[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01

Network Working Group                                        V. Vasiliev
Internet-Draft                                                    Google
Intended status: Standards Track                        November 2, 2019
Expires: May 5, 2020


                        WebTransport over HTTP/3
                    draft-vvv-webtransport-http3-01

Abstract

   WebTransport [OVERVIEW] is a protocol framework that enables clients
   constrained by the Web security model to communicate with a remote
   server using a secure multiplexed transport.  This document describes
   Http3Transport, a WebTransport protocol that is based on HTTP/3
   [HTTP3] and provides support for unidirectional streams,
   bidirectional streams and datagrams, all multiplexed within the same
   HTTP/3 connection.

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/.

   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 May 5, 2020.

Copyright Notice

   Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of



Vasiliev                   Expires May 5, 2020                  [Page 1]


Internet-Draft               Http3Transport                November 2019


   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 . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Session IDs . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Session Establishment . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Establishing a Transport-Capable HTTP/3 Connection  . . .   4
     4.2.  Extended CONNECT in HTTP/3  . . . . . . . . . . . . . . .   4
     4.3.  Creating a New Session  . . . . . . . . . . . . . . . . .   5
     4.4.  Limiting the Number of Simultaneous Sessions  . . . . . .   5
   5.  WebTransport Features . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Unidirectional streams  . . . . . . . . . . . . . . . . .   6
     5.2.  Client-Initiated Bidirectional Streams  . . . . . . . . .   6
     5.3.  Server-Initiated Bidirectional Streams  . . . . . . . . .   7
     5.4.  Datagrams . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Session Termination . . . . . . . . . . . . . . . . . . . . .   8
   7.  Transport Properties  . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Upgrade Token Registration  . . . . . . . . . . . . . . .   9
     9.2.  QUIC Transport Parameter Registration . . . . . . . . . .   9
     9.3.  Frame Type Registration . . . . . . . . . . . . . . . . .  10
     9.4.  Stream Type Registration  . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   HTTP/3 [HTTP3] is a protocol defined on top of QUIC [QUIC-TRANSPORT]
   that can multiplex HTTP requests over a QUIC connection.  This
   document defines Http3Transport, a mechanism for multiplexing non-
   HTTP data with HTTP/3 in a manner that conforms with the WebTransport
   protocol framework [OVERVIEW].  Using the mechanism described here,
   multiple Http3Transport instances can be multiplexed simultaneously
   with regular HTTP traffic on the same HTTP/3 connection.

1.1.  Terminology

   The keywords "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




Vasiliev                   Expires May 5, 2020                  [Page 2]


Internet-Draft               Http3Transport                November 2019


   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document follows terminology defined in Section 1.2 of
   [OVERVIEW].  Note that this document distinguishes between a
   WebTransport server and an HTTP/3 server.  An HTTP/3 server is the
   server that terminates HTTP/3 connections; a WebTransport server is
   an application that accepts WebTransport sessions, which can be
   accessed via an HTTP/3 server.

2.  Protocol Overview

   Http3Transport servers are identified by a pair of authority value
   and path value (defined in [RFC3986] Sections 3.2 and 3.3
   correspondingly).

   When an HTTP/3 connection is established, the client and server have
   to negotiate a specific set of QUIC transport parameters that
   indicate support for various Http3Transport features.  Most notably,
   the "http3_transport_support" parameter signals Http3Transport
   support to the peer.

   Http3Transport sessions are initiated inside a given HTTP/3
   connection by the client, who sends an extended CONNECT request
   [RFC8441].  If the server accepts the request, an Http3Transport
   session is established.  As a part of this process, the client
   proposes, and the server confirms, a session ID.  A session ID (SID)
   is unique within a given HTTP/3 connection, and is used to associate
   all of the streams and datagrams with the specific session.

   After the session is established, the peers can exchange data using
   the following mechanisms:

   o  A client can create a bidirectional stream using a special
      indefinite-length HTTP/3 frame that transfers ownership of the
      stream to Http3Transport.

   o  A server can create a bidirectional stream, which is possible
      since HTTP/3 does not define any semantics for server-initiated
      bidirectional streams.

   o  Both client and server can create a unidirectional stream using a
      special stream type.

   o  A datagram can be sent using a QUIC DATAGRAM frame
      [QUIC-DATAGRAM].





