Network Working Group                                             L. Jin
Internet-Draft                                                 F. Jounay
Intended status: Informational                            France Telecom
Expires: November 14, 2013                                     M. Bhatia
                                                                 S. Kini
                                                            May 13, 2013

          Hub and Spoke Multipoint Label Switched Path Tunnels


   There are applications that require bi-directional, co-routed and
   guaranteed communication from a root node to several leaf nodes in a
   hub and spoke fashion.  To meet such application requirements in a
   Multi-protocol Label Switching (MPLS) network this draft defines a
   Hub and Spoke Multipoint Traffic Engineered Label Switched Path (HSMP
   TE LSP) with resource reservations for guaranteed communication.
   This draft also defines a protocol to setup such LSPs by re-using and
   extending P2MP RSVP-TE.

Status of This Memo

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   This Internet-Draft will expire on November 14, 2013.

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   Provisions Relating to IETF Documents

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   ( in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Abbreviations and Terminology . . . . . . . . . . . . . . . .   3
   3.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Time Synchronization  . . . . . . . . . . . . . . . . . .   3
     3.2.  P2MP pseudowire . . . . . . . . . . . . . . . . . . . . .   3
   4.  Scalability issues  . . . . . . . . . . . . . . . . . . . . .   3
   5.  Hub and Spoke Multipoint LSP  . . . . . . . . . . . . . . . .   4
     5.1.  Data plane  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Control Plane . . . . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   There are many applications that require one-to-many bi-directional
   communication.  Some of these applications are described in Section 3
   along with their requirements from the network.  Making such
   applications work over a MPLS network by using both P2MP and P2P
   constructs results in scalability issues and these are discussed in
   Section 4.  This document defines a technique to do one-to-many bi-
   directional communication over an MPLS network that re-uses the
   existing P2MP and P2P constructs in MPLS but combines them in a
   scalable manner.  This technique re-uses and extends the current
   traffic-engineered P2MP and P2P constructs and protocols.  It is
   described in detail in Section 5.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  Abbreviations and Terminology

      HSMP LSP - Hub and Spoke Multipoint LSP

3.  Applications

   We describe two representative applications that require one-to-many
   bi-directional communication.  The first is the 'Time
   Synchronization' application described in 2.1.  The second is the
   P2MP pseudowire application and is described in section 2.

3.1.  Time Synchronization

   Time Synchronization [IEEE1588] over an MPLS network is being defined
   in [I-D.ietf-tictoc-1588overmpls].  A scalable time-sync architecture
   requires the master to provide time synchronization to a large number
   of slaves.  It requires the PTP messages to flow bi-directionally
   between master and slave in a hub and spoke manner.  More importantly
   these messages must have the same delay in both directions.  This
   requires the underlying network to reserve resources to transport PTP
   messages and also to co-route them in both directions to avoid any
   differences in the delays of the paths in both directions.

3.2.  P2MP pseudowire

   A P2MP PW [I-D.ietf-pwe3-p2mp-pw] is required for the VPMS service
   [I-D.ietf-l2vpn-vpms-frmwk-requirements].  In this application the
   root PE requires bi-directional communication with several leaf PEs.
   The underlying MPLS transport should support this type of
   communication for the P2MP PW in a reliable and efficient manner.

4.  Scalability issues

   A straightforward method to achieve one-to-many bi-directional
   communication with resource guarantees is to use a P2MP RSVP-TE
   tunnel from the hub PE to the spoke PEs and use P2P RSVP-TE tunnels
   from each spoke PE to the hub PE.  The spoke-to-hub P2P tunnels can
   be explicitly routed such that they are co-routed along the reverse
   direction of the P2MP tunnel.  In this model, scalability issues
   arise both in the data plane and the control plane as explained below
   in this section.

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   For the purpose of this discussion, an application with a hub and
   spoke bi-directional communication over a tree topology MPLS network
   is illustrated in Figure 1.  The hub LSR is A and the spoke LSRs are
   E, F, G, and H.  For communication from the hub to the spokes, a P2MP
   LSP can be setup with A as the root and E, F, G and H as leaves.  For
   communication from the spokes to the hub, a P2P LSP can be setup from
   each spoke to the hub.

                                     / \
                                    /   \
                                   /     \
                                  C       D
                                 / \     / \
                                E   F   G   H

             Figure 1: Hub and spoke LSP over a tree topology

   Each LSR along this tree will have to allocate a unique label for
   each of the P2P LSPs that go from spoke to hub through it.  This
   leads to a linear increase in forwarding state at each LSR in
   proportion to the number of spoke nodes that are in its sub-tree.
   This has poor scaling characteristics in the data plane as the number
   of spoke nodes increase.

