Internet DRAFT - draft-song-mpls-sr-eh
draft-song-mpls-sr-eh
Network Working Group H. Song, Ed.
Internet-Draft Futurewei Technologies
Intended status: Standards Track 15 May 2023
Expires: 16 November 2023
Segment Routing with MPLS Extension Header
draft-song-mpls-sr-eh-01
Abstract
This document describes an alternative approach to implement segment
routing in MPLS networks with the use of a post-stack MPLS extension
header under the MPLS network action framework. The idea is
applicable to other encoding styles for post-stack MPLS network
actions. The new approach reduces the MPLS label stack depth and
provide supports for advanced functions such as network programming
similar to SRv6.
Requirements Language
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 16 November 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Segment Routing with MPLS Extension Header . . . . . . . . . 3
3. Segment Routing Data Plane Processing . . . . . . . . . . . . 5
4. Support SFC using SR with MPLS Extension Header . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Motivation
Segment Routing (SR) [RFC8402] allows a node to steer a packet based
on an ordered list of segments. It can be applied on an MPLS data
plane (i.e., SR-MPLS) or an IPv6 data plane (i.e., SRv6). In SR-
MPLS, each segment identifier (SID) is encoded in an MPLS label
[RFC8660]. The SID list forms an MPLS label stack. Each hop will
pop the top label from a packet's label stack and forward the packet
based on the SID encoded in the label.
MPLS has a wide deployment base and SR-MPLS can be directly applied
on an MPLS data plane without any change. Meanwhile, SR-MPLS's SID
overhead is small and each SID in SR-MPLS is only 4 bytes.
However, SR-MPLS has several drawbacks:
* The SID label stack may be deep, which can hurt the forwarding
performance when the bottom of stack needs to be accessed or deep
packet inspection needs to be performed. For example, network
load balancing based on entropy label [RFC6790] or IP header, and
other network services such as In-situ OAM
[I-D.ietf-ippm-ioam-data], rely on headers deeply embedded in a
packet. A deep MPLS label stack is unfavorable in such occasions.
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* While the compactness of an MPLS label helps to reduce the header
overhead, it leaves no room to encode extra information other than
SID. Because of this, a noticeable missing feature of SR-MPLS is
the network programming [RFC8986]. Network programming is a
powerful feature of SRv6. It enables a network operator or an
application to specify a packet processing program by encoding a
sequence of instructions in the IPv6 packet header. Obviously, it
is preferred for SR-MPLS to support the same feature as well.
In MPLS networks, MPLS Network Action (MNA) [I-D.ietf-mpls-mna-fwk]
extends the MPLS label stack by supporting extra network actions
encoded both in stack and post stack. The post-stack actions are
encapsulated in MPLS extension headers
[I-D.song-mpls-extension-header]. SR in MPLS can be implemented with
an extension header. If other post-stack MNA encoding style is
adopted, the method is still valid. The new approach not only
retains MPLS's advantages but also overcomes its drawbacks.
2. Segment Routing with MPLS Extension Header
With the presence of MPLS extension header, the SID label stack is
kept in an extension header. Only the current SID label is copied to
the top of the MPLS label stack. An example for the packet format is
shown in Figure 1.
0 31
+--------+--------+--------+--------+ -
| SID Label | ^
+--------+--------+--------+--------+ | MPLS Label Stack
| Post-stack MNA Indicator | |
~ ~ V
+--------+--------+--------+--------+ -
| HEH | NH=SR | ^
+--------+--------+--------+--------+ |
| | | MPLS SR Extension Header
~ Segment Routing Header (SRH) ~ |
| | V
+--------+--------+--------+--------+ -
| | ^
~ Upper Layer Protocols/Payload ~ | Upper Layer Packet
| | V
+--------+--------+--------+--------+ -
Figure 1: SR in MPLS with Extension Header
The format of the extension header for the SID list, or the Segment
Routing Header (SRH), is shown in Figure 2.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NH=UL | HLEN | Segment Count |Segment Pointer|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID[0] | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| FUNCT and ARGS [0] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ ... ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SID[n-1] | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
| FUNCT and ARGS [n-1] |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Optional TLVs ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SRH Format
The meaning of the fields in an SRH is as follows:
NH: As defined in [I-D.song-mpls-extension-header].
HLEN: As defined in [I-D.song-mpls-extension-header].
Segment Count: 8-bit unsigned integer for the number of SIDs in the
segment list.
Segment Pointer: 8-bit index (zero based) of the current SID in the
segment list.
SID[i]: The i-th 20-bit SID. SID[0] is the first segment of the
path.
FUNCT and ARGS[i]: The i-th 108-bit instructions and parameters for
network programming at the i-th segment.
