Internet Engineering Task Force                                  N. Kuhn
Internet-Draft                                                      CNES
Intended status: Informational                                E. Stephan
Expires: 22 April 2022                                            Orange
                                                            G. Fairhurst
                                                                T. Jones
                                                  University of Aberdeen
                                                              C. Huitema
                                                    Private Octopus Inc.
                                                         19 October 2021


               Transport parameters for 0-RTT connections
                      draft-kuhn-quic-0rtt-bdp-10

Abstract

   QUIC 0-RTT transport features currently focuses on egress traffic
   optimization.  This draft proposes a QUIC extension that improves the
   performance of ingress traffic.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 22 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components



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   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notations and terms . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Safe jump start . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Rationale behind the safety guidelines  . . . . . . . . .   5
     2.2.  Rationale #1: Variable network conditions . . . . . . . .   5
     2.3.  Rationale #2: Malicious clients . . . . . . . . . . . . .   6
     2.4.  Trade-off between the different solutions . . . . . . . .   7
       2.4.1.  Security aspects  . . . . . . . . . . . . . . . . . .   7
       2.4.2.  Interoperability and use-cases  . . . . . . . . . . .   8
       2.4.3.  Summary . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Safety guidelines . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Implementation considerations . . . . . . . . . . . . . . . .  11
     4.1.  Rationale behind the different implementation options . .  11
     4.2.  Independent local storage of values . . . . . . . . . . .  11
     4.3.  Using NEW_TOKEN frames  . . . . . . . . . . . . . . . . .  12
     4.4.  BDP Frame . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.4.1.  BDP Frame Format  . . . . . . . . . . . . . . . . . .  13
       4.4.2.  Extension activation  . . . . . . . . . . . . . . . .  14
   5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  BDP extension protected as much as initial_max_data . . .  14
     5.2.  Other use-cases . . . . . . . . . . . . . . . . . . . . .  15
       5.2.1.  Optimizing client's requests  . . . . . . . . . . . .  15
       5.2.2.  Sharing transport information across multiple
               connections . . . . . . . . . . . . . . . . . . . . .  15
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   QUIC 0-RTT transport features currently focus on egress traffic
   optimization.  This draft proposes a QUIC extension to improve the
   performance of ingress traffic.







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   [RFC9000] mentions that "Generally, implementations are advised to be
   cautious when using previous values on a new path."  This draft
   proposes a discussion on how using previous values can be achieved in
   a interoperable manner and how it can be done safely.

   When clients resume a session to download a large document, the
   congestion control algorithms will require time to ramp-up the packet
   rate.  This document specifies a method that can improve traffic
   delivery and that allows a QUIC connection to avoid a slow Round-Trip
   Time (RTT)-based process to grow connection parameters such as the
   congestion window (CWND):

   1.  During a previous session, current RTT (current_rtt), bottleneck
       bandwidth (current_bb) and client's current IP
       (current_client_ip) are stored as saved_rtt, saved_bb and
       saved_client_ip;

   2.  When resuming a session, the server might set the current_rtt and
       the current_bb to the saved_rtt and saved_bb of a previous
       connection.

   This method applies to any QUIC resumed sessions: both saved_session
   and recon_session can be a 0-RTT QUIC connection or a 1-RTT QUIC
   connection.

   This draft consider different solutions: (1) the saved parameters are
   not sent to the client; (2) the saved parameters are sent to the
   client and the client can not read them; (3) the saved parameters are
   sent to the client and the client can read them.  There is no
   solution where the client can modify the parameters.

   Sometimes the parameters of a previous session are not relevant,
   e.g.: (1) network conditions can change where using a previously
   estimated bottleneck bandwidth could increase congestion; (2) a
   client could convince a server to use a CWND much larger than
   required.

   This draft:

   1.  proposes guidelines for how to safely apply the previously
       computed parameters to new sessions;

   2.  describes different implementation considerations in QUIC for the
       proposed method;

   3.  discusses the trade-off associated to the different
       implementation solutions.




