Overview of Mobile IP , Features of Mobile IP , Key Mechanism in Mobile IP , route Optimization , Overview of TCP/IP , Architecture of TCP/IP , Adaptation of TCP Window , Improvement in TCP Performance,mobile computing.

1. MOBILE COMPUTING
SARASWATHI R
SRI AKILANDESWARI WOMEN’S COLLEGE

2. UNIT
• Overview of Mobile IP – Features of
Mobile IP – Key Mechanism in Mobile
IP – route Optimization.
• Overview of TCP/IP – Architecture of
TCP/IP- Adaptation of TCP Window –
Improvement in TCP Performance.

3. Overview of Mobile IP

4. RECALL
• TCP
• IP

5. TCP
• TCP is a reliable and connection
oriented protocol.

6. IP
• Internet Protocol
is connectionless and unreliable pro
tocol. It ensures no guarantee of
successfully transmission of data.
• In order to make it reliable, it must be
paired with reliable protocol such as
TCP at the transport layer.

7. MOBILE IP
• Mobile IP is a communication
protocol (created by extending Internet
Protocol, IP) that allows the users to
move from one network to another
with the same IP address.
• It ensures that the communication
will continue without user’s sessions
or connections being dropped.

8. Terminologies
• Mobile Node (MN):
It is the hand-held communication
device that the user caries e.g. Cell
phone.
• The mobile node is an end system or
device such as a cell phone, PDA
(Personal Digital assistant), or laptop
whose software enables network roaming
capabilities.

9. Home Network
• It is a network to which the mobile
node originally belongs to as per its
assigned IP address (home address).
• The home network is the subset the
MN belongs to with respect to its IP
address. No mobile IP support is
needed within this network.

10. Home Agent (HA)
• It is a router in home network to which the
mobile node was originally connected.
• The home agent provides several services for the
mobile node and is located in the home network.
The tunnel for packets towards the mobile node
starts at home agent.
• The home agent maintains a location registry,
i.e. it is informed of the mobile node's location by
the current COA (care of address). Following
alternatives for the implementation of an HA
exist.

11. Home Address:
• It is the permanent IP address
assigned to the mobile node (within its
home network).

12. Foreign Network:
• It is the current network to which the
mobile node is visiting (away from its
home network).
• The foreign network is the current
subset the MN visits and which is not
the home network.

13. Foreign Agent (FA)
• It is a router in foreign network to which mobile node is currently
connected. The packets from the home agent are sent to the foreign
agent which delivers it to the mobile node.
• The foreign agent can provide several services to the mobile node
during its visit to the foreign network. The FA can have the COA
(care or address) acting as a tunnel endpoint and forwarding
packets to the MN. The foreign agent can be the default router for
the MN.
• Foreign agent can also provide security services because they belong
to the foreign network as opposed to the MN which is only visiting.
• In short, FA is a router that may function as the point of attachment
for the mobile node when it roams to a foreign network delivers
packets from the home agent to the mobile node.

14. Correspondent Node (CN)
• It is a device on the internet
communicating to the mobile node.

15. Care of Address (COA)
• It is the temporary address used by a mobile node while
it is moving away from its home network.
• The Care- of- address defines the current location of the
mobile node from an IP point of view. All IP packets sent
to the MN are delivered to the COA, not directly to the
IP address of the MN.
• Packet delivery toward the mobile node is done using a
tunnel. To be more precise, the COA marks the
endpoint of the tunnel, i.e. the address where packets
exit the tunnel.

16. MOBILE IP WORKS

17. WORKING MOBILE IP
• Correspondent node sends the data to the mobile node. Data
packets contains correspondent node’s address (Source) and home
address (Destination).
• Packets reaches to the home agent. But now mobile node is not in
the home network, it has moved into the foreign network.
• Foreign agent sends the care-of-address to the home agent to which
all the packets should be sent. Now, a tunnel will be established
between the home agent and the foreign agent by the process of
tunneling.
• Tunneling establishes a virtual pipe for the packets available
between a tunnel entry and an endpoint. It is the process of sending
a packet via a tunnel and it is achieved by a mechanism called
encapsulation.

