Computer Networks 2002/2003

1. Computer Networks2002/2003 The Data Link Layer Johan Lukkien Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

2. The OSI model PSDU SSDU TSDU NSDU DSDU PhSDU Johan J. Lukkien, j.j.lukkien@tue.nl 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 Johan J. Lukkien, j.j.lukkien@tue.nl 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

5. Some stacks and the OSI model Johan J. Lukkien, j.j.lukkien@tue.nl 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? Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

7. 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 1’s a 0 is stuffed • Use special, unused bit patterns in physical layer Johan J. Lukkien, j.j.lukkien@tue.nl 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

10. 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 Johan J. Lukkien, j.j.lukkien@tue.nl 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 3309-1979 • Point-to-point protocol • RFC 1661 (1662,1663) • The IEEE 802 LANs Johan J. Lukkien, j.j.lukkien@tue.nl 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 Johan J. Lukkien, j.j.lukkien@tue.nl 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

14. 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) Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

15. PPP: Point to Point Protocol • The Internet choice: • derived from HDLC Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

16. PPP (cnt’d) • 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, .... Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

17. PPP (cnt’d) • 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 Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking

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

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

26. Using your own SAP • Use locally assigned SAP’s • but that’s difficult to synchronize dynamically • Use header extension • both SAP’s: 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.... Johan J. Lukkien, j.j.lukkien@tue.nl TU/e Computer Science, System Architecture and Networking