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

Versions: (draft-hou-pim-drlb) 00 01 02 03 04 05 06 07 08 09 10

Network Working Group                                             Y. Cai
Internet-Draft                                                     H. Ou
Intended status: Standards Track                           Alibaba Group
Expires: April 25, 2019                                    S. Vallepalli
                                                               M. Mishra
                                                               S. Venaas
                                                     Cisco Systems, Inc.
                                                                A. Green
                                                         British Telecom
                                                        October 22, 2018


                  PIM Designated Router Load Balancing
                         draft-ietf-pim-drlb-09

Abstract

   On a multi-access network, one of the PIM routers is elected as a
   Designated Router (DR).  On the last hop LAN, the PIM DR is
   responsible for tracking local multicast listeners and forwarding
   traffic to these listeners if the group is operating in PIM-SM.  This
   document specifies a modification to the PIM-SM protocol that allows
   more than one of these last hop routers to be selected, so that the
   forwarding load can be distributed among these routers.

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 April 25, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





Cai, et al.              Expires April 25, 2019                 [Page 1]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Functional Overview . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  GDR Candidates  . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Hash Mask and Hash Algorithm  . . . . . . . . . . . . . .   7
     4.3.  Modulo Hash Algorithm . . . . . . . . . . . . . . . . . .   8
       4.3.1.  Limitations . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  PIM Hello Options . . . . . . . . . . . . . . . . . . . .   9
   5.  Hello Option Formats  . . . . . . . . . . . . . . . . . . . .  10
     5.1.  PIM DR Load Balancing Capability (DRLBC) Hello Option . .  10
     5.2.  PIM DR Load Balancing GDR (DRLBGDR) Hello Option  . . . .  10
   6.  Protocol Specification  . . . . . . . . . . . . . . . . . . .  11
     6.1.  PIM DR Operation  . . . . . . . . . . . . . . . . . . . .  11
     6.2.  PIM GDR Candidate Operation . . . . . . . . . . . . . . .  12
       6.2.1.  Router Receives New DRLBGDR . . . . . . . . . . . . .  13
       6.2.2.  Router Receives Updated DRLBGDR . . . . . . . . . . .  13
     6.3.  PIM Assert Modification . . . . . . . . . . . . . . . . .  14
   7.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.  Manageability Considerations  . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Initial registry  . . . . . . . . . . . . . . . . . . . .  16
     9.2.  Assignment of new message types . . . . . . . . . . . . .  16
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   On a multi-access LAN such as an Ethernet, one of the PIM routers is
   elected as a DR.  The PIM DR has two roles in the PIM-SM protocol.
   On the first hop LAN, the PIM DR is responsible for registering an
   active source with the Rendezvous Point (RP) if the group is



Cai, et al.              Expires April 25, 2019                 [Page 2]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   operating in PIM-SM.  On the last hop LAN, the PIM DR is responsible
   for tracking local multicast listeners and forwarding to these
   listeners if the group is operating in PIM-SM.

   Consider the following last hop LAN in Figure 1:

                             (core networks)
                              |     |     |
                              |     |     |
                             R1    R2     R3
                              |     |     |
                           --(last hop LAN)--
                                    |
                                    |
                            (many receivers)

                       Figure 1: Last Hop LAN

   Assume R1 is elected as the Designated Router.  According to
   [RFC7761], R1 will be responsible for forwarding traffic to that LAN
   on behalf of any local members.  In addition to keeping track of IGMP
   and MLD membership reports, R1 is also responsible for initiating the
   creation of source and/or shared trees towards the senders or the
   RPs.

   Forcing sole data plane forwarding responsibility on the PIM DR
   uncovers a limitation in the protocol.  In comparison, even though an
   OSPF DR or an IS-IS DIS handles additional duties while running the
   OSPF or IS-IS protocols, they are not required to be solely
   responsible for forwarding packets for the network.  On the other
   hand, on a last hop LAN, only the PIM DR is asked to forward packets
   while the other routers handle only control traffic (and perhaps drop
   packets due to RPF failures).  Hence the forwarding load of a last
   hop LAN is concentrated on a single router.

