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Versions: (draft-hou-pim-drlb) 00 01 02 03 04 05

Network Working Group                                          Yiqun Cai
Internet-Draft                                                 Microsoft
Intended status: Standards Track                          Sri Vallepalli
Expires: January 4, 2015                                        Heidi Ou
                                                     Cisco Systems, Inc.
                                                              Andy Green
                                                         British Telecom
                                                            July 3, 2014


                  PIM Designated Router Load Balancing
                       draft-ietf-pim-drlb-05.txt

Abstract

   On a multi-access network, one of the PIM routers is elected as a
   Designated Router (DR).  On the last hop network, the PIM DR is
   responsible for tracking local multicast listeners and forwarding
   traffic to these listeners if the group is operating in PIM-SM.  In
   this document, we propose 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 http://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 January 4, 2015.

Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Functional Overview . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  GDR Candidates  . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Hash Mask and Hash Algorithm  . . . . . . . . . . . . . .   7
     4.3.  Modulo Hash Algorithm . . . . . . . . . . . . . . . . . .   8
     4.4.  PIM Hello Options . . . . . . . . . . . . . . . . . . . .   9
   5.  Hello Option Formats  . . . . . . . . . . . . . . . . . . . .   9
     5.1.  PIM DR Load Balancing Capability (DRLBC) Hello Option . .   9
     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
     6.3.  PIM Assert Modification . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  14
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  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 [RFC4601].

   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 for the corresponding multicast flow.



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   o  GDR Candidate: a last hop router that has 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.

2.  Introduction

   On a multi-access network 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 network, the PIM DR is responsible for registering
   an active source with the Rendezvous Point (RP) if the group is
   operating in PIM-SM.  On the last hop network, 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 network in Figure 1:


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


                        Figure 1: Last Hop Network

   Assume R1 is elected as the Designated Router.  According to
   [RFC4601], 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
   proves 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




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   packets due to RPF failures).  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.  These days, it is very common that the
   last hop LAN usually 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, some times by a large number, that of a single link.  For
   example, let us modify Figure 1 and introduce an Ethernet switch in
   Figure 2.



                          ( 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
   data and different traffic is requested by each host.  This
   represents a total 1.5 Gbps of data, which is under 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 router, the
   PIM DR elected using the procedure defined by [RFC4601], 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.



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   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).

   The problem may also manifest itself in a different way.  For
   example, R1 happens to forward 500 Mbps worth of unicast data to H1,
   and at the same time, H2 and H3 each requests 300 Mbps of different
   multicast data.  Once again packet drop happens on R1 while in the
   mean time, there is sufficient forwarding capacity left on R2 and R3
   and link capacity between the switch and R2/R3.

   Another important issue is related to failover.  If R1 is the only
   forwarder on the last hop network, in the event of a failure when R1
   goes out of service, multicast forwarding for the entire network has
   to be rebuilt by the newly elected PIM DR.  However, if there was a
   way that allowed multiple routers to forward to the network 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.

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

3.  Applicability

   The proposed change described in this specification 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).  And distributing the load to do
   registering does not justify the additional complexity required to
   support it.

4.  Functional Overview

   In the existing PIM DR election, when multiple last hop routers are
   connected to a multi-access network (for example, an Ethernet), one
   of them is selected to act as PIM DR.  The PIM DR is responsible for
   sending local Join/Prune messages towards the RP or source.  To elect
   the PIM DR, each PIM router on the network examines the received PIM
   Hello messages and compares its DR priority and IP address with those
   of its neighbors.  The router with the highest DR priority is the PIM



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   DR.  If there are multiple such routers, their IP addresses are used
   as the tie-breaker, as described in [RFC4601].

   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 network.  There is only one PIM DR on the multi-access
   network, 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
   Candidates' Addresses are announced by DR in a new DR Load Balancing
   GDR (DRLBGDR) PIM Hello Option.

   For each multicast flow, a hash algorithm is used to select one of
   the routers to be the GDR.  Hash Masks are defined for Source, Group
   and RP separately, in order to handle PIM ASM/SSM.  The masks are
   announced in PIM Hello by DR as a DR Load Balancing GDR (DRLBGDR)
   Hello Option.  Besides that, a DR Load Balancing Capability (DRLBC)
   Hello Option, which contains hash algorithm type, is also announced
   by the router 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.

   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 on the LAN.  R1, R2 and R3



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   have the same DR priority while R4's DR priority is less preferred.
   In this 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.

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

   As a DR, R1 will include its own Load Balancing Hash Masks, and also
   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 MUST be configured on the PIM routers that can
   potentially become a PIM DR.

   o  If the group is ASM, and if the RP Hash Mask announced by the PIM
      DR is not 0, calculate the value of hashvalue_RP to determine GDR.

   o  If the group is ASM and if the RP Hash Mask announced by the PIM
      DR is 0, obtain the value of hashvalue_Group to to determine GDR.

   o  If the group is SSM, use hashvalue_SG to determine GDR.

   A simple Modulo hash algorithm will be discussed in this document.
   However, to allow other hash algorithm to be used, a 4-bytes "Hash
   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



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   DR (and having the same DR priority as the DR) are eligible for GDR
   election.

4.3.  Modulo Hash Algorithm

   Modulo hash algorithm is discussed here as an example, with detailed
   description on hashvalue_RP.

   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 zeros, 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:

      (((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.

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




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      *  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.

   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

4.4.  PIM Hello Options

   When a last hop PIM router sends a PIM Hello from 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
   the sorted list of all GDR Candidates' Address on the last hop
   network.

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

5.  Hello Option Formats

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
















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       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            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Hash Algorithm Type                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                     Figure 3: Capability Hello Option

   Type:   TBD.

   Length:   4 octets

   Hash Algorithm Type:   0 for Modulo hash algorithm

   This DRLBC Hello Option SHOULD be advertised by last hop routers from
   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:




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   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 from high to low.  The order is
      converted to the ordinal number associated with each GDR candidate
      in hash value calculation.  For example, addresses advertised are
      R3, R2, R1, the ordinal number assigned to R3 is 0, to R2 is 1 and
      to R1 is 2.

      If "Interface ID" option, as described in [RFC6395], presents in a
      GDR Candicate's PIM Hello message, and the "Router ID" portion is
      non-zero,

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

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

      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"Router ID" field is zero, the "GDR Candidate Address" will be
      the IPv4 or IPv6 source address from PIM Hello message.

   This DRLBGDR Hello Option SHOULD 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 [RFC4601].  A
   DR that has this specification enabled on the interface, advertises
   the new LBGRD Hello Option, which contains value of masks from user
   configuration, followed by a sorted list of all GDR Candidates'
   Addresses, from high to low.  Moreover, same as non-DR routers, DR
   also advertises DRLBC Hello Option to indicate its capability of



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   supporting this specification and the type of its GDR election hash
   algorithm.

   If a PIM DR receives a PIM Hello with DRLBGRD 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 DRLBGRD Option.

6.2.  PIM GDR Candidate Operation

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

   A router interface where this protocol is enabled advertises DRLBC
   Hello Option in its PIM Hello, even if the router may not be
   considered as a GDR Candidate, for example, due to low DR priority.

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

   A GDR Candidate may receive a DRLBGDR Hello Option from a PIM router
   which is not DR.  The GDR Candidate must ignore such DRLBGDR Hello
   Option.

   A GDR Candidate may receive a Hello from the elected PIM DR, and the
   PIM DR does not support this specification.  The GDR election
   described by this specification will not take place, that is only the
   PIM DR joins the multicast 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 this could happen include:

   o  When a new PIM router comes up

   o  When a GDR restarts





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   When the GDR changes, existing traffic might be disrupted.
   Duplicates or packet loss might be observed.  To illustrate the case,
   consider the following scenario: there are two streams 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 it still might happen.
   Here we describe a mechanism to minimize the impact.  The motivation
   is that we want to minimize packet loss.  And therefore, we would
   allow a small amount of duplicates and depend on PIM Assert to
   minimize the duplication.

   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.

   Due to the introduction of GDR, this document suggests the following
   modification to the Assert packet: if a router enables this
   specification on its downstream interface, but it is not a 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 little bit longer time for R2 to find out
   that R3 is the new GDR and still thinks 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 the metric and IP address.  If R3 has
   the better routing metric, or same metric but better tie-breaker, the
   result will be consistent with GDR selection.  If unfortunately, R2
   has the better metric or same metric but 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 is seen that 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 being
   the GDR, and set 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



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   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.  IANA Considerations

   Two new PIM Hello Option Types have been assigned to the DR Load
   Balancing messages.  [HELLO-OPT], this document recommends 34(0x22)
   as the new "PIM DR Load Balancing Capability Hello Option", and
   35(0x23) as the new "PIM DR Load Balancing GDR Hello Option".

8.  Security Considerations

   Security of the new DR Load Balancing PIM Hello Options is only
   guaranteed by the security of PIM Hello message, so the security
   considerations for PIM Hello messages as described in PIM-SM
   [RFC4601] apply here.

9.  Acknowledgement

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

10.  References

10.1.  Normative References

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

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC6395]  Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
              Hello Option for PIM", RFC 6395, October 2011.

   [RFC4291]  Hinden, R. and L. S., "IP Version 6 Addressing
              Architecture", RFC 6890, February 2006.

10.2.  Informative References

   [HELLO-OPT]
              IANA, , "PIM Hello Options", PIM-HELLO-OPTIONS
              http://www.iana.org/, March 2007.



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Authors' Addresses

   Yiqun Cai
   Microsoft
   La Avenida
   Mountain View, CA  94043
   USA

   Email: yiqunc@microsoft.com


   Sri Vallepalli
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: svallepa@cisco.com


   Heidi Ou
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: hou@cisco.com


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

   Email: andy.da.green@bt.com















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