SGSN Selection procedure

As I said the last time, all points of the list below will be covered.
Until now I've covered the MME Selection, PGW and SGW Selection - both can be found through links.
So Now I will start with SGSN Selection (5th point on the list below), and later today will try also to publish the idea of HSS Selection.

1. As part of the UE Attach the eNB will query the iDNS Server using the TAI for the address of the MME it should provide to that UE.
2. During UE attach the MME query the iDNS Server to select the PDN-GW (Packet Data Network Gateway, PGW) where a requested (subscribed) PDN connectivity (APN) is located. Selection can be based on the information provided to the MME, when the UE attaches to the network.
3. Following the PGW selection, the MME query the DNS Server to select an available SGW to serve the UE using the TAC, which in most cases is based on network topology and the location of the UE within the network, so that the best SGW is selected.
4. SGSN will query the DNS Server to resolve the Old MME using LAC, and RAC (taken from Old GUTI received from UE) during LTE to 3G/2G Handover.
5. MME will query the DNS Server to resolve the Old SGSN using NRI, LAC, and RAC (taken from P-TMSI received from UE) during 3G/2G to LTE Handover.
6. During attach the MME is configured to support the HSS Peer Service and Interface Associations towards the Diameter proxy/edge agent. These are static configurations in the MME towards the Diameter Proxy and no DNS query is initiated by the MME to select the Diameter Proxy. 

The SGSN Selection is required when the UE handovers from 3G to 4G. There are procedures where the source SGSN must be contacted by the target MME.

During a mobility procedure towards a new core network node (MME), a UE served by a SGSN has a previously assigned P-TMSI by the source SGSN. The target MME extracts the source's NRI, RAC, LAC, MNC and MCC from the P-TMSI.

The S-NAPTR procedure for finding the old SGSN services and interfaces from the P-TMSI  is started with "Service Parameters" of "x-3gpp-sgsn:x-gn", "x-3gpp-sgsn:x-gp", as defined in  3GPP TS 23.003  and setting the Application-Unique String to the NRI-RAI FQDN based on NRI, RAC, LAC, MNC and MCC.
The FQDN based on NRI, RAC, LAC, MNC and MCC known as the NRI-RAI FQDN.

Gn/Gp interfaces are provisioned redundantly for both ".gprs" and ".3gppnetwork.org" top level domains in the DNS.
Below the example of a part of DNS configuration file for the SGSN section:

nri-sgsn<SGSN>.rac<RAC>.lac<LAC>.rac.epc.mnc<MNC>.mcc<MCC>.3gppnetwork.org. IN NAPTR 1  1  "a"   "x-3gpp-sgsn:x-gn" ""  nri<NRI>.rac<RAC>.lac<LAC>.mnc<MNC>.mcc<MCC>.gprs.

nri<NRI>.rac<RAC>.lac<LAC>.mnc<MNC>.mcc<MCC>.gprs. A <SGSN_IP_ADDRESS>

Source(s):
from my own experience
3GPP TS 23.003