One document matched: draft-ietf-avt-tfrc-profile-06.txt
Differences from draft-ietf-avt-tfrc-profile-05.txt
Internet Engineering Task Force AVT WG
INTERNET-DRAFT Ladan Gharai
draft-ietf-avt-tfrc-profile-06.txt USC/ISI
6 September 2006
Expires: March 2007
RTP Profile for TCP Friendly Rate Control
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This memo specifies a profile called "RTP/AVPFCC" for the use of the
real-time transport protocol (RTP) and its associated control
protocol, RTCP, with TCP Friendly Rate Control (TFRC). This profile
extends the RTP/AVPF profile with RTP data header additions and new
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AVPF feedback packets, in order to support TFRCs integration with
RTP. TFRC is an equation based congestion control scheme for unicast
flows operating in a best effort Internet environment. This profile
provides RTP flows with the mechanism to use congestion control in
best effort IP networks.
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 defines a profile called "RTP/AVPFCC" for the use of the
real-time transport protocol (RTP) [RTP] and its associated control
protocol, RTCP, with the TCP Friendly Rate Control (TFRC) [TFRC].
RTP/AVPFCC extends the RTP profile for RTCP-based feedback (RTP/AVPF)
to provide support for TFRC.
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 [TFRC].
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. The RTP/AVPFCC profile,
extends the RTP/AVPF profile with RTP data header additions and new
AVPF feedback packets to support the TFRC feedback and information
exchange requirements.
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 [2119].
3. Relation to the Datagram Congestion Control Protocol
The TFRC congestion control mechanism is also supported by the
Datagram Congestion Control Protocol (DCCP). In this section we
detail the pros and cons of using TFRC with RTP versus DCCP.
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DCCP is a minimal general purpose transport-layer protocol with
unreliable yet congestion controlled packet delivery semantics and
reliable connection setup and teardown. DCCP currently supports both
TFRC and TCP-like congestion control, and the protocol is structured
to support new congestion control mechanisms defined in the future.
In addition DCCP supports a host of other features, such as: use of
Explicit Congestion Notification (ECN) and the ECN Nonce, reliable
option negotiation and Path Maximum Transfer Unit (PMTU). 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 RTP/AVPFCC profile:
o Media applications lacking congestion control can incorporate
congestion controlled transport without delay by using the
RTP/AVPFCC profile. The DCCP protocol is currently under
development and widespread deployment is not yet in place.
o Use of the RTP/AVPFCC profile is not contingent on any OS level
changes and can be quickly deployed, as the AVPFCC profile is
implemented at the application layer.
o AVPFCC/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 the RTP/AVPFCC profile 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.
Overall, the RTP/AVPFCC profile provides an immediate means for
congestion control in media streams, in the time being 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 the RTP/AVPFCC profile:
o A RTP/AVPFCC sender transmits a send timestamp to the RTP/AVPFCC
receiver with every data packet. In addition to congestion
control the send timestamp can be used by the receiver for
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jitter calculations.
In contrast DCCP with CCID3 transmits a quad round trip
counter to the receiver.
o A RTP/AVPFCC receiver only provides the RTP/AVPFCC 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 [SRTP] profile in conjunction with the
AVPFCC profile.
4. RTP and RTCP Packet Forms and Protocol Behavior
The section "RTP Profiles and Payload Format Specifications" of RFC
3550 enumerates a number of items that can be specified or modified
in a profile. This section addresses each of these items and states
which item is modified by the RTP/AVPFCC profile:
RTP data header: The standard format of the fixed RTP data
header has been modified (see Section 6).
Payload types: The payload type in the RTP data header is
reduced to 6 bits, therefore payload types are restricted to
values in the range of 0 to 63.
RTP data header additions: Two 32 bit fixed fields, send
timestamp and round trip time (RTT), are added to the RTP
data header. The send time stamp is always present and
the RTT field is present if the R bit is set.
RTP data header extensions: No RTP header extensions are
defined, but applications operating under this profile
MAY use such extensions. Thus, applications SHOULD NOT
assume that the RTP header X bit is always zero and SHOULD
be prepared to ignore the header extension. If a header
extension is defined in the future, that definition MUST
specify the contents of the first 16 bits in such a way
that multiple different extensions can be identified.
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RTCP packet types: Additional RTCP packet types are defined
by this profile per the RTP/AVPF profile [AVPF].
RTCP report interval: This profile is restricted to unicast
flows, therefore at all times there is only one active sender
and one receiver. Sessions operating under this profile MAY
specify a separate parameter for the RTCP traffic bandwidth
rather than using the default fraction of the session
bandwidth. In particular this may be necessary for data
flows were the the RTCP recommended reduced minimum interval
is still greater than the RTT.
SR/RR extension: The SR/RR extensions are defined per RFC 3550.
No changes are specified.
SDES use: Applications MAY use any of the SDES items described
in the RTP specification.
Security: This profile is subject to the security considerations
discussed in the TFRC and RTP specifications [TFRC][RTP]. This
profile does not specify any additional security services.
String-to-key mapping: No mapping is specified by this profile.
Congestion: This profile specifies how to use RTP/RTCP with TFRC
congestion control.
Underlying protocol: The profile specifies the use of RTP over
unicast UDP flows only, multicast MUST NOT be used.
Transport mapping: The standard mapping of RTP and RTCP to
transport-level addresses is used.
Encapsulation: This profile is defined for encapsulation
over UDP only.
5. 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 [TFRC]. We note how the RTP/AVPFCC profile accommodates these
exchanges.
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5.1. Data Packets
As stated in [TFRC] 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 TFRCs functionality.
