Internet DRAFT - draft-dong-spring-srv6-inter-layer-programming
draft-dong-spring-srv6-inter-layer-programming
SPRING Working Group L. Han
Internet-Draft China Mobile
Intended status: Standards Track J. Dong
Expires: 14 September 2023 Huawei Technologies
Z. Du
M. Wang
China Mobile
13 March 2023
SRv6 for Inter-Layer Network Programming
draft-dong-spring-srv6-inter-layer-programming-05
Abstract
This document defines a new SRv6 function which can be used for SRv6
based inter-layer network programming. It is a variant of the SRv6
End.X behavior which is called "End.XU". Instead of pointing to an
interface with layer-3 adjacency, the End.XU behavior points to an
underlay interface which connects to a remote layer-3 node via
underlying paths or connections that may be invisible in the L3
topology.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Use Cases of SRv6 Inter-Layer Programming . . . . . . . . . . 3
2.1. IP and Optical Inter-layer Programming . . . . . . . . . 3
2.2. IP and MTN Inter-layer Programming . . . . . . . . . . . 4
3. SRv6 END.XU Behavior . . . . . . . . . . . . . . . . . . . . 4
4. Application of SRv6 End.XU . . . . . . . . . . . . . . . . . 5
4.1. IP and Optical Integration . . . . . . . . . . . . . . . 6
4.2. MTN Networks . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
In many network scenarios, operator owns a multi-layered network. In
layer-3, the technology has converged to IP, while there can be
different technologies in the layer-2 and layer-1. In such networks,
the cross-layer planning and optimization is considered more
efficient than independent planning and operation of the layer-3 and
the underlying networks in terms of resource utilization and SLA
assurance, but are also considered more complicated. Thus a
mechanism for flexible inter-layer network integration is desired.
Segment Routing over IPv6 (SRv6) [RFC8986] enables a network operator
or an application to specify a packet processing program by encoding
a sequence of instructions in the IPv6 packet header. Currently SRv6
does not consider about the network layers under the IP layer.
However, with the capability of SRv6 network programming, it is
possible to achieve seamless integration between IP (layer-3) and the
underlying (layer-2 and layer-1) networks.
Following the SRv6 network programming concept, a new SRv6 behavior
is defined for sending packet through an underlay interface, which
connects to underlay links or connections between two layer-3 nodes.
The underlay link or connection may be realized using either a Metro
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Transport Network (MTN) path[ITU-T_G.8310], an ODUk or a DWDM
connection. Such a SRv6 behavior can be considered as a variant of
the SRv6 END.X behavior as defined in [RFC8986]. The End.XU behavior
points to an underlay interface which connects to a remote layer-3
node via an underlying path or connection. Different from an L3
adjacency, the underlay path or connection can be unidirectional and
does not require bidirectional check. Thus the underlay path or
connection is invisible in the L3 topology and will not be used for
layer-3 route computation (e.g. SPF). However, this may be the
expected behavior when the underlay paths or connections are
provisioned for carrying specific types of services. The SRv6 End.XU
SIDs can be used together with other types of SRv6 SIDs to build SRv6
SID lists for inter-layer network programming.
This document first describes the typical use cases of SRv6 inter-
layer network programming, then new SRv6 End.XU behavior for inter-
layer network programming is defined. The application of SRv6 End.XU
in typical scenarios is also illustrated with examples.
1.1. Requirements Language
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.
2. Use Cases of SRv6 Inter-Layer Programming
2.1. IP and Optical Inter-layer Programming
In many network scenarios, the underlay of the IP network is an
optical network. The IP network and optical network are usually
managed separately, the optical network works as an underlay which is
invisible to the IP network. In some cases, the optical path
resource and the IP path resource may not be one-to-one mapping, the
redundant optical paths may not be fully used by the IP layer. In
some other cases, there may be optical paths between non-adjacent IP
nodes thus they are not visible in the L3 topology, and thus they can
not be used for carrying traffic based on layer-3 routing.