Vasiliev                   Expires May 5, 2020                  [Page 3]


Internet-Draft               Http3Transport                November 2019


   An Http3Transport session is terminated when the CONNECT stream that
   created it is closed.

3.  Session IDs

   In order to allow multiple Http3Transport sessions to occur within
   the same HTTP/3 connection, Http3Transport assigns every session a
   unique ID, further referred to as session ID.  A session ID is a
   62-bit number that is unique within the scope of an HTTP/3
   connection, and is never reused even after the session is closed.
   The client unilaterally picks the session ID.  As the IDs are encoded
   using QUIC variable length integers, the client SHOULD start with
   zero and then sequentially increment the IDs.  A session ID is
   considered to be used, and thus ineligible for new transports, as
   soon as the client sends a request proposing it.  These reuse
   requirements guarantee that both HTTP/3 endpoints have a consistent
   view of the session ID space.

   The Session ID is a hop-by-hop property: if Http3Transport is
   proxied, the same session can have different IDs from the client's
   and server's perspective.  Because of that, session IDs SHOULD NOT be
   exposed to the application.

4.  Session Establishment

4.1.  Establishing a Transport-Capable HTTP/3 Connection

   In order to indicate support for Http3Transport, both the client and
   server MUST send an empty "http3_transport_support" transport
   parameter.  Endpoints MUST NOT use any Http3Transport-related
   functionality unless the parameter has been negotiated.  The
   negotiation is done through a QUIC transport parameter instead of an
   HTTP/3-level setting as it allows the server to customize the
   transport parameters it intends to send based on whether the client
   has indicated support for Http3Transport.

   If "http3_transport_support" is negotiated, support for the QUIC
   DATAGRAM extension MUST be negotiated.  The
   "initial_max_bidi_streams" MUST be greater than zero, overriding the
   existing requirement in [HTTP3].

4.2.  Extended CONNECT in HTTP/3

   [RFC8441] defines an extended CONNECT method in Section 4, enabled by
   the SETTINGS_ENABLE_CONNECT_PROTOCOL parameter.  That parameter is
   only defined for HTTP/2.  This document does not create a new multi-
   purpose parameter to indicate support for extended CONNECT in HTTP/3;




Vasiliev                   Expires May 5, 2020                  [Page 4]


Internet-Draft               Http3Transport                November 2019


   instead, the "http3_transport_support" transport parameter implies
   that an endpoint supports extended CONNECT.

4.3.  Creating a New Session

   As Http3Transport sessions are established over HTTP/3, they are
   identified using the "https" URI scheme [RFC7230].

   In order to create a new Http3Transport session, a client can send an
   HTTP CONNECT request.  The ":protocol" pseudo-header field MUST be
   set to "webtransport".  The ":scheme" field MUST be "https".  Both
   the ":authority" and the ":path" value MUST be set; those fields
   indicate the desired WebTransport server.  The client MUST pick a new
   session ID as described in Section 3 and send it encoded as a
   hexadecimal literal in ":sessionid" header.  An "Origin" header
   [RFC6454] MUST be provided within the request.

   Upon receiving an extended CONNECT request with a ":protocol" field
   set to ":webtransport", the HTTP/3 server can check if it has a
   WebTransport server associated with the specified ":authority" and
   ":path" values.  If it does not, it SHOULD reply with status code 404
   (Section 6.5.4, [RFC7231]).  If it does, it MAY accept the session by
   replying with status code 200.  Before accepting it, the HTTP/3
   server MUST verify that the proposed session ID does not conflict
   with any currently open sessions, and it MAY verify that it was not
   used ever before on this connection.  The WebTransport server MUST
   verify the "Origin" header to ensure that the specified origin is
   allowed to access the server in question.

   From the client's perspective, an Http3Transport session is
   established when the client receives a 200 response.  From the
   server's perspective, a session is established once it sends a 200
   response.  Both endpoints MUST NOT open any streams or send any
   datagrams on a given session before that session is established.
   Http3Transport does not support 0-RTT.

4.4.  Limiting the Number of Simultaneous Sessions

   From the flow control perspective, Http3Transport sessions count
   against the stream flow control just like regular HTTP requests,
   since they are established via an HTTP CONNECT request.  This
   document does not make any effort to introduce a separate flow
   control mechanism for sessions, nor to separate HTTP requests from
   WebTransport data streams.  If the server needs to limit the rate of
   incoming requests, it has alternative mechanisms at its disposal:






Vasiliev                   Expires May 5, 2020                  [Page 5]


Internet-Draft               Http3Transport                November 2019


   o  "HTTP_REQUEST_REJECTED" error code defined in [HTTP3] indicates to
      the receiving HTTP/3 stack that the request was not processed in
      any way.

   o  HTTP status code 429 indicates that the request was rejected due
      to rate limiting [RFC6585].  Unlike the previous method, this
      signal is directly propagated to the application.

