Internet DRAFT - draft-dunbar-rtgwg-5g-edge-metadata-bgp-usage
draft-dunbar-rtgwg-5g-edge-metadata-bgp-usage
Network Working Group L. Dunbar
Internet Draft Futurewei
Intended status: Informational K. Majumdar
Expires: January 3, 2024 Microsoft
H. Wang
Huawei
G. Mishra
Verizon
July 3, 2023
BGP Usage for 5G Edge Computing Service Metadata
draft-dunbar-rtgwg-5g-edge-metadata-bgp-usage-01
Abstract
This draft describes the problems in the 5G Edge computing
environment and how BGP can be used to propagate additional IP
layer detectable information about the 5G edge data centers so
that the ingress routers in the 5G Local Data Network can make
path selections based on not only the routing distance but
also the IP Layer relevant metrics of the destinations. The
goal is to improve latency and performance for 5G Edge
Computing (EC) services even when the detailed servers running
status are unavailable.
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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Table of Contents
1. Introduction.............................................. 3
1.1. 5G Edge Computing Background......................... 3
1.2. 5G Edge Computing Network Properties................. 4
1.3. ANYCAST in 5G EC Environment......................... 5
1.4. Problem of Unbalanced Anycast Distribution........... 7
1.5. Problem of Application Service instance Relocation... 7
2. Conventions used in this document......................... 7
3. Destination Metadata for 5G Edge Computing................ 8
3.1. Assumptions.......................................... 8
3.2. IP Layer Metrics to Gauge Traffic Load............... 9
3.3. Metadata Constrained Optimal Path Selection......... 10
4. Soft Anchoring of an ANYCAST Flow........................ 11
5. Manageability Considerations............................. 13
6. Security Considerations.................................. 13
7. IANA Considerations...................................... 13
8. References............................................... 13
8.1. Normative References................................ 13
8.2. Informative References.............................. 14
9. Acknowledgments.......................................... 14
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1. Introduction
This document describes the problems in the 5G Edge computing
environment and how BGP can be used to propagate additional
IP-layer relevant information about the destination so that
the ingress routers in the 5G Local Data Network can make path
selections based on not only the routing distance but also the
IP Layer relevant metrics of the destinations. The goal is to
improve latency and performance for 5G Edge Computing (EC)
services even when the detailed servers running status are
unavailable.
1.1. 5G Edge Computing Background
In 5G Edge Computing (EC), one application can have multiple
instances hosted in different edge data centers that are close
in proximity. The 5G Local Data Networks (LDN)that connect the
edge data centers with the 5G Base stations (a.k.a. UPFs)
consist of a small number of dedicated routers.
When a User Equipment (UE) sends packets using the destination
address from a DNS reply or its cache, the packets from the UE
are carried in a PDU session through 5G Core [5GC] to the 5G
UPF-PSA (User Plane Function - PDU Session Anchor). The UPF-
PSA decapsulates the 5G GTP outer header and forwards the
packets from the UE to its directly connected Ingress router
of the 5G LDN. The LDN for 5G EC is responsible for forwarding
the packets to the intended destination(s).
When the UE moves out of coverage of its current gNB (next-
generation Node B) and anchors to a new gNB, the 5G SMF
(Session Management Function) could select the same UPF or a
new UPF for the UE per standard handover procedures described
in 3GPP TS 23.501 and TS 23.502. If the UE is anchored to a
new UPF-PSA when the handover process is complete, the packets
to/from the UE is carried by a GTP tunnel to the new UPF-PSA.
Per TS 23.501-h20 Section 5.8.2, the UE may maintain its IP
address when anchored to a new UPF-PSA unless the new UFP-PSA
belongs to different mobile operators. 5GC may maintain a path
from the old UPF to the new UPF for a short time for the SSC
[Session and Service Continuity] mode 3 to make the handover
process more seamless.
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1.2. 5G Edge Computing Network Properties
In this document, 5G Edge Computing Network refers to multiple
Local Data Networks (LDN) in one region that interconnect the
Edge Computing data centers. Those (IP) LDN networks are the
N6 interfaces from 3GPP 5G perspective.
