path: root/doc/rfc/rfc8169.txt

Internet Engineering Task Force (IETF)                         G. Mirsky
Request for Comments: 8169                                     ZTE Corp.
Category: Standards Track                                     S. Ruffini
ISSN: 2070-1721                                                  E. Gray
                                                                Ericsson
                                                                J. Drake
                                                        Juniper Networks
                                                               S. Bryant
                                                                  Huawei
                                                           A. Vainshtein
                                                             ECI Telecom
                                                                May 2017


              Residence Time Measurement in MPLS Networks

Abstract

   This document specifies a new Generic Associated Channel (G-ACh) for
   Residence Time Measurement (RTM) and describes how it can be used by
   time synchronization protocols within an MPLS domain.

   Residence time is the variable part of the propagation delay of
   timing and synchronization messages; knowing this delay for each
   message allows for a more accurate determination of the delay to be
   taken into account when applying the value included in a Precision
   Time Protocol event message.

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 7841.

   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/rfc8169.










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Copyright Notice

   Copyright (c) 2017 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.





































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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   4
       1.1.1.  Terminology . . . . . . . . . . . . . . . . . . . . .   4
       1.1.2.  Requirements Language . . . . . . . . . . . . . . . .   5
   2.  Residence Time Measurement  . . . . . . . . . . . . . . . . .   5
     2.1.  One-Step Clock and Two-Step Clock Modes . . . . . . . . .   6
       2.1.1.  RTM with Two-Step Upstream PTP Clock  . . . . . . . .   7
       2.1.2.  Two-Step RTM with One-Step Upstream PTP Clock . . . .   8
   3.  G-ACh for Residence Time Measurement  . . . . . . . . . . . .   8
     3.1.  PTP Packet Sub-TLV  . . . . . . . . . . . . . . . . . . .  10
     3.2.  PTP Associated Value Field  . . . . . . . . . . . . . . .  11
   4.  Control-Plane Theory of Operation . . . . . . . . . . . . . .  11
     4.1.  RTM Capability  . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  RTM Capability Sub-TLV  . . . . . . . . . . . . . . . . .  12
     4.3.  RTM Capability Advertisement in Routing Protocols . . . .  13
       4.3.1.  RTM Capability Advertisement in OSPFv2  . . . . . . .  13
       4.3.2.  RTM Capability Advertisement in OSPFv3  . . . . . . .  14
       4.3.3.  RTM Capability Advertisement in IS-IS . . . . . . . .  14
       4.3.4.  RTM Capability Advertisement in BGP-LS  . . . . . . .  14
     4.4.  RSVP-TE Control-Plane Operation to Support RTM  . . . . .  15
       4.4.1.  RTM_SET TLV . . . . . . . . . . . . . . . . . . . . .  16
   5.  Data-Plane Theory of Operation  . . . . . . . . . . . . . . .  20
   6.  Applicable PTP Scenarios  . . . . . . . . . . . . . . . . . .  21
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  New RTM G-ACh . . . . . . . . . . . . . . . . . . . . . .  22
     7.2.  New MPLS RTM TLV Registry . . . . . . . . . . . . . . . .  22
     7.3.  New MPLS RTM Sub-TLV Registry . . . . . . . . . . . . . .  23
     7.4.  RTM Capability Sub-TLV in OSPFv2  . . . . . . . . . . . .  23
     7.5.  RTM Capability Sub-TLV in IS-IS . . . . . . . . . . . . .  24
     7.6.  RTM Capability TLV in BGP-LS  . . . . . . . . . . . . . .  24
     7.7.  RTM_SET Sub-object RSVP Type and Sub-TLVs . . . . . . . .  25
     7.8.  RTM_SET Attribute Flag  . . . . . . . . . . . . . . . . .  26
     7.9.  New Error Codes . . . . . . . . . . . . . . . . . . . . .  26
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  27
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30










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1.  Introduction

   Time synchronization protocols, e.g., the Network Time Protocol
   version 4 (NTPv4) [RFC5905] and the Precision Time Protocol version 2
   (PTPv2) [IEEE.1588], define timing messages that can be used to
   synchronize clocks across a network domain.  Measurement of the
   cumulative time that one of these timing messages spends transiting
   the nodes on the path from ingress node to egress node is termed
   "residence time" and is used to improve the accuracy of clock
   synchronization.  Residence time is the sum of the difference between
   the time of receipt at an ingress interface and the time of
   transmission from an egress interface for each node along the network
   path from an ingress node to an egress node.  This document defines a
   new Generic Associated Channel (G-ACh) value and an associated
   Residence Time Measurement (RTM) message that can be used in a
   Multiprotocol Label Switching (MPLS) network to measure residence
   time over a Label Switched Path (LSP).

   This document describes RTM over an LSP signaled using RSVP-TE
   [RFC3209].  Using RSVP-TE, the LSP's path can be either explicitly
   specified or determined during signaling.  Although it is possible to
   use RTM over an LSP instantiated using the Label Distribution
   Protocol [RFC5036], that is outside the scope of this document.

   Comparison with alternative proposed solutions such as
   [TIMING-OVER-MPLS] is outside the scope of this document.

1.1.  Conventions Used in This Document

1.1.1.  Terminology

   MPLS:   Multiprotocol Label Switching

   ACH:    Associated Channel Header

   TTL:    Time to Live

   G-ACh:  Generic Associated Channel

   GAL:    Generic Associated Channel Label

   NTP:    Network Time Protocol

   ppm:    parts per million

   PTP:    Precision Time Protocol

   BC:     boundary clock



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   LSP:    Label Switched Path

   OAM:    Operations, Administration, and Maintenance

   RRO:    Record Route Object

   RTM:    Residence Time Measurement

   IGP:    Internal Gateway Protocol

   BGP-LS: Border Gateway Protocol - Link State

1.1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.


