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Network Working Group                                            W. Eddy
Internet-Draft                                               MTI Systems
Intended status: Experimental                                    L. Wood
Expires: May 5, 2016                                       Surrey alumni
                                                              W. Ivancic
                                                        November 2, 2015

               TFRC-based Congestion Control for Saratoga


   This document specifies the use of TCP-Friendly Rate Control (TFRC)
   with the Saratoga data transfer protocol.  The necessary conventions
   that a Saratoga sender and receiver implementation must follow if
   they wish to enable the use of TFRC are described.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on May 5, 2016.

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   This document may not be modified, and derivative works of it may not
   be created, except to format it for publication as an RFC or to
   translate it into languages other than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Receiver-Side Behavior  . . . . . . . . . . . . . . . . . . .   3
   3.  Sender-Side Behavior  . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Differences from Shahriar et al. . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In order to support use of the Saratoga protocol
   [draft-wood-tsvwg-saratoga] on networks with multiple data flows,
   multiple hops, and/or experimentally on the Internet, some form of
   congestion control is required.  Particularly, for use on the
   Internet, rather than the private links and networks that Saratoga
   was originally developed for, congestion control is a requirement, as
   indicated by the UDP Guidelines [RFC5405].

   This document provides the specification of one type of congestion
   control, based on TCP-Friendly Rate Control (TFRC), which is intended
   to be fairly conservative in terms of performance.  Better-performing
   congestion-control algorithms are possible in terms of throughput,
   but this TFRC-based algorithm has the advantages of being relatively
   simple and based on the well-established TFRC components, rather than
   on a newer and less well-tested mechanism.

   Saratoga data flow occurs within uniquely identified transactions
   between a sender and receiver.  Saratoga primitively supports link-
   local multicast to a set of receivers, but that use case is not
   considered further in this specification and is considered out of
   scope for this document.  The congestion control mechanism specified
   in this document operates within a single transaction between a
   sender and a receiver.  Subsequent or concurrent transactions between
   the same nodes use distinct congestion control state.

   TFRC has "receiver-based" and "sender-based" variations, as
   descripted in [RFC5348].  In either case, a node with knowledge of
   the loss-event rate and round-trip time (RTT) uses this information

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   in the TFRC throughput equation in order to compute an allowed
   transmit rate for the sender.  A sender-based variant of TFRC for use
   with Saratoga has been described in the academic literature
   [Shahriar11], and the mechanism described in this document is derived
   to some extent from that previous work.

   Saratoga allows the receiver to generate, at any time, a STATUS
   packet containing a "Progress Indicator" cumulative acknowledgement
   (PI cumack) and a list of "holes" in the sequence space of data bytes
   received.  The STATUS packet can also be requested to be sent by
   setting a flag in the DATA packet header.  A DATA packet can
   optionally include a timestamp field, which is echoed in any STATUS
   packet that it may trigger.  This enables the sender to estimate the
   Round-Trip Time (RTT), required as part of TFRC equation.  The TFRC
   adaptation for Saratoga relies on the regular reception of STATUS
   packets at the sender, in order for the sender to use its knowledge
   of the RTT, the packets sent within an interval, and the packets
   received (implicit from the holes and PI cumack), to run the TFRC
   equation to determine a reasonable sending rate.

   Generally, Saratoga is designed to enable high-performance transfers
   over highly asymmetric and lossy links, which may also have high
   latencies.  The rate-control, in the base specification, has been
   permitted to be open-loop with the sender configured to saturate the
   network capacity that it expects exclusive access to.  However, in
   order to permit sender-based TFRC congestion control for shared
   capacity, accurate and timely feedback is necessary in order to
   create a functioning closed-loop control system.  This limits the
   applicability of TFRC extensions to Saratoga to networks with RTTs
   more typical of the Internet than, for instance, interplanetary
   microwave communications links.  Supporting congestion control also
   limits the asymmetry that can be supported between the data path
   capacity and the feedback path capacity, as more regular STATUS
   updates are required for the congestion control loop.

2.  Receiver-Side Behavior

   The receiver MUST echo timestamps in non-voluntary (solicited) STATUS
   packets.  It SHOULD generate and transmit these STATUS packets
   without unnecessary implementation delay.  The sender will typically
   request mandatory STATUS packets at least once per its estimated RTT.
   If multiple STATUS packets within the same transaction are queued for
   transmission within the receiver, it MAY choose to only send the most
   recent STATUS and clear others from its queue.  This may help to
   mitigate asymmetry issues.