The 5G Edge Computing Network's ingress routers are directly
connected to the 5G UPFs. The egress routers to the 5G Edge
Computing [EC] Network are the routers directly connected to
the EC service instances. The EC service instances and the
egress routers are co-located. Some Edge Computing Data
centers may have virtual switches or Top of Rack [ToR]
switches between the egress routers and the service instances.
But transmission delay between the egress routers and the EC
service instances is negligible, which is too small to be
considered in this document.
For an application that has multiple service instances
clustered together behind an application layer load balancer,
it is usually the load balancer's IP address(es) visible to
the 5G LDNs. How an application layer load balancer
distributes traffic to a group of service instances is out of
the scope of the network layer. This document is only for
optimizing the traffic delivery from UEs to the IP addresses
visible to the 5G LDN, which can be application layer load
balancers or the actual service instances.
The 5G EC services are registered premium services that
require super-low latency and very high SLA. Most services by
the UEs are not part of the registered 5G EC Services.
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+--+
|UE|---\+---------+ +------------------+
+--+ | 5G | +--------+ | S1: aa08::4450 |
+--+ | Site +--+-+---+ +----+ |
|UE|----| A |PSA1| Ra| | R1 | S2: aa08::4460 |
+--+ | +----+---+ +----+ |
+---+ | | | | | S3: aa08::4470 |
|UE1|---/+---------+ | | +------------------+
+---+ |IP Network | L-DN1
|(3GPP N6) |
| | | +------------------+
| UE1 | | | S1: aa08::4450 |
| moves to | +----+ |
| Site B | | R3 | S2: aa08::4460 |
v | +----+ |
| | | S3: aa08::4470 |
| | +------------------+
| | L-DN3
+--+ | |
|UE|---\+---------+ | | +------------------+
+--+ | 5G | | | | S1: aa08::4450 |
+--+ | Site +--+--+---+ +----+ |
|UE|----| B |PSA2| Rb | | R2 | S2: aa08::4460 |
+--+ | +--+-+----+ +----+ |
+--+ | | +-----------+ | S3: aa08::4470 |
|UE|---/+---------+ +------------------+
+--+ L-DN2
Figure 1: Service instances in multiple edge DCs
1.3. ANYCAST in 5G EC Environment
Increasingly, Anycast is used by various application providers
and CDNs because Anycast provides better and faster resiliency
to failover events than geo database DNS-based load balancing,
which relies on DNS to provide a different IP based on source
address.
Anycast address leverages the proximity information present in
the network (routing) layer. It eliminates the single point of
failure and bottleneck at the DNS resolvers. Anycast address
can be assigned to instances located in multiple data centers
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to leverage network condition for balanced forwarding. Another
benefit of using the ANYCAST address is removing the
dependency on UEs. Some UEs (or clients) might use their
cached IP addresses for an extended period instead of querying
DNS.
Client using Virtual IP address is a common practice in Cloud
Native networking, e.g., Kubernetes, to scale dynamic changes
of application instances. Virtual IP requires the destination
gateway node to perform address translation for return
traffic, which is unsuitable for underlay network nodes with
millions of flows passing by. The Cloud Native network can
also leverage network conditions to balance forwarding among
multiple Cloud Gateway nodes by assigning the same virtual IP
address.
Having multiple locations of the same IP address in the 5G EC
LDN can be problematic if path selection is solely based on
routing cost as the routing cost differences to reach
different egress routers can be very small. This list
elaborates the issues in detail:
a) Path Selection: When a new flow comes to an ingress node
(Ra), avoiding instability with Anycast flipping among
paths to the same address can be an issue. The problem
also exists in the BGP multipath environment, with the
optimal path selected based on routing cost metrics.
The ingress node needs to forward the packets from one
flow to the same service instance, a.k.a. Flow Affinity
or Flow-based load balancing. The ingress node (Ra/Rb)
can use Flow ID (in IPv6 header) or UDP/TCP port number
combined with the source address to enforce packets in
one flow being placed in one tunnel to one Egress router.