2.  Residence Time Measurement

   "Packet Loss and Delay Measurement for MPLS Networks" [RFC6374] can
   be used to measure one-way or two-way end-to-end propagation delay
   over an LSP or a pseudowire (PW).  But these measurements are
   insufficient for use in some applications, for example, time
   synchronization across a network as defined in the PTP.  In PTPv2
   [IEEE.1588], the residence time is accumulated in the correctionField
   of the PTP event message, which is defined in [IEEE.1588] and
   referred to as using a one-step clock, or in the associated follow-up
   message (or Delay_Resp message associated with the Delay_Req
   message), which is referred to as using a two-step clock (see the
   detailed discussion in Section 2.1).

   IEEE 1588 uses this residence time to correct for the transit times
   of nodes on an LSP, effectively making the transit nodes transparent.

   This document proposes a mechanism that can be used as one type of
   on-path support for a clock synchronization protocol or can be used
   to perform one-way measurement of residence time.  The proposed
   mechanism accumulates residence time from all nodes that support this
   extension along the path of a particular LSP in the Scratch Pad field
   of an RTM message (Figure 1).  This value can then be used by the
   egress node to update, for example, the correctionField of the PTP
   event packet carried within the RTM message prior to performing its
   PTP processing.




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2.1.  One-Step Clock and Two-Step Clock Modes

   One-step mode refers to the mode of operation where an egress
   interface updates the correctionField value of an original event
   message.  Two-step mode refers to the mode of operation where this
   update is made in a subsequent follow-up message.

   Processing of the follow-up message, if present, requires the
   downstream endpoint to wait for the arrival of the follow-up message
   in order to combine correctionField values from both the original
   (event) message and the subsequent (follow-up) message.  In a similar
   fashion, each two-step node needs to wait for the related follow-up
   message, if there is one, in order to update that follow-up message
   (as opposed to creating a new one).  Hence, the first node that uses
   two-step mode MUST do two things:

   1.  Mark the original event message to indicate that a follow-up
       message will be forthcoming.  This is necessary in order to

       *  Let any subsequent two-step node know that there is already a
          follow-up message, and

       *  Let the endpoint know to wait for a follow-up message.

   2.  Create a follow-up message in which to put the RTM determined as
       an initial correctionField value.

   IEEE 1588v2 [IEEE.1588] defines this behavior for PTP messages.

   Thus, for example, with reference to the PTP protocol, the PTPType
   field identifies whether the message is a Sync message, Follow_up
   message, Delay_Req message, or Delay_Resp message.  The 10-octet-long
   Port ID field contains the identity of the source port [IEEE.1588],
   that is, the specific PTP port of the boundary clock (BC) connected
   to the MPLS network.  The Sequence ID is the sequence ID of the PTP
   message carried in the Value field of the message.

   PTP messages also include a bit that indicates whether or not a
   follow-up message will be coming.  This bit MAY be set by a two-step
   mode PTP device.  The value MUST NOT be unset until the original and
   follow-up messages are combined by an endpoint (such as a BC).

   For compatibility with PTP, RTM (when used for PTP packets) must
   behave in a similar fashion.  It should be noted that the handling of
   Sync event messages and of Delay_Req/Delay_Resp event messages that
   cross a two-step RTM node is different.  The following outlines the
   handling of a PTP Sync event message by the two-step RTM node.  The
   details of handling Delay_Resp/Delay_Req PTP event messages by the



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   two-step RTM node are discussed in Section 2.1.1.  As a summary, a
   two-step RTM-capable egress interface will need to examine the S bit
   in the Flags field of the PTP sub-TLV (for RTM messages that indicate
   they are for PTP), and -- if it is clear (set to zero) -- it MUST set
   the S bit and create a follow-up PTP Type RTM message.  If the S bit
   is already set, then the RTM-capable node MUST wait for the RTM
   message with the PTP type of follow-up and matching originator and
   sequence number to make the corresponding residence time update to
   the Scratch Pad field.  The wait period MUST be reasonably bounded.

   Thus, an RTM packet, containing residence time information relating
   to an earlier packet, also contains information identifying that
   earlier packet.

   In practice, an RTM node operating in two-step mode behaves like a
   two-step transparent clock.

   A one-step-capable RTM node MAY elect to operate in either one-step
   mode (by making an update to the Scratch Pad field of the RTM message
   containing the PTP event message) or two-step mode (by making an
   update to the Scratch Pad of a follow-up message when presence of a
   follow-up is indicated), but it MUST NOT do both.

   Two main subcases identified for an RTM node operating as a two-step
   clock are described in the following sub-sections.

2.1.1.  RTM with Two-Step Upstream PTP Clock

   If any of the previous RTM-capable nodes or the previous PTP clock
   (e.g., the BC connected to the first node) is a two-step clock and if
   the local RTM-capable node is also operating a two-tep clock, the
   residence time is added to the RTM packet that has been created to
   include the second PTP packet (i.e., the follow-up message in the
   downstream direction).  This RTM packet carries the related
   accumulated residence time, the appropriate values of the Sequence ID
   and Port ID (the same identifiers carried in the original packet),
   and the two-step flag set to 1.

   Note that the fact that an upstream RTM-capable node operating in
   two-step mode has created a follow-up message does not require any
   subsequent RTM-capable node to also operate in two-step mode, as long
   as that RTM-capable node forwards the follow-up message on the same
   LSP on which it forwards the corresponding previous message.

   A one-step-capable RTM node MAY elect to update the RTM follow-up
   message as if it were operating in two-step mode; however, it MUST
   NOT update both messages.




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   A PTP Sync packet is carried in the RTM packet in order to indicate
   to the RTM node that RTM must be performed on that specific packet.

   To handle the residence time of the Delay_Req message in the upstream
   direction, an RTM packet must be created to carry the residence time
   in the associated downstream Delay_Resp message.

   The last RTM node of the MPLS network, in addition to updating the
   correctionField of the associated PTP packet, must also react
   properly to the two-step flag of the PTP packets.

2.1.2.  Two-Step RTM with One-Step Upstream PTP Clock

   When the PTP network connected to the MPLS operates in one-step clock
   mode and an RTM node operates in two-step mode, the follow-up RTM
   packet must be created by the RTM node itself.  The RTM packet
   carrying the PTP event packet needs now to indicate that a follow-up
   message will be coming.

