| MPLS Working Group | G. Mirsky |
| Internet-Draft | ZTE Corp. |
| Intended status: Standards Track | S. Ruffini |
| Expires: August 3, 2017 | E. Gray |
| Ericsson | |
| J. Drake | |
| Juniper Networks | |
| S. Bryant | |
| Huawei | |
| A. Vainshtein | |
| ECI Telecom | |
| January 30, 2017 |
Residence Time Measurement in MPLS network
draft-ietf-mpls-residence-time-13
This document specifies a new Generic Associated Channel for Residence Time Measurement and describes how it can be used by time synchronization protocols within a MPLS domain.
Residence time is the variable part of propagation delay of timing and synchronization messages and knowing what this delay is for each message allows for a more accurate determination of the delay to be taken into account in applying the value included in a Precision Time Protocol event message.
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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 http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on August 3, 2017.
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
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Time synchronization protocols, e.g., Network Time Protocol version 4 (NTPv4) [RFC5905] and Precision Time Protocol (PTP) Version 2 [IEEE.1588.2008], 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 it 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 Multi-Protocol 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. Althugh it is possible to use RTM over an LSP instantiated using LDP, that is outside the scope of this document.
Comparison with alternative proposed solutions such as [I-D.ietf-tictoc-1588overmpls] is outside the scope of this document.
MPLS: Multi-Protocol Label Switching
ACH: Associated Channel
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
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
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 [RFC2119].
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 LSP or PW. But these measurements are insufficient for use in some applications, for example, time synchronization across a network as defined in the Precision Time Protocol (PTP). In PTPv2 [IEEE.1588.2008], residence time is accumulated in the correctionField of the PTP event message, as defined in [IEEE.1588.2008] 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), 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 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.
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 end-point 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:
IEEE 1588v2 [IEEE.1588.2008] 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.1588.2008], that is, the specific PTP port of the boundary clock 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, once it is set by a two-step mode device, MUST stay set accordingly until the original and follow-up messages are combined by an end-point (such as a Boundary Clock).
Thus, an RTM packet, containing residence time information relating to an earlier packet, also contains information identifying that earlier packet.
For compatibility with PTP, RTM (when used for PTP packets) must behave in a similar fashion. Without loss of generality should note that handling of Sync event messages and handling of Delay_Req/Delay_Resp event messages that cross a two-step RTM node is different. Following outlines handling of PTP Sync event message by the two-step RTM node. Details of handling Delay_Resp/Delay_Req PTP event messages by the two-step RTM node are in Section 2.1.1. To do this, 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 it 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 bound.
In practice an RTM operating according to two-step clock behaves like a two-steps 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 in two-step mode (by making an update to the Scratch Pad of a follow-up message when its presence is indicated), but MUST NOT do both.
Two main subcases identified for an RTM node operating as a two-step clock described in the following sub-sections.
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, the residence time is added to the RTM packet that has been created to include the associated PTP packet (i.e. follow-up message in the downstream direction), if the local RTM-capable node is also operating as a two-step clock. This RTM packet carries the related accumulated residence time and the appropriate values of the Sequence Id and Port Id (the same identifiers carried in the packet processed) and the Two-step Flag set to 1.
Note that the fact that an upstream RTM-capable node operating in the two-step mode has created a follow-up message does not require any subsequent RTM capable node to also operate in the 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.
A PTP event packet (sync) is carried in the RTM packet in order for an RTM node to identify that residence time measurement must be performed on that specific packet.
To handle the residence time of the Delay_Req message on the upstream direction, an RTM packet must be created to carry the residence time on 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 properly handle the two-step flag of the PTP packets.
When the PTP network connected to the MPLS and RTM node, operates in one-step clock mode, the associated RTM packet must be created by the RTM node itself. The associated RTM packet including 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 be removing RTM encapsulation and, in case of two-step clock mode being indicated, will generate PTP messages as appropriate (according to the [IEEE.1588.2008]). In this case, the common header of the PTP packet carrying the synchronization message would have to be modified in the twoStepFlag field indicating that there is now a follow up message associated to that.
RFC 5586 [RFC5586] and RFC 6423 [RFC6423] define the G-ACh to extend the applicability of the PW Associated Channel (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.