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2.2. IP and MTN Inter-layer Programming
The architecture of Metro Transport Network (MTN) is defined in
[ITU-T_G.8310]. In an MTN based network, network nodes can support
two forwarding modes: per-hop IP packet forwarding and the MTN Path
(MTNP) layer cross-connect. An MTN path is a multi-hop transport
path which may be established between any two nodes in the MTN
network, and the intermediate nodes of the MTN path will forward the
traffic solely based on the pre-established MTN cross-connect without
IP layer lookup. Thus an MTN path is an underlay connection between
two remote MTN nodes. Although in some cases it is possible to set
up a layer-3 adjacency between the two endpoints of the MTN path, it
will make the provisioning of MTN path complicated. Moreover, in
some cases the two endpoints may reside in different IGP areas or
ASes, which makes a layer-3 adjacency between them more challenging.
Since the MTN paths are not visible in the L3 topology, it is
difficult to compute and establish an inter-layer path which consists
of both the layer-3 network segments and the MTN paths.
3. SRv6 END.XU Behavior
This section defines a new SRv6 behavior for the underlay cross-
connect.
The "Endpoint with Underlay cross-connect" behavior ("End.XU" for
short) is a variant of the End.X behavior defined in [RFC8986]. Its
main use is for inter-layer network programming and traffic
engineering.
Any SID instance of this behavior is associated with an underlay
interface, which connects to one or more underlay links or
connections.
When N receives a packet destined to S and S is a local End.XU SID, N
does the following:
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S01. When an SRH is processed {
S02. If (Segments Left == 0) {
S03. Stop processing the SRH, and proceed to process the next
header in the packet, whose type is identified by
the Next Header field in the routing header.
S04. }
S05. If (IPv6 Hop Limit <= 1) {
S06. Send an ICMP Time Exceeded message to the Source Address
with Code 0 (Hop limit exceeded in transit),
interrupt packet processing, and discard the packet.
S07. }
S08. max_LE = (Hdr Ext Len / 2) - 1
S09. If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
S10. Send an ICMP Parameter Problem to the Source Address
with Code 0 (Erroneous header field encountered)
and Pointer set to the Segments Left field,
interrupt packet processing, and discard the packet.
S11. }
S12. Decrement IPv6 Hop Limit by 1
S13. Decrement Segments Left by 1
S14. Update IPv6 DA with Segment List[Segments Left]
S15. Forward the packet through the underlay interface associated
with SID S
S16. }
Note that the underlay interface and the associated connection in
step 15 SHOULD be established before the associated End.XU SID is
announced into the network.
End.XU SIDs MAY be announced using IGP or BGP-LS in a similar way to
the announcement of the End.X SIDs, while they need to be
distinguished from the End.X SID by both the network nodes and the
network controller. The detailed protocol extension will be
described in a separate document. Then the network controller or a
headend node could use the End.XU SIDs together with other types of
SRv6 SIDs to build SID lists for inter-layer network paths.
4. Application of SRv6 End.XU
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4.1. IP and Optical Integration
Assuming that an operator owns both the IP and optical network, and
the operator needs to deploy E2E service across IP and optical
network, with traditional approaches the planning and service
provisioning would be complex and time consuming due of the manual
synergy needed between the operator's IP team and optical team. With
the introduction of SRv6 and the End.XU behavior, one simplified
approach for IP and optical integration is to build a SID list that
integrates the path in both the IP layer and the optical layer.
As the optical layer is not packet based, source routing mechanism
can not be directly used in the optical network. However, the
abstracted optical paths (e.g., with ODUk or DWDM) could be exposed
to the control system of the IP network using the SRv6 End.XU SIDs,
and some of the attributes of the optical paths may also be provided.
Based on this information, IP-optical inter-layer paths can be
programmed to meet some specific service requirements, such as low
latency.