   The egress RTM-capable node of the LSP will remove RTM encapsulation
   and, in case of two-step clock mode being indicated, will generate
   PTP messages to include the follow-up correction as appropriate
   (according to [IEEE.1588]).  In this case, the common header of the
   PTP packet carrying the synchronization message would have to be
   modified by setting the twoStepFlag field indicating that there is
   now a follow-up message associated to the current message.

3.  G-ACh for Residence Time Measurement

   [RFC5586] and [RFC6423] define the G-ACh to extend the applicability
   of the Pseudowire Associated Channel Header (ACH) [RFC5085] to LSPs.
   G-ACh provides a mechanism to transport OAM and other control
   messages over an LSP.  Processing of these messages by selected
   transit nodes is controlled by the use of the Time-to-Live (TTL)
   value in the MPLS header of these messages.

   The message format for RTM is presented in Figure 1.














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     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 1|Version|   Reserved    |           RTM G-ACh           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                        Scratch Pad                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Type               |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Value (optional)                        |
    ~                                                               ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 1: RTM G-ACh Message Format for Residence Time Measurement

   o  The first four octets are defined as a G-ACh header in [RFC5586].

   o  The Version field is set to 0, as defined in [RFC4385].

   o  The Reserved field MUST be set to 0 on transmit and ignored on
      receipt.

   o  The RTM G-ACh field (value 0x000F; see Section 7.1) identifies the
      packet as such.

   o  The Scratch Pad field is 8 octets in length.  It is used to
      accumulate the residence time spent in each RTM-capable node
      transited by the packet on its path from ingress node to egress
      node.  The first RTM-capable node MUST initialize the Scratch Pad
      field with its RTM.  Its format is a 64-bit signed integer, and it
      indicates the value of the residence time measured in nanoseconds
      and multiplied by 2^16.  Note that depending on whether the timing
      procedure is a one-step or two-step operation (Section 2.1), the
      residence time is either for the timing packet carried in the
      Value field of this RTM message or for an associated timing packet
      carried in the Value field of another RTM message.

   o  The Type field identifies the type and encapsulation of a timing
      packet carried in the Value field, e.g., NTP [RFC5905] or PTP
      [IEEE.1588].  Per this document, IANA has created a sub-registry
      called the "MPLS RTM TLV Registry" in the "Generic Associated
      Channel (G-ACh) Parameters" registry (see Section 7.2).

   o  The Length field contains the length, in octets, of any Value
      field defined for the Type given in the Type field.




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   o  The TLV MUST be included in the RTM message, even if the length of
      the Value field is zero.

3.1.  PTP Packet Sub-TLV

   Figure 2 presents the format of a PTP sub-TLV that MUST be included
   in the Value field of an RTM message preceding the carried timing
   packet when the timing packet is PTP.

     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              |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Flags                         |PTPType|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Port ID                            |
    |                                                               |
    |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                               |           Sequence ID         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 2: PTP Sub-TLV Format

   where the Flags field has the following format:

     0                   1                   2
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |S|                      Reserved                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 3: Flags Field Format of PTP Packet Sub-TLV

   o  The Type field identifies the PTP packet sub-TLV and is set to 1
      according to Section 7.3.

   o  The Length field of the PTP sub-TLV contains the number of octets
      of the Value part of the TLV and MUST be 20.

   o  The Flags field currently defines one bit, the S bit, that defines
      whether the current message has been processed by a two-step node,
      where the flag is cleared if the message has been handled
      exclusively by one-step nodes and there is no follow-up message
      and is set if there has been at least one two-step node and a
      follow-up message is forthcoming.





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   o  The PTPType field indicates the type of PTP packet to which this
      PTP sub-TLV applies.  PTPType is the messageType field of a PTPv2
      packet with possible values defined in Table 19 of [IEEE.1588].

   o  The 10-octet-long Port ID field contains the identity of the
      source port.

   o  The Sequence ID is the sequence ID of the PTP message to which
      this PTP sub-TLV applies.

   A tuple of PTPType, Port ID, and Sequence ID uniquely identifies the
   PTP timing message included in an RTM message and is used in two-step
   RTM mode; see Section 2.1.1.

3.2.  PTP Associated Value Field

   The Value field (see Figure 1) -- in addition to the PTP sub-TLV --
   MAY carry a packet of the PTP Time synchronization protocol (as was
   identified by the Type field).  It is important to note that the
   timing message packet may be authenticated or encrypted and carried
   over this LSP unchanged (and inaccessible to intermediate RTM capable
   LSRs) while the residence time is accumulated in the Scratch Pad
   field.

   The LSP ingress RTM-capable LSR populates the identifying tuple
   information of the PTP sub-TLV (see section 3.1) prior to including
   the (possibly authenticated/encrypted) PTP message packet after the
   PTP sub-TLV in the Value field of the RTM message for an RTM message
   of the PTP Type (Type 1; see Section 7.3).

4.  Control-Plane Theory of Operation

   The operation of RTM depends upon TTL expiry to deliver an RTM packet
   from one RTM-capable interface to the next along the path from
   ingress node to egress node.  This means that a node with RTM-capable
   interfaces MUST be able to compute a TTL, which will cause the expiry
   of an RTM packet at the next node with RTM-capable interfaces.

4.1.  RTM Capability

   Note that the RTM capability of a node is with respect to the pair of
   interfaces that will be used to forward an RTM packet.  In general,
   the ingress interface of this pair must be able to capture the
   arrival time of the packet and encode it in some way such that this
   information will be available to the egress interface of a node.






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   The supported mode (one-step or two-step) of any pair of interfaces
   is determined by the capability of the egress interface.  For both
   modes, the egress interface implementation MUST be able to determine
   the precise departure time of the same packet and determine from
   this, and the arrival time information from the corresponding ingress
   interface, the difference representing the residence time for the
   packet.

   An interface with the ability to do this and update the associated
   Scratch Pad in real time (i.e., while the packet is being forwarded)
   is said to be one-step capable.

