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Internet Engineering Task Force (IETF)
Request for Comments: 7394
Category: Standards Track
ISSN: 2070-1721
S. Boutros
S. Sivabalan
G. Swallow
S. Saxena
Cisco Systems
V. Manral
Ionos Networks
S. Aldrin
Huawei Technologies, Inc.
November 2014

Definition of Time to Live TLV for LSP-Ping Mechanisms

Abstract

LSP-Ping is a widely deployed Operation, Administration, and Maintenance (OAM) mechanism in MPLS networks. However, in the present form, this mechanism is inadequate to verify connectivity of a segment of a Multi-Segment Pseudowire (MS-PW) and/or bidirectional co-routed Label Switched Path (LSP) from any node on the path of the MS-PW and/or bidirectional co-routed LSP. This document defines a TLV to address this shortcoming.

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/rfc7394.

Copyright Notice

Copyright © 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 (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 ....................................................2
   2. Terminology .....................................................3
   3. Time To Live TLV ................................................4
      3.1. TTL TLV Format .............................................4
      3.2. Usage ......................................................4
   4. Operation .......................................................5
      4.1. Traceroute Mode ............................................6
      4.2. Error Scenario .............................................6
   5. Security Considerations .........................................6
   6. IANA Considerations .............................................7
   7. References ......................................................7
      7.1. Normative References .......................................7
   Acknowledgements ...................................................7
   Contributors .......................................................7
   Authors' Addresses .................................................8

1. Introduction

An MS-PW may span across multiple service provider networks. In order to allow Service Providers (SPs) to verify segments of such MS-PWs from any node on the path of the MS-PW, any node along the path of the MS-PW, should be able to originate an MPLS Echo Request packet to any other node along the path of the MS-PW and receive the corresponding MPLS Echo Reply. If the originator of the MPLS Echo Request is at the end of a MS-PW, the receiver of the request can send the reply back to the sender without knowing the hop-count distance of the originator. The reply will be intercepted by the originator regardless of the TTL value on the reply packet. But, if the originator is not at the end of the MS-PW, the receiver of the MPLS Echo Request may need to know how many hops away the originator of the MPLS Echo Request is so that it can set the TTL value on the MPLS header for the MPLS Echo Reply to be intercepted at the originator node.

In MPLS networks, for bidirectional co-routed LSPs, if it is desired to verify connectivity from any intermediate node Label Switching Router (LSR) on the LSP to the any other LSR on the LSP the receiver may need to know the TTL to send the MPLS Echo Reply with, so as the packet is intercepted by the originator node.

A new optional TTL TLV is defined in this document. This TLV will be added by the originator of the MPLS Echo Request to inform the receiver how many hops away the originator is on the path of the MS-PW or bidirectional LSP.

This mechanism only works if the MPLS Echo Reply is sent down the co-routed LSP; hence, the scope of this TTL TLV is currently limited to MS-PW or bidirectional co-routed MPLS LSPs. The presence of the TLV implies the use of the return path of the co-routed LSP, if the return path is any other mechanism, then the TLV in the MPLS Echo Request MUST be ignored.

2. Terminology

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].

LSR: 

        Label Switching Router

MPLS-TP: 

            MPLS Transport Profile

MS-PW: 

          Multi-Segment Pseudowire

PW: 

       Pseudowire

TLV: 

        Type Length Value

TTL: 

        Time To Live

3. Time To Live TLV

3.1. TTL TLV 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type = 32769                 |   Length = 8                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Value       |   Reserved    |   Flags                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: Time To Live TLV Format

The TTL TLV has the format shown in Figure 1. 

   Value

The value of the TTL as specified by this TLV 

   Flags

The Flags field is a bit vector with the following format: 

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             MBZ             |R|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

One flag is defined for now, the R flag. The rest of the flags are Reserved - MUST be zero (MBZ) when sending and ignored on receipt.

The R flag (Reply TTL) is set signify that the value is meant to be used as the TTL for the reply packet. Other bits may be defined later to enhance the scope of this TLV.

3.2. Usage

The TTL TLV MAY be included in the MPLS Echo Request by the originator of the request.

If the TTL TLV is present and the receiver does not understand TTL TLVs, it will simply ignore the TLV, as is the case for all optional TLVs. If the TTL TLV is not present or is not processed by the receiver, any determination of the TTL value used in the MPLS label on the LSP-Ping echo reply is beyond the scope of this document.

If the TTL TLV is present and the receiver understands TTL TLVs, one of the following two conditions apply:

  • If the TTL TLV value field is zero, the LSP-Ping echo request packet SHOULD be dropped.
  • Otherwise, the receiver MUST use the TTL value specified in the TTL TLV when it creates the MPLS header of the MPLS Echo Reply. The TTL value in the TTL TLV takes precedence over any TTL value determined by other means, such as from the Switching Point TLV in the MS-PW. This precedence will aid the originator of the LSP- Ping echo request in analyzing the return path.