18. WORKING MOBILE IP
• Now, home agent encapsulates the data packets into
new packets in which the source address is the home
address and destination is the care-of-address and
sends it through the tunnel to the foreign agent.
• Foreign agent, on other side of the tunnel receives the
data packets, decapsulates them and sends them to the
mobile node.
• Mobile node in response to the data packets received,
sends a reply in response to foreign agent.
• Foreign agent directly sends the reply to the
correspondent node.

19. Features of Mobile IP
• Mobile Internet Protocol (Mobile IP) was created in order
to provide better mobile connectivity without
interrupting computers that are already connected to a
network.
• When mobile devices were introduced, there was no
network technology in place for these devices to connect
to the Internet.
• Mobile IP created a new subset of IP connectivity that
worked within the already established system, keeping
network engineers from having to scrap and reinvent
the way Internet connection works.

20. ROAMING CONNECTIVITY
• Mobile IP allows mobile devices to connect to the Internet
when they are not at their home network.
• This lets laptops connect to hotspots and it lets phones
connect through 3G and other Internet network sources.
• An IP address lets a network know where to send and receive
information from on a network.
• Mobile IP uses an address that references its home network
while finding a location on the new network.
• This keeps Mobile IP from knocking other computers off of a
network, because each computer comes from a unique
network and has a unique number.

21. COMPATIBILITY
• Mobile IP is compatible with most networks that offer
the Internet.
• This include the 3G network used for mobile
televisions; Internet hotspots found in cafes, airports
and book stores; and all home network devices.
• Early attempts at Mobile IP would only work with
certain routers or certain types of networks.
• Mobile IP today has no special requirements because
the system is universal and fits within the original IP
infrastructure.

22. TUNNELLING AND REVERSE TUNNELLING
• The method by which mobile IP receives information from a
network is called tunnelling.
• A network cannot directly send information to a mobile IP
device. In order to get this information the mobile device
must create an IP address within its new IP address.
• This allows the network to send information to the IP address
through the ―tunnel‖ of the two new IPs.
• Firewalls and routers can sometimes block tunnelling by
enabling what is called ingress filtering.
• Mobile IP also can use the process of reverse tunnelling,
which is a similar process that reverses the flow of
information to achieve the same result as tunnelling.

23. CORDLESS
• The greatest feature of Mobile IP is that there are no
cords needed to complete the network connection.
• The standard IP required that networks be connected by
a phone line or Ethernet cord.
• With Mobile IP, the device finds the network
automatically and attempts to establish a connection.
• Some mobile capable devices like laptop computers
have the ability to connect using the Mobile IP or using
the standard IP with an Ethernet or phone cord.

24. KEY MECHANISM IN MOBILE IP
• Discovering the care-of-address /
Agent Discovery
• Registering the care-of-address /
Agent Registration
• Tunneling to the care-of- address

25. DISCOVERING THE CARE-OF-ADDRESS
/ AGENT DISCOVERY
• Agents advertise their presence by
periodically broadcasting their agent
advertisement messages.
• The mobile node receiving the agent
advertisement messages observes
whether the message is from its own
home agent and determines whether it is
in the home network or foreign network.

26. DISCOVERING THE CARE-OF-ADDRESS
/ AGENT DISCOVERY
• Discovery Protocol is used to identify the
respective home & Foreign agents
• The steps involved in Discovery of Care-of-
address are,
• 1. The mobile node must obtain mobile IP from
DHCP server and this is done by Router & Agent.
• 2. Mobile node get the IP address from the Agent
discovery process which involves
• Agent advertisement
• Agent solicitation

27. AGENT ADVERTISEMENT
• An agent advertisement message has one or
more care-of-addresses and a flag indicating
whether it is a home agent or foreign agent.

