Comparing ALPS and Half-RTT Data

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Author David Benjamin 
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TLS                                                          D. Benjamin
Internet-Draft                                                Google LLC
Intended status: Informational                           3 December 2020
Expires: 6 June 2021

                    Comparing ALPS and Half-RTT Data


   This document compares the Application Layer Protocols Settings
   extension with the half-RTT feature in TLS 1.3.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on 6 June 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Using Half-RTT Data . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Half-RTT Delimiter  . . . . . . . . . . . . . . . . . . .   3
     2.2.  Non-Integer HTTP Settings . . . . . . . . . . . . . . . .   4
     2.3.  Early Data and Session Tickets  . . . . . . . . . . . . .   4
     2.4.  Client Certificates . . . . . . . . . . . . . . . . . . .   5
     2.5.  TLS Terminators . . . . . . . . . . . . . . . . . . . . .   6
     2.6.  TCP Flow Control  . . . . . . . . . . . . . . . . . . . .   7
   3.  Using ALPS  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Half-RTT Delimiter  . . . . . . . . . . . . . . . . . . .   8
     3.2.  Non-Integer HTTP Settings . . . . . . . . . . . . . . . .   8
     3.3.  Early Data and Session Tickets  . . . . . . . . . . . . .   8
     3.4.  Client Certificates . . . . . . . . . . . . . . . . . . .   8
     3.5.  TLS Terminators . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  TCP Flow Control  . . . . . . . . . . . . . . . . . . . .   9
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   An application-layer protocol often starts with both parties
   negotiating parameters under which the protocol operates; for
   instance, HTTP/2 [RFC7540] and HTTP/3 [I-D.ietf-quic-http] use a
   SETTINGS frame to exchange the list of protocol parameters supported
   by each endpoint.  This can achieved by waiting for TLS handshake
   [RFC8446] to complete and then performing the application-layer
   handshake within the application protocol itself.

   This approach, however, means application protocols must wait for a
   secondary negotiation to complete, often incurring network round-
   trip.  HTTP/2 and HTTP/3 mitigate this with a best-effort negotiation
   scheme: clients do not wait for server SETTINGS before sending a
   request.  But then, by the time the client applies the setting, it
   has already sent the first request based on the default values.  This
   limits the kinds of extensions possible.  For example, the SETTINGS
   frame cannot support negotiate header compression [QUIC-3622] or a
   different static table [HTTP2-788] without changing the protocol to
   disable compression by default and switch partway through.

   Protocol selection is another example of application-level
   negotiation with these trade-offs.  The Application Layer Protocol
   Negotiation (ALPN) extension [RFC7301] adds protocol selection into
   the TLS handshake.  ALPN is instead consistently ordered before all

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   application data, including TLS 1.3 early data, without either a
   round-trip penalty or the need to send initial pre-negotiation data
   (see Section 3.2 of [RFC7540]).

   The Application Layer Protocol Settings (ALPS) extension
   [I-D.vvv-tls-alps] implements [QUIC-3086-COMMENT] and adds a similar
   mechanism for settings within the protocol.  It sends ALPN-specific
   protocol settings strings in the handshake, which can be ordered
   correctly relative to application data and integrated with TLS 1.3
   early data negotiation.

   As an alternative, Section 4.4.4 of [RFC8446] allows a server to send
   application data after the server Finished message, often referred to
   as half-RTT data.  Half-RTT data is not a complete solution to the
   settings problem, however.  This document describes the other changes
   necessary and compares the approach to ALPS.

2.  Using Half-RTT Data

   Although not currently widely-implemented, half-RTT data can be used
   to deliver HTTP/2 SETTINGS and other values at the right round-trip.
   This would result in a handshake flow like the following.

       Client                                              Server

       ClientHello               -------->
                                 <--------       [HTTP/2 SETTINGS]
       [HTTP/2 SETTINGS]         -------->
       [HTTP/2 requests]         <------->      [HTTP/2 responses]

   The approach, however, requires a number of additional changes and
   protocol interactions to work correctly.

