NewReno Modification for Smooth Recovery After Fast Retransmission
draft-nishida-newreno-modification-01

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

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] (Floyd, S., Henderson, T., and A. Gurtov, “The NewReno Modification to TCP's Fast Recovery Algorithm,” April 2004.). This document describes the point in Fast Recovery and presents possible scenarios. This memo also propose a simple modification to fix this problem.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

Since this document describes a potential risk in NewReno, it uses the same terminology and definitions in RFC3782 [RFC3782] (Floyd, S., Henderson, T., and A. Gurtov, “The NewReno Modification to TCP's Fast Recovery Algorithm,” April 2004.). Which means this documents assumes that the reader is familiar with the terms SENDER MAXIMUM SEGMENT SIZE (SMSS), CONGESTION WINDOW (cwnd), and FLIGHT SIZE (FlightSize) defined in [RFC2581] (Allman, M., Paxson, V., and W. Stevens, “TCP Congestion Control,” April 1999.).

3.  Problem Description

This section describes a potential risk 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 cwnd 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.

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.

      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 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 14)

4.3.  Case 3: Lost of ACK segments

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

      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)

5.  Discussion

Some TCP implementations such as Linux, NS-2 Network simulator do not have this issue. This is because these implementations always transmit more than 1 MSS right after fast recovery. In these implementations, when TCP exits Fast Recovery (when the first FULL ACK is received) it also calls "open cwnd" function at the same time and performs Slow Start or Congestion Avoidance algorithm. Hence, even though cwnd is set to 1 MSS after Fast Recovery as described in Section 3, the cwnd will be increased by 1 MSS by Slow Start. (Since ssthresh should be bigger than 1 MSS at this moment, Slow Start is always used to increase cwnd)

However, this behavior can be controversial because it enters Slow-Start after Fast Recovery without receiving any packets. Although this point is unclear in RFC3782, we believe that this is rather aggressive behavior and TCP should not open cwnd after Fast Recovery without receiving another ACKs. In fact, several implementation do not perform Slow Start right after Fast Recovery. With these implementations, severe performance degradations can be observed over lossy networks.

6.  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)

7.  Security Considerations

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

8.  IANA Considerations

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

9. Normative References

Author's Address