TLVs: Optional Type-Length-Value, TBD.
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The operator is free to partition the FUNCT and ARGS field to encode
the function and parameters at a segment.
3. Segment Routing Data Plane Processing
The SR source node generates the SRH. The Segment Pointer field of
the SRH is initialized to 0. The SRH is inserted into the MPLS
packet as an MPLS extension header. The SID[0] in the SRH is copied
into an MPLS label. The TTL field in the label is initialized to a
configured value. The label is pushed to the top of the label stack.
The packet is then forwarded based on the top label.
At each node, if the SID in the top label matches the local SID, the
function associated with the SID in the SRH is executed. If there
are still segment(s) left in the segment list (i.e., Segment Count >
Segment Pointer + 1), then the Segment Pointer in the SRH is
incremented by 1, and the SID pointed by the Segment Pointer is
copied to the top label in the MPLS label stack. The TTL field in
the top label is decremented by 1. The packet is then forwarded
based on the top label.
If the current segment is the last segment, the top label is popped
and the SRH extension header is deleted. The packet is then
forwarded based on the header of the remaining packet.
4. Support SFC using SR with MPLS Extension Header
The document [I-D.ietf-spring-sr-service-programming] describes how
the SFC [RFC7665] can be achieved through SR-MPLS. Similarly, the
Segment Routing with MPLS Extension Header can also realize the
service chaining, with additional advantages over the previous
proposal.
A noticeable issue of the SR-MPLS based SFC is its lack of metadata
carrying capability. Metadata may be critical for message passing
and information sharing between service functions. This drawback
limits the applicability of SR-MPLS for SFC. In our solution, the
metadata can be carried in the optional TLVs in the SRH.
Another document [I-D.ietf-spring-nsh-sr] proposes to integrate the
SR and the NSH [RFC8300] to better support SFC, in which NSH provides
a service plane and SR supports transport. Again, in our proposal,
the NSH can be encapsulated into a TLV in the SRH.
5. Security Considerations
This document shares the same security considerations with the SR-
MPLS, network-programming, and SFC.
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6. IANA Considerations
This document requires IANA to assign a new protocol number (TBA1) to
indicate the SID list.
7. Contributors
TBD.
8. Acknowledgments
TBD.
9. References
9.1. 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>.
9.2. Informative References
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In Situ Operations, Administration, and Maintenance
(IOAM)", Work in Progress, Internet-Draft, draft-ietf-
ippm-ioam-data-17, 13 December 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-ippm-
ioam-data-17>.
[I-D.ietf-mpls-mna-fwk]
Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
Network Actions Framework", Work in Progress, Internet-
Draft, draft-ietf-mpls-mna-fwk-03, 11 March 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
mna-fwk-03>.
[I-D.ietf-spring-nsh-sr]
Guichard, J. and J. Tantsura, "Integration of Network
Service Header (NSH) and Segment Routing for Service
Function Chaining (SFC)", Work in Progress, Internet-
Draft, draft-ietf-spring-nsh-sr-13, 27 April 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
nsh-sr-13>.
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[I-D.ietf-spring-sr-service-programming]
Clad, F., Xu, X., Filsfils, C., Bernier, D., Li, C.,
Decraene, B., Ma, S., Yadlapalli, C., Henderickx, W., and
S. Salsano, "Service Programming with Segment Routing",
Work in Progress, Internet-Draft, draft-ietf-spring-sr-
service-programming-07, 15 February 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
sr-service-programming-07>.
[I-D.song-mpls-extension-header]
Song, H., Zhou, T., Andersson, L., Zhang, Z. J., and R.
Gandhi, "MPLS Network Actions using Post-Stack Extension
Headers", Work in Progress, Internet-Draft, draft-song-
mpls-extension-header-12, 14 April 2023,
<https://datatracker.ietf.org/doc/html/draft-song-mpls-
extension-header-12>.
[RFC6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC6790, November 2012,
<https://www.rfc-editor.org/info/rfc6790>.
[RFC7665] Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
Chaining (SFC) Architecture", RFC 7665,
DOI 10.17487/RFC7665, October 2015,
<https://www.rfc-editor.org/info/rfc7665>.
[RFC8300] Quinn, P., Ed., Elzur, U., Ed., and C. Pignataro, Ed.,
"Network Service Header (NSH)", RFC 8300,
DOI 10.17487/RFC8300, January 2018,
<https://www.rfc-editor.org/info/rfc8300>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC8660, December 2019,
<https://www.rfc-editor.org/info/rfc8660>.
[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>.
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[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>.
Author's Address
Haoyu Song (editor)
Futurewei Technologies
2330 Central Expressway
Santa Clara,
United States of America
Email: haoyu.song@futurewei.com
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