28. AGENT SOLICITATION
• An agent solicitation message will be
sent by Mobile Node(MN) ,when the
MN does not receive any COA to
search for a foreign agent(FA).

29. REGISTERING THE CARE-OF-ADDRESS / AGENT
REGISTRATION
• After receiving COA from Foreign agent, it must register the
new COA in Home Agent.
• Mobile node contacts with home agent with COA for
registration using registration request.
• Home Agent obtains the COA from received request and
record with IP table , which is known as Mobility Binding.
• Current address of the mobile node is bond with new
address (COA) of mobile node is known as mobility binding.
• The Foreign agent in turn updates its visitors list by inserting
the entry for the mobile node and relays the reply to the
mobile node.

30. REGISTERING THE CARE-OF-ADDRESS / AGENT
REGISTRATION
• The Registration process consists of the following steps:
– 1. If the mobile node travels to a foreign network, it registers with the foreign
agent by sending a registration request message which includes the
permanent IP address of the mobile host and the IP address of its home agent.
– 2. The foreign agent in turn performs the registration process on behalf of the
mobile host by sending a registration request containing the permanent IP
address of the mobile node and the IP address of the foreign agent to the
home agent.
• Registration request flag bits
– 3. When the home agent receives the Registration Request, it updates the
mobility binding by associating the care-of-address of the mobile node with its
home address
– 4. The home agent then sends an acknowledgement to the foreign agent.
– 5. The foreign agent in turn updates its visitors list by inserting the entry for
the mobile node and relays the reply to the mobile node.

31. REGISTERING THE CARE-OF-ADDRESS / AGENT
REGISTRATION

33. TUNNELING TO THE CARE-OF- ADDRESS
• Tunneling takes place to forward an IP datagram from the
home agent to a care-of-address as follows:
– 1. When a home agent receives a packet addressed to a mobile
host, it forwards the packet to the care-of- address using IP-
within-IP.
– 2. Using IP-within-IP, the home agent inserts a new IP header in
front of the IP header of any datagram.
– 3. Destination address is set to the care-of-address.
– 4. Source address is set to the home agent’s address.
– 5. After stripping out the first header, IP processes the packet
again.

35. TUNNELING TO THE CARE-OF- ADDRESS
• The Mobile Node sends packets using its home IP address, effectively
maintaining the appearance that it is always on its home network.
• Even while the Mobile Node is roaming on foreign networks, its movements
are transparent to correspondent nodes.
• Data packets addressed to the Mobile Node are routed to its home network,
where the Home Agent now intercepts and tunnels them to the care-of
address toward the Mobile Node.
• Tunnelling has two primary functions: encapsulation of the data packet to
reach the tunnel endpoint, and decapsulation when the packet is delivered
at that endpoint.
• The default tunnel mode is IP Encapsulation within IP Encapsulation.
• Optionally, GRE and minimal encapsulation within IP may be used.Typically,
the Mobile Node sends packets to the Foreign Agent, which routes them to
their final destination, the Correspondent Node, as shown in Figure 2.

36. TUNNELING TO THE CARE-OF- ADDRESS

37. TUNNELING TO THE CARE-OF- ADDRESS

38. ROUTE OPTIMIZATION
Mobile IPv4 route optimization
• Mobile IPv4 route optimization is a proposed
extension to the Mobile IPv4 protocol.
• It provides enhancements to the routing of data
grams between the mobile node and to the
correspondent node.
• The enhancements provide means for a
correspondent node to tunnel data grams directly
to the mobile node or to its foreign agent care-of
address.

39. ROUTE OPTIMIZATION
Route optimization messages and data structures
• The route optimization extension adds a conceptual data structure, the binding
cache, to the correspondent node and to the foreign agent.
• The binding cache contains bindings for mobile nodes' home addresses and their
current care-of addresses.
• With the binding the correspondent node can tunnel data grams directly to the
mobile node's care-of address.
• Every time the home agent receives a datagram that is destined to a mobile node
currently away from home, it sends a binding update to the correspondent node to
update the information in the correspondent node's binding cache.
• After this the correspondent node can directly tunnel packets to the mobile node.
Thus direct bi-directional communication is achieved with route optimization.

