[SOLVED] CS algorithm flex dns DHCP ER data mining 2/25/21

2/25/21
Chapter 6
The Link Layer
and LANs
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All material copyright 1996-2020
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A
Top-Down Approach
8th edition
Jim Kurose, Keith Ross Pearson, 2020
Link layer and LANs: our goals
understand principles behind link layer services:
error detection, correction
sharing a broadcast channel:
multiple access
link layer addressing
local area networks:
Ethernet, VLANs datacenter networks
instantiation, implementation of various link layer technologies
Link Layer: 6-2
12
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-3
Link layer: introduction
terminology:
hosts and routers: nodes
communication channels that connect adjacent nodes along communication path: links
wired
wireless LANs
layer-2 packet: frame, encapsulates datagram
mobile network
enterprise network
national or global ISP
da er ne
Link Layer: 6-4
tacent twork
link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
34
1

Link layer: services
framing, link access:
encapsulate datagram into frame, adding
header, trailer
channel access if shared medium
MAC addresses in frame headers identify
source, destination (different from IP address!)
reliable delivery between adjacent nodes we already know how to do this!
seldom used on low bit-error links
wireless links: high error rates
Q: why both link-level and end-end reliability?

Link Layer: 6-6
56
Link layer: services (more)
flow control:
pacing between adjacent sending and receiving nodes
error detection:
errors caused by signal attenuation, noise. receiver detects errors, signals
retransmission, or drops frame
error correction:
receiver identifies and corrects bit error(s)
without retransmission
half-duplex and full-duplex:
with half duplex, nodes at both ends of link can transmit, but not at same time

Link Layer: 6-7
Where is the link layer implemented?
in each-and-every host
link layer implemented in network interface card (NIC) or on a chip
Ethernet, WiFi card or chip
implements link, physical layer
attaches into hosts system buses
combination of hardware, software, firmware
host bus (e.g., PCI)
nterface
Link Layer: 6-8
application transport
network
link
cpu
memory
controller
link
physical
physical
network i
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Link layer: context
datagram transferred by different link protocols over differentlinks:
e.g., WiFi on first link, Ethernet onnextlink
each link protocol provides different services
e.g., may or may not provide reliable data transfer over link
transportation analogy:
trip from Princeton to Lausanne limo:PrincetontoJFK
plane: JFK to Geneva
train:GenevatoLausanne
tourist = datagram transport segment =
communication link
transportation mode = link-layer protocol
travel agent = routing algorithm
Link Layer: 6-5
2

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Interfaces communicating
datagram network
application transport
network
link
application transport
cpu m
emory
memory
CPU
link
l
datagram
datagram
receiving side:
looks for errors, reliable data
transfer, flow control, etc.
extracts datagram, passes to upper layer at receiving side
l
i
i
n
n
k
k
h
controller
controller
h
link
physical
link
physical
physical
physical
sending side:
encapsulates datagram in frame
adds error checking bits, reliable data
transfer, flow control, etc.
Link Layer: 6-9
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols
LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-10
9 10
Error detection
EDC: error detection and correction bits (e.g., redundancy)
D: data protected by error checking, may include header fields
datagram
d data bits
D EDC
bit-error prone link
all
bits in D N
OK detected ? error
D EDC
datagram
otherwise
Error detection not 100% reliable!
protocol may miss some errors, but rarely
larger EDC field yields better detection and correction
Link Layer: 6-11
Parity checking
single bit parity:
detect single bit errors
d data bits
parity bit
Even parity: set parity bit so there is an even number of 1s
two-dimensional bit parity:
detect and correct single bit errors row parity
0111000110101011
1
d1,1 . . . d2,1 . . .
d1,j d1,j+1 d2,j d2,j+1
di,j di,j+1 di+1,j di+1,j+1
detected 10101 1 and
column parity
noerrors: 10101 1 11110 0 01110 1 10101 0
di,1 di+1,1
. . .
. . .
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
Link Layer: 6-12
parity correctable 10110 0 error
single-bit error:
01110 1 10101 0
parity error
11 12
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Internet checksum (review)
Goal: detect errors (i.e., flipped bits) in transmitted segment
sender:
treat contents of UDP segment (including UDP header fields and IP addresses) as sequence of 16-bit integers
checksum: addition (ones complement sum) of segment content
checksum value put into UDP checksum field
receiver:
compute checksum of received segment
check if computed checksum equals checksum field value:
not equal error detected
equal no error detected. But maybe
errors nonetheless? More later .
Transport Layer: 3-13
Cyclic Redundancy Check (CRC)
more powerful error-detection coding
D: data bits (given, think of these as a binary number) G: bit pattern (generator), of r+1 bits (given)
r CRC bits d data bits
D R bit pattern
= D*2r XOR R formula for bit pattern
goal: choose r CRC bits, R, such that exactly divisible by G (mod 2) receiver knows G, divides 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)
Link Layer: 6-14
13 14
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols
LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-16
Multiple access links, protocols
two types of links:
point-to-point
point-to-point link between Ethernet switch, host
PPP for dial-up access
broadcast (shared wire or medium)
old-fashioned Ethernet
upstream HFC in cable-based access network
802.11 wireless LAN, 4G/4G. satellite
shared wire (e.g., shared radio: 4G/5G shared radio: WiFi cabled Ethernet)
shared radio: satellite
humans at a cocktail party (shared air, acoustical)
Link Layer: 6-17
16 17
4