   Each LSR also has to allocate control plane state for each of the P2P
   LSPs that go from spoke to hub through it.  Each P2P LSP will need a
   separate path state block (PSB) and a reservation state block (RSB)
   and these will store additional information on signaling attributes.
   This state is in addition to the state maintained for the P2MP LSP.
   Clearly this state too increases linearly with the number of spoke
   nodes that are in its sub-tree.  This too has poor scaling
   characteristics in the control plane as the number of spoke nodes
   increase.  Also the number of signaling messages increases linearly
   though some of it may be mitigated by using refresh reduction

5.  Hub and Spoke Multipoint LSP

   To solve the issues identified in Section 4 this document defines a
   hub and spoke traffic-engineered multipoint LSP (HSMP TE LSP) with
   resource reservations.  Such an LSP is a combination of an explicitly
   routed uni-directional traffic-engineered P2MP LSP from the hub to
   the spokes and a co-routed uni-directional MP2P LSP from the spokes
   to the hub.  The data plane for a HSMP TE LSP is explained in
   Section 5.1 and the control plane is explained in Section 5.2

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5.1.  Data plane

   In the direction from hub-to-spoke the data plane processing is the
   same as that of a P2MP LSP.  In the direction from the spokes to the
   hub, each LSR allocates labels for its upstream LSRs.  Each LSR
   merges the traffic received from multiple upstream LSRs before
   forwarding it on the LSP towards the hub.  It should be noted that
   due to label merging the GAL processing in the direction from spoke
   to hub is not defined.

5.2.  Control Plane

   The signaling protocol to setup a HSMP TE LSP can re-use the
   signalling protocol for P2MP RSVP-TE [RFC4875] with some extensions.
   The hub and spokes of a HSMP LSP can be modeled the same as the
   source and leaves respectively of a P2MP LSP.  A source-to-leaf (S2L)
   sub-LSP defined in [RFC4875] for the P2MP LSP is used to represent a
   hub-to-spoke communication of the HSMP LSP.  To signal the bi-
   directional co-routed nature of the communication from the hub to the
   spoke, the extensions defined in section 3 of [RFC3473] must be used.
   Each Path message of a HSMP LSP LSR MUST have a Upstream_Label
   object.  If a PathErr is received in response with a "Routing problem
   /Unacceptable label value" indication then the Acceptable Label Set
   (if present) must be examined to allocate a label for the
   Upstream_Label object.  If an LSR signaling an HSMP LSP receives
   PathErr messages with different Acceptable Label Sets from different
   neighboring LSRs then it may need to allocate more than one label to
   satisfy all the Acceptable Label Sets.  The LSR should try to
   minimize the number of unique labels allocated for a HSMP LSP in such
   a case.

   Pruning and grafting for a HSMP LSP follow the same procedures as for
   a P2MP LSP.  During re-merge in addition to the procedures in section
   18.1.1 of [RFC4875] the ingress or transit LSR that creates the
   branch would also be a re-merge LSR for the traffic from the spokes
   towards the hub.  Also the re-merge node for the traffic from hub to
   spoke would be a branching node for traffic from the spokes to the
   hub.  The LSR that is branching the traffic from the spokes to the
   hub would duplicate the traffic whereas the LSR that is re-merging
   the traffic should forward traffic only from one the incoming

   The HSMP LSP also requires bandwidth allocation that is asymmetric
   between the hub-to-spoke and the spoke-to-hub direction.  At the same
   time it requires the spokes to be able to request different amounts
   of bandwidth towards the hub.  The protocol extensions defined in
   [RFC6387] are used for asymmetric bandwidth allocation between the
   hub-to-spoke and spoke-to-hub directions.  The UPSTREAM_TSPEC,

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   UPSTREAM_ADSPEC and UPSTREAM_FLOWSPEC objects are also used in a HSMP
   LSP.  However in case of a HSMP LSP an intermediate LSR can receive
   different UPSTREAM_TSPECs in the Resv messages from neighboring LSRs.
   Combining these Tspecs and generating an appropriate
   UPSTREAM_FLOWSPEC towards each spoke is still under discussion.
   Also, processing a received UPSTREAM_FLOWSPEC and generating
   appropriate UPSTREAM FLOWSPECs in the hub to spoke direction is also
   under discussion.

6.  Acknowledgements

   We would like to thank Dimitri Papadimitriou, Yuji Kamite, Sebastien
   Jobert for their comments and feedback on the document.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   The same security considerations apply as for the RSVP-TE P2MP LSP
   [RFC4875] specification.

9.  References

9.1.  Normative References

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

   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 2007.

   [RFC6387]  Takacs, A., Berger, L., Caviglia, D., Fedyk, D., and J.
              Meuric, "GMPLS Asymmetric Bandwidth Bidirectional Label
              Switched Paths (LSPs)", RFC 6387, September 2011.

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9.2.  Informative References

              Kamite, Y., JOUNAY, F., Niven-Jenkins, B., Brungard, D.,
              and L. Jin, "Framework and Requirements for Virtual
              Private Multicast Service (VPMS)", draft-ietf-l2vpn-vpms-
              frmwk-requirements-05 (work in progress), October 2012.

              Sivabalan, S., Boutros, S., and L. Martini, "Signaling
              Root-Initiated Point-to-Multipoint Pseudowire using LDP",
              draft-ietf-pwe3-p2mp-pw-04 (work in progress), March 2012.

              Davari, S., Oren, A., Bhatia, M., Roberts, P., and L.
              Montini, "Transporting Timing messages over MPLS
              Networks", draft-ietf-tictoc-1588overmpls-04 (work in
              progress), February 2013.

              IEEE 1588-2008, "IEEE Standard for a Precision Clock
              Synchronization Protocol for Networked Measurement and
              Control Systems ", 2008.

   [RFC2961]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F.,
              and S. Molendini, "RSVP Refresh Overhead Reduction
              Extensions", RFC 2961, April 2001.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090, May

Authors' Addresses

   Lizhong Jin


   Frederic Jounay
   France Telecom


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   Manav Bhatia


   Sriganesh Kini


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