Internet Engineering Task Force N. Akiya
Internet-Draft G. Swallow
Updates: 4379,6424,6790 (if approved) C. Pignataro
Intended status: Standards Track Cisco Systems
Expires: June 12, 2015 A. Malis
S. Aldrin
Huawei Technologies
December 9, 2014

Label Switched Path (LSP) and Pseudowire (PW) Ping/Trace over MPLS Network using Entropy Labels (EL)


The Multiprotocol Label Switching (MPLS) Label Switched Path (LSP) Ping and Traceroute are used to exercise specific paths of Equal-Cost Multipath (ECMP). When LSP is signaled to use Entropy Label (EL) described in RFC6790, the ability for LSP Ping and Traceroute operation to discover and exercise ECMP paths has been lost in scenarios which LSRs apply deviating load balance techniques. One such scenario is when some LSRs apply EL based load balancing while other LSRs apply non-EL based load balancing (ex: IP). Another scenario is when EL based LSP is stitched with another LSP which can be EL based or non-EL based.

This document extends the MPLS LSP Ping and Traceroute mechanisms to restore the ability of exercising specific paths of ECMP over LSP which make use of Entropy Label. This document updates RFC4379, RFC6424 and RFC6790.

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].

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

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."

This Internet-Draft will expire on June 12, 2015.

Copyright Notice

Copyright (c) 2014 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. 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

1.1. Terminology

The following acronyms/terminologies are used in this document:

1.2. Prerequisite

MPLS implementations employ wide variety of load balancing techniques in terms of fields used for hash "keys". [RFC4379] and [RFC6424] are designed to provide multipath support for subset of techniques. Intent of this document is to restore multipath support for those supported techniques which have been compromised by the introduction of [RFC6790] (i.e. Entropy Labels). Section 10 describes supported and unsupported cases, and it may be useful for one to visit this section first.

1.3. Background

Section 3.3.1 of [RFC4379] specifies multipath information encoding in Downstream Mapping TLV (Section 3.3 of [RFC4379]) and Downstream Detailed Mapping TLV (Section 3.3 of [RFC6424]) which can be used by LSP Ping initiator to trace and validate all ECMP paths between ingress and egress. These encodings are sufficient when all the LSRs along the path(s), between ingress and egress, consider same set of "keys" as input for load balancing algorithm: all IP based or all label based.

With introduction of [RFC6790], it is quite normal to see set of LSRs performing load balancing based on EL/ELI while others still follow the traditional way (IP based). This results in LSP Ping initiator not be able to trace and validate all ECMP paths in following scenarios: [RFC6790] will invariably end up with nodes that support ELI/EL and nodes that do not. There will typically be areas that support ELI/EL and areas that do not.

These scenarios will be quite common because every deployment of

As pointed out in [RFC6790] the procedures of [RFC4379] with respect to multipath information type {9} are incomplete. However [RFC6790] does not actually update [RFC4379]. Further the specific EL location is not clearly defined, particularly in the case of Flow Aware Pseudowires [RFC6391]. This document defines a new FEC Stack sub-TLV for the Entropy Label. Section 3 of this document updates the procedures for multipath information type {9} described in [RFC4379]. Rest of this document describes extensions required to restore ECMP discovery and tracing capabilities for scenarios described.

2. Overview

[RFC4379] describes LSP traceroute as an operation where the initiating LSR send a series of MPLS echo requests towards the same destination. The first packet in the series have the TTL set to 1. When the echo reply is received from the LSR one hop away the second echo request in the series is sent with the TTL set to 2, for each echo request the TLL is incremented by one until a response is received from the intended destination. Initiating LSR discovers and exercises ECMP by obtaining multipath information from each transit LSR and using specific destination IP address or specific entropy label.

Notion of {x, y, z} from here on refers to Multipath information types x, y or z.

LSP Ping initiating LSR sends MPLS echo request with multipath information. This multipath information is described in DSMAP/DDMAP TLV of echo request, and may contain set of IP addresses or set of labels. Multipath information types {2, 4, 8} carry set of IP addresses and multipath information type {9} carries set of labels. Responder LSR (receiver of MPLS echo request) will determine the subset of initiator specified multipath information which load balances to each downstream (outgoing interface). Responder LSR sends MPLS echo reply with resulting multipath information per downstream (outgoing interface) back to the initiating LSR. Initiating LSR is then able to use specific IP destination address or specific label to exercise specific ECMP path on the responder LSR.