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1.1.  Notations and terms

   *  IW: Initial window (e.g. from [RFC6928]);

   *  current_iw: Current Initial window;

   *  recom_iw: Recommended Initial window - it seems important to note
      that some Content Delivery Networks (CDNs) currently exploit a
      very high Initial Window (IW) [TMA18] for a local path;

   *  BDP: defined below;

   *  CWND: congestion window used by server (bytes allowed in flight by
      CC);

   *  current_bb : Current estimated bottleneck bandwidth;

   *  saved_bb: Estimated bottleneck bandwidth preserved from a previous
      connection;

   *  RTT: Round-Trip Time;

   *  current_rtt: Current RTT;

   *  saved_rtt: RTT preserved from a previous connection.

   *  client_ip : IP address of the client

   *  current_client_ip : Current IP address of the client

   *  saved_client_ip : IP address of the client preserved from a
      previous connection;

   *  remembered BDP parameters: combination of saved_rtt and saved_bb.

   *  ITT : Interpacket Transmission Time

   *  MSS : Maximum Message Size

   *  AEAD : Authenticated Encryption with Associated Data

   *  LRU : Least Recently Used

   [RFC6349] defines the BDP as follows: "Derived from Round-Trip Time
   (RTT) and network Bottleneck Bandwidth (BB), the Bandwidth-Delay
   Product (BDP) determines the Send and Received Socket buffer sizes
   required to achieve the maximum TCP Throughput."  This draft
   considers the Bandwidth-Delay Product (BDP) as estimated by the



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   server which includes all buffering along the network path.  A QUIC
   connection might not exactly reproduce the procedure detailed in
   [RFC6349] to measure the BDP.  The server can exploit internal
   evaluations of the Bottleneck Bandwidth and the to assess the BDP.

   This document refers to the saved_bb and current_bb for the
   previously estimated bottleneck bandwidth.  This value may be easily
   reached for protocols suc as BBR.  Other protocols, such as CUBIC or
   RENO, may estimate this value by exploiting the congestion window and
   the RTT.  This approach may result in over estimating the bottleneck
   bandwidth and should be considered with caution.

1.2.  Requirements Language

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Safe jump start

2.1.  Rationale behind the safety guidelines

   The previously measured saved_rtt and saved_bb should not be used as-
   is to avoid potential congestion collapse:

   *  Rationale #1: An Internet method needs to be robust to network
      conditions that can differ between sessions.

   *  Rationale #2: Information sent by a malicious client would not be
      relevant since it might try to convince servers to use a CWND
      higher than required.  This could increase congestion.

2.2.  Rationale #1: Variable network conditions

   The server MUST check the validity of the saved_rtt and saved_bb
   parameters, whether they are sent by a client or stored at the
   server.  Indeed, the following events make use of these parameters
   inappropriate:

   *  IP address changes: If the client changes its IP address (i.e.
      saved_client_ip is different from current_client_ip), the
      different address indicates a different network path.  This new
      path does not necessarily exhibit the same characteristics as the
      old one.  If the server changes its IP address after a migration,
      it may not be safe to exploit previously estimated parameters.




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   *  Lifetime of the extension: If the network conditions change, e.g.,
      the path was not congested when BDP parameters were evaluated, but
      later the path experiences congestion for the next connection, the
      previously estimated parameters would not be valid.

   *  Relevance of the BB estimation: There are cases where the
      congestion window is exploited to assess the bottleneck bandwidth.
      However, at the end of slow start its value may be significantly
      larger than the value on which the connection converges after a
      few rounds.  Directly exploiting such value for the bottleneck
      bandwith estimation may be inappropriate.

   There are different solutions for the variable network conditions:

   *  Rationale #1 - Solution #1 : When resuming a session, set the
      current_bb and current_rtt to the saved_bb and saved_rtt
      parameters estimated from a previous connection.

   *  Rationale #1 - Solution #2 : When resuming a session, implement a
      safety check to measure whether using the saved_bb and saved_rtt
      parameters would not cause congestion over the path.  In this
      case, the current_bb and current_rtt might not be set directly to
      the saved_bb and saved_rtt: the server might wait for the
      completion of the safety check before doing so.

