Session Description Protocol

When a call is initiated between two endpoints it is important to have information of calling and called party IP address both for signaling and media, codec they are going to use and several other important information. In a SIP based call (call where SIP protocol is used) we use SDP to advertise media information like transport address, transport protocol, codec, ports and other session description information. SDP works at application layer.  SDP is defined in RFC 2327, 4566 (newer). Normally we see SDP in Invite, 200 OK or ACK depending on the call type which we will discuss soon.

SIP Response Codes

In this post I am going to discuss about SIP Response codes. In the previous post we discussed about SIP messages. SIP Response codes as the name says is a kind of response to any SIP message. It can be a response to Invite message, Bye message or an Options message. We have. 6 different classes SIP Response codes which we will discuss now.

1XX - INFORMATIONAL RESPONSE

2XX - SUCCESSFUL RESPONSES

3XX - REDIRECTION RESPONSES

4XX - CLIENT FAILURE RESPONSES

5XX - SERVER FAILURE RESPONSES

6XX - GLOBAL FAILURE RESPONSES

SIP Messages

When we talk about SIP call flow there are two important things that need to be understood SIP messages and response code. SIP messages and responses are used for setup , in session changes, info and for  tearing down a session. A response code is a response which is sent in response to a SIP message, and we can also get a SIP message in acknowledgement of response code.We will look into some quite frequent SIP messages in this post and response codes in the next post.

SIP Messages:

INVITE: Invite is very first message to set up a connection. It is also used to modify a session as re-invite (which we will discuss in later posts). User Agent sending an Invite is known as User Agent Client.

ACK: ACK is sent in response to a 200 OK message. ACK is used only with Invite or a Re-Invite message.

SIP CALL FLOW

In SIP Basics we discussed about components of SIP. It's time to understand a sample call flow. Before we start I would like to discuss few components of call flow:

Request: When a UAC (User Agent Client) wants to initiate a call it sends an Invite to the UAS (User Agent Server). In this case Invite is a request .

Response: A reply to UAC's request is called a response. In this case it would be 100 Trying, which we will discuss in a moment.

SIP BASICS

SIP BASICS

SIP:

SIP Stands for Session Initiation Protocol. SIP is becoming a global standard for VoIP. SIP is a protocol which is used for signalling, controlling multimedia sessions such as voice and video between two endpoints or through a proxy server. SIP is also used for instant messaging, client presence, user mobility based on its current location.

  

SIP has been developed by IETF: Internet Engineering Task Force. This group is responsible for adding new features and modifying existing features in SIP. All concepts used in SIP are defined under RFC’s i.e. RFC 3261 which is one of the most important RFC for VoIP.