One of the marketing reasons why one should prefer VoLTE instead of common VoIP is the Quality of Service. The framework for Policy and Charging Control is described in its own post. Here we’ll focus QoS support in SIP/SDP protocols. Note that this post goes in details and expects that you’re familiar with SIP/SDP session negotiation procedure. When we want to support QoS during the session negotiation we need to add so-called preconditions in the SIP INVITE:
Require: precondition
The details can be found in the RFC 4032 and RFC 3312. The preconditions can be of several types as qos, connectivity or security. In order to support QoS there are dedicated fields in SDP:
current-status = "a=curr:" precondition-type
SP status-type SP direction-tag
desired-status = "a=des:" precondition-type
SP strength-tag SP status-type
SP direction-tag
confirm-status = "a=conf:" precondition-type
SP status-type SP direction-tag
precondition-type = "qos" | token
strength-tag = ("mandatory" | "optional" | "none"
= | "failure" | "unknown")
status-type = ("e2e" | "local" | "remote")
direction-tag = ("none" | "send" | "recv" | "sendrecv")
The logic is very simple. Each participant will define whether he/she has some desired status he/she wants to reach. Then during the SDP offer/answer negotiation they are simply checking if the current state >= desired state. Continue reading
