Internet DRAFT - draft-xiao-6man-srv6-checksum
draft-xiao-6man-srv6-checksum
6MAN Working Group X. Min
Internet-Draft Y. Liu
Intended status: Standards Track ZTE Corp.
Expires: 19 November 2022 C. Xie
China Telecom
18 May 2022
SRv6 Upper-Layer Checksum
draft-xiao-6man-srv6-checksum-00
Abstract
This document provides a unified mechanism that makes the upper-layer
checksum computation rule defined in IPv6 Specification applicable,
whether SRv6 SIDs or SRv6 compressed SIDs are used.
Status of This Memo
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This Internet-Draft will expire on 19 November 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
3. Unified Mechanism for Upper-Layer Checksum in SRv6 . . . . . 4
3.1. C-flag in Segment Routing Header . . . . . . . . . . . . 4
3.2. C-flag Processing . . . . . . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
IPv6 Specification [RFC8200] defines how upper-layer checksum is
computed. Specifically, a "pseudo-header" for IPv6 is constructed as
a portion of fields included in upper-layer (e.g., TCP, UDP, ICMPv6,
OSPF) checksum computation. As defined in Section 8.1 of [RFC8200],
if the IPv6 packet doesn't contain Routing header, the Destination
Address used in the pseudo-header will be in the Destination Address
field of the IPv6 header; if the IPv6 packet contains a Routing
header, the Destination Address used in the pseudo-header is that of
the final destination. In the latter case, at the originating node,
that address will be in the last element of the Routing header; at
the recipient(s), that address will be in the Destination Address
field of the IPv6 header. As also defined in Section 8.1 of
[RFC8200], any node implementing zero-checksum mode of UDP tunnel
must follow the requirements specified in "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums" [RFC6936], and
it's outside the scope of this document.
Segment Routing over IPv6 (SRv6) [RFC8754] defines an IPv6 Routing
header called Segment Routing Header (SRH). To comply with the
upper-layer checksum computation rule defined in [RFC8200], at the
SRv6 ingress node, the last element of the SRH, i.e., the last
Segment Identifier (SID), will become the Destination Address used in
the pseudo-header for upper-layer checksum computation; at the SRv6
egress node, after SRH processing is finished, the Destination
Address in the IPv6 header will become the Destination Address used
in the pseudo-header for upper-layer checksum computation. Note that
even at the SRv6 egress node, SRH processing may still invoke IPv6
Destination Address substitution.
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The C-SID document [I-D.ietf-spring-srv6-srh-compression] defines
SRv6 compressed SIDs, which use 16-bit or 32-bit SRv6 C-SID to
substitute 128-bit SRv6 SID. The NEXT-C-SID flavor and REPLACE-C-SID
flavor are defined in the C-SID document. In one case of NEXT-C-SID
flavor, at the SRv6 ingress node, the IPv6 packet doesn't contain
Routing header, more than one C-SIDs are included in IPv6 Destination
Address, the upper-layer checksum computation rule defined in
[RFC8200] doesn't apply anymore. In another case of REPLACE-C-SID
flavor, at the SRv6 ingress node, the IPv6 packet contains an SRH,
the last element of the SRH is not a 128-bit IPv6 address, but a
16-bit or 32-bit C-SID, the upper-layer checksum computation rule
defined in [RFC8200] doesn't apply anymore.
This document provides a unified mechanism that makes the upper-layer
checksum computation rule defined in IPv6 Specification applicable,
whether SRv6 SIDs or SRv6 compressed SIDs are used.
2. Conventions
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.
2.2. Abbreviations
SR: Segment Routing
SRv6: Segment Routing with IPv6 data plane
SID: Segment ID
C-SID: Compressed Segment ID [I-D.ietf-spring-srv6-srh-compression]
SRH: Segment Routing Header [RFC8754]
PSP: Penultimate Segment Pop of the SRH [RFC8986]
TCP: Transmission Control Protocol [RFC0793]
UDP: User Datagram Protocol [RFC0768]
ICMPv6: Internet Control Message Protocol for IPv6 [RFC4443]
OSPF: Open Shortest Path First protocol [RFC2328]
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3. Unified Mechanism for Upper-Layer Checksum in SRv6
This section defines a unified mechanism for upper-layer checksum in
SRv6 networks. This mechanism utilizes a new SRH flag and requests
all SRv6 nodes along the transport path to act on the new SRH flag.
3.1. C-flag in Segment Routing Header
[RFC8754] describes the Segment Routing Header (SRH) and how SRv6
capable nodes use it. The SRH contains an 8-bit "Flags" field.
This document defines the following bit in the SRH Flags field to
carry the C-flag:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |C| |
+-+-+-+-+-+-+-+-+
Where:
C-flag: Checksum flag in the SRH Flags field defined in [RFC8754].
When C-flag is set, the last element of the SRH MUST be set to an
IPv6 address of the final destination.
3.2. C-flag Processing
The C-flag in SRH is used as a marking-bit in the SRv6 packets using
upper-layer checksum, each segment endpoint would process the C-flag
as defined in this document, to make the SRv6 upper-layer checksum
computation smooth and complied to [RFC8200].
