Network Working Group L. Dunbar
Internet Draft H. Chen
Intended status: Standard Futurewei
Expires: June 16, 2022 Aijun Wang
China Telecom
January 7, 2022
IGP Extension for 5G Edge Computing Service
draft-dunbar-lsr-5g-edge-compute-03
Abstract
Routers in 5G Local Data Network (LDN) can use additional
site-costs, preference, and other application related
metrics in addition to the network routing distance to
compute constraint-based SPF within the 5G LDN to enhance
performance for selected services. This draft describes
using application server related metrics to influence the
SPF and Flexible Algorithms to indicate the constraints.
Status of this Memo
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Table of Contents
1. Introduction........................................... 3
1.1. Unbalanced Distribution due to UE Mobility........ 3
1.2. ANYCAST in 5G EC Environment...................... 4
2. Conventions used in this document...................... 4
3. Solution Overview...................................... 6
4. New Flags added to FAD Flags Sub-TLV................... 6
5. "Site-Cost" Advertisement in OSPF...................... 7
6. "Site-Cost" Advertisement in IS-IS..................... 7
7. Alternative method for Distributing Aggregated Cost.... 7
8. Manageability Considerations........................... 8
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9. Security Considerations................................ 8
10. IANA Considerations................................... 8
11. References............................................ 8
11.1. Normative References............................. 9
11.2. Informative References........................... 9
12. Appendix:5G Edge Computing Background................ 11
13. 5G EC LDN Characteristics for the Constraint SPF..... 12
13.1. IP Layer Metrics to Gauge EC Server Running Status
...................................................... 12
13.2. App Metrics Constrained Shortest Path First..... 14
13.3. Reason for using IGP Based Solution............. 15
13.4. Flow Affinity to an ANYCAST server.............. 15
14. Acknowledgments...................................... 16
1. Introduction
In 5G Edge Computing (EC) environment, it is common for an
application that needs low latency to be instantiated on
multiple servers close in proximity to UEs (User
Equipment). When those multiple server instances share one
IP address (ANYCAST), the transient network and load
conditions can be incorporated in selecting an optimal path
among server instances for UEs.
Flexible algorithms provide mechanisms for topologies to
use different IGP path algorithms. This draft describes
using Flexible Algorithms [LSR-FlexAlgo] to indicate the
desired constrained SPF behavior for a subset of prefixes,
in addition to the encodings for advertising the IP Layer
App related metrics that can impact application servers'
performance.
1.1. Unbalanced Distribution due to UE Mobility
UEs' frequent moving from one 5G site to another can make
it difficult to plan where the App Servers should be
hosted. When one App server is heavily utilized, other App
servers of the same address close by can be under-utilized.
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The difference in the routing distance to reach multiple
Application Servers might be relatively small. The traffic
load at the router where the App Server is attached and the
site capacity, when combined, can be more significant than
the routing distance from the latency and performance
perspective.
Since the condition can be short-lived, it is difficult for
the application controller to anticipate the moving and
adjusting.
1.2. ANYCAST in 5G EC Environment
ANYCAST is assigning the same IP address for multiple
servers in different locations. Using ANYCAST can eliminate
the single point of failure and bottleneck at load
balancers or DNS. Another benefit is removing the
dependency on how UEs resolve IP addresses for their
applications. Some UEs (or clients) might use stale cached
IP addresses for an extended period.
But having the same IP address in multiple locations of the
5G Edge Computing environment can be problematic because
all those locations can be close in proximity. There might
be a very small difference in the routing distance to reach
an Application Server attached to a different edge router.
Note: for the ease of description, the EC (Edge Computing)
server, Application server, App server are used
interchangeably throughout this document.
2. Conventions used in this document
A-ER: Egress Edge Router to an Application Server,
[A-ER] is used to describe the last router that
the Application Server is attached. For 5G EC
environment, the A-ER can be the gateway router
to a (mini) Edge Computing Data Center.
Application Server: An application server is a physical or
virtual server that hosts the software system
for the application.
