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PWE3                                                      S. Bryant, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                           Cisco Systems
Expires: April 25, 2011                                         U. Drafz
                                                        Deutsche Telekom
                                                             V. Kompella
                                                                J. Regan
                                                               S. Amante
                                                  Level 3 Communications
                                                        October 22, 2010

          Flow Aware Transport of Pseudowires over an MPLS PSN


   Where the payload carried over a pseudowire carries a number of
   identifiable flows it can in some circumstances be desirable to carry
   those flows over the equal cost multiple paths (ECMPs) that exist in
   the packet switched network.  Most forwarding engines are able to
   hash based on label stacks and use this to balance flows over ECMPs.
   This draft describes a method of identifying the flows, or flow
   groups, to the label switched routers by including an additional
   label in the label stack.

Requirements Language

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

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."

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   This Internet-Draft will expire on April 25, 2011.

Copyright Notice

   Copyright (c) 2010 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
   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  ECMP in Label Switched Routers . . . . . . . . . . . . . .  5
     1.2.  Flow Label . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Native Service Processing Function . . . . . . . . . . . . . .  6
   3.  Pseudowire Forwarder . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Encapsulation  . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Signaling the Presence of the Flow Label . . . . . . . . . . .  8
     4.1.  Structure of Flow Label Sub-TLV  . . . . . . . . . . . . .  9
   5.  Multi-Segment Pseudowires  . . . . . . . . . . . . . . . . . . 10
   6.  OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Applicability of FAT PWs . . . . . . . . . . . . . . . . . . . 11
     7.1.  Equal Cost Multiple Paths  . . . . . . . . . . . . . . . . 12
     7.2.  Link Aggregation Groups  . . . . . . . . . . . . . . . . . 13
     7.3.  Multiple RSVP-TE Paths . . . . . . . . . . . . . . . . . . 13
     7.4.  The Single Large Flow Case . . . . . . . . . . . . . . . . 13
     7.5.  Applicability to MPLS-TP . . . . . . . . . . . . . . . . . 15
     7.6.  Asymmetric Operation . . . . . . . . . . . . . . . . . . . 15
   8.  Applicability to MPLS  . . . . . . . . . . . . . . . . . . . . 15
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   11. Congestion Considerations  . . . . . . . . . . . . . . . . . . 16
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     13.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18

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1.  Introduction

   A pseudowire (PW) [RFC3985] is normally transported over one single
   network path, even if multiple Equal Cost Multiple Paths (ECMP) exit
   between the ingress and egress PW provider edge (PE)
   equipments[RFC4385] [RFC4928].  This is required to preserve the
   characteristics of the emulated service (e.g. to avoid misordering
   SAToP pseudowire packets [RFC4553] or subjecting the packets to
   unusable inter-arrival times ).  The use of a single path to preserve
   order remains the default mode of operation of a pseudowire (PW).
   The new capability proposed in this document is an OPTIONAL mode
   which may be used when the use of ECMP paths for is known to be
   beneficial (and not harmful) to the operation of the PW.

   Some pseudowires are used to transport large volumes of IP traffic
   between routers at two locations.  One example of this is the use of
   an Ethernet pseudowire to create a virtual direct link between a pair
   of routers.  Such pseudowire's may carry from hundred's of Mbps to
   Gbps of traffic.  Such pseudowire's do not require strict ordering to
   be preserved between packets of the pseudowire.  They only require
   ordering to be preserved within the context of each individual
   transported IP flow.  Some operators have requested the ability to
   explicitly configure such a pseudowire to leverage the availability
   of multiple ECMP paths.  This allows for better capacity planning as
   the statistical multiplexing of a larger number of smaller flows is
   more efficient than with a smaller set of larger flows.  Although
   Ethernet is used as an example above, the mechanisms described in
   this draft are general mechanisms that may be applied to any
   pseudowire type in which there are identifiable flows, and in which
   there is no requirement to preserve the order between those flows.

