QUIC                                                             M. Duke
Internet-Draft                                         F5 Networks, Inc.
Intended status: Experimental                            30 October 2020
Expires: 3 May 2021

                         QUIC Version Aliasing


   The QUIC transport protocol [QUIC-TRANSPORT] preserves its future
   extensibility partly by specifying its version number.  There will be
   a relatively small number of published version numbers for the
   foreseeable future.  This document provides a method for clients and
   servers to negotiate the use of other version numbers in subsequent
   connections and encrypts Initial Packets using secret keys instead of
   standard ones.  If a sizeable subset of QUIC connections use this
   mechanism, this should prevent middlebox ossification around the
   current set of published version numbers and the contents of QUIC
   Initial packets, as well as improving the protocol's privacy

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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   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 3 May 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.

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   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 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
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
   3.  The Version Alias Transport Parameter . . . . . . . . . . . .   4
     3.1.  Version Number Generation . . . . . . . . . . . . . . . .   4
     3.2.  Initial Token Extension (ITE) Generation  . . . . . . . .   5
     3.3.  Salt and Packet Length Offset Generation  . . . . . . . .   5
     3.4.  Expiration Time . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.6.  Multiple Servers for One Domain . . . . . . . . . . . . .   7
   4.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Server Actions on Aliased Version Numbers . . . . . . . . . .   9
   6.  Considerations for Retry Packets  . . . . . . . . . . . . . .  10
   7.  Security and Privacy Considerations . . . . . . . . . . . . .  10
     7.1.  Version Downgrade . . . . . . . . . . . . . . . . . . . .  10
     7.2.  Retry Injection . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  Increased Linkability . . . . . . . . . . . . . . . . . .  12
     7.4.  Salt Polling Attack . . . . . . . . . . . . . . . . . . .  12
     7.5.  Increased Processing of Garbage UDP Packets . . . . . . .  12
     7.6.  Increased Retry Overhead  . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  14
     B.1.  since draft-duke-quic-version-aliasing-01 . . . . . . . .  14
     B.2.  since draft-duke-quic-version-aliasing-00 . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The QUIC version number is critical to future extensibility of the
   protocol.  Past experience with other protocols, such as TLS1.3
   [RFC8446], shows that middleboxes might attempt to enforce that QUIC
   packets use versions known at the time the middlebox was implemented.
   This has a chilling effect on deploying experimental and standard
   versions on the internet.

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   Each version of QUIC has a "salt" [QUIC-TLS] that is used to derive
   the keys used to encrypt Initial packets.  As each salt is published
   in a standards document, any observer can decrypt these packets and
   inspect the contents, including a TLS Client Hello.  A subsidiary
   mechanism like Encrypted SNI [ENCRYPTED-SNI] might protect some of
   the TLS fields inside a TLS Client Hello.

   This document proposes "QUIC Version Aliasing," a standard way for
   servers to advertise the availability of other versions inside the
   cryptographic protection of a QUIC handshake.  These versions are
   syntactically identical to the QUIC version in which the
   communication takes place, but use a different salt.  In subsequent
   communications, the client uses the new version number and encrypts
   its Initial packets with a key derived from the provided salt.  These
   version numbers and salts are unique to the client.

   If a large subset of QUIC traffic adopts his technique, middleboxes
   will be unable to enforce particular version numbers or policy based
   on Client Hello contents without incurring unacceptable penalties on
   users.  This would simultaneously protect the protocol against
   ossification and improve its privacy properties.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.

   A "standard version" is a QUIC version that would be advertised in a
   QUIC version negotiation and conforms to a specification.  Any
   aliased version corresponds to a standard version in all its formats
   and behaviors, except for the version number field in long headers.

   An "aliased version" is a version with a number generated in
   accordance with this document.  Except for the version field in long
   headers, it conforms entirely to the specification of the standard

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2.  Protocol Overview

   When they instantiate a connection, servers select an alternate
   32-bit version number, and optionally an initial token extension, for
   the next connection at random and securely derive a salt and Packet
   Length Offset from those values using a repeatable process.  They
   communicate this using a transport parameter extension including the
   version, initial token extension, salt, Packet Length Offset, and an
   expiration time for that value.

