TCP/IP Over Lossy Links -

TCP SACK without Congestion Control

1.The History of TCP

2.Current TCP Congestion Control

3.Design Ideas: no congestion control at all

4.Measurement Results

5.Future TCP Congestion Control?

6.Conclusion

Organization

„Old Tahoe“: slow start and congestion avoidance after a lost packet, wait for a timeout, then perform slowstart to recover

„Tahoe“ (`88):also contains fast recovery. After the reception of a triple duplicate ACKs, performs fast retransmit followed by a slowstart [Jaco88].

„Reno“ (`90):fast retransmit extended by fast recovery.

1. The History of TCP (incomplete)

[Jaco88] Van Jacobson, “Congestion Avoidance and Control”, ACM SIGCOMM '88[Jaco90] Van Jacobson, “Modified TCP Congestion Avoidance Algorithm”, email to end2end- interest@ISI.EDU, April 1990

in case multiple packets are lost from a window of data, very likely, TCP „Reno“ has to wait for a timeout followed by slow start to recover [Jaco90].

But

Vegas (`94, never actually implemented): modified congestion behavior. By measuring the output queue size, equilibrium is detemined (Little‘s Law of queuing theory) [BraMalPet94].

„New Reno“ (~`94): small optimization of TCP Reno, uses partial ACKs as an indication that the following packet got lost and immediately retransmits it without leaving fast recovery. [Hoe96].

TCP SACK(~`95): added the SACK option within ACKs. Allows receiver to specify the range of packets that were received out of order. [MatmahFlRo96].

[BraMalPet94] Lawrence S. Brakmo, Sean W. O'Malley, Larry L. Peterson, „TCP Vegas: New Techniques for Congestion Detection and Avoidance“, Sigcomm 1994[MatMahFlRo96] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, „TCP Selective Acknowledgement Options“, RFC 2018, April 1996[Hoe96] Janey C. Hoe, “Improving the start-up behavior of a Congestion Control Scheme for TCP, Sigcomm

1996

1. The History of TCP (incomplete)

In contrast to all previous flavors, more than one packet per RTT during fast recovery can be send

FACK (`96): In fast recovery, congestion window is fixed. TCP FACK uses „pipe“ variable to estimate the data in the network by taking the transmitted out-of-order segments into account, thus preventing premature timeouts [MatMah96].

[MatMah96] M. Mathis and J. Mahdavi, "Forward acknowledgement: Refining TCP congestion control“, ACM Computer Communication Review, Oct 1996.

1. The History of TCP (incomplete)

Slow Start: with every received ACK, double the number of packets sent.

Slow start adds a window to the sender's TCP: the congestion window, called cwnd as well as a variable called ssthres

2. Congestion Control: slow start

The congestion window is flow control imposed by the sender.It is based on the sender's educated guess of perceived network congestion. Congestion Control assumes that packets are only lost due to overfull queues.

Figure taken from [Jaco88]

exponential growth ofthe Congestion Windowup to ssthres, thenlinear growth

2. Congestion Control: congestion avoidance in TCP Reno

SStime

window

CA

SS: Slow StartCA: Congestion Avoidance Fast retransmission/fast recovery

2. TCP Congestion Control

TCP send rate is determined by three windows:

win=min(snd_cwnd,snd_wnd,snd_bwnd)

Congestion window

assumed bottlenecks: queue sizes in the network

Advertised window

assumed bottleneck: receiver’s buffer

Bandwidth window, “ACK clock”

assumed bottleneck: link capacity

3. Design Idea: no congestion control at all

In contrast to UDP, the protocol will still guarantee for in-order delivery and will adopt to the link capacity.

UDP?

Without SACK, this flavor of TCP will perform poorly (waste of bandwidth on duplicate ACKs that can lead to timeouts)

The sending rate is given by:

win=min(snd_cwnd,snd_wnd,snd_bwnd) Now:

win=min(snd_bwnd,snd_wnd)

SACK gives us control over the now “static” window

4. Measurements: the emulation environment

[emu] www.emulab.net

Senderwith modified TCP

Router 2

Receiverwith modified TCP

Senderwith original TCP

Receiverwith original TCP

Router 1100Mbps 100Mbps

10Mbps

delays for each link:10ms, 100ms and 400ms

resulting RTTs: 60ms, 600ms, 2.4 s

Node 1

Node 0, „base“ Node 3, „base“

Node 2

On all links, delay=10ms.Loss rates varied from p=0, p=0.001, p=0.01, p=0.1 to p=0.2Packt loss events are uniformly distributed.