No new features are needed.
b) When a UE moves to a new 5G site in the middle of a
communication session with an EC service instance, a
method is needed to stick the flow to the same EC service
instance, which is required by 5G Edge Computing: 3GPP TR
23.748. [5g-edge-compute-sticky-service] describes
several approaches to achieve stickiness in the IPv6
domain.
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Note: most EC services have shorter sessions, e.g.,
shorter TCP sessions. Most likely, when a UE is moving to
a new 5G site, the TCP session via the old UPF to an EC
service instance is already finished. Only a very small
percentage of registered EC services need to stick to the
original service instance when handover to a new cell
tower.
From BGP perspective, the multiple service instances with the
same IP address (ANYCAST)attached to different egress routers
is the same as multiple next hops for the IP address.
This draft describes using BGP UPDATE to propagate some
metrics about the destination data centers to the ingress
routers so that both network and destination conditions can be
considered when computing the optimal path to the egress
routers.
1.4. Problem of Unbalanced Anycast Distribution
It is common to have higher capacity EC service instances
placed in a metro data center to accommodate more UEs in
proximity and fewer placed in remote sites. Sometimes, UEs
swarm to a specific site unexpectedly, e.g., a special event
at a remote site for a short period, e.g., 1~2 days. The EC
service instances in the remote site might be heavily
utilized. In contrast, the EC service instances of the same
app in the metro DC can be under-utilized. Since the condition
can be short-lived or unexpected, it might not make business
sense to adjust EC capacity among DCs.
1.5. Problem of Application Service instance Relocation
When an EC service instance is added to, moved, or deleted
from a 5G EC Data Center, it is useful to propagate the
changes to 5G PSA or the PSA adjacent routers. After the
change, the cost associated with the site might change as
well.
2. Conventions used in this document
A-ER: Egress Router to an Application Service instance,
[A-ER] is used to describe the last router that
the Application Service instance is attached. For
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a 5G EC environment, the A-ER can be the gateway
router to a (mini) Edge Computing Data Center.
EC: Edge Computing
Edge DC: Edge Data Center, which provides the Edge
Computing Hosting Environment. An Edge DC might
host 5G core functions in addition to the
frequently used application service instances.
gNB next generation Node B
LDN: Local Data Network
PSA: PDU Session Anchor (UPF)
SSC: Session and Service Continuity
UE: User Equipment
UPF: User Plane Function
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in BCP 14 [RFC2119] [RFC8174] when,
and only when, they appear in all capitals, as shown here.
3. Destination Metadata for 5G Edge Computing
The destination metadata consists of metrics about the running
environment at the egress routers to which EC service
instances are directly attached.
3.1. Assumptions
From the IP Layer, the EC service instances or their
respective load balancers are identified by their IP
addresses. Those IP addresses are the identifiers to the EC
service instances throughout this document. Here are some
assumptions about the 5G EC services:
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- Only the registered EC services, which are only a small
portion of the services, need to incorporate the
destination related metrics for optimal forwarding.
- The 5G EC controller or management system can send those
EC service identifiers to relevant routers.
- The ingress routers' local BGP path compute algorithm
includes a special plugin that can compute the path to
the optimal Next Hop (egress router) based on the BGP
Metadata TLV received for the registered EC services.
The proposed solution is for the egress routers, a.k.a. A-ERs
in this document, that have direct links to the EC Service
instances to collect various measurements about the Service
instances' running status [5G-EC-Metrics] and advertise the
metrics to the ingress routers in 5G EC LDN (Local Data
Network).
3.2. IP Layer Metrics to Gauge Traffic Load
[5G-EC-Metrics] describes the IP Layer Metrics that can be
used to estimate the service instances running status and
environment:
- IP-Layer Metric for Load Measurement:
The Load Measurement is a weighted combination of the number
of packets/bytes to the IP address and the number of
packets/bytes from the address which are collected by the A-
ER to which the Service instance is directly attached.
The A-ER is configured with an ACL that can filter out the
packets for the Application Service instance.
- Site Degradation Index
a numeric number, representing the percentage of the site
being functional. When a data center goes dark (i.e., lost
power), the A-ER can announce the capacity index being 0 for
all the IP addresses reached by the A-ER.
