Internet DRAFT - draft-yang-spring-ach6-sr
draft-yang-spring-ach6-sr
SPRING Working Group F. Yang
Internet-Draft T. Zhou
Intended status: Standards Track Huawei Technologies
Expires: January 13, 2022 July 12, 2021
ACH6 in Segment Routing
draft-yang-spring-ach6-sr-00
Abstract
Associated Channel over IPv6 (ACH6) provides a control channel to one
specific IPv6 forwarding path for control and management purpose.
When ACH6 is used in a Segment Routing network, it provides a control
channel to an SRv6 path. This document specifies an SRv6 ACH6
mechanism and describes how ACH6 is applied in a Segment Routing
network.
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.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. ACH6 in Segment Routing . . . . . . . . . . . . . . . . . . . 3
2.1. ACH6 Network Reference Model in Segment Routing . . . . . 3
2.2. Identification of ACH6 in Segment Routing . . . . . . . . 4
2.3. ACH6 TLV Format in Segment Routing . . . . . . . . . . . 4
2.4. Encapsulation of ACH6 TLV in Segment Routing . . . . . . 5
3. Use Case of ACH6 in Segment Routing . . . . . . . . . . . . . 6
3.1. OAM to an SRv6 Path . . . . . . . . . . . . . . . . . . . 6
3.2. Protection to an SRv6 Path . . . . . . . . . . . . . . . 7
3.3. Resource Reservation to an SRv6 Path . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Segment Routing [RFC8402] leverages the source routing paradigm. By
leveraging SR into IPv6 network, an ordered list of SRv6 SIDs
provides the certainty of a packet forwarding path as restricted to a
specific topological path. The Function part in SRv6 SIDs indicates
instructions to be executed on network nodes to achieve network
programming to an IPv6 forwarding path.
[I-D.yang-rtgwg-ipv6-associated-channel] proposes an Associated
Channel over IPv6 (ACH6) to provide a control channel to one specific
IPv6 forwarding path for control and management purpose. When ACH6
is used in a Segment Routing network, it provides a control channel
to an SRv6 path. This document specifies an SRv6 ACH6 mechanism and
describes how ACH6 is applied in a Segment Routing network.
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2. ACH6 in Segment Routing
SRv6 ACH6 provides a control channel to carry control and management
messages to an SRv6 path separately from data forwarding. It is a
method to provide distributed control and management capabilities to
an SRv6 path, which complements the SDN centerialized control plane.
In SRv6 ACH6 control channel, different types of control and
management messages to an SRv6 path are carried.
2.1. ACH6 Network Reference Model in Segment Routing
In SRv6 network, IPv6 packet is generated and transported from one
SRv6 endpoint to another with an ordered list of SRv6 SIDs in Segment
Routing Header (SRH) [RFC8754]. SRv6 ACH6 is an inband path-based
control channel from one SRv6 endpoint to another. SRv6 ACH6 packet
is also encapsulated with an Segment Routing Header. To guarantee
ACH6 control packet is transported in the same path as data packets
forward, ACH6 packet uses the same SRv6 SID list with the one in SRH
of data packets associated with.
Figure 1 shows an ACH6 network reference model used in an SRv6
network.
SRv6 Endpoint SRv6 Endpoint SRv6 Endpoint
+----+ +-------+ +---------+ +------+ +----+
----| Ex |-----| ACH6 |------| ACH6 |------| ACH6 |------| Ey |----
| | |Ingress| |Mid-Point| |Egress| | |
+----+ +-------+ +---------+ +------+ +----+
|<-------------SRv6 Path-------------->|
|<-------------SRv6 ACH6 ------------->|
|<---------------------- SRv6 Domain ------------------------>|
Figure 1 ACH6 Network Reference Model in SRv6
Ex/Ey: SRv6 endpoint
ACH6 Ingress Node: is the node indicates the entering of control and
management channel over an SRv6 path, where control and management
messages are generated and encapsulated. ACH6 ingress node sets its
local IPv6 address as source address of ACH6 packet.
ACH6 Mid-Point Node: the SRv6 endpoints on SRv6 SID list of ACH6
control packet are ACH6 Mid-Point Node, which would process ACH6
packet when hop-by-hop processing on SRv6 endpoints is required by
ACH6 control channel.
ACH6 Egress Node: indicates the exiting of control and management
channel over an SRv6 path, where the control and management messages
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are extracted and delivered to control or management plane for
further process. ACH6 egress node sets its local IPv6 address as
destination address of ACH6 packet.
