Internet DRAFT - draft-gharai-avtcore-rtp-tfrc
draft-gharai-avtcore-rtp-tfrc
Network Working Group L. Gharai
Internet-Draft
Expires: March 9, 2012 C. Perkins
University of Glasgow
September 6, 2011
RTP with TCP Friendly Rate Control
draft-gharai-avtcore-rtp-tfrc-01
Abstract
This memo specifies how the TCP Friendly Rate Control (TFRC) of RTP
flows can be supported using the RTP/AVPF profile and the general RTP
header extension mechanism. AVPF feedback packets and RTP header
extensions are defined to support the exchange of control information
between RTP TFRC senders and receivers. TFRC is an equation-based
congestion control scheme for unicast flows operating in a best
effort Internet environment.
Status of this Memo
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Relation to the Datagram Congestion Control Protocol . . . . . 3
4. The TFRC Information Exchange Loop . . . . . . . . . . . . . . 5
4.1. Data Packets . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Feedback Packets . . . . . . . . . . . . . . . . . . . . . 5
5. The Header Extension . . . . . . . . . . . . . . . . . . . . . 6
6. TFRC-FB: A New AVPF Transport Layer Feedback Message . . . . . 7
7. RTCP Transmission Intervals and Bandwidth Requirements . . . . 8
8. SDP Usoage . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Usage with the SDP Offer/Answer Model . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 11
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 11
12. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
[Note to RFC Editor: All references to RFC XXXX are to be replaced
with the RFC number of this memo, when published]
This memo specifies how the TCP Friendly Rate Control (TFRC) of RTP
flows can be supported using the RTP/AVPF [RFC3550][RFC4585] profile
and RTP header extensions, by defining a new header extension and
AVPF feedback packet, and related parameters. Any of the AVPF based
RTP profiles, such as SAVPF, can be used to support TFRC RTP flows.
TFRC is an equation-based congestion control scheme for unicast flows
operating in a best effort Internet environment and competing with
TCP traffic. TFRC computes a TCP-friendly data rate based on current
network conditions, as represented by the latest round trip time and
packet loss calculations. The complete TFRC mechanism is described
in detail in [RFC3448].
To calculate a TCP-friendly data rate and keep track of round trip
times and packet losses, TFRC senders and receivers rely on
exchanging specific information between each other, i.e: the sender
provides the receiver with the latest updates to round trip time
calculations, while the receiver provides feedback needed to compute
round trip times and on packet losses. This memo defines how this
information can be exchanged between TFRC senders and receiver with
RTP header extensions and an AVPF feedback packet.
2. Terminology
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 RFC 2119 [RFC2119].
3. Relation to the Datagram Congestion Control Protocol
The TFRC congestion control mechanism is one of a set of congestion
control methods provided by the Datagram Congestion Control Protocol
(DCCP) [RFC4340] In this section we detail the pros and cons of using
TFRC with RTP versus DCCP.
DCCP is a minimal general purpose transport-layer protocol with
unreliable yet congestion controlled packet delivery semantics and
reliable connection setup and teardown [RFC4336]. DCCP currently
supports both TFRC [RFC4342] and TCP-like [RFC4341] congestion
control, and the protocol is structured to support new congestion
control mechanisms defined in the future. A DCCP mapping for RTP has
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been standardized for media applications [RFC5762]. In addition DCCP
supports a host of other features, such as: use of Explicit
Congestion Notification (ECN) and the ECN Nonce, flexible options
processing, reliable option negotiation, DTLS [RFC5238], Path Maximum
Transfer Unit (PMTU) and Service Codes. Naturally an application
using RTP/DCCP as its transport protocol will benefit from the
protocol features supported by DCCP.
