Internet DRAFT - draft-ietf-avtext-rtp-duplication
draft-ietf-avtext-rtp-duplication
AVTEXT A. Begen
Internet-Draft Cisco
Intended status: Standards Track C. Perkins
Expires: August 24, 2014 University of Glasgow
February 20, 2014
Duplicating RTP Streams
draft-ietf-avtext-rtp-duplication-06
Abstract
Packet loss is undesirable for real-time multimedia sessions, but can
occur due to a variety of reasons including unplanned network
outages. In unicast transmissions, recovering from such an outage
can be difficult depending on the outage duration due to the
potential large number of missing packets. In multicast
transmissions, recovery is even more challenging as many receivers
could be impacted by the outage. One solution to this challenge
without incurring unbounded delay is to duplicate the packets and
send them in separate redundant streams, provided that the underlying
network satisfies certain requirements. This document explains how
Real-time Transport Protocol (RTP) streams can be duplicated without
breaking RTP or RTP Control Protocol (RTCP) rules.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 24, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Requirements Notation . . . . . . . . . . . . 3
3. Dual Streaming Use Cases . . . . . . . . . . . . . . . . . . 3
3.1. Temporal Redundancy . . . . . . . . . . . . . . . . . . . 4
3.2. Spatial Redundancy . . . . . . . . . . . . . . . . . . . 4
3.3. Dual Streaming over a Single Path or Multiple Paths . . . 5
3.4. Requirements . . . . . . . . . . . . . . . . . . . . . . 6
4. Use of RTP and RTCP with Temporal Redundancy . . . . . . . . 6
4.1. RTCP Considerations . . . . . . . . . . . . . . . . . . . 6
4.2. Signaling Considerations . . . . . . . . . . . . . . . . 7
5. Use of RTP and RTCP with Spatial Redundancy . . . . . . . . . 8
5.1. RTCP Considerations . . . . . . . . . . . . . . . . . . . 8
5.2. Signaling Considerations . . . . . . . . . . . . . . . . 9
6. Use of RTP and RTCP with Temporal and Spatial Redundancy . . 9
7. Congestion Control Considerations . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
11.1. Normative References . . . . . . . . . . . . . . . . . . 11
11.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Real-time Transport Protocol (RTP) [RFC3550] is widely used today
for delivering IPTV traffic, and other real-time multimedia sessions.
Many of these applications support very large numbers of receivers,
and rely on intra-domain UDP/IP multicast for efficient distribution
of traffic within the network.
While this combination has proved successful, there does exist a
weakness. As [RFC2354] noted, packet loss is not avoidable. This
loss might be due to congestion; it might also be a result of an
unplanned outage caused by a flapping link, link or interface
failure, a software bug, or a maintenance person accidentally cutting
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the wrong fiber. Since UDP/IP flows do not provide any means for
detecting loss and retransmitting packets, it is left up to the RTP
layer and the applications to detect, and recover from, packet loss.
In a carefully managed network, congestion should not normally
happen, however, network outages can still happen due to the reasons
listed above. In such a managed network, one technique to recover
from packet loss without incurring unbounded delay is to duplicate
the packets and send them in separate redundant streams. As
described later in this document, the probability that two copies of
the same packet are lost in cases of non-congestive packet loss is
quite small.
Variations on this idea have been implemented and deployed today
[IC2011]. However, duplication of RTP streams without breaking the
RTP and RTCP functionality has not been documented properly. This
document discusses the most common use cases and explains how
duplication can be achieved for RTP streams in such use cases to
address the immediate market needs. In the future, if there will be
a different use case, which is not covered by this document, a new
specification that explains how RTP duplication should be done in
such a scenario may be needed.
Stream duplication offers a simple way to protect media flows from
packet loss. It has a comparatively high bandwidth overhead, since
everything is sent twice, but with a low processing overhead. It is
also very predictable in its overheads. Alternative approaches, for
example, retransmission-based recovery [RFC4588] or Forward Error
Correction [RFC6363], may be suitable in some other cases.
