SPRING Working Group Louis Chan
INTERNET-DRAFT
Intended status: Standard Track Juniper Networks
Expires: Jan 4, 2023 Jul 4, 2022
Sub-slicing for SRv6
draft-chan-spring-srv6-sub-slice-00.txt
Abstract
This document describes how to achieve further slicing or traffic engineering
interoperability between vendors without the use of SRH.
Slicing or traffic engineering information is encapsulated as part of the SRv6 SID.
Use of IP longest prefix match approach to identify the further slicing via sub-
slice identifier.
The traffic engineering from one end to another end is seen as segment by segment
approach. This approach could solve the scalability of traffic engineering tunnels
required in a huge network, which order of N^2 has be considered.
Status of this Memo
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This Internet-Draft will expire on Jan 4, 2023.
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Table of Contents
1. Introduction...................................................2
2. Conventions used in this document..............................2
3. Encoding sub-slice in FUNCT:ARG................................3
4. Example of operation...........................................3
5. Solution to possible looping issue.............................4
6. Compatibility with SRv6 compression............................4
7. Interoperability consideration.................................5
8. Multi-level sub-slicing........................................5
9. Security Considerations........................................5
10. Others........................................................6
11. References....................................................6
11.1. Normative References.....................................6
11.2. Informative References...................................6
12. Acknowledgments...............................................6
1. Introduction
The purpose of this document is to describe to a way to signal the desired slicing
or sub-slicing information with the SRv6 endpoint behavior SID.
The FUNCT:ARG portion of SRv6 SID is encoded with certain format to achieve such.
In the transit router, when the SRv6 packet is received, it is processed with IPv6
longest prefix match (LPM) approach, which in turn, could point the packet to
another tunnel, likely a SRv6-TE tunnel. The effect is similar to a binding
SID approach.
The benefits of this approach are
- Provide an easy interoperability method between vendors for slicing without the
full SRH header. This format looks legitimate to any vendors.
- Reduce the tunnels to be provisioned in the network based on tactical TE
strategy. It would give less work to controller to handle huge number of tunnels
in a big scale network.
- Provide an intrinsic backup path. Secondary path provisioning is not a
requirement.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119 [RFC2119].
In this document, these words will appear with that interpretation only when in ALL
CAPS. Lower case uses of these words are not to be interpreted as carrying
significance described in RFC 2119.
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3. Encoding sub-slice in FUNCT:ARG
The method is to encode the sub-slice information into the FUNCT:ARG or the
endpoint portion
+------------+---------------+--------------------------+
| Locator | Sub-slice ID | Remainder for behavior |
+------------+---------------+--------------------------+
|<- Endpoint Behavior ->|
This format could be used in conjunction WITH or WITHOUT Flex-Algo. If it is used
with Flex-Algo, network slicing is expected. Therefore, the use of the term, sub-
slice, is to allow further level of slicing within Flex-Algo.
The encoding of sub-slice information is right from the originator of endpoint
behavior. For example, the sub-slice information is encoded in END.DT4 and END.DT6
from the originator of the VPN.
No SRH is required in the first ingress PE to send out the packet.
Each node in transit would evaluate the IPv6 header according to longest prefix
match rule as normal IP processing. It will forward the packet according to locator
routes it learnt from routing protocol.
When a more specific IPv6 route with "locator + sub-slice", which is a longer
prefix, is programmed in the routing table, the forwarding decision would be
redirected to another TE tunnel. In this case, sub-slice is achieved.
4. Example of operation
R1--/--C2--/--C3--/--R4 ; "/ " means network in between
Between R1 and R4, it runs two L3VPN with END.DT4 SID.
Sub-slice TE tunnel is created between C2 and C3 for one of the VPN above.
The locator is encoded as
FC00:xxxx:nnnn::/48, where xxxx is the Flex-Algo portion. Here is 0200 as example
nnnn is the locator portion. R1: 0001, R4: 0004
For the END.DT4 announced from R4
VPN1: FC00:0200:0004::1000
VPN2: FC00:0200:0004:0008::2000 ; 0008 here is a sub-slice ID
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From R1, it sends packets for both VPNs without any SRH. Below example only shows
the headers but not the payload.
In C2, its routing table has two entries.
a) FC00:0200:0004::0/48 - This is learnt via routing protocol
b) FC00:0200:0004:0008::0/64 - This entry is programmed by other means for TE
For VPN1 traffic, C2 will forward the packet using (a) route.
