Standard Communication with Network Elements (SCONE) Protocol
draft-ietf-scone-protocol-02

SCONE                                                         M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Informational                                C. Huitema
Expires: 8 January 2026                             Private Octopus Inc.
                                                        奥 一穂 (K. Oku)
                                                                  Fastly
                                                                M. Joras
                                                                    Meta
                                                                M. Ihlar
                                                                Ericsson
                                                             7 July 2025

     Standard Communication with Network Elements (SCONE) Protocol
                      draft-ietf-scone-protocol-02

Abstract

   This document describes a protocol where on-path network elements can
   give endpoints their perspective on what the maximum achievable
   throughput might be for QUIC flows.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at https://ietf-wg-
   scone.github.io/scone/draft-ietf-scone-protocol.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-scone-protocol/.

   Discussion of this document takes place on the SCONE Working Group
   mailing list (mailto:scone@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/scone/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/scone/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-scone/scone.

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/.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Independent of Congestion Signals . . . . . . . . . . . .   5
     3.2.  Unspecified Scope . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Per-Flow Signal . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Undirectional Signal  . . . . . . . . . . . . . . . . . .   6
     3.5.  Advisory Signal . . . . . . . . . . . . . . . . . . . . .   6
   4.  Conventions and Definitions . . . . . . . . . . . . . . . . .   6
   5.  SCONE Packet  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Rate Signals  . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Endpoint Processing of SCONE Packets  . . . . . . . . . .   9
   6.  Negotiating SCONE . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Deployment  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Applying Throughput Advice Signals  . . . . . . . . . . .  10
   8.  Version Interaction . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Providing Opportunities to Apply Throughput Advice
           Signals . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.2.  Feedback To Sender About Signals  . . . . . . . . . . . .  11
     8.3.  Interactions with congestion control  . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     9.1.  Flooding intermediaries with fake packets . . . . . . . .  13
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
     10.1.  Passive Attacks  . . . . . . . . . . . . . . . . . . . .  15
     10.2.  Active Attacks . . . . . . . . . . . . . . . . . . . . .  15

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   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  SCONE Versions . . . . . . . . . . . . . . . . . . . . .  16
     11.2.  scone_supported Transport Parameter  . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   Many access networks apply rate limits to constrain the data rate of
   attached devices.  This is often done without any indication to the
   applications running on devices.  The result can be that application
   performance is degraded, as the manner in which rate limits are
   enforced can be incompatible with the rate estimation or congestion
   control algorithms used at endpoints.

   Having the network indicate what its rate limiting policy is, in a
   way that is accessible to endpoints, allows applications to use this
   information when adapting their send rate.

   Network elements are not limited to communicating information about
   rate limiting policies.  Network elements in access networks could
   provide information to endpoints that can help account for changes in
   network capacity that are not suited to congestion control feedback.
   This might include reduced capacity due to overuse, equipment faults,
   or other transient issues; conversely, networks might choose to
   signal increased availability of capacity.

   The Standard Communication with Network Elements (SCONE) protocol is
   negotiated by QUIC endpoints.  This protocol provides a means for
   network elements to signal the maximum available sustained
   throughput, or rate limits, for flows of UDP datagrams that transit
   that network element to a QUIC endpoint.

   Networks with rate limiting policies use SCONE to send throughput
   advice to cooperating endpoints to limit overall network usage.
   Where congestion control signals -- such as ECN, delays and loss --
   operate on a time scale of a round trip time, throughput advice
   operates over a much longer period.  This has benefits in some
   networks as endpoints can fully consume network capacity in bursts,
   rather than extending network interaction at lower rates.

   For endpoints, SCONE throughput advice makes network policies
   visible, which can reduce wasteful probing beyond those limits.

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2.  Overview

   QUIC endpoints can negotiate the use of SCONE by including a
   transport parameter (Section 6) in the QUIC handshake.  Endpoints
   then occasionally coalesce a SCONE packet with ordinary QUIC packets
   that they send.

   Network elements that have rate limiting policies can detect flows
   that include SCONE packets.  The network element can indicate a
   maximum sustained throughput by modifying the SCONE packet as it
   transits the network element.