Current behavior is problematic in following scenarios: [RFC6790]. Therefore, this document defines a multipath information type to be used in the DSMAP/DDMAP of MPLS echo request/reply packets in Section 9.

The above scenarios point to how the existing multipath information is insufficient when LSP traceroute is operated on an LSP with Entropy Labels described by

In addition, responder LSR can reply with empty multipath information if no IP address set or label set from received multipath information matched load balancing to a downstream. Empty return is also possible if initiating LSR sends multipath information of one type, IP address or label, but responder LSR load balances on the other type. To disambiguate between the two results, this document introduces new flags in the DSMAP/DDMAP TLV to allow responder LSR to describe the load balance technique being used.

It is required that all LSRs along the LSP understand new flags as well as new multipath information type. It is also required that initiating LSR can select both IP destination address and label to use on transmitting MPLS echo request packets. Two additional DS Flags are defined for the DSMAP and DDMAP TLVs in Section 8. These two flags are used by the responder LSR to describe its load balance behavior on received MPLS echo request.

Note that the terms "IP Based Load Balancer", "Label Based Load Balancer" and "Label Based Load Balancer" are in context of how received MPLS echo request is handled by the responder LSR.

3. Multipath Type 9

This section defines to which labels multipath type {9} applies.

[RFC4379] defined multipath type {9} for tracing of LSPs where label based load-balancing is used. However, as pointed out in [RFC6790], the procedures for using this type are incomplete as the specific location of the label was not defined. It was assumed that the presence of multipath type {9} implied the value of the bottom-of-stack label should be varied by the values indicated by multipath to determine their respective out-going interfaces.

Section 7 defines a new FEC-Stack sub-TLV to indicate an entropy label. These labels may appear anywhere in a label stack.

Multipath type {9} applies to the first label in the label-stack that corresponds to an EL-FEC. If no such label is found, it applies to the label at the bottom of the label stack.

4. Pseudowire Tracing

This section defines procedures for tracing pseudowires. These procedures pertain to the use of multipath information type {9} as well as type {TBD4}. In all cases below, when a control word is in use the N-flag in the DDMAP or DSMAP MUST be set. Note that when a control word is not in use the returned DDMAPs or DSMAPs may not be accurate.

In order to trace a non Flow-Aware Pseudowire the initiator includes an EL-FEC instead of the appropriate PW-FEC at the bottom of the FEC-Stack. Tracing in this way will cause compliant routers to return the proper outgoing interface. Note that this procedure only traces to the end of the MPLS LSP that is under test and will not verify the PW FEC. To actually verify the PW-FEC or in the case of a MS-PW, to determine the next pseudowire label value, the initiator MUST repeat that step of the trace, (i.e., repeating the TTL value used) but with the FEC-Stack modified to contain the appropriate PW-FEC. Note that these procedures are applicable to scenarios which an initiator is able to vary the bottom label (i.e. pseudowire label). Possible scenarios are tracing multiple non Flow-Aware Pseudowires on the same endpoints or tracing a non Flow-Aware Pseudowire provisioned with multiple pseudowire labels.

In order to trace a Flow Aware Pseudowire, the initiator includes an EL-FEC at the bottom of the FEC-Stack and pushes the appropriate PW-FEC onto the FEC-Stack.

In order to trace through non-compliant routers the initiator forms an MPLS echo request message and includes a DDMAP or DSMAP with multipath type {9}. For a non Flow-Aware Pseudowire it includes the appropriate PW-FEC in the FEC-Stack. For a Flow Aware Pseudowire, the initiator includes a NIL-FEC at the bottom of the FEC-Stack and pushes the appropriate PW-FEC onto the FEC-Stack.

5. Initiating LSR Procedures

In order to facilitate the flow of the following text we speak in terms of a boolean called EL_LSP maintained by the initiating LSR. This value controls the multipath information type to be used in transmitted echo request packets. When the initiating LSR is transmitting an echo request packet with DSMAP/DDMAP with a non-zero multipath information type, then EL_LSP boolean MUST be consulted to determine the multipath information type to use.

In addition to procedures described in [RFC4379] as updated by Section 3 and [RFC6424], initiating LSR MUST operate with following procedures.

In following conditions, the initiating LSR may have lost the ability to exercise specific ECMP paths. The initiating LSR MAY continue with "best effort".

6. Responder LSR Procedures

Common Procedures: The responder LSR receiving an MPLS echo request packet with multipath information type {TBD4} MUST validate following contents. Any deviation MUST result in the responder LSR to consider the packet as malformed and return code 1 (Malformed echo request received) in the MPLS echo reply packet.

