UNIT IV

Mobile Transport Layer

• Conventional TCP/IP Protocols
• Indirect TCP
• Snooping TCP
• Mobile TCP
• Other transport Protocols for Mobile Networks
Transport Layer Importance in Mobile Networks
• In mobile networks, the transport layer plays a crucial role
in ensuring reliable and efficient data communication.
• While the network layer handles routing and addressing, the
transport layer where protocols like TCP and UDP operate,
manages end-to-end data transmission.
TCP vs. UDP

TCP (Transmission Control Protocol):

• Reliability: TCP provides reliable data delivery by ensuring
that all packets are received in the correct order and
retransmitting any lost packets.
• Connection-oriented: Establishes a connection before data
transmission begins and maintains that connection throughout
the data exchange.
• In-order delivery: Guarantees that packets are delivered in the
sequence they were sent.
• Congestion control: Adjusts the rate of data transmission
based on network congestion, reducing packet loss and
improving network stability.
UDP (User Datagram Protocol):
• Simplicity: UDP is a simpler protocol that does not provide
reliability or order guarantees.
• Connectionless: Sends data without establishing a
connection, which can reduce latency.
• No congestion control: Does not adjust the transmission rate
based on network conditions, which can lead to packet loss
if the network is congested.
• Challenges in Mobile Environments: TCP mechanisms,
designed for stable networks, face issues in mobile settings
due to frequent connectivity changes, which can disrupt data
transmission.
• Solutions for Mobile TCP: Various solutions have been
proposed to adapt TCP for mobile networks, addressing
problems like connection interruptions and performance
degradation.
• Current Adaptations: Recent adaptations focus on refining
TCP and boosting performance via proxies to better suit
mobile environments.
Traditional TCP
• Traditional TCP (Transmission Control Protocol) is a
connection-oriented protocol designed for reliable data
transmission over a network.
• It ensures that data is delivered in order, without errors, and
handles congestion control to prevent network overload.
• TCP uses mechanisms like slow start, congestion control, and
fast retransmit/recovery to manage data flow and maintain
connection stability.
Congestion Control
• Prevents network congestion from worsening by controlling
the data transmission rate.
• TCP reduces the sending rate and the size of the congestion
window when packet loss is detected, assuming it’s due to
congestion.
Slow Start
• Gradually finds the network’s capacity without
overwhelming it.
• Starts with a small congestion window and doubles its size
each round-trip time (RTT) until a threshold is reached, then
increases linearly.
Fast Retransmit/Fast Recovery
• Purpose: Quickly recovers from packet loss without waiting
for a timeout.
• Detects packet loss via multiple duplicate acknowledgments
and retransmits the lost packet immediately, keeping the
congestion window size unchanged.
Implications on Mobility
• TCP often misinterprets packet loss in mobile networks,
thinking it's caused by congestion.
• This leads to unnecessary slowdowns. In reality, packet loss
might come from errors or network changes due to mobility.
• New methods like Explicit Congestion Notification (ECN)
aim to improve how TCP handles these issues, but adapting
TCP for mobile environments remains complex.
Classical TCP improvements
• Classical TCP improvements are techniques and
modifications designed to enhance the performance,
reliability, and efficiency of the traditional TCP protocol.
– Indirect TCP
– Snooping TCP
– Mobile TCP
– Fast retransmit/fast recovery
– Transmission/time-out freezing
– Selective retransmission
– Transaction-oriented TCP
Indirect TCP (I-TCP):
• Divides the TCP connection into separate segments for
wired and wireless networks.
• This approach optimizes performance by handling the wired
and wireless parts of the connection independently,
improving overall efficiency in heterogeneous network
environments.
Snooping TCP:
• Employs a network node to intercept and examine TCP
traffic between sender and receiver.
• The node can cache packets and manage retransmissions
locally, which reduces redundant retransmissions and
enhances performance in wireless networks.
Mobile TCP:
• Enhances TCP to address the challenges of mobility, such as
handovers and variable network conditions.
• It ensures a more stable connection and reliable data transfer
by adapting to the dynamic nature of mobile networks.
Fast Retransmit/Fast Recovery:
• Accelerates recovery from packet loss by retransmitting
packets promptly upon receiving duplicate acknowledgments.
• Fast Recovery adjusts the congestion window size to quickly
resume data transmission and mitigate the impact of packet
loss.
Transmission/Time-out Freezing:
• Temporarily halts data transmission during periods of
significant congestion or network delay.
• This technique prevents further packet loss and congestion
collapse by pausing transmission until network conditions
improve.
Selective Retransmission:
• Retransmits only the packets that are lost or corrupted,
rather than all subsequent packets.
• This selective approach improves efficiency by targeting
only the specific data that needs to be resent.
Selective Retransmission:
• Retransmits only the packets that are lost or corrupted,
rather than all subsequent packets.
• This selective approach improves efficiency by targeting
only the specific data that needs to be resent.
Indirect TCP (I-TCP)
• Indirect TCP (I-TCP) addresses mobile network challenges
by dividing the end-to-end TCP connection into two
segments: one between the mobile device and a fixed proxy,
and another between the proxy and the destination.
• This approach reduces the impact of wireless link variations
and handoffs. The proxy manages the wireless link,
improving performance and reliability for mobile users.
• It helps mitigate issues like frequent reconnections and
variable bandwidth.
• I-TCP was created to improve TCP performance over
wireless networks where traditional TCP struggles due to
higher error rates and varying connection quality.

