Network Working Group L. Dunbar
Internet Draft Futurewei
Intended status: Standard K. Majumdar
Expires: October 25, 2023 Microsoft
G. Mishra
Verizon
H. Song
Futurewei
April 25, 2023
IP Layer Metrics for Edge Services
draft-dunbar-cats-edge-service-metrics-00
Abstract
This draft describes the IP Layer metrics and methods to
measure the Edge Services' running status and environment for
IP network to dynamically optimize the forwarding of low
latency edge services without any knowledge above the IP
layer.
Status of this Memo
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Table of Contents
1. Introduction..............................................3
1.1. Use Case: 5G Edge Computing..........................3
1.2. Problem 1: Selecting 5G Edge Service Instance
Location..................................................4
1.3. Problem 2: UE mobility creates unbalanced anycast
distribution..............................................5
2. Conventions used in this document.........................6
3. IP-Layer Metrics Definitions for 5G Edge Services........11
3.1. IP-Layer Service ID.................................11
3.2. IP-Layer metric for Service Instances Load
Measurement..............................................11
3.3. Capacity Index in the overall cost..................14
3.4. Site Preference Index in the overall cost...........14
3.5. RTT to an ANYCAST Address in 5G EC..................15
4. Algorithm in Selecting the optimal Target Location.......16
5. Scope of IP Layer Metrics Advertisement..................17
6. Manageability Considerations.............................17
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7. Security Considerations..................................17
8. IANA Considerations......................................18
9. References...............................................18
9.1. Normative References................................18
9.2. Informative References..............................18
10. Acknowledgments.........................................19
1. Introduction
1.1. Use Case: 5G Edge Computing
In the 5G Edge Computing environment [3GPP-EdgeComputing],
one application or service can have multiple instances hosted
in different Edge Computing data centers. Those Edge
Computing (mini) data centers are usually very close to, or
co-located with, 5G base stations to minimize latency and
optimize the user experience.
When a UE (User Equipment) initiates the packets using the
destination address from a DNS reply or its own cache, the
packets from the UE are carried in a PDU session through the
5G Core [5GC] to the 5G UPF-PSA (User Plan Function - PDU
Session Anchor). The UPF-PSA decapsulates the 5G GTP outer
header and forwards the packets from the UEs to the Ingress
router of the Edge Computing (EC) Local Data Network (LDN).
The LDN for 5G EC, which is the IP Networks from 5GC
perspective, is responsible for forwarding the packets to the
intended destinations.
Routers in the local IP network should be able to select the
"best" or "closest" location out of many service instances.
However, simply using distance alone as a metric may not be
sufficient as there may be many locations in close proximity.
Moreover, one of the main aims of locating the service
instance close to the user is to provide lower latency. When
a UE moves and attaches to another UPF, the packets from the
UE can enter the IP network from a different ingress router.
It is desirable if the IP network can continue forwarding the
packets from the UE to the established service instance. As a
user keeps moving further away, a closer service instance
might be able to serve the UE better. Network measurements,
including latency of various paths are provided to the
ingress router to assist in re-selection.
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1.2. Problem 1: Selecting 5G Edge Service Instance Location
Having multiple locations closer to UEs to host one service
can greatly improve the user experience. But selecting an
optimal location for the service traffic from a UE may not be
that simple.
Using DNS to reply with the address of the service instance
location closest to the requesting UE can encounter issues
like:
- UE can cache results indefinitely, when the UE moves to a
5G cell site very far away, the cached address may still
be used, which can incur large network delay.
- The service instance at a specific location whose address
replied by the DNS might be heavily loaded causing slow
or no response, when there are available low utilized
service instances, for the same service, at different
locations very close in proximity.
- No inherent leverage of proximity information present in
the network (routing) layer, resulting in loss of
performance
- Local DNS resolver become the unit of traffic management
Increasingly, Anycast is used extensively by various
application providers and CDNs because ANYCAST makes it
possible to dynamically load balance across locations that
host the application/service instances based on network
conditions. Service instances' location selection using
Anycast address leverages the proximity information present
in the network (routing) layer and eliminates the single
point of failure and bottleneck at the DNS resolvers and
application layer load balancers. Another benefit of using
ANYCAST address is removing the dependency on UEs that use
their cached destination IP addresses for extended period.
