One document matched: draft-yang-avt-selective-transmission-00.txt
Audio/Video Transport Y. Hu
Internet-Draft Huawei Technologies Co., Ltd.
Intended status: Standards Track July 31, 2009
Expires: February 1, 2010
Selective transmission of RTP Session
draft-yang-avt-selective-transmission-00.txt
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Abstract
Selective transmission is an effective technique implementation for
media stream transport when network congestion occurs or dynamic
transient burst stream transfers on a bandwidth constrained network.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Selective transmission Mechanism Description . . . . . . . . . 4
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Procedure specification . . . . . . . . . . . . . . . . . . 5
4. RTCP message extension . . . . . . . . . . . . . . . . . . . . 6
4.1. RTCP Retransmission Suppression Indication . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
When a congestion event in the network occurs, it tends to result in
long bursts of consecutive packet loss. For real-time video
services, long bursts of consecutive packet loss will cause mosaic-
screen, even black screen whether a packet drop management policy is
first-in first-out (FIFO) or random early detection (RED). Though
retransmission can partly solve this issue, the retransmission
results may cause more serious congestion if network congestion is
still there. Therefore, it is suggested to utilize a new method to
solve this problem. Considering the inherent characteristic of media
codec, some packets of video stream are sensitive, important for
video decoding and playback, and some packets of video stream are
unimportant and less sensitive. For example, packets of I-frame and
P-frame are sensitive, important for video stream, but packets of
B-frame are less important and sensitive than packets of I-frame and
P-frame. Sometimes, the packets of the last P-frame of GOP is
unimportant and less sensitive. We consider using these inherent
characteristic of media codec to selectively transmit sensitive and
important packets and drop unimportant and less sensitive packets of
media stream when network congestion occurs.
If selectively transmitting sensitive, important packets and dropping
unimportant and less sensitive packets of media stream in the
network, it will result in non-consecutive sequence number of media
stream packets. Due to non-consecutive sequence number of media
stream packets, for RTP receivers, they will think these packets
having been drop and will request the retransmission for those
packets. However, those packets have been dropped actively by the
network and RTP receivers don't need to request retransmission to
these packets having been dropped. Therefore, intermediary network
element need a method to suppress these retransmission request.
Actually, this method of selective transmission can also be used in
the case of "Unicast-Based Rapid Acquisition of Multicast RTP
Sessions"[draft-ietf-avt-rapid-acquisition-for-rtp]. In this case,
in order to reduce the acquisition delay of the reference information
when RTP receivers switch among multicast sessions, an intermediary
network element transfers a unicast transient burst stream from the
reference information to RTP receivers on a bandwidth constrained
network. In order to decrease the impact of dynamic transient burst
stream to the network, the intermediary network element may also
adopt the method of this selective transmission to reduce the
transmission of burst media packets.
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1.1. 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 [RFC2119].
2. Definitions
This document uses the following acronyms and definitions frequently:
Media packet sensitivity: Importance of a video packet to the decoder
if the packet is lost. A packet is unimportant and less sensitive ,
such as a B-frame packet, if the packet is used to decode the current
video frame. A packet is sensitive to loss and important, such as a
packet of I-frame, if the packet is needed to decode several video
frames.
Selective transmission: An intermediary network element can drop less
important video packets and transmit the rest of the packets.
Retransmission Suppression: Intermediary network element needs to
suppress the retransmission request of RTP receivers to packet
dropped according to the selective transmission policy.
3. Selective transmission Mechanism Description
The section describes an overview and procedure specification of
selective transmission mechanism.
3.1. Overview
Before having selectively transmitted media stream packets, the
intermediary network element inspects sensitivity and importance of
each media stream packet. When network congestion occurs, the
intermediary network element drops less important and less sensitive
media stream.
The number of dropped packets depends on the size of network
congestion. Heavier network congestion, will result in dropping of
more selective media stream packets. In the case of Unicast-Based
Rapid Acquisition of Multicast RTP Sessions, we can also utilize this
technique method of selective transmission to reduce the transmission
size of burst media packets allowing faster synchronization.
In the case of Unicast-Based Rapid Acquisition of Multicast RTP
Sessions, we can also utilize this technique method of selective
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transmission to reduce the transmission of burst media packets.
3.2. Procedure specification
The procedure of selective transmission mainly include the following
steps:
1. The network element inspects and records sensitivity and
importance of each media stream packet. It records less important
and less sensitive packets in order to prepare for selectively
dropping these packets.
