Network Working Group L. Dunbar
Internet Draft H. Chen
Intended status: Standard Futurewei
Expires: April 11, 2022 Aijun Wang
China Telecom
October 11, 2021
Flex Algo Extension for 5G Edge Computing Service
draft-dunbar-lsr-5g-edge-compute-01
Abstract
Routers in 5G Local Data Network (LDN) can use additional
site-costs, preference, and other application related
metrics on top of the network condition to compute
constraint-based SPF within the 5G LDN to enhance
performance for selected services. This draft describes
those application server related metrics to be used in
Flexible Algorithms.
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Table of Contents
1. Introduction........................................... 3
1.1. 5G Edge Computing Background...................... 3
1.2. Problem#1: ANYCAST in 5G EC Environment........... 5
1.3. Problem #2: Unbalanced Anycast Distribution due to
UE Mobility............................................ 5
2. Conventions used in this document...................... 5
3. Solution Overview...................................... 7
3.1. Flow Affinity to an ANYCAST server................ 7
3.2. IP Layer Metrics to Gauge EC Server Running Status 8
3.3. App Metrics Constrained Shortest Path First....... 9
3.4. Reason for using IGP Based Solution.............. 10
4. IS-IS FAD with AppMetaData Constraint sub-TLVs........ 10
4.1. ISIS FAD sub-TLV for the Aggregated cost......... 10
4.2. IS-IS FAD for AppMetaData Metrics Advertisements. 11
4.3. ISIS AppMetaData Sub-TLV......................... 13
4.4. OSPF AppMetaData Sub-TLV......................... 14
5. AppMetaData SubSub-TLVs............................... 15
6. AppMetaData Metric Advertisement...................... 17
7. Aggregated Cost Advertisement in ISIS................. 18
8. Aggregated Cost Advertisement in OSPF................. 19
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8.1. OSPFv3 LSA to carry the Aggregated Cost.......... 19
8.2. OSPFv2 LSA to carry the Aggregated Cost.......... 19
9. Alternative method for Distributing Aggregated Cost... 20
10. Manageability Considerations......................... 20
11. Security Considerations.............................. 20
12. IANA Considerations.................................. 20
13. References........................................... 21
13.1. Normative References............................ 21
13.2. Informative References.......................... 22
14. Acknowledgments...................................... 22
1. Introduction
In 5G Edge Computing (EC) environment, it is common for one
application to be instantiated on multiple servers close in
proximity. Those multiple server instances can share one IP
address (ANYCAST) so that the transient network and load
conditions can be considered when computing the IGP path.
Flexible algorithms provide mechanisms to create
constraint-based paths in IGP. This draft describes some
specific metrics, that can impact application servers'
performance, to be used in the Flexible Algorithms.
1.1. 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.
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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.
+--+
|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 1: App Servers in different edge DCs
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1.2. Problem#1: ANYCAST in 5G EC Environment
ANYCAST makes it possible to load balance across server
locations based on network conditions dynamically. With
multiple servers having the same IP address, it eliminates
the single point of failure and bottleneck at the
application layer load balancers. Another benefit of using
ANYCAST address is removing the dependency on how UEs get
the IP addresses for their applications. Some UEs (or
clients) might use stale cached IP addresses for an
extended period.
But, having multiple locations of the same IP address in
the 5G Edge Computing environment can be problematic
because all those edge computing Data Centers can be close
in proximity. There might be very little difference in the
routing distance to reach the Application Servers attached
to a different edge router, which can cause packets from
one flow to be forwarded to different locations, resulting
in service glitches.
1.3. Problem #2: Unbalanced Anycast 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.
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 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.
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
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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.
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
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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. Solution Overview
The proposed solution is for the egress edge router (A-ER)
with the EC Servers directly attached to advertise the
"Site-Cost" [Section 3.2] within the 5G EC LDN via Flexible
algorithms [LSR-FlexAlgo], so that constrained IGP path can
be computed.
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.
3.1. 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.
3.2. 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.
Here are some IP Layer 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 Site-Cost throughout the
document.
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3.3. 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 App
Servers attached to each A-ER in addition to TE metrics to
compute the shortest path to the A-ER.
Here is one algorithm that computes the cost to reach the
App Servers 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 the reference site (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 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 link cost between the
egress edge nodes 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.
3.4. Reason for using IGP Based Solution
Here are some benefits of using IGP to propagate the IP
Layer App-Metrics:
- Intermediate routers can derive 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.
