Download Network review

Cosc 4765
Networking
overview
Bandwidth Review
• Bit (b) = a unit of information, 0 or 1
– 10 bits can represent 1024 different messages
– 20 bits represent > 1 million
– 30 bits > 1 billion messages
• The bandwidth of a communication channel =
number of bits per second it transmits
• All channels have limited bandwidth
• One byte (B) = 8 bits (an octet)
• Transmitting 1 MB at 56K bps takes 143 sec.
• 1 GB = gigabyte takes 40 hours
– at 7Mbps 19 minutes; at 1 Gbps takes 8 seconds
• Latency = delay from first bit transmitted to first
received
Progress of Technology
• Have more disk storage
IMPROVEMENT: 8000 x
– 1971:
10 MB
– 2001:
80,000 MB (soon 1 terabyte =
1000GB)
• Higher communication speeds
–
–
–
–
–
–
1971-2001
IMPROVEMENT:
3 BILLION x
Human speech:
30 bits/sec
1971 Modem
300 bits/sec
2001 Modem:
56,000 bits/sec
T1 line:
1,544,000 bits/sec
Internet 2:
1,000,000,000 bits/sec
Nortel:
1,000,000,000,000 bits/sec in 1 fiber
(entire U.S. telephone
traffic)
BANDWIDTH
APPLICATION
TECHNOLOGY
Experimental
1 terabit
All U.S. telephone conversations simultaneously
Gigabit
Ethernet
1 gigabit
Full-motion HDTV
OC12 = 622 Mb
FDDI
OC3 = 155 Mb
Virtual Reality, Medical Imaging
T3/E3
T3 = 44.7 Mb
Video Conferencing, Multimedia
DSL ~ 7 Mb
Streaming Video + Voice
T1 = 1.544 Mb
ADSL
T1/E1
ISDN
128K
Fiber
Copper
Browsing, Audio
New Modem
56K
E-mail, FTP
In Kbps
19.2
Old Modem
Telnet
4.8
Wireless WAN
Paging
Human speech = 30 bps
BANDWIDTH LIST
Some humor first
http://www.the5thwave.com/images/cartoons_computer/large/training/631lg.gif
IEEE Standards for networking
There are 7 layers in the OSI network model
• Layer 7: Application
– mechanisms to support end-user applications such
as mail, ftp, etc.
•
Layer 6: Presentation
– mechanisms for dealing with data representation
•
Layer 5: Session
– mechanisms for establishing reliable
communications between cooperating applications
IEEE Standards for networking (2)
• Layer 4: Transport layer
– provides reliable end-to-end error recovery
mechanisms and flow control in the higher networking
software
– Firewall work here (an up to layer 7)
• Layer 3: Network (hardware)
– Establishes communication from station to station
– Most high level Network protocols are in this layer
– Routers work at this layer
IEEE Standards for networking (3)
• Layer 2: Data link (hardware)
– transmits and receives frames, MAC protocol belongs
to this layer
– Switches and bridges work at this layer.
• layer 1: Physical (hardware)
– standardizes the electrical, mechanical, and
functional control of data circuits that connect to the
transmission media
– Hubs and repeaters work at this layer.
• Layer 0: transmission media
– cables between two network stations (includes
wireless transmissions)
How OSI layer works.
– A program, which "functions" at layer 7,
(application layer) passes the message down
to the next OSI levels. Each layer changes
and adds information as needed.
– the message goes out onto the network.
– then back up the OSI levels (stripping off
information needed only at that layer and
changing the message as needed) until it
reaches the application layer of the receiving
program, which then reads the message
based on the protocol used.
• We are going to look at each layer, starting at
layer 7 and working our way down to layer 0
• But remember, each layer is dependent on the
one above it and below it.
• An important concept in OSI is data
encapsulation. Layer 7 data is encapsulated by
Layer 4, then layer 3 encapsulates layer 4 data
(which is encapsulated layer 7 data with more
information), continuing this process down to
layer 2.
– Each layer also uses different terms as well.
Ethernet
• Uses the OSI network model, but with
different names.
– Also compresses layer 5, 6, 7 into the same
layer. Most network applications are written
to the Ethernet standards (including O/Ss),
• Because of the different names, which
confuses people, the layer number is
normally used as the name.