   If a client next connects to that server within the indicated
   expiration time, it MAY use the provided version number and encrypt
   its Initial Packets using a key derived from the provided salt.  It
   adds the Packet Length Offset to the true packet length when encoding
   it in the long header.  If the server provided an Initial Token
   Extension, the client puts it in the Initial Packet token field.  If
   there is another token the client wishes to include, it appends the
   Initial Token Extension to that token.  The server can reconstruct
   the salt and Packet Length Offset from the requested version and
   token, and proceed with the connection normally.

   The Packet Length Offset is provides a low-cost way for the server to
   verify it can derive a valid salt from the inputs without trial
   decryption.  This has important security implications, as described
   in Section 7.2.

   When generating a salt and Packet Length Offset, servers can choose
   between doing so randomly and storing the mapping, or using a
   cryptographic process to transform the aliased version number and
   token extension into the salt.  The two options provide a simple
   tradeoff between computational complexity and storage requirements.

   Note that clients and servers MUST implement
   [QUIC-VERSION-NEGOTIATION] to use this specification.  Therefore,
   servers list supported versions in Version Negotiation Packets.  Both
   clients and servers list supported versions in Version Negotiation
   Transport Parameters.

3.  The Version Alias Transport Parameter

3.1.  Version Number Generation

   Servers MUST use a random process to generate version numbers.  This
   version number MUST NOT correspond to a QUIC version the server
   advertises in QUIC Version Negotiation packets or transport
   parameters.  Servers SHOULD also exclude version numbers used in
   known specifications or experiments to avoid confusion at clients,
   whether or not they have plans to support those specifications.

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   Servers MUST NOT use client-controlled information (e.g. the client
   IP address) in the random process, see Section 7.4.

   Servers MUST NOT advertise these versions in QUIC Version Negotiation

3.2.  Initial Token Extension (ITE) Generation

   Servers SHOULD generate an Initial Token Extension (ITE) to provide
   additional entropy in salt generation.  Two clients that receive the
   same version number but different extensions will not be able to
   decode each other's Initial Packets.

   Servers MAY choose any length that will allow client Initial Packets
   to fit within the minimum QUIC packet size of 1200 octets.  A four-
   octet extension is RECOMMENDED.  The ITE MUST appear to be random to

   If a server supports multiple standard versions, it MUST either
   encode the standard version of the current connection in the ITE or
   store it in a lookup table.

   If the server chooses to encode the standard version, it MUST be
   cryptographically protected.

   Encoded standard versions MUST be robust to false positives.  That
   is, if decoded with a new key, the version encoding must return as
   invalid, rather than an incorrect value.

   Alternatively, servers MAY store a mapping of unexpired aliased
   versions and ITEs to standard versions.  This mapping SHOULD NOT
   create observable patterns, e.g. one plaintext bit indicates if the
   standard version is 1 or 2.

   The server MUST be able to distinguish ITEs from Resumption and Retry
   tokens in incoming Initial Packets that contain an aliased version
   number.  As the server controls the lengths and encoding of each,
   there are many ways to guarantee this.

3.3.  Salt and Packet Length Offset Generation

   The salt is an opaque 20-octet field.  It is used to generate Initial
   connection keys using the process described in [QUIC-TLS].

   The Packet Length Offset is a 64-bit unsigned integer with a maximum
   value of 2^62 - 1.  Clients MUST ignore a transport parameter with a
   value that exceeds this limit.

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   To reduce header overhead, servers MAY consistently use a Packet
   Length Offset of zero if and only if it either (1) never sends Retry
   packets, or (2) can guarantee, through the use of persistent storage
   or other means, that it will never lose the cryptographic state
   required to generate the salt before the promised expiration time.
   Section 7.2 describes the implications if it uses zero without
   meeting these conditions.

   Servers MUST either generate a random salt and Packet Length Offset
   and store a mapping of aliased version and ITE to salt and offset, or
   generate the salt and offset using a cryptographic method that uses
   the version number, ITE, and only server state that is persistent
   across connections.

   If the latter, servers MUST implement a method that it can repeat
   deterministically at a later time to derive the salt and offset from
   the incoming version number and ITE.  It MUST NOT use client
   controlled information other than the version number and ITE; for
   example, the client's IP address and port.

