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QUIC-LB: Generating Routable QUIC Connection IDs
draft-ietf-quic-load-balancers-13

Document Type Active Internet-Draft (quic WG)
Authors Martin Duke , Nick Banks , Christian Huitema
Last updated 2022-03-28
Replaces draft-duke-quic-load-balancers
Stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Sep 2021
QUIC-LB: Generating Routable QUIC Connection IDs to IESG
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draft-ietf-quic-load-balancers-13
QUIC                                                             M. Duke
Internet-Draft                                                    Google
Intended status: Standards Track                                N. Banks
Expires: 29 September 2022                                     Microsoft
                                                              C. Huitema
                                                    Private Octopus Inc.
                                                           28 March 2022

            QUIC-LB: Generating Routable QUIC Connection IDs
                   draft-ietf-quic-load-balancers-13

Abstract

   QUIC address migration allows clients to change their IP address
   while maintaining connection state.  To reduce the ability of an
   observer to link two IP addresses, clients and servers use new
   connection IDs when they communicate via different client addresses.
   This poses a problem for traditional "layer-4" load balancers that
   route packets via the IP address and port 4-tuple.  This
   specification provides a standardized means of securely encoding
   routing information in the server's connection IDs so that a properly
   configured load balancer can route packets with migrated addresses
   correctly.  As it proposes a structured connection ID format, it also
   provides a means of connection IDs self-encoding their length to aid
   some hardware offloads.

Note to Readers

   Discussion of this document takes place on the QUIC Working Group
   mailing list (quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/quic/
   (https://mailarchive.ietf.org/arch/browse/quic/).

   Source for this draft and an issue tracker can be found at
   https://github.com/quicwg/load-balancers (https://github.com/quicwg/
   load-balancers).

Status of This Memo

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

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

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

   This Internet-Draft will expire on 29 September 2022.

Copyright Notice

   Copyright (c) 2022 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
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  First CID octet . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Config Rotation . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Configuration Failover  . . . . . . . . . . . . . . . . .   7
     2.3.  Length Self-Description . . . . . . . . . . . . . . . . .   7
     2.4.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Load Balancing Preliminaries  . . . . . . . . . . . . . . . .   8
     3.1.  Unroutable Connection IDs . . . . . . . . . . . . . . . .   8
     3.2.  Fallback Algorithms . . . . . . . . . . . . . . . . . . .  10
     3.3.  Server ID Allocation  . . . . . . . . . . . . . . . . . .  10
   4.  Server ID Encoding in Connection IDs  . . . . . . . . . . . .  11
     4.1.  CID format  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Configuration Agent Actions . . . . . . . . . . . . . . .  11
     4.3.  Server Actions  . . . . . . . . . . . . . . . . . . . . .  11
       4.3.1.  Special Case: Single Pass Encryption  . . . . . . . .  12
       4.3.2.  General Case: Four-Pass Encryption  . . . . . . . . .  12
     4.4.  Load Balancer Actions . . . . . . . . . . . . . . . . . .  14
       4.4.1.  Special Case: Single Pass Encryption  . . . . . . . .  15
       4.4.2.  General Case: Four-Pass Encryption  . . . . . . . . .  15
   5.  Per-connection state  . . . . . . . . . . . . . . . . . . . .  15
   6.  Additional Use Cases  . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Load balancer chains  . . . . . . . . . . . . . . . . . .  16
     6.2.  Moving connections between servers  . . . . . . . . . . .  17

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   7.  Version Invariance of QUIC-LB . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
     8.1.  Attackers not between the load balancer and server  . . .  19
     8.2.  Attackers between the load balancer and server  . . . . .  19
     8.3.  Multiple Configuration IDs  . . . . . . . . . . . . . . .  19
     8.4.  Limited configuration scope . . . . . . . . . . . . . . .  19
     8.5.  Stateless Reset Oracle  . . . . . . . . . . . . . . . . .  20
     8.6.  Connection ID Entropy . . . . . . . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  QUIC-LB YANG Model . . . . . . . . . . . . . . . . .  23
     A.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  29
   Appendix B.  Load Balancer Test Vectors . . . . . . . . . . . . .  29
     B.1.  Unencrypted CIDs  . . . . . . . . . . . . . . . . . . . .  30
     B.2.  Encrypted CIDs  . . . . . . . . . . . . . . . . . . . . .  30
   Appendix C.  Interoperability with DTLS over UDP  . . . . . . . .  30
     C.1.  DTLS 1.0 and 1.2  . . . . . . . . . . . . . . . . . . . .  30
     C.2.  DTLS 1.3  . . . . . . . . . . . . . . . . . . . . . . . .  31
     C.3.  Future Versions of DTLS . . . . . . . . . . . . . . . . .  32
   Appendix D.  Acknowledgments  . . . . . . . . . . . . . . . . . .  32
   Appendix E.  Change Log . . . . . . . . . . . . . . . . . . . . .  32
     E.1.  since draft-ietf-quic-load-balancers-12 . . . . . . . . .  32
     E.2.  since draft-ietf-quic-load-balancers-11 . . . . . . . . .  32
     E.3.  since draft-ietf-quic-load-balancers-10 . . . . . . . . .  32
     E.4.  since draft-ietf-quic-load-balancers-09 . . . . . . . . .  33
     E.5.  since draft-ietf-quic-load-balancers-08 . . . . . . . . .  33
     E.6.  since draft-ietf-quic-load-balancers-07 . . . . . . . . .  33
     E.7.  since draft-ietf-quic-load-balancers-06 . . . . . . . . .  33
     E.8.  since draft-ietf-quic-load-balancers-05 . . . . . . . . .  33
     E.9.  since draft-ietf-quic-load-balancers-04 . . . . . . . . .  34
     E.10. since-draft-ietf-quic-load-balancers-03 . . . . . . . . .  34
     E.11. since-draft-ietf-quic-load-balancers-02 . . . . . . . . .  34
     E.12. since-draft-ietf-quic-load-balancers-01 . . . . . . . . .  34
     E.13. since-draft-ietf-quic-load-balancers-00 . . . . . . . . .  35
     E.14. Since draft-duke-quic-load-balancers-06 . . . . . . . . .  35
     E.15. Since draft-duke-quic-load-balancers-05 . . . . . . . . .  35
     E.16. Since draft-duke-quic-load-balancers-04 . . . . . . . . .  35
     E.17. Since draft-duke-quic-load-balancers-03 . . . . . . . . .  35
     E.18. Since draft-duke-quic-load-balancers-02 . . . . . . . . .  35
     E.19. Since draft-duke-quic-load-balancers-01 . . . . . . . . .  36
     E.20. Since draft-duke-quic-load-balancers-00 . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

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1.  Introduction

   QUIC packets [RFC9000] usually contain a connection ID to allow
   endpoints to associate packets with different address/port 4-tuples
   to the same connection context.  This feature makes connections
   robust in the event of NAT rebinding.  QUIC endpoints usually
   designate the connection ID which peers use to address packets.
   Server-generated connection IDs create a potential need for out-of-
   band communication to support QUIC.

   QUIC allows servers (or load balancers) to designate an initial
   connection ID to encode useful routing information for load
   balancers.  It also encourages servers, in packets protected by
   cryptography, to provide additional connection IDs to the client.
   This allows clients that know they are going to change IP address or
   port to use a separate connection ID on the new path, thus reducing
   linkability as clients move through the world.

   There is a tension between the requirements to provide routing
   information and mitigate linkability.  Ultimately, because new
   connection IDs are in protected packets, they must be generated at
   the server if the load balancer does not have access to the
   connection keys.  However, it is the load balancer that has the
   context necessary to generate a connection ID that encodes useful
   routing information.  In the absence of any shared state between load
   balancer and server, the load balancer must maintain a relatively
   expensive table of server-generated connection IDs, and will not
   route packets correctly if they use a connection ID that was
   originally communicated in a protected NEW_CONNECTION_ID frame.

   This specification provides common algorithms for encoding the server
   mapping in a connection ID given some shared parameters.  The mapping
   is generally only discoverable by observers that have the parameters,
   preserving unlinkability as much as possible.

   As this document proposes a structured QUIC Connection ID, it also
   proposes a system for self-encoding connection ID length in all
   packets, so that crypto offload can efficiently obtain key
   information.

   While this document describes a small set of configuration parameters
   to make the server mapping intelligible, the means of distributing
   these parameters between load balancers, servers, and other trusted
   intermediaries is out of its scope.  There are numerous well-known
   infrastructures for distribution of configuration.

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1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

   In this document, "client" and "server" refer to the endpoints of a
   QUIC connection unless otherwise indicated.  A "load balancer" is an
   intermediary for that connection that does not possess QUIC
   connection keys, but it may rewrite IP addresses or conduct other IP
   or UDP processing.  A "configuration agent" is the entity that
   determines the QUIC-LB configuration parameters for the network and
   leverages some system to distribute that configuration.

   Note that stateful load balancers that act as proxies, by terminating
   a QUIC connection with the client and then retrieving data from the
   server using QUIC or another protocol, are treated as a server with
   respect to this specification.

   For brevity, "Connection ID" will often be abbreviated as "CID".

1.2.  Notation

   All wire formats will be depicted using the notation defined in
   Section 1.3 of [RFC9000].  There is one addition: the function len()
   refers to the length of a field which can serve as a limit on a
   different field, so that the lengths of two fields can be concisely
   defined as limited to a sum, for example:

   x(A..B) y(C..B-len(x))

   indicates that x can be of any length between A and B, and y can be
   of any length between C and B provided that (len(x) + len(y)) does
   not exceed B.

   The example below illustrates the basic framework:

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Example Structure {
  One-bit Field (1),
  7-bit Field with Fixed Value (7) = 61,
  Field with Variable-Length Integer (i),
  Arbitrary-Length Field (..),
  Variable-Length Field (8..24),
  Variable-Length Field with Dynamic Limit (8..24-len(Variable-Length Field)),
  Field With Minimum Length (16..),
  Field With Maximum Length (..128),
  [Optional Field (64)],
  Repeated Field (8) ...,
}

                       Figure 1: Example Format

2.  First CID octet

   The first octet of a Connection ID is reserved for two special
   purposes, one mandatory (config rotation) and one optional (length
   self-description).

   Subsequent sections of this document refer to the contents of this
   octet as the "first octet."

2.1.  Config Rotation

   The first two bits of any connection ID MUST encode an identifier for
   the configuration that the connection ID uses.  This enables
   incremental deployment of new QUIC-LB settings (e.g., keys).

   When new configuration is distributed to servers, there will be a
   transition period when connection IDs reflecting old and new
   configuration coexist in the network.  The rotation bits allow load
   balancers to apply the correct routing algorithm and parameters to
   incoming packets.

   Configuration Agents SHOULD deliver new configurations to load
   balancers before doing so to servers, so that load balancers are
   ready to process CIDs using the new parameters when they arrive.

   A Configuration Agent SHOULD NOT use a codepoint to represent a new
   configuration until it takes precautions to make sure that all
   connections using CIDs with an old configuration at that codepoint
   have closed or transitioned.

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   Servers MUST NOT generate new connection IDs using an old
   configuration after receiving a new one from the configuration agent.
   Servers MUST send NEW_CONNECTION_ID frames that provide CIDs using
   the new configuration, and retire CIDs using the old configuration
   using the "Retire Prior To" field of that frame.

   It also possible to use these bits for more long-lived distinction of
   different configurations, but this has privacy implications (see
   Section 8.3).

2.2.  Configuration Failover

   If a server has not received a valid QUIC-LB configuration, and
   believes that low-state, Connection-ID aware load balancers are in
   the path, it SHOULD generate connection IDs with the config rotation
   bits set to '11' and SHOULD use the "disable_active_migration"
   transport parameter in all new QUIC connections.  It SHOULD NOT send
   NEW_CONNECTION_ID frames with new values.

   A load balancer that sees a connection ID with config rotation bits
   set to '11' MUST revert to 5-tuple routing.  These connection IDs may
   be of any length; however, see Section 8.6 for limits on this length.

2.3.  Length Self-Description

   Local hardware cryptographic offload devices may accelerate QUIC
   servers by receiving keys from the QUIC implementation indexed to the
   connection ID.  However, on physical devices operating multiple QUIC
   servers, it is impractical to efficiently lookup these keys if the
   connection ID does not self-encode its own length.

   Note that this is a function of particular server devices and is
   irrelevant to load balancers.  As such, load balancers MAY omit this
   from their configuration.  However, the remaining 6 bits in the first
   octet of the Connection ID are reserved to express the length of the
   following connection ID, not including the first octet.

   A server not using this functionality SHOULD make the six bits appear
   to be random.

2.4.  Format

   First Octet {
     Config Rotation (2),
     CID Len or Random Bits (6),
   }

                        Figure 2: First Octet Format

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   The first octet has the following fields:

   Config Rotation: Indicates the configuration used to interpret the
   CID.

   CID Len or Random Bits: Length Self-Description (if applicable), or
   random bits otherwise.  Encodes the length of the Connection ID
   following the First Octet.

3.  Load Balancing Preliminaries

   In QUIC-LB, load balancers do not generate individual connection IDs
   for servers.  Instead, they communicate the parameters of an
   algorithm to generate routable connection IDs.

   The algorithms differ in the complexity of configuration at both load
   balancer and server.  Increasing complexity improves obfuscation of
   the server mapping.

   This section describes three participants: the configuration agent,
   the load balancer, and the server.  For any given QUIC-LB
   configuration that enables connection-ID-aware load balancing, there
   must be a choice of (1) routing algorithm, (2) server ID allocation
   strategy, and (3) algorithm parameters.

   Fundamentally, servers generate connection IDs that encode their
   server ID.  Load balancers decode the server ID from the CID in
   incoming packets to route to the correct server.

   There are situations where a server pool might be operating two or
   more routing algorithms or parameter sets simultaneously.  The load
   balancer uses the first two bits of the connection ID to multiplex
   incoming DCIDs over these schemes (see Section 2.1).