2.1.  Half-RTT Delimiter

   In this design, the client waits to receive the HTTP/2 SETTINGS frame
   before sending requests.  However, HTTP/2 servers are not required to
   send SETTINGS in half-data today, and most existing ones do not.
   [[TODO: Did I ever write this down anywhere I can link to?  When I
   probed TLS 1.3 HTTP/2 servers, I found none that send half-RTT
   data.]] Without a new signal to the client, waiting would add a
   latency penalty to existing servers.  TLS 1.3 does not include a
   delimiter between half-RTT data and the rest of the server

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   application stream, so the client does not know a priori when it is
   done reading.

   One option would be a TLS extension that adds a delimiter between
   half-RTT and normal server application data.  The client would then
   wait for that delimiter without round-trip penalty and proceed.  This
   would not work in QUIC because QUIC does not use TLS for application
   data at all.  Instead, half-RTT data would need to be lifted into the
   handshake, which is the ALPS extension.

   Alternatively, the client could rely on application protocol
   semantics, and assume the protocol defines exactly what is sent in
   half-RTT.  However, HTTP/2 does not do this today.  This would
   require defining new HTTP/2.1 and HTTP/3.1 protocols with a MUST-
   level requirement to send half-RTT SETTINGS.  HTTP/2.1 and HTTP/3.1
   would be negotiated via ALPN.  Note both HTTP/2 and HTTP/3 must be
   updated because, per Section 3.2 of [I-D.ietf-quic-http],
   connectivity problems can break QUIC and clients are encouraged to
   fall back to a TCP-based version of HTTP.

2.2.  Non-Integer HTTP Settings

   The HTTP/2 and HTTP/3 SETTINGS frame can only carry integer values,
   but extensions may need to carry variable-length data.  For example,
   [I-D.davidben-http-client-hint-reliability] uses a string value.

   [I-D.bishop-httpbis-extended-settings] proposes an EXTENDED_SETTINGS
   frame to fix this.  If defining HTTP/2.1 and HTTP/3.1,
   EXTENDED_SETTINGS can be added as a mandatory component of the new

   If EXTENDED_SETTINGS is left optional, the client needs to know
   whether to expect a half-RTT EXTENDED_SETTINGS frame after half-RTT
   SETTINGS, to avoid the issues discussed in Section 2.1.  Thus
   SETTINGS would need to contain a SETTINGS_EXTENDED_SETTINGS setting
   to indicate more half-RTT frames are coming.

   [HTTPWG-COMMENT] suggested tabling EXTENDED_SETTINGS in favor of
   extensions defining new HTTP frames.  That would not work here,
   absent each extension additionally defining an analog to
   SETTINGS_EXTENDED_SETTINGS, to signal to the client to expect a new

2.3.  Early Data and Session Tickets

   TLS 1.3 introduces early data, which allows clients to send
   application data before receiving a ServerHello from the server.
   [RFC8470] describes how to use it in HTTP.

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   Application-level connection properties additionally must be
   established before the client sends early data.  Otherwise if, for
   instance, HPACK static tables are negotiated, the client will not be
   able to encode the early request.  Note Section 2 of [RFC8470] says
   early data in HTTP is conceptually concatenated with other
   application data, so early data and 1-RTT data in HTTP must share
   decoding rules.

   Early data is sent before any response from the server, so connection
   properties are typically carried over from the ticket.
   Section 4.2.10 of [RFC8446] describes the mechanism for the ALPN
   extension: Each PSK has an associated ALPN protocol, determined from
   the previous connection.  The client sends early data assuming that
   protocol was used.  If the server negotiates a different value, it
   rejects early data.

   Reliably-ordered protocol settings would require a similar
   construction.  However TLS does not define ALPN's early data behavior
   generally, so every application protocol would need to define it
   themselves and, when implementing, rely on various callback
   interfaces in the TLS implementation.

   This also introduces a dependency between NewSessionTicket and the
   server application data stream: the NewSessionTicket is not
   meaningful without part of the server application data (here, the
   SETTINGS and EXTENDED_SETTINGS frames).  Moreover, in QUIC and DTLS,
   post-handshake messages are not ordered relative to application data,
   so the client may receive NewSessionTicket messages in the wrong
   order.  The client then cannot store sessions in the TLS session
   until some application-defined point.  This requires further
   integration between TLS and the application protocol.

