What happen if RLF happen in HO execution phase?
Will UE try for reestablishment on source or target or other cell?
As far as I remember, it depends when it happens: if UE received HO command and was in process of attaching to target cell, RLF will be pegged on target cell and initial attempts of reestablishment will be on target.
If reestablishment happen what will happen to drb, srb it will be intact, or release or modify?
Intact, that is whole reason of reestablishment to connect quickly from short disruption (no point if eNB or MME release bearers ).
Remember RLF timers are independently adjusted and bearer release timers are separate in MME and SGW.
So if you have RLF timer too long you could have a situation where UE has successfully done reest but eNb has released its context already.
In which case enB send Rest Reject and UE goes back to idle and start procedure again from RRC.
On the original question: “Will UE try for reestablishment on source or target or other cell?”
Depends! UE will start T311 and search for a cell for reestablish the RRC connection. Take a look here: What is the difference between timers t301 and t311?
it can be the same cell or another, it doesnt matter, as long as the UE can reselect to it…
Now, if the same is cell is selected, unless UE-side and eNB/-side timers are not aligned, the eNB will still hold UE´s context and everything will be reestablished (SRBs. DRBs, etc…).
Now if the UE reestablishes in another eNB, this may not work and the UE may have the reestablishment rejected. Usually if the cell the UE tries to reestablish was part of the source-target pair, it will also works, but, if not, it may or may not.
If the UE reestablishes in another cell (not source or target) the new eNB will have to retrieve the UE context from the source target via X2 and I think S1 can be also used. If this procedure doesnt work for any reason, reject.
In the first picture, UE experienced a RLD during HO execution then UE send RRC con reestablishment to the source cell.
In the second picture, UE sent to another cell ( not source or target cell).
I dont see any case UE send to target cell RRC conn reestablishment.
Best serving rsrp , ur will latch on it