Problem Statement with Aggregate Header Limit
draft-liu-6man-aggregate-header-limit-problem-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Yao Liu , Yisong Liu | ||
| Last updated | 2024-09-29 | ||
| Replaces | draft-liu-spring-aggregate-header-limit-problem | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-liu-6man-aggregate-header-limit-problem-00
6MAN Y. Liu
Internet-Draft ZTE
Updates: RFC8883 (if approved) Y. Liu
Intended status: Informational China Mobile
Expires: 2 April 2025 29 September 2024
Problem Statement with Aggregate Header Limit
draft-liu-6man-aggregate-header-limit-problem-00
Abstract
This document first updates the concept "Aggregate header limit"(AHL)
which is originally proposed in RFC8883 to indicate the total header
size that a router is able to process at full forwarding rate. Then
this document describes the problems for path calculation and
function enablement without the awareness of AHL in IPv6, and the
considerations for AHL collection are also included.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 2 April 2025.
Copyright Notice
Copyright (c) 2024 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 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. Conventions used in this document . . . . . . . . . . . . . . 3
2.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. AHL Collection Considerations . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
As in [RFC8883], some hardware devices implement a parsing buffer of
a fixed size to process packets. The parsing buffer is expected to
contain all the headers that a device needs to examine. If the
aggregate length of headers in a packet exceeds the size of the
parsing buffer, a device will either discard the packet or defer
processing to a software slow path. [RFC8883] proposes the concept
"aggregate header limit" to indicate this size limit. As per
[RFC8883], an ICMPv6 Destination Unreachable error with code for
"Headers too long" SHOULD be sent when a node discards a packet
because the aggregate length of the headers in the packet exceeds the
processing limits of the node.
[RFC9098] also mentions that due to packet-forwarding engine
constraints, if an IPv6 header chain is sufficiently long such that
it exceeds the packet lookup capacity of the router, the router might
be unable to determine how the packet should be handled and thus
could resort to dropping the packet. And some packet-forwarding
engines manage IPv6 header chains using recirculation, but
recirculation can impact the forwarding capacity of hardware, as each
packet will pass through the processing engine multiple times.
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[I-D.ietf-6man-eh-limits] provides the minimum baseline of support
for use of extension headers on the Internet. The default limit for
the IPv6 header chain is 104 bytes,including 64 bytes of extension
headers.
Considering that there's not a clearer definition of Aggregate Header
Limit/Header Chain Size Limit yet. Before discussing the problem,
this document updates RFC8883 by proposing the definition of
Aggregate Header Limit as below:
"Aggregate Header Limit is the total header size(in IPv6, it comprise
the IPv6 header chain as well as any headers that are part of network
encapsulation that precedes the innermost transport layer) that a
router is able to process at full forwarding rate(e.g, at fast path).
For some devices designed with parsing buffer, this limit is related
with its buffer size and buffer design."
The introduction of IPv6 extension headers, especially SRH, and some
advanced features/functions have increased the total packet header
chain size greatly, which may cause inefficient packet processing due
to aggregate header limit exceeding.
This document describes the problems for path calculation and
function enablement without the awareness of AHL, and provides some
usecases with AHL, the considerations for AHL collection are also
included.
2. Conventions used in this document
2.1. Terminology
MSD: Maximum SID Depth as in [RFC8491].
AHL: Aggregate header limit. It's the total header size(in IPv6, it
comprise the IPv6 header chain as well as any headers that are part
of network encapsulation that precedes the innermost transport layer)
that a router is able to process at full forwarding rate(e.g, at fast
path). For some device designed with parsing buffer, this limit is
related with its buffer size and buffer design.
The terminology defined in [I-D.ietf-6man-hbh-processing] are used in
this document as below:
Forwarding Plane: IPv6 routers exchange user or applications data
through the forwarding plane. Routers process fields contained in
IPv6 packet headers. However, they do not process information
contained in packet payloads.
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Control Plane: Routers exchange management and routing information.
They also exchange routing information with one another. Management
and routing information are processed by its recipient. Management
and control information can be forwarded by a router that process
fields contained in packet headers.
Fast Path: A path through a router that is optimized for forwarding
packets. The Fast Path might be supported by Application Specific
Integrated Circuits (ASICS), Network Processor (NP), or other special
purpose hardware. This is the usual processing path within a router
taken by the forwarding plane.
