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

Versions: (draft-mcbride-bier-ipv6-requirements) 00

BIER                                                          M. McBride
Internet-Draft                                                 Futurewei
Intended status: Standards Track                                  J. Xie
Expires: November 30, 2019                                   S. Dhanaraj
                                                                  Huawei
                                                                R. Asati
                                                                   Cisco
                                                            May 29, 2019


                         BIER IPv6 Requirements
                  draft-ietf-bier-ipv6-requirements-00

Abstract

   The BIER WG has a charter item to work on mechanisms which use BIER
   natively in IPv6.  This document is intended to help the WG with this
   effort by specifying requirements for transporting packets, with Bit
   Index Explicit Replication (BIER) headers, in an IPv6 environment.
   There will be a need to send IPv6 payloads, to multiple IPv6
   destinations, using BIER.  There have been several proposed solutions
   in this area.  But there hasn't been a document which describes the
   problem and lists the requirements.  The goal of this document is to
   describe the BIER IPv6 requirements and summarize the pro's and con's
   of the proposed solutions.

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 November 30, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




McBride, et al.         Expires November 30, 2019               [Page 1]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  BIER IPv6 Scenario's  . . . . . . . . . . . . . . . . . . . .   3
     3.1.  BIERv6 for Access Network . . . . . . . . . . . . . . . .   4
     3.2.  BIERv6 for Data Center  . . . . . . . . . . . . . . . . .   4
     3.3.  BIERv6 for Core Networks  . . . . . . . . . . . . . . . .   5
     3.4.  Implications for BIER in SRv6 . . . . . . . . . . . . . .   5
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  L2 Agnostic . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Hop by hop DA modification  . . . . . . . . . . . . . . .   5
     4.3.  L4 Inspection . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Multicast address in SA field . . . . . . . . . . . . . .   5
     4.5.  Incorrect bits  . . . . . . . . . . . . . . . . . . . . .   6
     4.6.  SA filtering  . . . . . . . . . . . . . . . . . . . . . .   6
     4.7.  BIER architecture support . . . . . . . . . . . . . . . .   6
     4.8.  Keep it simple  . . . . . . . . . . . . . . . . . . . . .   6
     4.9.  Hardware fast path  . . . . . . . . . . . . . . . . . . .   6
   5.  Solutions Evaluation  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  BIER-ETH encapsulation in IPv6 networks . . . . . . . . .   6
     5.2.  Encode Bitstring in IPv6 destination address  . . . . . .   8
     5.3.  Add BIER header into IPv6 Extension Header  . . . . . . .   8
     5.4.  Transport BIER as IPv6 payload  . . . . . . . . . . . . .   9
     5.5.  Tunneling BIER in a IPv6 tunnel . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Bit Index Explicit Replication (BIER) [RFC8279] is an architecture
   that provides optimal multicast forwarding, without requiring
   intermediate routers to maintain per-flow state, through the use of a



McBride, et al.         Expires November 30, 2019               [Page 2]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   multicast-specific BIER header.  [RFC8296] defines two types of BIER
   encapsulation to run on physical links: one is BIER MPLS
   encapsulation to run on various physical links that support MPLS, the
   other is non-MPLS BIER Ethernet encapsulation to run on ethernet
   links, with an ethertype 0xAB37.  This document describes using BIER
   in non-MPLS IPv6 environments.  We explain the requirements of
   transporting IPv4/IPv6 multicast payloads, from an IPv6 router (BFIR)
   to multicast IPv6 destinations (BFERs), using BIER.  This can include
   native IPv6 encapsulation and generic tunneling.  The goal of this
   document is to help the BIER WG evaluate the BIER v6 requirements and
   solutions.

1.1.  Requirements Language

   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].

1.2.  Terminology

   o  BIER: Bit Index Explicit Replication.  Provides optimal multicast
      forwarding through adding a BIER header and removing state in
      intermediate routers.

   o  BUM: Broadcast, Unknown Unicast, Multicast.  Term used to describe
      the three types of Ethernet modes that will be forwarded to
      multiple destinations

2.  Problem Statement

   The problem is the ability of the network to transport BUM packets,
   with BIER headers, in an IPv6 environment.  In an IPv6 network, many
   deployments consider using a non-MPLS encapsulation for unicast as
   the data-plane.  In such case, it may be expected to have a BIER IPv6
   encapsulation which is compliant with various kinds of physical
   links, perhaps in a hop-by-hop manner, and maintain the benefit of
   "fast reroute" of an IPv6 tunnel.  Evaluating the BIER IPv6
   requirements will help determine the best solutions to address these
   problems.

