| Internet-Draft | Truncated SID Inter Domain | July 2021 |
| Liu, et al. | Expires 26 January 2022 | [Page] |
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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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 BLOCK:NODE where BLOCK is the SRv6 SID block (IPv6 prefix allocated for SRv6 SIDs by the operator) and NODE is the identifier of the node instantiating the SID. Note that "SID Locator Block" and "BLOCK" are used interchangeably throughout the document.¶
Referring to the scheme described in [RFC6554], when all segments in SRH share the common prefix (i.e., SRv6 SID Locator Block), SRH only needs to store the difference between SIDs (i.e., NODE:FUNCT:ARGS). In this scheme, SRH Segments Left field must represent the number of explicitly listed intermediate nodes (but not 128-bits items) still to be visited before reaching the final destination as specified in [RFC8200]. However, [RFC6554] mainly focus on the case where all segments share the common prefix and their difference parts are the same length. To meet more complex scenarios, [I-D.mirsky-6man-unified-id-sr] introduces a stiching mode to permit SIDs with different common prefix and different length of difference parts. In detailed, [I-D.mirsky-6man-unified-id-sr] introduces several new SID flavors to current SID, to indicate the type/length of the next SID in the SRH.¶
An SRv6 path could traverse domains that use different block prefixes and achieving different compressed SIDs lengths. Although it is possible to encapsulate the complete 128-bits SID of the boundary node where the block swapping occurs in the SRH, the compression efficiency is low. For higher compression efficiency in such cases, this document continues to introduce a new SID flavor to indicate the block swapping information of the next SID in the SRH. The required control plane extensions are also defined.¶
The flavor defined in this document can be used in combination with any other flavors, and is applicable to any SID behaviors such as END, END.X, and SFC related behaviors, etc.¶
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.¶
The interworking of different domains is illustrated in Figure 1.¶
................................. .......................
: : : :
+----+ +----+ +----+ +----+ +----+ +----+
+ S +-----+ A +-----+ B +-----+ABR +-----+ C +-----+ D +
+----+ +----+ +----+ +----+ +----+ +----+
: : : :
.......... domain 1.............. .......domain2.........
This section describes the process of new flavor based on the SRv6 compression scheme defined in [I-D.mirsky-6man-unified-id-sr]. However, this new flavor is an extension of SRv6 programming, and is independent with any specific SRv6 compression schemes.¶
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>.¶
Typically, this segment list is selected/computed by a Path Computation Element (PCE) system or a controller based on the structure and flavor of each SID.¶
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 its length is maintained in the local SID entry of the current SID.¶
So in ABR, a 128bits-BLOCK1-ABR-32 SID with BS flavor is allocated. 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 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 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.¶
For the inter-AS scenario, the processing is similar, especially an 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.¶
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 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: TBA¶
Length: 2 octet field, indicate the length of Swapped Block.¶
Swapped Block: This field encodes the advertised swapped block prefix information.¶
Different nodes in the same IGP domain may have different block prefixes. So IGP extensions are also needed in this scenario.¶
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].¶
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 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.¶
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 //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.¶
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.¶
+-----------+---------------------+--------------+------------------+ | TLV Code | Description | IS-IS TLV/ | Reference | | Point | | Sub-TLV | | +-----------+---------------------+--------------+------------------+ | TBA1 | SRv6 SID Swapped | --- | This document | | | Block | | | +-----------+---------------------+--------------+------------------+¶
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. Referring to section "12.5. Sub-Sub-TLVs for SID Sub-TLVs" of [I-D.ietf-lsr-isis-srv6-extensions], the registration procedure of "Sub-Sub-TLVs for SID Sub-TLVs" is still "Expert Review" as defined in [RFC8126].¶
+----------+----------------------------------------+---------------+ | TLV Code | Description | Reference | | Point | | | +----------+----------------------------------------+---------------+ | TBA2 | SRv6 SID Swapped Block | this document | +----------+----------------------------------------+---------------+¶
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.¶
+----------+----------------------------------------+---------------+ | TLV Code | Description | Reference | | Point | | | +----------+----------------------------------------+---------------+ | TBA3 | SRv6 SID Swapped Block | this document | +----------+----------------------------------------+---------------+¶