   +--------+    +---------+     +----------+
   |  QUIC  |    | Network |     |   QUIC   |
   | Sender |    | Element |     | Receiver |
   +---+----+    +----+----+     +----+-----+
       |              |               |
       +--- SCONE --->|   SCONE+rate  |
       |    +QUIC     +---- +QUIC --->|
       |              |               |  Validate QUIC packet
       |              |               |  and record rate
       |              |               |

   QUIC endpoints that receive modified SCONE packets observe the
   indicated version, process the QUIC packet, and then record the
   indicated rate.

   Indicated rate limits apply only in a single direction.  Separate
   indications can be sent for the client-to-server direction and
   server-to-client direction.  The indicated rates do not need to be
   the same.

   Indicated rate limits only apply to the path on which they are
   received.  A connection that migrates or uses multipath [QUIC-MP]
   cannot assume that rate limit indications from one path apply to new
   paths.

3.  Applicability

   This protocol only works for flows that use the SCONE packet
   (Section 5).

   The protocol requires that packets are modified as they transit a
   network element, which provides endpoints strong evidence that the
   network element has the power to apply rate limiting; though see
   Section 9 for potential limitations on this.

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   The throughput advice signal that this protocol carries is
   independent of congestion signals, limited to a single path and UDP
   packet flow, unidirectional, and strictly advisory.

3.1.  Independent of Congestion Signals

   Throughput advice signals are not a substitute for congestion
   feedback.  Congestion signals, such as acknowledgments, provide
   information on loss, delay, or ECN markings [ECN] that indicate the
   real-time condition of a network path.  Congestion signals might
   indicate a throughput that is different from the signaled rate limit.

   Endpoints cannot assume that a signaled rate limit is achievable if
   congestion signals indicate otherwise.  Congestion could be
   experienced at a different point on the network path than the network
   element that indicates a rate limit.  Therefore, endpoints need to
   respect the send rate constraints that are set by a congestion
   controller.

3.2.  Unspecified Scope

   Modifying a packet does not prove that the throughput that is
   indicated would be achievable.  A signal that is sent for a specific
   flow is likely enforced at a different scope.  The extent of that
   scope is not carried in the signal.

   For instance, limits might apply at a network subscription level,
   such that multiple flows receive the same signal.

   Endpoints can therefore be more confident in the throughput signal as
   an indication of the maximum achievable throughput than as any
   indication of expected throughput.  That throughput will only be
   achievable when there is no significant data flowing in the same
   scope.  In the presence of other flows, congestion limits are likely
   to determine actual throughput.

   This makes the application of signals most usefully applied to a
   downlink flow in access networks, close to an endpoint.  In that
   case, capacity is less likely to be split between multiple active
   flows.

3.3.  Per-Flow Signal

   The same UDP address tuple might be used for multiple QUIC
   connections.  A single signal might be lost or only reach a single
   application endpoint.  Network elements that signal about a flow
   might choose to send additional signals, using connection IDs to
   indicate when new connections could be involved.

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3.4.  Undirectional Signal

   The endpoint that receives a throughput advice signal is not the
   endpoint that might adapt its sending behavior as a result of
   receiving the signal.  This ensures that the throughput advice signal
   is attached to the flow that it is mostly likely to apply to.

   An endpoint might need to communicate the value it receives to its
   peer in order to ensure that the limit is respected.  This document
   does not define how that signaling occurs as this is specific to the
   application in use.

3.5.  Advisory Signal

   A signal does not prove that a higher rate would not be successful.
   Endpoints that receive this signal therefore need to treat the
   information as advisory.

   The fact that an endpoint requests bitrate signals does not
   necessarily mean that it will adhere to them; in some cases, the
   endpoint cannot.  For example, a flow may initially be used to serve
   video chunks, with the client selecting appropriate chunks based on
   bitrate signals, but later switch to a bulk download for which
   bitrate adaptation is not applicable.  Composite flows from multiple
   applications, such as tunneled flows, might only have a subset of the
   involved applications that are capable of handling SCONE signals.
   Therefore, when a network element detects a flow using more bandwidth
   than advertised via SCONE, it might switch to applying its policies
   for non-SCONE flows, using congestion control signals.

