Internet DRAFT - draft-ietf-avtext-framemarking
draft-ietf-avtext-framemarking
Network Working Group M. Zanaty
Internet-Draft E. Berger
Intended status: Experimental S. Nandakumar
Expires: 5 September 2024 Cisco Systems
4 March 2024
Video Frame Marking RTP Header Extension
draft-ietf-avtext-framemarking-16
Abstract
This document describes a Video Frame Marking RTP header extension
used to convey information about video frames that is critical for
error recovery and packet forwarding in RTP middleboxes or network
nodes. It is most useful when media is encrypted, and essential when
the middlebox or node has no access to the media decryption keys. It
is also useful for codec-agnostic processing of encrypted or
unencrypted media, while it also supports extensions for codec-
specific information.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Key Words for Normative Requirements . . . . . . . . . . . . 4
3. Frame Marking RTP Header Extension . . . . . . . . . . . . . 4
3.1. Long Extension for Scalable Streams . . . . . . . . . . . 5
3.2. Short Extension for Non-Scalable Streams . . . . . . . . 7
3.3. Layer ID Mappings for Scalable Streams . . . . . . . . . 7
3.3.1. VP9 LID Mapping . . . . . . . . . . . . . . . . . . . 8
3.3.2. H265 LID Mapping . . . . . . . . . . . . . . . . . . 8
3.3.3. H264-SVC LID Mapping . . . . . . . . . . . . . . . . 9
3.3.4. H264 (AVC) LID Mapping . . . . . . . . . . . . . . . 10
3.3.5. VP8 LID Mapping . . . . . . . . . . . . . . . . . . . 10
3.3.6. Future Codec LID Mapping . . . . . . . . . . . . . . 11
3.4. Signaling Information . . . . . . . . . . . . . . . . . . 11
3.5. Usage Considerations . . . . . . . . . . . . . . . . . . 11
3.5.1. Relation to Layer Refresh Request (LRR) . . . . . . . 12
3.5.2. Scalability Structures . . . . . . . . . . . . . . . 12
4. Security Considerations and Privacy Considerations . . . . . 12
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
Many widely deployed RTP [RFC3550] topologies [RFC7667] used in
modern voice and video conferencing systems include a centralized
component that acts as an RTP switch. It receives voice and video
streams from each participant, which may be encrypted using SRTP
[RFC3711], or extensions that provide participants with private media
[RFC8871] via end-to-end encryption where the switch has no access to
media decryption keys. The goal is to provide a set of streams back
to the participants which enable them to render the right media
content. In a simple video configuration, for example, the goal will
be that each participant sees and hears just the active speaker. In
that case, the goal of the switch is to receive the voice and video
streams from each participant, determine the active speaker based on
energy in the voice packets, possibly using the client-to-mixer audio
level RTP header extension [RFC6464], and select the corresponding
video stream for transmission to participants; see Figure 1.
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In this document, an "RTP switch" is used as a common short term for
the terms "switching RTP mixer", "source projecting middlebox",
"source forwarding unit/middlebox" and "video switching MCU" as
discussed in [RFC7667].
+---+ +------------+ +---+
| A |<---->| |<---->| B |
+---+ | | +---+
| RTP |
+---+ | Switch | +---+
| C |<---->| |<---->| D |
+---+ +------------+ +---+
Figure 1: RTP switch
In order to properly support switching of video streams, the RTP
switch typically needs some critical information about video frames
in order to start and stop forwarding streams.
* Because of inter-frame dependencies, it should ideally switch
video streams at a point where the first frame from the new
speaker can be decoded by recipients without prior frames, e.g
switch on an intra-frame.
* In many cases, the switch may need to drop frames in order to
realize congestion control techniques, and needs to know which
frames can be dropped with minimal impact to video quality.
* For scalable streams with dependent layers, the switch may need to
selectively forward specific layers to specific recipients due to
recipient bandwidth or decoder limits.
Furthermore, it is highly desirable to do this in a payload format-
agnostic way which is not specific to each different video codec.
Most modern video codecs share common concepts around frame types and
other critical information to make this codec-agnostic handling
possible.
It is also desirable to be able to do this for SRTP without requiring
the video switch to decrypt the packets. SRTP will encrypt the RTP
payload format contents and consequently this data is not usable for
the switching function without decryption, which may not even be
possible in the case of end-to-end encryption of private media
[RFC8871].
