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Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit Replication (BIER) Layer

Document Type Active Internet-Draft (bier WG)
Authors Greg Mirsky , Tony Przygienda , Andrew Dolganow
Last updated 2023-09-20
Replaces draft-mirsky-bier-path-mtu-discovery
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status (None)
Additional resources Mailing list discussion
Stream WG state WG Consensus: Waiting for Write-Up
Document shepherd Hooman Bidgoli
IESG IESG state I-D Exists
Consensus boilerplate Yes
Telechat date (None)
Responsible AD (None)
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BIER Working Group                                             G. Mirsky
Internet-Draft                                                  Ericsson
Intended status: Standards Track                           T. Przygienda
Expires: 23 March 2024                                  Juniper Networks
                                                             A. Dolganow
                                                       20 September 2023

Path Maximum Transmission Unit Discovery (PMTUD) for Bit Index Explicit
                        Replication (BIER) Layer


   This document describes Path Maximum Transmission Unit Discovery
   (PMTUD) in Bit Indexed Explicit Replication (BIER) layer.

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

   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 23 March 2024.

Copyright Notice

   Copyright (c) 2023 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 (
   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
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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions used in this document . . . . . . . . . . . .   2
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   2
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   3.  PMTUD Mechanism for BIER  . . . . . . . . . . . . . . . . . .   4
     3.1.  Data TLV for BIER Ping  . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgment  . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In packet switched networks, when a host seeks to transmit data to a
   target destination, the data is transmitted as a set of packets.  In
   many cases, it is more efficient to use the largest size packets that
   are less than or equal to the least Maximum Transmission Unit (MTU)
   for any forwarding device along the routed path to the IP destination
   for these packets.  Such "least MTU" is known as Path MTU (PMTU).
   Fragmentation or packet drop, silent or not, may occur on hops along
   the route where an MTU is smaller than the size of the datagram.  To
   avoid any of the listed above behaviors, the packet source must find
   the value of the least MTU, i.e., PMTU, that will be encountered
   along the route that a set of packets will follow to reach the given
   set of destinations.  Such MTU determination along a specific path is
   referred to as path MTU discovery (PMTUD).

   [RFC8279] introduces and explains Bit Index Explicit Replication
   (BIER) architecture and how it supports the forwarding of multicast
   data packets.  [I-D.ietf-bier-ping] introduced BIER Ping as a
   transport-independent OAM mechanism to detect and localize failures
   in the BIER data plane.  This document specifies how BIER Ping can be
   used to perform efficient PMTUD in the BIER domain.

1.1.  Conventions used in this document

1.1.1.  Terminology

   This document uses terminology defined in [RFC8279].  Familiarity
   with this specification and the terminology used is expected.

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1.1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Problem Statement

   [I-D.ietf-bier-oam-requirements] sets forth the requirement to define
   PMTUD protocol for BIER domain.  This document describes the
   extension to [I-D.ietf-bier-ping] for use in the BIER PMTUD solution.

   Current PMTUD mechanisms ([RFC1191], [RFC8201], and [RFC4821]) are
   primarily targeted to work on point-to-point, i.e. unicast paths.
   These mechanisms use packet fragmentation control by disabling
   fragmentation of the probe packet.  As a result, a transient node
   that cannot forward a probe packet that is bigger than its link MTU
   sends to the packet source an error notification, otherwise the
   packet destination may respond with a positive acknowledgment.  Thus,
   possibly through a series of iterations, varying the size of the
   probe packet, the packet source discovers the PMTU of the particular

   Applying such existing PMTUD solutions are inefficient for point-to-
   multipoint paths constructed for multicast traffic.  Probe packets
   must be flooded through the whole set of multicast distribution paths
   over and over again until the very last egress responds with a
   positive acknowledgment.  Consider the multicast network presented in
   Figure 1, where MTU on all links but one (B, D) is the same.  If MTU
   on the link (B, D) is smaller than the MTU on the other links, using
   existing PMTUD mechanism probes will unnecessarily flood to leaf
   nodes E, F, and G for the second and consecutive times and positive
   responses will be generated and received by root A repeatedly.

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                         --| D |
                 -----  /  -----
               --| B |--
              /  -----  \  -----
             /           --| E |
   -----    /              -----
   | A |---                -----
   -----    \            --| F |
             \  -----   /  -----
              --| C |--
                -----   \  -----
                         --| G |

                        Figure 1: Multicast network

3.  PMTUD Mechanism for BIER

   A BFIR selects a set of BFERs for the specific multicast
   distribution.  Such a BFIR determines, by explicitly controlling a
   subset of targeted BFERs and transmitting a series of probe packets,
   the MTU of that multicast distribution tree.  In the case of ECMP,
   BFIR MAY test each path by variating the value in the Entropy field.
   The critical step is that in case of failure at an intermediate BFR
   to forward towards the subset of targeted downstream BFERs, the BFR
   responds with a partial (compared to the one it received in the
   request) bitmask towards the originating BFIR in error notification.
   That allows for retransmission of the next probe with a smaller MTU
   address only towards the failed downstream BFERs instead of all BFERs
   addressed in the previous probe.  In the scenario discussed in
   Section 2 the second and all following (if needed) probes will be
   sent only to the node D since MTU discovery of E, F, and G has been
   completed already by the first probe successfully.