3.1.  Unroutable Connection IDs

   QUIC-LB servers will generate Connection IDs that are decodable to
   extract a server ID in accordance with a specified algorithm and
   parameters.  However, QUIC often uses client-generated Connection IDs
   prior to receiving a packet from the server.

   These client-generated CIDs might not conform to the expectations of
   the routing algorithm and therefore not be routable by the load
   balancer.  Those that are not routable are "unroutable DCIDs" and
   receive similar treatment regardless of why they're unroutable:

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   *  The config rotation bits (Section 2.1) may not correspond to an
      active configuration.  Note: a packet with a DCID that indicates
      5-tuple routing (see Section 2.2) is always routable.

   *  The DCID might not be long enough for the decoder to process.

   *  The extracted server mapping might not correspond to an active
      server.

   All other DCIDs are routable.

   Load balancers MUST forward packets with routable DCIDs to a server
   in accordance with the chosen routing algorithm.  Exception: if the
   load balancer can parse the QUIC packet and makes a routing decision
   depending on the contents (e.g., the SNI in a TLS client hello), it
   MAY route in accordance with this instead.  However, load balancers
   MUST always route long header packets it cannot parse in accordance
   with the DCID (see Section 7).

   Load balancers SHOULD drop short header packets with unroutable
   DCIDs.

   When forwarding a packet with a long header and unroutable DCID, load
   balancers MUST use a fallback algorithm as specified in Section 3.2.

   Load balancers MAY drop packets with long headers and unroutable
   DCIDs if and only if it knows that the encoded QUIC version does not
   allow an unroutable DCID in a packet with that signature.  For
   example, a load balancer can safely drop a QUIC version 1 Handshake
   packet with an unroutable DCID, as a version 1 Handshake packet sent
   to a QUIC-LB routable server will always have a server-generated
   routable CID.  The prohibition against dropping packets with long
   headers remains for unknown QUIC versions.

   Furthermore, while the load balancer function MUST NOT drop packets,
   the device might implement other security policies, outside the scope
   of this specification, that might force a drop.

   Servers that receive packets with unroutable CIDs MUST use the
   available mechanisms to induce the client to use a routable CID in
   future packets.  In QUIC version 1, this requires using a routable
   CID in the Source CID field of server-generated long headers.

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3.2.  Fallback Algorithms

   There are conditions described below where a load balancer routes a
   packet using a "fallback algorithm."  It can choose any algorithm,
   without coordination with the servers, but the algorithm SHOULD be
   deterministic over short time scales so that related packets go to
   the same server.  The design of this algorithm SHOULD consider the
   version-invariant properties of QUIC described in [RFC8999] to
   maximize its robustness to future versions of QUIC.

   A fallback algorithm MUST NOT make the routing behavior dependent on
   any bits in the first octet of the QUIC packet header, except the
   first bit, which indicates a long header.  All other bits are QUIC
   version-dependent and intermediaries SHOULD NOT base their design on
   version-specific templates.

   For example, one fallback algorithm might convert a unroutable DCID
   to an integer and divided by the number of servers, with the modulus
   used to forward the packet.  The number of servers is usually
   consistent on the time scale of a QUIC connection handshake.  Another
   might simply hash the address/port 4-tuple.  See also Section 7.

3.3.  Server ID Allocation

   Load Balancer configurations include a mapping of server IDs to
   forwarding addresses.  The corresponding server configurations
   contain one or more unique server IDs.

   The configuration agent chooses a server ID length for each
   configuration that MUST be at least one octet.

   A QUIC-LB configuration MAY significantly over-provision the server
   ID space (i.e., provide far more codepoints than there are servers)
   to increase the probability that a randomly generated Destination
   Connection ID is unroutable.

   The configuration agent SHOULD provide a means for servers to express
   the number of server IDs it can usefully employ, because a single
   routing address actually corresponds to multiple server entities (see
   Section 6.1).

   Conceptually, each configuration has its own set of server ID
   allocations, though two static configurations with identical server
   ID lengths MAY use a common allocation between them.

   A server encodes one of its assigned server IDs in any CID it
   generates using the relevant configuration.

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4.  Server ID Encoding in Connection IDs

4.1.  CID format

   All connection IDs use the following format:

   QUIC-LB Connection ID {
       First Octet (8),
       Server ID (8..152-len(Nonce)),
       Nonce (32..152-len(Server ID)),
   }

                            Figure 3: CID Format

4.2.  Configuration Agent Actions

   The configuration agent assigns a server ID to every server in its
   pool in accordance with Section 3.3, and determines a server ID
   length (in octets) sufficiently large to encode all server IDs,
   including potential future servers.

   Each configuration specifies the length of the Server ID and Nonce
   fields, with limits defined for each algorithm.

   Optionally, it also defines a 16-octet key.  Note that failure to
   define a key means that observers can determine the assigned server
   of any connection, significantly increasing the linkability of QUIC
   address migration.

   The nonce length MUST be at least 4 octets.  The server ID length
   MUST be at least 1 octet.

   As QUIC version 1 limits connection IDs to 20 octets, the server ID
   and nonce lengths MUST sum to 19 octets or less.

4.3.  Server Actions

   The server writes the first octet and its server ID into their
   respective fields.

   If there is no key in the configuration, the server MUST fill the
   Nonce field with bytes that appear to be random.  If there is a key,
   the server fills the nonce field with a nonce of its choosing.  See
   Section 8.6 for details.

   The server MAY append additional bytes to the connection ID, up to
   the limit specified in that version of QUIC, for its own use.  These
   bytes MUST NOT provide observers with any information that could link

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   two connection IDs to the same connection, client, or server.  In
   particular, all servers using a configuration MUST consistently add
   the same length to each connection ID, to preserve the linkability
   objectives of QUIC-LB.  Any additional bytes SHOULD appear random
   unless individual servers are not distinguishable (e.g. any server
   using that configuration appends identical bytes to every connection
   ID).

   If there is no key in the configuration, the Connection ID is
   complete.  Otherwise, there are further steps, as described in the
   two following subsections.

   Encryption below uses the AES-128-ECB cipher.  Future standards could
   add new algorithms that use other ciphers to provide cryptographic
   agility in accordance with [RFC7696].  QUIC-LB implementations SHOULD
   be extensible to support new algorithms.

4.3.1.  Special Case: Single Pass Encryption

   When the nonce length and server ID length sum to exactly 16 octets,
   the server MUST use a single-pass encryption algorithm.  All
   connection ID octets except the first form an AES-ECB block.  This
   block is encrypted once, and the result forms the second through
   seventeenth most significant bytes of the connection ID.

4.3.2.  General Case: Four-Pass Encryption

   Any other field length requires four passes for encryption and at
   least three for decryption.  To understand this algorithm, it is
   useful to define four functions that minimize the amount of bit-
   shifting necessary in the event that there are an odd number of
   octets.