Layer 7: Application
• Application protocol defines:
– types of messages to be exchanged
• requests and response messages
– The syntax of the messages, fields and how
they are delineated.
– semantics of the fields (ie what they mean)
– rules for when and how a program sends
messages and replies to messages.
• Such as the HTTP protocol.
Layer 6: Presentation layer
• Deals with data representation
– since UNIX, windows, Mac, the Internet, etc
do not agree on what the data should look
like, this layer deals with the multiple data
representation standards.
– These include whether the O/S uses ASCII
standards or another character standard,
big/little Endian byte ordering standards, etc.
Layer 5: Session layer
• when a program asks for a network connection,
it is this layer than establishes and maintains the
connection
– Sockets are created on this layer
• These layer makes a request to Layer 4 for
protocol. Layer 5 is a virtual layer in most
respects. It's standard interface into layer 4.
– A socket is created on this layer, but how all the
communication is done is left up layer 4 and below.
Layer 4: Transport
• Provides logical communication between
application processes on different hosts.
– Not a physical connection, but applications think so.
– Applications don’t need to worry about physical
infrastructure.
• Two protocols provided and developer must
choose one.
– UDP (User Datagram Protocol)
– TCP (Transmission control Protocol)
• Other non-common transport protocols exist
here
UDP in detail
• [RFC 768]
– multiplex/demultiplexing and error checking.
– No connection establishment
– No connection state
– small packet header overhead
• UDP adds 8B of header, while TCP adds 20B
– Unregulated send rate
UDP segment
• Contains
–
–
–
–
–
Source Port #
Destination Port #
Length of entire segment (including header)
Checksum
Application data or message.
• No IP number, contained in the network layer
header information. IP are layer 3 information.
TCP: Overview
RFCs: 793, 1122, 1323,
2018, 2581
• point-to-point:
• full duplex data:
– one sender, one receiver
– bi-directional data flow in
same connection
– MSS: maximum segment
size
• reliable, in-order byte
steam:
– no “message boundaries”
• pipelined:
• connection-oriented:
– handshaking (exchange
of control msgs) init’s
sender, receiver state
before data exchange
– TCP congestion and flow
control set window size
• send & receive buffers
socket
door
application
writes data
application
reads data
TCP
send buffer
TCP
receive buffer
segment
• flow controlled:
socket
door
– sender will not overwhelm
receiver
TCP segment structure
32 bits
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
rcvr window size
ptr urgent data
Options (variable length)
application
data
(variable length)
counting
by bytes
of data
(not segments!)
# bytes
rcvr willing
to accept
Data Reliability
• Why does TCP provide reliable data
transfer and UDP does not?
– In the Network layer (Layer 3), Best-effect
delivery service is provided
– Meaning the best attempt to deliver is made,
but no guarantees, no orderly deliver, and no
guarantee on the integrity of the data.
Layer 3: Network layer
Host, router network layer functions:
Transport layer: TCP, UDP
Network
layer
IP protocol
•addressing conventions
•datagram format
•packet handling conventions
Routing protocols
•path selection
•RIP, OSPF, BGP
routing
table
ICMP protocol
•error reporting
•router “signaling”
Link layer
physical layer
Network layer functions
• transport packet from sending
to receiving hosts
• network layer protocols in
every host, router
application
transport
network
data link
physical
network
data link
physical
network
data link
physical
three important functions:
• path determination: route taken
by packets from source to dest.
Routing algorithms
• switching: move packets from
router’s input to appropriate
router output
• call setup: some network
architectures require router call
setup along path before data
flows
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
network
data link
physical
application
transport
network
data link
physical
Network service model
Q: What service model
for “channel”
transporting packets
from sender to
receiver?
• guaranteed bandwidth?
• preservation of interpacket timing (no jitter)?
• loss-free delivery?
• in-order delivery?
• congestion feedback to
sender?
The most important
abstraction provided
by network layer:
? ?
?
virtual circuit
or
datagram?
