Internet DRAFT - draft-tjiang-dmm-5g-dupf-5mbs
draft-tjiang-dmm-5g-dupf-5mbs
dmm T. Jiang
Internet-Draft China Mobile
Intended status: Informational L. Han
Expires: 10 September 2023 Futurewei Technologies, Inc
9 March 2023
5G Distributed UPFs for 5G Multicast and Broadcast Services (5MBS)
draft-tjiang-dmm-5g-dupf-5mbs-01
Abstract
The drafts [I-D.zzhang-dmm-5g-distributed-upf] and
[I-D.zzhang-dmm-mup-evolution] have described the 5G mobile user
plane (MUP) via the refinement of distributed UPFs and a more radical
proposal by integrating gNB & UPF as a single network function (NF).
Some user plane implementation requirements that vendors and
operators are exploring are not introducing changes to 3GPP
architecture & signaling, if possible. The document 3GPP TS 23.247
[_3GPP-23.247] for 5G multicast and broadcast services, or 5MBS,
specifies the 5GS architecture to support MBS communication. Thanks
to the addition of new 5GS network functions (NFs) and MB-interfaces
on 5G CP & UP, specifically if coupled with the increasingly popular
satellite-related requirements, these would certainly post additional
provisioning & implementation challenges to the underlay transport
infrastructure.
This document is not an attempt to do 3GPP SDO work in IETF.
Instead, it discusses how to potentially integrate distributed UPFs
with the delivery of 5MBS communication, as well as the benefits of
using distributed UPFs to handle 5MBS traffic delivery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 10 September 2023.
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Table of Contents
1. Distributed UPFs in 5G User Plane . . . . . . . . . . . . . . 2
2. 5G Multicast and Broadcast Services (5MBS) . . . . . . . . . 4
3. Challenges in 5G MBS Communication . . . . . . . . . . . . . 5
3.1. 5MBS Transport Challenges . . . . . . . . . . . . . . . . 5
3.2. 5MBS UP Signaling Challenges . . . . . . . . . . . . . . 5
3.3. 5MBS Challenges in Satellite Communication . . . . . . . 6
4. 5G Distributed UPF for 5G MBS Implementation . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Distributed UPFs in 5G User Plane
Mobile User Plane (MUP) in 5G has two distinct parts: the Access
Network part between UE and gNB, and the Core Network part between
gNB and UPF. UPFs are traditionally deployed at central locations,
with UEs' PDU sessions encapsulated and extended thru GTP-U tunnels
via the N3 (and potentially N9) interfaces in 5GS. The interface N6
supports fundamentally a direct IP or Ethernet connection to the data
network or DNN.
Actually, UPFs could be distributed & deployed closer to gNBs.
The draft [I-D.zzhang-dmm-5g-distributed-upf] has described the 5G
mobile user plane (MUP) via the refinement of distributed UPFs or
dUPFs. The following picture shows the dUPF architecture:
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N3 N6
UE1 gNB1 | dUPF1 |
+---------+ |+------+-----+|
| PDU | || PDU | || PE1
+---------+ +------+------+|+------+ IP/ || +-----+--+
| | | |GTP-U |||GTP-U | ||----+ IP/ | |
| 5G-AN | |5G-AN +------+|+------+Ether|| |Ether| |
| xHaul | |xHaul |L3/2/1|||L3/2/1| || +-----+--+
+---------+ +------|------+|+------------+| ( )
| | ( Transport ) PE3
| | ( Network +--+-----+
UE2 gNB2 | dUPF2 | ( | | IP/ |
+---------+ |+------+-----+| ( (DN) | |Ether|
| PDU | || PDU | || ( +--+-----+
+---------+ +------+------+|+------+ IP/ || +-----+--+
| | | |GTP-U |||GTP-U | || | IP/ | |
| 5G-AN | |5G-AN +------+|+------+Ether|| |Ether| |
| xHaul | |xHaul |L3/2/1|||L3/2/1| || +-----+--+
+---------+ +-------------+|+------------+| PE2
In distributed UPF architecture, the central (PSA) UPF is no longer
needed. dUPF1 and UPF2 connect via PE1 and PE2, respectively, to the
DN VPN (or network instance/NI) that UE1 and UE2 intend to access.
There could exist other PEs, like PE3 in the picture, for other sites
of the same network domain(VPN or NI) or for global Internet access.