   The Saratoga receiver MUST track the time when it last sent a STATUS
   packet, and periodically send voluntary (unsolicited) STATUS packets

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   to the sender, even if no DATA packets requesting STATUS have been
   received.  This is needed to provide feedback of progress (or lack of
   progress) when there is heavy loss in the DATA path, and the sender
   needs to be informed of this in order to adjust its sending rate.
   This timer for sending unsolicited STATUS packets MAY be set to the
   minimum among the the last three measured interarrival times between
   consecutive DATA packets bearing STATUS requests within the

   The receiver SHOULD generate voluntary STATUS packets with an updated
   holes list when it receives an incoming DATA packets with offset
   beyond the current cumack (i.e. not advancing the Progress
   Indicator).  This creates a hole in the STATUS packet and provides a
   timely notification of loss to the sender.  Possible exceptions to
   this behavior would be for highly-asymmetric links, where the
   feedback traffic needs to be minimized, or for cases where
   significant reordering is expected and a more intelligent strategy
   for generating STATUS packets is implemented.

3.  Sender-Side Behavior

   As the envisioned use of Saratoga implementations with TFRC is for
   bulk-transfer applications that will not be "data-limited" in
   [RFC5348] terms, tracking of data-limited periods and adjusting the
   sending rate based on them is not required by the specification in
   this document.

   The "nofeedback" timer defined in [RFC5348] is reset based on the
   reception of Saratoga STATUS packets, as these provide the feedback

   It is assumed that the Saratoga sender MUST implement a configurable
   "Maximum Payload Size" (MPS), which limits the total number of bytes
   of user data per Saratoga packet.  This assumption is used to
   simplify tracking of lost packets based on STATUS feedback that only
   provides ranges of DATA holes, and not the detail of individual lost
   or received packets.  The MPS MAY be adjusted downwards by an
   implementation based on PMTUD feedback, but the details for this are
   not within scope of this document, and do not impact the TFRC
   mechanism defined here, as long as the current MPS for a transaction
   is known by the implementation.

   The initial sending rate for DATA packets within a Saratoga
   transaction from the Saratoga sender implementing TFRC is initialized

   X = min(4*MPS, max(2*MPS, 4380))

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   This is as already defined for TFRC, with the Saratoga transaction
   MPS used as the Maximum Segment Size that TFRC is based on.  During
   the course of the transaction, this is adjusted as described in the
   remainder of this section.

   The Saratoga sender implementing TFRC-based congestion control
   specified in this document MUST use timestamps on its DATA packets.
   For each DATA packet that the Saratoga sender releases onto the
   network, it MUST temporarily store the timestamp, offset, and packet
   size in a packet history list.  The history is accessed by looking
   for packets sent at or before a given timestamp, and if organized
   this way by timestamp, continuous portions of the packet history list
   are periodically cleared during processing of STATUS packets,
   described later in this section.

   The sender MUST keep track of a per-transaction estimated RTT based
   on observance of timestamp echo fields in STATUS packets.  On
   receiving a STATUS packet, an RTT sample is computed via subracting
   the echoed timestamp from the current clock time.  [RFC5348]
   nominally includes t_delay; no t_delay is employed (or t_delay is 0)
   as Saratoga does not have a way to convey t_delay.  The collection of
   RTT samples SHOULD be smoothed via an algorithm such as the EWMA
   described in [RFC6298].  In this document, the use of "RTT" generally
   refers to the smoothed RTT computed via this process rather than an
   instantaneous RTT sample, unless clearly noted.

   When sending DATA packets, the sender MUST periodically request
   STATUS packets using the available DATA packet flag for this purpose.
   The time period for making these requests is a minimum of once per
   the current estimated RTT.

   There is no assumption of the ordering which a Saratoga sender
   delivers particular chunks of a file, nor to the order and algorithms
   which it uses to respond to and repair the list of holes conveyed in
   the STATUS packets.  However, the sender must keep track of the
   comprehensive set of holes within the transaction that have been seen
   most recently in STATUS feedback.