   The expand_left() function outputs 16 octets, with its first argument
   in the most significant bits, its second argument in the least
   significant byte, and zeros in all other positions.  Thus,

   expand_left(0xaaba3c, 0x13) = 0xaaba3c00000000000000000000000013

   expand_right() is similar, except that the second argument is in the
   most significant byte, and the first argument is in the least
   significant bits.  Therefore,

   expand_right(0xaaba3c, 0x13) = 0x13000000000000000000000000aaba3c

   Similarly, truncate_left() and truncate_right() take the most
   significant and least significant bits, respectively, from a
   ciphertext.  For example, to take 28 bits of a ciphertext:

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   truncate_left(0x2094842ca49256198c2deaa0ba53caa0, 28) = 0x2094842
   truncate_right(0x2094842ca49256198c2deaa0ba53caa0, 28) = 0xa53caa0

   The example at the end of this section helps to clarify the steps
   described below.

   1.  The server concatenates the server ID and nonce to create
       plaintext_CID.

   2.  The server splits plaintext_CID into components left_0 and
       right_0 of equal length, splitting an odd octet in half if
       necessary.  For example, 0x7040b81b55ccf3 would split into a
       left_0 of 0x7040b81 and right_0 of 0xb55ccf3.

   3.  Encrypt the result of expand_left(left_0) to obtain a ciphertext.

   4.  XOR the least significant bits of the ciphertext with right_0 to
       form right_1.

       Thus steps 3 and 4 can be expressed as right_1 = right_0 ^
       truncate_right( AES_ECB(key, expand_left(left_0, 0x01)),
       len(right_0))

   5.  Repeat steps 3 and 4, but use them to compute left_1 by expanding
       and encrypting right_1 with the most significant octet as 0x02
       and XOR the results with left_0.

       left_1 = left_0 ^ truncate_left( AES_ECB(key,
       expand_right(right_1, 0x02)), len(left_0))

   6.  Repeat steps 3 and 4, but use them to compute right_2 by
       expanding and encrypting left_1 with the least significant octet
       as 0x03 and XOR the results with right_1.

       right_2 = right_1 ^ truncate_right( AES_ECB(key,
       expand_left(left_1, 0x03)), len(right_1))

   7.  Repeat steps 3 and 4, but use them to compute left_2 by expanding
       and encrypting right_2 with the most significant octet as 0x04
       and XOR the results with left_1.

       left_2 = left_1 ^ truncate_left( AES_ECB(key,
       expand_right(right_2, 0x04)), len(left_1))

   8.  The server concatenates left_2 with right_2 to form the
       ciphertext CID, which it appends to the first octet.

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   The following example executes the steps for the provided inputs.
   Note that the plaintext is of odd octet length, so the middle octet
   will be split evenly left_0 and right_0.

   server_id = 0x31441a
   nonce = 0x9c69c275
   key = 0xfdf726a9893ec05c0632d3956680baf0

   // step 1
   plaintext_CID = 0x31441a9c69c275

   // step 2
   left_0 = 0x31441a9
   right_0 = 0xc69c275

   // step 3
   aes_input = 0x31441a90000000000000000000000001
   ciphertext = 0x4d140de42d0b85bdf554ba35c1d5c653

   // step 4
   right_1 = 0xc69c275 ^ 0x1d5c653 = 0xdbc0426

   // step 5
   aes_input = 0x0200000000000000000000000dbc0426
   aes_output = 0x7e99160f3cf5b89c70584ccd2c2cd24b
   left_1 = 0x31441a9 ^ 0x7e99160 = 0x4fdd0c9

   // step 6
   AES input = 0x4fdd0c90000000000000000000000003
   AES output = 0x26c1d5a3a5e31ff8e3ca505da6061ac6
   right_2 = 0xdbc0426 ^ 0x6061ac6 = 0xbba1ee0

   // step 7
   AES input = 0x0400000000000000000000000bba1ee0
   AES output = 0xade1b8b25b436a94007d80cf3704377b
   left_2 = 0x4fdd0c9 ^ 0xade1b8b = 0xe23cb42

   // step 8
   cid = first_octet || left_2 || right_2 = 0x07e23cb42bba1ee0

4.4.  Load Balancer Actions

   On each incoming packet, the load balancer extracts consecutive
   octets, beginning with the second octet.  If there is no key, the
   first octets correspond to the server ID.

   If there is a key, the load balancer takes one of two actions:

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4.4.1.  Special Case: Single Pass Encryption

   If server ID length and nonce length sum to exactly 16 octets, they
   form a ciphertext block.  The load balancer decrypts the block using
   the AES-ECB key and extracts the server ID from the most significant
   bytes of the resulting plaintext.

4.4.2.  General Case: Four-Pass Encryption

   First, split the ciphertext CID (excluding the first octet) into its
   equal- length components left_2 and right_2.  Then follow the process
   below:

   left_1 = left_2 ^ truncate_left(AES_ECB(key, expand_right(right_2), 0x04))
   right_1 = right_2 ^ truncate_right(AES_ECB(key, expand_left(left_1, 0x03))
   left_0 = left_1 ^ truncate_left(AES_ECB(key, expand_right(right_1), 0x02))

   As the load balancer has no need for the nonce, it can conclude after
   3 passes as long as the server ID is entirely contained in left_0
   (i.e., the nonce is at least as large as the server ID).  If the
   server ID is longer, a fourth pass is necessary:

   right_0 = right_1 ^ truncate_right(AES_ECB(key, expand_left(left_0,
   0x01)))

   and the load balancer has to concatenate left_0 and right_0 to obtain
   the complete server ID.

5.  Per-connection state

   QUIC-LB requires no per-connection state at the load balancer.  The
   load balancer can extract the server ID from the connection ID of
   each incoming packet and route that packet accordingly.

   However, once the routing decision has been made, the load balancer
   MAY associate the 4-tuple with the decision.  This has two
   advantages:

   *  The load balancer only extracts the server ID once per incoming
      4-tuple.  When the CID is encrypted, this substantially reduces
      computational load.

   *  Incoming Stateless Reset packets and ICMP messages are easily
      routed to the correct origin server.

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   In addition to the increased state requirements, however, load
   balancers cannot detect the CONNECTION_CLOSE frame to indicate the
   end of the connection, so they rely on a timeout to delete connection
   state.  There are numerous considerations around setting such a
   timeout.

   In the event a connection ends, freeing an IP and port, and a
   different connection migrates to that IP and port before the timeout,
   the load balancer will misroute the different connection's packets to
   the original server.  A short timeout limits the likelihood of such a
   misrouting.

   Furthermore, if a short timeout causes premature deletion of state,
   the routing is easily recoverable by decoding an incoming Connection
   ID.  However, a short timeout also reduces the chance that an
   incoming Stateless Reset is correctly routed.

   Servers MAY implement the technique described in Section 14.4.1 of
   [RFC9000] in case the load balancer is stateless, to increase the
   likelihood a Source Connection ID is included in ICMP responses to
   Path Maximum Transmission Unit (PMTU) probes.  Load balancers MAY
   parse the echoed packet to extract the Source Connection ID, if it
   contains a QUIC long header, and extract the Server ID as if it were
   in a Destination CID.

6.  Additional Use Cases

   This section discusses considerations for some deployment scenarios
   not implied by the specification above.