By providing meta-information about the RTP streams outside the
encrypted media payload, an RTP switch can do codec-agnostic
selective forwarding without decrypting the payload. This document
specifies the necessary meta-information in an RTP header extension.
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2. Key Words for Normative Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Frame Marking RTP Header Extension
This specification uses RTP header extensions as defined in
[RFC8285]. A subset of meta-information from the video stream is
provided as an RTP header extension to allow an RTP switch to do
generic selective forwarding of video streams encoded with
potentially different video codecs.
The Frame Marking RTP header extension is encoded using the one-byte
header or two-byte header as described in [RFC8285]. The one-byte
header format is used for examples in this memo. The two-byte header
format is used when other two-byte header extensions are present in
the same RTP packet, since mixing one-byte and two-byte extensions is
not possible in the same RTP packet.
This extension is only specified for Source (not Redundancy) RTP
Streams [RFC7656] that carry video payloads. It is not specified for
audio payloads, nor is it specified for Redundancy RTP Streams. The
(separate) specifications for Redundancy RTP Streams often include
provisions for recovering any header extensions that were part of the
original source packet. Such provisions can be followed to recover
the Frame Marking RTP header extension of the original source packet.
Source packet frame markings may be useful when generating Redundancy
RTP Streams; for example, the I (Independent Frame) and D
(Discardable Frame) bits, defined in Section 3.1, can be used to
generate extra or no redundancy, respectively, and redundancy schemes
with source blocks can align source block boundaries with independent
frame boundaries as marked by the I bit.
A frame, in the context of this specification, is the set of RTP
packets with the same RTP timestamp from a specific RTP
synchronization source (SSRC). A frame within a layer is the set of
RTP packets with the same RTP timestamp, SSRC, Temporal ID (TID), and
Layer ID (LID).
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3.1. Long Extension for Scalable Streams
The following RTP header extension is RECOMMENDED for scalable
streams. It MAY also be used for non-scalable streams, in which case
TID, LID and TL0PICIDX MUST be 0 or omitted. The ID is assigned per
[RFC8285], and the length is encoded as L=2 which indicates 3 octets
of data when nothing is omitted, or L=1 for 2 octets when TL0PICIDX
is omitted, or L=0 for 1 octet when both LID and TL0PICIDX are
omitted.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID | LID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
or
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=1 |S|E|I|D|B| TID | LID | (TL0PICIDX omitted)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
or
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=0 |S|E|I|D|B| TID | (LID and TL0PICIDX omitted)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
* S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame within a layer; otherwise MUST be 0.
* E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame
within a layer; otherwise MUST be 0. Note that the RTP header
marker bit MAY be used to infer the last packet of the highest
enhancement layer, in payload formats with such semantics.
* I: Independent Frame (1 bit) - MUST be 1 for a frame within a
layer that can be decoded independent of temporally prior frames,
e.g. intra-frame, VPX keyframe, H.264 IDR [RFC6184], H.265
IDR/CRA/BLA/RAP [RFC7798]; otherwise MUST be 0. Note that this
bit only signals temporal independence, so it can be 1 in spatial
or quality enhancement layers that depend on temporally co-located
layers but not temporally prior frames.
* D: Discardable Frame (1 bit) - MUST be 1 for a frame within a
layer the sender knows can be discarded, and still provide a
decodable media stream; otherwise MUST be 0.
* B: Base Layer Sync (1 bit) - When TID is not 0, this MUST be 1 if
the sender knows this frame within a layer only depends on the
base temporal layer; otherwise MUST be 0. When TID is 0 or if no
scalability is used, this MUST be 0.
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* TID: Temporal ID (3 bits) - Identifies the temporal layer/sub-
layer encoded, starting with 0 for the base layer, and increasing
with higher temporal fidelity. If no scalability is used, this
MUST be 0. It is implicitly 0 in the short extension format.
* LID: Layer ID (8 bits) - Identifies the spatial and quality layer
encoded, starting with 0 for the base layer, and increasing with
higher fidelity. If no scalability is used, this MUST be 0 or
omitted to reduce length. When omitted, TL0PICIDX MUST also be
omitted. It is implicitly 0 in the short extension format or when
omitted in the long extension format.