2. Terminology and Requirements Notation
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
[RFC2119].
3. Dual Streaming Use Cases
Dual streaming refers to a technique that involves transmitting two
redundant RTP streams (the original plus its duplicate) of the same
content, with each stream capable of supporting the playback when
there is no packet loss. Therefore, adding an additional RTP stream
provides a protection against packet loss. The level of protection
depends on how the packets are sent and transmitted inside the
network.
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It is important to note that dual streaming can easily be extended to
support cases when more than two streams are desired. However, using
three or more streams is rare in practice, due to the high overhead
that it incurs and the little additional protection it provides.
3.1. Temporal Redundancy
From a routing perspective, two streams are considered identical if
the following two IP header fields are the same (in addition to the
transport ports), since they will be both routed over the same path:
o IP Source Address
o IP Destination Address
Two routing-plane identical RTP streams might carry the same payload,
but can use different Synchronization Sources (SSRC) to differentiate
the RTP packets belonging to each stream. In the context of dual RTP
streaming, we assume that the sender duplicates the RTP packets and
sends them in separate RTP streams, each with a unique SSRC. All the
redundant streams are transmitted in the same RTP session.
For example, one main stream and its duplicate stream can be sent to
the same IP destination address and UDP destination port with a
certain delay between them [I-D.ietf-mmusic-delayed-duplication].
The streams carry the same payload in their respective RTP packets
with identical sequence numbers. This allows receivers (or other
nodes responsible for gap filling and duplicate suppression) to
identify and suppress the duplicate packets, and subsequently produce
a hopefully loss-free and duplication-free output stream. This
process is commonly called stream merging or de-duplication.
3.2. Spatial Redundancy
An RTP source might be associated with multiple network interfaces,
allowing it to send two redundant streams from two separate source
addresses. Such streams can be routed over diverse or identical
paths depending on the routing algorithm used inside the network. At
the receiving end, the node responsible for duplicate suppression can
look into various RTP header fields, for example SSRC and sequence
number, to identify and suppress the duplicate packets.
If source-specific multicast (SSM) transport is used to carry such
redundant streams, there will be a separate SSM session for each
redundant stream since the streams are sourced from different
interfaces (i.e., IP addresses). Thus, the receiving host has to
join each SSM session separately.
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Alternatively, destination host could also have multiple IP addresses
for an RTP source to send the redundant streams to.
3.3. Dual Streaming over a Single Path or Multiple Paths
Having described the characteristics of the streams, one can reach
the following conclusions:
1. When two routing-plane identical streams are used, the flow
labels will be the same. This makes it impractical to forward
the packets onto different paths. In order to minimize packet
loss, the packets belonging to one stream are often interleaved
with packets belonging to its duplicate stream, and with a delay,
so that if there is a packet loss, such a delay would allow the
same packet from the duplicate stream to reach the receiver
because the chances that the same packet is lost in transit again
is often small. This is what is also known as Time-shifted
Redundancy, Temporal Redundancy or simply Delayed Duplication
[I-D.ietf-mmusic-delayed-duplication] [IC2011]. This approach
can be used with both types of dual streaming, described in
Section 3.1 and Section 3.2.
2. If the two streams have different IP headers, an additional
opportunity arises in that one is able to build a network, with
physically diverse paths, to deliver the two streams concurrently
to the intended receivers. This reduces the delay when packet
loss occurs and needs to be recovered. Additionally, it also
further reduces chances for packet loss. An unrecoverable loss
happens only when two network failures happen in such a way that
the same packet is affected on both paths. This is referred to
as Spatial Diversity or Spatial Redundancy [IC2011]. The
techniques used to build diverse paths are beyond the scope of
this document.
Note that spatial redundancy often offers less delay in
recovering from packet loss provided that the forwarding delay of
the network paths are more or less the same (This is often made
sure through careful network design). For both temporal and
spatial redundancy approaches, packet misordering might still
happen and needs to be handled using the sequence numbers of some
sort (e.g., RTP sequence numbers).