Experiment has been set up in the emulab environment [emu].

4. Measurements: collection of Data

1. Initialize tcpdump on the to-be-observed node:

sudo tcpdump -c num -w file -i if &

2. Start ttcp on the receiving node

ttcp -r -s src

3. Start ttcp on the sending node

ttcp -t -s -n num dst

num - number of packets to be capturedfile - name of the dump fileif - interface to be listened tosrc - IP address of the sending nodedst - IP address of the receiving node

[eth] www.etheral.com, packet sniffer and analyzer

Traces have been analyzed off-line with ethereal [eth].

4. Measurements: SACKBASE, lossless link

tcpdump started on the sender, zoomed into connection „set up“ phase

advertised receiverWindow+Seq # of last ACK

seq # ACKs received

size of the send window in caseof a link bottleneck: bandwidth-delay product

4. Measurements: SACKEXP, lossless link

tcpdump started on the sender, zoomed into connection „set up“ phase

4. Definitions: Throughput and Goodput

Goodput:The number of packets per unit of timeforwarded from sender to receiver, minus any packets lost or retransmitted, minus ACKs.

Throughput:The number of packets per unit of timetransmitted between sender and receiver.

4. Measurements: summary of results, competing flows

[KemXinKas04] R. Kempter, B. Xin, S. Kumar Kasera, “Towards a Composable Transport Protocol: TCP without Congestion Control”, SIGCOMM 2004 poster presentation

4. Measurements: summary of results, competing flows

[KemXinKas04] R. Kempter, B. Xin, S. Kumar Kasera, “Towards a Composable Transport Protocol: TCP without Congestion Control”, SIGCOMM 2004 poster presentation

5. Future TCP Congestion Control? ECN bitAnother way to do congestion control: the ECN bit

[RamFloyd99] Ramakrishnan, K.K., and Floyd, S., A Proposal to add Explicit Congestion Notification (ECN) to IP. RFC 2481, January 1999

Instead of dropping packets, a router sends a TCP an explicit message stating that the network is becoming congested. The network determines an explicit rate for a sender [RamFloyd99].

1

IP Packet 0 IP Header

ECN bit

1 IP Header

TCP ACK

ECN Echo

Hop-by-Hop vs. End-to-end congestion control

6. Conclusion

• Ambiguity in packet loss current TCP congestion control low throughput over lossy links

• TCP w/o congestion control and without SACK is inefficient

• p=0.1%, SACKEXP = 91% of goodput of lossless link, TCP SACK = 65% (at identical efficiencies of 91%)

• As losses increase to 20%, SACKEXP = goodputs ~ 700% at similar efficiencies compared to TCP SACK

• Plan: investigate performance of SACKEXP with Congestion Control based on the ECN bit/ICMP Source Quench

• Performance of SACKEXP has to be compared to a TCP that can resort to a Link Layer retransmission scheme.

• What about TCP fairness????

Questions arewelcome!

THE END

[Jaco88] Van Jacobson, “Congestion Avoidance and Control”, ACM SIGCOMM '88[Jaco90] Van Jacobson, “Modified TCP Congestion Avoidance Algorithm”, email to end2end- interest@ISI.EDU, April 1990[BraMalPet94] Lawrence S. Brakmo, Sean W. O'Malley, Larry L. Peterson, „TCP Vegas: New Techniques for Congestion Detection and Avoidance“, Sigcomm 1994[MatMahFlRo96] M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, „TCP Selective Acknowledgement Options“, RFC 2018, April 1996[Hoe96] Janey C. Hoe, “Improving the start-up behavior of a Congestion Control Scheme for TCP, Sigcomm 1996[MatMah96] M. Mathis and J. Mahdavi, "Forward acknowledgement: Refining TCP congestion control“, ACM Computer Communication Review, Oct 1996.