Fix max hop limit for GBC packets

[Hugo-Domingos]

Fix: MHL hardcoded to 1 for GBC packets (DENM, GeoBroadcast) and comparison of the wrong field on the reception side.

Summary

The Maximum Hop Limit (MHL) field in the GeoNetworking Common Header was hardcoded to 1 for all non-Single Hop Broadcast packets, including GeoBroadcast (GBC) packets such as DENMs.

According to ETSI EN 302 636-4-1 V1.4.1 (2020-01), Section 10.3.4 Table 20, the MHL should be set from the max_hop_limit parameter in the GN-DATA.request primitive.

On the receiver side there was also a verification to check if the RHL is greater than MHL, which was being done on the 'itsGnDefaultHopLimit' instead of on the actual packet data.

Changes

• geonet/common_header.py: Replaced hardcoded mhl = 1 with mhl = request.max_hop_limit for non-SHB packets.
• geonet/service_access_point.py: Added max_hop_limit field to GNDataRequest (default: 10).
• btp/service_access_point.py: Added max_hop_limit field to BTPDataRequest (default: 10), allowing upper layers to configure the hop limit.
• btp/router.py: Passes max_hop_limit from the BTP request down to the GN request.
• geonet/router.py: Fixed the RHL validation check on reception to compare basic_header.rhl > common_header.mhl instead of basic_header.rhl > mib.itsGnDefaultHopLimit, as specified in the standard.

Before (MHL = 1)

After (MHL = 10)

Test Plan

• Verified on remote device that DENM packets are now sent with MHL = 10 and RHL = 10
• Confirmed via packet capture (Wireshark) that the Common Header MHL field is correctly set

[Copilot]

Pull request overview

Fixes GeoNetworking Maximum Hop Limit (MHL) handling for GeoBroadcast (GBC) packets by sourcing MHL from the request (instead of hardcoding 1) and corrects the receive-side hop limit validation to compare against the packet’s own MHL.

Changes:

• Add max_hop_limit to GNDataRequest (default 10) to allow configuring MHL per request.
• Set Common Header mhl from request.max_hop_limit for non-SHB packets.
• Validate received packets using basic_header.rhl > common_header.mhl instead of comparing to MIB default.
• Attempt to propagate max_hop_limit from BTP into GeoNetworking requests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/flexstack/geonet/service_access_point.py	Adds max_hop_limit to GNDataRequest.
src/flexstack/geonet/common_header.py	Uses request.max_hop_limit to initialize CommonHeader.mhl for non-SHB.
src/flexstack/geonet/router.py	Fixes receive-side RHL/MHL validation to use packet MHL.
src/flexstack/btp/router.py	Passes max_hop_limit from BTP request into GN request.

Comments suppressed due to low confidence (1)

src/flexstack/geonet/service_access_point.py:430

• max_hop_limit was added to GNDataRequest, but it is not included in to_dict() / from_dict(). Any request serialized/deserialized (e.g., via JSON) will silently drop the configured hop limit and revert to the default. Add this field to the dict representation and parse it back (with a default for backward compatibility).

    max_hop_limit: int = 10

    def to_dict(self) -> dict:
        """
        Returns a dictionary representation of the GN Data Request.

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