Virtual circuits
“source-to-dest path behaves much like
telephone circuit”
– performance-wise
– network actions along source-to-dest path
• call setup, teardown for each call before data can flow
• each packet carries VC identifier
• every router on source-dest path s maintain “state” for
each passing connection
– transport-layer connection only involved two end systems
• link, router resources (bandwidth, buffers) may be
allocated to VC
– to get circuit-like performance.
Virtual circuits: signaling protocols
• used to setup, maintain teardown VC
• used in ATM, frame-relay, X.25
• not used in today’s Internet
application
transport 5. Data flow begins
network 4. Call connected
data link 1. Initiate call
physical
6. Receive data application
3. Accept call transport
2. incoming call network
data link
physical
Datagram networks: the Internet model
• no call setup at network layer
• routers: no state about end-to-end connections
– no network-level concept of “connection”
• packets typically routed using destination host ID
– packets between same source-dest pair may take different
paths
application
transport
network
data link 1. Send data
physical
application
transport
2. Receive data network
data link
physical
Datagram or VC network: why?
Internet
• data exchange among
computers
ATM (asynchronous transfer mode)
• evolved from telephony
• human conversation:
– “elastic” service, no strict
– strict timing,
timing req.
reliability
• “smart” end systems
requirements
(computers)
– need for guaranteed
– can adapt, perform control,
service
error recovery
– simple inside network,
• “dumb” end systems
complexity at “edge”
– telephones
• many link types
– complexity inside
– different characteristics
network
– uniform service difficult
Routing
Routing protocol
5
Goal: determine “good” path
(sequence of routers) thru
network from source to dest.
Graph abstraction for
routing algorithms:
• graph nodes are
routers
• graph edges are
physical links
– link cost: delay, $
cost, or congestion
level
2
A
B
2
1
D
3
C
F
1
3
1
5
E
2
• “good” path:
– typically means
minimum cost path
– other def’s possible
IP Addressing: introduction
• IP address: 32-bit
identifier for host, router
interface
• interface: connection
between host, router and
physical link
– router’s typically have
multiple interfaces
– host may have multiple
interfaces
– IP addresses associated
with interface, not host,
router
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4
223.1.1.3
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
223.1.3.1
223.1.1.1 = 11011111 00000001 00000001 00000001
223
1
1
1
IP datagram format
IP protocol version
number
header length
(bytes)
“type” of data
max number
remaining hops
(decremented at
each router)
upper layer protocol
to deliver payload to
32 bits
type of
ver head.
len service
length
fragment
16-bit identifier flgs
offset
time to upper
Internet
layer
live
checksum
total datagram
length (bytes)
for
fragmentation/
reassembly
32 bit source IP address
32 bit destination IP address
Options (if any)
data
(variable length,
typically a TCP
or UDP segment)
E.g. timestamp,
record route
taken, specify
list of routers
to visit.
Getting a datagram from source to
dest.
routing table in A
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
IP datagram:
misc source dest
fields IP addr IP addr
data
• datagram remains
unchanged, as it
travels source to
destination
• addr fields of interest
here
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
E
Getting a datagram from source to
dest.
misc
data
fields 223.1.1.1 223.1.1.3
Starting at A, given IP
datagram addressed to
B:
• look up net. address of B
• find B is on same net. as A
• link layer will send
datagram directly to B
inside link-layer frame
– B and A are directly
connected
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
A
223.1.1.4
223.1.1.4
1
2
2
223.1.1.1
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
E
Getting a datagram from source to
dest.
misc
data
fields 223.1.1.1 223.1.2.3
Dest. Net. next router Nhops
223.1.1
223.1.2
223.1.3
Starting at A, dest. E:
• look up network address of E
• E on different network
A
223.1.1.4
223.1.1.4
223.1.1.1
– A, E not directly attached
• routing table: next hop router to
E is 223.1.1.4
• link layer sends datagram to
router 223.1.1.4 inside linklayer frame
• datagram arrives at 223.1.1.4
• continued…..
