1、 International Telecommunication Union ITU-T H.248.85TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (03/2013) SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMSInfrastructure of audiovisual services Communication procedures Gateway control protocol: Usage of loopback in ITU-T H.248 Recommendation ITU-T H
2、.248.85 ITU-T H-SERIES RECOMMENDATIONS AUDIOVISUAL AND MULTIMEDIA SYSTEMS CHARACTERISTICS OF VISUAL TELEPHONE SYSTEMS H.100H.199 INFRASTRUCTURE OF AUDIOVISUAL SERVICES General H.200H.219 Transmission multiplexing and synchronization H.220H.229 Systems aspects H.230H.239 Communication procedures H.24
3、0H.259Coding of moving video H.260H.279 Related systems aspects H.280H.299 Systems and terminal equipment for audiovisual services H.300H.349 Directory services architecture for audiovisual and multimedia services H.350H.359 Quality of service architecture for audiovisual and multimedia services H.3
4、60H.369 Supplementary services for multimedia H.450H.499 MOBILITY AND COLLABORATION PROCEDURES Overview of Mobility and Collaboration, definitions, protocols and procedures H.500H.509 Mobility for H-Series multimedia systems and services H.510H.519 Mobile multimedia collaboration applications and se
5、rvices H.520H.529 Security for mobile multimedia systems and services H.530H.539 Security for mobile multimedia collaboration applications and services H.540H.549 Mobility interworking procedures H.550H.559 Mobile multimedia collaboration inter-working procedures H.560H.569 BROADBAND, TRIPLE-PLAY AN
6、D ADVANCED MULTIMEDIA SERVICES Broadband multimedia services over VDSL H.610H.619 Advanced multimedia services and applications H.620H.629 Ubiquitous sensor network applications and Internet of Things H.640H.649 IPTV MULTIMEDIA SERVICES AND APPLICATIONS FOR IPTV General aspects H.700H.719 IPTV termi
7、nal devices H.720H.729 IPTV middleware H.730H.739 IPTV application event handling H.740H.749 IPTV metadata H.750H.759 IPTV multimedia application frameworks H.760H.769 IPTV service discovery up to consumption H.770H.779 Digital Signage H.780H.789 For further details, please refer to the list of ITU-
8、T Recommendations. Rec. ITU-T H.248.85 (03/2013) i Recommendation ITU-T H.248.85 Gateway control protocol: Usage of loopback in ITU-T H.248 Summary Recommendation ITU-T H.248.85 clarifies the interaction between the use of the ITU-T H.248 “loopback“ and the Session Description Protocol (SDP) loopbac
9、k attribute. Loopback is the process where media (including transport packets) received (Local Descriptor) on a Termination is sent from the sending side (Remote Descriptor) of the Termination without the media being passed between that Termination and other Terminations in the Context. History Edit
10、ion Recommendation Approval Study Group Unique ID*1.0 ITU-T H.248.85 2013-03-16 16 11.1002/1000/11864 _ *To access the Recommendation, type the URL http:/handle.itu.int/ in the address field of your web browser, followed by the Recommendations unique ID. For example, http:/handle.itu.int/11.1002/100
11、0/11830-en. ii Rec. ITU-T H.248.85 (03/2013) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanen
12、t organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes
13、 the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards
14、 are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommen
15、dation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are
16、used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL PROPERTY RIGHTS ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intel
17、lectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not receiv
18、ed notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/
19、. ITU 2014 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. Rec. ITU-T H.248.85 (03/2013) iii Table of Contents Page 1 Scope 1 2 References. 1 3 Definitions 1 3.1 Terms defined elsewhere 1 3.2 Terms defined in t
20、his Recommendation . 1 4 Abbreviations and acronyms 2 5 Conventions 2 6 Loopback functionality in ITU-T H.248 2 6.1 Precedence 3 6.2 Sending of media 4 6.3 Monitoring media quality . 4 Appendix I Example use cases for loopback-based performance monitoring in RTP networks 5 I.1 Overview 5 I.2 Use cas
