MPLS H. Song, Ed.
Internet-Draft Futurewei Technologies
Intended status: Standards Track Z. Li
Expires: September 11, 2021 T. Zhou
Huawei
L. Andersson
Bronze Dragon Consulting
March 10, 2021
MPLS Extension Header
draft-song-mpls-extension-header-03
Abstract
Motivated by the need to support multiple in-network services and
functions in an MPLS network, this document describes a generic and
extensible method to encapsulate extension headers into MPLS packets.
The encapsulation method allows stacking multiple extension headers
and quickly accessing any of them as well as the original upper layer
protocol header and payload. We show how the extension header can be
used to support several new network applications and optimize some
existing network services.
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.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 11, 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. MPLS Extension Header . . . . . . . . . . . . . . . . . . . . 4
3. Type of MPLS Extension Headers . . . . . . . . . . . . . . . 7
4. Operation on MPLS Extension Headers . . . . . . . . . . . . . 7
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Motivation
Some applications require adding instructions and/or metadata to user
packets within a network. Such examples include In-situ OAM (IOAM)
[I-D.ietf-ippm-ioam-data] and Service Function Chaining (SFC)
[RFC7665]. New applications are emerging. It is possible that the
instructions and/or metadata for multiple applications are stacked
together in one packet to support a compound service.
Conceivably, such instructions and/or metadata would be encoded as
new headers and encapsulated in user packets. Such headers may
require to be processed in fast path or in slow path. Moreover, such
headers may require being attended at each hop on the forwarding path
(i.e., hop-by-hop or HBH) or at designated end nodes (i.e., end-to-
end or E2E).
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The encapsulation of the new header(s) poses some challenges to MPLS
networks, because the MPLS protocol header contains no explicit
indicator for the upper layer protocols by design. We leave the
discussion on the indicator of new header(s) in an MPLS packet to
another companion document [I-D.song-mpls-eh-indicator]. In this
document, we focus on the encode and encapsulation of new headers in
an MPLS packet.
The similar problem has been tackled for some particular application
before. However, the solutions have some drawbacks:
o These solutions rely on either the built-in next-protocol
indicator in the header or the knowledge of the format and size of
the header to access the following packet data. The node is
required to be able to parse the new header, which is unrealistic
in an incremental deployment environment.
o A piecemeal solution often assumes the new header is the only
extra header and its location in the packet is fixed by default.
It is impossible or difficult to support multiple new headers in
one packet due to the conflicted assumption.
To solve these issues, we propose to introduce extension header as a
general and extensible means to support new in-network functions and
applications in MPLS networks. The idea is similar to IPv6 extension
headers which offer a huge innovation potential (e.g, network
security, SRv6 [RFC8754], network programming
[I-D.ietf-spring-srv6-network-programming], SFC
[I-D.xu-clad-spring-sr-service-chaining], etc.). Thanks to the
existing of extension headers, it is straightforward to introduce new
in-network services into IPv6 networks. For example, it has been
proposed to carry IOAM header [I-D.brockners-inband-oam-transport] as
a new extension header in IPv6 networks.
Nevertheless, IPv6 is not perfect either. It has two main issues.
First, IPv6's header is large compared to MPLS, claiming extra
bandwidth overhead and complicating the packet processing. We prefer
to retain the header compactness in MPLS networks. Second, IPv6's
extension headers are chained with the original upper layer protocol
headers in a flat stack. One must scan all the extension headers to
access the upper layer protocol headers and the payload. This is
inconvenient and raises some performance concerns for some
applications (e.g., DPI and ECMP). The new scheme for MPLS header
extension needs to address these issues too.
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2. MPLS Extension Header
From the previous discussion, we have laid out the design
requirements to support extension headers in MPLS networks:
Performance: If possible, unnecessary label stack scanning for a
label and extension header stack scanning for the upper layer
protocol should be avoided.
Scalability: New applications can be easily supported by introducing
new extension headers. Multiple extension headers can be easily
stacked together to support multiple services simultaneously.
Backward Compatibility: Legacy devices which do not recognize the
extension header option should still be able to forward the
packets as usual. If a device recognize some of the extension
headers but not the others in an extension header stack, it can
process the known headers only while ignoring the others.
We assume the MPLS label stack has included some indicator of the
extension header(s). The actual extension headers are inserted
between the MPLS label stack and the original upper layer packet
header. The format of the MPLS packets with extension headers is
shown in Figure 1.
