| Internet-Draft | DetNet Options | August 2021 |
| Thubert | Expires 13 February 2022 | [Page] |
RFC 8938, the Deterministic Networking Data Plane Framework relies on the 6-tuple to identify an IPv6 flow. But the full DetNet operations require also the capabilities to signal meta-information such as a sequence within that flow, and to transport different types of packets along the same path with the same treatment, e.g., Operations, Administration, and Maintenance packets and/or multiple flows with fate and resource sharing. This document introduces new IPv6 options that signal that path and redundancy information to the intermediate DetNet relay and forwarding nodes.¶
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This Internet-Draft will expire on 13 February 2022.¶
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Section 2 of the Deterministic Networking Problem Statement [DetNet-PBST] introduces the concept of Deterministic Networking (DetNet) to the IETF. DetNet extends the reach of lower layer technologies such as Time-Sensitive Networking (TSN) [IEEE 802.1 TSN] and Timeslotted Channel Hopping (TSCH) [IEEE Std. 802.15.4] over IPv6 and MPLS [RFC8938], to provide bounded latency and reliability guarantees over an end-to-end layer-3 nailed-down path.¶
The "Deterministic Networking Architecture" [DetNet-ARCH] details the contribution of layer-3 protocols, and defines three planes: the Application (User) Plane, the Controller Plane, and the Network Plane. [DetNet-ARCH] places an emphasis on the centralized model whereby a controller instantiates a DetNet state in the routers that is located based on matching information in the packet. For IPv6 flows, this document proposes a layer-3 signaling to index that state, using an IPv6 Extension Header (EH).¶
The "6TiSCH Architecture" [6TiSCH-ARCH] leverages RPL, the "Routing Protocol for Low Power and Lossy Networks" [RPL] and introduces concept of a Track as a highly redundant RPL Destination Oriented Directed Acyclic Graph (DODAG) rooted at the Track Ingress.¶
A Track may for instance be installed using RPL route projection [RPL-PDAO]. In that case, the TrackId is an index from a namespace associated to one IPv6 address of the Track Ingress node, and the Track that an IPv6 packet follows is signaled by the combination of the source address (of the Track Ingress node), and the TrackID placed in a RPL Option [RFC6553] located in an IPv6 Hop-by-Hop (HbH) Options Header [IPv6] in the IPv6 packet.¶
The "Reliable and Available Wireless (RAW) Architecture/Framework" [RAW-ARCH], extends the DetNet Network Plane to accomodate one or multiple hops of homogeneous or heterogeneous wireless technologies, e.g. a Wi-Fi6 Mesh or parallel radio access links combining Wi-Fi and 5G. The RAW Architecture reuses the concept of Track and introduces a new dataplane component, the Path Selection Engine (PSE), to dynamically select a subpath and maintain the required quality of service within a Track in the face of the rapid evolution of the medium properties.¶
With [IPv6], the behavior of a router upon an IPv6 packet with a HbH Options Header has evolved, making the examination of the header by routers along the path optional, as opposed to previously mandatory. Additionally, the Option Type for any option in a HbH Options Header encodes in the leftmost bits whether a router that inspects the header should drop the packet or ignore the option when encountering an unknown option. Combined, these capabilities enable a larger use of the header beyond the boundaries of a limited domain, as examplified by the change of behavior of the RPL data plane, that was changed to allow a packet with a RPL option to escape the RPL domain in the larger Internet [RFC9008].¶
"IPv6 Hop-by-Hop Options Processing Procedures" [HbH-UPDT] further specifies the procedures for how IPv6 Hop-by-Hop options are processed to make their processing even more practical and increase their use in the Internet. In that context, it makes sense to consider Hop-by-Hop Options to transport the information that is relevant to DetNet.¶
