Network Working Group                                      M. Kuehlewind
Internet-Draft                                                  Ericsson
Intended status: Informational                               B. Trammell
Expires: 26 July 2021                                             Google
                                                         22 January 2021

              Manageability of the QUIC Transport Protocol


   This document discusses manageability of the QUIC transport protocol,
   focusing on caveats impacting network operations involving QUIC
   traffic.  Its intended audience is network operators, as well as
   content providers that rely on the use of QUIC-aware middleboxes,
   e.g. for load balancing.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 July 2021.

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

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Features of the QUIC Wire Image . . . . . . . . . . . . . . .   4
     2.1.  QUIC Packet Header Structure  . . . . . . . . . . . . . .   4
     2.2.  Coalesced Packets . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Use of Port Numbers . . . . . . . . . . . . . . . . . . .   6
     2.4.  The QUIC Handshake  . . . . . . . . . . . . . . . . . . .   7
     2.5.  Integrity Protection of the Wire Image  . . . . . . . . .  11
     2.6.  Connection ID and Rebinding . . . . . . . . . . . . . . .  11
     2.7.  Packet Numbers  . . . . . . . . . . . . . . . . . . . . .  12
     2.8.  Version Negotiation and Greasing  . . . . . . . . . . . .  12
   3.  Network-visible Information about QUIC Flows  . . . . . . . .  12
     3.1.  Identifying QUIC Traffic  . . . . . . . . . . . . . . . .  13
       3.1.1.  Identifying Negotiated Version  . . . . . . . . . . .  13
       3.1.2.  Rejection of Garbage Traffic  . . . . . . . . . . . .  14
     3.2.  Connection Confirmation . . . . . . . . . . . . . . . . .  14
     3.3.  Application Identification  . . . . . . . . . . . . . . .  14
       3.3.1.  Extracting Server Name Indication (SNI)
               Information . . . . . . . . . . . . . . . . . . . . .  15
     3.4.  Flow Association  . . . . . . . . . . . . . . . . . . . .  16
     3.5.  Flow teardown . . . . . . . . . . . . . . . . . . . . . .  16
     3.6.  Flow Symmetry Measurement . . . . . . . . . . . . . . . .  16
     3.7.  Round-Trip Time (RTT) Measurement . . . . . . . . . . . .  17
       3.7.1.  Measuring Initial RTT . . . . . . . . . . . . . . . .  17
       3.7.2.  Using the Spin Bit for Passive RTT Measurement  . . .  17
   4.  Specific Network Management Tasks . . . . . . . . . . . . . .  19
     4.1.  Stateful Treatment of QUIC Traffic  . . . . . . . . . . .  19
     4.2.  Passive Network Performance Measurement and
           Troubleshooting . . . . . . . . . . . . . . . . . . . . .  19
     4.3.  Server Cooperation with Load Balancers  . . . . . . . . .  20
     4.4.  DDoS Detection and Mitigation . . . . . . . . . . . . . .  20
     4.5.  UDP Policing  . . . . . . . . . . . . . . . . . . . . . .  21
     4.6.  Distinguishing Acknowledgment traffic . . . . . . . . . .  21
     4.7.  Quality of Service handling and ECMP  . . . . . . . . . .  22
     4.8.  QUIC and Network Address Translation (NAT)  . . . . . . .  22
       4.8.1.  Resource Conservation . . . . . . . . . . . . . . . .  23
       4.8.2.  "Helping" with routing infrastructure issues  . . . .  23
     4.9.  Filtering behavior  . . . . . . . . . . . . . . . . . . .  24
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  25
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  25
   9.  Appendix  . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Distinguishing IETF QUIC and Google QUIC Versions . . . .  26
     9.2.  Extracting the CRYPTO frame . . . . . . . . . . . . . . .  27
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  28

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     10.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   QUIC [QUIC-TRANSPORT] is a new transport protocol encapsulated in UDP
   and encrypted by default.  QUIC integrates TLS [QUIC-TLS] to encrypt
   all payload data and most control information.  The design focused on
   support of semantics for HTTP, which required changes to HTTP known
   as HTTP/3 [QUIC-HTTP].

   Given that QUIC is an end-to-end transport protocol, all information
   in the protocol header, even that which can be inspected, is not
   meant to be mutable by the network, and is therefore integrity-
   protected.  While less information is visible to the network than for
   TCP, integrity protection can also simplify troubleshooting, because
   none of the nodes on the network path can modify the transport layer

   This document provides guidance for network operations that manage
   QUIC traffic.  This includes guidance on how to interpret and utilize
   information that is exposed by QUIC to the network, requirements and
   assumptions that the QUIC design with respect to network treatment,
   and a description of how common network management practices will be
   impacted by QUIC.

   Since QUIC's wire image [WIRE-IMAGE] is integrity protected and not
   modifiable on path, in-network operations are not possible without
   terminating the QUIC connection, for instance using a back-to-back
   proxy.  Proxy operations are not in scope for this document.  A proxy
   can either explicit identify itself as providing a proxy service, or
   may share the TLS credentials to authenticate as the server and (in
   some cases) client acting as a front-facing instance for the endpoint

   Network management is not a one-size-fits-all endeavour: practices
   considered necessary or even mandatory within enterprise networks
   with certain compliance requirements, for example, would be
   impermissible on other networks without those requirements.  This
   document therefore does not make any specific recommendations as to
   which practices should or should not be applied; for each practice,
   it describes what is and is not possible with the QUIC transport
   protocol as defined.

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2.  Features of the QUIC Wire Image

   In this section, we discuss those aspects of the QUIC transport
   protocol that have an impact on the design and operation of devices
   that forward QUIC packets.  Here, we are concerned primarily with the
   unencrypted part of QUIC's wire image [WIRE-IMAGE], which we define
   as the information available in the packet header in each QUIC
   packet, and the dynamics of that information.  Since QUIC is a
   versioned protocol, the wire image of the header format can also
   change from version to version.  However, the field that identifies
   the QUIC version in some packets, and the format of the Version
   Negotiation Packet, are both inspectable and invariant

   This document describes version 1 of the QUIC protocol, whose wire
   image is fully defined in [QUIC-TRANSPORT] and [QUIC-TLS].  Features
   of the wire image described herein may change in future versions of
   the protocol, except when specified as an invariant
   [QUIC-INVARIANTS], and cannot be used to identify QUIC as a protocol
   or to infer the behavior of future versions of QUIC.

   Section 9.1 provides non-normative guidance on the identification of
   QUIC version 1 packets compared to some pre-standard versions.

2.1.  QUIC Packet Header Structure

   QUIC packets may have either a long header, or a short header.  The
   first bit of the QUIC header is the Header Form bit, and indicates
   which type of header is present.  The purpose of this bit is
   invariant across QUIC versions.