- Site preference index:
is used to describe some sites are more preferred than
others. For example, a site with higher bandwidth has a
higher preference number than other.
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In this document, the term "Application Service instance
Egress Router" [A-ER] is used to describe the last router that
application Service instance are attached. For the 5G EC
environment, the A-ER can be the gateway router to the EC DC
where application service instances are hosted.
3.3. Metadata Constrained Optimal Path Selection
The main benefit of using ANYCAST is to leverage the network
layer conditions to select an optimal path to the application
instantiated in multiple locations.
When the ingress routers to the 5G LDN are informed of the
Load and Capacity Index of the App Service instances at
different EC data centers, they can incorporate those metrics
with the network path conditions for the path selections.
Here is an algorithm that computes the cost to reach the App
Service instances attached to Site-i relative to another site,
say Site-b. When the reference site, Site-b, is plugged in the
formula, the cost is 1. So, if the formula returns a value
less than 1, the cost to reach Site-i is less than reaching
Site-b.
CP-b * Load-i Pref-b * Network-Delay-i
Cost-i= (w *(----------------) + (1-w) *(-------------------------))
CP-i * Load-b Pref-i * Network-Delay-b
Load-i: Load Index at Site-i, it is the weighted
combination of the total packets or/and bytes sent to and
received from the Application Service instance at Site-i
during a fixed time period.
CP-i: degraded capacity index at Site-i, a higher value
means higher capacity.
Delay-i: Network latency measurement (RTT) to the A-ER that
has the Application Service instance attached at the site-
i.
Pref-i: Preference index for the Site-i, a higher value
means higher preference.
w: Weight for load and site information, which is a value
between 0 and 1. If smaller than 0.5, Network latency and
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the site Preference have more influence; otherwise, Service
instance load and its capacity have more influence.
4. Soft Anchoring of an ANYCAST Flow
"Sticky Service" in the 3GPP Edge Computing specification
(3GPP TR 23.748) is about flows from a UE sticking to a
specific location when the UE moves from one 5G Site to
another.
"Soft Anchoring" is a mechanism for ingress routers to apply
preference to the path towards the previous service instance
location when the UE is anchored to a new UPF and continue
using its cached IP for the EC service instance.
Let's assume one application "App.net" is instantiated on
four service instances that are attached to four different
routers R1, R2, R3, and R4 respectively. It is desired for
packets to the "App.net" from UE-1 to stick with one service
instance, say the App Service instance attached to R1, even
when the UE moves from one 5G site to another. However, when
there is a failure reaching R1 or the Application Service
instance attached to R1, the packets of the flow "App.net"
from UE-1 need to be forwarded to the Application Service
instance attached to R2, R3, or R4.
We call this kind of sticky service "Soft Anchoring", meaning
that anchoring to the site of R1 is preferred, but other
sites can be chosen when the preferred site encounters a
failure.
Here is a mechanism to achieve Soft Anchoring:
- Assign a group of ANYCAST addresses to one application.
For example, "App.net" is assigned with 4 ANYCAST
addresses, L1, L2, L3, and L4. L1/L2/L3/L4 represents
the location preferred ANYCAST addresses.
- For the App.net Service instance attached to a router,
the router has four Stub links to the same Service
instance, L1, L2, L3, and L4 respectively. The cost to
L1, L2, L3, and L4 is assigned differently for different
egress routers. For example,
o When attached to R1, the L1 has the lowest cost,
say 10, when attached to R2, R3, and R4, the L1 can
have a higher cost, say 30.
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o ANYCAST L2 has the lowest cost when attached to R2,
higher cost when attached to R1, R3, R4
respectively.
o ANYCAST L3 has the lowest cost when attached to R3,
higher cost when attached to R1, R2, R4
respectively, and
o ANYCAST L4 has the lowest cost when attached to R4,
higher cost when attached to R1, R2, R3
respectively
- When a UE queries for the "App.net" for the first time,
the DNS reply has the location preferred ANYCAST
address, say L1, based on where the query is initiated.