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0 31
+--------+--------+--------+--------+ \
| | |
~ MPLS Label Stack ~ |
| | |
+--------+--------+--------+--------+ |
| EH Indicator (TBD) | > MPLS Label Stack
+--------+--------+--------+--------+ | (extended with EHI)
| | |
~ MPLS Label Stack ~ |
| | |
+--------+--------+--------+--------+ <
| Header of Extension Headers (HEH) | |
+--------+--------+--------+--------+ |
| | |
~ Extension Header (EH) 1 ~ |
| | |
+--------+--------+--------+--------+ > MPLS EH Fields
~ ~ | (new)
+--------+--------+--------+--------+ |
| | |
~ Extension Header (EH) N ~ |
| | |
+--------+--------+--------+--------+ <
| | |
~ Upper Layer Headers/Payload ~ > MPLS Payload
| | | (as is)
+--------+--------+--------+--------+ /
Figure 1: MPLS with Extension Headers
Following the MPLS label stack is the 4-octet Header of Extension
Headers (HEH), which indicates the total number of extension headers
in this packet, the overall length of the extension headers, and the
type of the next header. The format of the HEH is shown in Figure 2.
0 1 2 3
0123 45678901 234567890123 45678901
+----+--------+------------+--------+
| R | EHCNT | EHTLEN | NH |
+----+--------+------------+--------+
Figure 2: HEH Format
The meaning of the fields in an HEH is as follows:
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R: 4-bit reserved.
EHCNT: 8-bit unsigned integer for the Extension Header Counter.
This field keeps the total number of extension headers included in
this packet. It does not count the original upper layer protocol
headers.
EHTLEN: 12-bit unsigned integer for the Extension Header Total
Length in 4-octet units. This field keeps the total length of the
extension headers in this packet, not including the HEH itself.
NH: 8-bit selector for the Next Header. This field identifies the
type of the header immediately following the HEH.
The EHCNT field can be used to keep track of the number of extension
headers when some headers are inserted or removed at some network
nodes. The EHLEN field can help to skip all the extension headers in
one step if the original upper layer protocol headers or payload need
to be accessed.
The format of an Extension Header (EH) is shown in Figure 3.
0 1 2 3
01234567 89012345 6789012345678901
+--------+--------+----------------+
| NH | HLEN | |
+--------+--------+ +
| |
~ Header Specific Data ~
| |
+--------+--------+----------------+
Figure 3: EH Format
The meaning of the fields in an EH is as follows:
NH: 8-bit selector for the Next Header. This field identifies the
type of the EH immediately following this EH.
HLEN: 8-bit unsigned integer for the Extension Header Length in
4-octet units, not including the first 4 octets.
Header Specific Data: Variable length field for the specification of
the EH. This field is 4-octet aligned.
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The extension headers as well as the first original upper layer
protocol header are chained together through the NH field in HEH and
EHs. The encoding of NH uses the same values as the IPv4 protocol
field. Values for new EH types shall be assigned by IANA.
Specifically, the NH field of the last EH in a chain can have two
special values, which shall be assigned by IANA:
NONE (No Next Header): Indicates that there is no other header and
payload after this header. This can be used to transport packets
with only extension header(s).
UNKNOWN (Unknown Next Header): Indicates that the type of the header
after this header is unknown. This is intended to be compatible
with the original MPLS design in which the upper layer protocol
type is unknown from the MPLS header alone.
3. Type of MPLS Extension Headers
Basically, there are two types of MPLS EHs: HBH and E2E. E2E means
that the EH is only supposed to be inserted/removed and processed at
the MPLS tunnel end points where the MPLS header is inserted or
removed. The EHs that are inserted or removed within the MPLS tunnel
are of the HBH type. However, any node in the tunnel can be
configured to ignore an HBH EH, even if it is capable of processing
the EH.
If there are two types of EHs in a packet, the HBH EHs must take
precedence over the E2E EHs.
Making a distinction of the EH types and ordering the EHs in a packet
help improve the forwardidng performance. For example, if a node
within an MPLS tunnel finds only E2E EHs in a packet, it can avoid
scanning the EH list.
4. Operation on MPLS Extension Headers
When the first EH X needs to be added to an MPLS packet, an EH
indicator is inserted into the proper location in the MPLS label
stack. A HEH is then inserted after the MPLS label stack, in which
EHCNT is set to 1, EHTLEN is set to the length of X in 4-octet units,
and NH is set to the header value of X. At last, X is inserted after
the HEH, in which NH and HELN are set accordingly. Note that if this
operation happens at a PE device, the upper layer protocol is known
before the MPLS encapsulation, so its value can be saved in the NH
field if desired. Otherwise, the NH field is filled with the value
of "UNKNOWN".