But selection of an ANYCAST location purely based on the
network condition can encounter issue of the location
selected by network routing information being overutilized
while there are available underutilized locations close by.
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1.3. Problem 2: UE mobility creates unbalanced anycast
distribution
Another problem of using ANYCAST address for multiple
locations of one service in 5G environment is that UEs'
frequent moving from one 5G site to another. The frequent
move of UEs can make it difficult to plan where the service
instances should be hosted. When a large number of UEs using
a particular service congregate together unpredictably, the
ANYCAST location selected based on routing distance can be
heavily utilized, while the instances of the same service at
other locations close-by are underutilized.
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+--+
|UE|---\+---------+ +------------------+
+--+ | 5G | +-----------+ | S1: aa08::4450 |
+--+ | Site A +----+ +----+ |
|UE|----| | Ra | | R1 | S2: aa08::4460 |
+--+ | +----+ +----+ |
+---+ | | | | | S3: aa08::4470 |
|UE1|---/+---------+ | | +------------------+
+---+ |IP Network | L-DN1
|(3GPP N6) |
| | | +------------------+
| | | | S1: aa08::4450 |
| | +----+ |
| | | R3 | S2: aa08::4460 |
v | +----+ |
| | | S3: aa08::4470 |
| | +------------------+
| | L-DN3
+--+ | |
|UE|---\+---------+ | | +------------------+
+--+ | 5G | | | | S1: aa08::4450 |
+--+ | Site B +----+ +----+ |
|UE|----| | Rb | | R2 | S2: aa08::4460 |
+--+ | +----+ +----+ |
+--+ | | +-----------+ | S3: aa08::4470 |
|UE|---/+---------+ +------------------+
+--+ L-DN2
Figure 1: multiple ANYCAST instances in different edge DCs
This document describes the measurements at the IP Layer that
can reflect the service instances running status and environment
at the specific locations. This document also describes the
method of incorporating those measurements with IP routing cost
to come up with a more optimal criteria in selecting the service
instance locations.
2. Conventions used in this document
CATS: Computing-Aware Traffic Steering takes into
account the dynamic nature of computing resource
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metrics and network state metrics to steer
service traffic to a service instance.
Service: A monolithic function. A composite service can be
built by orchestrating monolithic services.
Service instance: A run-time environment (e.g., a server or a
process on a server) that makes the functionality
of a service available. One service can have
multiple instances running at the same or
different network locations.
CS-ID: The CATS Service ID is an identifier representing
a service, which the clients use to access said
service. Such an identifier identifies all of the
instances of the same service, no matter on where
they are actually running. The CS-ID is
independent of which service instance serves the
service demand. Usually multiple instances
provide a (logically) single service, and service
demands are dispatched to the different instance
by choosing one instance among all available
instances.
CB-ID: The CATS Binding ID is an identifier of a single
service instance of a given CS-ID. Different
service instances provide the same service
identified through a single CS-ID, but with
different CATS Binding IDs.
Service demand: The demand for a specific service identified
by a specific CS-ID.
Service request: The request for a specific service instance.
CATS-router: A network device (usually at the edge of the
network) that makes forwarding decisions based on
CATS information to steer traffic belonging to
the same service demand to the same chosen
service instance.
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Ingress CATS-Router: A network edge router that serves as a
service access point for CATS clients. It steers
the service packets onto an overlay path to an
Egress CAN-Router linked to the most suitable
edge site to access a service instance.
CATS-ER: CATS-ER is an egress CATS-Router, i.e., the
egress endpoint of an overlay path to a service
instance. CATS-ER is used to describe the last
router that the service instances are attached.
In a 5G EC environment, the CATS-ER can be the
gateway router to the Edge Computing Data Center.
C-SMA: The CATS Service Metric Agent responsible for
collecting service capabilities and status, and
for reporting them to the C-PS.
