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Lecture 8: Naming

Challenge: how do we efficiently locate wide area resources? Naming: name -> IP address Routing: IP address -> packet delivery

More generic challenge: how do we build wide area services? Cache and replica coherence Computation in the network

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Domain Name System (DNS)

Design Assumptions Names have global scope -- they mean the

same thing everywhere Names are organized hierarchically

– ex: cnn.com, june.cs.washington.edu– each subtree can be assigned to different org

Caching and replication for performance, availability

Weak consistency to simplify implementation

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Hierarchical Name Space

Ex: june.cs.washington.edu, june.com OK to reuse names in different parts of tree

root

edumilorg ukcom ca

gwu wustl washington wsu mit

june ward beaver

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Name Space Delegation

Each organization controls its own name space (“zone” = subtree of global tree) each org has its own name servers

– replicated for availability

name servers translate only org names– client lookup proceeds step-by-step

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DNS Lookup Example

client local DNS proxy

Root&edu DNS server

washington DNS server

cs DNSserver

cs.washington.edu

washingt

on=IPadd

rcs.w

ashingto

n.edu

cs=IPaddr’june=IPaddr’’

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Performance and Availability

DNS servers are replicated name service available if >= one replica is up queries load balanced between replicas

DNS queries are cached at local proxy quick response for repeated translations partial name caching (edu, washington.edu,

cs.washington.edu, june.cs.washington.edu)

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DNS Cache EffectivenessDestination Host Locality

(time since host last accessed)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.1 1 10 100Seconds

Cu

mu

lati

ve

Fra

cti

on

All Flows

Inter-hos t only

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Replica/Cache Coherence

How do we keep replicas/cached copies up to date?

DNS solution: eventual consistency periodic downloads from master to replicas cached data periodically times out

– proxy must re-query server

ok if old data is provided temporarily?

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Eventual Consistency Example

Serverreplicas

clients

t+k:x’

x’

t+1:x ok

t+1:reget x

t:x’

t+q:x’

t+q:reget x

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Coherence Models

Sequential consistency all operations appear as if to a single, shared

memory Causal consistency

concurrent ops can occur in any order Processor consistency

all ops by same host performed in order Eventual consistency

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Implementing Cache Coherence

User managed cache ex: hit reload to get up-to-date version

Timeouts (NFS, WWW, DNS) client periodically polls server for newest

version Callbacks (AFS, SMPs, transactions)

server notifies client whenever data changes invalidation or update

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Sequential Consistency Example

Serverreplicas

clients

t+1:x’

x’

t+2:x’ t:x’

t+2:ack

t+1:x’

t+3:ack t+5:ack

t+4:ack

Write doesn’t complete until all copies invalidated or updated

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DNS Experience?

Single instance of globally distributed database in widespread use

Caching/replication crucial for performance Simple consistency model

easy to implement, crucial for heterogeneity System is difficult to evolve Surprises:

negative caching, ...

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Active Networks

Motivation Flexibility: can take decades to deploy new

protocols Performance: need to put functionality at most

effective point in network Approaches:

active packets -- programs in IP packets active routers -- programmable routers active services -- put servers in network

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Active Names Motivation

Services have become complex, distributed programs

Clients have become heterogeneous Implies need for:

Flexibility– client/service should be able to use any protocol

Performance– client/service should be able to move functionality

into network

DNS Server

URL

Host BindingIP Address

Image (1024X768)Proxy Server

•Static name -> IP address binding

•Naming and transport separate

•Names have global scope

DNS Naming Reconsidered

•Not flexible or extensible

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Adding Flexibility to Naming

Replica management HTTP redirect

– front end replies with server name– client reissues request to server

DNS round robin– rotate order of replicas provided to query

Cisco Local Director/Distributed Director– transparent redirection in routers

load balancing vs. distance, failover?

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More Naming Flexibility

Customization server customizes content to specific client

– mycnn.com, advertisement rotation

client customizes content– format to display

Deployment of new IP features mobility, larger address space

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More Naming Flexibility

Location independent objects Universal resource names (URNs)

– URLs that can change servers– proposal: sed scripts to mangle names

Global object systems (e.g., Globe, Legion)– how do you scalably locate mobile objects?