3.4.  Expiration Time

   Servers should select an expiration time in seconds, measured from
   the instant the transport parameter is first sent.  This time SHOULD
   be less than the time until the server expects to support new QUIC
   versions, rotate the keys used to encode information in the version
   number, or rotate the keys used in salt generation.

   Furthermore, the expiration time SHOULD be short enough to frustrate
   a salt polling attack ({salt-polling}})

   Conversely, an extremely short expiration time will often force the
   client to use standard QUIC version numbers and salts.

3.5.  Format

   This document defines a new transport parameter extension for QUIC
   with identifier 0x5641.  The contents of the value field are
   indicated below.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                           Version (32)                        |
   |                                                               |
   +                                                               +
   |                                                               |
   +                                                               +
   |                            Salt (160)                         |
   +                                                               +
   |                                                               |
   +                                                               +
   |                                                               |
   |                     Packet Length Offset (i)                  |
   |                         Expiration (i)                        |
   |                Initial Token Extension (variable)             |

             Figure 1: Version Alias Transport Parameter value

   The definition of the fields is described above.  Note that the
   "Expiration" field is in seconds, and its length is encoded using the
   Variable Length Integer encoding from Section 16 of [QUIC-TRANSPORT].

   The Packet Length Offset is also encoded as a Variable Length

   Clients can determine the length of the Initial Token Extension by
   subtracting known and encoded field lengths from the overall
   transport parameter length.

3.6.  Multiple Servers for One Domain

   If multiple servers serve the same entity behind a load balancer, all
   such servers SHOULD either have a common configuration for encoding
   standard versions and computing salts, or share a common database of
   mappings.  They MUST NOT generate version numbers that any of them
   would advertise in a Version Negotiation Packet or Transport

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4.  Client Behavior

   When a client receives the Version Alias Transport Parameter, it MAY
   cache the version number, ITE, salt, Packet Length Offset, and the
   expiration of these values.  It MAY use the version number and ITE in
   a subsequent connection and compute the initial keys using the
   provided salt.

   Clients MUST NOT advertise aliased versions in the Version
   Negotiation Transport Parameter unless they support a standard
   version with the same number.  Including that number signals support
   for the standard version, not the aliased version.

   Clients SHOULD NOT attempt to use the provided version number and
   salt after the provided Expiration time has elapsed.

   Clients MAY decline to use the provided version number or salt in
   more than one connection.  It SHOULD do so if its IP address has
   changed between two connection attempts.  Using a consistent version
   number can link the client across connection attempts.

   Clients MUST use the same standard version to format the Initial
   Packet as the standard version used in the connection that provided
   the aliased version.

   If the server provided an ITE, the client MUST append it to any
   Initial Packet token it is including from a Retry packet or NEW_TOKEN
   frame, if it is using the associated aliased version.  If there is no
   such token, it simply includes the ITE as the entire token.

   The QUIC Token Length field MUST include the length of both any Retry
   or NEW_TOKEN token and the ITE.

   The Length fields of all Initial, Handshake, and 0-RTT packets in the
   connection are set to the value described in [QUIC-TRANSPORT] plus
   the provided Packet Length Offset, modulo 2^62.

   If the response to an Initial packet using the provided version is a
   Version Negotiation Packet, the client SHOULD cease attempting to use
   that version and salt to the server unless it later determines that
   the packet was the result of a version downgrade, see Section 7.1.

   If a client receives an aliased version number that matches a
   standard version that the client supports, it SHOULD assume the
   server does not support the standard version and MUST use aliased
   version behaviors in any connection with the server using that
   version number.

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   If a client receives a Version Negotiation packet or Version
   Negotiation transport parameter advertising a version number the
   server previously sent as an aliased version, and the client verifies
   any Version Negotiation Packet is not a Version Downgrade attack
   (Section 7.1), it MUST discard the aliased version number, ITE,
   packet length offset, and salt and not use it in future connections.

5.  Server Actions on Aliased Version Numbers

   When a server receives an Initial Packet with an unsupported version
   number, it SHOULD send a Version Negotiation Packet if it is
   specifically configured not to generate that version number at

   Otherwise, it extracts the ITE, if any, and either looks up the
   corresponding salt in its database or computes it using the technique
   originally used to derive the salt from the version number and ITE.