   The long header exposes more information.  It is used during
   connection establishment, including version negotiation, retry, and
   0-RTT data.  It contains a version number, as well as source and
   destination connection IDs for grouping packets belonging to the same
   flow.  The definition and location of these fields in the QUIC long
   header are invariant for future versions of QUIC, although future
   versions of QUIC may provide additional fields in the long header

   Short headers are used after connection establishment, and contain
   only an optional destination connection ID and the spin bit for RTT

   The following information is exposed in QUIC packet headers:

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   *  "fixed bit": the second most significant bit of the first octet
      most QUIC packets of the current version is currently set to 1,
      for endpoints to demultiplex with other UDP-encapsulated
      protocols.  Even thought this bit is fixed in the QUICv1
      specification, endpoints may use a version or extension that
      varies the bit.  Therefore, observers cannot reliably use it as an
      identifier for QUIC.

   *  latency spin bit: the third most significant bit of first octet in
      the short packet header.  The spin bit is set by endpoints such
      that tracking edge transitions can be used to passively observe
      end-to-end RTT.  See Section 3.7.2 for further details.

   *  header type: the long header has a 2 bit packet type field
      following the Header Form and fixed bits.  Header types correspond
      to stages of the handshake; see Section 17.2 of [QUIC-TRANSPORT]
      for details.

   *  version number: the version number is present in the long header,
      and identifies the version used for that packet.  During Version
      Negotiation (see Section 2.8 and Section 17.2.1 of
      [QUIC-TRANSPORT]), the version number field has a special value
      (0x00000000) that identifies the packet as a Version Negotiation
      packet.  Many QUIC versions that start with 0xff implement IETF
      drafts.  QUIC versions that start with 0x0000 are reserved for
      IETF consensus documents.  For example, QUIC version 1 uses
      version 0x00000001.  Operators should expect to observe packets
      with other version numbers as a result of various internet
      experiments and future standards.

   *  source and destination connection ID: short and long packet
      headers carry a destination connection ID, a variable-length field
      that can be used to identify the connection associated with a QUIC
      packet, for load-balancing and NAT rebinding purposes; see
      Section 4.3 and Section 2.6.  Long packet headers additionally
      carry a source connection ID.  The source connection ID
      corresponds to the destination connection ID the source would like
      to have on packets sent to it, and is only present on long packet
      headers.  On long header packets, the length of the connection IDs
      is also present; on short header packets, the length of the
      destination connection ID is implicit.

   *  length: the length of the remaining QUIC packet after the length
      field, present on long headers.  This field is used to implement
      coalesced packets during the handshake (see Section 2.2).

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   *  token: Initial packets may contain a token, a variable-length
      opaque value optionally sent from client to server, used for
      validating the client's address.  Retry packets also contain a
      token, which can be used by the client in an Initial packet on a
      subsequent connection attempt.  The length of the token is
      explicit in both cases.

   Retry (Section 17.2.5 of [QUIC-TRANSPORT]) and Version Negotiation
   (Section 17.2.1 of [QUIC-TRANSPORT]) packets are not encrypted or
   obfuscated in any way.  For other kinds of packets, other information
   in the packet headers is cryptographically obfuscated:

   *  packet number: All packets except Version Negotiation and Retry
      packets have an associated packet number; however, this packet
      number is encrypted, and therefore not of use to on-path
      observers.  The offset of the packet number is encoded in long
      headers, while it is implicit (depending on destination connection
      ID length) in short headers.  The length of the packet number is
      cryptographically obfuscated.

   *  key phase: The Key Phase bit, present in short headers, specifies
      the keys used to encrypt the packet to support key rotation.  The
      Key Phase bit is cryptographically obfuscated.

2.2.  Coalesced Packets

   Multiple QUIC packets may be coalesced into a UDP datagram, with a
   datagram carrying one or more long header packets followed by zero or
   one short header packets.  When packets are coalesced, the Length
   fields in the long headers are used to separate QUIC packets; see
   Section 12.2 of [QUIC-TRANSPORT].  The length header field is
   variable length, and its position in the header is also variable
   depending on the length of the source and destination connection ID;
   see Section 17.2 of [QUIC-TRANSPORT].

2.3.  Use of Port Numbers

   Applications that have a mapping for TCP as well as QUIC are expected
   to use the same port number for both services.  However, as with TCP-
   based services, especially when application layer information is
   encrypted, there is no guarantee that a specific application will use
   the registered port, or the used port is carrying traffic belonging
   to the respective registered service.  For example, [QUIC-HTTP]
   specifies the use of Alt-Svc for discovery of HTTP/3 services on
   other ports.

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   Further, as QUIC has a connection ID, it is also possible to maintain
   multiple QUIC connections over one 5-tuple.  However, if the
   connection ID is not present in the packet header, all packets of the
   5-tuple belong to the same QUIC connection.

2.4.  The QUIC Handshake

   New QUIC connections are established using a handshake, which is
   distinguishable on the wire and contains some information that can be
   passively observed.

   To illustrate the information visible in the QUIC wire image during
   the handshake, we first show the general communication pattern
   visible in the UDP datagrams containing the QUIC handshake, then
   examine each of the datagrams in detail.

   In the nominal case, the QUIC handshake can be recognized on the wire
   through at least four datagrams we'll call "QUIC Client Hello", "QUIC
   Server Hello", and "Initial Completion", and "Handshake Completion",
   for purposes of this illustration, as shown in Figure 1.

   Packets in the handshake belong to three separate cryptographic and
   transport contexts ("Initial", which contains observable payload, and
   "Handshake" and "1-RTT", which do not).  QUIC packets in separate
   contexts during the handshake are generally coalesced (see
   Section 2.2) in order to reduce the number of UDP datagrams sent
   during the handshake.

   As shown here, the client can send 0-RTT data as soon as it has sent
   its Client Hello, and the server can send 1-RTT data as soon as it
   has sent its Server Hello.

   Client                                    Server
     |                                          |
     +----QUIC Client Hello-------------------->|
     +----(zero or more 0RTT)------------------>|
     |                                          |
     |<--------------------QUIC Server Hello----+
     |<---------(1RTT encrypted data starts)----+
     |                                          |
     +----Initial Completion------------------->|
     +----(1RTT encrypted data starts)--------->|
     |                                          |
     |<-----------------Handshake Completion----+
     |                                          |

   Figure 1: General communication pattern visible in the QUIC handshake

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   A typical handshake starts with the client sending of a QUIC Client
   Hello datagram as shown in Figure 2, which elicits a QUIC Server
   Hello datagram as shown in Figure 3 typically containing three
   packets: an Initial packet with the Server Hello, a Handshake packet
   with the rest of the server's side of the TLS handshake, and initial
   1-RTT data, if present.

