Converged Datapath

Feature Summary and Revision History

Summary Data

Table 1. Summary Data

Applicable Product(s) or Functional Area

5G-UPF

Applicable Platform(s)

VPC-SI

Feature Default Setting

Disabled – Configuration Required

Related Changes in this Release

Not Applicable

Related Documentation

UCC 5G UPF Configuration and Administration Guide

Revision History

Table 2. Revision History
Revision Details Release

First introduced.

2021.01.0

Feature Description

The Converged Datapath feature allows interconnection of the same UE’s session at UPF instance with SGW-C/cnSGW and Session Management Function (SMF)/Inter-Working Function (IWF) to build converged/collapsed datapath and achieve higher throughput. With this feature:

  • The UP/UPF selection logic is enhanced to aide same node selection on SGW-C/cnSGW and SMF.

  • The SxDemux selects the same Session Manager (SessMgr) instance based on existing session of N4 or Sxa respectively.

  • The Sxa session and N4 session correlation is done at SessMgr.

  • The datapath is allowed to be collapsed in the forwarding plane.

  • Extra hop in subscriber’s datapath is eliminated, resulting in reduced latency and improved user experience.

Architecture

As part of this feature, there are two sessions on the same UPF instance established by SGW-C/cnSGW and SMF. Once they are established, the software logic determines the peer session so that the converged/collapsed datapath for packet processing is possible at the UPF node.

How it Works

This section describes how the feature works.

SxDemuxMgr

In distributed architecture of UP/UPF, sessions (Sxa or N4) run on different SessMgr instances. To support collapsing/converging the sessions to a single SessMgr, the SessMgr instance is selected by both sessions during establishment.

At SxDemux, when Sx Establishment Request (Sxa or N4) is received for selecting the SessMgr instance, it’s parsed for finding the SessMgr instance from remote F-TEID, where corresponding sessions (N4 or Sxa respectively) are established. The F-TEID, that contains the Tunnel Identifier embedded with SessMgr instance, is extracted.

SessMgr

There are two sessions, Sxa and N4, that exist on the same SessMgr instance. To converge them, the following logic is used to identify the session:

  • For Uplink Packet: Egress FARs F-TEID matches with Ingress PDRs F-TEID.

  • For Downlink Packet: Ingress FAR’s F-TEID should match with Egress PDR’s F-TEID.

The F-TEID includes both Tunnel Identifier and the endpoint IP address. After the session is identified, the required information is used in datapath to build the converged datapath.

Datapath

After convergence of session occurs at SessMgr, the SessMgr removes the existing Bearer stream (3 tuple) from Fast Path that is installed for Sxa session. It’s established only when flow-level stream (6 tuple), based on received packet, is analyzed.

The uplink packet is received by S-GW ingress PDR endpoint. The downlink packet is forwarded using S-GW Ingress FAR-based outer header.

Charging

Charging of SMF leg (N4 leg) is supported.


Note
S-GW charging is not supported.

Call Flows

This section describes the call flows associated with Converged Datapath feature.

Initial Attach with SGW-C/cnSGW and SMF/IWF

The following illustration describes the initial attach call flow with collapsed UPF.

Table 3. Initial Attach with SGW-C/cnSGW and SMF/IWF Call Flow Description
Step Description
1 Create Session Request (CSReq) is received by SGW-C/cnSGW and it selects the UPF.
2

The SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. The UPF:

  • Allocates Sxa session

  • Allocates S-GW Ingress and Egress local F-TEID
3 The UPF sends Sx Establishment Response (Sxa) back to SGW-C/cnSGW.
4

The SGW-C/cnSGW sends CSReq to SMF/IWF.

The SMF/IWF selects the same UPF that is selected by SGW-C/cnSGW.

5 The SMF/IWF sends Sx Establishment Request (N4) to the UPF.
6 The UPF sends Sx Establishment Response (N4) to the SMF/IWF.
7 The SMF/IWF sends Create Session Response to the SGW-C/cnSGW.
8

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF.

The UPF updates P-GW data F-TEID as part of Egress FAR.

The UPF also interconnects Sxa and N4 session using internal logic and removes already-created Bearer Stream (3 tuple).

9 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
10 The SGW-C/cnSGW sends Create Session Response to the MME.
11 The MME sends Modify Bearer Request to the SGW-C/cnSGW.
12

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF.

