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 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on February 1, 2010. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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. Hu Expires February 1, 2010 [Page 1] Internet-Draft Selective transmission July 2009 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 Hu Expires February 1, 2010 [Page 2] Internet-Draft Selective transmission July 2009 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. Hu Expires February 1, 2010 [Page 3] Internet-Draft Selective transmission July 2009 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 Hu Expires February 1, 2010 [Page 4] Internet-Draft Selective transmission July 2009 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 Hu Expires February 1, 2010 [Page 5] Internet-Draft Selective transmission July 2009 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 Hu Expires February 1, 2010 [Page 6] Internet-Draft Selective transmission July 2009 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. Hu Expires February 1, 2010 [Page 7] Internet-Draft Selective transmission July 2009 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 Hu Expires February 1, 2010 [Page 8]