* TL0PICIDX: Temporal Layer 0 Picture Index (8 bits) - When TID is 0
and LID is 0, this is a cyclic counter labeling base layer frames.
When TID is not 0 or LID is not 0, this indicates a dependency on
the given index, such that this frame within this layer depends on
the frame with this label in the layer with TID 0 and LID 0. If
no scalability is used, or the cyclic counter is unknown, this
MUST be omitted to reduce length. Note that 0 is a valid index
value for TL0PICIDX.
The layer information contained in TID and LID convey useful aspects
of the layer structure that can be utilized in selective forwarding.
Without further information about the layer structure, these TID/LID
identifiers can only be used for relative priority of layers and
implicit dependencies between layers. They convey a layer hierarchy
with TID=0 and LID=0 identifying the base layer. Higher values of
TID identify higher temporal layers with higher frame rates. Higher
values of LID identify higher spatial and/or quality layers with
higher resolutions and/or bitrates. Implicit dependencies between
layers assume that a layer with a given TID/LID MAY depend on
layer(s) with the same or lower TID/LID, but MUST NOT depend on
layer(s) with higher TID/LID.
With further information, for example, possible future RTCP SDES
items that convey full layer structure information, it may be
possible to map these TIDs and LIDs to specific absolute frame rates,
resolutions and bitrates, as well as explicit dependencies between
layers. Such additional layer information may be useful for
forwarding decisions in the RTP switch, but is beyond the scope of
this memo. The relative layer information is still useful for many
selective forwarding decisions even without such additional layer
information.
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3.2. Short Extension for Non-Scalable Streams
The following RTP header extension is RECOMMENDED for non-scalable
streams. It is identical to the shortest form of the extension for
scalable streams, except the last four bits (B and TID) are replaced
with zeros. It MAY also be used for scalable streams if the sender
has limited or no information about stream scalability. The ID is
assigned per [RFC8285], and the length is encoded as L=0 which
indicates 1 octet of data.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=0 |S|E|I|D|0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The following information are extracted from the media payload and
sent in the Frame Marking RTP header extension.
* S: Start of Frame (1 bit) - MUST be 1 in the first packet in a
frame; otherwise MUST be 0.
* E: End of Frame (1 bit) - MUST be 1 in the last packet in a frame;
otherwise MUST be 0. SHOULD match the RTP header marker bit in
payload formats with such semantics for marking end of frame.
* I: Independent Frame (1 bit) - MUST be 1 for frames that can be
decoded independent of temporally prior frames, e.g. intra-frame,
VPX keyframe, H.264 IDR [RFC6184], H.265 IDR/CRA/BLA/IRAP
[RFC7798]; otherwise MUST be 0.
* D: Discardable Frame (1 bit) - MUST be 1 for frames the sender
knows can be discarded, and still provide a decodable media
stream; otherwise MUST be 0.
* The remaining (4 bits) - are reserved/fixed values and not used
for non-scalable streams; they MUST be set to 0 upon transmission
and ignored upon reception.
3.3. Layer ID Mappings for Scalable Streams
This section maps the specific Layer ID information contained in
specific scalable codecs to the generic LID and TID fields.
Note that non-scalable streams have no Layer ID information and thus
no mappings.
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3.3.1. VP9 LID Mapping
The VP9 [I-D.ietf-payload-vp9] Spatial Layer ID (SID, 3 bits) and
Temporal Layer ID (TID, 3 bits) in the VP9 payload descriptor are
mapped to the generic LID and TID fields in the header extension as
shown in the following figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID |0|0|0|0|0| SID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The S bit MUST match the B bit in the VP9 payload descriptor.
The E bit MUST match the E bit in the VP9 payload descriptor.
The I bit MUST match the inverse of the P bit in the VP9 payload
descriptor.
The D bit MUST be 1 if the refresh_frame_flags in the VP9 payload
uncompressed header are all 0, otherwise it MUST be 0.
The B bit MUST be 0 if TID is 0; otherwise, if TID is not 0, it MUST
match the U bit in the VP9 payload descriptor. Note: When using
temporally nested scalability structures as recommended in
Section 3.5.2, the B bit and VP9 U bit will always be 1 if TID is not
0, since it is always possible to switch up to a higher temporal
layer in such nested structures.
