.

» The client resends the Anycast request to J.» Replica lookup: Core node J finds

replicas near the client:– J extracts the request’s servicename and maps the client’s IPaddress to coordinates.– J queries rendezvous nodefor service coralcdn, call this SJ.– SJ responds with the bestcoralcdn replicas, which Jreturns to the client.

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Evaluation

Replicas on 250 PlanetLab nodes around the world Oasis core on 37 nodes

Meridian: Uses on-demand probing (geographic coordinates) Vivaldi: Uses on-demand probing (virtual coordinates)

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Evaluation

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Evaluation

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Evaluation

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Deployment till Date

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Discussion

Is it OK to incorporate Anycast capability within the IP protocol?» If yes, then we do not need Oasis like infrastructure» However, like multicast, feature rich anycast may not be efficiently

incorporated within the IP protocol

Is Oasis much different from what Akamai may already be doing?

One of the motivations behind Oasis is Intrusion-detection alerts» Such a motivation appears unconvincing, as there are many other

ways to work around this problem

What happens if prefixes are not as geographically stable?» Will they become less stable in mobile context?

What is your general opinion about Oasis» To me the results appear surprisingly good