40. ROUTE OPTIMIZATION

41. ROUTE OPTIMIZATION

42. ROUTE OPTIMIZATION
Route optimization adds four new UDP-messages to the Mobile IPv4 protocol:
• Binding update informs the correspondent node or foreign agent of the mobile node's
new location. It is sent by the home agent or in the case of previous foreign agent
notification, by the new foreign agent, as shown in Figure 4.
• The binding update contains the care-of address and the home address of the mobile
node and also the lifetime of the binding. It also must contain a mobile IP
authentication extension. An identification number may also be present to provide a
way of matching updates with acknowledgements and to protect against replay attacks.
• Binding acknowledgement is sent by the correspondent node or the foreign agent in
response to the binding update. It contains the mobile node's home address and a
status code. It also contains an identification number, if there was one in the
corresponding binding update.

43. ROUTE OPTIMIZATION
• Binding request is sent by the correspondent node to the
home agent to request a binding update. It contains the home
address of the queried mobile node and possibly an
identification number.
• Binding warning is sent by the previous foreign agent in
response to receiving a tunnelled datagram for a mobile node
for which it has a binding and for which it is not acting as the
current foreign agent.
• The binding warning is sent to the home agent. It contains
the home address of the mobile node and the address of the
correspondent node that does not have up to date
information of the mobile node's current care-of address.
With this information the home agent can send a binding
update to the correspondent node.

44. ROUTE OPTIMIZATION

45. ROUTE OPTIMIZATION
The effect on static routes
• As the correspondent node learns the care-of address of the mobile node from the binding
update, it can tunnel data grams directly to the mobile node's care-of address .
• Thus only the first data grams are routed via the home agent. This reduces the network load
and also reduces the delays caused by routing.
• Thus the optimization is valuable to mobile nodes that visit networks located far from their
home agent.
• However, the overhead caused by tunnelling is not decreased. The correspondent node's use of
minimal encapsulation is a partial remedy, if both the encapsulator and the decapsulator
support it.
• Ingress filtering may also prevent the mobile node from sending data grams directly to the
correspondent node. The use of direct reverse tunnelling from the care-of address to the
correspondent node's address is a possible solution to ingress filtering.
• However, it is not possible with foreign agent care-of addresses, since the current reverse
tunnelling standard requires the foreign agent to tunnel all packets to the home agent of the
mobile node.

46. ROUTE OPTIMIZATION
Smooth handoffs with route optimization
• In the static case the protocol is fairly simple, but handoffs somewhat complicate
the situation. When the correspondent node has an out of date entry for the
mobile node's care-of address it tries to send the tunnelled datagram to the mobile
node's previous location and the datagram is lost. To solve this problem the
protocol includes the previous foreign agent notification mechanism, which adds a
binding cache to the foreign agent.
• When a mobile node moves to a new sub network it sends a registration request to
the new foreign agent. The registration request may contain a previous foreign
agent notification extension. Upon receiving such a request the foreign agent
builds a binding update and sends it to the previous foreign agent. The previous
foreign agent can then, after authenticating the update, create a binding for the
mobile node. With this binding it can re-tunnel data grams to the mobile node's
new care-of address. The re-tunnelling requires foreign agent care-of addresses in
order for the agents to act as tunnel endpoints.
• The previous foreign agent notification mechanism provides temporary localization
of the handoffs. It does not reduce the signalling load between the home agent and
the mobile node, but reduces the number of data grams lost due to correspondent
nodes with out-of date bindings.

47. ROUTE OPTIMIZATION
General deployment requirements
• In order to make use of the binding updates the
correspondent nodes must be able to process and
authenticate them and be able to encapsulate data grams.
• To establish this, the network stacks of the operating systems
require changes.
• Since correspondent nodes need to establish a security
association with the home agent and foreign agents need to
establish one with the mobile node, a widely deployed key
management system is obviously needed.
• Otherwise only nodes with statically configured security
associations can benefit from the binding updates.