5.  WebTransport Features

   Http3Transport provides the following features described in
   [OVERVIEW]: unidirectional streams, bidirectional streams and
   datagrams, initiated by either endpoint.

   Session IDs are used to demultiplex streams and datagrams belonging
   to different Http3Transport sessions.  On the wire, session IDs are
   encoded using the QUIC variable length integer scheme described in
   [QUIC-TRANSPORT].

5.1.  Unidirectional streams

   Once established, both endpoints can open unidirectional streams.
   The HTTP/3 control stream type SHALL be 0x54.  The body of the stream
   SHALL be the stream type, followed by the session ID, encoded as a
   variable-length integer, followed by the user-specified stream data
   (Figure 1).

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           0x54 (i)                          ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Session ID (i)                       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Stream Body                         ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 1: Unidirectional Http3Transport stream format

5.2.  Client-Initiated Bidirectional Streams

   Http3Transport clients can initiate bidirectional streams by opening
   an HTTP/3 bidirectional stream and sending an HTTP/3 frame with type
   "WEBTRANSPORT_STREAM" (type=0x41).  The format of the frame SHALL be
   the frame type, followed by the session ID, encoded as a variable-
   length integer, followed by the user-specified stream data
   (Figure 2).  The frame SHALL last until the end of the stream.




Vasiliev                   Expires May 5, 2020                  [Page 6]


Internet-Draft               Http3Transport                November 2019


     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           0x41 (i)                          ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Session ID (i)                       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Stream Body                         ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 2: WEBTRANSPORT_STREAM frame format

5.3.  Server-Initiated Bidirectional Streams

   Http3Transport servers can initiate bidirectional streams by opening
   a bidirectional stream within the HTTP/3 connection.  Note that since
   HTTP/3 does not define any semantics for server-initiated
   bidirectional streams, this document is a normative reference for the
   semantics of such streams for all HTTP/3 connections in which the
   "http3_transport_support" option is negotiated.  The format of those
   streams SHALL be the session ID, encoded as a variable-length
   integer, followed by the user-specified stream data (Figure 3).

     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Session ID (i)                       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Stream Body                         ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Figure 3: Server-initiated bidirectional stream format

5.4.  Datagrams

   Datagrams can be sent using the DATAGRAM frame as defined in
   [QUIC-DATAGRAM] and [H3-DATAGRAM].  For all HTTP/3 connections in
   which the "http3_transport_support" option is negotiated, the Flow
   Identifier is set to the session ID.  In other words, the format of
   datagrams SHALL be the session ID, followed by the user-specified
   payload (Figure 4).










Vasiliev                   Expires May 5, 2020                  [Page 7]


Internet-Draft               Http3Transport                November 2019


     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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Session ID (i)                       ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Datagram Body                        ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: Datagram format

   In QUIC, a datagram frame can span at most one packet.  Because of
   that, the applications have to know the maximum size of the datagram
   they can send.  However, when proxying the datagrams, the hop-by-hop
   MTUs can vary.  TODO: Describe how the path MTU can be computed,
   specifically propagation across HTTP proxies.

6.  Session Termination

   An Http3Transport is terminated when either endpoint closes the
   stream associated with the CONNECT request that initiated the
   session.  Upon learning about the session being terminated, the
   endpoint MUST stop sending new datagrams and reset all of the streams
   associated with the session.

7.  Transport Properties

   Http3Transport supports most of the WebTransport features described
   in Table 1.

            +---------------------+--------------------------+
            | Property            | Support                  |
            +---------------------+--------------------------+
            | Stream independence | Always supported         |
            |                     |                          |
            | Partial reliability | Always supported         |
            |                     |                          |
            | Pooling support     | Always supported         |
            |                     |                          |
            | Connection mobility | Implementation-dependent |
            +---------------------+--------------------------+

              Table 1: Transport properties of Http3Transport

8.  Security Considerations

   Http3Transport satisfies all of the security requirements imposed by
   [QUIC-TRANSPORT] on WebTransport protocols, thus providing a secure
   framework for client-server communication in cases when the client is



Vasiliev                   Expires May 5, 2020                  [Page 8]


Internet-Draft               Http3Transport                November 2019


   potentially untrusted.  Since HTTP/3 is QUIC-based, a lot of the
   analysis in [WEBTRANSPORT-QUIC] applies here.