   The time and scope over which throughput advice applies is not
   specified.  Network conditions and rate-limit policies can change in
   ways that make previously signaled advice obsolete, and there are no
   guarantees that updated advice will be sent at such events.  The
   signaled advice can be assumed to apply to the flow of packets on the
   same UDP address tuple for the duration of that flow.  For rate
   limiting networks, rate limiting policies often apply on the level of
   a device or subscription, but endpoints cannot assume that this is
   the case.  A separate signal can be sent for each flow.

4.  Conventions and Definitions

   The key words "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 [BCP14] when, and only when, they appear in all capitals, as
   shown here.

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5.  SCONE Packet

   A SCONE packet is a QUIC long header packet that follows the QUIC
   invariants; see Section 5.1 of [INVARIANTS].

   Figure 1 shows the format of the SCONE packet using the conventions
   from Section 4 of [INVARIANTS].

   SCONE Packet {
     Header Form (1) = 1,
     Reserved (1),
     Rate Signal (6),
     Version (32) = 0xSCONE1 or 0xSCONE2,
     Destination Connection ID Length (8),
     Destination Connection ID (0..2040),
     Source Connection ID Length (8),
     Source Connection ID (0..2040),
   }

                       Figure 1: SCONE Packet Format

   The most significant bit (0x80) of the packet indicates that this is
   a QUIC long header packet.  The next bit (0x40) is reserved and can
   be set according to [QUIC-BIT].

   The low 6 bits (0x3f) of the first byte contain the Rate Signal
   field.  Values for this field are described in Section 5.1.

   This packet includes a Destination Connection ID field that is set to
   the same value as other packets in the same datagram; see
   Section 12.2 of [QUIC].

   The Source Connection ID field is set to match the Source Connection
   ID field of any packet that follows.  If the next packet in the
   datagram does not have a Source Connection ID field, which is the
   case for packets with a short header (Section 5.2 of [INVARIANTS]),
   the Source Connection ID field is empty.

   SCONE packets SHOULD be included as the first packet in a datagram.
   This is necessary in many cases for QUIC versions 1 and 2 because
   packets with a short header cannot precede any other packets.

5.1.  Rate Signals

   The Rate Signal field in SCONE uses the low 6 bits (0x3f) of the
   first byte.  This field is encoded as a logarithmically spaced
   distribution over a range defined by the SCONE protocol version.

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   When sent by a QUIC endpoint, the Version field of a SCONE packet is
   set to 0xSCONE2 and the Rate Signal field is set to 0x3F (63),
   indicating no rate limit is in place or that the SCONE protocol is
   not supported by network elements on the path.  All other values
   (0x00 through 0x3F for protocol version 0xSCONE1 and 0x00 through
   0x3E for protocol version 0xSCONE2) represent the ceiling of rates
   being advised by the network element(s) on the path.

   For SCONE protocol version 0xSCONE1, the rate limits use a
   logarithmic scale with:

   *  Base rate (b_min) = 100 Kbps

   *  Bitrate at value n = b_min * 10^(n/20)

   For SCONE protocol version 0xSCONE2, the rate limits use a
   logarithmic scale with:

   *  Bitrate at value n = b_min * 10^((n + 64)/20)

   With two versions combined, bitrates between 100 Kbps and 199.5 Gbps
   can be expressed.

