Network Working Group                                      M. Kuehlewind
Internet-Draft                                                  Ericsson
Intended status: Informational                               B. Trammell
Expires: 6 March 2022                            Google Switzerland GmbH
                                                        2 September 2021


              Manageability of the QUIC Transport Protocol
                    draft-ietf-quic-manageability-13

Abstract

   This document discusses manageability of the QUIC transport protocol,
   focusing on the implications of QUIC's design and wire image on
   network operations involving QUIC traffic.  Its intended audience is
   network operators and equipment vendors who rely on the use of
   transport-aware network functions.

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 6 March 2022.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
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   as described in Section 4.e of the Trust Legal Provisions and are
   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 . . . . . . . . . . . . . . .   3
     2.1.  QUIC Packet Header Structure  . . . . . . . . . . . . . .   4
     2.2.  Coalesced Packets . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Use of Port Numbers . . . . . . . . . . . . . . . . . . .   6
     2.4.  The QUIC Handshake  . . . . . . . . . . . . . . . . . . .   6
     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  . . . . . . . .  13
     3.1.  Identifying QUIC Traffic  . . . . . . . . . . . . . . . .  13
       3.1.1.  Identifying Negotiated Version  . . . . . . . . . . .  13
       3.1.2.  First Packet Identification for Garbage Rejection . .  14
     3.2.  Connection Confirmation . . . . . . . . . . . . . . . . .  14
     3.3.  Distinguishing Acknowledgment Traffic . . . . . . . . . .  15
     3.4.  Server Name Indication (SNI)  . . . . . . . . . . . . . .  15
       3.4.1.  Extracting Server Name Indication (SNI)
               Information . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Flow Association  . . . . . . . . . . . . . . . . . . . .  16
     3.6.  Flow Teardown . . . . . . . . . . . . . . . . . . . . . .  17
     3.7.  Flow Symmetry Measurement . . . . . . . . . . . . . . . .  17
     3.8.  Round-Trip Time (RTT) Measurement . . . . . . . . . . . .  17
       3.8.1.  Measuring Initial RTT . . . . . . . . . . . . . . . .  18
       3.8.2.  Using the Spin Bit for Passive RTT Measurement  . . .  18
   4.  Specific Network Management Tasks . . . . . . . . . . . . . .  20
     4.1.  Passive Network Performance Measurement and
            Troubleshooting  . . . . . . . . . . . . . . . . . . . .  20
     4.2.  Stateful Treatment of QUIC Traffic  . . . . . . . . . . .  20
     4.3.  Address Rewriting to Ensure Routing Stability . . . . . .  22
     4.4.  Server Cooperation with Load Balancers  . . . . . . . . .  22
     4.5.  Filtering Behavior  . . . . . . . . . . . . . . . . . . .  23
     4.6.  UDP Blocking, Throttling, and NAT Binding . . . . . . . .  23
     4.7.  DDoS Detection and Mitigation . . . . . . . . . . . . . .  24
     4.8.  Quality of Service Handling and ECMP Routing  . . . . . .  25
     4.9.  Handling ICMP Messages  . . . . . . . . . . . . . . . . .  26
     4.10. Guiding Path MTU  . . . . . . . . . . . . . . . . . . . .  26
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  28
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33




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1.  Introduction

   QUIC [QUIC-TRANSPORT] is a new transport protocol that is
   encapsulated in UDP.  QUIC integrates TLS [QUIC-TLS] to encrypt all
   payload data and most control information.  QUIC version 1 was
   designed primarily as a transport for HTTP, with the resulting
   protocol being known as HTTP/3 [QUIC-HTTP].

   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 of the QUIC design with respect to network treatment, and
   a description of how common network management practices will be
   impacted by QUIC.

   QUIC is an end-to-end transport protocol.  No information in the
   protocol header, even that which can be inspected, is mutable by the
   network.  This is achieved through integrity protection of the wire
   image [WIRE-IMAGE].  Encryption of most control signaling means that
   less information is visible to the network than is the case with TCP.

   Integrity protection can also simplify troubleshooting, because none
   of the nodes on the network path can modify transport layer
   information.  However, it means in-network operations that depend on
   modification of data are not possible without the cooperation of an
   QUIC endpoint.  This might be possible with the introduction of a
   proxy which authenticates as an endpoint.  Proxy operations are not
   in scope for this document.

   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.

2.  Features of the QUIC Wire Image

   This section discusses those aspects of the QUIC transport protocol
   that have an impact on the design and operation of devices that
   forward QUIC packets.  This section is therefore primarily
   considering the unencrypted part of QUIC's wire image [WIRE-IMAGE],
   which is defined 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



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   the QUIC version in some packets, and the format of the Version
   Negotiation Packet, are both inspectable and invariant
   [QUIC-INVARIANTS].

   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.

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 contains a version
   number, as well as source and destination connection IDs for
   associating packets with a QUIC connection.  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 [QUIC-INVARIANTS].

   In version 1 of QUIC, the long header is used during connection
   establishment to transmit crypto handshake data, perform version
   negotiation, retry, and send 0-RTT data.