   Http3Transport requires explicit opt-in through the use of a QUIC
   transport parameter; this avoids potential protocol confusion attacks
   by ensuring the HTTP/3 server explicitly supports it.  It also
   requires the use of the Origin header, providing the server with the
   ability to deny access to Web-based clients that do not originate
   from a trusted origin.

   Just like HTTP/3 itself, Http3Transport pools traffic to different
   origins within a single connection.  Different origins imply
   different trust domains, meaning that the implementations have to
   treat each transport as potentially hostile towards others on the
   same connection.  One potential attack is a resource exhaustion
   attack: since all of the transports share both congestion control and
   flow control context, a single client aggressively using up those
   resources can cause other transports to stall.  The user agent thus
   SHOULD implement a fairness scheme that ensures that each transport
   within connection gets a reasonable share of controlled resources;
   this applies both to sending data and to opening new streams.

9.  IANA Considerations

9.1.  Upgrade Token Registration

   The following entry is added to the "Hypertext Transfer Protocol
   (HTTP) Upgrade Token Registry" registry established by [RFC7230]:

   The "webtransport" label identifies HTTP/3 used as a protocol for
   WebTransport:

   Value:  webtransport

   Description:  WebTransport over HTTP/3

   Reference:  This document

9.2.  QUIC Transport Parameter Registration

   The following entry is added to the "QUIC Transport Parameter
   Registry" registry established by [QUIC-TRANSPORT]:

   The "http3_transport_support" parameter indicates that the specified
   HTTP/3 connection is Http3Transport-capable.

   Value:  0x????




Vasiliev                   Expires May 5, 2020                  [Page 9]


Internet-Draft               Http3Transport                November 2019


   Parameter Name:  http3_transport_support

   Specification:  This document

9.3.  Frame Type Registration

   The following entry is added to the "HTTP/3 Frame Type" registry
   established by [HTTP3]:

   The "WEBTRANSPORT_STREAM" frame allows HTTP/3 client-initiated
   bidirectional streams to be used by WebTransport:

   Code:  0x54

   Frame Type:  WEBTRANSPORT_STREAM

   Specification:  This document

9.4.  Stream Type Registration

   The following entry is added to the "HTTP/3 Stream Type" registry
   established by [HTTP3]:

   The "WebTransport stream" type allows unidirectional streams to be
   used by WebTransport:

   Code:  0x41

   Stream Type:  WebTransport stream

   Specification:  This document

   Sender:  Both

10.  References

10.1.  Normative References

   [H3-DATAGRAM]
              Schinazi, D., "Using QUIC Datagrams with HTTP/3", draft-
              schinazi-quic-h3-datagram-01 (work in progress), October
              2019.

   [HTTP3]    Bishop, M., Ed., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", draft-ietf-quic-http (work in progress).






Vasiliev                   Expires May 5, 2020                 [Page 10]


Internet-Draft               Http3Transport                November 2019


   [OVERVIEW]
              Vasiliev, V., "The WebTransport Protocol Framework",
              draft-vvv-webtransport-overview-01 (work in progress).

   [QUIC-DATAGRAM]
              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", draft-pauly-quic-datagram
              (work in progress).

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", draft-ietf-quic-
              transport (work in progress).

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,
              <https://www.rfc-editor.org/info/rfc6454>.

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
              <https://www.rfc-editor.org/info/rfc6585>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.






Vasiliev                   Expires May 5, 2020                 [Page 11]


Internet-Draft               Http3Transport                November 2019


   [RFC8441]  McManus, P., "Bootstrapping WebSockets with HTTP/2",
              RFC 8441, DOI 10.17487/RFC8441, September 2018,
              <https://www.rfc-editor.org/info/rfc8441>.

10.2.  Informative References

   [WEBTRANSPORT-QUIC]
              Vasiliev, V., "WebTransport over QUIC", draft-vvv-
              webtransport-quic-01 (work in progress).

Author's Address

   Victor Vasiliev
   Google

   Email: vasilvv@google.com



































Vasiliev                   Expires May 5, 2020                 [Page 12]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/