2.2. Identification of ACH6 in Segment Routing
The Associated Channel ID is the identifier of ACH6 control channel,
and indicates the path which control channel is associated with. In
SRv6, Path Segment [I-D.ietf-spring-srv6-path-segment] is used to
identify a specific SRv6 path. It can also be used as Associated
Channel ID to identify the control channel of an SRv6 path. The
encoding of Path Segment and how Path Segment is allocated keeps same
specifications defined in [I-D.ietf-spring-srv6-path-segment].
2.3. ACH6 TLV Format in Segment Routing
ACH6 TLV in Segment Routing is defined as:
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=TBD | length | Channel Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ Value ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 ACH6 TLV Format in SR
Type: 8 bits, indicates it is an ACH6 TLV.
Length: 8 bits, defines the length of Value field in bytes.
Channel Type: is a 16-bit-length fixed portion as a part of Value
field. It indicates the specific type of messages carried in SRv6
ACH6 control channel. Note that a new ACH TLV Channel Type Registry
would be requested to IANA. In later documents which specify
application protocols of associated channel, MUST also specify the
applicable Channel Type field value assigned by IANA.
Value: is a variable portion of Value field. It specifies the
messages indicated by Channel Type and carried in associated channel.
Note that the Value field of ACH6 TLV MAY contain sub-TLVs to provide
additional context information to ACH6 TLV.
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2.4. Encapsulation of ACH6 TLV in Segment Routing
In SRv6, ACH6 control channel is used in either an end-to-end or a
hop-by-hop approach.
Regarding an end-to-end case, messages in ACH6 is encapsulated at
ACH6 ingress node and decapsulated at ACH6 egress node. ACH6 TLV is
recommended to be encapsulated in IPv6 Destination Options Header
places after the Segment Routing Header. An alternative way to carry
ACH6 TLV is using IPv6 payload. When ACH6 TLV format is encapsulated
in payload, TLV Type and Length can be omitted. The method of taking
advantage of SRH Flag field to indicate active probing packet
[I-D.song-spring-siam] can be used for ACH6 too.
Regarding a hop-by-hop case, messages in ACH6 is encapsulated at ACH6
ingress node. ACH6 mid-points decapsulate and re-capsulate every
ACH6 packet. At last, ACH6 egress node decapsulates ACH6 packet and
delivers control and management messages for further process. In
this case, ACH6 TLV is recommended to be encapsulated in IPv6
Destination Options Header preceding the Segment Routing Header.
The encapsulation of ACH6 in IPv6 Destination Options Header is
defined as:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | Next Header | Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Source Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Destination Address //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ <---+
| DOH TLV(ACH6) | Hdr Ext Len | Channel Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ DOH1
| ~ (HbH
~ Value (depends on the specific protocol) ~ case)
~ | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ <---+
| Next Header | Hdr Ext Len | Routing Type | Segments Left | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Last Entry | Flags | Tag | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Segment List[0] (128 bits) ~ SRH
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ... ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Segment List[n] (128 bits) ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ Path Segment (128 bits) ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ SRH TLV (Optional,variable) ~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ <---+
| DOH TLV(ACH6) | Hdr Ext Len | Channel Type | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ DOH2
| ~ (E2E
~ Value (depends on the specific protocol) ~ case)
~ | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ <---+
Figure 3 ACH6 TLV Encapsulation in SRv6
3. Use Case of ACH6 in Segment Routing
3.1. OAM to an SRv6 Path
In SRv6, several works are carrying on to establish an SRv6 OAM
toolset. [I-D.ietf-6man-spring-srv6-oam] provides the mechanisms of
continuity check, path discovery by reusing Ping and Traceroute, and
defines a sampling flag for flow information telemetry. Simple Two-
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way Active Measurement Protocol (STAMP) [RFC8762] is encapsulated
after UDP header to measure performance metrics in SRv6 network.
[I-D.ietf-ippm-ioam-data] supports extensible data collection for
SRv6 network monitor and measurement.
ACH6 provides another method of supporting a group of OAM tools in a
unified TLV format. In this method, a toolset of OAM functions is
classified into three types of messages, including on-demand echo
request/reply, proactive continuity check, and performance
measurement. By using ACH6 to carry OAM messages, continuity check
and performance management can be monitored either hop-by-hop on
every SR endpoint or end-to-end from the first endpoint to the last.
Leveraging IPv6 extension headers to carry OAM messages can
facilitate data plane processing on OAM messages, and further improve
processing efficiency and accuracy. At last, by leveraging the
native semantics of IPv6 extension headers, this method can naturally
reduce OAM configuration and session management on SRv6 endpoints.
Figure 4 gives the example format of ACH6 OAM TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Type = ODERR/PCC/PM |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ~
~ OAM Message Body (Variable) ~
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 ACH6 OAM TLV
ACH6 Channel Type to indicate which type of OAM message is
encapsulated in the following OAM message body, for example on demand
echo request/reply. OAM message can also re-utilize the format of
existing protocols. For example, BFD or STAMP protocol formats can
be encapsulated in IPv6 payload field after UDP header.
