Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)
An RTCP report block that tells a sender which RTP packets were still missing after every repair method had run.
Begen, A., Hsu, D., and M. Lague, "Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)", RFC 5725, DOI 10.17487/RFC5725, February 2010, <https://www.rfc-editor.org/info/rfc5725>.
@misc{rfc5725,
series = {Request for Comments},
number = 5725,
howpublished = {RFC 5725},
publisher = {RFC Editor},
doi = {10.17487/RFC5725},
url = {https://www.rfc-editor.org/info/rfc5725},
author = {A. Begen and D. Hsu and M. Lague},
title = {{Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)}},
pagetotal = 9,
year = 2010,
month = feb,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 5725 D. Hsu
Category: Standards Track M. Lague
ISSN: 2070-1721 Cisco
February 2010
Post-Repair Loss RLE Report Block Type for
RTP Control Protocol (RTCP) Extended Reports (XRs)
Abstract
This document defines a new report block type within the framework of
RTP Control Protocol (RTCP) Extended Reports (XRs). One of the
initial XR report block types is the Loss Run Length Encoding (RLE)
Report Block. This report conveys information regarding the
individual Real-time Transport Protocol (RTP) packet receipt and loss
events experienced during the RTCP interval preceding the
transmission of the report. The new report, which is referred to as
the Post-repair Loss RLE report, carries information regarding the
packets that remain lost after all loss-repair methods are applied.
By comparing the RTP packet receipts/losses before and after the loss
repair is completed, one can determine the effectiveness of the loss-
repair methods in an aggregated fashion. This document also defines
the signaling of the Post-repair Loss RLE report in the Session
Description Protocol (SDP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5725.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 4
3. Post-Repair Loss RLE Report Block . . . . . . . . . . . . . . . 4
4. Session Description Protocol Signaling . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
1. Introduction
The RTP Control Protocol (RTCP) is the out-of-band control protocol
for applications that are using the Real-time Transport Protocol
(RTP) for media delivery and communications [RFC3550]. RTCP allows
RTP entities to monitor data delivery and provides them minimal
control functionality via sender and receiver reports as well as
other control packets. [RFC3611] expands the RTCP functionality
further by introducing the RTCP Extended Reports (XRs).
One of the initial XR report block types defined in [RFC3611] is the
Loss Run Length Encoding (RLE) Report Block. This report conveys
information regarding the individual RTP packet receipt and loss
events experienced during the RTCP interval preceding the
transmission of the report. However, the Loss RLE in an RTCP XR
report is usually collected only on the primary source stream before
any loss-repair method is applied. Once one or more loss-repair
methods, e.g., Forward Error Correction (FEC) [RFC5109] and/or
retransmission [RFC4588], are applied, some or all of the lost
packets on the primary source stream may be recovered. However, the
pre-repair Loss RLE cannot indicate which source packets were
recovered and which are still missing. Thus, the pre-repair Loss RLE
cannot specify how well the loss repair performed.
This issue can be addressed by generating an additional report block
(within the same or a different RTCP XR report), which reflects the
packet receipt/loss events after all loss-repair methods are applied.
This report block, which we refer to as the post-repair Loss RLE,
indicates the remaining missing, i.e., unrepairable, source packets.
When the pre-repair and post-repair Loss RLEs are compared, the RTP
sender or another third-party entity can evaluate the effectiveness
of the loss-repair methods in an aggregated fashion. To avoid any
ambiguity in the evaluation, it is RECOMMENDED that the post-repair
Loss RLE be generated for the source packets that have no further
chance of being repaired. If the loss-repair method(s) may still
recover one or more missing source packets, the post-repair Loss RLE
SHOULD NOT be sent until the loss-recovery process has been
completed. However, a potential ambiguity may result from sequence-
number wrapping in the primary source stream. Thus, the Post-repair
Loss RLE reports may not be delayed arbitrarily. In case of an
ambiguity in the incoming reports, it is the sender's or the
monitoring entity's responsibility to understand which packets the
Post-repair Loss RLE report is related to.