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Application Server Location: Represent a cluster of servers
at one location serving the same Application.
One application may have a Layer 7 Load
balancer, whose address(es) are reachable from
an external IP network, in front of a set of
application servers. From IP network
perspective, this whole group of servers is
considered as the Application server at the
location.
Edge Application Server: used interchangeably with
Application Server throughout this document.
EC: Edge Computing
Edge Hosting Environment: An environment providing the
support required for Edge Application Server's
execution.
NOTE: The above terminologies are the same as
those used in 3GPP TR 23.758
Edge DC: Edge Data Center, which provides the Edge
Computing 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.
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
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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. Solution Overview
The proposed solution is for the egress edge router (A-ER)
with the EC Servers directly attached to
- advertise the "Site-Cost" via IP prefix reachability
TLV associated with the (anycast) prefix.
- use a Flag in the Flexible Algorithm TLV to indicate
that "site-cost" needs to be included for the
constrained SPF to reach the Prefix
The "Site-Cost" associated with an EC server (i.e., ANYCAST
prefix) is computed based on the IP layer App-related
metrics [Section 12.1], such as Load Measurement, the
Capacity Index, the Preference Index, and other constraints
by a consistent algorithm across all A-ERs.
The solution assumes that the 5G EC controller or
management system is aware of the EC ANYCAST addresses that
need optimized forwarding. To minimize the processing, only
the addresses that match with the ACLs configured by the 5G
EC controller will have their Site-Cost collected and
advertised.
4. New Flags added to FAD Flags Sub-TLV
A New flag is added to indicate a constrained SPF compute
method is needed for the prefix.
Flags:
0 1 2 3 4 5 6 7...
+-+-+-+-+-+-+-+-+...
|M|P| | ...
+-+-+-+-+-+-+-+-+...
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P-flag: Site-Cost Metrics is included in deriving
Constrained IGP path to the prefix.
5. "Site-Cost" Advertisement in OSPF
- IPv4: OSPFv2
A new Aggregated Cost Sub-TLV needs to be added to
OSPFv2 Extended Prefix TLV [RFC7684]
- IPv6: OSPFv3
A new sub-TLV can be appended to the E-Intra-Area-
Prefix-LSA, E-Inter-Area-Prefix-LSA, E-AS-External-
LSA, and E-Type-7-LSA [RFC8362] to carry the
Aggregated Cost.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AggCostSubTLV | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AggCost to the App Server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Aggregated cost Advertisement in IS-IS
6. "Site-Cost" Advertisement in IS-IS
Aggregated Cost appended to the IP Reachability TLV: 128,
130, or 135.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AggCostSubTLV | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AggCost to the App Server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Aggregated cost Advertisement in IS-IS
7. Alternative method for Distributing Aggregated Cost
Section 6 and Section 7 demonstrate different ways for
OSPFv2, OSPFv3, and ISIS to propagate the aggregated cost.
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It would be better if the aggregated cost could be
advertised the same way, regardless of OSPFv2, OSPFv3, or
ISIS.
Draft [draft-wang-lsr-stub-link-attributes] introduces the
Stub-Link TLV for OSPFv2/v3 and ISIS protocol respectively.
Considering the interfaces on an edge router that connects
to the EC servers are normally configured as passive
interfaces, these IP-layer App-metrics can also be
advertised as the attributes of the passive/stub link. The
associated prefixes can then be advertised in the "Stub-
Link TLV" that is defined in [draft-wang-lsr-stub-link-
attributes]. All the associated prefixes share the same
characteristic of the link. Other link related sub-TLVs
defined in [RFC8920] can also be attached and applied to
the calculation of path to the associated prefixes."
Section 6 for the advertisement of AppMetaData Metric can
also utilize the Stub-Link TLV that defined in [draft-wang-
lsr-stub-link-attributes]
8. Manageability Considerations
To be added.
9. Security Considerations
To be added.
10. IANA Considerations
The following Sub-TLV types need to be added by IANA to
FlexAlgo.
