Network Working Group C. Li
Internet-Draft Huawei
Intended status: Standards Track W. Cheng
Expires: February 4, 2021 China Mobile
M. Chen
S. Previdi
Huawei
Z. Li
China Mobile
August 3, 2020
A Light Weight IOAM for SRv6 Network Programming
draft-li-spring-light-weight-srv6-ioam-02
Abstract
In-situ OAM(IOAM) records OAM information within the packet while the
packet traverses a particular network domain. This document defines
a light weight IOAM method for SRv6 network programming. In this
method, IOAM information is carried in the data packet by the SIDs in
the SRH.
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 February 4, 2021.
Copyright Notice
Copyright (c) 2020 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
3. SRv6 Light Weight IOAM . . . . . . . . . . . . . . . . . . . 4
3.1. The FLAG Field of SID . . . . . . . . . . . . . . . . . . 4
4. Capabilities and SID Format Advertisement . . . . . . . . . . 5
5. SIDs Distribution . . . . . . . . . . . . . . . . . . . . . . 5
6. Packet Processing . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Source SR Node . . . . . . . . . . . . . . . . . . . . . 6
6.1.1. Reduced SRH . . . . . . . . . . . . . . . . . . . . . 6
6.2. Transit Node . . . . . . . . . . . . . . . . . . . . . . 6
6.3. SR Segment Endpoint Node . . . . . . . . . . . . . . . . 7
6.3.1. Egress Node . . . . . . . . . . . . . . . . . . . . . 7
6.4. Instructions for FLAG . . . . . . . . . . . . . . . . . . 8
7. Illustrations . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Timestamp Recording Procedures . . . . . . . . . . . . . 9
8. Backward Compatibility Considerations . . . . . . . . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
10. Security Considerations . . . . . . . . . . . . . . . . . . . 10
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Segment routing (SR) [RFC8402] is a source routing paradigm that
explicitly indicates the forwarding path for packets at the source
node by inserting an ordered list of instructions, called segments.
A segment can represent a topological or service-based instruction.
When segment routing is deployed on IPv6 dataplane, called SRv6
[RFC8754], a segment is a 128 bit value, and it can be an IPv6
address of a local interface but it does not have to. As defined in
[I-D.ietf-spring-srv6-network-programming], a segment has the format
of LOC: FUNCT. The most significant L bits is LOC that is routable
and leads packets to the SID originating node. The least significant
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128-L bits is the value of FUNCT that defines the local actions
associated to the SID. L is the length of LOC and it is flexible.
For supporting SR, a new type of routing header, called Segment
Routing Header (SRH), which contains a list of SIDs and other needed
information , has been defined in [RFC8754].
In-situ OAM(IOAM) records OAM information within the packet while the
packet traverses a particular network domain. A discussion of the
motivation and requirements for in-situ OAM can be found in
[I-D.brockners-inband-oam-requirements].
[I-D.ietf-6man-spring-srv6-oam] defines an IOAM mechanism for SRv6.
However, recording IOAM data in SRH TLVs will bring bigger overhead,
which will reduce transport efficiency. In addition, the length of
the header will increase in the incremental trace option
[I-D.ietf-ippm-ioam-data], which may bring MTU problem and increase
the difficulty of packet processing.
This document introduces a light weight IOAM method for SRv6 network
programming. In this method, the OAM information is in the segment
list of the SRH once the segment is processed.
In most cases, an SRv6 segment will not be reused again after it has
been processed for updating the IPv6 destination address (as part of
the SRH procedures described in [RFC8754]. However, these processed
SIDs will still be carried in the SRH to the destination of the
packet (or penultimate node if PSP is enabled). Therefore, these
processed SIDs (i.e.: the 128 bit space they occupy) can be reused
for other purposes such as carrying performance measurement
information or IOAM [I-D.ietf-ippm-ioam-data] information.
Using the SID in order to carry OAM information allows not to
increase the size of the packet header and, of course, will cause the
loss of the original SID value.
2. Terminology
This memo makes use of the terms defined in [RFC8402],
[I-D.ietf-ippm-ioam-data] and
[I-D.ietf-spring-srv6-network-programming].
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.
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3. SRv6 Light Weight IOAM
This document defines a light weight IOAM model for SRv6 network
programming.
In SRv6, a SID will not be used again after it has been processed
according to SRH procedures described in [RFC8754]. Therefore, the
128 bits of the used SID are reused in order to store metadata such
as IOAM data.
