WiFi OffLoad : Overall Procedure of Untrusted Data Path Setup

This page will show you he overall procedure of UE getting access to a Service or application via Untruted Non-3GPP network (mostly WiFi). The steps described at step (5), (6) is well defined in 3GPP, but you may see some variations at other steps.. but I think the procedure shown in this page would be common in most cases. It would be helpful to get more meanins out of this if you read WiFi Offload CheckList part before you start reading this page. If you are working on the development or testing on WiFi Offloading or VoWiFi (Voice Over WiFi), the detailed understanding of this procedure greatly help you to troubleshoot various issues you would come accross during the test. I personally checks each and every steps of the procedure described here when I am troubleshooting WiFi Offload/VoWiFi. Also, recommend other engineers to check every steps... but in reality, most of them just say "Something does not work. Just help me fix it" :)

Following is overall sequence diagram of ePDG Discovery and IKE procedure. This is the case where WiFi AP and UE WiFi module is based on IPv4 (It doesn't matter whether UE is using IPv4 or IPv6 for application layer in this diagram). I will post another section later for the case when UE and AP is using IPv6). Actually the only difference is whether they use DHCP or DHCPv6, but there would be something I need to add in detail for IPv6 case).

(1) 802.11

(1.a) and (1.b) belongs to 802.11 protocol. Details of this step is out of the scope of this page. If you want to know the details of this step, refer to step (1)~(7) in WLAN Protocol page.

(2) SSID Selection

Once UE (WiFi device) decode Beacon, it will show all the SSID it detected. Then you can select a specific SSID manually or the device automatically select the SSID in a certain order you have configured.

(3) IP Allocaion for UE WiFi

(3.a) DHCP Discover

Bootstrap Protocol (Discover)

    Message type: Boot Request (1)

    Hardware type: Ethernet (0x01)

    Hardware address length: 6

    Hops: 0

    Transaction ID: 0xff18ecc3

    Seconds elapsed: 1

    Bootp flags: 0x0000 (Unicast)

        0... .... .... .... = Broadcast flag: Unicast

        .000 0000 0000 0000 = Reserved flags: 0x0000

    Client IP address: 0.0.0.0 (0.0.0.0)

    Your (client) IP address: 0.0.0.0 (0.0.0.0)

    Next server IP address: 0.0.0.0 (0.0.0.0)

    Relay agent IP address: 0.0.0.0 (0.0.0.0)

    Client MAC address: aa:bb:cc:dd:ee:ff

    Client hardware address padding: 00000000000000000000

    Server host name not given

    Boot file name not given

    Magic cookie: DHCP

    Option: (53) DHCP Message Type (Discover)

        Length: 1

        DHCP: Discover (1)

    Option: (61) Client identifier

        Length: 7

        Hardware type: Ethernet (0x01)

        Client MAC address: aa:bb:cc:dd:ee:ff

    Option: (57) Maximum DHCP Message Size

        Length: 2

        Maximum DHCP Message Size: 1500

    Option: (60) Vendor class identifier

        Length: 12

        Vendor class identifier: dhcpcd-5.5.6

    Option: (12) Host Name

        Length: 24

        Host Name: android-21b08ec0480ae8b6

    Option: (55) Parameter Request List

        Length: 10

        Parameter Request List Item: (1) Subnet Mask

        Parameter Request List Item: (33) Static Route

        Parameter Request List Item: (3) Router

        Parameter Request List Item: (6) Domain Name Server

        Parameter Request List Item: (15) Domain Name

        Parameter Request List Item: (26) Interface MTU

        Parameter Request List Item: (28) Broadcast Address

        Parameter Request List Item: (51) IP Address Lease Time

        Parameter Request List Item: (58) Renewal Time Value

        Parameter Request List Item: (59) Rebinding Time Value

    Option: (255) End

        Option End: 255

(3.b) DHCP Offer

Bootstrap Protocol (Offer)

    Message type: Boot Reply (2)

    Hardware type: Ethernet (0x01)

    Hardware address length: 6

    Hops: 0

    Transaction ID: 0xff18ecc3

    Seconds elapsed: 1

    Bootp flags: 0x0000 (Unicast)

        0... .... .... .... = Broadcast flag: Unicast

        .000 0000 0000 0000 = Reserved flags: 0x0000

    Client IP address: 0.0.0.0 (0.0.0.0)

    Your (client) IP address: 192.168.0.1 (192.168.0.1)

    Next server IP address: 192.168.0.254 (192.168.0.254)

    Relay agent IP address: 0.0.0.0 (0.0.0.0)

    Client MAC address: aa:bb:cc:dd:ee:ff

    Client hardware address padding: 00000000000000000000

    Server host name not given

    Boot file name not given

    Magic cookie: DHCP

    Option: (53) DHCP Message Type (Offer)

        Length: 1

        DHCP: Offer (2)

    Option: (54) DHCP Server Identifier

        Length: 4

        DHCP Server Identifier: 192.168.0.254 (192.168.0.254)

    Option: (51) IP Address Lease Time

        Length: 4

        IP Address Lease Time: (3600s) 1 hour

    Option: (58) Renewal Time Value

        Length: 4

        Renewal Time Value: (1800s) 30 minutes

    Option: (59) Rebinding Time Value

        Length: 4

        Rebinding Time Value: (3200s) 53 minutes, 20 seconds

    Option: (1) Subnet Mask

        Length: 4

        Subnet Mask: 255.255.255.0 (255.255.255.0)