4. IS-IS FAD with AppMetaData Constraint sub-TLVs
4.1. ISIS FAD sub-TLV for the Aggregated cost
If egress edge routers with EC servers directly attached
can compute the aggregated cost, they can append the
Aggregated Cost sub-sub-TLV directly to the IS-IS FAD Sub-
TLV:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Aggregated cost Sub-sub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flex-Algorithm: SPF.
Metric-Type:
A new value to be assigned by IANA to indicate the
Aggregated Cost AppMetaData Metrics included in computing
the constrained SPF.
Calc-Type:
A value chosen by the IGP operator to indicate a
constrained SPF algorithm that takes the Aggregated Cost
into the SPF computation across the routers in the 5G
LDN.
The aggregated cost is computed based on the Load
Measurement, the Capacity value, the Preference value and
other constraints by a consistent algorithm across all A-
ERs.
0 1 2 3
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 3: AppMetaData Aggregated Cost subsub TLV
4.2. IS-IS FAD for AppMetaData Metrics Advertisements
This section describes the sub-sub TLVs that carry the
detailed IP Layer Metrics for other routers in the 5G LDN
to compute the constrained SPF.
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Type | Length |Flex-Algorithm | Metric-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Calc-Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AppMetaData Metrics SubSub-TLVs |
+ +
| ... |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Flex-Algorithm: SPF.
Metric-Type:
A new value to be assigned by IANA to indicate the
additional AppMetaData Metrics included in computing the
constrained SPF.
Calc-Type:
A value chosen by the IGP operator to indicate a specific
constrained SPF algorithm that takes the AppMetaData
attributes into the path computation across the routers
in the IGP domain.
It worth noting that not all hosts (prefix) attached to an
A-ER are EC servers that need network optimization. An A-
ER only needs to advertise the site-Cost Metrics for the
EC server addresses requested by the Controller.
Draft [draft-wang-lsr-passive-interface-attribute]
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 Prefix Sub-TLV" that is
defined in [draft-wang-lsr-passive-interface-attribute].
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.
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4.3. ISIS AppMetaData Sub-TLV
For EC Servers using IPv6, the AppMetaData Sub-TLV is
encoded as below:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AppMetaDataType| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 or IPv4 AppServer (ANYCAST) address |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Load measurement SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capability SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: AppMetaData sub TLV
AppMetaData Type (TBD1): ISIS-IPv4 or ISIS-IPv6.
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4.4. OSPF AppMetaData Sub-TLV
For EC Servers using IPv6, the AppMetaData Sub-TLV is
encoded as below:
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|AppMetaDataType | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv6 or IPv4 AppServer (ANYCAST) address |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Load measurement SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capability SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference SubSub-TLV |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: AppMetaData sub TLV
AppMetaData Type (TBD2): OSPF-IPv4 or OSPF-IPv6.
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5. AppMetaData SubSub-TLVs
Two types of Load Measurement SubSub-TLVs are specified:
a) The Aggregated Load Index based on a weighted
combination of the collected measurements.
b) The raw measurements of packets/bytes to/from the App
Server address. The raw measurement is useful when the
egress edge routers cannot be configured with a
consistent algorithm to compute the aggregated load
index or the raw measurements are needed by a central
analytic system.
The Aggregated Load Index Sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD3) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measurement Period |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Aggregated Load Index to reach the App Server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Aggregated Load Index Sub-TLV
Type=TBD2 (to be assigned by IANA) indicates that the
sub-TLV carries the Load Measurement Index derived from
the Weighted combination of bytes/packets sent
to/received from the App server:
Index=w1*ToPackets+w2*FromPackes+w3*ToBytes+w4*FromBytes
Where wi is a value between 0 and 1; w1+ w2+ w3+ w4 = 1.
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The Raw Load Measurement sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD4) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Measurement Period |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| total number of packets to the AppServer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| total number of packets from the AppServer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| total number of bytes to the AppServer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| total number of bytes from the AppServer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Raw Load Measurement Sub-TLV
Type= TBD3 (to be assigned by IANA) indicates that the
sub-TLV carries the Raw measurements of packets/bytes
to/from the App Server ANYCAST address.
Measurement Period: A user-specified period in seconds,
default is 3600 seconds.
The Capacity Index sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD5) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capacity Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Capacity Index Sub-TLV
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The Preference Index sub-TLV has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (TBD6) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Preference Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Preference Index Sub-TLV
Note: "Capacity Index" and "Site preference" can be
different for different attached server addresses. For
Figure 1, the address S1 can have higher Site Preference
when attached to R1 than R2.
6. AppMetaData Metric Advertisement
With Flex-Algorithm, the network administrator can define a
function that compute the SPF with consideration of the
AppMetaData metrics advertised by the routers to which the
EC servers are directly attached.