3.  BIER IPv6 Scenario's










McBride, et al.         Expires November 30, 2019               [Page 3]


Internet-Draft           BIER IPv6 Requirements                 May 2019


         +--------------------------------------------+
         |                                            |
         |                                         +------+
         |                                         | BFER |
     +------+                IPv6                  +------+
     | BFIR |                                         |
     +------+               Network                +------+
         |                                         | BFER |
         |                                         +------+
         |                                            |
         +--------------------------------------------+

   This basic scenario depicts the need to replicate bier packets from a
   BFIR to BFERs across an IPv6 core.  The IPv6 environment may include
   a variety of link types, may be entirely IPv6, may be dual stack or
   any type of combination which includes IPv6.  Regardless of the
   environment, there are times when a BIER header, including the BIER
   bitstring used to determine the set of BIER forwarding egress
   routers, will need to traverse a IPv6 domain.  The ways in which BIER
   will function in an IPv6 environment is the problem that needs to be
   solved.  [RFC8354] lists some good IPv6 related use cases which we
   will similarly reference in this document.

3.1.  BIERv6 for Access Network

   Access networks deliver a variety of types of multicast video traffic
   from the service provider's network to the home (or Enterprise)
   environment and from the home towards the service provider's network.

   There will be a need to send traffic from the IPv4 access towards the
   service provider's IPv6 network and vice versa.  A packet could be
   mapped into a providers IPv6 network through the use of a BIERv6
   header.  The access devices would not need to know specific details
   about the packet to perform this mapping; instead the access device
   would only need to know how to process a BIER header unless there is
   end to end IPv6.

3.2.  BIERv6 for Data Center

   Some Data Center operators are transitioning their Data Center
   infrastructure from IPv4 to native IPv6 only, in order to cope with
   IPv4 address depletion and to achieve larger scale.  In such
   environment, BIERv6, can be used to natively steer multicast data
   across an IPv6 data center.







McBride, et al.         Expires November 30, 2019               [Page 4]


Internet-Draft           BIER IPv6 Requirements                 May 2019


3.3.  BIERv6 for Core Networks

   While the overall amount of traffic offered to the network continues
   to grow and considering that multiple types of traffic with different
   characteristics and requirements are quickly converging over single
   network architecture, the network operators are starting to face new
   challenges.

   Some operators are currently building, or plan to build in the near
   future, an IPv6 only native infrastructure for their core network.
   Having a native BIERv6 infrastructure will help maintain simplicity
   of the network and reduce state versus traditional IP Multicast.

3.4.  Implications for BIER in SRv6

   The Source Packet Routing in Networking (SPRING) architecture
   describes how Segment Routing can be used to steer packets through an
   IPv6 or MPLS network using the source routing paradigm.  [RFC8354]
   focuses on use cases for Segment Routing in an IPv6 only environment,
   something which is equially important for BIER in an IPv6 only
   environment.

4.  Requirements

   There have been several suggested requirements, on the BIER email
   list, which we will use to form the BIER IPv6 requirements and to
   help evaluate the proposed solutions:

4.1.  L2 Agnostic

   The solution should be agnostic to the underlying L2 data link type.

4.2.  Hop by hop DA modification

   The solution should not require hop-by-hop modification of the IP
   destination address field.

4.3.  L4 Inspection

   The solution should not require the BFRs to inspect layer 4 or
   require any changes to layer 4.

4.4.  Multicast address in SA field

   The solution should not allow a multicast address to be put in the IP
   source address field.





McBride, et al.         Expires November 30, 2019               [Page 5]


Internet-Draft           BIER IPv6 Requirements                 May 2019


4.5.  Incorrect bits

   The solution should not assume that bits never get set incorrectly.

4.6.  SA filtering

   The solution should not require changes in source address filtering
   procedures.

4.7.  BIER architecture support

   The solution should be possible to be used to support the entire BIER
   architecture.

