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PWE3                                                         Y(J). Stein
Internet-Draft                                             I. Mendelsohn
Intended status: Standards Track                               R. Insler
Expires: May 6, 2009                             RAD Data Communications
                                                        November 2, 2008

                               PW Bonding

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   There are times when pseudowires must be transported over physical
   links with limited bandwidth.  We shall use the term "bonding" (also
   variously known as inverse multiplexing, link aggregation, trunking,
   teaming, etc.) to mean an efficient mechanism for separating the PW
   traffic over several links.  Unlike load balancing and equal cost
   multipath, bonding makes no assumption that the PW traffic can be
   decomposed into distinguishable flows, and thus bonding requires

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   delay compensation and packet reordering.  Furthermore, PW bonding
   can optionally track bandwidth constraints in order to minimize
   packet loss.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  PW Bonding mechanism . . . . . . . . . . . . . . . . . . . . .  5
   3.  PW Dynamic Bandwidth Allocation  . . . . . . . . . . . . . . .  6
   4.  Protocol Extensions  . . . . . . . . . . . . . . . . . . . . .  7
   5.  Partial Path PW Bonding  . . . . . . . . . . . . . . . . . . .  8
   6.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
     10.2.  Informative References  . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Introduction

   Inverse multiplexing is any mechanism for transporting a single high
   capacity traffic flow over multiple lower capacity paths.  Inverse
   multiplexing is also known as bonding, link load balancing, link
   aggregation, trunking, teaming, concatenation, and multipath.  In the
   context of pseudowires we will use the term bonding.

   Bonding has been defined for many transport technologies (and often
   more than one mechanism has been developed for a single technology)
   including TDM (continguous and virtual concatenation VCAT), ATM (ATM
   forum's IMA and ITU's G.998.1 multi-pair bonding), Ethernet (802.3
   link aggregation LAG and EFM PME aggregation), xDSL (the previous two
   and G.998.3 time domain inverse multiplexing TDIM), PPP (MLPPP), and
   in the context of IP transport, equal cost multiplath (ECMP).

   Regardless of the transport infrastructure, all bonding mechanisms
   must confront a fundamental problem, namely that the constituent
   paths will in general have different (and not necessarily constant)
   propagation delays.  Thus a mechanism must be employed to ensure in-
   order delivery of the data units.  Two solutions have been proposed
   for this problem, namely performing differential delay compensation,
   and decomposing the input into mutually distinct flows.  Methods
   using the former solution (e.g., VCAT, TDIM) buffer the data from
   each path at egress (e.g., VCAT buffers up to 1/2 second), and
   introduce protocol elements to synchronize the paths before
   recombining them.  Methods using the latter soution (LAG, ECMP) skirt
   the problem by consistently mapping data units from a given flow onto
   the same constituent path, assuming that there is only the need to
   maintain order inside each flow, and not across flows.

   Methods employing differential delay compensation tend to more
   complex and to require large buffers, but are universally applicable.
   Methods decomposing the input into flows depend on the existence of
   such flows and sniffing the input for their identification.  Thus if
   the input is a single large flow, or if it is not possible to
   identify flows (e.g., due to lower layer encryption), or if it is
   undesirably complex to do so, these methods may not be applicable.

   Furthermore, methods decomposing the input into flows tacitly assume
   that the hashing of flow identifiers onto tunnels results in fair
   distribution of traffic.  This is generally a good assumption when
   there are a very large number of independent flows.  Incorrect
   distribution causes some underlying paths to become congested and
   drop packets, while others are relatively underutlized.  Direct
   inverse multiplexing with differential delay compensation one can
   ensure fairness, and in fact can adapt to underlying paths with
   unequal and even time varying capacity.

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   In the context of pseudowires a decomposition mechanism has been
   previously proposed [5].  The present draft proposes a PW bonding
   mechanism based on direct inverse multiplexing with differential
   delay compensation.  In particular, the proposed mechanism may be
   used when PWs are supported by DSL links.

   The simplest scenario for PW-bonding is depicted in Figure 1.  Here
   the entire PW is transported edge to edge over separate PW
   components, each inside a distinct transport tunnel.  A somewhat more
   complex scenario is partial path bonding, as depicted in Figure 2,
   where only a portion of the PW path is bandwidth restricted.  Here
   only the PW components are shown, and not the tunnels into which they
   are placed.  Here it is required to separate the PW into components
   in separate tunnels at some point inside the network.  However, since
   P device where this happens is not PW aware, the PW components must
   still be defined by the ingress PE.

                 +--------+                        +--------+
                 |   PE   |                        |   PE   |
                 |        |         tunnel 1       |        |
                 |        X========================X        |
                 |        |      PW component 1    |        |
                 |        X------------------------X        |
                 |        |                        |        |
                 |        X========================X        |
                 |        |                        |        |
            AC   |        |                        |        |   AC
          -------o        |                        |        o-------
                 |        |                        |        |
                 |        |         tunnel 2       |        |
                 |        X========================X        |
                 |        |      PW component 2    |        |
                 |        X------------------------X        |
                 |        |                        |        |
                 |        X========================X        |
                 |        |                        |        |
                 +--------+                        +--------+

      Figure 1. edge-to-edge PW bonding - 2 PW components in tunnels

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              +------+          +-----+                +------+
              |  PE  |          |  P  |                |  PE  |
              |      |          |     |  PW component  |      |
              |      |          |     X================X      |
              |      |          |     |                |      |
          AC  |      |          |     |                |      |  AC
        ------o      |    PW    |     |  PW component  |      o------
              |      X==========X     X================X      |
              |      |          |     |                |      |
              |      |          |     |                |      |
              |      |          |     |  PW component  |      |
              |      X          |     X================X      |
              |      |          |     |                |      |
              +------+          +-----+                +------+

            Figure 2. partial path PW bonding - 3 PW components

   Each PW component will normally receive a distinct PW label, and thus
   seem to the network to be a distinct PW.  Furthermore, PW components
   MUST use the PW control word [2].  However, as we shall see in the
   next section, the sequence number generation and processing is
   different for PW components that for true PWs.

2.  PW Bonding mechanism

   As discussed in the previous section, at the egress PE the traffic
   from each PW component is buffered, and the protocol is responsible
   for ensuring that packets constituting the PW are reassembled in
   correct order.  This is accomplished by mandating use of the PW
   control word, and sharing the same sequence number sequence for all
   PW components making up the PW.  The sequence numbers are used by the
   egress PE to ensure properly ordering.  The idea is depicted in
   Figure 3, for the simple case of edge-to-edge bonding.  Here eight
   packets are divided amongst three PW components by the ingress PE,
   according to a bandwidth allocation algorithm to be described later.
   Due to different link latencies, the packets arrive at the egress out
   of order, but are easily reordered by the egress PE by observing the
   sequence number.

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                    +------+          +---------------+
                    |  PE  |          |        PE     |
                    |      | 1 2   7  |               |
                    |      X==========X               |
                    |      |          |               |
     1 2 3 4 5 6 7 8|      |          |1 3 2 4 5 7 6 8|1 2 3 4 5 6 7 8
     ---------------o      |   3 4  8 |               o---------------
          PW        |      X==========X               |
                    |      |          |               |
                    |      |          |               |
                    |      |     5 6  |               |
                    |      X==========X               |
                    |      |          |               |
                    +------+          +---------------+

   Figure 3.  Use of sequence numbers to ensure correct packet ordering

   In order to enable reordering, the egress PE must allocate sufficient
   buffer memory to sustain the largest expected differential delay.
   The differential delay is added to the latencies of all packets,
   making the effective latency equal to that of the slowest PW

3.  PW Dynamic Bandwidth Allocation

   In the simplest case, all packets to be sent over the various PW
   components are of the same size, and all PW components support the
   same data rates.  For this case (but only for this case), a simple
   round-robin algorithm for distributing the packets onto PW components
   is optimal in the sense that it minimizes the probability of packet
   loss due to buffer exhaustion.

   The simple round-robin algorithm is not optimal when the packets are
   not all of the same size, or when the PW components do not all
   support the same data rate, or both.  In such cases we need to fairly
   distribute data bytes over the components in such fashion as to
   minimize the probability that a packet will be dropped due to over-
   run of a component's buffer.  While the packet sizes are always known
   before transmission, the state of the buffers are usually unknown,
   and in some cases the supported data rates may be unknown.  The
   following discussion will be for the edge-to-edge component case; the
   partial path case is similar, but requires separate consideration of
   the two directions.

   If the packet size is not constant, and the component rates are
   known, but we have no further information (e.g., we do not know the
   size of the buffers, nor do we have feedback from the egress PE on

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   the actual fill states) the best algorithm for an ingress PE is based
   on a leaky bucket scheme.  In this scheme the ingress PE maintains,
   for each PW component, a variable Bn that approximately tracks the
   fill state of the egress PE's buffer for this component.  The
   variable Bn is continually decreased at a rate equal to the data rate
   of the component n, but always remains non-negative.  Each time a
   packet is sent over PW component n, its size in bytes is added to Bn.
   When a new packet needs to be sent, the ingress PE sends it on the PW
   component with minimal Bn.  This algorithm can also be used when it
   can be assumed that the component rates are equal, or approximately

   If in addition to packet size and PW component date rates, the
   ingress PE knows the buffer size used for differential compensation,
   a similar, but somewhat better, algorithm can be used.  When deciding
   over which component to send the packet, rather than choosing the
   minimal Bn, the ingress PE chooses the maximal Bn to which the packet
   size can be added without overflowing the given buffer size.  In
   practice some extra margin must be applied in order to account for

   Finally, if the egress PE can send information on the actual state of
   its buffers back to the ingress PE, then an algorithm that uses these
   buffer states instead of the approximated leaky bucket ones can be

   Any implementation MUST support the round-robin method, and SHOULD
   support the first leaky bucket mode.  Control protocol extensions are
   needed to enable communication from egress back to ingress of the
   additional information needed to support more optimal modes.  If the
   rates can be accurately known the first leaky bucket mode MUST be
   used, and if further information is available then other mechanisms
   MAY be used.

4.  Protocol Extensions

   In order to set up the PW components using the PWE3 control protocol
   [3] a single PWid or generalized PWid is assigned to the logical PW,
   and additional PWids or generalized PWids are allocated for the PW
   components.  All PW components are assigned an identical group ID, in
   order to indicate their relationship, and to enable easy withdrawal
   of the logical PW.  First the logical PW is set up using a label
   mapping message containing the interface parameters, and a new
   "bonding" sub-TLV containing the group ID.  Subsequently the PW
   components are configured.  Each PW component is assigned to a
   distinct transport tunnel by mechanisms not specified here.

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   Attachment circuit faults are signaled via PW status messages
   associated with the PWid or generalized PWid of the logical PW.  PW
   component faults and capacity indicators are sent via status messages
   per PW component PWid or generalized PWid.

   Enhancements to the PWE3 control protocol are needed in order to
   associate PW components with distinct labels in distinct tunnels to a
   single logical PW, and to communicate component capacity and status
   information.  The format of these LDP extensions will be detailed in
   the next version of this draft.

   Standard VCCV mechanisms [4] may be used independently for each PW
   component, and the resulting connectivity information may be used by
   the ingress PE in the process of distributing traffic over PW
   components.  VCCV for the partial path scenario is for further study.

5.  Partial Path PW Bonding

   When only a portion of the PW's path suffers from bandwidth
   constriction, the partial path bonding scenario depicted in Figure 2
   is used.  As for the regular bonding case, the ingress PE decomposes
   the input into multiple PW components, and performs the same
   algorithm to decide into which component to send a given packet.  For
   those portions of the network where a single tunnel can support the
   entire service bandwidth, the PW components may all be all placed in
   the same transport tunnel.  For constricted bandwidth segments, each
   PW component must be placed in a distinct tunnel.  The distinct
   transport tunnels are merged into the single tunnel using label
   merging, per section 3.26.2 of [1].