   The Initial Completion datagram contains at least one Handshake
   packet and some also include an Initial packet.

   Datagrams that contain a QUIC Initial Packet (Client Hello, Server
   Hello, and some Initial Completion) must be at least 1200 octets
   long.  This protects against amplification attacks and verifies that
   the network path meets minimum Maximum Transmission Unit (MTU)
   requirements.  This is usually accomplished with either the addition
   of PADDING frames to the Initial packet, or coalescing of the Initial
   Packet with packets from other encryption contexts.

   The content of QUIC Initial packets are encrypted using Initial
   Secrets, which are derived from a per-version constant and the
   client's destination connection ID; they are therefore observable by
   any on-path device that knows the per-version constant.  We therefore
   consider these as visible in our illustration.  The content of QUIC
   Handshake packets are encrypted using keys established during the
   initial handshake exchange, and are therefore not visible.

   Initial, Handshake, and the Short Header packets transmitted after
   the handshake belong to cryptographic and transport contexts.  The
   Initial Completion Figure 4 and the Handshake Completion Figure 5
   datagrams finish these first two contexts, by sending the final
   acknowledgment and finishing the transmission of CRYPTO frames.

   | UDP header (source and destination UDP ports)            |
   | QUIC long header (type = Initial, Version, DCID, SCID) (Length)
   +----------------------------------------------------------+  |
   | QUIC CRYPTO frame header                                 |  |
   +----------------------------------------------------------+  |
   | TLS Client Hello (incl. TLS SNI)                         |  |
   +----------------------------------------------------------+  |
   | QUIC PADDING frames                                      |  |

     Figure 2: Typical QUIC Client Hello datagram pattern with no 0-RTT

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   The Client Hello datagram exposes version number, source and
   destination connection IDs in the clear.  Information in the TLS
   Client Hello frame, including any TLS Server Name Indication (SNI)
   present, is obfuscated using the Initial secret.  Note that the
   location of PADDING is implementation-dependent, and PADDING frames
   may not appear in a coalesced Initial packet.

   | UDP header (source and destination UDP ports)              |
   | QUIC long header (type = Initial, Version, DCID, SCID)   (Length)
   +------------------------------------------------------------+  |
   | QUIC CRYPTO frame header                                   |  |
   +------------------------------------------------------------+  |
   | TLS Server Hello                                           |  |
   +------------------------------------------------------------+  |
   | QUIC ACK frame (acknowledging client hello)                |  |
   | QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
   +------------------------------------------------------------+  |
   | encrypted payload (presumably CRYPTO frames)               |  |
   | QUIC short header                                          |
   | 1-RTT encrypted payload                                    |

            Figure 3: Typical QUIC Server Hello datagram pattern

   The Server Hello datagram also exposes version number, source and
   destination connection IDs and information in the TLS Server Hello
   message which is obfuscated using the Initial secret.

   | UDP header (source and destination UDP ports)              |
   | QUIC long header (type = Initial, Version, DCID, SCID)   (Length)
   +------------------------------------------------------------+  |
   | QUIC ACK frame (acknowledging Server Hello Initial)        |  |
   | QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
   +------------------------------------------------------------+  |
   | encrypted payload (presumably CRYPTO/ACK frames)           |  |
   | QUIC short header                                          |
   | 1-RTT encrypted payload                                    |

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         Figure 4: Typical QUIC Initial Completion datagram pattern

   The Initial Completion datagram does not expose any additional
   information; however, recognizing it can be used to determine that a
   handshake has completed (see Section 3.2), and for three-way
   handshake RTT estimation as in Section 3.7.

   | UDP header (source and destination UDP ports)              |
   | QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
   +------------------------------------------------------------+  |
   | encrypted payload (presumably ACK frame)                   |  |
   | QUIC short header                                          |
   | 1-RTT encrypted payload                                    |

        Figure 5: Typical QUIC Handshake Completion datagram pattern

   Similar to Initial Completion, Handshake Completion also exposes no
   additional information; observing it serves only to determine that
   the handshake has completed.

   When the client uses 0-RTT connection resumption, 0-RTT data may also
   be seen in the QUIC Client Hello datagram, as shown in Figure 6.

   | UDP header (source and destination UDP ports)            |
   | QUIC long header (type = Initial, Version, DCID, SCID) (Length)
   +----------------------------------------------------------+  |
   | QUIC CRYPTO frame header                                 |  |
   +----------------------------------------------------------+  |
   | TLS Client Hello (incl. TLS SNI)                         |  |
   | QUIC long header (type = 0RTT, Version, DCID, SCID)    (Length)
   +----------------------------------------------------------+  |
   | 0-rtt encrypted payload                                  |  |

         Figure 6: Typical 0-RTT QUIC Client Hello datagram pattern

   In a 0-RTT QUIC Client Hello datagram, the PADDING frame is only
   present if necessary to increase the size of the datagram with 0RTT
   data to at least 1200 bytes.  Additional datagrams containing only
   0-RTT protected long header packets may be sent from the client to

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   the server after the Client Hello datagram, containing the rest of
   the 0-RTT data.  The amount of 0-RTT protected data is limited by the
   initial congestion window, typically around 10 packets [RFC6928].

2.5.  Integrity Protection of the Wire Image

   As soon as the cryptographic context is established, all information
   in the QUIC header, including exposed information, is integrity
   protected.  Further, information that was sent and exposed in
   handshake packets sent before the cryptographic context was
   established are validated later during the cryptographic handshake.
   Therefore, devices on path cannot alter any information or bits in
   QUIC packet headers, except specific parts of Initial packets, since
   alteration of header information will lead to a failed integrity
   check at the receiver, and can even lead to connection termination.

2.6.  Connection ID and Rebinding

   The connection ID in the QUIC packet headers allows routing of QUIC
   packets at load balancers on other than five-tuple information,
   ensuring that related flows are appropriately balanced together; and
   to allow rebinding of a connection after one of the endpoint's
   addresses changes - usually the client's.  Client and server
   negotiate connection IDs during the handshake; typically, however,
   only the server will request a connection ID for the lifetime of the
   connection.  Connection IDs for either endpoint may change during the
   lifetime of a connection, with the new connection ID being negotiated
   via encrypted frames.  See Section 5.1 of [QUIC-TRANSPORT].
   Therefore, observing a new connection ID does not necessary indicate
   a new connection.

   Server-generated connection IDs should seek to obscure any encoding,
   of routing identities or any other information.  Exposing the server
   mapping would allow linkage of multiple IP addresses to the same host
   if the server also supports migration.  Furthermore, this opens an
   attack vector on specific servers or pools.