6.1.  Load balancer chains

   Some network architectures may have multiple tiers of low-state load
   balancers, where a first tier of devices makes a routing decision to
   the next tier, and so on, until packets reach the server.  Although
   QUIC-LB is not explicitly designed for this use case, it is possible
   to support it.

   If each load balancer is assigned a range of server IDs that is a
   subset of the range of IDs assigned to devices that are closer to the
   client, then the first devices to process an incoming packet can
   extract the server ID and then map it to the correct forwarding
   address.  Note that this solution is extensible to arbitrarily large
   numbers of load-balancing tiers, as the maximum server ID space is
   quite large.

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   If the number of necessary server IDs per next hop is uniform, a
   simple implementation would use successively longer server IDs at
   each tier of load balancing, and the server configuration would match
   the last tier.  The forward load balancers would simply treat the
   least significant bits of the server ID as part of the nonce.

6.2.  Moving connections between servers

   Some deployments may transparently move a connection from one server
   to another.  The means of transferring connection state between
   servers is out of scope of this document.

   To support a handover, a server involved in the transition could
   issue CIDs that map to the new server via a NEW_CONNECTION_ID frame,
   and retire CIDs associated with the new server using the "Retire
   Prior To" field in that frame.

   Alternately, if the old server is going offline, the load balancer
   could simply map its server ID to the new server's address.

7.  Version Invariance of QUIC-LB

   The server ID encodings, and requirements for their handling, are
   designed to be QUIC version independent (see [RFC8999]).  A QUIC-LB
   load balancer will generally not require changes as servers deploy
   new versions of QUIC.  However, there are several unlikely future
   design decisions that could impact the operation of QUIC-LB.

   The maximum Connection ID length could be below the minimum necessary
   for one or more encoding algorithms.

   Section 3.1 provides guidance about how load balancers should handle
   unroutable DCIDs.  This guidance, and the implementation of an
   algorithm to handle these DCIDs, rests on some assumptions:

   *  Incoming short headers do not contain DCIDs that are client-
      generated.

   *  The use of client-generated incoming DCIDs does not persist beyond
      a few round trips in the connection.

   *  While the client is using DCIDs it generated, some exposed fields
      (IP address, UDP port, client-generated destination Connection ID)
      remain constant for all packets sent on the same connection.

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   While this document does not update the commitments in [RFC8999], the
   additional assumptions are minimal and narrowly scoped, and provide a
   likely set of constants that load balancers can use with minimal risk
   of version- dependence.

   If these assumptions are invalid, this specification is likely to
   lead to loss of packets that contain unroutable DCIDs, and in extreme
   cases connection failure.

   Some load balancers might inspect elements of the Server Name
   Indication (SNI) extension in the TLS Client Hello to make a routing
   decision.  Note that the format and cryptographic protection of this
   information may change in future versions or extensions of TLS or
   QUIC, and therefore this functionality is inherently not version-
   invariant.  See also Section 3.1 for other considerations about this
   case.  Note that an SNI-aware load balancer, faced with an unknown
   QUIC version, might misdirect initial packets to the wrong tenant.
   While inefficient, this preserves the ability for tenants to deploy
   new versions provided they have an out-of-band means of providing a
   connection ID for the client to use.

8.  Security Considerations

   QUIC-LB is intended to prevent linkability.  Attacks would therefore
   attempt to subvert this purpose.

   Note that without a key for the encoding, QUIC-LB makes no attempt to
   obscure the server mapping, and therefore does not address these
   concerns.  Without a key, QUIC-LB merely allows consistent CID
   encoding for compatibility across a network infrastructure, which
   makes QUIC robust to NAT rebinding.  Servers that are encoding their
   server ID without a key algorithm SHOULD only use it to generate new
   CIDs for the Server Initial Packet and SHOULD NOT send CIDs in QUIC
   NEW_CONNECTION_ID frames, except that it sends one new Connection ID
   in the event of config rotation Section 2.1.  Doing so might falsely
   suggest to the client that said CIDs were generated in a secure
   fashion.

   A linkability attack would find some means of determining that two
   connection IDs route to the same server.  As described above, there
   is no scheme that strictly prevents linkability for all traffic
   patterns, and therefore efforts to frustrate any analysis of server
   ID encoding have diminishing returns.

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8.1.  Attackers not between the load balancer and server

   Any attacker might open a connection to the server infrastructure and
   aggressively simulate migration to obtain a large sample of IDs that
   map to the same server.  It could then apply analytical techniques to
   try to obtain the server encoding.

   An encrypted encoding provides robust protection against this.  An
   unencrypted one provides none.

   Were this analysis to obtain the server encoding, then on-path
   observers might apply this analysis to correlating different client
   IP addresses.

8.2.  Attackers between the load balancer and server

   Attackers in this privileged position are intrinsically able to map
   two connection IDs to the same server.  The QUIC-LB algorithms do
   prevent the linkage of two connection IDs to the same individual
   connection if servers make reasonable selections when generating new
   IDs for that connection.

8.3.  Multiple Configuration IDs

   During the period in which there are multiple deployed configuration
   IDs (see Section 2.1), there is a slight increase in linkability.
   The server space is effectively divided into segments with CIDs that
   have different config rotation bits.  Entities that manage servers
   SHOULD strive to minimize these periods by quickly deploying new
   configurations across the server pool.

8.4.  Limited configuration scope

   A simple deployment of QUIC-LB in a cloud provider might use the same
   global QUIC-LB configuration across all its load balancers that route
   to customer servers.  An attacker could then simply become a
   customer, obtain the configuration, and then extract server IDs of
   other customers' connections at will.

   To avoid this, the configuration agent SHOULD issue QUIC-LB
   configurations to mutually distrustful servers that have different
   keys for encryption algorithms.  In many cases, the load balancers
   can distinguish these configurations by external IP address.

   However, assigning multiple entities to an IP address is
   complimentary with concealing DNS requests (e.g., DoH [RFC8484]) and
   the TLS Server Name Indicator (SNI) ([I-D.ietf-tls-esni]) to obscure
   the ultimate destination of traffic.  While the load balancer's

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   fallback algorithm (Section 3.2) can use the SNI to make a routing
   decision on the first packet, there are three ways to route
   subsequent packets:

   *  all co-tenants can use the same QUIC-LB configuration, leaking the
      server mapping to each other as described above;

   *  co-tenants can be issued one of up to three configurations
      distinguished by the config rotation bits (Section 2.1), exposing
      information about the target domain to the entire network; or

   *  tenants can use 4-tuple routing in their CIDs (in which case they
      SHOULD disable migration in their connections), which neutralizes
      the value of QUIC-LB but preserves privacy.

   When configuring QUIC-LB, administrators must evaluate the privacy
   tradeoff considering the relative value of each of these properties,
   given the trust model between tenants, the presence of methods to
   obscure the domain name, and value of address migration in the tenant
   use cases.

   As the plaintext algorithm makes no attempt to conceal the server
   mapping, these deployments SHOULD simply use a common configuration.

