computer networksnemo/cen5501/slides/chapter1.pdfcen 5501c - computer networks - spring 2007 -...
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![Page 1: Computer Networksnemo/cen5501/slides/Chapter1.pdfCEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 40 Reliable Data Xfer – GBN ARQ Src Dest *-D 0 A C K D 1 Missing D3](https://reader033.vdocuments.site/reader033/viewer/2022042804/5f56bf68c1efc343f30b194d/html5/thumbnails/1.jpg)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 1
Computer Networks
Chapter 1 - Fundamentals
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 2
Computer Networks
• Need to share– Information– Resources
• Communication vs. StorageTransmission across:– Space (communication)– Time (storage)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 3
Virtuality• Architecture
– Layered– Hierarchical
• Algorithms– Information hiding, ADTs, objects
• Protocols – Distributed coordination algorithms
• Programs– Modularity
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 4
Layered vs. Hierarchical
• Both– Peer-to-peer communication– Encapsulation– Protocol = common language/behaviors
• Layered– Layer i serves layer i+1 ONLY– Layer i gets service from layer i-1 ONLY
• Hierarchical– Lower layers serve higher layers
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 5
Layered vs. Hierarchical
• Issues– Flexibility– Efficiency– Modularity– Maintainability– Scalability/manageability– Future adaptability
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 6
Comparative Architectures7 - Application
6 - Presentation
5 - Session
4 - Transport
3 - Network
2 – Data Link
1 - Physical
UserApplication
Data Link ControlDDCMP
Physical 1 - Physical
Path Control
Transmission Ctl
Data Flow Control
Transport
Functional Management Data Services
Network Services Protocol
NetworkApplication
ISO - OSI IBM - SNA DEC - DECnet
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 7
OSI Reference Architecture
• Physical (L1 = PHY)• Data Link (L2 = MAC/Link)• Network (L3)• Transport (L4)• Session (L5)• Presentation (L6)• Application (L7)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 8
PHY Layer
• Hardware• Physical manipulation of medium
(modulation)• Physical sensing of medium (detection)• Low level synchronization
(bits/symbols/frames)• Forward error correction/error detection• Mechanical/electrical interconnect and
medium
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 9
Link Layer• Firmware• Framing• Addressing• Medium access control (MAC)• Backward error detection/correction• Reliable delivery of frames from one STA
to a directly connected STA• Pacing• Upward multiplexing
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 10
Network Layer
• Software/firmware• Packets/cells• Routing• Packet fragmentation/reassembly• Backward error correction• Delivery of frames from source to an
indirectly connected destination• Congestion control
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 11
Transport Layer• Software on end host: End-to-end layer• Reliable communication stream
– Messages– Byte stream– Ordering– BEC
• Upward Multiplexing• Delivery of messages/byte stream from
source process to destination process• Congestion control
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 12
Session Layer
• Software on end host: end-to-end layer• Stream management• Dialog control• Packet chaining (atomic delivery)• Downward Multiplexing• Authentication• Connection-oriented
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 13
Presentation Layer
• Software on end host• Common utilities
– Encryption– Compression– Uniform formatting (XML, ASN.1,…)
• Standardized representations• Interfacing to local resources
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 14
Application Layer
• Software on end host• Specific application programs
– FTP– Remote terminal (rlogin, telnet, ssh,…)– Email– HTTP
• May also be layered in distributed software system
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 15
Encapsulation
applicationtransportnetwork
linkphysical
applicationtransportnetwork
linkphysical
source destinationMMMM
Ht
HtHnHtHnHl
MMMM
Ht
HtHnHtHnHl
messagesegmentdatagramframe
(thanks – Kurose & Ross)
• Receive SDU from higher layer• Hide uninterpreted SDU as payload of PDU
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 16
End-to-end Data Transport
(thanks – Kurose & Ross)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 17
Service Models
• Interface– Connectionless– Connection-oriented
• Reliability– Best effort– Reliable
• Combinations
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 18
Service Interface
• Interface– Connectionless: memoryless
• Send packet• Receive packet
– Connection-oriented: stateful• Initialize (set up connection)• Use (send/receive)• Close (release state)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 19
Service Reliability
• Reliability– Best effort
• Lost packets• Duplicate packets• Delayed/reordered delivery• Damaged packets
– Reliable• Undamaged packets• All packets sent delivered in timely fashion• Delivered in order sent
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 20
Message Conventions
1-REQ
2,3
2 - Tx 3 - Rx
4 - IND5 - RSP8 - CNF
6 - Tx7 - Rx6,7
Request/Confirm Indication/Response
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 21
Combining Service Models
Best Effort
Reliable
Connectionless Connection-Oriented
UDP, IP, IPX, CLNP, DECnet, Appletalk, CLNS,…
ATM
TCP, X.25, CONS???
