Transcript Ethernet, IP and TCP

LIS508 lecture 10:
Ethernet and TCP/IP
Thomas Krichel
2002-11-24
Structure
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General things about networks
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LANs and Ethernet
IP
TCP
More general things
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History of Internet
the TCP/IP model
discussion mostly based on Tanenbaum’s
“computer networks” classic book….
Origins of TCP/IP
• 1957: USSR launches the Sputnik
• US worried about command and control
structure after a nuclear attack
• Early 60s Paul Baran promotes packet
switching rather than circuit switching.
• Mid 60s: Pentagon says that it wants this,
gives grants to ARPA.
ARPAnet original design
• Network nodes have host computers and router
computer
• Hosts sends messages smaller than 8061 bits
• routers breaks it up into packets smaller than
1008 bits
• all router are connected by transmission lines
• each router connected to 2 others
• store and forward principle
implementation
• 12k times 16 bit words memory minicomputer
without hard disk as routers connected by
56kbps leased phone lines
• life with four hosts in 1969–12
• over 30 hosts in 1972–09
• protocol research leads to TCP/IP in 1974
• integrated into Berkeley UNIX, freely available
• Internet research group developing protocols
• 1979 Internet research group reorganized to
Internet Control and Configuration Board
More history
• 1980: MILNET split off the ARPAnet
• CSNET set up
– links researchers at non-ARPA contract institutions to
the ARPAnet
– ran on a single box with dial-up lines since late 70s
• 1984 NSF links adds a router to it
supercomputers, sets up NSFnet,
– the first purely TCP/IP network, on 56kbps.
– additional funding for (eventually 20) regional
networks connect to the backbone
• 1990 ANS (MERIT,MCI,IBM) take over NSFnet
LAN
• A LAN generally operates a broadcasting
network.
• Something that is transmitted by one
station is heard by all other stations
• An individual message is called a frame.
• This creates a media access control (mac)
problem. When one station issues a
frame, it destroys a frame from another
station, through frame collision.
Ethernet
• comes from the term "luminiferous ether".
• Today the name for a family of LAN
protocols implementing CSMA/CD
protocol for the mac problem.
• Initially only referred to IEEE 802.3
standard
• Can run over a variety of physical devices.
10baseT
• This is most widely used physical
infrastructure today.
• It runs over twisted-pair cable and RJ-45
jack.
• Cables are cheap, transmission is fast
• Maximum length of cable about 150m,
thus not good for bus architecture.
other devices
• “repeater” is a device that amplifies an electric
signal so that it can travel over some more wire.
• “hub” is a repeater with a lot of outgoing wires.
Hubs are dump, or smart. A smart hub can be
inspected by a monitoring device to see what the
traffic is doing there.
• A “bridge” or “switch” is a device that links
different LANs together. In normal “promiscuous”
mode, it receives signals from all LANs. It will
know which MAC address is on which LAN and
transmit echo signal to the right LAN.
MAC address
• A physical address burned onto each
network interface card used on a LAN.
• Forms an address for your computer on a
LAN
• It is 6 bytes long, written in hex, usually
written with each byte separated by “:”.
• 2 power 48 addresses, that is about 11000
addresses for every inhabitant of the
planet.
Internet Protocol IP
• LAN protocols are not part of the Internet.
• IP operates on top of LANs to make them
interoperable.
• IP sets up a set of logical addresses that locate
a physical device over the whole range of
locations over the Internet.
• It provides for the means to send datagrams
from one address to the other…
• Such datagrams are called packets. The IP
service is not reliable!
in the words of rfc791
• “The Internet protocol is designed for use in
interconnected systems of packet-switched
computer connection networks. The Internet
protocol provides for transmitting blocks of data
called datagrams from sources to destinations.
The Internet protocol also provides for
fragmentation and reassembly of long datagram,
if necessary, for transmission through 'small
packet' networks.”
• “There are no mechanisms to augment end-toend data reliability, flow control, sequencing, or
other services commonly found in host-to-host
protocols.”
IP address
• 32 bit address, usually written as a
sequence of four decimal numbers
between 0 and 255.
• Contains a network part and a host part
– dependent on the network mask
– depending on the class of the network
• All traffic that is not on the local network is
sent to a gateway
finding the IP address
• A host is on the Internet when it has an IP
address that others can reach. Usually
that implies knowing
– IP address of the machine
– network mask
– IP address of the gateway
• There can be written into a file on the
machine. This is usually referred to as a
static address.
static and dynamic addresses
• Static IP addresses have problems
– waste a lot of address space on machines
that are switched off
– have to be rewritten when device moves
• dynamic addressing has become more
common
• the most widely used protocol is the
dynamic host configuration protocol
dhcp
• When connected to a network a client software
on a host machine sends out a request on the
LAN it is attached to.
• A dhcp server will answer and lease an IP
address to the host, sending it the network mask
and the gateway address as well.
• Internet providers work like that to save address
space.
• Problem: some services require IP addresses to
be stable. They can not be run in such a setting.
routing
• using IP, all datagrams travel individually.
• they may travel through many networks in
order to get to their destination
• networks are interconnected through
routers who make the decision where to
send the packet to next.
• Such decision are based on routing
protocols such as OSPF or BGP etc. This
is a very complicated piece of engineering.
frame / packet / segment
• The Ethernet frame on the LAN contains the IP
packet.
• A gateway between two networks takes the
packet out of the Ethernet frame of one network,
and “wraps” it in a frame used on another
network.
• Inside the IP packet, there is a TCP segment.
The TCP segment tells the destination machine
what to do with the data.
transmission control protocol
• provides a reliable service of communication
– stream orientation: bytes come out from the sender,
arrive in the same order at the receiver
– packet buffering: fast arrived data is stored at the
destination until it can be processed
– full duplex: communication in two ways.
• if a packet has gone astray, TCP retransmit it.
• uses port numbers as addresses to tell the host
what to do with arriving packets.
applications and ports
• TCP uses port numbers to detect which
application protocol to send the data to.
• Some ports are well-known
– 80 for http
– 23 for smtp
-- 20 for ssh
-- 53 for dns
• firewalls can block traffic for specific ports
on specific machines and therefore allow
or prevent services.
summary : TCP/IP “model”
• Application layer
– http, smtp, ftp, dns, ntp
• Transport layer
– tcp, udp
• Network layer
– IP
• Host-to-host layer
– LAN and WAN protocols
Critique of tcp/ip
• Not a model but a protocol stack
• No proper distinction between
– Service
– Protocol
– Interface
• Ad hoc set of application protocols
http://openlib.org/home/krichel
Thank you for your attention!