Mobile Transport Layer March 2nd

Wireless Networks and Mobile Computing (CSI 5169)

Breeson Francis

March 2nd 2011

Mobile Transport Layer

Outline

1. Introduction to TCP

2. TCP Mechanisms

3. Problems in TCP

4. Classical Approaches

5. Latest Techniques

6. References


Introduction to TCP• TCP is a connection oriented protocol

• 3 way Hand-shake • TCP is a reliable protocol

• achieved by means of retransmissions• TCP enables data to be received in an ordered way• End-to-end semantics

• Acknowledgements sent to TCP sender to confirm delivery of data received by TCP receiver

• Ack for data sent only after data has reached receiver

• TCP detects data duplication• TCP provides flow and congestion control


Introduction to TCP (ctnd)TCP Client/Server Connection

Connectionsetup

Datatransmission

TCP SYN

TCP SYN/ACK

TCP ACK

HTTP request

HTTP response

HTTP responseFIN

ACK

FIN

ACK

ConnectionTermination

Client Server


Sent, not acked

TCP Mechanisms

• Sliding Window

• Slow Start• Is triggered at the beginning of connection or

when a timeout(RTO) occurs • Sender calculates congestion window for

receiver• Congestion window (cwnd) is set to 1 initially• cwnd is increased exponentially for every ACK till

ssthresh• Slow start ends when cwnd reaches ssthresh,

congestion avoidance then onwards

4 5 6 7 8 9 10 11 12 13 153 14

Offered Window

Sent and ack received

Usable window

16 17 18 2019

Can’t send until window moves

21


TCP Mechanisms (ctnd)

• Congestion Avoidance• cwnd increased 1 segment for each RTT

(additive increase)• Multiplicative decrease, cwnd set to 1/2 of its

value when congestion loss occurs• Sender can send up to minimum of advertised

window and cwnd

0

2

4

6

8

10

12

14

0 1 2 3 4 5 6 7 8

Time (round trips)

Con

gest

ion

Win

do

w s

ize

(seg

men

ts)

Slow start

Congestionavoidance

Slow start threshold

ssthresh = maximum of { min (cwnd, receiver’s advertised window)/2 and 2 segment size }


TCP Mechanisms (ctnd)

• Fast Retransmission and Fast Recovery• Fast retransmit occurs when a packet is lost, but

latter packets get through• When 3 or more dupacks are received, send the

missing segment immediately• Start congestion avoidance(Fast Recovery)• The receiver got all packets up to the lost packet

and is actually receiving packets• cwnd not reduced to 1 MSS

cwnd = ssthresh = current_cwnd / 2 + dup_acks * MSS


Problems in Wireless Networks• Random Losses

• Channel fading, high error, interfernce• Higher Link Latencies

• Difficult to estimate time-out, RTT, bandwidth limitation• Hand-Offs

• Mobile users switch base stations• Multipath Routing

• Multiple paths lead to significant amount of out-of-order packets, which in turn generates duplicate acks

• Asymmetric Channel Allocation• sending entity gets more channel time/bandwidth

compared to the receiving entity resulting in – queuing up of acknowledgements– larger RTTs – traffic becoming bursty in nature.


Problems in Wireless Networks

• Route Instability• Leads to OOO packets


Problems in Wireless Networks

• Network Partitioning• Exponential back off of TCP’s RTO mechanism• RTO doubled after every timeout• Periods of inactivity even when the network is

connected


Classical Approaches

• Split connection approach• TCP-Aware link layer (Snooping)• Feedback based mechanism



• Split Connection Approach• End-to-end TCP connection is broken into wired

part and wireless part• Connection between mobile host(MH) and fixed

host(FH) through base station(BS) is split into 2 TCP connections

FH-MH = FH-BS + BS-MHMobile

Host Access Point (Base

Station)

Wired Network

Wireless TCP

Standard TCP

Fixed Host



• Split Connection Approach• Split connection results in independent flow/error

control, packet size, timeouts at each part• Optimized TCP protocol used in the wireless

segment

FH MHBSwireless

physical

link

network

transport

application

physical

link

network

transport

application

physical

link

network

transport

application rxmt

TCP connection TCP connection


Classical Approaches• Split Connection Approach

• Buffering at base station• During hand-offs BSs should do state transfer

along with the buffers

Access Point (BS2)

Wired Network

Access Point (BS1)