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 22
Network Service vs. Implementation
Service
Implementation
Connectionless Connection-Oriented
UDP, CLNS
TCP, DNA
???Connectionless
Connection-Oriented X.25, ATM, CONS, SNA
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 23
Network Properties
• Scope• Scalability• Robustness• Autoconfigurability• Tweakability• Determinism• Migration
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 24
Network Properties - Robustness
• Types of Errors– Link/node failure– Data errors (esp. undetected!)– S/W errors– H/W errors– Human Errors
• Features
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 25
Network Properties - Robustness
• Types of Errors• Features
– Safety Barriers– Self-stabilization– Fault detection– Byzantine robustness
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 26
Reliable Data Transfer
• Models– Errors– Receiver capacity
• Requirements– Duplexity– Timers– State
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 27
Data Transfer Model• Events
– What can happen at node, channel• Frames
– What do they hold• Duplexity
– Simplex, half duplex, full duplex• Time costs
– What does it take to complete transfer• Metrics
– How do we measure the costs
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 28
Data Transfer - Events
• Node– New frame to send from HLE– Frame/ACK arrival – good frame– Frame/ACK arrival – damaged frame– Timeout– Attempt to receive next frame by HLE
• Channel– Error – damage frame– Error – lose frame
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 29
Data Transfer - Frames• Forward Control Info
– Type– Sequence number– Timestamp– Length– Addressing– Error Detection (FCS)
• Reverse Control Info– ACKs– Flow control/pacing– Piggybacking
• Information– payload
Dst Src Type SN TS Len … Payload FCS
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 30
Channel Model - Duplexity
• Simplex – only one way• Half duplex – one way at a time• Full duplex – simultaneously both ways
A BA B A B
SimplexHalf Duplex Full Duplex
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 31
Data Transfer - Delays
• Processing– Source– Destination
• Transmission– Time to put bits on wire
• Propagation– Time for bit to traverse
channel
Src Dest
Src Proc
Transmission
Propagation
Propagation
Src Proc
Dest Proc
Data frame
ACK frameACK Tx
Tx REQ
Rx REQ
Rx CNFTx CNF
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 32
Data Transfer - Metrics• Utilization
– Time sending info/total time• Storage requirements
– At source– At destination
• Channel type– duplexity
• Timers– Retransmission– ACK transmission
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 33
Reliable Data Xfer - Utopia• Infinitely fast receiver• Simplex channel• No errors• 100% utilization by
protocol
Src Dest
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 34
Stop&Wait Data Xfer - Pacing• Finitely fast receiver• Half duplex channel• No errors• <100% utilization by
protocol– UProtocol = TTx/Tcycle– TTx= L (bits)/R (bps)– Tcycle = TTx+Tprop+Tproc+
Tprop+Tproc
Src Dest
OK
OK
Data time
Cycle time
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 35
Reliable Data Xfer – PAR(Positive ACK and Retransmit)
• Finitely fast receiver• Full duplex channel• Channel errors• Utilization factor due to
errors– Uerrors = Tgood/ Tgood+ Tbad
• U = Uprotocol x Uerrors
Src Dest
ACK-
*
ACK-
X
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 36
Reliable Data Xfer – PARneed for sequence numbers
Src Dest
ACK-
*
ACK-
X
Src Dest
ACK-
*
ACK-
X
First Frame
Second Frame
Second Frame - duplicate
Missing ACK Missing ACK
ACK
Frame 1
Frame 2
Frame 2 - duplicate
Missing ACK
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 37
Reliable Data Xfer – 1-bit ARQSrc Dest
ACK 1-
*
ACK 0-
X
Src Dest
ACK 1-
*
ACK 0-
X
Frame 0
Frame 1
Frame 1
Missing ACK Missing ACK
Frame 0
Frame 1
Frame 1
Missing ACK
ACK 0
accept accept
accept
accept
discardduplicate
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 38
Protocol Utilization – 1-bit ARQ• Protocol util.
