1. The Data Link Layer Johan Lukkien
Computer Networks 2002/2003
The Data Link Layer
Johan Lukkien
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

2. The OSI model 13-Jul-19 PSDU SSDU TSDU NSDU DSDU PhSDU
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

3. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

4. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

5. Some stacks and the OSI model
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

6. Data Link Layer issues (cnt’d)
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: don’t make a low level service unnecessarily complicated
e.g. is reliability useful at this level? When? Why?
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

7. Framing methods Byte count Flags
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
after each occurrence of 5 1’s a 0 is stuffed
Use special, unused bit patterns in physical layer
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

8. Byte count A stream encoded with byte count
without errors
with errors
Note: difficult to resynchronize
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

9. Stuffing 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

10. Flow control policies Feed-back based Rate-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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

11. Example data link protocols
Point-to-point (tradionally, reliable service)
High level data link control protocol (HDLC)
ISO
Point-to-point protocol
RFC 1661 (1662,1663)
The IEEE 802 LANs
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

12. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

13. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

14. Frame types Three types, encoded in Control P/F: poll/final
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)
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

15. PPP: Point to Point Protocol
The Internet choice:
derived from HDLC
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

16. PPP (cnt’d) Byte oriented – byte stuffing Derived from HDLC Protocol
so it works on both bit- and byte oriented ‘wires’
Derived from HDLC
address = (broadcast...)
mostly, control = (unnumbered frame, no reliability) but may use HDLC mechanism
Protocol
type of packet in payload
e.g. LCP, NCP, AUTHENTICATE, IP, IPX, AppleTalk, ....
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

17. PPP (cnt’d) Link Control Protocol: Authenticate
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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

18. PPP: LCP frame types 13-Jul-19 Johan J. Lukkien, j.j.lukkien@tue.nl
TU/e Computer Science, System Architecture and Networking

19. Dial-up scenario Physical connection Line negotiation Authenticate
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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

20. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

21. Multiple Access Protocols
ALOHA
Carrier Sense Multiple Access Protocols
Collision-Free Protocols
Limited-Contention Protocols
Wavelength Division Multiple Access Protocols
Wireless LAN Protocols
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

22. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

23. 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 1’s: broadcast – not really used
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

24. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

25. 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
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

26. Using your own SAP Use locally assigned SAP’s Use header extension
but that’s difficult to synchronize dynamically
Use header extension
both SAP’s: (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....
13-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking