Transcript 3rd Edition, Chapter 5 - University of Delaware

Chapter 5: The Data Link Layer




Application
Transport
Network
data link layer service








Moving data between nearby network elements
•
•
•
Move data between end-host and router
Move data between end-hosts
Move data between routers
•
There are many types of physical layer
error detection, correction
Encryption
sharing a broadcast channel: multiple access
link layer addressing and routing
reliable data transfer, flow control
Interact/act as a bridge between the network layer and the physical layer
Which services that the link layer provides do other layers provide?
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link virtualization:
ATM, MPLS
Link Layer: Introduction
Some terminology:
 hosts and routers are nodes
 communication channels that
connect adjacent nodes along
communication path are links



wired links
wireless links
LANs
 layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
- This is not entirely correct.
Link layer: context
 datagram transferred by
different link protocols
over different links:

e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11
on last link
 each link protocol
provides different
services

e.g., may or may not
provide reliability over link
transportation analogy
 trip from Princeton to Lausanne



limo: Princeton to JFK
plane: JFK to Geneva
train: Geneva to Lausanne
 tourist = datagram
 transport segment =
communication link
 transportation mode = link
layer protocol

Note that a bus or plane trip
might contain many changes of
the bus or plane, but this
seems like a single hop
 travel agent = routing
algorithm
Link Layer Services

framing, link access:





encapsulate datagram into frame, adding header, trailer
channel access if shared medium
“MAC” addresses used in frame headers to identify
source, dest
• different from IP address!
Routing
reliable delivery between adjacent nodes



we learned how to do this already (chapter 3)!
seldom used on low bit-error link (fiber, some twisted
pair)
wireless links: high error rates
• Q: why both link-level and end-end reliability?
Link Layer Services (more)

flow control:


Encryption


pacing between adjacent sending and receiving nodes
Some links can easily be tapped, so encryption is needed for privacy
error detection:


errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame
 error correction:


receiver identifies and corrects bit error(s) without resorting to
retransmission
half-duplex and full-duplex

with half duplex, nodes at both ends of link can transmit, but not at
same time
Where is the link layer implemented?
 in each and every host in the
network

Which other layers are
implemented in every host?
 link layer implemented in
“adaptor” (aka network interface
card NIC)


Ethernet card, PCMCI card,
802.11 card
implements link, physical layer
 attaches into host’s system
buses
 combination of hardware,
software, firmware
host schematic
application
transport
network
link
cpu
memory
controller
link
physical
host
bus
(e.g., PCI)
physical
transmission
network adapter
card
Adaptors Communicating
datagram
datagram
controller
controller
receiving host
sending host
datagram
frame
 sending side:
 encapsulates datagram in
frame
 adds error checking bits,
rdt, flow control, etc.
 receiving side
 looks for errors, rdt, flow
control, etc
 extracts datagram
• passes to upper layer at
receiving side
• Moves frame to another
link
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM. MPLS
Error Detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
otherwise
Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
0
0
Internet checksum (review)
Goal: detect “errors” (e.g., flipped bits) in transmitted
packet (note: used at transport layer only)
Sender:
 treat segment contents
as sequence of 16-bit
integers
 checksum: addition (1’s
complement sum) of
segment contents
 sender puts checksum
value into UDP checksum
field
Receiver:
 compute checksum of
received segment
 check if computed checksum
equals checksum field value:
 NO - error detected
 YES - no error detected.
But maybe errors
nonetheless?
Checksumming: Cyclic Redundancy Check
 view data bits, D, as a binary number
 choose r+1 bit pattern (generator), G
 goal: choose r CRC bits, R, such that



<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits
 widely used in practice (Ethernet, 802.11 WiFi, ATM)
CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R
R = remainder[
D.2r
G
]
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
Multiple Access Links and Protocols
Two types of “links”:
 point-to-point
 PPP for dial-up access
 point-to-point link between Ethernet switch and host
 broadcast (shared wire or medium)
 old-fashioned Ethernet
 802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
Multiple Access Control (MAC) protocols
 single shared broadcast channel
 two or more simultaneous transmissions by
nodes: interference

collision if node receives two or more signals at the
same time
multiple access protocol
 An algorithm that determines how nodes share
channel, i.e., determine when node can transmit
 communication about channel sharing must use
channel itself!

