SPRING Working Group Shaofu. Peng
Internet-Draft ZTE Corporation
Intended status: Standards Track January 4, 2021
Expires: July 8, 2021
Truncated SID Inter Domain Considerations
draft-peng-spring-truncated-sid-inter-domain-00
Abstract
This document introduces a method for interworking between domains of
Segment Routing in IPv6 network that use different levels of Segment
Identifier's compression.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions used in this document . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
3. Interworking Between Different Domains . . . . . . . . . . . 3
4. Control Plane Extensions . . . . . . . . . . . . . . . . . . 5
4.1. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . 5
4.2. Extensions to IGP . . . . . . . . . . . . . . . . . . . . 5
4.2.1. Extensions to IS-IS . . . . . . . . . . . . . . . . . 5
4.2.2. Extensions to OSPFv3 . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6.1. BGP-LS SRv6 SID Swapped Block Sub-TLV . . . . . . . . . . 7
6.2. IS-IS SRv6 SID Swapped Block Sub-TLV . . . . . . . . . . 7
6.3. OSPFv3 SRv6 SID Swapped Block Sub-TLV . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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-srv6-network-programming] proposes
the Segment Routing in IPv6 (SRv6) Network Programming to specify a
packet processing program by encoding a sequence of instructions in
the IPv6 packet's extension header.
[I-D.ietf-spring-srv6-network-programming] defined a Segment
Identifier (SID) as consisting of LOC:FUNCT:ARG, where a locator
(LOC) is encoded in the L most significant bits of the SID, followed
by F bits of function (FUNCT) and A bits of arguments (ARG). A
locator may be represented as B:N where B is the SRv6 SID block (IPv6
prefix allocated for SRv6 SIDs by the operator) and N is the
identifier of the parent node instantiating the SID. Note that "SID
Locator Block" and "BLOCK" are used interchangeably throughout the
document.
As demonstrated in several proposals, such as
[I-D.cl-spring-generalized-srv6-for-cmpr] and
[I-D.mirsky-6man-unified-id-sr], SIDs could be allocated by SRv6
nodes from within the same SRv6 SID Locator Block, and referring to
the idea described in [RFC6554] SRH only needs to store the
difference between SIDs, such as NODE:FUNCT:ARGS, the non-BLOCK part,
and does not need to contain the SRv6 SID Locator Block information.
In this case, some or all SIDs encoded in the SRH are the non-BLOCK
part of the 128-bit SIDs.
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An SRv6 path could traverse domains that use different block prefixes
and achieving different compressed SIDs lengths. This document
provides a method of higher compression efficiency for interworking
between domains. It introduces a new SID flavor and defines the
required control plane extensions.
2. Conventions used in this document
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.
3. Interworking Between Different Domains
The interworking of different domains is illustrated in Figure 1.
................................. .......................
: : : :
+----+ +----+ +----+ +----+ +----+ +----+
+ S +-----+ A +-----+ B +-----+ABR +-----+ C +-----+ D +
+----+ +----+ +----+ +----+ +----+ +----+
: : : :
.......... domain 1.............. .......domain2.........
Figure 1: Interworking between domains
An end-to-end SRv6 path from A to D passes through two domains. The
block prefix of Domain 1 is BLOCK1, and the compressed SID's length
is 32 bits. The block prefix of Domain 2 is BLOCK2, and the
compressed SID's length is 16 bits.
The following notation is used in the document, 128bits-BLOCK1-SID-
A-32 means that this is a 128-bit SID of node A whose block is BLOCK1
and itself can be compressed to 32 bits if needed. The original
segment list before the compression is <128bits-BLOCK1-SID-A-32,
128bits-BLOCK1-SID-B-32, 128bits-BLOCK1-SID-ABR-32, 128bits-BLOCK2-
SID-C-16, 128bits-BLOCK2-SID-D-16>.
This segment list is selected/computed by a Path Computation Element
(PCE) system or a controller and the details are for further
discussion.
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A new SID flavor is defined in this document to make the entire
segment list more compressible.
Block Swapping (BS) Flavor: it indicates that SRv6 SID Locator Block
will be swapped. And the information about the new block with
block's length is maintained in the local SID entry.
So in ABR, a 128bits-BLOCK1-ABR-32 SID with BS flavor is allocated.
It is within 128-bit SID with block1 and has BS Flavor, and the local
SID entry will swap BLOCK1 to BLOCK2. Thus, the next short 16 bit
SID can be stitched with BLOCK2 to generate a full 128 bit SID. With
the help of the BS flavor, the corresponding compressed segment list
could be <32bits-SID-A,32bits-SID-B,32bits-SID-ABR(BS), 16bits-SID-C,
16bits-SID-D>.
32bits-SID-A means this is a 32-bit compressed SID of node A.
32bits-SID-ABR(BS) means this is a 32-bit compressed SID of node ABR
with BS flavor.
At the headend S, the packet sent to node A is (SA,DA=BLOCK1+32bits-
SID-A)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B,
32bits-SID-A; SL=3), i.e., only one single 128 segment entry is
needed in SRH. Note that SL=3 means there are three 32-bits segment
items within SRH remaining to be visited.
The packet sent from node A to node B is (SA,DA=BLOCK1+32bits-SID-
B)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B, 32bits-
SID-A; SL=2).