Following subsections describe expected responder LSR procedures when echo reply is to include DSMAP/DDMAP TLVs, based on local load balance technique being employed. In case the responder LSR performs deviating load balance techniques per downstream basis, appropriate procedures matching to each downstream load balance technique MUST be operated.

6.1. IP Based Load Balancer & Not Pushing ELI/EL

6.2. IP Based Load Balancer & Pushes ELI/EL

6.3. Label Based Load Balancer & Not Pushing ELI/EL

6.4. Label Based Load Balancer & Pushes ELI/EL

6.5. Flow Aware MS-PW Stitching LSR

Stitching LSR that cross-connects Flow Aware Pseudowires behave in one of two ways:

7. Entropy Label FEC

Entropy Label Indicator (ELI) is a reserved label that has no explicit FEC associated, and has label value 7 assigned from the reserved range. Use Nil FEC as Target FEC Stack sub-TLV to account for ELI in a Target FEC Stack TLV.

Entropy Label (EL) is a special purpose label with label value being discretionary (i.e. label value may not be from the reserved range). For LSP verification mechanics to perform its purpose, it is necessary for a Target FEC Stack sub-TLV to clearly describe EL, particularly in the scenario where label stack does not carry ELI (ex: Flow Aware Pseudowire [RFC6391]). Therefore, this document defines a EL FEC to allow a Target FEC Stack sub-TLV to be added to the Target FEC Stack to account for EL.

The Length is 4. Labels are 20-bit values treated as numbers.

 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
|                 Label                 |          MBZ          |

        Figure 1: Entropy Label FEC

8. DS Flags: L and E

Two flags, L and E, are added in DS Flags field of the DSMAP/DDMAP TLVs. Both flags MUST NOT be set in echo request packets when sending, and ignored when received. Zero, one or both new flags MUST be set in echo reply packets.

 DS Flags

     0 1 2 3 4 5 6 7
    |  MBZ  |L|E|I|N|

 Flag  Name and Meaning
 ----  ----------------
    L  Label based load balance indicator
       This flag MUST be set to zero in the echo request. LSR
       which performs load balancing on a label MUST set this
       flag in the echo reply. LSR which performs load
       balancing on IP MUST NOT set this flag in the echo

    E  ELI/EL push indicator
       This flag MUST be set to zero in the echo request. LSR
       which pushes ELI/EL MUST set this flag in the echo
       reply. LSR which does not push ELI/EL MUST NOT set
       this flag in the echo reply.

9. New Multipath Information Type: TBD4

One new multipath information type is added to be used in DSMAP/DDMAP TLVs. New multipath type has value of TBD4.

  Key   Type                  Multipath Information
  ---   ----------------      ---------------------
 TBD4   IP and label set      IP addresses and label prefixes

Multipath type TBD4 is comprised of three sections. One section to describe IP address set. One section to describe label set. One section to describe another label set which associates to either IP address set or label set specified in the other section.

Multipath information type TBD4 has following format:

 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
|IPMultipathType| Reserved(MBZ) |     IP Multipath Length       |
~                                                               ~
|                  (IP Multipath Information)                   |
~                                                               ~
|LbMultipathType| Reserved(MBZ) |    Label Multipath Length     |
~                                                               ~
|                 (Label Multipath Information)                 |
~                                                               ~
|         Reserved(MBZ)         |  Assoc Label Multipath Length |
~                                                               ~
|            (Associated Label Multipath Information)           |
~                                                               ~

        Figure 2: Multipath Information Type TBD4

10. Supported and Unsupported Cases

MPLS architecture never defined strict rules on how implementations are to identify hash "keys" for load balancing purpose. As result, implementations may be of following load balancer types:

  1. IP Based Load Balancer.
  2. Label Based Load Balancer.
  3. Label and IP Based Load Balancer.

For cases (2) and (3), implementation can include different sets of labels from the label stack for load balancing purpose. Thus following sub-cases are possible:

  1. Entire label stack.
  2. Top N labels from label stack where number of labels in label stack is >N.
  3. Bottom N labels from label stack where number of labels in label stack is >N.

In a scenario where there is one Flow Label or Entropy Label present in the label stack, following further cases are possible for (2b), (2c), (3b) and (3c):

  1. N labels from label stack include Flow Label or Entropy Label.
  2. N labels from label stack does not include Flow Label or Entropy Label.