TID, SID and TL0PICIDX MUST match the correspondingly named fields in
the VP9 payload descriptor, with SID aligned in the least significant
3 bits of the 8-bit LID field and zeros in the most significant 5
bits.
3.3.2. H265 LID Mapping
The H265 [RFC7798] LayerID (6 bits) and TID (3 bits) from the NAL
unit header are mapped to the generic LID and TID fields in the
header extension as shown in the following figure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID |0|0| LayerID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The S and E bits MUST match the correspondingly named bits in
PACI:PHES:TSCI payload structures.
The I bit MUST be 1 when the NAL unit type is 16-23 (inclusive) or
32-34 (inclusive), or an aggregation packet or fragmentation unit
encapsulating any of these types, otherwise it MUST be 0. These
ranges cover intra (IRAP) frames as well as critical parameter sets
(VPS, SPS, PPS).
The D bit MUST be 1 when the NAL unit type is 0, 2, 4, 6, 8, 10, 12,
14, or 38, or an aggregation packet or fragmentation unit
encapsulating only these types, otherwise it MUST be 0. These ranges
cover non-reference frames as well as filler data.
The B bit can not be determined reliably from simple inspection of
payload headers, and therefore is determined by implementation-
specific means. For example, internal codec interfaces may provide
information to set this reliably.
TID and LayerID MUST match the correspondingly named fields in the
H265 NAL unit header, with LayerID aligned in the least significant 6
bits of the 8-bit LID field and zeros in the most significant 2 bits.
3.3.3. H264-SVC LID Mapping
The following shows H264-SVC [RFC6190] Layer encoding information (3
bits for spatial/dependency layer, 4 bits for quality layer and 3
bits for temporal layer) mapped to the generic LID and TID fields.
The S, E, I and D bits MUST match the correspondingly named bits in
PACSI payload structures.
The I bit MUST be 1 when the NAL unit type is 5, 7, 8, 13, or 15, or
an aggregation packet or fragmentation unit encapsulating any of
these types, otherwise it MUST be 0. These ranges cover intra (IDR)
frames as well as critical parameter sets (SPS/PPS variants).
The D bit MUST be 1 when the NAL unit header NRI field is 0, or an
aggregation packet or fragmentation unit encapsulating only NAL units
with NRI=0, otherwise it MUST be 0. The NRI=0 condition signals non-
reference frames.
The B bit can not be determined reliably from simple inspection of
payload headers, and therefore is determined by implementation-
specific means. For example, internal codec interfaces may provide
information to set this reliably.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID |0| DID | QID | TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.4. H264 (AVC) LID Mapping
The following shows the header extension for H264 (AVC) [RFC6184]
that contains only temporal layer information.
The S bit MUST be 1 when the timestamp in the RTP header differs from
the timestamp in the prior RTP sequence number from the same SSRC,
otherwise it MUST be 0.
The E bit MUST match the M bit in the RTP header.
The I bit MUST be 1 when the NAL unit type is 5, 7, or 8, or an
aggregation packet or fragmentation unit encapsulating any of these
types, otherwise it MUST be 0. These ranges cover intra (IDR) frames
as well as critical parameter sets (SPS/PPS).
The D bit MUST be 1 when the NAL unit header NRI field is 0, or an
aggregation packet or fragmentation unit encapsulating only NAL units
with NRI=0, otherwise it MUST be 0. The NRI=0 condition signals non-
reference frames.
The B bit can not be determined reliably from simple inspection of
payload headers, and therefore is determined by implementation-
specific means. For example, internal codec interfaces may provide
information to set this reliably.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID |0|0|0|0|0|0|0|0| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.5. VP8 LID Mapping
The following shows the header extension for VP8 [RFC7741] that
contains only temporal layer information.
The S bit MUST match the correspondingly named bit in the VP8 payload
descriptor when PID=0, otherwise it MUST be 0.
The E bit MUST match the M bit in the RTP header.
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The I bit MUST match the inverse of the P bit in the VP8 payload
header.
The D bit MUST match the N bit in the VP8 payload descriptor.
The B bit MUST match the Y bit in the VP8 payload descriptor. Note:
When using temporally nested scalability structures as recommended in
Section 3.5.2, the B bit and VP8 Y bit will always be 1 if TID is not
0, since it is always possible to switch up to a higher temporal
layer in such nested structures.
