Network Working Group                                         Y. Nishida
Internet-Draft                                              WIDE Project
Intended status: Standards Track                          April 23, 2009
Expires: October 25, 2009


   NewReno Modification for Smooth Recovery After Fast Retransmission
                 draft-nishida-newreno-modification-00

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   This Internet-Draft will expire on October 25, 2009.

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Abstract

   This memo describes a feeble point in Fast Recovery algorithm in
   NewReno defined in RFC3782 and proposes a simple modification to
   solve the problem.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  4
   3.  Problem Description  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Possible Scenarios . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Case 1: Small Sending Window Size at Sender  . . . . . . .  6
     4.2.  Case 2: Zero Window Advertisement from Receiver  . . . . .  6
     4.3.  Case 3: Lost of ACK segments . . . . . . . . . . . . . . .  7
   5.  Proposed Fix . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13






























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1.  Introduction

   There are some situations that NewReno cannot recover quickly after
   the success of fast retransmission.  This issue is resulted from a
   feeble point in Fast Recovery algorithm in NewReno defined in RFC3782
   [RFC3782].  This document describes the point in Fast Recovery and
   presents possible scenarios.  This memo also propose a simple
   modification to fix this problem.











































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2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document assumes that the reader is familiar with the terms
   SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and
   FLIGHT SIZE (FlightSize) defined in [RFC2581].  FLIGHT SIZE is
   defined as in [RFC2581]. as follows:


      FLIGHT SIZE:
        The amount of data that has been sent but not yet acknowledged.





































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3.  Problem Description

   This section describes a feeble point in Fast Retransmit and Fast
   Recovery Algorithm in RFC3782.

   Section 3 in RFC3782 describes the Fast Retransmit and Fast Recovery
   Algorithm in NewReno.  The algorithm consists of 6 steps.  The
   following lines are the description of the fifth steps which
   describes the behavior for the arrival of the first Full ACK after
   first retransmission.


   5)  When an ACK arrives that acknowledges new data, this ACK could be
       the acknowledgment elicited by the retransmission from step 2, or
       elicited by a later retransmission.

       Full acknowledgements:

       If this ACK acknowledges all of the data up to and including
       "recover", then the ACK acknowledges all the intermediate
       segments sent between the original transmission of the lost
       segment and the receipt of the third duplicate ACK.  Set cwnd to
       either (1) min (ssthresh, FlightSize + SMSS) or (2) ssthresh
       where ssthresh is the value set in step 1; this is termed
       "deflating" the window.  (We note that "FlightSize" in step 1
       referred to the amount of data outstanding in step 1, when Fast
       Recovery was entered, while "FlightSize" in step 5 refers to the
       amount of data outstanding in step 5, when Fast Recovery is
       exited.)

   According to this description, the behavior after the first FULL ACK
   reception will be one of the followings.


            (1) min (ssthresh, FlightSize + SMSS)
            (2) ssthresh

   However, there is a risk in (1) which can cause performance
   degradation.  In (1), if FlightSize is zero, the result of (1) will
   be 1 SMSS. (ssthresh should be bigger than 1) This means TCP can
   transmit only 1 segment in this case.  This can cause the delay in
   ACK transmission at the receiver side if the receiver use delayed ACK
   algorithm.  The FlightSize in (1) represents the amount of data
   outstanding in the fifth step: the moment when the new Full ACK
   arrives.  The next section describes several scenarios where the
   FlightSize becomes zero.





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4.  Possible Scenarios

   There are several possible situations that FlightSize becomes zero
   when the first new full ACK arrives after fast retransmission.  This
   section describe several possible cases.

4.1.  Case 1: Small Sending Window Size at Sender

   This is the tcpdump example of the case.  This log is recorded at A.


         1  10:41:00.000001 A > B: . 1000:2000(1000) ack 1 win 32768
         2  10:41:00.001001 A > B: . 2000:3000(1000) ack 1 win 32768
         3  10:41:00.002001 A > B: . 3000:4000(1000) ack 1 win 32768
         4  10:41:00.003001 A > B: . 4000:5000(1000) ack 1 win 32768
         5  10:41:00.010001 B > A: . ack 1000 win 16384
         6  10:41:00.011001 B > A: . ack 1000 win 16384
         7  10:41:00.012001 B > A: . ack 1000 win 16384
         8  10:41:00.013001 A > B: . 1000:2000(1000) ack 1 win 32768
         9  10:41:00.014001 A > B: . 5000:6000(1000) ack 1 win 32768
        10  10:41:00.024001 B > A: . ack 6000 win 16384
        11  10:41:00.025001 A > B: . 6000:7000(1000) ack 1 win 32768


   In this example, A sends data segments to B. At the beginning of the
   log, the cwnd of A is 4 SMSS, hence A sends 4 segments to B (line
   1-4).  Here, if the segment sent in line 1 (segment 1000:2000) is
   lost, B sends 3 duplicated ACKs for the lost segment (line 5-7) to
   ask retransmission.  At line 8, A receives 3 duplicated ACKs then it
   transmits the lost segment.  At line 9, A sets cwnd to ssthresh plus
   3*SMSS (as defined in the second steps in NewReno algorithm) and cwnd
   becomes 5 SMSS as the result.  This window inflation allows A to
   transmit one new segment.

   Since the two segments in line 8 and 9 are usually transmitted almost
   at the same time, the receiver may send back only one ACK for these
   two segments (line 10) The ACK received in line 10 is the first Full
   ACK and there is no out-standing data in this moment.  Hence, new
   cwnd is set to 1 SMSS and only one new segment is sent (line 11)

4.2.  Case 2: Zero Window Advertisement from Receiver

   This is the tcpdump example of the case.  This log is recorded at A.








