Internet Engineering Task Force                                  N. Kuhn
Internet-Draft                                                      CNES
Intended status: Informational                                E. Stephan
Expires: November 19, 2020                                        Orange
                                                            G. Fairhurst
                                                                T. Jones
                                                  University of Aberdeen
                                                            May 18, 2020

               Transport parameters for 0-RTT connections


   0-RTT mechanisms reduce the time it takes for the first bytes of
   application data to be processed in a transport connection and can
   greatly reduce connection latency during setup.  The 0-RTT transport
   features described by quic-transport help clients establish secure
   connections with a minimal number of round-trips.

   This document describes a generic method to exchange path parameters
   relating to transport.  The additional transport parameters can help
   a connection that continues after an interruption or restarts by
   sharing connection properties.  They can be used to increase the
   performance for a path with large RTT.

Status of This Memo

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   This Internet-Draft will expire on November 19, 2020.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  BDP metadata parameters . . . . . . . . . . . . . . . . . . .   4
   4.  Extension activation  . . . . . . . . . . . . . . . . . . . .   5
   5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Example of server solution . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Each transport connection typically starts without knowledge of the
   path between the endpoints.  Transport protocols use implicit signals
   from the network to discover the properties of the path.  This
   information is used to adapt the transport mechanisms to the network
   path.  For example, an Internet transport endpoint is unable to
   determine a safe rate at which to start or continue their
   transmission, and uses slow-start to determine a safe rate.  This
   applies to the 1-RTT mode of QUIC.

   QUIC supports the sending of data in two different modes, after the
   transport handshake has completed, 1-RTT mode, and sending data along
   with handshake packets, 0-RTT mode.  Using 0-RTT data an application
   is able to send transport parameters with the handshake packets,
   making it possible to reduce the latency of the connection setup.

   In 0-RTT mode, a QUIC server must store a copy of a number of flow
   control related transport parameters, or receives an integrity-

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   protected copy of these values in the ticket the client includes in
   the first message of the handshake, to enable the use of 0-RTT data.
   The setting or omission of one of these parameters can result in QUIC
   creating a connection, but flow control can still prevent any data
   being sent by the client.

   For 0-RTT data to be sent, the QUIC server must record the values of:

   o  initial_max_data

   o  initial_max_stream_data_bidi_local

   o  initial_max_stream_data_bidi_remote

   o  initial_max_stream_data_uni

   o  initial_max_streams_bidi

   o  initial_max_streams_uni

   These values set the flow control limits within which a connection
   must operate.  The server has to store these parameters for a client
   to send data when resuming during 0-RTT.  The stored values are used
   for any data that is transmitted before the handshake has completed
   and 1-RTT data is able to be sent on the connection.  Once the
   handshake has completed, these values are discarded and the values
   established during the handshake are used.

   This document proposes an extension to the transport parameters that
   are shared during the 0-RTT phase to allow resumption using
   additionnal transport and connection properties that were discovered
   in previous connections.  These additional parameters aim to provide
   faster startup of flows and better buffer management.  The BDP
   metadata extension is proposed and candidate parameters are discussed
   in Section 3.

2.  Motivation

   Reducing the number of round-trips required to start a connection is
   an important way to reduce setup time and lower overall connection
   latency. 0-RTT mechanisms that allow a client to feed requests to a
   server in the first RTT do not alone improve the total time-to-
   service.  The BDP extension described in this document aims to
   improve traffic delivery by allowing the connection to short-cut slow
   RTT-based processes that grow connection parameters.

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   Currently each side has a proprietary solution to measure and to
   store path characteristics.  Recalling the parameters of a previous
   connection would allow:

   o  Server and client to re-initialise the state from previously
      established parameters (i.e., minRTT, MTU, bottleneck capacity,

   o  The client to prepare the right resources

   o  The server to adapt to non-default path characteristics

   There is an interest in sharing transport information across multiple
   connections.  As one example, [I-D.ietf-tcpm-2140bis] considers the
   sharing of transport parameters between connections originating from
   the same host.  The proposal in this document has the advantage of
   storing the information at the client and not requiring the server to
   retain additional state for each client.

3.  BDP metadata parameters

   Section 7.3.1 of [I-D.ietf-quic-transport] describes the parameters
   that must be remembered if a client wishes to send 0-RTT data.  Both
   endpoints store the value of the server transport parameters from a
   connection and apply them to any 0-RTT packets that are sent in
   subsequent connections to the same peer.  Of the six mandatory
   parameters, only initial_max_data improves the time-to-service of the
   0-RTT connection.  The BDP metadata extension augments the list of
   server transport parameters that are shared with the client to
   improve the time-to-service and save resources such as CPU, memory
   and power.

   The BDP extension proposes three new parameters that help a
   connection startup in a minimal number of RTTs:

   o  recon_bytes_in_flight (0x000X): The bytes in flight measured on
      the previous connection by the server.  Integer number of bytes.
      Using the bytes_in_flight defined in [I-D.ietf-quic-recovery],
      recon_bytes_in_flight can be set to bytes_in_flight.

   o  recon_min_rtt (0x000X): The minimum RTT measured on the previous
      connection by the server.  Integer number of milliseconds.  Using
      the min_rtt defined in [I-D.ietf-quic-recovery], recon_min_rtt can
      be set to min_rtt.  The min_rtt parameter may not track a
      decreasing RTT: the min_rtt that is reported here may not be the
      actual minimum RTT measured during the 1-RTT connection, but still
      reflects the characteristics of the latency on the network.

