Source note · SDP offer answer and codec negotiation
An SDP sent to indicate capabilities rather than to make an offer sets every port to zero precisely so that it cannot establish media if something interprets it as an offer or answer, and its session id must be unique for each such message.
Checked 2026-09-15
The usage of port zero makes sure that an SDP formatted for capabilities does not cause media streams to be established if it is interpreted as an offer or answer.
- Vendor
- IETF
- Product
- SIP
- Subsystem
- SDP offer/answer
- Deployment
- not restricted
- Region
- not restricted
- Release range
- RFC 3264
- Checked
- 2026-09-15
Sources
- RFC 3264 — An Offer/Answer Model with the Session Description Protocol (SDP) — IETF / RFC Editor · tier 1 standards and regulators · RFC 3264 section 9 Indicating Capabilities
Cite this note
APA
WarmTransfer. (2026, September 15). SDP offer answer and codec negotiation: source note sdp-oa-capability-sdp-uses-port-zero. WarmTransfer. https://warmtransfer.net/knowledge/claims/sdp-oa-capability-sdp-uses-port-zero
BibTeX
@misc{warmtransfer-claim-sdp-oa-capability-sdp-uses-port-zero,
title = {SDP offer answer and codec negotiation: source note sdp-oa-capability-sdp-uses-port-zero},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/sdp-oa-capability-sdp-uses-port-zero},
note = {Source note sdp-oa-capability-sdp-uses-port-zero, checked 2026-09-15}
}