IS-IS Extensions for
Segment RoutingHuaweiITstefano@previdi.netCisco Systems, Inc.USAginsberg@cisco.comCisco Systems, Inc.BrusselsBEcfilsfil@cisco.comArrcusabashandy.ietf@gmail.comRtBrick Inc.hannes@rtbrick.comOrangeFRbruno.decraene@orange.com
Routing
IS-IS for IP InternetsMPLSSIDIGPIS-ISLabel advertisementSegment RoutingSegment 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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP 14
when, and only when,
they appear in all capitals, as shown here.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). Prefix segments
represent an ECMP-aware shortest-path to a prefix (or a node), 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 does 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.There are additional segment types, e.g., Binding SID defined in
. This document also defines an advertisement
for one type of Binding SID: the Mirror Context segment.This draft describes the necessary IS-IS extensions that need to be
introduced for Segment Routing operating on an MPLS data-plane.The Segment Routing architecture is described in .Segment Routing use cases are described in .The Segment Routing architecture defines
different types of Segment Identifiers (SID). This document defines the
IS-IS encodings for the IGP-Prefix Segment, the IGP-Adjacency Segment,
the IGP-LAN-Adjacency Segment and the Binding Segment.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 . The 'Prefix SID' MUST be
unique within a given IGP domain (when the L-flag is not set).A Prefix-SID sub-TLV is associated to a prefix advertised by a node
and MAY be present in any of the following TLVs: TLV-135 (Extended IPv4 reachability) defined in .TLV-235 (Multitopology IPv4 Reachability) defined in .TLV-236 (IPv6 IP Reachability) defined in .TLV-237 (Multitopology IPv6 IP Reachability) defined in .Binding-TLV and Multi-Topology Binding-TLV defined in and
respectively.The Prefix-SID sub-TLV has the following format:Type: 3Length: 5 or 6 depending on the size of the SID (described
below)Flags: 1 octet field of following flags: where: R-Flag: Re-advertisement flag. If set, then the prefix to
which this Prefix-SID is attached, has been propagated by the
router either from another level (i.e., from level-1 to
level-2 or the opposite) or from redistribution (e.g.: from
another protocol).N-Flag: Node-SID flag. If set, then the Prefix-SID refers
to the router identified by the prefix. Typically, the N-Flag
is set on Prefix-SIDs attached to a router loopback address.
The N-Flag is set when the Prefix-SID is a Node-SID as
described in .P-Flag: no-PHP flag. If set, then the penultimate hop MUST
NOT pop the Prefix-SID before delivering the packet to the
node that advertised the Prefix-SID.E-Flag: Explicit-Null Flag. If set, any upstream neighbor
of the Prefix-SID originator MUST replace the Prefix-SID with
a Prefix-SID having an Explicit-NULL value (0 for IPv4 and 2
for IPv6) before forwarding the packet.V-Flag: Value flag. If set, then the Prefix-SID carries a
value (instead of an index). By default the flag is UNSET.L-Flag: Local Flag. If set, then the value/index carried by
the Prefix-SID has local significance. By default the flag is
UNSET.Other bits: MUST be zero when originated and ignored when
received.Algorithm: the router may use various algorithms when
calculating reachability to other nodes or to prefixes attached to
these nodes. Algorithm identifiers are defined in . Examples of these algorithms are metric
based Shortest Path First (SPF), various sorts of Constrained SPF,
etc. The algorithm field of the Prefix-SID contains the identifier
of the algorithm the router uses to compute the reachability of
the prefix to which the Prefix-SID is associated.At origination, the Prefix-SID algorithm field MUST be set to 0
or to any value advertised in the SR-Algorithm sub-TLV ().A router receiving a Prefix-SID from a remote node and with an
algorithm value that such remote node has not advertised in the
SR-Algorithm sub-TLV () MUST ignore
the Prefix-SID sub-TLV.SID/Index/Label as defined in .When the Prefix SID is an index (the V-flag is not set) the value
is used to determine the actual label value inside the set of all
advertised label ranges of a given router. This allows a receiving
router to construct forwarding state to a particular destination
router.In many use-cases a 'stable transport' address is overloaded as an
identifier of a given node. Because 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 Prefix to node mappings. Furthermore if a given node has
several 'stable transport' addresses there are flags to differentiate
those among other Prefixes advertised from a given node.The V-flag indicates whether the SID/Index/Label field is a
value or an index.The L-Flag indicates whether the value/index in the
SID/Index/Label field has local or global significance.The following settings for V and L flags are valid:V-flag is set to 0 and L-flag is set to 0: The SID/Index/Label
