Home Realm
This section explains how to configure a home realm. The home realm applies only to a SIP configuration. It represents the internal default realm or network for the Oracle Communications Session Border Controller and is where the Oracle Communications Session Border Controller’s SIP proxy is located.
Overview
You primarily use a home realm when using the SIP NAT function to connect multiple realms/networks to the Oracle Communications Session Border Controller. You define the home realm defined as either public or private for the purposes of using the SIP NAT function. If the home realm is public, all external realms are considered private. If the home realm is private, all external networks are considered public. Usually the home realm is public.
Messages are encoded (for example, the topology is hidden) when they pass from a private to a public realm. Messages are decoded when the pass from a public realm to a private realm.
These external realms/networks might have overlapping address spaces. Because SIP messages contain IP addresses, but no layer 2 identification (such as a VLAN tag), the SIP proxy must use a single global address space to prevent confusing duplicate IP addresses in SIP URIs from different realms.
SIP NAT Function
The SIP NAT function converts external addresses in SIP URIs to an internal home realm address. Usually the external address is encoded into a cookie that is added to the userinfo portion of the URI and the external address is replaced with a home realm address unique to the SIP NAT (the SIP NAT home address).
URIs are encoded when they pass from a private realm to a public realm. When an encoded URI passes back to the realm where it originated, it is decoded (the original userinfo and host address are restored). The encoding/decoding process prevents the confusion of duplicate addresses from overlapping private addresses. It can also be used to hide the private address when a SIP message is traversing a public network. Hiding the address occurs when it is a private address; or when the owner of the private network does not want the IP addresses of their equipment exposed on a public network or on other private networks to which the Oracle Communications Session Border Controller connects.
Home Realm’s Purpose
A home realm is required because the home address for SIP NATs is used to create a unique encoding of SIP NAT cookies. You can define the home realm as a network internal to the Oracle Communications Session Border Controller, which eliminates the need for an actual home network connected to the Oracle Communications Session Border Controller. You can define this virtual home network if the supply of IP addresses is limited (because each SIP NAT requires a unique home address), or if all networks to which the Oracle Communications Session Border Controller is connected must be private to hide addresses.
For example, you can define a public home realm using the loopback network (127.0.0.0) and using the home realm address prefix (for example, 127.0.0.0/8) for encoding addresses that do not match (all addresses outside 127.0.0.0/8) in SIP NAT cookies. The SIP NAT address prefix field can be used to accomplish this while keeping the ability to define an address prefix for the ream for ingress realm determination and admission control. By defining the SIP NAT address prefix as 0.0.0.0, the home realm address prefix is used to encode addresses that do not match.