   The server similarly obtains the Packet Length Offset and subtracts
   it from the provided Length field, modulo 2^62.  If the resulting
   value is larger than the entire UDP datagram, the server discards the
   packet and SHOULD send a Version Negotiation Packet.

   If the server supports multiple standard versions, it uses the
   standard version extracted by the ITE or stored in the mapping to
   parse the decrypted packet.

   In all packets with long headers, the server uses the aliased version
   number and adds the Packet Length Offset to the length field.

   In the extremely unlikely event that the Packet Length Offset
   resulted in a legal value but the salt is incorrect, the packet may
   fail authentication.  If so, or the encoded standard version is not
   supported at the server, the server SHOULD send a Version Negotiation

   To reduce linkability for the client, servers SHOULD provide a new
   Version Alias transport parameter, with a new version number, ITE,
   salt, and Packet Length Offset, each time a client connects.
   However, issuing version numbers to a client SHOULD be rate-limited
   to mitigate the salt polling attack Section 7.4.

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6.  Considerations for Retry Packets

   QUIC Retry packets reduce the load on servers during periods of
   stress by forcing the client to prove it possesses the IP address
   before the server decrypts any Initial Packets or establishes any
   connection state.  Version aliasing substantially complicates the

   If a server has to send a Retry packet, the required format is
   ambiguous without understanding which standard version to use.  If
   all supported standard versions use the same Retry format, it simply
   uses that format with the client-provided version number.

   If the supported standard versions use different Retry formats, the
   server obtains the standard version via lookup or decoding and
   formats a Retry containing the aliased version number accordingly.

   Servers generate the Retry Integrity Tag of a Retry Packet using the
   procedure in Section 5.8 of [QUIC-TLS].  However, for aliased
   versions, the secret key K uses the first 16 octets of the aliased
   salt instead of the key provided in the specification.

   Clients MUST ignore Retry packets that contain a QUIC version other
   than the version it used in its Initial Packet.

   Servers MUST NOT reply to a packet with an incorrect Length field in
   its long header with a Retry packet; it SHOULD reply with Version
   Negotiation as described above.

7.  Security and Privacy Considerations

   This document intends to improve the existing security and privacy
   properties of QUIC by dramatically improving the secrecy of QUIC
   Initial Packets.  However, there are new attacks against this

7.1.  Version Downgrade

   A countermeasure against version aliasing is the downgrade attack.
   Middleboxes may drop a packet containing a random version and imitate
   the server's failure to correctly process it.  Clients and servers
   are required to implement [QUIC-VERSION-NEGOTIATION] to detect

   Note that downgrade detection only works after receiving a response
   from the server.  If a client immediately responds to a Version
   Negotiation Packet with an Initial Packet with a standard version
   number, it will have exposed its request in a format readable to

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   observers before it discovers if the Version Negotiation Packet is
   authentic.  A client SHOULD wait for an interval to see if a valid
   response comes from the server before assuming the version
   negotiation is valid.  The client MAY also alter its Initial Packet
   (e.g., its ALPN field) to sanitize sensitive information and obtain
   another aliased version before proceeding with its true request.

   Servers that support version aliasing SHOULD be liberal about the
   Initial Packet content they receive, keeping the connection open long
   enough to deliver their transport parameters, to support this

7.2.  Retry Injection

   QUIC Version 1 Retry packets are spoofable, as they follow a fixed
   format, are sent in plaintext, and the integrity protection uses a
   widely known key.  As a result, QUIC Version 1 has verification
   mechanisms in subsequent packets of the connection to validate the
   origin of the Retry.

   Version aliasing largely frustrates this attack.  As the integrity
   check key is derived from the secret salt, packets from attackers
   will fail their integrity check and the client will ignore them.

   The Packet Length Offset is important in this framework.  Without
   this mechanism, servers would have to perform trial decryption to
   verify the client was using the correct salt.  As this does not occur
   before sending Retry Packets, servers would not detect disagreement
   on the salt beforehand and would send a Retry packet signed with a
   different salt than the client expects.  Therefore, a client that
   received a Retry packet with an invalid integrity check would not be
   able to distinguish between the following possibilities:

   *  a Retry packet corrupted in the network, which should be ignored;

   *  a Retry packet generated by an attacker, which should be ignored;

   *  a Retry packet from a server that lost its cryptographic state,
      meaning that further communication with aliased versions is
      impossible and the client should revert to using a standard

   The Packet Length Offset introduces sufficient entropy to make the
   third possibility exceedingly unlikely.