field is a 4 octet index defining the offset in the SID/Label
space advertised by this router using the encodings defined in
.V-flag is set to 1 and L-flag is set to 1: The SID/Index/Label
field is a 3 octet local label where the 20 rightmost bits are
used for encoding the label value.All other combinations of V-flag and L-flag are invalid and any
SID advertisement received with an invalid setting for V and L
flags MUST be ignored.The R-Flag MUST be set for prefixes that are not local to the
router and either:advertised because of propagation (Level-1 into
Level-2);advertised because of leaking (Level-2 into Level-1);advertised because of redistribution (e.g.: from another
protocol).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 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 prefix to which the Prefix-SID is attached is local to
the router (i.e., the prefix is configured on one of the local
interfaces, e.g., a 'stable transport' loopback).The prefix to which the Prefix-SID is attached has a Prefix
length of either /32 (IPv4) or /128 (IPv6).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).The Prefix Attributes Flags sub-TLV
also defines the N and R flags and with the same semantics of the
equivalent flags defined in this document. Whenever the Prefix
Attributes Flags sub-TLV is present for a given prefix the values
of the N and R flags advertised in that sub-TLV MUST be used and
the values in a corresponding Prefix SID sub-TLV (if present) MUST
be ignored.The following behavior is associated with the settings of the E
and P flags:If the P-flag is not set then any upstream neighbor of the
Prefix-SID originator MUST pop the Prefix-SID. This is
equivalent to the penultimate hop popping mechanism used in
the MPLS dataplane which improves performance of the ultimate
hop. MPLS EXP bits of the Prefix-SID are not preserved to the
ultimate hop (the Prefix-SID being removed). If the P-flag is
unset the received E-flag is ignored.If the P-flag is set then:If the E-flag is not set then any upstream neighbor of
the Prefix-SID originator MUST keep the Prefix-SID on top
of the stack. This is useful when, e.g., the originator of
the Prefix-SID must stitch the incoming packet into a
continuing MPLS LSP to the final destination. This could
occur at an inter-area border router (prefix propagation
from one area to another) or at an inter-domain border
router (prefix propagation from one domain to
another).If the E-flag is set then any upstream neighbor of the
Prefix-SID originator MUST replace the PrefixSID with a
Prefix-SID having an Explicit-NULL value. This is useful,
e.g., when the originator of the Prefix-SID is the final
destination for the related prefix and the originator
wishes to receive the packet with the original EXP
bits.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 Prefix-SID sub-TLV MUST be included when the associated
Prefix Reachability TLV is propagated across level boundaries.The level-1-2 router that propagates the Prefix-SID sub-TLV
between levels maintains the content (flags and SID) except as noted
in and .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 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:TLV-22 (Extended IS reachability)TLV-222 (Multitopology IS)TLV-23 (IS Neighbor Attribute)TLV-223 (Multitopology IS Neighbor Attribute)TLV-141 (inter-AS reachability information)Multiple Adj-SID sub-TLVs MAY be associated with a single
IS-neighbor.The following format is defined for the Adj-SID sub-TLV:Type: 31Length: 5 or 6 depending on size of the SIDFlags: 1 octet field of following flags: where: F-Flag: Address-Family flag. If unset, then the Adj-SID
is used when forwarding IPv4 encapsulated traffic to the
neighbor. If set then the Adj-SID is used when forwarding
IPv6 encapsulated traffic to the neighbor.B-Flag: Backup flag. If set, the Adj-SID is eligible for
protection (e.g.: using IPFRR or MPLS-FRR) as described in
.V-Flag: Value flag. If set, then the Adj-SID carries a
value. By default the flag is SET.L-Flag: Local Flag. If set, then the value/index carried
by the Adj-SID has local significance. By default the flag
is SET.S-Flag. Set flag. When set, the S-Flag indicates that the
Adj-SID refers to a set of adjacencies (and therefore MAY be
assigned to other adjacencies as well).P-Flag. Persistent flag. When set, the P-Flag indicates
that the Adj-SID is persistently allocated, i.e., the
Adj-SID value remains consistent across router restart
and/or interface flap.Other bits: MUST be zero when originated and ignored when
received.Weight: 1 octet. The value represents the weight of the
Adj-SID for the purpose of load balancing. The use of the weight
is defined in .SID/Index/Label as defined in .An SR capable router MAY allocate an Adj-SID for each of its
adjacenciesAn SR capable router MAY allocate more than one Adj-SID to an
adjacency.An SR capable router MAY allocate the same Adj-SID to
different adjacencies.When the P-flag is not set, the Adj-SID MAY be persistent.