   Typically, forwarding hardware can deduce that an IP payload is being
   directly carried by an MPLS label stack, and is capable of looking at
   some fields in packets to construct hash buckets for conversations or
   flows.  However, an intermediate node has no information on the type
   pseudowire being carried in the packet.  This limits the forwarder at
   the intermediate node to only being able to make an ECMP choice based
   on a hash of the label stack.  In the case of a pseudowire emulating
   a high bandwidth trunk, the granularity obtained by hashing the
   default label stack is inadequate for satisfactory load-balancing.
   The ingress node, however, is in the special position of being able
   to look at the un-encapsulated packet and spread flows amongst any
   available ECMP paths, or even any Loop-Free Alternates [RFC5286] .
   This draft proposes a method to introduce granularity on the hashing
   of traffic running over pseudowires by introducing an additional
   label, chosen by the ingress node, and placed at the bottom of the
   label stack.

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   In addition to providing an indication of the flow structure for use
   in ECMP forwarding decisions, the mechanism described in the document
   may also be used to select flows for distribution over an 802.1ad
   link aggregation group that has been used in an MPLS network.

1.1.  ECMP in Label Switched Routers

   Label switched routers commonly hash the label stack or some elements
   of the label stack as a method of discriminating between flows, in
   order to distribute those flows over the available equal cost
   multiple paths that exist in the network.  Since the label at the
   bottom of stack is usually the label most closely associated with the
   flow, this normally provides the greatest entropy, and hence is
   usually included in the hash.  This draft describes a method of
   adding an additional label at the bottom of stack in order to
   facilitate the load balancing of the flows within a pseudowire over
   the available ECMPs.  A similar design for general MPLS use has also
   been proposed [I-D.kompella-mpls-entropy-label], however that is
   outside the scope of this draft.

   An alternative method of load balancing by creating a number of
   pseudowires and distributing the flows amongst them was considered,
   but was rejected because:

   o  It did not introduce as much entropy as the load balance label

   o  It required additional pseudowires to be set up and maintained.

1.2.  Flow Label

   An additional label [RFC3032] is interposed between the pseudowire
   label and the control word, or if the control word is not present,
   between the pseudowire label and the pseudowire payload.  This
   additional label is called the flow label.  Indivisible flows within
   the pseudowire MUST be mapped to the same flow label by the ingress
   PE.  The flow label stimulates the correct ECMP load balancing
   behaviour in the packet switched network (PSN).  On receipt of the
   pseudowire packet at the egress PE (which knows this additional label
   is present) the flow label is discarded without processing.

   Note that the flow label MUST NOT be an MPLS reserved label (values
   in the range 0..15) [RFC3032], but is otherwise unconstrained by the

   Considerations of the TTL value are described in the Security section
   of this document.  The flow label can never become the top label in
   normal operation, and hence the TTL in the flow label is never used

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   to determine whether the packet should be discarded due to TTL
   expiry.  Therefore there are no lower restrictions on the TTL value.

   The use of the TC bits (formerly known as the EXP bits) in the flow
   label is outside the scope of this document.  Unless otherwise agreed
   by the ingress and egress PEs these bits MUST be set to zero by the
   ingress PE and MUST be ignored by the egress PE.

2.  Native Service Processing Function

   The Native Service Processing (NSP) function [RFC3985] is a component
   of a PE that has knowledge of the structure of the emulated service
   and is able to take action on the service outside the scope of the
   pseudowire.  In this case it is required that the NSP in the ingress
   PE identify flows, or groups of flows within the service, and
   indicate the flow (group) identity of each packet as it is passed to
   the pseudowire forwarder.  As an example, where the PW type is an
   Ethernet, the NSP might parse the ingress Ethernet traffic and
   consider all of the IP traffic.  This traffic could then be
   categorised into flows by considering all traffic with the same
   source and destination address pair to be a single indivisible flow.
   Since this is an NSP function, by definition, the method used to
   identify a flow is outside the scope of the pseudowire design.
   Similarly, since the NSP is internal to the PE, the method of flow
   indication to the pseudowire forwarder is outside the scope of this

3.  Pseudowire Forwarder

   The pseudowire forwarder must be provided with a method of mapping
   flows to load balanced paths.