out-of-band channel for coordination is difficult
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. when one node wants to transmit, it can send at
rate R.
2. when M nodes want to transmit, each can send at
average rate R/M
3. fully decentralized:



no special node to coordinate transmissions
no synchronization of clocks, slots
Generally, centralized MAC are much more efficient
4. simple
MAC Protocols: a taxonomy
Three broad classes:
 Channel Partitioning



divide channel into smaller “pieces” (time slots, frequency, code)
allocate piece to node for exclusive use
this approach is difficult since we know that statistical
multiplexing can support more users
 Random Access




channel not divided, allow collisions
Detect and recover from collisions
Detection and recovery (e.g., retransmission) can be inefficient
Predictable/guaranteed performance is difficult to achieve
 Centralized/taking turns
Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
 access to channel in "rounds"
 each station gets fixed length slot (length = pkt
trans time) in each round
 unused slots go idle
 GSM (some cell phones) uses TDMA


Why?
So service is predictable and calls can be rejected if
there is not enough bandwidth
 example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle
6-slot
frame
1
3
4
1
3
4
Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access
 channel spectrum divided into frequency bands
 each station assigned fixed frequency band
 unused transmission time in frequency bands go idle
 GSM also uses FDMA
 example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6
FDM cable
frequency bands
idle
Random Access Protocols
 When node has packet to send
 transmit at full channel data rate R.
 no a priori coordination among nodes
 two or more transmitting nodes ➜ “collision”,
 random access MAC protocol specifies:
 how to detect collisions
 how to recover from collisions (e.g., via delayed
retransmissions)
 Examples of random access MAC protocols:
 slotted ALOHA
 ALOHA
 CSMA, CSMA/CD, CSMA/CA
The ALOHA Protocol
 Developed @ U of Hawaii in early 70’s.
 Packet radio networks.
 “Free for all”: whenever station has a frame to send,
it does so.
 Aloha is the simplest of MAC protocols
 Aloha is old but still widely used


As will be seen, many protocols have a period of time where
nodes transmits when they want.
During such periods of time, the MAC essentially Aloha
Collisions
 Invalid frames may be caused by channel noise or
 Because other station(s) transmitted at the same
time: collision.
 Collisions and other link layer losses must be
detected and corrected


Question 1. Where are all the places that losses can
occur?
Question 2: where can errors be detected and corrected
 Roughly speaking, a collision happens even when
the last bit of a frame overlaps with the first bit
of the next frame.
ALOHA’s Performance 1
If another node transmits here,
then there is a collision
t0
t0+t
t0+2t
t0+3t
Time
vulnerable
If another node selects to transmit
during this vulnerable period, then a
collision will occur
ALOHA’s Performance
 Assume that users try to send frames at random times
(Poisson events).
 Let G be the average rate that users try to send frames per
frame time (G is the utilization).
 The probability of trying to send k frames in TWO frame
time is
k

2G  e 2G
Pk  
k!
The probability zero other frames are sent is P(0)=e-2G.
The throughput is the rate that frames are sent multiplied
by the probability that the transmission is successful
G e-2G
ALOHA’s Performance
0.184
G e
0.2
 2 G 0.1
0
0
0
0
1
2
G
3
3
The best throughput occurs for what value of G?
What is this best throughput?
Slotted Aloha – frames are only transmitted during
slots, they cannot cross slot boundaries
But this will only happen if a
packet arrives at the MAC
layer during this period
t0
t0+t
If a frame is transmitted
here, then a collision occurs
t0+2t
t0+3t
vulnerable
If another node selects to transmit during this
vulnerable period, then a collision will occur
The vulnerable period is half
the size of unslotted aloha
Time
Slotted Aloha
 Vulnerable period is halved.
 Doubles performance of ALOHA.
 S = G e-G.
 S = Smax = 1/e = 0.368 for G = 1.
Slotted Aloha Performance
0.368
G e
0.4
 G 0.2
0
0
0
2
4
0
G
4
Slotted Aloha Performance
How long does it take to send a frame?
Slotted Aloha Performance
How long does it take to send a frame?
T heprobability thatit takesexactlyk tries: pk   e
G
1  e 
G k 1
one success k-1 failures
Expected
number of
transmissions

 k r 
k 1
k 1


k 0
k 0

E   kpk    keG 1  e G
e
G
 k 1  e 

k 1
G k 1
e
G

k 1
1
1  1  e 
G
2
e G
  2G  eG
e

d
r k
k 1 dr

d 
d  1 
1
k
  r   

dr k 1
dr  1  r  1  r 2
This analysis is funny because it does not account for
the fact that if packets are not successfully
transmitted, then the rate at which transmissions are
attempted increases.
ALOHA and Slotted ALOHA
Pros
 single active node can
continuously transmit at full
rate of channel
 highly decentralized
 simple
Cons
 Collisions


wasting slots
Inefficient
 idle slots
 nodes may be able to detect
collision in less than time to
transmit packet
 Slotted aloha requires clock
synchronization