21、e #1: IP-IP ITU-T H.248 MG and remote SIP UE Loopback point located at MG (“local LP“) 5 I.3 Use case #2: IP-IP ITU-T H.248 MG and remote SIP UE Loopback point located at remote RTP entity (“remote LP“) . 7 I.4 Use case #3: Multiple IP network domains segment-based measurements using loopback 8 Bibl
22、iography. 10 Rec. ITU-T H.248.85 (03/2013) 1 Recommendation ITU-T H.248.85 Gateway control protocol: Usage of loopback in ITU-T H.248 1 Scope This Recommendation clarifies the interaction between and use of the ITU-T H.248 “loopback“ mode with the SDP Media Loopback functionality described by IETF R
23、FC 6849. It only describes the interactions between these functionalities rather than re-describing the entire loopback procedures in IETF RFC 6849. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions o
24、f this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and
25、other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T H.248.1 Recommendation ITU-T H.248.1 (2013), Gateway Co
26、ntrol Protocol: Version 3. ITU-T H.248.30 Recommendation ITU-T H.248.30 (2007), Gateway Control Protocol: RTCP extended performance metrics packages. ITU-T H.248.48 Recommendation ITU-T H.248.48 (2012), Gateway Control Protocol: RTCP XR Block Reporting Package. IETF RFC 3550 IETF RFC 3550 (2003), RT
27、P: A Transport Protocol for Real-Time Applications. IETF RFC 3611 IETF RFC 3611 (2003), RTP Control Protocol Extended Reports (RTCP XR). IETF RFC 6849 IETF RFC 6849 (2013), An Extension to the Session Description Protocol (SDP) and Real-time Transport Protocol (RTP) for Media Loopback. 3 Definitions
28、 3.1 Terms defined elsewhere None. 3.2 Terms defined in this Recommendation This Recommendation defines the following terms: 3.2.1 ITU-T H.248 loopback: The process where media (including transport packets) received (Local Descriptor) on a Termination is sent from the sending side (Remote Descriptor
29、) of the Termination without the media being passed between that Termination and other Terminations in the Context. 3.2.2 ITU-T H.248 media loopback: The media traffic covers all types of media which may be described by the ITU-T H.248 Media Descriptor. This is a superset with regards to IETF loopba
30、ck media traffic. The term media covers therefore not only packet-switched media traffic, but also media related to ITU-T H.248 physical terminations. 2 Rec. ITU-T H.248.85 (03/2013) 3.2.3 IETF media loopback: The media traffic is limited to “Media-over-RTP“ only. The loopbacked media portion relate
31、s to the application data encapsulated in RTP packets (i.e., the RTP-SDU part). 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: BGW Border gateway BR Basic Reports CP Collection Point IP Internet Protocol LP Loopback Point MG Media Gateway MGC Media Ga
32、teway Controller MP Measurement Point OP Originating Point RD Remote Descriptor RP Reporting Point RPLR RTP Packet Loss Rate RTCP RTP Control Protocol RTP Real-Time Protocol SDP Session Description Protocol SDU Service Data Unit SIP Session Initiation Protocol SR Sender Receiver UE User Equipment XR
33、 Extended Report 5 Conventions Elements of the ITU-T H.248 protocol model, e.g., Context, Termination, Stream, Event, are represented using the first letter capitalized. ITU-T H.248 Property, Event, Signal and Parameter identities are given in italics. When used in the main text, SDP syntax is highl
34、ighted in bold i.e, “m=“. 6 Loopback functionality in ITU-T H.248 ITU-T H.248 loopback functionality is supported through the use of the LocalControl Descriptor Mode Property (clause 7.1.7 of ITU-T H.248.1). The mode property is applicable to all transport types and is independent of any call contro
35、l protocol used by the MGC. When a mode property is set to “loopback“, the MG shall behave according to clause 7.1.7 of ITU-T H.248.1. The ITU-T H.248 loopback is a media level loopback in that the received media stream is processed according to the received Termination and may be sent in the Contex
36、t (but not to other Terminations). Rec. ITU-T H.248.85 (03/2013) 3 The mode property, whilst being independent of the call control variant and transport used, is used within an overall network procedure for initiating and co-ordinating loopback. Where the real-time protocol (RTP) is used, IETF RFC 6
