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rfc:rfc6391

Internet Engineering Task Force (IETF) S. Bryant, Ed. Request for Comments: 6391 C. Filsfils Category: Standards Track Cisco Systems ISSN: 2070-1721 U. Drafz

                                                      Deutsche Telekom
                                                           V. Kompella
                                                              J. Regan
                                                        Alcatel-Lucent
                                                             S. Amante
                                           Level 3 Communications, LLC
                                                         November 2011

Flow-Aware Transport of Pseudowires over an MPLS Packet Switched Network

Abstract

 Where the payload of a pseudowire comprises a number of distinct
 flows, it can 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 generate a hash of the MPLS label
 stack and use this mechanism to balance MPLS flows over ECMPs.
 This document describes a method of identifying the flows, or flow
 groups, within pseudowires such that Label Switching Routers can
 balance flows at a finer granularity than individual pseudowires.
 The mechanism uses an additional label in the MPLS label stack.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6391.

Bryant, et al. Standards Track [Page 1] RFC 6391 FAT-PW November 2011

Copyright Notice

 Copyright (c) 2011 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.

Table of Contents

 1. Introduction ....................................................3
    1.1. Requirements Language ......................................4
    1.2. ECMP in Label Switching Routers ............................4
    1.3. Flow Label .................................................4
 2. Native Service Processing Function ..............................5
 3. Pseudowire Forwarder ............................................6
    3.1. Encapsulation ..............................................7
 4. Signalling the Presence of the Flow Label .......................8
    4.1. Structure of Flow Label Sub-TLV ............................9
 5. Static Pseudowires ..............................................9
 6. Multi-Segment Pseudowires .......................................9
 7. Operations, Administration, and Maintenance (OAM) ..............10
 8. Applicability of PWs Using Flow Labels .........................11
    8.1. Equal Cost Multiple Paths .................................12
    8.2. Link Aggregation Groups ...................................13
    8.3. Multiple RSVP-TE Paths ....................................13
    8.4. The Single Large Flow Case ................................14
    8.5. Applicability to MPLS-TP ..................................15
    8.6. Asymmetric Operation ......................................15
 9. Applicability to MPLS LSPs .....................................15
 10. Security Considerations .......................................16
 11. IANA Considerations ...........................................16
 12. Congestion Considerations .....................................16
 13. Acknowledgements ..............................................17
 14. References ....................................................17
    14.1. Normative References .....................................17
    14.2. Informative References ...................................18

Bryant, et al. Standards Track [Page 2] RFC 6391 FAT-PW November 2011

1. Introduction

 A pseudowire (PW) [RFC3985] is normally transported over one single
 network path, even if multiple Equal Cost Multiple Paths (ECMPs)
 exist between the ingress and egress PW provider edge (PE) equipment
 [RFC4385] [RFC4928].  This is required to preserve the
 characteristics of the emulated service (e.g., to avoid misordering
 Structure-Agnostic Time Division Multiplexing over Packet (SAToP) PW
 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 PW.  The new capability proposed in
 this document is an OPTIONAL mode that may be used when the use of
 ECMPs is known to be beneficial (and not harmful) to the operation of
 the PW.
 Some PWs are used to transport large volumes of IP traffic between
 routers.  One example of this is the use of an Ethernet PW to create
 a virtual direct link between a pair of routers.  Such PWs may carry
 from hundreds of Mbps to Gbps of traffic.  These PWs only require
 packet ordering to be preserved within the context of each individual
 transported IP flow.  They do not require packet ordering to be
 preserved between all packets of all IP flows within the pseudowire.
 The ability to explicitly configure such a PW to leverage the
 availability of multiple ECMPs 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.
 Typically, forwarding hardware can deduce that an IP payload is being
 directly carried by an MPLS label stack, and it is capable of looking
 at some fields in packets to construct hash buckets for conversations
 or flows.  However, when the MPLS payload is a PW, an intermediate
 node has no information on the type of PW 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 MPLS label
 stack.  In the case of a PW emulating a high-bandwidth trunk, the
 granularity obtained by hashing the label stack is inadequate for
 satisfactory load balancing.  The ingress node, however, is in the
 special position of being able to understand the unencapsulated
 packet header to assist with spreading flows among any available
 ECMPs, or even any Loop-Free Alternates [RFC5286].  This document
 defines a method to introduce granularity on the hashing of traffic
 running over PWs by introducing an additional label, chosen by the
 ingress node, and placed at the bottom of the label stack.