Lose synchronization requires
guard times, which reduces
efficiency
CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
 If channel sensed busy, defer transmission
 human analogy: don’t interrupt others!
Question
 For 10 Mbps ethernet, the maximum cable
length is 2000m
 For 100Mbps ethernet, the maximum cable
length is 200m
 Why is the maximum length for 100Mbps
10 times shorter than 10Mbps?
CSMA collisions
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
spatial layout of nodes
CSMA/CD collision detection
Transmitter 1
Transmitter 2
Receiver 1
Propagation delay
Transmission time
time
Collision detected
by transmitter 1.
When is it detected?
Collision detected
by transmitter 2
Position on wire
Receiver 1 receives
garbled signal
CSMA/CD collision detection
Transmitter 1
Transmission time
time
Collision NOT detected
by transmitter 1
Transmitter 2
Receiver 1
Propagation delay
Position on wire
Receiver 1 receives
garbled signal
Collision detected
by transmitter 2
What are the requirements to ensure that collisions are detected?
The transmitter must transmit for 2*Tpropagation + epsilon
The transmit time is frame length / bit rate
Therefore
2*CableLength/speed of propagation + epsilon < FrameLength/bit-rate
CSMA/CD
What are the requirements to ensure that collisions are detected?
The transmitter must transmit for 2*Tpropagation + epsilon
The transmit time is frame length / bit rate
Therefore
2*CableLength/speed of propagation + epsilon < FrameLength/bit-rate
If frame length can be arbitrarily small, then the cable length must be very short
Thus, frames cannot be arbitrarily small. Minimum frame length in Ethernet is 64B.
The minimum frame length in Ethernet is independent of bit-rate.
Why is the maximum cable length of a 10Mbps ethernet cable 10 times
longer than the maximum cable length of a 100Mbps ethernet?
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in
CSMA


collisions detected within short time
colliding transmissions aborted, reducing channel
wastage
 collision detection:
 easy in wired LANs: measure signal strengths,
compare transmitted, received signals
 Difficult/impossible in wireless LANs: received
signal strength overwhelmed by local transmission
strength
 human analogy: the polite conversationalist
persistent
What to do when the link is found to be busy?
 1-persistent


If medium is idle, then transmit.
If medium is not idle, then wait until it is and then transmit.
• In this case, all nodes that desire to transmit during the period
when a node is transmitting will collide!
 p-persistent



If medium is idle, then transmit.
If medium is not idle, then wait until it is idle
Once idle then transmit with probability p. And wait for the
next slot with probability 1-p and repeat.
• Here slot does not have to be the time to send a full frame, but
just enough time to let other hosts start sending.
 Exponential Backoff

Next slide
Exponential Backoff
1.
2.
3.
4.
Upon desiring to transmit a frame, set BackOff = BO (some
starting value, 4 and 8 are common)
If medium is idle, then transmit.
If medium is not idle, then wait until it is idle
Once idle,
a.
b.
pick an integer, r, between 0 and BO-1
Wait r time slots
1.
2.
5.
A time slot is long enough so that if a node begins to trasnmit at the
beginning of the time slot, then all nodes will hear the transmission before
the time slot end
Give an equation for the length of a time slot
c.
If no other transmission begins before the r time slots, then transmit
a.
b.
Continue to transmit so that all nodes will know that a collision
occurred, then stop
Set BO = min( 2 * BO , BO_Max )
c.
Go to step 4
If a collision is detected,
a.
In ethernet BO_max = 1024
Question: discuss the different ways in which backoff is used in network protocols
“Taking Turns” MAC protocols
channel partitioning MAC protocols:
 share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node!
Random access MAC protocols
 efficient at low load: single node can fully utilize channel
 high load: collision overhead
• Be careful. Here we say that high load is when the number
of users increases. If the number of users is fixed (and
small), then the efficiency under high load is not as bad
 “taking turns” protocols




look for best of both worlds!
Use in mobile phones data access
802.16 aka WiMax partly uses this approach
802.11 specifies this capability, but it is not widely
deployed YET
“Taking Turns” MAC protocols
Polling:
 master node “invites” slave nodes
to transmit in turn
 typically used with “dumb” slave
devices
 concerns:




data
polling overhead
latency
single point of failure (master)
master
QoS guarantees can be made

If a VoIP call requires 12bps.
The master can determine if
the call will receive the desire
quality and ensure that it
does.
•
•