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7.3.  Increased Linkability

   As each version number and ITE is unique to each client, if a client
   uses one twice, those two connections are extremely likely to be from
   the same host.  If the client has changed IP address, this is a
   significant increase in linkability relative to QUIC with a standard
   version numbers.

7.4.  Salt Polling Attack

   Observers that wish to decode Initial Packets might open a large
   number of connections to the server in an effort to obtain part of
   the mapping of version numbers and ITEs to salts for a server.  While
   storage-intensive, this attack could increase the probability that at
   least some version-aliased connections are observable.  There are
   three mitigations servers can execute against this attack:

   *  use a longer ITE to increase the entropy of the salt,

   *  rate-limit transport parameters sent to a particular client, and/

   *  set a low expiration time to reduce the lifetime of the attacker's

   Segmenting the version number space based on client information, i.e.
   using only a subset of version numbers for a certain IP address
   range, would significantly amplify an attack.  Observers will
   generally be on the path to the client and be able to mimic having an
   identical IP address.  Segmentation in this way would dramatically
   reduce the search space for attackers.  Thus, servers are prohibited
   from using this mechanism.

7.5.  Increased Processing of Garbage UDP Packets

   As QUIC shares the UDP protocol number with other UDP applications,
   in some deployments it may be possible for traffic intended for other
   UDP applications to arrive at a QUIC server endpoint.  When servers
   support a finite set of version numbers, a valid version number field
   is a strong indicator the packet is, in fact, QUIC.  If the version
   number is invalid, a QUIC Version Negotiation is a low-cost response
   that triggers very early in packet processing.

   However, a server that provides version aliasing is prepared to
   accept almost any version number.  As a result, many more
   sufficiently sized UDP payloads with the first bit set to '1' are
   potential QUIC Initial Packets that require generation of a salt and
   Packet Length Offset.

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   Note that a nonzero Packet Length Offset will allow the server to
   drop all but approximately 1 in every 2^49 packets, so trial
   decryption is unnecessary.

   While not a more potent attack then simply sending valid Initial
   Packets, servers may have to provision additional resources to
   address this possibility.

7.6.  Increased Retry Overhead

   This document requires two small cryptographic operations to build a
   Retry packet instead of one, placing more load on servers when
   already under load.

8.  IANA Considerations

   This draft chooses a transport parameter (0x5641) to minimize the
   risk of collision.  IANA should assign a permanent value from the
   QUIC Transport Parameter Registry.

9.  References

9.1.  Normative References

   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using Transport
              Layer Security (TLS) to Secure QUIC", Work in Progress,
              Internet-Draft, draft-ietf-quic-tls-latest,

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

              Schinazi, D., Ed. and E. Rescorla, Ed., "Compatible
              Version Negotiation for QUIC", Work in Progress, Internet-
              Draft, draft-ietf-quic-version-negotiation-latest,

9.2.  Informative References

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              Rescorla, E., Ed., Oku, K., Ed., Sullivan, N., Ed., and
              C.A. Wood, Ed., "Encrypted Server Name Indication for TLS
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-

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

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

Appendix A.  Acknowledgments

   Marten Seemann was the original progenitor of the version aliasing

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-version-aliasing-01

   *  Fixed all references to "seed" where I meant "salt."

   *  Added the Packet Length Offset, which eliminates Retry Injection

B.2.  since draft-duke-quic-version-aliasing-00

   *  Added "Initial Token Extensions" to increase salt entropy and make
      salt polling attacks impractical.

   *  Allowed servers to store a mapping of version number and ITE to
      salt instead.

   *  Made standard version encoding mandatory.  This dramatically
      simplifies the new Retry logic and changes the security model.

   *  Added references to Version Negotiation Transport Parameters.

   *  Extensive readability edit.

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Author's Address

   Martin Duke
   F5 Networks, Inc.

   Email: martin.h.duke@gmail.com

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