There are some benefits of distributed UPFs:
* The N3 interface becomes very simple - over a direct or short
transport connection between gNB and dUPF.
* The transport infrastructure off N3/N9 and N6 are straightforward,
most likely over the same underlay VPN (MPLS, SR-MPLS or SRv6)
supporting the traditional N3/N9 tunneling as in centralized PSA
UPF case.
* MEC becomes much simpler since no need to deploy centralized PSA
UPF plus ULCL UPFs; UE-UE traffic can be optimized for LAN-type
services (via host-route).
In short, the distributed UPFs model achieves "N3/N9/N6 shortcut and
central UPF bypass", which is desired by many operators.
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2. 5G Multicast and Broadcast Services (5MBS)
The 3GPP document TS 23.247 [_3GPP-23.247] for 5G multicast and
broadcast services, or 5MBS, specifies the 5GS architecture to
support MBS communication. The following picture shows the brief
system architecture of 5MBS:
----+----------(SBA for 5GC) ---------+-----
| | |
+--+--+ +---+---+ +---+----+
| AMF | | SMF | | MB-SMF |
+--+--+ +-+-+-+-+ +---+----+
/ | |
N2 / N4 | N4mb|
/ | |
/ N3 +-+-+---+ N19mb +---+----+ N6mb +----+
+-----+---------+ UPF +--------------| MB-UPF |------| DN |
+----+ | | +-------+ (Individual) +---+----+ +----+
| UE +---+ gNB | |
+----+ +-----+ |
|_________N3mb (shared delivery)_____|
TS 23.247 [_3GPP-23.247] adds new 5GS network functions (NFs) on both
5G control-plane (CP) and user-plane (UP). For example, the CP NF
MB-SMF is, in collaboration with the regular SMF, to provision and
signal to the UP NF MB-UPF (via the interface N4mb) for setting up
MBS delivery path.
5MBS has specified two data delivery modes, individual delivery vs.
shared delivery:
* Individual delivery: When the (downlink or DL) MBS packets are
received by the MB-UPF from the interface N6mb, MB-UPF replicates
& forwards those packets towards (multiple) UPFs, via the
interface N19mb, through either unicast (requiring multiple GTP
tunnels if unicast underlay transport is applied) or multicast (if
multicast underlay transport over N19mb is applied) transmission.
* Shared delivery: When the (DL) MBS packets are received by the MB-
UPF from N6mb, MB-UPF replicates & forwards those packets towards
(multiple) gNBs, via the interface N3mb (the lower-path in the
picture), through either (multiple) separate GTP tunnels if
unicast underlay transport over N3mb is applied, or a single GTP
tunnel if multicast underlay over N3mb is supported.
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3. Challenges in 5G MBS Communication
3.1. 5MBS Transport Challenges
The 5MBS architecture in TS 23.247 [_3GPP-23.247] introduces some
network challenges:
* Because of the addition of new CP and UP NFs, this will post
additional provisioning & implementation challenges to the
underlay transport infrastructure. For example, in the individual
delivery mode, both SMF and MB-SMF have to synchronize with each
other to help set up the relay/stitching path between UPF, MB-UPF
and DN.
* The picture in previous section shows three new interface types
corresponding to three different segments: N3mb, N6mb and N19mb.
Based on the traffic delivery mode, once MB-UPF receives DL
traffic from N6mb, it will have to do either individual or shared
delivery.
* In accordance with TS 23.247 [_3GPP-23.247], the underlay
transport infrastructure of all three segments can use either
unicast or multicast transmission, based on the capabilities of
underlay networks. For example, for the DL _shared_ delivery from
MB-UPF to gNB via the interface N3mb, 5G MBS packets can be
transmitted to multiple gNBs via multicast transmission if the
underlay network supports. Otherwise, MB-UPF will have to use
unicast to transmit separately to (multiple) gNBs. Considering
that this unicast/multicast flexibility is applicable to all the
three above-mentioned segments, the implementation will have to
face more challenges.