When congested, new calls
are rejected, but existing call
continue to receive good
performance
Consider the difference
between the demands by
VoIP and services provided
by TCP
Guarantees are worth much
more money than nonguarantees
poll
data
slaves
“Taking Turns” MAC protocols
Token passing:
 control token passed
from one node to next
sequentially.
 token message
 concerns:



token overhead
Latency
single point of failure
(token)
T
(nothing
to send)
T
data
Summary of MAC protocols

channel partitioning, by time, frequency or code


random access (dynamic),





Time Division, Frequency Division
ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technologies (wire), hard in
others (wireless)
CSMA/CD used in Ethernet
CSMA/CA used in 802.11 (We’ll study it when we talk
about wireless)
taking turns


polling from central site, token passing
Bluetooth, FDDI, IBM Token Ring
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
MAC Addresses and ARP
 32-bit IP address:


network-layer address
used to get datagram to destination IP subnet
 MAC (or LAN or physical or Ethernet)
address:

function: get frame from one interface to another
physically-connected interface (same network)
• The textbook is wrong about this. Today, host are almost
never physically connected

48 bit MAC address (for most LANs)
• burned in NIC ROM, also sometimes software settable
LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
LAN Address (more)
 MAC address allocation administered by IEEE
 manufacturer buys portion of MAC address space (to assure
uniqueness)

Check OUI lookup
• Google OUI lookup
• Enter MAC address
• See manufacture
 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


address depends on IP subnet to which node is attached
If a NIC is changed, then the MAC is changed
• Whereas, the IP address can stay the same
ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
 Each IP node (host,
router) on LAN has
ARP table

137.196.7.78
1A-2F-BB-76-09-AD
137.196.7.23
 ARP table: IP/MAC
137.196.7.14
137.196.7.88
address mappings for
some LAN nodes
< IP address; MAC address; TTL>
LAN
71-65-F7-2B-08-53
At prompt, >> arp -a

58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
ARP protocol: Same LAN (network)

A wants to send datagram to C



Check if C’s IP address is in the
same subnet
Use subnet mask and compare this
nodes IP to C’s IP
E.g.,
•
•
•
•
•
•
my IP=128.4.35.67
B’s IP=128.5.19.12
Subnet mask is 255.255.0.0 => the
first 8 bytes define the subnet
So in this case, A and B are in
different subnets
Thus, the datagram is sent to the
gateway, which must be in the
same subnet.
Suppose that the B is the
gateway, but only the IP address
of B is known



Suppose a host wants to send to B and only B’s IP
address is know and B is in the same subnet
and B’s MAC address not in A’s ARP table.
A broadcasts ARP query packet, containing B's IP
address
 dest MAC address = FF-FF-FF-FF-FF-FF
 Ethernet frame type = ARP query
•
Other types include datagram
all machines on LAN receive ARP query
B receives ARP packet, replies to A with its (B's)
MAC address




A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes old (times
out)


frame sent to A’s MAC address (unicast)
soft state: information that times out (goes away)
unless refreshed
ARP is “plug-and-play”:

nodes create their ARP tables without intervention
from net administrator
Addressing: routing to another LAN
walkthrough: send datagram from A to B via R
assume A knows B’s IP address
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
111.111.111.111
E6-E9-00-17-BB-4B
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
111.111.111.112
R
222.222.222.221
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
 two ARP tables in router R, one for each IP
network (LAN)
 A creates IP datagram with source A, destination B
 A uses ARP to get R’s MAC address for 111.111.111.110
 A creates link-layer frame with R's MAC address as dest,





frame contains A-to-B IP datagram
This is a really important
A’s NIC sends frame
example – make sure you
understand!
R’s NIC receives frame
R removes IP datagram from Ethernet frame, sees its
destined to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
E6-E9-00-17-BB-4B
111.111.111.111
222.222.222.220
111.111.111.110
111.111.111.112
CC-49-DE-D0-AB-7D
222.222.222.221
1A-23-F9-CD-06-9B
R
222.222.222.222
B
49-BD-D2-C7-56-2A
ARP
 Watch wireshark without any connections
 What happens if I set an entry in the ARP table
with the IP address of my gateway, but my MAC
address?
 E.g., take two machines A and B on the same LAN
(what does this mean? How can you tell if two
machines are on the same LAN).






Let P be a nonexistent IP address in the LAN.
On machine A ping P.
• Use wireshark on B to see no evidence of the ping.
On A, set an arp entry on A with IP = P and MAC = B’s
MAC
Then ping P
Watch ping messages appear in wireshark on B
But still, no response.
ARP spoofing – man-in-the-middle attack
 If the medium is shared, then a node can
eavesdrop on transmissions
Wireless uses link layer encryption
 These days, wired ethernet used a dedicate
wires from the switch (link layer router) to
each host

• But ARP attack still works
 Goal: intercept messages between the
victim and anyone else
I record the real MAC address of the victim
 When an ARP query request is made for the
victim, I respond with my MAC

