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 The OSI model is a layered framework for the design of network systems
that allows communication between all types of data systems. The OSI
model is composed of seven ordered layers:
 Layer 1: Physical Layer : The physical layer supports the electrical or
mechanical interface to the physical medium and performs services
requested by the data link layer. The major functions and services
performed by the physical layer are as follows:
1. Establishment and termination of a connection to a communications medium.
2. Participation in the process whereby the communication resources are
effectively shared among multiple users (e.g., contention resolution and flow
control).
3. Conversion between the representation of digital data in the end user’s
equipment and the corresponding signals transmitted over a communications
channel .
 The physical layer is concerned with the following:
1. Physical characteristics of interfaces and media
2. Representation of bits, transmission rate, synchronization of bits
3. Link configuration
4. Physical topology, and transmission mode
Introduction

 Layer 2: Data Link Layer : The data link layer provides the functional and
procedural means to transfer data between network entities and to
detect and possibly correct errors that may occur in the physical layer.
This layer responds to service requests from the network layer and issues
service requests to the physical layer. Specific responsibilities of the data
link layer include the following:
1. Framing
2. Physical addressing
3. Flow control
4. Error control
5. Access control
 Layer 3: Network Layer : The network layer provides the functional and
procedural means of transferring variable-length data sequences from a
source to a destination via one or more networks while maintaining the
QoS requested by the transport layer. The network layer performs
network routing, flow control, segmentation and reassembly, and error
control functions. This layer responds to service requests from the
transport layer and issues service requests to the data link layer. Specific
responsibilities of the network layer include the following:
1. Logical addressing
2. Routing
Introduction

 Layer 4: Transport Layer : The purpose of the transport layer is to provide
transparent transfer of data between end users, thus relieving the upper
layers from any concern with providing reliable and cost-effective data
transfer.
 This layer responds to service requests from the session layer and issues
service requests to the network layer. Specific responsibilities of the
transport layer include the following:
1. Service-point addressing
2. Segmentation and reassembly
3. Connection control and flow control
4. Error control
 Layer 5: Session Layer : The session layer provides the mechanism for
managing a dialog between end user application processes. It supports
either duplex or half-duplex operations and establishes check pointing,
adjournment, termination, and restart procedures. This layer responds to
service requests from the presentation layer and issues service requests
to the transport layer. Specific responsibilities of the session layer include
the following:
1. Dialog control
2. Synchronization
Introduction

 Layer 6: Presentation Layer : The presentation layer relieves the
application layer of concern regarding syntactical differences in data
representation within the end-user systems. This layer responds to
service requests from the application layer and issues service requests to
the session layer. Specific responsibilities of the presentation layer include
the following:
1. Translation
2. Encryption
3. Compression
 Layer 7: Application Layer : The application layer is the highest layer. This
layer interfaces directly to and performs common application services for
the application processes and also issues requests to the presentation
layer.
 The common application services provide semantic conversion between
associated application processes. Specific services provided by the
application layer include the following:
1. Network virtual terminal
2. File transfer, access, and management
3. Mail services
4. Directory services
Introduction

 The TCP/IP protocol suite provides service to transfer data from one network
device to another using the Internet. The TCP/IP protocol suite is composed of
five layers:
 Physical and Data Link Layers : The physical and data link layers are responsible
for communicating with the actual network hardware (e.g., the Ethernet card).
Data received from the physical medium are handed over to the network layer,
and data received from the network layer are sent to the physical medium. The
TCP/IP does not specify any specific protocol at this layer and supports all
standard and proprietary protocols.
 Network Layer : The network layer is responsible for delivering data to the
destination. It does not guarantee the delivery of data and assumes that the
upper layer will handle this issue. This layer consists of several supporting
protocols :
 Internet Protocol (IP) : is a network layer protocol that provides a
connectionless, “best effort” delivery of packets through an internetwork. The
term best effort means that there is no error checking or tracking done for the
sequence of packets being transmitted. It assumes that the higher-layer protocol
takes care of the reliability of packet delivery. The packets being transmitted are
called datagrams. Each of these datagrams is transmitted independently and
may take different routes to reach the same destination. IP supports a
mechanism of fragmentation and reassembly of datagrams to handle data links
with different maximum-transmission unit (MTU) sizes.
TCP/IP Protocol

