MPLS Extension Header
draft-song-mpls-extension-header-00
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| Authors | Haoyu Song , Zhenbin Li , Tianran Zhou | ||
| Last updated | 2018-07-15 | ||
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draft-song-mpls-extension-header-00
Network Working Group H. Song, Ed.
Internet-Draft Z. Li
Intended status: Standards Track T. Zhou
Expires: January 16, 2019 Huawei
July 15, 2018
MPLS Extension Header
draft-song-mpls-extension-header-00
Abstract
Motivated by the need to support multiple in-network services and
functions in an MPLS network, this document describes a generic
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.
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
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 January 16, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. MPLS Extension Header . . . . . . . . . . . . . . . . . . . . 4
3. Operation on MPLS Extension Headers . . . . . . . . . . . . . 7
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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.brockners-inband-oam-requirements] 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.
However, the encapsulation of the new header(s) poses some challenges
to the ubiquitous MPLS networks. A problem of the MPLS protocol
header is that there is no explicit indicator for the upper layer
protocols. The succinct MPLS header provide little room to encode
any extra information. Moreover, the backward compatibility issue
discourages any attempts trying to overload the semantics of the
existing MPLS header fields.
The similar "header extension" requirement for MPLS has led to
several proposals. A special Generic Associated Channel Label (GAL)
[RFC5586] is assigned to support the identification of an Associated
Channel Header (ACH). Later, it was proposed to use GAL to indicate
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the presence of a Metadata Channel Header (MCH)
[I-D.guichard-sfc-mpls-metadata] as well.
GAL has several limitations:
o It must be located at the bottom of a label stack for its chief
use case of MPLS-TP. An LSR needs to scan the entire label stack
to be able to identify the presence of a new header. This can
impact the performance when the label stack is deep.
o When GAL is present, the first nibble of the word following the
GAL needs to be checked to determine the header type. Since the
value of the nibble cannot be greater than 3, this approach is
neither scalable nor reliable.
o By design, GAL can only indicate the presence of a single header.
Therefore, the solution alone is not sufficient to support adding
multiple headers at the same time.
o The presence of GAL makes the network load balancing or deep
packet inspection based on upper layer protocol headers and
payload difficult.
In addition to the above limitations, it is not desirable to keep
overloading GAL with new semantics. Instead of trying to patch on
existing schemes, we propose a general mechanism to solve the above
mentioned issues and create new innovation opportunities. We derive
our scheme from the experience of the IPv4 to IPv6 evolution. The
adoption of IPv6 is gaining its momentum. Ironically, this is not
due much to the extended address space over IPv4. One true power of
IPv6 is that it supports extension headers, which offer a huge
innovation potential (e.g, network security, SRv6
[I-D.ietf-spring-segment-routing], network programming
[I-D.filsfils-spring-srv6-network-programming], SFC
[I-D.xu-clad-spring-sr-service-chaining], etc.). It is
straightforward to introduce new in-network services into IPv6
networks through extension headers. For example, it has been
proposed to carry IOAM header [I-D.brockners-inband-oam-transport]
and NSH as new extension headers in IPv6 networks.
Nevertheless, IPv6 is not perfect either. For one thing, IPv6's
header overhead is large compared to MPLS. We would like to retain
the header compactness in MPLS networks. On the other hand, IPv6's
extension headers are chained with the original upper layer protocol
headers in a flat stack. One has to 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
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applications (e.g., DPI and ECMP). The new scheme for MPLS header
extension needs to address these issues too.
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 and
extension header scanning 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.
To support the extension header in MPLS, we need to assign a new
special label, namely the Extension Header Label (EHL). So far 8
special label values are left unsigned by IANA (which are 4 to 6 and
8 to 12). We believe this use case is significant enough to deserve
one dedicated special label. Alternatively, a two label scheme with
the use of the extension label (XL) plus an EHL is possible, but it
does use one more label. It is also possible to use FEC labels to
indicate the presence of extension headers. Although this approach
avoid the need of a new special label, it introduces a good deal of
complexity into the control plane. In the remaining of the document,
we assume a special EHL is assigned.
The format of the MPLS packets with extension headers is shown in
Figure 1. An EHL can be located in anywhere in an MPLS label stack.
However, if there are legacy devices which do not recognize the EHL
in the network, then for backward compatibility, the EHL must be
located at the bottom of the stack (i.e., only the MPLS tunnel ends
and EHL-aware nodes will look up and process it). Otherwise, the EHL
can be located close to the top of the stack for better lookup
performance.
The format of an EHL is the same as an MPLS label. The first 20-bit
label value will be assigned by IANA. The BoS bit is used to
indicate the location of the label. The other fields, CoS and TTL,
are unused in the context of EHL.
