Transcript
Chapter 5 Link Layer and LANs
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Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
Thanks and enjoy! JFK/KWR All material copyright 1996-2007 J.F Kurose and K.W. Ross, All Rights Reserved 5: DataLink Layer
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Chapter 5: The Data Link Layer Our goals: understand principles behind data link layer
services:
error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done!
instantiation and implementation of various link
layer technologies
5: DataLink Layer
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link virtualization:
ATM, MPLS
5: DataLink Layer
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Link Layer: Introduction Some terminology: hosts and routers are nodes communication channels that
connect adjacent nodes along communication path are links
wired links wireless links LANs
layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 5: DataLink Layer
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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 5: DataLink Layer
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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? 5: DataLink Layer
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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 5: DataLink Layer
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Where is the link layer implemented? in each and every host link layer implemented in
“adaptor” (aka network interface card NIC)
Ethernet card, PCMCI card, 802.11 card implements link, physical layer
attaches into host’s
system buses combination of hardware, software, firmware
host schematic application transport network link
cpu
memory
controller
link physical
host bus (e.g., PCI)
physical transmission
network adapter card
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Adaptors Communicating datagram
datagram controller
controller
receiving host
sending host datagram
frame
sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc.
receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to upper layer at receiving side 5: DataLink Layer
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization:
ATM. MPLS
5: DataLink Layer
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Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction
otherwise
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Parity Checking Single Bit Parity: Detect single bit errors
Two Dimensional Bit Parity: Detect and correct single bit errors
0
0
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Internet checksum (review) Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Sender: treat segment contents
as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field
Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless?
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Checksumming: Cyclic Redundancy Check view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that
exactly divisible by G (modulo 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, ATM)
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CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R
R = remainder[
D.2r G
]
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization:
ATM, MPLS
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Multiple Access Links and Protocols Two types of “links”: point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 802.11 wireless LAN
shared wire (e.g., cabled Ethernet)
shared RF (e.g., 802.11 WiFi)
shared RF (satellite)
humans at a cocktail party (shared air, acoustical) 5: DataLink Layer
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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
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Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized:
no special node to coordinate transmissions no synchronization of clocks, slots
4. simple
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MAC Protocols: a taxonomy Three broad classes: Channel Partitioning
divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use
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
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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 6-slot frame 1
3
4
1
3
4
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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 pkt, frequency
FDM cable
frequency bands
bands 2,5,6 idle
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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 5: DataLink Layer
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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!
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CSMA collisions
spatial layout of nodes
collisions can still occur: propagation delay means two nodes may not hear each other’s transmission
collision:
entire packet transmission time wasted
note:
role of distance & propagation delay in determining collision probability
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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 5: DataLink Layer
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CSMA/CD collision detection
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“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! 5: DataLink Layer
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“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)
data
poll
master data
slaves
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“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 5: DataLink Layer
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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, IBM Token Ring
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization:
ATM, MPLS
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MAC Addresses and ARP 32-bit IP address: network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address:
function: get frame from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs)
• burned in NIC ROM, also sometimes software settable
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LAN Addresses and ARP Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN (wired or wireless)
Broadcast address = FF-FF-FF-FF-FF-FF
= adapter 58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
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LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space
(to assure uniqueness) analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address ➜ portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is attached
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ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 137.196.7.78 1A-2F-BB-76-09-AD 137.196.7.23
Each IP node (host,
router) on LAN has ARP table ARP table: IP/MAC address mappings for some LAN nodes
137.196.7.14
LAN 71-65-F7-2B-08-53
137.196.7.88
< IP address; MAC address; TTL>
58-23-D7-FA-20-B0
TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)
0C-C4-11-6F-E3-98
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ARP protocol: Same LAN (network) A wants to send datagram
to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address dest MAC address = FFFF-FF-FF-FF-FF all machines 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
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Addressing: routing to another LAN walkthrough: send datagram from A to B via R assume A knows B’s IP address 88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
111.111.111.111
E6-E9-00-17-BB-4B 1A-23-F9-CD-06-9B
222.222.222.220 111.111.111.110 111.111.111.112
R
222.222.222.221
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
two ARP tables in router R, one for each IP
network (LAN)
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A creates IP datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest,
frame contains A-to-B IP datagram This is a really important A’s NIC sends frame example – make sure you understand! R’s NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B 88-B2-2F-54-1A-0F
74-29-9C-E8-FF-55
A
E6-E9-00-17-BB-4B
111.111.111.111
222.222.222.220 111.111.111.110 111.111.111.112
222.222.222.221
1A-23-F9-CD-06-9B
R
222.222.222.222
B
49-BD-D2-C7-56-2A
CC-49-DE-D0-AB-7D
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization:
ATM and MPLS
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Ethernet “dominant” wired LAN technology: cheap $20 for NIC first widely used LAN technology simpler, cheaper than token LANs and ATM kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet sketch
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Star topology bus topology popular through mid 90s all nodes in same collision domain (can collide with each other)
today: star topology prevails active switch in center each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other)
switch
bus: coaxial cable
star 5: DataLink 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
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Ethernet Frame Structure (more) Addresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame Type: indicates higher layer protocol (mostly IP
but others possible, e.g., Novell IPX, AppleTalk) CRC: checked at receiver, if error is detected, frame is dropped
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Ethernet: Unreliable, connectionless connectionless: No handshaking between sending and
receiving NICs unreliable: receiving NIC doesn’t send acks or nacks to sending NIC
stream of datagrams passed to network layer can have gaps (missing datagrams) gaps will be filled if app is using TCP otherwise, app will see gaps
Ethernet’s MAC protocol: unslotted CSMA/CD
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Ethernet CSMA/CD algorithm 1. NIC receives datagram 4. If NIC detects another from network layer, transmission while creates frame transmitting, aborts and sends jam signal 2. If NIC senses channel idle, starts frame transmission 5. After aborting, NIC If NIC senses channel enters exponential busy, waits until channel backoff: after mth idle, then transmits collision, NIC chooses K at random from 3. If NIC transmits entire {0,1,2,…,2m-1}. NIC waits frame without detecting K·512 bit times, returns to another transmission, NIC Step 2 is done with frame ! 5: DataLink Layer
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Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec
See/interact with Java applet on AWL Web site: highly recommended !
Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K· 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten collisions, choose K from {0,1,2,3,4,…,1023}
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CSMA/CD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
efficiency
1 1 5t prop /ttrans
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA: and simple,
cheap, decentralized!
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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
application transport network link physical
MAC protocol and frame format 100BASE-TX
100BASE-T2
100BASE-FX
100BASE-T4
100BASE-SX
100BASE-BX
copper (twister pair) physical layer
fiber physical layer 5: DataLink Layer
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Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other
no need for a centralized, global clock among nodes!
Hey, this is physical-layer stuff! 5: DataLink Layer
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3 Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet
5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization:
ATM, MPLS
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Hubs … physical-layer (“dumb”) repeaters: bits coming in one link go out all other links at same rate all nodes connected to hub can collide with one another no frame buffering no CSMA/CD at hub: host NICs detect collisions twisted pair
hub
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Switch link-layer device: smarter than hubs, take
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 5: DataLink Layer
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Switch: allows multiple simultaneous transmissions A
hosts have dedicated,
direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex
each link is its own collision domain
switching: A-to-A’ and B-
to-B’ simultaneously, without collisions
not possible with dumb hub
C’
B
6
1 5
2
3
4 C
B’
A’ switch with six interfaces (1,2,3,4,5,6)
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Switch Table Q: how does switch know that
A’ reachable via interface 4, B’ reachable via interface 5? A: each switch has a switch table, each entry:
C’
B
6
Q: how are entries created,
maintained in switch table? something like a routing protocol?
1 5
(MAC address of host, interface to reach host, time stamp)
looks like a routing table!
A
2
3
4 C
B’
A’ switch with six interfaces (1,2,3,4,5,6)
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Switch: self-learning switch learns which hosts
can be reached through which interfaces
Source: A Dest: A’
A A A’ C’
when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table
B 1
6
5
2
3
4 C
B’
A’
MAC addr interface TTL
A
1
60
Switch table (initially empty) 5: DataLink Layer
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Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived
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Self-learning, forwarding: example
Source: A Dest: A’
A A A’ C’
B
frame destination unknown: flood
A6A’
1
2 4
5
destination A
location known: selective send
C
A’ A B’
3
A’
MAC addr interface TTL
A A’
1 4
60 60
Switch table (initially empty) 5: DataLink Layer
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Interconnecting switches switches can be connected together S4 S1
S2
A B
S3
C
F
D E
I
G
H
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!) 5: DataLink Layer
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Self-learning multi-switch example Suppose C sends frame to I, I responds to C S4
1 S1
S2
A B
C
2
S3
F
D E
I
G
H
Q: show switch tables and packet forwarding in S1,
S2, S3, S4
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Institutional network to external network
mail server router
web server
IP subnet
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Switches vs. Routers both store-and-forward devices routers: network layer devices (examine network layer headers) switches are link layer devices routers maintain routing tables, implement routing
algorithms switches maintain switch tables, implement filtering, learning algorithms
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet
5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization:
ATM
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Point to Point Data Link Control one sender, one receiver, one link: easier than
broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point DLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!
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PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer
datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address 5: DataLink Layer
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PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering all relegated to higher layers!
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PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible
multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)
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PPP Data Frame info: upper layer data being carried check: cyclic redundancy check for error
detection
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Byte Stuffing “data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?
Sender: adds (“stuffs”) extra < 01111110> byte
after each < 01111110> data byte Receiver: two 01111110 bytes in a row: discard first byte, continue data reception single 01111110: flag byte 5: DataLink Layer
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Byte Stuffing flag byte pattern in data to send
flag byte pattern plus stuffed byte in transmitted data 5: DataLink Layer
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PPP Data Control Protocol Before exchanging networklayer data, data link peers must configure PPP link (max. frame length, authentication) learn/configure network layer information for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address 5: DataLink Layer
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Link Layer 5.1 Introduction and
services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet
5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization:
ATM and MPLS
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Virtualization of networks Virtualization of resources: powerful abstraction in systems engineering: computing examples: virtual memory, virtual devices Virtual machines: e.g., java IBM VM os from 1960’s/70’s layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly
5: DataLink Layer
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The Internet: virtualizing networks 1974: multiple unconnected nets ARPAnet data-over-cable
networks packet satellite network (Aloha) packet radio network
ARPAnet "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648.