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Multiple access 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
distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit
communication about channel sharing must use channel itself! no out-of-band channel for coordination
Link Layer: 6-18
An ideal multiple access protocol
given: multiple access channel (MAC) of rate R bps
desiderata:
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 4. simple
Link Layer: 6-19
MAC protocols: taxonomy
three broad classes:
channel partitioning
divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use
random access
channel not divided, allow collisions recover from collisions
taking turns
nodes take turns, but nodes with more to send can take longer turns
Link Layer: 6-20
Channel partitioning MAC protocols: TDMA
TDMA: time division multiple access
access to channel in rounds
each station gets fixed length slot (length = packet transmission time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have packets to send, slots 2,5,6 idle
6-slot 6-slot frame frame
134134
Link Layer: 6-21
20 21
5

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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
example: 6-station LAN, 1,3,4 have packet to send, frequency bands 2,5,6 idle
FDM cable
Link Layer: 6-22
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:
ALOHA, slotted ALOHA
CSMA, CSMA/CD, CSMA/CA
Link Layer: 6-23
Slotted ALOHA
assumptions:
all frames same size
time divided into equal size
slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot, all nodes detect collision
operation:
when node obtains fresh frame, transmits in next slot
if no collision: node can send new frame in next slot
if collision: node retransmits frame in each subsequent slot with probability p until success
randomization why?
Link Layer: 6-24
Slotted ALOHA
node111 11 node 2 2 2 2
C: collision S: success E: empty
node3 3 CECSECESS
3 3
Pros:
single active node can continuously transmit at full rate of channel
highly decentralized: only slots in nodes need to be in sync
simple
Cons:
collisions, wasting slots
idle slots
nodes may be able to detect collision in less than time to transmit packet
clock synchronization
Link Layer: 6-25
24 25
6
time
frequency bands

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Slotted ALOHA: efficiency
efficiency: long-run fraction of successful slots (many nodes, all with many frames to send)
suppose: N nodes with many frames to send, each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency: find p* that maximizes Np(1-p)N-1
for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives:
max efficiency = 1/e = .37
at best: channel used for useful transmissions 37% of time!
Link Layer: 6-26
Pure ALOHA
unslotted Aloha: simpler, no synchronization when frame first arrives: transmit immediately
collision probability increases with no synchronization:
frame sent at t0 collides with other frames sent in [t0-1,t0+1]
will overlap with start of is frame
will overlap
with end of is frame
pure Aloha efficiency: 18% !
t0 -1
t0
t0 +1
Link Layer: 6-27
26 27
CSMA (carrier sense multiple access)
simple CSMA: listen before transmit:
if channel sensed idle: transmit entire frame if channel sensed busy: defer transmission
human analogy: dont interrupt others!
CSMA/CD: CSMA with collision detection
collisions detected within short time
colliding transmissions aborted, reducing channel wastage collision detection easy in wired, difficult with wireless
human analogy: the polite conversationalist
Link Layer: 6-28
CSMA: collisions
collisions can still occur with carrier sensing:
propagation delay means two nodes may not hear each others just- started transmission
collision: entire packet transmission time wasted
distance & propagation delay play role in in determining collision probability
spatial layout of nodes
Link Layer: 6-29
28 29
7