   Short headers contain only an optional destination connection ID and
   the spin bit for RTT measurement.  In version 1 of QUIC, they are
   used after connection establishment.

   The following information is exposed in QUIC packet headers in all
   versions of QUIC:

   *  version number: the version number is present in the long header,
      and identifies the version used for that packet.  During Version
      Negotiation (see Section 17.2.1 of [QUIC-TRANSPORT] and
      Section 2.8), the version number field has a special value
      (0x00000000) that identifies the packet as a Version Negotiation
      packet.  QUIC version 1 uses version 0x00000001.  Operators should
      expect to observe packets with other version numbers as a result
      of various Internet experiments, future standards, and greasing
      ([RFC7801]).  All deployed versions are maintained in an IANA
      registry (see Section 22.2 of [QUIC-TRANSPORT]).





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   *  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 4.4 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.

   In version 1 of QUIC, the following additional information is
   exposed:

   *  "fixed bit": The second-most-significant bit of the first octet of
      most QUIC packets of the current version is set to 1, enabling
      endpoints to demultiplex with other UDP-encapsulated protocols.
      Even though this bit is fixed in the version 1 specification,
      endpoints might use an 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 the first
      octet in the short packet header for version 1.  The spin bit is
      set by endpoints such that tracking edge transitions can be used
      to passively observe end-to-end RTT.  See Section 3.8.2 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.

   *  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).

   *  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, version 1 of QUIC
   cryptographically obfuscates other information in the packet headers:



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   *  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 can be decoded 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 single 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 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 for all
   other IETF transports [RFC7605], there is no guarantee that a
   specific application will use a given registered port, or that a
   given port carries traffic belonging to the respective registered
   service, especially when application layer information is encrypted.
   For example, [QUIC-HTTP] specifies the use of Alt-Svc for discovery
   of HTTP/3 services on other ports.

   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 zero-length, all packets of the 5-tuple likely
   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.







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   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.

   The QUIC handshake can normally be recognized on the wire through
   four flights of datagrams labelled "Client Initial", "Server
   Initial", "Client Completion", and "Server Completion", in the
   illustration 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 can be coalesced (see Section 2.2) in
   order to reduce the number of UDP datagrams sent during the
   handshake.  QUIC packets can be lost and reordered, so packets within
   a flight might not be sent close in time, though the sequence of the
   flights will not change, because one flight depends upon the peer's
   previous flight.

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

   Figure 1: General communication pattern visible in the QUIC handshake

   A handshake starts with the client sending one or more datagrams
   containing Initial packets as shown in Figure 2, which elicits the
   Server Initial response as shown in Figure 3 typically containing
   three types of packets: Initial packet(s) with the beginning of the
   server's side of the TLS handshake, Handshake packet(s) with the rest
   of the server's portion of the TLS handshake, and 1-RTT packet(s), if
   present.




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   The Client Completion flight contains at least one Handshake packet
   and could also include an Initial packet.

   Datagrams that contain an Initial packet (Client Initial, Server
   Initial, and some Client Completion) contain at least 1200 octets of
   UDP payload.  This protects against amplification attacks and
   verifies that the network path meets the requirements for the minimum
   QUIC IP packet size; see Section 14 of [QUIC-TRANSPORT].  This is
   accomplished by either adding PADDING frames within the Initial
   packet, coalescing other packets with the Initial packet, or leaving
   unused payload in the UDP packet after the Initial packet.  A network
   path needs to be able to forward at least this size of packet for
   QUIC to be used.

   The content of Initial packets is 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 and considered
   visible in this 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 1-RTT packets belong to different
   cryptographic and transport contexts.  The Client Completion Figure 4
   and the Server Completion Figure 5 flights conclude the Initial and
   Handshake contexts, by sending final acknowledgments and 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: Example Client Initial datagram without 0-RTT

   A Client Initial packet exposes the version, source and destination
   connection IDs without encryption.  The payload of the Initial packet
   is obfuscated using the Initial secret.  The complete TLS Client
   Hello, including any TLS Server Name Indication (SNI) present, is
   sent in one or more CRYPTO frames across one or more QUIC Initial
   packets.



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   +------------------------------------------------------------+
   | 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: Coalesced Server Initial datagram pattern

   The Server Initial datagram also exposes version number, source and
   destination connection IDs in the clear; the payload of the Initial
   packet(s) 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 Initial)              |  |
   +------------------------------------------------------------+<-+
   | QUIC long header (type = Handshake, Version, DCID, SCID) (Length)
   +------------------------------------------------------------+  |
   | encrypted payload (presumably CRYPTO/ACK frames)           |  |
   +------------------------------------------------------------+<-+
   | QUIC short header                                          |
   +------------------------------------------------------------+
   | 1-RTT encrypted payload                                    |
   +------------------------------------------------------------+

           Figure 4: Coalesced Client Completion datagram pattern








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   The Client Completion flight does not expose any additional
   information; however, as the destination connection ID is server-
   selected, it usually is not the same ID than in the Client Initial.
   Client Completion flights contain 1-RTT packets which indicate the
   handshake has completed (see Section 3.2) on the client, and for
   three-way handshake RTT estimation as in Section 3.8.

