- This chapter is about how networks can be joined together into an
internet.
- It is natural that there will be very different networks for
different needs.
- It is also natural that people want to
interconnect these different networks -- no one wants to be
an "island" all of the time.
- Workers are more productive if they can achieve all
"connectivity" using a single computer.
- The concept of universal service
is to allow any two computers to communicate.
- Problem: different WAN's have incompatible packet formats,
addressing schemes, and protocols.
- It takes more than splicing wires together or the use of bridges to
connect incompatible networks.
- On the other hand, people can and do create hardware and software
tools that allow them to connect diverse networks together.
- An internetwork or internet
is a heterogeneous set of interconnected networks -- different kinds
of networks all connected together.
-
Two different kinds of networks can be joined by connecting each
of them to a special kind of packet switch called a router.
- A router is like a bridge in that each network connection is a
standard NIC -- the kind that would be used by any other computer
on the same network.
- A router is unlike a bridge in that it often connects
two or more completely different kinds of networks -- like a fast
Ethernet and a Token Ring.
- An internet is
a bunch of networks connected by routers.
-
The layout of the connections is chosen to achieve an adequate
tradeoff among performance, reliability, and cost.
- Redundant connections are desirable to improve performance and
reliability.
- The job of a router is CPU-intensive so it should not be
burdened with having a large number of network connections.
- There should be enough extra connections so that the network
will not be partitioned if just a few routers fail.
- The job of a router is CPU-intensive so it should not be
burdened with having a large number of network connections.
- Besides routers the other major ingredient
required for building an internet is internet protocol
software.
- Internet protocol software is discussed in detail later.
-
Through the use of software a collection of
dissimilar networks can be made to appear to be one large network
offering universal service with a common interface to all users -- a
virtual network.
- TCP/IP is the standout protocol
family for internetworking.
- However other protocols have been used.
- TCP/IP was first and was developed by the same group that created
the internetworking concept.
- The original work on TCP/IP was funded by the (Defense) Advanced
Research Projects Agency (DARPA) and then by the National Science
Foundation (NSF) and other U.S. government agencies.
- The great success of the Internet has influenced network software
companies to make products that are compatible with internetworking.
- For example it is very common now for computer protocols to have the
ability to work with more than one network technology.
-
The TCP/IP layering model is:
- Layer 5: Application: Each layer 5 protocol specifies how
one application uses an internet.
- Layer 4: Transport: Specify how to ensure reliable transfer.
- Layer 3: Internet: Specifies format of internet packets and
rules for forwarding internet packets from router to router.
- Layer 2: Network Interface: specify format of LAN frames and
transmission details.
- Layer 1: Physical Layer: basic level of bit-by-bit
transmission.
- Layer 5: Application: Each layer 5 protocol specifies how
one application uses an internet.
- (Parts of) layers in the TCP/IP stack are similar to (parts of)
layers in the OSI stack:
TCP/IP OSI Application Application, Presentation Transport Transport Internet Network Network Interface Data Link Physical Physical
- The ISO stack does not have an Internet Layer, but the ISO
Network Layer is similar to parts of the TCP/IP Internet Layer.
- The TCP/IP stack does not have a Session Layer, but parts of the
TCP/IP Transport and Application Layers contain some of the
functionality of a Session Layer.
- A host is anything that is connected to the Internet and
runs applications software.
- Most routers are dedicated routers. A dedicated router is not a
host. It does not run applications software. It only performs
routing tasks.
- To make communication across the Internet
possible, all routers and hosts must run Internet Protocol
software.
- Dedicated routers do not need layer 5 software.
- Hosts usually must run software at all five levels.