   Some notable values in these ranges include:

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                  +==========+=============+============+
                  | Version  | Rate Signal | Bitrate    |
                  +==========+=============+============+
                  | 0xSCONE1 | 0           | 100 Kbps   |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 10          | 316 Kbps   |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 20          | 1 Mbps     |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 30          | 3.16 Mbps  |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 40          | 10 Mbps    |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 50          | 31.6 Mbps  |
                  +----------+-------------+------------+
                  | 0xSCONE1 | 60          | 100 Mbps   |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 6           | 316 Mbps   |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 16          | 1 Gbps     |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 26          | 3.16 Gbps  |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 36          | 10 Gbps    |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 46          | 31.6 Gbps  |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 56          | 100 Gbps   |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 62          | 199.5 Gbps |
                  +----------+-------------+------------+
                  | 0xSCONE2 | 63          | No limit   |
                  +----------+-------------+------------+

                                  Table 1

5.2.  Endpoint Processing of SCONE Packets

   Processing a SCONE packet involves reading the value from the Rate
   Signal field.  However, this value MUST NOT be used unless another
   packet from the same datagram is successfully processed.  Therefore,
   a SCONE packet always needs to be coalesced with other QUIC packets.

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   A SCONE packet is defined by the use of the longer header bit (0x80
   in the first byte) and the SCONE protocol version (0xTBD in the next
   four bytes).  A SCONE packet MAY be discarded, along with any packets
   that come after it in the same datagram, if the Source Connection ID
   is not consistent with those coalesced packets, as specified in
   Section 5.

   A SCONE packet MUST be discarded if the Destination Connection ID
   does not match one recognized by the receiving endpoint.

6.  Negotiating SCONE

   A QUIC endpoint indicates that it is willing to receive SCONE packets
   by including the scone_supported transport parameter (0xTBD).

   This transport parameter is valid for QUIC versions 1 [QUIC] and 2
   [QUICv2] and any other version that recognizes the versions,
   transport parameters, and frame types registries established in
   Sections 22.2, 22.3, and 22.4 of [QUIC].

7.  Deployment

   QUIC endpoints can enable the use of the SCONE protocol by sending
   SCONE packets Section 5.  Network elements then apply or replace the
   Rate Signal field (Section 7.1) according to their policies.

7.1.  Applying Throughput Advice Signals

   A network element detects a SCONE packet by observing that a packet
   has a QUIC long header and one of the SCONE protocol versions
   (0xSCONE1 or 0xSCONE2).

   A network element then conditionally replaces the Version field and
   the Rate Signal field with values of its choosing.

   A network element might receive a packet that already includes a rate
   signal.  The network element replaces the rate signal if it wishes to
   signal a lower rate limit; otherwise, the original values are
   retained, preserving the signal from the network element with the
   lower policy.

   The following pseudocode indicates how a network element might detect
   a SCONE packet and replace an existing rate signal, given throughput
   advice (target_throughput).

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   is_long = packet[0] & 0x80 == 0x80
   packet_version = ntohl(packet[1..5])
   if is_long and (packet_version == SCONE1_VERSION or
                   packet_version == SCONE2_VERSION):
     packet_throughput = \
       signal_to_throughput(packet_version, packet[0] & 0x3f)

     if target_throughput < packet_throughput:
       target_version, target_signal = \
         throughput_to_signal(target_throughput)
       packet[0] = packet[0] & 0xc0 | target_signal
       if target_version != packet_version:
         packet[1..5] = htonl(target_version)

8.  Version Interaction

   The SCONE protocol defines two versions (0xSCONE1 and 0xSCONE2) that
   cover different ranges of bitrates.  This design allows for:

   *  Support for both very low bitrates (down to 100 Kbps) and very
      high bitrates (up to 199.5 Gbps)

   *  Graceful handling of network elements that might only recognize
      one version.

8.1.  Providing Opportunities to Apply Throughput Advice Signals

   Endpoints that wish to offer network elements the option to add
   throughout advice signals can send SCONE packets at any time.  This
   is a decision that a sender makes when constructing datagrams.  It is
   recommended that endpoints promptly send an initial SCONE packet once
   the peer confirms its willingness to receive them.

   Endpoints MUST send any SCONE packet they send as the first packet in
   a datagram, coalesced with additional packets.  An endpoint that
   receives and discards a SCONE packet without also successfully
   processing another packet from the same datagram SHOULD ignore any
   throughput advice signal.  Such a datagram might be entirely spoofed.

   A network element that wishes to signal an updated rate limit waits
   for the next SCONE packet in the desired direction.