3.2. 5MBS UP Signaling Challenges
The user plane from the MB-UPF to gNB directly (i.e., the lower-path
in the above figure for the shared delivery) and the user plane from
the MB-UPF to UPFs then to gNB (i.e., the upper path in the figure
for individual delivery) may use IP multicast transport via a common
GTP-U tunnel per MBS session, or use unicast transport via separate
GTP-U tunnels at gNB or at UPF per MBS session. When using the IP
multicast transport, GTP-U Multicast Tunnels shall be used for
unidirectional transfer of the encapsulated T-PDUs from one GTP-U
Tunnel Endpoint (i.e., acting as the sender) to multiple GTP-U Tunnel
Endpoints (i.e., acting as receivers). The Common Tunnel Endpoint ID
(C-TEID) which is present in the GTP header shall indicate which
tunnel a particular T-PDU belongs to. The C-TEID value to be used in
the TEID field shall be allocated at the source Tunnel Endpoint
(e.g., the sender) and signaled to the destination Tunnel Endpoints
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(e.g., receivers) using a control plane protocol, e.g., GTPv1-C &
GTPv2-C. One C-TEID shall be allocated per MBMS bearer service or
per MBS session [_3GPP-23.247][_3GPP-29.281]. As we have explained
in the draft [I-D.zzhang-dmm-mup-evolution], the signaling overhead
to establish a N3 GTP unicast tunnel has reached seven steps, let
alone the case of the more complicated MBS tunnel creation.
3.3. 5MBS Challenges in Satellite Communication
The 5G service via the satellite constellation has become a popular
topic in 3GPP. There are currently three major satellite-related
projects in SA workgroups, i.e., the satellite access (SAT_Ph2)
[_3GPP-23.700-28] and the satellite backhaul (SATB) [_3GPP-23.700-27]
in SA2 as well as the Phase-3 enhancement via the satellite-based
store-and-forward technology (SAT_Ph3) in SA1 WG [_3GPP-22.865].
These projects study various 5GS requirements when either a gNB or a
UPF or both are on-board satellites. Evidently, the continuously-
moving satellite constellations introduce another dimension of
challenges to UE registration, session management and traffic
routing. The GTP-U tunnel end points have to be changed frequently
when the satellite providing the on-board service for a UPF rotates
away from the corresponding gNB of the same GTP-U tunnel. For the
SAT_access case, the ground station (GS) has to find a new gNB on-
board another satellite every couple of minutes (e.g., being around
7-8 minutes for the LEO category) to hand over UEs. There are
significantly large amount of singalling messages involved even for
unicast case via satellite constellation, let alone if we extend the
similar scenarios to 5G MBS communication.
4. 5G Distributed UPF for 5G MBS Implementation
The REQ8 of [RFC7333] talks about the multicast efficiency between
non-optimal and optimal routes, where it states that, in term of
multicast considerations, DMM SHOULD enable multicast solutions to be
developed to avoid network inefficiency in multicast traffic
delivery.
The current 5MBS architecture requires all DL multicast traffic go
through the (centralized) MB-UPF, regardless of using the individual
or shared delivery. In many operators' networks, 5GS might be
deployed in a location that is distant from customer sites. If the
deployed site happens to be on-board satellites, the additional
complexities and moving dynamics will certainly worsen the
operations. In these scenarios, the efficiency of multicast
transmission will be compromised. On the other aspect, a 5G dUPF
deployed closer to gNB, or even more radically applying 'ANUP' via
the possible integration of gNB & UPF [I-D.zzhang-dmm-mup-evolution],
might lead to more efficient implementation:
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* For shared delivery, the MB-UPF can be distributed closer to or
integrated with gNB, i.e., either dUPF or ANUP-like. The N6mb is
a normal IP interface which is connected to DN over underlay
network. This transport connection will most likely use the VPN
infrastructure that has been provisioned by operators for 5GS. As
a dUPF or ANUP, the N3mb tunnel off MB-UPF could be made much
simpler. In some field edge sites, a dUPF could co-locate on-prem
with gNB, which can even remove the usage of complex (inter-site)
VPN to favor native IP transport.
* For individual delivery, it involves two UPFs, one regular UPF and
one MB-UPF. To follow the current 3GPP specification, we can
distribute and deploy both UPFs closer to gNB. While the DL
traffic off the N6mb interface may achieve the same gain as in the
shared-delivery mode, the transport for the N19mb tunnel and the
(regular) N3 tunnel can be significantly simplified. Remember we
have mentioned previously that either unicast or multicast
(underlay) transmission can be used for N19mb (and actually also
for N6mb and N3mb). Therefore, applying dUPF or, possibly ANUP in
future, will help simplify the N19mb VPN transmission.