   +------------------------------------------------------------+
   | 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: Coalesced Server Completion datagram pattern

   Similar to Client Completion, Server 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, the Client Initial
   flight can also include one or more 0-RTT packets, 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: Coalesced 0-RTT Client Initial datagram

   When a 0-RTT packet is coalesced with an Initial packet, the datagram
   will be padded to 1200 byes.  Additional datagrams containing only
   0-RTT packets with long headers can be sent after the client Initial



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   packet(s), containing more 0-RTT data.  The amount of 0-RTT protected
   data that can be sent in the first flight is limited by the initial
   congestion window, typically to around 10 packets (see Section 7.2 of
   [QUIC-RECOVERY]).

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 exposed in packets sent
   before the cryptographic context was established is validated during
   the cryptographic handshake.  Therefore, devices on path cannot alter
   any information or bits in QUIC packets.  Such alterations would
   cause the integrity check to fail, which results in the receiver
   discarding the packet.  Some parts of Initial packets could be
   altered by removing and re-applying the authenticated encryption
   without immediate discard at the receiver.  However, the
   cryptographic handshake validates most fields and any modifications
   in those fields will result in connection establishment failing
   later.

2.6.  Connection ID and Rebinding

   The connection ID in the QUIC packet headers allows association of
   QUIC packets using information independent of the five-tuple.  This
   allows rebinding of a connection after one of the endpoints
   experienced an address change - usually the client.  Further it can
   be used by in-network devices to ensure that related 5-tuple flows
   are appropriately balanced together.

   Client and server each choose a connection ID during the handshake;
   for example, a server might request that a client use a connection
   ID, whereas the client might choose a zero-length value.  Connection
   IDs for either endpoint may change during the lifetime of a
   connection, with the new connection ID being supplied via encrypted
   frames (see Section 5.1 of [QUIC-TRANSPORT]).  Therefore, observing a
   new connection ID does not necessarily indicate a new connection.

   [QUIC_LB] specifies algorithms for encoding the server mapping in a
   connection ID in order to share this information with selected on-
   path devices such as load balancers.  Server mappings should only be
   exposed to selected entities.  Uncontrolled exposure would allow
   linkage of multiple IP addresses to the same host if the server also
   supports migration that opens an attack vector on specific servers or
   pools.  The best way to obscure an encoding is to appear random to
   any other observers, which is most rigorously achieved with
   encryption.  As a result, any attempt to infer information from
   specific parts of a connection ID is unlikely to be useful.



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2.7.  Packet Numbers

   The Packet Number field is always present in the QUIC packet header
   in version 1; however, it is always encrypted.  The encryption key
   for packet number protection on Initial packets -- which are 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 used by the server to indicate that a
   requested version from the client is not supported (see Section 6 of
   [QUIC-TRANSPORT].  Version Negotiation packets are not intrinsically
   protected, but future QUIC versions will use later encrypted messages
   to verify that they were authentic.  Therefore, any modification 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 an Initial packet with a reserved version
   number to trigger version negotiation.  In the Version Negotiation
   packet, the connection IDs of the client's 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 on 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.











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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,
   but typically without access to any keying information.

3.1.  Identifying QUIC Traffic

   The QUIC wire image is not specifically designed to be
   distinguishable from other UDP traffic by a passive observer in the
   network.

   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/3
   uses UDP port 443 by convention but various methods can be used to
   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 Section 5 of [RFC7605].

   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
   and Section 17 of [QUIC-TRANSPORT]), this method of recognizing QUIC
   traffic is not reliable.  First, it only provides one bit of
   information and is prone to collision with UDP-based protocols other
   than those considered in [RFC7983].  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 an extension.

   Even though transport parameters transmitted in the client's Initial
   packet are observable by the network, they cannot be modified by the
   network without causing connection failure.  Further, the 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 might infer the version number in use by observing the
   handshake: for QUIC version 1, if the version number in the Initial
   packet from a client is the same as the version number in the Initial
   packet of the server response, that version has been accepted by both
   endpoints to be used for the rest of the connection.



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   The negotiated version cannot be identified for flows for which a
   handshake is not observed, such as in the case of connection
   migration; however, it might be possible to associate a flow with a
   flow for which a version has been identified; see Section 3.5.

3.1.2.  First Packet Identification for Garbage Rejection

   A related question is whether the first packet of a given flow on a
   port known to be associated with QUIC is a valid QUIC packet.  This
   determination supports in-network filtering of garbage UDP packets
   (reflection attacks, random backscatter, etc.).  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 bits in the first byte may
   have different meanings in future versions of the protocol.

3.2.  Connection Confirmation

   This document focuses on QUIC version 1, and this Connection
   Confirmation section applies only to packets belonging to QUIC
   version 1 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.

   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 connection may also send data in 0-RTT
   packets directly after the Initial packet containing the TLS Client
   Hello.  Since packets may be reordered or lost in the network, 0-RTT
   packets could be seen before the Initial packet.