4.8.  Keep it simple

   The solution should avoid having to use different encapsulation
   types, or use complex tunneling techniques, to support BIER as a E2E
   multicast transport.

4.9.  Hardware fast path

   The solution should enable the processing and forwarding of BIER
   packets in hardware fast path.

5.  Solutions Evaluation

   The following are solutions that have been proposed to solve BIER in
   IPv6 environments.

   As illustrated in these examples, the BIER header, or the BitString,
   may appear in the IPv6 Header, IPv6 Extension Header, IPv6 Payload,
   or IPv6 Tunnel Packet:

5.1.  BIER-ETH encapsulation in IPv6 networks

         +---------------+-----------------+-------------------
         |   Ethernet    |   BIER header   | payload
         |  (ethType =   | (BIFT-id, ...)  |
         |    0xAB37)    |                 |
         |               |  Next Header    |
         +---------------+-----------------+-------------------

   BIER-ETH encapsulation (BIER header for Non-MPLS networks as defined
   in [RFC8296]) can be used to transport the multicast data in the IPv6
   network by encapsulating the multicast user data payload within the
   BIER-ETH header.  However, using BIER-ETH in IPv6 networks is not
   considered to be a native IPv6 solution which utilizes the IPv6



McBride, et al.         Expires November 30, 2019               [Page 6]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   header to forward the packet.  Below listed are some of the
   properties of BIER-ETH encapsulation which could be seen as the
   reasons for the same,

   o  BIER-ETH is not agnostic to the underlying (L2) data link type.
      It can be deployed only in the networks with Ethernet data link
      and cannot be deployed in an network which deploys any other data
      link types.  Use of BIER-ETH in IPv6 networks might also result in
      using different BIER encapsulations, when BIER is used as a E2E
      multicast transport across a larger heterogeneous IPv6 networks
      with different data link types used in different layers of the
      network.

   o  BIER-ETH in IPv6 networks is considered similar to 6PE solution
      where-in the multicast user data packet is encapsulated with-in
      the BIER-MPLS header.

      *  It is worth noting that the only major difference between BIER-
         MPLS and BIER-Non-MPLS header is that BIER-MPLS uses downstream
         assigned MPLS label while BIER-Non-MPLS header uses a domain-
         wide-unique BIFT-id.  While the use of domain-wide-unique BIFT-
         id in BIER-ETH header takes away the complexity of allocation
         and state maintenance from the network, it still requires some
         sort of ID (similar to label) to identify the application
         context after the decapsulation of BIER header (example: MVPN
         VRF Label).  Encoding of such an ID/LABEL before encapsulating
         the multicast user data payload with BIER-ETH header cannot be
         avoided.

      *  The absence of an IPv6 header, and the optional IPv6 extension
         headers, deprives BIER of some of the useful cases (ex: Use of
         IPv6 address for identification of network function or service
         mapping) that is otherwise possible in native IPv6
         encapsulation which utilizes a IPv6 header.

      *  Tunneling of BIER packets is one common technique used for FRR,
         to tunnel over BIER incapable nodes etc.  While it is possible
         for the BIER-ETH encapsulated packet to be further encapsulated
         within a GRE6 or SRv6, etc tunnel, it might not be possible to
         parse and decapsulate different types of tunnel headers and
         forward the BIER packet completely in hardware fast path
         similar to the label stack processing in BIER-MPLS networks.
         It would be useful to select an encapsulation which could help
         in processing the tunnel and BIER header and make the
         forwarding decision completely in hardware fast path, which is
         lacking in BIER-ETH encapsulation if chosen to be deployed in
         pure IPv6 networks.




McBride, et al.         Expires November 30, 2019               [Page 7]


Internet-Draft           BIER IPv6 Requirements                 May 2019


5.2.  Encode Bitstring in IPv6 destination address

      +---------------+-------------------
      |  IPv6 header  | payload
      | (BitString in |
      | DA lower bits)|
      |  Next Header  |
      +---------------+-------------------

   As described in [I-D.pfister-bier-over-ipv6], The information
   required by BIER is stored in the destination IPv6 address.  The BIER
   BitString is encoded in the low-order bits of the IPv6 destination
   address of each packet.  The high-order bits of the IPv6 destination
   address are used by intermediate routers for unicast forwarding,
   deciding whether a packet is a BIER packet, and if so, to identify
   the BIER Sub-Domain, Set Identifier and BitString length.  No
   additional extension or encapsulation header is required.  Instead of
   encapsulating the packet in IPv6, the payload is attached to the BIER
   IPv6 header and the IPv6 protocol number is set to the type of the
   payload.  If the payload is UDP, the UDP checksum needs to change
   when the BitString in the IPv6 destination address changes.

