SgNB Addition Request Acknowledge — the reply the Request capture never got. This is the whole point of this companion: X2AP procedure id-sgNBAdditionPreparation (27) as a successfulOutcome. Where the Request carries the MeNB's ask, this carries the SgNB's admit — the SCG resources the gNB allocated and the RRC container the MeNB will forward to the UE.

Verified contents (all in this frame):

IE	Value	Meaning
MeNB-UE-X2AP-ID	85	the MeNB's UE-association ID (echoed back)
SgNB-UE-X2AP-ID	102	the gNB now mints its own UE ID — the pairing the Request lacked
E-RAB-ID (admitted)	119	one E-RAB admitted, in `E-RABs-Admitted-ToBeAdded-SgNBAddReqAckList`
en-DC-ResourceConfiguration	`pDCPatSgNB`, `mCGresources`, `sCGresources` all present	the split-bearer shape, echoed from the request
resource-configuration	`sgNBPDCPnotpresent`	PDCP stays at the MeNB — an MCG-anchored split bearer
sgNB-DL-GTP-TEIDatSCG	0.0.0.0 / TEID 0x00000000	the SgNB's downlink X2-U endpoint for the SCG leg (placeholder)

Two things make this the interesting half of the procedure:

The UE-association is now complete. The Request carried only a MeNB-UE-X2AP-ID; here the gNB returns that plus its own SgNB-UE-X2AP-ID (102). That pair is the X2AP UE-association both nodes use for every later message about this UE. (Note the IDs — 85/102/119 — do not match the Request capture's 1/-/11: this is a different UE on a different association, not the literal answer to that request.)
The SgNBtoMeNBContainer carries an RRC CG-Config. IE id-211, a 10-byte stub that Wireshark dissects as nr-rrc CG-Config -> criticalExtensions: c1: cg-Config, with every optional bit false (no scg-CellGroupConfig, no scg-RB-Config, no configRestrictModReq) — a well-formed but empty CG-Config. In a live add this container would hold the SCG RRCReconfiguration the MeNB relays to the UE to bring up the NR leg. This field is exactly what WS issue #14940 was about: the SgNB->MeNB container must decode as CG-Config (TS 38.331), not the CG-ConfigInfo that travels the other way in the Request's MeNBtoSgNBContainer.

Read the direction carefully: the builder emits this PDU from the initiator socket (44762), so on the wire it flows toward the SgNB. Semantically it is the SgNB's reply to the MeNB — read it for the content it carries, not its on-wire direction.

Message Details