4G – IMS – take 2
Doing some more reading in the TS 29.061, I ended up in some other dilemmas.
The following information results from this spec so far (as per my understanding):
1. the PGW acts as a “proxy” for the SIP-IMS messages, encapsulating them in GTPv1-U
2. in order for the PGW to locate the P-CSCF, this PGW can have a pre-configured list of P-CSCFs
3. when the UE connects to that APN, the PGW must look through the list of pre-configured Ps, verify which ones are still up (by using ICMP, for example) and send to the UE a list of Ps; if there are multiple Ps in the list, the PGW will use the PCO IE to provide to the UE a prioritized list of Ps
Now, the dilemma comes. As the PGW is a control-plane entity and a user-plane entity in the 4G world, it can send both 4G control-plane messages (to the SGW – that may propagate or not till the MME – GTPv2-C messages) and user-plane messages (which are GTPv1-U messages encapsulating SIP, DHCP, whatever protocol).
TS 29.061 states the following, about how the PGW sends the IP / IPs of the P-CSCF to the UE: – section 13a.2.2 IMS Specific Procedures in the GGSN/P-GW:
The GGSN/P-GW shall then provide only those P-CSCF address(es) that are available in a Create PDP Context Response/Create Bearer Response.
Now, there are 2 issues with this statement:
1. the Create Bearer Response message is actually sent FROM the MME/SGW TO the PGW; the PGW is the one sending the Create Bearer Request message
2. disregarding item 1 and only thinking about the fact that the PGW will send the IP of the P-CSCF as part of the GTPv2-C signaling (rather the proxying it via the GTPv1-U tunnel), TS 29.274 leaves no room for more IEs in the Bearer Context grouped IE:
Table 7.2.3-2: Bearer Context within Create Bearer Request
| Octets 1 | Bearer Context IE Type = 93 (decimal) | |||
| Octets 2 and 3 | Length = n | |||
| Octets 4 | Spare and Instance fields | |||
| Information elements | P | Condition / Comment | IE Type | Ins. |
| EPS Bearer ID | M | This IE shall be set to 0. | EBI | 0 |
| TFT | M | This IE can contain both uplink and downlink packet filters to be sent to the UE. Downlink packet filters are also used by SGW for PMIP based S5/8 interfaces. | Bearer TFT | 0 |
| S1-U SGW F-TEID | C | This IE shall be sent on the S11 interface if the S1-U interface is used. | F-TEID | 0 |
| S5/8-U PGW F-TEID | C | This IE shall be sent on the S4, S5/S8 and S11 interfaces. | F-TEID | 1 |
| S12 SGW F-TEID | C | This IE shall be sent on the S4 interface if the S12 interface is used. | F-TEID | 2 |
| S4-U SGW F-TEID | C | This IE shall be sent on the S4 interface if the S4-U interface is used. | F-TEID | 3 |
| Bearer Level QoS | M | Bearer QoS | 0 | |
| Charging Id | C | This IE shall be sent on the S5/S8 interface. | Charging Id | 0 |
| Bearer Flags | O | Applicable flags are:
- PPC (Prohibit Payload Compression) |
Bearer Flags | 0 |
So, if the 3GPP guys actually claim to configure multiple P-CSCF addresses in the above grouped IE, where are they putting those IP addresses?
where:
IMS – IP Multimedia Subsystem
P-CSCF – Proxy Call Session Control Function
PGW – PDN Gateway
PDN – Packet Data Network
SGW – Serving Gateway
MME – Mobility Management Entity
UE – User Equipment
IE – Information Element
GTP – GPRS Tunneling Protocol
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