5.3.  Add BIER header into IPv6 Extension Header

      +---------------+-----------------+-------------------
      |  IPv6 header  | IPv6 Ext header | payload
      |(Multicast DA) | (BIER header in |
      |               |  TLV Type = X)  |
      | Next Header   |   Next Header   |
      +---------------+-----------------+-------------------

   According to [RFC8200] In IPv6, optional internet-layer information
   is encoded in separate headers that may be placed between the IPv6
   header and the upper- layer header in a packet.  There is a small
   number of such extension headers, each one identified by a distinct
   Next Header value.  An IPv6 packet may carry zero, one, or more
   extension headers, each identified by the Next Header field of the
   preceding header.  Extension headers (except for the Hop-by-Hop
   Options header) are not processed, inserted, or deleted by any node
   along a packet's delivery path, until the packet reaches the node (or
   each of the set of nodes, in the case of multicast) identified in the
   Destination Address field of the IPv6 header.  The Hop-by-Hop Options
   header is not inserted or deleted, but may be examined or processed
   by any node along a packet's delivery path, until the packet reaches
   the node (or each of the set of nodes, in the case of multicast)
   identified in the Destination Address field of the IPv6 header.  The
   Hop-by-Hop Options header, when present, must immediately follow the




McBride, et al.         Expires November 30, 2019               [Page 8]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   IPv6 header.  Its presence is indicated by the value zero in the Next
   Header field of the IPv6 header.

   Two of the currently-defined extension headers are the Hop-by-Hop
   Options header and the Destination Options header which carry a
   variable number of type-length-value (TLV) encoded "options".

   In [I-D.xie-bier-ipv6-encapsulation] an IPv6 BIER Destination Option
   is carried by the IPv6 Destination Option Header (indicated by a Next
   Header value 60).  It is initialized in a packet sent by an IPv6 BFIR
   router to inform the following BFR routers in an IPv6 BIER domain to
   replicate to destination BFER routers hop-by-hop.  BIER is generally
   a hop-by-hop and one-to-many architecture and it is required for a
   BIER IPv6 encapsulation to include the BIER Header ([RFC8296]) as an
   IPv6 Extension Header, to pilot the hop-by-hop BIER replication.

   Hop by hop Options Headers may be considered.  The Hop-by-Hop Options
   header is used to carry optional information that may be examined and
   processed by every node along a packet's delivery path.  The Hop-by-
   Hop Options header is identified by a Next Header value of 0 in the
   IPv6 header.

   Defining New Extension Headers and Options may also be considered, if
   the IPv6 Destination Option Header is not good enough and new
   extension headers can solve the problem better.

   Such proposals may include requests to IANA to allocate a "BIER
   Option" code from "Destination Options and Hop-by-Hop Options", and/
   or a "BIER Option Header" code from "IPv6 Extension Header Types".

5.4.  Transport BIER as IPv6 payload

      +---------------+-----------------+-------------------
      |  IPv6 header  | IPv6 Ext header | BIER Hdr + payload
      |               |    (optional)   | as IPv6 payload
      |               |                 |
      | Next Header   | Next Header = X |
      +---------------+-----------------+-------------------

   There is a proposal for a transport-independent BIER encapsulation
   header which is applicable regardless of the underlying transport
   technology.  As described in [I-D.xu-bier-encapsulation] and
   [I-D.zhang-bier-bierin6], the BIER header, and the payload following
   it, can be combined as an IPv6 payload, and be indicated by a new
   Upper-layer IPv6 Next-Header value.  A unicast IPv6 destination
   address is used for the replication and changes when replicating a
   packet out to a neighbor.