   Hence, while both ingress and egress interfaces are required to
   support RTM for the pair to be RTM capable, it is the egress
   interface that determines whether or not the node is one-step or two-
   step capable with respect to the interface pair.

   The RTM capability used in the sub-TLV shown in Figures 4 and 5 is
   thus a non-routing-related capability associated with the interface
   being advertised based on its egress capability.  The ability of any
   pair of interfaces on a node that includes this egress interface to
   support any mode of RTM depends on the ability of the ingress
   interface of a node to record packet arrival time and convey it to
   the egress interface on the node.

   When a node uses an IGP to support the RTM capability advertisement,
   the IGP sub-TLV MUST reflect the RTM capability (one-step or two-
   step) associated with the advertised interface.  Changes of RTM
   capability are unlikely to be frequent and would result, for example,
   from the operator's decision to include or exclude a particular port
   from RTM processing or switch between RTM modes.

4.2.  RTM Capability Sub-TLV

   [RFC4202] explains that the Interface Switching Capability Descriptor
   describes the switching capability of an interface.  For
   bidirectional links, the switching capabilities of an interface are
   defined to be the same in either direction, that is, for data
   entering the node through that interface and for data leaving the
   node through that interface.  That principle SHOULD be applied when a
   node advertises RTM capability.

   A node that supports RTM MUST be able to act in two-step mode and MAY
   also support one-step RTM mode.  A detailed discussion of one-step
   and two-step RTM modes is contained in Section 2.1.






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4.3.  RTM Capability Advertisement in Routing Protocols

4.3.1.  RTM Capability Advertisement in OSPFv2

   The format for the RTM Capability sub-TLV in OSPF is presented in
   Figure 4.

     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             |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | RTM |  Value       ...
    +-+-+-+-+-+-+-+-+-+- ...

                Figure 4: RTM Capability Sub-TLV in OSPFv2

   o  Type value (5) has been assigned by IANA in the "OSPFv2 Extended
      Link TLV Sub-TLVs" registry (see Section 7.4).

   o  Length value equals the number of octets of the Value field.

   o  Value contains a variable number of bitmap fields so that the
      overall number of bits in the fields equals Length * 8.

   o  Bits are defined/sent starting with Bit 0.  Additional bitmap
      field definitions that may be defined in the future SHOULD be
      assigned in ascending bit order so as to minimize the number of
      bits that will need to be transmitted.

   o  Undefined bits MUST be transmitted as 0 and MUST be ignored on
      receipt.

   o  Bits that are NOT transmitted MUST be treated as if they are set
      to 0 on receipt.

   o  RTM (capability) is a 3-bit-long bitmap field with values defined
      as follows:

      *  0b001 - one-step RTM supported

      *  0b010 - two-step RTM supported

      *  0b100 - reserved

   The capability to support RTM on a particular link (interface) is
   advertised in the OSPFv2 Extended Link Opaque LSA as described in
   Section 3 of [RFC7684] via the RTM Capability sub-TLV.



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4.3.2.  RTM Capability Advertisement in OSPFv3

   The capability to support RTM on a particular link (interface) can be
   advertised in OSPFv3 using LSA extensions as described in
   [OSPFV3-EXTENDED-LSA].  The sub-TLV SHOULD use the same format as in
   Section 4.3.1.  The type allocation and full details of exact use of
   OSPFv3 LSA extensions is for further study.

4.3.3.  RTM Capability Advertisement in IS-IS

   The capability to support RTM on a particular link (interface) is
   advertised in a new sub-TLV that may be included in TLVs advertising
   Intermediate System (IS) Reachability on a specific link (TLVs 22,
   23, 222, and 223).

   The format for the RTM Capability sub-TLV is presented in Figure 5.

     0                   1                   2
     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 ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
    |      Type     |     Length    | RTM |   Value      ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...

                     Figure 5: RTM Capability Sub-TLV

   o  Type value (40) has been assigned by IANA in the "Sub-TLVs for
      TLVs 22, 23, 141, 222, and 223" registry for IS-IS (see
      Section 7.5).

   o  Definitions, rules of handling, and values for the Length and
      Value fields are as defined in Section 4.3.1.

   o  RTM (capability) is a 3-bit-long bitmap field with values defined
      in Section 4.3.1.

4.3.4.  RTM Capability Advertisement in BGP-LS

   The format for the RTM Capability TLV is presented in Figure 4.

   Type value (1105) has been assigned by IANA in the "BGP-LS Node
   Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
   sub-registry (see Section 7.6).

   Definitions, rules of handling, and values for fields Length, Value,
   and RTM are as defined in Section 4.3.1.






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   The RTM capability will be advertised in BGP-LS as a Link Attribute
   TLV associated with the Link NLRI as described in Section 3.3.2 of
   [RFC7752].

4.4.  RSVP-TE Control-Plane Operation to Support RTM

   Throughout this document, we refer to a node as an RTM-capable node
   when at least one of its interfaces is RTM capable.  Figure 6
   provides an example of roles a node may have with respect to RTM
   capability:

    -----     -----     -----     -----     -----     -----     -----
    | A |-----| B |-----| C |-----| D |-----| E |-----| F |-----| G |
    -----     -----     -----     -----     -----     -----     -----

                        Figure 6: RTM-Capable Roles

   o  A is a boundary clock with its egress port in Master state.  Node
      A transmits IP-encapsulated timing packets whose destination IP
      address is G.

   o  B is the ingress Label Edge Router (LER) for the MPLS LSP and is
      the first RTM-capable node.  It creates RTM packets, and in each
      it places a timing packet, possibly encrypted, in the Value field
      and initializes the Scratch Pad field with its RTM.

   o  C is a transit node that is not RTM capable.  It forwards RTM
      packets without modification.

   o  D is an RTM-capable transit node.  It updates the Scratch Pad
      field of the RTM packet without updating the timing packet.

   o  E is a transit node that is not RTM capable.  It forwards RTM
      packets without modification.

   o  F is the egress LER and the last RTM-capable node.  It removes the
      RTM ACH encapsulation and processes the timing packet carried in
      the Value field using the value in the Scratch Pad field.  In
      particular, the value in the Scratch Pad field of the RTM ACH is
      used in updating the Correction field of the PTP message(s).  The
      LER should also include its own residence time before creating the
      outgoing PTP packets.  The details of this process depend on
      whether or not the node F is itself operating as a one-step or
      two-step clock.

   o  G is a boundary clock with its ingress port in Slave state.  Node
      G receives PTP messages.