8.5.  Stateless Reset Oracle

   Section 21.9 of [RFC9000] discusses the Stateless Reset Oracle
   attack.  For a server deployment to be vulnerable, an attacking
   client must be able to cause two packets with the same Destination
   CID to arrive at two different servers that share the same
   cryptographic context for Stateless Reset tokens.  As QUIC-LB
   requires deterministic routing of DCIDs over the life of a
   connection, it is a sufficient means of avoiding an Oracle without
   additional measures.

   Note also that when a server starts using a new QUIC-LB config
   rotation codepoint, new CIDs might not be unique with respect to
   previous configurations that occupied that codepoint, and therefore
   different clients may have observed the same CID and stateless reset
   token.  A straightforward method of managing stateless reset keys is
   to maintain a separate key for each config rotation codepoint, and
   replace each key when the configuration for that codepoint changes.
   Thus, a server transitions from one config to another, it will be
   able to generate correct tokens for connections using either type of
   CID.

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8.6.  Connection ID Entropy

   If a server ever reuses a nonce in generating a CID for a given
   configuration, it risks exposing sensitive information.  Given the
   same server ID, the CID will be identical (aside from a possible
   difference in the first octet).  This can risk exposure of the QUIC-
   LB key.  If two clients receive the same connection ID, they also
   have each other's stateless reset token unless that key has changed
   in the interim.

   The encrypte mode needs to generate different cipher text for each
   generated Connection ID instance to protect the Server ID.  To do so,
   at least four octets of the CID are reserved for a nonce that, if
   used only once, will result in unique cipher text for each Connection
   ID.

   If servers simply increment the nonce by one with each generated
   connection ID, then it is safe to use the existing keys until any
   server's nonce counter exhausts the allocated space and rolls over.
   To maximize entropy, servers SHOULD start with a random nonce value,
   in which case the configuration is usable until the nonce value wraps
   around to zero and then reaches the initial value again.

   Whether or not it implements the counter method, the server MUST NOT
   reuse a nonce until it switches to a configuration with new keys.

   If the nonce is sent in plaintext, servers MUST generate nonces so
   that they appear to be random.  Observable correlations between
   plaintext nonces would provide trivial linkability between individual
   connections, rather than just to a common server.

   For any algorithm, configuration agents SHOULD implement an out-of-
   band method to discover when servers are in danger of exhausting
   their nonce space, and SHOULD respond by issuing a new configuration.
   A server that has exhausted its nonces MUST either switch to a
   different configuration, or if none exists, use the 4-tuple routing
   config rotation codepoint.

   When sizing a nonce that is to be randomly generated, the
   configuration agent SHOULD consider that a server generating a N-bit
   nonce will create a duplicate about every 2^(N/2) attempts, and
   therefore compare the expected rate at which servers will generate
   CIDs with the lifetime of a configuration.

9.  IANA Considerations

   There are no IANA requirements.

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10.  References

10.1.  Normative References

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

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

10.2.  Informative References

   [I-D.draft-ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://www.ietf.org/archive/id/draft-ietf-tls-
              dtls13-43.txt>.

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., Fossati, T., and A. Kraus,
              "Connection Identifier for DTLS 1.2", Work in Progress,
              Internet-Draft, draft-ietf-tls-dtls-connection-id-13, 22
              June 2021, <https://www.ietf.org/archive/id/draft-ietf-
              tls-dtls-connection-id-13.txt>.

   [I-D.ietf-tls-esni]
              Rescorla, E., Oku, K., Sullivan, N., and C. A. Wood, "TLS
              Encrypted Client Hello", Work in Progress, Internet-Draft,
              draft-ietf-tls-esni-14, 13 February 2022,
              <https://www.ietf.org/archive/id/draft-ietf-tls-esni-
              14.txt>.

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

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
              <https://www.rfc-editor.org/info/rfc4347>.

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   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

   [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/info/rfc7983>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

Appendix A.  QUIC-LB YANG Model

   These YANG models conform to [RFC6020] and express a complete QUIC-LB
   configuration.  There is one model for the server and one for the
   middlebox (i.e the load balancer and/or Retry Service).

  module ietf-quic-lb-server {
    yang-version "1.1";
    namespace "urn:ietf:params:xml:ns:yang:ietf-quic-lb";
    prefix "quic-lb";

    import ietf-yang-types {
      prefix yang;
      reference
        "RFC 6991: Common YANG Data Types.";
    }

    import ietf-inet-types {
      prefix inet;
      reference

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        "RFC 6991: Common YANG Data Types.";
    }

    organization
      "IETF QUIC Working Group";

    contact
      "WG Web:   <http://datatracker.ietf.org/wg/quic>
       WG List:  <quic@ietf.org>

       Authors: Martin Duke (martin.h.duke at gmail dot com)
                Nick Banks (nibanks at microsoft dot com)
                Christian Huitema (huitema at huitema.net)";

    description
      "This module enables the explicit cooperation of QUIC servers with
       trusted intermediaries without breaking important protocol
       features.

       Copyright (c) 2022 IETF Trust and the persons identified as
       authors of the code.  All rights reserved.

       Redistribution and use in source and binary forms, with or
       without modification, is permitted pursuant to, and subject to
       the license terms contained in, the Simplified BSD License set
       forth in Section 4.c of the IETF Trust's Legal Provisions
       Relating to IETF Documents
       (https://trustee.ietf.org/license-info).

       This version of this YANG module is part of RFC XXXX
       (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
       for full legal notices.

       The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
       NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
       'MAY', and 'OPTIONAL' in this document are to be interpreted as
       described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
       they appear in all capitals, as shown here.";

    revision "2022-02-11" {
      description
        "Updated to design in version 13 of the draft";
      reference
        "RFC XXXX, QUIC-LB: Generating Routable QUIC Connection IDs";
    }

    container quic-lb {
      presence "The container for QUIC-LB configuration.";

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      description
        "QUIC-LB container.";

      typedef quic-lb-key {
        type yang:hex-string {
          length 47;
        }
        description
          "This is a 16-byte key, represented with 47 bytes";
      }

      leaf config-id {
        type uint8 {
          range "0..2";
        }
        mandatory true;
        description
          "Identifier for this CID configuration.";
      }

      leaf first-octet-encodes-cid-length {
        type boolean;
        default false;
        description
          "If true, the six least significant bits of the first CID
           octet encode the CID length minus one.";
      }

      leaf server-id-length {
        type uint8 {
          range "1..15";
        }
        must '. <= (19 - ../nonce-length)' {
          error-message
            "Server ID and nonce lengths must sum to no more than 19.";
        }
        mandatory true;
        description
          "Length (in octets) of a server ID. Further range-limited
           by nonce-length.";
      }

      leaf nonce-length {
        type uint8 {
          range "4..18";
        }
        mandatory true;
        description

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          "Length, in octets, of the nonce. Short nonces mean there will
           be frequent configuration updates.";
      }

      leaf cid-key {
        type quic-lb-key;
        description
          "Key for encrypting the connection ID.";
      }

      leaf server-id {
        type yang:hex-string;
        must "string-length(.) = 3 * ../../server-id-length - 1";
        mandatory true;
        description
          "An allocated server ID";
      }
    }
  }

 module ietf-quic-lb-middlebox {
   yang-version "1.1";
   namespace "urn:ietf:params:xml:ns:yang:ietf-quic-lb";
   prefix "quic-lb";

   import ietf-yang-types {
     prefix yang;
     reference
       "RFC 6991: Common YANG Data Types.";
   }

   import ietf-inet-types {
     prefix inet;
     reference
       "RFC 6991: Common YANG Data Types.";
   }

   organization
     "IETF QUIC Working Group";

   contact
     "WG Web:   <http://datatracker.ietf.org/wg/quic>
      WG List:  <quic@ietf.org>

      Authors: Martin Duke (martin.h.duke at gmail dot com)
               Nick Banks (nibanks at microsoft dot com)
               Christian Huitema (huitema at huitema.net)";

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   description
     "This module enables the explicit cooperation of QUIC servers with
      trusted intermediaries without breaking important protocol
      features.

