Visual RFC.
ALL RFCs / RFC 8285
Read the RFC ↗

RFC 8285 / START HERE

RTP header extensions, before the details

RTP means Real-time Transport Protocol. This RFC defines a general mechanism for adding header metadata to RTP media packets.

Small metadata elements share one outer header-extension block. Compact local numbers name the elements, lengths find their data, and setup information tells a receiver what those numbers mean. The media payload stays separate.

Start the guide See the 6 chapters ↓

The idea in one picture

  1. Agree on meaning

  2. Carry short elements

  3. Interpret metadata

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

RFC 8285 is an October 2017 Proposed Standard that obsoletes RFC 5285. It adds negotiated use of one-byte and two-byte element forms in the same stream, across different packets. It does not replace RTP or define every possible metadata element.

A familiar example

Imagine a fictional video call whose setup maps a short number to an agreed metadata URI. One packet carries a tiny value and another uses a longer form after mixed-mode agreement. Loss may require repetition; encrypted media alone does not make the header metadata secret.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

RTP / RTCPRFC 3550 · beginner overview ↗SDPRFC 8866 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Locate extensions without changing RTP meaningRTP carries media packets. One outer header-extension block can contain several separately identified metadata elements.
  2. Decode the compact one-byte element headerFour ID bits and four length bits describe a small element. The length nibble encodes the data length minus one.
  3. Use the two-byte form for larger valuesThe ID and length each occupy a full octet. Zero-length data is representable, and appbits is a separate outer field.
  4. Bind local identifiers to negotiated URI meaningsAn extmap attribute binds a compact session-local number to an absolute URI and, optionally, direction and configuration.
  5. Negotiate mixed streams and extended offersOne RTP stream can alternate the two formats only with support from every recipient. Extended negotiation IDs are not packet IDs.
  6. Budget delivery and protect metadata deliberatelyConfigured metadata is not promised in every packet. Repetition improves a modelled delivery chance; security requires explicit protection.
Start with chapter one →

This introduction explains RFC 8285 (2017). Read the original RFC ↗