ECE 741 Wireless Networks
Term Paper
Spring 2009
M-TCP over wireless networks
Nikhil Bonthapally
G00398744
George Mason University
Fairfax, VA
M-TCP over Wireless Networks
Nikhil Bonthapally
Abstract
Transmission Control Protocol (TCP) is the de facto network transport protocol,
predecessor of Network Control Protocol (NCP) gives us a fair advantage of reliable data
delivery, flow-control associated with both the transmitter and receiver. TCP’s
performance has been improved over the years on wireline networks. General cause of
congestion and packet losses are due to network congestion in wireline networks. The
primary reason for packet losses in wireless networks being high bit error rate (BER) and
hand-offs. Traditional TCP does not account to these and thus the performance is
degraded. I propose a solution to the above problem by introducing M-TCP [1](Kevin
Brown and Suresh Singh).
Introduction
Transmission Control Protocol/ Internet Protocol originated out of the investigative
research into networking protocols that the Networks Department of Defense (DoD)
initiated in 1969. TCP is a flow controlled, connection-oriented, end-to-end reliable
protocol designed to fit into a layered hierarchy of protocols supporting multi-network
applications. TCP provides for reliable inter-process communications between pairs of
processes in host computers attached to distinct but interconnected computer
communication [5].
The current cellular network infrastructure uses the concept of dividing the geographical
area into regions called cells. Each cell has its own base station that provides an end-toend connection between the users. Channel reuse, frequency reuse, signal to interference
ratio (SIR) and many such performance factors would require to have a small cell size.
Small cell latencies, frequent disconnections during handoff are the results of such small
cell size. And worst of all, the serial time outs at TCP sender is more because of these
reasons.
TCP, already said provides a reliable data transfer by the process of acknowledging the
packets received back by the sender. TCP sender keeps a track of the packets and
acknowledgement and when the acknowledgement is not received before the Timeout
expires, it retransmits the packets. This multiple serial timeout increases exponentially
after each failure [4]. In a wireless environment, this serial timeout feature proves to be
more fatal to the performance of TCP than losses due to bit errors or small congestion
windows. Continuous timeouts could leave a mobile user disconnected for as long as one
minute.
The modified TCP implementation proposes an end-to-end retransmission scheme that
can reduce the pauses to a low level and make it more suitable for human interaction.
Transmission Control Protocol (TCP)
Transmission Control Protocol relies on the implicit information it receives from the
network and remote host. It makes a well-informed guess about the state of the network
and trusts the information it receives from the remote host. TCP being a reliable protocol
has the method of assigning timers to each packet it sends out, for an acknowledgement
to be returned. This way, TCP keeps track of packets successfully transmitted. This timer
is called Retransmission Timer.
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When an acknowledgement is received before the timer expires, the timer is reset without
any consequence but if no acknowledgement is received, sender would retransmit the
segment and double the retransmission timer value for each consecutive timeout up to a
maximum of about 64 seconds [4]. Retransmission time and timeout basically depend on
the Round Trip Time of the packet in a network. The total time taken by the packet to
reach the receiver and the acknowledgement packet to arrive at the sender is defined as
the round trip time.
In order to control the timeout, sliding window protocol is used to acknowledge a
window of packets instead of acknowledging each packet. As data is sent by the sender
and then acknowledged by the receiver, the window slides forward to cover more data in
the byte stream. The sender congestion window should be made equal to the
corresponding receiver window. Another timer called the persistence timer is used when
the receiver sends an acknowledgement with a window size of 0. It means, the receiver
telling the sender to wait, and then sender goes into persistence mode and sets its
persistence timer. After the expiration of the timer, the sender will send a one-byte
segment as a probe to the receiver. The response from the receiver to the probe gives the
window size. If it is still zero, the persistence timer is set again and the cycle repeats. If a
non-zero number is received, sender will go out of persistence mode and new data can be
sent. In a general scenario, a sender runs exponential back off algorithm if it does not
receive any acknowledgements for round trip time. This way, the sender increases the
back off time, thereby delaying the next transmission. The persistence timer disables the
above algorithm to be run on the sender. In wireless networks, this acts as an advantage
and we use it to increase the efficiency of data traffic on wireless networks.
M-TCP
M-TCP, modified TCP is designed to suit wireless connections. Cellular
communications field is more suitable for M-TCP deployment. Mobile users connect to
the Internet via cellular network. A geographic unit called cell has base stations to
provide connection end-points for mobile hosts. In order to provide high-bandwidth
wireless connections, cell sizes will have to be kept small which in turn causes cell
latency. Cell latency is the cause of frequent disconnections as a user roams among cells.
