Data Link Control DLC

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Data Link Control (DLC)

• Prof. A. Sahoo
• KReSIT

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DLC topics

• Sliding Window Mechanism
• DLC functionality
• Well known DLC protocols HDLC, LAPB, SLIP, PPP
• Davie, Tanenbaum

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Data Link
LayerHow to provide a reasonable link
abstraction to the higher layers?Reliable
communication over a single unreliable physical
link.
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Data Link Layer Requirements

• Control or Manage the exchange of data between
  two points.
• Problems with a communication link
• Frame Synchronization
• Flow Control
• Error Control
• Detection
• Correction
• Optimal Use of Link

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Data link Layer Services to upper layers

• Unacknowledged connectionless service
• No acks, no connection
• Error recovery up to higher layers
• For low error-rate links or voice traffic
• Acknowledged connectionless service
• Acks improve reliability
• For unreliable channels. E.g. Wireless Systems
• Acknowledged connection-oriented service
• Equivalent of reliable bit-stream
• Connection establishment
• Packets Delivered In-Order
• Connection Release
• Inter-Router Traffic
• Typically implemented by network adaptor
• Adaptor fetches (deposits) frames out of (into)
  host memory

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Data link layer functions

• Data Link layer functions ( in short)
• Provide service interface to the network layer
• Framing
• Dealing with transmission errors
• Regulating data flow
• Slow receivers not swamped by fast senders
• Addressing
• For multi-point links
• Link Management ( connection Initiate,
  Disconnect)
• Optimal link uses

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High level view
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Data Transfer

• Simple Stop and Wait
• Sliding Window (Peterson Davie)
• Stop and Wait
• Send data frame A --gtB
• B send acknowledgement frame (ACK), B--gtA
• A waits till ACK comes
• On No receipt of ACK, A times-out and resend data
  frame.
• Ensures reliability
• Controlled flow
• Problems lower link utilization ( see figure in
  next slide)
• Longer frame to increase link utilization---gt not
  always possible
• Buffer overflow
• Unavailability of data for bursty traffic
• Link capture by one node
• Easier error detection, smaller frame transmit on
  error
• So, limit on frame size required, but,
• Small frames make Stop Wait bad for high delay
  links

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Stop and Wait link utilization
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Sliding Window

• Increased link utilization ( send more number of
  packets before expecting ack
• Reliable transfer ( ack-ed reception)
• Flow control
• For ordered packet sequence, receiver needs to
  have some buffer to hold received packets in
  unordered manner
• Flow control controlled by receiver buffer size
• Error detection
• Sliding Window
• 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 ( sequence number)
• ACK includes number of next frame expected

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Sliding Window
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Sliding window Sender

• Sender maintains following three variable
• Send window size ( SWS)
• Upper bound on number of unacknowledged frame
• Last Frame acknowledged
• Sequence number of last frame acknowledged
• Last Frame sent ( LFS)
• Sequence number of last frame sent
• Maintain invariant LFS - LAR lt SWS
• When ACK arrives, advance LAR ( by above
  invariant), thereby opening window
• Buffer up to SWS frames

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Sliding window Receiver

• Receiver maintains following three variable
• Receive window size ( RWS)
• Upper bound on number of out-of-order frames
  that the receiver is willing to accept (buffer
  limitation)
• Largest acceptable frame (LAR)
• Sequence number of largest acceptable frame
• Last Frame Received ( LFR)
• Sequence number of last frame received
• Maintain Invariant LAF - LFR lt RWS
• Frame SeqNum arrives
• if SeqNum lt LFR or SeqNum gt LAF, then the
  frame is outside the receiver window, hence
  reject.
• if LFR lt SeqNum lt LAF, then the frame is
  within the receiver window, hence accept.

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Sliding window Receiver

• Send Cumulative ACK.
• Ack contain the next sequence no n (NFE)
  expected.
• This means till (n-1) no (LFR) is correctly
  received
• Window is adjusted by LAF LAR RWS

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SWS, RWS and Maximum Sequence Number

• Setting SWS and RWS
• SWS - should be guided by BW delay product
• RWS -
• 1 --gt no buffering, no ordering ( assuming
  receiver processor is much faster)
• RWS SWS ---gt can order all data sent in one
  shot by sender
• Sequence Number
• In previous 2 slides, sequence number assumed to
  be infinite (not possible)
• Has to be finite ( 3 bits in HDLC)
• What is the relationship between SWS, RWS and
  maximum sequence number (MaxSeqNo)?
• Rule received frame numbers and expected frame
  numbers should be mutually exclusive
• So what should be relationship between MaxSeqNo,
  SWS and RWS?