However there are a number of benefits to be gained by the
development and standardization of the use RTP with TFRC:
o Media applications lacking congestion control can incorporate
congestion controlled transport without delay by using RTP with
TFRC. Widespread deployment of the DCCP protocol is not currently
in place.
o Use of RTP with TFRC is not contingent on any OS level changes and
can be quickly deployed, because RTP is implemented at the
application layer.
o RTP/UDP flows face the same restrictions in firewall traversal as
do UDP flows and do not require NATs and firewall modifications.
DCCP flows, on the other hand, do require NAT and firewall
modifications, however once these modifications are in place, they
can result in easier NAT and firewall traversal for RTP/DCCP flows
in the future.
o Use of RTP with TFRC with various media applications will give
researchers, implementors and developers a better understanding of
the intricate relationship between media quality and equation-
based congestion control. Hopefully this experience with
congestion control and TFRC will ease the migration of media
applications to DCCP once DCCP is deployed.
Using the AVPF/RTP profile and header extension to support TFRC
provides an immediate means for congestion control in media streams,
in the time until DCCP is deployed.
Additionally, there are also a number of technical differences as to
how (and which) congestion control information is exchanged between
DCCP with CCID3 and RTP:
o Using header extensions the RTP TFRC sender transmits a send
timestamp to the RTP TFRC receiver with every data packet. This
send timestamp is used by TFRC congestion control, but may also be
used by the receiver for jitter calculation. In contrast DCCP
with CCID3 transmits a quad round trip counter to the receiver.
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o The RTP TFRC receiver only provides the RTP TFRC sender with the
loss event rate as computed by the receiver. In contrast DCCP
with CCID3, provides 2 other options for the transport of loss
event rate. A sender may choose to receive loss intervals or an
Ack Vector. These two options provide the sender with the
necessary information to compute the loss event rate.
o Sequence number: DCCP supports a 48 bit and a 24 bit sequence
number, whereas RTP only supports a 16 bit sequence number. While
this makes RTP susceptible to data injection attacks, it can be
avoided by using the SRTP [RFC3711] profile.
4. The TFRC Information Exchange Loop
TFRC depends on the exchange of congestion control information
between a sender and receiver. In this section we reiterate which
items are exchanged between a TFRC sender and receiver as discussed
in [RFC3448]. We note how RTP can accommodates these exchanges.
4.1. Data Packets
As stated in [RFC3448] a TFRC sender transmits the following
information in each data packet to the receiver:
o A sequence number, incremented by one for each data packet
transmitted.
o A timestamp indicating the packet send time and the sender's
current estimate of the round-trip time, RTT. This information is
then used by the receiver to compute the TFRC loss intervals. - or
- A course-grained timestamp incrementing every quarter of a round
trip time, which is then used to determine the TFRC loss
intervals.
The standard RTP sequence number suffices for the functionality
provided by TFRC. A RTP header extension [hdrtxt] is used to
transmit the send timestamp and RTT. This extension is defined in
Section 5.
4.2. Feedback Packets
As stated in [RFC3448] a TFRC receiver provides the following
feedback to the sender at least once per RTT or per data packet
received (which ever time interval is larger):
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o The send timestamp of the last data packet received, t_i.
o The amount of time elapsed between the receipt of the last data
packet at the receiver, and the generation of this feedback
report, t_delay. This is used by the sender for RTT computations.
o The rate at which the receiver estimates that data was received
since the last feedback report was sent, x_recv.
o The receiver's current estimate of the loss event rate, p, a real
value between 0 and 1.0.
To accommodate the feedback of these values a new AVPF transport
layer feedback message is defined, as detailed in Section 6. The
timing interval between the feedback packets is discussed in
Section 7.
5. The Header Extension
The form of the extension block is depicted in Figure 1. The length
field for the extension takes the value 6 to indicate that the
payload is 7 bytes. Two header extension fields are defined and used
as follows:
Send timestamp (t_i): 32 bits
The timestamp indicating when the packet is sent. This timestamp
is measured in microseconds and is used for round trip time
calculations.
