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Versions: 00 draft-ietf-pim-drlb

Network Working Group                                          Yiqun Cai
Internet-Draft                                            Sri Vallepalli
Intended status: Standards Track                                Heidi Ou
Expires: April 17, 2012                              Cisco Systems, Inc.
                                                              Andy Green
                                                         British Telecom
                                                        October 15, 2011


            Protocol Independent Multicast DR Load Balancing
                       draft-hou-pim-drlb-00.txt

Abstract

   On a multi-access network such as an Ethernet, one of the PIM routers
   is elected as a Designated Routers (DR).  The PIM DR has two roles in
   the PIM protocol.  On the first hop network, the PIM DR is
   responsible for registering an active source to the RP if the group
   is operated in PIM SM.  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 operated in PIM SM/SSM/DM.
   In this document, we propose a modification to the PIM protocol that
   allows multiple of these last hop routers to be selected so that the
   forwarding load can be distributed to and handled among these
   routers.  A router responsible for forwarding for a particular group
   is called a Group Designated Router (GDR).

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 April 17, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



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   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
   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.  Terminologies  . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Functional Overview  . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  GDR Candidates . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Hash Mask  . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  PIM Hello Options  . . . . . . . . . . . . . . . . . . . .  8
   5.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . . .  8
     5.1.  PIM DR Load Balancing GDR (LBGDR) Hello TLV  . . . . . . .  8
     5.2.  PIM DR Load Balancing Hash Masks (LBM) Hello TLV . . . . .  9
   6.  Protocol Specification . . . . . . . . . . . . . . . . . . . .  9
     6.1.  PIM DR Operation . . . . . . . . . . . . . . . . . . . . . 10
     6.2.  PIM GDR Candidate Operation  . . . . . . . . . . . . . . . 10
     6.3.  PIM Assert Modification  . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 12
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     10.1. Normative Reference  . . . . . . . . . . . . . . . . . . . 12
     10.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

















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1.  Terminologies

   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
      group, a hash algorithm (described below) is used to select one of
      the routers as GDR.  The GDR is responsible for initiating the
      forwarding tree building for the corresponding group.

   o  GDR Candidate: a last hop router that has potential to become a
      GDR.  A GDR Candidate must have the same DR priority as the DR
      router.  It must send and process received 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 group.


2.  Introduction

   On a multi-access network such as an Ethernet, one of the PIM routers
   is elected as a Designated Routers (DR).  The PIM DR has two roles in
   the PIM protocol.  On the first hop network, the PIM DR is
   responsible for registering an active source to the RP if the group
   is operated 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 operated in PIM SM/SSM/DM.

   Considering the following last hop network in Figure 1.
















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


                        Figure 1: Last Hop Network

   Assuming R1 is elected as the Designated Router.  According to
   [RFC4601], R1 will be responsible for forwarding to the last hop LAN.
   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
   dropping 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.











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                          ( 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
   that can 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 RFC 4601, 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).

   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 to one of the routers would only lead to



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   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 protocol that
   allows multiple of these routers, called Group Designated Router
   (GDR) to be selected so that the forwarding load can be distributed
   to and handled by a number of routers.


3.  Applicability

   The proposed change described in this draft applies to PIM last hop
   routers only.

   It doesn't 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 doesn't justify the additional complexity required to
   support it.


4.  Functional Overview

   In 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 Join/Prune messages to 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
   DR.  If there are multiple such routers, IP address is 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 group, 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 different PIM modes.  The masks
   are announced in PIM Hello as a new Load Balancing Hash Mask TLV (LBM
   TLV).  Last hop routers with this new TLV and with the same DR
   priority as the PIM DR are GDR candidates.




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   A simple hash algorithm based on the announced Source, Group or RP
   masks allow one GDR to be assigned to a corresponding multicast
   group, and that GDR is responsible for initiating the creation of
   multicast forwarding tree for the group.