   Note that in this version of QUIC, clients send Initial packets
   before servers do, servers send Handshake packets before clients do,
   and only clients send Initial packets with tokens.  Therefore, an
   endpoint can be identified as a client or server by an on-path
   observer.  An attempted connection after Retry can be detected by
   correlating the contents of the Retry packet with the Token and the
   Destination Connection ID fields of the new Initial packet.











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3.3.  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 [RFC3449].  Distinguishing
   ACK packets is possible in TCP, but is 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 not
   recommended to use as a way to construe internals of QUIC's operation
   as those mechanisms can change, e.g., due to the use of extensions.

3.4.  Server Name Indication (SNI)

   The client's TLS ClientHello may contain a Server Name Indication
   (SNI) [RFC6066] extension, 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 an
   Application-Layer Protocol Negotiation (ALPN) [RFC7301] extension, 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.

   Work is currently underway in the TLS working group to encrypt the
   contents of the ClientHello in TLS 1.3 [TLS-ECH].  This would make
   SNI-based application identification impossible by on-path
   observation for QUIC and other protocols that use TLS.

3.4.1.  Extracting Server Name Indication (SNI) Information

   If the ClientHello is not encrypted, SNI can be derived from the
   client's Initial packet(s) by calculating the Initial secret to
   decrypt the packet payload and parsing the QUIC CRYPTO frame(s)
   containing the TLS ClientHello.

   As both the derivation of the Initial secret and the structure of the
   Initial packet itself are version-specific, the first step is always
   to parse the version number (the second through fifth bytes of the
   long header).  Note that only long header packets carry the version
   number, so it is necessary to also check if the 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
   packet to 1.  However, it is expected that these versions will
   gradually disappear over time.



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   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 bits of the header are both set to 0.  Then the
   Destination Connection ID needs to be extracted from the packet.  The
   Initial secret is calculated using the version-specific Initial salt,
   as described in Section 5.2 of [QUIC-TLS].  The length of the
   connection ID is indicated in the 6th byte of the header followed by
   the connection ID itself.

   Note that subsequent Initial packets might contain a Destination
   Connection ID other than the one used to generate the Initial secret.
   Therefore, attempts to decrypt these packets using the procedure
   above might fail unless the Initial secret is retained by the
   observer.

   To determine the end of the packet 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 Section 5.4 of [QUIC-TLS].  The source connection ID length is
   specified in the byte after the destination connection ID.  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 client's Initial packet(s) can be parsed to
   detect the CRYPTO frame(s) that contains the TLS ClientHello, which
   then can be parsed similarly to TLS over TCP connections.  Note that
   there can be multiple CRYPTO frames spread out over one or mor
   Initial packets, and they might not be in order, so reassembling the
   CRYPTO stream by parsing offsets and lengths is required.  Further,
   the client's Initial packet(s) may contain other frames, so the first
   bytes of each frame need to be checked to identify the frame type,
   and if needed skipped over it.  Note that the length of the frames is
   dependent on the frame type; see Section 18 of [QUIC-TRANSPORT].
   E.g.  PADDING frames, each consisting of a single zero byte, may
   occur before, after, or between CRYPTO frames.  However, extensions
   might define additional frame types.  If an unknown frame type is
   encountered, it is impossible to know the length of that frame which
   prevents skipping over it, and therefore parsing fails.

3.5.  Flow Association

   The QUIC connection ID (see Section 2.6) is designed to allow a
   coordinating on-path device, such as a load-balancer, to associate
   two flows when one of the endpoints changes address.  This change can
   be due to NAT rebinding or address migration.




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   The connection ID must change upon intentional address change by an
   endpoint, and connection ID negotiation is encrypted, so it is not
   possible for a passive observer to link intended changes of address
   using the connection ID.

   When one endpoint's address unintentionally changes, as is the case
   with NAT rebinding, an on-path observer may be able to use the
   connection ID to associate the flow on the new address with the flow
   on the old address.

   A network function that attempts to use the connection ID to
   associate flows must be robust to the failure of this technique.
   Since the connection ID may change multiple times during the lifetime
   of a connection, packets with the same five-tuple but different
   connection IDs might or might not belong to the same connection.
   Likewise, packets with the same connection ID but different five-
   tuples might not belong to the same connection, either.

   Connection IDs should be treated as opaque; see Section 4.4 for
   caveats regarding connection ID selection at servers.

3.6.  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 Section 4.2.

3.7.  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.

3.8.  Round-Trip Time (RTT) Measurement

   The round-trip time (RTT) 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 Section 17.3.1 of
   [QUIC-TRANSPORT].



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3.8.1.  Measuring Initial RTT

   In the common case, the delay between the client's Initial packet
   (containing the TLS ClientHello) and the server's Initial packet
   (containing the TLS ServerHello) represents the RTT component on the
   path between the observer and the server.  The delay between the
   server's first Handshake packet and the Handshake packet sent by the
   client represents the RTT component on the path between the observer
   and the client.  While the client may send 0-RTT packets after the
   Initial packet during 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.

