Segment Routing Path MTU in BGP
draft-ietf-idr-sr-policy-path-mtu-10
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Cheng Li , Yongqing Zhu , Ahmed El Sawaf , Zhenbin Li | ||
| Last updated | 2024-09-24 | ||
| Replaces | draft-li-idr-sr-policy-path-mtu | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-idr-sr-policy-path-mtu-10
Interdomain Routing Working Group C. Li
Internet-Draft Huawei Technologies
Intended status: Standards Track Y. Zhu
Expires: 28 March 2025 China Telecom
A. Sawaf
Saudi Telecom Company
Z. Li
Huawei Technologies
24 September 2024
Segment Routing Path MTU in BGP
draft-ietf-idr-sr-policy-path-mtu-10
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 SR Policy candidate paths consisting of one or
more segments with the appropriate SR path attributes. BGP
distributes each SR Policy candidate path as combination of an prefix
plus a the BGP Tunnel Encapsulation(Tunnel-Encaps) attribute
containing an SR Policy Tunnel TLV with information on the SR Policy
candidate path as a tunnel. However, the path maximum transmission
unit (MTU) information for a segment list for SR path is not
currently passed in the BGP Tunnel-Encaps attribute. . This document
defines extensions to BGP to distribute path MTU information within
SR policies.
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 28 March 2025.
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Copyright Notice
Copyright (c) 2024 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 publication of this document.
Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
3. SR Policy for Path MTU . . . . . . . . . . . . . . . . . . . 5
3.1. Path MTU Sub-TLV . . . . . . . . . . . . . . . . . . . . 6
4. Operations . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 7
5.1. Huawei's Commercial Delivery . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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 [RFC9256].
In order to distribute SR policies to the headend,
[I-D.ietf-idr-sr-policy-safi] specifies a BGP mechanism to pass SR
Policies and Candidate SR Policies in BGP UPDATE message. Each SR
Candidate Path is passed as combination of a specific type of NLRI
and BGP Tunnel Encapsulation Attribute (Tunnel-Encaps) with SR Policy
Tunnel type tunnel. The NLRI must contain either be the IPv4 Unicast
AFI with SR Policy SAFI (AFI=1/SAFI=73), the IPv6 Unicast AFI with
the SR Policy SAFI (AFI=2/SAFI=73).
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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
this type of traffic loss, because the control plane for the LSP
would still function normally. [RFC3209] specifies the mechanism of
MTU signaling in RSVP. Similarly, the SRv6 packets will be dropped
if the packet size is larger than the path MTU, since IPv6 packet
cannot be fragmented on transmission [RFC8200].
The host may discover the PMTU by Path MTU Discovery (PMTUD)
[RFC8201] or other mechanisms. But the ingress router 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 router.
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 currently distributed in the BGP
Tunnel-Encaps attribute TLV for the SR Policy Tunnel.
This document defines a new sub-TLV for the BGP Tunnel-Encaps
attribute for the SR Policy Tunnel type to specify Maximum Path MTU
for a Segment list (Sub-TLV). The Maximum Path MTU can be calculated
as the maximum of individual Link MTU information. The Link MTU
information can be obtained via BGP-LS [I-D.ietf-idr-bgp-ls-link-mtu]
or some other means. based on all Link MTUs, 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].
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MTU: Maximum Transmission Unit, the size in bytes of the largest IP
packet, including the IP header and payload, that can be
transmitted on a link or path. Note that this could more properly
be called the IP MTU, to be consistent with how other standards
organizations use the acronym MTU.
Link MTU: The Maximum Transmission Unit, i.e., maximum IP packet
size in bytes, that can be conveyed in one piece over a link. Be
aware that this definition is different from the definition used
by other standards organizations.
For IETF documents, link MTU is uniformly defined as the IP MTU
over the link. This includes the IP header, but excludes link
layer headers and other framing that is not part of IP or the IP
payload.
Be aware that other standards organizations generally define link
MTU to include the link layer headers.
For the MPLS data plane, this size includes the IP header and data (or
other payload) and the label stack but does not include any lower-layer
headers. A link may be an interface (such as Ethernet or Packet-over-
SONET), a tunnel (such as GRE or IPsec), or an LSP.
Path: The set of links traversed by a packet between a source node
and a destination node.
Path MTU, or PMTU: The minimum link MTU of all the links in a path
between a source node and a destination node.
For the MPLS data plane, it is the MTU of an LSP from a given LSR to
the egress(es), over each valid (forwarding) path. This size includes
the IP header and data (or other payload) and any part of the label
stack that was received by the ingress LSR before it placed the packet
into the LSP (this part of the label stack is considered part of the
payload for this LSP). The size does not include any lower-level
headers.
Note that: The PMTU value may be modified by subtracting some overhead
introduced by protection mechanism, like TI-LFA. Therefore, the value
of PMTU dilivered to the ingress node MAY be smaller than the minimum
link MTU of all the links in a path between a source node and a
destination node.
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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-sr-policy-safi] , the SR policy encoding
structure is as follows:
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
...
...
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3.1. Path MTU Sub-TLV
A 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 NLRI MUST
be treated as a malformed NLRI.