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         1  11:42:00.000001 A > B: . 1000:2000(1000) ack 1 win 32768
         2  11:42:00.001001 A > B: . 2000:3000(1000) ack 1 win 32768
         3  11:42:00.002001 A > B: . 3000:4000(1000) ack 1 win 32768
         4  11:42:00.003001 A > B: . 4000:5000(1000) ack 1 win 32768
         5  11:42:00.004001 A > B: . 5000:6000(1000) ack 1 win 32768
         6  11:42:00.005001 A > B: . 6000:7000(1000) ack 1 win 32768
         7  11:42:00.010001 B > A: . ack 1000 win 0
         8  11:42:00.011001 B > A: . ack 1000 win 0
         9  11:42:00.012001 B > A: . ack 1000 win 0
        10  11:42:00.012201 A > B: . 1000:2000(1000) ack 1 win 32768
        11  11:42:00.013001 B > A: . ack 1000 win 0
        12  11:42:00.014001 B > A: . ack 1000 win 0
        13  11:42:00.022001 B > A: . ack 7000 win 16384
        14  11:42:00.023001 A > B: . 7000:8000(1000) ack 1 win 32768


   In this example, A sends data segments to B. At the beginning of the
   log, the cwnd of A is 6 SMSS, hence A sends 6 segments to B (line
   1-6).  Here, if the segment sent in line 1 (segment 1000:2000) is
   lost, B sends duplicated ACKs for the lost segment (line 7-9 and
   11-12) to ask retransmission.  However, these duplicated ACKs sent
   from B have zero advertised window because of buffer overflow.  In
   this case, although the cwnd at A is inflated at the reception of the
   duplicated ACKs, it cannot transmit new segments.  Hence, only the
   lost segment is retransmitted (line 10).  When B receives
   retransmitted segment, the buffer becomes empty, then B sends a Full
   ACK with non-zero advertised window.  The ACK received in line 13 is
   the fisrt Full ACK and there is no out-standing data in this moment.
   Hence, new cwnd is set to 1 SMSS and only one new segment is sent
   (line 14)

4.3.  Case 3: Lost of ACK segments

   This is the tcpdump example of the case.  This log is recorded at A.

















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         1  12:43:00.000001 A > B: . 1000:2000(1000) ack 1 win 32768
         2  12:43:00.001001 A > B: . 2000:3000(1000) ack 1 win 32768
         3  12:43:00.002001 A > B: . 3000:4000(1000) ack 1 win 32768
         4  12:43:00.003001 A > B: . 4000:5000(1000) ack 1 win 32768
         5  12:43:00.004001 A > B: . 5000:6000(1000) ack 1 win 32768
         6  12:43:00.005001 A > B: . 6000:7000(1000) ack 1 win 32768
         7  12:43:00.010001 B > A: . ack 1000 win 16384
         8  12:43:00.011001 B > A: . ack 1000 win 16384
         9  12:43:00.012001 B > A: . ack 1000 win 16384
        10  12:43:00.012201 A > B: . 1000:2000(1000) ack 1 win 32768
        11  12:43:00.022001 B > A: . ack 7000 win 16384
        12  12:43:00.023001 A > B: . 7000:8000(1000) ack 1 win 32768

   In this example, A sends data segments to B. At the beginning of the
   log, the cwnd of A is 6 SMSS, hence A sends 6 segments to B (line
   1-6).  Here, if the segment sent in line 1 (segment 1000:2000) is
   lost, B generates 5 duplicated ACKS, however 2 ACK segments are lost
   in this case.  Then, only 3 duplicated ACKs arrives at A (line 7-9).
   At line 10, A transmits the lost segment and sets cwnd to ssthresh
   plus 3*SMSS.  As the result, the cwnd becomes 6 SMSS.  However, this
   cwnd does not allow A to transmit new segments.  At line 11, A
   receives the first Full ACK and there is no out-standing data in this
   moment.  Hence, new cwnd is set to 1 SMSS and only one new segment is
   sent (line 12)



























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5.  Proposed Fix

   To solve the problem mentioned above, we propose a simple fix to the
   fifth step in NewReno.

   The proposed solution is modifying the current cwnd adjustment:

              (1) min (ssthresh, FlightSize + SMSS)
      to
              (1) min (ssthresh, max(FlightSize, SMSS) + SMSS)

   This fix ensures that cwnd is always larger than 1 SMSS.  Hence,
   sender TCP can always transmit at least two segments right after the
   first Full ACK reception.  This can avoid the delay of ACK
   transmissions caused by delayed ACK algorithm.  The new algorithm
   increases 1 SMSS only when FlightSize becomes zero and behaves
   completely the same as the previous algorithm does in other
   situations.  The new algorithm might add slight burstness since it
   requires additional increase of cwnd.  However, we believe this
   burstness can be almost negligible.































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6.  Security Considerations

   This document only propose simple modification in RFC3782.  There are
   no known additional security concerns for this algorithm.















































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7.  IANA Considerations

   This document does not create any new registries or modify the rules
   for any existing registries managed by IANA.















































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8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2581]  Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
              Control", RFC 2581, April 1999.

   [RFC3782]  Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
              Modification to TCP's Fast Recovery Algorithm", RFC 3782,
              April 2004.








































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Author's Address

   Yoshifumi Nishida
   WIDE Project
   Endo 5322
   Fujisawa, Kanagawa  252-8520
   Japan

   Email: nishida@wide.ad.jp










































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