3.8.2.  Using the Spin Bit for Passive RTT Measurement

   The spin bit provides a version-specific method to measure per-flow
   RTT from observation points on the network path throughout the
   duration of a connection.  See Section 17.4 of [QUIC-TRANSPORT] for
   the definition of the spin bit in Version 1 of QUIC.  Endpoint
   participation in spin bit signaling is optional.  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.

   When in use, the latency spin bit in each direction changes value
   once per RTT any time that both endpoints are sending packets
   continuously.  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.  This
   mechanism follows the principles of protocol measurability laid out
   in [IPIM].




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   Note that this measurement, as with passive RTT measurement for TCP,
   includes any transport protocol delay (e.g., delayed sending of
   acknowledgments) 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.

   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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   Raw RTT samples generated using these techniques can be processed in
   various ways to generate useful network performance metrics.  A
   simple linear smoothing or moving minimum filter can be applied to
   the stream of RTT samples to get a more stable estimate of
   application-experienced RTT.  RTT samples measured from the spin bit
   can also be used to generate RTT distribution information, including
   minimum RTT (which approximates network RTT over longer time windows)
   and RTT variance (which approximates jitter as seen by the
   application).

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.  Passive Network Performance Measurement and Troubleshooting

   Limited RTT measurement is possible by passive observation of QUIC
   traffic; see Section 3.8.  No passive measurement of loss is possible
   with the present wire image.  Limited observation of upstream
   congestion may be possible via the observation of CE markings on ECN-
   enabled QUIC traffic.

   On-path devices can also make measurements of RTT, loss and other
   performance metrics when information is carried in an additional
   network-layer packet header (Section 6 of
   [I-D.ietf-tsvwg-transport-encrypt] describes use of operations,
   administration and management (OAM) information).  Using network-
   layer approaches also has the advantage that common observation and
   analysis tools can be consistently used by multiple transport
   protocols, however, these techniques are often limited to
   measurements within one or multiple cooperating domains.

4.2.  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 3.6) means that this state must be purged
   either through timers or through least-recently-used eviction,
   depending on application requirements.









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   While QUIC has no clear network-visible end-of-connection signal and
   therefore does require timer-based state removal, the QUIC handshake
   indicates confirmation by both ends of a valid bidirectional
   transmission.  As soon as the handshake completed, timers should be
   set long enough to also allow for short idle time during a valid
   transmission.

   [RFC4787] requires a network state timeout that is not less than 2
   minutes for most UDP traffic.  However, in practice, a QUIC endpoint
   can experience lower timeouts, in the range of 30 to 60 seconds.

   In contrast, [RFC5382] recommends a state timeout of more than 2
   hours for TCP, given that TCP is a connection-oriented protocol with
   well- defined closure semantics.  Even though QUIC has explicitly
   been designed to tolerate NAT rebindings, decreasing the NAT timeout
   is not recommended, as it may negatively impact application
   performance or incentivize endpoints to send very frequent keep-alive
   packets.

   The recommendation is therefore that, even when lower state timeouts
   are used for other UDP traffic, a state timeout of at least two
   minutes ought to be used for QUIC traffic.

   If state is removed too early, this could lead to black-holing of
   incoming packets after a short idle period.  To detect this
   situation, a timer at the client needs to expire before a re-
   establishment can happen (if at all), which would lead to unnecessary
   long delays in an otherwise working connection.

   Furthermore, not all endpoints use routing architectures where
   connections will survive a port or address change.  So even when the
   client revives the connection, a NAT rebinding can cause a routing
   mismatch where a packet is not even delivered to the server that
   might support address migration.  For these reasons, the limits in
   [RFC4787] are important to avoid black-holing of packets (and hence
   avoid interrupting the flow of data to the client), especially where
   devices are able to distinguish QUIC traffic from other UDP payloads.

   The QUIC header optionally contains a connection ID which could
   provide additional entropy beyond the 5-tuple.  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 after the handshake, and this renegotiation is not
   visible to the path.  Therefore, using the connection ID as a flow
   key field for stateful treatment of flows is not recommended as
   connection ID changes will cause undetectable and unrecoverable loss
   of state in the middle of a connection.  Specially, the use of the
   connection ID for functions that require state to make a forwarding



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   decison is not viable as it will break connectivity or at minimum
   cause long timeout-based delays before this problem is detected by
   the endpoints and the connection can potentially be re-established.

   Use of connection IDs is specifically discouraged for NAT
   applications.  If a NAT hits an operational limit, it is recommended
   to rather drop the initial packets of a flow (see also Section 4.5),
   which potentially triggers a fallback to TCP.  Use of the connection
   ID to multiplex multiple connections on the same IP address/port pair
   is not a viable solution as it risks connectivity breakage, in case
   the connection ID changes.

4.3.  Address Rewriting to Ensure Routing Stability

   While QUIC's migration capability makes it possible for a connection
   to survive client address changes, this does not work if the routers
   or switches in the server infrastructure route using the address-port
   4-tuple.  If infrastructure routes on addresses only, NAT rebinding
   or address migration will cause packets to be delivered to the wrong
   server.  [QUIC_LB] describes a way to addresses this problem by
   coordinating the selection and use of connection IDs between load-
   balancers and servers.