30 31
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CSMA/CD:
CSMA/CS reduces the amount of time wasted in collisions
transmission aborted on collision detection
spatial layout of nodes
Link Layer: 6-30
Ethernet CSMA/CD algorithm
1. NIC receives datagram from network layer, creates frame
2. If NIC senses channel:
if idle: start frame transmission.
if busy: wait until channel idle, then transmit
3. If NIC transmits entire frame without collision, NIC is done with frame !
4. If NIC detects another transmission while sending: abort, send jam signal
5. After aborting, NIC enters binary (exponential) backoff:
aftermthcollision,NICchoosesKatrandomfrom{0,1,2,,2m-1}.NICwaits
K512 bit times, returns to Step 2
morecollisions:longerbackoffinterval
Link Layer: 6-31
CSMA/CD efficiency
Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency = 1 1+5tprop/ttrans
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA: and simple, cheap, decentralized!
Link Layer: 6-32
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
taking turns protocols
look for best of both worlds!
Link Layer: 6-33
32 33
8

Taking turns MAC protocols
polling:
master node invites other nodes to transmit in turn
typically used with dumb devices
concerns:
polling overhead
latency
single point of failure (master)
data
poll
master
data
slaves
Link Layer: 6-34
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
Link Layer: 6-35
34 35
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Cable access network: FDM, TDM and random access! Internet frames, TV channels, control transmitted
cable headend
downstream at different frequencies

CMTS

splitter
cable modem
IS
P
cable modem termination system
multiple downstream (broadcast) FDM channels: up to 1.6 Gbps/channel single CMTS transmits into channels
multiple upstream channels (up to 1 Gbps/channel)
multiple access: all users contend (random access) for certain upstream channel time slots; others assigned TDM
Link Layer: 6-36
Cable access network:
MAP frame for Interval [t1, t2]
Downstream channel i Upstream channel j
CMTS
cable headend
t1
Minislots containing minislots request frames
Residences with cable modems
Assigned minislots containing cable modem upstream data frames
DOCSIS: data over cable service interface specificaiton FDM over upstream, downstream frequency channels
TDM upstream: some slots assigned, some have contention
downstream MAP frame: assigns upstream slots
request for upstream slots (and data) transmitted random access (binary backoff) in selected slots
Link Layer: 6-37
t2
36 37
9

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Summary of MAC protocols
channel partitioning, by time, frequency or code Time Division, Frequency Division
random access (dynamic),
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
taking turns
polling from central site, token passing Bluetooth, FDDI, token ring
Link Layer: 6-38
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-39
38 39
MAC addresses
32-bit IP address:
network-layer address for interface
used for layer 3 (network layer) forwarding e.g.: 128.119.40.136
MAC (or LAN or physical or Ethernet) address:
function: used locally to get frame from one interface to another
physically-connected interface (same subnet, in IP-addressing sense)
48-bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable
e.g.: 1A-2F-BB-76-09-AD
hexadecimal (base 16) notation (each numeral represents 4 bits)
Link Layer: 6-40
MAC addresses
each interface on LAN
has unique 48-bit MAC address
has a locally unique 32-bit IP address (as weve seen)
137.196.7.78
1A-2F-BB-76-09-AD
LAN (wired or wireless)
137.196.7/24
71-65-F7-2B-08-53
137.196.7.23
58-23-D7-FA-20-B0
137.196.7.14
0C-C4-11-6F-E3-98
137.196.7.88
Link Layer: 6-41
40 41
10

ARP: address resolution protocol
Question: how to determine interfaces MAC address, knowing its IP address?
ARP table: each IP node (host, router) on LAN has 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)
Link Layer: 6-43
ARP
ARP
137.196.7.78
1A-2F-BB-76-09-AD LAN
ARP
71-65-F7-2B-08-53
58-23-D7-FA-20-B0
137.196.7.23
137.196.7.14
ARP
0C-C4-11-6F-E3-98
137.196.7.88
42 43
1
A broadcasts ARP query, containing Bs IP addr
destination MAC address = FF-FF-FF-FF-FF-FF allnodesonLANreceiveARPquery
C
Ethernet frame (sent to FF-FF-FF-FF-FF-FF)
ARP protocol in action
example: A wants to send datagram to B
BsMACaddressnotinAsARPtable,soAusesARPtofindBsMACaddress
ARP table in A
Source MAC: 71-65-F7-2B-08-53 Source IP: 137.196.7.23
Target IP address: 137.196.7.14