Temporal and spatial redundancy deal with different patterns of
packet loss. The former helps with transient loss (within the
duplication window), while the latter helps with longer-term packet
loss that affects only one of the two redundant paths.
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To summarize, dual streaming allows an application and a network to
work together to provide a near zero-loss transport with a bounded or
minimum delay. The additional advantage includes a predictable
bandwidth overhead that is proportional to the minimum bandwidth
needed for the multimedia session, but independent of the number of
receivers experiencing a packet loss and requesting a retransmission.
For a survey and comparison of similar approaches, refer to [IC2011].
3.4. Requirements
One of the following conditions is currently REQUIRED to hold in
applications using this specification:
o The original and duplicate RTP streams are carried (with their own
SSRCs) in the same "m" line (There could be other RTP streams
listed in the same "m" line).
o The original and duplicate RTP streams are carried in separate "m"
lines and there is no other RTP stream listed in either "m" line.
When the original and duplicate RTP streams are carried in separate
"m" lines in a Session Description Protocol (SDP) description and if
the SDP description has one or more other RTP streams listed in
either "m" line, duplication grouping is not trivial and further
signaling will be needed, which is left for future standardization.
4. Use of RTP and RTCP with Temporal Redundancy
To achieve temporal redundancy, the main and duplicate RTP streams
SHOULD be sent using the sample 5-tuple of transport protocol, source
and destination IP addresses, and source and destination transport
ports. Due to the possible presence of network address and port
translation (NAPT) devices, load balancers, or other middleboxes, use
of anything other than an identical 5-tuple and flow label might also
cause spatial redundancy (which might introduce an additional delay
due to the delta between the path delays), and so is NOT RECOMMENDED
unless the path is known to be free of such middleboxes.
Since the main and duplicate RTP streams follow an identical path,
they are part of the same RTP session. Accordingly, the sender MUST
choose a different SSRC for the duplicate RTP stream than it chose
for the main RTP stream, following the rules in [RFC3550] Section 8.
4.1. RTCP Considerations
If RTCP is being sent for the main RTP stream, then the sender MUST
also generate RTCP for the duplicate RTP stream. The RTCP for the
duplicate RTP stream is generated exactly as-if the duplicate RTP
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stream were a regular media stream. The sender MUST NOT duplicate
the RTCP packets sent for the main RTP stream when sending the
duplicate stream, instead it MUST generate new RTCP reports for the
duplicate stream. The sender MUST use the same RTCP CNAME in the
RTCP reports it sends for both streams, so that the receiver can
synchronize them.
The main and duplicate streams are conceptually synchronized using
the standard RTCP Sender Report-based mechanism, deriving a mapping
between their timelines. However, the RTP timestamps and sequence
numbers MUST be identical in the main and duplicate streams, making
the mapping quite trivial.
Both the main and duplicate RTP streams, and their corresponding RTCP
reports, will be received. If RTCP is used, receivers MUST generate
RTCP reports for both the main and duplicate streams in the usual
way, treating them as entirely separate media streams.
4.2. Signaling Considerations
Signaling is needed to allow the receiver to determine that an RTP
stream is a duplicate of another, rather than a separate stream that
needs to be rendered in parallel. There are two parts to this: an
SDP extension is needed in the offer/answer exchange to negotiate
support for temporal redundancy; and signaling is needed to indicate
which stream is the duplicate (the latter can be done in-band using
an RTCP extension, or out-of-band in the SDP description).
Out-of-band signalling is needed for both features. The SDP
attribute to signal duplication in the SDP offer/answer exchange
('duplication-delay') is defined in
[I-D.ietf-mmusic-delayed-duplication]. The required SDP grouping
semantics are defined in [RFC7104].
