Transcript The Data Link Layer Chapter 3

The Data Link Layer
Chapter 3
Hybrid Reference Model REMINDER
Application
Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Data Link Layer
• “Virtual communication” takes place in the
data link layer (in additional to the other
layers).
• Provides services to the network layer
– Framing (sending machine breaks input data
into input frames). Need to recognize framing
boundaries.
– Error control - noise burst can ruin a
transmission.
Data Link Layer
• Provides svcs to the network layer (contd)
– Flow control - how to stop a fast transmitter
from overwhelming a slow one.
Data Link Layer
• Services Provided to Network Layer
– Transfer data from network layer on source
machine to network layer on destination
machine through “virtual communication.”
– Actual path is through all lower layers.
– Services are 1) Unacknowledged
connectionless service 2) Acknowledged
connectionless service 3) Ack. Connectionoriented service
Data Link Layer
Unacknowledged connectionless
service
• Source machine sends frames to destination
machine. Destination machine does not ack.
• No connection estd. before hand
• Frame loss due to noise is not recovered in
data link layer but maybe above.
• Appropriate for
– Low error rate systems
– real time traffic (speech) - bad data is worse
than late data.
Acknowledged connectionless
service
• Each frame sent is acknowledged but no
connections are set up.
• If frame does not arrive, sender can wait
and retransmit.
• Can be used over unreliable channels.
Acknowledged connectionoriented service
• Connection estd. Before any data are
transferred.
• Each frame sent is numbered and the data
link layer guarantees that each frame sent is
indeed received.
• Each frame is recd. Only once (guaranteed)
and that order is maintained (guaranteed) (is
frame receivable multiple times in
connectionless service?)
Connection-oriented service
• Three phases to transfer:
– Connection is established by initializing
counters for received/unreceived frames
– One or more frames are transmitted
– Connection is released freeing up resources
(variables, buffers)
• Example - next slide
Placement of data link protocol
• WAN subnet consisting of routers
connected by point-to-point leased tel.
Lines.
• Frame arrives at router - it is checksummed
and is passed to data link layer software.
• DLL soft. Checks to see if frame was
expected. If so gives packet in payload field
to routing software.
• Routing software chooses appropriate
outgoing line and passes the packet to DLL
software which transmits it.
Placement of data link protocol
Routing Code (box 3, prev. slide)
• Routing code wants reliable sequenced
connections on each of the Pt-to-Pt lines.
• Data link protocol must make unreliable
lines look good (good for wireless links).
Framing
• In order to provide service to the network
layer, DLL must use services provided by
the physical layer.
• Bit stream delivered by physical layer may
not be error free.
• DLL must correct for errors
• DLL breaks bit stream into discrete frames
and creates checksum for each frame.
Framing (cont’d)
• Transmission is tricky. One possibility is to
insert gaps between frames but there is no
guarantee about timing from the network.
• Start and end of frame marked by
–
–
–
–
Character count
Starting and ending chars with char stuffing
Starting & ending flags with bit stuffing
Physical layer coding violations
Character count
Even if checksum is incorrect, the destination does not
know where the bad frame starts.
Starting and ending chars w/
character stuffing
• Use ASCII chars DLE STX (Data Link
Escape, Start TeXt) and DLE ETX (End
TeXt). If receiver loses frame boundaries it
should look for DLE STX and DLE ETX
• Problem: Binary data. DLE STX/ETX
could occur and ruin framing.
• Stuffing: Add an ASCII DLE before each
DLE in the binary data.
Starting and ending chars w/
character stuffing
Data from Network Layer
Data after stuffing
Data after de-stuffing that is passed to network
layer on destination end
DISADVANTAGE: Reliance on 8 bits per character
Start & end flags w/ bit stuffing
• Frame begins and ends with 01111110
called flag byte.
• When senders DLL senses 5 consecutive 1’s
it “stuffs” a 0 into the bit stream.
• Receiver sees the 5 consecutive 1’s
followed by a 0 and destuffs it (i.e., deletes
the 0).
• Bit stuffing unambiguously locates frame
boundaries.
Start & end flags w/ bit stuffing
Physical Layer Coding Violations
Read on your own
Error Control
• How to ensure that all frames are sent to
network layer at dest. in proper order?
• This is important for reliable connectionoriented service.
• Receiver sends back responses (positive or
negative) to incoming frames. Positive
means frame OK, negative means
retransmit.
Error Control
• Question: What happens when an error
burst destroys a whole frame?
• Answer: Receiver knows not that an error
has occurred. It wouldn’t send and
acknowledgement. Sender would hang
forever in such a case.
• To solve above, DLL’s have timers. On
sending frame a timer is set off ….
Error Control
• … if acknowledgement is not received (or
lost) before timer goes off, sender is alerted
to retransmit frame.
• Problem: What to do if frames are
submitted multiple times?
• Answer: Assign a sequence number to
frame. Therefore receiver will be able to
distinguish original and retransmitted frame.
Flow control
• Problem: What to do when sender sends
frames faster than receiver can accept them?
• Solution: Have rules as to when a sender
can send the next frame. The rule could say
“send n frames now but do not send any
more after that until I ask for them.”
• More on flow control later.
Error detection and correction
• Two strategies
– Include enough redundant information to
deduce what the error was - error correcting
codes
– Use redundant info. So that the receiver knows
that an error occurred but not which error. In
any case it can request a retransmission - error
detecting codes
Error correcting codes
• Terminology:
– Frame consists of m message bits + r redundant
or checkbits. n = r + m
– n bit unit consisting of data + checkbits is
called a codeword
– Hamming distance - 0010 XOR 1101 = 1111,
i.e., the number of bit positions at which two
codewords differ is called the Hamming
distance.
– If two codewords are a Hamming distance d
apart, it takes d single bit errors to convert one
to another.
Error correcting codes
• 2^m possible data messages are legal but
2^n possible codewords are not. This is
because of how check bits are calculated.
• Having an algo. to calculate legal
codewords we can compute exhaustive list
of them. From this list two codewords
whose Hamming Distance is minimum can
be calculated.
• This distance is called the Hamming
Distance of the Complete Code.
Error correcting codes
• To detect d bit errors we need a distance
d+1 code because there is no way d single
bit errors can change one valid code word
into another valid code word.
• To correct d errors we need a distance
2d+1 code because legal codewords are so
far apart so that with even d changes - the
original codeword and the modified
codeword are still closest.
Error correcting codes
• Parity Bit: is appended to the data and is
chosen so that the number of 1 bits in the
code word is even (or odd).
– Example (even parity): 10110101 -> 101101011
and 10110001 -> 101100010
• A code with a single parity bit has a
distance 2 since any single bit error
produces a word with the wrong parity. It is
used to detect single errors.
Error correcting codes
• Example: Code consisting of 4 code words
– 0000000000, 0000011111, 1111100000,
1111111111
– Code has distance 5. Therefore double errors
can be corrected
– If 0000000111 arrives, the receiver knows that
the original must have been 0000011111.
– If triple error changes 0000000000 into
0000000111 then by a minimum distance
approach we will not correct the code word.
Error correcting codes
• Given m message bits and r checkbits.
Design a code that will allow all single bit
errors to be corrected.
– Each of the 2^m legal messages has n illegal
codewords at a distance 1 from it (just invert
each of the n bits in the n-bit codeword)
– Thus each 2^m legal messages requires (n+1)
bit patterns dedicated to it
– (n+1)2^m <= 2^n but n = m + r so (m+r+1) <=
2^r. Given m we get a lower limit on r (number
of check bits) to correct single bit errors.
Error correcting codes- Hamming
• Bits of codeword are numbered from L-R
starting with 1.
• Bits that are powers of 2 (1, 2, 4, 8 etc) are
check bits. The rest (3,5,6,7,9, etc) are filled
with the m data bits.
• Each check bit forces the parity of some
collection of bits (including itself) to be
even (or odd).
• A bit may be included in several parity
computations ...
Error correcting codes- Hamming
• To see which check bits the data bit in
position k contributes to, rewrite k as a sum
of powers of 2. k = 15 = 1 + 2 + 4 + 8 or k =
29 = 1 + 4 + 8 = 16.
• A bit is checked by only those check bits
occurring in its power of two expansion.
• On codeword receipt, receiver initializes
counter to 0. It checks each check bit (k = 1,
2, 4, 8, …) to see if it has the correct parity
(even or odd). If not, it adds k to the
counter.
Error correcting codes- Hamming
• If counter is non zero, it contains the
number of the incorrect bit. E.g. if check
bits 1, 2, 8 are in error, the bit in error is 11
Error-Detecting Codes
• Let the block size be 1000 bits. To provide
error correction for such blocks 10 check
bits are needed.
• Why? (m+r+1) <= 2^r.
• 1Mbit requires 10^4 checkbits.
• To detect a block with a single bit error one
parity bit per block will suffice.
• Once every 1000 blocks an extra block is
reqd.
Error-Detecting Codes
• With a single parity bit, if a block is garbled
by a burst error, what is the probability that
the error will be detected?
• Solution:Take a block and break into an
nxm matrix. Compute parity bit for each
column and place at the bottom of matrix.