 Internet Control Message Protocol (ICMP) : is a companion protocol to IP that
provides a mechanism for error reporting and query to a host or a router. The
query message is used to probe the status of host or a router by the network
manager whereas the error-reporting message is used by the host and routers to
report errors.
 Internet Group Management Protocol (IGMP) : is used to maintain multicast
group membership within a domain. Similar to ICMP, it uses query and reply
messages to maintain multicast group membership in its domain. A multicast
router sends a periodic IGMP query message to find out the multicast session
members in its domain. If a new host wants to join a multicast group, it sends an
IGMP join message to its neighboring multicast router, which takes care of
adding the host to the multicast delivery tree.
 Dynamic Host Configuration Protocol (DHCP) : is designed to handle dynamic
assignments of IP addresses in a domain. This protocol is an extension of the
bootstrap protocol (BOOTP) and provides a way for the mobile nodes to request
an IP address from a DHCP server in case nodes move to a different network.
 This dynamic assignment of IP address is also applicable to the hosts that attach
to the network occasionally. It saves precious IP address space by utilizing the
same IP address for needed hosts. DHCP is fully compatible with BOOTP, which
supports only static binding of physical address to IP address.
TCP/IP Protocol

 Internet Routing Protocols : Some of the widely used routing protocols at
the network layer are routing information protocol (RIP) , open shortest
path first (OSPF) , and border gateway protocol (BGP) .
 Routing information protocol (RIP): RIP is a distance vector–based
interior routing protocol. It uses the Bellman-Ford algorithm (discussed in
the following subsection) to calculate routing tables. In distance vector
routing, each router periodically shares its knowledge about other routers
in the network with its neighbors. Each router also maintains a routing
table consisting of each destination IP address, the shortest distance to
reach the destination in terms of hop count, and the next hop to which
the packet must be forwarded. The current RIP message contains the
minimal amount of information necessary for routers to route messages
through a network and is meant for small networks.
 RIP version 2 enables RIP messages to carry more information, which
permits the use of a simple authentication mechanism to update routing
tables securely. More important, RIP version 2 supports subnet masks, a
critical feature that was not available in RIP
TCP/IP Protocol

Open shortest path first (OSPF): OSPF is an interior routing protocol developed
for IP networks. This protocol is based on the shortest path first (SPF)
algorithm, which sometimes is referred to as the Dijkstra algorithm.
OSPF supports hierarchical routing, in which hosts are partitioned into
autonomous systems (AS). Based on the address range, an AS is further split into
OSPF areas that help border routers to identify every single node in the area. The
concept of OSPF area is similar to subnetting in IP networks. Routing can be
limited to a single OSPF or can cover multiple OSPFs. OSPF is a link-state
routing protocol that requires sending link-state advertisements (LSAs) to all
other routers within the same hierarchical area. As OSPF routers accumulate
link-state information, they use the SPF algorithm to calculate the shortest path
to each node. As a link-state routing protocol, OSPF contrasts with RIP, which
is a distance vector routing protocol.
Routers running the distance vector algorithm send all or a portion of their
routing tables in routing-update messages to their neighbors.
TCP/IP Protocol

 Border gateway protocol (BGP): BGP is an interdomain or
interautonomous system routing protocol. Using BGP, interautonomous
systems communicate with each other to exchange reachability
information.
 BGP is based on the Path Vector Routing Protocol, wherein each entry in
the routing table contains the destination network, the next router, and
the path to reach the destination. The path is an ordered list of
autonomous systems that a packet should travel to reach the destination.
 TCP : is a connection-oriented reliable transport protocol that sends data
as a stream of bytes. At the sending end, TCP divides the stream of data
into smaller units called segments. TCP marks each segment with a
sequence number.
 The sequence number helps the receiver to reorder the packets and
detect any lost packets. If a segment has been lost in transit from source
to destination, TCP retransmits the data until it receives a positive
acknowledgment from the receiver.
 TCP can also recognize duplicate messages and can provide flow control
mechanisms in case the sender is transmitting at a faster speed than the
receiver can handle.
TCP/IP Protocol

 Application Layer : In TCP/IP the top three layers of OSI—session,
presentation, and application layers—are merged into a single layer called
the application layer. Some of the applications running at this layer are
domain name server (DNS), simple mail transfer protocol (SMTP), Telnet,
file transfer protocol (FTP), remote login (Rlogin), and network file
system (NFS).
 Routing Using Bellman-Ford Algorithm : One step that can take a
substantial amount of time is the selection of a route between the source
and destination. This is important as appropriate path selection is critical
for minimizing communication delays.
 The Bellman-Ford algorithm is one of the
routing algorithms designed to find shortest
paths between two nodes of a given graph
(Figure 9.3).
TCP/IP Protocol

Figure 9.4 Steps in the Bellman-Ford algorithm for the sample network.