• Or better yet, I I w
ARP spoofing – man-in-the-middle attack
Victim:
MAC=00:12:12:12:12:12
IP: 1.2.3.4
Who has IP address 1.2.3.4
switch
Who has IP address 1.2.3.4
attacker:
MAC=00:11:11:11:11:11
IP= 5.6.7.8
Some other host
ARP spoofing – man-in-the-middle attack
Victim:
MAC=00:12:12:12:12:12
IP: 1.2.3.4
MAC 00:12:12:12:12 has IP address 1.2.3.4
MAC 00:12:12:12:12 has IP address 1.2.3.4
switch
attacker:
MAC=00:11:11:11:11:11
IP= 5.6.7.8
Attacker knows the
MAC of victim
Some other host
Save MAC/IP
mapping in cache for
20 minutes
ARP spoofing – man-in-the-middle attack
Victim:
MAC=00:12:12:12:12:12
IP: 1.2.3.4
Later (like the next day at 2AM), when
all caches have been cleared
Confused… but
ignores it
MAC 00:11:11:11:11 has IP address 1.2.3.4
switch
Some other host
MAC 00:11:11:11:11 has IP address 1.2.3.4
attacker:
MAC=00:11:11:11:11:11
IP= 5.6.7.8
Attacker knows the
MAC of victim
Save IP/ARP
mapping in cache
ARP spoofing – man-in-the-middle attack
Victim:
MAC=00:12:12:12:12:12
IP: 1.2.3.4
Later (like the next day at 2AM), when
all caches have been cleared
Ahh, I got the
secret plan I was
expecting
switch
Some other host
MAC 00:11:11:11:11: IP:1.2.3.4: The secret plan is …..
attacker:
MAC=00:11:11:11:11:11
IP= 5.6.7.8
MAC 00:12:12:12:12: IP:1.2.3.4 The secret plan is …..
Attacker knows the
secret plan
Changed MAC address to correct address
ARP spoofing – man-in-the-middle attack
 Some new switches can protect against these
attacks


How can these attacks be detected and stopped?
One way is to detect a attacker is to look at ARP tables
and see is a single IP has two MACs
• Is real IP and the victims IP
• But if a machine has wired and wireless NICs and is running
microsoft OS, the OS will sometimes send a frame with the
wireless IP as source address over the wired LAN and
hence with the wired MAC address
• Then tables will record the mapping between the MAC and
IP, and there will be two IPs for a single MAC
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM and MPLS
Ethernet
“dominant” wired LAN technology:
 cheap $20 for NIC
 first widely used LAN technology
 simpler, cheaper than token LANs and ATM
 kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
Star topology
 bus topology popular through mid 90s

all nodes in same collision domain (can collide with each other)
 star topology


active switch in center
each “spoke” runs a (separate) Ethernet protocol (nodes do not
collide with each other)
 LAN

Multiple stars connected (we’ll see later)
switch
bus: coaxial cable
star
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other
network layer protocol packet) in Ethernet frame
Preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates
Ethernet Frame Structure (more)
 Addresses: 6 bytes


if adapter receives frame with matching destination address, or with
broadcast address (eg ARP packet), it passes data in frame to network
layer protocol
otherwise, adapter discards frame (unless in promiscuous modes)
 Type:



ARP query/response
LAN routing
higher layer protocol (mostly IP but others possible, e.g., Novell IPX,
AppleTalk)
 CRC: checked at receiver, if error is detected, frame is dropped
Ethernet: Unreliable, connectionless
 connectionless: No handshaking between sending and
receiving NICs
 unreliable: receiving NIC doesn’t send acks or nacks
to sending NIC



stream of datagrams passed to network layer can have gaps
(missing datagrams)
gaps will be filled if app is using TCP
otherwise, app will see gaps
 Ethernet’s MAC protocol: unslotted CSMA/CD
Ethernet CSMA/CD algorithm
1. NIC receives datagram
4. If NIC detects another
from network layer,
transmission while
creates frame
transmitting, aborts and
sends jam signal
2. If NIC senses channel idle,
starts frame transmission 5. After aborting, NIC
If NIC senses channel
enters exponential
busy, waits until channel
backoff: after mth
idle, then transmits
collision, NIC chooses K at
random from
3. If NIC transmits entire
{0,1,2,…,2m-1}. NIC waits K
frame without detecting
slots where one slot is 512
another transmission, NIC
bit times, returns to Step
is done with frame !
2
Ethernet’s CSMA/CD (more)
Jam Signal: make sure all
other transmitters are
aware of collision; 48 bits
Bit time: .1 microsec for 10
Mbps Ethernet ;
for K=1023, wait time is
about 50 msec
Exponential Backoff:
 Goal: adapt retransmission
attempts to estimated
current load
 heavy load: random wait
will be longer
 first collision: choose K from
{0,1}; delay is K· 512 bit
transmission times
 after second collision: choose
K from {0,1,2,3}…
 after ten or more collisions,
choose K from
{0,1,2,3,4,…,1023}
CSMA/CD efficiency
 Tprop = max prop delay between 2 nodes in LAN
 ttrans = time to transmit max-size frame
efficiency
 efficiency goes to 1
 as tprop goes to 0
 as ttrans goes to infinity
1
1  5t prop /ttrans
 better performance than ALOHA: and simple,
cheap, decentralized!
802.3 Ethernet Standards: Link & Physical Layers