Bryant, et al. Standards Track [Page 3] RFC 6391 FAT-PW November 2011

 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 IEEE
 802.1AX-2008 (originally specified as IEEE 802.3ad-2000) Link
 Aggregation Group (LAG) that has been used in an MPLS network.
 NOTE: Although Ethernet is frequently referenced as a use case in
 this RFC, the mechanisms described in this document are general
 mechanisms that may be applied to any PW type in which there are
 identifiable flows, and in which there is no requirement to preserve
 the order between those flows.

1.1. 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 RFC 2119 [RFC2119].

1.2. ECMP in Label Switching Routers

 Label Switching Routers (LSRs) commonly generate a hash of the label
 stack or some elements of the label stack as a method of
 discriminating between flows and use this to distribute those flows
 over the available ECMPs that exist in the network.  Since the label
 at the bottom of the 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 document
 describes a method of adding an additional Label Stack Entry (LSE) at
 the bottom of the stack in order to facilitate the load balancing of
 the flows within a PW over the available ECMPs.  A similar design for
 general MPLS use has also been proposed [MPLS-ENTROPY]; see Section 9
 of this document.
 An alternative method of load balancing by creating a number of PWs
 and distributing the flows amongst them was considered, but was
 rejected because:
 o  It did not introduce as much entropy as can be introduced by
    adding an additional LSE.
 o  It required additional PWs to be set up and maintained.

1.3. Flow Label

 An additional LSE [RFC3032] is interposed between the PW LSE and the
 control word, or if the control word is not present, between the PW
 LSE and the PW payload.  This additional LSE is called the flow LSE,
 and the label carried by the flow LSE is called the flow label.

Bryant, et al. Standards Track [Page 4] RFC 6391 FAT-PW November 2011

 Indivisible flows within the PW 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 PW packet at the egress PE (which knows a flow LSE is
 present), the flow LSE 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
 protocol.
 It is useful to give consideration to the choice of Time to Live
 (TTL) value in the flow LSE [RFC3032].  The flow LSE is at the bottom
 of the label stack; therefore, even when penultimate hop popping is
 employed, it will always be preceded by the PW label on arrival at
 the PE.  If, due to an error condition, the flow LSE becomes the top
 of the stack, it might be examined as if it were a normal LSE, and
 the packet might then be forwarded.  This can be prevented by setting
 the flow LSE TTL to 1, thereby forcing the packet to be discarded by
 the forwarder.  Note that setting the TTL to 1 regardless of the
 payload may be considered a departure from the TTL procedures defined
 in [RFC3032] that apply to the general MPLS case.
 This document does not define a use for the Traffic Class (TC) field
 [RFC5462] (formerly known as the Experimental Use (EXP) bits
 [RFC3032]) in the flow label.  Future documents may define a use for
 these bits; therefore, implementations conforming to this
 specification MUST set the TC field to zero at the ingress and MUST
 ignore them at the egress.

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
 PW.  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 PW design.  Similarly,
 since the NSP is internal to the PE, the method of flow indication to
 the PW forwarder is outside the scope of this document.

Bryant, et al. Standards Track [Page 5] RFC 6391 FAT-PW November 2011

3. Pseudowire Forwarder

 The PW 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 flow label values are determined is outside the scope of this
 document; however, the flow 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 introduce a high degree of entropy in their values,
 to maximise the entropy presented to the ECMP selection mechanism in
 the LSRs in the PSN, and hence distribute the flows as evenly as
 possible over the available PSN ECMP.  The forwarder at the ingress
 PE prepends the PW control word (if applicable), and then pushes the
 flow label, followed by the PW label.
 NOTE: Although this document does not attempt to specify any hash
 algorithms, it is suggested that any such algorithm should be based
 on the assumption that there will be a high degree of entropy in the
 values assigned to the flow labels.
 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 LSE is present.  If
 a flow LSE is present, it MUST be checked to determine whether it
 carries a reserved label.  If it is a reserved label, the packet is
 processed according to the rules associated with that reserved label;
 otherwise, the LSE is discarded.
 All other PW forwarding operations are unmodified by the inclusion of
 the flow LSE.