Matrix is transmitted one row at a time.
Receiver checks parity bits, if any one is
wrong it requests transmission again.
Error-Detecting Codes
• Burst Error: In a number of bits of length l,
at least the first bit and the last bit are
wrong.
• In the n (cols) x m (rows) matrix method a
single burst of length n (in any row) can be
fixed since the parity will be violated.
• However if a burst of length of n+1 occurs
(where the error is in the first bit and the
n+1th bit) then the error will be undected.
• Another way is Cyclic Redundancy Code
Error-Detecting Codes (CRC)
• Bit strings represented as polynomials
– 110001 = 1x^5 + 1x^4 + 0x^3 + 0x^2 +0x^1 +
1x^0
– Highest order bit (of k bits) is the coefficient of
x^(k-1).
– The polynomial is said to be of degree k-1
– Polynomial addition and subtraction is done
modulo 2
• 10011011 + 11001010 = 01010001
– Division is same as binary but subtraction is
done modulo 2.
Error-Detecting Codes (CRC)
• Sender and receiver use an agreed upon
generator polynomial G(x). The high and
low order bits of G(x) are 1.
• To compute a checksum for a frame with m
bits (whose polynomial is M(x)), the frame
must be longer than the generator poly.
• Take the checksum and place it at the end of
the frame so that the poly represented by the
new frame (with checksums at the end of it)
is divisible by G(x)
Error-Detecting Codes (CRC)
• When receiver gets checksummed frame, it
divides by G(x). If remainder exists, then
error occurred.
• Algorithm
– r = degree of G(x). Append r zero bits to the
lower-end of frame. Now we have m+r bits and
this corresponds to the polynomial x^rM(x).
– Divide bit string corresponding to G(x) into the
bit string corresponding to x^rM(x) using
modulo 2 division.
Error-Detecting Codes (CRC)
• Algorithm (contd)
– Subtract the remainder (always r or fewer bits)
from the bit string corresponding to x^rM(x)
using modulo 2 subtraction.
– Result of above operation is the checksummed
frame to be transmitted. Call its polynomial
T(x).
– T(x) is divisible by G(x).
G(x) = x^4 +x + 1
Error-Detecting Codes (CRC)
• Suppose error occurs i.e., T(x) + E(x). Each
1 bit in E(x) is an error.
• A single burst error is indicated by 1xxxxx1
where xxxx could be any combination of 0’s
and 1’s.
• Receiver does [T(x) + E(x)]/G(x). What is
left is E(x)/G(x).
• If E(x) = x^i, i shows which bit is in error
then if G(x) has >= 2 terms it will never
divide E(x) so single bit errors are detected.
Error-Detecting Codes (CRC)
• Two isolated single bit errors: E(x) = x^i +
x^j = x^j(x^(i - j) + 1), i > j.
– Assume that G(x) is not divisible by x, a
sufficient condition for double errors to be
detected is that G(x) does not divide x^k+1
upto the maximum of i-j.
• Odd number of bits in error: Make x+1 a
factor of G(x) & we can catch all errors
consisting of an odd number of inverted
bits.
• READ PARA starting bottom of p189 on
own.
Error-Detecting Codes (CRC)
• Burst errors of length <= r: r is the number
of checkbits.
– Burst error of length k < r represented by x^i
(x^(k-1) + … + 1); i determines how far from
the right hand of the frame the burst is located.
– If G(x) has an x^0 term it will not have x^i as a
factor so if degree of (x^(k-1) + … 1) is less
than the degree of G(x), the remainder can
never be 0.
• Burst length of length = r+1: Pr(Accept) =
1/2^(r-1)
Error-Detecting Codes (CRC)
• Burst length of length > r+1 then then
Pr(Accept) = 1/2^r
• Example Polynomials
– CRC-12: x^12 + x^11 + x^3 + x^2 + x^1 + 1
– CRC-16: x^16 + x^15 + x^2 + 1
– CRC-CCITT: x^16 + x^12 + x^5 + 1
• CRC 12 used for 6 bit chars. CRC-16 and
CCITT used for 8 bit characters.
Elementary data link protocols
• Assumptions:
– Physical, data link layer and network layer
engage in “peer to peer” communication, i.e.,
each layer has physically independent proc.
– Machine A sends data to machine B using a
reliable connection oriented service. Also we
will consider the case where B wants to send to
A simultaneously.
– A has ‘infinite bit bucket’, i.e., no waiting for
data production by Network Layer - i.e., data is
always available.
Elementary data link protocols
• Assumptions:
– The DLL on source machine A assumes that
data from Network layer (on A) is all data
every bit of which is to be passed to the
network layer on destination machine B.
– When DLL accepts a packet it creates a frame
by putting a data link header and trailer to it.
SEE FIGURE ON NEXT SLIDE.
Elementary data link protocols
Elem. Data link protocols
• DLL Interaction with the Physical Layer
– to_physical_layer to transmit frame
– from_physical_layer to receive frame
• DLL Interaction with the Network Layer
– to_network_layer to pass packet upwards
– from_network_layer: get pkt from network
layer
• wait-for-event(&event) This procedure
only returns when something has
happened, for example a frame arrives.
event says what happened (e.g., event
Elementary data link protocols
• Frame contents - kind, seq_no, ack, info
– First 3 contain control info and the last
contains actual data.
– kind lets us know if frame contains only
control info. or if it contains control info. +
data.
– seq_no is used to number frames to tell
them apart
– ack is used for acknowledgements
– info field of data frame contains a single
packet. For a control frame info field is not
used.
Elementary data link protocols
• Packet and Frame:
– Network layer constructs a packet by taking a
message from the transport layer and adds the
network layer header to it.
– Packet is then passed to DLL and put into the
info field of the outgoing frame.
– The destination DLL extracts the packet from
the frame and sends packet to network layer
(above it).
Elementary data link protocols
• Some Listed Procedures + more:
– wait_for_event waits in a loop for events to
occur. For example when a frame is sent by
DLL, it starts a timer (in order to account for
lost frames/replies). wait_for_event returns
event = timeout.
– to_network_layer, from_network_layer used to
pass packets between network layer and DLL
– to_physical_layer, from_physical_layer used to
pass frames between DLL and physical layer.
– Frame number is counted from 0 - MAX_SEQ
Unrestricted simplex protocol
• Assumptions:
– Data transfer is unidirectional (MAX_SEQ is
not used).
– Transmitting and receiving network layers are
always ready
– Processing time of frames can be ignored
– Buffer space is infinite
– The communication channel between DLL’s is
always noiseless
Unrestricted simplex protocol
• Protocol has two procedures - sender1 (runs
in DLL of source machine) and receiver1
(runs in DLL of destination machine).
– sender1 - fetch packet from network layer,
create a frame and send frame to physical layer.
– receiver1- Await arrival of frame (assumed
undamaged). Take frame from physical layer
and send data portion to network layer.
Simplex stop-and-wait protocol
• Eliminates the assumption that there is an
infinite buffer space in the DLL in which to
store all incoming frames.
• Assume communication channel is error
free and data traffic is one way (more later).
• Therefore we only have to worry about how
not to flood the receiver.
• If the receiver requires Δt (seconds,
microseconds etc) from_physical_layer and
to_network_layer then sender must send
less than one frame per Δt.
Simplex stop-and-wait protocol
• Assuming no automatic buffering and
queuing are done in hardware, sender must
never send a new frame until receiver has
completely taken old one through the use of
from_physical_layer.
• Delay is not feasible if the receiver has
several lines to attend to.
• Accounting for worst case behavior of the
receiver is too conservative.
• Thus it is preferable to have receiver send
feedback to sender.
Simplex stop-and-wait protocol
• After sending a packet to network layer,
receiver sends a dummy frame back to
sender - which effectively tells sender to
send next frame.
• Thus sender is required by protocol to wait
for some time - i.e., until dummy frame is
received. This is known as stop-and-wait.
• Data traffic is simplex but frame traffic is
duplex. Since strict alternation is followed,
half-duplex is permissible.
Simplex protocol for noisy
channel
• Assume-if frame damaged, CRC catches it.
• SCENARIO (receiver = A, sender = B)
– Network layer on A gives packet to its DLL. B
receives packet correctly and is sent to its
Network layer. B sends ack frame back to A.
– Ack frame gets lost.
– DLL on A times out. A takes lost ack to mean
that frame never made it to B. Sends frame
containing packet 1 again.
– B receives packet 1 twice and may process it
twice. Protocol fails.
Simplex protocol for noisy
channel
• Solution: Mechanism to distinguish
between a first- time and a retransmitted
frame.
– Place sequence numbers in header of each
frame.
• Max number of bits needed for sequence
numbers? ANSWER: 1 (0 or 1).
• This kind of protocol is called ARQ
(Automatic Repeat reQuest)
• Read code on own. Exam??
Sliding Window Protocols
• Full duplex communication, i.e., same ckt is
used to send data/ack in both directions.
• Piggybacking: When data frame arrives,
receiver (B) instead of sending an ack to A
immediately, waits till it has a packet to
send to (A) from its network layer. The ack
is put in the frame header. Advantage:
Better bandwidth utilization. Disadvantage:
Waiting for packet could take for ever.
