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Network Layers
COMT 625
COMT 625
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Overview
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IP and general Internet Operations
Address Mapping
ATM LANs
Other network protocols
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The Network Layer
• Impose a global addressing scheme
• May also
– Provide virtual circuit connections
– Segment and re-assemble packets
– Provide “user-friendly” name-based
addressing
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An Example
Web Server:
www.lerc.nasa.gov
Web
Browser
“Backbone”
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The Request
• User types into a web browser:
• “http://www.lerc.nasa.gov/index.htm”
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Browser Interpretation
• “http://www.lerc.nasa.gov/index.htm”
• Protocol to use is HTTP (HyperText
Transfer Protocol); the transport protocol
is TCP
• The requested server is named
www.lerc.nasa.gov
• The Service Access Point (SAP)is 80
• The requested file is “index.htm”
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Name Resolution
• www.lerc.nasa.gov needs name to
address mapping
• Make a call to the “resolver” (or, in most
PCs, the “stub resolver”) to get a
numeric address.
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Name Resolution cont.
• The answer may come from local
storage, or may itself require a network
transmission using the Domain Name
System (DNS) protocols.
• The name space and the address space
are unrelated, except that they can be
translated into each other.
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HTTP Message
• The browser assembles a text message:
• First line:
–GET index.htm
• Optional additional lines:
–Browser type
–HTTP version number
–Transmission option negotiation
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Request to TCP
• Browser makes a call to TCP, asking to
open a connection to the numeric
browser address
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Request to IP
• TCP creates a header with the origin
and destination SAPs
• TCP sets a “flag” (bit) to request a new
connection (SYN)
• TCP may set other parameters and
options
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Locate the Gateway
• IP needs to find a way to get the TCP
PDU (called a segment) to the
destination address.
• First decision: is the (IP) address in the
current broadcast domain?
• Since in this case the destination is on a
different network, it needs a gateway.
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Locate the Physical Gateway
Address
• IP knows the global (IP) address of the
gateway from the local configuration
storage.
• To make a request of the data link layer,
it needs the physical (MAC - Media
Access Control) address of the gateway.
• The same mapping would be needed if
the target is on the same LAN
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Physical Gateway Address
cont.
• The ARP protocol is used to broadcast
a request for the “owner” of the IP
gateway address.
• The gateway responds to the broadcast;
IP learns the MAC address from the
response.
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Dispatch the SYN Packet
• IP creates one or more packets (usually
one in this case).
• The header has the origin and
destination IP addresses and other
options.
• The packet goes to the Ethernet driver,
which attaches the MAC origin address
and the gateway MAC destination.
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Routing
• The gateway receives the packet, and
determines that it is not:
–addressed to the gateway itself (at the IP
level)
–addressed to any networks the gateway is
attached to
• The gateway examines its routing table,
and selects a circuit to send the packet
out on.
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Receipt
• www.lerc.nasa.gov receives the packet.
• IP removes its headers after errorchecking the header
• TCP removes its headers, after errorchecking the entire packet.
• TCP consults the owner of SAP 80, and
determines that the web server is willing
to create the connection.
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Response
• TCP acknowledges the receipt of the
SYN (technically it could send data in
this packet).
• TCP responds to any options that may
have been requested in the SYN
• Browser actually transmits its request
and the data flow starts.
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Standards Setting
ISOC
IANA
IESG
IAB
IETF
Area Directors
Areas
Internet
Routing
Operations and Management
General
Security
User Services
Applications
Transport
Working Groups
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The Domain Name System
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Rationale
• People remember names better that
numbers (think 1-800-flowers)
• Names should be assigned and
controlled within the organization that
“owns” the named hosts
• Names must be unique
A Distributed Database is Required
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Name Structure
• abc.xyc.foo.bar
• Some portion (or all) of this name
designates the “domain”
• If the name designates a host, the
leftmost portion of the name designates
the host
• www.ohiou.edu points to a host
• csm.ohiou.edu is a domain
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Naming Hierarchy
• The Root Domain, “.”
• International Top Level
Domains (TLDs)
.
EDU
COM
–ISO Country Codes, .INT
• Generic TLDs
ohiou.edu
osu.edu
–.COM, .NET, .ORG, .GOV,
csm.ohiou.edu
.MIL, .EDU
• Special Purpose - .ARPA
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cs.ohiou.edu
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Zones of Authority
• A Name-Server is said to be
“Authoritative” for the
domains it has actual database records for
• A Name Server can service
more than one domain.
• Name Servers must be
redundant
.
EDU
ohiou.edu
COM
osu.edu
csm.ohiou.edu
cs.ohiou.edu
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Query Structure
• The Domain Name system is designed to hold
multiple pieces of information under any given name;
a request must designate the Query Type
• Technically, information for multiple protocol stacks
can be stored; the normal Query Class is “Internet”.
• A query can request that the contacted name server
complete a query, even if it has to contact other Name
Servers – a Recursive query
• A Non-Recursive, or Iterative query will provide the
address of the next name server instead.
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Query Process
• A “Resolver” must know at least on Name Server.
• A Name Server must at least know one Root Name
Server.
• The Resolver formulates the query and contacts its
know Name Server.
• Name Servers store, or cache, answers to recent
queries, and supply those when the same question is
asked again (e.g., by another host).
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Adminstration
• Allocation of IP addresses
– Address availability
• Allocation of Domain Names
– Trademark issues
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Other Protocols
• Appletalk
– Used in the Mac OS
– Routable
• NETBIOS
– Windows operating syste,s
– uses NETBEUI (non-routable) or
– TCP/IP
– Incorporates name mapping
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