Round trip time (RTT): 24 bits
The round trip time as measured by the RTP TFRC sender in
microseconds.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0xBE | 0xDE | length=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID | len=6 | RTT |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send timestamp (t_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP Sender Header Extension Block
Figure 1
6. TFRC-FB: A New AVPF Transport Layer Feedback Message
A new transport layer AVPF feedback message is defined to support
feedback from the receivers: TFRC-FB. Figure 2 depicts the both the
common packet format (the first 12 octets) and the feedback control
information (FCI) for the TFRC-FB packet.
We note that the TFRC related feedback, is specific to one media
stream sender, therefore all messages in the compound RTCP packet
MUST share the same media source SSRC. In the case where a sender is
sending multiple media streams to a receiver, each media flow will be
allocated its own AVPF feedback flow.
We define four FCI fields for the TFRC-FB message as follows:
Send timestamp (t_i): 32 bits
The send timestamp of the last data packet received by the RTP
TFRC receiver, t_i, in microseconds.
Delay (t_delay): 32 bits
The amount of time elapsed between the receipt of the last data
packet at the RTP TFRC receiver, and the generation of this
feedback report in microseconds. This is used by the RTP TFRC
sender for RTT computations.
Data rate (X_recv): 32 bits
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The rate at which the receiver estimates that data was received
since the last feedback report was sent in bytes per second.
X_recv is computed per [RFC3448].
Loss event rate (p): 32 bits
The receiver's current estimate of the loss event rate, p,
expressed as a fixed point number with the binary point at the
left edge of the field. (That is equivalent to taking the integer
part after multiplying the loss event rate by 2^32.) The value of
the loss event rate is computed per [RFC3448] Section 5.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P| FMT=2 | PT=RTPFB | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| send timestamp (t_i) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay (t_delay) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data rate at the receiver (x_recv) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| loss event rate (p) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 2: The AVPF TFRC-FB RTCP Transport Layer Feedback Message
Figure 2
7. RTCP Transmission Intervals and Bandwidth Requirements
When using TFRC rate controlled RTP, the RTCP transmission intervals
must be set according to the requirements of the TFRC algorithm.
TFRC requires a receiver to generate a feedback ack packet at least
once per RTT or per packet received (based on the larger time
interval). These requirements are to ensure timely reaction to
congestion.
The TFRC requirements of receiving feedback once per RTT can at times
conflict with the AVP RTCP bandwidth constraints, particularly at
small RTTs of 20 ms or less. Assuming only one TFRC-FB report per
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RTCP compound packet, Figure 3 lists the RTCP bandwidths at RTTs of
2, 5, 10 and 20 ms and the minimum corresponding RTP data rates,
where RTCP(X) <= (0.05)*RTP(X) is true. For example, according to
Table 1, a TFRC RTP flow of less than 3.2 Mbps and a RTT of 5 ms, can
not comply with the 5% RTCP bandwidth constraints (Table 1 assumes
each RTCP packet is 100 bytes). RTP flows facing such circumstances
should take into account the additional RTCP bandwidth needed when
signaling their bandwidth information in SDP [RFC4566].
Based on initial assumptions on round trip time if more than the
recommended 5% is needed for RTCP bandwidth, the applications SHOULD
use the SDP bandwidth modifiers RS and RR [RFC3556] to signal the
amount of RTCP bandwidth needed. If the round trip time assumptions
change after the RTP flows start running, the application MAY
recalculate the amount of RTCP bandwidth needed and re-signal this
new value using its signaling protocol of choice.
RTT RTCP(X) RTP(X)
+------------------------------+
| 20 ms | 40 kbps | 0.8 Mbps |
| 10 ms | 80 kbps | 1.6 Mbps |
| 5 ms | 160 kbps | 3.2 Mbps |
| 2 ms | 400 kbps | 8.0 Mbps |
+------------------------------+
RTCP bandwidth for TFRC flows with corresponding RTTs of 20, 10, 5
and 2 ms. Assuming, 100 byte RTCP packets and one RTCP packet per
RTT.