Also in a scenario where there are multiple Entropy Labels present in the label stack, it is possible for implementations to employ deviating techniques:

Furthermore, handling of reserved (i.e. special) labels varies among implementations:

It is important to point this out since presence of GAL will affect those implementations which include reserved labels for load balancing purpose.

As can be seen from above, there are many flavors of potential load balancing implementations. Attempting for any OAM tools to support ECMP discovery and traversal over all flavors of such will require fairly complex procedures and implementations to support those complex procedures. Complexities in OAM tools will produce minimal benefits if majority of implementations are expected to employ small subset of cases described above. [RFC4379], [RFC6424] and this document will support cases (1) and (2a1), where only the first (top-most) Entropy Label is included when there are multiple Entropy Labels in the label stack.


11. Security Considerations

This document extends LSP Traceroute mechanism to discover and exercise ECMP paths when LSP uses ELI/EL in label stack. Additional processings are required for responder and initiator nodes. Responder node that pushes ELI/EL will need to compute and return multipath data including associated EL. Initiator node will need to store and handle both IP multipath and label multipath information, and include destination IP addresses and/or ELs in MPLS echo request packet as well as in carried multipath information to downstream nodes. Due to additional processing, it is critical that proper security measures described in [RFC4379] and [RFC6424] are followed.

12. IANA Considerations

12.1. DS Flags

The IANA is requested to assign new bit numbers from the "DS flags" sub-registry from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry ([IANA-MPLS-LSP-PING]).

Note: the "DS flags" sub-registry is created by [I-D.ietf-mpls-lsp-ping-registry].

 Bit number Name                                        Reference
 ---------- ----------------------------------------    ---------
       TBD2 E: ELI/EL push indicator                    this document
       TBD3 L: Label based load balance indicator       this document

12.2. Multpath Type

The IANA is requested to assign a new value from the "Multipath Type" sub-registry from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry ([IANA-MPLS-LSP-PING]).

Note: the "Multipath Type" sub-registry is created by [I-D.ietf-mpls-lsp-ping-registry].

 Value      Meaning                                  Reference
 ---------- ---------------------------------------- ---------
  TBD4      IP and label set                         this document

12.3. Entropy Label FEC

The IANA is requested to assign a new sub-TLV from the "Sub-TLVs for TLV Types 1 and 16" section from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry ([IANA-MPLS-LSP-PING]).

 Sub-Type Sub-TLV Name          Reference
 -------- ------------          ---------
     TBD1 Entropy Label FEC     this document

13. Acknowledgements

Authors would like to thank Loa Andersson, Curtis Villamizar, Daniel King and Sriganesh Kini for performing thorough review and providing valuable comments.

14. Contributing Authors

Nagendra Kumar
Cisco Systems

15. References

15.1. Normative References

[I-D.ietf-mpls-lsp-ping-registry] Decraene, B., Akiya, N., Pignataro, C., Andersson, L. and S. Aldrin, "IANA registries for LSP ping Code Points", Internet-Draft draft-ietf-mpls-lsp-ping-registry-00, November 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures", RFC 4379, February 2006.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W. and L. Yong, "The Use of Entropy Labels in MPLS Forwarding", RFC 6790, November 2012.

15.2. Informative References

[I-D.ietf-mpls-forwarding] Villamizar, C., Kompella, K., Amante, S., Malis, A. and C. Pignataro, "MPLS Forwarding Compliance and Performance Requirements", Internet-Draft draft-ietf-mpls-forwarding-09, March 2014.
[I-D.ravisingh-mpls-el-for-seamless-mpls] Singh, R., Shen, Y. and J. Drake, "Entropy label for seamless MPLS", Internet-Draft draft-ravisingh-mpls-el-for-seamless-mpls-04, October 2014.
[IANA-MPLS-LSP-PING] IANA, "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters", .
[RFC6391] Bryant, S., Filsfils, C., Drafz, U., Kompella, V., Regan, J. and S. Amante, "Flow-Aware Transport of Pseudowires over an MPLS Packet Switched Network", RFC 6391, November 2011.
[RFC6424] Bahadur, N., Kompella, K. and G. Swallow, "Mechanism for Performing Label Switched Path Ping (LSP Ping) over MPLS Tunnels", RFC 6424, November 2011.

Authors' Addresses

Nobo Akiya Cisco Systems EMail:
George Swallow Cisco Systems EMail:
Carlos Pignataro Cisco Systems EMail:
Andrew G. Malis Huawei Technologies EMail:
Sam Aldrin Huawei Technologies EMail: