[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03

PWE3                                                      S. Bryant, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                           Cisco Systems
Expires: September 3, 2009                                      U. Drafz
                                                        Deutsche Telekom
                                                             V. Kompella
                                                                J. Regan
                                                          Alcatel-Lucent
                                                               S. Amante
                                                  Level 3 Communications
                                                           March 2, 2009


                Flow Aware Transport of MPLS Pseudowires
                    draft-bryant-filsfils-fat-pw-03

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 3, 2009.

Copyright Notice

   Copyright (c) 2009 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 in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights



Bryant, et al.          Expires September 3, 2009               [Page 1]


Internet-Draft                   FAT-PW                       March 2009


   and restrictions with respect to this document.

Abstract

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119 [1].

































Bryant, et al.          Expires September 3, 2009               [Page 2]


Internet-Draft                   FAT-PW                       March 2009


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  . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Signaling the Presence of the Flow Label . . . . . . . . . . .  7
     4.1.  Structure of Flow Label TLV  . . . . . . . . . . . . . . .  8
   5.  OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  ECMP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     6.2.  Link Aggregation Groups  . . . . . . . . . . . . . . . . . 12
     6.3.  The Single Large Flow Case . . . . . . . . . . . . . . . . 12
   7.  Applicability to MPLS  . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   10. Congestion Considerations  . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     12.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16



























Bryant, et al.          Expires September 3, 2009               [Page 3]


Internet-Draft                   FAT-PW                       March 2009


1.  Introduction

   A pseudowire [11] is normally transported over one single network
   path, even if multiple Equal Cost Multiple Paths (ECMP) exit between
   the ingress and egress PEs[2] [3].  This is required to preserve the
   characteristics of the emulated service (e.g. to avoid misordering
   SAToP pseudowire's [4]).  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
   the 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 an
   available ECMP paths, or even Loop-Free Alternates I [12] .  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.

   In addition to providing an indication of the flow structure for use



Bryant, et al.          Expires September 3, 2009               [Page 4]


Internet-Draft                   FAT-PW                       March 2009


   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 [13], 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
      method.

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

1.2.  Flow Label

   An additional label 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
   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) [5], but is otherwise unconstrained by the
   protocol.

   Considerations of the TTL value are described in the Security section
   of this document.  In the case of a pseudowire there are no lower
   restrictions on the label value since the TTL is never the top label.
   The designers of the generalized solution [13].




Bryant, et al.          Expires September 3, 2009               [Page 5]


Internet-Draft                   FAT-PW                       March 2009


2.  Native Service Processing Function

   The Native Service Processing (NSP) function 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.  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 document


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
   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






Bryant, et al.          Expires September 3, 2009               [Page 6]


Internet-Draft                   FAT-PW                       March 2009


      +-------------+                                +-------------+
      |  Emulated   |                                |  Emulated   |
      |  Ethernet   |                                |  Ethernet   |
      | (including  |         Emulated Service       | (including  |
      |  VLAN)      |<==============================>|  VLAN)      |
      |  Services   |                                |  Services   |
      +-------------+                                +-------------+
      |    Flow     |                                |    Flow     |
      +-------------+            Pseudowire          +-------------+
      |Demultiplexer|<==============================>|Demultiplexer|
      +-------------+                                +-------------+
      |    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

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


      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 [6], there is a Pseudowire
   Interface Parameter Sub-TLV type used to synchronize the flow label
   states between the ingress and egress PEs.



Bryant, et al.          Expires September 3, 2009               [Page 7]


Internet-Draft                   FAT-PW                       March 2009


   The absence of a flow label (FL) TLV by either party indicates that
   the PE concerned is unable to recognize this TLV and the sender of
   the FL TLV MUST send a new label mapping without the FL TLV.  This
   preserves backwards compatibility with existing PEs that do not
   understand the FL TLV or that cannot, do not wish to, process the
   flow label.

   A PE that wishes to use a flow label sends an FL TLV with the F bit
   set.  A PE that can correctly process a flow label and is willing to
   receive on, but does not wish to send a flow label sends an FL TLV
   with the F bit clear.  A PE that sends an FL TLV with the F bit set
   and receives an FL TLV with or without the F bit set MUST include the
   flow label between the pseudowire label and the control word (or is
   the control word is not present between the pseudowire label and the
   pseudowire payload).

   If PWE3 signalling [6] 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 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     |F|       must be zero          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: Multiple VC TLV

   Where:

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

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

   o  When F=1 a flow label will be pushed.  When F=0 a flow label will
      not be pushed.