1
2
2
223.1.2.1
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
E
Getting a datagram from source to
dest.
misc
data
fields 223.1.1.1 223.1.2.3
• Arriving at 223.1.4, destined for
223.1.2.2
• look up network address of E
• E on same network as router’s
interface 223.1.2.9
Dest.
next
network router Nhops interface
223.1.1
223.1.2
223.1.3
A
1
1
1
223.1.1.4
223.1.2.9
223.1.3.27
223.1.1.1
223.1.2.1
– router, E directly attached
• link layer sends datagram to
223.1.2.2 inside link-layer frame
via interface 223.1.2.9
• datagram arrives at 223.1.2.2!!!
(hooray!)
-
B
223.1.1.2
223.1.1.4
223.1.1.3
223.1.3.1
223.1.2.9
223.1.3.27
223.1.2.2
223.1.3.2
E
IP Fragmentation & Reassembly
• network links have MTU
(max.transfer size) - largest
possible link-level frame.
– different link types,
different MTUs
• large IP datagram divided
(“fragmented”) within net
– one datagram becomes
several datagrams
– “reassembled” only at
final destination
– IP header bits used to
identify, order related
fragments
fragmentation:
in: one large datagram
out: 3 smaller datagrams
reassembly
IP Fragmentation and Reassembly
length ID fragflag offset
=4000 =x
=0
=0
One large datagram becomes
several smaller datagrams
length ID fragflag offset
=1500 =x
=1
=0
length ID fragflag offset
=1500 =x
=1
=1480
length ID fragflag offset
=1040 =x
=0
=2960
MTU is min. of 576 bytes, so if MSS is 536b, fragmentation can eliminated
RIP ( Routing Information Protocol)
• Distance vector algorithm
• Included in BSD-UNIX Distribution in 1982
• Distance metric: # of hops (max = 15 hops)
– Can you guess why?
• Distance vectors: exchanged every 30 sec via
Response Message (also called advertisement)
• Each advertisement: route to up to 25
destination nets
RIP: Link Failure and Recovery
If no advertisement heard after 180 sec -->
neighbor/link declared dead
– routes via neighbor invalidated
– new advertisements sent to neighbors
– neighbors in turn send out new
advertisements (if tables changed)
– link failure info quickly propagates to entire
net
– poison reverse used to prevent ping-pong
loops (infinite distance = 16 hops)
RIP Table processing
• RIP routing tables managed by applicationlevel process called route-d (daemon)
• advertisements sent in UDP packets,
periodically repeated
RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination
-------------------127.0.0.1
192.168.2.
193.55.114.
192.168.3.
224.0.0.0
default
Gateway
Flags Ref
Use
Interface
-------------------- ----- ----- ------ --------127.0.0.1
UH
0 26492 lo0
192.168.2.5
U
2
13 fa0
193.55.114.6
U
3 58503 le0
192.168.3.5
U
2
25 qaa0
193.55.114.6
U
3
0 le0
193.55.114.129
UG
0 143454
• Three attached class C networks (LANs)
• Router only knows routes to attached LANs
• Default router used to “go up”
• Route multicast address: 224.0.0.0
• Loopback interface (for debugging)
• ASUWLINK: netstat –rn will show the route table
ICMP: Internet Control Message
Protocol
• used by hosts, routers, gateways to communication
network-level information
– error reporting: unreachable host, network, port,
protocol
– echo request/reply (used by ping)
• network-layer “above” IP:
– ICMP msgs carried in IP datagrams
• ICMP message: type, code plus first 8 bytes of IP
datagram causing error
ICMP
• Reason for ICMP:
– provides a mechanism for IP devices to use
when they need to exchange information
about network problems that are preventing
delivery
• Normally semi-permanent and/or non-transient
errors.
• problems that prevent all datagrams through to
their destination.
ICMP (2)
• IP datagram failed to be delivered
because
– next-hop router is unavailable
– non-existent destination IP address or Port
• ICMP error message are returned if is a
semi-permanent or non-transient error
– transient errors are ignored and left to the
application or TCP to deal with.
• why? the next packet should not have the same
problem.
ICMP (3)
• non-transient and semi-permanent errors
– a fundamental problem with the network itself
– a problem in the way that the sender is trying
to use the network
– destination becomes unreachable
– IP Time-to-Live value reaching zero
• Time-to-Live value based on hops, not actual time.
ICMP (4)
• Also used for
– exchanging general information about the
network
• Essentially ICMP is a collection of
predefined messages
– system chooses a message from a dictionary,
places the code for the message into an
ICMP-specific datagram and then sends it.
