| IS-IS for IP Internets | S. Previdi, Ed. |
| Internet-Draft | L. Ginsberg, Ed. |
| Intended status: Standards Track | C. Filsfils |
| Expires: June 18, 2018 | A. Bashandy |
| Cisco Systems, Inc. | |
| H. Gredler | |
| RtBrick Inc. | |
| S. Litkowski | |
| B. Decraene | |
| Orange | |
| J. Tantsura | |
| Individual | |
| December 15, 2017 |
IS-IS Extensions for Segment Routing
draft-ietf-isis-segment-routing-extensions-14
Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF).
This draft describes the necessary IS-IS extensions that need to be introduced for Segment Routing operating on an MPLS data-plane.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 18, 2018.
Copyright (c) 2017 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 (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Segment Routing (SR) allows for a flexible definition of end-to-end paths within IGP topologies by encoding paths as sequences of topological sub-paths, called "segments". These segments are advertised by the link-state routing protocols (IS-IS and OSPF). Two types of segments are defined, Prefix segments and Adjacency segments. Prefix segments represent an ecmp-aware shortest-path to a prefix, as per the state of the IGP topology. Adjacency segments represent a hop over a specific adjacency between two nodes in the IGP. A prefix segment is typically a multi-hop path while an adjacency segment, in most of the cases, is a one-hop path. SR's control-plane can be applied to both IPv6 and MPLS data-planes, and do not require any additional signaling (other than the regular IGP). For example, when used in MPLS networks, SR paths do not require any LDP or RSVP-TE signaling. Still, SR can interoperate in the presence of LSPs established with RSVP or LDP.
This draft describes the necessary IS-IS extensions that need to be introduced for Segment Routing operating on an MPLS data-plane.
Segment Routing architecture is described in [I-D.ietf-spring-segment-routing].
Segment Routing use cases are described in [RFC7855].
Segment Routing architecture ([I-D.ietf-spring-segment-routing]) defines different types of Segment Identifiers (SID). This document defines the IS-IS encodings for the IGP-Prefix-SID, the IGP-Adjacency-SID, the IGP-LAN-Adjacency-SID and the Binding-SID.
A new IS-IS sub-TLV is defined: the Prefix Segment Identifier sub-TLV (Prefix-SID sub-TLV).
The Prefix-SID sub-TLV carries the Segment Routing IGP-Prefix-SID as defined in [I-D.ietf-spring-segment-routing]. The 'Prefix SID' MUST be unique within a given IGP domain (when the L-flag is not set). The 'Prefix SID' MUST carry an index (when the V-flag is not set) that determines the actual SID/label value inside the set of all advertised SID/label ranges of a given router. A receiving router uses the index to determine the actual SID/label value in order to construct forwarding state to a particular destination router.
In many use-cases a 'stable transport' IP Address is overloaded as an identifier of a given node. Because the IP Prefixes may be re-advertised into other levels there may be some ambiguity (e.g. Originating router vs. L1L2 router) for which node a particular IP prefix serves as identifier. The Prefix-SID sub-TLV contains the necessary flags to disambiguate IP Prefix to node mappings. Furthermore if a given node has several 'stable transport' IP addresses there are flags to differentiate those among other IP Prefixes advertised from a given node.
A Prefix-SID sub-TLV is associated to a prefix advertised by a node and MAY be present in any of the following TLVs:
When the IP Reachability TLV is propagated across level boundaries, the Prefix-SID sub-TLV SHOULD be kept.
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 | Flags | Algorithm | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Index/Label (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |R|N|P|E|V|L| | +-+-+-+-+-+-+-+-+
The Prefix-SID sub-TLV has the following format:
The R-Flag MUST be set for prefixes that are not local to the router and either:
In the case where a Level-1-2 router has local interface addresses configured in one level, it may also propagate these addresses into the other level. In such case, the Level-1-2 router MUST NOT set the R bit. The R-bit MUST be set only for prefixes that are not local to the router and advertised by the router because of propagation and/or leaking.
The N-Flag is used in order to define a Node-SID. A router MAY set the N-Flag only if all of the following conditions are met:
The router MUST ignore the N-Flag on a received Prefix-SID if the prefix has a Prefix length different than /32 (IPv4) or /128 (IPv6).
[RFC7794] also defines the N and R flags and with the same semantics of the equivalent flags defined in this document. There will be a transition period where both sets of flags will be used and eventually only the flags of the Prefix Attributes will remain. During the transition period implementations supporting the N and R flags defined in this document and the N and R flags defined in [RFC7794] MUST advertise and parse all flags. In case the received flags have different values, the value of the flags defined in [RFC7794] prevails.