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   An ingress node that is configured to perform RTM along a path
   through an MPLS network to an egress node MUST verify that the
   selected egress node has an interface that supports RTM via the
   egress node's advertisement of the RTM Capability sub-TLV, as covered
   in Section 4.3.  In the Path message that the ingress node uses to
   instantiate the LSP to that egress node, it places an LSP_ATTRIBUTES
   object [RFC5420] with an RTM_SET Attribute Flag set, as described in
   Section 7.8, which indicates to the egress node that RTM is requested
   for this LSP.  The RTM_SET Attribute Flag SHOULD NOT be set in the
   LSP_REQUIRED_ATTRIBUTES object [RFC5420], unless it is known that all
   nodes recognize the RTM attribute (but need not necessarily implement
   it), because a node that does not recognize the RTM_SET Attribute
   Flag would reject the Path message.

   If an egress node receives a Path message with the RTM_SET Attribute
   Flag in an LSP_ATTRIBUTES object, the egress node MUST include an
   initialized RRO [RFC3209] and LSP_ATTRIBUTES object where the RTM_SET
   Attribute Flag is set and the RTM_SET TLV (Section 4.4.1) is
   initialized.  When the Resv message is received by the ingress node,
   the RTM_SET TLV will contain an ordered list, from egress node to
   ingress node, of the RTM-capable nodes along the LSP's path.

   After the ingress node receives the Resv, it MAY begin sending RTM
   packets on the LSP's path.  Each RTM packet has its Scratch Pad field
   initialized and its TTL set to expire on the closest downstream RTM-
   capable node.

   It should be noted that RTM can also be used for LSPs instantiated
   using [RFC3209] in an environment in which all interfaces in an IGP
   support RTM.  In this case, the RTM_SET TLV and LSP_ATTRIBUTES object
   MAY be omitted.

4.4.1.  RTM_SET TLV

   RTM-capable interfaces can be recorded via the RTM_SET TLV.  The
   RTM_SET sub-object format is a generic TLV format, presented in
   Figure 7.

     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      |     Length    |I|         Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                             Value                             ~
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7: RTM_SET TLV Format



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   Type value (5) has been assigned by IANA in the RSVP-TE "Attributes
   TLV Space" sub-registry (see Section 7.7).

   The Length contains the total length of the sub-object in bytes,
   including the Type and Length fields.

   The I bit indicates whether the downstream RTM-capable node along the
   LSP is present in the RRO.

   The Reserved field must be zeroed on initiation and ignored on
   receipt.

   The content of an RTM_SET TLV is a series of variable-length
   sub-TLVs.  Only a single RTM_SET can be present in a given
   LSP_ATTRIBUTES object.  The sub-TLVs are defined in Section 4.4.1.1.

   The following processing procedures apply to every RTM-capable node
   along the LSP.  In this paragraph, an RTM-capable node is referred to
   as a node for sake of brevity.  Each node MUST examine the Resv
   message for whether the RTM_SET Attribute Flag in the LSP_ATTRIBUTES
   object is set.  If the RTM_SET flag is set, the node MUST inspect the
   LSP_ATTRIBUTES object for presence of an RTM_SET TLV.  If more than
   one is found, then the LSP setup MUST fail with generation of the
   ResvErr message with Error Code "Duplicate TLV" (Section 7.9) and
   Error Value that contains the Type value in its 8 least significant
   bits.  If no RTM_SET TLV is found, then the LSP setup MUST fail with
   generation of the ResvErr message with Error Code "RTM_SET TLV
   Absent" (Section 7.9).  If one RTM_SET TLV has been found, the node
   will use the ID of the first node in the RTM_SET in conjunction with
   the RRO to compute the hop count to its downstream node with a
   reachable RTM-capable interface.  If the node cannot find a matching
   ID in the RRO, then it MUST try to use the ID of the next node in the
   RTM_SET until it finds the match or reaches the end of the RTM_SET
   TLV.  If a match has been found, the calculated value is used by the
   node as the TTL value in the outgoing label to reach the next RTM-
   capable node on the LSP.  Otherwise, the TTL value MUST be set to
   255.  The node MUST add an RTM_SET sub-TLV with the same address it
   used in the RRO sub-object at the beginning of the RTM_SET TLV in the
   associated outgoing Resv message before forwarding it upstream.  If
   the calculated TTL value has been set to 255, as described above,
   then the I flag in the node's RTM_SET TLV MUST be set to 1 before the
   Resv message is forwarded upstream.  Otherwise, the I flag MUST be
   cleared (0).

   The ingress node MAY inspect the I bit received in each RTM_SET TLV
   contained in the LSP_ATTRIBUTES object of a received Resv message.
   The presence of the RTM_SET TLV with the I bit set to 1 indicates
   that some RTM nodes along the LSP could not be included in the



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   calculation of the residence time.  An ingress node MAY choose to
   resignal the LSP to include all RTM nodes or simply notify the user
   via a management interface.

   There are scenarios when some information is removed from an RRO due
   to policy processing (e.g., as may happen between providers) or the
   RRO is limited due to size constraints.  Such changes affect the core
   assumption of this method and the processing of RTM packets.  RTM
   SHOULD NOT be used if it is not guaranteed that the RRO contains
   complete information.

4.4.1.1.  RTM_SET Sub-TLVs

   The RTM Set sub-object contains an ordered list, from egress node to
   ingress node, of the RTM-capable nodes along the LSP's path.

   The contents of an RTM_SET sub-object are a series of variable-length
   sub-TLVs.  Each sub-TLV has its own Length field.  The Length
   contains the total length of the sub-TLV in bytes, including the Type
   and Length fields.  The Length MUST always be a multiple of 4, and at
   least 8 (smallest IPv4 sub-object).