B
58-23-D7-FA-20-B0
137.196.7.14
IP addr
MAC addr
TTL
A
1
71-65-F7-2B-08-53 137.196.7.23
D
Link Layer: 6-44
ARP protocol in action
example: A wants to send datagram to B
BsMACaddressnotinAsARPtable,soAusesARPtofindBsMACaddress
ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)
B
2
D
Target IP address: 137.196.7.14 Target MAC address:
ARP table in A
C A

58-23-D7-FA-20-B0
IP addr
MAC addr
TTL
71-65-F7-2B-08-53 137.196.7.23
58-23-D7-FA-20-B0
137.196.7.14
B replies to A with ARP response, giving its MAC address
Link Layer: 6-45
2
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MAC addresses
MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to
assure uniqueness)
analogy:
MAC address: like Social Security Number IP address: like postal address
MAC flat address: portability
can move interface from one LAN to another
recall IP address not portable: depends on IP subnet to which
node is attached
Link Layer: 6-42
11

ARP protocol in action
example: A wants to send datagram to B
BsMACaddressnotinAsARPtable,soAusesARPtofindBsMACaddress
ARP table in A
C A
71-65-F7-2B-08-53 137.196.7.23
IP addr
MAC addr
TTL
137.196. 7.14
58-23-D7-FA-20-B0
500
B
58-23-D7-FA-20-B0
137.196.7.14
3 A receives Bs reply, adds B entry into its local ARP table
D
Link Layer: 6-46
Routing to another subnet: addressing
walkthrough: sending a datagram from A to B via R
focus on addressing at IP (datagram) and MAC layer (frame) levels assume that:

A knows Bs IP address
A knows IP address of first hop router, R (how?) A knows Rs MAC address (how?)
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer: 6-47
46 47
Routing to another subnet: addressing
A creates IP datagram with IP source A, destination B
A creates link-layer frame containing A-to-B IP datagram
RsMACaddressisframesdestination
MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B
c: 111.111.111.111 dest: 222.222.222.222
IP sr IP
IP
Eth
Phy
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer: 6-48
Routing to another subnet: addressing
frame sent from A to R
frame received at R, datagram removed, passed up to IP
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B
dest: 222.222.222.222
IP src: 111.111.111.111
IP dest: 222.222.222.222 c: 111.111.111.111
IP sr IP
IP
Eth
Phy
IP
Eth
Phy
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
Link Layer: 6-49
48 49
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12

Routing to another subnet: addressing
R determines outgoing interface, passes datagram with IP source A, destination B to link layer
R creates link-layer frame containing A-to-B IP datagram. Frame destination address:
Bs MAC address
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer: 6-50
Routing to another subnet: addressing
R determines outgoing interface, passes datagram with IP source A, destination B to link layer
R creates link-layer frame containing A-to-B IP datagram. Frame destination address:
Bs MAC address
transmits link-layer frame
MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A
IP src: 111.111.111.111
IP dest: 222.222.222.222
IP
Eth
Phy
IP
Eth
Phy
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer: 6-51
50 51
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Routing to another subnet: addressing
B receives frame, extracts IP datagram destination B B passes datagram up protocol stack to IP
IP
Eth
Phy
IP
Eth
Phy
A
111.111.111.111 74-29-9C-E8-FF-55
111.111.111.112 CC-49-DE-D0-AB-7D
R
222.222.222.220 1A-23-F9-CD-06-9B
111.111.111.110 E6-E9-00-17-BB-4B
IP src: 111.111.111.111
IP dest: 222.222.222.222
B
222.222.222.222 49-BD-D2-C7-56-2A
222.222.222.221 88-B2-2F-54-1A-0F
Link Layer: 6-52
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-53
52 53
13