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   o  recon_max_pkt_number (0x000X): The maximum amount of packets that
      the server is allowed to send when reconnecting.  Integer unit of

4.  Extension activation

   The BDP extension is protected by the same mechanism that protects
   the exchange of the 0-RTT transport parameters.  A client that
   activates 0-RTT data sends back the transport parameters received
   from the server during the previous connection (see Section 7.3.1 of

   The client reads the parameters in the BDP metadata extension, but
   can not change them.

   Accept: A client MAY use the extension parameters.  Then, it
   activates ingress optimization and sends back the transport
   parameters of the BDP metadata extension that it received from the
   server during the previous connection.

   Refuse: A client could choose not to use there parameters.  Then, it
   does not support ingress optimisation and drops the extension signal.
   A client that disagrees with the extension parameters received from
   the server refuses the optimization.

5.  Discussion

   The recon_bytes_in_flight parameter is higher than the number of
   bytes in the actual BDP since it may include bytes in buffers along
   the path.

   The recon_bytes_in_flight parameter is an indication of the end-to-
   end BDP that is experienced by the congestion and flow control.  If
   the recon_bytes_in_flight is high, the server may decide to increase
   the maximum amount of packets it will send when reconnecting using
   the recon_max_pkt_number parameter.

   The maximum number of initial data packets that can be sent without
   acknowledgment needs to be chosen to avoid congestion collapse.  An
   example of a server solution is proposed in Appendix A.  In short:

   o  recon_bytes_in_flight indicates the characteristics of the network
      underneath to both peers that can adapt their buffer sizes or
      parameter tuning.

   o  recon_min_rtt lets both a client and a server know the minimum RTT
      of the previous connection.  Parameters can then be adapted (e.g.,

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      to adapt usage of kInitialRtt in "Setting the Loss Detection
      Timer", see section of [I-D.ietf-quic-recovery]).

   o  recon_max_pkt_number lets the server warn the client that it may
      increase the amount of packets that it expects to send when
      reconnecting.  This value is negotiated with the client and result
      in better buffer management and reduced flow start up.

   Other parameters can contribute to the optimization of 0-RTT
   connection.  There are good candidates, like max_ack_delay, in the
   Appendix of [I-D.ietf-quic-recovery].

6.  Acknowledgments

   The authors would like to thank Gabriel Montenegro, Patrick McManus,
   Ian Swett, Igor Lubashev, Christian Huitema and Tom Jones for their
   fruitful comments on earlier versions of this document.

7.  IANA Considerations

   TBD: Text is required to register the extension BDP_metadata field.
   Parameters are registered using the procedure defined in

8.  Security Considerations

   The BDP metadata parameters are measured by the server during a
   previous connection.

   The BDP extension is protected by the mechanism that protects the
   exchange of the 0-RTT transport parameters.  For the version 1 of
   QUIC, the BDP extension is protected using the mechanism that already
   protects the "initial_max_data" parameter.  This is defined in
   sections 4.5 to 4.7 of [I-D.ietf-quic-tls].  It provides the server
   with a way to check the parameters proposed by the client are those
   that the server sent to the client during the previous connexion.

9.  Informative References

              Cardwell, N., Cheng, Y., Yeganeh, S., and V. Jacobson,
              "BBR Congestion Control", draft-cardwell-iccrg-bbr-
              congestion-control-00 (work in progress), July 2017.

              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", draft-ietf-quic-recovery-27 (work in
              progress), March 2020.

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              Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              draft-ietf-quic-tls-27 (work in progress), February 2020.

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-27 (work
              in progress), February 2020.

              Touch, J., Welzl, M., and S. Islam, "TCP Control Block
              Interdependence", draft-ietf-tcpm-2140bis-05 (work in
              progress), April 2020.

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, DOI 10.17487/RFC2914, September 2000,

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782,
              January 2007, <>.

Appendix A.  Example of server solution

   This section details a solution at the server to safely increase the
   maximum amount of packets that the server sends when receiving a
   0-RTT packet from a client.

   The initial window is considered a safe starting point for an unknown
   path to avoid adding congestion to a congested network.  The general
   assumption is that a path does not currently suffer persistent
   congestion, and therefore the initial window is applicable until
   feedback about the path is received.  The resulting initial sending
   rate is only tentative until the capacity is confirmed to be
   available.  If there is loss within this initial transmission, then
   this could be evidence that the path is congested, and the sender
   needs to adjust to this congestion.

   Significant loss could be an indication of congestion collapse - i.e.
   persistent loss, requiring back-off of the sending rate.

   If however, the reception of IW is confirmed for the first RTT of
   data, and also the path is determined to be similar to that of a
   recent previous session (e.g., similar RTT), the method permits the
   sender to use the previous path information as an input to help
   determine a new safe rate.  One possibility is to immediately jump to
   a new sending rate that is derived from the previously sustained

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   rate.  This follows the ideas of
   [I-D.cardwell-iccrg-bbr-congestion-control] and [RFC4782].

   The QoS mechanisms that are deployed in the networks can help in
   prevent the congestion collapse from occurring.  However, the sender
   must provide a significant reduction if there is evidence of
   potential congestion collapse [RFC2914] from his point of view.
   Precautions can be taken to guarantee that it is reasonably safe to
   jump to a high sending rate : measuring that network conditions did
   not change, allowing some space for other flows that have started and
   pace transmission of packets.  The sender might need to rapidly
   reduce its rate, if the higher sending rate does not prove to be
   supported.  It if is supported, the sender can resume standard
   congestion control.

Authors' Addresses

   Nicolas Kuhn


   Emile Stephan


   Gorry Fairhurst
   University of Aberdeen


   Tom Jones
   University of Aberdeen


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