   Section 3 describes various approaches for Rationale #1 - Solution
   #2.

2.3.  Rationale #2: Malicious clients

   The server MUST check the integrity of the saved_rtt and saved_bb
   parameters received from a client.

   There are different solutions to avoid attacks by malicious clients:

   *  Rationale #2 - Solution #1 : The server stores a local estimation
      of the bottleneck bandwidth and RTT parameters as saved_bb and
      saved_rtt.

   *  Rationale #2 - Solution #2 : The server sends the estimation of
      the bottleneck bandwidth and RTT parameters to the client as
      saved_bb and saved_rtt.  The information is encrypted by the
      server.  The client resends the information when resuming a
      connection.  The client can neither read nor modify the saved_rtt
      and saved_bb parameters.






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   *  Rationale #2 - Solution #3 : The server sends the estimation of
      the saved_rtt and saved_bb parameters to the client.  The
      information includes integrity protection.  The client resends the
      information when resuming a connection.  The client can read, but
      can not modify, the saved_rtt and saved_bb parameters.

   Section 4 describes various implementation approaches for each of
   these solutions using local storage ( Section 4.2 for Rationale #2 -
   Solution #1), NEW_TOKEN Frame ( Section 4.3 for Rationale #2 -
   Solution #2), BDP extension Frame ( Section 4.4 for Rationale #2 -
   Solution #3).

2.4.  Trade-off between the different solutions

   This section provides a description of different implementation
   options and discusses their respective advantages and drawbacks.
   While there are some discussions for the solutions regarding
   Rationale #2, the server MUST consider Rationale #1 - Solution #2 and
   avoid Rationale #1 - Solution #1: the server MUST implement a safety
   check to measure whether the remembered BDP parameters (i.e.
   saved_rtt and saved_bb) are relevant or check that their usage would
   not cause congestion over the path.

2.4.1.  Security aspects

   The client may send information related to the saved_rtt and saved_bb
   to the server with the BDP Frame extension using either Rationale #2
   - Solution #2 or Rationale #2 - Solution #3.  However, the server may
   not trust the client.  Indeed, even if 0-RTT packets containing the
   BDP Frame are encrypted, a client could modify the values within the
   extension and encrypt the 0-RTT packet.  Authentication mechanisms
   might not guarantee that the values are safe.  The server could then
   need to also store the saved_rtt and saved_bb parameters.

   A malicious client might modify the saved_bb parameter to convince
   the server to use a CWND much larger than required.  Using the
   algorithms proposed in Section 3, the server may reduce any intended
   harm and can check that part of the information provided by the
   client are valid.  A supplementary check could decide not to use
   values that would be higher than those currently used by CDNs
   [TMA18].

   Storing the BDP parameters locally at the server reduces the
   associated risks by allowing the client to transmit information
   related to the BDP of the path.






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2.4.2.  Interoperability and use-cases

   If the server stores a resumption ticket for each client to protect
   against replay on a third party IP, it could also store the IP
   address (i.e.  saved_client_ip) and BDP parameters (i.e. saved_rtt
   and saved_bb) of the previous session of the client.

   In cases where the BDP Frame extension is exploited, the approach of
   storing the BDP parameters locally at the server can provide a cross-
   check of the BDP parameters sent by a client.  The server can anyway
   enable a safe jump start, but without the BDP Frame extension, the
   client does not have the choice of accepting it or not.

   While storing local values related to the BDP would help in improving
   the ingress for 0-RTT connections, not using a BDP Frame extension
   may reduce the interest of the approach where (1) the client knows
   the BDP estimations done at the server, (2) the client decides to
   accept or reject ingress optimization, (3) the client tunes
   application level requests.

2.4.3.  Summary

   As a summary, the approach of local storage of values is more secure
   and the BDP Frame extension provides more information to the client
   and more interoperability.  The Figure 1 provides a summary of the
   advantages and drawbacks of each approach.