As per [I-D.ietf-idr-sr-policy-safi], when the error determined
allows for the router to skip the malformed NLRI(s) and continue
processing of the rest of the update message, then it MUST handle
such malformed NLRIs as 'Treat-as-withdraw'. This document does not
define new error handling rules for Path MTU sub-TLV, and the error
handling rules defined in [I-D.ietf-idr-sr-policy-safi] apply to this
document.
A Path MTU sub-TLV is associated with an SR path specified by a
segment list sub-TLV or a path segment [RFC9545]
[I-D.ietf-spring-srv6-path-segment]. The Path MTU sub-TLV has the
following format:
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 in octets the value field not including Type
and Length fields. The value must be 6.
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.
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4. Operations
The document does not bring new operation beyond the description of
operations defined in [I-D.ietf-idr-sr-policy-safi]. The existing
operations defined in [I-D.ietf-idr-sr-policy-safi] 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-sr-policy-safi], 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.
5. Implementation Status
[Note to the RFC Editor - remove this section before publication, as
well as remove the reference to [RFC7942].
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC7942], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
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5.1. Huawei's Commercial Delivery
The feature has been implemented on Huawei VRP8.
* Organization: Huawei
* Implementation: Huawei's Commercial Delivery implementation based
on VRP8.
* Description: The implementation has been done.
* Maturity Level: Product
* Contact: guokeqiang@huawei.com
6. IANA Considerations
This document defines a new Sub-TLV in registries "SR Policy List
Sub- TLVs" [I-D.ietf-idr-sr-policy-safi]:
Value Description Reference
---------------------------------------------------------------------
TBA Path MTU sub-TLV This document
7. Security Considerations
This document defines the extension to BGP to distribute path MTU
information within SR policies. Therefore, the security mechanisms
of the base BGP security model [RFC4271] and the security
considereations in [I-D.ietf-idr-sr-policy-safi] apply to this
document. The path MTU extension is included in the SR Policy
extension [I-D.ietf-idr-sr-policy-safi], so it does not introduce
extra security problems comparing the existing SR policy entension.
The path MTU information is critical to the path, and a wrong path
MTU may cause packet dropping in the forwarding. An implementation
needs to make sure that the value of the link MTU is correctly
collected from some means, such as BGP-LS. It also must ensure the
processing and calculation of path MTU is correct to avoid packet
dropping in forwarding. In addition, the path MTU distribution from
a controller to an ingress router has to be protected. The security
considereations in [I-D.ietf-idr-sr-policy-safi] apply to this
distribution procedure.
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8. Contributors
Jun Qiu
Huawei Technologies
China
Email: qiujun8@huawei.com
9. Acknowledgements
Authors would like to thank Ketan Talaulikar, Aijun Wang, Weiqiang
Cheng, Huanan Chen, Chongfeng Xie, Stefano Previdi, Taishan Tang,
Keqiang Guo, Chen Zhang, Susan Hares, Weiguo Hao, Gong Xia, Bing
Yang, Linda Dunbar, Shunwan Zhuang, Huaimo Chen, Mach Chen, Jingring
Xie, Zhibo Hu, Jimmy Dong and Jianwei Mao for their proprefessional
comments and help.
10. References
10.1. Normative References
[I-D.ietf-idr-sr-policy-safi]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P., and
D. Jain, "Advertising Segment Routing Policies in BGP",
Work in Progress, Internet-Draft, draft-ietf-idr-sr-
policy-safi-06, 26 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-sr-
policy-safi-06>.
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[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>.
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[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>.
[RFC9256] Filsfils, C., Talaulikar, K., Ed., Voyer, D., Bogdanov,
A., and P. Mattes, "Segment Routing Policy Architecture",
RFC 9256, DOI 10.17487/RFC9256, July 2022,
<https://www.rfc-editor.org/info/rfc9256>.
10.2. Informative References
[I-D.ietf-idr-bgp-ls-link-mtu]
Zhu, Y., Hu, Z., Peng, S., and R. Mwehair, "Signaling
Maximum Transmission Unit (MTU) using BGP-LS", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-ls-link-mtu-
08, 17 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-
ls-link-mtu-08>.
[I-D.ietf-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., and Y. Zhu, "Path
Segment Identifier (PSID) in SRv6 (Segment Routing in
IPv6)", Work in Progress, Internet-Draft, draft-ietf-
spring-srv6-path-segment-11, 18 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
srv6-path-segment-11>.
[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>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[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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[RFC9545] Cheng, W., Ed., Li, H., Li, C., Ed., Gandhi, R., and R.
Zigler, "Path Segment Identifier in MPLS-Based Segment
Routing Networks", RFC 9545, DOI 10.17487/RFC9545,
February 2024, <https://www.rfc-editor.org/info/rfc9545>.
Authors' Addresses
Cheng Li
Huawei Technologies
Huawei Campus, No. 156 Beiqing Rd.
Beijing
100095
China
Email: c.l@huawei.com
YongQing Zhu
China Telecom
109, West Zhongshan Road, Tianhe District.
Guangzhou
China
Email: zhuyq8@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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