When calculating the outgoing label for the prefix, the router MUST take into account E and P flags advertised by the next-hop router, if next-hop router advertised the SID for the prefix. This MUST be done regardless of next-hop router contributing to the best path to the prefix or not.
When propagating (either from Level-1 to Level-2 or vice versa) a reachability advertisement originated by another IS-IS speaker, the router MUST set the P-flag and MUST clear the E-flag of the related Prefix-SIDs.
The following behavior is associated with the settings of the E and P flags:
The Prefix-SID sub-TLV MUST be preserved when the IP Reachability TLV gets propagated across level boundaries.
The level-1-2 router that propagates the Prefix-SID sub-TLV between levels MUST set the R-flag.
If the Prefix-SID contains a global index (L and V flags unset) and it is propagated as such (with L and V flags unset), the value of the index MUST be preserved when propagated between levels.
The level-1-2 router that propagates the Prefix-SID sub-TLV between levels MAY change the setting of the L and V flags in case a local label value is encoded in the Prefix-SID instead of the received value.
A new IS-IS sub-TLV is defined: the Adjacency Segment Identifier sub-TLV (Adj-SID sub-TLV).
The Adj-SID sub-TLV is an optional sub-TLV carrying the Segment Routing IGP-Adjacency-SID as defined in [I-D.ietf-spring-segment-routing] with flags and fields that may be used, in future extensions of Segment Routing, for carrying other types of SIDs.
IS-IS adjacencies are advertised using one of the IS-Neighbor TLVs below:
Multiple Adj-SID sub-TLVs MAY be associated with a single IS-neighbor.
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 | Flags | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|B|V|L|S|P| |
+-+-+-+-+-+-+-+-+
The following format is defined for the Adj-SID sub-TLV:
In LAN subnetworks, the Designated Intermediate System (DIS) is elected and originates the Pseudonode-LSP (PN-LSP) including all neighbors of the DIS.
When Segment Routing is used, each router in the LAN MAY advertise the Adj-SID of each of its neighbors. Since, on LANs, each router only advertises one adjacency to the DIS (and doesn't advertise any other adjacency), each router advertises the set of Adj-SIDs (for each of its neighbors) inside a newly defined sub-TLV part of the TLV advertising the adjacency to the DIS (e.g.: TLV-22).
The following new sub-TLV is defined: LAN-Adj-SID (Type: TBD, suggested value 32) containing the set of Adj-SIDs the router assigned to each of its LAN neighbors.
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 | Flags | Weight | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | System-ID (6 octets) | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label/Index (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|F|B|V|L|S|P| |
+-+-+-+-+-+-+-+-+
The format of the LAN-Adj-SID sub-TLV is as follows:
Multiple LAN-Adj-SID sub-TLVs MAY be encoded. Note that this sub-TLV MUST NOT appear in TLV 141.
When the P-flag is not set, the LAN-Adj-SID MAY be persistent. When the P-flag is set, the LAN-Adj-SID MUST be persistent.
In case one TLV-22/23/222/223 (reporting the adjacency to the DIS) can't contain the whole set of LAN-Adj-SID sub-TLVs, multiple advertisements of the adjacency to the DIS MUST be used and all advertisements MUST have the same metric.
Each router within the level, by receiving the DIS PN LSP as well as the non-PN LSP of each router in the LAN, is capable of reconstructing the LAN topology as well as the set of Adj-SID each router uses for each of its neighbors.
A label is encoded in 3 octets (in the 20 rightmost bits).
An index is encoded in 4 octets.
The SID/Label sub-TLV may be present in the following TLVs/sub-TLVs defined in this document:
SR Capability (Section 3.1)
SRLB (Section 3.3)
Binding TLV (Section 2.4)
MT Binding TLV (Section 2.5)
Note that when used in the SR Capability and SRLB sub-TLVs the code point used is the same as the sub-TLV code point assigned by IANA in the sub-TLVs for TLV 149 and 150 registry.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SID/Label (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ where:
The SID/Label sub-TLV contains a SID or a MPLS Label. The SID/Label sub-TLV has the following format:
The SID/Label Binding TLV MAY be originated by any router in an IS-IS domain.
The SID/Label Binding TLV is used to advertise prefixes to SID/Label mappings. This functionality is called the Segment Routing Mapping Server (SRMS). The behavior of the SRMS is defined in [I-D.ietf-spring-segment-routing-ldp-interop].
