IDR Working Group Yao. Liu
Internet-Draft Shaofu. Peng
Intended status: Standards Track ZTE Corporation
Expires: May 27, 2021 November 23, 2020
BGP Extensions for Unified SID in TE Policy
draft-liu-idr-segment-routing-te-policy-complement-04
Abstract
This document defines extensions to BGP in order to advertise Unified
SIDs in SR-TE policies.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SR policy with Unified SID . . . . . . . . . . . . . . . . . 2
2.1. Advertisement of SID Attribute . . . . . . . . . . . . . 4
2.1.1. Option 1: Advertising SID Attribute within existing
sub-TLVs . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2. Option 2: Introducing a new U-segment list sub-TLV . 7
2.2. Controller Processing . . . . . . . . . . . . . . . . . . 7
2.3. Headend Processing . . . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Normative References . . . . . . . . . . . . . . . . . . 9
5.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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
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SID is generally composed of two parts, that is, LOC:FUNCT, or may
carry arguments at the same time, that is, LOC:FUNCT:ARGS.
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.
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3)For the inter-domain border node, it can obtain the new SRv6 SID
Locator Block information from the local SID entry.
2.1. Advertisement of SID Attribute
The U-SID solution defined in [I-D.mirsky-6man-unified-id-sr] reply
two attributes of SID, they are: SID structure attribute and Endpoint
Behavior attribute. However,
[I-D.ietf-idr-segment-routing-te-policy] does not provide these
information now. This document discusses two options to supplement
these information.
2.1.1. Option 1: Advertising SID Attribute within existing sub-TLVs
In this section, a new sub-sub-TLV is introduced in each segment sub-
TLV(type B/I/J/K) [I-D.ietf-idr-segment-routing-te-policy] to offer
the SID structure information.
Since the new compression information-related sub-sub-TLV is included
in segment List sub-TLV, the meaning of the whole segment list will
be changed, that is, the headend cannot regard this segment list as a
classic segment list to process and encapsulate the classic 128 bit
SRH. Therefore, the controller must know the compression capability
supported by the head node when delivering SR policy to the it.
There are two ways to do this.
Opntion 1, negotiate of compression capacity through BGP session.
The controller only sends the Segment List Sub-TLV with compression
information to the BGP neighbors with compression capability.
It is necessary to consider the scenario with a route reflector. The
BGP session is not directly established between the controller and
the head node. One or more RT Extended Community can be carried in
the SR policy UPDATE announcement message to contain the specific
head node Router-ID information.
If the controller learns that the head node has the compression
capability by some means (such as collecting through BGP-LS), but the
RR does not have the abilities , then the controller can still choose
to send to the RR according to the actual destination node notified
by UPDATE.
If the reflector does not recognize the newly added sub-TLV / sub-
sub-TLV compression information, it is necessary to decide whether to
unconditionally transmit it to the head node according to the
positive bit in the top-level TLV (that is, the Tunnel Encapsulation
Attribute).
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If the reflector recognizes the newly added sub-TLV / sub-sub-TLV, it
is necessary to check whether the headend has compression capability.
If not, RR will not reflect the Segment List Sub-TLV containing
compressed information to the head node.
Opntion 2, the controller collects the compression capability of the
head node through BGP-LS. If the head node has compression
capability, the controller can deliver an segment list Sub-TLV
containing compression information to the head node. Otherwise, only
an Segment List sub-TLV containing 128-bit SIDs can be delivered.
If there is a RR, it only needs to decide whether to transmit it
unconditionally to the head node according to transitive bit in the
top-level TLV (that is, Tunnel Encapsulation Attribute).
The first method is too complicated, so option 2 is recommended.
Figure 2 uses the type B segment sub-TLV as an example, other types
of segment sub-TLV are similar.