Uproto= T/(T+2τ)T = Tx time = D/RD = size (bits/frame)R = data rate (bps)τ = propagation delay
• Uproto= 1/(1+2α)α = τ /T = τ /(D/R)
τ
Src Dest
ACK 1
Frame 0
T
τ
ACK 0
Frame 1
Cycle Tim
e
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 39
Protocol Utilization – 1-bit ARQ1-bit ARQ Efficiency
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.25 0.5 1 2 4 8 16 32 64
Normalize d de lay
Effic
ienc
y
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 40
Reliable Data Xfer – GBN ARQSrc Dest
*
-
D 0D 1ACK 1
Missing D3
Accept D0D 2
D 3 (resent)
D 4D 5
ACK 2ACK 3
ACK 3ACK 3 Discard D4
Discard D5
D 4D 5
ACK 6ACK 5
Accept D1Accept D2
Accept D3
Accept D4Accept D5
ACK 4
• Multiple sequence #’s• Channel errors• Discard out of order
frames on Rx• Resend all frames
from missing on forward
• Rx buffer size of 1• Tx buffer size of N
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 41
Reliable Data Xfer – SR ARQSrc Dest
*
-
Missing D3
D 0D 1ACK 1 Accept D0D 2
D 3 (resent)
D 4D 5
ACK 2ACK 3
ACK 3ACK 3 Buffer D4
Buffer D5
D 6D 7
ACK 8ACK 7
Accept D1Accept D2
Accept D3
Accept D6Accept D7
ACK 6
• Multiple sequence #’s• Channel errors• Buffer out of order
frames on Rx• Resend only missing
frame• Rx buffer size of N• Tx buffer size of k
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 42
Protocol Utilization – ARQSrc Dest
kT data
T cycle
• Protocol util.depends on kUproto= kT/(T+2τ)T = Tx time = D/Rτ = propagation delay
• Uproto= min(1,k/(1+2α))α = τ /T = τ /(D/R)
• Utilization factor due to errors different for GBN and SR
τ
τ
T
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 43
Protocol Utilization –ARQARQ Protocol Efficiency
0
0.2
0.4
0.6
0.8
1
1.2
0.25 0.5 1 2 4 8 16 32 64
a = normalized delay
effic
ienc
y
k=1k=2k=3k=5k=9k=17
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 44
Error Utilization Factor• SR-ARQ
Only resend missing framesUerr,SR= 1-pp = frame error prob
• GBN-ARQSend all frames after missing frameUerr,GBN=(1-p)/(1+2αp) when k>1+2αUerr,GBN=(1-p)/(1-p+kp) when k<1+2αDepends on k
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 45
Error Utilization FactorEfficiency Loss due to Errors
0
0.2
0.4
0.6
0.8
1
1.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Frame Error Rate p
Effic
ienc
y
GBN k=1GBN k=2GBN k=4GBN k=8GBN k=16GBN k=32
(large a)
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 46
Overall ARQ Utilization• SR-ARQ
USR= min{(1-p),k(1-p)/(1+2α)}
• GBN-ARQUGBN= (1-p)/(1+2αp) when k>1+2αUGBN= k(1-p)/(1+2α)(1-p+kp) when k<1+2α
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 47
Overall UtilizationARQ Efficiency, p=0.001
0
0.2
0.4
0.6
0.8
1
1.2
0.25 0.5 1 2 4 8 16 32 641282565121024
Normalized delay (a)
Effic
ienc
y
both k=1GBN k=9GBN k=65GBN k=129SR k=9SR k=65SR k=129
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CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman 48
Sequence Numbers• Sequence numbers are finite (usually)
Assume n bits• Hence, they wrap around
i.e., 0 follows 2n-1• Need to use circular “<“ relation
i.e., 0 > 2n-1• Need to limit range usable by sender and
acceptable to receiver (SW and RW)|SW| + |RW| <= 2n
0 12
2n-1
2n-1-12n-1
2n-1+1