This causes serial timeouts at the TCP sender. And because the sender doubles the
retransmission timer with each unsuccessful retransmission, even after the mobile is
reconnected it is feasible that in the protocol implementation, no data could be
transmitted for as long as up to one minute.
M-TCP Working
Network architecture for M-TCP is shown in Figure-1. As it shows, it has a three level
hierarchy. Mobile hosts (MH) are at the lowest level. They communicate with other
mobiles within the cell with the help of Mobile Support Stations (MSS). A Supervisor
Host (SH) is assigned to a group of MSSs [1]. Supervisor Hosts are connected to the
wireline network. SHs are responsible for handling most routing and other protocol
details for the mobile users.
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Figure-1
M-TCP forces the sender to be in persist state in the case of any disconnection. While in
this state, the sender will not suffer any retransmission timeout i.e., it will not run the
exponential back off algorithm and this would preserve the current size of the congestion
window. And when the connection recovers upon the receiving a notification from MTCP, the sender will be able to transmit at full speed.
A simplified model of the network is shown below in Figure 2. The TCP implementation
is split into two at the Super Host (SH). SH-TCP receives the data packets transmitted by
the sender (FH) in the wireline network and passes these packets to the M-TCP client for
delivery to MH. Acknowledgement packets received by the M-TCP at the SH are
forwarded to SH-TCP for delivery to the TCP sender. When SH-TCP receives a packet
from a TCP sender, it passes the packet to the M-TCP client, and acknowledges packet
when its ACK is received from the MH. However, the protocol should make sure that the
sender does not trigger congestion control when the MH is temporarily disconnected
(unavailable) or a packet is lost between SH & MH.
Figure-2
As we know, only an ACK packet with cwnd equals to zero can force the sender into
persist state, the last ACK number has to be kept in memory in order to be able to create
such a new ACK packet. When such situation arises (retransmission time out occurs),
SH-TCP creates and sends this new ACK packet to FH. With this, the FH is forced into
persist state.
In this state, the FH will not run the back-off algorithm and hence will not suffer
retransmission timeout. It will not close its congestion window and remain in its state
until it hears back from the MH.
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SH keeps on checking for the ACK packets to determine if MH is reconnected.
A reconnection ACK is sent to SH, after MH reconnects. Upon receiving this, the SH will
retransmit all the packets that have not been acknowledged by MH. The newly received
ACK packets will be forwarded to FH via SH-TCP. FH exits the persist state and now
resumes its transmission. Since the sender never times out, it never performs congestion
control. And thus, the sender can resume transmission at the rate before the packet loss
occurred.
High BER is not taken into consideration by M-TCP protocol because it assumes, mobile
environments with good wireless link layer protocol will ensure small BER at the TCP
layer. However, with M-TCP, even in mobile environment with high BER, the sender can
still leave persist state quickly and resume regular data transmissions.
OPNET Implementation
OPNET solutions model communications devices, protocols, technologies, and
architectures, and simulate their performance in a dynamic virtual network environment.
Integrated code debugging and data analysis features facilitate the design process. I used
standard WLAN Ethernet Router model (wlan_ethernet_router) to model the SH, WLAN
Workstation (wlan_wkstn_adv) for MH, and Ethernet Server (Ethernet_server_adv) for
FH. These nodes are chosen from two standard lists of OPNET models: Wireless LAN
(WLAN) and Ethernet.
Ethernet Server is a model with server applications running over TCP/IP and UDP/IP. It
has an interface to one Ethernet connection at 10 Mbps, 100 Mbps, or 1,000 Mbps. These
meet our requirements to have a TCP connection to SH via Ethernet. WLAN Workstation
is chosen for the mobile host for similar reasons. It represents a workstation with
applications running over TCP/IP and UDP/IP. It supports one underlying WLAN
connection at 1 Mbps, 5.5 Mbps, or 11 Mbps. Lastly, WLAN Ethernet Router was used
to establish connection between FH and MH because it has the capability of connecting
to both Ethernet (one Ethernet port available) and wireless LAN (one wireless LAN port
available).
M-TCP Scenario
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One of the major advantages of using M-TCP is that we do not have to modify the
existing internet components i.e., the wired LAN domain including the fixed host does
not need any modifications. TCP on the mobile host is modified into M-TCP client.