   Applying address translation at a middlebox to maintain a stable
   address-port mapping for flows based on connection ID might seem like
   a solution to this problem.  However, hiding information about the
   change of the IP address or port conceals important and security-
   relevant information from QUIC endpoints and as such would facilitate
   amplification attacks (see Section 9 of [QUIC-TRANSPORT]).  A NAT
   function that hides peer address changes prevents the other end from
   detecting and mitigating attacks as the endpoint cannot verify
   connectivity to the new address using QUIC PATH_CHALLENGE and
   PATH_RESPONSE frames.

   In addition, a change of IP address or port is also an input signal
   to other internal mechanisms in QUIC.  When a path change is
   detected, path-dependent variables like congestion control parameters
   will be reset protecting the new path from overload.

4.4.  Server Cooperation with Load Balancers

   In the case of networking architectures that include load balancers,
   the connection ID can be used as 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
   [QUIC-APPLICABILITY].  [QUIC_LB] describes a system for coordinating
   selection and use of connection IDs between load-balancers and
   servers.



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4.5.  Filtering Behavior

   [RFC4787] describes possible packet filtering behaviors that relate
   to NATs but is often also used is other scenarios where packet
   filtering is desired.  Though the guidance there holds, a
   particularly unwise behavior admits a handful of UDP packets and then
   makes a decision to whether or not filter later packets in the same
   connection.  QUIC applications are encouraged to fail over to TCP if
   early packets do not arrive at their destination
   [I-D.ietf-quic-applicability], as QUIC is based on UDP and there are
   known blocks of UDP traffic (see Section 4.6).  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 connection.

4.6.  UDP Blocking, Throttling, and NAT Binding

   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.  Some
   networks therefore block UDP traffic.  With increased deployment of
   QUIC, there is also an increased need to allow UDP traffic on ports
   used for QUIC.  However, if UDP is generally enabled on these ports,
   UDP flood attacks may also use the same ports.  One possible response
   to this threat is to throttle UDP traffic on the network, allocating
   a fixed portion of the network capacity to UDP and blocking UDP
   datagrams over that cap.  As the portion of QUIC traffic compared to
   TCP is also expected to increase over time, using such a limit is not
   recommended but if done, limits might need to be adapted dynamically.

   Further, if UDP traffic is desired to be throttled, it is recommended
   to block individual QUIC flows entirely rather than dropping packets
   indiscriminately.  When the handshake is blocked, QUIC-capable
   applications may fail over to TCP.  However, blocking a random
   fraction of QUIC packets across 4-tuples will allow many QUIC
   handshakes to complete, preventing a TCP failover, but these
   connections will suffer from severe packet loss (see also
   Section 4.5).  Therefore, UDP throttling should be realized by per-
   flow policing, as opposed to per-packet policing.  Note that this
   per-flow policing should be stateless to avoid problems with stateful
   treatment of QUIC flows (see Section 4.2), for example blocking a
   portion of the space of values of a hash function over the addresses
   and ports in the UDP datagram.  While QUIC endpoints are often able
   to survive address changes, e.g. by NAT rebindings, blocking a
   portion of the traffic based on 5-tuple hashing increases the risk of
   black-holing an active connection when the address changes.




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   Note that some source ports are assumed to be reflection attack
   vectors by some servers; see Section 8.1 of
   [I-D.ietf-quic-applicability].  As a result, NAT binding to these
   source ports can result in that traffic being blocked.

4.7.  DDoS Detection and Mitigation

   On-path observation of the transport headers of packets can be used
   for various security functions.  For example, Denial of Service (DOS)
   and Distributed DOS (DDOS) attacks against the infrastructure or
   against an endpoint can be detected and mitigated by characterising
   anomalous traffic.  Other uses include support for security audits
   (e.g., verifying the compliance with ciphersuites); client and
   application fingerprinting for inventory; and to provide alerts for
   network intrusion detection and other next generation firewall
   functions.

   Current practices in detection and mitigation of 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 [DOTS-ARCH].

   Efficient classification of this DDoS traffic in the mitigation
   environment is key to the success of this approach.  Limited first-
   packet garbage detection as in Section 3.1.2 and stateful tracking of
   QUIC traffic as in Section 4.2 above may be useful during
   classification.

   Note that the use of a connection ID to support connection migration
   renders 5-tuple based filtering insufficient to detect active flows
   and requires more state to be maintained by DDoS defense systems if
   support of migration of QUIC flows is desired.  For the common case
   of NAT rebinding, where the client's address changes without the
   client's intent or knowledge, DDoS defense systems can detect a
   change in the client's endpoint address by linking flows based on the
   server's connection IDs.  However, QUIC's linkability resistance
   ensures that a deliberate connection migration is accompanied by a
   change in the connection ID.  In this case, the connection ID can not
   be used to distinguish valid, active traffic from new attack traffic.

   It is also possible for endpoints to directly support security
   functions such as DoS classification and mitigation.  Endpoints can
   cooperate with an in-network device directly by e.g. sharing
   information about connection IDs.