48. ROUTE OPTIMIZATION
Security considerations
• Since the correspondent nodes and foreign agents have
binding caches, which change the routing of data grams
destined to mobile nodes, the binding updates must be
authenticated.
• The authentication is performed in a similar manner as in
base Mobile IPv4. All binding updates contain a route
optimization or smooth handoff authentication extension.
• This extension contains a hash, which is calculated from the
datagram and the shared secret.

49. ROUTE OPTIMIZATION
• The correspondent node and the mobile node's
home agent need a security association.
• This association is used for the authentication of
the binding updates.
• Since the mobile node sends a binding update
directly to its previous foreign agent, they also
need a security association. If the security
associations are not preconfigured they can be
established via a key management protocol such
as ISAKMP or SKIP.

50. Overview of TCP/IP

51. Network Access Layer
• A network layer is the combination of the Physical layer and
Data Link layer defined in the OSI reference model.
• It defines how the data should be sent physically through the
network.
• This layer is mainly responsible for the transmission of the
data between two devices on the same network.
• The functions carried out by this layer are encapsulating the
IP datagram into frames transmitted by the network and
mapping of IP addresses into physical addresses.
• The protocols used by this layer are ethernet, token ring,
FDDI, X.25, frame relay.

52. Internet Layer
IP Protocol
• IP protocol is used in this layer, and it is
the most significant part of the entire
TCP/IP suite.
– IP Addressing
– Host-to-host communication
– Data Encapsulation and Formatting
– Fragmentation and Reassembly
– Routing

53. Internet Layer
ARP Protocol
• ARP stands for Address Resolution Protocol.
• ARP is a network layer protocol which is used to find
the physical address from the IP address.
• The two terms are mainly associated with the ARP
Protocol:
– ARP request: When a sender wants to know the physical
address of the device, it broadcasts the ARP request to the
network.
– ARP reply: Every device attached to the network will
accept the ARP request and process the request, but only
recipient recognize the IP address and sends back its
physical address in the form of ARP reply. The recipient
adds the physical address both to its cache memory and
to the datagram header

54. Internet Layer
ICMP Protocol
• ICMP stands for Internet Control Message Protocol.
• It is a mechanism used by the hosts or routers to send notifications
regarding datagram problems back to the sender.
• A datagram travels from router-to-router until it reaches its
destination. If a router is unable to route the data because of some
unusual conditions such as disabled links, a device is on fire or
network congestion, then the ICMP protocol is used to inform the
sender that the datagram is undeliverable.
• ICMP can send the messages only to the source, but not to the
intermediate routers because the IP datagram carries the addresses
of the source and destination but not of the router that it is passed
to.

55. Transport Layer
• The transport layer is responsible for
the reliability, flow control, and
correction of data which is being sent
over the network.
• The two protocols used in the
transport layer are
– User Datagram protocol and
– Transmission control protocol.

56. Transport Layer
• User Datagram Protocol (UDP)
– It provides connectionless service and end-to-end
delivery of transmission.
– It is an unreliable protocol as it discovers the errors
but not specify the error.
– User Datagram Protocol discovers the error, and
ICMP protocol reports the error to the sender that
user datagram has been damaged.
– UDP consists of the following fields:
Source port address
Destination port address
Total length
Checksum

57. Transport Layer
• It creates a virtual circuit between the sender and receiver,
and it is active for the duration of the transmission.
• TCP is a reliable protocol as it detects the error and
retransmits the damaged frames. Therefore, it ensures all the
segments must be received and acknowledged before the
transmission is considered to be completed and a virtual
circuit is discarded.
• At the sending end, TCP divides the whole message into
smaller units known as segment, and each segment contains
a sequence number which is required for reordering the
frames to form an original message.
• At the receiving end, TCP collects all the segments and
reorders them based on sequence numbers.