ICMP Message Formats
Ping and ICMP
• Uses ICMP messages to test basic
connectivity between two devices
• The message created is for ICMP itself, so
no other protocols are involved
– ICMP is not a transport protocol and as such
can not be used to deliver application data.
• ICMP receives the message, which is an
"echo request", then generates a response
"echo reply" and sends it.
UNIX ping
• example (ping k2 from meru)
seker>ping k2
PING k2.cs.uwyo.edu (129.72.216.12): 56 data bytes
64 bytes from 129.72.216.12: icmp_seq=0 ttl=64 time=2.773 ms
64 bytes from 129.72.216.12: icmp_seq=1 ttl=64 time=1.720 ms
----k2.cs.uwyo.edu PING Statistics---2 packets transmitted, 2 packets received, 0.0% packet loss
round-trip min/avg/max = 1.720/2.246/2.773 ms
• MEANS:
• icmp_seq is the sequence number from the icmp packet
• ttl is time-to-live, time is the round trip time for the packet, so 2.773
milliseconds for the first line
• And summary information
ICMP message types
• Three main message types:
– ICMP error message
• There a problem to report
– ICMP query message
• Asking for information
– ICMP query reply message
• response to query
• It is really a ICMP query message, it is easier to
think about separately.
When not to send ICMP Messages
• An ICMP error message in response to
another ICMP error message.
– would create a message loop and case a
network/broadcast storm.
• ICMP Error messages to broadcast or
multicast address
– It could generate thousands of messages.
• ICMP query response messages may be sent.
• Optional in the RFC 1122
Common Message Types
Type
0
3
4
5
8
9
10
11
12
13
14
17
18
Code
0
0-15
0
0-3
0
0
0
0-1
0-2
0
0
0
0
description
echo reply (ping)
dest. network unreachable
source quench
Redirect
echo request (ping)
route advertisement
router Solicition
Time-To-Live expired
bad IP header
Timestamp Request
Timestamp reply
Address Mask request
Address Mask Reply
Message Family
Query (reply)
Error
Error
Error
Query (request)
Query (reply)
Query (request)
Error
Error
Query (request)
Query (reply)
Query (request) (obsolete)
Query (reply) (obsolete)
traceroute
• allows you to identify the route that datagrams
are taking to a remote device.
• How it works:
– sends a set of packets with incrementally larger Timeto-Live (hops) values, checking ICMP time exceeded
error messages as packets expire getting to their
distintation
– first packet ttl =1, router sets it to zero, and returns an
ICMP error, traceroute notes the router and time
– second packet ttl=2, so the second router sets it to
zero, and returns an ICMP error, traceroute notes the
routers and time,
– etc, until it reaches it destination.
Traceroute Example 1
>traceroute k2
traceroute to k2 (129.72.216.12), 30 hops max, 60 byte packets
1 k2.cs.uwyo.edu (129.72.216.12) 3 ms 2 ms 2 ms
• Only 1 hop, because there is no router in between
• this version sends 3 messages to get a better idea of time.
>traceroute arthur.uwyo.edu
traceroute to arthur.uwyo.edu (129.72.10.203), 30 hops max, 60 byte
packets
1 129.72.216.1 11 ms 7 ms 9 ms
2 quark.uwyo.edu (129.72.62.70) 1 ms 5 ms 2 ms
3 arthur.uwyo.edu (129.72.10.203) 2 ms 2 ms 2 ms
• the 216 "gateway", internal uwyo router, then arthur.