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Ethernet
dominant wired LAN technology:
first widely used LAN technology
simpler, cheap
kept up with speed race: 10 Mbps 400 Gbps
single chip, multiple speeds (e.g., Broadcom BCM5761)
Metcalfes Ethernet sketch
https://www.uspto.gov/learning-and-resources/journeys-innovation/audio-stories/defying-doubters
Link Layer: 6-54
Ethernet: physical topology
bus: popular through mid 90s
all nodes in same collision domain (can collide with each other)
switched: prevails today
active link-layer 2 switch in center
each spoke runs a (separate) Ethernet protocol (nodes do not collide with each other)
bus: coaxial cable switched
Link Layer: 6-55
Ethernet frame structure
sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
type
preamble
dest. address
source address
data (payload)
CRC
preamble:
used to synchronize receiver, sender clock rates
7 bytes of 10101010 followed by one byte of 10101011
Link Layer: 6-56
Ethernet frame structure (more)
type
addresses: 6 byte source, destination MAC addresses
if adapter receives frame with matching destination address, or with broadcast
address (e.g., ARP packet), it passes data in frame to network layer protocol
otherwise, adapter discards frame
preamble
dest. address
source address
data (payload)
CRC
type: indicates higher layer protocol
mostly IP but others possible, e.g., Novell IPX, AppleTalk used to demultiplex up at receiver
CRC: cyclic redundancy check at receiver error detected: frame is dropped
Link Layer: 6-57
56 57
14

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Ethernet: unreliable, connectionless
connectionless: no handshaking between sending and receiving NICs
unreliable: receiving NIC doesnt send ACKs or NAKs to sending NIC
data in dropped frames recovered only if initial sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost
Ethernets MAC protocol: unslotted CSMA/CD with binary backoff
Link Layer: 6-58
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, 10 Gbps, 40 Gbps different physical layer media: fiber, cable
application
MAC protocol and frame format
transport
network
100BASE-TX
100BASE-T2
100BASE-FX
link physical
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister pair) physical layer fiber physical layer
Link Layer: 6-59
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-60
Ethernet switch
Switch is a link-layer device: takes an active role
store, forward Ethernet frames
examine incoming frames 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
transparent: hosts unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured
Link Layer: 6-61
60 61
15

Switch: multiple simultaneous transmissions
hosts have dedicated, direct connection to switch
switches buffer packets C
Ethernet protocol used on each
incoming link, so:
no collisions; full duplex
each link is its own collision B domain
switching: A-to-A and B-to-B can transmit simultaneously, without collisions
A
2
3
C switch with six
Link Layer: 6-62
B
1
6 5
4
A
interfaces (1,2,3,4,5,6)
Switch: multiple simultaneous transmissions
hosts have dedicated, direct connection to switch
switches buffer packets C
Ethernet protocol used on each
incoming link, so:
no collisions; full duplex
each link is its own collision B domain
switching: A-to-A and B-to-B can transmit simultaneously, without collisions
butA-to-AandCtoAcannothappen simultaneously
A
2
3
C switch with six
Link Layer: 6-63
6 5
4
1
B
A
interfaces (1,2,3,4,5,6)
62 63
Switch forwarding table
Q: how does switch know A reachable via interface 4, B reachable via interface 5? C
A: each switch has a switch table, each entry:
(MAC address of host, interface to reach host, time stamp)
looks like a routing table! B
Q: how are entries created, maintained in switch table?
something like a routing protocol?
A
2 3
B
1 6
54
A C
Link Layer: 6-64
Switch: self-learning
switch learns which hosts
can be reached through
which interfaces C
when frame received, switch learns location of sender: incoming LAN segment
records sender/location pair B in switch table
Source: A Dest: A
A A A
2 3
A C
Switch table (initially empty)
Link Layer: 6-65
B
1 6
54
MAC addr
interface
TTL
A
1
60
64 65
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16