- AggCostSubTLV Type for ISIS, OSPF (TBD1): IPv4 or IPv6
P-flag added to FAD Flags Sub-TLV to indicate that the
Site-Cost Metrics is included in deriving Constrained IGP
path to the prefix.
11. References
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11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC2328] J. Moy, "OSPF Version 2", RFC 2328, April 1998.
[RFC5521] P. Mohapatra, E. Rosen, "The BGP Encapsulation
Subsequent Address Family Identifier (SAFI) and
the BGP Tunnel Encapsulation Attribute", April
2009.
[RFC7684] P. Psenak, et al, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, Nov. 2015.
[RFC8200] S. Deering R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", July 2017.
[RFC8326] A. Lindem, et al, "OSPFv3 Link State
advertisement (LSA0 Extensibility", RFC 8362,
April 2018.
[RFC9012] E. Rosen, et al "The BGP Tunnel Encapsulation
Attribute", April 2021.
11.2. 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-StickyService] 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.
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[BGP-5G-AppMetaData] L. Dunbar, K. Majumdar, H. Wang, "BGP
App Metadata for 5G Edge Computing Service",
draft-dunbar-idr-5g-edge-compute-app-meta-data-
03, work-in-progress, Sept 2020.
[LSR-Flex-Algo] P. Psenak, et al, "IGP Flexible Algorithm",
draft-ietf-lsr-flex-algo-17, July 2021.
[LSR-Flex-Algo-BW] S. Hegde, et al, "Flexible Algorithms:
Bandwidth, Delay, Metrics and Constraints",
draft-ietf-lsr-flex-algo-bw-con-01, July 2021.
[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.
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12. Appendix:5G Edge Computing Background
The network connecting the 5G EC servers with the 5G Base
stations consists of a small number of dedicated routers
that form the 5G Local Data Network (LDN) to enhance the
performance of the EC services.
When a User Equipment (UE) initiates application 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 Plan Function
- PDU Session Anchor). The UPF-PSA decapsulates the 5G GTP
outer header, performs NAT sometimes, before handing the
packets from the UEs to the adjacent router, also known as
the ingress router to the EC LDN, which is responsible for
forwarding the packets to the intended destinations.
When the UE moves out of coverage of its current gNB (next-
generation Node B) (gNB1), the handover procedure is
initiated, which includes the 5G SMF (Session Management
Function) selecting a new UPF-PSA [3GPP TS 23.501 and TS
23.502]. When the handover process is complete, the IP
point of attachment is to the new UPF-PSA. The UE's IP
address stays the same unless moving to different operator
domain. 5GC may maintain a path from the old UPF to the new
UPF for a short time for SSC [Session and Service
Continuity] mode 3 to make the handover process more
seamless.
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+--+
|UE|---\+---------+ +------------------+
+--+ | 5G | +---------+ | S1: aa08::4450 |
+--+ | Site +--++---+ +----+ |
|UE|----| A |PSA| 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 |PSA| Rb | | R2 | S2: aa08::4460 |
+--+ | +--++----+ +----+ |
+--+ | | +-----------+ | S3: aa08::4470 |
|UE|---/+---------+ +------------------+
+--+ L-DN2
Figure 10: App Servers in different edge DCs
13. 5G EC LDN Characteristics for the Constraint SPF
13.1. IP Layer Metrics to Gauge EC Server Running Status
Most applications do not expose their internal logic to the
network. Their communications are generally encrypted. Most
of them do not even respond to PING or ICMP messages
initiated by routers.
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Here are some IP Layer App related Metrics that can gauge
the servers running status and environment:
- Capacity Index:
a numeric number, configured on all A-ERs in the
domain consistently, is used to represent the capacity
of an EC server attached to an A-ER. The IP addresses
exposed to the A-ER can be the App Layer Load
balancers that have many instances attached. At other
sites, the IP address exposed is the server itself.
- Site preference index:
Is used to describe some sites are more preferred than
others. For example, a site with less leasing cost has
a higher preference value. Note: the preference value
is configured on all A-ERs in the domain consistently
by the Domain Controller.