   This leads to several issues.  One of the issues is that the
   aggregated bandwidth will be limited to what R1 can handle towards
   this particular interface.  It is very common that the last hop LAN
   consists of switches that run IGMP/MLD or PIM snooping.  This allows
   the forwarding of multicast packets to be restricted only to segments
   leading to receivers who have indicated their interest in multicast
   groups using either IGMP or MLD.  The emergence of the switched
   Ethernet allows the aggregated bandwidth to exceed, sometimes by a
   large number, that of a single link.  For example, let us modify
   Figure 1 and introduce an Ethernet switch in Figure 2.






Cai, et al.              Expires April 25, 2019                 [Page 3]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


                            (core networks)
                             |     |     |
                             |     |     |
                            R1    R2     R3
                             |     |     |
                          +=gi0===gi1===gi2=+
                          +                 +
                          +      switch     +
                          +                 +
                          +=gi4===gi5===gi6=+
                             |     |     |
                            H1    H2     H3


               Figure 2: Last Hop Network with Ethernet Switch




   Let us assume that each individual link is a Gigabit Ethernet.  Each
   router, R1, R2 and R3, and the switch have enough forwarding capacity
   to handle hundreds of Gigabits of data.

   Let us further assume that each of the hosts requests 500 Mbps of
   unique multicast data.  This totals to 1.5 Gbps of data, which is
   less than what each switch or the combined uplink bandwidth across
   the routers can handle, even under failure of a single router.

   On the other hand, the link between R1 and switch, via port gi0, can
   only handle a throughput of 1Gbps.  And if R1 is the only DR (the PIM
   DR elected using the procedure defined by [RFC7761]) at least 500
   Mbps worth of data will be lost because the only link that can be
   used to draw the traffic from the routers to the switch is via gi0.
   In other words, the entire network's throughput is limited by the
   single connection between the PIM DR and the switch (or the last hop
   LAN as in Figure 1).

   Another important issue is related to failover.  If R1 is the only
   forwarder on the last hop router for a shared LAN, when R1 goes out
   of service, multicast forwarding for the entire LAN has to be rebuilt
   by the newly elected PIM DR.  However, if there was a way that
   allowed multiple routers to forward to the LAN for different groups,
   failure of one of the routers would only lead to disruption to a
   subset of the flows, therefore improving the overall resilience of
   the network.






Cai, et al.              Expires April 25, 2019                 [Page 4]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   There is a limitation in the hash algorithm used in this document,
   but this document provides the option to have different and more
   consistent hash algorithms in the future.

   This document specifies a modification to the PIM-SM protocol that
   allows more than one of these routers, called Group Designated
   Routers (GDR) to be selected so that the forwarding load can be
   distributed among a number of routers.

2.  Terminology

   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 [RFC2119].

   With respect to PIM, this document follows the terminology that has
   been defined in [RFC7761].

   This document also introduces the following new acronyms:

   o  GDR: GDR stands for "Group Designated Router".  For each multicast
      flow, either a (*,G) for ASM, or an (S,G) for SSM, a hash
      algorithm (described below) is used to select one of the routers
      as a GDR.  The GDR is responsible for initiating the forwarding
      tree building process for the corresponding multicast flow.

   o  GDR Candidate: a last hop router that has the potential to become
      a GDR.  A GDR Candidate must have the same DR priority and must
      run the same GDR election hash algorithm as the DR router.  It
      must send and process new PIM Hello Options as defined in this
      document.  There might be more than one GDR Candidate on a LAN,
      but only one can become GDR for a specific multicast flow.