----- ----- -----
| P1 |--------| P2 |--------| P3 |
----- ----- -----
/ |. |. |. \
----- / | . | . | . \ -----
| P7 | | . | . | . | P8 |
----- \ | . | . | ./ -----
\ | . | . | / .
----- . ----- . ----- .
| P4 |-------| P5 |--------| P6 | .
----- . ----- . ----- .
. . . . . .
. ===== . ===== . =====
. | O1 |----------| O2 |--------| O3 |
. ===== . ===== . =====
. | . | . |
. | . | . |
. | . | . |
. | . | . |
.| .| .|
===== ===== =====
| O4 |----------| O5 |--------| O6 |
===== ===== =====
Figure 1. IP and Optical Layered Network Topology
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In Figure 1, P1 to P8 are IP nodes, and O1 to O6 are optical nodes.
Assume the operator needs to deploy a low latency path between P7 and
P8. With normal segment routing, an IP layer path with the segment
list {P7, P1, P2, P3, P8} can be used. But if an optical path from
O1 to O3 exists, and the End.XU SID defined in this document is used
to announce this optical path as an underlay connection with specific
attributes into the IP network, the headend node or the controller in
IP layer can program an inter-layer path along {P7, P1, End.XU (O1,
O2, O3), P3, P8} which may provide lower latency.
The optical path between O1 and O3 may be created in advance or as a
result of the request from the IP layer. The creation should be done
by the optical network controller (not shown in the Figure). The
details of the process are out of scope of this document, and may
refer to [I-D.ietf-teas-actn-poi-applicability].
There is also another case of IP and Optical integration. Assume
there are two optical paths between P1 and P2. One is {P1, O1, O2,
P2} , and the other is {P1, O1, O4, O5, O2, P2}. Two separate End.XU
SIDs are allocated for these two underlay connections separately.
One is End.XU P1::C2 for the underlay path {P1, O1, O2, P2}, and the
other is End.XU P1::C45 for the path {P1, O1, O4, O5, O2, P2}. The
headend P7 or the IP network controller will be informed about these
two SRv6 End.XU SIDs and the associated path attributes, so that the
headend or the controller can program different end-to-end inter-
layer paths using SID lists with different End.XU SIDs for services
with different SLA requirements.
4.2. MTN Networks
Assuming that an operator owns both an MTN network domain and an IP
network domain. In the MTN network, each MTN node has both the
layer-3 functionality and the MTN Path layer functionality. In
layer-3, all the MTN nodes are in a layer-3 network topology, which
connects to the IP network domain. In the MTN Path Layer, a set of
MTN paths are provisioned between the selected pairs of MTN nodes.
In the MTN network, different types of services may be carried using
either a layer-3 path, or an MTN path, or an inter-layer path
comprising of both the layer-3 links and the MTN path as different
segments. In addition, For some type of services, end-to-end paths
across the IP domain and the MTN domain are needed, which is
comprised of both the layer-3 paths and the MTN path as different
segments.
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.......................................... ...........................
. . . .
. +----+ +----+ +----+ . . +----+ +----+ .
. | M1 |-----| M2 |-----| M3 |------| P1 |-----| P2 | .
. +----+ +----+ +----+ . . +----+ +----+ .
. / | | | . . | | \ .
. +----+ / | | | . . | | \+----+.
. | M7 |/ | | | . . | | | P5 |.
. +----+\ | | | . . | | /+----+.
. \ | | | . . | | / .
. \+----+ +----+ +----+ . . +----+ +----+ .
. | M4 |-----| M5 |-----| M6 |------| P3 |-----| P4 | .
. +----+ +----+ +----+ . . +----+ +----+ .
. . . .
. Layer-3 Topology MTN Network . . IP Network .
. . ...........................
----------------------------------------------------------------------
. MTN Path Layer Topology .
. .
. +----+ +----+ +----+ .
. | M1'|################| M3'| .
. +----+ ## +----+ ## +----+ .
. ## ## .
. +----+ ## ## .
. | M7'| ## .