   The forwarder must generate a label for the flow or group of flows.
   How the load balance label values are determined is outside the scope
   of this document, however the load balance label allocated to a flow
   MUST NOT be an MPLS reserved label and SHOULD remain constant for the
   life of the flow.  It is recommended that the method chosen to
   generate the load balancing labels introduces a high degree of
   entropy in their values, to maximise the entropy presented to the
   ECMP path selection mechanism in the LSRs in the PSN, and hence
   distribute the flows as evenly as possible over the available PSN
   ECMP paths.  The forwarder at the ingress PE prepends the pseudowire
   control word (if applicable), and then pushes the flow label,
   followed by the pseudowire label.

   The forwarder at the egress PE uses the pseudowire label to identify

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   the pseudowire.  From the context associated with the pseudowire
   label, the egress PE can determine whether a flow label is present.
   If a flow label is present, the label is discarded.

   All other pseudowire forwarding operations are unmodified by the
   inclusion of the flow label.

3.1.  Encapsulation

   The PWE3 Protocol Stack Reference Model modified to include flow
   label is shown in Figure 1 below

      +-------------+                                +-------------+
      |  Emulated   |                                |  Emulated   |
      |  Ethernet   |                                |  Ethernet   |
      | (including  |         Emulated Service       | (including  |
      |  VLAN)      |<==============================>|  VLAN)      |
      |  Services   |                                |  Services   |
      +-------------+                                +-------------+
      |    Flow     |                                |    Flow     |
      +-------------+            Pseudowire          +-------------+
      +-------------+                                +-------------+
      |    PSN      |            PSN Tunnel          |    PSN      |
      |   MPLS      |<==============================>|   MPLS      |
      +-------------+                                +-------------+
      |  Physical   |                                |  Physical   |
      +-----+-------+                                +-----+-------+

               Figure 1: PWE3 Protocol Stack Reference Model

   The encapsulation of a pseudowire with a flow label is shown in
   Figure 2 below

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    |                               |
    |            Payload            |
    |                               |  n octets
    |                               |
    |   Optional Control Word       |  4 octets
    |      Flow label               |  4 octets
    |      PW label                 |  4 octets
    |      MPLS Tunnel label(s)     | n*4 octets (four octets per label)

      Figure 2: Encapsulation of a pseudowire with a pseudowire load
                              balancing label

4.  Signaling the Presence of the Flow Label

   When using the signalling procedures in [RFC4447], a Pseudowire
   Interface Parameter Flow Label Sub-TLV (FL Sub-TLV) type is used to
   synchronise the flow label states between the ingress and egress PEs.

   The absence of a FL Sub-TLV indicates that the PE is unable process
   flow labels.  A PE that is using PW signalling and that does not send
   a FL Sub-TLV MUST NOT include a flow label in the PW packet.  A PE
   that is using PW signalling and which does not receive a FL Sub-TLV
   from its peer MUST NOT include a flow label in the PW packet.  This
   preserves backwards compatibility with existing PW specifications.

   A PE that wishes to send a flow label in a PW packet MUST include in
   its label mapping message a FL Sub-TLV with T = 1 (see Section 4.1).

   A PE that is willing to receive a flow label MUST include in its
   label mapping message a FL Sub-TLV with R = 1 (see Section 4.1).

   A PE that receives a label mapping message a FL Sub-TLV with R = 0
   MUST NOT include a flow label in the PW packet.

   Thus a PE sending a FL Sub-TLV with T = 1 and receiving a FL Sub-TLV
   with R = 1 MUST include a flow label in the PW packet.  Under all
   other combinations of FL Sub-TLV signalling a PE MUST NOT include a
   flow label in the PW packet.

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   The signalling procedures in [RFC4447] state that "Processing of the
   interface parameters should continue when unknown interface
   parameters are encountered, and they MUST be silently ignored."  The
   signalling procedure described here is therefore backwards compatible
   with existing implementations.

   If PWE3 signalling [RFC4447] is not in use for a pseudowire, then
   whether the flow label is used MUST be identically provisioned in
   both PEs at the pseudowire endpoints.  If there is no provisioning
   support for this option, the default behaviour is not to include the
   flow label.

   Note that what is signalled is the desire to include the flow label
   in the label stack.  The value of the label is a local matter for the
   ingress PE, and the label value itself is not signalled.