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 | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTM G-ACh message format for Residence Time Measurement
The message format for Residence Time Measurement (RTM) is presented in Figure 1
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
Figure 2 presents 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 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
where Flags field has format
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.
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.
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 Figure 4 and Figure 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 the 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 operator's decision to include or exclude a particular port from RTM processing or switch between RTM modes.
[RFC4202] explains that the Interface Switching Capability Descriptor describes the switching capability of an interface. For bi-directional links, the switching capabilities of an interface are defined to be the same in either direction. I.e., 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. Detailed discussion of one-step and two-step RTM modes is contained in Section 2.1.
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: RTM Capability sub-TLV in OSPFv2
The format for the RTM Capability sub-TLV in OSPF is presented in Figure 4
The capability to support RTM on a particular link (interface) is advertised in the OSPFv2 Extended Link Opaque LSA described in Section 3 [RFC7684] via the RTM Capability sub-TLV.
Its Type value will be assigned by IANA from the OSPF Extended Link TLV Sub-TLVs registry Section 7.4, that will be created per [RFC7684] request.
The capability to support RTM on a particular link (interface) can be advertised in OSPFv3 using LSA extensions as described in [I-D.ietf-ospf-ospfv3-lsa-extend]. Exact use of OSPFv3 LSA extensions is for further study.
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 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: RTM Capability sub-TLV for the Extended IS Reachability TLV
The format for the RTM Capabilities sub-TLV is presented in Figure 5
The capability to support RTM on a particular link (interface) is advertised in the Extended IS Reachability TLV described in [RFC5305] via the RTM Capability sub-TLV.
The format for the RTM Capabilities TLV is as presented in Figure 4.
Type value TBA9 will be assigned by IANA from the BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs sub-registry Section 7.6.
Length, RTM, and Reserved fields as defined in Section 4.3.
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].
Throughout this document we refer to a node as 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
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. In the Path message that the ingress node uses to instantiate the LSP to that egress node it places the LSP_ATTRIBUTES Object [RFC5420] with RTM_SET Attribute Flag set, as described in Section 7.8, which indicates to the egress node that RTM is requested for this LSP. RTM_SET Attribute Flag SHOULD NOT be set in the LSP_REQUIRED_ATTRIBUTES object [RFC5420], unless it is known that all nodes support RTM, because a node that does not recognize RTM_SET Attribute Flag would reject the Path message.
If an egress node receives a Path message with RTM_SET Attribute Flag in LSP_ATTRIBUTES object, it MUST include initialized RRO [RFC3209] and LSP_ATTRIBUTES object where RTM_SET Attribute Flag is set and RTM_SET TLV Section 4.8 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.
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
RTM capable interfaces can be recorded via RTM_SET TLV. The RTM_SET sub-object format is of generic Type, Length, Value (TLV), presented in Figure 7 .
Type value (TBA4) will be assigned by IANA from its Attributes TLV Space sub-registry Section 7.7.
The Length contains the total length of the sub-object in bytes, including the Type and Length fields.
The I bit flag indicates whether the downstream RTM capable node along the LSP is present in the RRO.
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 the LSP_ATTRIBUTES object. The sub-TLVs are defined in Section 4.8.1 below.
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 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 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 Type value in its 8 least significant bits. If no RTM_SET TLV has been 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 reachable RTM capable interface. If the node cannot find a matching ID in 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 RTM_SET sub-TLV with the same address it used in 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 been set to 255, as described above, then the I flag in node RTM_SET TLV MUST be set to 1 before Resv message forwarded upstream. Otherwise, the I flag MUST be cleared (0).
The ingress node MAY inspect the I bit flag received in each RTM_SET TLV contained in the LSP_ATTRIBUTES object of a received Resv message. Presence of the RTM_SET TLV with I bit field set to 1 indicates that some RTM nodes along the LSP could be included in the 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 RRO is limited due to size constraints . Such changes affect the core assumption of this method and processing of RTM packets. RTM SHOULD NOT be used if it is not guaranteed that the RRO contains complete information.
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 a 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. 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 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 Error Value contains 16-bit value composed of (Type of TLV, Type of sub-TLV).
