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QUIC                                                             M. Duke
Internet-Draft                                         F5 Networks, Inc.
Intended status: Informational                              29 July 2020
Expires: 30 January 2021

              Network Address Translation Support for QUIC


   Network Address Translators (NATs) are widely deployed to share
   scarce public IPv4 addresses among multiple end hosts.  They
   overwrite IP addresses and ports in IP packets to do so.  QUIC is a
   protocol on top of UDP that provides transport-like services.  QUIC
   is better-behaved in the presence of NATs than older protocols, and
   existing UDP NATs should operate without incident if unmodified.
   QUIC offers additional features that may tempt NAT implementers as
   potential optimizations.  However, in practice, leveraging these
   features will lead to new connection failure modes and security

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 30 January 2021.

Copyright Notice

   Copyright (c) 2020 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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  QUIC and NAT Rebinding  . . . . . . . . . . . . . . . . . . .   3
   4.  The Lure of the Connection ID . . . . . . . . . . . . . . . .   4
     4.1.  Resource Conservation . . . . . . . . . . . . . . . . . .   4
     4.2.  "Helping" with routing infrastructure issues  . . . . . .   5
   5.  Filtering behavior  . . . . . . . . . . . . . . . . . . . . .   5
   6.  QUIC Detection  . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .   7
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .   7
     B.1.  since draft-duke-quic-natsupp-02  . . . . . . . . . . . .   7
     B.2.  since draft-duke-quic-natsupp-01  . . . . . . . . . . . .   7
     B.3.  since draft-duke-quic-natsupp-00  . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   Network Address Translators (NATs) are a widely deployed means of
   multiplexing multiple private IP addresses over scarce IPv4 public
   address space by replacing those addresses and using ports to
   distinguish those connections.  The new address can also guarantee
   that packets move through a proxy throughout the life of a
   connection, so that the connection can continue with the required
   state at that proxy.

   This document 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.

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   QUIC [QUIC-TRANSPORT] is a protocol, operating over UDP, that
   provides many transport-like services to the application layer.
   Among these services is the mapping of multiple endpoint IP addresses
   to a single connection through use of a Connection ID (CID).
   Connection IDs are opaque byte fields that are expressed consistently
   across all QUIC versions [QUIC-INVARIANTS].  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.

   The remainder of this document 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.  The conclusion of this document is that
   NATs should retain their existing 4-tuple-based operation and refrain
   from parsing or otherwise using QUIC connection IDs.

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

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  QUIC and NAT Rebinding

   An explicit goal of QUIC is to be robust to NAT rebinding.  When a
   connection is idle for a long time, the NAT may guess it has
   terminated and assign the client port to a new connection.  As TCP
   defines a connection by its address and port 4-tuple, a TCP packet
   will not appear to belong to any existing connection at the receiver.

   QUIC endpoints identify their connections using a CID that is encoded
   in every packet.  If the client attempts to resume communication, the
   packet will be assigned a new source IP and/or port.  Incoming
   packets from the server will be misrouted and dropped until the
   client sends a packet from its new address.

   Therefore, 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.

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4.  The Lure of the Connection ID

   There are a few reasons that CID-aware NATs could seemingly appear

4.1.  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 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.

   While mid-connection failure in some cases may seem superior to
   rejecting QUIC outright, HTTP/3 over QUIC falls back to TCP.  This is
   preferable to a connection suddenly black holing and timing out.
   Furthermore, wide deployment of NATs with this behavior would make it
   risky to change Connection IDs in the internet, which would thwart
   various important protocol properties.

   It is possible, in principle, to encode the client's identity in a
   connection ID using [QUIC-LB] and explicit coordination with the NAT.
   However, QUIC-LB makes assumptions about endpoint mobility and common
   configuration in server infrastructure that are almost never valid in
   client/NAT architectures.  Deploying such a system would include the
   administrative overhead while not solving the problem described in
   this section if the client changes networks.

   Note that using connection IDs in this manner would anyway violate
   the best common practice to avoid "port overloading" as described in

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4.2.  "Helping" with routing infrastructure issues

   One problem in QUIC deployment is router and switch server
   infrastructures 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.  For the reasons described above, routers and switches will
   not have access to negotiated CIDs.  This is a particular problem for
   low-state load balancers, and a QUIC extension exists [QUIC-LB] to
   allow some server-load balancer coordination for routable CIDs.

   A NAT at the front of this infrastructure might 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

5.  Filtering behavior

   [RFC4787] describes possible packet filtering behaviors that relate
   to NATs.  Though thes 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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6.  QUIC Detection

   Beyond the above difficulties, merely identifying that a UDP packet
   is part of a QUIC connection is not straightforward.  Due to address
   migration, NATs cannot assume that QUIC version 1 application traffic
   is preceeded by a handshake on the path.  The short header prepended
   to version 1 application traffic has few consistent codepoints that
   reliably identify it as QUIC.  Moreover, the protocol is designed to
   be extensible.  [QUIC-INVARIANTS] describes the small set of QUIC
   protocol properties that will remain stable across versions.

   For these reasons, applying generalized UDP policies will prevent
   accidental breakage of QUIC features and mishandled non-QUIC UDP

7.  Security Considerations

   This document proposes no change in behavior in the internet, so
   there are no new security implications.  However, ignoring the
   recommendations here could prevent existing security mechanisms in
   QUIC from working properly.

8.  IANA Considerations

   There are no IANA requirements.

9.  Informative References

              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-latest, <https://tools.ietf.org/html/draft-

   [QUIC-LB]  Duke, M. and N. Banks, "QUIC-LB: Generating Routable QUIC
              Connection IDs", Work in Progress, Internet-Draft, draft-

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

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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

Appendix A.  Acknowledgments

   Thanks to Dmitri Tikhonov, who first recognized that certain NAT
   behaviors could create problems for QUIC.

Appendix B.  Change Log

      *RFC Editor's Note:* Please remove this section prior to$
      publication of a final version of this document.$

B.1.  since draft-duke-quic-natsupp-02

   *  Added discussion of QUIC identification

B.2.  since draft-duke-quic-natsupp-01

   *  Added brief discussion of impact of filtering.

   *  Added references to RFC 4787.

   *  Corrected normative reference to be informative.

B.3.  since draft-duke-quic-natsupp-00

   *  Tightened NAT terminology

   *  Added additional clarfying examples

   *  Added warning against using QUIC-LB for NATs that front clients.

Author's Address

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   Martin Duke
   F5 Networks, Inc.

   Email: martin.h.duke@gmail.com

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