37、849 describes additional SDP parameters and procedures for negotiating and initiating loopback functionality. As the ITU-T H.248.1 Local and Remote Descriptors contain SDP, this introduces an alternate means of specifying loopback functionality. Figure 1 illustrates the example of an ITU-T H.248 Str
38、eam endpoint for traffic processing following a layered protocol architecture. Figure 1 shows that the loopback path may be “located“ on different protocol layers. Such a protocol layer-dependent loopback path is either: implicitly given by the available media and transport information in the Local
39、and Remote Descriptors; or may be explicitly indicated, like e.g., by the SDP parameters referred to in clause 6.1. H.248.85(13)_F01ITU-T H.248 Stream endpointLayer 7 (application)Layer 6 (presentation)Layer 5 (session)Layer 4 (transport)Layer 3 (network)Layer 2 (data link)Layer 1 (physical)Loopback
40、 (media) pathIETF media loopbackProtocol layer-dependentloopback (media) path options(some typical examples are shown)Layer 3 media loopbackRD traffic directionIncomingpacketsOutgoingpacketsFigure 1 Protocol layer-dependent Loopback (media) path options 6.1 Precedence As indicated in clause 7.1.7 of
41、 ITU-T H.248.1, there may be an interaction between the mode property and the contents of the Local and Remote Descriptors. The mode property is described as taking precedence over any mode specified by the contents of the Local and Remote Descriptors. IETF RFC 6849 defines procedures and attributes
42、 for both the initiating and the answering side. For the initiating side, these procedures relate to the sending of a certain RTP payload that would be controlled through the normal ITU-T H.248 “Mode“ property (i.e., Send, Send/Receive) and setting the appropriate media and connection information in
43、 the Remote Descriptor. However, for the answering side, the procedures of IETF RFC 6849 result in a change of mode behaviour and thus may conflict with the setting of the ITU-T H.248 mode property. Consequently, the precedence described by clause 7.1.7 of ITU-T H.248.1 is only applicable to the “an
44、swerer“ (i.e., the Termination performing the loopback, or providing the mirror). Therefore, if the ITU-T H.248 mode property is set to “Inactive“, “Send“, “Receive“, or “Send/Receive“, this setting will take precedence over any SDP defined by IETF RFC 6849 set on the answering Termination, even if
45、a=loopback-mirror is set. As a result, the media or RTP packets will not be looped back (mirrored). However, if the ITU-T H.248 mode property is set to “Loopback“, then any SDP defined by IETF RFC 6849 is deemed to be complementary to the ITU-T H.248 mode. Media or RTP packets will be looped back ac
46、cording to the SDP contained in the Local and Remote Descriptors. This enables ITU-T H.248 to discriminate between a Packet or Media loopback as defined in clause 4.1 of IETF RFC 6849. 4 Rec. ITU-T H.248.85 (03/2013) 6.2 Sending of media IETF RFC 6849 assumes that an SDP offer/answer exchange occurs
47、 to negotiate the use of the loopback functionality. On receipt of an answer, the MGC may then provide the Remote Descriptor of the Termination/Stream in question with the necessary media/connection information at which point the MG will start to generate packets. 6.3 Monitoring media quality Clause
48、 9 of IETF RFC 6849 indicates that the use of the loopback attribute is intended for monitoring of media quality of the session. Appendix I illustrates some example use cases. Furthermore, it states that an offering or answering entity that is compliant to this specification SHOULD support RTCP per
49、IETF RFC 3550 and RTCP-XR per IETF RFC 3611. However, this may not always be the case. Appendix I has examples of use cases that do not use RTCP and RTCP-XR, due to the fact that the measurement and reporting points are at the same location. It then details which metrics/blocks should be supported. In order to request an MG to collect statistics regarding these metrics, the MGC should set the appropriate statistics via ITU-T H.248.30 and/or ITU-T H.248.48. Rec. IT
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