The UPF updates eNodeB F-TEID as part of Ingress FAR.
13 The UPF sends Sx Modify Response to the SGW-C/cnSGW.
14 The SGW-C/cnSGW sends Modify Bearer Response to the MME.

5G to 4G Handover with Collapsed UPF

The following illustration describes the 5G to 4G handover call flow with collapsed UPF.

Table 4. 5G to 4G Handover with Collapsed UPF Call Flow Description
Step Description
1

As part of UE initial attach, N4 session is already established with SMF and UPF.

The MME sends Create Session Request (CSReq) to SGW-C/cnSGW. If it’s a handover request, the SGW-C/cnSGW selects the same UPF that is selected by the SMF.

2

The SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. At UPF:

  • SxDemux selects the same SessMgr instance extracted from the P-GW F-TEID that is received in FAR. Both Sxa and N4 session are on the same SessMgr.

  • Allocates Sxa session

  • Allocates S-GW Ingress and Egress local F-TEID

  • Interconnects Sxa and N4 session using internal logic and doesn’t install Bearer Stream (3 tuple)

3 The UPF sends Sx Establishment Response (Sxa) back to SGW-C/cnSGW.
4 The SGW-C/cnSGW sends Create Session Response to the MME.
5 The MME sends Modify Bearer Request to the SGW-C/cnSGW.
6

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF.

The UPF updates eNodeB F-TEID for downlink data.
7 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
8 The SGW-C/cnSGW sends Modify Bearer Request to the SMF/IWF.
9

The SMF/IWF sends Sx Modify Bearer Request (N4) to the UPF. The UPF:

  • Updates N4 session FAR towards S-GW with F-TEID

  • Updates TEP entries at VPP with new F-TEID
10 The UPF sends Sx Modify Response (N4) to the SMF/IWF.
11 The SMF/IWF sends Sx Modify Response to the SGW-C/cnSGW.
12 The SGW-C/cnSGW sends Modify Bearer Response to the MME.

Intra S-GW Handover with Collapsed UPF

The following illustration describes the intra-SGW handover call flow with collapsed UPF.

Table 5. Intra S-GW Handover with Collapsed UPF Call Flow Description
Step Description
1

As part of UE initial attach, N4 and Sxa session is already established with SMF and UPF. At UPF, N4+Sxa session exists and are interconnected.

The MME sends Modify Bearer Request to the SGW-C/cnSGW.

The SGW-C/cnSGW updates eNodeB F-TEID in FAR.
2

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF:

  • Updates eNodeB F-TEID for downlink data

  • Updates TEP entries at VPP with new Remove TEID
3 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
4 The SGW-C/cnSGW sends Modify Bearer Response to the MME.

Idle/Active DDN Handling with Collapsed UPF

The following illustration describes the Idle/Active DDN handling with collapsed UPF.

Table 6. Idle/Active DDN Handling with Collapsed UPF Call Flow Description
Step Description
1

As part of UE initial attach, N4 and Sxa session is already established with SMF and UPF. At UPF, N4+Sxa session exists and are interconnected. At MME, the UE goes from Active to Ide state.

The MME sends Release Access Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW informs UPF to buffer packets.

2 The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF updates FAR action towards eNodeB to buffer state. 6 tuple flows are onloaded for buffering.
3 The UPF sends Sx Modify Response (Sxa) back to SGW-C/cnSGW.
4 The MME sends Release Access Bearer Response to the SGW-C/cnSGW.
5 The ISP/Gi sends the received downlink packet to the UPF. The packet received by UPF at N4 session is passed to interconnect Sxa session for buffering.
6 The UPF sends Usage Report Request (Sxa) to the SGW-C/cnSGW.
7 The SGW-C/cnSGW sends Usage Report Response (Sxa) to the UPF.
8 The SGW-C/cnSGW sends DDN notification to the MME.
9 The MME sends DDN Acknowledgment back to the SGW-C/cnSGW.
10

At MME, the UE moves from Idle to Active. The MME sends Modify Bearer Request to the SGW-C/cnSGW. The SGW-C/cnSGW:

  • Updates eNodeB F-TEID in FAR

  • Updates FAR action to Forward
11

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. The UPF:

  • Updates eNodeB F-TEID for downlink data

  • Updates TEP entries at VPP with new Remove TEID

  • Releases buffered packets by finding respective 6 tuple streams

12 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
13 The SGW-C/cnSGW sends Modify Bearer Response to the MME.

