Interdomain Routing Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Standards Track Y. Zhu
Expires: May 6, 2020 China Telecom
A. Sawaf
Saudi Telecom Company
Z. Li
Huawei Technologies
November 03, 2019
Segment Routing Path MTU in BGP
draft-li-idr-sr-policy-path-mtu-03
Abstract
Segment Routing is a source routing paradigm that explicitly
indicates the forwarding path for packets at the ingress node. An SR
policy is a set of candidate SR paths consisting of one or more
segment lists with necessary path attributes. However, the path
maximum transmission unit (MTU) information for SR path is not
available in the SR policy since the SR does not require signaling.
This document defines extensions to BGP to distribute path MTU
information within SR policies.
Status of This Memo
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Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
3. SR Policy for Path MTU . . . . . . . . . . . . . . . . . . . 3
3.1. SR Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . . 4
4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the ingress
node. The ingress node steers packets into a specific path according
to the Segment Routing Policy ( SR Policy) as defined in
[I-D.ietf-spring-segment-routing-policy]. In order to distribute SR
policies to the headend, [I-D.ietf-idr-segment-routing-te-policy]
specifies a mechanism by using BGP.
The maximum transmission unit (MTU) is the largest size packet or
frame, in bytes, that can be sent in a network. An MTU that is too
large might cause retransmissions. Too small an MTU might cause the
router to send and handle relatively more header overhead and
acknowledgments.
When an LSP is created across a set of links with different MTU
sizes, the ingress router needs to know what the smallest MTU is on
the LSP path. If this MTU is larger than the MTU of one of the
intermediate links, traffic might be dropped, because MPLS packets
cannot be fragmented. Also, the ingress router may not be aware of
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this type of traffic loss, because the control plane for the LSP
would still function normally. [RFC3209] specify the mechanism of
MTU signaling in RSVP. Likewise, SRv6 pakcets will be dropped if the
packet size is larger than path MTU, since IPv6 packet can not be
fragmented on transmission [RFC8200] .
The host may discover the PMTU by Path MTU Discovery (PMTUD)
[RFC8201] or other mechanisms. But the ingress still needs to
examine the packet size for dropping too large packets to avoid
malicious traffic or error traffic. Also, the packet size may
exceeds the PMTU because of the new encapsulation of SR-MPLS or SRv6
packet at the ingress.
In order to check whether the Packet size exceeds the PMTU or not,
the ingress node needs to know the Path MTU associated to the
forwarding path. However, the path maximum transmission unit (MTU)
information for SR path is not available since the SR does not
require signaling.
This document defines extensions to BGP to distribute path MTU
information within SR policies. The Link MTU information can be
obtained via BGP-LS [I-D.zhu-idr-bgp-ls-path-mtu] or some other
means. With the Link MTU, the controller can compute the PMTU and
convey the information via the BGP SR policy.
2. Terminology
This memo makes use of the terms defined in [RFC8402] and [RFC3209].
2.1. 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.
3. SR Policy for Path MTU
As defined in [I-D.ietf-idr-segment-routing-te-policy] , the SR
policy encoding structure is as follows:
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SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Segment
Segment
...
...
As introduced in Section 1, each SR path has it's path MTU. SR
policy with SR path MTU information is expressed as below:
SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
Attributes:
Tunnel Encaps Attribute (23)
Tunnel Type: SR Policy
Binding SID
Preference
Priority
Policy Name
Explicit NULL Label Policy (ENLP)
Segment List
Weight
Path MTU
Segment
Segment
...
...
3.1. SR Path MTU Sub-TLV
An SR Path MTU sub-TLV is an Optional sub-TLV. When it appears, it
must appear only once at most within a Segment List sub-TLV. If
multiple Path MTU sub-TLVs appear within a Segment List sub-TLV, the
first one will be processed, and the rest will be ignored. An SR
Path MTU sub-TLV is associated with an SR path specified by a segment
list sub-TLV or path segment as defined in
[I-D.ietf-spring-mpls-path-segment] and
[I-D.li-spring-srv6-path-segment]. It has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Path MTU sub-TLV
Where:
Type: to be assigned by IANA.
Length: the total length of the value field not including Type and
Length fields.
Reserved: 16 bits reserved and MUST be set to 0 on transmission and
MUST be ignored on receipt.
Path MTU: 4 bytes value of path MTU in octets. The value can be
calculated by a central controller or other devices based on the
information that learned via IGP of BGP-LS or other means.
Whenever the path MTU of a physical or logical interface is changed,
a new SR policy with new path MTU information should be updated
accordingly by BGP.
4. Operations
The document does not bring new operation beyong the description of
operations defined in [I-D.ietf-idr-segment-routing-te-policy]. The
existing operations defined in
[I-D.ietf-idr-segment-routing-te-policy] can apply to this document
directly.
Typically but not limit to, the SR policies carrying path MTU
infomation are configured by a controller.
After configuration, the SR policies carrying path MTU infomation
will be advertised by BGP update messages. The operation of
advertisement is the same as defined in
[I-D.ietf-idr-segment-routing-te-policy], as well as the receiption.
The consumer of the SR policies is not the BGP process. The
operation of sending information to consumers is out of scope of this
document.
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5. IANA Considerations
This document defines a new Sub-TLV in registries "SR Policy List
Sub- TLVs" [I-D.ietf-idr-segment-routing-te-policy]:
Value Description Reference
---------------------------------------------------------------------
TBA Path MTU sub-TLV This document
6. Security Considerations
TBA
7. Contributors
Jun Qiu
Huawei Technologies
China
Email: qiujun8@huawei.com
8. Acknowledgements
TBA
9. References
9.1. Normative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Mattes, P., Rosen, E., Jain,
D., and S. Lin, "Advertising Segment Routing Policies in
BGP", draft-ietf-idr-segment-routing-te-policy-07 (work in
progress), July 2019.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-03 (work in progress),
May 2019.
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[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>.
[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>.
9.2. Informative References
[I-D.ietf-spring-mpls-path-segment]
Cheng, W., Li, H., Chen, M., Gandhi, R., and R. Zigler,
"Path Segment in MPLS Based Segment Routing Network",
draft-ietf-spring-mpls-path-segment-01 (work in progress),
September 2019.
[I-D.li-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., Li, Z., Dong, J.,
and R. Gandhi, "Path Segment for SRv6 (Segment Routing in
IPv6)", draft-li-spring-srv6-path-segment-03 (work in
progress), August 2019.
[I-D.zhu-idr-bgp-ls-path-mtu]
Zhu, Y., Hu, Z., Yan, G., and J. Yao, "BGP-LS Extensions
for Advertising Path MTU", draft-zhu-idr-bgp-ls-path-
mtu-01 (work in progress), July 2019.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
<https://www.rfc-editor.org/info/rfc3209>.
[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>.
[RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
"Path MTU Discovery for IP version 6", STD 87, RFC 8201,
DOI 10.17487/RFC8201, July 2017,
<https://www.rfc-editor.org/info/rfc8201>.
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Authors' Addresses
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: chengli13@huawei.com
YongQing Zhu
China Telecom
109, West Zhongshan Road, Tianhe District.
Guangzhou
China
Email: zhuyq.gd@chinatelecom.cn
Ahmed El Sawaf
Saudi Telecom Company
Riyadh
Saudi Arabia
Email: aelsawaf.c@stc.com.sa
Zhenbin Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing 100095
China
Email: lizhenbin@huawei.com
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