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0 31
+--------+--------+--------+--------+
| |
~ MPLS Label Stack ~
| |
+--------+--------+--------+--------+
| EHL |
+--------+--------+--------+--------+
| |
~ MPLS Label Stack ~
| |
+--------+--------+--------+--------+
| Header of Extension Headers (HEH) |
+--------+--------+--------+--------+
| |
~ Extension Header (EH) 1 ~
| |
+--------+--------+--------+--------+
~ ~
+--------+--------+--------+--------+
| |
~ Extension Header (EH) N ~
| |
+--------+--------+--------+--------+
| |
~ Upper Layer Protocols/Payload ~
| |
+--------+--------+--------+--------+
Figure 1: MPLS with Extension Header
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: Indicates that there is no any other header and payload after
this header. This can be used to transport packets with only
extension header(s).
UNKNOWN: 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. Operation on MPLS Extension Headers
When the first EH X needs to be added to an MPLS packet, an EHL 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".
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 EHL and HEH can also be deleted.
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.
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4. 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.
NSH: Network Service Header (NSH) [RFC8300] provides a service plane
for Service Function Chaining (SFC). NSH maintains the SFC
context and metadata. If MPLS is used as the transport protocol
for NSH, NSH 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: 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.
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.
5. Summary
Evidenced by the existing and emerging use cases, MPLS networks need
a standard way to support extension headers. In Figure 4, we
summarize the potential schemes that allow MPLS packets to carry
extension headers and list the main issues for each scheme.
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+---+---------------------------+---------------------------------+
|No.| Description | Issues |
+---+---------------------------+---------------------------------+
| 1 | GAL + MCH with multi- |- Label location limitation lead |
| | header extension | to performance concern |
| | |- Interfere with load balancing |
| | | and DPI functions |
| | |- Overlaod GAL semantics |
| | |- Need standard extension |
+---+---------------------------+---------------------------------+
| 2 | GAL + another nibble value|- Same as above |
| | to encode the EHs (e.g., | |
| | "0010") | |
+---+---------------------------+---------------------------------+
| 3 | FEC label to indicate EH |- Complex control plane |
+---+---------------------------+---------------------------------+
| 4 | XL(15) + EHL |- One extra label |
| | |- Need standard extension |
+---+---------------------------+---------------------------------+
| 5 | Sepcial EHL |- Need standard extension |
+---+---------------------------+---------------------------------+
Figure 4: Potential Schemes for MPLS Extension Headers
Through comprehensive considerations on the pros and cons of each
scheme, we currently recommend the scheme No.5. The proposed MPLS
extension header scheme provides a generic way to support in-network
services and functions in MPLS networks.
6. Security Considerations
TBD
7. IANA Considerations
This document requires IANA to assign a new special MPLS label value
("EHL") which is dedicated to indicate the presence of MPLS extension
header(s).
This document also requires IANA to assign two new protocol numbers
which are used to indicate no next header ("NONE") or an unknown next
header ("UNKNOWN").
The new header type values shall be assigned by IANA on a case-by-
case basis.
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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>.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<https://www.rfc-editor.org/info/rfc5586>.
[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>.
[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>.
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10.2. Informative References
[I-D.brockners-inband-oam-requirements]
Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
Gredler, H., Leddy, J., Youell, S., Mozes, D., Mizrahi,
T., <>, P., and r. remy@barefootnetworks.com,
"Requirements for In-situ OAM", draft-brockners-inband-
oam-requirements-03 (work in progress), March 2017.
[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.filsfils-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J.,
daniel.voyer@bell.ca, d., Matsushima, S., and Z. Li, "SRv6
Network Programming", draft-filsfils-spring-srv6-network-
programming-05 (work in progress), July 2018.
[I-D.guichard-sfc-mpls-metadata]
Guichard, J., Pignataro, C., Spraggs, S., and S. Bryant,
"Carrying Metadata in MPLS Networks", draft-guichard-sfc-
mpls-metadata-00 (work in progress), September 2013.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., Pignataro, C., Gredler, H.,
Leddy, J., Youell, S., Mizrahi, T., Mozes, D., Lapukhov,
P., Chang, R., daniel.bernier@bell.ca, d., and J. Lemon,
"Data Fields for In-situ OAM", draft-ietf-ippm-ioam-
data-03 (work in progress), June 2018.
[I-D.ietf-spring-segment-routing]
Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", draft-ietf-spring-segment-routing-15 (work
in progress), January 2018.
[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)
Huawei
2330 Central Expressway
Santa Clara
USA
Email: haoyu.song@huawei.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
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