CS计算机代考程序代写 dns flex data mining DHCP algorithm PowerPoint Presentation

PowerPoint Presentation

Computer Networking: A Top-Down Approach
8th edition
Jim Kurose, Keith Ross
Pearson, 2020
Chapter 6
The Link Layer
and LANs
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Thanks and enjoy! JFK/KWR

Version History

8.0 (May 2020)
All slides reformatted for 16:9 aspect ratio
All slides updated to 8th edition material
Use of Calibri font, rather that Gill Sans MT
Add LOTS more animation throughout
lighter header font
Updated datacenter slides, day-in-the-life
1

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

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

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

datacenter
network

link layer has responsibility of
transferring datagram from one node
to physically adjacent node over a link

Link Layer: 6-4

Link layer: context
datagram transferred by different link protocols over different links:
e.g., WiFi on first link, Ethernet on next link
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: Princeton to JFK
plane: JFK to Geneva
train: Geneva to Lausanne
tourist = datagram
transport segment = communication link
transportation mode = link-layer protocol
travel agent = routing algorithm

Link Layer: 6-5

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

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 host’s system buses
combination of hardware, software, firmware

controller
physical

cpu
memory

host bus
(e.g., PCI)

network interface

application
transport
network
link

link
physical

Link Layer: 6-8

controller
physical

memory
CPU

Interfaces communicating

controller
physical

cpu
memory

application
transport
network
link

link
physical

application
transport
network
link

link
physical

sending side:
encapsulates datagram in frame
adds error checking bits, reliable data transfer, flow control, etc.
receiving side:
looks for errors, reliable data transfer, flow control, etc.
extracts datagram, passes to upper layer at receiving side

linkh

datagram

datagram
Link Layer: 6-9

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-10

Error detection
Link Layer: 6-11
EDC: error detection and correction bits (e.g., 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

datagram

D
EDC

d data bits

bit-error prone link

D’
EDC’

all
bits in D’
OK
?
N
detected
error

otherwise

datagram

Parity checking
Link Layer: 6-12
single bit parity:
detect single bit errors

0111000110101011
1
parity
bit
d data bits
two-dimensional bit parity:
detect and correct single bit errors
d1,1
d2,1
di,1
. . .
d1,j+1
d2,j+1
di,j+1
. . .
. . .
d1,j
d2,j
di,j
. . .
di+1,1
di+1,j+1
di+1,j
. . .
. . .
. . .
. . .
row parity
column
parity
0 1 1 1 0 1
1 0 1 0 1 1
1 1 1 1 0 0
1 0 1 0 1 0
no errors:
0 1 1 1 0 1
1 0 1 0 1 1
1 0 1 1 0 0
1 0 1 0 1 0
parity
error
parity
error
detected
and
correctable
single-bit
error:
Even parity: set parity bit so there is an even number of 1’s
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/

Internet checksum (review)
sender:
treat contents of UDP segment (including UDP header fields and IP addresses) as sequence of 16-bit integers
checksum: addition (one’s 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 ….

Goal: detect errors (i.e., flipped bits) in transmitted segment

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)

Link Layer: 6-14
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)

r CRC bits

d data bits

D
R
= D 2r XOR R
*
bit pattern
formula for bit pattern

Link Layer: 6-15
Cyclic Redundancy Check (CRC): example
We want:
D.2r XOR R = nG
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
D.2r
G

R = remainder [ ]
or equivalently:
D.2r = nG XOR R
or equivalently:
if we divide D.2r by G, want remainder R to satisfy:
1 0 0 1
1 0 1 0
1 0 1
0 0 0
1 0 0 1
1 0 0 1
1 0 0 1
0 0 0
1 1 0
1 1 0 0
1 0 1 0
0 1 1
0 1 1
D
R

1 0 0 1
G

0 0 0
1 0 1 1 1 0

2r
*
1
0
1

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-16

Multiple access links, protocols
Link Layer: 6-17
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.,
cabled Ethernet)
shared radio: WiFi
shared radio: satellite
humans at a cocktail party
(shared air, acoustical)

shared radio: 4G/5G

Multiple access protocols
Link Layer: 6-18
single shared broadcast channel
two or more simultaneous transmissions by nodes: interference
collision if node receives two or more signals at the same time
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

multiple access protocol

An ideal multiple access protocol
Link Layer: 6-19
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