Slow Path: A path through a router that is capable of general purpose
processing and is not optimized for any particular function. This
processing path is used for packets that require special processing
or differ from assumptions made in Fast Path heuristics or to process
router control protocols used by the control plane.
Full Forwarding Rate: This is the rate that a router can forward
packets without adversely impacting the aggregate forwarding rate.
For example, a router could process packets at a rate that allows it
to maintain the full speed on its outgoing interfaces, which is
sometimes called "wire speed".
2.2. 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. Problem Statement
The introduction of IPv6 extension headers, especially SRH, and some
further features/functions have increased the total packet header
chain size greatly. Following are some possible scenarios that would
greatly increase the difficulty of efficient packet processing from
the aspects of total header size increasing.
* Unlike MPLS, even as an intermediate endpoint, the total SRH
should be withine the processing buffer to achieve efficient
packet forwarding. And SRH itself may carry additional TLVs for
additional functions, e.g, the SRH Opaque Metadata TLV and NSH
Carrier TLV for SR service programming
[I-D.ietf-spring-sr-service-programming]. Besides the headend
node, the intermediate nodes may push extra header to the packet
as well. For example, for Binding SID, an SR Segment Endpoint
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nodes would push a new IPv6 header with its own SRH containing an
segment list above the original IPv6 header. And in the case of
TI-LFA [I-D.ietf-rtgwg-segment-routing-ti-lfa], the PLR node may
push a repair SID list to the original packet.
* For network performance measurement, several functions have been
defined. For IPv6 IOAM pre-allocated trace, the headend attachs
the hop-by-hop/destination options header with the IOAM data
fields as introduced in [RFC9486]. And to implement the
Alternate-Marking Method in IPv6, the AltMark Option is carried by
the Hop-by-Hop Options Header or the Destination Options
Header[RFC9343].
* To improve the service capability of the network, features like
network slicing and detnet are proposed. For network slicing, the
VTN Option in IPv6 Hop-by-Hop option are provided
[I-D.ietf-6man-enhanced-vpn-vtn-id]. For detnet, there're
discussion on carrying detnet related within the IPv6 extension
headers, either as SRH TLV [draft-wang-detnet-tsn-over-srv6] or as
Destination Options and Hop-by-Hop Options [draft-xiong-detnet-
6man-queuing-option].
Most of the functions mentioned above are not mutually-exclusive, the
possibility of combination of extension headers/TLVs would make total
header size even bigger. Normally, there're different models/
versions of network devices(i.e, switches, routers) from multiple
vendors in an operator's network, different devices may have
different aggregate header limits and different behaviors after
aggregate header limit exceeding. As more and more functions
mentioned above are superimposed in the operator's network, packet
dropping or rate limiting due to AHL exceeding is a potential risk,
which makes it difficult to management the network.
Path calculation, whether by the controller or the headend, without
the awareness of AHL of the nodes in the network and the prediction
on which features would be enabled along the path, may result in a
path with nodes with lower AHLs than required. If the controller is
aware of aforesaid information, e.g, when calculation certain path
for SR Policy, and the controller knows that and per-hop IOAM and
network slicing would be enabled for this SR Policy, the controller
would leave out the space for HBH header with options for IOAM and
VTN-ID and to ensure that the packet header size wouldn't exceed the
AHLs of the intermediate segment endpoints along the list.
The situation is similar for packet encapsulation triggered by
function enablement, whether on the headend or the intermediate
nodes, packets may be encapsulated with larger header size than the
downstream nodes able to process. If AHL information of the nodes/
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path can be obtained in advance, when the node needs to attach extra
data along the existing path, and the aggregate header limit of the
downstream nodes along the path are not sufficient to process the
headers, the node may choose to not to use the related function and
log an error.
4. AHL Collection Considerations
As per [RFC8883], an ICMPv6 Destination Unreachable error with code
for "Headers too long" SHOULD be sent when a node discards a packet
because the aggregate length of the headers in the packet exceeds the
processing limits of the node. Base on this definition, obtaining
the minimum AHL along the path can be achieved by sending detection
messages of certain size and receiving the ICMPv6 error messages,
which is similar with path MTU discovery for IPv6 in RFC8201.
This may work when the network is small, with not many static paths.