4.1.  Structure of Flow Label Sub-TLV

   The structure of the flow label TLV is shown in Figure 3.

    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
   | FL            |    Length     |T|R|      Reserved             |

                       Figure 3: Flow Label Sub-TLV


   o  FL is the flow label sub-TLV identifier assigned by IANA.

   o  Length is the length of the TLV in octets and is 4.

   o  When T=1 the PE is requesting the ability to send a PW packet that
      includes a flow label.  When T= 0, the PE is indicating that it
      will not send a PW packet containing a flow label.

   o  When R=1 the PE is able to receive a PW packet with a flow label
      present.  When R=0 the PE is unable to receive a PW packet with
      the flow label present.

   o  Reserved bits MUST be zero on transmit and MUST be ignored on

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5.  Multi-Segment Pseudowires

   The flow label mechanism described in this document works on multi-
   segment PWs without requiring modification to the Switching PEs
   (S-PEs).  This is because the flow label is transparent to the label
   swap operation, and because interface parameter Sub-TLV signalling is

6.  OAM

   The following OAM considerations apply to this method of load

   Where the OAM is only to be used to perform a basic test that the
   pseudowires have been configured at the PEs, VCCV [RFC5085] messages
   may be sent using any load balance pseudowire path, i.e. using any
   value for the flow label.

   Where it is required to verify that a pseudowire is fully functional
   for all flows, VCCV [RFC5085] connection verification message MUST be
   sent over each ECMP path to the pseudowire egress PE.  This problem
   is difficult to solve and scales poorly.  We believe that this
   problem is addressed by the following two methods:

   1.  If a failure occurs within the PSN, this failure will normally be
       detected by the PSN's Interior Gateway protocol (IGP) link/node
       failure detection mechanism (loss of light, bidirectional
       forwarding detection [RFC5880] or IGP hello detection), and the
       IGP convergence will naturally modify the ECMP set of network
       paths between the Ingress and Egress PE's.  Hence the PW is only
       impacted during the normal IGP convergence time.

   2.  If the failure is related to the individual corruption of an
       Label Forwarding Information database (LFIB) entry in a router,
       then only the network path using that specific entry is impacted.
       If the PW is load balanced over multiple network paths, then this
       failure can only be detected if, by chance, the transported OAM
       flow is mapped onto the impacted network path, or all paths are
       tested.  This type of error may be better solved be solved by
       other means such as LSP self test [I-D.ietf-mpls-lsr-self-test].

   To troubleshoot the MPLS PSN, including multiple paths, the
   techniques described in [RFC4378] and [RFC4379] can be used.

   Where the pseudowire OAM is carried out of band (VCCV Type 2)
   [RFC5085] it is necessary to insert an "MPLS Router Alert Label" in
   the label stack.  The resultant label stack is a follows:

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    |                               |
    |            Payload            |
    |                               |  n octets
    |                               |
    |   Optional Control Word       |  4 octets
    |      Flow label               |  4 octets
    |      PW label                 |  4 octets
    |      Router Alert label       |  4 octets
    |      MPLS Tunnel label(s)     | n*4 octets (four octets per label)

                    Figure 4: Use of Router Alert LAbel

7.  Applicability of FAT PWs

   A node within the PSN is not able to perform deep-packet-inspection
   (DPI) of the PW as the PW technology is not self-describing: the
   structure of the PW payload is only known to the ingress and egress
   PE devices.  The method proposed in this document provides a
   statistical mitigation of the problem of load balance in those cased
   where a PE is able to discern flows embedded in the traffic received
   on the attachment circuit.

   The methods describe in this document are transparent to the PSN and
   as such do not require any new capability from the PSN.

   The requirement to load-balance over multiple PSN paths occurs when
   the ratio between the PW access speed and the PSN's core link
   bandwidth is large (e.g. >= 10%).  ATM and FR are unlikely to meet
   this property.  Ethernet may have this property, and for that reason
   this document focuses on Ethernet.  Applications for other high-
   access-bandwidth PW's (e.g.  Fibre Channel) may be defined in the

   This design applies to MPLS pseudowires where it is meaningful to de-
   construct the packets presented to the ingress PE into flows.  The
   mechanism described in this document promotes the distribution of
   flows within the pseudowire over different network paths.  This in
   turn means that whilst packets within a flow are delivered in order
   (subject to normal IP delivery perturbations due to topology

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   variation), order is not maintained amongst packets of different
   flows.  It is not proposed to associate a different sequence number
   with each flow.  If sequence number support is required this
   mechanism is not applicable.