The SID/Label Binding TLV has Type TBD (suggested value 149), and 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 | Flags | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Range | Prefix Length | Prefix | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Prefix (continued, variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SubTLVs (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: SID/Label Binding TLV format
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |F|M|S|D|A| | +-+-+-+-+-+-+-+-+
Flags: 1 octet field of following flags:
The 'Range' field provides the ability to specify a range of addresses and their associated Prefix SIDs. This advertisement supports the SRMS functionality. It is essentially a compression scheme to distribute a continuous Prefix and their continuous, corresponding SID/Label Block. If a single SID is advertised then the range field MUST be set to one. For range advertisements > 1, the number of addresses that need to be mapped into a Prefix-SID and the starting value of the Prefix-SID range.
Router-A: 192.0.2.1/32, Prefix-SID: Index 1
Router-B: 192.0.2.2/32, Prefix-SID: Index 2
Router-C: 192.0.2.3/32, Prefix-SID: Index 3
Router-D: 192.0.2.4/32, Prefix-SID: Index 4
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 |0|0| | RESERVED | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Range = 4 | /32 | 192 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | .0 | .2 | .1 |Prefix-SID Type| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-TLV Length| Flags | Algorithm | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example 1: if the following router addresses (loopback addresses) need to be mapped into the corresponding Prefix SID indexes.
10.1.1/24, Prefix-SID: Index 51 10.1.2/24, Prefix-SID: Index 52 10.1.3/24, Prefix-SID: Index 53 10.1.4/24, Prefix-SID: Index 54 10.1.5/24, Prefix-SID: Index 55 10.1.6/24, Prefix-SID: Index 56 10.1.7/24, Prefix-SID: Index 57
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 |0|0| | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Range = 7 | /24 | 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| .1 | .1 |Prefix-SID Type| sub-TLV Length|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Algorithm | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 51 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example-2: If the following prefixes need to be mapped into the corresponding Prefix-SID indexes:
It is not expected that a network operator will be able to keep fully continuous Prefix / SID/Index mappings. In order to support noncontinuous mapping ranges an implementation MAY generate several instances of Binding TLVs.
For example if a router wants to advertise the following ranges:
A router would need to advertise three instances of the Binding TLV.
The 'Prefix' represents the Forwarding equivalence class at the tail-end of the advertised path. The 'Prefix' does not need to correspond to a routable prefix of the originating node.
The 'Prefix Length' field contains the length of the prefix in bits. Only the most significant octets of the Prefix are encoded. (i.e., 1 octet for prefix length 1 up to 8, 2 octets for prefix length 9 to 16, 3 octets for prefix length 17 up to 24 and 4 octets for prefix length 25 up to 32, ...., 16 octets for prefix length 113 up to 128).
The Prefix-SID sub-TLV (suggested value 3) is defined in Section 2.1 and contains the SID/index/label value associated with the prefix and range. The Prefix-SID SubTLV MUST be present in the SID/Label Binding TLV.
A node receiving a MS entry for a prefix MUST check the existence of such prefix in its link-state database prior to consider and use the associated SID.
The Prefix-SID flags are defined in Section 2.1. The Mapping Server MAY advertise a mapping with the N flag set when the prefix being mapped is known in the link-state topology with a mask length of 32 (IPv4) or 128 (IPv6) and when the prefix represents a node. The mechanisms through which the operator defines that a prefix represents a node are outside the scope of this document (typically it will be through configuration).
The other flags defined in Section 2.1 are not used by the Mapping Server and MUST be ignored at reception.
As the mapping server does not specify the originator of a prefix advertisement it is not possible to determine PHP behavior solely based on the Mapping Server Advertisement. However, if additional information is available PHP behavior may safely be done. The required information consists of:
In the absence of an Extended Reachability Attribute Flags sub-TLV ([RFC7794]) the A flag in the binding TLV indicates that the originator of a prefix reachability advertisement is directly connected to the prefix and thus PHP MUST be done by the neighbors of the router originating the prefix reachability advertisement. Note that A-flag is only valid in the original area in which the Binding TLV is advertised.
The algorithm field contains the identifier of the algorithm the router MUST use in order to compute reachability to the range of prefixes. Use of the algorithm field is described in Section 2.1.
The SID/Label sub-TLV (Type: TBD, suggested value 1) contains the SID/Label value as defined in Section 2.3. It MUST be present in the SID/Label Binding TLV when the M-flag is set in the Flags field of the parent TLV.