When the P-flag is set, the Adj-SID MUST be persistent.Examples of use of the Adj-SID sub-TLV are described in .The F-flag is used in order for the router to advertise the
outgoing encapsulation of the adjacency the Adj-SID is attached
to.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 containing the
set of Adj-SIDs the router assigned to each of its LAN
neighbors.The format of the LAN-Adj-SID sub-TLV is as follows:Type: 32Length: variable.Flags: 1 octet field of following flags: where F, B, V, L, S and P flags are defined in . Other bits: MUST be zero when
originated and ignored when received.Weight: 1 octet. The value represents the weight of the
Adj-SID for the purpose of load balancing. The use of the weight
is defined in .Neighbor System-ID: IS-IS System-ID of length "ID Length" as
defined in .SID/Index/Label as defined in .Multiple LAN-Adj-SID sub-TLVs MAY be encoded.Note that this sub-TLV MUST NOT appear in TLV 141.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-SIDs each
router uses for each of its neighbors.The SID/Label sub-TLV may be present in the following TLVs/sub-TLVs
defined in this document:SR-Capabilities Sub-TLV ()SR Local Block Sub-TLV ()SID/Label Binding TLV ()Multi-Topology SID/Label Binding TLV ()Note that the code point used in all of the above cases is the
SID/Label Sub-TLV code point specified in the new “sub-TLVs for
TLV 149 and 150” registry created by this document.The SID/Label sub-TLV contains a SID or a MPLS Label. The SID/Label
sub-TLV has the following format: Type: 1Length: 3 or 4SID/Label: if length is set to 3 then the 20 rightmost bits
represent a MPLS label. If length is set to 4 then the value is a
32 bit indexThe SID/Label Binding TLV MAY be originated by any router in an
IS-IS domain. There are multiple uses of the SID/Label Binding
TLV.The SID/Label Binding TLV may be 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 .The SID/Label Binding TLV may also be used to advertise a Mirror
SID to advertise the ability to process traffic originally destined to
another IGP node. This behavior is defined in .The SID/Label Binding TLV has the following format:Type: 149Length: variable.1 octet of flags1 octet of RESERVED (SHOULD be transmitted as 0 and MUST be
ignored on receipt)2 octets of Range1 octet of Prefix Length0-16 octets of Prefixsub-TLVs, where each sub-TLV consists of a sequence of: 1 octet of sub-TLV type1 octet of length of the value field of the sub-TLV0-243 octets of valueFlags: 1 octet field of following flags: where: F-Flag: Address Family flag. If unset, then the Prefix
carries an IPv4 Prefix. If set then the Prefix carries an IPv6
Prefix.M-Flag: Mirror Context flag. Set if the advertised SID
corresponds to a mirrored context. The use of a mirrored context
is described in .S-Flag: If set, the SID/Label Binding TLV SHOULD be flooded
across the entire routing domain. If the S flag is not set, the
SID/Label Binding TLV MUST NOT be leaked between levels. This
bit MUST NOT be altered during the TLV leaking.D-Flag: when the SID/Label Binding TLV is leaked from level-2
to level-1, the D-Flag MUST be set. Otherwise, this flag MUST be
clear. SID/Label Binding TLVs with the D-Flag set MUST NOT be
leaked from level-1 to level-2. This is to prevent TLV looping
across levels.A-Flag: Attached flag. The originator of the SID/Label
Binding TLV MAY set the A bit in order to signal that the
prefixes and SIDs advertised in the SID/Label Binding TLV are
directly connected to their originators. The mechanisms through
which the originator of the SID/Label Binding TLV can figure out
if a prefix is attached or not are outside the scope of this
document (e.g.: through explicit configuration). If the Binding
TLV is leaked to other areas/levels the A-flag MUST be
cleared.An implementation may decide not to honor the S-flag in order
not to leak Binding TLV's between levels (for policy
reasons).Other bits: MUST be zero when originated and ignored when
received.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 range field MUST be set to the number of addresses that need to
be mapped into a Prefix-SID. In either case the prefix is the first
address to which a SID is to be assigned.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 is defined in and contains the SID/index/label value
associated with the prefix and range. The Prefix-SID Sub-TLV MUST be
present in the SID/Label Binding TLV when the M-flag is clear. The
Prefix-SID Sub-TLV MUST NOT be present when the M-flag is set.The Prefix-SID flags are defined in . 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 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:A prefix reachability advertisement for the prefix has been
received which includes the Prefix Attribute Flags sub-TLV
.X and R flags are both set to 0 in the Prefix Attribute
Flags sub-TLV.In the absence of an Prefix Attribute Flags sub-TLV 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
associated with the SIDs for the prefix(es) in the range. Use of
the algorithm field is described in .The SID/Label sub-TLV (Type: 1) contains the SID/Label value as
defined in . It MUST be present in