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

      Redistribution and use in source and binary forms, with or
      without modification, is permitted pursuant to, and subject to
      the license terms contained in, the Simplified BSD License set
      forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents
      (https://trustee.ietf.org/license-info).

      This version of this YANG module is part of RFC XXXX
      (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself
      for full legal notices.

      The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
      NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
      'MAY', and 'OPTIONAL' in this document are to be interpreted as
      described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
      they appear in all capitals, as shown here.";

   revision "2021-02-11" {
     description
       "Updated to design in version 13 of the draft";
     reference
       "RFC XXXX, QUIC-LB: Generating Routable QUIC Connection IDs";
   }

   container quic-lb {
     presence "The container for QUIC-LB configuration.";

     description
       "QUIC-LB container.";

     typedef quic-lb-key {
       type yang:hex-string {
         length 47;
       }
       description
         "This is a 16-byte key, represented with 47 bytes";
     }

     list cid-configs {
       key "config-rotation-bits";

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       description
         "List up to three load balancer configurations";

       leaf config-rotation-bits {
         type uint8 {
           range "0..2";
         }
         mandatory true;
         description
           "Identifier for this CID configuration.";
       }

       leaf server-id-length {
         type uint8 {
           range "1..15";
         }
         must '. <= (19 - ../nonce-length)' {
           error-message
             "Server ID and nonce lengths must sum to no more than 19.";
         }
         mandatory true;
         description
           "Length (in octets) of a server ID. Further range-limited
            by nonce-length.";
       }

       leaf cid-key {
         type quic-lb-key;
         description
           "Key for encrypting the connection ID.";
       }

       leaf nonce-length {
         type uint8 {
           range "4..18";
         }
         mandatory true;
         description
           "Length, in octets, of the nonce. Short nonces mean there
            will be frequent configuration updates.";
       }

       list server-id-mappings {
         key "server-id";
         description "Statically allocated Server IDs";

         leaf server-id {
           type yang:hex-string;

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           must "string-length(.) = 3 * ../../server-id-length - 1";
           mandatory true;
           description
             "An allocated server ID";

         }

         leaf server-address {
           type inet:ip-address;
           mandatory true;
           description
             "Destination address corresponding to the server ID";
         }
       }
     }
   }
 }

A.1.  Tree Diagram

   This summary of the YANG models uses the notation in [RFC8340].

   module: ietf-quic-lb-server
     +--rw quic-lb!
        +--rw config-id                         uint8
        +--rw first-octet-encodes-cid-length?   boolean
        +--rw server-id-length                  uint8
        +--rw nonce-length                      uint8
        +--rw cid-key?                          quic-lb-key
        +--rw server-id                         yang:hex-string

   module: ietf-quic-lb-middlebox
     +--rw quic-lb!
        +--rw cid-configs* [config-rotation-bits]
        |  +--rw config-rotation-bits    uint8
        |  +--rw server-id-length        uint8
        |  +--rw cid-key?                quic-lb-key
        |  +--rw nonce-length            uint8
        |  +--rw server-id-mappings* [server-id]
        |     +--rw server-id         yang:hex-string
        |     +--rw server-address    inet:ip-address

Appendix B.  Load Balancer Test Vectors

   This section uses the following abbreviations:

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   cid      Connection ID
   cr_bits  Config Rotation Bits
   LB       Load Balancer
   sid      Server ID

   In all cases, the server is configured to encode the CID length.

B.1.  Unencrypted CIDs

   cr_bits sid nonce cid
   0 c4605e 4504cc4f 07c4605e4504cc4f
   1 350d28b420 3487d970b 40a350d28b4203487d970b

B.2.  Encrypted CIDs

   The key for all of these examples is
   8f95f09245765f80256934e50c66207f.  The test vectors include an
   example that uses the 16-octet single-pass special case, as well as
   an instance where the server ID length exceeds the nonce length,
   requiring a fourth decryption pass.

   cr_bits sid nonce cid
   0 ed793a ee080dbf 07fbfe05f731b425
   1 ed793a51d49b8f5fab65 ee080dbf48 4f010956fb5c1d4d86e010183e0b7d1e
   2 ed793a51d49b8f5f ee080dbf48c0d1e5 904dd2d05a7b0de9b2b9907afb5ecf8cc3
   0 ed793a51d49b8f5fab ee080dbf48c0d1e55d 127a285a09f85280f4fd6abb434a7159e4d3eb

Appendix C.  Interoperability with DTLS over UDP

   Some environments may contain DTLS traffic as well as QUIC operating
   over UDP, which may be hard to distinguish.

   In most cases, the packet parsing rules above will cause a QUIC-LB
   load balancer to route DTLS traffic in an appropriate way.  DTLS 1.3
   implementations that use the connection_id extension
   [I-D.ietf-tls-dtls-connection-id] might use the techniques in this
   document to generate connection IDs and achieve robust routability
   for DTLS associations if they meet a few additional requirements.
   This non-normative appendix describes this interaction.

C.1.  DTLS 1.0 and 1.2

   DTLS 1.0 [RFC4347] and 1.2 [RFC6347] use packet formats that a QUIC-
   LB router will interpret as short header packets with CIDs that
   request 4-tuple routing.  As such, they will route such packets
   consistently as long as the 4-tuple does not change.  Note that DTLS
   1.0 has been deprecated by the IETF.

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   The first octet of every DTLS 1.0 or 1.2 datagram contains the
   content type.  A QUIC-LB load balancer will interpret any content
   type less than 128 as a short header packet, meaning that the
   subsequent octets should contain a connection ID.

   Existing TLS content types comfortably fit in the range below 128.
   Assignment of codepoints greater than 64 would require coordination
   in accordance with [RFC7983], and anyway would likely create problems
   demultiplexing DTLS and version 1 of QUIC.  Therefore, this document
   believes it is extremely unlikely that TLS content types of 128 or
   greater will be assigned.  Nevertheless, such an assignment would
   cause a QUIC-LB load balancer to interpret the packet as a QUIC long
   header with an essentially random connection ID, which is likely to
   be routed irregularly.

   The second octet of every DTLS 1.0 or 1.2 datagram is the bitwise
   complement of the DTLS Major version (i.e. version 1.x = 0xfe).  A
   QUIC-LB load balancer will interpret this as a connection ID that
   requires 4-tuple based load balancing, meaning that the routing will
   be consistent as long as the 4-tuple remains the same.