   Consider the network displayed in Figure 1 to be a presentation of a
   BIER domain and all nodes to be BFRs.  To discover MTU over BIER
   domain to BFERs D, F, E, and G BFIR A will use BIER Ping with Data
   TLV, defined in Section 3.1.  Size of the first probe set to M_max
   determined as minimal MTU value of BFIR's links to BIER domain.  As
   has been assumed in Section 2, MTUs of all links but the link (B, D)
   are the same.  Thus BFERs E, F, and G would receive BIER Echo Request
   and will send their respective replies to BFIR A.  BFR B may pass the
   packet which is too large to forward over egress link (B, D) to the
   appropriate network layer for error processing where it would be
   recognized as a BIER Echo Request packet.  BFR B MUST send BIER Echo
   Reply to BFIR A and MUST include Downstream Mapping TLV, defined in
   [I-D.ietf-bier-ping] setting its fields in the following fashion:

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   *  MTU SHOULD be set to the minimal MTU value among all egress BIER
      links, logical links between this and downstream BFRs, that could
      be used to reach B's downstream BFERs;

   *  Address Type MAY be set to any value defined in Section 3.3.4

   *  I flag MUST be cleared to direct the responding BFR not to include
      the Incoming SI-BitString TLV in the BIER Echo Response.

   *  Downstream Interface Address field MUST be zeroed.

   *  List of Sub-TLVs MUST include the Egress Bitstring sub-TLV with
      the list of all BFERs that cannot be reached because the egress
      MTU turned out to be too small.

   The BFIR will receive either of the two types of packets:

   *  a positive Echo Reply from one of BFERs to which the probe has
      been sent.  In this case, the bit corresponding to the BFER MUST
      be cleared from the bitmask string (BMS);

   *  a negative Echo Reply with bit string listing unreached BFERs and
      recommended MTU value MTU'.  The BFIR MUST add the bit string to
      its BMS and set the size of the next probe as min(MTU, MTU')

   If a negative Echo Reply is received, the BFIR MUST wait for the
   expiration of the Echo Request before transmitting the updated Echo
   Request.  If upon expiration of the Echo Request timer BFIR didn't
   receive any Echo Replies, then the size of the probe SHOULD be
   decreased.  There are scenarios when an implementation of the PMTUD
   would not decrease the size of the probe.  For example, suppose upon
   expiration of the Echo Request timer BFIR didn't receive any Echo
   Reply.  In that case, BFIR MAY continue to retransmit the probe using
   the initial size and MAY apply probe delay retransmission procedures.
   The algorithm used to delay retransmission procedures on BFIR is
   outside the scope of this specification.  The BFIR sends probes using
   BMS and locally defined retransmission procedures, but not more
   frequently than after the Echo Request timer expired, until either
   the bit string is clear, i.e., contains no set bits, or until the
   BFIR retransmission procedure terminates and PMTU discovery is
   declared unsuccessful.  In the case of convergence of the procedure,
   the size of the last probe indicates the PMTU size that can be used
   for all BFERs in the initial BMS without incurring fragmentation.

   Thus we conclude that in order to comply with the requirement in

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   *  a BFR SHOULD support PMTUD;

   *  a BFR MAY use defined per BIER sub-domain MTU value as initial MTU
      value for discovery or use it as MTU for this BIER sub-domain to
      reach BFERs;

   *  a BFIR MUST have a locally defined PMTUD probe retransmission

3.1.  Data TLV for BIER Ping

   There needs to be a control for probe size in order to support the
   BIER PMTUD.  Data TLV format is presented in Figure 2.

     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
    |          Type  (TBA1)         |             Length            |
    |                              Data                             |
    ~                                                               ~
    |                                                               |

                         Figure 2: Data TLV format

   *  Type: indicates Data TLV, to be allocated by IANA Section 4.

   *  Length: the length of the Data field in octets.

   *  Data: n octets (n = Length) of arbitrary data.  The receiver
      SHOULD ignore it.

4.  IANA Considerations

   IANA is requested to assign a new Type value for Data TLV Type from
   its registry of TLV and sub-TLV Types of BIER Ping as follows:

                  | Value | Description | Reference     |
                  | TBA1  |     Data    | This document |

                           Table 1: Data TLV Type

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5.  Security Considerations

   Routers that support PMTUD based on this document are subject to the
   same security considerations as defined in [I-D.ietf-bier-ping]

6.  Acknowledgment

   Authors greatly appreciate thorough review and the most detailed
   comments by Eric Gray.

7.  References

7.1.  Normative References

              Nainar, N. K., Pignataro, C., Chen, M., and G. Mirsky,
              "BIER Ping and Trace", Work in Progress, Internet-Draft,
              draft-ietf-bier-ping-12, 29 July 2023,

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              DOI 10.17487/RFC1191, November 1990,

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

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,

7.2.  Informative References

              Mirsky, G., Nainar, N. K., Chen, M., and S. Pallagatti,
              "Operations, Administration and Maintenance (OAM)

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              Requirements for Bit Index Explicit Replication (BIER)
              Layer", Work in Progress, Internet-Draft, draft-ietf-bier-
              oam-requirements-13, 10 August 2023,

   [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,

Authors' Addresses

   Greg Mirsky

   Tony Przygienda
   Juniper Networks

   Andrew Dolganow

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