White Rabbit High-availability Seamless Redundancy (WR-HSR)

WR-HSR is a research project to implement the High availability Seamless Ring (HSR) protocol (IEC 62439-3 Clause 5) on White Rabbit switches and dual-port end nodes.
The implementation is not part of the roadmap of the White Rabbit project.

Introduction

HSR guarantees zero-time recovery in case of single point of failure. Including the protocol in WR elements, we could extend HSR features to time and frequency distribution in WR ring networks.

The nodes (devices) in an HSR network are attached by two Ethernet ports. A source node sends the same frame over both ports. A destination should receive, in the fault-free state, two identical frames within a certain time skew, forward the first frame to the application and discard the second frame when (and if) it comes. A sequence number is used to recognize such duplicates.

Fig1: HSR typical network topology. (non-wr)*

HSR nodes are arranged into a ring, which allows the network to operate without dedicated switches, since every node is able to forward frames from port to port. HSR originally meant "High-availability Seamless Ring", but HSR is not limited to a simple ring topology. Redundant connections to other HSR rings and to PRP networks are possible.
Since the forwarding delay of every node in a HSR ring adds to the total network latency, it is important that frames are forwarded quickly. In practice, special hardware support is required to bring down the per-hop latency to a reasonable value, often using cut-through switching. Another property of a HSR ring is that only about half of the network bandwidth is available to applications (compared to RSTP). This is because all frames are sent twice over the same network, even when there is no failure.

WR-HSR Implementation

The implementation of WR-HSR relies on the development of a peer-to-peer mechanism instead of the current end-to-end method to measure the link delay between the master node of the ring and end-nodes. The nodes that conform the ring must implement a Transparent Clock (TC), able to forward and consume sync and follow_up messages with an HSR tag. In case an untag PTP message reaches a TC, it assumes it comes from a master clock so that it tags the frame, duplicates it, and sends it out through the two HSR ports following different paths in the ring.

One of the challenges of this project is the sintonization process involved in the synchronization carried out by Synchronous Ethernet (SynqE), which current implementation requires of Master/Slave states for the frequency distribution and shall be adapted to TC.

To sum up, this development implies:

• the development of peer delay message exchange to measure the link delay between two adjacent nodes (Fig.2 & Fig.3).
• the development of peer-to-peer for sync and follow_up messages over TC (Fig.4).
• computing the residence time of sync messages on each node to be added in the correction field of follow_up messages. This correction field, accumulated from all nodes the frame passes by, together with the link delay measured, will be used by the end node to get synchronized with the master (Fig.5).
• each PTP message includes a HSR tag, used to drop duplicated messages from the network and checks possible errors in transmission.
• PTP is computed per port, applying a Best Master Clock algorithm on each.
• in case of node failure, a switchover to the other path must be performed to keep sinchronization to the master node active.
• adaptation of SyncE to TC for the syntonization process.

Fig2: Link delay measurement using peer delay mechanism*

Fig3: Peer Delay message exchange diagram*

Fig4: Peer-to-Peer Sync/Follow_up message exchange*

Fig5: Residence Time Measurement for PTP Correction Field*

Project information

• Software
• Frequently Asked Questions
• Users

Contacts

• José Luis Gutiérrez - University of Granada - General questions about the project.
• Javier Díaz - University of Granada - General questions about the project.

Project Status

October 12th, 2017