MAC protocols: taxonomy
Link Layer: 6-20
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

Channel partitioning MAC protocols: TDMA
Link Layer: 6-21
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

1
3
4

6-slot
frame

Channel partitioning MAC protocols: FDMA
Link Layer: 6-22
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

frequency bands
time

FDM cable

Random access protocols
Link Layer: 6-23
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

Slotted ALOHA
Link Layer: 6-24
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?

Slotted ALOHA
Link Layer: 6-25
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

3
node 1
node 2
node 3

C
C
C
S
S
S
E
E
E
C: collision
S: success
E: empty

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!

Slotted ALOHA: efficiency
Link Layer: 6-26

Pure ALOHA
Link Layer: 6-27
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]

t0 + 1
t0 – 1
t0

will overlap
with end of
i’s frame
will overlap
with start of
i’s frame
pure Aloha efficiency: 18% !

CSMA (carrier sense multiple access)
Link Layer: 6-28
simple CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
if channel sensed busy: defer transmission
human analogy: don’t 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

CSMA: collisions
Link Layer: 6-29
collisions can still occur with carrier sensing:
propagation delay means two nodes may not hear each other’s just-started transmission
collision: entire packet transmission time wasted
distance & propagation delay play role in in determining collision probability

spatial layout of nodes

CSMA/CD:
Link Layer: 6-30
CSMA/CS reduces the amount of time wasted in collisions
transmission aborted on collision detection

spatial layout of nodes

Ethernet CSMA/CD algorithm
Link Layer: 6-31
NIC receives datagram from network layer, creates frame
If NIC senses channel:
if idle: start frame transmission.
if busy: wait until channel idle, then transmit
If NIC transmits entire frame without collision, NIC is done with frame !
If NIC detects another transmission while sending: abort, send jam signal
After aborting, NIC enters binary (exponential) backoff:
after mth collision, NIC chooses K at random from {0,1,2, …, 2m-1}. NIC waits K·512 bit times, returns to Step 2
more collisions: longer backoff interval

CSMA/CD efficiency
Link Layer: 6-32
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame

efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA: and simple, cheap, decentralized!

“Taking turns” MAC protocols
Link Layer: 6-33
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!

“Taking turns” MAC protocols
Link Layer: 6-34

polling:
master node “invites” other nodes to transmit in turn
typically used with “dumb” devices
concerns:
polling overhead
latency
single point of failure (master)

master
slaves

poll

data

“Taking turns” MAC protocols
Link Layer: 6-35
token passing:
control token passed from one node to next sequentially.
token message
concerns:
token overhead
latency
single point of failure (token)

T
data
(nothing
to send)
T

Cable access network: FDM, TDM and random access!
Link Layer: 6-36

cable headend
CMTS

ISP

cable modem
termination system

cable
modem
splitter

…

…
Internet frames, TV channels, control transmitted
downstream at different frequencies

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

Cable access network:
Link Layer: 6-37
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

Residences with cable modems

Downstream channel i
Upstream channel j

MAP frame for
Interval [t1, t2]

t1
t2
Assigned minislots containing cable modem
upstream data frames
Minislots containing
minislots request frames

cable headend
CMTS

Summary of MAC protocols
Link Layer: 6-38
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, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-39

MAC addresses
Link Layer: 6-40
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

hexadecimal (base 16) notation
(each “numeral” represents 4 bits)

e.g.: 1A-2F-BB-76-09-AD

MAC addresses
Link Layer: 6-41
each interface on LAN
has unique 48-bit MAC address
has a locally unique 32-bit IP address (as we’ve seen)

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53
LAN
(wired or wireless)
137.196.7/24

137.196.7.78
137.196.7.14
137.196.7.88
137.196.7.23

MAC addresses
Link Layer: 6-42
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