At the upper-layer checksum originating node, if the IPv6 packet
contains an SRH, the SRH C-flag MUST be set and the Segment List[0]
MUST be set to a 128-bit IPv6 address of the final destination; if
the IPv6 packet doesn't contain an SRH while the Destination Address
field contains more than one compressed SID, an SRH MUST be added
with C-flag set and Segment List[0] set to a 128-bit IPv6 address of
the final destination. When the upper-layer checksum originating
node knows more than one IPv6 address of the final destination, e.g.,
a local interface address of the final destination, a 128-bit SID
locally instantiated at the final destination, and an IPv6 address
transformed from a 16-bit or 32-bit compressed SID locally
instantiated at the final destination, the originating node needs to
select one of them as the last element of SRH, how the originating
node makes the choice is beyond the scope of this document.
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When the penultimate segment of a segment-list is a Penultimate
Segment Pop (PSP) SID, the SRH is removed at the penultimate segment
and the C-flag is not processed at the ultimate segment. The
penultimate segment as a PSP SID MUST copy Segment List[0] from the
SRH to the Destination Address field of the IPv6 header, then the
ultimate segment can still compute the upper-layer checksum with
correct IPv6 Destination Address even without SRH.
When an SRv6 node receives a packet destined to S and S is a local
SID, the line S01 of the pseudo-code associated with the SID S, as
defined in Section 4.3.1.1 of [RFC8754], is appended to as follows
for the C-flag processing.
S01.2. IF C-flag is set and local configuration permits
C-flag processing {
If (Segment List[0] is locally instantiated or represents
a local interface) {
a. Set Segments Left to 0.
b. Update IPv6 DA with Segment List[0].
}
Else {
If (IPv6 DA is locally instantiated as a PSP SID) {
a. Update IPv6 DA with Segment List[0].
b. Submit the packet to the egress IPv6 FIB lookup for
transmission to the new destination.
}
}
Note that the C-flag processing happens before execution of regular
processing of the local SID S. Specifically, the line S01.2 of the
pseudo-code specified in this document is inserted between line S01
and S02 of the pseudo-code defined in Section 4.3.1.1 of [RFC8754].
When the C-flag defined in this document and the O-flag defined in
Section 2.1 of [I-D.ietf-6man-spring-srv6-oam] are both set, the
C-flag processing happens after O-flag processing. Specifically, the
line S01.2 of the pseudo-code specified in this document is inserted
between line S01.1 of the pseudo-code defined in Section 2.1.1 of
[I-D.ietf-6man-spring-srv6-oam] and line S02 of the pseudo-code
defined in Section 4.3.1.1 of [RFC8754].
Also note that if the final destination needs to be reached more than
once on the programmed transport path, the SRv6 packets with C-flag
set would be terminated at the first time the final destination is
reached. If it's deemed necessary for the SRv6 packets with C-flag
set to reach the final destination more than once, more judgment
conditions may be added to the pseudo-code of C-flag processing.
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4. IANA Considerations
In the "Segment Routing Header Flags" registry created for [RFC8754],
a new Checksum Flag is requested from IANA as follows:
+==============+========+=============+============+===========+
| Bit Position | Symbol | Description | Semantics | Reference |
| | | | Definition | |
+==============+========+=============+============+===========+
| 3 | C | Checksum | Section | This |
| | | Flag | 3.1 | Document |
+--------------+--------+-------------+------------+-----------+
Table 1: New SRH Flag
5. Security Considerations
This document does not raise additional security issues beyond those
of the specifications referred to in the list of references.
6. Acknowledgements
TBA.
7. References
7.1. Normative References
[I-D.ietf-6man-spring-srv6-oam]
Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
Chen, "Operations, Administration, and Maintenance (OAM)
in Segment Routing Networks with IPv6 Data plane (SRv6)",
Work in Progress, Internet-Draft, draft-ietf-6man-spring-
srv6-oam-13, 23 January 2022,
<https://www.ietf.org/archive/id/draft-ietf-6man-spring-
srv6-oam-13.txt>.
[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>.
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[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>.
[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>.
7.2. Informative References
[I-D.ietf-spring-srv6-srh-compression]
Cheng, W., Filsfils, C., Li, Z., Decraene, B., Cai, D.,
Voyer, D., Clad, F., Zadok, S., Guichard, J. N., Aihua,
L., Raszuk, R., and C. Li, "Compressed SRv6 Segment List
Encoding in SRH", Work in Progress, Internet-Draft, draft-
ietf-spring-srv6-srh-compression-01, 21 March 2022,
<https://www.ietf.org/archive/id/draft-ietf-spring-srv6-
srh-compression-01.txt>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<https://www.rfc-editor.org/info/rfc6936>.
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[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>.
Authors' Addresses
Xiao Min
ZTE Corp.
Nanjing
China
Phone: +86 25 88013062
Email: xiao.min2@zte.com.cn
Yao Liu
ZTE Corp.
Nanjing
China
Email: liu.yao71@zte.com.cn
Chongfeng Xie
China Telecom
Beijing
China
Email: xiechf@chinatelecom.cn
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