2. When network congestion occurs, intermediary network element
drops the whole or parts of the less important and less sensitive
media packets instead of dropping packets on first-in first-out
(FIFO) or random early detection (RED) based on the state of network
congestion. That is to say, when the network has a light congestion
level, it drops only some of those media packets while when network
congestion is heavy, it may drop all these media packets and only
transmits the media packets with sensitivity and importance. This
policy will preventshowing mosaic pictures in receivers' screen that
may happen if packets are randomly dropped.
3. After receiving the media stream packets which have been
transmitted selectively, receivers will identify some packet loss and
request retransmission of those lost packets. In fact, it is
unnecessary for receives to request those packets because
intermediary network element took the initiative to drop them. In
order to avoid this issue, intermediary network element can readjust
non-consecutive sequence numbers of these packets to be consecutive
sequence numbers.
...I1(1000-1299)B2(1300-1349)B3(1350-1399)P4(1400-1579)B5(1580-1629)
B6(1630-1679)P7(1680-1859)B8(1860-1909)B9(1910-1959)P10(1960-2139)
B11(2140-2189)B12(2190-2239)P13(2240-2419)I14(2420-2719)...
For example, a video frame sequence and sequence number of its
packets. In this example, for the sake of convenience, I-frame
consists of 300 RTP packets, B-frame consists of 50 RTP packets,
P-frame consists of 180 RTP packets.
...I1(1000-1299)B2(1300-1349)B3(1350-1399)P4(1400-1579)
B6(1580-1629)P7(1630-1809)B8(1810-1859)P10(1860-2039)
B11(2040-2089)B12(2090-2139)P13(2140-2319)I14(2420-2719)...
Intermediary network element select B-frame as low sensitivity, and
selectively drops packets of B5 and B9 and transmits the rest of
frames. Since packets of B5 and B9 have been dropped, there are
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missing sequence number of packets from 1579 to 1630 and 1909 to
1960. When receivers receive these RTP packets which have been
transmitted the video frame sequence except B5 and B9 frames, they
will request the retransmission of RTP packets B5 and B9 frames. In
order to avoid this unnecessary retransmission, intermediary network
element can rearrange its sequence number as follows:
After intermediary network element transmits the last RTP packet of
P13, it will transmits the first RTP packet of I14. Between the last
RTP packet of P13 and the first RTP packet of I14, there is a gap of
100 in the sequence numbers because intermediary network element
dropped the RTP packets of B5 and B9 frames. In order to suppress
this virtual retransmission due to readjust sequence number,
intermediary network element send an extended RTCP transport-layer
feedback message to notify receivers that there are missing sequence
numbers.
4. RTCP message extension
This section defines the format of RTCP transport-layer feedback
message that intermediary network element notifies receivers to
suppress their retransmission request when selectively transmitting
media stream.
4.1. RTCP Retransmission Suppression Indication
The RTCP Retransmission Suppression Indication Message is identified
by PT=RTPFB and FMT=TBD.
The RTCP RSI Message is used by Receiver to suppress its
retransmission request.
The RTCP RSI field has the structure depicted in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| the Starting Sequence Number | the Ending Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Syntax for the RSI message
0: unassigned
1: network congestion
2: rapid acquisition
3-255: unassigned
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Type (8 bits): The Type field indicates the type of selective
transmission, such as network congestion, or unicast-based rapid
acquisition of multicast RTP sessions, etc.
Reserved (24 bits): The Reserved fields are one octet and are set to
zero on transmission, and ignored on reception.
The Starting Sequence Number (16 bits): The SSN field indicates a
starting sequence number. After the starting sequence number, all
RTP packets retransmission request are suppressed.
The Ending Sequence Number (16 bits): The ESN field indicates an
ending sequence number. Before the ending sequence number, all RTP
packets retransmission request are suppressed.
5. Security Considerations
TBC.
6. Normative References
[I-D.ietf-avt-rapid-acquisition-for-rtp]
Steeg, B., Begen, A., Caenegem, T., and Z. Vax, "Unicast-
Based Rapid Acquisition of Multicast RTP Sessions",
draft-ietf-avt-rapid-acquisition-for-rtp-01 (work in
progress), June 2009.
[I-D.ietf-avt-rtcp-guidelines]
Ott, J. and C. Perkins, "Guidelines for Extending the RTP
Control Protocol (RTCP)",
draft-ietf-avt-rtcp-guidelines-01 (work in progress),
March 2009.
[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.
[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.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
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Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
Author's Address
Yinling Hu
Huawei Technologies Co., Ltd.
No.91 Baixia Road
Nanjing 210001
P.R.China
Email: yangpeilin@huawei.com
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