Two Protocol Case Studies HDLC & Kermit

HDLC A multi-functional protocol. Works in lots of modes. Forms the basis for MANY of the current protocols in use. When studying this protocol consider it is like a vehicle that you can use to fly, drive, or ride on the water; you may not see how to use all of the parts.

Classifications of stations Primary –tells other stations what to do Secondary –more reactive –doesn’t initiate on it’s own Combined –same station can be either

Basic Configurations PrimarySecondary Combined Point to Point Primary Secondary Multipoint

Basic Modes - NRM NRM - Normal Response Mode Primary Secondary Whether in point-to-point or multipoint, the primary dictates/controls response of secondary.

Basic Modes - ABM ABM - Asynchronous Balanced Mode Using combined stations, the relationship is peer to peer. Neither is on control of the other Combined Point to Point ARM - Asynchronous Response Mode Similar to ABM but primary/secondary roles exist. Secondary can initiate certain types of responses. Ignore this.

HDLC Frame Format Focus on what you can learn about the protocol from the frame! 88/16 variable16/328 flagCRCINFOcontroladdressflag Typically source & destination Status or commands Not always present

Flag Field & Bit Stuffing Marking beginning and end is important! headtail frame contents flag What do you do if the sequence appears in the frame?

Bit Stuffing In this case 6 ones in a row is the sequence to consider. When sending the flag, nothing special is done. When sending data, look for the sequence of 6 ones. When found, insert an extra Data intended Data sent Bit stuffed If receiver gets 5 ones followed by a 0, throw away the 0. Can you figure out what happens if the sender sends ?

HDLC Frame Types 0 N(S) P/F N(R) 0 1 S P/F N(R) 1 1 M P/F M Information Supervisory Unnumbered

Information Frames 0 N(S) P/F N(R) Information See Figure 5.30 part b This is the MIDDLE of the exchange setup TALK disconnect Examine the symmetric nature of the sequence numbers Is the scenario using go-back-n or selective repeat? RR is a supervisory frame to send a reverse ACK when no reverse data exists

Supervisory Frames 0 1 S P/F N(R) Supervisory 2 bits RR : Receive ready (ACK when no returning data) REJ : Reject (NAK) RNR : Receive not ready Flow control (buffers full) SREJ : Selective Reject Resend that one frame number ( not exactly as done in the general case )

Unnumbered Frames 1 1 M P/F M Unnumbered 2 3 Although broken, total field size of M is 5 bits -> 32 values. This identifies the frame type. See Table 5.2 This defines most of the message types/interactions. Classified as C (command) or R (response).

Beginning and Ending The entire exchange has to also have a beginning and an end. See Figure 5.30 again Connection Establishment (a) Data Exchange (b) Disconnect (c)

Connect Station AStation B Send RIM Send SIM Send SARM Send UA

Information Exchange Station AStation B SEND I Frames N(R)=0 N(S)=0 SEND I Frames N(R)=0 N(S)=0 N(R)=0 N(S)=1 Frame Damaged N(R)=1 N(S)=2 Frame Rejected Send REJ with N(R)=1 SEND I Frames N(R)=2 N(S)=1 N(R)=2 N(S)=2 N(R)=2 N(S)=3 (no activity) Send RRJ with N(R)=3

Disconnect Station AStation B Send UA Send DISC

Kermit A quick look

What is Kermit? Before FTP Used to transfer files in an interactive session Just focus on the higher level interaction. Run telnet-like program Log in and go to the directory where file is stored run KERMIT go to your window on local machine and set in receive mode (protocols connect and download) log off

A typical interaction Station AStation B S Y F Z B Y Y Y S - Send Initiation Y - ACK F - File Header D - Data Frame Z - End of File B - End of transmission DATA