The Layer: Links, Access Networks and LANs

UNIT-5

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

By

​

G. Fayaz Hussain

Assistant Professor

Department of CSE

​

Ravindra College of Engineering for Women

Kurnool – 518452, Andhra Pradesh, India

Unit 5: Link layer

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
• instantiation, implementation of various link layer technologies

Link Layer

5-2

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer, LANs: outline

5.1 Introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-3

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer: introduction

terminology:

• hosts and routers: nodes
• communication channels that connect adjacent nodes along communication path: links
• wired links
• wireless links
• LANs
• layer-2 packet: frame, encapsulates datagram

​

​

Link Layer

5-4

data-link layer has responsibility of

transferring datagram from one node

to physically adjacent node over a link

global ISP

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer: context

• datagram transferred by different link protocols over different links:
• e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link
• each link protocol provides different services
• e.g., may or may not provide rdt 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

5-5

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer services

• framing, link access:
• encapsulate datagram into frame, adding header, trailer
• channel access if shared medium
• “MAC” addresses used in frame headers to identify source, dest
• different from IP address!
• reliable delivery between adjacent nodes
• we learned how to do this already (chapter 3)!
• seldom used on low bit-error link (fiber, some twisted pair)
• wireless links: high error rates
• Q: why both link-level and end-end reliability?

Link Layer

5-6

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer services (more)

• flow control:
• pacing between adjacent sending and receiving nodes
• error detection:
• errors caused by signal attenuation, noise.
• receiver detects presence of errors:
• signals sender for retransmission or drops frame
• error correction:
• receiver identifies and corrects bit error(s) without resorting to retransmission
• half-duplex and full-duplex
• with half duplex, nodes at both ends of link can transmit, but not at same time

Link Layer

5-7

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Where is the link layer implemented?

• in each and every host
• link layer implemented in “adaptor” (aka network interface card NIC) or on a chip
• Ethernet card, 802.11 card; Ethernet chipset
• implements link, physical layer
• attaches into host’s system buses
• combination of hardware, software, firmware

​

Link Layer

5-8

controller

physical

transmission

cpu

memory

host

bus

(e.g., PCI)

network adapter

card

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Adaptors communicating

• sending side:
• encapsulates datagram in frame
• adds error checking bits, rdt, flow control, etc.

• receiving side
• looks for errors, rdt, flow control, etc
• extracts datagram, passes to upper layer at receiving side

Link Layer

5-9

controller

controller

sending host

receiving host

datagram

datagram

datagram

frame

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-10

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Error detection

Link Layer

5-11

EDC= Error Detection and Correction bits (redundancy)

D = Data protected by error checking, may include header fields �

• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction

otherwise

Data Link Layer

5-12

Parity checking

Link Layer

5-13

single bit parity:

• detect single bit errors

two-dimensional bit parity:

• detect and correct single bit errors

0

0

Internet checksum (review)

sender:

• treat segment contents as sequence of 16-bit integers
• checksum: addition (1’s complement sum) of segment contents
• sender puts checksum value into UDP checksum field

​

​

receiver:

• compute checksum of received segment
• check if computed checksum equals checksum field value:
• NO - error detected
• YES - no error detected. But maybe errors nonetheless?

Link Layer

5-14

goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)

​

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Data Link Layer

5-15

Cyclic redundancy check

• more powerful error-detection coding
• view data bits, D, as a binary number
• choose r+1 bit pattern (generator), G
• goal: choose r CRC bits, R, such that
• <D,R> exactly divisible by G (modulo 2)
• receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!
• can detect all burst errors less than r+1 bits
• widely used in practice (Ethernet, 802.11 WiFi, ATM)

Link Layer

5-16

Data Link Layer

5-17

Data Link Layer

5-18

Data Link Layer

5-19

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-20

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Multiple access links, protocols

two types of “links”:

• point-to-point
• PPP for dial-up access
• point-to-point link between Ethernet switch, host
• broadcast (shared wire or medium)
• old-fashioned Ethernet
• upstream HFC
• 802.11 wireless LAN

​

Link Layer

5-21

shared wire (e.g.,

cabled Ethernet)

shared RF

(e.g., 802.11 WiFi)

shared RF

(satellite)

humans at a

cocktail party

(shared air, acoustical)

Multiple access 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

5-22

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

An ideal multiple access protocol

given: broadcast channel 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

5-23

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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

5-24

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Channel partitioning MAC protocols: TDMA

TDMA: time division multiple access

• access to channel in "rounds"
• each station gets fixed length slot (length = pkt trans time) in each round
• unused slots go idle
• example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

Link Layer

5-25

1

3

4

1

3

4

6-slot

frame

6-slot

frame

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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 pkt, frequency bands 2,5,6 idle

Link Layer

5-26

frequency bands

time

FDM cable

Random access protocols

• when node has packet to send
• transmit at full channel data rate R.
• no a priori coordination among nodes
• two or more transmitting nodes ➜ “collision”,
• random access MAC protocol specifies:
• how to detect collisions
• how to recover from collisions (e.g., via delayed retransmissions)
• examples of random access MAC protocols:
• slotted ALOHA
• ALOHA
• CSMA, CSMA/CD, CSMA/CA

Link Layer

5-27

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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 prob. p until success

Link Layer

5-28

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Slotted ALOHA

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

5-29

Pure (unslotted) ALOHA

• unslotted Aloha: simpler, no synchronization
• when frame first arrives
• transmit immediately
• collision probability increases:
• frame sent at t₀ collides with other frames sent in [t₀-1,t₀+1]

Link Layer

5-30

CSMA (carrier sense multiple access)

CSMA: listen before transmit:

if channel sensed idle: transmit entire frame

• if channel sensed busy, defer transmission ��
• human analogy: don’t interrupt others!