3.  Applicability

   The extension specified in this document applies to PIM-SM last hop
   routers only.  It does not alter the behavior of a PIM DR on the
   first hop network.  This is because the source tree is built using
   the IP address of the sender, not the IP address of the PIM DR that
   sends the registers towards the RP.  The load balancing between first
   hop routers can be achieved naturally if an IGP provides equal cost
   multiple paths (which it usually does in practice).  Also
   distributing the load to do registering does not justify the
   additional complexity required to support it.







Cai, et al.              Expires April 25, 2019                 [Page 5]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


4.  Functional Overview

   In the PIM DR election as defined in [RFC7761], when multiple last
   hop routers are connected to a multi-access LAN (for example, an
   Ethernet), one of them is elected to act as PIM DR.  The PIM DR is
   responsible for sending local Join/Prune messages towards the RP or
   source.  In order to elect the PIM DR, each PIM router on the LAN
   examines the received PIM Hello messages and compares its own DR
   priority and IP address with those of its neighbors.  The router with
   the highest DR priority is the PIM DR.  If there are multiple such
   routers, their IP addresses are used as the tie-breaker, as described
   in [RFC7761].

   In order to share forwarding load among last hop routers, besides the
   normal PIM DR election, the GDR is also elected on the last hop
   multi-access LAN.  There is only one PIM DR on the multi-access LAN,
   but there might be multiple GDR Candidates.

   For each multicast flow, that is, (*,G) for ASM and (S,G) for SSM, a
   hash algorithm is used to select one of the routers to be the GDR.  A
   new DR Load Balancing Capability (DRLBC) PIM Hello Option, which
   contains hash algorithm type, is announced by routers on interfaces
   where this specification is enabled.  Last hop routers with the new
   DRLBC Option advertised in its Hello, and using the same GDR election
   hash algorithm and the same DR priority as the PIM DR, are considered
   as GDR Candidates.

   Hash Masks are defined for Source, Group and RP separately, in order
   to handle PIM ASM/SSM.  The masks, as well as a sorted list of GDR
   Candidate Addresses, are announced by the DR in a new DR Load
   Balancing GDR (DRLBGDR) PIM Hello Option.

   A hash algorithm based on the announced Source, Group, or RP masks
   allows one GDR to be assigned to a corresponding multicast state.
   And that GDR is responsible for initiating the creation of the
   multicast forwarding tree for multicast traffic.

4.1.  GDR Candidates

   GDR is the new concept introduced by this specification.  GDR
   Candidates are routers eligible for GDR election on the LAN.  To
   become a GDR Candidate, a router MUST support this specification,
   have the same DR priority and run the same GDR election hash
   algorithm as the DR on the LAN.

   For example, assume there are 4 routers on the LAN: R1, R2, R3 and
   R4, which all support this specification.  R1, R2 and R3 have the
   same DR priority while R4's DR priority is less preferred.  In this



Cai, et al.              Expires April 25, 2019                 [Page 6]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   example, R4 will not be eligible for GDR election, because R4 will
   not become a PIM DR unless all of R1, R2 and R3 go out of service.

   Furthermore, assume router R1 wins the PIM DR election, R1 and R2 run
   the same hash algorithm for GDR election, while R3 runs a different
   one.  In this case, only R1 and R2 will be eligible for GDR election,
   while R3 will not.

   As a DR, R1 will include its own Load Balancing Hash Masks and the
   identity of R1 and R2 (the GDR Candidates) in its DRLBGDR Hello
   Option.

4.2.  Hash Mask and Hash Algorithm

   A Hash Mask is used to extract a number of bits from the
   corresponding IP address field (32 for v4, 128 for v6) and calculate
   a hash value.  A hash value is used to select a GDR from GDR
   Candidates advertised by PIM DR.  For example, 0.0.255.0 defines a
   Hash Mask for an IPv4 address that masks the first, the second, and
   the fourth octets.