4.1.  GDR Candidates

   GDR is the new concept introduced by this draft.  To become a
   candidate GDR, a router must have the same DR priority as the DR.
   For example, if there are 4 routers on the LAN: R1, R2, R3 and R4.
   R1, R2 and R3 have the same DR priority while R4's DR priority is
   less preferred.  In this example, only R1, R2 and R3 will be eligible
   for GDR election.  R4 is not because R4 will not become a PIM DR
   unless all of R1, R2 and R3 go out of service.

   Further assuming router R1 wins the PIM DR election.  In its Hello
   packet, R1 will include the identity of R1, R2 and R3 (the GDR
   candidates) besides its own Load Balancing Hash Mask TLV.  The order
   of the GDR candidates is converted to the ordinal number associated
   with each GDR candidate.  For example, addresses advertised by R1 is
   R1, R2, R3, the ordinal number assigned to R1 is 0, to R2 is 1 and to
   R3 is 2.

4.2.  Hash Mask

   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 GDR from GDR Candidates
   advertised in order by PIM DR.  For example, 0.255.0.0 defines a Hash
   Mask for an IPv4 address that masks the first, the third 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.

   For ASM groups, a hash value is calculated using the following
   formula:

   o  hashvalue_RP = ((RP_address & RP_hashmask) >> N ) % M

   RP_address is the address of the RP defined for the group.  N is the
   number of bits that are 0 from the right.  M is the number of GDR



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   candidates as described above.

   If RP_hashmask is 0, a hash value is also calculated using the group
   Hash Mask in a similar fashion

   o  hashvalue_Group = ((Group_address & Group_hashmask) >> N) % M

   For SSM groups, a hash value is calculated using both the source and
   group Hash Mask

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

4.3.  PIM Hello Options

   When a non-DR PIM router that supports this draft sends a PIM Hello,
   it includes a new option, called "Load Balancing Hash Masks TLV (LBM
   TLV)".  The LBM TLV consists of three Hash Masks as defined above.

   Besides this new LBM TLV, the elected PIM DR router also includes a
   "Load Balancing GDR TLV (LBGDR TLV)" in its PIM Hello.  The LBGDR TLV
   consists of the sorted addresses of all GDR candidates on the last
   hop network.

   The elected PIM DR router uses LBM TLV to calculate its LBGDR TLV.
   The GDR candidates use LBM TLV and LBGDR TLV advertised by DR PIM
   router to calculate hash value.


5.  Packet Format

5.1.  PIM DR Load Balancing GDR (LBGDR) Hello TLV




       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                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        GDR Address(es)                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




                          Figure 3: GDR Hello TLV



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   Type:   TBD
   Length:
   GDR Address (32/128 bits):   Address(es) of GDR candidates.  All
      addresses must be in the same address family.  The addresses are
      sorted from high to low.  The order is used as the ordinal number,
      starting from 0, in hash value calculation.

   This LBGDR TLV should only be advertised by the elected PIM DR
   router.

5.2.  PIM DR Load Balancing Hash Masks (LBM) Hello TLV



       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                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




                      Figure 4: Hash Masks Hello TLV

   Type:   TBD.
   Length:
   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, with the same length.

   This LBM TLV should be advertised by last hop routers, which support
   this draft.


6.  Protocol Specification







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6.1.  PIM DR Operation

   The DR elect process is still the same as defined in [RFC4601].  A DR
   supports this draft advertises a new Hello Option LBGRD TLV to
   includes all GDR candidates.  Moreover, same as non-DR routers, DR
   also advertises LBM TLV Hello Option to indicate its capability of
   supporting this draft.

   LBGRD TLV is composed by sorting the addresses of all GDR candidates.
   LBM TLV on PIM DR contains value of masks from user configuration.

   If a PIM DR receives a neighbor Hello with LBGRD TLV, the PIM DR
   should ignore the TLV.

   If a PIM DR receives a neighbor Hello with LBM TLV, and the neighbor
   has the same DR priority as PIM DR itself, the PIM DR should consider
   the neighbor as a GDR candidate and insert the neighbor's address
   into the sorted list of LBGRD TLV.