   The best way to obscure an encoding is to appear random to observers,
   which is most rigorously achieved with encryption.  Even when
   encrypted, a scheme could embed the unencrypted length of the
   connection ID in the connection ID itself, instead of remembering it.

   [QUIC_LB] further specified possible algorithms to generate
   connection IDs at load balancers.

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2.7.  Packet Numbers

   The packet number field is always present in the QUIC packet header;
   however, it is always encrypted.  The encryption key for packet
   number protection on handshake packets sent before cryptographic
   context establishment is specific to the QUIC version, while packet
   number protection on subsequent packets uses secrets derived from the
   end-to-end cryptographic context.  Packet numbers are therefore not
   part of the wire image that is visible to on-path observers.

2.8.  Version Negotiation and Greasing

   Version Negotiation packets are not intrinsically protected, but QUIC
   versions can use later encrypted messages to verify that they were
   authentic.  Therefore any manipulation of this list will be detected
   and may cause the endpoints to terminate the connection attempt.

   Also note that the list of versions in the Version Negotiation packet
   may contain reserved versions.  This mechanism is used to avoid
   ossification in the implementation on the selection mechanism.
   Further, a client may send a Initial Client packet with a reserved
   version number to trigger version negotiation.  In the Version
   Negotiation packet the connection ID and packet number of the Client
   Initial packet are reflected to provide a proof of return-
   routability.  Therefore changing this information will also cause the
   connection to fail.

   QUIC is expected to evolve rapidly, so new versions, both
   experimental and IETF standard versions, will be deployed in the
   Internet more often than with traditional Internet- and transport-
   layer protocols.  Using a particular version number to recognize
   valid QUIC traffic is likely to persistently miss a fraction of QUIC
   flows and completely fail in the near future, and is therefore not
   recommended.  In addition, due to the speed of evolution of the
   protocol, devices that attempt to distinguish QUIC traffic from non-
   QUIC traffic for purposes of network admission control should admit
   all QUIC traffic regardless of version.

3.  Network-visible Information about QUIC Flows

   This section addresses the different kinds of observations and
   inferences that can be made about QUIC flows by a passive observer in
   the network based on the wire image in Section 2.  Here we assume a
   bidirectional observer (one that can see packets in both directions
   in the sequence in which they are carried on the wire) unless noted.

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3.1.  Identifying QUIC Traffic

   The QUIC wire image is not specifically designed to be
   distinguishable from other UDP traffic.

   The only application binding defined by the IETF QUIC WG is HTTP/3
   [QUIC-HTTP] at the time of this writing; however, many other
   applications are currently being defined and deployed over QUIC, so
   an assumption that all QUIC traffic is HTTP/3 is not valid.  HTTP
   over QUIC uses UDP port 443 by default, although URLs referring to
   resources available over HTTP/3 may specify alternate port numbers.
   Simple assumptions about whether a given flow is using QUIC based
   upon a UDP port number may therefore not hold; see also [RFC7605]
   section 5.

   While the second most significant bit (0x40) of the first octet is
   set to 1 in most QUIC packets of the current version (see
   Section 2.1), this method of recognizing QUIC traffic is NOT
   RECOMMENDED.  First, it only provides one bit of information and is
   quite prone to collide with UDP-based protocols other than those that
   this static bit is meant to allow multiplexing with.  Second, this
   feature of the wire image is not invariant [QUIC-INVARIANTS] and may
   change in future versions of the protocol, or even be negotiated
   during the handshake via the use of transport parameters.

   Even though transport parameters transmitted in the client initial
   are obserable by the network, they cannot be modified by the network
   without risking connection failure.  Further, the negotiated reply
   from the server cannot be observed, so observers on the network
   cannot know which parameters are actually in use.

3.1.1.  Identifying Negotiated Version

   An in-network observer assuming that a set of packets belongs to a
   QUIC flow can infer the version number in use by observing the
   handshake: an Initial packet with a given version from a client to
   which a server responds with an Initial packet with the same version
   implies acceptance of that version.

   Negotiated version cannot be identified for flows for which a
   handshake is not observed, such as in the case of connection
   migration; however, these flows can be associated with flows for
   which a version has been identified; see Section 3.4.

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   This document focuses on QUIC Version 1, and this section applies
   only to packets belonging to Version 1 QUIC flows; for purposes of
   on-path observation, it assumes that these packets have been
   identified as such through the observation of a version number
   exchange as described above.

3.1.2.  Rejection of Garbage Traffic

   A related question is whether a first packet of a given flow on a
   known QUIC-associated port is a valid QUIC packet, to support in-
   network filtering of garbage UDP packets (reflection attacks, random
   backscatter).  While heuristics based on the first byte of the packet
   (packet type) could be used to separate valid from invalid first
   packet types, the deployment of such heuristics is not recommended,
   as packet types may have different meanings in future versions of the

3.2.  Connection Confirmation

   Connection establishment uses Initial and Handshake packets
   containing a TLS handshake, and Retry packets that do not contain
   parts of the handshake.  Connection establishment can therefore be
   detected using heuristics similar to those used to detect TLS over
   TCP.  A client initiating a 0-RTT connection may also send data
   packets in 0-RTT Protected packets directly after the Initial packet
   containing the TLS Client Hello.  Since these packets may be
   reordered in the network, 0-RTT Protected data packets could be seen
   before the Initial packet.

   Note that clients send Initial packets before servers do, servers
   send Handshake packets before clients do, and only clients send
   Initial packets with tokens.  Therefore, the role as a client or
   server can generally be confirmed by an on- path observer.  An
   attempted connection after Retry can be detected by correlating the
   token on the Retry with the token on the subsequent Initial packet
   and the destination connection ID of the new Initial packet.

3.3.  Application Identification

   The cleartext TLS handshake may contain Server Name Indication (SNI)
   [RFC6066], by which the client reveals the name of the server it
   intends to connect to, in order to allow the server to present a
   certificate based on that name.  It may also contain information from
   Application-Layer Protocol Negotiation (ALPN) [RFC7301], by which the
   client exposes the names of application-layer protocols it supports;
   an observer can deduce that one of those protocols will be used if
   the connection continues.

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   Work is currently underway in the TLS working group to encrypt the
   SNI in TLS 1.3 [TLS-ESNI].  This would make SNI-based application
   identification impossible through passive measurement for QUIC and
   other protocols that use TLS.

3.3.1.  Extracting Server Name Indication (SNI) Information

   If the SNI is not encrypted it can be derived from the QUIC Initial
   packet by calculating the Initial Secret to decrypt the packet
   payload and parse the QUIC CRYPTO Frame containing the TLS

   As both the initial salt for the Initial Secret as well as CRYPTO
   frame itself are version-specific, the first step is always to parse
   the version number (second to sixth byte of the long header).  Note
   that only long header packets carry the version number, so it is
   necessary to also check the if first bit of the QUIC packet is set to
   1, indicating a long header.

