SIP 100 Trying
The next hop received the request and is working on it. It stops retransmissions.
1xx Provisional · RFC 3261 · Answer to OPTIONS: Not a final answer. The check waits for the final response.
What SIP 100 means
100 Trying is the first answer most calls get. It tells the sender that the next hop has received the request and taken responsibility for it, so the sender can stop retransmitting. It is a hop-by-hop message: each proxy or SBC on the path sends its own 100 to the hop before it, and none of them is forwarded end to end.
You will see 100 almost only on INVITE, because a call can take seconds to route and ring. Over UDP it stops the caller's retransmission timer, which would otherwise resend the INVITE until something comes back. A 100 says nothing about whether the call will succeed: it is never a final answer, and a final response from 200 to 699 must still follow.
Example response
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 198.51.100.10:5060;branch=z9hG4bK-100-a1;rport=5060
From: <sip:+[email protected]>;tag=9f2e
To: <sip:+[email protected]>
Call-ID: [email protected]
CSeq: 1 INVITE
Content-Length: 0Common causes of SIP 100
- The SBC accepted the INVITE and is still waiting for the carrier or PBX behind it, which is normal for the first few hundred milliseconds.
- The next hop forwarded the INVITE to a destination that never responds, so no 180, 183 or final response comes back.
- A firewall or NAT lets the 100 through but drops the later responses, which are often larger or take a different path.
- Responses from further away go to the wrong place because a Via or Contact header advertises a private IP address.
- DNS lookups for the next hop are slow or failing, so the proxy holds the call while it tries each SRV target in turn.
How to fix SIP 100
- Take a packet capture at the SBC and find which hop sent the 100: each hop sends its own, so the Via branch tells you who holds the call.
- Follow the INVITE to the next hop and see whether it got a response there; the stall is usually one hop beyond the 100.
- Check that later responses are allowed back through the firewall, and not only the first packet of the exchange.
- Confirm that Via and Contact carry an address the far side can reach, so its responses can find their way back.
SIP 100 as an answer to OPTIONS
A 100 to OPTIONS is rare, because a server can usually answer OPTIONS at once. If one arrives it is not an answer: PingVo keeps waiting for the final response, and a probe that gets only a 100 before its timeout counts as no answer, the same as silence. Any final response that follows, including an error, counts as reachable by default.
Questions about SIP 100
Does 100 Trying mean the call is ringing?
No. 100 only means the next hop has the request. Ringing is 180, and early ringback or an announcement usually arrives with 183.
Why does a 100 Trying have no To tag?
A 100 never creates a dialog, so the server doesn't need to add a To tag. Tags appear with the first response that can start a dialog, such as 180, 183 or 200.