Protocols and Interfaces - IPv4, IPv6, X.25, X.75

Module 4
Protocols and Interfaces
By: Ms. Pradnya Saval
Asst. Prof. TCET
Mumbai

Contents
• Issues in IPV4
• IPV6 protocol
• Mature Packet Switching Protocols:
• ITU Recommendation X.25,
• User Connectivity,
• Theory of Operations,
• Network Layer Functions,
• X.75 Internetworking Protocol

IPV4
• 32-Bit Addressing (2^32 Addresses = 4,294,967,296)
• Studies say that there will not be enough addresses anymore in future due to
the increase in mobile IP generation

Issues in IPV4
• Less Address Space
• IP address starvation
• Distribution of addresses (USA >50%)
• Routing is complicated
• Realization of new technologies (Mobile computing, real time services,
multicast, security, QOS, etc.)

IPV6
• 128-bit addresses (2^128 Addresses = 3.4*10^38)
• IPv6 resolves the following issues:
• Larger address space
• Better header format
• New options
• Allowance for extension
• Support for resource allocation
• Support for more security

Extension Headers (Cont..)
 Hop-by-Hop Option: The hop-by-hop option is used when the source needs to pass information to all
routers visited by the datagram.
 Source Routing: The source routing extension header combines the concepts of the strict source route and
the loose source route options
 Fragmentation: The source or a router is required to fragment if the size of the datagram is larger than the
MTU of the network over which the datagram travels.
 Authentication: The authentication extension header has a dual purpose: it validates the message sender and
ensures the integrity of data
 Encrypted Security Payload: The encrypted security payload (ESP) is an extension that provides
confidentiality and guards against eavesdropping.
 Destination Option: The destination option is used when the source needs to pass information to the
destination only. Intermediate routers are not permitted access to this information

TRANSITION FROM IPv4 TO IPv6 (Cont..)
• Dual Stack:
• To determine which version to use when sending a packet to a destination, the source host queries the
DNS. If the DNS returns an IPv4 address, the source host sends an IPv4 packet. If the DNS returns
an IPv6 address, the source host sends an IPv6 packet.

• Tunneling:
• Tunneling is a strategy used when two computers using IPv6 want to
communicate with each other and the packet must pass through a region that
uses IPv4.

• Header Translation:
• Header translation is necessary when the majority of the Internet has moved to IPv6 but some systems
still use IPv4.

Switched Communication Network
• Three Types of Switched Communication Network
• Circuit Switched
• Message Switched
• Packet Switched
• Datagram Approach
• Virtual Circuit Approach

Packet Switched Datagram and Virtual Circuit Approach

X.25 Network
• X.25 is a standard reliable protocol for WAN communications
• It is typically used in the packet-switched networks (PSNs) of common carriers, such as the
telephone companies.
• Subscribers are charged based on their use of the network.
• Uses Virtual Circuit Approach
• The devices used in X.25 network fall into three general categories:
• data terminal equipment (DTE),
• data circuit-terminating equipment (DCE),
• packet-switching exchange (PSE) 15

ITU Recommendation X.25 Network
16

ITU Recommendation X.25 Network (Cont..)
• Data Terminal Equipment (DTE)
• Devices are end systems that communicate across the X.25 network.
• They are usually terminals, personal computers, or network hosts, and are located on the
premises of individual subscribers.
• Data communication Equipment (DCEs)
• are communications devices, such as modems and packet switches that provide the interface
between DTE devices and a PSE, and are generally located in the carrier's facilities.
• Packet-switching Exchange (PSEs)
• are switches that compose the bulk of the carrier's network. They transfer data from one
DTE device to another through the X.25 PSN. 17

ITU Recommendation X.25 Network (Cont..)
Packet Assembler/Disassembler
• The packet assembler/disassembler (PAD) is a device commonly found in X.25
networks.
• The PAD is located between a DTE device and a DCE device and it performs
three primary functions:
• buffering (storing data until a device is ready to process it),
• packet assembly,
• and packet disassembly 18

X.25 Network - Protocol Suite
19
The X.25 protocol suite maps to the lowest three layers of the OSI reference model. The layers are:
• Physical Level: Deals with the physical interface between an attached station and the link that
attaches that station to the packet-switching node. X.21 is the most commonly used physical
layer standard.

