BGP Extensions for Unified SID in TE Policy
draft-liu-idr-segment-routing-te-policy-complement-01
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| Authors | Yao Liu , Shaofu Peng | ||
| Last updated | 2020-03-27 (Latest revision 2020-03-07) | ||
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draft-liu-idr-segment-routing-te-policy-complement-01
IDR Working Group Yao. Liu
Internet-Draft Shaofu. Peng
Intended status: Standards Track ZTE Corporation
Expires: September 28, 2020 March 27, 2020
BGP Extensions for Unified SID in TE Policy
draft-liu-idr-segment-routing-te-policy-complement-01
Abstract
This document defines extensions to BGP in order to advertise Unified
SIDs in SR-TE policies.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SR policy with Unified SID . . . . . . . . . . . . . . . . . 2
2.1. BGP Extensions . . . . . . . . . . . . . . . . . . . . . 4
2.2. Controller Processing . . . . . . . . . . . . . . . . . . 5
2.3. Head-end Processing . . . . . . . . . . . . . . . . . . . 6
3. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
Segment Routing [RFC8402] leverages the source routing paradigm. An
ingress node steers a packet through an ordered list of
instructions,called segments.
[I-D.ietf-spring-segment-routing-policy] details the concepts of SR
Policy and steering into an SR Policy.
[I-D.ietf-idr-segment-routing-te-policy] specifies the way to use BGP
to distribute one or more of the candidate paths of an SR Policy to
the headend of that policy.
With increasing requirements for a shortened identifier in a segment
routing network with the IPv6 data plane,
[I-D.mirsky-6man-unified-id-sr] proposed an extension of SRH that
enables the use of a shorter segment identifier, such as 32-bits
Label format SID or 32-bits IP address format SID.
This document defines extensions to BGP in order to advertise Unified
SIDs in SR-TE policies.
Firstly, we focus on how to carry 32-bits IP address format U-SID,
other type of U-SID (such as 16-bits) will be considered in future
version.
2. SR policy with Unified SID
As discussed in [I-D.ietf-spring-srv6-network-programming], the node
with the SRv6 capability will maintain its local SID table. A Local
SID is generally composed of two parts, that is, LOC:FUNCT, or may
carry arguments at the same time, that is, LOC:FUNCT:ARGS.
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FUNCT indicates the local function of the packet on the node that
generates the LOC.ARGS may contain information related to traffic and
services, or any other information required for executing the
function.LOC indicates locator. In most cases, other nodes in the
network can forward packets to the node that generates this LOC
according to the corresponding routing table entries.
The controller plane protocol can also use B:N to represent an LOC,
where B is SRv6 SID Locator Block and N to represent node N. In
other words, the structure of a complete SID is B:N:FUNCT:ARGS.
[I-D.ietf-lsr-isis-srv6-extensions] defines the extension of ISIS to
support SRv6, and each node can announce the SID assigned by itself.
In particular, SRv6 SID Structure Sub-Sub-TLV is defined and the
specific structure of the corresponding SID is provided, including
the length of SRv6 SID Locator Block, the length of SRv6 SID Locator
Node, the length of SRv6 SID Function, and the length of SRv6 SID
Arguments.
Similarly, [I-D.ietf-bess-srv6-services] also provide the SID
structure information for L3VPN or EVPN service related SID.
Thus, it can be seen that the existing control plane protocol reveals
a very intuitive method to reduce the size of SRH. That is , under
the specific address planning(the SIDs allocated by all SRv6 nodes
are in the same SRv6 SID Locator Block), SRH only needs to store the
difference between SIDs (N:FUNCT:ARGS), and does not need to contain
the SRv6 SID Locator Block information. In a 128-bit classic SRv6
SID, the highest part is SRv6 SID Locator Block, and the following 32
bits are composed of SRv6 SID Locator Node, SRv6 SID Function and
SRv6 SID Arguments, and the rest bits are zeros.
As for how to obtain the SRv6 SID Locator Block information during
packet forwarding, there maybe three cases:
1)For the head-end node, when the node sends a packet along the
segment list to the first segment, it already knows the 128-bit
classical SID before truncating. The head node copies it directly to
the DA of IPv6 Header, but the SRH carries the 32-bit truncatured
SIDs.
