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Internet Engineering Task Force (IETF)                        S. BoutrosRequest for Comments: 7394                                  S. SivabalanCategory: Standards Track                                     G. SwallowISSN: 2070-1721                                                S. Saxena                                                           Cisco Systems                                                               V. Manral                                                          Ionos Networks                                                               S. Aldrin                                               Huawei Technologies, Inc.                                                           November 2014Definition of Time to Live TLV for LSP-Ping MechanismsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7394.Boutros, et al.              Standards Track                    [Page 1]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. Terminology .....................................................33. Time To Live TLV ................................................43.1. TTL TLV Format .............................................43.2. Usage ......................................................44. Operation .......................................................54.1. Traceroute Mode ............................................64.2. Error Scenario .............................................65. Security Considerations .........................................66. IANA Considerations .............................................77. References ......................................................77.1. Normative References .......................................7   Acknowledgements ...................................................7   Contributors .......................................................7   Authors' Addresses .................................................81.  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 originatorBoutros, et al.              Standards Track                    [Page 2]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 2014   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 inRFC 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 LiveBoutros, et al.              Standards Track                    [Page 3]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 20143.  Time To Live TLV3.1.  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.Boutros, et al.              Standards Track                    [Page 4]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 2014   If the TTL TLV is present and the receiver understands TTL TLVs, one   of the following two conditions apply:   o  If the TTL TLV value field is zero, the LSP-Ping echo request      packet SHOULD be dropped.   o  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".Boutros, et al.              Standards Track                    [Page 5]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 20144.1.  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) + 15.  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.Boutros, et al.              Standards Track                    [Page 6]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 20146.  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 (seeSection 3).   IANA has allocated the value 32769.7.  References7.1  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.comBoutros, et al.              Standards Track                    [Page 7]

RFC 7394             TTL TLV for LSP-Ping Mechanisms       November 2014Authors' 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.comBoutros, et al.              Standards Track                    [Page 8]