   +---------+-----------+----------------+---------------+-----------+
   |Rationale| Solution  |    Advantage   |    Drawback   |  Comment  |
   +---------+-----------+----------------+---------------+-----------+
   |#1       |#1         |                |               |           |
   |Variable |set        |Ingress optim.  |Risks of adding|MUST NOT   |
   |Network  |current_*  |                | congestion    |implement  |
   |         |to saved_* |                |               |           |
   |         +-----------+----------------+---------------+-----------+
   |         |#2         |                |               |           |
   |         |Implement  |Reduce risks of |Negative impact|MUST       |
   |         |safety     | adding         | on ingress    |implement  |
   |         |check      | congestion     | optim.        |Section 3  |
   +---------+-----------+----------------+---------------+-----------+
   |#2       |#1         |                |               |           |
   |Malicious|Local      |Enforced        |Client can not |           |
   |client   |storage    | security       | decide to     |           |
   |         |           |                | reject        |           |
   |         |           |                |Malicious      |           |
   |         |           |                | server could  |           |
   |         |           |                | fill client's |           |
   |         |           |                | buffer        |           |



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   |         |           |                |Limited        |           |
   |         |           |                | use-cases     |Section 4.2|
   |         +-----------+----------------+---------------+-----------+
   |         |#2         |                |               |           |
   |         |NEW_TOKEN  |Save resource   |Malicious      |           |
   |         |           | at server      | client may    |           |
   |         |           |Opaque token    | change token  |           |
   |         |           | protected      | even if       |           |
   |         |           |                | protected     |           |
   |         |           |                |Malicious      |           |
   |         |           |                | server could  |           |
   |         |           |                | fill client's |           |
   |         |           |                | buffer        |           |
   |         |           |                |Server may not |           |
   |         |           |                | trust client  |Section 4.3|
   |         +-----------+----------------+---------------+-----------+
   |         |#3         |                |               |           |
   |         |BDP        |Extended        |Malicious      |           |
   |         |extension  | use-cases      | client may    |           |
   |         |           |Save resource   | change BDP    |           |
   |         |           | at server      | even if       |           |
   |         |           |Client can      | protected     |           |
   |         |           | read and decide|Server may not |           |
   |         |           | to reject      | trust client  |           |
   |         |           |BDP extension   |               |           |
   |         |           | protected      |               |           |
   |         |           |                |               |Section 4.4|
   +---------+-----------+----------------+---------------+-----------+

                       Figure 1: Comparing solutions

3.  Safety guidelines

   The safety guidelines are designed to avoid a server adding excessive
   congestion to an already congested path.  The following mechanisms
   should help in fulfilling this objective:

   *  The server SHOULD compare the measured transport parameters (in
      particular current_rtt) of the 0-RTT connection with those of the
      1-RTT connection (in particular saved_rtt);

   *  The server SHOULD NOT consider the saved_bb parameter if there is
      any loss of packet during the first transmission of data;

   *  The server MUST NOT send more than a recommended maximum IW
      (recom_iw) in the first transmission of data.  This value could be
      based on a local understanding of the path characteristics and
      what is deployed in CDNs [TMA18].



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   *  The server SHOULD NOT store and/or send information related to the
      previously estimated bottleneck bandwidth (saved_bb) if this
      estimation has not been computed after some rounds.

   The proposed mechanisms SHOULD be limited by any rate-limitation
   mechanisms of QUIC, such as flow control mechanisms or amplification
   attacks prevention.  In particular, it may be necessary to issue
   proactive MAX_DATA frames to increase the flow control limits of the
   connection.  In particular, the maximum number of packets that can be
   sent without acknowledgment needs to be chosen to avoid the creation
   and the increase of congestion for the path.  Moreover, this
   extension should not be an opportunity for the current connection to
   be a vector of an amplification attack.  The address validation
   process, used to prevent amplification attacks, SHOULD be performed
   [RFC9000].

