Bidirectional Tunnel Probing for Multi-Realm Return Path Faults

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Solution Overview

Problem

Traditional probe packets in packet-switched networks fail to return due to faults in the return path defined by an interior gateway protocol (IGP), leading to false indications of tunnel failure and unnecessary disruption of network traffic.

Innovation Solution

Implement bi-directional tunnel probing using probe packets with a forward tunnel label, a reverse tunnel label, and an IP packet header, allowing probe packets to traverse tunnels in both directions, thereby avoiding false failure indications.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If traditional unidirectional probe packets are used to measure tunnel liveness, then the tunnel availability can be monitored, but false failure indications occur when the return path has faults

Engineering Contradiction:

Improvetunnel availability monitoring accuracyVSAvoidfalse failure indications

Core Design Contradiction:

ReliabilityVSLoss of information

Solution Approach 1:

The patent applies inversion by making the probe packet traversal bidirectional instead of unidirectional. The probe packet travels from source to destination and then returns from destination to source, both times using the tunnel. This reverses the traditional approach where only forward traversal was performed, allowing detection of faults in both forward and return paths of the tunnel.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent implements feedback by requiring the probe packet to return to the source router after reaching the destination. The source router monitors whether the probe packet successfully returns, providing feedback about the health of both the forward and reverse tunnel paths. This closed-loop feedback mechanism eliminates false failure indications by confirming bidirectional functionality.

Inventive Principle:

Principle #23Feedback

2Adaptability or versatility

If probe packets use IGP routes for return path, then routing flexibility is provided, but faults in IGP return paths cause false tunnel failure indications

Engineering Contradiction:

Improverouting flexibilityVSAvoidtunnel liveness measurement accuracy

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The patent inverts the return path mechanism by having the probe packet return through the tunnel itself rather than using IGP routes. This is achieved by configuring the destination router to forward the probe packet back through the tunnel to the source, ensuring that both forward and return paths are tested using the same tunnel routing.

Inventive Principle:

Principle #13The other way round (Inversion)

3Adaptability or versatility

If tunnels are limited to boundary routers in multi-realm networks, then inter-realm communication is enabled, but tunnel probing cannot verify end-to-end connectivity across realms

Engineering Contradiction:

Improveinter-realm communication capabilityVSAvoidend-to-end connectivity verification

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

The patent applies universality by making the probe packet mechanism applicable to both intra-realm and inter-realm tunnels. The same bidirectional probe packet approach works regardless of whether the tunnel spans a single realm or multiple realms, providing a universal solution for tunnel liveness measurement across different network scopes.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentUS12609881B2Bi-directional tunnel probing in a multi-realm network

Publication Date: 2026.04.21 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

Bi-directional tunnel probing in a network may be extended to multi-realm networks, providing bi-directional probing in a multi-realm network. Bi-directional probing uses probe packets with a forward tunnel label, a reverse tunnel label, and an IP packet header. Bi-directional probing in a multi-realm network uses a forward tunnel label, an SID, a reverse tunnel label, and an IP packet header. Penultimate hop popping strips the outermost labels in a specific order. The SID gets the probe packet back into the originating realm, and the reverse tunnel label returns the probe packet along the reverse direction of the tunnel. No SID is needed for intra-realm probing, and the IP packet header is used to return the probe packet if the reverse tunnel label is absent.