many different Ethernet standards





common MAC protocol and frame format
different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps
different physical layer media: fiber, cable
Very large ethernets are possible
MPLS runs over ethernet (so traffic engineering is possible)
application
transport
network
link
physical
MAC protocol
and frame format
100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister
pair) physical layer
fiber physical layer
Manchester encoding
 used in 10BaseT
 each bit has a transition
 allows clocks in sending and receiving nodes to
synchronize to each other

no need for a centralized, global clock among nodes!
 Hey, this is physical-layer stuff!
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3 Multiple access
protocols
5.4 Link-layer
Addressing
5.5 Ethernet
 5.6 Link-layer switches
 5.7 PPP
 5.8 Link Virtualization:
ATM, MPLS
Hubs
… physical-layer (“dumb”) repeaters:
 bits coming in one link go out all other links at
same rate
 all nodes connected to hub can collide with one
another
 no frame buffering
 no CSMA/CD at hub: host NICs detect
collisions
twisted pair
hub
Switch
 link-layer device: smarter than hubs, take
role

active
Store and forward Ethernet frames
• Question: do switches in circuit switching networks store and
forward?


transparent


examine incoming frame’s MAC address, selectively
forward frame to one-or-more outgoing links when frame
is to be forwarded on segment, uses CSMA/CD to access
segment
hosts are unaware of presence of switches
plug-and-play, self-learning

switches do not need to be configured
Switch: allows multiple simultaneous
transmissions
A
 hosts have dedicated,
direct connection to switch
 switches buffer packets
 Ethernet protocol used on
each incoming link, but no
collisions; full duplex


each link is its own collision
domain
switching: A-to-A’ and Bto-B’ simultaneously,
without collisions

not possible with dumb hub
C’
B
6
1
5
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
Switch Table

Q: how does switch know that
A’ reachable via interface 4,
B’ reachable via interface 5?
 A: each switch has a switch
table, each entry:

Q: how are entries created,
maintained in switch table?

C’
B
6
something like a routing
protocol?
1
5
(MAC address of host, interface
to reach host, time stamp)
 looks like a routing table!

A
2
3
4
C
B’
A’
switch with six interfaces
(1,2,3,4,5,6)
Switch: frame filtering/forwarding
When frame received:
1. record link associated with sending host
2. index switch table using MAC dest address
3. if entry found for destination
then {
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
forward on all but the interface
else flood
on which the frame arrived
4. periodically, purge all old table entries
Self-Learning
MAC
Interface
MAC
Interface
MAC
Interface
A
1
3
1
2
1
2
2
3
2
B
1
3
MAC
Interface
3
Self-Learning
MAC
Interface
MAC
Interface
MAC
Interface
A
Dest=B; Source=A
1
3
1
2
1
2
2
3
2
B
1
3
MAC
Interface
3
Self-Learning
MAC
Interface
A
1
MAC
Interface
MAC
Interface
A
Dest=B; Source=A
1
3
1
2
1
2
2
3
Make table entry for A
No table entry for B, so flood
Note: if the switch
has ports manually
configured, then the
frame is not flooded
to a host.
But they are flooded
to other switches
2
B
1
3
MAC
Interface
3
Self-Learning
MAC
Interface
A
1
Make table entry for A
No table entry for B, so flood
MAC
Interface
A
1
MAC
Interface
A
1
Dest=B; Source=A
3
1
2
1
2
2
3
2
B
1
3
MAC
Interface
3
Self-Learning
MAC
Interface
A
1
Make table entry for A
No table entry for B, so flood
MAC
Interface
MAC
Interface
A
1
A
2
A
1
3
2
Dest=B; Source=A
1
2
1
2
3
Dest=B; Source=A
1
2
B
3
MAC
Interface
A
1
3
Make table entry for A
No table entry for B, so flood
Self-Learning
MAC
Interface
A
1
MAC
Interface
MAC
Interface
A
1
A
2
A
1
3
1
2
1
2
2
3
2
Dest=A; Source=B
B
1
3
MAC
Interface
A
1
3
Self-Learning
MAC
Interface
A
1
MAC
Interface
MAC
Interface
A
1
A
2
A
1
3
2
1
2
1
2
3
1
Dest=A; Source=B
2
B
3
MAC
Interface
A
1
B
2
3
Make table entry for B
Have a table entry for A, so forward
Self-Learning
MAC
Interface
A
1
Make table entry for B
Have a table entry for A, so forward
A
1
2
MAC
Interface
MAC
Interface
A
1
A
2
B
3
3
1
2
1
2
3
Dest=A; Source=B
2
B
1
3
MAC
Interface
A
1
B
2
3
Self-Learning
A
MAC
Interface
A
1
B
3
MAC
Interface
MAC
Interface
A
1
A
2
B
Make table entry for B
Have a table entry for A, so forward
3
1
3
1
2
1
2
Dest=A;
Source=B
2
3
2
B
1
3
MAC
Interface
A
1
B
2
3
Self-Learning
MAC
20 minutes later, all table entries are deleted
Interface
MAC
Interface
MAC
Interface
A
1
3
1
2
1
2
2
3
2
B
1
3
MAC
Interface
3
Poorly Designed Institutional
network. Why?
to external
network
mail server
router
web server
IP subnet
Institutional network without a
single point of failure
to external
network
mail server
router
web server
IP subnet
A
Explain self learning on this network
Suppose that A sends a frame to the mail server and all tables are empty?
Due to the loops, the frames will loop and overwhelm the network.
Loops provide robustness, but have to be eliminated.
Loop Resolution
 Goal: remove “extra” paths by removing
“extra” bridges.
 Spanning tree:
Given graph G(V,E), there exists a tree that
spans all nodes where there is only one path
between any pair of nodes, i.e., NO loops.
 LANs are represented by nodes and bridges by
edges.