6.2.  PIM GDR Candidate Operation

   When an IGMP join is received, without this proposal, router R1 (the
   PIM DR) will handle the join and potentially run into the issues
   described earlier.  Using this proposal, a simple algorithm is used
   to determine which router is going to be responsible for building
   forwarding trees on behalf of the host.

   The algorithm works as follows, assuming the router in question is X
   and a GDR Candidate:

   o  If the group is ASM, and if the RP Hash Mask announced by the PIM
      DR is not 0.0.0.0, calculate the value of hashvalue_RP.  If
      hashvalue_RP is the same as the ordinal number assigned implicitly
      to X by PIM DR, X becomes the 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.  And compare that to
      the ordinal value of X assigned by the PIM DR to decide if X is
      the GDR
   o  If the group is SSM, then use hashvalue_SG to determine if X is
      the GDR.

   If X is the GDR for the group, X will be responsible for building the
   forwarding tree.

   A router that supports this draft advertises LBM TLV in its Hello,
   even the router may not be a GDR candidate.

   A GDR candidate may receive a LBM TLV from PIM DR router, with a



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   different Hash Masks as advertised in its own Hello LBM TLV.  The GDR
   candidate must use the Hash Masks advertised by the PIM DR Hello to
   calculate the hash value.

   A GDR candidate may receive a LBGDR TLV from a non-DR PIM router.
   The GDR candidate must ignore such LBGDR TLV.

   A GDR candidate may receive Hello from the elected PIM DR, and the
   PIM DR doesn't support this draft.  The GDR election described by
   this draft will not take place, that is only the PIM DR joins the
   multicast tree.

6.3.  PIM Assert Modification

   When routers restart, GDR may change for a specific group, which
   might cause packet drops.

   For example, if there are two streams G1 and G2, and R1 is the GDR
   for G1 and R2 is the GDR for G2.  When R3 comes up online, it is
   possible that R2 becomes GDR for G1 and R3 becomes GDR for G2, and
   rebuilding of the forwarding trees for G1 and G2 will lead to
   potential packet loss.

   This is not a typical deployment scenario but it still might happen.
   Here we describe a mechanism to minimize the impact.

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

   The same tie breakers are used to select an Assert winner with one
   modification.  That is, instead of comparing IP addresses as the last
   resort, a router considers whether the sender of an Assert is a GDR.
   In this example, R1 will let R2 be the assert winner for G1, and R2
   will do the same for R3 for G2.  This will cause some duplicates in
   the network while minimizing packet loss.

   If a router on the LAN doesn't support this draft, the Assert
   modification described above will not take place, that is only the IP
   address of an Assert sender is used as the tie breaker.  Fore
   example, if R4, with preferred IP address, doesn't understand GDR and
   sends Assert for G1, R2, the GDR for G1, will grant R4 as the Assert
   winner, clear OIF on R2.







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7.  IANA Considerations

   Two new PIM Hello Option Types are required to assign to the DR Load
   Balancing messages.  According to [HELLO-OPT], this document
   recommends 32(0x20) as the new "PIM DR Load Balancing GDR Hello
   Option", and 33(0x21) as the new "PIM DR Load Balancing Hash Masks
   Hello Option" .


8.  Security Considerations

   Security of the PIM DR Load Balancing Hello message is only
   guaranteed by the security of PIM Hello packet, so the security
   considerations for PIM Hello packets 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, ??? for their review comments.


10.  References

10.1.  Normative Reference

   [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.

10.2.  Informative References

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, January 2005.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, October 2007.

   [HELLO-OPT]
              IANA, "PIM Hello Options", PIM-HELLO-OPTIONS per
              RFC4601 http://www.iana.org/assignments/pim-hello-options,
              March 2007.



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Authors' Addresses

   Yiqun Cai
   Cisco Systems, Inc.
   Tasman Drive
   San Jose, CA  95134
   USA

   Email: ycai@cisco.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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