8.2.  Feedback To Sender About Signals

   Information about throughout advice is intended for the sending
   application.  Any signal from network elements can be propagated to
   the receiving application using an implementation-defined mechanism.

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   This document does not define a means for indicating what was
   received.  That is, the expectation is that any signal is propagated
   to the application for handling, not handled automatically by the
   transport layer.  How a receiving application communicates the
   throughput advice signal to a sending application will depend on the
   application in use.

   Different applications can choose different approaches.  For example,
   in an application where a receiver drives rate adaptation, it might
   not be necessary to define additional signaling.

   A sender can use any acknowledgment mechanism provided by the QUIC
   version in use to learn whether datagrams containing SCONE packets
   were likely received.  This might help inform whether to send
   additional SCONE packets in the event that a datagram is lost.
   However, rather than relying on transport signals, an application
   might be better able to indicate what has been received and
   processed.

   SCONE packets could be stripped from datagrams in the network, which
   cannot be reliably detected.  This could result in a sender falsely
   believing that no network element applied a throughput advice signal.

8.3.  Interactions with congestion control

   SCONE and congestion control both provide the application with
   estimates of a path capacity.  They are complementary.  Congestion
   control algorithms are typically designed to quickly detect and react
   to congestion, i.e., to the "minimum" capacity of a path.  SCONE
   informs the endpoint of the maximum capacity of a path based on
   network rate limit policy, network conditions, or a combination of
   the two.

   Consider for example a path in which the bottleneck router implements
   Early Congestion Notification as specified in the L4S architecture
   [RFC9330].  If the path capacity diminishes, queues will build up and
   the router will immediately start increasing the rate at which
   packets are marked as "Congestion Experienced".  The receiving
   endpoint will notice these marks, and inform its peer.  The incoming
   congestion will be detected within 1 round trip time (RTT).  This
   scenario will play out whatever the reason for the change in
   capacity, whether due to increased competition between multiple
   applications or, for example, to a change in capacity of a wireless
   channel.

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9.  Security Considerations

   The modification of packets provides endpoints proof that a network
   element is in a position to drop datagrams and could apply a rate
   limit policy.  Section 8.1 states that endpoints only accept signals
   if the datagram contains a packet that it accepts to prevent an off-
   path attacker from inserting spurious throughput advice signals.

   Some off-path attackers may be able to both observe traffic and
   inject packets.  Attackers with such capabilities could observe
   packets sent by an endpoint, create datagrams coalescing an arbitrary
   SCONE packet and the observed packet, and send these datagrams such
   that they arrive at the peer endpoint before the original packet.
   Spoofed packets that seek to advertise a higher limit than might
   otherwise be permitted also need to bypass any rate limiters.  The
   attacker will thus get arbitrary SCONE packets accepted by the peer,
   with the result being that the endpoint receives a false or
   misleading rate limit.

   The recipient of a throughput advice signal therefore cannot
   guarantee that the signal was generated by an on-path network
   element.  However, the capabilities required of an off-path attacker
   are substantially similar to those of on path elements.

   The actual value of the throughput advice signal is not
   authenticated.  Any signal might be incorrectly set in order to
   encourage endpoints to behave in ways that are not in their
   interests.  Endpoints are free to ignore limits that they think are
   incorrect.  The congestion controller employed by a sender provides
   real-time information about the rate at which the network path is
   delivering data.

   Similarly, if there is a strong need to ensure that a rate limit is
   respected, network elements cannot assume that the signaled limit
   will be respected by endpoints.

9.1.  Flooding intermediaries with fake packets

   Attackers that can inject packets may compose arbitrary "SCONE-like"
   packets by selecting a pair of IP addresses and ports, an arbitrary
   rate signal, a valid SCONE version number, an arbitrary "destination
   connection ID", and an arbitrary "source connection ID".  The SCONE
   packet will carry these information.  A coalesced "1RTT" packet will
   start with a plausible first octet, and continue with the selected
   destination connection ID followed by a sufficiently long series of
   random bytes, mimicking the content of an encrypted packets.