3.2. Protection to an SRv6 Path
Protection State Coordination (PSC) Protocol [RFC6378] provides a
single-phased coordination mechanism used for linear protection
between two endpoints. This coordination mechanism is useful when
there is a need of traffic to be transported on two co-routed paths.
In SRv6, active and backup candidate paths in SR policy can provide
an end-to-end protection to a specific SRv6 path. However, without a
coordination mechanism like PSC, SR policy cannot guarantee the
bidirectional traffics are transported on co-routed paths.
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ACH6 extends PSC protocol to exchange notification and coordination
messages between SRv6 endpoints. An ordered segment list of working
path and an ordered segment list of backup path are separately pre-
created at the source and destination of an SRv6 path. Working paths
on two SRv6 endpoints are co-routed, so does backup paths. When
there is failure to indicate protection switchover on working path,
ACH6 exchanges protection state coordination messages between SRv6
endpoints to indicate synchronized switchover. When two SRv6
endpoints accomplish the switchover, the protection paths are co-
routed for bidirectional traffics.
Figure 5 gives the example format of ACH6 protection state
coordination TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Type = PSC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver|Request|PT |R| Reserved1 | FPath | Path |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional TLVs |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5 ACH6 PSC TLV
The definition and usage of Request, PT, R, FPath and Path fields are
referenced to [RFC6378].
3.3. Resource Reservation to an SRv6 Path
In current practice of SRv6, a distributed resource reservation
protocol like RSVP-TE is not used. SDN controller plays the role of
calculating forwarding path and reserving relevant resources to the
path. It is feasible for controller to calculate bandwidth and send
path setup information to headend via PCEP. But for the other
metrics e.g. queues, same parameter may have different formats and
values on each node. It is not effective for controller to
separately establish PCE session with each node to provision the
metrics.
The second reason to use a distributed messages exchange mechanism
among SRv6 endpoints is that modifications of path-based resource
reservation are required to be accomplished fast enough to guarantee
service's SLA when network failures happen, especially in the case
when thousands of services with independent resource reservations are
affected by the same failure on physical path.
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A proposed hybrid structure of resource reservation in SRv6 network
is comprised of distributed ACH6 resource reservation mechanism and a
centralized stateful PCE [RFC8231].
4. IANA Considerations
o This document requests IANA to assign a codepoint of Destination
Options Header TLVs to indicate ACH6 TLV.
o This document request IANA to create a new registry of ACH6
Channel Types to identify the usage of associated channel.
5. Security Considerations
TBD
6. Acknowledgements
TBD
7. References
7.1. 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>.
[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>.
7.2. Informative 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)",
draft-ietf-6man-spring-srv6-oam-10 (work in progress),
April 2021.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", draft-ietf-ippm-ioam-data-12 (work in
progress), February 2021.
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[I-D.ietf-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., and R. Gandhi,
"Path Segment for SRv6 (Segment Routing in IPv6)", draft-
ietf-spring-srv6-path-segment-00 (work in progress),
November 2020.
[I-D.song-spring-siam]
Song, H. and T. Pan, "SRv6 In-situ Active Measurement",
draft-song-spring-siam-00 (work in progress), December
2020.
[I-D.yang-rtgwg-ipv6-associated-channel]
Yang, F., Chen, M., and T. Zhou, "Associated Channel over
IPv6", draft-yang-rtgwg-ipv6-associated-channel-00 (work
in progress), February 2021.
[RFC6378] Weingarten, Y., Ed., Bryant, S., Osborne, E., Sprecher,
N., and A. Fulignoli, Ed., "MPLS Transport Profile (MPLS-
TP) Linear Protection", RFC 6378, DOI 10.17487/RFC6378,
October 2011, <https://www.rfc-editor.org/info/rfc6378>.
[RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
Computation Element Communication Protocol (PCEP)
Extensions for Stateful PCE", RFC 8231,
DOI 10.17487/RFC8231, September 2017,
<https://www.rfc-editor.org/info/rfc8231>.
[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>.
[RFC8762] Mirsky, G., Jun, G., Nydell, H., and R. Foote, "Simple
Two-Way Active Measurement Protocol", RFC 8762,
DOI 10.17487/RFC8762, March 2020,
<https://www.rfc-editor.org/info/rfc8762>.
Authors' Addresses
Fan Yang
Huawei Technologies
Beijing
China
Email: shirley.yangfan@huawei.com
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Tianran Zhou
Huawei Technologies
Beijing
China
Email: zhoutianran@huawei.com
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