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
Primary
Secondary
Combined
Combined
Point to Point
Primary
Secondary
Multipoint
Secondary
Basic Modes - NRM
NRM - Normal Response Mode
Primary
Secondary
Secondary
Whether in point-to-point or multipoint, the primary
dictates/controls response of secondary.
Basic Modes - ABM
ABM - Asynchronous Balanced Mode
Combined
Combined
Point to Point
Using combined stations, the relationship is peer to peer.
Neither is on control of the other
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!
8
8/16
8/16
flag address control
Typically
source &
destination
Status
or
commands
variable
16/32
8
INFO
CRC
flag
Not always
present
Flag Field & Bit Stuffing
Marking beginning and end is important!
flag
01111110
head
frame contents
01111110
tail
What do you do if the sequence 01111110 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 0.
Data intended
01111110
01111110
Data sent
01111110
011111010
01111110
01111110
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 0111110 ?
HDLC Frame Types
0
N(S)
P/F
N(R)
Information
0
1
S
P/F
N(R)
Supervisory
1
1
M
P/F
Unnumbered
M
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
Supervisory
0
1
S
P/F
N(R)
•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
3
2
1
1
M
P/F
M
Unnumbered
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
•Data Exchange (b)
•Disconnect (c)
(a)
Connect
Station A
Station B
Send RIM
Send SIM
Send UA
Send SARM
Send UA
Information Exchange
Station A
Station B
SEND I Frames
N(R)=0 N(S)=0
SEND I Frames
N(R)=0 N(S)=1
N(R)=0 N(S)=0
N(R)=0 N(S)=1
N(R)=1 N(S)=2
Frame Damaged
Frame Rejected
Send REJ with N(R)=1
SEND I Frames
N(R)=2 N(S)=1
(no activity)
N(R)=2 N(S)=2
N(R)=2 N(S)=3
Send RRJ with N(R)=3
Disconnect
Station A
Station B
Send DISC
Send UA
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 A
S
Y
S - Send Initiation
Y - ACK
F - File Header
D - Data Frame
Z - End of File
B - End of transmission
F
Y
DATA
Z
Y
B
Y
Station B