The "Deterministic Networking Data Plane Framework" [RFC8938] relies on the 6-tuple to identify an IPv6 flow. But the full DetNet operations require also the capabilities to signal meta-information such as a sequence within that flow, and to transport different types of packets along the same path with the same treatment. For instance, it is required that Operations, Administration, and Maintenance (OAM) [RFC6291] packets and/or multiple flows share the same fate and resource sharing over the same Track or the same Traffic Engineered (TE) [RFC3272] DetNet path.¶
As opposed to the HbH EH, the Destination Option Header (DOH) is only read by the destination of the packet, which can be one at a time the collection of nodes listed in a Routing Extension Header (RH) if the DOH is placed before the RH.¶
This document introduces new IPv6 Options, the DetNet Redundancy Information Option and the DetNet Path Options, that signal the DetNet information to the intermediate DetNet nodes in an abstract form, that is pure layer-3 and agnostic of the transport layer. The options are placed in either a HbH EH or in a DOH, which happens when the next node that needs to process the option is the IPv6 destination in the IPv6 header.¶
This pure layer-3 technique alines DetNet with the IPv6 architecture and opens to the progress / extensions done elsewhere for IPv6; e.g., if the DetNet path leverages Segment routing (SRv6) [RFC8402] for some reason - there are plausible ones in RAW -, the Segment Routing Header (SRH) [RFC8754] is inserted after the HbH and/or DOH by the PE and both are readily accessible for the on-path routers without the need of a deeper inspection of the packet (up to and beyond the transport header).¶
For instance, the DetNet Redundancy Information Option may be placed in a DOH EH before an SRH that signals the exhaustive list of the Detnet relays along the path of the packet, so every relay can process the redundancy information therein, while the DetNet Strict Path Option would be placed in an HbH EH to be read by every DeNet fowarding node, and intercepted should it strays away from its path.¶
Timestamp semantics and timestamp formats used in this document are defined in "Guidelines for Defining Packet Timestamps" [RFC8877].¶
The Deterministic Networking terms used in this document are defined in the "Deterministic Networking Architecture" [DetNet-ARCH].¶
The terms Track and TrackID are defined in the "6TiSCH Architecture" [6TiSCH-ARCH].¶
Transported in IPv6 HbH Options, the DetNet options are available early in the header chain of the packet. A DetNet-aware end system (see section 4.2 of [DetNet-ARCH]) may place the options in the header chain when constructing the packet, in which case there is no need of an encapsulation.¶
Alternatively, the source end system may signal the flow information some other way, or it may lack the full DetNet awareness; in that case the DetNet path endpoints are the provider Edge (PE) routers (see Figure 1 reproducing figure 5 of [DetNet-ARCH]) and the Ingress PE needs to encapsulate the packets to add the HbH options.¶
In Figure 1, the DetNet end systems may be f-aware and signal an IPv6 flow using the 6-tuple for the End-to-End service, but may not be s-aware, and may not sequence the packets for Packet Replication, Elimination, and Ordering Functions (PREOF), which operate at the detNet Service Layer. In that case, the Ingress PE will encapsulate the packets for this and possibly other flows to provide a common DetNet Service with OAM and PREOF, across the DetNet-1 service provider network, terminating the tunnel at the Egress PE router.¶
_
/ \ +----DetNet-UNI (U) / \
/App\ | /App\
/-----\ | /-----\
| NIC | v ________ | NIC |
+--+--+ _____ / \ DetNet-UNI (U) --+ +--+--+
| / \__/ \ | |
| / +----+ +----+ \_____ | |
| / | | | | \_______ | |
+------U PE +----+ P +----+ \ _ v |
| | | | | | | ___/ \ |
| +--+-+ +----+ | +----+ | / \_ |
\ | | | | | / \ |
\ | +----+ +--+-+ +--+PE |------ U-----+
\ | | | | | | | | | \_ _/
\ +---+ P +----+ P +--+ +----+ | \____/
\___ | | | | /
\ +----+__ +----+ DetNet-1 DetNet-2
| \_____/ \___________/ |
| |
| | End-to-End Service | | | |
<------------------------------------------------------------->
| | DetNet Service | | | |
| <------------------------------------------------> |
| | | | | |