ARP: address resolution protocol
Link Layer: 6-43
ARP table: each IP node (host, router) on LAN has table
Question: how to determine interface’s MAC address, knowing its IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53
LAN

137.196.7.78
137.196.7.14
137.196.7.88
137.196.7.23

ARP

ARP
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 in action
Link Layer: 6-44

58-23-D7-FA-20-B0

137.196.7.14
B
C
D
TTL

71-65-F7-2B-08-53

137.196.7.23
A

ARP table in A
IP addr
MAC addr
TTL
example: A wants to send datagram to B
B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address
A broadcasts ARP query, containing B’s IP addr
destination MAC address = FF-FF-FF-FF-FF-FF
all nodes on LAN receive ARP query

Source MAC: 71-65-F7-2B-08-53
Source IP: 137.196.7.23
Target IP address: 137.196.7.14
…

1
Ethernet frame (sent to FF-FF-FF-FF-FF-FF)

ARP protocol in action
Link Layer: 6-45

58-23-D7-FA-20-B0

137.196.7.14
B
C
D
TTL

71-65-F7-2B-08-53

137.196.7.23
A

ARP table in A
IP addr
MAC addr
TTL
example: A wants to send datagram to B
B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address
B replies to A with ARP response, giving its MAC address

Target IP address: 137.196.7.14
Target MAC address:
58-23-D7-FA-20-B0
…

2
ARP message into Ethernet frame (sent to 71-65-F7-2B-08-53)

ARP protocol in action
Link Layer: 6-46

58-23-D7-FA-20-B0

137.196.7.14
B
C
D
TTL

71-65-F7-2B-08-53

137.196.7.23
A

ARP table in A
IP addr
MAC addr
TTL
example: A wants to send datagram to B
B’s MAC address not in A’s ARP table, so A uses ARP to find B’s MAC address
A receives B’s reply, adds B entry into its local ARP table

137.196.
7.14
58-23-D7-FA-20-B0
500

Routing to another subnet: addressing
Link Layer: 6-47
walkthrough: sending a datagram from A to B via R
focus on addressing – at IP (datagram) and MAC layer (frame) levels
R

1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

222.222.222.222
49-BD-D2-C7-56-2A
222.222.222.221
88-B2-2F-54-1A-0F
assume that:
A knows B’s IP address
A knows IP address of first hop router, R (how?)
A knows R’s MAC address (how?)

Routing to another subnet: addressing
Link Layer: 6-48
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

A
222.222.222.222
49-BD-D2-C7-56-2A

222.222.222.221
88-B2-2F-54-1A-0F

IP
Eth
Phy

IP src: 111.111.111.111
IP dest: 222.222.222.222

A creates IP datagram with IP source A, destination B
A creates link-layer frame containing A-to-B IP datagram
R’s MAC address is frame’s destination
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B

Routing to another subnet: addressing
Link Layer: 6-49
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

A
222.222.222.222
49-BD-D2-C7-56-2A

222.222.222.221
88-B2-2F-54-1A-0F

IP
Eth
Phy

frame sent from A to R

IP
Eth
Phy

frame received at R, datagram removed, passed up to IP
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B

IP src: 111.111.111.111
IP dest: 222.222.222.222

Routing to another subnet: addressing
Link Layer: 6-50
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

A
222.222.222.222
49-BD-D2-C7-56-2A

222.222.222.221
88-B2-2F-54-1A-0F

IP src: 111.111.111.111
IP dest: 222.222.222.222

MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A

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: B’s MAC address

IP
Eth
Phy

Routing to another subnet: addressing
Link Layer: 6-51
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

A
222.222.222.222
49-BD-D2-C7-56-2A

222.222.222.221
88-B2-2F-54-1A-0F

IP
Eth
Phy

IP src: 111.111.111.111
IP dest: 222.222.222.222
MAC src: 1A-23-F9-CD-06-9B
MAC dest: 49-BD-D2-C7-56-2A

transmits link-layer frame
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: B’s MAC address

Routing to another subnet: addressing
Link Layer: 6-52
R
1A-23-F9-CD-06-9B
222.222.222.220
111.111.111.110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55

A
222.222.222.222
49-BD-D2-C7-56-2A

222.222.222.221
88-B2-2F-54-1A-0F

IP
Eth
Phy

B receives frame, extracts IP datagram destination B
B passes datagram up protocol stack to IP

IP src: 111.111.111.111
IP dest: 222.222.222.222

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-53

Ethernet
Link Layer: 6-54
“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)

Metcalfe’s Ethernet sketch
https://www.uspto.gov/learning-and-resources/journeys-innovation/audio-stories/defying-doubters

Ethernet: physical topology
Link Layer: 6-55
bus: popular through mid 90s
all nodes in same collision domain (can collide with each other)

bus: coaxial cable

switched

switched: prevails today
active link-layer 2 switch in center
each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)

Ethernet frame structure
Link Layer: 6-56
sending interface encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

dest.
address
source
address
data (payload)
CRC
preamble
type
preamble:
used to synchronize receiver, sender clock rates
7 bytes of 10101010 followed by one byte of 10101011

Ethernet frame structure (more)
Link Layer: 6-57

dest.
address
source
address
data (payload)
CRC
preamble
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
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

Ethernet: unreliable, connectionless
Link Layer: 6-58
connectionless: no handshaking between sending and receiving NICs
unreliable: receiving NIC doesn’t 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
Ethernet’s MAC protocol: unslotted CSMA/CD with binary backoff

802.3 Ethernet standards: link & physical layers
Link Layer: 6-59
different physical layer media: fiber, cable

application
transport
network
link
physical

MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX

100BASE-T2
100BASE-SX
100BASE-BX

fiber physical layer

copper (twister pair) physical layer
many different Ethernet standards
common MAC protocol and frame format
different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10 Gbps, 40 Gbps

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-60

Ethernet switch
Link Layer: 6-61
Switch is a link-layer device: takes an active role
store, forward Ethernet frames
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
transparent: hosts unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured

Switch: multiple simultaneous transmissions
Link Layer: 6-62
switch with six interfaces (1,2,3,4,5,6)
A
A’
B
B’
C
C’

1
2
3
4
5
6

hosts have dedicated, direct connection to switch
switches buffer packets
Ethernet protocol used on each incoming link, so:
no collisions; full duplex
each link is its own collision domain
switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions

Switch: multiple simultaneous transmissions
Link Layer: 6-63
switch with six interfaces (1,2,3,4,5,6)
A
A’
B
B’
C
C’

1
2
3
4
5
6

Switch forwarding table
Link Layer: 6-64
A
A’
B
B’
C
C’

1
2
3
4
5
6

Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?
A: each switch has a switch table, each entry:
(MAC address of host, interface to reach host, time stamp)
looks like a routing table!
Q: how are entries created, maintained in switch table?
something like a routing protocol?

Switch: self-learning
Link Layer: 6-65
A
A’
B
B’
C
C’

1
2
3
4
5
6

switch learns which hosts can be reached through which interfaces

A A’

Source: A
Dest: A’

MAC addr interface TTL

Switch table
(initially empty)
A
1
60
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table

Switch: frame filtering/forwarding
Link Layer: 6-66
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 */

A
A’
B
B’
C
C’

1
2
3
4
5
6

Self-learning, forwarding: example
Link Layer: 6-67

A A’

Source: A
Dest: A’

MAC addr interface TTL

switch table
(initially empty)
A
1
60

A A’

A’ A

A’
4
60
frame destination, A’, location unknown:
flood
destination A location known:
selectively send
on just one link

Interconnecting switches
Link Layer: 6-68
self-learning switches can be connected together:
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!)