Similar to the pre-repair Loss RLE, the post-repair Loss RLE conveys
the receipt/loss events at the packet level and considers partially
repaired packets as unrepaired. Thus, the methods that can partially
recover the missing data SHOULD NOT be evaluated based on the
information provided by the Post-repair Loss RLE reports since such
information may underestimate the effectiveness of such methods.
Note that the idea of using pre-repair and post-repair Loss RLEs can
be further extended when multiple sequential loss-repair methods are
applied to the primary source stream. Reporting the Loss RLEs before
and after each loss-repair method can provide specific information
about the individual performances of these methods. However, it can
be a difficult task to quantify the specific contribution made by
each loss-repair method in hybrid systems, where different methods
collectively work together to repair the lost source packets. Thus,
in this specification we only consider reporting the Loss RLE after
all loss-repair methods have been applied.
This document registers a new report block type to cover the post-
repair Loss RLE within the framework of RTCP XR. Applications that
are employing one or more loss-repair methods MAY use Post-repair
Loss RLE reports for every packet they receive or for a set of
specific packets they have received. In other words, the coverage of
the post-repair Loss RLEs may or may not be contiguous.
2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Post-Repair Loss RLE Report Block
The Post-repair Loss RLE Report Block is similar to the existing Loss
RLE Report Block defined in [RFC3611]. The report format is shown in
Figure 1. Using the same structure for reporting both pre-repair and
post-repair Loss RLEs allows the implementations to compare the Loss
RLEs very efficiently.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=10 | rsvd. | T | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: ... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format for the Post-repair Loss RLE Report Block
o block type (BT): 8 bits
A Post-repair Loss RLE Report Block is identified by the constant
10.
o rsvd.: 4 bits
This field is reserved for future definition. In the absence of
such definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver.
o thinning (T): 4 bits
The amount of thinning performed on the sequence-number space.
Only those packets with sequence numbers 0 mod 2^T are reported by
this block. A value of 0 indicates that there is no thinning and
all packets are reported. The maximum thinning is one packet in
every 32,768 (amounting to two packets within each 16-bit sequence
space).
If thinning is desired, it is RECOMMENDED to use the same thinning
value in the Pre-repair and Post-repair Loss RLE reports. This
will allow easier report processing and correlation. However,
based on the specific needs of the application or the monitoring
entity, different values of thinning MAY be used for Pre-repair
and Post-repair Loss RLE reports.
o block length: 16 bits
The length of this report block, including the header, in 32-bit
words minus one.
o SSRC of source: 32 bits
The SSRC of the RTP data packet source being reported upon by this
report block.
o begin_seq: 16 bits
The first sequence number that this block reports on.
o end_seq: 16 bits
The last sequence number that this block reports on plus one.
o chunk i: 16 bits
There are three chunk types: run length, bit vector, and
terminating null. These are defined in Section 4 of [RFC3611].
If the chunk is all zeroes, then it is a terminating null chunk.
Otherwise, the left-most bit of the chunk determines its type: 0
for run length and 1 for bit vector.
Note that the sequence numbers that are included in the report refer
to the primary source stream.
When using Post-repair Loss RLE reports, the amount of bandwidth
consumed by the detailed reports should be considered carefully. The
bandwidth usage rules, as they are described in [RFC3611], apply to
Post-repair Loss RLE reports as well.
4. Session Description Protocol Signaling
A new parameter is defined for the Post-repair Loss RLE Report Block
to be used with Session Description Protocol (SDP) [RFC4566] using
the Augmented Backus-Naur Form (ABNF) [RFC5234]. It has the
following syntax within the "rtcp-xr" attribute [RFC3611]:
pkt-loss-rle-post = "post-repair-loss-rle" ["=" max-size]
max-size = 1*DIGIT ; maximum block size in octets
Refer to Section 5.1 of [RFC3611] for a detailed description and the
full syntax of the "rtcp-xr" attribute. The "pkt-loss-rle-post"
parameter is compatible with the definition of "format-ext" in the
"rtcp-xr" attribute.
5. Security Considerations
The security considerations of [RFC3611] apply in this document as
well. Additional security considerations are briefly mentioned
below.