   Sub-TLVs are organized as a last-in-first-out stack.  The first-out
   sub-TLV relative to the beginning of RTM_SET TLV is considered the
   top.  The last-out sub-TLV is considered the bottom.  When a new
   sub-TLV is added, it is always added to the top.

   The RTM_SET TLV is intended to include the subset of the RRO sub-TLVs
   that represent those egress interfaces on the LSP that are RTM
   capable.  After a node chooses an egress interface to use in the RRO
   sub-TLV, that same egress interface, if RTM capable, SHOULD be placed
   into the RTM_SET TLV using one of the following: IPv4 sub-TLV, IPv6
   sub-TLV, or Unnumbered Interface sub-TLV.  The address family chosen
   SHOULD match that of the RESV message and that used in the RRO; the
   unnumbered interface sub-TLV is used when the egress interface has no
   assigned IP address.  A node MUST NOT place more sub-TLVs in the
   RTM_SET TLV than the number of RTM-capable egress interfaces the LSP
   traverses that are under that node's control.  Only a single RTM_SET
   sub-TLV with the given Value field MUST be present in the RTM_SET
   TLV.  If more than one sub-TLV with the same value (e.g., a
   duplicated address) is found, the LSP setup MUST fail with the
   generation of a ResvErr message with the Error Code "Duplicate
   sub-TLV" (Section 7.9) and the Error Value containing a 16-bit value
   composed of (Type of TLV, Type of sub-TLV).

   Three kinds of sub-TLVs for RTM_SET are currently defined.





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4.4.1.1.1.  IPv4 Sub-TLV

     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     |     Length    |            Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       IPv4 address                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 8: IPv4 Sub-TLV Format

   Type
      0x01 IPv4 address.

   Length
      The Length contains the total length of the sub-TLV in bytes,
      including the Type and Length fields.  The Length is always 8.

   IPv4 address
      A 32-bit unicast host address.

   Reserved
      Zeroed on initiation and ignored on receipt.

4.4.1.1.2.  IPv6 Sub-TLV

     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     |     Length    |            Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                         IPv6 address                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 9: IPv6 Sub-TLV Format

   Type
      0x02 IPv6 address.

   Length
      The Length contains the total length of the sub-TLV in bytes,
      including the Type and Length fields.  The Length is always 20.





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   IPv6 address
      A 128-bit unicast host address.

   Reserved
      Zeroed on initiation and ignored on receipt.

4.4.1.1.3.  Unnumbered Interface Sub-TLV

     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     |     Length    |            Reserved             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Node ID                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Interface ID                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 10: IPv4 Sub-TLV Format

   Type
      0x03 Unnumbered interface.

   Length
      The Length contains the total length of the sub-TLV in bytes,
      including the Type and Length fields.  The Length is always 12.

   Node ID
      The Node ID interpreted as the Router ID as discussed in Section 2
      of [RFC3477].

   Interface ID
      The identifier assigned to the link by the node specified by the
      Node ID.

   Reserved
      Zeroed on initiation and ignored on receipt.

5.  Data-Plane Theory of Operation

   After instantiating an LSP for a path using RSVP-TE [RFC3209] as
   described in Section 4.4, the ingress node MAY begin sending RTM
   packets to the first downstream RTM-capable node on that path.  Each
   RTM packet has its Scratch Pad field initialized and its TTL set to
   expire on the next downstream RTM-capable node.  Each RTM-capable
   node on the explicit path receives an RTM packet and records the time
   at which it receives that packet at its ingress interface as well as
   the time at which it transmits that packet from its egress interface.



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   These actions should be done as close to the physical layer as
   possible at the same point of packet processing, striving to avoid
   introducing the appearance of jitter in propagation delay whereas it
   should be accounted as residence time.  The RTM-capable node
   determines the difference between those two times; for one-step
   operation, this difference is determined just prior to or while
   sending the packet, and the RTM-capable egress interface adds it to
   the value in the Scratch Pad field of the message in progress.  Note,
   for the purpose of calculating a residence time, a common free
   running clock synchronizing all the involved interfaces may be
   sufficient, as, for example, 4.6 ppm accuracy leads to a 4.6
   nanosecond error for residence time on the order of 1 millisecond.
   This may be acceptable for applications where the target accuracy is
   in the order of hundreds of nanoseconds.  As an example, several
   applications being considered in the area of wireless applications
   are satisfied with an accuracy of 1.5 microseconds [ITU-T.G.8271].

   For two-step operation, the difference between packet arrival time
   (at an ingress interface) and subsequent departure time (from an
   egress interface) is determined at some later time prior to sending a
   subsequent follow-up message, so that this value can be used to
   update the correctionField in the follow-up message.

   See Section 2.1 for further details on the difference between one-
   step and two-step operation.

   The last RTM-capable node on the LSP MAY then use the value in the
   Scratch Pad field to perform time correction, if there is no
   follow-up message.  For example, the egress node may be a PTP
   boundary clock synchronized to a Master Clock and will use the value
   in the Scratch Pad field to update PTP's correctionField.

6.  Applicable PTP Scenarios

   This approach can be directly integrated in a PTP network based on
   the IEEE 1588 delay request-response mechanism.  The RTM-capable
   nodes act as end-to-end transparent clocks, and boundary clocks, at
   the edges of the MPLS network, typically use the value in the Scratch
   Pad field to update the correctionField of the corresponding PTP
   event packet prior to performing the usual PTP processing.