- Load Measurement for gauging the load of the attached
prefix (i.e., EC Server):
The Load Measurement for an EC Server is a weighted
combination of the number of packets/bytes to the EC
server (i.e., its IP address) and the number of
packets/bytes from the EC server. The Load Measurement
are collected by the A-ER that has the EC Server
directly attached.
An A-ER only collects those measurement for the
prefixes instructed by the Domain Controller.
For ease of description, those metrics with more to be
added later are called IP Layer App Metrics (or Site-Cost)
throughout the document.
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13.2. App Metrics Constrained Shortest Path First
The main benefit of using ANYCAST is to leverage the
network layer information to balance the traffic among
multiple locations of one application server.
For the 5G EC environment, the routers in the LDN need to
take consideration of various measurements of the EC
servers attached to each A-ER in addition to TE metrics to
compute ECMP paths to the servers.
Here is one algorithm that computes the cost to reach the
App Servers attached to Site-i relative to another site,
say Site-j. When the reference site, Site-j, 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 the reference site (Site-j).
CP-j * Load-i Pref-j * Network-Delay-i
Cost-i= (w *(----------------) + (1-w) *(-------------------------))
CP-i * Load-j Pref-i * Network-Delay-j
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 Server at Site-i
during a fixed time period.
CP-i: capacity index at site i, a higher value means
higher capacity.
Network Delay-i: Network latency measurement (RTT) to
the A-ER that has the Application Server attached at the
site-i.
Noted: Ingress nodes can easily measure RTT to all the
egress edge nodes by existing IPPM metrics. But it is
not so easy for ingress nodes to measure RTT to all the
App Servers. Therefore, "Network-Delay-i", a.k.a.
Network latency measurement (RTT), is between the
Ingress and egress edge nodes. The cost for the egress
edge nodes to reach to their attached servers is
embedded in the "capacity index".
Pref-i: Preference index for site-i, a higher value
means higher preference. Preference can be derived from
the total path cost to reach the A-ER [RFC5305], as
calculated below: 1/(total-path-cost).
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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.
13.3. Reason for using IGP Based Solution
Here are some benefits of using IGP to propagate the IP
Layer App-Metrics:
- Intermediate routers can utilize the aggregated cost to
reach the EC Servers attached to different egress edge
nodes, especially:
- The path to the optimal egress edge node can be
more accurate or shorter.
- Convergence is shorter when there is any failure
along the way towards the optimal ANYCAST server.
- When there is any failure at the intended ANYCAST
server, all the packets in transit can be optimally
forwarded to another App Server attached to a
different egress edge router.
- Doesn't need the ingress nodes to establish tunnels with
egress edge nodes.
There are limitations of using IGP too, such as:
- The IGP approach might not suit well to 5G EC LDN
operated by multiple ISPs.
For LDN operated by multiple IPSs, BGP should be used.
[BGP-5G-AppMetaData] describes the BGP UPDATE message to
propagate IP Layer App-Metrics crossing multiple ISPs.
13.4. Flow Affinity to an ANYCAST server
When multiple servers with the same IP address (ANYCAST)
are attached to different A-ERs, Flow Affinity means
routers sending the packets of the same flow to the same A-
ER even if the cost towards the A-ER is no longer optimal.
Many commercial routers support some forms of flow affinity
to ensure packets belonging to one flow be forwarded along
the same path.
Editor's note: for IPv6 traffic, Flow Affinity can be
achieved by routers forwarding the packets with the same
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Flow Label extracted from the IPv6 Header along the same
path.
14. Acknowledgments
Acknowledgements to Peter Psenak, Acee Lindem, Shraddha
Hegde, Tony Li, Gyan Mishra, Jeff Tantsura, and Donald
Eastlake for their review and suggestions.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Linda Dunbar
Futurewei
Email: ldunbar@futurewei.com
Huaimo Chen
Futurewei
Email: huaimo.chen@futurewei.com
Aijun Wang
China Telecom
Email: wangaj3@chinatelecom.cn
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