NOTE: The above terminologies are the same as
those used in 3GPP TR 23.758
C-NMA: The CATS Network Metric Agent responsible for
collecting network capabilities and status, and
for reporting them to the C-PS
C-PS: The CATS Path Selector determines the path toward
the appropriate service location and service
instances to meet a service demand given the
service status and network status information.
C-TC: The CATS Traffic Classifier is responsible for
determining which packets belong to a traffic
flow for a particular service demand, and for
steering them on the path to the service instance
as determined by the C-PS.
Edge DC: Edge Data Center, which provides the Hosting
Environment for the edge services. An Edge DC
might host 5G core functions in addition to the
frequently used application servers.
gNB next generation Node B
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PSA: PDU Session Anchor (UPF)
SSC: Session and Service Continuity
UE: User Equipment
UPF: User Plane Function
ANYCAST Instance: refer to the service instance at a specific
location which is reachable by the ANYCAST
address.
Service Instance Location: Represent a cluster of servers at
one location serving the same Service. One
service may have a Layer 7 Load balancer, whose
address(es) are reachable from external IP
network, in front of a set of service instances.
From the IP network perspective, this whole group
of instances are considered as one service
instance at the location.
EC: Edge Computing
Edge Computing Hosting Environment: An environment, such as
psychical or virtual machines, host the service
instances.
NOTE: The above terminologies are the same as
those used in 3GPP TR 23.758
Edge DC: Edge Data Center, which provides the Edge Hosting
Environment. It might be co-located with 5G Base
Station and not only host 5G core functions, but
also host frequently used Edge server instances.
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L-DN: Local Data Network
PSA: PDU Session Anchor (UPF)
RTT: Round Trip Time
RTT-ANYCAST: A list of Round trip times to a group of
routers that have the ANYCAST instances directly
attached.
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.
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3. IP-Layer Metrics Definitions for 5G Edge Services
3.1. IP-Layer Service ID
From network perspective, a service identifier, or IP Layer
Service ID, is an ANYCAST address that can represent multiple
service instances at different locations that host the
service.
3.2. IP-Layer metric for Service Instances Load Measurement
There are many network techniques and protocols to optimize
forwarding and ensure QoS, such as DSCP/DiffServ, Traffic
Engineered (TE) solutions, Segment Routing, etc. But most
applications and services don't expose their internal logic
to network operators. Their communications are generally
encrypted. Most do not respond to PING or ICMP messages
initiated by routers or network elements.
This document specifies the IP Layer metrics and algorithms
that enable the IP networks to dynamically optimize the
forwarding of 5G edge computing service without any knowledge
above the IP layer.
Without knowledge of application internal logics, network
layer or IP Layer can monitor the traffic patterns to/from
the service instances at each location to gauge the running
status of the service at the location. The proposed IP Layer
Metrics and algorithm enable the IP networks to be more aware
of the service behavior without dependency on getting
information from the services themselves.
First, the network needs to discover which router(s) has the
service instances attached. Those routers are called CATS
Egress Router, or CATS-ER for short. CATS-ER is usually the
Gateway Router to an Edge Computing Data Center. To discover
if a router is the CATS-ER for a specific edge service, the
router can periodically send reverse ARP (IPv4) or Neighbor
Discovery scan with the address of the Service ID to discover
if there are the service instances hosted in its edge
computing data center. If yes, the router or routers are
identified as the CATS-ER for the Service ID. For one Service
ID, there can be many CATS-ERs at different EC Data Centers.
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For a service instance at a specific location, which is
identified by the address of the service instance at the IP
layer, the CATS-ER can measure the amount of traffic destined
towards the address & the amount of the traffic from the
specific address, such as:
- Total number of packets to the attached service instance
(ToPackets);
- Total number of packets from the attached service
instance (FromPackets);
- Total number of Bytes to the attached service instance
(ToBytes);
- Total number of bytes from the attached service instance
(FromBytes);
The actual load measurement to the service instance attached
to an CATS-ER can be based on one of the metrics above or
including all four metrics with different weights applied to
each, such as:
LoadIndex =
w1*ToPackets+w2*FromPackes+w3*ToBytes+w4*FromBytes
Where 0<= wi <=1 and w1+ w2+ w3+ w4 = 1.