2)For the normal transit node, it can obtain the SRv6 SID Locator
Block information from the DA of the received IPv6 packet.
3)For the inter-domain border node, it can obtain the new SRv6 SID
Locator Block information from the local SID entry.
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2.1. BGP Extensions
This document defines two flags in the segment-list sub-TLV
[I-D.ietf-idr-segment-routing-te-policy] RESERVED field, where,
T-Flag: Truncatured-Flag, one bit, when set, it indicates there are
truncated piece information of classical IPv6 SID in the SR path.
FSU-Flag: First SID UET flag, two bits, it indicates the UET type of
the first SID, in other words, indicates the UET domain constructed
by the headend and the first segment node.
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 |T|FSU|RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// sub-TLVs //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: T-Flag in Segment List sub-TLV
In this document, the Flags field of each segment sub-TLV(type B/I/J/
K) [I-D.ietf-idr-segment-routing-te-policy] is extended to indicate
the block length (BL) and non-block length (NBL) of a 128-bit SID,
that is a simple representation of SID structure information.
Figure 2 uses the type B segment sub-TLV as an example to illustrate
the extended SSI field. Other types of segment sub-TLV are similar.
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 |V|A| SSI |UET| | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 SID (16 octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Length Type Field in Segment sub-TLV
Where
SSI: SID Structure Indication, 3-bit field with the following values:
000 unknown
001 BL=96bits, NBL=32bits,
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010 BL=64bits, NBL=32bits,
011 BL=32bits, NBL=32bits,
Other values are reserved for future use.
It should be noted that NBL represents the length of the
Node:Func:ARGs that is immediately followed the block.
UET: U-SID Encapsulation Type, 2-bit field, it indicates the UET type
of the next SID, in other words, indicates the UET domain constructed
by the current segment node and the next segment node. The value
refers to [I-D.mirsky-6man-unified-id-sr].
2.2. Controller Processing
Controller can collect UET capability information of all nodes, see
[I-D.mirsky-6man-unified-id-sr], each node can support one or more
than one UET capabilities. In general, a border node that belongs to
multiple UET domain will support multiple UET capabilities, while
other nodes can only support a single UET capability.
Controller can also collect SID per UET of all nodes. If a node
support an UET capability, it will also allocate related SIDs for
this UET Flavor.
When controller computed an SR path, it can check the UET capability
of each segment node within the segment list, to outline which UET
domains the SR path crosses. For example, from Headend H to endpoint
E, a segment list <X1, X2, X3, B, Y1, Y2, Y3, E> may cross two UET
domains, the node H, X1, X2, X3, B all support UET-1, and the node B,
Y1, Y2, Y3, E all support UET-2. In this case, the FSU-flag will be
set to UET-1, it indicates the UET domian which the first SID X1
belongs to. At the same time, the controller will select UET related
SID for each segment according to the UET domain which the segment
node belongs to, i.e., the UET Flag of SID X1, X2, X3 will be set to
UET-1, and the UET Flag of SID B, Y1, Y2, Y3, E will be se to UET-2.
Note that in this case, SID B with UET-2 Flavor, but not UET-1
Flavor, is inserted in ths list for the purpose of seamless splicing.
Then, controller need to check the structure information of each
selected SID, to ensure they can safely construct an SID list with
UET information. For example, the structure information of SID X1
(with UET-1 Flavor), SID X2 (with UET-1 Flavor), SID X3 (with UET-1
Flavor), SID B (with UET-2 Flavor), MUST support to get UET-1
(because the UET of prev SID is UET-1) related truncated piece
information (Node:Func:ARGS) from the original IPv6 SID. Similarly,
the structure information of SID Y1 (with UET-2 Flavor), SID Y2 (with
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UET-2 Flavor), SID Y3 (with UET-2 Flavor), SID E (with UET-2 Flavor),
MUST support to get UET-2 (because the UET of prev SID is UET-2)
related truncated piece information from the original IPv6 SID.