TID and TL0PICIDX MUST match the correspondingly named fields in the
VP8 payload descriptor.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID=? | L=2 |S|E|I|D|B| TID |0|0|0|0|0|0|0|0| TL0PICIDX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.3.6. Future Codec LID Mapping
The RTP payload format specification for future video codecs SHOULD
include a section describing the LID mapping and TID mapping for the
codec.
3.4. Signaling Information
The URI for declaring this header extension in an extmap attribute is
"urn:ietf:params:rtp-hdrext:framemarking". It does not contain any
extension attributes.
An example attribute line in SDP:
a=extmap:3 urn:ietf:params:rtp-hdrext:framemarking
3.5. Usage Considerations
The header extension values MUST represent what is already in the RTP
payload.
When an RTP switch needs to discard a received video frame due to
congestion control considerations, it is RECOMMENDED that it
preferably drop frames marked with the D (Discardable) bit set, or
the highest values of TID and LID, which indicate the highest
temporal and spatial/quality enhancement layers, since those
typically have fewer dependenices on them than lower layers.
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When an RTP switch wants to forward a new video stream to a receiver,
it is RECOMMENDED to select the new video stream from the first
switching point with the I (Independent) bit set in all spatial
layers and forward the same. An RTP switch can request a media
source to generate a switching point by sending Full Intra Request
(RTCP FIR) as defined in [RFC5104], for example.
3.5.1. Relation to Layer Refresh Request (LRR)
Receivers can use the Layer Refresh Request (LRR)
[I-D.ietf-avtext-lrr] RTCP feedback message to upgrade to a higher
layer in scalable encodings. The TID/LID values and formats used in
LRR messages MUST correspond to the same values and formats specified
in Section 3.1.
Because frame marking can only be used with temporally-nested
streams, temporal-layer LRR refreshes are unnecessary for frame-
marked streams. Other refreshes can be detected based on the I bit
being set for the specific spatial layers.
3.5.2. Scalability Structures
The LID and TID information is most useful for fixed scalability
structures, such as nested hierarchical temporal layering structures,
where each temporal layer only references lower temporal layers or
the base temporal layer. The LID and TID information is less useful,
or even not useful at all, for complex, irregular scalability
structures that do not conform to common, fixed patterns of inter-
layer dependencies and referencing structures. Therefore it is
RECOMMENDED to use LID and TID information for RTP switch forwarding
decisions only in the case of temporally nested scalability
structures, and it is NOT RECOMMENDED for other (more complex or
irregular) scalability structures.
4. Security Considerations and Privacy Considerations
In the Secure Real-Time Transport Protocol (SRTP) [RFC3711], RTP
header extensions are authenticated and optionally encrypted
[RFC9335]. When unencrypted header extensions are used, some
metadata is exposed and visible to middle boxes on the network path,
while encrypted media data and metadata in encrypted header
extensions are not exposed.
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The primary utility of this specification is for RTP switches to make
proper media forwarding decisions. RTP switches are the SRTP peers
of endpoints, so they can access encrypted header extensions, but not
end-to-end encrypted private media payloads. Other middle boxes on
the network path can only access unencrypted header extensions, since
they are not SRTP peers.
RTP endpoints which negotiate this extension should consider whether
this video frame marking metadata needs to be exposed to the SRTP
peer only, in which case the header extension can be encrypted; or
whether other middle boxes on the network path also need this
metadata, for example, to optimize packet drop decisions that
minimize media quality impacts, in which case the header extension
can be unencrypted, if the endpoint accepts the potential privacy
leakage of this metadata. For example, it would be possible to
determine keyframes and their frequency in unencrypted header
extensions. This information can often be obtained via statistical
analysis of encrypted data. For example, keyframes are usually much
larger than other frames, so frame size alone can leak this in the
absence of any unencrypted metadata. However, unencrypted metadata
provides a reliable signal rather than a statistical probability; so
endpoints should take that into consideration to balance the privacy
leakage risk against the potential benefit of optimized media
delivery when deciding whether to negotiate and encrypt this header
extension.
5. Acknowledgements
Many thanks to Bernard Aboba, Jonathan Lennox, Stephan Wenger, Dale
Worley, and Magnus Westerlund for their inputs.