IDFT Handling during S1 Handover

The following illustration describes the IDFT handling during S1 handover with collapsed UPF.

Table 7. IDFT Handling during S1 Handover Call Flow Description
Step Description
1

As part of initial attach, N4 and Sxa session is already established with SMF and UPF. S1 handoff is triggered.

The MME sends Indirect Tunnel Request Message to the SGW-C/cnSGW.

2

The SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF. At UPF:

  • IDFT PDR is detected at SessMgr

  • New F-TEID is allocated for IDFT tunnel

  • Converged datapath is not required and traffic goes through Slowpath

3 The UPF sends Sx Modify Response (Sxa) to the SGW-C/cnSGW.
4 The SGW-C/cnSGW sends Create Indirect Tunnel Response Message to the MME.
5 The Source eNodeB sends IDFT data packet to the UPF. If there’s no matching 3 tuple stream at UPF, the packet is forwarded to SessMgr.
6 The UPF sends IDFT data packet to the destination eNodeB.

S-GW Relocation with Same SGW-U

The following illustration describes the S-GW relocation with destination S-GW selecting the same UPF.

Table 8. S-GW Relocation with Same SGW-U Call Flow Description
Step Description
1

As part of initial attach, N4 and Sxa session is already established with Source SGW-C/cnSGW, SMF/IWF, and UPF.

The MME sends Create Session Request to the destination SGW-C/cnSGW.

2 The destination SGW-C/cnSGW sends Sx Establishment Request (Sxa) to the UPF. The UPF links new Sxa session with the N4 session for uplink packets (Slowpath).
3 The UPF sends Sx Establishment Response (Sxa) to the destination SGW-C/cnSGW.
4 The destination SGW-C/cnSGW sends Create Session Response to the MME.
5 The MME sends Modify Bearer Request (eNodeB F-TEID) to the destination SGW-C/cnSGW.
6 The destination SGW-C/cnSGW sends Sx Modify Request (Sxa) to the UPF.
7 The UPF sends Sx Modify Response (Sxa) to the destination SGW-C/cnSGW.
8 The destination SGW-C/cnSGW sends Modify Bearer Request to the SMF/IWF.
9 The SMF/IWF sends Sx Modify Request (N4) to the UPF. The UPF switches downlink tunnel and links the N4 session with the new Sxa session.
10 The UPF sends Sx Modify Response (N4) to the SMF/IWF.
11 The SMF/IWF sends Modify Bearer Response to the destination SGW-C/cnSGW.
12 The destination SGW-C/cnSGW sends Modify Bearer Response to the MME.
13 The MME sends Delete Session Request to the source SGW-C/cnSGW.
14 The source SGW-C/cnSGW sends Sx Delete Session Request to the UPF.
15 The UPF sends Sx Delete Session Response to the source SGW-C/cnSGW.
16 The source SGW-C/cnSGW sends Delete Session Request to the MME.

Limitations

The following are the known limitations of the feature:

  • If Sxa leg is of one user-plane-service and N4 leg is of another user-plane-service, then datapath won't be collapsed.

  • If Sxa leg is under one context and N4 leg is in another context, then datapath can't be collapsed.

  • The S-GW part of the show subscribers user-plane-only full all CLI output doesn't display ToS-marked Uplink and Downlink packets.

  • For the S-GW part of the show user-plane-service statistics rat all CLI output, the session statistics for Unknown is incremented, however, the data statistics aren't incremented under RAT-type Unknown.

  • Lawful Intercept for S-GW isn't supported.

  • S-GW charging isn't supported.

  • S-GW bearer inactivity timeout isn't honored, as it's determined by S-GW URR for which processing isn't done for collapsed datapath.

  • If S-GW leg of call is locally purged for converged session, then the UPF continues to send data toward eNodeB.

Monitoring and Troubleshooting

Show Commands and/or Outputs

This section provides information about the show CLI commands and/or outputs available in support of the Converged Datapath feature.

show subscribers user-plane-only full all

The output of this CLI command is enhanced to display the following fields:

  • Converged Session

  • Converged Peer Callid

show user-plane-service statistics all

The output of this CLI command is enhanced to display the following fields:

  • Converged Data Session PDNs:

    • Active

    • Setup

    • Released