Computer networks

1. UNIT II

2. HDLC
• HDLC is a bit oriented data link protocol.
• Designed to support both half duplex and full
duplex communication over point to point and
multi point links.
• Systems using HDLC can be characterized by
their station types, their configurations and
their response modes

3. HDLC – Station Types
• Three types of stations: Primary and
Secondary.
• Primary Station: Sends commands to
secondary stations.
• Secondary Station: Issues responds to
commands issued by primary station.
• Combined station: Acts as both primary or
secondary station.

4. HDLC – Configurations
• Configuration refers to the relationship of hardware
devices on a link.
• Stations can be configured in three ways: Unbalanced,
Symmetric and Balanced.
• Unbalanced Configuration: Master – Slave
Configuration. One device is primary and other devices
are secondary devices.
• Symmetric Configuration: Each physical station on a
link consists of two logical stations: One primary and
secondary.
• Balanced Configuration: Both stations in a Point to
Point topology are of combined type.

5. HDLC – Modes
• 3 Modes: NRM, ARM, ABM
• Normal Response Mode: Refers to standard
primary – secondary relationship. Secondary
device must have permission from the primary
device before transmitting. Once permission
has been granted, the secondary may initiate
a response transmission of one or more
frames.

6. HDLC – Modes
• Asynchronous Response Mode: A Secondary
device may initiate a transmission without
permission from the primary device.
• Asynchronous Balanced Mode: All stations are
equal and therefore only combined stations
are used.

7. HDLC – Frame Format
01111110 01111110
Defines 3 types of frames
(I,S,U frames)

8. HDLC – Frame
• Delimit frame at both ends
• 01111110
• Receiver hunts for flag sequence to synchronize
• Bit stuffing used to avoid confusion with data
containing 01111110
– The transmitter inserts 0 bit after every sequence of
five 1s with the exception of flag fields
– If receiver detects five 1s it checks next bit
• If 0, it is deleted
• If 1 and seventh bit is 0 (i.e., 10), accept as flag
• If sixth and seventh bits 1 (i.e., 11), sender is indicating abort

9. Bit Stuffing
• Example with
possible errors

10. Bit Stuffing
• Example with
possible errors

11. Control Field
• Different for different frame type
– I-frame (information frame)
• data to be transmitted to user (next layer up)
• Flow and error control piggybacked on information frames
– S-frame (Supervisory frame)
• Used for flow and error control
– U-frame (Unnumbered frame)
• supplementary link control
• First one or two bits of control filed identify
frame type

12. Control Field Diagram

13. Poll/Final Bit
• Use depends on context
• Command frame
– P bit : used for poll from primary
– 1 to solicit (poll) response from peer
• Response frame
– F bit : used for response from secondary
– 1 indicates response to soliciting command

14. I-frame
• Contains the sequence number of
transmitted frames and a piggybacked ACK
1 2 3 4 5 6 7 8
0 N(S) P/F N(R)
•I,0,0
•I,1,0
•I,2,0,P

15. S-frame
• Used for flow and error control
1 2 3 4 5 6 7 8
1 S P/F N(R)
•RR --- receive ready
•RNR --- receive not ready
•REJ --- reject on frame N(R)
•SREJ --- selective reject on N(R)
0

16. U-frame
• Mode setting, recovery, connect/diconnect
1 2 3 4 5 6 7 8
1 M P/F M
1
Unnumbered
function bits

17. Unnumberred frames
• Set normal response mode (SNRM)
• Set asynchronous response mode (SARM)
• Set asynchronous balanced mode (SABM)
• Disconnect (DISC)
• Unnumberred acknowledgement (UA)
• Disconnect mode (DM)
• Request disconnect (RD)
• Unnumberred poll (UP)
• Reset (RSET)
• Exchange identification (XID)
• Test (TEST)
• Frame reject (FRMR)

18. Information Field
• Only in information and some unnumbered
frames
• Must contain integral number of octets
• Variable length

19. Frame Check Sequence Field
• FCS
• Error detection
• 16 bit CRC
• Optional 32 bit CRC

20. HDLC Operation
• Exchange of information, supervisory and
unnumbered frames
• Three phases
– Initialization
– Data transfer
– Disconnect

21. Examples of Operation (1)

22. Examples of Operation (2)

23. Point to Point Protocol
• Common protocol for point to point access is
PPP.
• Millions of internet users who need to
connect their home computers to the server
of an ISP use PPP.
• To control and manage transfer of data, PPP is
used.