    Option: (28) Broadcast Address

        Length: 4

        Broadcast Address: 192.168.0.255 (192.168.0.255)

    Option: (6) Domain Name Server

        Length: 4

        Domain Name Server: 192.168.0.254 (192.168.0.254)

    Option: (255) End

        Option End: 255

    Padding

(3.c) DHCP Request

Bootstrap Protocol (Request)

    Message type: Boot Request (1)

    Hardware type: Ethernet (0x01)

    Hardware address length: 6

    Hops: 0

    Transaction ID: 0xff18ecc3

    Seconds elapsed: 1

    Bootp flags: 0x0000 (Unicast)

        0... .... .... .... = Broadcast flag: Unicast

        .000 0000 0000 0000 = Reserved flags: 0x0000

    Client IP address: 0.0.0.0 (0.0.0.0)

    Your (client) IP address: 0.0.0.0 (0.0.0.0)

    Next server IP address: 0.0.0.0 (0.0.0.0)

    Relay agent IP address: 0.0.0.0 (0.0.0.0)

    Client MAC address: aa:bb:cc:dd:ee:ff

    Client hardware address padding: 00000000000000000000

    Server host name not given

    Boot file name not given

    Magic cookie: DHCP

    Option: (53) DHCP Message Type (Request)

        Length: 1

        DHCP: Request (3)

    Option: (61) Client identifier

        Length: 7

        Hardware type: Ethernet (0x01)

        Client MAC address: aa:bb:cc:dd:ee:ff

    Option: (50) Requested IP Address

        Length: 4

        Requested IP Address: 192.168.0.1 (192.168.0.1)

    Option: (54) DHCP Server Identifier

        Length: 4

        DHCP Server Identifier: 192.168.0.254 (192.168.0.254)

    Option: (57) Maximum DHCP Message Size

        Length: 2

        Maximum DHCP Message Size: 1500

    Option: (60) Vendor class identifier

        Length: 12

        Vendor class identifier: dhcpcd-5.5.6

    Option: (12) Host Name

        Length: 24

        Host Name: android-21b08ec0480ae8b6

    Option: (55) Parameter Request List

        Length: 10

        Parameter Request List Item: (1) Subnet Mask

        Parameter Request List Item: (33) Static Route

        Parameter Request List Item: (3) Router

        Parameter Request List Item: (6) Domain Name Server

        Parameter Request List Item: (15) Domain Name

        Parameter Request List Item: (26) Interface MTU

        Parameter Request List Item: (28) Broadcast Address

        Parameter Request List Item: (51) IP Address Lease Time

        Parameter Request List Item: (58) Renewal Time Value

        Parameter Request List Item: (59) Rebinding Time Value

    Option: (255) End

        Option End: 255

(3.d) DHCP Ack

Bootstrap Protocol (ACK)

    Message type: Boot Reply (2)

    Hardware type: Ethernet (0x01)

    Hardware address length: 6

    Hops: 0

    Transaction ID: 0xff18ecc3

    Seconds elapsed: 1

    Bootp flags: 0x0000 (Unicast)

        0... .... .... .... = Broadcast flag: Unicast

        .000 0000 0000 0000 = Reserved flags: 0x0000

    Client IP address: 0.0.0.0 (0.0.0.0)

    Your (client) IP address: 192.168.0.1 (192.168.0.1)

    Next server IP address: 192.168.0.254 (192.168.0.254)

    Relay agent IP address: 0.0.0.0 (0.0.0.0)

    Client MAC address: aa:bb:cc:dd:ee:ff

    Client hardware address padding: 00000000000000000000

    Server host name not given

    Boot file name not given

    Magic cookie: DHCP

    Option: (53) DHCP Message Type (ACK)

        Length: 1

        DHCP: ACK (5)

    Option: (54) DHCP Server Identifier

        Length: 4

        DHCP Server Identifier: 192.168.0.254 (192.168.0.254)

    Option: (51) IP Address Lease Time

        Length: 4

        IP Address Lease Time: (3600s) 1 hour

    Option: (58) Renewal Time Value

        Length: 4

        Renewal Time Value: (1800s) 30 minutes

    Option: (59) Rebinding Time Value

        Length: 4

        Rebinding Time Value: (3200s) 53 minutes, 20 seconds

    Option: (1) Subnet Mask

        Length: 4

        Subnet Mask: 255.255.255.0 (255.255.255.0)

    Option: (28) Broadcast Address

        Length: 4

        Broadcast Address: 192.168.0.255 (192.168.0.255)

    Option: (6) Domain Name Server

        Length: 4

        Domain Name Server: 192.168.0.254 (192.168.0.254)

    Option: (255) End

        Option End: 255

    Padding

 

(4) ePDG Discovery

Refer to ePDG Discovery Page for the details.

(5) IKEv2

This is very long/complicated process and described in a separate page. Refer to Overall Sequence Flow - 3GPP 33.402 of IKE page.

(6) CSCF Discovery

Actually this is not an independen procedure. This is a part of IKEv2 procedure and described in detail in CSCF Descovery page.