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Sequence Number

• Example (MaxSeqNo 7, SWS RWS 8)
• Frames sent 0 to 7
• Frames received 0 to 6. Receiver expecting 7, 0,
  1, 2, 3, 4,5 , 6 ( 0-6 next cycle)
• Cumulative Ack sent for 6, but Ack is lost on the
  way
• Sender resends 0 to 7
• Receiver accepts all frames, thinking they are
  from next cycle.
• This is a wrong reception, generating duplicate
  packets to upper layers

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Error Control

• When a sequence of frames/packets are transmitted
  over a lossy link the reception faces problems of
  frame damage, loss, reordering,
  duplication,insertion
• Error Detection
• Using acknowledgements
• Positive if good frame
• Negative if bad frame detected using error
  detecting codes
• Using timers
• For lost frames

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Error Control

• Error Recovery
• Re-transmission
• ARQ ( Automatic Repeat Request) primarily based
  on sliding window mechanism
• Stop and wait
• Go back N
• Selective reject (selective retransmission)
• Forward Error Correction ( FEC) Use of
  Redundancy for packet level error correction (
  known as Erasure codes) See Keshav, Error control
  chapter

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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

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Stop and Wait -Diagram
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Go Back N

• 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

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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
• Lost Frame
• Frame i lost
• Transmitter sends i1
• Receiver gets frame i1 out of sequence
• Receiver send reject i
• Transmitter goes back to frame i and retransmits

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Go Back N

• Lost Frame
• 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

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Go Back N

• Damaged Acknowledgement
• Receiver gets frame i and send acknowledgement
  (i1) which is lost
• Acknowledgements are cumulative, so next
  acknowledgement (in) 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
• Damaged Rejection
• As for lost frame

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Selective Reject

• Also called selective retransmission
• Only rejected frames are retransmitted
• Subsequent frames are accepted by the receiver
  and buffered
• Minimizes retransmission
• Receiver must maintain large enough buffer
• More complex logic in transmitter

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High Level Data Link Control

• Derived from SDLC (synchronous DLC) developed by
  IBM in the 1970s
• SDLC modified by ISO became HDLC
• Basis for some protocols that are used widely
• LAP-B in X.25 networks (the original packet
  switched network)
• Defined for both point-to-point and multi-drop
  lines

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HDLC Station Types

• Primary station
• Controls operation of link
• Frames issued are called commands
• Maintains separate logical link to each secondary
  station
• Secondary station
• Under control of primary station
• Frames issued called responses
• Design takes care of one host computer and many
  terminal configuration
• Combined station
• May issue commands and responses

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HDLC Link Configurations

• Unbalanced
• One primary and one or more secondary stations
• Supports full duplex and half duplex
• Balanced
• Two combined stations
• Supports full duplex and half duplex

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HDLC Transfer Modes

• Two widely used mode
• Normal Response Mode (NRM)
• Unbalanced configuration
• Primary initiates transfer to secondary
• Secondary may only transmit data in response to
  command from primary
• Used on multi-drop lines
• Host computer as primary
• Terminals as secondary
• Asynchronous Balanced Mode (ABM)
• Balanced configuration
• Either station may initiate transmission without
  receiving permission
• Most widely used
• No polling overhead

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HDLC Transfer Modes

• Asynchronous Response Mode (ARM)
• Unbalanced configuration
• Secondary may initiate transmission without
  permission form primary
• Primary responsible for line
• rarely used

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Frame Structure

• All transmissions in frames
• Single frame format for all data and control
  exchanges
• Flag field used for framing, use of bit
  stuffing technique
• 8 bit link layer address
• Control field describes various operation modes
  and packet type. Describes various phase of ARQ
  system

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Control Field

• Normally uses Go back N with MaxSeqNo 8
• Different for different frame type. Frame types
  are
• Information - data to be transmitted to user
  (next layer up)
• Flow and error control piggybacked on information
  frames
• contains the sequence number
• Supervisory - ARQ when piggyback not used ( no
  reverse data packet to send)
• Unnumbered - supplementary link control for
  Initiation and Termination of link
• First one or two bits of control field identify
  frame type

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Control Field Diagram (Tanenbaum)
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Poll/Final Bit

• Use depends on context
• Command frame
• P bit
• 1 to solicit (poll) response from peer
• Response frame
• F bit
• 1 indicates response to soliciting command

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HDLC Operation

• Exchange of information, supervisory and
  unnumbered frames
• Three phases
• Initialization
• Data transfer
• Disconnect

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Other DLC Protocols (LAPB,LAPD)

• Link Access Procedure, Balanced (LAPB)
• Part of X.25 (ITU-T)
• Subset of HDLC - ABM
• Point to point link between system and packet
  switching network node
• Link Access Procedure, D-Channel
• ISDN (ITU-D)
• ABM
• Always 7-bit sequence numbers (no 3-bit)
• 16 bit address field contains two sub-addresses
• One for device and one for user (next layer up)

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Other DLC Protocols SLIP PPP ( Tanenbaum pg
229)

• SLIP- Serial Line IP
• No error detection or correction
• Works only for IP
• Needs fixed IP no ---gt DHCP does not work
• Not Standardized
• No Authentication

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PPP Point to Point Protocol

• Works for any network layer
• IP address can be configured
• No error/ flow control
• Authentication possible

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Other DLC Protocols (Frame Relay) (2)

• ABM, 7-bit sequence numbers, 16 bit CRC
• Address field (2, 3 or 4 octet)
• Data link connection identifier (DLCI)
• Identifies logical connection

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Other DLC Protocols (ATM)

• Asynchronous Transfer Mode
• Streamlined capability across high speed networks
• Not HDLC based
• Frame format called cell
• Fixed 53 octet (424 bit)
• Details later