 Need for TCP over Wireless : The existing Internet employs TCP/IP as its protocol
stack. Many of the existing applications require TCP as the transport layer for
reliable transfer of data packets.
 Accessing the Internet is essential for commercial applications, while voice and
other data communications utilize the underlying Internet backbone. For
wireless networks to become popular, support for the existing applications and
compatibility with the wired Internet must be provided. Therefore, it is
imperative that wireless networks also adopt and support TCP for reliable
transfer of data.
 Limitations of Wired Version of TCP : The primary concern in the use of
conventional TCP over wireline networks is packet loss, because congestion can
be present at various nodes in the network.
 In such systems where congestion is the only source for errors, TCP congestion
avoidance mechanisms are extremely useful. However, the same cannot be said
about wireless networks, as errors can be introduced due to inherent use of air
as a medium of packet transport. Errors can also be attributed to the mobility of
users in the network. In such cases, TCP’s congestion-avoidance and error-
recovery mechanisms lead to unnecessary retransmissions, thereby leading to
inefficient use of available wireless bandwidth. In the following subsection, a
summary of the various approaches used to improve the efficiency of TCP over
wireless networks is given. These strategies range from modifying link layer
modules to using split TCP.
TCP over Wireless

 Solutions for Wireless Environment : The scarce spectrum imposes
a fundamental limit on the performance of the wireless channel,
and MSs have limited computing resources and severe energy
constraints. Due to these characteristics, a lot of work has been
done to optimize the performance of the protocol stack.
 Some of the approaches to improving the performance of TCP over
wireless links are as follows:
 End-to-end protocols attempt to make the TCP sender handle
losses through the use of two techniques :
 First, they use some form of selective acknowledgments to allow
the sender to recover from multiple packet losses in a window,
without resorting to a coarse timeout.
 Second, they attempt to have the sender distinguish between
congestion and other forms of losses using an explicit loss
notification (ELN) mechanism.
TCP over Wireless

 TCP–SACK : Standard TCP uses a cumulative acknowledgment scheme, which
does not provide the sender with sufficient information to recover quickly from
multiple packet losses within a single transmission window.
 A selective acknowledgment (SACK) mechanism, combined with a selective
repeat retransmission policy, can help to overcome these limitations. The
receiving TCP sends back SACK packets to the sender, informing the sender of
the data that have been received. The sender can then retransmit only the
missing data segments. If the duplicate segment is received and is part of a
larger block of noncontiguous data in the receiver’s data queue, then the next
SACK block should be used to specify this larger block.
 Wireless wide-area transmission control protocol (WTCP) : WTCP protocol is a
reliable transport layer protocol for a network with wireless links. WTCP runs on
the BS that is involved in the TCP connection. In this protocol, the BS buffers
data from the fixed host and uses separate flow and congestion control
mechanisms for the link between itself and the MS. It temporarily hides the fact
that a mobile link breakage has occurred by using local retransmissions of the
data for which the MS has not sent an ACK. Once it has received an ACK from the
MS, it sends this ACK to the fixed host, but only after changing the timestamp
value in the ACK, so that the TCP’s round-trip estimation at the fixed sender is
not affected. This mechanism effectively hides the wireless link errors from the
fixed sender.
TCP over Wireless