This document defines new IPv6 options for DetNet to signal path and a reliability information (e.g., sequencing) to the DetNet layers. Those options are to be placed in the IPv6 HbH Options Header, which is found right after the outer IPv6 header in the DetNet packet and immediately reachable for the forwarding engine. The format of the options follow the generic definition in section 4.2 of [IPv6]. For each tyoe of option, the draft allows to express the information in different fashions, depending on the use case, and possibly carrying an information that plays the same role at another layer, in which case the format of the information is opaque.¶
The reliability information may be inherited from another layer as long as the value is guaranteed to be unique within a reasonable set of sequential packet so all packets with the same value are redundant. Timestamping can be used as an alternate sequencing technique, that avoids maintaining per-path state at the path ingress, which is feasible for nodes that maintain a very precise sense of time (e.g., from GPS or PTP) for their DetNet operations. As long as the time granularity is in the order of a few bytes transmission, the system timestamp provides an absolute sense of ordering over a very long period across all paths for which this node is ingress, and thus within any of those. Alternatively, the draft allows to combine a rough time stamp (e.g., from a system clock synchronized by NTP) and a sequence counter that differntiates the packets that are stamped within the timer resolution.¶
If a DetNet Path option (see Section 4.2), including the RPL Option, is present in the same HbH Option Header as a DetNet Redundancy Information option (see Section 4.1), then the redundancy information applies to the signaled path across all flows that traverse that path; else the redundancy information applies to the flow indicated by the 6-tuple [RFC8938].¶
The DetNet Redundancy Information Option helps discriminate copies of a same packet vs. different packets, and is useful for service-sublayer Packet Replication Elimination and Ordering Functions (PREOF). The option may be placed either in an HbH or a DoH EH, e.g., prior to a Segment Routing Header (SRH) [RFC8754] that lists the DetNet relays. A sequence counter is probably the most typical expression of the redundancy information, but it is not the only way to identify a packet and/or enable reordering, e.g., a timestamp can be seen as a large sequence counter with gaps.¶
It is also possible that a packet is divided in elements such as network-coded fragments. In that case, the pieces are discriminated with an opaque 8-bit fragment tag. The goal is to retain one copy of each fragment but not reorder them.¶
A packet sequence can be expressed uniquely as a wrapping counter, represented as an unsigned integer in the option. In that case, the size of the representation MUST be large enough to cover at least 3 times the upper bound on out-of-order packet delivery in terms of number of packets. The sequence counter may be copied from a field in another protocol, and it is possible that the value 0 is reserved when wrapping, to the option offers both possibilities, wrapping to either 0 or to 1.¶
This specification also allows to use a time stamp for the packet redundancy information, in conformance with the recommendations in [RFC8877]. This can be accomplished by utilizing the Precision Time Protocol (PTP) format defined in IEEE Std. 1588 [IEEE Std. 1588] or Network Time Protocol (NTP) [RFC5905] formats. In that case, the timestamp resolution at the origin node that builds the option MUST be fine enough to ensure that two consecutive packets are never stamped with the same value. There is no requirement for this particular stamping function that the sense of time at the origin node is synchronized with the rest of the DetNet network.¶
IEEEE TSN [IEEE 802.1 TSN] defined a redundancy tag (R-Tag) for the IEEE Std. 802.1CB Frame Replication and Elimination for Reliability (FRER). The R-Tag is a structured field and its content is subject to evolve; but the expectation for this specification is that the overall size remains 48 bits and that the 48-bit value is different for a large number of contiguous frames. When transporting TSN frames in a DetNet packet, it is possible to leverage the R-Tag as Redundancy information, though it cannot be assumed that the R-Tag is sequentially incremented; so it can be used for packet duplicate elimination but it is not suitable not for packet re-ordering.¶