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   Another potential method could use an on-path network device that
   relies on pattern inferences in the traffic and heuristics or machine
   learning instead of processing observed header information.

   However, it is questionable whether connection migrations must be
   supported during a DDoS attack.  While unintended migration without a
   connection ID change can be more easily supported, it might be
   acceptable to not support migrations of active QUIC connections that
   are not visible to the network functions performing the DDoS
   detection.  As soon as the connection blocking is detected by the
   client, the client may be able to 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.

   Beyond in-network DDoS protection mechanisms, 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.

4.8.  Quality of Service Handling and ECMP Routing

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








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4.9.  Handling ICMP Messages

   Datagram Packetization Layer PMTU Discovery (PLPMTUD) can be used by
   QUIC to probe for the supported PMTU.  PLPMTUD optionally uses ICMP
   messages (e.g., IPv6 Packet Too Big messages).  Given known attacks
   with the use of ICMP messages, the use of PLPMTUD in QUIC has been
   designed to safely use but not rely on receiving ICMP feedback (see
   Section 14.2.1. of [QUIC-TRANSPORT]).

   Networks are recommended to forward these ICMP messages and retain as
   much of the original packet as possible without exceeding the minimum
   MTU for the IP version when generating ICMP messages as recommended
   in [RFC1812] and [RFC4443].

4.10.  Guiding Path MTU

   Some network segments support 1500-byte packets, but can only do so
   by fragmenting at a lower layer before traversing a network segment
   with a smaller MTU, and then reassembling within the network segment.
   This is permissible even when the IP layer is IPv6 or IPv4 with the
   DF bit set, because fragmention occurs below the IP layer.  However,
   this process can add to compute and memory costs, leading to a
   bottleneck that limits network capacity.  In such networks this
   generates a desire to influence a majority of senders to use smaller
   packets, to avoid exceeding limited reassembly capacity.

   For TCP, MSS clamping (Section 3.2 of [RFC4459]) is often used to
   change the sender's TCP maximum segment size, but QUIC requires a
   different approach.  Section 14 of [QUIC-TRANSPORT] advises senders
   to probe larger sizes using Datagram Packetization Layer PMTU
   Discovery ([DPLPMTUD]) or Path Maximum Transmission Unit Discovery
   (PMTUD: [RFC1191] and [RFC8201]).  This mechanism encourages senders
   to approach the maximum packet size, which could then cause
   fragmentation within a network segment of which they may not be
   aware.

   If path performance is limited when forwarding larger packets, an on-
   path device should support a maximum packet size for a specific
   transport flow and then consistently drop all packets that exceed the
   configured size when the inner IPv4 packet has DF set, or IPv6 is
   used.

   Networks with configurations that would lead to fragmentation of
   large packets within a network segment should drop such packets
   rather than fragmenting them.  Network operators who plan to
   implement a more selective policy may start by focusing on QUIC.





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   QUIC flows cannot always be easily distinguished from other UDP
   traffic, but we assume at least some portion of QUIC traffic can be
   identified (see Section 3.1).  For networks supporting QUIC, it is
   recommended that a path drops any packet larger than the
   fragmentation size.  When a QUIC endpoint uses DPLPMTUD, it will use
   a QUIC probe packet to discover the PMTU.  If this probe is lost, it
   will not impact the flow of QUIC data.

   IPv4 routers generate an ICMP message when a packet is dropped
   because the link MTU was exceeded.  [RFC8504] specifies how an IPv6
   node generates an ICMPv6 Packet Too Big message (PTB) in this case.
   PMTUD relies upon an endpoint receiving such PTB messages [RFC8201],
   whereas DPLPMTUD does not reply upon these messages, but still can
   optionally use these to improve performance Section 4.6 of
   [DPLPMTUD].

   A network cannot know in advance which discovery method is used by a
   QUIC endpoint, so it should send a PTB message in addition to
   dropping an oversized packet.  A generated PTB message should be
   compliant with the validation requirements of Section 14.2.1 of
   [QUIC-TRANSPORT], otherwise it will be ignored for PMTU discovery.
   This provides a signal to the endpoint to prevent the packet size
   from growing too large, which can entirely avoid network segment
   fragmentation for that flow.

   Endpoints can cache PMTU information, in the IP-layer cache.  This
   short-term consistency between the PMTU for flows can help avoid an
   endpoint using a PMTU that is inefficient.  The IP cache can also
   influence the PMTU value of other IP flows that use the same path
   [RFC8201][RFC8899], including IP packets carrying protocols other
   than QUIC.  The representation of an IP path is implementation-
   specific [RFC8201].

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.



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   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
   mechanism.

   Version Negotiation packets do not contain any mechanism to prevent
   version downgrade attacks.  However, future versions of QUIC that use
   Version Negotiation packets are required 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 significant text to and/or
   feedback on this document:

   *  Chris Box

   *  Dan Druta

   *  David Schinazi

   *  Gorry Fairhurst

   *  Ian Swett

   *  Igor Lubashev

   *  Jana Iyengar

   *  Jared Mauch

   *  Lars Eggert

   *  Lucas Purdue

   *  Marcus Ilhar

   *  Mark Nottingham

   *  Martin Duke

   *  Martin Thomson

   *  Matt Joras

   *  Mike Bishop



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   *  Nick Banks

   *  Thomas Fossati

   *  Sean Turner

8.  Acknowledgments

   This work was 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.  References

9.1.  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, <https://datatracker.ietf.org/doc/html/
              draft-ietf-quic-tls-34>.