In this document, we assume that the rewritable length of a SID is 64
bits at least, since IOAM data need to meet the accuracy requirement
as defined at [I-D.ietf-ippm-ioam-data]. The rewritable part
consists of the last 64 bits of the SID, which MAY be the FUNCT part.
In order to determine to which node the IOAM data is related, the LOC
that identifies the node must remain, especially when the type of
IOAM data is not node ID [I-D.ietf-ippm-ioam-data]. For getting rid
of the limitation of keeping LOC part of the SID, a Path Segment
[I-D.li-spring-srv6-path-segment] is included in the SID list.
In order to indicate the type of IOAM data, the document defines a
new field of the SID called the FLAG. With the FLAG field, the
format of the SID is LOC:FLAG:FUNCT. The new FLAG field is used in
order to indicate additional operations, such as IOAM operations.
The IOM data stored in the SID is structured in the following format:
<FLAG><IOAMdata>. The procedure will be described in section 5.
3.1. The FLAG Field of SID
In order to indicate the type of IOAM data, this document defines a
new field of the SID, called FLAG.
This document does not limit the offset and length of the FLAG field,
and it can be configured by the operator. For instance, a FLAG field
can be a 8 bits value between the LOC and the FUNCT fields. The
offset can be the 48th bit. In other words, 0-47 bit is the LOC,
48-55 bit is the FLAG, and 56-127 bit is the FUNCT. The value of the
FLAG field indicates the IOAM processing and IOAM data type, and may
have the following values:
o 0: Non IOAM
o 1: Timestamp(32-bit seconds and 32-bit subseconds)
o 2: Packet counter
o 3: Queue depth
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o 4: Ingress_if_id and egress_if_id (short format)
o 5: Hop_Lim and node_id
o 6: Namespace specific data
o 7: Buffer occupancy
o 8: Checksum Complement
o 9-255: Reserved
The IOAM data reuses the format defined in [I-D.ietf-ippm-ioam-data].
A packet counter is a 64-bit value that records the total number of
packets in a flow, path or SR policy received by the node. It can be
used for use cases like packet loss measurement.
4. Capabilities and SID Format Advertisement
In order to support light weight IOAM, nodes SHOULD advertise the
IOAM capability to other nodes within an SR domain via, e.g., IGP
extensions. The definition, advertisement and processing of such
capability advertisement is out of scope of this document, and will
be described in a separate document.
The format of the SID SHOULD also be advertised to the other nodes in
the SR domain so that each node having to insert IOAM data, know
which format of the SID it has to use (i.e.: the size of the LOC,
FLAG and FUNC fields). The description of the advertisement of the
SID format is out of scope of this document, and will be described in
another document.
5. SIDs Distribution
It has to be noted that the FLAG field does not introduce any new
type of SID in the SR architecture.
A node can distribute all variants of SIDs with different FLAG
values. For example, Node A instantiates an End SID
[I-D.ietf-spring-srv6-network-programming] as
A::0::100(LOC:FLAG:FUNCT), and it supports adding IOAM data of
timestamp and queue depth. Then, node A can distribute the SIDs
A::0::100, A::1::100, and A::3::100 in order to support respectively
non-IOAM End SID, End SID with timestamp recording, and End SID with
Queue depth recording. An Ingress node B can use these three
variants of SIDs according to the SR policy in order to achieve IOAM
with the specific node data.
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Alternatively, a node can distribute only a default variant of SID in
order to reduce the SID distribution flooding traffic. The other
variants can be generated and used by the ingress node according to
the capabilities information of the SID endpoint node A. The details
will be discussed in another document.
6. Packet Processing
[RFC8754] describes SRv6 packet processing at the SR source, Transit
and SR segment endpoint nodes. This section describes the SRv6
packet processing with the new FLAG field in the SID.
6.1. Source SR Node
This document assumes that, in an operator network, the packet
received at the ingress node is encapsulated into an outer IPv6+SRH
header. Therefore, the term "ingress" and "source" are to be
intended as the same node.
A source node steers a packet into an SR Policy that consists of a
segment list. When deploying IOAM, the source node SHOULD insert the
associated SID variant into the segment list as illustrated in
section 5. The variants of the SID can be learned from SIDs
distribution or generated according to node's capabilities.
After the first SID (SID-List[n-1], last entry in the SID list)is
updated to the IPv6 DA, the source node SHOULD rewrite the SID with
IOAM data according to the value of FLAG field and then send it to
the next hop.
6.1.1. Reduced SRH
Reduced SRH cannot support light weight IOAM since there is no space
for carrying IOAM data of the source node.