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   The injected packets will travel towards the destination.  The final
   destination will reject such packets because the destination ID is
   invalid or because decryption fail, but network elements cannot do
   these checks, and will have to process the packets.  All the network
   elements between the injection point and the destination will have to
   process these packets.

   Attackers could send a high volume of these "fake" SCONE packets in a
   denial of service (DOS) attempt against network elements.  The attack
   will force the intermediaries to process the fake packets.  If
   network elements are keeping state for ongoing SCONE flows, the
   attack can cause the excessive allocation of memory resource.  The
   mitigation there will be the same as mitigation of other distributed
   DOS attacks: limit the rate of SCONE packets that a network element
   is willing to process; possibly, implement logic to distinguish valid
   SCONE packets from fake packets; or, use generic protection against
   Distributed DOS attacks.

   Attackers could also try to craft the fake SCONE packets in ways that
   trigger a processing error at network elements.  For example, they
   might pick connection identifiers of arbitrary length.  Network
   elements can mitigate these attacks with implementations that fully
   conform to the specification of Section 5.

10.  Privacy Considerations

   The focus of this analysis is the extent to which observing SCONE
   packets could be used to gain information about endpoints.  This
   might be leaking details of how applications using QUIC operate or
   leaks of endpoint identity when using additional privacy protection,
   such as a VPN.

   Any network element that can observe the content of that packet can
   read the rate limit that was applied.  Any signal is visible on the
   path, from the point at which it is applied to the point at which it
   is consumed at an endpoint.  On path elements can also alter the
   SCONE signal to try trigger specific reactions and gain further
   knowledge.

   In the general case of a client connected to a server through the
   Internet, we believe that SCONE does not provide much advantage to
   attackers.  The identities of the clients and servers are already
   visible through their IP addresses.  Traffic analysis tools already
   provide more information than the data rate limits set by SCONE.

   There are two avenues of attack that require more analysis:

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   *  that the passive observation of SCONE packets might help identify
      or distinguish endpoints; and

   *  that active manipulation of SCONE signals might help reveal the
      identity of endpoints that are otherwise hidden behind VPNs or
      proxies.

10.1.  Passive Attacks

   If only few clients and server pairs negotiate the usage of SCONE,
   the occasional observation of SCONE packets will "stick out".  That
   observation, could be combined with observation of timing and volume
   of traffic to help identify the endpoint or categorize the
   application that they are using.

   A variation of this issue occurs if SCONE is widely implemented, but
   only used in some specific circumstances.  In that case, observation
   of SCONE packets reveals information about the state of the endpoint.

   If multiple servers are accessed through the same front facing
   server, Encrypted Client Hello (ECH) may be used to prevent outside
   parties to identify which specific server a client is using.
   However, if only a few of these servers use SCONE, any SCONE packets
   will help identify which specific server a client is using.

   This issue will be mitigated if SCONE becomes widely implemented, and
   if the usage of SCONE is not limited to the type of applications that
   make active use of the signal.

   QUIC implementations are therefore encouraged to make the feature
   available unconditionally.  Endpoints might send SCONE packets
   whenever a peer can accept them.

10.2.  Active Attacks

   Suppose a configuration in which multiple clients use a VPN or proxy
   service to access the same server.  The attacker sees the IP
   addresses in the packets behind VPN and proxy and also between the
   users and the VPN, but it does not know which VPN address corresponds
   to what user address.

   Suppose now that the attacker selects a flow on the link between the
   VPN/proxy and server.  The attacker applies throughput advice signals
   to SCONE packets in that flow.  The attacker chooses a bandwidth that
   is lower than the "natural" bandwidth of the connection.  A reduction
   in the rate of flows between client and VPN/proxy might allow the
   attacker to link the altered flow to the client.

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   +--------+
   | Client |------.
   +--------+       \      +-------+
                     '---->|       |            +--------+
   +--------+              |  VPN  |<==========>|        |
   | Client |------------->|   /   |<==========>| Server |
   +--------+              | Proxy |<==========>|        |
                     .---->|       |     ^      +--------+
   +--------+       /      +-------+     |
   | Client |======'                     |
   +--------+      ^           Apply throughput advice signal
                    \
                     \
                  Observe change

   An attacker that can manipulate SCONE headers can also simulate
   congestion signals by dropping packets or by setting the ECN CE bit.
   That will also likely result in changes in the congestion response by
   the affected client.