- When the UE moves from one 5G site-A to Site-B, UE
continues sending packets of the "App.net" to ANYCAST
address L1. The routers will continue sending packets to
R1 because the total cost for the App.net instance for
ANYCAST L1 is lowest at R1. If any failure occurs making
R1 not reachable, the packets of the "App.net" from UE-1
will be sent to R2, R3, or R4 (depending on the total
cost to reach L1 attached to R2/R3/R4).
If the Application Service instance supports the HTTP
redirect, more optimal forwarding can be achieved.
- When a UE queries for the "App.net" for the first time,
the global DNS reply has the ANYCAST address G1, which
has the same cost regardless of where the Application
service instances are attached.
- When the UE initiates the communication to G1, the
packets from the UE will be sent to the Application
Service instance that has the lowest cost, say the
Service instance attached to R1. The Application service
instance is instructed with HTTPs Redirect to reply with
a location-specific URL, say App.net-Loc1. The client on
the UE will query the DNS for App.net-Loc1 and get the
response of ANYCAST L1. The subsequent packets from the
UE-1 for App.net are sent to L1.
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5. Manageability Considerations
The Edge Service Metadata described in this document are only
intended for propagating between Ingress and egress routers of
one single BGP domain, i.e., the 5G Local Data Networks, which
is a limited domain with edge services a few hops away from
the ingress nodes. Only the selective services by UEs are
considered as 5G Edge Services. The 5G LDN is usually managed
by one operator, even though the routers can be by different
vendors.
6. Security Considerations
The proposed Edge Service Metadata are advertised within the
trusted domain of 5G LDN's ingress and egress routers. There
are no extra security threats compared with iBGP.
7. IANA Considerations
This document doesn't require any IANA action.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4364] E. rosen, Y. Rekhter, "BGP/MPLS IP Virtual Private
networks (VPNs)", Feb 2006.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174, DOI
10.17487/RFC8174, May 2017, <https://www.rfc-
editor.org/info/rfc8174>.
[RFC8200] s. Deering R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", July 2017
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8.2. Informative References
[3GPP-EdgeComputing] 3GPP TR 23.748, "3rd Generation
Partnership Project; Technical Specification Group
Services and System Aspects; Study on enhancement of
support for Edge Computing in 5G Core network
(5GC)", Release 17 work in progress, Aug 2020.
[5G-EC-Metrics] L. Dunbar, H. Song, J. Kaippallimalil, "IP
Layer Metrics for 5G Edge Computing Service", draft-
dunbar-ippm-5g-edge-compute-ip-layer-metrics-00,
work-in-progress, Oct 2020.
[5G-Edge-Sticky] L. Dunbar, J. Kaippallimalil, "IPv6 Solution
for 5G Edge Computing Sticky Service", draft-dunbar-
6man-5g-ec-sticky-service-00, work-in-progress, Oct
2020.
[RFC5521] P. Mohapatra, E. Rosen, "The BGP Encapsulation
Subsequent Address Family Identifier (SAFI) and the
BGP Tunnel Encapsulation Attribute", April 2009.
[BGP-SDWAN-Port] L. Dunbar, H. Wang, W. Hao, "BGP Extension
for SDWAN Overlay Networks", draft-dunbar-idr-bgp-
sdwan-overlay-ext-03, work-in-progress, Nov 2018.
[SDWAN-EDGE-Discovery] L. Dunbar, S. Hares, R. Raszuk, K.
Majumdar, "BGP UPDATE for SDWAN Edge Discovery",
draft-dunbar-idr-sdwan-edge-discovery-00, work-in-
progress, July 2020.
[Tunnel-Encap] E. Rosen, et al "The BGP Tunnel Encapsulation
Attribute", draft-ietf-idr-tunnel-encaps-10, Aug
2018.
9. Acknowledgments
Acknowledgements to Sue Hares and Donald Eastlake for their
review and contributions.
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This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Linda Dunbar
Futurewei
Email: ldunbar@futurewei.com
Kausik Majumdar
Microsoft
Email: kmajumdar@microsoft.com
Haibo Wang
Huawei
Email: rainsword.wang@huawei.com
Gyan Mishra
Verizon
Email: gyan.s.mishra@verizon.com
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