58. Application Layer
• An application layer is the topmost layer in the TCP/IP model.
• It is responsible for handling high-level protocols, issues of
representation.
• This layer allows the user to interact with the application.
• When one application layer protocol wants to communicate with
another application layer, it forwards its data to the transport layer.
• There is an ambiguity occurs in the application layer. Every
application cannot be placed inside the application layer except
those who interact with the communication system.
• For example: text editor cannot be considered in application layer
while web browser using HTTP protocol to interact with the network
where HTTP protocol is an application layer protocol.

59. Application Layer
• HTTP: HTTP stands for Hypertext transfer protocol. This protocol allows us to access the
data over the world wide web. It transfers the data in the form of plain text, audio, video. It is
known as a Hypertext transfer protocol as it has the efficiency to use in a hypertext
environment where there are rapid jumps from one document to another.
• SNMP: SNMP stands for Simple Network Management Protocol. It is a framework used for
managing the devices on the internet by using the TCP/IP protocol suite.
• SMTP: SMTP stands for Simple mail transfer protocol. The TCP/IP protocol that supports the
e-mail is known as a Simple mail transfer protocol. This protocol is used to send the data to
another e-mail address.
• DNS: DNS stands for Domain Name System. An IP address is used to identify the connection
of a host to the internet uniquely. But, people prefer to use the names instead of addresses.
Therefore, the system that maps the name to the address is known as Domain Name System.
• TELNET: It is an abbreviation for Terminal Network. It establishes the connection between
the local computer and remote computer in such a way that the local terminal appears to be a
terminal at the remote system.
• FTP: FTP stands for File Transfer Protocol. FTP is a standard internet protocol used for
transmitting the files from one computer to another computer.

60. Adaptation of TCP Window
• The first phase of a TCP session is
establishment of the connection.
• This requires a three-way handshake,
ensuring that both sides of the
connection have an unambiguous
understanding of the sequence
number space of the remote side for
this session.

61. Adaptation of TCP Window
• The operation of the connection is as follows:
– The local system sends the remote end an initial sequence
number to the remote port, using a SYN packet.
– The remote system responds with an ACK of the initial
sequence number and the initial sequence number of the
remote end in a response SYN packet.
– The local end responds with an ACK of this remote
sequence number.
– The performance implication of this protocol exchange is
that it takes one and a half round-trip times (RTTs) for the
two systems to synchronize state before any data can be
sent.

62. Adaptation of TCP Window

63. Adaptation of TCP Window
• After the connection has been established, the TCP
protocol manages the reliable exchange of data between
the two systems.
• The algorithms that determine the various
retransmission timers have been redefined numerous
times.
• TCP is a sliding-window protocol, and the general
principle of flow control is based on the management of
the advertised window size and the management of
retransmission timeouts, attempting to optimize
protocol performance within the observed delay and
loss parameters of the connection.

64. Adaptation of TCP Window
• Tuning a TCP protocol stack for optimal performance
over a very low-delay, high-bandwidth LAN requires
different settings to obtain optimal performance over a
dialup Internet connection, which in turn is different for
the requirements of a high-speed wide-area network.
• Although TCP attempts to discover the delay bandwidth
product of the connection, and attempts to
automatically optimize its flow rates within the
estimated parameters of the network path, some
estimates will not be accurate, and the corresponding
efforts by TCP to optimize behavior may not be
completely successful.

65. Adaptation of TCP Window
• Another critical aspect is that TCP is an adaptive
flow-control protocol.
• TCP uses a basic flow-control algorithm of
increasing the data-flow rate until the network
signals that some form of saturation level has
been reached (normally indicated by data loss).
• When the sender receives an indication of data
loss, the TCP flow rate is reduced; when reliable
transmission is reestablished, the flow rate
slowly increases again.

66. Adaptation of TCP Window
• If no reliable flow is reestablished, the flow rate backs
further off to an initial probe of a single packet, and the
entire adaptive flow-control process starts again.
• This process has numerous results relevant to service
quality. First, TCP behaves adaptively , rather than
predictively .
• The flow-control algorithms are intended to increase the
data-flow rate to fill all available network path capacity,
but they are also intended to quickly back off if the
available capacity changes because of interaction with
other traffic, or if a dynamic change occurs in the end-
to-end network path.