   Note that proprietary QUIC versions, that have been deployed before
   standardization, might not set the first bit in a QUIC long header
   packets to 1.  To parse these versions, example code is provided in
   the appendix (see Section 9.1), however, it is expected that these
   versions will gradually disappear over time.

   When the version has been identified as QUIC version 1, the packet
   type needs to be verified as an Initial packet by checking that the
   third and fourth bit of the header are both set to 0.  Then the
   client destination connection ID needs to be extracted to calculate
   the Initial Secret together with the version specific initial salt,
   as described in [QUIC-TLS].  The length of the connection ID is
   indicated in the 6th byte of the header followed by the connection ID

   To determine the end of the header and find the start of the payload,
   the packet number length, the source connection ID length, and the
   token length need to be extracted.  The packet number length is
   defined by the seventh and eight bits of the header as described in
   section 17.2. of [QUIC-TRANSPORT], but is obfuscated as described in
   [QUIC-TLS].  The source connection ID length is specified in the byte
   after the destination connection ID.  And the token length, which
   follows the source connection ID, is a variable length integer as
   specified in Section 16 of [QUIC-TRANSPORT].

   After decryption, the Initial Client packet can be parsed to detect
   the CRYPTO frame that contains the TLS Client Hello, which then can
   be parsed similarly to TLS over TCP connections.  The Initial client
   packet may contain other frames, so the first bytes of each frame

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   need to be checked to identify the frame type, and if needed skip
   over it.  Note that the length of the frames is dependent on the
   frame type.  In QUIC version 1, the packet is expected to only carry
   the CRYPTO frame and optionally padding frames.  However, PADDING
   frames, which are each one byte of zeros, may also occur before or
   after the CRYPTO frame.

   Note that client Initial packets after the first do not always use
   the destination connection ID that was used to generate the Initial
   keys.  Therefore, attempts to decrypt these packets using the
   procedure above might fail.

3.4.  Flow Association

   The QUIC connection ID (see Section 2.6) is designed to allow an on-
   path device such as a load-balancer to associate two flows as
   identified by five-tuple when the address and port of one of the
   endpoints changes; e.g. due to NAT rebinding or server IP address
   migration.  An observer keeping flow state can associate a connection
   ID with a given flow, and can associate a known flow with a new flow
   when when observing a packet sharing a connection ID and one endpoint
   address (IP address and port) with the known flow.

   However, since the connection ID may change multiple times during the
   lifetime of a flow, and the negotiation of connection ID changes is
   encrypted, packets with the same 5-tuple but different connection IDs
   may or may not belong to the same connection.

   The connection ID value should be treated as opaque; see Section 4.3
   for caveats regarding connection ID selection at servers.

3.5.  Flow teardown

   QUIC does not expose the end of a connection; the only indication to
   on-path devices that a flow has ended is that packets are no longer
   observed.  Stateful devices on path such as NATs and firewalls must
   therefore use idle timeouts to determine when to drop state for QUIC
   flows, see further section Section 4.1.

3.6.  Flow Symmetry Measurement

   QUIC explicitly exposes which side of a connection is a client and
   which side is a server during the handshake.  In addition, the
   symmetry of a flow (whether primarily client-to-server, primarily
   server-to-client, or roughly bidirectional, as input to basic traffic
   classification techniques) can be inferred through the measurement of
   data rate in each direction.  While QUIC traffic is protected and
   ACKs may be padded, padding is not required.

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3.7.  Round-Trip Time (RTT) Measurement

   Round-trip time of QUIC flows can be inferred by observation once per
   flow, during the handshake, as in passive TCP measurement; this
   requires parsing of the QUIC packet header and recognition of the
   handshake, as illustrated in Section 2.4.  It can also be inferred
   during the flow's lifetime, if the endpoints use the spin bit
   facility described below and in [QUIC-TRANSPORT], section 17.3.1.

3.7.1.  Measuring Initial RTT

   In the common case, the delay between the Initial packet containing
   the TLS Client Hello and the Handshake packet containing the TLS
   Server Hello represents the RTT component on the path between the
   observer and the server.  The delay between the TLS Server Hello and
   the Handshake packet containing the TLS Finished message sent by the
   client represents the RTT component on the path between the observer
   and the client.  While the client may send 0-RTT Protected packets
   after the Initial packet during 0-RTT connection re-establishment,
   these can be ignored for RTT measurement purposes.

   Handshake RTT can be measured by adding the client-to-observer and
   observer-to-server RTT components together.  This measurement
   necessarily includes any transport and application layer delay (the
   latter mainly caused by the asymmetric crypto operations associated
   with the TLS handshake) at both sides.

3.7.2.  Using the Spin Bit for Passive RTT Measurement

   The spin bit provides an additional method to measure per-flow RTT
   from observation points on the network path throughout the duration
   of a connection.  Endpoint participation in spin bit signaling is
   optional in QUIC.  That is, while its location is fixed in this
   version of QUIC, an endpoint can unilaterally choose to not support
   "spinning" the bit.  Use of the spin bit for RTT measurement by
   devices on path is only possible when both endpoints enable it.  Some
   endpoints may disable use of the spin bit by default, others only in
   specific deployment scenarios, e.g. for servers and clients where the
   RTT would reveal the presence of a VPN or proxy.  To avoid making
   these connections identifiable based on the usage of the spin bit,
   all endpoints randomly disable "spinning" for at least one eighth of
   connections, even if otherwise enabled by default.  An endpoint not
   participating in spin bit signaling for a given connection can use a
   fixed spin value for the duration of the connection, or can set the
   bit randomly on each packet sent.

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   When in use and a QUIC flow sends data continuously, the latency spin
   bit in each direction changes value once per round-trip time (RTT).
   An on-path observer can observe the time difference between edges
   (changes from 1 to 0 or 0 to 1) in the spin bit signal in a single
   direction to measure one sample of end-to-end RTT.

   Note that this measurement, as with passive RTT measurement for TCP,
   includes any transport protocol delay (e.g., delayed sending of
   acknowledgements) and/or application layer delay (e.g., waiting for a
   response to be generated).  It therefore provides devices on path a
   good instantaneous estimate of the RTT as experienced by the
   application.  A simple linear smoothing or moving minimum filter can
   be applied to the stream of RTT information to get a more stable

   However, application-limited and flow-control-limited senders can
   have application and transport layer delay, respectively, that are
   much greater than network RTT.  When the sender is application-
   limited and e.g. only sends small amount of periodic application
   traffic, where that period is longer than the RTT, measuring the spin
   bit provides information about the application period, not the
   network RTT.