   The following mechanisms could be implemented:

   *  Exploit a standard IW:

      1.  The server sends the first data packet using the IW - this can
          be considered a safe starting point for an unknown path, which
          avoids adding congestion to the path;

      2.  If the reception of IW exhibits characteristics that resemble
          those of a recent previous session from the client (i.e.
          current_rtt < 1.2*saved_rtt and all the data was
          acknowledged), the method permits the sender to consider the
          saved_bb as an input to adapt current_bb and help rapidly
          determine a new safe rate;

      3.  The sender needs to avoid a burst of packets being sent as a
          result of a step-increase in the congestion window [RFC9000].
          Pacing the packets as a function of the current_rtt can
          provide this additional safety during the period in which the
          CWND is increased by the method.

   *  Identify a relevant pacing rhythm:

      -  The server estimates the pacing rhythm using saved_rtt and
         saved_bb.  The Interpacket Transmission Time (ITT) is
         determined by the ratio between the current Maximum Message
         Size (MSS) for packets and the ratio between the saved_bb and
         saved_rtt.  A tunable safety margin might be introduced to
         avoid sending more than a recommended maximum IW (recom_iw):

         o  current_iw = min(recom_iw,saved_bb)




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         o  ITT = MSS/(current_iw/saved_rtt)

      -  When the IW is acknowledged, the server falls back to a
         standard slow-start mechanism.

   *  Tune slow-start mechanisms.  After transport parameters are set to
      previously estimated bottleneck bandwidth, if slow-start
      mechanisms continue, important packets overshoot may be
      transmitted from the server.  This can occur even if the safety
      check described in this section is implemented.

      -  For NewReno and CUBIC, it is recommended to exit slow-start and
         enter in congestion avoidance phase.

      -  For BBR, it is recommended to move to the "probe bandwidth"
         state.

   This follows the idea of [RFC4782],
   [I-D.irtf-iccrg-sallantin-initial-spreading] and [CONEXT15].

   While safety recommendations are necessary, it seems important to
   note that some Content Delivery Networks (CDNs) currently exploit a
   very high Initial Window (IW) [TMA18] for a local path.

4.  Implementation considerations

4.1.  Rationale behind the different implementation options

   Using NewSessionTickets messages of TLS is a solution that could have
   been envisioned.  The idea would have been to add a 'bdp_metada'
   field in the NewSessionTickets that the client could read.  The sole
   extension currently defined in TLS1.3 that can be seen by the client
   is max_early_data_size (see section 4.6.1 of [RFC8446]).  However, in
   the general design of QUIC, TLS sessions are managed by the TLS
   stacks.

   Three distinct approaches are presented: sending an opaque blob to
   the client that it may retransmit for future connection (see
   Section 4.3), enable a local storage of BDP related values (see
   Section 4.2) and a BDP Frame extension (see Section 4.4).

4.2.  Independent local storage of values

   This approach independently lets both a client and a server remember
   their BDP parameters:






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   *  During a 1-RTT session, the endpoint stores the RTT (as the
      saved_rtt) and bottleneck bandwidth (as the saved_bb) together
      with the session resume ticket.  The client can also store the IP
      address of the server.

   *  The server maintains a table of previously issued tickets, indexed
      by the random ticket identifier that is used to guarantee
      uniqueness of the Authenticated Encryption with Associated Data
      (AEAD) encryption.  Old tokens are removed from the table using
      the Least Recently Used (LRU) logic.  For each ticket identifier,
      the table holds the RTT and bottleneck bandwidth (i.e. saved_rtt
      and saved_bb), and also the IP address of the client (i.e.
      saved_client_ip).

   During the 0-RTT session, the endpoint wait for the first RTT
   measurement from the peer's IP address.  This is used to verify that
   the current_rtt has not significantly changed from the saved_rtt, and
   hence is an indication that the BDP information applies to the path
   that is currently being used.

   If this RTT is confirmed (e.g. current_rtt < 1.2*saved_rtt, the
   endpoint also verifies that an initial window of data has been
   acknowledged without requiring retransmission.  This second check is
   used to detect a path with significant incipient congestion (i.e.
   where it would not be safe to update the CWND based on the saved_bb).
   In practice, this could be realized by a proportional increase in the
   CWND, where the increase is (saved_bb/IW)*proportion_of_IW_currently-
   ACKed.

