PowerPoint Presentation
Link Layer
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
George Parisis
School of Engineering and Informatics
University of Sussex
Link Layer
5-*
Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
Link Layer
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Link Layer
5-*
MAC addresses and ARP
32-bit IP address:
network-layer address for interface
used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address:
function: used ‘locally” to get frame from one interface to another physically-connected interface (same network, 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 “number” represents 4 bits)
Link Layer
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Link Layer
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LAN addresses and ARP
each adapter on LAN has unique LAN address
adapter
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)
Link Layer
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Link Layer
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LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
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
Link Layer
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Link Layer
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ARP: address resolution protocol
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)
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
137.196.7.23
137.196.7.78
137.196.7.14
137.196.7.88
Question: how to determine
interface’s MAC address, knowing its IP address?
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Link Layer
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ARP protocol: same LAN
A wants to send datagram to B
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
all nodes on LAN receive ARP query
B receives ARP packet, replies to A with its (B’s) MAC address
frame sent to A’s MAC address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)
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
Link Layer
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Link Layer
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walkthrough: send datagram from A to B via R
focus on addressing – at IP (datagram) and MAC layer (frame)
assume A knows B’s IP address (how?)
assume A knows IP address of first hop router, R (how?)
assume A knows R’s MAC address (how?)
Addressing: routing to another LAN
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
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Link Layer
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Addressing: routing to another LAN
A creates IP datagram with IP source A, destination B
A creates link-layer frame with R’s MAC address as dest, frame contains A-to-B IP datagram
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
IP
Eth
Phy
IP src: 111.111.111.111
IP dest: 222.222.222.222
MAC src: 74-29-9C-E8-FF-55
MAC dest: E6-E9-00-17-BB-4B
Link Layer
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Link Layer
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Addressing: routing to another LAN
frame sent from A to R
frame received at R, datagram removed, passed up to IP
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
IP
Eth
Phy
IP
Eth
Phy
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
IP src: 111.111.111.111
IP dest: 222.222.222.222
Link Layer
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Link Layer
5-*
Addressing: routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B’s MAC address as dest, frame contains A-to-B IP datagram
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
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
IP
Eth
Phy
IP
Eth
Phy
Link Layer
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Link Layer
5-*
Addressing: routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B’s MAC address as dest, frame contains A-to-B IP datagram
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
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
IP
Eth
Phy
IP
Eth
Phy
Link Layer
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R
1A-23-F9-CD-06-9B
222.222.222.220
CC-49-DE-D0-AB-7D
111.111.111.112
111.111.111.111
74-29-9C-E8-FF-55
A
222.222.222.221
88-B2-2F-54-1A-0F
B
Link Layer
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Addressing: routing to another LAN
R forwards datagram with IP source A, destination B
R creates link-layer frame with B’s MAC address as dest, frame contains A-to-B IP datagram
111.111.111.110
E6-E9-00-17-BB-4B
222.222.222.222
49-BD-D2-C7-56-2A
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
IP
Eth
Phy
Link Layer
*
Link Layer
5-*
Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
Link Layer
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Link Layer
5-*
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 – 100 Gbps
Metcalfe’s Ethernet sketch
Link Layer
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Link Layer
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Ethernet: physical topology
bus: popular through mid 90s
all nodes in same collision domain (can collide with each other)
star: prevails today
active switch in center
each link runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus: coaxial cable
star
Link Layer
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Data Link Layer
5-*
Ethernet: physical topology
Data Link Layer
Link Layer
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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
dest.
address
source
address
data (payload)
CRC
preamble
type
Link Layer
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Ethernet frame structure (more)
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 (IP)
CRC: cyclic redundancy check at receiver
error detected: frame is dropped
dest.
address
source
address
data (payload)
CRC
preamble
type
Link Layer
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Ethernet: unreliable, connectionless
connectionless: no handshaking between sending and receiving NICs
unreliable: receiving NIC doesnt send acks or nacks 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
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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
MAC protocol
and frame format
100BASE-TX
100BASE-T4
100BASE-FX
100BASE-T2
100BASE-SX
100BASE-BX
application
transport
network
link
physical
fiber physical layer
copper (twister
pair) physical layer
Link Layer
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Link Layer
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Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
Link Layer
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Link Layer
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Ethernet switch
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 are unaware of presence of switches
plug-and-play, self-learning
switches do not need to be configured
Link Layer
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Link Layer
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Switch: multiple simultaneous transmissions
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 B-to-B’ can transmit simultaneously, without collisions
A
A’
B
B’
C
C’
1
2
3
4
5
6
switch with six interfaces
(1,2,3,4,5,6)
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Switch forwarding table
Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?
A: each switch has a switching 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?
A
A’
B
B’
C
C’
1
2
3
4
5
6
switch with six interfaces
(1,2,3,4,5,6)
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Switch: self-learning
switch learns which hosts can be reached through which interfaces
when frame received, switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table
Switch table
(initially empty)
A
A’
B
B’
C
C’
1
2
3
4
5
6
A A’
Source: A
Dest: A’
MAC addr interface TTL
A
1
60
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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
{
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 */
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Link Layer
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Self-learning, forwarding: example
switch table
(initially empty)
frame destination, A’, locaton unknown:
flood
destination A location known:
selectively send
on just one link
A
A’
B
B’
C
C’
1
2
3
4
5
6
A A’
Source: A
Dest: A’
MAC addr interface TTL
A
1
60
A A’
A A’
A A’
A A’
A A’
A’ A
A’
4
60
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Interconnecting switches
switches can be connected together
Q: sending from A to G – how does S1 know to forward frame destined to F 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
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Link Layer
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Institutional network
to external
network
router
IP subnet
mail server
web server
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Link Layer
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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
application
transport
network
link
physical
switch
application
transport
network
link
physical
network
link
physical
link
physical
datagram
frame
frame
frame
datagram
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Summary
principles behind data link layer services:
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
instantiation and implementation of various link layer technologies
Ethernet
switched LANs
Link Layer
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