   See related discussion in [QUIC-436], [QUIC-2790], and [QUIC-2945].
   Note HTTP/3 addressed the ordering issue by making associating
   settings with the ticket optional on the client [QUIC-2972], while a
   solution to this problem makes it mandatory.

2.4.  Client Certificates

   TLS APIs are often structured around the following sequence of

   1.  The calling application configures TLS parameters.  This may
       include preferred cipher suites, client certificate requirements,
       callbacks to defer some configuration, etc.

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   2.  The calling application runs the handshake to some notion of
       completion.  Before the handshake completes, connection
       properties are not established, the peer is not authenticated,
       and the application does not read or write data.

   3.  The calling application queries handshake properties.  It may
       query the negotiated ALPN protocol to determine how to proceed in
       the application protocol.  It may query the peer certificate for
       application-level access checks.

   4.  The calling application reads and writes data according to the
       application protocol.

   This is analogous to many TCP socket APIs, where there is a "connect"
   or "accept" operation that completes before "recv" and "send"
   operations are available.

   In server connections that do not resume a session, the TLS 1.3 half-
   RTT point has different semantics from a complete TLS handshake.  The
   client's identity has not been established yes, so TLS
   implementations cannot transparently report the connection as ready
   to the calling application.  Doing so risks security issues (the
   application's client certificate requirements are not yet checked)
   and compatibility breaks (the application cannot usefully query the
   peer certificate).

   Instead, the TLS implementation might expose a separate interface for
   an earlier partial completion state.  The application would then
   write half-RTT data, knowing that client authentication requirements
   are not yet met.  This complicates the interface and the above
   structure.  Alternatively, the TLS implementation may require the
   application configure a byte string to send as half-RTT data during
   the handshake, but note this risks the deadlocks described in
   Section 2.6.

2.5.  TLS Terminators

   Some server deployments use a TLS terminator which then makes a TCP
   connection to some backend application server.  These deployments
   would need to preserve any MUST-level requirements to send SETTINGS
   in half-RTT data.  A TLS terminator which completes the handshake and
   then proxies data from the backend server would inadvertently add a
   round-trip delay to the SETTINGS frame, delaying HTTP requests.
   However, a TLS terminator which begins proxying data at the half-RTT
   point instead risks skipping client certificate authentication.

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   Instead, the TLS terminator must coordinate with the backend server
   to determine what data may be sent early to unauthenticated clients,
   and what data is bulk application traffic.

2.6.  TCP Flow Control

   If not implemented properly, this design risks deadlocks with TCP
   flow control [TCP-TLS].  It is possible for both the client Finished
   flight and the server half-RTT data to exceed transport buffers.  The
   server must read the client Finished flight and complete the
   handshake, even if the half-RTT data has not been written to the

   In particular, a TLS implementation may try to avoid the issues in
   Section 2.4 by treating half-RTT as a configured string sent as part
   of the handshake, rather than exposing a writable stream to the
   calling application.  This strategy must still write half-RTT data in
   parallel with completing the handshake to avoid a deadlock.  Many TLS
   implementations are layered on top of non-blocking TCP socket APIs,
   which means the calling application would still be responsible for
   driving these parallel operations.  This changes the I/O patterns the
   application expects from TLS.

3.  Using ALPS

   The ALPS strategy is described in [I-D.vvv-tls-alps] and
   [I-D.vvv-httpbis-alps].  It implements [QUIC-3086-COMMENT], sending
   application settings in the EncryptedExtensions on both client and
   server.  The client half is not strictly necessary (TLS 1.3 is always
   writable on the client first), but simplifies server implementations
   in QUIC, where application data streams are not ordered relative to
   each other.

       Client                                               Server

       ClientHello               -------->
                                           + alps(HTTP/2 SETTINGS)
                                 <--------              {Finished}
       + alps(HTTP/2 SETTINGS)
       [HTTP/2 requests]         <------->      [HTTP/2 responses]

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3.1.  Half-RTT Delimiter

   ALPS does not require a half-RTT delimiter.  The entire payload is
   sent in the EncryptedExtensions message, which includes a common
   framing for extension values.