An attacker who monitors the regular Pre-repair Loss RLE reports sent
by a group of receivers in the same multicast distribution network
may infer the network characteristics (Multicast Inference of Network
Characteristics). However, monitoring the Post-repair Loss RLE
reports will not reveal any further information about the network.
Without the regular Pre-repair Loss RLE reports, the Post-repair ones
will not be any use to attackers. Even when used with the regular
Pre-repair Loss RLE reports, the Post-repair Loss RLE reports only
reveal the effectiveness of the repair process. However, this does
not enable any new attacks, nor does it provide information to an
attacker that could not be similarly obtained by watching the RTP
packets fly by himself, performing the repair algorithms and
computing the desired output.
An attacker may interfere with the repair process for an RTP stream.
In that case, if the attacker is able to see the post-repair Loss
RLEs, the attacker may infer whether or not the attack is effective.
If not, the attacker may continue attacking or alter the attack. In
practice, however, this does not pose a security risk.
An attacker may put incorrect information in the regular Pre-repair
and Post-repair Loss RLE reports such that it impacts the proactive
decisions made by the sender in the repair process or the reactive
decisions when responding to the feedback messages coming from the
receiver. A sender application should be aware of such risks and
should take the necessary precautions to minimize the chances for
(or, better, eliminate) such attacks.
Similar to other RTCP XR reports, the Post-repair Loss RLE reports
MAY be protected by using the Secure RTP (SRTP) and Secure RTP
Control Protocol (SRTCP) [RFC3711].
6. IANA Considerations
New block types for RTCP XR are subject to IANA registration. For
general guidelines on IANA considerations for RTCP XR, refer to
[RFC3611].
This document assigns the block type value 10 in the RTCP XR Block
Type Registry to "Post-repair Loss RLE Report Block". This document
also registers the SDP [RFC4566] parameter "post-repair-loss-rle" for
the "rtcp-xr" attribute in the RTCP XR SDP Parameters Registry.
The contact information for the registrations is:
Ali Begen
abegen@cisco.com
170 West Tasman Drive
San Jose, CA 95134 USA
7. Acknowledgments
The authors would like to thank the members of the VQE Team at Cisco
and Colin Perkins for their inputs and suggestions.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Caceres, R., and A. Clark, "RTP Control
Protocol Extended Reports (RTCP XR)", RFC 3611,
November 2003.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
8.2. Informative References
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
Authors' Addresses
Ali Begen
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: abegen@cisco.com
Dong Hsu
Cisco
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
EMail: dohsu@cisco.com
Michael Lague
Cisco
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
EMail: mlague@cisco.com
AI ELI5
On this page
§1Why a Post-Repair Report
RTCP is the control protocol that accompanies RTP media streams, and RTCP Extended Reports (XRs) from RFC 3611 add more detailed reports. One of them is the Loss RLE report block, which describes individual packet receipts and losses over the RTCP interval before the report was sent.
That report is usually collected on the primary source stream before any repair. Loss-repair methods such as forward error correction (RFC 5109) or retransmission (RFC 4588) may recover some lost packets, but the pre-repair report cannot show which ones, so it cannot show how well repair performed.
This RFC fixes that with a second block, the Post-repair Loss RLE, which reports the source packets that remain missing.
A first report says which video pieces went missing. A second report says which are still missing after you tried to fix them.
§1.1Comparing Before and After
When the pre-repair and post-repair reports are compared, the RTP sender or another third-party entity can judge the loss-repair methods in aggregate. To avoid ambiguity, it is RECOMMENDED that the post-repair report cover source packets that have no further chance of repair. If repair may still recover packets, the report SHOULD NOT be sent until recovery is complete.
The wait cannot be unlimited. Sequence-number wrapping in the primary stream could make a late report ambiguous, so Post-repair Loss RLE reports may not be delayed arbitrarily. If the reports are ambiguous, the sender or monitor must work out which packets a report refers to.
Like the pre-repair report, it works at packet level and counts partially repaired packets as unrepaired. Methods that only partially recover data SHOULD NOT be evaluated with it, since that may underestimate their effectiveness. The RFC only considers reporting after all repair methods have been applied, not after each method in a hybrid system.
Applications MAY report for every received packet or only for a set of specific packets, so coverage need not be contiguous.
Wait until you have finished fixing before you report what is still broken. If you wait too long, nobody can tell which pieces the report is about.
§3The Report Block Format
The block deliberately uses the same structure as the Loss RLE block in RFC 3611, so implementations can compare the two efficiently. Its fields are:
BT(8 bits): the constant10identifies a Post-repair Loss RLE block.rsvd.(4 bits): reserved, MUST be set to zero and MUST be ignored by the receiver.T, thinning (4 bits): only packets with sequence numbers 0 mod 2^T are reported.0means no thinning. The maximum thinning is one packet in every 32,768.block length(16 bits): the length of the block including the header, in 32-bit words minus one.SSRC of source(32 bits): the RTP source being reported on.begin_seqandend_seq(16 bits each): the first sequence number reported, and the last one plus one.chunk 1tochunk n(16 bits each): defined in Section 4 of RFC 3611. An all-zero chunk is a terminating null chunk. Otherwise the left-most bit gives the type, 0 for run length and 1 for bit vector.
If thinning is used, it is RECOMMENDED to use the same value in the pre-repair and post-repair reports, though different values MAY be used. The sequence numbers refer to the primary source stream. The bandwidth usage rules of RFC 3611 apply here as well.
The report is a short form with a label saying what it is, which stream it is about, the range of packet numbers, and a compact list of which ones are missing.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
BT | BT | 1 byte | 10 | Block type, the constant 10. |
RSVD | rsvd. | 4 bits | Reserved; set to zero and ignored by the receiver. | |
THINNING | T | 4 bits | Thinning of the sequence-number space; 0 means none. | |
BLOCK-LENGTH | block length | 2 bytes | Block length in 32-bit words minus one. | |
SSRC | SSRC of source | 4 bytes | The RTP source being reported on. | |
BEGIN-SEQ | begin_seq | 2 bytes | First sequence number reported. | |
END-SEQ | end_seq | 2 bytes | Last sequence number reported plus one. | |
CHUNK-1 | chunk 1 | 2 bytes | Run length, bit vector or terminating null chunk. | |
CHUNK-2 | chunk 2 | 2 bytes | Further chunks follow up to chunk n. |
§4Signaling in SDP
Endpoints announce support in the Session Description Protocol through a new parameter of the rtcp-xr attribute. The syntax, in ABNF, is pkt-loss-rle-post = "post-repair-loss-rle" ["=" max-size], where max-size is one or more digits giving the maximum block size in octets.
The full syntax of the rtcp-xr attribute is in Section 5.1 of RFC 3611, and the new parameter is compatible with its format-ext definition.
Before a call, each side can say in its setup note that it is willing to send these reports, and how big they may get.
pkt-loss-rle-post = "post-repair-loss-rle" ["=" max-size]
max-size = 1*DIGIT ; maximum block size in octets§5Security Considerations
The security considerations of RFC 3611 apply. The RFC adds that an attacker who watches pre-repair reports from receivers in a multicast network may infer network characteristics, but post-repair reports reveal nothing more, and without the pre-repair reports they are of no use to an attacker. They only show how effective repair was, which an attacker could work out from watching the RTP packets and running the repair algorithms.
An attacker interfering with repair could see from the reports whether the attack works. The RFC says this does not pose a security risk in practice. A forged report in either block could mislead the sender's proactive or reactive repair decisions, so a sender application should be aware of this and take precautions. Like other RTCP XR reports, the block MAY be protected with SRTP and SRTCP.
The report mostly tells a spy what they could have worked out anyway. The bigger worry is someone sending a fake report to confuse the sender, so senders should be careful.
§6IANA Registrations
The RFC assigns block type value 10 in the RTCP XR Block Type Registry to the Post-repair Loss RLE Report Block. It also registers the SDP parameter post-repair-loss-rle for the rtcp-xr attribute in the RTCP XR SDP Parameters Registry. General guidance for new XR block types is in RFC 3611.
The number 10 and the name for this report were written in the official list so nobody else uses them for something different.