   [QUIC-TRANSPORT]
              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,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              transport-34>.

9.2.  Informative References

   [DOTS-ARCH]
              Mortensen, A., Ed., Reddy.K, T., Ed., Andreasen, F.,
              Teague, N., and R. Compton, "DDoS Open Threat Signaling
              (DOTS) Architecture", RFC 8811, DOI 10.17487/RFC8811,
              August 2020, <https://www.rfc-editor.org/rfc/rfc8811>.

   [DPLPMTUD] Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/rfc/rfc8899>.









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   [I-D.ietf-quic-applicability]
              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-applicability-12, 30 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              applicability-12>.

   [I-D.ietf-tsvwg-transport-encrypt]
              Fairhurst, G. and C. Perkins, "Considerations around
              Transport Header Confidentiality, Network Operations, and
              the Evolution of Internet Transport Protocols", Work in
              Progress, Internet-Draft, draft-ietf-tsvwg-transport-
              encrypt-21, 20 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-
              transport-encrypt-21>.

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

   [QUIC-APPLICABILITY]
              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-applicability-12, 30 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              applicability-12>.

   [QUIC-HTTP]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-34, 2 February 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              http-34>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-13, 14 January 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              invariants-13>.

   [QUIC-RECOVERY]
              Iyengar, J. and I. Swett, "QUIC Loss Detection and
              Congestion Control", Work in Progress, Internet-Draft,
              draft-ietf-quic-recovery-34, 14 January 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              recovery-34>.




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   [QUIC_LB]  Duke, M. and N. Banks, "QUIC-LB: Generating Routable QUIC
              Connection IDs", Work in Progress, Internet-Draft, draft-
              ietf-quic-load-balancers-07, 9 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              load-balancers-07>.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,
              <https://www.rfc-editor.org/rfc/rfc1191>.

   [RFC1812]  Baker, F., Ed., "Requirements for IP Version 4 Routers",
              RFC 1812, DOI 10.17487/RFC1812, June 1995,
              <https://www.rfc-editor.org/rfc/rfc1812>.

   [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,
              <https://www.rfc-editor.org/rfc/rfc2475>.

   [RFC3449]  Balakrishnan, H., Padmanabhan, V., Fairhurst, G., and M.
              Sooriyabandara, "TCP Performance Implications of Network
              Path Asymmetry", BCP 69, RFC 3449, DOI 10.17487/RFC3449,
              December 2002, <https://www.rfc-editor.org/rfc/rfc3449>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/rfc/rfc4443>.

   [RFC4459]  Savola, P., "MTU and Fragmentation Issues with In-the-
              Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
              2006, <https://www.rfc-editor.org/rfc/rfc4459>.

   [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, <https://www.rfc-editor.org/rfc/rfc4787>.

   [RFC4937]  Arberg, P. and V. Mammoliti, "IANA Considerations for PPP
              over Ethernet (PPPoE)", RFC 4937, DOI 10.17487/RFC4937,
              June 2007, <https://www.rfc-editor.org/rfc/rfc4937>.

   [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,
              <https://www.rfc-editor.org/rfc/rfc5382>.




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   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/rfc/rfc6066>.

   [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, <https://www.rfc-editor.org/rfc/rfc7301>.

   [RFC7605]  Touch, J., "Recommendations on Using Assigned Transport
              Port Numbers", BCP 165, RFC 7605, DOI 10.17487/RFC7605,
              August 2015, <https://www.rfc-editor.org/rfc/rfc7605>.

   [RFC7801]  Dolmatov, V., Ed., "GOST R 34.12-2015: Block Cipher
              "Kuznyechik"", RFC 7801, DOI 10.17487/RFC7801, March 2016,
              <https://www.rfc-editor.org/rfc/rfc7801>.

   [RFC7983]  Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,
              <https://www.rfc-editor.org/rfc/rfc7983>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/rfc/rfc8201>.

   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/rfc/rfc8504>.

   [RFC8899]  Fairhurst, G., Jones, T., Tüxen, M., Rüngeler, I., and T.
              Völker, "Packetization Layer Path MTU Discovery for
              Datagram Transports", RFC 8899, DOI 10.17487/RFC8899,
              September 2020, <https://www.rfc-editor.org/rfc/rfc8899>.

   [TLS-ECH]  Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-13, 12 August 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              esni-13>.

   [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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   [WIRE-IMAGE]
              Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <https://www.rfc-editor.org/rfc/rfc8546>.

Authors' Addresses

   Mirja Kuehlewind
   Ericsson

   Email: mirja.kuehlewind@ericsson.com


   Brian Trammell
   Google Switzerland GmbH
   Gustav-Gull-Platz 1
   CH- 8004 Zurich
   Switzerland

   Email: ietf@trammell.ch































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