This specification also allows for an hybrid model with a coarse grained packet sequence within a coarse grained time stamp. In that case, both a time stamp option and a wrapping counter options are found, and the counter is used to compare packets with the same time stamp and ignored otherwise In that case, the size of the representation of the counter MUST be large enough to cover at least 3 times the number of packets that may be sent with the same value of time stamp.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | R.I. Type | Fragment Tag | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Redundancy Information (variable Size) . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Redundancy Information Option fields:¶
8-bit length of the option data.¶
8-bit field, set to 0 when the packet is sent in entirety; packets with the same Redundancy Information and different fragments tags MUST be considered as different by the elimination function and are not subject to ordering based on the Tag.¶
8-bit identifier of the type of Redundancy information. Value to be confirmed by IANA.¶
| Seq. Type Value | Category | Common Name | Redundancy Information Format |
|---|---|---|---|
| 1 | Wrapping Counter | Basic Sequence Counter | 32-bit unsigned integer |
| 2 | Wrapping Counter | Zero-avoiding Sequence Counter | 32-bit unsigned integer, wraps to 1 |
| 3 | Wrapping Counter | RPL Sequence Counter | 8-bit RPL sequence, see section 7. of [RPL] |
| 11 | Time Stamp | Fractional NTP | NTP 64-bit Timestamp Format, see section 4.2.1. of [RFC8877] |
| 12 | Time Stamp | Short NTP | NTP 32-bit Timestamp Format, see section 4.2.2. of [RFC8877] |
| 13 | Time Stamp | PTP | PTP 80-bit Timestamp Format, see [IEEE Std. 1588] |
| 14 | Time Stamp | Short PTP | PTP 64-bit Truncated Timestamp Format, see section 4.3. of [RFC8877] |
| 24 | Structured Unique Tag | TSN Redundancy Tag | 48-bit opaque |
The DetNet Architecture [DetNet-ARCH] assigns a DetNet flow "to specific paths through a network", but is not specific on how the path is then signaled in the packet. The DetNet Data Plane Framework [RFC8938] relies on the 6-tuple to identify an IPv6 flow and implicitely the path could be indexed by the flow identification. But this requires to maintain one path per flow and makes it difficult to assign other traffic such as OAM to the same path.¶
This draft provides aditional means to signal the path in which the flow is placed separately from the flow indentification, and independantly of the transport layer, so a path can be shared between one or more flows and OAM packets across IP address families. All the packets that are assigned to the same path are subject to the same DetNet forwarding treatment.¶
the DetNet expectation is that a PCE sets up a state at the DetNet forwarding sublayer to instruct each hop on how to process the DetNet flows. The DetNet Path Options when present contains information that MUST be used to select the DetNet state installed and if the DetNet state does not exist then the packet cannot be forwarded.¶
In complement to the RPL option, this specification defines a protocol-independent Strict Path Identifier, which is also taken from a namespace indicated by the IPv6 source address of the packet.¶
The DetNet Strict Path Option is to be used in a limited domain and all routers along the path are expected to support the option. The option is placed in an HbH EH to be seen by all routers on path. The path indicated therein may also be used by the service sublayer, to signal the scope where the redundancy information is unique across a number of packets large enough to ensure that a forwarding node never has to handle different packets with the same redundancy information, though the same value may be found for packets with a different path information.¶
The typical DetNet path is typically contained under a single administrative control or within a closed group of administrative control; these include campus-wide networks and private WANs [DetNet-ARCH]. The typical expectation is that all nodes along a DetNet path are aware of the path and actively maintain a forwarding state for it. The DetNet Strict Path Option (see Section 4.2.1) is designed for that environment; if a packet escapes the local domain, a router that does not support the option will intercept it and return an error to the source.¶