   This solution does not allow the client to refuse the exploitation of
   the BDP parameters.  If the server does not want to store the metrics
   from previous connections, an equivalent of the tcp_no_metrics_save
   for QUIC may be necessary.  This option could be negociable so that a
   client can refuse the exploitation of previous sessions.

4.3.  Using NEW_TOKEN frames

   Using NEW_TOKEN Frames, the server could send a token to the client
   through a NEW_TOKEN Frame.  The token is an opaque blob and the
   client can not read its content (see section 19.7 of [RFC9000]).  The
   client sends the received token in the header of an Initial packet
   for future connection.









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4.4.  BDP Frame

   This section proposes the exploitation of a new Frame, the BDP Frame.
   The BDP Frame MUST be contained in 0-RTT packets if sent by the
   client.  The BDP Frame MUST be contained in 1-RTT packets if sent by
   the server.  The BDP Frame MUST be considered in the congestion
   control and its data may not be limited by flow control limits.  The
   server MAY send multiple BDP Frames in both 1-RTT and 0-RTT
   connections.  The client may send BDP Frames during 1-RTT and 0-RTT
   connections.

4.4.1.  BDP Frame Format

   A BDP Frame is formatted as shown in Figure 2.

   BDP Frame {
     Type (i) = 0xXXX,
     Lifetime (i),
     Saved BB (i),
     Saved RTT (i),
     Saved IP length (i),
     Saved IP (...)
   }

                         Figure 2: BDP Frame Format

   A BDP Frame contains the following fields:

   *  Lifetime (extension_lifetime): The extension_lifetime is a value
      in milliseconds, encoded as a variable length integer.  This
      follows the idea of NewSessionTicket of TLS [RFC8446].  This
      represents the validity in time of this extension.

   *  Saved BB (saved_bb): The saved_bb is a value in bytes, encoded as
      a variable length integer.  The bottleneck bandwidth estimated on
      the previous connection by the server.  Using the previous values
      of bytes_in_flight defined in [RFC9002] can result in overshoot
      and is not advised.

   *  Saved RTT (saved_rtt): The saved_rtt is a value in milliseconds,
      encoded as a variable length integer.  This could be set to the
      min_rtt defined in [RFC9002], saved_rtt can be set to min_rtt.
      The min_rtt parameter might not track a decreasing RTT: the
      min_rtt that is reported here might not be the actual minimum RTT
      measured during the 1-RTT connection, but usually reflects the
      characteristics of the path latency.





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   *  Saved IP length (saved_ip_lenght) : The length of the IP address
      set to either 4 (IPv4) or 16 (IPv6).

   *  Saved IP (saved_client_ip) : The saved_client_ip could be set to
      the IP address of the client.

4.4.2.  Extension activation

   The client can accept the transmission of BDP Frames from the server
   by using the following enable_bdp transport extension.

   enable_bdp (0xTBD): in the 1-RTT connection, the client indicates to
   the server that it wishes to receive BDP extension Frames for
   improving ingress of 0-RTT connection.  The default value is 0.
   Values strictly above 3 are invalid, and receipt of these values MUST
   be treated as a connection error of type TRANSPORT_PARAMETER_ERROR.

   *  0: Default value.  If the client does not send this parameter, the
      server considers that the client does not support or does not wish
      to activate the BDP extension.

   *  1: The client indicates to the server that it wishes to receive
      BDP Frame and activates the ingress optimization for the 0-RTT
      connection.

   *  2: The client indicates that it does not wish to receive BDP
      Frames but activates ingress optimization.

   *  3: The client indicates that it wishes to receive BDP Frames but
      does not activate ingress optimization.

   This Transport Parameter is encoded as per Section 18 of [RFC9000].

5.  Discussion

5.1.  BDP extension protected as much as initial_max_data

   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
   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 [RFC9001].  This provides the
   server with a way to verify that the parameters proposed by the
   client are the same as those that the server sent to the client
   during the previous connection.