The weights of each metric contributing to the load index of
the service instance attached to a CATS-ER can be configured
or learned by self-adjusting based on user feedbacks.
The raw measurement is useful when the CATS-ER routers cannot
be configured with a consistent algorithm to compute the
aggregated load index and the raw measurements are needed by
a central analytic system.
The CATS-ER can advertise either the aggregated Load Index or
the raw measurements periodically, by BGP UPDATE messages
(in-band) or BGP-LS (via controller), to a group of routers
that have traffic destined towards the ANYCAST addresses of
those services.
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It is better to have applications or their controllers
directly reporting their own workload running status to the
network. When it is not feasible to have the third-party
application controller provide the workload information to
the network operators, the proposed IP layer Load
Measurements provide an intelligent estimate of the instance
running status at a specific location.
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3.3. Capacity Index in the overall cost
Capacity Index indicates the capacity value for a site or a
pod where the edge services are hosted. One Edge Site can
be in full capacity, reduced capacity, or completely out of
service.
Cloud Site/Pod failures and degradation include, but not
limited to, a site capacity degradation or entire site
going down caused by a variety of reasons, such as fiber
cut connecting to the site or among pods within one site,
cooling failures, insufficient backup power, cyber threats
attacks, too many changes outside of the maintenance
window, etc. Fiber-cut is not uncommon within a Cloud site
or between sites.
When those failure events happen, the Edge (egress) router
visible to the ingress routers can be running fine.
Therefore, the ingress routers with paths to the egress
routers can't use BFD to detect the failures.
When there is a failure occurring at an edge site (or pod),
many instances can be impacted. In addition, the routes
(i.e., the IP addresses) in an Edge Cloud Site might not be
aggregated nicely. Instead of many BGP UPDATE messages for
each instance to the impacted ingress routers, the egress
router can send one single BGP UPDATE indicating the
capacity of the site. The ingress routers can switch all or
a portion of the instances that are associated with the
site depending on how much the site is degraded.
Site Capacity should be represented as the percentage of
the site availability, e.g., 100%, 50%, or 0%. When a site
goes dark, the Index is set to 0. 50 means 50% capacity
functioning.
3.4. Site Preference Index in the overall cost
As described in [IPv6-StickyService] and [ISPF-EXT-EC], an
EC sticky service needs to connect a UE to the service
instance that has been serving the UE before the UE moves
to a new 5G Site, unless there is failure to that location.
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To achieve the goal of sticking a flow from one specific UE
to a specific site, a "site Preference Index" is created.
The value of the Site Preference Index can be manipulated
for packets of some flows to be steered towards a instance
location farther away in routing distance. The "Site
Preference Index" enables some sites to be more preferred
for handling the UE traffic to a instance than others.
3.5. RTT to an ANYCAST Address in 5G EC
ANYCAST used in 5G Edge computing environment is slightly
different from the typical ANYCAST address being deployed.
Typical ANYCAST address is used to represent instances in
vast different geographical locations, such as different
continents. ANCAST address for "app.net" for Asia lead
packets to a server instance of "app.net" hosted in Asia.
Therefore, the RTT for "app.net" in Asia, is a single value
that represent the round time trip to the server in Asia
that host the "app.net".
5G Edge Computing environment can have one service hosted
in multiple Edge Computing DCs close in proximity. Routers,
i.e. the ingress router to 5G LDN (Local Data Network), can
forward packets for the ANYCAST address of "app.net" to
different egress routers that have "app.net" instances
attached.
If "app.net" is hosted in four different 5G Edge Computing
Data Centers. All those DCs have the same ANYCAST address
for the "app.net". The RTT to "app.net" ANYCAST address
need to be a group of values (instead of one RTT value to a
unicast address). The RTT group value should include the
CATS-ER router's specific unicast address (e.g., the
loopback address) to which the service instance is
attached.
