Johan J. Lukkien, j.j.lukkientue.nl

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Computer Networks2002/2003

• The Data Link Layer
• Johan Lukkien

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The OSI model
PSDU
SSDU
TSDU
NSDU
DSDU
PhSDU
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Data Link Layer services

• Provide packet oriented service (frames)
• send/receive frames between machines
• deal with transmission errors
• support addressing, in the broadcast domain
• support sharing by several network layer
  protocols
• Services provided by physical layer
• put bits (signals) on (p2p or broadcast) wires
• diverse technologies
• Note
• quality of service differs per technology
• e.g. reliability
• provided service itself as well
• e.g. connection orientation

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Data Link Layer issues

• Address LAN and point-2-point issues
• define DPDU and DSDU
• More or less stack-independent, see picture
• Part of the link layer is technology dependent
  the medium access (MAC) sublayer
• structure transmissions into frames (DPDU)
• policy for shared medium access
• e.g. dealing with collisions
• error detection
• flow control
• addressing

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Some stacks and the OSI model
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Data Link Layer issues (cntd)

• The other part (Logical Link Control, LLC)
  concerns
• the connection to multiple network layers
• additional functionality to improve quality of
  provided services
• reliability, connection orientation
• Note these quality issues re-
  occur in network and transport layer
• e.g., flow control, error checking
• Rule of thumb dont make a low level service
  unnecessarily complicated
• e.g. is reliability useful at this level? When?
  Why?

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Framing methods

• Byte count
• include length in header
• not used very much difficult to re-sync
• Flags
• dedicated start and stop byte (sequence)
• often chosen to be identical
• need to be escaped when it occurs as part of
  payload
• byte stuffing add an escape byte in front
• used the escape to escape itself as well
• generalize to bit sequence, HDLC
• special pattern for start/stop, e.g. 01111110
• after each occurrence of 5 1s a 0 is stuffed
• Use special, unused bit patterns in physical layer

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Byte count

• A stream encoded with byte count
• without errors
• with errors
• Note difficult to resynchronize

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Stuffing
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Flow control policies

• Feed-back based
• communicate receiver state to sender
• so sender can take decision
• explicit start/stop commands
• ask for another fixed maximum amount of data
• more smoothly
• Rate-based
• internal mechanism to adjust the rate
• Data-Link mainly feed-back based methods

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Example data link protocols

• Point-to-point (tradionally, reliable service)
• High level data link control protocol (HDLC)
• ISO 3309-1979
• Point-to-point protocol
• RFC 1661 (1662,1663)
• The IEEE 802 LANs

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High-Level Data Link Control Protocol

• Background
• master and tributaries sharing a multi-access
  link
• controller for several terminals
• concentrator
• Developed by IBM SDLC
• adapted by ISO HDLC
• adapted by CCITT to support X.25 LAP, LAPB
• ....collection of bit-oriented link-layer
  protocols

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High-Level Data Link Control Protocol

• Frame format
• Point to many-point and vice versa
• use flags for separation of frames
• Control sequence numbers, acks, ....
• Data payload, any length
• Checksum CRC
• Address the involved tributary

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Frame types

• Three types, encoded in Control
• Information frame.
• A supervisory frame
• info concerning the communication status
• An unnumbered frame
• no numbering
• P/F poll/final
• Next expected
  frame
• Seq serial
  number (sliding
  window)

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PPP Point to Point Protocol

• The Internet choice
• derived from HDLC

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PPP (cntd)

• Byte oriented byte stuffing
• so it works on both bit- and byte oriented
  wires
• Derived from HDLC
• address 11111111 (broadcast...)
• mostly, control 00000011 (unnumbered frame, no
  reliability) but may use HDLC mechanism
• Protocol
• type of packet in payload
• e.g. LCP, NCP, AUTHENTICATE, IP, IPX, AppleTalk,
  ....

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PPP (cntd)

• Link Control Protocol
• setup line and bring it down
• evaluate line performance
• negotiate communication options in dependent of
  network protocol, e.g. header compression
• Authenticate
• optional
• Network Control Protocol
• negotiate details of supported network layer
• e.g. IP address
• more than one may be supported

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PPP LCP frame types
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Dial-up scenario

• Physical connection
• PC user initiates connection to ISP router
• modems synchronize and negotiate
• Line negotiation
• LCP packets are sent via PPP frames to router
• finalize with this phase with connection-ack
• Authenticate
• using an authentication protocol, e.g.
  challenge/response passwd
• Network
• IP-NCP packets are sent to negotiate IP as
  network protocol
• dynamically assigned IP address is returned from
  router
• PC operates as a part of the Internet
• User is done with connection
• NCP tears down the network connection (freeing
  the assigned IP address for re-use)
• LCP tears down the data link connection
• Modem is told to hangup

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Local area networks IEEE 8-2

• Communication based on broadcasting
• multi-access links
• or just packet-level broadcasting
• LAN technologies define
  both the physical and
  the MAC layer
• Particular issue
• addressing
• including multi-cast

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Multiple Access Protocols

• ALOHA
• Carrier Sense Multiple Access Protocols
• Collision-Free Protocols
• Limited-Contention Protocols
• Wavelength Division Multiple Access Protocols
• Wireless LAN Protocols

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Name, address, location, route....

• Context
• a source sends a message to a destination both
  connect to the network via an access point at a
  certain location (identifier)
• Access point
• a fixed physical location to connect to the
  network
• Name
• an identification that is independent of access
  point and communicating partner
• Address
• the name of an access point
• Route
• a path from source to destination dependent on
  the access points of both

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IEEE 802 MAC addresses

• In fact identifiers
• Attached to a Network Interface Card
• 48 bit numbers, with some structure
• globally unique global authority (first Xerox,
  then IEEE), 1250/224
• 3 bytes fixed Organizational Unique Identifier
• G/L bit global unique-ness
• G/I bit multicast?
• all 1s broadcast not really used

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Logical Link Control (802.2)

• Standardizes towards network layer
• Adds quality to the service
• type 1
• datagram service best effort packet delivery
• type 2
• reliable connection oriented service
• HDLC-like
• type ...
• achieve some reliability without the complete
  overhead of type 2

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802.2 Format

• DSAP destination access point (1byte)
• SSAP source access point (1 byte)
• Control 1 or 2 bytes
• (Data, i.e. the packet)
• Access points protocols used at both sides
• usually identical
• two bits reserved
• group/individual (not really useful to address
  multiple protocols though....)
• global/local
• only 802-approved numbers are admitted
• and those have both SAP fields equal

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Using your own SAP

• Use locally assigned SAPs
• but thats difficult to synchronize dynamically
• Use header extension
• both SAPs 10101010 (aa)
• a.k.a. SNAP, subnetwork access protocol
• indicates a header of 5 extra bytes
• to make headers have an even length
• to use the OUI of MAC addresses to standardize
• buying a block of addresses also brings a block
  of protocols....