Three kinds of sub-TLVs for RTM_SET are currently defined.
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
Length
IPv4 address
Reserved
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
Length
IPv6 address
Reserved
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
Length
Node ID
Interface ID
Reserved
After instantiating an LSP for a path using RSVP-TE [RFC3209] as described in Section 4.7, 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; this should be done as close to the physical layer as possible to ensure precise accuracy in time determination. 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 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 ns. 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.
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 typically boundary clocks, at the edges of the MPLS network, 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.
IANA is requested to reserve a new G-ACh as follows:
| Value | Description | Reference |
|---|---|---|
| TBA1 | Residence Time Measurement | This document |
IANA is requested to create sub-registry in Generic Associated Channel (G-ACh) Parameters Registry called "MPLS RTM TLV Registry". All code points in the range 0 through 127 in this registry shall be allocated according to the "IETF Review" procedure as specified in [RFC5226] . Code points 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 RTM TLV type s:
| 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 |
| 6-127 | Unassigned | |
| 128 - 191 | Unassigned | |
| 192 - 254 | Private Use | This document |
| 255 | Reserved | This document |
IANA is requested to create sub-registry in MPLS RTM TLV Registry, requested in Section 7.2, called "MPLS RTM Sub-TLV Registry". All code points in the range 0 through 127 in this registry shall be allocated according to the "IETF Review" procedure as specified in [RFC5226]. Code points 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 RTM sub-TLV types:
| Value | Description | Reference |
|---|---|---|
| 0 | Reserved | This document |
| 1 | PTP | This document |
| 2-127 | Unassigned | |
| 128 - 191 | Unassigned | |
| 192 - 254 | Private Use | This document |
| 255 | Reserved | This document |
IANA is requested to assign a new type for RTM Capability sub-TLV from OSPFv2 Extended Link TLV Sub-TLVs registry as follows:
| Value | Description | Reference |
|---|---|---|
| TBA2 | RTM Capability | This document |
IANA is requested to assign a new Type for 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 |
|---|---|---|---|---|---|---|---|
| TBA3 | RTM | y | n | n | n | n | This document |
IANA is requested to assign a new code point for RTM Capability TLV from BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs sub-registry in its Border Gateway Protocol - Link State (BGP-LS) Parameters registry as follows:
| TLV Code Point | Description | IS-IS TLV/Sub-TLV | Reference |
|---|---|---|---|
| TBA9 | RTM Capability | 22/TBA3 | This document |
IANA is requested to assign a new Type for RTM_SET sub-object from Attributes TLV Space sub-registry as follows:
| Type | Name | Allowed on LSP_ATTRIBUTES | Allowed on LSP_REQUIRED_ATTRIBUTES | Allowed on LSP Hop Attributes | Reference |
|---|---|---|---|---|---|
| TBA4 | RTM_SET sub-object | Yes | No | No | This document |
IANA requested to create new sub-registry for sub-TLV types of RTM_SET sub-object. All code points in the range 0 through 127 in this registry shall be allocated according to the "IETF Review" procedure as specified in [RFC5226] . Code points 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 |
| 4-127 | Unassigned | |
| 128 - 191 | Unassigned | |
| 192 - 254 | Private Use | This document |
| 255 | Reserved | This document |
IANA is requested to assign new flag from Attribute Flags registry
| Bit No | Name | Attribute Flags Path | Attribute Flags Resv | RRO | ERO | Reference |
|---|---|---|---|---|---|---|
| TBA5 | RTM_SET | Yes | Yes | No | No | This document |
IANA is requested to assign new Error Codes from Error Codes and Globally-Defined Error Value Sub-Codes registry
| Error Code | Meaning | Reference |
|---|---|---|
| TBA6 | Duplicate TLV | This document |
| TBA7 | Duplicate sub-TLV | This document |
| TBA8 | RTM_SET TLV Absent | This document |
Routers that support Residence Time Measurement 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.
The ability for potentially authenticating and/or encrypting RTM and PTP data for scenarios both with and without participation of intermediate RTM/PTP-capable nodes is 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].
Authors want to thank Loa Andersson, Lou Berger and Acee Lindem for their thorough reviews, thoughtful comments and, most of all, patience.