In the following SDP example, a video stream is duplicated, and the
main and duplicate streams are transmitted in two separate SSRCs
(1000 and 1010):
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v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=Delayed Duplication
t=0 0
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
a=rtpmap:100 MP2T/90000
a=ssrc:1000 cname:ch1a@example.com
a=ssrc:1010 cname:ch1a@example.com
a=ssrc-group:DUP 1000 1010
a=duplication-delay:50
a=mid:Ch1
Section 3.2 of [RFC7104] states that it is advisable that the SSRC
listed first in the "a=ssrc-group:" line (i.e., SSRC of 1000) is sent
first, with the other SSRC (i.e., SSRC of 1010) being the time-
delayed duplicate. This is not critical, however, and a receiving
host should size its playout buffer based on the 'duplication-delay'
attribute, and play the stream that arrives first in preference, with
the other stream acting as a repair stream, irrespective of the order
in which they are signaled.
5. Use of RTP and RTCP with Spatial Redundancy
Assuming the network is structured appropriately, when using spatial
redundancy, the duplicate RTP stream is sent using a different source
and/or destination address/port pair. This will be a separate RTP
session to the session conveying the main RTP stream. Thus, the
SSRCs used for the main and duplicate streams MUST be chosen
randomly, following the rules in Section 8 of [RFC3550].
Accordingly, they will almost certainly not match each other. The
sender MUST, however, use the same RTCP CNAME for both the main and
duplicate streams. An "a=group:DUP" line or "a=ssrc-group:DUP" line
is used to indicate duplication.
5.1. RTCP Considerations
If RTCP is being sent for the main RTP stream, then the sender MUST
also generate RTCP for the duplicate RTP stream. The RTCP for the
duplicate RTP stream is generated exactly as-if the duplicate RTP
stream were a regular media stream. The sender MUST NOT duplicate
the RTCP packets sent for the main RTP stream when sending the
duplicate stream, instead it MUST generate new RTCP reports for the
duplicate stream. The sender MUST use the same RTCP CNAME in the
RTCP reports it sends for both streams, so that the receiver can
synchronize them.
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The main and duplicate streams are conceptually synchronized using
the standard RTCP Sender Report-based mechanism, deriving a mapping
between their timelines. However, the RTP timestamps and sequence
numbers MUST be identical in the main and duplicate streams, making
the mapping quite trivial.
Both the main and duplicate RTP streams, and their corresponding RTCP
reports, will be received. If RTCP is used, receivers MUST generate
RTCP reports for both the main and duplicate streams in the usual
way, treating them as entirely separate media streams.
5.2. Signaling Considerations
The required SDP grouping semantics have been defined in [RFC7104].
In the following example, the redundant streams have different IP
destination addresses. The example shows the same UDP port number
and IP source address for each stream, but either or both could have
been different for the two streams.
v=0
o=ali 1122334455 1122334466 IN IP4 dup.example.com
s=DUP Grouping Semantics
t=0 0
a=group:DUP S1a S1b
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
a=rtpmap:100 MP2T/90000
a=mid:S1a
m=video 30000 RTP/AVP 101
c=IN IP4 233.252.0.2/127
a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
a=rtpmap:101 MP2T/90000
a=mid:S1b
6. Use of RTP and RTCP with Temporal and Spatial Redundancy
This uses the same RTP/RTCP mechanisms from Sections Section 4 and
Section 5, plus a combination of both sets of signaling.
7. Congestion Control Considerations
Duplicating RTP streams has several considerations in the context of
congestion control. First of all, RTP duplication MUST NOT be used
in cases where the primary cause of packet loss is congestion since
duplication can make congestion only worse. Furthermore, RTP
duplication SHOULD NOT be used where there is a risk of congestion
upon duplicating an RTP stream. Duplication is RECOMMENDED only to
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be used for protection against network outages due to a temporary
link or network element failure and where it is known (e.g., through
explicit operator configuration) that there is sufficient network
capacity to carry the duplicated traffic. The capacity requirement
constrains the use of duplication to managed networks, and makes it
unsuitable for use on unmanaged public networks.
It is essential that the nodes responsible for the duplication and
de-duplication are aware of the original stream's requirements and
the available capacity inside the network. If there is an adaptation
capability for the original stream, these nodes have to assume the
same adaptation capability for the duplicated stream, too. For
example, if the source doubles the bitrate for the original stream,
the bitrate of the duplicate stream will also be doubled.