But there may be some problems in the following scenarios:
* When the number of paths increases, more and more detection
messages need to be sent, and the burden of processing the
received ICMPv6 error messages also increases
* For SR dynamic candidate paths [RFC9256], the segment lists of the
paths may change over time, which makes it more difficult to
detect the AHLs.
[RFC9268] leverages an Hop-by-Hop Option to collect the limit(i.e,
minimum path MTU) along the path. An Hop-by-Hop Option for AHL
collection purpose might be another option taking example by RFC9268.
This mechanism works for some cases. But in SRv6, there might be
transit routers who just forward the SRv6 packets like normal IPv6
packets without inspecting into the extension header chain. Even if
the AHLs of these nodes have smaller AHLs than other nodes, the SRv6
packets would still be processed normally along the path. In this
case, if mechanism like RFC9268 is used, the AHL of the path be
collected would be one of the AHLs of these transit nodes, but
actually packets with larger header chain size could be sent and
processed normally.
Signaling could be another option. Considering that there're already
mechanisms like IGP-MSD [RFC8491][RFC8476] to advertise certain size
limit at per node and per link basis. The mechanism for advertising
AHL is similar with IGP-MSD. In the inter-domain scenario, the BGP
signaling may help as well.For the controller, it can get the AHLs of
the nodes in the network via BGP-LS, YANG or other south-north
mechanisms.The details of signaling mechanism is out of the scope of
the document and would be discussed in separated documents.
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5. IANA Considerations
This document makes no request of IANA.
6. Security Considerations
This document does not introduce any new security issues.
7. Acknowledgement
The authors would like to thank Tom Herbert, Alvaro Retana, Eric
Vyncke, Jeff Tantsura, Sasha Vainshtein and Acee Lindem for their
helpful comments and suggestions.
8. References
8.1. Normative References
[I-D.ietf-6man-hbh-processing]
Hinden, R. M. and G. Fairhurst, "IPv6 Hop-by-Hop Options
Processing Procedures", Work in Progress, Internet-Draft,
draft-ietf-6man-hbh-processing-20, 5 June 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
hbh-processing-20>.
[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>.
[RFC8883] Herbert, T., "ICMPv6 Errors for Discarding Packets Due to
Processing Limits", RFC 8883, DOI 10.17487/RFC8883,
September 2020, <https://www.rfc-editor.org/info/rfc8883>.
8.2. Informative References
[I-D.ietf-6man-eh-limits]
Herbert, T., "Limits on Sending and Processing IPv6
Extension Headers", Work in Progress, Internet-Draft,
draft-ietf-6man-eh-limits-15, 6 September 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-
limits-15>.
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[I-D.ietf-6man-enhanced-vpn-vtn-id]
Dong, J., Li, Z., Xie, C., Ma, C., and G. S. Mishra,
"Carrying Network Resource Partition (NRP) Information in
IPv6 Extension Header", Work in Progress, Internet-Draft,
draft-ietf-6man-enhanced-vpn-vtn-id-07, 8 July 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-6man-
enhanced-vpn-vtn-id-07>.
[I-D.ietf-rtgwg-segment-routing-ti-lfa]
Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
Decraene, B., and D. Voyer, "Topology Independent Fast
Reroute using Segment Routing", Work in Progress,
Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
17, 5 July 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rtgwg-segment-routing-ti-lfa-17>.
[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-10, 23 August 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-spring-
sr-service-programming-10>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[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>.
[RFC9098] Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
G., and W. Liu, "Operational Implications of IPv6 Packets
with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
September 2021, <https://www.rfc-editor.org/info/rfc9098>.
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[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>.
[RFC9268] Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-
by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August
2022, <https://www.rfc-editor.org/info/rfc9268>.
[RFC9343] Fioccola, G., Zhou, T., Cociglio, M., Qin, F., and R.
Pang, "IPv6 Application of the Alternate-Marking Method",
RFC 9343, DOI 10.17487/RFC9343, December 2022,
<https://www.rfc-editor.org/info/rfc9343>.
[RFC9486] Bhandari, S., Ed. and F. Brockners, Ed., "IPv6 Options for
In Situ Operations, Administration, and Maintenance
(IOAM)", RFC 9486, DOI 10.17487/RFC9486, September 2023,
<https://www.rfc-editor.org/info/rfc9486>.
Authors' Addresses
Yao Liu
ZTE
China
Email: liu.yao71@zte.com.cn
Yisong Liu
China Mobile
China
Email: liuyisong@chinamobile.com
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