INTERNET PROTOCOLS

Class 9

CSCI 6433

David C. Roberts

Entire contents copyright 2011, David C. Roberts, all rights reserved

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Agenda• Name systems• DNS

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Why Name Systems• Computers are happy with numbers• Important reasons why IP addresses have numbers• However, humans like to deal with names• Especially in the case of IPv6• Name systems can provide names for humans to deal

with• But there’s another reason too—names can be made

more persistent than IP addresses

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Internet Name Spaces• MAC Addresses

• Identities of NICs• Unique across a LAN, at least• Usually 48 bits

• IP Addresses• Routable address of a host• Can change frequently• 32 or 128 bits• Unique across the Internet

• Domain Names• English language names• Generally tied to an activity of people• Changes slowly, may have years of duration

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Internet with Names, without DNS

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With DNS

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Basic Name System Functions• Name space: rules how names are structured and used• Name registration: a method for linking specific names to specific

devices so that names are unique across the entire Internet• Name resolution: mechanism for translating a symbolic name into an

IP address

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Principal RFCs for DNS

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Goals for DNS• Global, scalable, consistent name space• Local control over local resources• Distributed design to avoid bottlenecks• Application universality• Support for multiple underlying protocols• Hardware universality

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DNS Functions

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Name Space

Each node in the DNS tree is identified by a label

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Name Syntax• Length: each label can be up to 63 characters long• Symbols: letters, numbers, dash are allowed• Case: labels are not case-sensitive• Every label must be unique within its parent domain

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Domain Name Construction

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Resource Specification• Can specify a resource within a domain name by

providing a directory structure after the name• For example, could have

• Salt.crystal.rocks/Essay

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Uniqueness• Every label must be unique within its parent domain

Names can be up to 255 characters long

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Management of the Name Space• ICANN and IANA are responsible for management of the

name space• They delegate responsibility for parts of the name space

to other organizations• Different parts of the name space may be managed

differently

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Original TLDs• .ARPA• .COM• .EDU• .GOV• .MIL• .NET• .ORG

The three TLDs for most names became very crowded very quickly

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Current TLDs

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Country Code TLD Authorities• Each country has authority to set up and manage its TLD• Many country codes have been defined• Country codes have two letters• Some country codes are used for other purposes• Some TLDs are restricted

Examples:

Scoop.It uses TLD for Italy to spell “scoop it”

Bit.ly spells out “bitly” using TLD for Lybia

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DNS Name Servers• Each DNS server is a database server • Resource record stores a type of information about a

node in the DNS tree. It has general format for part of record, specialized format for information for its type

• Master file representation is text representation for resource record, editable by humans

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RR Types

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Root Name Servers• Today there are 13 different root name servers• Each of these has multiple physical servers

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DNS ResolversFunctions:• Standard name resolution: given a DNS name, find the IP

address• Reverse name resolution: take an IP address and

determine what name is associated with it• Electronic mail resolution: determine where to send email

based on the email address used in a message

Actions:• Provide user interface: allow name to be used in place of IP

address• Forming and sending queries: sends queries to DNS• Processing responses: accept response from DNS, decide

what do with it

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Iterative Resolution

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Recursive Resolution

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DNS Resource Records and Classes• Each DNS server is a type of database server• The database has entries called Resource Records for

domains• RRs are stored in binary but have text versions that are

shown to humans

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Common DNS Resource Records

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Caching• Caching is very important to DNS• Names that are referenced are often referenced again• There is extensive caching at all levels of DNS• When a cached value is returned, the address of the

authoritative server is also returned• Each RR has a TTL specified. Servers that cache the

record discard it when the time interval expires

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DNS Message Format

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Details

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Examples of Object Types

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Persistence• Persistence of DNS names and IP addresses is different• How?

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Summary • DNS is a crucial Internet service• It allows names to be established for persistent areas of

influence• The convenience of names is secondary to its importance• DNS is the Internet name space that best represents

human activities