Protocols for Improving Performance of TCP
over Wireless Links
Presented by
Syed Natif Nawaz
Joseph Toney
Introduction
 Advances in wireless communication and the decreasing size
and the cost of computing units have triggered an explosive
growth for the mobile computing market.
 A mobile host is connected to a base station through a low
bandwidth, high latency and error prone wireless channel.
 Base stations are connected to the rest of the network through
the high bandwidth, low latency and relatively error free wired
links.
 Bit Error Rate(BER) a wireless channel is 10-3 - 10-5, while that
of wired link is 10-8 - 10-10 or better
Contd...
 The transmission quality is further degraded by hand-off related
intermitted delay or loss of the connection as the mobile user
moves from the transmission range of one base station to that of
another.
 Design of TCP has to take into account the heterogeneous nature
of the network.
Necessity to improve the performance of TCP
in a wireless environment
 When used in wireless environment, the TCP developed for
fixed networks with relatively reliable links wrongly assumes
segment loss due to wireless link errors to be a sign of network
congestion and invokes congestion control mechanisms that
curb the flow of segments on that connection, because of which
the throughput is affected badly.
 For a better throughput, a segment loss due to wireless link
errors must be detected and retransmitted as quickly as possible.
Protocols
 Many approaches have been proposed to improve the
performance of TCP over networks with wireless links. These
have been broadly categorized as Transport Level Proposals and
Link Level Proposals.
 Various Protocols considered

Transport level proposals
• Indirect TCP (I-TCP)
• MTCP
• ESBN
• Fast Retransmission
contd ...

Link Level proposals
• Snoop Scheme
• CSDP
 TCP unaware technique

Delayed DUPACK’S scheme
Indirect TCP




I-TCP overcomes by utilizing the resources of MSR’s to
provide transport layer communication between mobile hosts
and fixed hosts.
I-TCP is fully compatible with TCP/IP on fixed network
Any interaction between MH & FH are split into two
separate interactions.
Using I-TCP MH sends a request to current MSR to
establish a connection.
 It uses special I-TCP calls, instead of regular system calls.
 If the MF switches cells during the life time of an I-TCP
connection the center point of the connection moves to new
MSR.
 When the MH switches cells, the states associated with two
sockets is handed over.
 The FH is completely unaware of the indirection.
 The connection is not re-established at the new MSR.
ITCP Architecture
Limitations of I-TCP



I-TCP violates the semantics of end to end reliability.
Application re-linking.
Software overhead.
Snoop Protocol
 Proposed by Elan Amir, Hari Balakrishnan, Srinivasan Seshnan
& Randy H. Katz - University of California, Berkeley.
 Improves on Split connection approach by retaining the end to
end reliability semantics.
 Wireless errors are handled by local retransmission at the base
station.
 Reduces interference between TCP retransmission and link level
retransmission.
 The probability of time out at the TCP sender is assumed small
 Snoop uses features like link level retransmission time out.
Snoop Architecture
Snoop functions
 Snoop_output()

Types of packets from the fixed host
• A new packet in the normal TCP sequence.
• An out of sequence packet that has been cached earlier.
• An out of sequence packet that has not been cached earlier.
State machine for snoop_output
Out of Sequence.
Already in cache
IDLE
In sequence/ Send pkt
Recv pkt/In cache
SEND
 Snoop_input()

Types of ACK’s from the Mobile host
•
•
•
•
A spurious ACK
A DUPACK
A New ACK
Time Out
State machine for snoop_input
Recv Dupack
NEWACK
New Ack
IDLE
SPURIOUS ACK
TIMEOUT
DUPACK
DELAYED DUPACK’S SCHEME
 It is preferred over snoop when encryption is used.
 It is a TCP unaware scheme.
 The base station implements a link level retransmission scheme
for lost packets .
 Link level ack’s are used to trigger link level retransmission
 TCP receiver reduces interference between TCP and link level
retransmission by delaying third and subsequent dupacks for
interval d.
 TCP receiver responds to the first two OOO packets by sending
dupacks immediately.
 Dupacks for further consecutive packets are delayed for
duration d
Delayed Dupack Scheme
 Two types of packet losses and the selection of d


A Packet is lost due to transmission errors on the wireless
link.
A packet is lost due to congestion before reaching the
wireless link.
Performance results of various schemes
WTCP
 Proposed by Karunaharan Ratnam & Ibrahim Matta Northeastern University.
 Main feature:


Requires no modification to the TCP code that runs in Mobile
host/Fixed Host. This is an important advantage because in a fixed
network, modifying the TCP installations in a millions of existing
hosts is not possible.
TCP end to end semantics is maintained.
WTCP Architecture
 A WTCP buffer is maintained in the base station.
 TCP segments that arrive for a mobile host are first queued in
the WTCP buffer.
 If the segment is the next segment expected, then it is stored in
the WTCP buffer along with it’s arrival time.
 The sequence number for the next segment expected from the
fixed host is increased by the number of bytes received.
 If the segment received has a larger sequence number than what
is expected, the segment is buffered.
 The received segment is discarded if it’s sequence number is
smaller than what is expected.
 WTCP maintains the state information for the wireless
connection such as transmission window, sequence number of
last acknowledgement received from mobile host and the
sequence number of the last segment that was sent to the mobile
host.
 Each time, WTCP locally retransmits, the time stamp is
incremented by the amount of time, the segment has spent in the
WTCP buffer.
 WTCP acknowledges a segment sent to the fixed host only after
the mobile host actually receives and acknowledges the
segment.
 The round trip time seen by the source is the actual round trip
time taken by a segment to reach and return from a mobile host.
Performance of WTCP during retransmissions
 Based on duplicated acknowledgment or time out, the base
station locally retransmits lost segments. In case of time -outs
transmission window size is reduced to one segment and so
potentially wasteful wireless transmission is avoided and
interference with other channels is reduced.
 In the regular TCP, the transmission window is opened, based on
whether the connection is in slow start or a congestion
avoidance phase.
 WTCP however, opens the wireless transmission completely
assuming that an acknowledgement indicates that wireless link
is in a good state.
Contd ...
 When the base station detects a lost segment, it retransmits,
(without invoking congestion control, unlike normal TCP).
However, until the mobile host receives the lost segment, the
reception of each our of order segment will generate a duplicate
acknowledgement, the number of which can be determined by
the base station and it ceases to retransmit.
Effect of Local Retransmissions on RTT
Estimation
 Inaccurate RTT estimation can cause unnecessary
retransmissions by triggering time outs.
 Sender writes the current time stamp of the segment that
generated it and the receiver echoes it back, without modifying
it.
 However when an intermediate host buffers the segments and
locally retransmits them, RTT computation can be affected
 If a segment gets retransmitted at the base station, RTT value
computed at the source for this segment will overshoot by the
residence time in the WTCP buffer.
 For hiding the LRT, there are two approaches.

Base station subtracts the actual LRT by modifying the time stamp
field of each acknowledgement forwarded to the source.
• But this is not feasible!, as the clock granularity of source and base
station may not be identical.

After retransmission from BS, time stamp of packets transmitted to
the MH
• replaced by time stamp of most recent packet received from source
after latest retransmission.
• Invalidated, if no packet is received from source after retransmission.
Simulation Model
 Tool Used: ns1.2a3 from LBL
 Simulated Network
10 Mbps
800 kbps
10 ms
100 ms
Fixed Host
(Source)
Base Station
Mobile Host
Simulation parameters







Parameter
Value
Max window size
Buffer Size at BS
Segment Size(including header)
BER in good state
BER in bad state
Mean good period
10 Kb
20 Kb
1Kb
10-2
10-6
1 sec
Simulation Results
Effect of hiding LRT
Explicit Bad State Notification
 General Scenario:
In most bad periods, local recovery at the base station
prevents packet losses on the wireless link. However the
source may still time out waiting for acknowledgement of
packets that have already been sent.
 Explicit Feedback


Explicit feedback from the base station can completely eliminate
the possibility of time outs occuring at the source, while the
wireless link is in a bad state.
The use of explicit feedback for congestion control in the internet
is known to work well in the form of ICMP source quench
messages.





