DNS Record Types used in EPC

It's been a while, so I will better start.
Previously, I hope, I've managed to shed some light on the DNS in EPC in general, from high above, form. Today I would like to present a records that are used in the LTE EPC deployments I've been involved into.

DNS A & AAAA records

A records (or ‘Address Records’) return an IPv4 address. This is most commonly used to map hostnames to an IPv4 address. Alternatively it is also used by some applications for subnet masks.

AAAA records return an IPv6 address. This is most commonly used to map hostnames to an IPv6 address of the host.

Example of an A record:

topon.nodes.pgw.be.epc  IN   A       212.183.156.177
topon.nodes.pgw.ms.epc  IN   A       212.183.156.179

where IN = Internet, A = A-record.

Example of an AAAA record:

topon.nodes.pgw.be.epc  IN   AAAA     2600:1800:5::10
topon.nodes.pgw.ms.epc  IN   AAAA     2600:1800:5::20

where IN = Internet, AAAA = AAAA-record.

DNS SRV records

DNS Resource Records (RR) for specifying the location of services (DNS SRV). Clients can ask for a specific service/protocol for a specific domain, and get back the names of any available servers.

Example of an SRV record:

; SRV Records
; _Service._proto.Name    TTL    Class    SRV    Priority    Weight  Port    Target
; SRV records for SGW
; There are multiple A records for each site to account for future nodes as there will be two PGWs at xBE and xMS.
topon.nodes.sgw.be.epc    IN    SRV    1    100    2123  node1.sgw.be.epc.mnc01.mcc234.3gppnetwork.org.
;topon.nodes.sgw.be.epc    IN    SRV    1    100    2123  node2.sgw.be.epc.mnc01.mcc234.3gppnetwork.org.
topon.nodes.sgw.ms.epc    IN    SRV    1    100    2123  node1.sgw.ms.epc.mnc01.mcc234.3gppnetwork.org.
;topon.nodes.sgw.ms.epc    IN    SRV    1    100    2123  node2.sgw.ms.epc.mnc01.mcc234.3gppnetwork.org.

The SRV record is formed of the following fields:

Priority – (=1 in the above example) a client must attempt to contact the target host with the lowest-numbered priority it can reach. Target hosts with the same priority should be tried in an order defined by the weight field.

Weight – (=100 in the above example) specifies a relative weight for entries with the same priority. Larger weights should be given a proportionally higher probability of being selected. A target with a weight of 0 corresponds to a backup.
Port – (=2123 [GTP] in the above example) is the port number assigned to the symbolic service specified in the record.
The system should compute the sum of all weights of those RRs having the lowest, identical precedence. Then choose a random number between 0 and the sum computed.  Select the target associated to this random value.

DNS NAPTR records

Name Authority Pointer (NAPTR) resource records specify lookup services for a wide variety of resources names. When applied to an existing string, NAPTR will produce a new domain label or URI.
With EPC, we will use a variation called S-NAPTR where the regular-expression is empty. Straightforward NAPTR (S-NAPTR) is used to add particular services to a DNS entry. The result of this is that the DNS server can reply back to the query with a service-protocol specific entry. S-NAPTR records are particularly common with mail-reading applications whereby the DNS server can have separate S-NAPTR records for the POP3 and IMAP4 protocols.
When using S-NAPTR, the DNS query does not indicate the ‘service:protocol’ needed. The DNS server will provide all S-NAPTR requested and the DNS client will consider the ones matching only the desired ‘service:protocol’.

Example of a NAPTR record

wap.operator.com.apn.epc.mnc01.mcc234.3gppnetwork.org. IN NAPTR 1 5  "a"  "x-3gpp-pgw:x-gn"   "" topon.nodes.pgw.be.epc.mnc01.mcc234.3gppnetwork.org.

The NAPTR record is formed of the following fields:

Order – (=1 in above example) a client must attempt to contact the target host with the lowest-numbered order it can reach. Target hosts with the same order should be tried in an order defined by the preference field. The DNS client should not consider any NAPTR with higher value for order.

Preference – (=5 in above example) specifies how NAPTR with equal order should be processed. Low numbers are being processed before high numbers.  A client may look at record with higher preference values if it has good reasons to do so such as not understanding the preferred protocol or service.
The important difference between Order and Preference is that once a match is found, the client must not consider records with a different order but they may process records with the same Order but different Preferences.
In other words, Preference is used to give weight to rules that are considered the same from an authority standpoint but not from a simple load-balancing standpoint.