Self-learning, forwarding: example
Source: A Dest: A
A A A
B
C
(initially empty)
Link Layer: 6-67
frame destination, A, location unknown: flood
destination A location known: selectively send
on just one link
C
B
12 A A 3
6 54
A AA
switch table
MAC addr
interface
TTL
A A
1 4
60 60
66 67
Interconnecting switches
self-learning switches can be connected together:
S4
S3
DFI
GH
Q: sending from A to G how does S1 know to forward frame destined to G via S4 and S3?
A: self learning! (works exactly the same as in single-switch case!)
Link Layer: 6-68
S1
S2
A
BC
E
Self-learning multi-switch example
Suppose C sends frame to I, I responds to C
S4
S3
DFI
GH
Q: show switch tables and packet forwarding in S1, S2, S3, S4
Link Layer: 6-69
S1
S2
A
BC
E
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Switch: frame filtering/forwarding
when frame received at switch:
1. record incoming link, MAC address of sending host 2. index switch table using MAC destination address 3. if entry found for destination
then {
if destination on segment from which frame arrived
then drop frame
else forward frame on interface indicated by entry
}
else flood /* forward on all interfaces except arriving interface */
Link Layer: 6-66
17

Small institutional network
to external network
mail server
web server
IP subnet
router
Link Layer: 6-70
Switches vs. routers
both are store-and-forward:
routers: network-layer devices (examine network-layer headers)
switches: link-layer devices (examine link-layer headers)
both have forwarding tables:
routers: compute tables using routing algorithms, IP addresses
switches: learn forwarding table using flooding, learning, MAC addresses
datagram frame
application
transport
network
link
physical
switch
link frame
network datagram link frame
Link Lay6e-r7:16-71
physical
physical
application
transport
network
link
physical
70 71
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Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-72
Virtual LANs (VLANs): motivation
Q: what happens as LAN sizes scale, users change point of attachment? single broadcast domain:
Computer Science
EE
scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN
efficiency, security, privacy issues
Link Layer: 6-73
72 73
18

Virtual LANs (VLANs): motivation
Q: what happens as LAN sizes scale, users change point of attachment? single broadcast domain:
Computer Science
EE
scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN
efficiency, security, privacy, efficiency issues
administrative issues:
CS user moves office to EE physically attached to EE switch, but wants to remain logically attached to CS switch
Link Layer: 6-74
Port-based VLANs
Virtual Local Area Network (VLAN)
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.
port-based VLAN: switch ports grouped (by switch management software) so that single physical switch

EE (VLAN ports 1-8) CS (VLAN ports 9-15)
operates as multiple virtual switches

EE (VLAN ports 1-8) CS (VLAN ports 9-15)
Link Layer: 6-75
1
79
15
2
8 10
16
9
15
10
16
17
28
74 75
Port-based VLANs
traffic isolation: frames to/from ports 1-8 can only reach ports 1-8
can also define VLAN based on MAC addresses of endpoints, rather than switch port
dynamic membership: ports can be dynamically assigned among VLANs
forwarding between VLANS: done via
routing (just as with separate switches)
in practice vendors sell combined switches plus routers

1 79 15
2 810 16
EE (VLAN ports 1-8)
CS (VLAN ports 9-15)
Link Layer: 6-76
VLANS spanning multiple switches
1
79
15
2
8 10
16
1357
2468

EE (VLAN ports 1-8) CS (VLAN ports 9-15) Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN
trunk port: carries frames between VLANS defined over multiple physical switches
frames forwarded within VLAN between switches cant be vanilla 802.1 frames (must carry VLAN ID info)
802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports
Link Layer: 6-77
76 77
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19

2/25/21
802.1Q VLAN frame format
type
type
Tag Control Information
(12 bit VLAN ID field, 3 bit priority field like IP TOS)
preamble
dest. address
source address
data (payload)
CRC
preamble
dest. address
source address
data (payload)
CRC
2-byte Tag Protocol Identifier (value: 81-00)
802.1 Ethernet frame
802.1Q frame
Link Layer: 6-78
Recomputed CRC
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-79
78 79
Multiprotocol label switching (MPLS)
goal: high-speed IP forwarding among network of MPLS-capable routers, using fixed length label (instead of shortest prefix matching)
faster lookup using fixed length identifier
borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address!
20 315
IP sses
Ethernet remaindrermoafinEdtheerronfeEt tfhraemrne,tinfrcalmudei,nigncIPluding
MPLS header
header header hweitahdIePr swoiuthrcIeP, sdoeusrtcinea, tdioenstaindadtrioenssaedsdre
label
Exp
S
TTL
Link Layer: 6-80
MPLS capable routers
a.k.a. label-switched router
forward packets to outgoing interface based only on label value (dont inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
flexibility: MPLSforwardingdecisionscandifferfrom those of IP
use destination and source addresses to route flows to same
destination differently (traffic engineering)
re-route flows quickly if link fails: pre-computed backup paths
Link Layer: 6-81
80 81
20