   Since the spin bit logic at each endpoint considers only samples from
   packets that advance the largest packet number, signal generation
   itself is resistant to reordering.  However, reordering can cause
   problems at an observer by causing spurious edge detection and
   therefore inaccurate (i.e., lower) RTT estimates, if reordering
   occurs across a spin-bit flip in the stream.

   Simple heuristics based on the observed data rate per flow or changes
   in the RTT series can be used to reject bad RTT samples due to lost
   or reordered edges in the spin signal, as well as application or flow
   control limitation; for example, QoF [TMA-QOF] rejects component RTTs
   significantly higher than RTTs over the history of the flow.  These
   heuristics may use the handshake RTT as an initial RTT estimate for a
   given flow.  Usually such heuristics would also detect if the spin is
   either constant or randomly set for a connection.

   An on-path observer that can see traffic in both directions (from
   client to server and from server to client) can also use the spin bit
   to measure "upstream" and "downstream" component RTT; i.e, the
   component of the end-to-end RTT attributable to the paths between the
   observer and the server and the observer and the client,
   respectively.  It does this by measuring the delay between a spin
   edge observed in the upstream direction and that observed in the
   downstream direction, and vice versa.

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4.  Specific Network Management Tasks

   In this section, we review specific network management and
   measurement techniques and how QUIC's design impacts them.

4.1.  Stateful Treatment of QUIC Traffic

   Stateful treatment of QUIC traffic (e.g., at a firewall or NAT
   middlebox) is possible through QUIC traffic and version
   identification (Section 3.1) and observation of the handshake for
   connection confirmation (Section 3.2).  The lack of any visible end-
   of-flow signal (Section 3.5) means that this state must be purged
   either through timers or through least-recently-used eviction,
   depending on application requirements.

   [RFC4787] recommends a 2 minute timeout interval for UDP.  However,
   timers can be lower, in the range of 15 to 30 seconds.  In contrast,
   [RFC5382] recommends a timeout of more than 2 hours for TCP, given
   that TCP is a connection-oriented protocol with well-defined closure
   semantics.  For network devices that are QUIC-aware, it is
   recommended to also use longer timeouts for QUIC traffic, as QUIC is
   connection-oriented.  As such, a handshake packet from the server
   indicates the willingness of the server to communicate with the

   The QUIC header optionally contains a connection ID which can be used
   as additional entropy beyond the 5-tuple, if needed.  The QUIC
   handshake needs to be observed in order to understand whether the
   connection ID is present and what length it has.  However, connection
   IDs may be renegotiated during a connection, and this renegotiation
   is not visible to the path.  Keying state off the connection ID may
   therefore cause undetectable and unrecoverable loss of state in the
   middle of a connection.  Use of connection ID specifically
   discouraged for NAT applications.

4.2.  Passive Network Performance Measurement and Troubleshooting

   Limited RTT measurement is possible by passive observation of QUIC
   traffic; see Section 3.7.  No passive measurement of loss is possible
   with the present wire image.  Extremely limited observation of
   upstream congestion may be possible via the observation of CE
   markings on ECN-enabled QUIC traffic.

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4.3.  Server Cooperation with Load Balancers

   In the case of content distribution networking architectures
   including load balancers, the connection ID provides a way for the
   server to signal information about the desired treatment of a flow to
   the load balancers.  Guidance on assigning connection IDs is given in

4.4.  DDoS Detection and Mitigation

   Current practices in detection and mitigation of Distributed Denial
   of Service (DDoS) attacks generally involve classification of
   incoming traffic (as packets, flows, or some other aggregate) into
   "good" (productive) and "bad" (DDoS) traffic, and then differential
   treatment of this traffic to forward only good traffic.  This
   operation is often done in a separate specialized mitigation
   environment through which all traffic is filtered; a generalized
   architecture for separation of concerns in mitigation is given in

   Key to successful DDoS mitigation is efficient classification of this
   traffic in the mitigation environment.  Limited first-packet garbage
   detection as in Section 3.1.2 and stateful tracking of QUIC traffic
   as in Section 4.1 above may be useful during classification.

   Note that the use of a connection ID to support connection migration
   renders 5-tuple based filtering insufficient and requires more state
   to be maintained by DDoS defense systems.  For the common case of NAT
   rebinding, DDoS defense systems can detect a change in the client's
   endpoint address by linking flows based on the server's connection
   IDs.  QUIC's linkability resistance ensures that a deliberate
   connection migration is accompanied by a change in the connection ID.

   It is questionable whether connection migrations must be supported
   during a DDoS attack.  If the connection migration is not visible to
   the network that performs the DDoS detection, an active, migrated
   QUIC connection may be blocked by such a system under attack.  As
   soon as the connection blocking is detected by the client, the client
   may rely on the fast resumption mechanism provided by QUIC.  When
   clients migrate to a new path, they should be prepared for the
   migration to fail and attempt to reconnect quickly.

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   TCP syncookies [RFC4937] are a well-established method of mitigating
   some kinds of TCP DDoS attacks.  QUIC Retry packets are the
   functional analogue to syncookies, forcing clients to prove
   possession of their IP address before committing server state.
   However, there are safeguards in QUIC against unsolicited injection
   of these packets by intermediaries who do not have consent of the end
   server.  See [QUIC_LB] for standard ways for intermediaries to send
   Retry packets on behalf of consenting servers.

4.5.  UDP Policing

   Today, UDP is the most prevalent DDoS vector, since it is easy for
   compromised non-admin applications to send a flood of large UDP
   packets (while with TCP the attacker gets throttled by the congestion
   controller) or to craft reflection and amplification attacks.
   Networks should therefore be prepared for UDP flood attacks on ports
   used for QUIC traffic.  One possible response to this threat is to
   police UDP traffic on the network, allocating a fixed portion of the
   network capacity to UDP and blocking UDP datagram over that cap.

   The recommended way to police QUIC packets is to either drop them all
   or to throttle them based on the hash of the UDP datagram's source
   and destination addresses, blocking a portion of the hash space that
   corresponds to the fraction of UDP traffic one wishes to drop.  When
   the handshake is blocked, QUIC-capable applications may failover to
   TCP (at least applications using well-known UDP ports).  However,
   blindly blocking a significant fraction of QUIC packets will allow
   many QUIC handshakes to complete, preventing a TCP failover, but the
   connections will suffer from severe packet loss.

