| DetNet | P. Thubert, Ed. |
| Internet-Draft | Cisco |
| Intended status: Standards Track | Z. Brodard |
| Expires: July 20, 2018 | Ecole Polytechnique |
| H. Jiang | |
| Telecom Bretagne | |
| January 16, 2018 |
BIER-TE-based OAM, Replication and Elimination
draft-thubert-bier-replication-elimination-02
This specification leverages Bit Index Explicit Replication - Traffic Engineering to control in the data plane the DetNet Replication and Elimination activities, and to provide traceability on links where replication and loss happen, in a manner that is abstract to the forwarding information.
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Deterministic Networking (DetNet) [I-D.ietf-detnet-problem-statement] provides a capability to carry unicast or multicast data flows for real-time applications with extremely low data loss rates and known upper bound maximum latency [I-D.ietf-detnet-architecture].
DetNet applies to multiple environments where there is a desire to replace a point to point serial cable or a multidrop bus by a switched or routed infrastucture, in order to scale, lower costs, and simplify management. One classical use case is found in particular in the context of the convergence of IT with Operational Technology (OT), also referred to as the Industrial Internet. But there are many others use cases [I-D.ietf-detnet-use-cases], for instance in in professional audio and video, automotive, radio fronthauls, etc..
The DetNet data plane alternatives studies the applicability of existing and emerging dataplane techniques that can be leveraged to enable DetNet properties in IP networks. One critical feature in the dataplane is traceability, the capability to control the activity of intermediate nodes on a packet. For instance, if Replication and Elimination is applied to a packet, then it is desirable to determine which node performed a certain copy of that packet that is circulating in the network.
Traceability belongs to Operations, Administration, and Maintenance (OAM) which is the toolset for fault detection and isolation, and for performance measurement. More can be found on OAM Tools in "An Overview of Operations, Administration and Maintenance (OAM) Tools".
This document proposes a new set to OAM tools based on Bit Indexed Explicit Replication (BIER) and more specifically BIER Traffic Engineering (BIER-TE) to control the process or Replication and Elimination, and provide traceability of these operations, in the DetNet dataplane. An adjacency, which is represented by a bit in the BIER header, can correspond in the dataplane to an Ethernet hop, a Label Switched Path, or it can correspond to an IPv6 loose or strict source routed path.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
BIER is a network plane replication technique that was initially intended as a new method for multicast distribution. In a nutshell, a BIER header includes a bitmap that explicitly signals the listeners that are intended for a particular packet, which means that 1) the sender is aware of the individual listeners and 2) the BIER control plane is a simple extension of the unicast routing as opposed to a dedicated multicast data plane, which represents a considerable reduction in OPEX. For this reason, the technology faces a lot of traction from Service Providers.
The simplicity of the BIER technology makes it very versatile as a network plane signaling protocol. Already, a new Traffic Engineering variation is emerging that uses bits to signal segments along a TE path. While BIER is mainly a multicast technology that typically leverages a unicast distributed control plane through IGP extensions, BIER-TE [I-D.eckert-bier-te-arch] is mainly a unicast technology that leverages a central computation to setup path, compute segments and install the mapping in the intermediate nodes.
BIER-TE supports a Traffic Engineered forwarding plane by explicit hop-by-hop forwarding and loose hop forwarding of packets.
From the BIER-TE architecture, the key differences over BIER are:
The generic view of an adjacency can be over a link, a tunnel or along a path segment.
With Segment Routing a segment can be signaled as an MPLS label, or an IPv6 routing header . A segment may be loosely of strictly source routed, depending on the need for full non-congruence and the confidence that loose routing may still achieve that need.