3.2.  Non-Integer HTTP Settings

   As in half-RTT data, an ALPS mechanism for HTTP/2 and HTTP/3 must
   handle the SETTINGS frame limitations.  [I-D.vvv-httpbis-alps] allows
   the ALPS payload to contain multiple frames, so either the
   [I-D.bishop-httpbis-extended-settings] or [HTTPWG-COMMENT] strategies
   may be used.  The payload is already framed in EncryptedExtensions,
   so there is no need for an indicator value like

3.3.  Early Data and Session Tickets

   As in the half-RTT strategy, ALPS requires early data and session
   ticket integration.  However, this behavior is part of the extension
   itself, so, like ALPN, there is no need to specify and implement this
   additional logic for each application protocol.

   Unlike the application-level integration for half-RTT data, this TLS-
   level integration for ALPS does not have ordering issues with
   NewSessionTicket.  NewSessionTicket messages are ordered relative to
   the handshake, so the ALPS values will always be available before a

3.4.  Client Certificates

   As in ALPN and the half-RTT strategy, the server ALPS value is sent
   before receiving the client certificate.  In ALPS, this would be part
   of the extension semantics exposed to application protocols, just as
   ALPN configuration is not protected by client certificates.

3.5.  TLS Terminators

   As in the half-RTT solution, ALPS requires a TLS terminator
   deployment to coordinate with its backend server to separate the
   early, unauthenticated SETTINGS data from the rest of the stream.
   However, the payload is already naturally kept separate from the rest
   of the application stream.  Instead, the settings values are an
   analog of the ALPN value, which already requires coordination.

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3.6.  TCP Flow Control

   ALPS sends the settings values in-band in the TLS handshake, rather
   than afterwards, so the deadlock risks described in [TCP-TLS] do not
   apply.  The client will read the entire EncryptedExtensions message
   (and more) before trying to send the client Certificate,
   CertificateVerify, and Finished.

4.  Security Considerations

   Any server information delivered in time for the client's first
   application data records must be sent before checking client
   certificates.  Section 2.4 and Section 3.4 discuss strategies for
   ensuring the calling application does not inadvertently reveal
   sensitive information to unauthenticated clients.

5.  IANA Considerations

   This document has no IANA considerations.

6.  Informative References

              "Update the HPACK static table", November 2020,

              Thomson, M., "draft-bishop-httpbis-extended-settings-00
              comments", 13 July 2016,

              Bishop, M., "HTTP/2 Extended SETTINGS Extension", Work in
              Progress, Internet-Draft, draft-bishop-httpbis-extended-
              settings-01, 15 November 2016, <

              Benjamin, D., "Client Hint Reliability", Work in Progress,
              Internet-Draft, draft-davidben-http-client-hint-
              reliability-02, 30 November 2020, <

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              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-32, 20 October 2020, <

              Vasiliev, V., "Using TLS Application-Layer Protocol
              Settings (ALPS) in HTTP", Work in Progress, Internet-
              Draft, draft-vvv-httpbis-alps-00, 6 July 2020,

              Benjamin, D. and V. Vasiliev, "TLS Application-Layer
              Protocol Settings Extension", Work in Progress, Internet-
              Draft, draft-vvv-tls-alps-01, 21 September 2020,

              Thomson, M., "Binding settings into session tickets", July
              2019, <>.

              Oku, K., "When to send the SETTINGS frame", July 2019,

              Bishop, M., "Send complete SETTINGS", August 2019,

              Bishop, M., "Add application parameters to QUIC handshake
              and use it for H3 SETTINGS (comment)", 17 October 2019,

              "Make using static table and Huffman encoding in QPACK
              opt-in", May 2020,

   [QUIC-436] Rescorla, E., "Move SETTINGS into TLS Handshake", April
              2017, <>.

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

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

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

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <>.

   [TCP-TLS]  Benjamin, D., "TLS 1.3 and TCP interactions", 29 May 2020,


   This document has benefited from contributions and suggestions from
   Victor Vasiliev.

Author's Address

   David Benjamin
   Google LLC


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