• Frame (Link) Level:
• Facilitates reliable transfer of data across the physical link by transmitting the data as
a sequence of frames.
• Uses Link Access Protocol Balanced (LAPB), bit oriented protocol.
• Link Layer performs following functions:
• Transfer of data in an efficient and timely fashion
• Synchronization of the link to ensure that the receiver is in step with the transmitter
• Detection of transmission errors and recovery of errors.
• Identification and reporting of procedural errors to higher level for recovery 20

Packet Level:
• Responsible for end-to-end
connection between two DTEs.
• Also called as Packet layer Protocol
(PLP)
• Procedures:
• SVC
• PVC
• DGs
• Fast Select 21
• Functions performed are:
• Establishing connection
• Transferring data
• Terminating a connection
• Error and flow control
• With the help of X.25 packet layer, data are
transmitted in packets over external virtual circuits.

Advantages and Drawbacks (Cont..)

User Connectivity
• ITU-T Recommendations X.3, X.28 and X.29 are defined as link-level
protocols that provide DTE with asynchronous terminal interfaces to X.25
networks.
• Communicate only via low-speed asynchronous mode.
• X.25 also provides synchronous user connectivity.
• There are three examples of asynchronous and BSC DTE terminal
connectivity options.

Interface Protocols of X.25 packet switching network

Interface Protocols of X.25 packet switching
network (Cont..)
• Recommendation X.121: Defines the international numbering plan for packet switching
networks.
• Recommendation X.28: Defines the operational control of these functions between the
character mode terminal DTE device and the DCE packet assembler/disassembler.
• Recommendation X.29: Defines the same controls, but for the host computer destination.
• Recommendation X.3: Defines a PAD concentrator function for start-mode or character-
mode DTE devices.
• Recommendation X.22: Defines X.25 synchronous dial-up mode for DTE services.

Theory of Operations
1. Traffic Characteristics
2. X.25 Packet Switching Operation

Traffic Characteristics
• To understand packet switching protocols following details must be known:
• Types of traffic for transport
• Characteristics of traffic
• Bursty Traffic:
• Data Transmission with duration of less than 5 seconds.
• Often travels in one direction rather than equally in both direction.
• Eg: HTTP browsing

Traffic Characteristics
• Packet Switching networks accommodate traffic that is not delay-
sensitive.
• Two primary requirements for packet networks are burstiness
and delay-insensitive traffic.
• Due to dynamic allocation of bandwidth technique used by
packet switching, bursty delay-insensitive traffic is ideal for
transport over a packet switching network.

• The figure shows that:
• Low speed terminal users at site A, B and C.
• High Speed user host.
• Terminals could use low speed X.3 circuits to
request information downloads from the host.
• Terminal traffic is very light (low-bandwidth
usage) compared to the data traffic from the
host (high-bandwidth usage).
• The single 56Kbps multiplexed X.25 line from
the host into the packet switching network
depicts the efficiency of asymmetrical traffic
patterns where the host will transmit much
more traffic than it receives.

X.25 Packet Switching Operation

X.25 Packet Switching Operation (Cont..)
• Packet Switching Network shows 6 nodes (1-6) each with 3 users (terminal A, B,
and C)
• Each user device is acting as a DTE and each network node is acting as a DCE.
• Packets are routed based on the routing table via different paths.
• Reassembly is done at the receiving node.
• The DTE device transmits data to the network via synchronous mode
X.25protocol.
• Error checking and retransmission is done at every node.

Network Layer Functions &
X.75 Internetworking
Protocol

Network Layer Functions
• It uses X.25 protocol as it defines the network layer features and functions.
• The concepts are used with respect to:
1. PVCs and VC/SVC
2. VC and LCNs
3. X.25 Control Packet Format
4. Normal Data Packet Format
5. Flow Control and Windowing
6. Fast Connect Option

PVCs and SVCs/VC
• Two types of X.25 virtual circuits exists:
• Switched Virtual Circuits (SVCs): These are temporary connections used for
irregular data transfers. They require 2 DTE devices establish, maintain, and terminate a
session each time the devices need to communicate. Eg: Telephonic Call
• Permanent Virtual Circuits (PVCs): These are permanently established connections
used for frequent and consistent data transfers. PVCs do not require that session be
established and terminated. Therefore, DTEs can begin transferring data whenever
necessary because the session is always active.

PVCs and SVCs/VC (Cont..)
• Three users of a public X.25
packet switching network.
• Each user has a single DS0
access circuit to network.
• Each PVCs carries traffic in both
the directions.
• SVCs operate in the same
manner, but are established and
terminated on demand.

PVCs and SVCs/VC (Cont..)
• Figure shows the standard SVCs X.25 packet
transfer sequence.
• The establishment of an SVC virtual call, data
transfer and then call clearing.
• Data transfer stage can last any amount of time.
• The delay factor must be taken into consideration
during the design.