The packet sent from B to ABR is (SA,DA=BLOCK1+32bits-SID-
ABR)(16bits-SID-D, 16bits-SID-C, 32bits-SID-ABR, 32bits-SID-B,
32bits-SID-A; SL=1).
When the packet arrives at ABR, it matches the local SID entry. The
endpoint behavior of that entry is END with BS Flavor, and the new
block with its block length is provided in the local SID entry. The
next short 16 bit SID can be retrieved from SRH and be stitched with
the new block to generate a full 128 bit SID to be copied to the DA
field. Thus, the packet sent from ABR to C could be
(SA,DA=BLOCK2+16bits-SID-C)(16bits-SID-D, 16bits-SID-C, 32bits-SID-
ABR, 32bits-SID-B, 32bits-SID-A; SL=1). Note that SL=1 means there
are one 16-bits segment items within SRH remaining to be visited.
Generally, multiple BS-flavored SIDs may be allocated on the boundary
node. For example, SID-12 with BS flavor indicates that SRv6 SID
Locator Block switches from BLOCK1 to BLOCK2, and SID-13 indicates
that SRv6 SID Locator Block switches from BLOCK1 to block3.
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For the inter-AS scenario, END.X SID allocated for SR-EPE or a direct
link can also have BS flavor to indicate the new block information of
the next AS.
4. Control Plane Extensions
4.1. Extensions to BGP-LS
This document introduces an SRv6 SID Swapped Block Sub-TLV. It is an
optional TLV for use in the BGP-LS Attribute for an SRv6 SID NLRI and
as a sub-TLV of the SRv6 End.X, IS-IS SRv6 LAN End.X and OSPFv3 SRv6
LAN End.X TLVs [I-D.ietf-idr-bgpls-srv6-ext]. This TLV is validate
only when the SID has a BS flavor.
The TLV has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Variable Swapped Block //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: BGP-LS SRv6 SID Swapped Block Sub-TLV
Type: TBA
Length: 2 octet field, indicate the length of Swapped Block.
Swapped Block: This field encodes the advertised swapped block prefix
information.
4.2. Extensions to IGP
Different nodes in the same IGP domain may have different block
prefixes. So IGP extensions are also needed in this scenario.
4.2.1. Extensions to IS-IS
SID Swapped Block Sub-Sub-TLV is introduced in this document. It is
an optional Sub-Sub-TLV of SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-
TLV and SRv6 LAN End.X SID Sub-TLV
[I-D.ietf-lsr-isis-srv6-extensions].
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Variable Swapped Block //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ISIS SRv6 SID Swapped Block Sub-Sub-TLV
Type: TBA, 1 octets.
Length: 1 octet field, indicate the length of Swapped Block.
Swapped Block: This field encodes the advertised swapped block prefix
information.
4.2.2. Extensions to OSPFv3
SID Swapped Block Sub-TLV is introduced in this document. It is used
as an optional Sub-TLV of SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-
TLV, SRv6 LAN End.X SID Sub-TLV
[I-D.ietf-lsr-ospfv3-srv6-extensions].
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// Variable Swapped Block //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: OSPFv3 SRv6 SID Swapped Block Sub-Sub-TLV
Type: TBA, 2 octets.
Length: 2 octet field, indicate the length of Swapped Block.
Swapped Block: This field encodes the advertised swapped block prefix
information.
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5. Security Considerations
TBD
6. IANA Considerations
6.1. BGP-LS SRv6 SID Swapped Block Sub-TLV
This document requests IANA to assign a new code point in the "BGP-LS
Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
TLVs" sub-registry under the "Border Gateway Protocol - Link State
(BGP-LS) Parameters" registry.
Type: TBA
6.2. IS-IS SRv6 SID Swapped Block Sub-TLV
This document requests IANA to assign a new code point in the "Sub-
Sub-TLVs for SID Sub-TLVs" registry under the IS-IS registry.
Type: TBA
6.3. OSPFv3 SRv6 SID Swapped Block Sub-TLV
This document requests IANA to assign a new code point in the "OSPFv3
Extended-LSA Sub-TLVs" registry under the "Open Shortest Path First
v3 (OSPFv3) Parameters" registry.
Type: TBA
7. References
7.1. Normative References
[I-D.ietf-idr-bgpls-srv6-ext]
Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
daniel.bernier@bell.ca, d., and B. Decraene, "BGP Link
State Extensions for SRv6", draft-ietf-idr-bgpls-
srv6-ext-05 (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.
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[I-D.ietf-lsr-ospfv3-srv6-extensions]
Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
"OSPFv3 Extensions for SRv6", draft-ietf-lsr-
ospfv3-srv6-extensions-01 (work in progress), August 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>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012,
<https://www.rfc-editor.org/info/rfc6554>.
[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>.
[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>.
7.2. Informative References
[I-D.cl-spring-generalized-srv6-for-cmpr]
Cheng, W., Li, Z., Li, C., Clad, F., Aihua, L., Xie, C.,
Liu, Y., and S. Zadok, "Generalized SRv6 Network
Programming for SRv6 Compression", draft-cl-spring-
generalized-srv6-for-cmpr-02 (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-28 (work in
progress), December 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.
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Author's Address
Peng Shaofu
ZTE Corporation
Email: peng.shaofu@zte.com.cn
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