>
 
   
 
Naming 
 
 
March 8, 2001
 
 
 
  
 
2  
 
 
Networks 
 
 
What is naming?
 
     Associations between 
     some elements in a set of names and some elements in a set of values
     
       Binding - One 
       such association
       
     Name resolution 
     - Determining the value associated with a name
     Context - A set 
     of name to value associations
     
         The same name 
         can be bound to different objects in different contexts
         
     Alias - a name that 
     references a value that already has a name
 
 
 
 
  
 
3  
 
 
Networks, cont. 
 
 
Name Services
 
     A service that resolves 
     names in a given context
     May or may not be 
     fully automated
     
       Examples:
       
         DNS on the Internet
         Grapevine 
         Ethernet cards
         
     Objectives: speed, 
     availability, scalability
 
 
 
 
  
 
4  
 
 
Networks, cont. 
 
 
Ethernet Card Naming
 
     Each Ethernet Card 
     has a 48-bit name
     
       e.g., C0 39 FB 
       23 5E 9A 
       
     Name used by link-layer 
     to differentiate between other Ethernet cards
     Manufacturers get 
     blocks of  addresses from a naming authority
     Issue - What happens 
     if we run out of names?  (How likely is this?)
 
 
 
 
  
 
5  
 
 
DNS
 
     Domain name - a 
     name associated with an IP address
     
       generically:  
       <name2>.<name1>
       <name1> is 
       a top-level domain
       <name3>.<name2>.<name1> 
       is a subdomain of <name2>.<name1>
       
     Use hierarchy to 
     reduce complexity and improve scalability of name resolution
     Use caching to improve 
     lookup speed and availability
     Typically implemented 
     on top of UDP
 
  
 
Networks, cont.
 
 
 
  
 
6  
 
 
DNS, cont.
 
     Name resolution 
     done hierarchically, split into zones
     A zone contains:
     
       Specific info 
       for some hosts in a domain (types)
       
         e.g., A, MX, NS, 
         CNAME
         
       Names of servers 
       providing DNS for sub-domains
       Zone governing properties 
       (caching, replication) 
 
  
 
Networks, cont.
 
 
 
  
 
7  
 
 
Name resolution in DNS
 
     Query - ask for 
     a type of info on a domain name
     
       e.g., photo.net 
       MX   (mail exchanger)
       
     Start with top-level, 
     well known root DNS server
     
       These root servers 
       service mit.edu, arsdigita.com, ...
       
     Get the name of 
     the DNS server with info about your sub-domain
     Resolve the name 
     of the DNS server
     Iterate - Send your 
     query to the new DNS server
 
  
 
Networks, cont.
 
 
 
  
 
8  
 
 
Name resolution in DNS, cont.
 
     Full name resolution 
     takes at least 2 steps
     Optimization: Use 
     caching 
     Three types of DNS 
     servers: authoritative, non-authoritative, caching
     
       Two authoritative 
       servers required for each domain name
 
  
 
Networks, cont.
 
 
 
  
 
9  
 
 
Name resolution in DNS, cont.
 
     Authoritative server 
     is primary source of info
     Non-authoritative 
     servers replicate data in authoritative servers
     
       e.g., 1-2 times 
       per day
       
     Caching servers 
     cache DNS info 
     Clients must be 
     told whether IP is from authoritative source or not
 
  
 
Networks, cont.
 
 
 
  
 
10  
 
 
Name resolution in DNS, cont.
 
     No mechanism to 
     guarantee consistency between DNS servers at all times
     To address cache 
     consistency, use Time To Live 
     
       DNS Server can 
       specify a TTL for a resolved domain name
       
     A caching server 
     invalidates an entry based on its TTL
 
  
 
Networks, cont.
 
 
 
  
 
11  
 
 
DNS issues  
 
     DNS changes can 
     take hours or days to propagate
     
       Partially depends 
       on TTL
       Clients must decide 
       how to determine whether an IP address is valid
       
     DNS can be spoofed
     
       Need higher-layer 
       security to ensure you are talking to the right host
       
     Simple errors can 
     create disasters
     
       One small change 
       to a root-level authoritative server can throw the Internet into chaos
 
  
 
Networks, cont.