. +----+ ## ## .
. ## ## .
. +----+ ## +----+ ## +----+ .
. | M4'|################| M6'| .
. +----+ +----+ +----+ .
. .
. .
..........................................
.
Figure 2. A network with MTN Domain and IP Domain
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Figure 2 gives an example of a network with a MTN domain and an IP
domain. M1 to M7 are MTN nodes, and P1 to P4 are IP nodes. The same
set of MTN nodes builds two separate network layers. The topology in
the IP layer shows the layer-3 connectivity between the MTN nodes and
the connectivity with the IP network domain, while the topology in
the MTN Path layer shows the MTN paths between the selected pair of
MTN nodes. An end-to-end path from M7 to P5 can be established in
layer-3 using a SID list representing the layer-3 path {M7, M1, M2,
M3, P1, P2, P5}. While for services which require low latency, an
end-to-end path consisting of both the layer-3 segments and MTN paths
could be established using an SRv6 SID list representing the path
{M7, M1::C3, P1, P2, P5}, where the End.XU SID M1::C3 represents the
MTN path M1'-M3'.
This shows that it is convenient to use an integrated SID list to
program an inter-layer path both within the MTN domain, and across
the IP and MTN domain using the combination of L3 SRv6 SIDs and the
End.XU SIDs.
5. Security Considerations
TBD
6. IANA Considerations
This document defines a new SRv6 Endpoint behavior called END.XU.
IANA has allocated the following code points for different flavors of
End.XU from the "SRv6 Endpoint Behaviors" sub-registry in the
"Segment-routing with IPv6 data plane (SRv6) Parameters" registry:
+------+--------+------------------------------------------+-----------+
| Value| Hex | Endpoint Behavior | Reference |
+------+--------+------------------------------------------+-----------+
| 150 | 0x0096 | End.XU | [This ID] |
| 151 | 0x0097 | End.XU with PSP | [This ID] |
| 152 | 0x0098 | End.XU with USP | [This ID] |
| 153 | 0x0099 | End.XU with USD | [This ID] |
| 154 | 0x009A | End.XU with PSP, USP & USD | [This ID] |
| 155 | 0x009B | End.XU with REPPLACE-CSID | [This ID] |
| 156 | 0x009C | End.XU with REPPLACE-CSID & PSP | [This ID] |
| 157 | 0x009D | End.XU with REPPLACE-CSID, PSP, USP & USD| [This ID] |
+------+--------+------------------------------------------+-----------+
7. Acknowledgements
The authors would like to thank Xiaodong Chang, Yongjian Hu, Ketan
Talaulikar and Zhibo Hu for their review and comments.
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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>.
[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>.
8.2. Informative References
[I-D.ietf-teas-actn-poi-applicability]
Peruzzini, F., Bouquier, J., Busi, I., King, D., and D.
Ceccarelli, "Applicability of Abstraction and Control of
Traffic Engineered Networks (ACTN) to Packet Optical
Integration (POI)", Work in Progress, Internet-Draft,
draft-ietf-teas-actn-poi-applicability-08, 11 January
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
teas-actn-poi-applicability-08>.
[ITU-T_G.8310]
ITU-T, "ITU-T G.8310: Architecture of the metro transport
network", https://www.itu.int/rec/T-REC-
G.8310-202012-I/en, December 2020.
Authors' Addresses
Liuyan Han
China Mobile
No.32 XuanWuMen West Street
Beijing, 100053
China
Email: hanliuyan@chinamobile.com
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Jie Dong
Huawei Technologies
Huawei Campus, No.156 Beiqing Road
Beijing, 100095
China
Email: jie.dong@huawei.com
Zongpeng Du
China Mobile
No.32 XuanWuMen West Street
Beijing, 100053
China
Email: duzongpeng@foxmail.com
Minxue Wang
China Mobile
No.32 XuanWuMen West Street
Beijing, 100053
China
Email: wangminxue@chinamobile.com
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