   Another case of practical interest is when the bandwidth is
   restricted in a non-MPLS access network, and the PE terminating the
   MPLS can not inverse multiplex the traffic onto low capacity links
   based on PW labels alone.  This case arises for a DSLAM terminating
   MPLS (or a PE terminating MPLS upstream from the DSLAM) and
   forwarding to customers solely based on Ethernet MAC address (and
   possibly VLAN ID).  For such a case a double PW encapsulation may be
   used.  Through the core network we tunnel an Ethernet PW, which
   itself carries the bonded PW components (which may be of any type
   supported by PWE encapsulations), see Figure 4.

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                             |  MPLS label stack  |
                             | exterior PW label  |
                             |  Ethernet header   |
                             | interior PW label  |
                             |   control word     |
                             |      payload       |

           Figure 4. packet format for DSL partial path scenario

   The DSLAM (or PE immediately upstream from the DSLAM) terminates the
   MPLS and exterior PW protocols, thus exposing the Ethernet header.
   Under the Ethernet header there MAY be an MPLS header (which the CE
   negotiates with the immediately upstream PE), and there MUST be an
   interior PW label (which the CE negotiates with the remote CE or PE).
   Based purely on the Ethernet addressing the DSLAM distributes the
   traffic over multiple DSL links following the partition crafted by
   the ingress PE.  All of these DSL links terminate on a single CE
   device which terminates the Ethernet, exposes the interior PW labels
   and sequence numbers in the control word.  Using these sequence
   numbers the CE can thus piece together the original traffic stream.

6.  Applicability

   PW bonding is a useful mechanism when the bandwidth of available
   physical links is insufficient to carry the user traffic, but several
   links can be dedicated.  Unlike load balancing and equal cost
   multipath mechanisms, PW bonding makes no assumption that the PW
   traffic can be decomposed into distinguishable flows.  It is fully
   applicable for non-IP or encrypted traffic.  By using mechanisms
   described above, PW bonding can approach full utilization of the
   aggregate link bandwidth.

   PW bonding involves delay compensation and packet reordering, and
   thus requires allocation of sufficient memory at the egress PE.  The
   amount of memory needed is proportional to the link speed and to the
   difference in propagation delay between the fastest and slowest
   links.  Thus PW bonding is most applicable when the link speeds are
   low (e.g., supported by DSL lines), and the delay differences are

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   Only the PEs need to know that the PW components are not full PWs
   (the only difference being the sequence number processing).  Thus PW
   bonding requires changes only to the PEs and does not require any
   changes to the intervening PSN.

7.  Security Considerations

   PW bonding does not introduce security considerations above those
   present for regular PWs.  In particular, attacks based on sequence
   number manipulation are of concern.  For partial path cases where CE
   devices participate in the PWE signaling, authentication is required.

8.  IANA Considerations

   Required extensions to the PWE3 control protocol, including the sub-
   TLV type code for the PW component label, and new PW status codes,
   will be detailed in the next version of this draft.

9.  Acknowledgments

   The authors would like to thank Gabriel Zigelboim for fruitful
   discussions on optimal dynamic allocation mechanisms.

10.  References

10.1.  Normative References

   [1]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol Label
        Switching Architecture", RFC 3031, January 2001.

   [2]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
        "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use
        over an MPLS PSN", RFC 4385, February 2006.

   [3]  Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G. Heron,
        "Pseudowire Setup and Maintenance Using the Label Distribution
        Protocol (LDP)", RFC 4447, April 2006.

   [4]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
        Connectivity Verification (VCCV): A Control Channel for
        Pseudowires", RFC 5085, December 2007.

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10.2.  Informative References

   [5]  Bryant, S., Filsfils, C., and U. Drafz, "Load Balancing Fat MPLS
        Pseudowires", draft-bryant-filsfils-fat-pw-02 (work in
        progress), July 2008.

Authors' Addresses

   Yaakov (Jonathan) Stein
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5389
   Email: yaakov_s@rad.com

   Itai Mendelsohn
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5761
   Email: itai_m@rad.com

   Ron Insler
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5445
   Email: ron_i@rad.com

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