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7.  IANA Considerations

7.1.  New RTM G-ACh

   IANA has assigned a new G-ACh as follows:

          +--------+----------------------------+---------------+
          | Value  |        Description         | Reference     |
          +--------+----------------------------+---------------+
          | 0x000F | Residence Time Measurement | This document |
          +--------+----------------------------+---------------+

                  Table 1: New Residence Time Measurement

7.2.  New MPLS RTM TLV Registry

   IANA has created a sub-registry in the "Generic Associated Channel
   (G-ACh) Parameters" registry called the "MPLS RTM TLV Registry".  All
   codepoints in the range 0 through 127 in this registry shall be
   allocated according to the "IETF Review" procedure as specified in
   [RFC5226].  Codepoints in the range 128 through 191 in this registry
   shall be allocated according to the "First Come First Served"
   procedure as specified in [RFC5226].  This document defines the
   following new RTM TLV types:

        +---------+-------------------------------+---------------+
        | Value   |          Description          | Reference     |
        +---------+-------------------------------+---------------+
        | 0       |            Reserved           | This document |
        | 1       |           No payload          | This document |
        | 2       | PTPv2, Ethernet encapsulation | This document |
        | 3       |   PTPv2, IPv4 encapsulation   | This document |
        | 4       |   PTPv2, IPv6 encapsulation   | This document |
        | 5       |              NTP              | This document |
        | 6-191   |           Unassigned          |               |
        | 192-254 |    Reserved for Private Use   | This document |
        | 255     |            Reserved           | This document |
        +---------+-------------------------------+---------------+

                          Table 2: RTM TLV Types











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7.3.  New MPLS RTM Sub-TLV Registry

   IANA has created a sub-registry in the "MPLS RTM TLV Registry" (see
   Section 7.2) called the "MPLS RTM Sub-TLV Registry".  All codepoints
   in the range 0 through 127 in this registry shall be allocated
   according to the "IETF Review" procedure as specified in [RFC5226].
   Codepoints in the range 128 through 191 in this registry shall be
   allocated according to the "First Come First Served" procedure as
   specified in [RFC5226].  This document defines the following new RTM
   sub-TLV types:

          +---------+--------------------------+---------------+
          | Value   |       Description        | Reference     |
          +---------+--------------------------+---------------+
          | 0       |         Reserved         | This document |
          | 1       |           PTP            | This document |
          | 2-191   |        Unassigned        |               |
          | 192-254 | Reserved for Private Use | This document |
          | 255     |         Reserved         | This document |
          +---------+--------------------------+---------------+

                         Table 3: RTM Sub-TLV Type

7.4.  RTM Capability Sub-TLV in OSPFv2

   IANA has assigned a new type for the RTM Capability sub-TLV in the
   "OSPFv2 Extended Link TLV Sub-TLVs" registry as follows:

                +-------+----------------+---------------+
                | Value |  Description   | Reference     |
                +-------+----------------+---------------+
                | 5     | RTM Capability | This document |
                +-------+----------------+---------------+

                      Table 4: RTM Capability Sub-TLV
















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7.5.  RTM Capability Sub-TLV in IS-IS

   IANA has assigned a new type for the RTM Capability sub-TLV from the
   "Sub-TLVs for TLVs 22, 23, 141, 222, and 223" registry as follows:

   +------+----------------+----+----+-----+-----+-----+---------------+
   | Type |  Description   | 22 | 23 | 141 | 222 | 223 | Reference     |
   +------+----------------+----+----+-----+-----+-----+---------------+
   | 40   | RTM Capability | y  | y  | n   | y   | y   | This document |
   +------+----------------+----+----+-----+-----+-----+---------------+

        Table 5: IS-IS RTM Capability Sub-TLV Registry Description

7.6.  RTM Capability TLV in BGP-LS

   IANA has assigned a new codepoint for the RTM Capability TLV from the
   "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
   Attribute TLVs" sub-registry in the "Border Gateway Protocol - Link
   State (BGP-LS) Parameters" registry as follows:

   +---------------+----------------+------------------+---------------+
   | TLV Code      |  Description   |  IS-IS TLV/Sub-  | Reference     |
   | Point         |                |       TLV        |               |
   +---------------+----------------+------------------+---------------+
   | 1105          | RTM Capability |      22/40       | This document |
   +---------------+----------------+------------------+---------------+

                   Table 6: RTM Capability TLV in BGP-LS























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7.7.  RTM_SET Sub-object RSVP Type and Sub-TLVs

   IANA has assigned a new type for the RTM_SET sub-object from the
   RSVP-TE "Attributes TLV Space" sub-registry as follows:

+------+------------+-----------+---------------+-----------+----------+
| Type |    Name    |  Allowed  | Allowed on    | Allowed   | Reference|
|      |            | on LSP_   | LSP_REQUIRED_ | on LSP    |          |
|      |            | ATTRIBUTES|   ATTRIBUTES  | Hop       |          |
|      |            |           |               | Attributes|          |
+------+------------+-----------+---------------+-----------+----------+
| 5    |  RTM_SET   |    Yes    |       No      |    No     | This     |
|      | sub-object |           |               |           | document |
+------+------------+-----------+---------------+-----------+----------+

                     Table 7: RTM_SET Sub-object Type

   IANA has created a new sub-registry for sub-TLV types of the RTM_SET
   sub-object called the "RTM_SET Object Sub-Object Types" registry.
   All codepoints in the range 0 through 127 in this registry shall be
   allocated according to the "IETF Review" procedure as specified in
   [RFC5226].  Codepoints in the range 128 through 191 in this registry
   shall be allocated according to the "First Come First Served"
   procedure as specified in [RFC5226].  This document defines the
   following new values of RTM_SET object sub-object types:

          +---------+--------------------------+---------------+
          | Value   |       Description        | Reference     |
          +---------+--------------------------+---------------+
          | 0       |         Reserved         | This document |
          | 1       |       IPv4 address       | This document |
          | 2       |       IPv6 address       | This document |
          | 3       |   Unnumbered interface   | This document |
          | 4-191   |        Unassigned        |               |
          | 192-254 | Reserved for Private Use | This document |
          | 255     |         Reserved         | This document |
          +---------+--------------------------+---------------+

                 Table 8: RTM_SET Object Sub-object Types












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7.8.  RTM_SET Attribute Flag

   IANA has assigned a new flag in the RSVP-TE "Attribute Flags"
   registry.