67. Adaptation of TCP Window
• For example, a single TCP flow across an otherwise idle network
attempts to fill the network path with data, optimizing the flow rate
within the available network capacity.
• If a second TCP flow opens up across the same path, the two flow-
control algorithms will interact so that both flows will stabilize to
use approximately half of the available capacity per flow.
• The objective of the TCP algorithms is to adapt so that the network
is fully used whenever one or more data flows are present.
• In design, tension always exists between the efficiency of network
use and the enforcement of predictable session performance.
• With TCP, you give up predictable throughput but gain a highly
utilized, efficient network.

68. Improvement in TCP Performance

69. Link-layer protocols
• There have been several proposals for reliable link-layer
protocols.
• The two main classes of techniques employed by these
protocols are:
• error correction (using techniques such as forward error
correction (FEC)), and
• retransmission of lost packets in response to automatic repeat
request (ARQ) messages.
• The link-layer protocols for the digital cellular systems in
the U.S. — both CDMA and TDMA — primarily use ARQ
techniques.
• While the TDMA protocol guarantees reliable, in-order
delivery of link-layer frames, the CDMA protocol only makes
a limited attempt and leaves it to the (reliable) transport
layer to recover from errors in the worst case.

70. Link-layer protocols
• The AIRMAIL[Asymmetric Reliable Mobile Access In Link-
layer] protocol employs a combination of FEC and ARQ
techniques for loss recovery.
• The main advantage of employing a link-layer protocol for
loss recovery is that it fits naturally into the layered structure
of network protocols.
• The link-layer protocol operates independently of higher-layer
protocols (which makes it applicable to a wide range of
scenarios), and consequently, does not maintain any per-
connection state.
• The main concern about link-layer protocols is the possibility
of adverse effect on certain transport-layer protocols such as
TCP.

71. Indirect-TCP (I-TCP) protocol
• This was one of the early protocols to use the split-connection
approach.
• It involves splitting each TCP connection between a sender and
receiver into two separate connections at the base station — one
TCP connection between the sender and the base station, and the
other between the base station and the receiver.
• In our classification of protocols, ITCP is a split-connection solution
that uses regular TCP for its connection over wireless link.
• I-TCP, like other split-connection proposals, attempts to separate
loss recovery over the wireless link from that across the wireline
network, thereby shielding the original TCP sender from the wireless
link.

72. I - TCP

73. Indirect-TCP (I-TCP) protocol
• However, as experiments indicate, the choice of TCP
over the wireless link results in several performance
problems.
• Since TCP is not well-tuned for the lossy link, the TCP
sender of the wireless connection often times out,
causing the original sender to stall.
• In addition, every packet incurs the overhead of going
through TCP protocol processing twice at the base
station (as compared to zero times for a non-split-
connection approach), although extra copies are
avoided by an efficient kernel implementation.

74. Indirect-TCP (I-TCP) protocol
• Another disadvantage of this approach is that
the end-to end semantics of TCP
acknowledgments is violated, since
acknowledgments to packets can now reach
the source even before the packets actually
reach the mobile host.
• Also, since this protocol maintains a
significant amount of state at the base
station per TCP connection, handoff
procedures tend to be complicated and slow.

75. Selective Acknowledgments
• Since standard TCP uses a cumulative acknowledgment scheme, it
often does not provide the sender with sufficient information to
recover quickly from multiple packet losses within a single
transmission window.
• Several studies have shown that TCP enhanced with selective
acknowledgments performs better than standard TCP in such
situations.
• SACKs were added as an option to TCP by RFC 1072.
• However, disagreements over the use of SACKs prevented the
specification from being adopted, and the SACK option was removed
from later TCP RFCs.
• Recently, there has been renewed interest in adding SACKs to TCP.