Supervisor Host needs most of the changes as the original TCP is modified to two layers,
SH-TCP and M-TCP. The SH-TCP on supervisor host is used to make the
communication between the supervisor host and the wired lab, the fixed host in the
Ethernet. The M-TCP on supervisor host is used to make communication between the
supervisor host and the wireless LAN, the mobile host in the wireless LAN.
M-TCP Node Models: Mobile Host
Shown below in Figure-3 is the MH node model for M-TCP. Process ip_encap is
modified to simulate MH’s temporary disconnection behavior. Packets handled by this
process are dropped periodically. This process is responsible for de-capsulation of IP
packets sent to higher layer and en-capsulation of packets into IP packets sent to the
lower layer.
Figure-3: Process Model of MH
Four configurable attributes are implemented to configure the m_ip_encap process
module:
BrokenEnable: If set to 1, periodical disconnection is enabled.
CycleTime: Indicates period (in seconds) between two disconnections.
BrokenStartTime: The start time of disconnection in each disconnection cycle.
BrokenEndTime: The end time of disconnection in each disconnection cycle.
Supervisor Host
Shown below in Figure-4 is the SH node model for M-TCP. Modifications are made in
processes ip and ip_encap. A new process model m_tcp is created to account for the
communication between SH and MH.
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Figure-4: Process model of SH
Simulation
Three scenarios are used and simulated to understand and compare the congestion
window size and goodput. FTP application is deployed.
Scenario 1. Regular TCP without disconnections: Ideal TCP conditions. Serves as base
for comparison.
Scenario 2. Regular TCP with disconnections: Disconnections are scheduled every 5
min. Each disconnection time is 30 sec at the end of each 5-min disconnection period.
Scenario 3. M-TCP with disconnections: M-TCP deployed in SH and MH.
Disconnections occur for 30 sec for every 5 minutes.
Simulation Results
Congestion Window Size
The figure below shows the similarity between regular TCP and M-TCP in their
congestion window size. In TCP with disconnections, the congestion window drastically
decreases with packet drops. M-TCP helps prevent congestion window from decreasing
during disconnections by forcing the sender into persist state.
Congestion Window Size Comparison
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Nikhil Bonthapally
Goodput
In a network with disconnections, M-TCP has a higher goodput than regular TCP. The
below graph is for a disconnection period of 30 seconds and yet the difference is visible.
In mobile network environment, the handoff time may vary from 10 seconds to a few
minutes. Disconnections are caused not only by handoffs but also because of the
retransmission timeout. The longer the disconnection is, the larger number of packets will
be lost and the longer the retransmission timeout will last.
Goodput Graph Comparison
Conclusion
In the present wireless network environment frequent disconnections, caused by signal
fading, hand-offs and BER are responsible for packet drops and the execution of
exponential back-off algorithm at the sender. This algorithm affects the throughput of the
system. M-TCP handles the frequent disconnections problem in wireless networks by
forcing the sender into persist state, which preserves the congestion window size. This
way, the algorithm is never executed and the congestion window, never reduced. M-TCP
is a solution to the above problems faced using regular TCP. M-TCP performance is
equivalent to that of regular TCP in wireline network and it actually outperforms the
regular TCP with disconnections in wireless networks.
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References
[1] W. G. Zeng, M. Zhan, Z. Li, and Lj. Trajkovic, ``Performance evaluation of M-TCP
over wireless links with periodic disconnections,'' OPNETWORK 2003, Washington,
DC, Aug. 2003
[2] K. Brown and S. Singh, “M-TCP: TCP for mobile cellular networks,” ACM
SIGCOMM Computer Communication Review, vol. 27, no. 5, pp. 19-42, October 1997.
[3] Improving the Performance of Reliable Transport Protocols in Mobile Computing
Environments Raman Caceres, Member, IEEE, and Liviu Iftode, IEEE JOURNAL ON
SELECTED AREAS IN COMMUNICATIONS. VOL. 13. NO. 5. JUNE 1995
[4] W. Richard Stevens. TCP/IP Illustrated, Volume 1: The Protocols.
Addison Wesley, ISBN: 0-201-63346-9. January 1994.
[5] Study of Time Based TCP in Synchronous Loss Environment
Nikhil Bonthapally, Shriraj Iyer, Students, MS EE, George Mason University, VA
[6] OPNET documentation V.8.0.B, OPNET Technologies Inc
Washington DC.
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