| Internet-Draft | Slice-Media-Service | July 2023 |
| Jiang & Wang | Expires 24 January 2024 | [Page] |
Extended Reality & multi-modality communication, or XRM, is a type of advanced service that has been studied and standardized in the 3GPP SA2 working group. It targets at achieving high data rate, ultra-low latency, and high reliability. The streams of an XRM service might be comprised of data from multiple modalities, namely, video, audio, ambient-sensor and haptic detection, etc. XRM service faces challenges on various aspects, e.g. accurate multi-modality data synchronization, QoS differentiation, large volume of packets, and etc. While a new 3GPP network slice type, HDLLC, has been recently introduced to handle the QoS requirements of XRM streams, the ubiquitously-existed encryption of packet payload posts additional challenges to the transport of encoded video packets via 5GS. We have then discussed two potential IETF schemes, e.g., IP-DSCP based or UDP-option extension, that could be applied to 'expose' XRM QoS 'metadata' to 5GS.¶
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Extended Reality & multi-modality communication, or XRM, is a type of advanced service that has been studied and standardized in the 3GPP SA2 working group. With the objective of achieving economical communication overhead, ultra-low latency, high reliability and top security, it features multi-modal interactions among a group of service entities that are distributed at the mobile network edges. The benefits of seamlessly integrating multiple types of inputs sourced via multiple channels make it widely applicable in fields, like AR/VR, telepresence, gaming, education, etc.¶
The XRM service can both consolidate the inputs from more than one source and disseminate information to multiple destinations. That is, input data from different kinds of devices/sensors or the output data to different types of destinations are required for the same task or application. This type of communication scheme possesses intrinsic advantages of providing services that would be complementary to each other, or even bearing progressive add-on gains, so that redundant delivery and information accuracy could be achieved effectively. In general sense, the streams of an XRM service might be comprised of data from four modalities, namely, video, audio, ambient-sensor and haptic detection.¶
Thanks to the unique nature and different requirements across modalities, and especially to address the 5G service requirements of different types of media steams with coordinated throughput, latency and reliability, XRM service faces challenges on various aspects, e.g. characteristics of generated data across modalities, accurate multi-modality data synchronization, QoS differentiation, large volume of small packets, and packet-size variation, etc. [_5G.TACMM]¶
XRM services use (multiple) IP PDU sessions to carry & transport data frames. With a PDU session corresponding to one modality stream, the coordinated transmission among multiple modality flows (or streams) needs to be warranted. The application client(s) of the different types of data of one application may be located at either one destination (e.g., a UE), or multiple destinations (e.g. having VR glasses, gloves, and more).¶
In current 5GS, the QoS Flow is the finest granularity of QoS differentiation in the PDU Session, which implies that each packet in a QoS flow is treated according to the same QoS requirements. Specifically for the XRM service, a group of packets are used to carry payloads of a PDU Set (e.g. a frame, video slice/tile). In media layer, packets in such a PDU Set are decoded/handled as a whole. For example, the frame/video slice may only be decoded in case all or certain amount of the packets carrying the frame/video slice are successfully delivered. For example, a frame within a GOP (Group of Pictures) can only be decoded by the client in case all frames on which that frame depends are successfully received. Hence the groups of packets within the PDU Set have inherent dependency on each other in media layer. Without considering such dependencies between the packets within the PDU set, 5GS may perform a scheduling with low efficiency. For example, the 5GS may randomly drop packet(s) but try to deliver other packets of the same PDU set which are useless to the client and thus waste radio resources [TS.23.501].¶
The similar benefits are also applicable to audio samples, haptics applications or remote control operations. The inter-dependency among packets of a PDU Set (e.g. a frame/video slice) can possibly help enhance the efficiency and promote user experience.¶
5G network slicing is a network architecture that enables the multiplexing of virtualized and independent logical networks on the same physical network infrastructure. Each network slice is an isolated end-to-end network tailored to fulfil diverse requirements requested by a particular application.¶
Network slices may differ between supported features and network function optimisations. This technology assumes a central role to support 5G mobile networks that are designed to efficiently embrace a plethora of services with very different service level requirements (SLR). The realization of this service-oriented view of the network leverages on the concepts of software-defined networking (SDN) and network function virtualization (NFV) that allow the implementation of flexible and scalable network slices on top of a common network infrastructure.¶
The business model adopted in Telco domain normally indicates that a network slice is administrated by a mobile virtual network operator (MVNO). The infrastructure provider (the owner of the Telco infrastructure) leases its physical resources to the MVNOs that share the underlying physical network. According to the availability of the assigned resources, a MVNO can autonomously deploy multiple network slices that are customized to the various applications provided to its own users.¶
As shown in the following Figure 1, 3GPP 5GS have defined 6 standard SSTs, covering eMBB, URLLC MIoT and more. Standardized SST values provide a way for establishing global interoperability for slicing so that Public Land Mobile Networks or PLMNs can support the roaming use case more efficiently for the most commonly used Slice/Service Types.¶