* For individual delivery, if we expand the scope beyond the current
3GPP spec., e.g., looking beyond the 5G or even 6G roadmap that
are already on the horizon of the 3GPP planning, we could
integrate the regular UPF and MB-UPF together as a distributed
UPF, and then deploy the dUPF closer to gNB. Of course, we might
even take one step further by integrating both UPFs (UPF and MB-
UPF) and gNB as a single 'logical' node, i.e., ANUP
[I-D.zzhang-dmm-mup-evolution]. Regardless, in either scenario,
both the N19mb and N3 tunnels can be simplified, or even
consolidated, significantly, TS 23.247 [_3GPP-23.247] specifies
the behaviors of MB-UPF, as a standalone NF. Indeed, all the
features and behaviors that would be implemented by a MB-UPF can
be collaboratively integrated into a regular UPF. This type of
'merging' should lead to more network efficiency and better
multicast traffic forwarding, conforming to the [RFC7333] REQ8.
When we take into consideration the above plausible arguments and
accordingly apply them to different 3GPP satellite-related projects,
e.g., SATB (backhaul), SAT_Ph2 & SAT_Ph3 (access), we can certainly
draw the conclusion that the extra burden of signalling messages, the
complexity of control plane as well as the excessive encapsulations
of user plane, as introduced by 5MBS, can be relieved dramatically.
Both drafts [I-D.zzhang-dmm-5g-distributed-upf]
[I-D.zzhang-dmm-mup-evolution] discussed and compared briefly
different tunneling mechanisms to implement the 3GPP GTP-U UP, i.e.,
SRv6, MPLS as the underlay, or in [I-D.mhkk-dmm-srv6mup-architecture]
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specifying a new SRv6 based MUP architecture to replace the GTP-U.
While these proposals may experience different issues upon 5MBS
transport implementation, the application of distributed or
'integrated' UPF might make it more feasible.
5. Security Considerations
TBD.
6. IANA Considerations
This document requests no IANA actions.
7. References
7.1. Normative References
[RFC7333] Chan, H., Ed., Liu, D., Seite, P., Yokota, H., and J.
Korhonen, "Requirements for Distributed Mobility
Management", RFC 7333, DOI 10.17487/RFC7333, August 2014,
<https://www.rfc-editor.org/info/rfc7333>.
7.2. Informative References
[I-D.mhkk-dmm-srv6mup-architecture]
Matsushima, S., Horiba, K., Khan, A., Kawakami, Y.,
Murakami, T., Patel, K., Kohno, M., Kamata, T., Camarillo,
P., Horn, J., Voyer, D., Zadok, S., Meilik, I., Agrawal,
A., and K. Perumal, "Segment Routing IPv6 Mobile User
Plane Architecture for Distributed Mobility Management",
Work in Progress, Internet-Draft, draft-mhkk-dmm-srv6mup-
architecture-04, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-mhkk-dmm-
srv6mup-architecture-04>.
[I-D.zzhang-dmm-5g-distributed-upf]
Zhang, Z. J., Patel, K., Jiang, T., and L. M. Contreras,
"5G Distributed UPFs", Work in Progress, Internet-Draft,
draft-zzhang-dmm-5g-distributed-upf-01, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-zzhang-dmm-
5g-distributed-upf-01>.
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[I-D.zzhang-dmm-mup-evolution]
Zhang, Z. J., Patel, K., Contreras, L. M., Islam, K.,
Mutikainen, J., Jiang, T., Jalil, L., and O. P. Sejati,
"Mobile User Plane Evolution", Work in Progress, Internet-
Draft, draft-zzhang-dmm-mup-evolution-03, 4 February 2023,
<https://datatracker.ietf.org/doc/html/draft-zzhang-dmm-
mup-evolution-03>.
[_3GPP-22.865]
"Study on satellite access - Phase 3; Rel-19; V0.3.0",
February 2023.
[_3GPP-23.247]
"Architectural enhancements for 5G multicast-broadcast
services; V18.0.0", December 2022.
[_3GPP-23.700-27]
"Study on 5G System with Satellite Backhaul; V18.0.0",
December 2022.
[_3GPP-23.700-28]
"Study on Integration of satellite components in the 5G
architecture; Phase 2; V18.0.0", December 2022.
[_3GPP-29.281]
"General Packet Radio System (GPRS) Tunnelling Protocol
User Plane (GTPv1-U); V17.4.0", September 2022.
Authors' Addresses
Tianji Jiang
China Mobile
Email: tianjijiang@chinamobile.com
Lin Han
Futurewei Technologies, Inc
Email: lhan@futurewei.com
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