traceroute Example 2
>traceroute www.netscape.com
traceroute to www.netscape.com (64.12.151.215), 30 hops max, 60 byte packets
1 129.72.216.1 5 ms 8 ms 8 ms
2 uwyo-router-subnet-062.uwyo.edu (129.72.62.1) 2 ms 1780 ms 1781 ms
3 frgp-gw-1.uwyo.edu (129.72.253.6) 12 ms 9 ms 6 ms
4 ucar.edu.ip.att.net (12.124.158.13) 24 ms 18 ms 16 ms
5 gbr1-p60.dvmco.ip.att.net (12.123.36.138) 27 ms 20 ms 25 ms
6 gbr4-p70.dvmco.ip.att.net (12.122.5.21) 19 ms 1697 ms 1781 ms
7 gbr4-p80.dlstx.ip.att.net (12.122.2.101) 29 ms 1703 ms 1781 ms
8 gbr6-p70.dlstx.ip.att.net (12.122.5.85) 31 ms 1708 ms 1781 ms
9 ***
10 tbr2-p013401.attga.ip.att.net (12.122.10.74) 52 ms 2764 ms 46 ms
11 tbr1-p012501.attga.ip.att.net (12.122.9.157) 47 ms 2763 ms 46 ms
12 tbr2-p013801.wswdc.ip.att.net (12.122.10.69) 56 ms 2775 ms 62 ms
13 ggr2-p390.wswdc.ip.att.net (12.123.9.85) 60 ms 1706 ms 1781 ms
• NOTE: * * * indicates the request timed out, since it is not receiving any response from either
the destination system or intermediary, but continues with the next ttl increment.
• It may be a firewall, instead of a network failure.
Multicast
• Normally, an IP number refers to 1 host,
but it can refer to many hosts on 1 or more
networks.
– Known as a multicast address
• Multicasting: Sending a packet from 1 host
to members of a multicast group
Multicast Examples
• Multimedia
– Users "tune in" a video or audio transmission from a single
source, but the source does not send to each individual.
• Teleconferencing
• Database
– replicated database are updated at the same time
• Distributed computation
– intermediate results are sent to all participants. The sender need
no even know who they are
• Real-time workgroup
– work is exchanged among active members in real time.
Broadcast (briefly)
• Broadcast sends data from one device to
every other device on a local network
– uses a broadcast specific address to a
network topology
– Devices MUST monitor and read any frame
that is marked for the broadcast address
– typically, 255 as the last octet.
• 10.216.218.255 is the broadcast address for cosc.
• 129.72.255.255 is the broadcast address for all of
uwyo.edu. (very bad to use!)
Vs Broadcast and Unicast
• Broadcast sends the message to everyone on
the network
• Unicast sends to an individual
• So if we have 5 members on 2 different
networks
– unicast must create and send 5 packets (1 for each
member) for each packet sent.
– broadcast must broadcast each packet to 2 networks.
– Multicast sends 1 packet and each member gets the
packet.
Vs Broadcast and Unicast (2)
• Unicast
– more work for the source host, must create
and send a packet for each member
• Broadcast
– Many (hundreds!) get "junk" packets.
• Multicast
– Source host sends only 1 packet to the group.
The work is done on the routers (if there is
more than 1 network/LAN involved).
Multicast addresses
• Multicast address are known as a Class D
addresses
– All IP address from 224.0.0.0 to 239.255.255.255
– there are ranges inside that are associated with a
specific application service
• All number 224.0.0.0 to 224.0.0.255 are
predefined and reserved addresses for routing
protocols and infrastructure services.
– http://www.isi.edu/in-notes/iana/assignments/mulitcastaddresses
Multicast addresses examples
• 224.0.0.1 all local multicast hosts
(including routers) and is never forwarded
• 224.0.0.2 all local multicast routers and is
never forwarded
• 224.0.1.1 Network Time Protocol
• 224.0.1.24 Microsoft's Windows Internet
Name Server locator services (WINS)
Layer 2: Data Link Layer
• link layer services
– error detection, correction
– multiple access protocols and LANs
– link layer addressing, ARP
Link Layer: setting the context
• two physically connected devices:
– host-router, router-router, host-host
• unit of data: frame
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
data link
protocol
phys. link
adapter card
network
link
physical
Hl Hn Ht M
frame
CSMA: Carrier Sense Multiple
Access
CSMA: listen before transmit:
• If channel sensed idle: transmit entire packet
• If channel sensed busy, defer transmission
– Persistent CSMA: retry immediately with
probability p when channel becomes idle (may
cause instability)
– Non-persistent CSMA: retry after random
interval
• human analogy: don’t interrupt others!