6.2. Transit Node
As specified in [RFC8200], the only node allowed to inspect the
Routing Extension Header (and therefore the SRH), is the node
corresponding to the DA of the packet. Any other transit node MUST
NOT inspect the underneath routing header and MUST forward the packet
toward the DA according to its IPv6 routing table. Thus, there is no
modification of packet processing on transit nodes.
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6.3. SR Segment Endpoint Node
As per [RFC8754], when an SRv6-capable node receives an IPv6 packet,
it performs a longest-prefix-match lookup on the packets destination
address. When this lookup returns A FIB entry that represents a
locally instantiated SRv6 SID, then the node should process the SRH
and related SIDs.
As per [I-D.ietf-spring-srv6-network-programming], if the IPv6 DA is
a local SID instantiated by the node, then it should be looked up in
"My local SID table " in order to execute the instruction bound to
it. The "My Local SID Table" matches on a per longest-prefix-match
basis.
In order to process FLAG field, there are two options:
o All variants of the SID should be generated by the endpoint node
and stored in the "My Local SID Table" so that the variants of the
SID in IPv6 DA can match the entry by longest-prefix-match basis.
The node looks up the variant SID and executes the associated
instruction. IOAM data will be rewritten into the SID after the
SID has been copied into the DA of the packet. In this way, all
variants will be instantiated in the SID table.
o The endpoint node only distributes a default variant SID (FLAG
value is 0) and stores it in the "My Local SID Table". Before
looking up the SID, the value of the FLAG part is obtained and
then make it as all zero. The node looks up the default variant
SID to get the instruction. The FLAG related instruction that
sets IOAM data into the SID is executed after the DA of the packet
is updated with the SID (as per SRH processing -
[I-D.ietf-spring-srv6-network-programming]).
6.3.1. Egress Node
In this document it is assumed that the egress node is the final
destination of the encapsulated packet. Therefore, the egress node
removes the outer IPv6 and SRH header.
As the last SRV6 endpoint node in the SID list, the egress node
cannot write any IOAM data into the SID since the segment list is
completed and there is no more SID to use. Therefore, the egress
node should record the related IOAM data and punt the data with a
copied packet to the CPU for further IOAM processing.
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6.4. Instructions for FLAG
In order to achieve lightweight IOAM, processing of the FLAG field is
required and illustrated by the pseudo code here below:
1.If FLAG == n:
2. Get the IOAM-data D1 of type n.
3. If SL=1 and DA is End.S or if SL=0: ;;Ref1
4. Punt a copied packet with D1 to CPU for further processing;;Ref2
5. Else:
6. Insert following code after the instruction of updating DA:
7. Update SRH[SL][64:] with D1 ;;Ref3
o Ref1: If SL is 0 or SL is 1 and the DA is an End.S
[I-D.ietf-spring-srv6-network-programming], it means that the
current node is the egress node. The node will punt a copied
packet with the D1 to the CPU for further processing.
o Ref2: At the egress node, in order not to affect the packet
forwarding, a copy of the data packet should be punted instead of
the data packet itself. However, this document does not limit
implementations to punt the entire packet or only headers of the
packet.
o Ref3: If the node is not the egress node the node only inserts
"updates the SID with D1" after the instruction of "update DA with
SRH[SL]". The last 64 bits of SID SRH[SL] is rewritten with D1
after updating SID SRH[SL] to IPv6 DA. If there is no Path
Segment in the segment list, the locator part needs to remain for
identifying a SID. The related IOAM data at the ingress node
should be written into the SID before sending the data packet.
The pseudo code below illustrates the example where the FLAG is set
to 1, indicating that the IOAM data is a timestamp:
1.If FLAG == 1:
2. Get the receiving timestamp T1.
3. If SL=1 and DA is End.S or if SL=0:
4. Punt a copy of the packet with T1 to CPU for further processing
5. Else:
6. Insert following code after the instruction of updating DA:
7. Update SRH[SL][64:] with T1
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7. Illustrations
For easy understanding, the following simple topology is used for
illustration.
CE-A ---- N1-----N2-----N3---- CE-B
Reference Topology
In the reference topology:
o Nodes N1, N2, and N3 are all SRv6 capable nodes.
o Node N1 and N3 are configured with a tenant 10, each connected to
CE-A and CE-B respectively.
o Node Nk has Ak::/48 for its local SID space from which Local SIDs
are explicitly allocated.
o A2::1::1 is an End [I-D.ietf-spring-srv6-network-programming]
function with timestamp recording allocated by N2
o A3::1::D10 is an End.DT4
[I-D.ietf-spring-srv6-network-programming] function with timestamp
recording bound to tenant IPv4 table 10 allocated by N3.
o Bk::/48 is the loopback address of node K for IGP.