   There are three Hash Masks defined:

   o  RP Hash Mask

   o  Source Hash Mask

   o  Group Hash Mask

   The hash masks need to be configured on the PIM routers that can
   potentially become a PIM DR, unless the implementation provides
   default Hash Mask values.  An implementation SHOULD provide masks
   with default values 255.255.255.255 (IPv4) and
   FFFF:FFFF:FFFF:FFFF:FFFFF:FFFF:FFFF:FFFF (IPv6).

   o  If the group is in ASM mode and the RP Hash Mask announced by the
      PIM DR is not 0, calculate the value of hashvalue_RP [Section 4.3]
      to determine GDR.

   o  If the group is in ASM mode and the RP Hash Mask announced by the
      PIM DR is 0, obtain the value of hashvalue_Group [Section 4.3 ] to
      determine GDR.

   o  If the group is in SSM mode, use hashvalue_SG [Section 4.3] to
      determine GDR.

   A simple Modulo hash algorithm is defined in this document.  However,
   to allow another hash algorithms to be used, a 1-octet "Hash



Cai, et al.              Expires April 25, 2019                 [Page 7]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   Algorithm Type" field is included in DRLBC Hello Option to specify
   the hash algorithm used by a last hop router.

   If different hash algorithm types are advertised among last hop
   routers, only last hop routers running the same hash algorithm as the
   DR (and having the same DR priority as the DR) are eligible for GDR
   election.

4.3.  Modulo Hash Algorithm

   The Modulo hash algorithm is discussed here with a detailed
   description on hashvalue_RP.  The same algorithm is described in
   brief for hashvalue_Group using the group address instead of the RP
   address for an ASM group with zero RP_hashmask, and also with
   hashvalue_SG for a the source address of an (S,G), instead of the RP
   address,

   o  For ASM groups, with a non-zero RP_Hash Mask, hash value is
      calculated as:

         hashvalue_RP = (((RP_address & RP_hashmask) >> N) & 0xFFFF) % M

         RP_address is the address of the RP defined for the group.  N
         is the number of zeroes, counted from the least significant bit
         of the RP_hashmask.  M is the number of GDR Candidates.

         For example, Router X with IPv4 address 203.0.113.1 receives a
         DRLBGDR Hello Option from the DR, which announces RP Hash Mask
         0.0.255.0 and a list of GDR Candidates, sorted by IP addresses
         from high to low: 203.0.113.3, 203.0.113.2 and 203.0.113.1.
         The ordinal number assigned to those addresses would be:

         0 for 203.0.113.3; 1 for 203.0.113.2; 2 for 203.0.113.1 (Router
         X)

         Assume there are 2 RPs: RP1 192.0.2.1 for Group1 and RP2
         198.51.100.2 for Group2.  Following the modulo hash algorithm:

         N is 8 for 0.0.255.0, and M is 3 for the total number of GDR
         Candidates.  The hashvalue_RP for RP1 192.0.2.1 is:

         (((192.0.2.1 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 2 % 3 = 2

         matches the ordinal number assigned to Router X.  Router X will
         be the GDR for Group1, which uses 192.0.2.1 as the RP.

         The hashvalue_RP for RP2 198.51.100.2 is:




Cai, et al.              Expires April 25, 2019                 [Page 8]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


         (((198.51.100.2 & 0.0.255.0) >> 8) & 0xFFFF % 3) = 100 % 3 = 1

         which is different from Router X's ordinal number(2) hence,
         Router X will not be GDR for Group2.

   o  If RP_hashmask is 0, a hash value for an ASM group is calculated
      using the Group Hash Mask:

         hashvalue_Group = (((Group_address & Group_hashmask) >> N) &
         0xFFFF) % M

         Compare hashvalue_Group with Ordinal number assigned to Router
         X, to decide if Router X is the GDR.

   o  For SSM groups, a hash value is calculated using both the Source
      and Group Hash Mask:

         hashvalue_SG = ((((Source_address & Source_hashmask) >> N_S) &
         0xFFFF) ^ (((Group_address & Group_hashmask) >> N_G) & 0xFFFF))
         % M

4.3.1.  Limitations

   The Modulo Hash Algorithm has poor failover characteristics when a
   shared LAN has more than two GDRs.  In the case of more than two GDRs
   on a LAN, when one GDR fails, all of the groups may be reassigned to
   a new GDR, even if they were not assigned to the failed GDR.
   However, many deployments use only two routers on a shared LAN for
   redundancy purposes.  Future work may define new hash algorithms
   where only groups assigned to the failed GDR get reassigned.