State TransferMobile Host

Fixed Host


Classical Approaches• Split Connection Approach

• No changes to the fixed network• Transmission errors on the wireless link do not

propagate into the fixed network• Retransmission at wireless segment can be done

independent of wired segment, Fast

• Loss of end-to-end semantics, an acknowledgement to a sender does not any longer mean that the intended receiver really got the packet

• Higher latency due to buffering at base station• Increased hand-off latency• BS Failure results in loss of data• Buffers tend to get full due to slower wireless link



• Split Connection in multihop wireless network• Many short TCP connections between proxies

along the connection



• Split Connection in multihop wireless network• Proxies buffer packets from the previous proxy or

the source and acknowledges their receipt with Local Acknowledgements (LACKs)

• Any dropped packets are recovered from the most recent proxy but not from the source

• Enhance parallelism. Reduce bandwidth consumption on retransmission

• Optimal frequency of proxy placement is not clear


• TCP Aware Link Layer• Retains local recovery of Split Connection

approach• Improves on split connection

– end-to-end semantics retained– soft state at base station, instead of hard state

MHwireless

physical

link

network

transport

application

physical

link

network

transport

application

physical

link

network

transport

application

rxmt

TCP connection

BSFH




• TCP Aware Link Layer• Buffers data at BS for link layer retransmission till

ACK received• When dupacks received by BS from MH,

retransmit on wireless link• Hides wireless losses from sender• Prevents fast retransmit at sender TCP by

dropping dupacks at BS• Requires modification at BS only• Link layer at base station needs to be TCP-aware• Not useful if TCP headers are encrypted (IPsec)• Cannot be used if TCP data and TCP acks traverse

different paths (both do not go through the same base station)



• Feedback based scheme in multihop wireless network

• Intermediate MH detects mobility of next MH along the path to destination

• Triggers Route Failure Notification(RFN) to source• Each intermediate MH validates RFN and

propagates to the source • On receiving RFN, source

– Stops sending further packets– Freezes all its timers– Stores Window size and packets to be sent

S

AD

CB

RFNRFN




• Source remains in the snoozed state until it receives Route Re-establishment(RRN) message

• A RRN is generated either by the node which generated RFN or an intermediate node which learned a new route to destination

• Source starts from the frozen state rather than restarting

S

AD

CBRRN

FE




• TCP has to be modified• Requires support from intermediate nodes• Requires support from underlying routing protocol


Latest Techniques

• Modified Fast Retransmission• Modified Congestion Control Mechanism• Reduced Control Traffic


Latest Techniques

• Modified Fast Retransmission [1] • For detecting retransmission packet loss• Max_Seq be the highest sequence number of packets

sent by the sender • Dupack_Lost be the duplicate ack of the packet that

was lost. • The outstanding number of packets in the network

which were not acknowledged Net_Outstand = Max_Seq – Dupack_Lost

• The sender retransmits the packet only after receiving three duplicate acks

• Loss detection point Loss_Detect_Point = Net_Outstand – Three_Dupacks

• For every additional duplicate acknowledgement (Add_Dupacks) cwnd is increased by one mss and packet send if the new value of cwnd allows it.


Latest Techniques

• Modified Fast Retransmission [1] • If Add_Dupacks is greater than Loss_Detect_point,

then that indicates that a new packet sent has reached the destination, whereas the retransmitted packet is lost.

• The TCP sender can retransmit the lost packet without waiting for the retransmission timeout.

• Simple and efficient• Requires changes at sending side only


Latest Techniques

• Modifying Congestion Control [3] • Bandwidth-Delay product, product of Link Bandwidth

and Round Trip Time (RTT)• It gives a clear picture of the network load• Sender side continuously computes BandWidth

Estimate (BWE) from Acks received• Based on the packet loss indication ssthresh and

cwnd are set as follows: – If packet loss is detected by three duplicate ACKs then ssthresh = (BWE * RTTmin) / segment size

cwnd = ssthresh if (cwnd > ssthresh) – If packet loss is detected by timeout then cwnd = 1 ssthresh = max{(BWE*RTTmin) / segment size, 2}


Latest Techniques

• Modifying Congestion Control [3] • Instead of blindly reducing cwnd and ssthresh,

current network condition taken into consideration

• Changes at sender side only


Latest Techniques

• Reducing Control Traffic [5]

» To avoid hidden/exposed terminal problem 802.11 introduced RTS/CTS mechanism

» Considering basic rate of 1 Mbps for and data rate of 11 Mbps the overhead is 3SIFS+RTS+CTS+ACKmac = 414ms.