Compromise wait a fixed time for packet to
send otherwise send separate ack.
Sliding Window Protocols
• Outbound frame has a sequence number
from 0 - 2^n -1 (therefore n bits suffice)
• Sender maintains a list of numbers in
sequence corresponding to frames it is
permitted to send. This set of frames falls
within the so-called sending window.
• Receiver maintains a similar list of frames it
is permitted to accept called a receiving
window.
• Sending & receiving window can have diff
sizes, upper/lower limits & can grow/shrink
Sliding Window Protocols
Frames sent but not acknowledged
S
e
n
d
e
r
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
When acknowledgement
arrives trailing edge is
advanced by p. (next slide)
When new pkt arrives from NL
it is given next highest seq. # and
window advanced by 1.
R
Last frame acked Frames receiver may accept
e
c
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6
e
i
v Frames outside
e window are
discarded without ack
r
Sliding Window Protocols
• SENDER:
– Sender maintains a list of consecutive sequence
numbers corresponding to frames it is permitted
to send. This list, termed the sending window,
represents frames sent but not yet ack’d.
– When an ack arrives, the lower edge of the
window is advanced to the corresponding
sequence number, thereby allowing the sender
to transmit new frames. (Note: When receiver
sends an ack for frame s, this is understood to
mean that all frames up to and including s have
been received.)
Sliding Window Protocols
• SENDER (continued):
– Let SWS be the maximum sender window size.
– Let LAR be the sequence number of the last
acknowledgment received.
– Let LFS be the sequence number of the last
frame sent.
– Therefore LFS - LAR + 1 <= SWS.
– The sender must have a buffer large enough to
hold as many frames as there are in its window.
– sender records the time at which every packet is
sent
Sliding Window Protocols
• SENDER (continued):
– If sender does not receive ack for packet before
timeout elapses, it retransmits original frame
• RECEIVER:
– The receiver also maintains a receiving
window, corresponding to the number of outof-order frames it can accept. Frames falling
outside the window are discarded without ack
– Let RWS be the maximum receiver window
size; HFA be the highest-numbered frame that
will be accepted & NFE be the next frame
expected
Sliding Window Protocols
• RECEIVER:
– HFA - NFE + 1 <= RWS.
– When frame with sequence number s arrives,
the receiver takes the following action. If s <
NFE or s > HFA, then the frame is outside the
receiver's window and so it is discarded. If NFE
<= s <= HFA, then the frame is accepted.
– An acknowledgment for s is sent if the
following conditions are met:
• s has not yet been acknowledged.
• all frames in the range from NFE to s have been
received.
Sliding Window Protocols
• RECEIVER:
– Thus the acknowledgments are cumulative.
When the acknowledgment is sent, the receiver
sets NFE = s + 1 and HFA = s + RWS.
Go back n
• Pipelining:
– Assumption was that trans. time is negligible
– Eg. 50Kbps channel, 500ms round trip
propagation delay. Send 1000bit frames. t=0 ms
the first fame is sent. t=270ms is when frame
fully arrives at receiver and only at t = 520ms
has the ack arrived back at the sender.
– Sender was blocked 500/520 or 96% of time.
– Solution: Choose w frames before blocking. In
example above, w = 26, i.e, by the time it has
sent 26 frames (t=520) the ack for 0 will have
arrived. This is known as pipelining.
Go back n/selective repeat
• Pipelining: (continued)
– If channel capacity is b bps, frame size l bits &
the round trip propagation time R sec, the time
required to send a single frame is l/b sec.
– There will be a total channel delay of R sec
– In stop-and-wait line is busy for l/b and idle for
R sec giving a line utilization for l/(l+bR).
– Thus pipelining can be used to keep the line
busy in the interval.
– What to do if errors occur during pipelining?
– Answer: go back n, or selective repeat
Go back n/selective repeat
• go back n :
– What to do when a frame in the middle of a
long stream is bad? Large # of succeeding
frames may come to receiver before sender is
told of error.
– On discovering error, a solution is go back n.
Here receiver discards all subsequent frames
(upto n) & sends no ack for the discarded
frames. Therefore the receiver DLL refuses all
frames but the next one it must give to the NL.
– Eventually sender will retransmit all frames
including lost/damaged one.
Go back n/selective repeat
• Selective repeat :
– Store all frames following the bad one.
– When sender notices that something is wrong it
only retransmits the bad frame.
– This is like having a receiver window larger
than 1
– Thus any frame within the window may be
accepted and buffered until all the preceding
ones have been passed to the network layer.
– Could need large amounts of memory in DLL