RTT to "app.net" ANYCAST Address is represented as:
List of {Egress Router address, RTT value}
This list is called "RTT-ANYCAST".
In order to better optimize the ANYCAST traffic, each
router adjacent to 5G PSA needs to periodically measure RTT
to a list of CATS-ER routers that advertise the ANYCAST
address. The RTT to egress router at Site-i is considered
as the RTT to the ANYCAST instance at the Site-i.
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4. Algorithm in Selecting the optimal Target Location
The goal of the algorithm is to equalize the traffic among
multiple locations of the same ANYCAST address.
The main benefit of using ANYCAST is to leverage the IP-layer
information to equalize the traffic among multiple locations
of the same service, usually identified by one or a group of
ANYCAST addresses.
For 5G Edge Computing environment, the ingress router to each
LDN needs to be notified of the Load Index and Capacity Index
of the service instances at different EC site to make the
intelligent decision on where to forward the traffic from UEs
for the service.
The Algorithm needs to take the following attributes into
consideration:
- Load Measurement Index [Section 3.2],
- capacity index [Section 3.3],
- Preference Index [Section 3.4], and
- network delay [Section 3.5].
Here is an algorithm for a router, e.g., the router directly
attached to the 5G PSA, to compare the cost to reach the
service instances at Site-i or Site-j:
Load-i * CP-j Pref-j * Delay-i
Cost-i=min(w *(----------------) + (1-w) *(------------------))
Load-j * CP-i Pref-i * Delay-j
Load-i: Load Index at Site-i, it is the weighted
combination of the total packets and bytes sent to and
received from the service instance at Site-i during a
fixed time period.
CP-i (Capacity-i) (higher value means higher capacity):
capacity index at the site i.
Delay-i: Network latency measurement (RTT) to the CATS-ER
that has the service instances attached at the site-i.
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Pref-i (Preference Index: higher value means higher
preference): Network Preference index for the site-I.
w: Weight for load and site information, which is a value
between 0 and 1. If smaller than 0.5, Network latency and
the site Preference have more influence; otherwise, Server
load and its capacity have more influence.
5. Scope of IP Layer Metrics Advertisement
Each service might be used by a small group of UEs.
Therefore, it is not necessary for CATS-ER router to
advertise the IP layer metrics to all other routers in the 5G
LDN. Likewise, each EC Data Center may only host a small
number of low latency services.
"Service ID Bound Group Routers" is used to refer a group of
routers that are interested in a group of specific ANYCAST
addresses. The IP Layer Metrics for a specific service ID
should be advertised among the routers in the "Service ID
bound Group Routers".
BGP RT Constrained Distribution [RFC4684] can be used to form
the "Service ID Bound Group Routers".
Since there are much more Service IDs than the number of
routers in 5G LDN, a more practical way to form the "Service
ID Bound Group of Routers" is for each ingress router to
query a network controller upon receiving the first packet to
a specific ANYCAST address to be included in the "Service ID
Bound Group Routers". There should be a timer associated with
Ingress router, as the UE that uses the service ID might move
away. Upon timer expires, the Ingress Router is removed from
the "Service ID Bound Group of Routers".
6. Manageability Considerations
To be added.
7. Security Considerations
To be added.
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8. IANA Considerations
To be added.
9. References
9.1. 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
9.2. Informative References
[3GPP-EdgeComputing] 3GPP TS 23.548 V18.1.1, "3rd Generation
Partnership Project; Technical Specification Group
Services and System Aspects; 5G System Enhancements
for Edge Computing; Stage 2", Release 18, April
2023.
[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.
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[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.
10. Acknowledgments
Acknowledgements to XXX for their review and contributions.
This document was prepared using 2-Word-v2.0.template.dot.
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Authors' Addresses
Linda Dunbar
Futurewei
Email: ldunbar@futurewei.com
Kausik Majumdar
Microsoft
Email: kmajumdar@microsoft.com
Gyan Mishra
Verizon
Email: gyan.s.mishra@verizon.com
HaoYu Song
Futurewei
Email: haoyu.song@futurewei.com
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