Transcript Naming - University of Georgia

Naming
CSCI 6900/4900
Mounting
• Mounting – Merging different namespaces
transparently
• File system example
– Directory node of one namespace stores identifier of
directory node of another namespace
– Mount Point – Directory node storing the foreign ID
– Mounting Point – Directory node in foreign space
– Resolution occurs by looking up mounting point and
then accessing its directory table
• Can be generalized to other namespaces as well
Mounting Foreign Namespaces
• Namespaces on different servers possibly on different
machines
– Mounting might require network communication
• Mounting requires at least three information
– Name of access protocol
– Name of server
– Name of mounting point in foreign space
• Each name needs to be resolved
– Mounting point needs to be resolved by the foreign server
– Three names are represented as a URL
– Example: nfs://flits.cs.vu.nl//home/steen
Mounting - Example
Mounting remote name spaces through a specific process protocol.
Tracing the resolution of /remote/vu/mbox
Mounting Alternative
• Pioneered by Global Name Service
• Add new root and make the existing root nodes the
children of new root
• Problem – Existing path names need to be changed
• Solution – The root id (from where the resolution
should normally start) is implicitly included in the
path
Global Name Service
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
– Caching can be done only at the server -> 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
Server for
node
Should
resolve
Looks up
Passes to
child
Receives
and caches
Returns 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>
Recursive name resolution of <nl, vu, cs, ftp>. Name servers
cache intermediate results for subsequent lookups.
Implementation of Name Resolution (4)
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
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
The most important types of resource records forming the
contents of nodes in the DNS name space.
DNS Implementation
• DNS namespace is divided into global layer and
administration layer
– Managerial layer no 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
Name
Record type
Record value
cs.vu.nl
NIS
solo.cs.vu.nl
solo.cs.vu.nl
A
130.37.21.1
Part of the description for the vu.nl domain
which contains the cs.vu.nl domain.