   A VPN or proxy could defend against this style of attack by removing
   SCONE (and ECN) signals.  There are few reasons to provide per-flow
   throughput advice signals in that situation.  Endpoints might also
   either disable this feature or ignore any signals when they are aware
   of the use of a VPN or proxy.

11.  IANA Considerations

   This document registers a new QUIC version (Section 11.1) and a QUIC
   transport parameter (Section 11.2).

11.1.  SCONE Versions

   This document registers the following entries to the "QUIC Versions"
   registry maintained at https://www.iana.org/assignments/quic
   (https://www.iana.org/assignments/quic), following the guidance from
   Section 22.2 of [QUIC].

   Value:  0xSCONE1
   Status:  permanent
   Specification:  This document
   Change Controller:  IETF (iesg@ietf.org)
   Contact:  QUIC Working Group (quic@ietf.org)
   Notes:  SCONE Protocol - Low Range

   Value:  0xSCONE2
   Status:  permanent
   Specification:  This document

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   Change Controller:  IETF (iesg@ietf.org)
   Contact:  QUIC Working Group (quic@ietf.org)
   Notes:  SCONE Protocol - High Range

11.2.  scone_supported Transport Parameter

   This document registers the scone_supported transport parameter in
   the "QUIC Transport Parameters" registry maintained at
   https://www.iana.org/assignments/quic
   (https://www.iana.org/assignments/quic), following the guidance from
   Section 22.3 of [QUIC].

   Value:  0xTBD
   Parameter Name:  scone_supported
   Status:  Permanent
   Specification:  This document
   Date:  This date
   Change Controller:  IETF (iesg@ietf.org)
   Contact:  QUIC Working Group (quic@ietf.org)
   Notes:  (none)

12.  References

12.1.  Normative References

   [BCP14]    Best Current Practice 14,
              <https://www.rfc-editor.org/info/bcp14>.
              At the time of writing, this BCP comprises the following:

              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>.

              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>.

   [INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,
              <https://www.rfc-editor.org/rfc/rfc8999>.

   [QUIC]     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/rfc/rfc9000>.

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   [QUIC-BIT] Thomson, M., "Greasing the QUIC Bit", RFC 9287,
              DOI 10.17487/RFC9287, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9287>.

   [QUICv2]   Duke, M., "QUIC Version 2", RFC 9369,
              DOI 10.17487/RFC9369, May 2023,
              <https://www.rfc-editor.org/rfc/rfc9369>.

12.2.  Informative References

   [ECN]      Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/rfc/rfc3168>.

   [QUIC-MP]  Liu, Y., Ma, Y., De Coninck, Q., Bonaventure, O., Huitema,
              C., and M. Kühlewind, "Multipath Extension for QUIC", Work
              in Progress, Internet-Draft, draft-ietf-quic-multipath-15,
              7 July 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-quic-multipath-15>.

   [RFC9330]  Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
              White, "Low Latency, Low Loss, and Scalable Throughput
              (L4S) Internet Service: Architecture", RFC 9330,
              DOI 10.17487/RFC9330, January 2023,
              <https://www.rfc-editor.org/rfc/rfc9330>.

Acknowledgments

   Jana Iyengar has made significant contributions to the original TRAIN
   specification that forms the basis for a large part of this document.

Authors' Addresses

   Martin Thomson
   Mozilla
   Email: mt@lowentropy.net

   Christian Huitema
   Private Octopus Inc.
   Email: huitema@huitema.net

   Kazuho Oku
   Fastly
   Email: kazuhooku@gmail.com

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   Additional contact information:

      奥 一穂
      Fastly

   Matt Joras
   Meta
   Email: matt.joras@gmail.com

   Marcus Ihlar
   Ericsson
   Email: marcus.ihlar@ericsson.com

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