SIP 488 Not Acceptable Here
Some part of the session description, often the codec list, is not acceptable at this endpoint.
4xx Client failure · RFC 3261 · Answer to OPTIONS: Up. It rejected the request.
What SIP 488 means
A 488 says the session description in the request isn't acceptable at this endpoint. The usual reason is that the SDP offer and the far end have no codec in common, but it also covers media encryption, transport profiles and other SDP parameters. The response may carry a Warning header with a code that names the problem.
488 shows up on the initial INVITE and on re-INVITEs that change media mid-call, for example a switch to T.38 fax. It applies to this location only; 606 Not Acceptable means the description is unacceptable everywhere. RFC 3398 maps Q.850 cause 65, bearer capability not implemented, to 488.
Example response
SIP/2.0 488 Not Acceptable Here
Via: SIP/2.0/UDP 198.51.100.29:5060;branch=z9hG4bK-488-w2
From: <sip:+[email protected]>;tag=c95d
To: <sip:+[email protected]>;tag=na-8f
Call-ID: [email protected]
CSeq: 1 INVITE
Warning: 305 sbc.example.net "Incompatible media format"
Content-Length: 0Common causes of SIP 488
- The offer lists only codecs the far end doesn't support, for example G.729 to a trunk that accepts only G.711.
- One side requires encrypted media and the other offers plain RTP, or the reverse.
- A re-INVITE for T.38 fax reached an endpoint that doesn't support it.
- An SBC removed codecs from the offer by policy, leaving none the far end accepts.
- An SDP attribute or parameter the far end can't handle made it reject the whole offer.
How to fix SIP 488
- Read the Warning header: 305 means incompatible media format, 304 media type not available, and 370 insufficient bandwidth.
- Compare the m= lines and payload types in the offer with the codecs the far end accepts.
- Check every SBC on the path for codec filtering, and make sure at least one common codec survives it.
- For fax failures, check whether both ends support T.38, or agree to fall back to G.711 pass-through.
- Match the media profile: plain RTP/AVP unless both ends support a secure profile such as RTP/SAVP.
SIP 488 as an answer to OPTIONS
OPTIONS from PingVo carries no SDP, so there is no offer to reject, and 488 isn't a usual answer to it. If a server sends one, it is up and processing SIP, and a PingVo Route counts it as reachable by default. Codec problems only show on a real call, which is why PingVo's call checks place one.
Questions about SIP 488
What is the difference between SIP 488 and 606?
488 means the offer isn't acceptable at this endpoint, so another endpoint might take it. 606 means it isn't acceptable anywhere.
What is the difference between SIP 488 and 415?
415 Unsupported Media Type rejects the type of the body, for example a format other than SDP. 488 understands the SDP but rejects what it asks for.
Why does a call fail with 488 when I enable fax?
The switch to fax is a re-INVITE to T.38, and the far end doesn't accept it. The voice part of the call was fine; the media change was refused.