A
B
S1
C

D
E
F
S2
S4
S3
H
I
G

Self-learning multi-switch example
Link Layer: 6-69
Suppose C sends frame to I, I responds to C
Q: show switch tables and packet forwarding in S1, S2, S3, S4

A
B
S1
C

D
E
F
S2
S4
S3
H
I
G

Small institutional network
Link Layer: 6-70

to external
network
router
IP subnet
mail server
web server

Switches vs. routers
Link Layer: 6-71

application
transport
network
link
physical

network
link
physical

link
physical
switch

datagram

application
transport
network
link
physical

frame

datagram

6-71

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

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-72

Virtual LANs (VLANs): motivation
Link Layer: 6-73

Computer
Science

Q: what happens as LAN sizes scale, users change point of attachment?
single broadcast domain:
scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN
efficiency, security, privacy issues

Virtual LANs (VLANs): motivation
Link Layer: 6-74
administrative issues:
CS user moves office to EE – physically attached to EE switch, but wants to remain logically attached to CS switch

Computer
Science

single broadcast domain:
scaling: all layer-2 broadcast traffic (ARP, DHCP, unknown MAC) must cross entire LAN
efficiency, security, privacy, efficiency issues
Q: what happens as LAN sizes scale, users change point of attachment?

8
2
7

9
16
10
15

Port-based VLANs
Link Layer: 6-75
switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.

Virtual Local Area Network (VLAN)

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

8
2
7

EE (VLAN ports 1-8)
…

9
16
10
15

…

CS (VLAN ports 9-15)

8
2
7

9
16
10
15

Port-based VLANs
Link Layer: 6-76

…

EE (VLAN ports 1-8)
CS (VLAN ports 9-15)
…

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

8
2
7

9
16
10
15

VLANS spanning multiple switches
Link Layer: 6-77

…

EE (VLAN ports 1-8)
CS (VLAN ports 9-15)
…

5
8
2
7

…

16
1

6
3
4

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 can’t be vanilla 802.1 frames (must carry VLAN ID info)
802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports

802.1Q VLAN frame format
Link Layer: 6-78
802.1 Ethernet frame

dest.
address
source
address
data (payload)
CRC
preamble
type

2-byte Tag Protocol Identifier
(value: 81-00)
Tag Control Information
(12 bit VLAN ID field, 3 bit priority field like IP TOS)

Recomputed
CRC

802.1Q frame

dest.
address
source
address
data (payload)
CRC
preamble
type

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-79

label
Exp
S
TTL

20
3
1
5
Multiprotocol label switching (MPLS)
Link Layer: 6-80
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!

remainder of Ethernet frame, including IP header with IP source, destination addresses

MPLS header

Ethernet
header

remainder of Ethernet frame, including IP header with IP source, destination addresses

MPLS capable routers
Link Layer: 6-81
a.k.a. label-switched router
forward packets to outgoing interface based only on label value (don’t inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
flexibility: MPLS forwarding decisions can differ from 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

MPLS versus IP paths
Link Layer: 6-82

R2
D
R3
R5
A
R6
R4

IP routing: path to destination determined by destination address alone

IP router

MPLS versus IP paths
Link Layer: 6-83

R2
D
R3
R5
A
R6
IP router
R4

IP routing: path to destination determined by destination address alone

IP/MPLS router

IP/MPLS entry router (R4) can use different MPLS routes to A based, e.g., on IP source address or other fields

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
R1

MPLS signaling
Link Layer: 6-84
modify OSPF, IS-IS link-state flooding protocols to carry info used by MPLS routing:
e.g., link bandwidth, amount of “reserved” link bandwidth

R2
D
R3
R5
A
R6
R4

modified
link state
flooding

RSVP-TE
entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers
R1

MPLS forwarding tables
Link Layer: 6-85

in out out
label label dest interface

6 – A 0

in out out
label label dest interface

10 6 A 1

12 9 D 0

in out out
label label dest interface

8 6 A 0

in out out
label label dest interface

10 A 0

12 D 0

8 A 1

R2
D
R3
R5
A
R6
R4

R1
0
1
0
0
1
0

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-86

Datacenter networks
Link Layer: 6-87
10’s to 100’s 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

Datacenter networks: network elements
Link Layer: 6-88
Server racks
20- 40 server blades: hosts
Top of Rack (TOR) switch
one per rack
40-100Gbps Ethernet to blades
Tier-2 switches
connecting to ~16 TORs below