   [I-D.ietf-tls-dtls-connection-id] defines an extension to add
   connection IDs to DTLS 1.2.  Unfortunately, a QUIC-LB load balancer
   will not correctly parse the connection ID and will continue 4-tuple
   routing.  An modified QUIC-LB load balancer that correctly identifies
   DTLS and parses a DTLS 1.2 datagram for the connection ID is outside
   the scope of this document.

C.2.  DTLS 1.3

   DTLS 1.3 [I-D.draft-ietf-tls-dtls13] changes the structure of
   datagram headers in relevant ways.

   Handshake packets continue to have a TLS content type in the first
   octet and 0xfe in the second octet, so they will be 4-tuple routed,
   which should not present problems for likely NAT rebinding or address
   change events.

   Non-handshake packets always have zero in their most significant bit
   and will therefore always be treated as QUIC short headers.  If the
   connection ID is present, it follows in the succeeding octets.
   Therefore, a DTLS 1.3 association where the server utilizes
   Connection IDs and the encodings in this document will be routed
   correctly in the presence of client address and port changes.

   However, if the client does not include the connection_id extension
   in its ClientHello, the server is unable to use connection IDs.  In
   this case, non- handshake packets will appear to contain random

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   connection IDs and be routed randomly.  Thus, unmodified QUIC-LB load
   balancers will not work with DTLS 1.3 if the client does not
   advertise support for connection IDs, or the server does not request
   the use of a compliant connection ID.

   A QUIC-LB load balancer might be modified to identify DTLS 1.3
   packets and correctly parse the fields to identify when there is no
   connection ID and revert to 4-tuple routing, removing the server
   requirement above.  However, such a modification is outside the scope
   of this document, and classifying some packets as DTLS might be
   incompatible with future versions of QUIC.

C.3.  Future Versions of DTLS

   As DTLS does not have an IETF consensus document that defines what
   parts of DTLS will be invariant in future versions, it is difficult
   to speculate about the applicability of this section to future
   versions of DTLS.

Appendix D.  Acknowledgments

   Manasi Deval, Erik Fuller, Toma Gavrichenkov, Jana Iyengar, Subodh
   Iyengar, Ladislav Lhotka, Jan Lindblad, Ling Tao Nju, Ilari
   Liusvaara, Kazuho Oku, Udip Pant, Ian Swett, Martin Thomson, Dmitri
   Tikhonov, Victor Vasiliev, and William Zeng Ke all provided useful
   input to this document.

Appendix E.  Change Log

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

E.1.  since draft-ietf-quic-load-balancers-12

   *  Separated Retry Service design into a separate draft.

E.2.  since draft-ietf-quic-load-balancers-11

   *  Fixed mistakes in test vectors

E.3.  since draft-ietf-quic-load-balancers-10

   *  Refactored algorithm descriptions; made the 4-pass algorithm
      easier to implement

   *  Revised test vectors

   *  Split YANG model into a server and middlebox version

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E.4.  since draft-ietf-quic-load-balancers-09

   *  Renamed "Stream Cipher" and "Block Cipher" to "Encrypted Short"
      and "Encrypted Long"

   *  Added section on per-connection state

   *  Changed "Encrypted Short" to a 4-pass algorithm.

   *  Recommended a random initial nonce when incrementing.

   *  Clarified what SNI LBs should do with unknown QUIC versions.

E.5.  since draft-ietf-quic-load-balancers-08

   *  Eliminate Dynamic SID allocation

   *  Eliminated server use bytes

E.6.  since draft-ietf-quic-load-balancers-07

   *  Shortened SSCID nonce minimum length to 4 bytes

   *  Removed RSCID from Retry token body

   *  Simplified CID formats

   *  Shrunk size of SID table

E.7.  since draft-ietf-quic-load-balancers-06

   *  Added interoperability with DTLS

   *  Changed "non-compliant" to "unroutable"

   *  Changed "arbitrary" algorithm to "fallback"

   *  Revised security considerations for mistrustful tenants

   *  Added retry service considerations for non-Initial packets

E.8.  since draft-ietf-quic-load-balancers-05

   *  Added low-config CID for further discussion

   *  Complete revision of shared-state Retry Token

   *  Added YANG model

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   *  Updated configuration limits to ensure CID entropy

   *  Switched to notation from quic-transport

E.9.  since draft-ietf-quic-load-balancers-04

   *  Rearranged the shared-state retry token to simplify token
      processing

   *  More compact timestamp in shared-state retry token

   *  Revised server requirements for shared-state retries

   *  Eliminated zero padding from the test vectors

   *  Added server use bytes to the test vectors

   *  Additional compliant DCID criteria

E.10.  since-draft-ietf-quic-load-balancers-03

   *  Improved Config Rotation text

   *  Added stream cipher test vectors

   *  Deleted the Obfuscated CID algorithm

E.11.  since-draft-ietf-quic-load-balancers-02

   *  Replaced stream cipher algorithm with three-pass version

   *  Updated Retry format to encode info for required TPs

   *  Added discussion of version invariance

   *  Cleaned up text about config rotation

   *  Added Reset Oracle and limited configuration considerations

   *  Allow dropped long-header packets for known QUIC versions

E.12.  since-draft-ietf-quic-load-balancers-01

   *  Test vectors for load balancer decoding

   *  Deleted remnants of in-band protocol

   *  Light edit of Retry Services section

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   *  Discussed load balancer chains

E.13.  since-draft-ietf-quic-load-balancers-00

   *  Removed in-band protocol from the document

E.14.  Since draft-duke-quic-load-balancers-06

   *  Switch to IETF WG draft.

E.15.  Since draft-duke-quic-load-balancers-05

   *  Editorial changes

   *  Made load balancer behavior independent of QUIC version

   *  Got rid of token in stream cipher encoding, because server might
      not have it

   *  Defined "non-compliant DCID" and specified rules for handling
      them.

   *  Added psuedocode for config schema

E.16.  Since draft-duke-quic-load-balancers-04

   *  Added standard for retry services

E.17.  Since draft-duke-quic-load-balancers-03

   *  Renamed Plaintext CID algorithm as Obfuscated CID

   *  Added new Plaintext CID algorithm

   *  Updated to allow 20B CIDs

   *  Added self-encoding of CID length

E.18.  Since draft-duke-quic-load-balancers-02

   *  Added Config Rotation

   *  Added failover mode

   *  Tweaks to existing CID algorithms

   *  Added Block Cipher CID algorithm

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   *  Reformatted QUIC-LB packets

E.19.  Since draft-duke-quic-load-balancers-01

   *  Complete rewrite

   *  Supports multiple security levels

   *  Lightweight messages

E.20.  Since draft-duke-quic-load-balancers-00

   *  Converted to markdown

   *  Added variable length connection IDs

Authors' Addresses

   Martin Duke
   Google
   Email: martin.h.duke@gmail.com

   Nick Banks
   Microsoft
   Email: nibanks@microsoft.com

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

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