Transcript Naming system

Naming
CSCI 4780/6780
Names & Naming System
• Names have unique importance
– Resource sharing
– Identifying entities
– Location reference
• Name can be resolved to the entity it refers to
• Naming system resolves names
• Naming system in distributed systems can itself
be distributed
Names, Entities & Addresses
• Name is a string of bits/characters used to refer to an entity
• Entity can resources/users/data/processes
– Web pages, files, hosts, printers, network connections
– Entities permit certain operations (reading/writing files)
• To operate we need to access the entity
• Access Point – A special kind of entity
– Host of a server
• Name of access point is called its Address
– Hosts IP address and port
• Address of access point is the address of the entity at that
access point
More on Entities and Addresses
• Entities can have multiple access points
– Multiple telephone numbers/email addresses
– Replicated services
• Entities might change access points
– Laptops get different IP addresses based on their
location
– New phone number/email when switching jobs
• Can we use the address as the name of an entity?
– Difficulties in relocation/replication transparency
• Name that is independent of address – Location
Independent Names
Identifiers
• An name with following properties
– Identifier refers to at most one entity
– Entity is referred to by at most one identifier
– Identifier always refers to same entity (persistence)
• Facilitates unambiguous reference
– Testing equality of identifiers suffices if entities being
referred to are the same
• Addresses cannot be identifiers if they can be reassigned
– Example: Telephone numbers
Flat Naming
• Identifiers are random bit strings
• No information on how to locate the entity
• Simple location resolution strategies
– Broadcasting (E.g. Address Resolution Protocol (ARP))
– Multicasting
• Forwarding Pointers
– A moving entity leaves behind a reference at its previous
location
– Long chains causes inefficiency
– Maintenance costs
– Vulnerability to broken links
Forwarding Pointers in SSP Chains
Figure 5-1. The principle of forwarding pointers
using (client stub, server stub) pairs.
Redirection by Shortcuts
Figure 5-2. Redirecting a forwarding pointer by
storing a shortcut in a client stub.
Shortcuts
Figure 5-2. Redirecting a forwarding pointer by
storing a shortcut in a client stub.
Home Based Approaches
• Each entity has a home location
– Permanent address of the entity
• Obtains temporary address when roaming
• Home location keeps track of the current address of
the entity
• Home agent forwards the packet to the entities
current location
• Drawbacks?
Home-Based Approaches
Figure 5-3. The principle of Mobile IP.