Tier-1 switches
connecting to ~16 T-2s below

Border routers
connections outside datacenter

…
…

Datacenter networks: network elements
Link Layer: 6-89

Facebook F16 data center network topology:
https://engineering.fb.com/data-center-engineering/f16-minipack/ (posted 3/2019)

Datacenter networks: multipath
Link Layer: 6-90

9
10
11
12
13
14
15
16
two disjoint paths highlighted between racks 1 and 11

rich interconnection among switches, racks:
increased throughput between racks (multiple routing paths possible)
increased reliability via redundancy

…
…

Datacenter networks: application-layer routing
Link Layer: 6-91

Load
balancer

Internet

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)

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
routing, management:
SDN widely used within/among organizations’ datacenters
place related services, data as close as possible (e.g., in same rack or nearby rack) to minimize tier-2, tier-1 communication

Datacenter networks: protocol innovations
Link Layer: 6-92

Link layer, LANs: roadmap

a day in the life of a web request

introduction
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
VLANs
link virtualization: MPLS
data center networking

Link Layer: 6-93

Synthesis: a day in the life of a web request
Link Layer: 6-94
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

A day in the life: scenario
Link Layer: 6-95

Comcast network
68.80.0.0/13

Google’s network
64.233.160.0/19

64.233.169.105
web server
DNS server

school network
68.80.2.0/24

browser

web page

arriving mobile client attaches to network …

requests web page: www.google.com

scenario:

Sounds
simple!

A day in the life: connecting to the Internet
Link Layer: 6-96

router has
DHCP server
arriving mobile:
DHCP client
connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP

DHCP
UDP
IP
Eth
Phy

DHCP

DHCP
UDP
IP
Eth
Phy

DHCP

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

A day in the life: connecting to the Internet
Link Layer: 6-97

router has
DHCP server
arriving mobile:
DHCP client

DHCP
UDP
IP
Eth
Phy

DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

DHCP

DHCP
encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client

Client now has IP address, knows name & addr of DNS
server, IP address of its first-hop router
DHCP client receives DHCP ACK reply

A day in the life… ARP (before DNS, before HTTP)
Link Layer: 6-98

router has
ARP server
arriving mobile:
ARP client

DNS
UDP
IP
Eth
Phy

Eth
Phy

ARP

before sending HTTP request, need IP address of www.google.com: DNS

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

ARP query

ARP

ARP reply

A day in the life… using DNS
Link Layer: 6-99

DNS
UDP
IP
Eth
Phy

DNS

Comcast network
68.80.0.0/13

DNS
server

DNS

IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router
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
demuxed to DNS
DNS replies to client with IP address of www.google.com

DNS
UDP
IP
Eth
Phy

DNS

A day in the life…TCP connection carrying HTTP
Link Layer: 6-100

DNS

Comcast network
68.80.0.0/13

64.233.169.105
Google web server

HTTP
TCP
IP
Eth
Phy

HTTP

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
TCP connection established!

SYN

TCP
IP
Eth
Phy

SYN

SYNACK

SYNACK
web server responds with TCP SYNACK (step 2 in TCP 3-way handshake)

100

A day in the life… HTTP request/reply
Link Layer: 6-101

DNS

Comcast network
68.80.0.0/13

64.233.169.105
Google web server

HTTP
TCP
IP
Eth
Phy

HTTP

HTTP request sent into TCP socket
IP datagram containing HTTP request routed to www.google.com
IP datagram containing HTTP reply routed back to client
web server responds with HTTP reply (containing web page)

HTTP

web page finally (!!!) displayed

101

Chapter 6: Summary
Link Layer: 6-102
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

102

Chapter 6: let’s take a breath
Link Layer: 6-103
journey down protocol stack complete (except PHY)
solid understanding of networking principles, practice!
….. could stop here …. but more interesting topics!
wireless
security

103

Additional Chapter 6 slides
Network Layer: 5-104

104

Pure ALOHA efficiency
Link Layer: 6-105
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!

105

trans
prop
/t
t
efficiency
5
1
1
+
=

/docProps/thumbnail.jpeg

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