Bryant, et al. Standards Track [Page 6] RFC 6391 FAT-PW November 2011

3.1. Encapsulation

 The PWE3 Protocol Stack Reference Model modified to include flow LSE
 is shown in Figure 1.
    +-------------+                                +-------------+
    |  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 PW with a flow LSE is shown in Figure 2.
     +---------------------------+
     |                           |
     |  Payload                  |
     |                           |  n octets
     |                           |
     +---------------------------+
     |  Optional Control Word    |  4 octets
     +---------------------------+
     |  Flow LSE                 |  4 octets
     +---------------------------+
     |  PW LSE                   |  4 octets
     +---------------------------+
     |  MPLS Tunnel LSE (s)      |  n*4 octets (four octets per LSE)
     +---------------------------+
  Figure 2: Encapsulation of a Pseudowire with a Pseudowire Flow LSE

Bryant, et al. Standards Track [Page 7] RFC 6391 FAT-PW November 2011

4. Signalling the Presence of the Flow Label

 When using the signalling procedures in [RFC4447], a new Pseudowire
 Interface Parameter Sub-TLV, the Flow Label Sub-TLV (FL Sub-TLV), is
 used to synchronise the flow label states between the ingress and
 egress PEs.
 The absence of an FL Sub-TLV indicates that the PE is unable to
 process flow labels.  An ingress PE that is using PW signalling and
 that does not send an FL Sub-TLV MUST NOT include a flow label in the
 PW packet.  An ingress PE that is using PW signalling and that does
 not receive an FL Sub-TLV from its egress 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 an 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 an FL Sub-TLV with R = 1 (see Section 4.1).
 A PE that receives a label mapping message containing an FL Sub-TLV
 with R = 0 MUST NOT include a flow label in the PW packet.
 Thus, a PE sending an FL Sub-TLV with T = 1 and receiving an 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.
 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.
 Note that what is signalled is the desire to include the flow LSE in
 the label stack.  The value of the flow label is a local matter for
 the ingress PE, and the label value itself is not signalled.

Bryant, et al. Standards Track [Page 8] RFC 6391 FAT-PW November 2011

4.1. Structure of Flow Label Sub-TLV

 The structure of the Flow Label Sub-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=0x17       |    Length     |T|R|      Reserved             |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 3: Flow Label Sub-TLV
 Where:
 o  FL (value 0x17) is the Flow Label Sub-TLV identifier assigned by
    IANA (see Section 11).
 o  Length is the length of the Sub-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
    receive.

5. Static Pseudowires

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

6. 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 LSE is transparent to the label
 swap operation, and because interface parameter Sub-TLV signalling is
 transitive.

Bryant, et al. Standards Track [Page 9] RFC 6391 FAT-PW November 2011

7. Operations, Administration, and Maintenance (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 to verify
 that the PWs have been configured at the PEs, Virtual Circuit
 Connectivity Verification (VCCV) [RFC5085] messages may be sent using
 any load balance PW 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, a VCCV [RFC5085] connectivity verification message
 MUST be sent over each ECMP path to the pseudowire egress PE.  This
 solution may be difficult to achieve and scales poorly.  Under these
 circumstances, it may be sufficient to send VCCV messages using any
 load balance pseudowire path, because if a failure occurs within the
 PSN, the failure will normally be detected and repaired by the PSN.
 That is, 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 PEs.  Hence, the PW is only impacted during the
 normal IGP convergence time.  Note that this period may be reduced if
 a fast re-route or fast convergence technology is deployed in the
 network [RFC4090] [RFC5286].
 If the failure is related to the individual corruption of a Label
 Forwarding Information Base (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 if all paths are tested.  Since testing
 all paths may present problems as noted above, other mechanisms to
 detect this type of error may need to be developed, such as a Label
 Switched Path (LSP) self-test technology.
 To troubleshoot the MPLS PSN, including multiple paths, the
 techniques described in [RFC4378] and [RFC4379] can be used.
 Where the PW 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 as follows:

Bryant, et al. Standards Track [Page 10] RFC 6391 FAT-PW November 2011

 +-------------------------------+
 |                               |
 |      VCCV Message             |  n octets
 |                               |
 +-------------------------------+
 |   Optional Control Word       |  4 octets
 +-------------------------------+
 |      Flow LSE                 |  4 octets
 +-------------------------------+
 |      PW LSE                   |  4 octets
 +-------------------------------+
 |      Router Alert LSE         |  4 octets
 +-------------------------------+
 |      MPLS Tunnel LSE(s)       |  n*4 octets (four octets per label)
 +-------------------------------+
                  Figure 4: Use of Router Alert Label
 Note that, depending on the number of labels hashed by the LSR, the
 inclusion of the Router Alert label may cause the OAM packet to be
 load-balanced to a different path from that taken by the data packets
 with identical flow and PW labels.