MPLS versus IP paths
R6
R4 R5
D R3
R2
IP router
A
IP routing: path to destination determined by destination address alone
Link Layer: 6-82
MPLS versus IP paths
IP/MPLS entry router (R4) can use different MPLS routes to A based, e.g., on IP source address or other fields
R6
R4 R5
MPLS routing: path to destination can be based on source and destination address
flavor of generalized forwarding (MPLS 10 years earlier)
fast reroute: precompute backup routes in case of link failure
D R3
IP router
IP/MPLS router
IP routing: path to destination determined by destination address alone
A
R2
R1
Link Layer: 6-83
82 83
MPLS signaling
modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing:
e.g., link bandwidth, amount of reserved link bandwidth entry MPLS router uses RSVP-TE signaling protocol to set up
MPLS forwarding at downstream routers
RSVP-TE
R2
R6
R4
R5 modified link state flooding
D R3
A
R1
Link Layer: 6-84
MPLS forwarding tables
R6
R4 R5
in label
out label
10
dest
A
out interface
0
12 8
D A
0 1
in label
out label
dest
out interface
10 12
6 9
A D
1 0
00 11
D R3
00 R2 R1
A
in label
out label
dest
out interface
8
6
A
0
in label
6
out label

dest
A
out interface
0
Link Layer: 6-85
84 85
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21

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Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
a day in the life of a web request
Link Layer: 6-86
Datacenter networks
10s to 100s of thousands of hosts, often closely coupled, in close proximity:
e-business (e.g. Amazon)
content-servers (e.g., YouTube, Akamai, Apple, Microsoft) search engines, data mining (e.g., Google)
challenges:
multiple applications, each serving
massive numbers of clients
reliability
managing/balancing load, avoiding
processing, networking, data bottlenecks
Inside a 40-ft Microsoft container, Chicago data center
Link Layer: 6-87
86 87
Datacenter networks: network elements

Border routers
connections outside datacenter Tier-1 switches
connectingto ~16T-2sbelow Tier-2 switches
connectingto ~16TORsbelow
Top of Rack (TOR) switch
one per rack
40-100Gbps Ethernet to blades
Server racks
20- 40 server blades: hosts
Link Layer: 6-88
Datacenter networks: network elements
Facebook F16 data center network topology:
https://engineering.fb.com/data-center-engineering/f16-minipack/ (posted 3/2019)
Link Layer: 6-89
88 89
22

90
91
92
93
Datacenter networks: multipath
rich interconnection among switches, racks:
increased throughput between racks (multiple routing paths possible) increased reliability via redundancy
9 10 11 12 13 14 15 16 two disjoint paths highlighted between racks 1 and 11
Datacenter networks: application-layer routing
Internet
Datacenter networks: protocol innovations
link layer:
RoCE: remote DMA (RDMA) over Converged Ethernet
transport layer:
ECN (explicit congestion notification) used in transport-layer congestion control (DCTCP, DCQCN)
experimentation with hop-by-hop (backpressure) congestion control
Link layer, LANs: roadmap
introduction
error detection, correction multiple access protocols LANs
addressing, ARP Ethernet
switches
VLANs
link virtualization: MPLS data center networking
routing, management:
SDN widely used within/among organizations datacenters
place related services, data as close as possible (e.g., in same rack or nearby
a day in the life of a web request
rack) to minimize tier-2, tier-1 communication
Link Layer: 6-90
Link Layer: 6-91
Link Layer: 6-92
Link Layer: 6-93