4.6.  Distinguishing Acknowledgment traffic

   Some deployed in-network functions distinguish pure-acknowledgment
   (ACK) packets from packets carrying upper-layer data in order to
   attempt to enhance performance, for example by queueing ACKs
   differently or manipulating ACK signaling.  Distinguishing ACK
   packets is trivial in TCP, but not supported by QUIC, since
   acknowledgment signaling is carried inside QUIC's encrypted payload,
   and ACK manipulation is impossible.  Specifically, heuristics
   attempting to distinguish ACK-only packets from payload-carrying
   packets based on packet size are likely to fail, and are emphatically

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4.7.  Quality of Service handling and ECMP

   It is expected that any QoS handling in the network, e.g. based on
   use of DiffServ Code Points (DSCPs) [RFC2475] as well as Equal-Cost
   Multi-Path (ECMP) routing, is applied on a per flow-basis (and not
   per-packet) and as such that all packets belonging to the same QUIC
   connection get uniform treatment.  Using ECMP to distribute packets
   from a single flow across multiple network paths or any other non-
   uniform treatment of packets belong to the same connection could
   result in variations in order, delivery rate, and drop rate.  As
   feedback about loss or delay of each packet is used as input to the
   congestion controller, these variations could adversely affect

   Depending of the loss recovery mechanism implemented, QUIC may be
   more tolerant of packet re-ordering than traditional TCP traffic (see
   Section 2.7).  However, it cannot be known by the network which exact
   recovery mechanism is used and therefore reordering tolerance should
   be considered as unknown.

4.8.  QUIC and Network Address Translation (NAT)

   QUIC Connection IDs are opaque byte fields that are expressed
   consistently across all QUIC versions [QUIC-INVARIANTS], see
   Section 2.6.  This feature may appear to present opportunities to
   optimize NAT port usage and simplify the work of the QUIC server.  In
   fact, NAT behavior that relies on CID may instead cause connection
   failure when endpoints change Connection ID, and disable important
   protocol security features.  NATs should retain their existing 4-
   tuple-based operation and refrain from parsing or otherwise using
   QUIC connection IDs.

   This section uses the colloquial term NAT to mean NAPT (section 2.2
   of [RFC3022]), which overloads several IP addresses to one IP address
   or to an IP address pool, as commonly deployed in carrier-grade NATs
   or residential NATs.

   The remainder of this section explains how QUIC supports NATs better
   than other connection-oriented protocols, why NAT use of Connection
   ID might appear attractive, and how NAT use of CID can create serious
   problems for the endpoints.

   [RFC4787] contains some guidance on building NATs to interact
   constructively with a wide range of applications.  This section
   extends the discussion to QUIC.

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   By using the CID, QUIC connections can survive NAT rebindings as long
   as no routing function in the path is dependent on client IP address
   and port to deliver packets between server and NAT.  Reducing the
   timeout on UDP NATs might be tempting in light of this property, but
   not all QUIC server deployments will be robust to rebinding.

4.8.1.  Resource Conservation

   NATs sometimes hit an operational limit where they exhaust available
   public IP addresses and ports, and must evict flows from their
   address/port mapping.  CIDs might appear to offer a way to multiplex
   many connections over a single address and port.

   However, QUIC endpoints may negotiate new connection IDs inside
   cryptographically protected packets, and begin using them at will.
   Imagine two clients behind a NAT that are sharing the same public IP
   address and port.  The NAT is differentiating them using the incoming
   Connection ID.  If one client secretly changes its connection ID,
   there will be no mapping for the NAT, and the connection will
   suddenly break.

   QUIC is deliberately designed to fail rather than persist when the
   network cannot support its operation.  For HTTP/3, this extends to
   recommending a fallback to TCP-based versions of HTTP rather than
   persisting with a QUIC connection that might be unstable.  And
   [I-D.ietf-quic-applicability] recommends TCP fallback for other
   protocols on the basis that this is preferable to sudden connection
   errors and time outs.  Furthermore, wide deployment of NATs with this
   behavior hinders the use of QUIC's migration function, which relies
   on the ability to change the connection ID any time during the
   lifetime of a QUIC connection.

   It is possible, in principle, to encode the client's identity in a
   connection ID using the techniques described in [QUIC_LB] and
   explicit coordination with the NAT.  However, this implies that the
   client shares configuration with the NAT, which might be logistically
   difficult.  This adds administrative overhead while not resolving the
   case where a client migrates to a point behind the NAT.

   Note that multiplexing connection IDs over a single port anyway
   violates the best common practice to avoid "port overloading" as
   described in [RFC4787].

4.8.2.  "Helping" with routing infrastructure issues

   Concealing client address changes in order to simplify operational
   routing issues will mask important signals that drive security
   mechanisms, and therefore opens QUIC up to various attacks.

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   One challenge in QUIC deployments that want to benefit from QUIC's
   migration capability is server infrastructures with routers and
   switches that direct traffic based on address-port 4-tuple rather
   than connection ID.  The use of source IP address means that a NAT
   rebinding or address migration will deliver packets to the wrong
   server.  As all QUIC payloads are encrypted, routers and switches
   will not have access to negotiated but not-yet-in-use CIDs.  This is
   a particular problem for low-state load balancers.  [QUIC_LB]
   addresses this problem proposing a QUIC extension to allow some
   server-load balancer coordination for routable CIDs.

   It seems that a NAT anywhere in the front of such an infrastructure
   setup could save the effort of converting all these devices by
   decoding routable connection IDs and rewriting the packet IP
   addresses to allow consistent routing by legacy devices.

   Unfortunately, the change of IP address or port is an important
   signal to QUIC endpoints.  It requires a review of path-dependent
   variables like congestion control parameters.  It can also signify
   various attacks that mislead one endpoint about the best peer address
   for the connection (see section 9 of [QUIC-TRANSPORT]).  The QUIC
   PATH_CHALLENGE and PATH_RESPONSE frames are intended to detect and
   mitigate these attacks and verify connectivity to the new address.
   This mechanism cannot work if the NAT is bleaching peer address

   For example, an attacker might copy a legitimate QUIC packet and
   change the source address to match its own.  In the absence of a
   bleaching NAT, the receiving endpoint would interpret this as a
   potential NAT rebinding and use a PATH_CHALLENGE frame to prove that
   the peer endpoint is not truly at the new address, thus thwarting the
   attack.  A bleaching NAT has no means of sending an encrypted
   PATH_CHALLENGE frame, so it might start redirecting all QUIC traffic
   to the attacker address and thus allow an observer to break the

4.9.  Filtering behavior

   [RFC4787] describes possible packet filtering behaviors that relate
   to NATs.  Though the guidance there holds, a particularly unwise
   behavior is to admit a handful of UDP packets and then make a
   decision as to whether or not to filter it.  QUIC applications are
   encouraged to fail over to TCP if early packets do not arrive at
   their destination.  Admitting a few packets allows the QUIC endpoint
   to determine that the path accepts QUIC.  Sudden drops afterwards
   will result in slow and costly timeouts before abandoning the

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5.  IANA Considerations

   This document has no actions for IANA.

6.  Security Considerations

   QUIC is an encrypted and authenticated transport.  That means, once
   the cryptographic handshake is complete, QUIC endpoints discard most
   packets that are not authenticated, greatly limiting the ability of
   an attacker to interfere with existing connections.