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When an EH Y needs to be added to an MPLS packet which already
contains extension header(s), the EHCNT and EHTLEN in the HEH are
updated accordingly (i.e., EHCNT is incremented by 1 and EHTLEN is
incremented by the size of Y in 4-octet units). Then a proper
location for Y in the EH chain is located. Y is inserted at this
location. The NH field of Y is copied from the previous EH's NH
field (or from the HEH's NH field, if Y is the first EH in the
chain). The previous EH's NH value, or, if Y is the first EH in the
chain, the HEH's NH, is set to the header value of Y.
Deleting an EH simply reverses the above operation. If the deleted
EH is the last one, the EH indicator and HEH can also be removed.
When processing an MPLS packet with extension headers, the node needs
to scan through the entire EH chain and process the EH one by one.
The node should ignore any unrecognized EH.
The EH can be categorized into HBH or E2E. If the EH indicator can
indicate the EH types and the EHs are ordered (i.e., HBH EHs are
located before E2E EHs), a node can avoid some unnecessary EH scan.
5. Use Cases
In this section, we show how MPLS extension header can be used to
support several new network applications.
In-situ OAM: In-situ OAM (IOAM) records flow OAM information within
user packets while the packets traverse a network. The
instruction and collected data are kept in an IOAM header
[I-D.ietf-ippm-ioam-data]. When applying IOAM in an MPLS network,
the IOAM header can be encapsulated as an MPLS extension header.
Network Telemetry and Measurement: A network telemetry and
instruction header can be carried as an extension header to
instruct a node what type of network measurements should be done.
For example, the method described in [RFC8321] can be implemented
in MPLS networks since the EH provides a natural way to color MPLS
packets.
Network Security: Security related functions often require user
packets to carry some metadata. In a DoS limiting network
architecture, a "packet passport" header is used to embed packet
authentication information for each node to verify.
Segment Routing and Network Programming: MPLS extension header can
support the implementation of a new flavor of the MPLS-based
segment routing, with better performance and richer
functionalities. The details will be described in another draft.
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With MPLS extension headers, multiple in-network applications can be
stacked together. For example, IOAM and SFC can be applied at the
same time to support network OAM and service function chaining. A
node can stop scanning the extension header stack if all the known
headers it can process have been located. For example, if IOAM is
the first EH in a stack and a node is configured to process IOAM
only, it will stop searching the EH stack when the IOAM EH is found.
6. Security Considerations
TBD
7. IANA Considerations
This document requests IANA to assign two new Internet Protocol
Numbers from the "Protocol Numbers" Registry to indicate "No Next
Header" or "Unknown Next Header".
This document does not create any new registries.
8. Contributors
The other contributors of this document are listed as follows.
o James Guichard
o Stewart Bryant
o Andrew Malis
9. Acknowledgments
TBD.
10. References
10.1. 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>.
[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>.
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[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>.
[RFC8321] Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate-Marking Method for Passive and Hybrid
Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
January 2018, <https://www.rfc-editor.org/info/rfc8321>.
[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>.
10.2. Informative References
[I-D.brockners-inband-oam-transport]
Brockners, F., Bhandari, S., Govindan, V., Pignataro, C.,
Gredler, H., Leddy, J., Youell, S., Mizrahi, T., Mozes,
D., Lapukhov, P., and R. Chang, "Encapsulations for In-
situ OAM Data", draft-brockners-inband-oam-transport-05
(work in progress), July 2017.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", draft-ietf-ippm-ioam-data-11 (work in
progress), November 2020.
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-28 (work in
progress), December 2020.
[I-D.song-mpls-eh-indicator]
Song, H., Li, Z., Zhou, T., and L. Andersson, "Options for
MPLS Extension Header Indicator", draft-song-mpls-eh-
indicator-00 (work in progress), February 2019.
[I-D.xu-clad-spring-sr-service-chaining]
Clad, F., Xu, X., Filsfils, C., daniel.bernier@bell.ca,
d., Decraene, B., Yadlapalli, C., Henderickx, W., Salsano,
S., and S. Ma, "Segment Routing for Service Chaining",
draft-xu-clad-spring-sr-service-chaining-00 (work in
progress), December 2017.
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Authors' Addresses
Haoyu Song (editor)
Futurewei Technologies
2330 Central Expressway
Santa Clara
USA
Email: haoyu.song@futurewei.com
Zhenbin Li
Huawei
156 Beiqing Road
Beijing, 100095
P.R. China
Email: lizhenbin@huawei.com
Tianran Zhou
Huawei
156 Beiqing Road
Beijing, 100095
P.R. China
Email: zhoutianran@huawei.com
Loa Andersson
Bronze Dragon Consulting
Stockholm
Sweden
Email: loa@pi.nu
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