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
– collisions detected within short time
– colliding transmissions aborted, reducing channel
wastage
– persistent or non-persistent retransmission
• collision detection:
– easy in wired LANs: measure signal strengths,
compare transmitted, received signals
– difficult in wireless LANs: receiver shut off while
transmitting
• human analogy: the polite conversationalist
LAN Addresses and ARP
32-bit IP address:
• network-layer address
• used to get datagram to destination network
(recall IP network definition)
LAN (or MAC or physical) address:
• used to get the frame from one interface to
another physically-connected interface (same
network)
• 48 bit MAC address (for most LANs)
burned in the adapter ROM
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
LAN Address (more)
• MAC address allocation administered by IEEE
• manufacturer buys portion of MAC address
space (to assure uniqueness)
• Analogy:
(a) MAC address: like Social Security
Number
(b) IP address: like postal address
• MAC flat address => portability
– can move LAN card from one LAN to another
• IP hierarchical address NOT portable
– depends on network to which one attaches
Recall earlier routing discussion
Starting at A, given IP
datagram addressed to
B:
A
• look up net. address of B,
find B on same net. as A
• link layer send datagram to
B inside link-layer frame
B
frame source,
dest address
B’s MAC A’s MAC
addr
addr
223.1.1.1
223.1.2.1
223.1.1.2
223.1.1.4 223.1.2.9
223.1.1.3
223.1.3.1
223.1.3.27
223.1.2.2
223.1.3.2
datagram source,
dest address
A’s IP
addr
B’s IP
addr
datagram
frame
IP payload
E
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
given B’s IP address?
• Each IP node (Host,
Router) on LAN has
ARP module, table
• ARP Table: IP/MAC
address mappings for
some LAN nodes
< IP address; MAC address;
TTL>
<
…………………………..
>
– TTL (Time To Live):
time after which
address mapping will
be forgotten (typically
20 min)
ARP protocol
• A knows B's IP address, wants to learn
physical address of B
• A broadcasts ARP query packet, containing
B's IP address
– all machines on LAN receive ARP query
• B receives ARP packet, replies to A with its
(B's) physical layer address
• A caches (saves) IP-to-physical address pairs
until information becomes old (times out)
– soft state: information that times out (goes
away) unless refreshed
Routing to another LAN
walkthrough: routing from A to B via R
A
R
B
• In routing table at source Host, find router
111.111.111.110
• In ARP table at source, find MAC address E6E9-00-17-BB-4B, etc
• A creates IP packet with source A, destination B
• A uses ARP to get R’s physical layer address for 111.111.111.110
• A creates Ethernet frame with R's physical address as dest,
Ethernet frame contains A-to-B IP datagram
• A’s data link layer sends Ethernet frame
• R’s data link layer receives Ethernet frame
• R removes IP datagram from Ethernet frame, sees its destined
to B
• R uses ARP to get B’s physical layer address
• R creates frame containing A-to-B IP datagram sends to B
A
R
B
Layer 1 and 0
• For our discussions we don’t have to worry to
much about layer 1 which is hardware NICs.
Also hubs and repeaters.
– Packet sniffing is done “mostly” at layer 2 and above.
• Layer 0 is transmission media such as wiring for
wired LANs.
– This would be physical security issue and less a
network issue.
– Wireless Technology will be covered separately.
References
• Computer Networking, A Top-Down Approach featuring the Internet,
Kurose and Ross, Addison Wesley, 2001
• Ethernet, The definitive Guide, Charles Spurgeon, O’Reilly, 2000.
• Internet Core Protocols, The Definitive Guide, Hall, O'Reilly, 2000.
• Cisco LAN Switch Configuration Guide, 1997
• Computer Networks, 3rd Edition, Andrew Tanenbaum, Prentice Hall,
1996
• Networking Essentials, 2nd Edition, Microsoft Press
• Computer Networking with Internet Protocols and Technology,
Stallings, Prentice Hall, 2003
• Computer Networks and Internets, 4th, Prentice Hall, 2003
• Internet Architectures, Minoli and Schmidt, Wiley, 1999
• Managing IP networks with Cisco Routers, Ballew, O'Reilly, 1997
• The Switch Book, The complete Guide to LAN Switching
Technology, Seifert, Wiley, 2000
• Numerous websites
Q&A