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)
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 32. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 43. Post-Repair Loss RLE Report Block . . . . . . . . . . . . . . . 44. Session Description Protocol Signaling . . . . . . . . . . . . 65. Security Considerations . . . . . . . . . . . . . . . . . . . . 66. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 77. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 88. References . . . . . . . . . . . . . . . . . . . . . . . . . . 88.1. Normative References . . . . . . . . . . . . . . . . . . . 88.2. Informative References . . . . . . . . . . . . . . . . . . 8
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
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
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
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
5. Security Considerations
The security considerations of [RFC3611] apply in this document as
well. Additional security considerations are briefly mentioned
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
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:
170 West Tasman Drive
San Jose, CA 95134 USA
The authors would like to thank the members of the VQE Team at Cisco
and Colin Perkins for their inputs and suggestions.
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,
[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,
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
170 West Tasman Drive
San Jose, CA 95134
1414 Massachusetts Ave.
Boxborough, MA 01719
1414 Massachusetts Ave.
Boxborough, MA 01719