Load balancer
load balancer: application-layer routing
receives external client requests
directs workload
within data center
returns results to
external client (hiding data center internals from client)
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A day in the life: scenario
browser
school network 68.80.2.0/24
web page
web server 64.233.169.105
DNS server
Comcast network 68.80.0.0/13
Googles network 64.233.160.0/19
scenario:
arriving mobile client attaches to network
requests web page:
www.google.com
Sounds simple!
Link Layer: 6-95
94 95
A day in the life: connecting to the Internet
DHCP
UDP
IP
Eth
Phy
DHCP DHCP DHCP DHCP
arriving mobile: DHCP client
router has DHCP server
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
Link Layer: 6-96
DHCP
DHCP
UDP
IP
Eth
Phy
DHCP DHCP DHCP DHCP
A day in the life: connecting to the Internet
DHCP
UDP
IP
Eth
Phy
DHCP DHCP DHCP DHCP
DHCP DHCP DHCP DHCP
DHCP
arriving mobile: DHCP client
router has DHCP server
DHCP server formulates DHCP ACK containing clients IP address, IP address of first-hop router for client, name & IP address of DNS server
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client
DHCP
UDP
IP
Eth
Phy
DHCP client receives DHCP ACK reply
Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router
Link Layer: 6-97
96 97
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Synthesis: a day in the life of a web request
our journey down the protocol stack is now complete! application, transport, network, link
putting-it-all-together: synthesis!
goal: identify, review, understand protocols (at all layers) involved in
seemingly simple scenario: requesting www page
scenario: student attaches laptop to campus network, requests/receives www.google.com
Link Layer: 6-94
24

A day in the life ARP (before DNS, before HTTP)
before sending HTTP request, need IP address
DNS
DNS DNS DNS
ARP query
ARP reply
ARP
arriving mobile: ARP client
router has ARP server
of www.google.com: DNS
DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP
ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface
client now knows MAC address of first hop router, so can now send frame containing DNS query
Link Layer: 6-98
UDP
ARPIP
Eth
Phy
Eth
Phy
A day in the life using DNS
DNS DNS DNS DNS
DNS
IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
DNS
demuxed to DNS
DNS replies to client
with IP address of www.google.com
DNS
UDP
IP
Eth
Phy
DNS
UDP
IP
Eth
Phy
DNS DNS DNS DNS
DNS server
Comcast network 68.80.0.0/13
IP datagram forwarded from campus network into Comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server
Link Layer: 6-99
98 99
100 101
H H
T T
HTTP request sent into TCP socket
IP datagram containing HTTP request routed to www.google.com
web server responds with HTTP reply (containing web page)
IP datagram containing HTTP reply routed back to client
Link Layer: 6-101
T
P
HTTP TCP IP Eth Phy
H
T
T
H
T T
HT
A day in the life HTTP request/reply
T T
P P
TT
TP
P
HTTP HTTP HTTP HTTP
HTTP TCP IP Eth Phy
Google web server 64.233.169.105
Comcast network 68.80.0.0/13
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A day in the lifeTCP connection carrying HTTP
HTTP
TCP
IP
Eth Phy
HTTP
S YSNYANC K SYSNYANCK S YS SNY YAN NC K
S Y SN YA NC K S YSNYANC K SYSNYANCK
Comcast network 68.80.0.0/13
to send HTTP request, client first opens TCP socket to web server
TCP SYN segment (step 1 in TCP 3-way handshake) inter- domain routed to web server
web server responds with TCP SYNACK (step 2 in TCP 3- way handshake)
TCP connection established! Link Layer: 6-100
TCP
IP Eth Phy
Google
web server 64.233.169.105
H H
T T
P P
H H
P P
web page finally (!!!) displayed
T T
T
25

102 103
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Chapter 6: Summary
principles behind data link layer services: error detection, correction
sharing a broadcast channel: multiple access link layer addressing
instantiation, implementation of various link layer technologies Ethernet
switched LANS, VLANs
virtualized networks as a link layer: MPLS
synthesis: a day in the life of a web request
Link Layer: 6-102
Chapter 6: lets take a breath
journey down protocol stack complete (except PHY)
solid understanding of networking principles, practice! .. could stop here . but more interesting topics!
wireless security
Link Layer: 6-103
Additional Chapter 6 slides
Network Layer: 5-104
Pure ALOHA efficiency
P(success by given node) = P(node transmits) *
P(no other node transmits in [t0-1,t0]* *
P(no other node transmits in [t0-1,t0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
choosing optimum p and then letting n
= 1/(2e) = .18
even worse than slotted Aloha!
Link Layer: 6-105
104 105
26