The Multi-Topology SID/Label Binding TLV allows the support of M-ISIS as defined in [RFC5120]. The Multi-Topology SID/Label Binding TLV has the same format as the SID/Label Binding TLV defined in Section 2.4 with the difference consisting of a Multitopology Identifier (MTID) as defined here below:
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 | MTID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Flags | RESERVED | Range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix Length | Prefix (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SubTLVs (variable) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Multi-Topology SID/Label Binding TLV format
0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RESVD | MTID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Segment Routing requires each router to advertise its SR data-plane capability and the range of MPLS label values it uses for Segment Routing in the case where global SIDs are allocated (i.e., global indexes). Data-plane capabilities and label ranges are advertised using the newly defined SR-Capabilities sub-TLV inserted into the IS-IS Router Capability TLV-242 that is defined in [RFC7981].
The Router Capability TLV specifies flags that control its advertisement. The SR Capabilities sub-TLV MUST be propagated throughout the level and MUST NOT be advertised across level boundaries. Therefore Router Capability TLV distribution flags are set accordingly, i.e., the S flag in the Router Capability TLV ([RFC7981]) MUST be unset.
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 | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // SID/Label Sub-TLV (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|I|V| |
+-+-+-+-+-+-+-+-+
The SR Capabilities sub-TLV has following format:
SID/Label sub-TLV contains the first value of the SRGB while the range contains the number of SRGB elements. The range value MUST be higher than 0.
The SR-Capabilities sub-TLV MAY be advertised in an LSP of any number but a router MUST NOT advertise more than one SR-Capabilities sub-TLV. A router receiving multiple SR-Capabilities sub-TLVs, from the same originator, SHOULD select the first advertisement in the lowest numbered LSP.
When multiple SRGB Descriptors are advertised the entries define an ordered set of ranges on which a SID index is to be applied. For this reason changing the order in which the descriptors are advertised will have a disruptive effect on forwarding.
When a router adds a new SRGB Descriptor to an existing SR-Capabilities sub-TLV the new Descriptor SHOULD add the newly configured block at the end of the sub-TLV and SHOULD NOT change the order of previously advertised blocks. Changing the order of the advertised descriptors will create label churn in the FIB and blackhole / misdirect some traffic during the IGP convergence. In particular, if a range which is not the last is extended it's preferable to add a new range rather than extending the previously advertised range.
The originating router MUST ensure the order is same after a graceful restart (using checkpointing, non-volatile storage or any other mechanism) in order to guarantee the same order before and after GR.
The originating router MUST NOT advertise overlapping ranges.
When a router receives multiple overlapping ranges, it MUST conform to the procedures defined in [I-D.ietf-spring-conflict-resolution].
The originating router advertises following ranges:
SR-Cap: range: 100, SID value: 100
SR-Cap: range: 100, SID value: 1000
SR-Cap: range: 100, SID value: 500
The receiving routers concatenate the ranges in the received
order and build the SRGB as follows:
SRGB = [100, 199]
[1000, 1099]
[500, 599]
The indexes span multiple ranges:
index=0 means label 100
...
index 99 means label 199
index 100 means label 1000
index 199 means label 1099
...
index 200 means label 500
...
Here follows an example of advertisement of multiple ranges:
The router may use various algorithms when calculating reachability to other nodes or to prefixes attached to these nodes. Examples of these algorithms are metric based Shortest Path First (SPF), various sorts of Constrained SPF, etc. The SR-Algorithm sub-TLV (Type: TBD, suggested value 19) allows the router to advertise the algorithms that the router is currently using. The following values have been defined:[RFC7981].
The SR-Algorithm sub-TLV is inserted into the IS-IS Router Capability TLV-242 that is defined in
The Router Capability TLV specifies flags that control its advertisement. The SR-Algorithm MUST be propagated throughout the level and MUST NOT be advertised across level boundaries. Therefore Router Capability TLV distribution flags are set accordingly, i.e., the S flag MUST be unset.
The SR-Algorithm sub-TLV is optional, it MAY only appear a single time inside the Router Capability TLV.
When the originating router does not advertise the SR-Algorithm sub-TLV, then all the Prefix-SIDs advertised by the router MUST have algorithm field set to 0. Any receiving router MUST assume SPF algorithm (i.e., Shortest Path First).
When the originating router does advertise the SR-Algorithm sub-TLV, then algorithm 0 MUST be present while algorithm 1 MAY be present.