8. Applicability of PWs Using Flow Labels

 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 cases
 where a PE is able to discern flows embedded in the traffic received
 on the attachment circuit.
 The methods described 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 Frame Relay 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 PWs may be defined in the future.
 This design applies to MPLS PWs 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 PW over different network paths.  In turn, this
 means that whilst packets within a flow are delivered in order

Bryant, et al. Standards Track [Page 11] RFC 6391 FAT-PW November 2011

 (subject to normal IP delivery perturbations due to topology
 variation), order is no longer maintained for all packets sent over
 the PW.  It is not proposed to associate a different sequence number
 with each flow.  If sequence number support is required, the flow
 label mechanism MUST NOT be used.
 Where it is known that the traffic carried by the Ethernet PW is IP,
 the flows can be identified and mapped to an ECMP.  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, Layer 2 Tunneling Protocol version 3 (L2TPv3) Session
 IDs, etc.
 Where it is known that the traffic carried by the Ethernet PW 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.

8.1. 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 defined 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 PW 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.
 There is no mechanism currently defined to indicate the bandwidths in
 use by specific flows using the fields of the MPLS shim header.
 Furthermore, since the semantics of the MPLS shim header are fully
 defined in [RFC3032] and [RFC5462], those fields cannot be assigned

Bryant, et al. Standards Track [Page 12] RFC 6391 FAT-PW November 2011

 semantics to carry this information.  This document does not define
 any semantic for use in the TTL or TC fields of the label entry that
 carries the flow label, but requires that the flow label itself be
 selected with a high degree of entropy suggesting that the label
 value should not be overloaded with additional meaning in any
 subsequent specification.
 A different type of load balancing is the desire to carry a PW over a
 set of PSN links in which the bandwidth of members of the link set is
 less than the bandwidth of the PW.  Proposals to address this problem
 have been made in the past [PWBONDING].  Such a mechanism can be
 considered complementary to this mechanism.

8.2. 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 coexist 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 set apply to a member of a LAG.

8.3. 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 Resource Reservation
 Protocol - Traffic Engineering (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 use 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 are
 outside the scope of this document.

Bryant, et al. Standards Track [Page 13] RFC 6391 FAT-PW November 2011

8.4. 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 do not exceed
 the maximum planned link capacity, unless it can be guaranteed that
 they conform to the Internet traffic profile of a very large number
 of small flows.
 If the NSP cannot access sufficient information to distinguish flows,
 perhaps because the protocol stack required parsing further into the
 packet than it is able, then the functionality described in this
 document does not give any benefits.  The most common case where a
 single flow dominates the traffic on a PW is when it is used to
 transport enterprise traffic.  Enterprise traffic may well consist of
 a single, large TCP flow, or encrypted flows that cannot be handled
 by the methods described in this document.
 An operator has four options under these circumstances:
 1.  The operator can choose to 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 of their access bandwidth, whilst
     preventing a 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 at the cost of out-of-order traffic to
     the user.
 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.

Bryant, et al. Standards Track [Page 14] RFC 6391 FAT-PW November 2011

 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 that use many
    parallel TCP sessions.  This same technique can be used by any
    transport protocol: multiple UDP ports, multiple L2TPv3 Session
    IDs, or multiple Generic Routing Encapsulation (GRE) keys may be
    used to decompose a large flow into smaller components.  This
    approach may be applied to IPsec [RFC4301] where multiple Security
    Parameter Indexes (SPIs) may be allocated to the same security
    association.

8.5. 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 Service Level Agreement (SLA).  Loss of integrity may be
 defined as packet corruption, reordering, or loss during normal
 network conditions".  In addition, MPLS-TP makes extensive use of the
 fate sharing between OAM and data packets, which is defeated by the
 flow LSE.  The flow-aware transport of a PW reorders packets and
 therefore MUST NOT be deployed in a network conforming to MPLS-TP,
 unless these integrity requirements specified in the SLA can be
 satisfied.

8.6. Asymmetric Operation

 The protocol defined in this document supports the asymmetric
 inclusion of the flow LSE.  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 packets 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).

9. Applicability to MPLS LSPs

 An extension of this technique is to create a basis for hash
 diversity without having to peek below the label stack for IP traffic
 carried over Label Distribution Protocol (LDP) LSPs.  The
 generalisation of this extension to MPLS has been described in
 [MPLS-ENTROPY].  This generalisation can be regarded as a

Bryant, et al. Standards Track [Page 15] RFC 6391 FAT-PW November 2011

 complementary, but distinct, approach from the technique described in
 this document.  While 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.

10. Security Considerations

 The PW generic security considerations described in [RFC3985] and the
 security considerations applicable to a specific PW type (for
 example, in the case of an Ethernet PW [RFC4448]) apply.  The
 security considerations in [RFC5920] also apply.
 Section 1.3 describes considerations that apply to the TTL value used
 in the flow LSE.  The use of a TTL value of one prevents the
 accidental forwarding of a packet based on the label value in the
 flow LSE.

11. IANA Considerations

 IANA maintains the registry "Pseudowire Name Spaces (PWE3)" with
 sub-registry "Pseudowire Interface Parameters Sub-TLV type Registry".
 IANA has registered the Flow Label Sub-TLV type in this registry.
    Parameter     ID Length     Description      Reference
    ------------------------------------------------------
    0x17             4           Flow Label       RFC 6391

12. Congestion Considerations

 The congestion considerations applicable to PWs as described in
 [RFC3985] apply to this design.
 The ability to explicitly configure a PW to leverage the availability
 of multiple ECMPs 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, PW congestion avoidance
 must be applied to each non-IP load balance group.

Bryant, et al. Standards Track [Page 16] RFC 6391 FAT-PW November 2011

13. Acknowledgements

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

14. References

14.1. 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., Ed., 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., Ed., Rosen, E., El-Aawar, N., and G. Heron,
             "Encapsulation Methods for Transport of Ethernet over
             MPLS Networks", RFC 4448, April 2006.
 [RFC4553]   Vainshtein, A., Ed., and YJ. Stein, Ed., "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., Ed., and C. Pignataro, Ed., "Pseudowire
             Virtual Circuit Connectivity Verification (VCCV): A
             Control Channel for Pseudowires", RFC 5085,
             December 2007.

Bryant, et al. Standards Track [Page 17] RFC 6391 FAT-PW November 2011

14.2. Informative References

 [MPLS-ENTROPY]
             Kompella, K., Drake, J., Amante, S., Henderickx, W., and
             L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
             Work in Progress, October 2011.
 [PWBONDING] Stein, Y(J)., Mendelsohn, I., and R. Insler, "PW
             Bonding", Work in Progress, November 2008.
 [RFC3985]   Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire Emulation
             Edge-to-Edge (PWE3) Architecture", RFC 3985, March 2005.
 [RFC4090]   Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
             Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
             May 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., Ed., and T. Nadeau, Ed., "A Framework for
             Multi-Protocol Label Switching (MPLS) Operations and
             Management (OAM)", RFC 4378, February 2006.
 [RFC5286]   Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
             for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
             September 2008.
 [RFC5462]   Andersson, L. and R. Asati, "Multiprotocol Label
             Switching (MPLS) Label Stack Entry: "EXP" Field Renamed
             to "Traffic Class" Field", RFC 5462, February 2009.
 [RFC5654]   Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M.,
             Ed., 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.
 [RFC5920]   Fang, L., Ed., "Security Framework for MPLS and GMPLS
             Networks", RFC 5920, July 2010.

Bryant, et al. Standards Track [Page 18] RFC 6391 FAT-PW November 2011

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
 Ulrich Drafz
 Deutsche Telekom
 Muenster
 Germany
 EMail: Ulrich.Drafz@telekom.de
 Vach Kompella
 Alcatel-Lucent
 EMail: vach.kompella@alcatel-lucent.com
 Joe Regan
 Alcatel-Lucent
 EMail: joe.regan@alcatel-lucent.com
 Shane Amante
 Level 3 Communications, LLC
 1025 Eldorado Blvd.
 Broomfield, CO  80021
 USA
 EMail: shane@level3.net

Bryant, et al. Standards Track [Page 19]

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