Link Layer

5-31

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

CSMA collisions

• collisions can still occur: propagation delay means two nodes may not hear each other’s transmission
• collision: entire packet transmission time wasted
• distance & propagation delay play role in in determining collision probability

​

Link Layer

5-32

spatial layout of nodes

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

CSMA/CD (collision detection)

CSMA/CD: carrier sensing, deferral as in CSMA

• collisions detected within short time
• colliding transmissions aborted, reducing channel wastage
• collision detection:
• easy in wired LANs: measure signal strengths, compare transmitted, received signals
• difficult in wireless LANs: received signal strength overwhelmed by local transmission strength
• human analogy: the polite conversationalist

Link Layer

5-33

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

CSMA/CD (collision detection)

Link Layer

5-34

spatial layout of nodes

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

“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

5-35

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

“Taking turns” MAC protocols

polling:

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

Link Layer

5-36

master

slaves

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

“Taking turns” MAC protocols

Link Layer

5-37

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

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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

5-38

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-39

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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

​

Link Layer

5-40

hexadecimal (base 16) notation

(each “number” represents 4 bits)

LAN addresses and ARP

Link Layer

5-41

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)

LAN addresses (more)

• 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 LAN card from one LAN to another
• IP hierarchical address not portable
• address depends on IP subnet to which node is attached

​

Link Layer

5-42

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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)

Link Layer

5-43

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

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

5-44

Addressing: routing to another LAN

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
• assume A knows IP address of first hop router, R (how?)
• assume A knows R’s MAC address (how?)

Link Layer

5-45

Addressing: routing to another LAN

Link Layer

5-46

• 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

Addressing: routing to another LAN

Link Layer

5-47

• 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

Addressing: routing to another LAN

Link Layer

5-48

• 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

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-49

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Ethernet

“dominant” wired LAN technology:

• cheap $20 for NIC
• first widely used LAN technology
• simpler, cheaper than token LANs and ATM
• kept up with speed race: 10 Mbps – 10 Gbps

​

Link Layer

5-50

Metcalfe’s Ethernet sketch

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 “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)

Link Layer

5-51

switch

bus: coaxial cable

star

Ethernet frame structure

sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

​

​

​

preamble:

• 7 bytes with pattern 10101010 followed by one byte with pattern 10101011
• used to synchronize receiver, sender clock rates

Link Layer

5-52

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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 (mostly IP but others possible, e.g., Novell IPX, AppleTalk)
• CRC: cyclic redundancy check at receiver
• error detected: frame is dropped

Link Layer

5-53

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

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 wth binary backoff

Link Layer

5-54

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-55

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

Link Layer

5-56

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Switch forwarding table

Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?

Link Layer

5-57

• 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?

Interconnecting switches

• switches can be connected together

Link Layer

5-58

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!)

Institutional network

Link Layer

5-59

to external

network

router

IP subnet

mail server

web server

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

Link Layer

5-60

application

transport

network

link

physical

switch

application

transport

network

link

physical

VLANs

port-based VLAN: switch ports grouped (by switch management software) so that single physical switch ……

​

Link Layer

5-61

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.

Virtual Local

Area Network

1

8

9

16

10

2

7

…

Electrical Engineering

(VLAN ports 1-8)

Computer Science

(VLAN ports 9-15)

15

…

Port-based VLAN

• 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

Link Layer

5-62

1

8

9

16

10

2

7

…

Electrical Engineering

(VLAN ports 1-8)

Computer Science

(VLAN ports 9-15)

15

…

• dynamic membership: ports can be dynamically assigned among VLANs

router

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-63

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Multiprotocol label switching (MPLS)

• initial goal: high-speed IP forwarding using fixed length label (instead of IP address)
• fast lookup using fixed length identifier (rather than shortest prefix matching)
• borrowing ideas from Virtual Circuit (VC) approach
• but IP datagram still keeps IP address!

​

Link Layer

5-64

PPP or Ethernet

header

IP header

remainder of link-layer frame

MPLS header

label

Exp

S

TTL

20

3

1

5

MPLS capable routers

• 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 (useful for VoIP)

​

Link Layer

5-65

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

MPLS versus IP paths

Link Layer

5-66

R2

D

R3

R5

A

R6

IP router

• IP routing: path to destination determined by destination address alone

R4

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

MPLS versus IP paths

Link Layer

5-67

R2

D

R3

R4

R5

A

R6

IP-only

router

• IP routing: path to destination determined by destination address alone

MPLS and

IP router

• MPLS routing: path to destination can be based on source and dest. address
• fast reroute: precompute backup routes in case of link failure

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

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

Link Layer

5-68

D

R4

R5

A

R6

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

MPLS forwarding tables

Link Layer

5-69

R1

R2

D

R3

R4

R5

0

1

0

0

A

R6

in out out

label label dest interface

10 A 0

12 D 0

1

0

8 A 1

Link layer, LANs: outline

5.1 introduction, services

5.2 error detection, correction

5.3 multiple access protocols

5.4 LANs

• addressing, ARP
• Ethernet
• switches
• VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

​

Link Layer

5-70

RCEW, Pasupula (V), Nandikotkur Road,

Near Venkayapalli, KURNOOL

Data center networks

• 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)

​

Link Layer

5-71

• challenges:
• multiple applications, each serving massive numbers of clients
• managing/balancing load, avoiding processing, networking, data bottlenecks

Inside a 40-ft Microsoft container,

Chicago data center

Link Layer

5-72

Server racks

TOR switches

Tier-1 switches

Tier-2 switches

Load

balancer

Load

balancer

B

1

2

3

4

5

6

7

8

A

C

Border router

Access router

Internet

Data center networks

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)