This document defines a new standard metric type,
AppMetaData, for this purpose. The AppMetaData Metric MAY
be advertised in the Generic Metric sub-TLV with the metric
type set to "AppMetaData Metric". ISIS and OSPF will
advertise this new type of metric in their link
advertisements. AppMetaData metric is a link attribute and
for advertisement and processing of this attribute for
Flex-algorithm purposes, MUST follow the section 12 of [I-
D.ietf-lsr-flex-algo]
Flex-Algorithm uses this metric type by specifying the
AppMetaData as the metric type in a FAD TLV. A FAD TLV may
also specify an automatic computation of the AppMetaData
metric based on a links advertised bandwidth. An explicit
advertisement of a link's AppMetaData metric using the
Generic Metric sub-TLV overrides this automatic
computation. The automatic AppMetaData metric calculation
sub-TLVs are advertised in FAD TLV and these parameters are
applicable to applications such as Flex-algorithm that make
use of the FAD TLV.
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7. Aggregated Cost Advertisement in ISIS
When A-ER can compute the aggregated cost to an attached EC
server, the Aggregated Cost sub-sub-TLV can be directly
appended to the App Specific Bit Mask. The aggregated cost
computing algorithm can be from EC controller that take the
Load Measurement, Capacity value, and Preference value into
consideration across all A-ERs.
The Application-Specific Link Attribute sub-TLV described
in [RFC8919] can be used to carry the "Aggregated-Cost" for
the EC server directly attached.
When carrying the "Aggregate-Cost" sub-sub TLVs, the App
specific Link Attribute sub-TLV can be included in TLV 22
(extended IS reachability), 23 (IS Neighbor Attribute), or
25(L2 bundle Member Attribute).
The Aggregate-Cost bit is added to the Standard
Applications Bit Mask (SABM).
0 1 2 3 4 5 6 7 ...
+-+-+-+-+-+-+-+-+...
|R|S|F|C| ...
+-+-+-+-+-+-+-+-+...
Figure 9: Extended Application Identifier Bit Mask
C-bit: set to specify the Site Cost related sub-sub TLVs,
described in the Section 3.2, are included in the App-
Specific Sub-TLV.
The R-bit, S-bit, F-bit are specified by the RFC8919.
The Extended App Specific Link Attributes Sub-TLV is as
following:
Type: 16
Length: (1 octet)
Value:
Extended Application Identifier Bit Mask [Figure 2]
Aggregate-Cost sub-sub-TLVs.
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0 1 2 3
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 EC Server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Aggregated Cost to EC Server
8. Aggregated Cost Advertisement in OSPF
When all egress edge routers with directly attached EC
servers can compute the aggregated cost that takes into
consideration the Load Measurement, Capacity value, and
Preference value, this aggregated cost can be considered as
the Metric of the link to the EC Server.
8.1. OSPFv3 LSA to carry the Aggregated Cost
For EC servers using IPv6 address, the aggregated cost
computed by the A-ERs can be encoded in the Metric field
[the interface cost] of Intra-Area-Prefix-LSA specified by
Section 3.7 of the [ RFC5340].
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 (Intra-Area Prefix) | TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | Aggregated Cost to the EC Server |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PrefixLength | PrefixOptions | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Prefix |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Aggregated Cost to App Server
8.2. OSPFv2 LSA to carry the Aggregated Cost
For EC servers in IPv4 address, the aggregated cost can be
encoded in the "Metric" field of the Stub Link LSA [Link
type =3] specified by Section 12.4 of the [RFC2328].
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9. Alternative method for Distributing Aggregated Cost
Section 7 and Section 8 demonstrate different ways for
OSPFv2, OSPFv3, and ISIS to propagate the aggregated cost.
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]
10. Manageability Considerations
To be added.
11. Security Considerations
To be added.
12. IANA Considerations
The following Sub-TLV types need to be added by IANA to
FlexAlgo.
- AppMetaData Type for ISIS (TBD1): IPv4 or IPv6
- AppMetaData Type for OSPF (TBD2): IPv4 or IPv6
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The following SubSub-TLV types need to be added by IANA, to
be included in FAD sub-TLV, ISIS Extended-LSA Sub-TLVs, or
OSPFv2 Extended Link Opaque LSA TLVs Registry.
- Aggregated Load Index Sub-TLV type (TBD3)
- Raw Load Measurement Sub-TLV type (TBD4)
- Capacity Index Sub-TLV type (TBD5)
- Preference Index Sub-TLV type (TBD6)
13. References
13.1. 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.
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13.2. 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.
[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.
14. Acknowledgments
Acknowledgements to 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.
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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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