Depending on where de-duplication takes place, there could be
different scenarios. When the duplication and de-duplication takes
place inside the network before the ultimate end-points that will
consume the RTP media, the whole process is transparent to these end-
points. Thus, these end-points will apply any congestion control, if
applicable, on the de-duplicated RTP stream. This output stream will
have less losses than either of the original and duplicated stream,
and the end-point will make congestion control decisions accordingly.
However, if de-duplication takes place at the ultimate end-point,
this end-point MUST consider the aggregate of the original and
duplicated RTP stream in any congestion control it wants to apply.
The end-point will observe the losses in each stream separately, and
this information can be used to fine-tune the duplication process.
For example, the duplication interval can be adjusted based on the
duration of a common packet loss in both streams. In these
scenarios, the RTP Monitoring Framework[RFC6792] can be used to
monitor the duplicated streams in the same way an ordinary RTP would
be monitored.
8. Security Considerations
The security considerations of [RFC3550],
[I-D.ietf-mmusic-delayed-duplication], [RFC7104], and any RTP
profiles and payload formats in use apply.
Duplication can be performed end-to-end, with the media sender
generating a duplicate RTP stream, and the receiver(s) performing de-
duplication. In such cases, if the original media stream is to be
authenticated (e.g., using SRTP [RFC3711]) then the duplicate stream
also needs to be authenticated, and duplicate packets that fail the
authentication check need to be discarded.
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Stream duplication and de-duplication can also be performed by in-
network middleboxes. Such middleboxes will need to rewrite the RTP
SSRC such that the RTP packets in the duplicate stream have a
different SSRC to the original stream, and will need to generate and
respond to RTCP packets corresponding to the duplicate stream. This
sort of in-network duplication service has the potential to act as an
amplifier for denial-of-service attacks if the attacker can cause
attack traffic to be duplicated. To prevent this, middleboxes
providing the duplication service need to authenticate the traffic to
be duplicated as being from a legitimate source, for example using
the secure RTP (SRTP) profile [RFC3711]. This requires the middlebox
to be part of the security context of the media session being
duplicated, so it has access to the necessary keying material for
authentication. To do this, the middlebox will need to be privy to
the session set-up signalling. Details of how that is done will
depend on the type of signalling used (SIP, RTSP, WebRTC, etc.), and
is not specified here.
Similarly, to prevent packet injection attacks, a de-duplication
middlebox needs to authenticate original and duplicate streams, and
ought not use non-authenticated packets that are received. Again,
this requires the middlebox to be part of the security context, and
have access to the appropriate signalling and keying material.
The use of the encryption features of SRTP does not affect stream de-
duplication middleboxes, since the RTP headers are sent in the clear.
9. IANA Considerations
No IANA actions are required.
10. Acknowledgments
Thanks to Magnus Westerlund for his suggestions.
11. References
11.1. Normative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[I-D.ietf-mmusic-delayed-duplication]
Begen, A., Cai, Y., and H. Ou, "Delayed Duplication
Attribute in the Session Description Protocol", draft-
ietf-mmusic-delayed-duplication-03 (work in progress),
December 2013.
[RFC7104] Begen, A., Cai, Y., and H. Ou, "Duplication Grouping
Semantics in the Session Description Protocol", RFC 7104,
January 2014.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
11.2. Informative References
[RFC2354] Perkins, C. and O. Hodson, "Options for Repair of
Streaming Media", RFC 2354, June 1998.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363, October 2011.
[RFC6792] Wu, Q., Hunt, G., and P. Arden, "Guidelines for Use of the
RTP Monitoring Framework", RFC 6792, November 2012.
[IC2011] Evans, J., Begen, A., Greengrass, J., and C. Filsfils,
"Toward Lossless Video Transport (to appear in IEEE
Internet Computing)", November 2011.
Authors' Addresses
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
CANADA
Email: abegen@cisco.com
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Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
UK
Email: csp@csperkins.org
URI: http://orcid.org/0000-0002-3404-8964
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