New Reno Sack

8/11/2019 New Reno Sack

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1Spring Semester 2009, Dept. of Computer Science, Technion

Internet Networking

recitation #9

TCP New Reno Vs. Reno


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Internet Networking

Introduction Packet Loss Management

TCP Reno (RFC 2581) can manage a loss of at most onepacket from a single window of data

TCP NewReno (RFC 2582) can manage a loss of morethan one packet without changing the TCP message

structure

TCP SACK (RFC 2018) enables to cope with a loss of morethan one packet by changing message structure (using

TCP options)


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Internet Networking

012

Ack(1)

3456

Ack(1)Ack(1)Ack(1)

78

Initial state

cwnd=7

Slow start

cwnd=8

Fast Retransmit

cwnd=8/2+3=7

ssthresh=8/2=4

=> Fast Recovery

Ack(1)Ack(1)

Ack(1)

Ack(1)

Ack(9)cwnd=8cwnd=9 9

cwnd=10 10cwnd=11 11

Exit Fast Recovery

cwnd=ssthresh=4

=> Congestion Avoidance

Ack(10)

1

12


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Limitation of TCP Reno algorithm

If cwnd size is too small (smaller than 4 packets) then its notpossible to get 3 duplicate acks and run the algorithm

The algorithm can not manage a loss of multiple packets from asingle window of data

It will cause a use of retransmission time out

The algorithm doesnt manage a loss of packets during the FastRecovery stage

Not a loss of the retransmitted packet

There is no recursive run of the Fast Retransmit


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Sender Receiver

012

Ack(1)

3456

Ack(1)

Ack(1)

78

Initial State

cwnd=7

Slow Start

cwnd=8

1Fast Retransmitcwnd=8/2+3=7

ssthresh=8/2=4

=> Fast Recovery

Ack(1)

Ack(1)Ack(1)

Ack(3)cwnd=8

Exit Fast Recovery

cwnd=ssthresh=4=> Congestion Avoidance

Ack(3)

Flight Size =No. of

Unacknowledged

segments

Flight Size > cwnd

=> No new segments

cwnd=9 9

The algorithm doesnt

know which segments

were acknowledged

What was happen

if this packet was lost?


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Internet Networking

TCP NewReno

The Idea: If the sender remembers the number of the last segmentthat was sent before entering the Fast Retransmit phase

Then it can deal with a situation when a new ACK (which is not

duplicate ACK) does not cover the last remembered segment

(partial ACK)

This is a situation when more packets were lost before entering the

Fast Retransmit.

After discovering such situation the sender will retransmit the newlost packet too and will stay at the Fast Recovery stage

The sender will finish the Fast Recovery stage when it will get ACKthat covers last segment sent before the Fast Retransmit


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TCP NewReno Retransmission Process (I)

Set ssthresh to max (FlightSize / 2, 2*MSS)

Record to Recovery variable the highest sequence numbertransmitted

Retransmit the lost segment and set cwnd to ssthresh + 3*MSS.

The congestion window is increased by the number of segments (three)

that were sent and buffered by the receiver.

For each additional duplicate ACK received, increment cwnd byMSS.

Thus, the congestion window reflects the additional segment that has

left the network.

Transmit a segment, if allowed by the new value of cwnd and thereceiver's advertised window.


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Internet Networking

TCP NewReno Retransmission Process (II)

When a partial ACK is received retransmit the first unacknowledged segment

deflate the congestion window by the amount of new acknowledged data,

then add back one MSS

send a new segment if permitted by the new value of cwnd

When an acknowledge of all of the data up to and including "recoverarrives:

In our example: Set cwnd to ssthresh


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When a sender should reset the Retransmit Timer ?

In the Impatient variant reset the timer only after the firstPartial ACK.

As the result, invoking Slow-Start when retransmit timer will

expire, especially when there are several packet lost and RTO

is not much larger than RTT

This approach causes fast retransmission of many packets

(Slow Start) and its appropriate for the case of multiple packet

drops

TCP NewReno

Resetting the Retransmit Timer


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Internet Networking

TCP NewReno

Resetting the Retransmit Timer

When a sender should reset the Retransmit Timer ?

In the Slow and Steady variant reset the timer after eachPartial ACK

As the result, remaining in Fast Recovery for a long time,especially when there are multiple packet drops and RTO is

much larger than RTT


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TCP NewReno Summary

Neither of the retransmit timers resetting variants is optimal.

If the SACK option is available, the TCP sender always has theinformation to make intelligent decisions about which packets to

retransmit and which packets not to retransmit during Fast

Recovery.

It is worthwhile to combine NewReno in the standard togetherwith the SACK option, because it demands the algorithm change

only at the sender side, without changing a message structure

and therefore enables to improve TCP effectiveness also for

receivers that dont support SACK.

New Reno Sack

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