the SID/Label Binding TLV when the M-flag is set in the Flags field
of the parent TLV.Example 1: if the following IPv4 router addresses (loopback
addresses) need to be mapped into the corresponding Prefix SID
indexes. Example-2: If the following IPv4 prefixes need to be mapped into
the corresponding Prefix-SID indexes: Example-3: If the following IPv6 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:
Range 16: { 192.0.2.1-15, Index 1-15 }Range 6: { 192.0.2.22-27, Index 22-27 }Range 41: { 192.0.2.44-84, Index 80-120 } A router would need to advertise three instances of the
Binding TLV.The Multi-Topology SID/Label Binding TLV allows the support of
M-ISIS as defined in . The Multi-Topology
SID/Label Binding TLV has the same format as the SID/Label Binding TLV
defined in with the difference consisting
of a Multitopology Identifier (MTID) as defined here below:where: Type: 150Length: variableMTID is the multitopology identifier defined as: RESVD: reserved bits. MUST be reset on transmission and
ignored on receive.MTID: a 12-bit field containing the non-zero ID of the
topology being announced. The TLV MUST be ignored if the ID is
zero. This is to ensure the consistent view of the standard
unicast topology.The other fields and Sub-TLVs are defined in .This section defines sub-TLVs which are inserted into the IS-IS
Router Capability TLV-242 that is defined in .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.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 MUST be unset.The SR Capabilities sub-TLV has following format:Type: 2Length: variable.Flags: 1 octet of flags. The following are defined: where: I-Flag: MPLS IPv4 flag. If set, then the router is capable
of processing SR MPLS encapsulated IPv4 packets on all
interfaces.V-Flag: MPLS IPv6 flag. If set, then the router is capable
of processing SR MPLS encapsulated IPv6 packets on all
interfaces.One or more SRGB Descriptor entries, each of which have the
following format:Range: 3 octets.SID/Label sub-TLV (as defined in ).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 unchanged after a
graceful restart (using checkpointing, non-volatile storage or any
other mechanism).The originating router MUST NOT advertise overlapping ranges.When a router receives multiple overlapping ranges, it MUST conform
to the procedures defined in .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 allows the
router to advertise the algorithms that the router is currently using.
Algorithm values are defined in the "IGP Algorithm Type" registry
defined in .
The following values have been defined:0: Shortest Path First (SPF) algorithm based on link metric.
This is the well-known shortest path algorithm as computed by the
IS-IS Decision process. Consistent with the deployed practice for
link-state protocols, algorithm 0 permits any node to overwrite
the SPF path with a different path based on local policy.1: Strict Shortest Path First (SPF) algorithm based on link
metric. The algorithm is identical to algorithm 0 but algorithm 1
requires that all nodes along the path will honor the SPF routing
decision. Local policy MUST NOT alter the forwarding decision
computed by algorithm 1 at the node claiming to support algorithm
1.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 MUST NOT be advertsied
more than once at a given level. A router receiving multiple
SR-Algorithm sub-TLVs from the same originator SHOULD select the first
advertisement in the lowest numbered LSP.When the originating router does not advertise the SR-Algorithm
sub-TLV, this implies that the only algorithm supported by routers
supporting the extensions defined in this document is Algorithm 0.When the originating router does advertise the SR-Algorithm
sub-TLV, then algorithm 0 MUST be present while non-zero algorithms
MAY be present.The SR-Algorithm sub-TLV has the following format: where: Type: 19Length: variable.Algorithm: 1 octet of algorithmThe 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 components other than the
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.The SRLB Sub-TLV is used for this purpose and has following
format:Type: 22Length: variable.Flags: 1 octet of flags. None are defined at this stage.One or more SRLB Descriptor entries, each of which have the
following format:Range: 3 octets.SID/Label sub-TLV (as defined in ).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.When a router receives multiple overlapping ranges, it MUST conform
to the procedures defined in .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 are outside the scope of this document.A router advertising the SRLB sub-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.The SRMS Preference sub-TLV has following format:Type: 24Length: 1.Preference: 1 octet. Unsigned 8 bit SRMS preference.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 This document requests allocation for the following TLVs and
Sub-TLVs.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:Intermediate system to Intermediate system intra-domain
routeing information exchange protocol for use in conjunction with
the protocol for providing the connectionless-mode Network Service
(ISO 8473)International Organization for
Standardization