… differing in: addressing
conventions packet formats error recovery routing
satellite net 5: DataLink Layer
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The Internet: virtualizing networks Internetwork layer (IP): addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity network of networks
Gateway: “embed internetwork packets in local packet format or extract them” route (at internetwork level) to next gateway
gateway
ARPAnet
satellite net 5: DataLink Layer
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Cerf & Kahn’s Internetwork Architecture What is virtualized? two layers of addressing: internetwork and local
network new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology cable satellite 56K telephone modem today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! 5: DataLink Layer
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ATM and MPLS ATM, MPLS separate networks in their own
right
different service models, addressing, routing from Internet
viewed by Internet as logical link connecting
IP routers
just like dialup link is really part of separate network (telephone network)
ATM, MPLS: of technical interest in their
own right
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Asynchronous Transfer Mode: ATM 1990’s/00 standard for high-speed (155Mbps to
622 Mbps and higher) Broadband Integrated Service Digital Network architecture Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world packet-switching (fixed length packets, called “cells”) using virtual circuits
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ATM architecture AAL
AAL ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system
adaptation layer: only at edge of ATM network
data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer
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ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” ATM is a network technology Reality: used to connect IP backbone routers “IP over ATM” ATM as switched link layer, connecting IP routers
IP network ATM network
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ATM Adaptation Layer (AAL) ATM Adaptation Layer (AAL): “adapts” upper
layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells analogy: TCP segment in many IP packets AAL
AAL
ATM
ATM
ATM
ATM
physical
physical
physical
physical
end system
switch
switch
end system 5: DataLink Layer
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ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams)
User data
AAL PDU
ATM cell 5: DataLink Layer
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ATM Layer Service: transport cells across ATM network analogous to IP network layer very different services than IP network layer Network Architecture Internet
Service Model
Guarantees ?
Congestion Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant rate guaranteed rate guaranteed minimum none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred via loss) no congestion no congestion yes
no
yes
no
no 5: DataLink Layer
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ATM Layer: Virtual Circuits VC transport: cells carried on VC from source to dest call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. Permanent VCs (PVCs)
long lasting connections typically: “permanent” route between to IP routers Switched VCs (SVC): dynamically set up on per-call basis
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ATM VCs Advantages of ATM VC approach:
QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic one PVC between each source/dest pair) does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived connections
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ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload
Why?: small payload -> short cell-creation delay for digitized voice halfway between 32 and 64 (compromise!)
Cell header
Cell format
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ATM cell header VCI: virtual channel ID
will change from link to link thru net PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum cyclic redundancy check
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ATM Physical Layer (more) Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Physical Medium Dependent: depends on physical medium being used TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send 5: DataLink Layer
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ATM Physical Layer Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a
container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps
TI/T3: transmission frame structure (old
telephone hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle)
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IP-Over-ATM Classic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses
IP over ATM replace “network” (e.g., LAN segment) with ATM network ATM addresses, IP addresses ATM network
Ethernet LANs
Ethernet LANs 5: DataLink Layer
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IP-Over-ATM app transport IP Eth phy
IP AAL Eth ATM phy phy ATM phy
ATM phy
app transport IP AAL ATM phy
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Datagram Journey in IP-over-ATM Network at Source Host: IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL5 AAL5 encapsulates data, segments cells, passes to ATM layer ATM network: moves cell along VC to destination at Destination Host:
AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP
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IP-Over-ATM Issues: IP datagrams into ATM AAL5 PDUs from IP addresses to ATM addresses just like IP addresses to 802.3 MAC addresses!
ATM network
Ethernet LANs
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Multiprotocol label switching (MPLS) initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do forwarding
borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address!
PPP or Ethernet header
MPLS header
label 20
IP header
remainder of link-layer frame
Exp S TTL 3
1
5 5: DataLink Layer
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MPLS capable routers a.k.a. label-switched router
forwards packets to outgoing interface based
only on label value (don’t inspect IP address)
MPLS forwarding table distinct from IP forwarding tables
signaling protocol needed to set up forwarding RSVP-TE forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! use MPLS for traffic engineering must co-exist with IP-only routers
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MPLS forwarding tables in label
out label dest
10 12 8
out interface
A D A
0 0 1
in label
out label dest
out interface
10
6
A
1
12
9
D
0
R6 0
0
D
1
1
R3
R4 R5
0
0
R2 in label
8
out label dest
6
A
out interface
in label
6
outR1 label dest
-
A
A out interface
0
0 5: DataLink Layer
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Chapter 5: 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 PPP virtualized networks as a link layer: ATM, MPLS
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Chapter 5: let’s take a breath journey down protocol stack complete
(except PHY) solid understanding of networking principles, practice ….. could stop here …. but lots of interesting topics! wireless multimedia security network management
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