SIP Fundamentals

SIP INVITE

INVITE is the SIP method that initiates a call (or a media session). It carries the SDP that describes the offered codecs and media addresses, and it triggers the full request/response sequence that culminates in 200 OK + ACK or a final error.

Anatomy of an INVITE

INVITE sip:+15557654321@sip.didhub.io SIP/2.0
Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bK-abc
From: <sip:trunk_x@sip.didhub.io>;tag=12345
To: <sip:+15557654321@sip.didhub.io>
Call-ID: a1b2c3d4@192.0.2.1
CSeq: 1 INVITE
Contact: <sip:trunk_x@192.0.2.1:5060>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 240

v=0
o=- 12345 1 IN IP4 192.0.2.1
s=-
c=IN IP4 192.0.2.1
t=0 0
m=audio 16384 RTP/AVP 0 8 9 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:9 G722/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16

Lifecycle

1. INVITE → 100 Trying → 180 Ringing → 200 OK → ACK: standard call setup.
2. Mid-call: re-INVITE can change media (hold, codec change, transfer).
3. Termination: BYE from either party.

Common failures

• 401 Unauthorized / 407 Proxy Authentication Required: re-send with Authorization header.
• 403 Forbidden: bad credentials or unauthorized destination.
• 503 Service Unavailable: upstream carrier rejected.
• 486 Busy Here: callee is on another call.

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