There maybe another segment list example, <B, Y1, Y2, Y3, E> also
cross two UET domains, that is, the node H, B all support UET-1, and
the node B, Y1, Y2, Y3, E all support UET-2. In this case, the FSU-
flag will be also set to UET-1, it indicates the UET domian which the
first SID B belongs to. At the same time, the controller will select
UET related SID for each segment according to the UET domain which
the segment node belongs to, i.e., the UET Flag of SID B, Y1, Y2, Y3,
E will be se to UET-2. Note that in this case, SID B with UET-2
Flavor, but not UET-1 Flavor, is inserted in ths list for the purpose
of seamless splicing. Then, the controller check the structure
information of each selected SID to ensure they can safely construct
an SID list with UET information. That is, the structure information
of SID B (with UET-2 Flavor), MUST support to get UET-1 (because the
UET of prev SID is UET-1) related truncated piece information from
the original IPv6 SID. Similarly, the structure information of SID
Y1 (with UET-2 Flavor), SID Y2 (with UET-2 Flavor), SID Y3 (with
UET-2 Flavor), SID E (with UET-2 Flavor), MUST support to get UET-2
(because the UET of prev SID is UET-2) related truncated piece
information from the original IPv6 SID.
If a SID can not support to get UET related truncated piece according
to the UET of prev SID, the controller MUST select another prev SID
with UET-0 flavor.
2.3. Head-end Processing
When headend received the SR policy, for each segment list of the
candidate path, if the Truncatured-Flag is set, the segment list
could try to be optimized to an SID list that contains short U-SIDs.
For each original SID within the received SID list, it will be
optimized to an U-SID according to the UET of prev SID. For example,
for the above segment list <X1, X2, X3, B, Y1, Y2, Y3, E>, the
original SID X1 could be optimized to U-SID X1 according the UET of
prev SID (in fact, it is FSU), get the truncated prev-UET related
piece from the original SID with the help of its SSI field.
Similarly, the original SID B could be optimized to U-SID B according
the UET of prev SID X3, get the truncated prev-UET related piece from
the original SID with the help of its SSI field.
The SRH will contain the optimized U-SIDs, and the initial SRH.UET
will be set as FSU. Other procedures refer to
[I-D.mirsky-6man-unified-id-sr].
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3. Security Considerations
Procedures and protocol extensions defined in this document do not
affect the security considerations discussed in
[I-D.ietf-idr-segment-routing-te-policy].
4. IANA Considerations
TBD
5. References
5.1. Normative References
[I-D.ietf-idr-segment-routing-te-policy]
Previdi, S., Filsfils, C., Talaulikar, K., Mattes, P.,
Rosen, E., Jain, D., and S. Lin, "Advertising Segment
Routing Policies in BGP", draft-ietf-idr-segment-routing-
te-policy-08 (work in progress), November 2019.
[I-D.ietf-lsr-isis-srv6-extensions]
Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
Z. Hu, "IS-IS Extension to Support Segment Routing over
IPv6 Dataplane", draft-ietf-lsr-isis-srv6-extensions-07
(work in progress), March 2020.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Sivabalan, S., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-06 (work in progress),
December 2019.
[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-14 (work in
progress), March 2020.
[I-D.mirsky-6man-unified-id-sr]
Cheng, W., Mirsky, G., Peng, S., Aihua, L., Wan, X., and
C. Wei, "Unified Identifier in IPv6 Segment Routing
Networks", draft-mirsky-6man-unified-id-sr-06 (work in
progress), March 2020.
[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>.
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5.2. Informative References
[I-D.ietf-bess-srv6-services]
Dawra, G., Filsfils, C., Raszuk, R., Decraene, B., Zhuang,
S., and J. Rabadan, "SRv6 BGP based Overlay services",
draft-ietf-bess-srv6-services-02 (work in progress),
February 2020.
Authors' Addresses
Liu Yao
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing
China
Email: liu.yao71@zte.com.cn
Peng Shaofu
ZTE Corporation
No. 50 Software Ave, Yuhuatai Distinct
Nanjing
China
Email: peng.shaofu@zte.com.cn
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