Spanning Tree Algorithm (1)
 When manufactured, each bridge is given a unique ID. The root is
the node with the smallest ID.
 Approach: Form a “routing” to the node with smallest ID
 By a routing, we mean that each switch knows which interface leads to
the shortest path to the switch with smallest ID
 This tree is formed by “disconnecting” switches from some LANs
 Note 1. here a LAN connects two switches. It is possible that end-host
are also attached to the same LAN (CSMA/CD will work), but today, the
“LAN” only has a switch on each end.
 Note 2. The switches are not physically disconnected, they simply never
flood packets over to the LAN.
• Of course, the spanning tree is recomputed often and if something breaks,
then the LAN might be “reconnected” to the switch
 Algorithm: If a LAN has more than one bridge, all the bridges on
that LAN must decide which bridge can route frames to and from
the LAN.


Switches remain connected if they are part of the shortest path to
switch with min ID
If there is a tie, then the one with smaller ID is selected.
Spanning Tree Algorithm (2)
Bridges exchange messages with the following information
 1. The ID of the bridge that is sending the message.
 2. The ID for what the sending bridge believes to be the root
bridge.
 3. The distance (hops) from the sending bridge to the root
bridge.
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
B2
B1
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
By rule 3, switch B5 will keep this interface
active since it uses this for its shortest path to
B1 (root)
B2
B1
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
By rule 1, switch B5 will keep this interface
active since it has a shorter path to the root
than B3
B2
B1
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B3’s has two shortest routes to the root.
One is through B2 and the other is through B5.
B2 has a lower ID than B5.
So this interface is not use to get to the root.
Thus, this interface is ignored.
B7
B5
B2
B1
Important: while we are talking about “getting
to the root” we are not talking about
forwarding packets to the root.
The forwarding tables (constructed with self
learning) will do this.
We use the idea of shortest path to determine
which interfaces to ignore.
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
B3 uses this interface to reach B1 along it path
via B2
B2
B1
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
Do the rest
B2
B1
B6
B4
Which interfaces to keep and which to ignore.
Pretend that the objective is to route to root (the one with smallest ID) and use least cost with
minimum ID to break ties.
A switch will keep an interface active if
1.
the switch has the shortest path to the
root
2.
The switch has a path of equal length as
other switches, but these other switches
have a higher ID
3.
The interface is used to route along the
shortest path to the root.
B3
B7
B5
B2
B1
B6
B4
Spanning Tree Routing
transparent bridges.
 Bridge routing table is automatically maintained
 Aka
(set up and updated as topology changes).
 3 mechanisms:



Loop resolution
Address learning
Frame forwarding
 Loop resolution must happen before address
learning.


