Delete Session Request ? Delete Bearer Request ? Delete Bearer Command

Hope you all are doing well...:-) I am feeling good to write something here after a long time. 

Have you ever faced any confusion when dealing with LTE Core Network Protocols? I think it will yes for almost 99% of the people, because confusion is part of our life when we are not very sure or confident about something. Similarly, When I started working on LTE EPC I was very confused about deleting a session and bearer in LTE. Now I got some clarity, which I want to share with you below about three GTPv2 control plane messages which are used in between MME and SGW/PGW very extensively to delete a session or bearer. 

1.       Delete Session Request

2.       Delete Bearer Request

3.       Delete Bearer Command

It will be a night mare for a newbie to know what these 3 messages are doing to delete a session/ bearer from the network. One type of delete message should be enough right? So when I have started working on EPC side, I am always confused about delete messages? To avoid any further confusion, I thought of clearing it by going through the 3GPP specs and sharing the same with you to avoid further confusion and I can save some precious time of the world. 

1.       Delete Session Request

      Direction of the Msg: MME/S4-SGSNà  SGW-Interface S11/S4

                                      SGW à  PGW-Interface S11/S8

                                      TWAN/ePDG à  PGW-Interface S2a/S2b

What Spec Says:

A Delete Session Request message shall be sent on the S11 interface by the MME to the SGW and on the S5/S8

interface by the SGW to the PGW as part of the procedures.

- EUTRAN Initial Attach

- UE, HSS or MME Initiated Detach

- UE or MME Requested PDN Disconnection

Again refer 7.2.9.1 29.274 you will get more no of procedures.

Linked EPS Bearer ID (LBI) This IE shall be included on the S4/S11, S5/S8 and S2a/S2b interfaces to indicate the default bearer associated with the PDN being disconnected unless in the handover/TAU/RAU with SGW relocation procedures.

2.       Delete Bearer Request

      Direction of the Msg: PGW à  SGW

                                           SGW à  MME/S4-SGSN

                                           PGWà  TWAN/ePDG

What Spec Says:

A Delete Bearer Request message shall be sent on the S5/S8 and S4/S11 interfaces as part of the following procedures:

- PGW or MME initiated bearer deactivation procedures,

- UE requested Bearer Resource Modification,

- MS and SGSN Initiated Bearer Deactivation procedure using S4 or

- PGW initiated bearer deactivation procedure using S4.

Again refer 7.2.9.2 29.274 you will get more details.

But question is still remain same when delete session request and delete bearer request and delete bearer command triggered by MME and GW ???

What I understood from spec and from network logs that delete session is for deleting the default bearer when MME is initiating the deletion and it is dependent on LBI.

But Delete Bearer Request dependent on both LBI and EBI. When GW wants to delete a single default bearer or default bearer with all the other bearers from GW side, it will do it based on LBI and if GW wants to delete only a single or multiple bearer it will do it based on EBI.(Using EBI here we cant delete the single default bearer) 

LBI If the request corresponds to the bearer deactivation procedure in case all bearers belonging to a PDN connection shall be released, then this IE shall be included on the S5/S8, S4/S11 and S2a/S2b interfaces to indicate the default bearer associated with the PDN being disconnected. This IE shall be included only when the EPS Bearer ID is not present in the message.

EBI This IE shall be used on S5/S8, S4/S11 and S2a/S2b interfaces for bearers different from the default one, i.e., for dedicated bearers. In this case at least one dedicated bearer shall be included. Several IEs with this type and instance values shall be included as necessary to represent a list of Bearers.

3.       Delete Bearer Command

      Direction of the Msg: MME/SGSN à  SGW, 

                                                                 SGW à PGW

What Spec Says:

A Delete Bearer Command message shall be sent on the S11 interface by the MME to the SGW and on the S5/S8

interface by the SGW to the PGW as a part of the eNodeB requested bearer release or MME-Initiated Dedicated Bearer Deactivation procedure.

It is dependent on a Bearer Context IE for any deletion. This IE carries all the EBI.

Bearer Contexts This IE shall be used to indicate dedicated bearers. When used, at least one dedicated bearer shall be present. Several IEs with this type and instance values shall be included as necessary to represent a list of Bearers

This deletion msg also initiated by MME, but here it is done for deleting the additional bearers. I am reminding you again here that delete session request is used for default bearer deletion.

Let’s discuss one small example to see how deletion of bearers really done in real core network.

Suppose you are creating a call using a single APN called “MYAPN”. Here we will create one default bearer with EBI 5 and two dedicated bearer with EBI 6& EBI 7 with same “MYAPN”.

a.        Suppose you want to delete the EBI 7 from MME side, then MME will trigger Delete Bearer command with bearer context EBI 7.

b.       Now you want to delete the EBI 6 from GW side, then GW will trigger Delete Bearer request with EBI 6.

c.        Finally you want to delete the default bearer.

·                        From MME side Delete session request will trigger with LBI 5.

·                        From GW side Delete bearer request will trigger with LBI 5.

Hope it will bring some clarity on deleting session request, delete bearer request and delete bearer command msgs to some extent. I know in real work environment you have to deal with different combination when you are designing/testing/ debugging the system. Still I hope above explanation will help you to provide some clarity.

If you are having any doubt then please write to me in below comment section.

Happy Reading!!!!!