LCN Assignment
• LCN: Logical Channel Number
• The following 5 events occur:
• Link is set up between local DTE & DCE node and also between the remote DTE &
DCE.
• VC is established between the local and remote DTE.
• Data is transferred between two DTEs.
• VC is released.
• Link is disconnected.

LCN Assignment (Cont..)
• Upto 4095 (2^12) multiplexed channels between each DTE and DCE are
provided.
• The calling and called hosts use different numbers.

X.25 Control Packet Formats
Figure depicts the packet format for call request and
incoming calls

X.25 Control Packet Formats (Cont..)
Control packet are used for VC setup and termination.
• The fields used are:
1. General format identifier: Indicates the general format of the rest of the header (call setup,
clearing, flow control, reset or data packet.)
2. Logical Channel Group Number (LCGN): Logical significance for each logical channel.
3. LCNs: Assigned for each incoming and outgoing virtual circuit for each DCE and DTE.
4. Control Packet Type Identifier: Related packet type (format identifier) from DCE to DTE
and from DTE to DCE.
5. Additional Bytes: Contain information that is packet specific (call request or incoming packets,
reset, etc.)

X.25 Control Packet Formats (Cont..)
• The various packet types in X.25 packet format are:
1. Clear Packet
2. Interrupt Packet
3. RR and RNR Packet
4. Reset Packet
5. Restart Packet
6. Diagnostic Packet

Normal Data Packet Format
Normal Data Packets are transferred
after the call setup and before call
termination.

Normal Data Packet Format
• The bits used in the Normal Data Packet Format are:
• Qualified bit (Q): Distinguish between user data or user device control data stream.
• Data bit (D): Set to 0 if the flow and acknowledgment have local significance. Set to 1 to designate
end-to-end-significance.
• LCGN and LCNs: Together provide 12 bits needed to form the VC.
• P(R) and P(S): Designated for the receive and send sequence count respectively.
• More (M) bit: Set to 0 throughout the length of the message, set to 1 for last packet of the message.
• Data field: Specifies the maximum size and contains the actual user data.

Flow Control and Windowing (Cont..)
• The send and receive sequence numbers in X.25 packet level are also used to
provide flow control between the packet level source and sink.
• Source is a transmitting device and sink is a device that receives.
• The sequence numbers are incremented modulo the maximum window size.
• The figure before uses Modulo 4 i.e. 0,1,2,3.
• The RSN in the acknowledgment indicates the next SSN expected in the next
packet from the other end of the virtual circuit.

• The RSN acts as an acknowledgement for all packets up to one less than the
RSN.
• The transmitter can send no more packets than modulo-1 without
acknowledgment.
• As shown in the figure three packets and then waits for the acknowledgment
before sending any additional packets. This is called as sliding window flow
control protocol.

• This process allows the receiver to control the maximum rate of
transmission over a virtual circuit, mainly used for traffic control.
• The RSN acknowledgment can be piggybacked in the packet header for a
packet headed in the opposite direction on a virtual circuit, or can be sent in
a separate acknowledgment packet.
• Window Size is directly proportional to traffic load on the logical
channel so recources should be wisely used.

Fast Connect Option
• Fast Connect Option is also available for fast packet transactions in X.25.
• Each packet has the call request format together with the data so the
establishment of a virtual circuit is not required.
• There are two types of fast connect option:
• Fast select call
• Fast select with immediate clear

Fast Connect Option (Cont..)
1. Fast select call:
• The packet select packet from user A has both call request and user data (upto 128bytes
of data) and user B can respond with a call-accept packet that also contains user data
from user B.
• The rest of the call connection and disconnect works the same as an SVC call
2. Fast select with immediate clear:
• This is similar to fast select with the call request packet establishing the connection and
the clear indication packet terminating the connection.

X.75 Internetworking Protocol (Cont..)
• ITU-T Recommendation X.75 defines the protocol structure and procedures
for internetworking multiple X.25 packet switching private data networks
(PSPDNs).
• X.75 can be used to:
• share network resources and data across the international or mutiprovider area.
• connect larger backbone packet switches to one another.
• Translate address in conjunction with the X.121 address protocol standard.
• Multilink procedures to support multiple links between signalling terminal exchange (STE).

X.75 Internetworking Protocol (Cont..)
• The figure shows DTEs from 2 separate X.25 networks located in US and
UK communicating via X.75 protocol.
• The X.75 link is transparent to the end user.
• The figure is an example of X.75 being used as an international packet
switching service.

Protocols and Interfaces - IPv4, IPv6, X.25, X.75

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