Link Layer Protocols

1. Link Layer Protocols Martin Weiss

2. Objectives of this Meeting • Explain what a protocol is • Compare connection-oriented with connectionless protocol types • Describe some protocol mechanisms • Describe different link layer protocols

3. Review of the Last Meeting • Error detection • Flow control • Performance of flow control techniques

4. Data Link Layer Issues • Type • Error detection and control • Flow control

5. OSI Reference Model End User Application Process OSI Layer Distributed Information Services Application Services: FTAM, MHS, EDI Application Syntax-Independent Message Interchange Data Representation, Transfer Syntax, Data Transformation Presentation Dialog Synchronization and Control Session Network Independent Message Interchange End-to-End Message Transfer Transport Network Routing and Addressing Network Data Link Control, Error Checking, Framing Link Mechanical and Electrical Interface Physical Physical Connection Data Communications Network

6. Application Process (AP) to AP Messages Source AP Desintation AP L_DATA.request L_DATA.indication Link Layer PDU’s Source Link Layer Desintation Link Layer Physical Transmission Medium Structure of Protocols

7. Link Management • Connection-Oriented • Connectionless

8. Review of Protocol Types • Protocol Environments • Byte oriented protocol • Bit oriented protocol

9. Kermit Protocol Structure SOH LEN SEQ TYPE data BCC CR

10. Kermit Details • SOH = Start of Header • LEN = Number of Bytes in the Frame • SEQ = Sequence Number of the Current Frame

11. Kermit Details • TYPE = Type of Frame • S: Send Initiation Parameters • F: Filename • D: File Data • Z: End of File • B: End of Transaction • Y: ACK • N: NAK • E: Error (Fatal)

12. Kermit Details • Data = Contents of Frame • BCC = Block Check Character • CR = End of Block Marker (ASCII CR Character)

13. User User Kermit Kermit Connect Connect Receive I[0,S,Init] Send (File) ACK[0,Y,Init] I[1,F,Filename] ACK[1,Y] Data Block [1] Sent Data Block [1] Received I[2,D,data] ACK[2,Y] Operation of Kermit Kermit Kermit Call Setup V(R) = 0 V(S) = 0 V(S) = 1 V(R) = 1 V(S) = 2 V(R) = 2 V(S) = 3 V(R) = 3 Time

14. User User Data Block [1] Sent Data Block [2] Sent Data Block [2] Received Data Block [1] Received I[3,D,data] I[3,D,data] I[2,D,data] NACK[3,N] ACK[2,Y] ACK[3,Y] Operation of Kermit (Error) V(R) = 2 V(S) = 2 Kermit Kermit V(S) = 3 V(R) = 3 V(S) = 4 V(R) = 3 V(R) = 4 Time

15. User User I[3,D,data] Data Block [1] Sent Data Block [2] Sent Data Block [2] Received Data Block [1] Received I[3,D,data] I[2,D,data] ACK[2,Y] ACK[3,Y] Operation of Kermit (Lost Frame) V(R) = 2 V(S) = 2 Kermit Kermit V(S) = 3 V(R) = 3 V(S) = 4 Timeout V(R) = 4 Time

16. User User I[3,D,data] ACK[3,Y] Data Block [1] Sent Data Block [2] Sent Data Block [2] Received Data Block [1] Received I[3,D,data] I[2,D,data] ACK[2,Y] ACK[3,Y] Operation of Kermit (Lost ACK) V(R) = 2 V(S) = 2 Kermit Kermit V(S) = 3 V(R) = 3 V(S) = 4 V(R) = 4 Timeout V(R) = 4 Time

17. User User Data Block [n*] Sent I[n*,D,data] Data Block [n*] Received ACK[n*,Y] I[(n+1)*,Z] EOF EOF ACK[(n+1)*,Y] I[(n+2)*,B] Exit EOT ACK[(n+2)*,Y] Exit Operation of Kermit Kermit Kermit V(S) = n* V(R) = n* Destination Computer Time

18. Sliding Window Protocols • Frequently assumes a peer to peer protocol • Acknowledgements can be “piggybacked”

19. Bit-Oriented Protocols • IBM’s Synchronous Data Link Control (SDLC) • ISO’s High-Level Data Link Control (HDLC) • ITU’s Link Access Protocol-Balanced (LAP-B) • ITU’s Link Access Protocol for the ISDN D channel (LAP-D) • ITU’s Link Access Protocol for Modems (LAP-M)

20. HDLC • Bit oriented protocol • Can operate in several modes • Normal Response Mode (NRM) • Asynchronous Response Mode (ARM) • Asynchronous Balanced Mode (ABM)

21. HDLC Frame Types • Unnumbered frames • Used for connection establishment and disconnection • Do not contain acknowledgements • Information frames • Carry information between nodes • Can use piggyback acknowledgements in ARM and ABM

22. HDLC Frame Types • Supervisory frames • Used for error and flow control • Contain sequence numbers

23. HDLC Frame Format Frame Format 8 8/16 8/16 0 to N 16/32 8 Flag Address Control Information FCS Flag Control Field for Normal Information Frame P/F = Poll/Final Bit N(S) = Send Sequence Number N(R) = Receive Sequence Number 0 N(S) P/F N(R) Control Field for Normal Supervisory Frame S: RR RNR REJ SREJ 0 1 S P/F N(R) Control Field for Normal Information Frame M: Defines 9 Commands and 4 Unnumbered Responses 1 0 M P/F M

24. Operation of HDLC • Normal mode • Components • Received frame counter (V(R)) • Sent frame counter (V(S)) • Contents of frame • V(R), S(R) • Frame type • Data • Unacknowledged list

25. I(0,0) RR(1) 1 0 0 1 I(1,0) 2 0 0 2 I(2,0) 3 0 0 3 RR(2) RR(3) Unidirectional Data (ACK and NAK) Sender Receiver (N(S), N(R)) UnACK’ed List V(S) V(R) V(S) V(R) UnACK’ed List 0 0 0 0 N(S) = V(R) => Frame OK 0 - 1 N(S) = V(R) => Frame OK 1,2 N(S) = V(R) => Frame OK 2 -

26. I(3,0) I(3,0) 3 0 4 0 I(4,0) I(4,0) 5 0 0 0 4 5 REJ(3) RR(5) Unidirectional Data (With Error) Sender Receiver (N(S), N(R)) UnACK’ed V(S) V(R) V(S) V(R) UnACK’ed List 0 3 - 3 N(S) = 4 V(R) = 3 ERROR! 3,4 3,4 N(S) = V(R) => Frame OK N(S) = V(R) => Frame OK -

27. I(0,0) I(0,1) 1 1 0 1 1 0 1 1 I(1,1) 2 1 1 1 2 3 I(2,1) 3 3 1 2 2 2 3 3 RR(2) I(1,3) Example 2: Bidirectional Data Sender Receiver (N(S), N(R)) UnACK’ed List V(S) V(R) V(S) V(R) UnACK’ed List 0 0 0 0 0 - 0 1 - 1,2 - 1 - -

28. Modems Error Correction (LAPM) Error Corr- ecting Part (ECP) User Interface Part (UIP) DTE Error Corr- ecting Part (ECP) User Interface Part (UIP) DTE Public Telephone Network Modem (DCE) Modem (DCE)

29. Point to Point Protocol (PPP) • Used as the link layer in dialup Internet access

30. Summary • Description of a protocol • Types of protocols • Protocol primitives and mechanisms • Examples