fix: AMF-UE-NGAP-ID shall be unique within an AMF Set as specified in TS 23.501

[louisroyer]

According to TS 38.401:

AMF UE NGAP ID
An AMF UE NGAP ID shall be allocated so as to uniquely identify the UE over the NG interface within the AMF. When a NG-RAN node receives an AMF UE NGAP ID it shall store it for the duration of the UE-associated logical NG-connection for this UE.
Once known to a NG-RAN node this ID is included in all UE associated NGAP signalling.

According to TS 23 501, section 5.9.9 (AMF UE NGAP ID):

An AMF UE NGAP ID is an identifier used to identify the UE in AMF on N2 reference point.
AMF allocates the AMF UE NGAP ID and send it to the 5G-AN.
For the following N2 signalling interaction sent from 5G-AN to AMF, AMF UE NGAP ID is used to identify the UE at the AMF.
AMF UE NGAP ID is unique per AMF set.
AMF UE NGAP ID may be updated without AMF change, or with AMF change as specified at clause 5.21.2.2

One of the main purpose of this ID is to ensure a proper coordination of UE-specific procedures (e.g., session setup, handovers, UE context release) between the AMF and RAN.

While implementing N2-based handover in UERANSIM, @Sid-Romero discovered the AMF was incrementing the AMF UE NGAP ID on reception of the "Handover Required" message despite this ID being theoretically unique for a given UE.

This pull request is intended to fix this interoperability issue.

Important: when merging this PR, the test TestN2Handover also needs to be updated in https://github.com/free5gc/free5gc repository by removing the following line: https://github.com/free5gc/free5gc/blob/228ac5f7b8b0c3d160bccb3470901b3e87e756c9/test/registration_test.go#L1993

[Copilot]

Pull Request Overview

This PR fixes an interoperability issue where AMF UE NGAP IDs were being regenerated during handover procedures instead of maintaining the same ID as required by 3GPP specifications. The fix ensures that the AMF UE NGAP ID remains unique and consistent within an AMF Set during N2-based handovers.

• Creates new target RAN UE entities using the existing AMF UE NGAP ID from the source UE
• Introduces specialized lookup functions to distinguish between source and target UE contexts during handover
• Updates handover and path switch procedures to use appropriate UE lookup methods

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
internal/ngap/message/send.go	Modified handover request to reuse existing AMF UE NGAP ID
internal/ngap/handler.go	Updated UE lookup calls to use source/target-specific methods
internal/context/context.go	Added separate lookup functions for source and target UE contexts
internal/context/amf_ran.go	Added new method to create RAN UE with predefined AMF UE NGAP ID

Comments suppressed due to low confidence (2)

internal/context/context.go:491

• [nitpick] The function name 'RanUeFindTargetByAmfUeNgapID' is ambiguous about what constitutes a 'target' UE. Consider a more descriptive name like 'RanUeFindByAmfUeNgapIDPreferTarget' to clarify that it returns the target UE if available, otherwise the base UE.

func (context *AMFContext) RanUeFindTargetByAmfUeNgapID(amfUeNgapID int64) *RanUe {

internal/context/context.go:480

• [nitpick] The function name 'RanUeFindSourceByAmfUeNgapID' is ambiguous about what constitutes a 'source' UE. Consider a more descriptive name like 'RanUeFindByAmfUeNgapIDPreferSource' to clarify that it returns the source UE if available, otherwise the base UE.

func (context *AMFContext) RanUeFindSourceByAmfUeNgapID(amfUeNgapID int64) *RanUe {

[Alonza0314]

Hi, @louisroyer

I've run the N2Handover test, it does not pass neither with removing targetUe.AmfUeNgapId = 1 nor keep the same code.

It would get a lot errors at AMF's log.

[louisroyer]

I've run the N2Handover test, it does not pass neither with removing targetUe.AmfUeNgapId = 1 nor keep the same code.

Hi @Alonza0314
Before I check again, is this a typo in your message or did you actually remove another line than the one I mentioned? because the related line is targetUe.AmfUeNgapId = 2, not targetUe.AmfUeNgapId = 1…

[Alonza0314]

Oh, sorry. It's typo.
I mean targetUe.AmfUeNgapId = 2.

[louisroyer]

uh, that's probably why I set this as draft 2 month ago… and I forgot to run this test last week, and made only a test with UERANSIM using the handover pull request, which worked fine.
I will look at it when I have more time. If by the meantime you have any idea on how to fix it, please notice me.