» Based on the above calculation time to send a 40 byte ACKtcp and 1040 byte data packets are 443 ms and 1170 ms

» Reduce ACKs! » The receiver waits for ACK delay

timeout to send the ACK leading to underutilization of the network.


Latest Techniques

• Reducing Control Traffic [5] • TCP sender to put the current value of cwnd in the

option field of TCP header• The receiver calculates the received-but-

unacknowledged packets • If equal to cwnd, it knows that the sender is

waiting for an ACK to proceed with sending the remaining packets

• Sends an ACK immediately • Self-triggered ACKs lessen the control traffic• Leads to increased TCP throughput performance


Scheme Sender Support Receiver Support Intermediate Node Support

Routing Protocol Support

Split Connection

No No Yes. Transport Level

No

TCP Aware Link Level Retransmission

No No Yes. Link Level No

Feedback Mechanism

Yes No Yes Yes

Modified Fast Retransmission

Yes No No No

Modified Congestion Control

Yes No No No

Reduced Control Traffic

Yes Yes No No

Conclusions


Questions1. At time t, a TCP connection has a congestion window of 4000

bytes. The maximum segment size used by the connection is 1000 bytes. What is the congestion window after it sends out 4 packets and receives acks for all of them in one time window? Suppose there is one ack per packet.

(a) If the connection is in slow-start?

Ans. 8000 bytes. In slow start, the sender increases its window for each ack successfully received, in this case 4 acks are received.Therefore, congestion window = 4000 + 4 * 1000 = 8000 bytes.

(b) If the connection is in congestion avoidance (linear mode)? Ans. 5000 bytes. The sender increases its window by one

segment each time window, in this its once.Therefore, congestion window = 4000 + 1 * 1000 = 5000 bytes.


Questions

2. A TCP machine is sending window of 10000 bytes over a 100 Mbps channel that has a 10 ms one-way delay. What is the maximum throughput achievable? What is the line efficiency?

Ans. One window can be sent every 20 msec. This gives 50 windows/sec. Throughput achieved = 50 * 10000 bytes / sec = 500000 bytes / sec = 4 Mbps Line efficieny = 4 Mbps / 100 Mbps = 4 percent


Questions

3. While it takes three segments to establish a TCP connection, it takes four to terminate a connection. Why?

Ans. This is caused by TCP's half-close. Since a TCP connection is full-duplex (that is, data can be flowing in each direction independent of the other direction), each direction must be shut down independently. The rule is that either end can send a FIN when it is done sending data. When a TCP receives a FIN, it must notify the application that the other end has terminated that direction of data flow. The sending of a FIN is normally the result of the application issuing a close.


References[1] S. Prasanthi, S. Chung, C. Ahn, "An Enhanced TCP Mechanism for

Detecting and Differentiating the Loss of Retransmissions over Wireless Networks", International Conference on Advanced Information Networking and Applications, 2011.

[2] S. Prasanthi, S. Chung, “An Efficient Algorithm for the Performance of TCP over Multi-hop Wireless Mesh Networks”, Seventh International Conference on Information Technology 2010.

[3] L. Yongmei, J. Zhigang, Z. Ximan, “A New Protocol to Improve Wireless TCP Performance and Its Implementation”, 5th International Conference on Wireless Communications, Networking and Mobile Computing, 2009.

[4] I.A. Rai, T. Hellen, "On improving the TCP performance in asymmetric wireless mesh networks", International Conference on Communications, Computing and Control Applications (CCCA), 2011.

[5] B. Chen, I. Marsic, R. Miller, “Issues and Improvements in TCP Performance over Multihop Wireless Networks”, IEEE Sarnoff Symposium, 2008.

[6] T. Shikama, "Mitigation of Bursty Packets by a TCP Proxy improving TCP Performance in a Wired and Wireless Network", IEEE Globecom 2010 Workshop on Complex Communication Networks.

[7] Nitin H. Vaidya, “TCP for Wireless and Mobile Hosts”.[8] Syed Natif Nawaz, Joseph Toney, “Protocols for Improving Performance of TCP over Wireless Links”.[9] Baruch Awerbuch, Dr. Amitabh Mishra, “Transport Layer for Mobile Ad hoc Networks”. Mobile Transport Layer

Questions


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Mobile Transport Layer March 2nd

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