   +-----+---------+-----------+-----------+-----+-----+---------------+
   | Bit | Name    | Attribute | Attribute | RRO | ERO | Reference     |
   | No  |         | Flags     | Flags     |     |     |               |
   |     |         | Path      | Resv      |     |     |               |
   +-----+---------+-----------+-----------+-----+-----+---------------+
   | 15  | RTM_SET | Yes       | Yes       | No  | No  | This document |
   +-----+---------+-----------+-----------+-----+-----+---------------+

                      Table 9: RTM_SET Attribute Flag

7.9.  New Error Codes

   IANA has assigned the following new error codes in the RSVP "Error
   Codes and Globally-Defined Error Value Sub-Codes" registry.

            +------------+--------------------+---------------+
            | Error Code | Meaning            | Reference     |
            +------------+--------------------+---------------+
            | 41         | Duplicate TLV      | This document |
            | 42         | Duplicate sub-TLV  | This document |
            | 43         | RTM_SET TLV Absent | This document |
            +------------+--------------------+---------------+

                         Table 10: New Error Codes

8.  Security Considerations

   Routers that support RTM are subject to the same security
   considerations as defined in [RFC4385] and [RFC5085].

   In addition -- particularly as applied to use related to PTP -- there
   is a presumed trust model that depends on the existence of a trusted
   relationship of at least all PTP-aware nodes on the path traversed by
   PTP messages.  This is necessary as these nodes are expected to
   correctly modify specific content of the data in PTP messages, and
   proper operation of the protocol depends on this ability.  In
   practice, this means that those portions of messages cannot be
   covered by either confidentiality or integrity protection.  Though
   there are methods that make it possible in theory to provide either
   or both such protections and still allow for intermediate nodes to
   make detectable but authenticated modifications, such methods do not
   seem practical at present, particularly for timing protocols that are
   sensitive to latency and/or jitter.



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   The ability to potentially authenticate and/or encrypt RTM and PTP
   data for scenarios both with and without participation of
   intermediate RTM-/PTP-capable nodes is left for further study.

   While it is possible for a supposed compromised node to intercept and
   modify the G-ACh content, this is an issue that exists for nodes in
   general -- for any and all data that may be carried over an LSP --
   and is therefore the basis for an additional presumed trust model
   associated with existing LSPs and nodes.

   Security requirements of time protocols are provided in RFC 7384
   [RFC7384].

9.  References

9.1.  Normative References

   [IEEE.1588]
              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              IEEE Std 1588-2008, DOI 10.1109/IEEESTD.2008.4579760.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <http://www.rfc-editor.org/info/rfc3209>.

   [RFC3477]  Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
              in Resource ReSerVation Protocol - Traffic Engineering
              (RSVP-TE)", RFC 3477, DOI 10.17487/RFC3477, January 2003,
              <http://www.rfc-editor.org/info/rfc3477>.

   [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, DOI 10.17487/RFC4385,
              February 2006, <http://www.rfc-editor.org/info/rfc4385>.

   [RFC5085]  Nadeau, T., Ed. and C. Pignataro, Ed., "Pseudowire Virtual
              Circuit Connectivity Verification (VCCV): A Control
              Channel for Pseudowires", RFC 5085, DOI 10.17487/RFC5085,
              December 2007, <http://www.rfc-editor.org/info/rfc5085>.





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   [RFC5420]  Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.
              Ayyangarps, "Encoding of Attributes for MPLS LSP
              Establishment Using Resource Reservation Protocol Traffic
              Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC5420,
              February 2009, <http://www.rfc-editor.org/info/rfc5420>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <http://www.rfc-editor.org/info/rfc5586>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <http://www.rfc-editor.org/info/rfc5905>.

   [RFC6423]  Li, H., Martini, L., He, J., and F. Huang, "Using the
              Generic Associated Channel Label for Pseudowire in the
              MPLS Transport Profile (MPLS-TP)", RFC 6423,
              DOI 10.17487/RFC6423, November 2011,
              <http://www.rfc-editor.org/info/rfc6423>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <http://www.rfc-editor.org/info/rfc7684>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <http://www.rfc-editor.org/info/rfc7752>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <http://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [ITU-T.G.8271]
              ITU-T, "Time and phase synchronization aspects of packet
              networks", ITU-T Recomendation G.8271/Y.1366, July 2016.

   [OSPFV3-EXTENDED-LSA]
              Lindem, A., Roy, A., Goethals, D., Vallem, V., and F.
              Baker, "OSPFv3 LSA Extendibility", Work in Progress,
              draft-ietf-ospf-ospfv3-lsa-extend-14, April 2017.




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   [RFC4202]  Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4202, DOI 10.17487/RFC4202, October 2005,
              <http://www.rfc-editor.org/info/rfc4202>.

   [RFC5036]  Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <http://www.rfc-editor.org/info/rfc5036>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <http://www.rfc-editor.org/info/rfc6374>.

   [RFC7384]  Mizrahi, T., "Security Requirements of Time Protocols in
              Packet Switched Networks", RFC 7384, DOI 10.17487/RFC7384,
              October 2014, <http://www.rfc-editor.org/info/rfc7384>.

   [TIMING-OVER-MPLS]
              Davari, S., Oren, A., Bhatia, M., Roberts, P., and L.
              Montini, "Transporting Timing messages over MPLS
              Networks", Work in Progress, draft-ietf-tictoc-
              1588overmpls-07, October 2015.

Acknowledgments

   The authors want to thank Loa Andersson, Lou Berger, Acee Lindem, Les
   Ginsberg, and Uma Chunduri for their thorough reviews, thoughtful
   comments, and, most of all, patience.

















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Authors' Addresses

   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com


   Stefano Ruffini
   Ericsson

   Email: stefano.ruffini@ericsson.com


   Eric Gray
   Ericsson

   Email: eric.gray@ericsson.com


   John Drake
   Juniper Networks

   Email: jdrake@juniper.net


   Stewart Bryant
   Huawei

   Email: stewart.bryant@gmail.com


   Alexander Vainshtein
   ECI Telecom

   Email: Alexander.Vainshtein@ecitele.com
          Vainshtein.alex@gmail.com














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