Bryant, et al.          Expires September 3, 2009               [Page 8]


Internet-Draft                   FAT-PW                       March 2009


5.  OAM

   The following OAM considerations apply to this method of load
   balancing.

   Where the OAM is only to be used to perform a basic test that the
   pseudowires have been configured at the PEs, VCCV [7] 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 [7] 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 IGP (link/node failure, link or BFD 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 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
       [14].

   To troubleshoot the MPLS PSN, including multiple paths, the
   techniques described in [8] and [9] can be used.

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













Bryant, et al.          Expires September 3, 2009               [Page 9]


Internet-Draft                   FAT-PW                       March 2009


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

                    Figure 4: Use of Router Alert LAbel


6.  Applicability

   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 does and this is the reason why this
   document focuses on Ethernet.  Applications for other high-access-
   bandwidth PW's (fiber-channel) may be defined in the future.

   This design applies to MPLS pseudowires where it is meaningful to
   deconstruct 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
   variation), order is not maintained amongst packets of different
   flows.  It is not proposed to associate a different sequence number



Bryant, et al.          Expires September 3, 2009              [Page 10]


Internet-Draft                   FAT-PW                       March 2009


   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.

6.1.  ECMP

   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.  A random allocation of a flow to a path provides a good
   approximation to an even spread provided polarization 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 [5] 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 values
   of the flow label by an LSR are undefined.




Bryant, et al.          Expires September 3, 2009              [Page 11]


Internet-Draft                   FAT-PW                       March 2009


   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 [15].  Such a mechanism can be considered
   complementary to this mechanism.

6.2.  Link Aggregation Groups

   Link Aggregation (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 signaling protocol
   adjacencies between devices, reducing control plane processing
   overhead.  As with ECMP key problem related to LAG is, due to
   inefficiencies in LAG load-distribution algorithms, a particular
   component- link may experience congestion, and 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.

6.3.  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
   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



Bryant, et al.          Expires September 3, 2009              [Page 12]


Internet-Draft                   FAT-PW                       March 2009


       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 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
   factors:

   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
      payload.

   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
      smaller components.  This approach may be applied to IPsec where
      multiple SPI's may be allocated to the same security association.


7.  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 generalization of this
   to MPLS has been described in draft-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.



Bryant, et al.          Expires September 3, 2009              [Page 13]


Internet-Draft                   FAT-PW                       March 2009


8.  Security Considerations

   The pseudowire generic security considerations described in [11] and
   the security considerations applicable to a specific pseudowire type
   (for example, in the case of an Ethernet pseudowire [10] apply.

   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 LSE.  Since the flow label is the bottom of stack and even
   when PHP is employed will on arrival at the egress PE be prepended by
   the PW label, the flow label TTL MAY 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.


9.  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.

   Parameter  Length       Description

   TBD         4            Load Balancing Label


10.  Congestion Considerations

   The congestion considerations applicable to pseudowires as described
   in [11] 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.



Bryant, et al.          Expires September 3, 2009              [Page 14]


Internet-Draft                   FAT-PW                       March 2009


11.  Acknowledgements

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


12.  References

12.1.  Normative References

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

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

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

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

   [6]   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.

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

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

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

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




Bryant, et al.          Expires September 3, 2009              [Page 15]


Internet-Draft                   FAT-PW                       March 2009


12.2.  Informative References

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

   [12]  Zinin, A., Torvi, R., Choudhury, G., Martin, C., Imhoff, B.,
         and D. Fedyk, "Basic Specification for IP Fast-Reroute: Loop-
         free Alternates", draft-ietf-rtgwg-ipfrr-spec-base-12 (work in
         progress), March 2008.

   [13]  Kompella, K. and S. Amante, "The Use of Entropy Labels in MPLS
         Forwarding", draft-kompella-mpls-entropy-label-00 (work in
         progress), July 2008.

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

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


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
   Brussels
   Belgium

   Email: cfilsfil@cisco.com











Bryant, et al.          Expires September 3, 2009              [Page 16]


Internet-Draft                   FAT-PW                       March 2009


   Ulrich Drafz
   Deutsche Telekom
   Muenster,
   Germany

   Phone:
   Fax:
   Email: Ulrich.Drafz@t-com.net
   URI:


   Vach Kompella
   Alcatel-Lucent


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


   Joe Regan
   Alcatel-Lucent


   Phone:
   Fax:
   Email: joe.regan@alcatel-lucent.comRegan
   URI:


   Shane Amante
   Level 3 Communications


   Phone:
   Fax:
   Email: shane@castlepoint.net
   URI:












Bryant, et al.          Expires September 3, 2009              [Page 17]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/