How It Works:
Segmentation:
• The TCP connection between a fixed network (e.g., a server)
and a mobile host (e.g., a laptop) is split into two segments:
• Segment 1: From the fixed network to an
intermediary access point (proxy).
• Segment 2: From the access point to the mobile host.
Roles of Access Point (Proxy):
• Proxy Duties: The access point (foreign agent) handles:
– The TCP connection with the fixed network using
standard TCP.
– The TCP connection with the mobile host, which may
use a modified TCP optimized for wireless links.
• Data Handling: The proxy acknowledges and retransmits
packets if they are lost on the wireless link.
Mobility Handling:
• Handover Process: When the mobile host moves to a new
access point:
– The old proxy forwards any buffered packets to the new
proxy.
– The new proxy continues the TCP session without
interruption.
Advantages:
• No Changes Needed for Fixed Network TCP: The fixed
network's TCP connections remain unchanged.
• Error Isolation: Errors on the wireless link are handled locally
by the proxy, preventing them from affecting the fixed
network.
• Improved Performance Testing: New TCP optimizations can
be tested between the mobile host and the proxy without
impacting the entire network.
• Faster Retransmissions: Shorter round-trip times between the
mobile host and the proxy can lead to faster retransmissions.
• Protocol Flexibility: The proxy can use different protocols or
compress headers between itself and the mobile host.
Disadvantages:
• Loss of End-to-End Guarantee: If the proxy fails, the
reliability of the connection might be affected, causing
potential issues for applications relying on end-to-end TCP
guarantees.
• Handover Delays: During handovers, there might be delays as
the old proxy forwards data to the new proxy.
• Security Concerns: The proxy must be trusted and integrated
with security mechanisms if encryption is used, since it
handles the end of the TCP connection.
Snooping TCP
• Snooping TCP involves a proxy that monitors TCP segments
and maintains a cache of data.
• This proxy, known as the snooper, can retransmit lost packets
locally without involving the end-to-end TCP connection,
reducing delays from wireless link losses.
• By managing retransmissions and maintaining state
information, Snooping TCP enhances performance in
environments with high packet loss.
• It helps improve the overall efficiency of data transmission in
mobile networks.
Key Features:
• Local Buffering: The foreign agent buffers packets destined for
the mobile host and retransmits them locally in case of packet
loss, reducing the delay compared to retransmissions from the
correspondent host.
• Acknowledgement Handling: The foreign agent snoops on packet
flows and acknowledgements to detect packet losses. It avoids
acknowledging data to prevent misleading the correspondent host
about the mobile host’s state.
• Negative Acknowledgements (NACKs): For data sent from the
mobile host to the correspondent host, the foreign agent detects
missing packets and sends NACKs to prompt immediate
retransmission by the mobile host.
Advantages:
• Maintains TCP Semantics: End-to-end connection is kept
intact.
• No Changes Needed: Only the foreign agent is modified; the
correspondent and mobile hosts remain unchanged.
• No Handover State Issues: Works even if the mobile host
switches foreign agents.
Disadvantages:
• Limited Wireless Isolation: Problems on the wireless link
can still affect the correspondent host.
• Requires Mobile Host Changes: Needs extra mechanisms on
the mobile host, reducing transparency.
• Encryption Issues: Encryption can block snooping, making
retransmissions problematic.
Mobile TCP (M-TCP)
• M-TCP is designed to improve TCP performance in
scenarios involving frequent or lengthy disconnections,
addressing the limitations of standard TCP and other
approaches like I-TCP and Snooping TCP.
How It Works:
• Connection Split: Similar to I-TCP, M-TCP splits the TCP
connection into two parts: one for the standard host-supervisory
host (SH) connection and another for the SH-mobile host (MH)
connection.
• Unmodified TCP: Used between the standard host and SH.
• Optimized TCP: Used between SH and MH, with adaptations
for frequent disconnections.
• Handling Disconnections: When the SH detects that the MH is
disconnected (no ACKs), it sets the sender’s window size to 0.
This puts the sender in persistent mode, avoiding unnecessary
retransmissions and slow start. Once connectivity is restored,
the window size is reset, allowing the sender to resume full-
speed data transmission.
Advantages:
• Maintains End-to-End Semantics: SH only forwards ACKs
from MH, preserving TCP’s end-to-end integrity.
• Efficient Disconnection Handling: Avoids buffering data at
SH, preventing the need to transfer buffer states during
handovers. Lost packets are retransmitted as needed.
Disadvantages:
• Propagation of Packet Loss: Packet loss due to bit errors on
the wireless link affects the sender, as M-TCP assumes a
low error rate.
• Requires New Protocols: Adaptations on the wireless side
require modifications to the MH protocol and additional
network elements like bandwidth managers.
Fast retransmit/Fast recovery
• To handle packet loss during mobile host handovers without
reducing transmission rates, the mobile host (MH) sends
three duplicate acknowledgements to the correspondent host
(CH) after moving to a new foreign agent (NFA).
• This triggers CH to use fast retransmit, quickly
retransmitting lost packets and maintaining the same data
rate.
Advantages:
• Simple implementation with minimal changes needed.
• No modifications required for foreign agents and
correspondent hosts.
Disadvantages:
• Does not address packet loss from other network issues, only
handovers.
• May be inefficient if the handover process is prolonged.
• Requires tight integration between mobile IP and TCP layers.
Transmission/Time-Out Freezing
• Transmission/Time-Out Freezing addresses long connection
interruptions, such as when moving into a tunnel or a cell
with no capacity.
• Unlike standard TCP, which disconnects after a timeout and
assumes congestion, this approach uses the MAC layer to
detect interruptions early and inform TCP.
• TCP then freezes its state, including the congestion window
and timers.
• When connectivity is restored, TCP resumes data transfer
from where it left off, without data loss.
Advantages:
• Maintains and resumes TCP connections after long
interruptions.
• Works with encrypted data.
Disadvantages:
• Requires updates to both the mobile host and correspondent
host software.
• Depends on the MAC layer’s ability to predict interruptions.
• May not work with encryption schemes that require time-
dependent resynchronization.
Selective Retransmission
• Selective Retransmission is a TCP feature that improves
efficiency by only retransmitting the packets that are lost,
instead of all packets after a loss.

