SMF support for multiple gnbs

weit · September 19, 2023, 11:04am

Hi,
I am trying to configure my SMF instance to support multiple gnbs (e.g. gNB1, gNB2) in its topology, either from default one (i.e. links:) in smfcfg.yaml or the UE specific one (i.e. topology:) under uerouting.yaml.

However, I can not find a way to do so.

From looking into the sources it seems that SMF selects the first gNB it encounters. Please find source links below.

If that is correct, I would like to ask whether there are plans to add a support for SMF to select UPFs path that starts from the gNB the UE is connected to i.e. the gNB, the PDU session request came from ?

github.com

free5gc/smf/blob/84c979a383434e5cdf44d884f11e3f66df836e49/internal/context/ue_defaultPath.go#L55


func findSourceInTopology(upi *UserPlaneInformation, topology []factory.UPLink) (string, error) {
	sourceList := make([]string, 0)
	for key, node := range upi.AccessNetwork {
		if node.Type == UPNODE_AN {
			sourceList = append(sourceList, key)
		}
	}
	for _, anName := range sourceList {
		for _, link := range topology {
			if link.A == anName || link.B == anName {
				// if multiple gNBs exist, select one according to some criterion
				logger.InitLog.Debugf("%s is AN", anName)
				return anName, nil
			}
		}
	}
	return "", errors.New("Not found AN node in topology")
}
func extractAnchorUPFForULCL(upi *UserPlaneInformation, source string, topology []factory.UPLink) ([]string, error) {
	upList := make([]string, 0)

github.com

free5gc/smf/blob/84c979a383434e5cdf44d884f11e3f66df836e49/internal/context/user_plane_information.go#L527


		for _, node := range upfList {
			if name == upi.GetUPFNameByIp(node.NodeID.ResolveNodeIdToIp().String()) {
				sortedUpList = append(sortedUpList, node)
			}
		}
	}
	return sortedUpList
}
func (upi *UserPlaneInformation) selectUPPathSource() (*UPNode, error) {
	// if multiple gNBs exist, select one according to some criterion
	for _, node := range upi.AccessNetwork {
		if node.Type == UPNODE_AN {
			return node, nil
		}
	}
	return nil, errors.New("AN Node not found")
}
func (upi *UserPlaneInformation) SelectUPFAndAllocUEIP(selection *UPFSelectionParams) (*UPNode, net.IP) {
	source, err := upi.selectUPPathSource()

Thanks,
Avi

meow0122 · September 19, 2023, 11:04am

Hi @weit,
Please refer to SMF Config · free5gc/free5gc Wiki · GitHub. You can try to add gNB2 in SMF configuration.

weit · September 19, 2023, 11:04am

Hi @meow0122, thanks for the reply.

I looked into the link you sent. I added gNB2 to both up_nodes and topology (e.g. links:) but SMF does not seem to distinguish between UEs connected to gNB1 and UEs connected to gNB2. I am using UERANSIM to simulate a gNB node.
SMF seems to select the first gNB it sees in the topology and it uses that to calculate the path for the PDU session establishment. I looked into the sources and I think I found the reason (I pasted source reference in my first post).

I would like to ask: how does SMF select the correct gNB (gNB1 or gNB2) in the topology?

Thanks again,
- Avi

free5GC · September 19, 2023, 11:04am