4. Operation

In this section, we explain a use case for the TTL TLV with an MPLS MS-PW.

<------------------MS-PW --------------------->

            A          B          C           D           E
            o -------- o -------- o --------- o --------- o
            
                       ---MPLS Echo Request--->
                       <--MPLS Echo Reply------
            
            Figure 2: Use-Case with MS-PWs

Let us assume an MS-PW going through LSRs A, B, C, D, and E. Furthermore, assume that an operator wants to perform a connectivity check between B and D, from B. Thus, an MPLS Echo Request with the TTL TLV is originated from B and sent towards D. The MPLS Echo Request packet contains the FEC of the PW Segment between C and D. The value field of the TTL TLV and the TTL field of the MPLS label are set to 2, the choice of the value 2 will be based on the operator input requesting the MPLS Echo Request or from the optional LDP switching point TLV. The MPLS Echo Request is intercepted at D because of TTL expiry. D detects the TTL TLV in the request and uses the TTL value (i.e., 2) specified in the TLV on the MPLS label of the MPLS Echo Reply. The MPLS Echo Reply will be intercepted by B because of TTL expiry.

The same operation will apply when we have a co-routed bidirectional LSP and we want to check connectivity from an intermediate LSR "B" to another LSR "D".

4.1. Traceroute Mode

In traceroute mode, the TTL value in the TLV is set to 1 for the first Echo Request, then to 2 for the next, and so on. This is similar to the TTL values used for the label set on the packet.

4.2. Error Scenario

It is possible that the MPLS Echo Request packet was intercepted before the intended destination for reasons other than label TTL expiry. This could be due to network faults, misconfiguration, or other reasons. In such cases, if the return TTL is set to the value specified in the TTL TLV, then the echo response packet will continue beyond the originating node. This becomes a security issue.

To prevent this, the label TTL value used in the MPLS Echo Reply packet MUST be modified by deducting the incoming label TTL on the received packet from TTL TLV value. If the MPLS Echo Request packet is punted to the CPU before the incoming label TTL is deducted, then another 1 MUST be added. In other words:

Return TTL Value on the MPLS Echo Reply packet = (TTL TLV Value) - (Incoming Label TTL) + 1

5. Security Considerations

This document allows the setting of the TTL value in the MPLS Label of an MPLS Echo Reply, so that it can be intercepted by an intermediate device. This can cause a device to get a lot of LSP- Ping packets that get redirected to the CPU.

However, the same is possible even without the changes mentioned in this document. A device should rate limit the LSP-Ping packets redirected to the CPU so that the CPU is not overwhelmed.

The recommendation in the Security Considerations of [RFC4379] applies, to check the source address of the MPLS Echo Request; however, the source address can now be any node along the LSP path.

A faulty transit node changing the TTL TLV value could make the wrong node reply to the MPLS Echo Request, and/or the wrong node to receive the MPLS Echo Reply. An LSP trace may help identify the faulty transit node.

6. IANA Considerations

IANA has assigned a TLV type value to the following TLV from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry in the "TLVs" subregistry.

Time To Live TLV (see Section 3).

IANA has allocated the value 32769.

7. References

7.1 Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997,
             <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
             Label Switched (MPLS) Data Plane Failures", RFC 4379,
             February 2006, <http://www.rfc-editor.org/info/rfc4379>.
   
   [RFC5085] Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire Virtual
             Circuit Connectivity Verification (VCCV): A Control Channel
             for Pseudowires", RFC 5085, December 2007,
             <http://www.rfc-editor.org/info/rfc5085>.

Acknowledgements

The authors would like to thank Greg Mirsky for his comments.

Contributors

Michael Wildt
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
United States
EMail: mwildt@cisco.com

Authors' Addresses

Sami Boutros
Cisco Systems, Inc.
3750 Cisco Way
San Jose, CA 95134
United States
EMail: sboutros@cisco.com 

   Siva Sivabalan
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario, K2K 3E8
   Canada
   EMail: msiva@cisco.com

George Swallow
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough, MA 01719
United States
EMail: swallow@cisco.com

Shaleen Saxena
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
United States
EMail: ssaxena@cisco.com

Vishwas Manral
Ionos Networks
4100 Moorpark Ave, Suite 122
San Jose, CA 95117
United States
EMail: vishwas@ionosnetworks.com 

   Sam Aldrin
   Huawei Technologies, Inc.
   1188 Central Express Way,
   Santa Clara, CA 95051
   United States
   EMail: aldrin.ietf@gmail.com