4.4.  PIM Hello Options

   When a last hop PIM router sends a PIM Hello for an interface with
   this specification enabled, it includes a new option, called "Load
   Balancing Capability (DRLBC)".

   Besides this DRLBC Hello Option, the elected PIM DR also includes a
   new "DR Load Balancing GDR (DRLBGDR) Hello Option".  The DRLBGDR
   Hello Option consists of three Hash Masks as defined above and also a
   sorted list of GDR Candidate addresses on the last hop LAN.

   The elected PIM DR uses DRLBC Hello Option advertised by all routers
   on the last hop LAN to compose the DRLBGDR Option.  The GDR
   Candidates use the DRLBGDR Hello Option advertised by the PIM DR to
   calculate the hash value.





Cai, et al.              Expires April 25, 2019                 [Page 9]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


5.  Hello Option Formats

5.1.  PIM DR Load Balancing Capability (DRLBC) Hello Option

      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 = TBD          |         Length = 4            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Reserved                  |   Hash Type   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


          Figure 3: Capability Hello Option

      Type: TBD

      Length: 4

      Hash Algorithm Type: 0 for Modulo hash algorithm

   This DRLBC Hello Option MUST be advertised by last hop routers on
   interfaces with this specification enabled.

5.2.  PIM DR Load Balancing GDR (DRLBGDR) Hello Option

     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 = TBD          |         Length                |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Group Mask                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Source Mask                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            RP Mask                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    GDR Candidate Address(es)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


          Figure 4: GDR Hello Option


      Type: TBD

      Length: (3 + n) x (4 or 16) where n is the number of GDR
      candidates.



Cai, et al.              Expires April 25, 2019                [Page 10]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


      Group Mask (32/128 bits): Mask

      Source Mask (32/128 bits): Mask

      RP Mask (32/128 bits): Mask



         All masks MUST be in the same address family as the Hello IP
         header.

      GDR Address (32/128 bits): Address(es) of GDR Candidate(s)

         All addresses must be in the same address family as the Hello
         IP header.  The addresses are sorted in descending order.  The
         order is converted to the ordinal number associated with each
         GDR candidate in hash value calculation.  For example, if
         addresses advertised are R3, R2, R1, the ordinal number
         assigned to R3 is 0, to R2 is 1 and to R1 is 2.

         If the "Interface ID" option, as specified in [RFC6395], is
         present in a GDR Candidate's PIM Hello message, and the "Router
         ID" portion is non-zero:

         +  For IPv4, the "GDR Candidate Address" will be set directly
            to the "Router ID".

         +  For IPv6, the "GDR Candidate Address" will be set to the
            IPv4-IPv6 translated address of the "Router ID", as
            described in [RFC4291], that is the "Router-ID" is appended
            to the prefix of 96 bits of zeroes.

         If the "Interface ID" option is not present in a GDR
         Candidate's PIM Hello message, or if the "Interface ID" option
         is present but the "Router ID" field is zero, the "GDR
         Candidate Address" will be the IPv4 or IPv6 source address of
         the PIM Hello message.

         This DRLBGDR Hello Option MUST only be advertised by the
         elected PIM DR.

6.  Protocol Specification

6.1.  PIM DR Operation

   The DR election process is still the same as defined in [RFC7761].  A
   DR that has this specification enabled on an interface advertises the
   new DRLBGDR Hello Option, which contains mask values from user



Cai, et al.              Expires April 25, 2019                [Page 11]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   configuration, followed by a sorted list of GDR Candidate Addresses,
   from the highest value to the lowest value.  Moreover, same as non-DR
   routers, the DR also advertises DRLBC Hello Option to indicate its
   capability of supporting this specification and the type of its GDR
   election hash algorithm.

   If a PIM DR receives a PIM Hello with the DRLBGDR Option, the PIM DR
   SHOULD ignore the TLV.

   If a PIM DR receives a neighbor DRLBC Hello Option, which contains
   the same hash algorithm type as the DR, and the neighbor has the same
   DR priority as the DR, PIM DR SHOULD consider the neighbor as a GDR
   Candidate and insert the GDR Candidate's Address into the sorted list
   of the DRLBGDR Option.  However, the DR MAY have policies limiting
   which GDR Candidates, or the number of GDR Candidates to include.

6.2.  PIM GDR Candidate Operation

   When an IGMP/MLD report is received, without this specification, only
   the PIM DR will handle the join and potentially run into the issues
   described earlier.  Using this specification, a hash algorithm is
   used by the GDR Candidates to determine which router is going to be
   responsible for building forwarding trees on behalf of the host.

   If this specification is enabled on an interface, the router MUST
   include the DRLBC Hello Option in its PIM Hello on the interface.
   Note that the presence of the DRLBC Option in PIM Hello does not
   guarantee that this router would be considered as a GDR candidate.
   Once DR election is done, the DRLBGDR Hello Option would be received
   from the current PIM DR on the link which would contain a list of
   GDRs selected by the PIM DR.

   A router only acts as a GDR candidate if it is included in the GDR
   list of the DRLBGDR Hello Option.

   A GDR Candidate may receive a DRLBGDR Hello Option from the PIM DR
   with different Hash Masks from those the candidate was configured
   with.  The GDR Candidate MUST use the Hash Masks advertised by the
   PIM DR to calculate the hash value.

   A GDR Candidate MUST ignore the DRLBGDR Hello Option if it is
   received from a PIM router which is not the DR.

   If the PIM DR does not support this specification, GDR election will
   not take place, and only the PIM DR joins the multicast tree.






Cai, et al.              Expires April 25, 2019                [Page 12]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


6.2.1.  Router Receives New DRLBGDR

   The first time a router receives a DRLBGDR option from the PIM DR, it
   MUST process the option and check if it is in the GDR list.

   1.  If a router is not listed as a GDR candidate in DRLBGDR, no
       action is needed.

   2.  If a router is listed as a GDR candidate in DRLBGDR, then it MUST
       process each of the groups, or source and group pairs if SSM, in
       the IGMP/MLD reports.  The masks are announced in the PIM Hello
       by the DR in the DRLBGDR Hello Option.  For each group in the
       reports that is in ASM mode, and each source and group pair if
       the group is in SSM mode, it (PIM Router) needs to run the hash
       algorithm (described in section 4.3) based on the announced
       Source, Group or RP masks to determine if it is the GDR for
       specified group, or source and group pair.  If the hash result is
       to be the GDR for the multicast flow, it does build the multicast
       forwarding tree.  If it is not the GDR for the multicast flow, no
       action is needed.

6.2.2.  Router Receives Updated DRLBGDR

   If a router (GDR or non GDR) receives an unchanged DRLBGDR from the
   current PIM DR, no action is needed.

   If a router (GDR or non GDR) receives a new or modified DRLBGDR from
   the current PIM DR, it requires processing as described below:

   1.  If it was included in the previous GDR list, and still is
       included in the new GDR list: It needs to process each of the
       groups, or source and group pairs if the group is in SSM mode,
       and run the hash algorithm to check if it is still the GDR for
       the given group, or source and group pair if SSM.

          If it was the GDR for a group, or source and group pair if
          SSM, and the new hash result chose it as the GDR, then no
          processing is required.

          If it was the GDR for a group, or source and group pair if
          SSM, earlier and now it is no longer the GDR, then it sets its
          assert metric for the multicast flow to be
          (PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.

          If it was not the GDR for a group, or source and group pair if
          SSM, earlier, and the new hash does not make it GDR, then no
          processing is required.




Cai, et al.              Expires April 25, 2019                [Page 13]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


          If it was not the GDR for an earlier group, or source and
          group pair if SSM, and now becomes the GDR, it starts building
          multicast forwarding tree for this flow.

   2.  If it was included in the previous GDR list, but is not included
       in the new GDR list: It needs to process each of the groups, or
       source and group pairs if the group is in SSM mode.

          If it was the GDR for a group, or source and group pair if
          SSM, it sets its assert metric for the multicast flow to be
          (PIM_ASSERT_INFINITY - 1), as explained in Section 6.3.

          If it was not the GDR, then no processing is required.

   3.  If it was not included in the previous GDR list, but is included
       in the new GDR list, the router MUST run the hash algorithm for
       each of the groups, source and group pairs if SSM.

          If it is not the GDR for a group, or source and group pair if
          SSM, no processing is required.

          If it is hashed as the GDR, it needs to build a multicast
          forwarding tree.

6.3.  PIM Assert Modification

   It is possible that the identity of the GDR might change in the
   middle of an active flow.  Examples when this could happen include:

      When a new PIM router comes up

      When a GDR restarts

   When the GDR changes, existing traffic might be disrupted.
   Duplicates or packet loss might be observed.  To illustrate the case,
   consider the following scenario where there are two flows G1 and G2.
   R1 is the GDR for G1, and R2 is the GDR for G2.  When R3 comes up
   online, it is possible that R3 becomes GDR for both G1 and G2, hence
   R3 starts to build the forwarding tree for G1 and G2.  If R1 and R2
   stop forwarding before R3 completes the process, packet loss might
   occur.  On the other hand, if R1 and R2 continue forwarding while R3
   is building the forwarding trees, duplicates might occur.

   This is not a typical deployment scenario but might still happen.
   Here we describe a mechanism to minimize the impact.  We essentially
   want to minimize packet loss.  Therefore, we would allow a small
   amount of duplicates and depend on PIM Assert to minimize the
   duplication.



Cai, et al.              Expires April 25, 2019                [Page 14]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   When the role of GDR changes as above, instead of immediately
   stopping forwarding, R1 and R2 continue forwarding to G1 and G2
   respectively, while, at the same time, R3 build forwarding trees for
   G1 and G2.  This will lead to PIM Asserts.

   With the introduction of GDR, the following modification to the
   Assert packet MUST be done: if a router enables this specification on
   its downstream interface, but it is not a GDR (before network event
   it was GDR), it would adjust its Assert metric to
   (PIM_ASSERT_INFINITY - 1).

   Using the above example, for G1, assume R1 and R3 agree on the new
   GDR, which is R3.  R1 will set its Assert metric as
   (PIM_ASSERT_INFINITY - 1).  That will make R3, which has normal
   metric in its Assert as the Assert winner.

   For G2, assume it takes a slightly longer time for R2 to find out
   that R3 is the new GDR and still considers itself being the GDR while
   R3 already has assumed the role of GDR.  Since both R2 and R3 think
   they are GDRs, they further compare their metric and IP addresses.
   If R3 has the better routing metric, or the same metric but a better
   tie-breaker, the result will be consistent during GDR selection.  If
   unfortunately, R2 has the better metric or the same metric but a
   better tie-breaker, R2 will become the Assert winner and continues to
   forward traffic.  This will continue until:

   The next PIM Hello Option from DR selects R3 as the GDR.  R3 will
   then build the forwarding tree and send an Assert.