 Freeze-TCP protocol [9.14]: The main idea behind freeze-TCP is to move the onus of
signaling an impending disconnection to the client. A mobile node can certainly
monitor signal strengths in wireless antennas and detect an impending handoff and,
in certain cases, might even be able to predict a temporary disconnection. In such a
case, it can advertise a zero window size, to force the sender into zero window probe
mode and prevent it from dropping its congestion window.
 Explicit bad state notification (EBSN) : Explicit bad state notification uses local
retransmission from the BS to shield the wireless link errors and improve
performance of TCP over the wireless link. However, while the BS is performing local
recovery, the source could still timeout, causing unnecessary source retransmission.
The EBSN approach avoids source timeout by using the EBSN message to the source
during local recovery. The EBSN message causes the source to reset its timeout
value. In this way, timeouts at the source during local recovery are eliminated.
 Fast retransmission approach : The fast retransmission approach tries to reduce the
effect of MS handoff. Regular TCP at the sender interprets the delay caused by a
handoff process to be due to congestion. Therefore, whenever a timeout occurs, its
TCP window size is reduced and these packets are retransmitted.
 The fast retransmission approach alleviates the retransmission problem by having
the MS send a certain number of duplicate acknowledgments to the sender
immediately after completing the handoff. This step causes TCP at the sender to
reduce its window size immediately and retransmit packets starting from the first
missing packet for which the duplicate acknowledgment has been sent, without
waiting for the timeout period to expire.
TCP over Wireless

 Link Layer Protocols : There are two main classes of techniques
employed for reliable link layer protocols:
1. Error correction using techniques such as FEC
2. Retransmission of lost packets in response to ARQ messages
 Transport unaware link improvement protocol (TULIP) : TULIP
provides a link layer that is transparent to the TCP, has no
knowledge of the TCP’s state, takes advantage of the TCP’s
generous timeouts, and makes efficient use of the bandwidth over
the wireless link. TULIP provides reliability only for packets
(frames) that require such service (service awareness), but it does
not know any details of the particular protocol to which it provides
reliable service for packets carrying TCP data traffic and unreliable
service for other packet types, such as user datagram protocol
(UDP) traffic. TULIP maintains local recovery of all lost packets at
the wireless link in order to prevent unnecessary and delayed
retransmission of packets over the entire path and a subsequent
 reduction in TCP’s congestion window.
TCP over Wireless

 AIRMAIL protocol : AIRMAIL is the abbreviation of Asymmetric Reliable Mobile
Access in Link Layer. This protocol employs a combination of FEC and ARQ
techniques for loss recovery. The BS sends an entire window of data before the
mobile receiver returns an acknowledgment. The rationale for this approach is
not to waste bandwidth on ACKs and to limit the amount of work done by the
mobile unit in order to conserve power.
 Snoop protocol : In the snoop protocol, a transport layer aware agent, called a
snoop agent, is introduced at the BS. The agent monitors the link interface for
any TCP segment destined for the MS and caches it if buffer space is available.
The BS also monitors the acknowledgments from the MS. A segment loss is
detected by the arrival of duplicate acknowledgments from the MS or by a local
timeout.
 The snoop agent retransmits the lost segment if it has been cached and
suppresses the duplicate acknowledgments. The snoop agent essentially hides
the link failures in the wireless link by using local retransmissions rather than
allowing the TCP sender to invoke congestion avoidance mechanisms and the
fast retransmission scheme.
 Split TCP Approach : Split connection protocols split each TCP connection
between a sender and receiver into two separate connections at the BS—one
TCP connection between the sender and the BS, and the other between the BS
and the MS. Over the wireless hop, a specialized protocol may be used that can
tune into the wireless environment.
TCP over Wireless

 Indirect-TCP (I-TCP) : I-TCP is a split connection solution that uses standard TCP
for its connection over the wireline link. The indirect protocol model for MSs
suggests that any interaction from a MS to a fixed host should be split into two
separate interactions—one between the MS and its mobile support router
(MSR) over the wireless medium and another between the MSR and the fixed
host over the fixed network. All the specialized support that is needed for the
mobile applications and low-speed and unreliable wireless medium can be built
into the wireless side of the interaction while the fixed side is left unchanged at
the transport layer. Handoff between two different MSRs is supported on the
wireless side without having to reestablish the connection at the new MSR.
 M-TCP protocol : In this approach, the BS relays ACKs back to the sender only
when the receiver (MS) has acknowledged data; therefore, the end-to-end
semantics is maintained, though it also splits up the connection between a
sender (fixed host) and a mobile receiver (MS) into two parts: one between fixed
host and BS and another between BS and MS, which uses a customized wireless
protocol. The receiver can make the sender enter the persist mode by
advertising a zero window size in the presence of frequent disconnections. In
this case, the sender freezes all packet retransmit timers and does not drop the
congestion window so that the idle time during the slow start phase can be
avoided. Whenever the BS detects a disconnection or packet loss, it sends back
an ACK with a zero window size to force the sender into persist mode and to
force it not to drop the congestion window.
TCP over Wireless