McBride, et al.         Expires November 30, 2019               [Page 9]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   Such proposals may include a request to IANA to allocate an IPv6
   Next-Header code from "Assigned Internet Protocol Numbers".

5.5.  Tunneling BIER in a IPv6 tunnel

      +---------------+-----------------+------------+----------------
      |  IPv6 header  | IPv6 Ext header | GRE header |
      |               |    (optional)   |            | BIER Hdr +
      |               |                 |            | payload as GRE
      | Next Header   |   Next Header   | Proto = X  | Payload
      +---------------+-----------------+------------+----------------

   A generic IPv6 Tunnel could be used to encapsulate the bier packet
   within an IPv6 domain.

   GRE is a mechanism by which any ethernet payload can be carried by an
   IP GRE tunnel due to the 16-bits 'Protocol Type' field.  Both IPv4
   and IPv6 can be used to carry GRE.  The Ethernet type codepoint
   0xAB37, defined for BIER, can be used in a GRE header to indicate the
   subsequent BIER header and payload in an IPv6 network.

      +---------------+-----------------+------------+----------------
      |  IPv6 header  | IPv6 Ext header | UDP header |
      |               |    (optional)   |            | BIER Hdr +
      |               |                 |            | payload as UDP
      | Next Header   |   Next Header   | DPort = X  | Payload
      +---------------+-----------------+------------+----------------

   UDP-based tunneling is another mechanism which uses a specific UDP
   port to indicate a UDP payload format.  Both IPv4 and IPv6 can
   support UDP.  Such UDP-based tunnels can be used for BIER in a IPv6
   network by defining a new UDP port to indicate the BIER header and
   payload.

6.  IANA Considerations

   Some BIERv6 encapsulation proposals do not require any action from
   IANA while other proposals require new BIER Destination Option
   codepoints from IPv6 sub-registries, new "Next header" values, or
   require new IP Protocol codes.  This document, however, does not
   require anything from IANA.

7.  Security Considerations

   There are no security issues introduced by this draft.






McBride, et al.         Expires November 30, 2019              [Page 10]


Internet-Draft           BIER IPv6 Requirements                 May 2019


8.  Acknowledgement

   Thank you to Eric Rosen for his listed set of requirements on the
   bier wg list.

9.  Normative References

   [I-D.pfister-bier-over-ipv6]
              Pfister, P. and I. Wijnands, "An IPv6 based BIER
              Encapsulation and Encoding", draft-pfister-bier-over-
              ipv6-01 (work in progress), October 2016.

   [I-D.xie-bier-ipv6-encapsulation]
              Xie, J., Geng, L., McBride, M., Dhanaraj, S., Yan, G., and
              Y. Xia, "Encapsulation for BIER in Non-MPLS IPv6
              Networks", draft-xie-bier-ipv6-encapsulation-00 (work in
              progress), March 2019.

   [I-D.xu-bier-encapsulation]
              Xu, X., somasundaram.s@alcatel-lucent.com, s., Jacquenet,
              C., Raszuk, R., and Z. Zhang, "A Transport-Independent Bit
              Index Explicit Replication (BIER) Encapsulation Header",
              draft-xu-bier-encapsulation-06 (work in progress),
              September 2016.

   [I-D.zhang-bier-bierin6]
              Zhang, Z. and T. Przygienda, "BIER in IPv6", draft-zhang-
              bier-bierin6-02 (work in progress), October 2018.

   [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>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.



McBride, et al.         Expires November 30, 2019              [Page 11]


Internet-Draft           BIER IPv6 Requirements                 May 2019


   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [RFC8354]  Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R.,
              Ed., and M. Townsley, "Use Cases for IPv6 Source Packet
              Routing in Networking (SPRING)", RFC 8354,
              DOI 10.17487/RFC8354, March 2018,
              <https://www.rfc-editor.org/info/rfc8354>.

Authors' Addresses

   Mike McBride
   Futurewei

   Email: michael.mcbride@futurewei.com


   Jingrong Xie
   Huawei

   Email: xiejingrong@huawei.com


   Senthil Dhanaraj
   Huawei

   Email: senthil.dhanaraj@huawei.com


   Rajiv Asati
   Cisco

   Email: rajiva@cisco.com















McBride, et al.         Expires November 30, 2019              [Page 12]


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