24. Point to Point Protocol - Services
• PPP defines the format of the frame to be
exchanged between devices.
• It also defines how two devices can negotiate
the establishment of the link and exchange of
data.
• New version of PPP provides connections over
multiple links.
• PPP provides network address configuration.

25. Point to Point Protocol - Services
• PPP does not provide flow control.
• Sender send several frames one after the
other with no concern about overwhelming
the receiver.
• PPP has error control. CRC mechanism is used.
If error found, the frames are discarded.

26. Point to Point Protocol - Framing
• PPP uses character oriented frame.

27. Point to Point Protocol - Framing
• Flag: Starts with 1 byte flag : 01111110
• Address: Constant value 11111111 –
broadcast address.
• Protocol: Defines what is being carried in data
field. Either user data or other information. 2
bytes long.
• Payload field: Carries either user data or other
information. Maximum of 1500 bytes.
• FCS: 2 byte or 4 byte used for CRC.

28. Point to Point Protocol – Transition
Phases
• Transition starts with dead state – No active
carrier.
• Establish state: When one of the two nodes
starts the communication, the connection
goes to establish state.
• In this state, options are negotiated between
two parties. Two parties agree that they need
authentication – required steps are
performed.

29. Point to Point Protocol – Transition Phases
• Data transfer takes place in open state.
• When connection reaches this state, the
exchange of data packets can be started.
• The connection remains in this state until one
of the endpoints wants to terminate the
connection.
• In this situation, the system goes to terminate
state.
• The system remains in this state until the
carrier is dropped which moves the system to
dead state.

30. Point to Point Protocol – Multiplexing
• PPP uses set of protocols to establish the link.
• Link control protocol and Network control
Protocol.
• Link control protocol: Responsible for
establishing, maintaining, configuring and
terminating links.
• Provides negotiation mechanism to set
options between two end points.
• Both endpoints of the link must reach an
agreement about the options before the link
can be established.

31. Point to Point Protocol – Multiplexing
• LCP packets includes Flag, address, control,
Payload, FCS, flag.
• Payload field consists of code, ID, length and
information.
• Code field defines the type of LCP packet.
• 0x01 – Configure request.
• 0x02 – Configure ack
• 0x03 – Configure nack
• 0x04 – Configure reject
• 0x05 – Terminate ack

32. Point to Point Protocol – Multiplexing
• 0x06 – Terminate ACK
• 0x07 - Code reject
• 0x08 – Protocol reject
• 0x09 – Echo request
• 0x0A – Echo reply
• 0x0B – Discard request
• ID field holds a value that matches a request
with reply.
• Length field defines the length of the entire
LCP packet. Information field contains data.

33. Point to Point Protocol – Authentication Protocol
• Authentication – Validating the identity of a
user who needs to access a set of resources.
• Two protocols: Password Authentication
Protocol; Challenge Handshake authentication
Protocol.
• PAP – Two step process
– User who wants to access a system sends an
authentication identification and a password.
– The system checks the validity of the identification
and the password and either accepts or denies the
connection.

34. Point to Point Protocol – Authentication Protocol
• Challenge Handshake Authentication Protocol is a three way
handshaking authentication protocol that provides security
than PAP.
• In this method password is kept secret.
• The system sends the user a challenge packet containing a
challenge value
• The user applies a predefined function that takes the
challenge value and the user’s own password and creates a
result. The user sends the result in the response packet to the
system.
• The system does the same. It applies the same function and
creates a result. If both results are same access is granted.

35. Point to Point Protocol – NCP
• One NCP protocol is Internet Protocol Control Protocol (IPCP).
• This protocol configures the link used to carry IP packets in
the internet.
• IPCP packets includes Flag, address, control, Payload, FCS,
flag. Payload field consists of code, ID, length and information.
Code field defines the type of LCP packet.
• 0x01 – Configure request.
• 0x02 – Configure ack
• 0x03 – Configure nack
• 0x04 – Configure reject
• 0x05 – Terminate request
• 0x06 – Terminate ack
• 0x07 – Code reject