2.1 Transport

This protocol uses Domain Name System (DNS) SRV, as specified in the format section in [RFC2782], and DNS A, as specified in [RFC1035] section 3.4.1, and SIP server Dynamic Host Configuration Protocol (DHCP) discovery<1>, as specified in [RFC3361] section 3, for automatic discovery of a SIP outbound proxy, and does not define any new message format for auto-discovery of SIP outbound proxy nodes.

This protocol uses a custom header field in SIP requests to support the keep-alive negotiation. The name of the new header field is Ms-Keep-Alive and the new header field can be used to specify proposed and supported keep-alive capabilities to keep the persistent, reliable, in-order transport from being disconnected because of inactivity.

All SIP traffic MUST be transported over TCP. TLS on the established TCP connection for added security is optional.