Flags – (=a in above example) controls aspects of rewriting and interpretation in the record. While other flags are allowed with NAPTR, only "S", "A" or "" are allowed with S-NAPTR.

• The "S" Flag means that the next lookup should be for SRV records.
• The "A" Flag means that the next lookup should be either for A, AAAA records.
•  An empty flag " " means that more NAPTR RR lookups are to be performed.

Source:
My own experience

DNS in Evolved Packet Core (EPC)

Lately the DNS topic was addressed here, I'm happy to get back to you with more on this.
Hope you will find more detailed description of usage of DNS in EPC. The below is detailed description of, I think, most important from the DNS topic with examples of dns zone files.

UPDATE: For more info on how the EPC uses the DNS please refer to DNS in LTE label.

Home Network Domain

The Home network Domain will be in the form ‘epc.mnc<mnc-val>.mcc<mcc-val>.3gppnetwork.org’

Both <mnc-val> and <mcc-val> are 3 digits long. If the MNC of the PLM is 2 digits, then a zero shall be added at the beginning.
The Home network domain for some example Operator will be:

epc.mnc456.mcc123.3gppnetworks.org

The Home Network Domain name of an IMSI can be derived by taking the first 5 or 6 digits (depending on whether a 2 or 3 digit mnc is used), use the MNC and MNC values derived to form the domain name, add the label epc at the beginning of the domain name.
For example an IMSI 1234560123456789 has an MSIN 0123456789 and a Home network domain name of epc.mnc456.mcc123.3gppnetworks.org.

Fig. 1. DNS subdomains in EPC

The below example shows how the APN-FQDN is formed:

APN FQDN = internet.apn.epc.mnc015.mcc234.3gppnetwork.org

which consists of:

• APN Network Identifier (APN-NI) - Basically it's the APN name, from UE point of view.

APN-NI = internet

• APN Operator Identifier (APN-OI) - levels equal and below to APN level from DNS configuration point of view
  Or just the rest what is left from APN-FQDN when you remove the APN (-NI) from it.

APN-OI = .apn.epc.mnc015.mcc234.3gppnetwork.

• APN Network Identifier (APN-NI) - Basiclly it's the APN name, from UE point of view.

APN-NI = internet

Whenever configuring the DNS, no matter if this is the one you will used by any Operator in EPC deployment you are doing or just for yourself, dont forget the dots "." they are freaking important.

HSS Selection

This is the last point to cover from the list I was presenting many times by now, and the section DNS in LTE will be finished.
Of course if I will face some interesting or will find somewhere example of DNS file will parse it and put it here, so any of you could reuse it.

Last but not least, the HSS Selection procedure.

The Diameter Proxy/Edge agent performs a NAPTR query (as to RFC 3403) for a HSS server in a particular diameter realm (for example, the HPMN or the roaming hub).
These NAPTR records provide a mapping from a domain to the SRV record for contacting a server with the specific transport protocol in the NAPTR services field.  The services relevant for the task of transport protocol selection are those with NAPTR service fields with values “AAA+D2x”, where x is a letter that corresponds to a transport protocol supported by the domain, i.e. D2S is for SCTP.

Source(s):
From my own experience 

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.

PGW and SGW Selection procedures in LTE

Last time it was about MME Selection, and today I will cover the PGW and SGW selection as well.
All other points will cover in details in few next days. Also, I'm aware that good example is worth few thousands words, that's why will provide a iDNS configuration as a separate article with all those NAPTR and so on explained.

Possible selection scenarios

As last time, same list of selection scenarios in which EPS Nodes send the DNS Query message to the Internal DNS (iDNS) for the selection of EPS nodes.

How MME is selected - MME Selection procedure

It's beed a while from last time, but here we go. Many times I saw people accesing this site looking for MME seletcion, or how LTE pick MME/SGW/PGW nodes. Hope the below clarifies.


In each LTE network the Internet Domain Name System (DNS) is widely used to handle the Dynamic Peer selection of the PDN-GW, SGW, MME, SGSN and HSS within the network. We could use statistic assingments but, hey! belive me, DNS is much easier if handled properly.

In a few days I will try to cover the details of DNS config for points mentioned below - stay tunned.

EPS Nodes sends the DNS Query message to the Internal DNS (iDNS) for the selection of EPS nodes for the following uses cases.

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.

Today it will about two of them referring to MME Selection, and the list above will be copied few times more when covering the SGW, PGW, SGSN and HSS selection.