   However, some information is still observerable, as supporting
   manageability of QUIC traffic inherently involves tradeoffs with the
   confidentiality of QUIC's control information; this entire document
   is therefore security-relevant.

   More security considerations for QUIC are discussed in
   [QUIC-TRANSPORT] and [QUIC-TLS], generally considering active or
   passive attackers in the network as well as attacks on specific QUIC

   Version Negotiation packets do not contain any mechanism to prevent
   version downgrade attacks.  However, future versions of QUIC that use
   Version Negotiation packets are require to define a mechanism that is
   robust against version downgrade attacks.  Therefore a network node
   should not attempt to impact version selection, as version downgrade
   may result in connection failure.

7.  Contributors

   The following people have contributed text to sections of this

   *  Dan Druta

   *  Martin Duke

   *  Marcus Ilhar

   *  Igor Lubashev

   *  David Schinazi

8.  Acknowledgments

   Special thanks to Martin Thomson and Martin Duke for the detailed
   reviews and feedback.

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   This work is partially supported by the European Commission under
   Horizon 2020 grant agreement no. 688421 Measurement and Architecture
   for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
   for Education, Research, and Innovation under contract no. 15.0268.
   This support does not imply endorsement.

9.  Appendix

   This appendix uses the following conventions: array[i] - one byte at
   index i of array array[i:j] - subset of array starting with index i
   (inclusive) up to j-1 (inclusive) array[i:] - subset of array
   starting with index i (inclusive) up to the end of the array

9.1.  Distinguishing IETF QUIC and Google QUIC Versions

   This section contains algorithms that allows parsing versions from
   both Google QUIC and IETF QUIC.  These mechanisms will become
   irrelevant when IETF QUIC is fully deployed and Google QUIC is

   Note that other than this appendix, nothing in this document applies
   to Google QUIC.  And the purpose of this appendix is merely to
   distinguish IETF QUIC from any versions of Google QUIC.

   Conceptually, a Google QUIC version is an opaque 32bit field.  When
   we refer to a version with four printable characters, we use its
   ASCII representation: for example, Q050 refers to {'Q', '0', '5',
   '0'} which is equal to {0x51, 0x30, 0x35, 0x30}. Otherwise, we use
   its hexadecimal representation: for example, 0xff00001d refers to
   {0xff, 0x00, 0x00, 0x1d}.

   QUIC versions that start with 'Q' or 'T' followed by three digits are
   Google QUIC versions.  Versions up to and including 43 are documented
   by <
   1WJvyZflAO2pq77yOLbp9NsGjC1CHetAXV8I0fQe-B_U/preview>.  Versions
   Q046, Q050, T050, and T051 are not fully documented, but this
   appendix should contain enough information to allow parsing Client
   Hellos for those versions.

   To extract the version number itself, one needs to look at the first
   byte of the QUIC packet, in other words the first byte of the UDP

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     first_byte = packet[0]
     first_byte_bit1 = ((first_byte & 0x80) != 0)
     first_byte_bit2 = ((first_byte & 0x40) != 0)
     first_byte_bit3 = ((first_byte & 0x20) != 0)
     first_byte_bit4 = ((first_byte & 0x10) != 0)
     first_byte_bit5 = ((first_byte & 0x08) != 0)
     first_byte_bit6 = ((first_byte & 0x04) != 0)
     first_byte_bit7 = ((first_byte & 0x02) != 0)
     first_byte_bit8 = ((first_byte & 0x01) != 0)
     if (first_byte_bit1) {
       version = packet[1:5]
     } else if (first_byte_bit5 && !first_byte_bit2) {
       if (!first_byte_bit8) {
         abort("Packet without version")
       if (first_byte_bit5) {
         version = packet[9:13]
       } else {
         version = packet[5:9]
     } else {
       abort("Packet without version")

9.2.  Extracting the CRYPTO frame

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     counter = 0
     while (payload[counter] == 0) {
       counter += 1
     first_nonzero_payload_byte = payload[counter]
     fnz_payload_byte_bit3 = ((first_nonzero_payload_byte & 0x20) != 0)

     if (first_nonzero_payload_byte != 0x06) {
       abort("Unexpected frame")
     if (payload[counter+1] != 0x00) {
       abort("Unexpected crypto stream offset")
     counter += 2
     if ((payload[counter] & 0xc0) == 0) {
       crypto_data_length = payload[counter]
       counter += 1
     } else {
       crypto_data_length = payload[counter:counter+2]
       counter += 2
     crypto_data = payload[counter:counter+crypto_data_length]

10.  References

10.1.  Normative References

   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-34,
              14 January 2021, <

              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-34, 14 January 2021,

10.2.  Informative References

   [Ding2015] Ding, H. and M. Rabinovich, "TCP Stretch Acknowledgments
              and Timestamps - Findings and Impliciations for Passive
              RTT Measurement (ACM Computer Communication Review)", July
              2015, <

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              Mortensen, A., Reddy.K, T., Andreasen, F., Teague, N., and
              R. Compton, "Distributed-Denial-of-Service Open Threat
              Signaling (DOTS) Architecture", Work in Progress,
              Internet-Draft, draft-ietf-dots-architecture-18, 6 March
              2020, <

              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-applicability-08, 2 November 2020,

   [IPIM]     Allman, M., Beverly, R., and B. Trammell, "In-Protocol
              Internet Measurement (arXiv preprint 1612.02902)", 9
              December 2016, <>.

              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-applicability-08, 2 November 2020,

              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-33, 15 December 2020, <

              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-13, 14 January 2021, <

   [QUIC_LB]  Duke, M. and N. Banks, "QUIC-LB: Generating Routable QUIC
              Connection IDs", Work in Progress, Internet-Draft, draft-
              ietf-quic-load-balancers-05, 30 October 2020,

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,

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   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <>.

   [RFC4937]  Arberg, P. and V. Mammoliti, "IANA Considerations for PPP
              over Ethernet (PPPoE)", RFC 4937, DOI 10.17487/RFC4937,
              June 2007, <>.

   [RFC5382]  Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
              Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
              RFC 5382, DOI 10.17487/RFC5382, October 2008,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <>.

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <>.

   [TLS-ESNI] Rescorla, E., Oku, K., Sullivan, N., and C. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-09, 16 December 2020,

   [TMA-QOF]  Trammell, B., Gugelmann, D., and N. Brownlee, "Inline Data
              Integrity Signals for Passive Measurement (in Proc. TMA
              2014)", April 2014.

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              Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <>.

Authors' Addresses

   Mirja Kuehlewind


   Brian Trammell
   Gustav-Gull-Platz 1
   CH- 8004 Zurich


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