   On receiving an STATUS packet, the Saratoga sender MUST compute a
   loss rate sample.  The loss rate in packets is computed via comparing
   the changes between the current snapshot and the prior one generated
   when the last STATUS packet was received.  Signs of positive progress
   include: (1) advancement of the cumack field by some multiple of the
   MPS (or less than one MPS in the case of the final packet of DATA
   within a transaction, (2) reduction in the size of any holes.  The
   number of MPS-sized chunks, packets_rcvd, covered by signs of
   positive progress in the current STATUS packet represents the number
   of non-duplicate packets received during the last reporting interval

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   (assuming a prior STATUS packet was not lost).  The sender uses its
   history of sent DATA packets and the timestamp echoed in the STATUS
   in order to determine how many DATA packets were sent within the
   interval.  It simply counts the number of packets, packets_sent, in
   the history with timestamps prior or equal to the echoed one.  At
   this point, the packet history at or below the echoed timestamp can
   be cleared or released.  packets_ecnd is always 0, as implementing
   support for ECN from a UDP application raises implementation
   difficulties.  The loss event rate sample for the TFRC equation, p,
   is then computed from packets_sent, packets_rcvd, and packets_ecnd as
   described below.  This loss rate sample, along with the current RTT
   measurement are fed into the TFRC equation to arrive at a sending
   rate for the Saratoga sender.

   The values needed to compute the TCP sending rate (X_Bps) from the
   TFRC equation, using the variable names from [RFC5348] are:

   o  s := the MPS value configured for the transaction

   o  R := the current smoothed RTT

   o  p := the computed loss event rate; 1 - ((packets_rcvd -
      packets_ecnd) / packets_sent)

   o  t_RTO := 4*R (as recommended by [RFC5348]

   o  b := 1 (as recommended by [RFC5348]

   The TFRC procedure for computing X_recv_set, recv_limit, X_Bps, and X
   are performed as defined in [RFC5348] per STATUS packet received,
   along with the rest of the TFRC feedback processing.  The new X value
   is immediately applied to the Saratoga implementation's outgoing
   packet clocking.

4.  Security Considerations

   The security considerations for use of TFRC in Saratoga are the same
   as those described in [RFC5348] for TFRC itself (but do not share the
   considerations listed there for TFRC's use in DCCP).

5.  IANA Considerations

   This document has no IANA considerations.

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6.  Acknowledgements

   We thank Cisco Systems for funding the early investigations into
   Saratoga congestion control that led to [Shahriar11], which has
   heavily influenced this document.

7.  References

7.1.  Normative References

              Wood, L., Eddy, W., Smith, C., Ivancic, W., and C.
              Jackson, "Saratoga: A Scalable Data Transfer Protocol",
              draft-wood-tsvwg-saratoga-18 (work in progress) , November

   [RFC5348]  Floyd, S., Handley, M., Padhye, J., and J. Widmer, "TCP
              Friendly Rate Control (TFRC): Protocol Specification",
              RFC 5348, DOI 10.17487/RFC5348, September 2008,

7.2.  Informative References

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              DOI 10.17487/RFC5405, November 2008,

   [RFC6298]  Paxson, V., Allman, M., Chu, J., and M. Sargent,
              "Computing TCP's Retransmission Timer", RFC 6298,
              DOI 10.17487/RFC6298, June 2011,

              Shahriar, A., Atiquzzaman, M., Ivancic, W., and L. Wood,
              "A sender-based TFRC for Saratoga: A rate control
              mechanism for a space-friendly transfer protocol", IEEE
              Aerospace Conference Big Sky, Montana, March 2011.

Appendix A.  Differences from Shahriar et al.

   The method described in this document does not involve keeping track
   of the "symmetry factor" that was a part of [Shahriar11].

   The method described in this document does not use the "dummy
   sequences" described as part of [Shahriar11], and instead infers lost
   packet counts from the number of bytes covered by holes above the PI
   cumack (the Progress Indicator).  This inference is possible whenever

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   the cumack is less than the highest sequence number expected (the In-
   Response-To field) within the last loss period.

Authors' Addresses

   Wesley M. Eddy
   MTI Systems

   Email: wes@mti-systems.com

   Lloyd Wood
   University of Surrey alumni
   Sydney, New South Wales

   Email: L.Wood@society.surrey.ac.uk

   Will Ivancic
   NASA Glenn Research Center
   21000 Brookpark Road, MS 54-5
   Cleveland, OH  44135

   Phone: +1-216-433-3494
   Email: William.D.Ivancic@grc.nasa.gov

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