 IPv6 also known as IPng (Internet Protocol next generation) has been
proposed to address the unforeseen growth of the Internet and the
limited address space provided by IPv4.
 Transition from IPv4 to IPv6 :
 IPv4 has extensively been used for data communication in wired
networks. We introduce this Internet protocol to understand its format.
This is important, because a large number of IPv4-hosts and IPv4-routers
have been installed and we need to maintain their compatibility.
 The IPv4 uses a 32-bit address to provide unreliable and connectionless
best effort delivery service. Datagrams (packets in the IP layer) may need
to be fragmented into smaller datagrams due to the maximum packet
size in some physical networks. It also depends on checksum to protect
corruption during the transmission. However, the following are some
disadvantages of IPv4:
1. Since the 32-bit address is not sufficient according to the rapidly increased size
of the Internet, more address space is needed.
2. Real-time audio and video transmissions are being used increasingly, and they
require strategies to minimize transmission delay and resource reservation.
Unfortunately, those features are neither provided nor supported by IPv4.
3. IPv4 does not have encryption or authentication.
Internet Protocol Version 6 (IPv6)

 The transition from IPv4 to IPv6 is supposed to be simple and
without any considerable (temporal) dependencies upon other
measures. The IETF plans the following transition mechanisms:
 The basic principle should be Dual-IP-Stack (i.e., IPv4 hosts and
IPv4 routers get an IPv6 stack in addition to their IPv4 stack). This
coexistence ensures full compatibility between not yet updated
systems, and already upgraded systems make it possible to employ
IPv6 for communication right away.
 IPv6-in-IPv4 encapsulation: IPv6 datagrams can get encapsulated
in IPv4 datagrams enabling IPv6 communication via pure IPv4
topologies. This so-called tunneling of IPv6 packets allows early
worldwide employment of IPv6, although not all networks that are
part of the communication path support IPv6. The tunnels between
two routers must be manually configured, whereas tunnels
between hosts and routers may be built up automatically.
Tunneling of IPv6 datagrams can be removed as soon as all routers
along the respective path have been upgraded with IPv6.

Figure 9.5
IPv4 header
format.
Figure 9.6
Format of IPv6.

 Features of IPv6 : IPv6 uses a 128-bit (16-byte) address to identify a host in the
Internet. Some of the salient features of IPv6 are as follows:
 Address space: An IPv6 address is 128 bits long, which can effectively handle the
problems created by a limited IPv4 address space.
 Resource allocation: IPv6 supports resource allocation by adding the mechanism of
flow label. By using flow label, a sender can request special handling of the packet in
the Internet.
 Modified header format: IPv6 separates options from the base header. This helps
speed up the routing process since most of the options need not be checked by
routers.
 Support for security: IPv6 supports encryption and decryption options, which
provide authentication and integrity.
 Differences between IPv6 and IPv4 : The main differences between IPv6 and IPv4 are
as follows:
 Expanded addressing capabilities: In IPv6 the address space is increased from 32 to
128 bits. This way, more hierarchical address levels are possible and address prefix
routing may be used more efficiently. Furthermore, the longer IPv6 addresses allow
more devices and simplify address auto-configuration. The multicast capabilities are
improved, and a new address type “anycast” is introduced for addressing the nearest
interface out of a group of interfaces.

 Simplified header format: To optimize the speed of processing an IPv6 packet
and to minimize its bandwidth requirements, some fields of the IPv4 header have
been eliminated for IPv6 or made optional.
 Improved support for options and extensions: A new design concept for IPv6 is
the extension header, which means that options and extensions can be more
efficiently added, transmitted, and processed. The size of options is not so
strictly limited as in IPv4, which facilitates flexibility for installing future options.
 Flow labeling capabilities: In IPv6, it is possible to label data flows, which
enables the sender to require a special treatment of packets (QoS) by routers on
the way to the destination. This may be a non-default QoS or a real-time service
for multimedia applications such as audio or video. In particular, the capabilities
of ATM can be used effectively.
 Support for authentication and encryption: IPv6 supports authentication of the
sender (i.e., a form of digital signature) and data encryption.
 IPv6 supports mobility and auto configuration. MSs such as laptops are
supposed to be reachable everywhere in the Internet with their home IP address,
and a computer that is connected to a network is supposed to configure its
correct address automatically.

Format of IPv6.

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