flow control and error control
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Flow control and Error control
ECS 152AXin Liu
Based on Kurose and Ross
The Data Link Layer
Our goals: understand principles behind data link layer
services: Flow control error detection, correction reliable data transfer
Link Layer: IntroductionSome 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
“link”
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
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 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 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
Adaptors Communicating
link layer implemented in “adaptor” (aka NIC) Ethernet card, PCMCI card,
802.11 card
sending side: encapsulates datagram in
a frame adds error checking bits,
rdt, flow control, etc.
receiving side looks for errors, rdt, flow
control, etc extracts datagram,
passes to rcving node
adapter is semi-autonomous
link & physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
Flow Control
Ensuring the sending entity does not overwhelm the receiving entity Preventing buffer overflow
Transmission time Time taken to emit all bits into medium
Propagation time Time for a bit to traverse the link
Model of Frame Transmission
Stop and Wait
Source transmits frame Destination receives frame and replies
with acknowledgement Source waits for ACK before sending
next frame Destination can stop flow by not send
ACK Works well for a few large frames
Fragmentation
Large block of data may be split into small frames Limited buffer size Errors detected sooner (when whole frame
received) On error, retransmission of smaller frames is
needed Prevents one station occupying medium for
long periods Stop and wait becomes inadequate
Stop and Wait Link Utilization
Sliding Windows Flow Control
Allow multiple frames to be in transit Receiver has buffer W long Transmitter can send up to W frames without
ACK Each frame is numbered ACK includes number of next frame expected Sequence number bounded by size of field (k)
Frames are numbered modulo 2k
Sliding Window Diagram
Example Sliding Window
Sliding Window Enhancements Receiver can acknowledge frames without
permitting further transmission (Receive Not Ready)
Must send a normal acknowledge to resume If duplex, use piggybacking
If no data to send, use acknowledgement frame If data but no acknowledgement to send, send last
acknowledgement number again, or have ACK valid flag (TCP)
Q: if the size of field is k, what is the maximum window size?
A: 2^k -1. Assuming 0,1,…2^k-1 (2^k window size) sent out, need to ack the last 2^k-1.
Error Detection and correction
Error DetectionEDC= 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
Parity Checking
Single Bit Parity:Detect single bit errors
Two Dimensional Bit Parity:Detect and correct single bit errors
0 0
UDP checksum
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? More later ….
Goal: detect “errors” (e.g., flipped bits) in transmitted segment
Internet Checksum Example Note
When adding numbers, a carryout from the most significant bit needs to be added to the result
Example: add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
wraparound
sumchecksum
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
<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 (ATM, HDCL)
CRC ExampleWant:
D.2r XOR R = nGequivalently:
D.2r = nG XOR R equivalently: if we divide D.2r by
G, want remainder R
R = remainder[ ]D.2r
G
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence
Transmit k+n bits which is exactly divisible by some number
Receive divides frame by that number If no remainder, assume no error
(x7 x6 1) (x6 x5 ) x7 (1 1)x6 x 5 1
x7 x5 1
(x 1)(x2 x 1) x3 x 2 x x2 x 1 x3 1
Addition:
Multiplication:
Division: x3 + x + 1 ) x6 + x5
x3 + x2 + x
x6 + x4 + x3
x5 + x4 + x3
x5 + x3 + x2
x4 + x2
x4 + x2 + xx
= q(x) quotient
= r(x) remainder
divisordividend
35 ) 1223
10517
Figure 3.55
Cyclic Redunancy check
Steps:
1) Multiply i(x) by xn-k (puts zeros in (n-k) low order positions)
2) Divide xn-k i(x) by g(x)
3) Add remainder r(x) to xn-k i(x) (puts check bits in the n-k low order positions):
quotient remainder
transmitted codewordb(x) = xn-ki(x) + r(x)
xn-ki(x) = g(x) q(x) + r(x)
Cyclic Redundancy Check
Cyclic Redundancy Check
Generator polynomial: g(x)= x3 + x + 1Information: (1,1,0,0) i(x) = x3 + x2
Encoding: x3i(x) = x6 + x5
1011 ) 1100000
1110
10111110101110101011
010
x3 + x + 1 ) x6 + x5
x3 + x2 + x
x6 + x4 + x3
x5 + x4 + x3
x5 + x3 + x2
x4 + x2
x4 + x2 + xx
Transmitted codeword:b(x) = x6 + x5 + xb = (1,1,0,0,0,1,0)
Capability of CRC
An error E(X) is undetectable if it is divisible by P(x). The following can be detected. All single-bit errors if P(x) has more than one nonzero
term All double-bit errors if P(x) has a factor with three
terms Any odd number of errors, if P(x) contain a factor x+1 Any burst with length less or equal to n-k A fraction of error burst of length n-k+1; the fraction
is 1-2^(-(-n-k-1)). A fraction of error burst of length greater than n-k+1;
the fraction is 1-2^(-(n-k)). Powerful error detection; more computation
complexity compared to Internet checksum
Error Correction
Correction of detected errors usually requires data block to be retransmitted Not appropriate for wireless applications Bit error rate is high
• Lots of retransmissions Propagation delay can be long (satellite) compared
with frame transmission time• Would result in retransmission of frame in error plus
many subsequent frames
Need to correct errors on basis of bits received
Basic idea: redundancy
Error Correction
Hamming distance: d(v1,v2) between two n-bit binary sequences v1 and v2 is the # of bits in which v1 and v2 disagree.