•Context-sensitive naming–User types cnn.com.

–If client is behind a modem, it gets back a b/w image.

–If client is a palm pilot, it gets a distilled image.

–If the client is in Europe, it goes to the European replica.

•Combine naming and transport in one framework.

•Provide flexibility and extensibility in the way wide area resources are accessed.

Active Names Scenario

Active Names: Basic Idea

DataName Programs

•Names resolved to mobile, secure programs Flexibility

•Active Names organized into hierarchical namespaces. A program is associated with each namespace.

•Namespace programs can be changed Extensibility.

•Active Names are connection oriented: better end-to-end semantics and performance.

Programming Model

Location independent programs» Programs may run on any AN node

Stream data model» Each program operates on a data stream

which is the result of the previous program Continuation passing style

» Control does not have to return to the caller

Request Program1 Program2

Client Proxy Server

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Performance Gains

Client Proxy Server

Server

Client Proxy

•Application customized transport protocols.

•Programs are location independent. Location can be chosen to optimally utilize resources (e.g., distillation).

•Customization can be performed close to client instead of at the server (e.g., to cache dynamic content).

•3 way RPC

Delegation Active names organized in a hierarchy of

namespaces

Namespace programs can delegate to subordinate namespaces

Composing Services

Root-HTTP

cnn yahoo ...

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Composing Services

After methods Continuation passing style programming Namespace programs bundle remaining work

into “after methods” before passing control – [Customizer|http|reply] -->[prefetch|http|distill|reply]

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Security

•Protection between active name programs provided by Java’s type safety mechanism.

•Caller passes a certificate to the callee granting it a subset of its rights (e.g., to translate name).

•Transitive closure of the certificates determines the rights of a principal (e.g., to provide service).

•For instance, each caller might grant its callee the right to respond to the client.

Microkernel Architecture

Virtual MachineResolverNW

$Local Resources

NamespacesHttp DNS

Distiller Hit Count

...

Utilities

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Application 1: Replica Selection

DNS Round-Robin» Randomly choose replica

» Avoid hot-spots

Distributed Director» Route to nearest replica

» Geographic locality

Active Naming» Previous performance, distance

» Adaptive

DNS Round-Robin» Randomly choose replica

» Avoid hot-spots

Distributed Director» Route to nearest replica

» Geographic locality

Active Naming» Previous performance, distance

» Adaptive

BerkeleyReplica

SeattleReplica

BerkeleyClients

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Replica Selection

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15

Offered Load

Res

pon

se T

ime

(s)

NearestRandomActive

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Application 2: Mobile Distillation

Clients name a single object Return object based on client

network connection, screen Current approach [Fox97]

Proxy maintains client profile

Requests object, distills Active naming

Transmit name + applet

Flexible distillation point

Tradeoff computation/bw

Support mobile clients

Client-Specific Naming

Variables: Network Screen

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Application 2: Mobile Distillation

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2 3 4 6 8 10 12 15 20

Number of Clients

Dis

tilla

tio

n L

ate

nc

y (

s)

Active

Server

Proxy

Distillation at

•Server: Saves bandwidth

•Proxy: Saves server CPU cycles

•Active: Cost estimate of both approaches

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Application 3: Customization

•Client customization: distilling the image

•Server customization: ad rotation, server-side includes.

•Composing these customizations using Active Names leads to 2-fold improvement in performance:

-Server customization can take place at proxy.

-Distillation further improves performance.

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Application 4: Active Caches

Reason Requests Possible Strategy

Dynamic 20.7% Replace CGI w/AN Program

Consistency Polling 9.9% Server driven consistency

“Uncachable” 9.2% AN for hit counting, customization

Compulsory 44.8% Prefetching

Redirect 3.7% AN load balancing

Misc. 11.5% Error handling, prefetching, etc.

How to improve on 50% cache hit rates?