For VPN2 traffic, C2 will forward the traffic to another path via (b) route, and
attach tunnel information, like SRv6-TE. An example of additional hops programmed
in SRH sending out from C2 to C3 could be
FC00:0200:pppp::1 ; add SRH with nodes
FC00:0200:qqqq::1 ; where pppp and qqqq represent intermediate nodes
The SRv6-TE tunnel could be in the form of either insert mode or encapsulation
mode. C3 could received the packet with or without SRH depending on the
configuration.
C3 should remove SRH header if it is the endpoint of the tunnel. It would continue
to forward the packet according to FC00:0200:0004:0008::2000.
Between C2 and C3, sub-slice is thus achieved for VPN2 traffic.
It depends on C3 or subsequent routers' forwarding table programming. Another TE
action could be imposed based on /64 LPM interpretation. This allows easy
interoperability between vendors. C3 might be the border router from another vendor
domain.
If the above C2 to C3 tunnel is down, C2 would forward the packet using /48 route,
which is the default Flex-Algo route. Hence, backup path is readily available.
Secondary tunnel provisioning is therefore optional.
5. Solution to possible looping issue
There is a possibility for routing loop in certain scenario. The options to
minimize the risk are
- Running OAM to detect the TE tunnel reachability up to the final end node.
- TE tunnel for sub-slice should be terminated on a trusted node, probably an ABR.
When the packet arrives this trusted node, either the packet is forwarded
properly, or discarded silently.
- Use of controller to detect the possibility of routing loop. Avoid or disable
the TE path for sub-slice when necessary.
6. Compatibility with SRv6 compression
TBD for sub-slice notation.
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For the TE tunnel portion, it would be compatible to any SRv6 compression scheme.
This sub-slice would provide an interoperability between different domains which
run different kind of SRv6 compression mechanism. The lowest denominator is an SRv6
packet without SRH.
7. Interoperability consideration
The benefit of this approach is to allow a clean handover of a SRv6 packet from one
domain to another domain.
With the use of Flex-Algo [FLEXALGO] in SRv6, there would be more granularity of
slicing information present in the header. A further example from section 4,
FC00:0200:0004:0008::2000 - 0200 means Flex-Algo A, sub-slice 0008
FC00:0300:0004:0009::2000 - 0300 means Flex-Algo B, sub-slice 0009
The two SRv6 connected domains do not require use of the same tunnel technology,
like plain Flex-Algo, SRv6-TE with or without compression. Each domain would have
enough information from the packet header to steer the traffic into specific
tunnel, if required.
On the other hand, the overhead of traffic engineering header is reduced from end
to end. If there is TE header overhead, it is reduced to local domain consumption
only.
8. Multi-level sub-slicing
Multi-level of sub-slice is easily achievable via different prefix length. For
example, /48 up to the locator, /56 for first level sub-slice and /64 for second
level sub-slice.
As an example, /56 case could be used as an aggregate for a group of the endpoint
behavior function, and /64 are used on individual.
a) FC00:0200:0004::0/48 - No sub-slice
b) FC00:0200:0004:01xx::0/56 - 01 denotes a sub-slice
c) FC00:0200:0004:0103::0/64 - 0103 denotes further sub-slice. A more specific TE
With a longer prefix length, the traffic could be directed to more specific traffic
engineering path. The TE path could use any kind of tunnel method, like SRv6-TE
without header compression, or with compression.
9. Security Considerations
TBD
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10. Others
This proposed method also allow SRv6 traffic to tunnel through non-SRv6 domain in
the middle. Router C2 in the above example could initiate other kinds of tunnel,
which could be RSVP LSP, SR-TE LSP and etc.
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels",
BCP 14, RFC 2119, March 1997.
11.2. Informative References
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[RFC8754] Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
(SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
<https://www.rfc-editor.org/info/rfc8754>.
[RFC8986] Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
(SRv6) Network Programming", RFC 8986,
DOI 10.17487/RFC8986, February 2021,
<https://www.rfc-editor.org/info/rfc8986>.
[FLEXAGLO] S. Hegde, P. Psenak and etc, IGP Flexible Algorithm
<https://datatracker.ietf.org/doc/draft-ietf-lsr-flex-algo>
12. Acknowledgments
The following people have contributed to this document:
Salih K A, Juniper Networks
Author Address
Louis Chan (editor)
Juniper Networks
2604, Cityplaza One, 1111 King's Road
Taikoo Shing
Hong Kong
Phone: +852-25876659
Email: louisc@juniper.net
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