Post on 05-Feb-2016
description
transcript
Naming
CSCI 4780/6780
Name Space Implementation• Naming service – A service that lets users to
add/delete and lookup names• In large distributed systems naming service has to
be distributed• For convenience namespace is partitioned into
logical layers– Global layer – Highest level nodes (root and first level
children)– Administration layer – Directory nodes managed by
single organization (Ex: Departmental nodes in UGA)– Managerial layer – Nodes of local network, shared files,
etc.
Name Space Distribution
An example partitioning of the DNS name space, including Internet-accessible files, into three layers.
Layers and their Characteristics• Global layer
– Stability – Directory tables rarely change– High availability is critical– Permits high levels of caching, throughput is critical
• Administrative Layer– Availability is critical – Permits high levels of caching– Response time should be fast
• Managerial level– Client-side caching is in general less effective– Fast response time is crucial
Comparison of Layers
Item Global Administrational Managerial
Geographical scale of network Worldwide Organization Department
Total number of nodes Few Many Vast numbers
Responsiveness to lookups Seconds Milliseconds Immediate
Update propagation Lazy Immediate Immediate
Number of replicas Many None or few None
Is client-side caching applied? Yes Yes Sometimes
Name Resolution - Implementation• Distribution affects name resolution
implementation• Example: root:<nl, vu, cs, ftp, pub, globe, index.txt>
• Two types – Iterative process and Recursive Process
• Iterative Process– Each node resolves as much as it can and sends result
to client, which contacts the next level of node
• Recursive Process– Nodes do not return intermediate results to client– Contact next level of nodes obtain addresses and send
complete results to the client
Iterative Name Resolution
Recursive Name Resolution (2)
Iterative Vs. Recursive Name Resolutions
• Iterative– Shorter connection durations -> Less load on servers– Less effective caching -> High latency
• Recursive– Longer connection duration -> High loads on servers– Permits caching at higher levels of nodes
• High-level nodes can “learn” about lower level nodes
• Permits shortcuts
– Cheaper with respect to communication latency
Caching in Recursive Name Resolution
Recursive name resolution of <nl, vu, cs, ftp>. Name servers cache intermediate results for subsequent lookups.
Server for node
Should resolve
Looks upPasses to
childReceives
and cachesReturns to requester
cs <ftp> #<ftp> -- -- #<ftp>
vu <cs,ftp> #<cs> <ftp> #<ftp> #<cs>#<cs, ftp>
ni <vu,cs,ftp> #<vu> <cs,ftp> #<cs>#<cs,ftp>
#<vu>#<vu,cs>#<vu,cs,ftp>
root <ni,vu,cs,ftp> #<nl> <vu,cs,ftp> #<vu>#<vu,cs>#<vu,cs,ftp>
#<nl>#<nl,vu>#<nl,vu,cs>#<nl,vu,cs,ftp>
Implementation of Name Resolution
The comparison between recursive and iterative name resolution with respect to communication costs.
The Domain Name Service• Largest distributed naming service
• Used for looking up host addresses and mail servers
• Hierarchically organized as a rooted tree– Labels – 63 Characters; Pathnames – 256 characters– String representation – Listing labels from right to left
separated by dots
• Domain – Subtree of nametree– Domain name – Path to the root of the domain
• Contents of node – Resource records
The DNS Name Space
The most important types of resource records forming the contents of nodes in the DNS name space.
Type of record
Associated entity
Description
SOA Zone Holds information on the represented zone
A Host Contains an IP address of the host this node represents
MX Domain Refers to a mail server to handle mail addressed to this node
SRV Domain Refers to a server handling a specific service
NS Zone Refers to a name server that implements the represented zone
CNAME Node Symbolic link with the primary name of the represented node
PTR Host Contains the canonical name of a host
HINFO Host Holds information on the host this node represents
TXT Any kind Contains any entity-specific information considered useful
DNS Implementation• DNS namespace is divided into global layer and
administration layer– Managerial layer not part of DNS
• Each zone implemented by a name server– Replicated for availability– Updates happen only at primary name server– Lazy replication – Zone transfer
• Database is made up of collection of files
DNS Implementation - Example
An excerpt from the
DNS database for the zone
cs.vu.nl.
Reference to Subdomains
Part of the description for the vu.nl domain which contains the cs.vu.nl domain.
Name Record type Record value
cs.vu.nl NIS solo.cs.vu.nl
solo.cs.vu.nl A 130.37.21.1