• Traditional TCP: Acknowledges packets cumulatively.

• If one packet is lost, TCP has to resend all subsequent
packets, wasting bandwidth.

• Selective Retransmission: The receiver acknowledges each

packet individually.
• This way, the sender knows exactly which packet is missing
and only resends that specific packet.
Advantages:
• Bandwidth Efficiency: Reduces the amount of data that needs
to be retransmitted, saving bandwidth.
• Useful in All Networks: Especially benefits slow or unreliable
connections, but improves performance in any network.

Disadvantages:
• Complex Receiver Software: Requires more memory and
processing on the receiver side to manage and reorder packets.
Transaction-Oriented TCP (T/TCP)
• Transaction-Oriented TCP (T/TCP) is designed to make
TCP more efficient for applications that send small,
occasional messages.

Traditional TCP involves several steps for each message:

• Connection Setup: 3 packets
• Data Transfer: 1 packet
• Connection Teardown: 3 packets

This adds up to 7 packets for a single data exchange, which is

inefficient for short transactions.
• T/TCP (RFC 1644) addresses this by combining the setup
and teardown of connections with data packets, potentially
reducing the total number of packets to just 2.
• This makes it more efficient for applications that only need
to send small amounts of data infrequently.
Advantages:
• Reduced Overhead: Decreases the number of packets
needed for establishing and closing connections.
• Efficiency: Useful for applications requiring minimal data
exchange and high latency networks.

Disadvantages:
• Compatibility Issues: Requires changes to both the mobile
host and all correspondent hosts.
• Security Concerns: T/TCP has several security issues.
• Mobility Visibility: It does not fully address mobility issues.