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5.2.  Other use-cases

5.2.1.  Optimizing client's requests

   In a case with Dynamic Adaptive Streaming over HTTPS (DASH), clients
   might encounter issues in knowing the available path capacity or DASH
   can encounter issues in reaching the best available video playback
   quality.  The client requests could then be adapted and specific
   traffic could utilize information from the path characteristics (such
   as encouraging the client to increase the quality of video chunks, to
   fill the buffers and avoid video blocking or to send high quality
   adds).

   In other cases, applications may provide additional services if
   clients can know the server's estimation of the path characteristics.

5.2.2.  Sharing transport information across multiple connections

   There can be benefit in sharing transport information across multiple
   connections.  [I-D.ietf-tcpm-2140bis] considers the sharing of
   transport parameters between TCP connections originating from the
   same host.  The proposal in this document has the advantage of
   storing server-generated information at the client and not requiring
   the server to retain additional state for each client.

6.  Acknowledgments

   The authors would like to thank Gabriel Montenegro, Patrick McManus,
   Ian Swett, Igor Lubashev, Robin Marx, Roland Bless and Franklin Simo
   for their fruitful comments on earlier versions of this document.

7.  IANA Considerations

   TBD: Text is required to register the BDP Frame and the enable_bdp
   transport parameter.  Parameters are registered using the procedure
   defined in [RFC9000].

8.  Security Considerations

   Security considerations are discussed in Section 5 and in Section 3.

9.  References

9.1.  Normative References

   [I-D.ietf-tcpm-2140bis]
              Touch, J., Welzl, M., and S. Islam, "TCP Control Block
              Interdependence", Work in Progress, Internet-Draft, draft-



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              ietf-tcpm-2140bis-11, 12 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tcpm-2140bis-
              11.txt>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4782]  Floyd, S., Allman, M., Jain, A., and P. Sarolahti, "Quick-
              Start for TCP and IP", RFC 4782, DOI 10.17487/RFC4782,
              January 2007, <https://www.rfc-editor.org/info/rfc4782>.

   [RFC6349]  Constantine, B., Forget, G., Geib, R., and R. Schrage,
              "Framework for TCP Throughput Testing", RFC 6349,
              DOI 10.17487/RFC6349, August 2011,
              <https://www.rfc-editor.org/info/rfc6349>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <https://www.rfc-editor.org/info/rfc6928>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/info/rfc9002>.

9.2.  Informative References

   [CONEXT15] Li, Q., Dong, M., and P B. Godfrey, "Halfback: Running
              Short Flows Quickly and Safely", ACM CoNEXT , 2015.



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   [I-D.irtf-iccrg-sallantin-initial-spreading]
              Sallantin, R., Baudoin, C., Arnal, F., Dubois, E., Chaput,
              E., and A. Beylot, "Safe increase of the TCP's Initial
              Window Using Initial Spreading", Work in Progress,
              Internet-Draft, draft-irtf-iccrg-sallantin-initial-
              spreading-00, 15 January 2014,
              <https://www.ietf.org/archive/id/draft-irtf-iccrg-
              sallantin-initial-spreading-00.txt>.

   [TMA18]    Ruth, J. and O. Hohlfeld, "Demystifying TCP Initial Window
              Configurations of Content Distribution Networks", 2018
              Network Traffic Measurement and Analysis Conference
              (TMA) , 2018.

Authors' Addresses

   Nicolas Kuhn
   CNES

   Email: nicolas.kuhn.ietf@gmail.com


   Emile Stephan
   Orange

   Email: emile.stephan@orange.com


   Godred Fairhurst
   University of Aberdeen
   Department of Engineering
   Fraser Noble Building
   Aberdeen

   Email: gorry@erg.abdn.ac.uk


   Tom Jones
   University of Aberdeen
   Department of Engineering
   Fraser Noble Building
   Aberdeen

   Email: tom@erg.abdn.ac.uk







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   Christian Huitema
   Private Octopus Inc.

   Email: huitema@huitema.net















































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