The session initiation protocol (SIP), an application-layer protocol in the Internet protocol (IP) model, is increasingly being used for signaling Internet telephony multimedia sessions. The widespread applicability and use of SIP has engendered the use of diverse transport-layer protocols underneath it (e.g., user datagram protocol [UDP], transmission control protocol [TCP], stream control transmission protocol [SCTP], transport layer security [TLS] over TCP, and TLS over SCTP). In carrier-grade networks, SIP must meet stringent scaling, reliability, and post-dial delay requirements. To some extent, these requirements can be met by optimally transporting SIP. The right choice of a transport protocol is primarily determined by the characteristics of the relevant transport protocols and by how those protocols interplay with individual SIP entities. Additionally, prudent management of transport-layer connections can help lower signaling latencies and facilitate scaling. This paper focuses on SIP transport as a decisive factor in engineering low-latency, scalable, carrier-grade SIP entities. It also presents the concept of SIP connection management and the persistent connection model.