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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Algorithm 1 | Algorithm 2 | Algorithm ... | Algorithm n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SR-Algorithm sub-TLV has following format:
The SR Local Block (SRLB) Sub-TLV contains the range of labels the node has reserved for local SIDs. Local SIDs are used, e.g., for Adjacency-SIDs, and may also be allocated by other components than IS-IS protocol. As an example, an application or a controller may instruct the router to allocate a specific local SID. Therefore, in order for such applications or controllers to know what are the local SIDs available in the router, it is required that the router advertises its SRLB.
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 | Flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Range | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // SID/Label Sub-TLV (variable) // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SRLB Sub-TLV is used for that purpose and has following format:
SID/Label sub-TLV contains the first value of the SRLB while the range contains the number of SRLB elements. The range value MUST be higher than 0.
The SRLB sub-TLV MAY be advertised in an LSP of any number but a router MUST NOT advertise more than one SRLB sub-TLV. A router receiving multiple SRLB sub-TLVs, from the same originator, SHOULD select the first advertisement in the lowest numbered LSP.
The originating router MUST NOT advertise overlapping ranges.
It is important to note that each time a SID from the SRLB is allocated, it SHOULD also be reported to all components (e.g.: controller or applications) in order for these components to have an up-to-date view of the current SRLB allocation and in order to avoid collision between allocation instructions.
Within the context of IS-IS, the reporting of local SIDs is done through IS-IS Sub-TLVs such as the Adjacency-SID. However, the reporting of allocated local SIDs may also be done through other means and protocols which mechanisms are outside the scope of this document.
A router advertising the SRLB TLV may also have other label ranges, outside the SRLB, for its local allocation purposes which are NOT advertised in the SRLB. For example, it is possible that an Adjacency-SID is allocated using a local label not part of the SRLB.
The Segment Routing Mapping Server (SRMS) Preference sub-TLV is used in order to associate a preference with SRMS advertisements from a particular source.
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 | Preference | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The SRMS Preference sub-TLV has following format:
The SRMS Preference sub-TLV MAY be advertised in an LSP of any number but a router MUST NOT advertise more than one SRMS Preference sub-TLV. A router receiving multiple SRMS Preference sub-TLVs, from the same originator, SHOULD select the first advertisement in the lowest numbered LSP.
The use of the SRMS Preference during the SID selection process is described in [I-D.ietf-spring-conflict-resolution].
This section describes the changes that have been applied to this document that are not backward compatible with previous versions.
Version -04 of this document introduced a change in Section 3.1 regarding the encoding method for multiple SRGBs in the SR-Cap SubTLV and made the support of multiple SRGBs REQUIRED.
The modified method consists of having a single SR-Cap Sub-TLV where all SRGBs are encoded. In previous versions (prior to version -04) of this document it was allowed to have multiple occurrences of the SR-Cap Sub-TLV.
At the time of writing this document, no existing implementations are affected by the change since no implementations actually (i.e., at the time of updating this document) encode multiple SRGBs anyway.
This documents request allocation for the following TLVs and subTLVs.
This document makes the following registrations in the "sub-TLVs for TLV 22, 23, 25, 141, 222 and 223" registry.
This document makes the following registrations in the "sub-TLVs for TLV 135,235,236 and 237" registry.
This document makes the following registrations in the "sub-TLVs for TLV 242" registry.
This document registers the following TLV:
This document creates the following sub-TLV Registry:
With the use of the extensions defined in this document, IS-IS carries information which will be used to program the MPLS data plane [RFC3031]. In general, the same types of attacks that can be carried out on the IP/IPv6 control plane can be carried out on the MPLS control plane resulting in traffic being misrouted in the respective data planes. However, the latter may be more difficult to detect and isolate. Existing security extensions as described in [RFC5304] and [RFC5310] apply to these segment routing extensions.
We would like to thank Dave Ward, Dan Frost, Stewart Bryant, Pierre Francois and Jesper Skrivers for their contribution to the content of this document.
The following people gave a substantial contribution to the content of this document and should be considered as co-authors:
Peter Psenak Cisco Systems Inc. US Email: ppsenak@cisco.com Martin Horneffer Deutsche Telekom DE Email: Martin.Horneffer@telekom.de Wim Henderickx Nokia BE Email: wim.henderickx@nokia.com Edward Crabbe Individual US Email: edward.crabbe@gmail.com Rob Shakir Google UK Email: robjs@google.com Igor Milojevic Individual RS Email: milojevicigor@gmail.com Saku Ytti TDC FI Email: saku@ytti.fi Steven Luong Cisco Systems Inc. US Email: sluong@cisco.com