In a nutshell, BIER-TE is used as follows:
===> (A) ====> (C) ====
// ^ | ^ | \\
ingress (I) | | | | (E) egress
\\ | v | v //
===> (B) ====> (D) ====
Figure 1: Ladder Shape with Replication and Elimination Points
===> (A) ====> (C) ====
// 1 ^ | 4 ^ | 7 \\
ingress (I) |2| |6| (E) egress
\\ 3 | v 5 | v 8 //
===> (B) ====> (D) ====
Figure 2: Assigning Bits
| Bit # | Adjacency | Owner | Example Bit Setting |
|---|---|---|---|
| 1 | I->A | I | 1 |
| 2 | A->B | A | 1 |
| B->A | B | ||
| 3 | I->C | I | 0 |
| 4 | A->C | A | 1 |
| 5 | B->D | B | 1 |
| 6 | C->D | C | 1 |
| D->C | D | ||
| 7 | C->E | C | 1 |
| 8 | D->E | D | 0 |
Replication and Elimination Protecting A->C
====> Repl ===> Elim ====
// | ^ \\
ingress | | egress
v |
Fwd ====> Fwd
Figure 3: Enabled Adjacencies
| Adjacency | BIER BitString |
|---|---|
| I->A | 01011110 |
| A->B | 00011110 |
| B->D | 00010110 |
| D->C | 00010010 |
| A->C | 01001110 |
BitString in BIER Header as Packet Progresses
| Operation | BIER BitString |
|---|---|
| D->C | 00010010 |
| A->C | 01001110 |
| -------- | |
| AND in C | 00000010 |
| C->E | 00000000 |
BitString Processing at Elimination Point C
| Failing Adjacency | Egress BIER BitString |
|---|---|
| I->A | Frame Lost |
| I->B | Not Tried |
| A->C | 00010000 |
| A->B | 01001100 |
| B->D | 01001100 |
| D->C | 01001100 |
| C->E | Frame Lost |
| D->E | Not Tried |
BitString indicating failures
In more details:
The BIER header is of variable size, and a DetNet network of a limited size can use a model with 64 bits if 64 adjacencies are enough, whereas a larger deployment may be able to signal up to 256 adjacencies for use in very complex paths. The format of this header is common to BIER and BIER-TE.
For the DetNet data plane, a replication point is an ingress point for more than one adjacency, and an elimination point is an egress point for more than one adjacency.
A pre-populated state in a replication node indicates which bits are served by this node and to which adjacency each of these bits corresponds. With DetNet, the state is typically installed by a controller entity such as a PCE. The way the adjacency is signaled in the packet is fully abstracted in the bit representation and must be provisioned to the replication nodes and maintained as a local state, together with the timing or shaping information for the associated flow.
The DetNet data plane uses BIER-TE to control which adjacencies are used for a given packet. This is signaled from the path ingress, which sets the appropriate bits in the BIER BitString to indicate which replication must happen.
The replication point clears the bit associated to the adjacency where the replica is placed, and the elimination points perform a logical AND of the BitStrings of the copies that it gets before forwarding.
As is apparent in the examples above, clearing the bits enables to trace a packet to the replication points that made any particular copy. BIER-TE also enables to detect the failing adjacencies or sequences of adjacencies along a path and to activate additional replications to counter balance the failures.
Finally, using the same BIER-TE bit for both directions of the steps of the ladder enables to avoid replication in both directions along the crossing adjacencies. At the time of sending along the step of the ladder, the bit may have been already reset by performing the AND operation with the copy from the other side, in which case the transmission is not needed and does not occur (since the control bit is now off).
BIER-TE occupies a particular position in the DetNet dataplane. In the one hand it is optional, and only useful if replication and elimination is taking place. In the other hand, it has unique capabilities to:
A research-stage implementation of the forwarding plane fir a 6TiSCH IOT use case was developed at Cisco's Paris Innovation Lab (PIRL) by Zacharie Brodard. It was implemented on OpenWSN Open-source firmware and tested on the OpenMote-CC2538 hardware. It implements the header types 15,16, 17, 18 and 19 (bit-by-bit encoding without group ID) in order to allow a BIER-TE protocol over IEE802.15.4e.
This work was complemented with a Controller-based control loop by Hao Jiang. The controller builds the complex paths (called Tracks in 6TiSCH) and decides the setting oif the BitStrings in real time in order to optimize the delivery ratio within a minimal energy budget.
Links:
TBD.
This document has no IANA considerations.
The method presented in this document was discussed and worked out together with the DetNet Data Plane Design Team:
The authors also like to thank the DetNet chairs Lou Berger and Pat Thaler, as well as Thomas Watteyne, 6TiSCH co-chair, for their contributions and support to this work.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |