Signaling Maximum SID
Depth (MSD) Using the Border Gateway Protocol - Link StateApstra, Inc.jefftant.ietf@gmail.comFuturewei Technologiesumac.ietf@gmail.comCisco Systemsketant@cisco.comZTE Corp.gregimirsky@gmail.comAmazon Web Servicesnikost@amazon.com
Routing
IDR Working GroupBGP-LSSIDMSDSRThis document defines a way for a Border Gateway Protocol - Link State
(BGP-LS) speaker to advertise multiple types of supported Maximum SID
Depths (MSDs) at node and/or link granularity.Such advertisements allow entities (e.g., centralized controllers) to
determine whether a particular Segment Identifier (SID) stack can be
supported in a given network.IntroductionWhen Segment Routing (SR) paths are computed
by a centralized controller, it is critical that the controller learns
the Maximum SID Depth (MSD) that can be imposed at each node/link on a
given SR path. This ensures that the Segment Identifier (SID) stack
depth of a computed path doesn't exceed the number of SIDs the node is
capable of imposing. defines how to signal
MSD in the Path Computation Element Protocol (PCEP). The OSPF and IS-IS
extensions for the signaling of MSD are defined in
and , respectively.However, if PCEP is not supported/configured on the head-end of an SR
tunnel or a Binding-SID anchor node, and the controller does not participate
in IGP routing, it has no way of learning the MSD of nodes and links.
BGP-LS defines a way to expose topology and
associated attributes and capabilities of the nodes in that topology to
a centralized controller. This document defines extensions to BGP-LS to advertise one or more
types of MSDs at node and/or link granularity. Other types of MSDs are
known to be useful. For example, and define Entropy Readable Label Depth (ERLD), which is
used by a head-end to insert an Entropy Label (EL) at a depth that can
be read by transit nodes.In the future, it is expected that new MSD-Types will be defined to
signal additional capabilities, e.g., ELs, SIDs that can be imposed
through recirculation, or SIDs associated with another data plane such
as IPv6. MSD advertisements may be useful even if SR itself is not
enabled. For example, in a non-SR MPLS network, MSD defines the maximum
label depth.Conventions Used in This DocumentTerminology
MSD:
Maximum SID Depth - the number of SIDs supported by a node or a link on a node
PCE:
Path Computation Element
PCEP:
Path Computation Element Protocol
SID:
Segment Identifier as defined in
SR:
Segment Routing
Label Imposition:
Imposition is the act of modifying and/or
adding labels to the outgoing label stack associated with a packet.
This includes:
replacing the label at the top of the label stack with a new
label
pushing one or more new labels onto the label stack
The number of labels imposed is then the sum of the number of labels
that are replaced and the number of labels that are pushed. See
for further details.
Requirements LanguageThe 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.Advertisement of MSD via BGP-LSThis document describes extensions that enable BGP-LS speakers to
signal the MSD capabilities of
nodes and their links in a network to a BGP-LS consumer of network
topology such as a centralized controller. The centralized controller
can leverage this information in computation of SR paths based on their
MSD capabilities. When a BGP-LS speaker is originating the topology
learnt via link-state routing protocols such as OSPF or IS-IS, the MSD
information for the nodes and their links is sourced from the underlying
extensions as defined in and
, respectively. The extensions introduced in this document allow for advertisement of
different MSD-Types, which are defined elsewhere and were introduced in .
This enables sharing of MSD-Types that may be defined in the future by the IGPs in BGP-LS. Node MSD TLVThe Node MSD () is encoded in a new Node Attribute TLV
to carry the provisioned SID depth of the router identified by the
corresponding Router-ID. Node MSD is the smallest MSD supported by the node
on the set of interfaces configured for use. MSD values may be learned via
a hardware API or may be provisioned. The following format is used:Where:
Type:
266
Length:
variable (multiple of 2); represents the total
length of
the value field in octets.
Value:
consists of one or more pairs of a 1-octet
MSD-Type and
1-octet MSD-Value.
MSD-Type:
one of the values defined in the "IGP
MSD-Types" registry defined in .
MSD-Value:
a number in the range of 0-255. For all
MSD-Types, 0 represents the lack of ability to impose an MSD stack
of any depth; any other value represents that of the node. This
value MUST represent the lowest value supported by
any link configured for use by the advertising protocol
instance.
Link MSD TLVThe Link MSD () is defined to
carry the MSD of the interface associated with the link.
It is encoded in a new Link Attribute TLV using the following format:Where:
Type:
267
Length:
variable (multiple of 2); represents the total
length of
the value field in octets.
Value:
consists of one or more pairs of a 1-octet
MSD-Type and
1-octet MSD-Value.
MSD-Type:
one of the values defined in
the "IGP MSD-Types" registry defined in .
MSD-Value:
a number in the range of 0-255. For all
MSD-Types, 0 represents the lack of ability to impose an MSD stack
of any depth; any other value represents that of the link when
used as an outgoing interface.
IANA ConsiderationsIANA has assigned code points from the registry
"BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
Attribute TLVs" based on the table below.
BGP-LS MSD TLV Code Points
TLV Code Point
Description
IS-IS TLV/Sub-TLV
Reference
266
Node MSD
242/23
This document
267
Link MSD
(22,23,25,141,222,223)/15
This document
Manageability ConsiderationsThe new protocol extensions introduced in this document augment the
existing IGP topology information that is distributed via . Procedures and protocol extensions
defined in this document do not affect the BGP protocol operations and
management other than as discussed in Section Manageability
Considerations of . Specifically, the malformed attribute tests for
syntactic checks in Section Fault Management of now encompass the new BGP-LS
Attribute TLVs defined in this document. The semantic or content
checking for the TLVs specified in this document and their association
with the BGP-LS Network Layer Reachability Information (NLRI) types or their BGP-LS Attribute is left to the
consumer of the BGP-LS information (e.g., an application or a controller)
and not the BGP protocol.A consumer of the BGP-LS information retrieves this information over
a BGP-LS session (refer to Sections and of ).This document only introduces new Attribute TLVs, and any syntactic
error in them would result in the BGP-LS Attribute being discarded .
The MSD information introduced in BGP-LS by this
specification, may be used by BGP-LS consumer applications like an SR PCE
to learn the SR SID stack handling
capabilities of the nodes in the topology. This can enable the SR PCE to
perform path computations taking into consideration the size of SID
stack that the specific head-end node may be able to impose. Errors in
the encoding or decoding of the MSD information may result in the
unavailability of such information to the SR PCE, or incorrect
information being made available to it. This may result in the head-end
node not being able to instantiate the desired SR path in its forwarding
and provide the SR-based optimization functionality. The handling of
such errors by applications like SR PCE may be implementation specific
and out of scope of this document.
The extensions specified in this document do not specify
any new configuration or monitoring aspects in BGP or BGP-LS.
The specification of BGP models is an
ongoing work based on the .Security ConsiderationsThe advertisement of an incorrect MSD value may have negative
consequences. If the value is smaller than supported, path computation
may fail to compute a viable path. If the value is larger than
supported, an attempt to instantiate a path that can't be supported by
the head-end (the node performing the SID imposition) may occur. The
presence of this information may also inform an attacker of how to
induce any of the aforementioned conditions.The procedures and protocol extensions defined in this document do not
affect the BGP security model. See the "Security Considerations" Section of
for a discussion of BGP security.
Also, refer to and for analyses of security issues for BGP.
Security considerations for acquiring and distributing BGP-LS information are discussed in .
The TLVs introduced in this document are used to propagate the MSD IGP
extensions defined in and .
It is assumed that the IGP
instances originating these TLVs will support all the required security (as
described in and ) in order to prevent any security
issues when propagating the TLVs into BGP-LS.
The advertisement of the node and link attribute information defined in this
document presents no significant additional risk beyond that associated with the
existing node and link attribute information already supported in .
ReferencesNormative ReferencesInformative ReferencesAcknowledgementsWe would like to thank , , ,
and for their reviews and valuable
comments.ContributorsCisco Systems Inc.Canadamsiva@cisco.com