In other environments such as RAW, it might be that the service-layer protection concentrates on just segments of the end-to-end path. In that case, the service-sublayer protection may require the signaling of both redundancy and path information, though the path information is potentially not used by some of the intermediate routers and may not be used for forwarding at all. The path information may also relate to segments that are installed along the path using a DetNet forwarding state as opposed to, say, source routing. In either case the DetNet Loose Path Option Section 4.2.2 can be used to signal the path without incurring an ICMP Error from an intermediate node.¶
An intermediate router that supports the DetNet Strict Path Option but is missing the necessary state to forward along the indicated path must drop the packet and return an ICMP error.code 0 pointing at the offset of the Strict Path ID in the DetNet Strict Path Option.¶
DetNet can also leverage the RPL Option that signals a Track in the RPL Packet Information (RPI) [RFC6553]. There are 2 versions of the RPL option, defined respectively in [RPL] with the act bits [IPv6] set to dropped the packet when the option is unknown, that defined in[RFC9008] which let the option be ignored.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Strict Path ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Redundancy Option fields:¶
8-bit length of the option data, set to 2.¶
16-bit identifier of the DetNet Path, taken from a local namespace associated with the IPv6 source address of the packet.¶
The DetNet Loose Path Option transports a Loose Path identifier which is taken from a namespace indicated by the Origin Autonomous System (AS). When the DetNet path is contained within a single AS, the Origin Autonomous System field can be left to 0 indicating local AS. The option may be placed either in an HbH or a DoH EH, but the preferred method is a DOH that precedes an RH such as SRH.¶
The DetNet Loose Path Option is to be used to signal a path that may be loose and may exceed the boundaries of a local domain; a portion of the hops may traverse routers in the wider internet that will not leverage the option and are expected to ignore it.¶
An intermediate router that supports the DetNet Loose Path Option but is missing the necessary state to forward along the indicated path must ignore the DetNet Loose Path Option.¶
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | Origin Autonomous System | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Loose Path ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Redundancy Option fields:¶
8-bit length of the option data, set to 6.¶
16-bit identifier of the Autonomous Systems (AS) that originates the path. The value of 0 signals a DetNet path that is constrained within the local AS or the local administrative DetNet domain.¶
32-bit identifier of the DetNet Path, taken from a local namespace associated with the origin AS of the DetNet path.¶
6TiSCH [6TiSCH-ARCH] and RAW [RAW-ARCH] signal a Track using a RPL Option [RFC6553] with a RPLInstanceID used as TrackID. This specification reuses the RPL option as a method to signal a DetNet path. In that case, the Projected-Route 'P' flag [RPL-PDAO] MUST be set to 1, and the O, R, F flags, as well as the Sender Rank field, MUST be set to 0 by the originator, forwarded as-is, and ignored on reception.¶
This specification creates a new Subregistry for the "Redundancy Type of the Redundancy Option" under the "Internet Protocol Version 6 (IPv6) Parameters" registry [IPV6-PARMS].¶
| Suggested Value | Meaning | Reference |
| 1 | Basic Sequence Counter | THIS RFC |
| 2 | Zero-avoiding Sequence Counter | THIS RFC |
| 3 | RPL Sequence Counter | THIS RFC |
| 11 | Fractional NTP time stamp | THIS RFC |
| 12 | Short NTP time stamp | THIS RFC |
| 13 | PTP time stamp | THIS RFC |
| 14 | Short PTP time stamp | THIS RFC |
| 24 | TSN Redundancy Tag | THIS RFC |
This specification updates the "Destination Options and Hop-by-Hop Options" under the "Internet Protocol Version 6 (IPv6) Parameters" registry [IPV6-PARMS] with the (suggested) values below:¶
| Hexa | act | chg | rest | Description | Reference |
| 0x12 | 00 | 0 | 10010 | DetNet Redundancy Information Option | THIS RFC |
| 0x93 | 10 | 0 | 10011 | DetNet Strict Path Option | THIS RFC |
| 0x14 | 00 | 0 | 10100 | DetNet Loose Path Option | THIS RFC |
TBD¶