(n,k) block code: map each k-bit sequence into a unique n-bit codeword (n>k).
K=2,n=5
Data code
00 00000
01 00111
10 11001
11 11110
Consider one bit of error, say 00000->00001.d(00000,00001)=1;d(00111,00001)=2;d(11001,00001)=2;d(11110,00001)=4;
Error Control
Detection and correction of errors Lost frames Damaged frames Automatic repeat request
Error detection Positive acknowledgement Retransmission after timeout Negative acknowledgement
Automatic Repeat Request (ARQ) Stop and wait Go back N Selective reject (selective
retransmission)
Stop and Wait
Source transmits single frame Wait for ACK If received frame damaged, discard it
Transmitter has timeout If no ACK within timeout, retransmit
If ACK damaged,transmitter will not recognize it Transmitter will retransmit Receive gets two copies of frame Use ACK0 and ACK1
Stop and Wait -Diagram
Stop and Wait - Pros and Cons Simple Inefficient
Go Back N (1)
Based on sliding window If no error, ACK as usual with next frame
expected Use window to control number of
outstanding frames If error, reply with rejection
Discard that frame and all future frames until error frame received correctly
Transmitter must go back and retransmit that frame and all subsequent frames
Go Back N - Damaged Frame
Receiver detects error in frame i Receiver sends rejection-i Transmitter gets rejection-i Transmitter retransmits frame i and all
subsequent
Go Back N - Lost Frame (1)
Frame i lost Transmitter sends i+1 Receiver gets frame i+1 out of
sequence Receiver send reject i Transmitter goes back to frame i and
retransmits
Go Back N - Lost Frame (2)
Frame i lost and no additional frame sent Receiver gets nothing and returns neither
acknowledgement nor rejection Transmitter times out and sends
acknowledgement frame with P bit set to 1
Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )
Transmitter then retransmits frame i
Go Back N - Damaged Acknowledgement Receiver gets frame i and send
acknowledgement (i+1) which is lost Acknowledgements are cumulative, so
next acknowledgement (i+n) may arrive before transmitter times out on frame i
If transmitter times out, it sends acknowledgement with P bit set as before
This can be repeated a number of times before a reset procedure is initiated
Go Back N - Damaged Rejection As for lost frame (2)
Go Back N - Diagram
Selective Repeat
receiver individually acknowledges all correctly received pkts buffers pkts, as needed, for eventual in-order
delivery to upper layer
sender only resends pkts for which ACK not received sender timer for each unACKed pkt
sender window N consecutive seq #’s again limits seq #s of sent, unACKed pkts
Selective repeat: sender, receiver windows
Selective repeat
data from above : if next available seq # in
window, send pkt
timeout(n): resend pkt n, restart
timer
ACK(n) in [sendbase,sendbase+N]:
mark pkt n as received if n smallest unACKed
pkt, advance window base to next unACKed seq #
senderpkt n in [rcvbase, rcvbase+N-
1]
send ACK(n) out-of-order: buffer in-order: deliver (also
deliver buffered, in-order pkts), advance window to next not-yet-received pkt
pkt n in [rcvbase-N,rcvbase-1]
ACK(n)
otherwise: ignore
receiver
Selective repeat in action
Selective repeat: dilemma
Example: seq #’s: 0, 1, 2, 3 window size=3
receiver sees no difference in two scenarios!
incorrectly passes duplicate data as new in (a)
Q: what relationship between seq # size and window size?
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