So when a wireless links enters a bad state, a base station
configured to function as a gateway supporting ICMP
messages, sends a source quench message to the source and
is expected to prevent unnecessary time outs.
This should improve the goodput as well as throughput, for
the connection but the results have been shown to be
otherwise.
A source quench message from the base station will not be
able to prevent time out of packets that are already on the
network. It will of course stem the flow of any further
packets and reduce the probability of timeouts.
An important observation here is the timeout of packets that
have already been sent to the base station have too be
avoided.
A mechanism to thwart the source's attempts to invoke
congestion control in response to delays on the wireless
links.
EBSN
 To avoid source time
out, the BS cannot send
acknowledgements of the packets not sent to the MH, back to
the source.
 Neither can acknowledgements of previously acknowledged
packets be sent, because they will be interpreted as duplicate
acknowledgements.
 Solution is to send a EBSN.
 EBSN causes the previous time out to be canceled and a new
time out put in place, based on the existing estimate of round
trip time and variance.
 The correct time out a value at the source is readjusted from the
MH following the receipt of the first new acknowledgement
from the MH, following any EBSN message received.
 A new acknowledgement is an indicator that the link has finally
arrived at a good state and this will ensure steady inflow of
acknowledgements.
Advantages & Disadvantages of EBSN
 Advantages:


Source time out is prevented during local recovery. Improves
goodput and throughput of the connection.
Effect of clock on performance is reduced.
 Disadvantages

Requires modification of TCP code at source.
Other schemes
 MTCP


Similar to I-TCP except that the last TCP byte of data is
acknowledged to the source only after it is received by the mobile
host.
Source falsely believes that every byte of data except the last byte
is received by the receiver, it can take remedial measures based on
whether the last byte is received or not.
 Fast Retransmission



Addresses the issue of mobile host hand off.
During mobile host hand off TCP segments can be lost or delayed.
The Mobile host has to unnecessarily wait for a long duration for
the source to retransmit.


In fast retransmit, the IP in mobile host triggers TCP to generate a
certain number of duplicate acknowledgements, causing the source
to retransmit the lost segment without waiting for the time out
period to expire.
This requires modification to the TCP code at the mobile host.
 Channel State Dependent Packet Scheduling



Pure link layer approach.
FIFO scheduling of data in links with bursty losses yields poor
utilization of the channel.
A set of algorithms are provided to enhance the channel utilization
based on the arrival time of the packet, whether it is a new packet
or retransmission and the length of each output queue.
Conclusion
 Various Link Layer and Transport Level proposals for
improving the performance of TCP over Wireless Links have
been studied.
 I-TCP
 Snoop Scheme
 WTCP
 EBSN
 Delayed Dupack Schemes.
References
 Karunaharan Ratnam and Ibrahim Matta. WTCP:An Efficient Transmission
Control Protocol for Networks with Wireless Links. Technical Report NUCCS-97-11, Northeastern University. July 1997.
 Nitin Vaidya, Miten Mehta, Charles Perkins and Gabriel Montenegro. Delayed
Duplicate Acknowledgements: A TCP-Unaware Approach to Improve
Performance of TCP over Wireless. Technical Report 99-003, February 1999.
 Ajay Bakre and B.R. Badrinath. I-TCP: Indirect TCP for Mobile Hosts.DCSTR-314, Rutgers University, October 1994.
 Elan Amir, Hari Balakrishnan, Srinivasan Seshan and Randy H. Katz. Efficient
TCP over Wireless Links.
 Bikram S. Bakshi, P. Krishna, N.H. Vaidya and D.K. Pradhan. Improving
Performance of TCP over Wireless Networks. Texas A & M University.