On the EECIS network, the link to the campus network
would go down for ~50ms.
This would trigger loop resolution
• During which time no packets were forwarded
Link Layer
 5.1 Introduction and




services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet
 5.6 Hubs and switches
 5.7 PPP
 5.8 Link Virtualization:
ATM and MPLS
ATM “link layer”
Vision: end-to-end
transport: “ATM from
desktop to desktop”
 ATM is a network
technology
Reality: used to connect
IP backbone routers
 “IP over ATM”
 ATM as switched
link layer,
connecting IP
routers
IP
network
ATM
network
ATM Physical Layer
Physical Medium Dependent (PMD) sublayer
 SONET/SDH: transmission frame structure (like a
container carrying bits);
 bit synchronization;
 bandwidth partitions (TDM);
 several speeds: OC3 = 155.52 Mbps; OC12 = 622.08
Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps
 TI/T3: transmission frame structure (old
telephone hierarchy): 1.5 Mbps/ 45 Mbps
 unstructured: just cells (busy/idle)
ATM architecture
AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
 adaptation layer: only at edge of ATM network
data segmentation/reassembly
 roughly analagous to Internet transport layer
 ATM layer: “network” layer
 cell switching, routing
 physical layer

IP-Over-ATM
Classic IP only
 3 “networks” (e.g.,
LAN segments)
 MAC (802.3) and IP
addresses
IP over ATM
 replace “network”
(e.g., LAN segment)
with ATM network
 ATM addresses, IP
addresses
ATM
network
Ethernet
LANs
Ethernet
LANs
IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
app
transport
IP
AAL
ATM
phy
Datagram Journey in IP-over-ATM Network
 at Source Host:
 IP layer maps between IP, ATM dest address (using ARP)
 passes datagram to AAL5
 AAL5 encapsulates data, segments cells, passes to ATM layer
 ATM network: moves cell along VC to destination
 at Destination Host:
AAL5 reassembles cells into original datagram
 if CRC OK, datagram is passed to IP

IP-Over-ATM
Issues:
 IP datagrams into ATM
AAL5 PDUs
 from next-hop IP
addresses to ATM
addresses
 just like IP addresses
to 802.3 MAC
addresses!
ATM
network
Ethernet
LANs
Multiprotocol label switching (MPLS)
 initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do
forwarding


borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
label
20
IP header
Exp S TTL
3
1
5
remainder of link-layer frame
MPLS capable routers
 a.k.a. label-switched router
 forwards packets to outgoing interface based only on
label value (don’t inspect IP address)

MPLS forwarding table distinct from IP forwarding tables
 signaling protocol needed to set up forwarding
 RSVP-TE
 forwarding possible along paths that IP alone would not allow
(e.g., source-specific routing) !!
 use MPLS for traffic engineering
 must co-exist with IP-only routers
MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R6
0
0
D
1
1
R3
R4
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
0
in
label
6
outR1
label dest
-
A
A
out
interface
0
VLAN (virtual LAN)
 Recall that ARPs are flooded throughout the entire LAN
 On a very large LAN, this flooding can be expensive
 There are security risks to having all end-hosts in the same
LAN

E.g., by listening to ARP messages, it is possible to passively
“scan” the network and learn about which hosts exist and are
active.
 VLANs can construct multiple LANs within a single LAN
 Frames are restricted to the VLAN




ARPs are only flooded in the VLAN
Passive listening is limited
Security between hosts can be done at the network layer
NetBios uses LAN only, so VLANs isolate NetBios host
• NetBios is used for sharing hard drives and printers
• It is possible to configure VLANs so that NetBios passes between
VLANs
VLAN
What is the difference
between a router and a
gateway?
Router/
gateway
Without VLAN all flooded frames go everywhere
Recall the tables empty periodically, so flooding is
frequent
switch
VLAN
Each color is a different
virtual LAN
Router/
gateway
• Note that each VLAN needs its
own gateway
• Each gateway must be in the same
subnet as the hosts in the VLAN
• So the single gateway must
have multiple IP addresses
• Hosts in the same VLAN need not
be attached to the same switch
• Hosts can move, and still be in the
same VLAN
• so they can still access their
shared hard drive
switch
VLAN Tagging
 To establish a packet’s association with a
particular VLAN, a tag is added
 802.1q – Specifies appending 32-bit VLAN tag
(field) into Ethernet Frame after Ethernet header
 12 bits are assigned to VLAN ID
 Usual Scenario





Packet enters switch from source host
Tag appended
Gets routed through the LAN and finally to a specific
port
Tag is stripped off
Original packet passed to destination host
Chapter 5: let’s take a breath
 journey down protocol stack
complete
(except PHY)
 solid understanding of networking principles,
practice
 ….. could stop here …. but lots of interesting
topics!
wireless
 multimedia
 security
 network management