+----------------------------------------------------------------+ | Types | Value | Characteristics | +-------+-------+------------------------------------------------+ | eMBB | 1 | Slice for 5G enhanced mobile broadband | +-------+-------+------------------------------------------------+ | URLLC | 2 | Slice for ultra-reliable low-latency comm. | +-------+-------+------------------------------------------------+ | MIoT | 3 | Slice for Massive IoT | +-------+-------+------------------------------------------------+ | V2X | 4 | Slice for V2X Services | +-------+-------+------------------------------------------------+ | HMTC | 5 | Slice for High-performance Machine-type comm. | +-------+-------+------------------------------------------------+ | HDLLC | 6 | Slice for High data-rate & low-latency comm. | +-------+-------+------------------------------------------------+
Specifically, the 6th SST in the table, i.e., HDLLC or the slice for High Data-rate & Low-Latency communication service, was just introduced as a new SST for the handling of the XRM service in May, 2023. XR media service or XRM is characterized by high data rate and low latency communication. As per [TS.23.501], the 5GS QoS framework has been enhanced to support different QoS handling for PDU Set. It supports differentiated QoS handling considering different importance of PDU Sets, e.g. by treating packets (i.e. PDUs) belonging to less important PDU Set(s) differently to reduce the resource consumption. One add-on benefit is the reduction of the complexity of roaming configuration between networks.¶
Similarly, another SDO, GSMA ENSWI, also believes that the Extended Reality and Media Services are promising services, and a new standardized SST can bring consistent user experience for XRM Services and enhance the user experience, especially in the roaming case. Currently, GSMA ENSWI is working on defining a new NEST (NEtwork Slice Template) for Extended Reality and Media Services [GSMAnewSSTforXRM].¶
Different SST maps to varied QoS requirements, e.g., guaranteed bandwidth, max-allowed bandwidth, max-data-burst, min-latency, max-latency, jitter variation, etc. Particularly for the 5G XRM service, thanks to the diversified settings of framing, slicing, encoding of video images, there invovle additional parameters like the relative importance among different data packets (or PDUs in 3GPP SA2 term) that are generated from differrent types of frames, e.g., the I, P, B frames which render tiered priorities among them.¶
Another factor impacting the transport of encoded video packets is the ubiquitously-existed encryption of packet payload. For example, supposing RTP is used to transport video data. If the data contents in a packet are encrypted at the video source (i.e., the UDP source), then the later-added UDP header would not be able to expose the above-mentioned QoS parameters to the routing entities in underlay transport networks until the same packet reaches the UDP destination. However, this posts somewhat great challenges to the 5GS-based XRM service.¶
...........................................
: /-----------\ :
: | 5GS(SBA) | :
: /\-----------/\ :
: / | \ :
: +-----+ +------+ +-----+ :
: | AMF |-----| SMF |----| PCF | :
: +-----+ +------+ +-----+ :
: / | | :
: N1 N2 N4 :
: / | | :
: / | +-------+ :
+----+ +-------+ N3 | | N6 (UP) : +-------------+
| UE |--| gNB |----| UPF |------------:---| IP-domain |
+----+ +-------+ | | : | Network(DNN)|
: +-------+ : +-------------+
: | | :
: +-N9-+ :
...........................................
For example, as shown in the Figure 2, the network function(NF) UPF is similar to an IP-domain router, which sends/receives IP packets off the N6 interface to/from the IP domain network DNN. In the XRM scenario, when a downlink packet (from the DNN) with encrypted video contents arrives at the UPF from the N6 interface, the UPF would generally only use the IP 5-tuple (or at most 6-tuple) for packet classfication and prioritization (i.e., PDR in the 3GPP 5G term [TS.23.501]). Unfortunately, the 5-tuple cannot expose all the QoS related information, or so-named 'metadata' for XRM in this draft, that would be required for the effective data processing of an XRM stream. The existence of encryption prevents a UPF from diving further into the data contents.¶
The challenges, revolving around the encrypted XRM service as described in Section 2.3, lead to the exploration of novel IETF mechanisms to convey these critical 'metadata' to a UPF (and then to the 5GS).¶
One scheme is to use the 6-bit DS field in an IP header [RFC2474]. The fundamental advantage is that the IP-DSCP bits are not normally subject to the encryption hurdle. However, the 6-bit DS field has only 64 DSCP combinations that could not provide better granularity which is an equal important factor for the further evolution of 5G advanced services. Another disadvantage is the DSCP does not have good hierarchy among its 6 bits. For example, while the objective to promote the just-standardized HDLLC (SST=6) was initially targetting at the XRM service, it could also be customized & applied later to Metaverse service, i.e., another type of HDLLC service. Metaverse service is being discussed in the 3GPP SA2 WG for the Rel-19 planning. Therefore, in this scenario (of HDLLC), any candidate novel scheme should have the capability and extensibity to accommodate the mappings for both the main SST, e.g., HDLLC itself, and the more granular sub-types, e.g., XRM, Metaverse, etc., as discussed previously.¶
Another novel scheme is to use the being-standardized UDP-option [transportUDPoption]. Actually, when compared to the requirements of encryption-handling, more-granular capability, as well as the extensibility that could be achieved via the new UDP-option structure, we think using UDP-option is a better alternative (than using the IP-DSCP). For example, according to [transportUDPoption], we may get a code from the 'Kind' range [10...126] to identify the main type being the '3GPP network slice'. Then, we could further define the sub-structure for more concrete SSTs as per the table in Figure 1.¶
Another advantage is that UDP is a layer-4 protocol and its header will normally not be processed by IP routers. Not only does this relieve the processing burden off IP transport devices, but also gives a clear demarcation of the TCP/IP layer structure.¶
Of course, though we do agree UDP datagrams should best be processed in the end-to-end way, i.e., encapsulated at UDP sources and decapsulated at UDP destinations, the 5GS-based XRM service is unique in that the 5GS is a composite system which owns abundant and powerful functionalities that might grant a 5GS UPF to 'intelligently' break the IP-UDP demarcation rule by peeking at the XRM 'metadata' in the UDP option field.¶
Generally, this function will not incur additional security issues.¶
A new authentication option or other signaling message option may be used based on the specific implementation.¶