   The process continues until R2 agrees to the selection of R3 as the
   GDR, and sets its own Assert metric to (PIM_ASSERT_INFINITY - 1),
   which will make R3 the Assert winner.  During the process, we will
   see intermittent duplication of traffic but packet loss will be
   minimized.  In the unlikely case that R2 never relinquishes its role
   as GDR (while every other router thinks otherwise), the proposed
   mechanism also helps to keep the duplication to a minimum until
   manual intervention takes place to remedy the situation.

7.  Compatibility

   In the case of a hybrid Ethernet shared LAN (where some PIM routers
   enable the specification defined in this document, and some do not)

   o  If a router which does not support this specification becomes the
      DR on the LAN, then it is the only router acting as a DR, and
      there will be no load-balancing.





Cai, et al.              Expires April 25, 2019                [Page 15]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   o  If a router which does not support this specification becomes a
      non-DR on link, then it acts as non-DR defined in [RFC7761], and
      it will not take part in any load-balancing.

8.  Manageability Considerations

   Only the routers announcing the same Hash Algorithm as the DR would
   be considered as GDR candidates.  Network administrators need to make
   sure that the desired set of routers announce the same algorithm.
   Migration between different algorithm types is not considered in this
   document.

9.  IANA Considerations

   IANA has temporarily assigned type 34 for the PIM DR Load Balancing
   Capability (DRLBC) Hello Option, and type 35 for the PIM DR Load
   Balancing GDR (DRLBGDR) Hello Option.  IANA is requested to make
   these assignments permanent when this document is published as an
   RFC.  The string TBD should be replaced by the assigned values
   accordingly.  This document requests IANA to create a DRLB hash type
   registry.  This should be placed in the "Protocol Independent
   Multicast (PIM)" branch of the tree.

9.1.  Initial registry

   The initial content of the registry should be as follows.



    Type   Name                                     Reference
    ------ ---------------------------------------- --------------------
    0      Hash algorithm modulo                    This document
    1-255  Unassigned


9.2.  Assignment of new message types

   Assignment of new message types is done according to the "IETF
   Review" model, see [RFC5226].

10.  Security Considerations

   Security of the new DR Load Balancing PIM Hello Options is only
   guaranteed by the security of PIM Hello messages, so the security
   considerations for PIM Hello messages as described in PIM-SM
   [RFC7761] apply here.





Cai, et al.              Expires April 25, 2019                [Page 16]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


11.  Acknowledgement

   The authors would like to thank Steve Simlo, Taki Millonis for
   helping with the original idea, Bill Atwood, Bharat Joshi for review
   comments, Toerless Eckert and Rishabh Parekh for helpful conversation
   on the document.

   Special thanks to Anish Kachinthaya, Anvitha Kachinthaya and Jake
   Holland for reviewing the document and providing comments.

12.  References

12.1.  Normative References

   [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>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [RFC6395]  Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
              Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,
              October 2011, <https://www.rfc-editor.org/info/rfc6395>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

12.2.  Informative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

Authors' Addresses

   Yiqun Cai
   Alibaba Group

   Email: yiqun.cai@alibaba-inc.com





Cai, et al.              Expires April 25, 2019                [Page 17]


Internet-Draft    PIM Designated Router Load Balancing      October 2018


   Heidi Ou
   Alibaba Group


   Sri Vallepalli
   Cisco Systems, Inc.
   3625 Cisco Way
   San Jose  CA 95134
   USA

   Email: svallepa@cisco.com


   Mankamana Mishra
   Cisco Systems, Inc.
   821 Alder Drive,
   Milpitas  CA 95035
   USA

   Email: mankamis@cisco.com


   Stig Venaas
   Cisco Systems, Inc.
   Tasman Drive
   San Jose  CA 95134
   USA

   Email: stig@cisco.com


   Andy Green
   British Telecom
   Adastral Park
   Ipswich  IP5 2RE
   United Kingdom

   Email: andy.da.green@bt.com













Cai, et al.              Expires April 25, 2019                [Page 18]


Html markup produced by rfcmarkup 1.128b, available from https://tools.ietf.org/tools/rfcmarkup/