   Where it is known that the traffic carried by the Ethernet pseudowire
   is IP the method of identifying the flows are well known and can be
   applied.  Such methods typically include hashing on the source and
   destination addresses, the protocol ID and higher-layer flow-
   dependent fields such as TCP/UDP ports, L2TPv3 Session ID's etc.

   Where it is known that the traffic carried by the Ethernet pseudowire
   is non-IP, techniques used for link bundling between Ethernet
   switches may be reused.  In this case however the latency
   distribution would be larger than is found in the link bundle case.
   The acceptability of the increased latency is for further study.  Of
   particular importance the Ethernet control frames SHOULD always be
   mapped to the same PSN path to ensure in-order delivery.

7.1.  Equal Cost Multiple Paths

   ECMP in packet switched networks is statistical in nature.  The
   mapping of flows to a particular path does not take into account the
   bandwidth of the flow being mapped or the current bandwidth usage of
   the members of the ECMP set.  This simplification works well when the
   distribution of flows is evenly spread over the ECMP set and there
   are a large number of flows that have low bandwidth relative to the
   paths.  The random allocation of a flow to a path provides a good
   approximation to an even spread of flows, provided that polarisation
   effects are avoided.  The method proposed in this document has the
   same statistical properties as an IP PSN.

   ECMP is a load-sharing mechanism that is based on sharing the load
   over a number of layer 3 paths through the PSN.  Often however
   multiple links exist between a pair of LSRs that are considered by
   the IGP to be a single link.  These are known as link bundles.  The
   mechanism described in this document can also be used to distribute
   the flows within a pseudowire over the members of the link bundle by
   using the flow label value to identify candidate flows.  How that
   mapping takes place is outside the scope of this specification.
   Similar considerations apply to link aggregation groups.

   In the ECMP case and the link bundling case the NSP may attempt to
   take bandwidth into consideration when allocating groups of flows to
   a common path.  That is permitted, but it must be borne in mind that
   the semantics of a label stack entry (LSE) as defined by [RFC3032]
   cannot be modified, the value of the flow label cannot be modified at
   any point on the LSP, and the interpretation of bit patterns in, or

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   values of, the flow label by an LSR are undefined.

   A different type of load balancing is the desire to carry a
   pseudowire over a set of PSN links in which the bandwidth of members
   of the link set is less than the bandwidth of the pseudowire.  This
   problem is addressed in [I-D.stein-pwe3-pwbonding].  Such a mechanism
   can be considered complementary to this mechanism.

7.2.  Link Aggregation Groups

   A Link Aggregation Group (LAG) is used to bond together several
   physical circuits between two adjacent nodes so they appear to
   higher-layer protocols as a single, higher bandwidth "virtual" pipe.
   These may co-exist in various parts of a given network.  An advantage
   of LAGs is that they reduce the number of routing and signalling
   protocol adjacencies between devices, reducing control plane
   processing overhead.  As with ECMP, the key problem related to LAGs
   is that due to inefficiencies in LAG load-distribution algorithms, a
   particular component of a LAG may experience congestion.  The
   mechanism proposed here may be able to assist in producing a more
   uniform flow distribution.

   The same considerations requiring a flow to go over a single member
   of an ECMP path set apply to a member of a LAG.

7.3.  Multiple RSVP-TE Paths

   In some networks it is desirable for a Label Edge Router (LER) to be
   able to load balance a PW across multiple RSVP-TE tunnels.  The flow
   label mechanism described in this document may be used to provide the
   LER with the required flow information, and necessary entropy to
   provide this type of load balancing.  An example of such a case is
   the of the flow label mechanism in networks using a link bundle with
   the all ones component [RFC4201].

   Methods by which the LER is configured to apply this type of ECMP is
   outside the scope of this document.