For the computation of the loss intervals the RTP/AVPFCC profile
extends the RTP data header as follows: a 32 bit field to transmit a
send timestamp and an additional 32 bit field, present only when the
RTT changes, to transmit the RTT. The presence of the RTT is
indicated by the R bit in the RTP header (see Section 6).
5.2. Feedback Packets
As stated in [TFRC] 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):
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 the RTP/AVPFCC profile
defines a new AVPF transport layer feedback message, as detailed in
Section 7.
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6. RTP Data Header Additions
The RTP/AVPFCC profile makes the following changes to the RTP header
(other fields of the payload header are defined as in RFC 3550
[RTP]):
o the profile uses a 6 bit payload type (PT) field,
o defines a 1 bit R field in the second octet, and
o defines two additional 32 bit fixed fields:
1. the packets send timestamp,
2. the round trip time (RTT) as measured by the sender. This
field is present if the R bit is set (see Figures 1 and 2).
The additional fields of the RTP header are defined and used as
follows:
Round trip time indicator (R): 1 bit
This field indicates the existence of the additional RTT field. If
the R bit is set, the RTP payload header includes a 32 bit field
representing the current round trip time (Figures 1 and 2).
Payload type (PT): 6 bits
The RTP/AVPFCC profile uses a 6 bit field for the payload type
(instead of a 7 bit field).
Send timestamp: 32 bits
The timestamp indicating when the packet is sent. This timestamp
is measured in microseconds and is used for round trip time
calculations. At microsecond granularity the send timestamp
wraps around in approximately 71 minutes.
Round trip time (RTT): 32 bits
The round trip time as measured by the RTP/AVPFCC sender in
microseconds. This field is only present if the R bit is set
(Figure 2).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X| CC |M|R| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| send timestamp |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: RTP header and additions with R=0, no RTT included.
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|X| CC |M|R| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| send timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTT |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: RTP header and additions with R=1, RTT included.
7. TFRC-FB: A New AVPF Transport Layer Feedback Message
To support feedback to the RTP/AVPFCC receivers a new transport layer
AVPF feedback message is defined: TFRC-FB. This message is depicted
in Figure 3. It is defined according to [AVPF] and includes the
following four fields:
Timestamp (t_i): 32 bits
The send timestamp of the last data packet received by the
RTP/AVPFCC receiver, t_i, in microseconds.
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Delay (t_delay): 32 bits
The amount of time elapsed between the receipt of the last data
packet at the RTP/AVPFCC receiver, and the generation of this
feedback report in microseconds. This is used by the TFRC RTP
sender for RTT computations.
Data rate (x_recv): 32 bits
The rate at which the receiver estimates that data was received
since the last feedback report was sent in bytes per second.
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.)
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 (SSRC of first source) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| t_i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| t_delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data rate at the receiver (x_recv) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| loss event rate (p) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
Figure 3
8. RTCP Transmission Intervals, TFRC and the AVPF Profile
The AVPFCC profile recommends setting RTCP transmission intervals
according to the requirements of the TFRC algorithm. TFRC requires a
receiver to generate a feedback 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.
Other requirements that AVPFCC must contend with are AVPF feedback
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rules and AVP RTCP bandwidth constraints. In general the AVPFCC
profile complies with the rules of AVPF for sending RTCP feedback
packets with the following two exceptions:
o allow_early is set to "true" at all times. Essentially, this means
that a receiver can always generate an early feedback packet, and
does not need to alternate between early and regular RTCP packets
(see RFC 4585, Section 3.4,k).
o T_rr_interval must not be set to a value larger than the current
round trip time, as this would prevent generating feedback packets
at least once per RTT (see RFC 4585, Section 3.4,m).
The TFRC requirements of receiving feedback once per RTT can at times
conflict with the AVP RTCP bandwidth constraints, particularly at
small RTTs of 20ms or less. Assuming only one TFRC-FB report per
RTCP compound packet, Table 1 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, an AVPFCC 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).
The correct operation of TFRC at RTTs of 20ms and less, at data rates
less than those list in Table 1 is an open issue and is TBD.
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 |
+------------------------------+
Table 1
9. SDP Definitions
Applications using RTP integrated with TFRC and sending or receiving
packets as defined in this document MUST use "AVPFCC" as part of
their session description. The session will inherit the properties of
the AVPF profile.
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10. IANA Considerations
In this section we detail IANA registry values that need to be
registered. In particular the new RTP/AVPF feedback packet, TFRC-FB.
For the RTPFB range of packets, the following format (FMT) value
needs to be registered:
Value name: TFRC-FB
Long name: TFRC feedback
Value: 2
Reference: RFC XXXX
11. Security Considerations
TBC
12. 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.
13. Author's Address
Ladan Gharai <ladan@isi.edu>
USC Information Sciences Institute
3811 N. Fairfax Drive, #200
Arlington, VA 22203
USA
Normative References
[RTP] H. Schulzrinne, S. Casner, R. Frederick and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
Internet Engineering Task Force, RFC 3550 (STD0064), July
2003.
[AVP] H. Schulzrinne and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control," RFC 3551 (STD0065),
July 2003.
[AVPF] J. Ott, S. Wenger, A. Sato, C. Burmeister and J. Ray,
"Extended RTP Profile for RTCP-based Feedback (RTP/AVPF)",
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RFC 4585, July 2006.
[2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", Internet Engineering Task Force,
RFC 2119, March 1997.
[2434] T. Narten and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", Internet Engineering Task
Force, RFC 2434, October 1998.
[TFRC] M. Handley, S. Floyed, J. Padhye and J. widmer,
"TCP Friendly Rate Control (TRFC): Protocol Specification",
Internet Engineering Task Force, RFC 3448, January 2003.
[SDP] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[SRTP] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman,
"The Secure Real-time Transport Protocol", RFC 3711, March
2004.
Informative References
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this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
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15. Full Copyright Statement
Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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