The SRv6 SID Structure Sub-Sub-TLV has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|sub-Type=STRUCT| Length | Count | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BL of SID 1 | TL of SID 1 | BL of SID 2 | TL of SID 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... ... | BL of SID N | TL of SID N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SRv6 SID Structure Sub-Sub-TLV format
where,
Count: 1 octet, the count of segments.The value of count must be
consistent with the number of Segment Sub-TLV contained in segment
list sub-TLV; otherwise, the whole segment list sub-TLV must be
ignored.
BL: block length of classical 128 bit SID in bits, value: 1~ 128. If
the corresponding SID is an MPLS label, BL is 0.
TL: truncated length of the compressed SID in bits, value: 1~ 128.
If a 128 bit SID is compressed to 32 bits, TL is 32. If a 128 bit
SID is not compressed, TL is 128. the TL of a 32-bit MPLS label is
32.
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As above, if the headend does not recognize the Segment Truncated
sub-TLV, the entire Segment List sub-TLV must be ignored.
A new flag is introduced in Segment Sub-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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |V|A| |UET| | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// SRv6 SID (16 octets) //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: UET-Flavor Flag in Segment sub-TLV
where,
UET: U-SID Encapsulation Type Flag, 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.
It could be the following values:
0: the UET domain following the current segment node is UET-128
domain, that means the next SID does not need compression and
remains 128 bits.
1: the UET domain following the current segment node is UET-32
domain, that means the next SID needs to be compressed to a 32-bit
IP address.
2: the UET domain following the current segment node is UET-
32-MPLS domain, that means the next SID needs to be compressed to
a 32-bit MPLS Label.
3: the UET domain following the current segment node is UET-16
domain, that means the next SID needs to be compressed to a 16-bit
IP address.
Currently, if only the 32-bit IP address compression mode is
considered, the UET-Flavor value is 0 or 1.
Two factors should be considered for the value of UET-Flavor: the
compression domain type composed of this segment node and the next
segment node, and the structure of the next selected SID can be
compressed in the way indicated by UET-Flavor.
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2.1.2. Option 2: Introducing a new U-segment list sub-TLV
In order to solve the forward compatibility problem more
conveniently, a new U-Segment List Sub-TLV is defined, which can
contain compressed information.
Similarly, the controller decides whether to send U-Segment List Sub-
TLV with compression information or Segment List Sub-TLV without
compression information to the head node based on whether the head
node has the compression capability.
The specific extension:
1) Add a UET-Flavor Flag to the existing Segment Sub-TLV.
2) In U-Segment List Sub-TLV, define Segment Truncated sub-TLV to
describe the compression result.
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
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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
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. Headend Processing
When the headend receives the SR policy, it obtains the compressed
information of each SID according to the TL field in the Segment
Truncated sub-TLV. The headend should identify the UET-Flavor of
each SID, which can be verified with the compression result, that is,
the UET-Flavor of a certain SID must be consistent with the
compression result of the next SID, otherwise the entire Segment List
sub-TLV must be ignored .
In particular, the UET-Flavor of the last SID can be used as a clear
basis to decide what compression method should be adopted for the
overlay SID, such as the VPN service.
Optionally, the headend can use reduced SRH that exclude the first
SID, to further reduce the cost of SRH.
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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-11 (work in progress), November 2020.
[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-11
(work in progress), October 2020.
[I-D.ietf-spring-segment-routing-policy]
Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
P. Mattes, "Segment Routing Policy Architecture", draft-
ietf-spring-segment-routing-policy-09 (work in progress),
November 2020.
[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-24 (work in
progress), October 2020.
[I-D.mirsky-6man-unified-id-sr]
Cheng, W., Mirsky, G., Peng, S., Aihua, L., and G. Mishra,
"Unified Identifier in IPv6 Segment Routing Networks",
draft-mirsky-6man-unified-id-sr-07 (work in progress),
July 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., Talaulikar, K., Raszuk, R.,
Decraene, B., Zhuang, S., and J. Rabadan, "SRv6 BGP based
Overlay services", draft-ietf-bess-srv6-services-05 (work
in progress), November 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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