6. IANA Considerations
This document defines a new extension URI to the RTP Compact
HeaderExtensions sub-registry of the Real-Time Transport Protocol
(RTP) Parameters registry, according to the following data:
Extension URI: urn:ietf:params:rtp-hdrext:framemarkinginfo
Description: Frame marking information for video streams
Contact: mzanaty@cisco.com
Reference: RFC XXXX
Note to RFC Editor: please replace RFC XXXX with the number of this
RFC.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8285] Singer, D., Desineni, H., and R. Even, Ed., "A General
Mechanism for RTP Header Extensions", RFC 8285,
DOI 10.17487/RFC8285, October 2017,
<https://www.rfc-editor.org/info/rfc8285>.
[RFC6184] Wang, Y.-K., Even, R., Kristensen, T., and R. Jesup, "RTP
Payload Format for H.264 Video", RFC 6184,
DOI 10.17487/RFC6184, May 2011,
<https://www.rfc-editor.org/info/rfc6184>.
[RFC6190] Wenger, S., Wang, Y.-K., Schierl, T., and A.
Eleftheriadis, "RTP Payload Format for Scalable Video
Coding", RFC 6190, DOI 10.17487/RFC6190, May 2011,
<https://www.rfc-editor.org/info/rfc6190>.
[RFC7741] Westin, P., Lundin, H., Glover, M., Uberti, J., and F.
Galligan, "RTP Payload Format for VP8 Video", RFC 7741,
DOI 10.17487/RFC7741, March 2016,
<https://www.rfc-editor.org/info/rfc7741>.
[RFC7798] Wang, Y.-K., Sanchez, Y., Schierl, T., Wenger, S., and M.
M. Hannuksela, "RTP Payload Format for High Efficiency
Video Coding (HEVC)", RFC 7798, DOI 10.17487/RFC7798,
March 2016, <https://www.rfc-editor.org/info/rfc7798>.
7.2. Informative References
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<https://www.rfc-editor.org/info/rfc7656>.
Zanaty, et al. Expires 5 September 2024 [Page 14]
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[RFC7667] Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
DOI 10.17487/RFC7667, November 2015,
<https://www.rfc-editor.org/info/rfc7667>.
[RFC6464] Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464,
DOI 10.17487/RFC6464, December 2011,
<https://www.rfc-editor.org/info/rfc6464>.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.
[RFC5104] Wenger, S., Chandra, U., Westerlund, M., and B. Burman,
"Codec Control Messages in the RTP Audio-Visual Profile
with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104,
February 2008, <https://www.rfc-editor.org/info/rfc5104>.
[RFC8871] Jones, P., Benham, D., and C. Groves, "A Solution
Framework for Private Media in Privacy-Enhanced RTP
Conferencing (PERC)", RFC 8871, DOI 10.17487/RFC8871,
January 2021, <https://www.rfc-editor.org/info/rfc8871>.
[RFC9335] Uberti, J., Jennings, C., and S. Murillo, "Completely
Encrypting RTP Header Extensions and Contributing
Sources", RFC 9335, DOI 10.17487/RFC9335, January 2023,
<https://www.rfc-editor.org/info/rfc9335>.
[I-D.ietf-avtext-lrr]
Lennox, J., Hong, D., Uberti, J., Holmer, S., and M.
Flodman, "The Layer Refresh Request (LRR) RTCP Feedback
Message", Work in Progress, Internet-Draft, draft-ietf-
avtext-lrr-07, 2 July 2017,
<https://datatracker.ietf.org/doc/html/draft-ietf-avtext-
lrr-07>.
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[I-D.ietf-payload-vp9]
Uberti, J., Holmer, S., Flodman, M., Hong, D., and J.
Lennox, "RTP Payload Format for VP9 Video", Work in
Progress, Internet-Draft, draft-ietf-payload-vp9-16, 10
June 2021, <https://datatracker.ietf.org/doc/html/draft-
ietf-payload-vp9-16>.
Authors' Addresses
Mo Zanaty
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
United States of America
Email: mzanaty@cisco.com
Espen Berger
Cisco Systems
Email: espeberg@cisco.com
Suhas Nandakumar
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
United States of America
Email: snandaku@cisco.com
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