7.4.  The Single Large Flow Case

   Clearly the operator should make sure that the service offered using
   PW technology and the method described in this document does not
   exceed the maximum planned link capacity, unless it can be guaranteed
   that it conforms to the Internet traffic profile of a very large
   number of small flows.

   If the payload on a PW is made of a single inner flow (i.e. an
   encrypted connection between two routers), or the flow identifiers

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   are too deeply buried in the packet, then the functionality described
   in this document does not give any benefits, though neither does it
   cause harm relative to the existing situation.  The most common case
   where a single flow dominated the traffic on a PW is when it is used
   to transport enterprise traffic.  Enterprise traffic may well consist
   of a large single TCP flows, or encrypted flows that cannot be
   handled by the methods described in this document.

   An operator has six options under these circumstances:

   1.  The operator can do nothing and the system will work as it does
       without the flow label.

   2.  The operator can make the customer aware that the service
       offering has a restriction on flow bandwidth and police flows to
       that restriction.  This would allow customers offering multiple
       flows to use a larger fraction their access bandwidth, whilst
       preventing an single flow from consuming a fraction of internal
       link bandwidth that the operator considered excessive.

   3.  The operator could configure the ingress PE to assign a constant
       flow label to all high bandwidth flows so that only one path was
       affected by these flows,

   4.  The operator could configure the ingress PE to assign a random
       flow label to all high bandwidth flows so as to minimise the
       disruption to the network as a cost of out of order traffic to
       the user.

   5.  The operator could configure the ingress to assign a label of
       special significance (such as a reserved label) to all high
       bandwidth flows so that some other action (not specified in this
       document) could be taken on the flow.

   The issues described above are mitigated by the following two

   o  Firstly, the customer of a high-bandwidth PW service has an
      incentive to get the best transport service because an inefficient
      use of the PSN leads to jitter and eventually to loss to the PW's

   o  Secondly, the customer is usually able to tailor their
      applications to generate many flows in the PSN.  A well-known
      example is massive data transport between servers which use many
      parallel TCP sessions.  This same technique can be used by any
      transport protocol: multiple UDP ports, multiple L2TPv3 Session
      ID's, multiple GRE keys may be used to decompose a large flow into

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      smaller components.  This approach may be applied to IPsec
      [RFC4301] where multiple Security Parameters Indexes (SPI's) may
      be allocated to the same security association.

7.5.  Applicability to MPLS-TP

   The MPLS Transport Profile (MPLS-TP) [RFC5654] requirement 44 states
   that "MPLS-TP MUST support mechanisms that ensure the integrity of
   the transported customer's service traffic as required by its
   associated SLA.  Loss of integrity may be defined as packet
   corruption, reordering, or loss during normal network conditions. "
   The flow aware transport of a PW reorders packets, therefore MUST NOT
   be deployed in a network conforming to the MPLS-TP unless these
   integrity requirements specified in the SLA can be satisfied.

7.6.  Asymmetric Operation

   The protocol defined in this document supports the asymmetric
   inclusion of the FAT label.  Asymmetric operation can be expected
   when there is asymmetry in the bandwidth requirements making it
   unprofitable for one PE to perform the flow classification, or when
   that PE is otherwise unable to perform the classification but is able
   to receive flow labeled packet from its peer.  Asymmetric operation
   of the PW may also be required when one PE has a high transmission
   bandwidth requirement, but has a need to receive the entire PW on a
   single interface in order to perform a processing operation that
   requires the context of the complete PW (for example policing of the
   egress traffic).

8.  Applicability to MPLS

   A further application of this technique would be to create a basis
   for hash diversity without having to peek below the label stack for
   IP traffic carried over LDP LSPs.  Work on the generalisation of this
   to MPLS has been described in [I-D.kompella-mpls-entropy-label].
   This is can be regarded as a complementary, but distinct, approach
   since although similar consideration may apply to the identification
   of flows and the allocation of flow label values, the flow labels are
   imposed by different network components, and the associated
   signalling mechanisms are different.

9.  Security Considerations

   The pseudowire generic security considerations described in [RFC3985]
   and the security considerations applicable to a specific pseudowire
   type (for example, in the case of an Ethernet pseudowire [RFC4448]

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   The ingress PE SHOULD take steps to ensure that the load-balance
   label is not used as a covert channel.

   It is useful to give consideration to the choice of TTL value in the
   flow label stack entry [RFC3032].  The flow label is at the bottom of
   label stack.  Therefore, even when penultimate hop popping is
   employed, it will always be will preceded by the PW label on arrival
   at the PE.  The flow label TTL should therefore never be considered
   by the forwarder, and hence SHOULD be set to a value of 1.  This will
   prevent the packet being inadvertently forwarded based on the value
   of the flow label.  Note that this may be a departure from
   considerations that apply to the general MPLS case.

10.  IANA Considerations

   IANA is requested to allocate the next available values from the IETF
   Consensus range in the Pseudowire Interface Parameters Sub-TLV type
   Registry as a Flow Label indicator.  The allocation of value 17 is

   Parameter  Length       Description

   17         4           Flow Label

11.  Congestion Considerations

   The congestion considerations applicable to pseudowires as described
   in [RFC3985] and any additional congestion considerations developed
   at the time of publication apply to this design.

   The ability to explicitly configure a PW to leverage the availability
   of multiple ECMP paths is beneficial to capacity planning as, all
   other parameters being constant, the statistical multiplexing of a
   larger number of smaller flows is more efficient than with a smaller
   number of larger flows.

   Note that if the classification into flows is only performed on IP
   packets the behaviour of those flows in the face of congestion will
   be as already defined by the IETF for packets of that type and no
   additional congestion processing is required.

   Where flows that are not IP are classified pseudowire congestion
   avoidance must be applied to each non-IP load balance group.

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12.  Acknowledgements

   The authors wish to thank Eric Grey, Kireeti Kompella, Joerg
   Kuechemann, Wilfried Maas, Luca Martini, Mark Townsley, and Lucy Yong
   for valuable comments on this document.

13.  References

13.1.  Normative References

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

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, January 2001.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4385]  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.

   [RFC4447]  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.

   [RFC4448]  Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
              "Encapsulation Methods for Transport of Ethernet over MPLS
              Networks", RFC 4448, April 2006.

   [RFC4553]  Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
              Division Multiplexing (TDM) over Packet (SAToP)",
              RFC 4553, June 2006.

   [RFC4928]  Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal
              Cost Multipath Treatment in MPLS Networks", BCP 128,
              RFC 4928, June 2007.

   [RFC5085]  Nadeau, T. and C. Pignataro, "Pseudowire Virtual Circuit
              Connectivity Verification (VCCV): A Control Channel for
              Pseudowires", RFC 5085, December 2007.

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13.2.  Informative References

              Swallow, G., "Label Switching Router Self-Test",
              draft-ietf-mpls-lsr-self-test-07 (work in progress),
              May 2007.

              Kompella, K. and S. Amante, "The Use of Entropy Labels in
              MPLS Forwarding", draft-kompella-mpls-entropy-label-01
              (work in progress), July 2010.

              Stein, Y., Mendelsohn, I., and R. Insler, "PW Bonding",
              draft-stein-pwe3-pwbonding-01 (work in progress),
              November 2008.

   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
              Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC4201]  Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling
              in MPLS Traffic Engineering (TE)", RFC 4201, October 2005.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC4378]  Allan, D. and T. Nadeau, "A Framework for Multi-Protocol
              Label Switching (MPLS) Operations and Management (OAM)",
              RFC 4378, February 2006.

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast
              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

   [RFC5654]  Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "Requirements of an MPLS Transport Profile",
              RFC 5654, September 2009.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

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Authors' Addresses

   Stewart Bryant (editor)
   Cisco Systems
   250 Longwater Ave
   Reading  RG2 6GB
   United Kingdom

   Phone: +44-208-824-8828
   Email: stbryant@cisco.com

   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com

   Ulrich Drafz
   Deutsche Telekom

   Email: Ulrich.Drafz@t-com.net

   Vach Kompella

   Email: Alcatel-Lucent vach.kompella@alcatel-lucent.com

   Joe Regan

   Email: joe.regan@alcatel-lucent.comRegan

   Shane Amante
   Level 3 Communications

   Email: shane@castlepoint.net

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