Skip to content

N3IWUE Design Document

Warning

This document doesn't include all design details about N3IWUE.

Info

GRE Tunnels

Note

Author: Andy Chen (CTFang)
Date: 2024/06/12

GRE Tunnel & Spec

Here is part of TS24.502 Access 5GCN via non-3GPP access networks spec:

8.3.2 Generic routing encapsulation (GRE)

If a user data packet message is transmitted over non-3GPP access between the UE and the N3IWF for untrusted non3GPP access, the user data packet message shall be encapsulated as a GRE user data packet.

In the GRE encapsulated user data packet:

​ b) the QFI field of the key field of the GRE header field is set to the QFI associated with the user data packet;

The GRE key field must use the QFI value as its key.

N3IWUE uses the Netlink Go library to create GRE Tunnel. Here is a segment of code for the new GRE Interface. 

greKeyField = (uint32(qfi) & 0x3F) << 24
newGreIfaceName := greIfaceName + "-" + strconv.Itoa(int(qfi))

// New GRE tunnel interface
newGRETunnel := &netlink.Gretun{
  LinkAttrs: netlink.LinkAttrs{
    Name: newGreIfaceName,
    MTU:  1438, // remain for endpoint IP header(most 40 bytes if IPv6) and ESP header (22 bytes)
  },
  Link:   uint32(parent.Attrs().Index), // PHYS_DEV in iproute2; IFLA_GRE_LINK in linux kernel
  Local:  ueTunnelAddr,
  Remote: n3iwfTunnelAddr,
  IKey:   greKeyField,
  OKey:   greKeyField,
}

And the default rule always has QFI = 1.

Procedure

During the N3IWUE PDU session Establishment procedure, when UE receives PDUSessionEstablishmentAccept, N3IWUE will try to do the following things: (refer to n3iwue/internal/nwucp/handler/handler.go > HandleDLNASTransport())

  • Get PDU Address from NAS
  • Setup GRE Tunnels
    • N3IWUE create per tunnel for each QFI
  • Add Route for GRE Tunnels
    • N3IWUE adds routes for each tunnel
    • Remote IPv4 address for each QFI retrieve from NAS message

Example Scenario

  • Two QoS rules
    • QFI = 2, address = 1.1.1.1
    • QFI = 3, address = 8.8.8.8

So, the N3IWUE would create 3 GRE tunnels and 3 routes.

result

After the ping test, use ifconfig to see the status for each GRE interfaces:

interfaces

  • ens18 is the only one interface on that computer
  • gretun-id-2-1 for QFI = 1, receive 5 ping reply from 9.9.9.9 and send 5 ping request with some others traffics.
  • gretun-id-2-2 for QFI = 2, send 5 ping request to 8.8.8.8 and receive 5 ping reply.
  • gretun-id-2-3 for QFI = 3, send 5 ping request to 1.1.1.1 and receive 5 ping reply.

fix: Add multiple GRE tunnel when have multiple QFI settings #7