Figure 3
Additionally, to support the transmission of a feedback packet once
per RTT, the AVPF T_rr_interval variable MUST NOT be set to a value
larger than the current round trip time, RTT, as this would prevent
generating feedback packets at least once per RTT (see [RFC4585],
Section 3.4).
8. SDP Usoage
RTP flows using TFRC congestion control MUST signal their use of the
AVPF profile and RTCP feedback packets, the round trip time (RTT) and
send timestamp extension, and MAY also signal an initial RTCP
bandwidth usage:
v=0
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o=alice 2890844526 2890844526 IN IP4 host.example.com
s=congestion control with TFRC
c=IN IP4 host.example.com
m=video 5400 RTP/AVPF 112
a=rtpmap:112 H261/90000
a=extmap:4 urn:ietf:params:rtp-hdtext:rtt-sendts
a=rtcp-fb * tfrc
b=AS:400
b=RS:800
b=RR:4000
8.1. Usage with the SDP Offer/Answer Model
TBC
9. IANA Considerations
In this section we detail IANA registry values that need to be
registered. Two new values must be reistered for the AVPF profile:
The new RTP/AVPF feedback packet, TFRC-FB. The following format
(FMT) values must be registerd in the FMT sub-registry of the RTPFB
payload type:
Value name:TFRC-FB
Long name: TFRC feedback
Value: 5
Reference: RFC XXXX
The new rtcp-fd-id "tfrc" must be registered with the "rtcp-fb"
attribute registry:
Value name: tfrc
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Long name: TFRC Feedback
Reference: RFC XXXX
For the new header extension, the name rtt-sendts must be registered
into the rtp-hdrext section of the urn:ietf: namespace, referring to
RFC XXXX.
10. Security Considerations
This memo defines how to use the RTP AVPF profile and the general RTP
header extensions to support TFRC congestion control. Therefore RTP
packets using these mechanisms are subject to the security
considerations discussed in the RTP specification [RFC3550], the RTP/
AVPF profile specification [RFC4585] and the general header
extensions mechanism [RFC5285]. Combining these mechanisms does not
pose any additional security implications. Applications requiring
authentication and integrity protection, or applictions operating in
environments that must strictly adhere to the TFRC send rate (and
fear manipulation of the feedback messsages) can use the SAVPF
[RFC5124] profile.
11. Acknowledgments
This memo is based upon work supported by the U.S. National Science
Foundation (NSF) under Grant No. 0334182. Any opinions, findings and
conclusions or recommendations expressed in this material are those
of the authors and do not necessarily reflect the views of NSF.
12. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3448] Handley, M., Floyd, S., Padhye, J., and J. Widmer, "TCP
Friendly Rate Control (TFRC): Protocol Specification",
RFC 3448, January 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, July 2003.
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[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4336] Floyd, S., Handley, M., and E. Kohler, "Problem Statement
for the Datagram Congestion Control Protocol (DCCP)",
RFC 4336, March 2006.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4341] Floyd, S. and E. Kohler, "Profile for Datagram Congestion
Control Protocol (DCCP) Congestion Control ID 2: TCP-like
Congestion Control", RFC 4341, March 2006.
[RFC4342] Floyd, S., Kohler, E., and J. Padhye, "Profile for
Datagram Congestion Control Protocol (DCCP) Congestion
Control ID 3: TCP-Friendly Rate Control (TFRC)", RFC 4342,
March 2006.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
July 2006.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5238] Phelan, T., "Datagram Transport Layer Security (DTLS) over
the Datagram Congestion Control Protocol (DCCP)",
RFC 5238, May 2008.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC5762] Perkins, C., "RTP and the Datagram Congestion Control
Protocol (DCCP)", RFC 5762, April 2010.
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Authors' Addresses
Ladan Gharai
Email: ladan@gharai.org
Colin Perkins
University of Glasgow
School of Computing Science
Glasgow G12 8QQ
United Kingdom
Email: csp@csperkins.org
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