It is assumed that the measured flow from CE-A to CE-B will travel
along the path <N1, N2, N3>.
When the ingress node N1 receives an IPv4 packet with SA:CE-A, and
DA:CE-B, the ingress node N1 will encapsulate the packet into an IPv6
header followed by an SRH with the SID list <A2::1::1, A3::1::D10>,
so that the packet header is now (B1, A2::1::1) (A3::1::D10,
A2::1::1, SL=2)(CE-A, CE-B). The DA of the IPv6 header is then
updated with the first segment.
7.1. Timestamp Recording Procedures
After updating the DA with A2::1::1, the ingress node N1 updates the
SID A2::1::1 in the segment list with current timestamp. Then it
forwards the packet to N2 according to its RIB/FIB. Assuming that
the timestamp value is T1, then the updated packet header becomes
(B1, A2::1::1) (A3::D10, A2::1::T1, SL=1) (CE-A, CE-B).
When N2 receives the packet with DA A2::1::1 which is an End SID with
timestamp recording originated by N2, N2 will insert "Update
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SRH[SL][64:] with the current timestamp" after instruction of "update
DA with SRH[SL]", and then process this End SID.
After updating IPv6 DA with SRH[0] A3::1::D10, the current timestamp
will be recorded at the last 64 bits of End SID A3::1::D10. Assuming
that the timestamp value is T2, the updated packet header becomes
(B1, A3::1::D10) (A3::1::T2, A2::1::T1, SL=0) (CE-A, CE-B).
According to the updated DA A3::1::D10, the packet will be forwarded
to N3.
When the egress node N3 receives the packet with header (B1,
A3::1::D10) (A3::1::T2, A2::1::T1, SL=0) (CE-A, CE-B), N3 should
timestamp the copied packet and punt it to CPU for further
processing. Then, N3 decapsulates the outer header and forwards the
inner packet to CE-B based on the routes in tenant-10 IPv4 table.
IOAM data processing can be implemented by a node or a remote
controller. In this solution, only one receiving timestamp or
sending timestamp can be recorded in the SID at each endpoint node.
Therefore, it is RECOMMENDED that the ingress node records the
sending timestamp while the other nodes record the receiving
timestamp.
8. Backward Compatibility Considerations
TBA
9. IANA Considerations
TBA
10. Security Considerations
TBA
11. Contributors
TBA
12. Acknowledgements
Many thanks to Stefano's professional comments.
13. References
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13.1. Normative References
[I-D.ietf-spring-srv6-network-programming]
Filsfils, C., Camarillo, P., Leddy, J., Voyer, D.,
Matsushima, S., and Z. Li, "SRv6 Network Programming",
draft-ietf-spring-srv6-network-programming-16 (work in
progress), June 2020.
[I-D.li-spring-srv6-path-segment]
Li, C., Cheng, W., Chen, M., Dhody, D., and R. Gandhi,
"Path Segment for SRv6 (Segment Routing in IPv6)", draft-
li-spring-srv6-path-segment-05 (work in progress), March
2020.
[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>.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374,
DOI 10.17487/RFC6374, September 2011,
<https://www.rfc-editor.org/info/rfc6374>.
[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>.
[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>.
[RFC8321] Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,
L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,
"Alternate-Marking Method for Passive and Hybrid
Performance Monitoring", RFC 8321, DOI 10.17487/RFC8321,
January 2018, <https://www.rfc-editor.org/info/rfc8321>.
[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>.
[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>.
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13.2. Informative References
[I-D.brockners-inband-oam-requirements]
Brockners, F., Bhandari, S., Dara, S., Pignataro, C.,
Gredler, H., Leddy, J., Youell, S., Mozes, D., Mizrahi,
T., Lapukhov, P., and r. remy@barefootnetworks.com,
"Requirements for In-situ OAM", draft-brockners-inband-
oam-requirements-03 (work in progress), March 2017.
[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-07 (work in progress),
July 2020.
[I-D.ietf-ippm-ioam-data]
Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
for In-situ OAM", draft-ietf-ippm-ioam-data-10 (work in
progress), July 2020.
Authors' Addresses
Cheng Li
Huawei
Email: c.l@huawei.com
Weiqiang Cheng
China Mobile
Email: chengweiqiang@chinamobile.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
Stefano Previdi
Huawei
Italy
Email: stefano@previdi.net
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Zhenqiang Li
China Mobile
32 Xuanwumen West Ave
Beijing, Xicheng District
China
Email: lizhenqiang@chinamobile.com
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