Signaling MSD (Maximum SID
Depth) using Border Gateway Protocol Link-StateApstra, Inc.jefftant.ietf@gmail.comFuturewei Technologiesumac.ietf@gmail.comCisco Systemsketant@cisco.comZTE Corp.gregimirsky@gmail.comApstra, Inc.nikos@apstra.com
Routing
IDR Working GroupInternet-DraftBGP-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.When 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 signaling of MSD are defined in
and respectively.However, if PCEP is not supported/configured on the head-end of a SR
tunnel or a Binding-SID anchor node, and 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 advertise 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 MSD are known to be useful. For example, and define Readable Label Depth Capability
(RLDC) that 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.BGP-LS: Distribution of Link-State and TE Information using
Border Gateway ProtocolMSD: Maximum SID DepthPCC: Path Computation ClientPCE: Path Computation ElementPCEP: Path Computation Element ProtocolSID: Segment IdentifierSR: Segment routingLabel 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.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 .This document describes extensions that enable BGP-LS speakers to
signal the MSD capabilities (described in )
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 and their instantiation on network nodes based on their MSD
capabilities. When a BGP-LS speaker is originating the topology learnt
via link-state routing protocols like 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 BGP-LS speaker may also advertise the MSD information
for the local node and its links when not running any link-state IGP
protocol e.g. when running BGP as the only routing protocol.The extensions introduced in this document allow for advertisement of
different MSD-Types. This document does not define these MSD-Types but
leverages the definition, guidelines and the code-point registry
specified in . This enables sharing of MSD-Types
that may be defined in the future by the IGPs in BGP-LS.Node MSD is encoded in a new Node Attribute TLV using the following format:Where:Type: 266Length: 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 IANA registry
titled "IGP MSD-Types" under the "Interior Gateway Protocol
(IGP) Parameters" registry created by .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 is encoded in a new Link Attribute TLV using the following format:Where:Type: 267Length: 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 IANA registry
titled "IGP MSD-Types" under the "Interior Gateway Protocol
(IGP) Parameters" registry created by .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.This document requests assigning code-points from the registry
"BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and
Attribute TLVs" based on table below. Early allocation for these
code-points have been done by IANA.The 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 the Manageability Considerations section of . Specifically, the malformed attribute tests for
syntactic checks in the Fault Management section 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 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 Section 1 and 2 of ).
The handling of semantic or content errors by the consumer would be
dictated by the nature of its application usage and hence is beyond the
scope of this document.This document only introduces new Attribute TLVs and any syntactic
error in them would result in the BGP-LS Attribute being discarded with
an error log. The MSD information introduced in BGP-LS by this
specification, may be used by BGP-LS consumer applications like a SR
path computation engine (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 headend 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 headend
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 BGP and BGP-LS models is an
ongoing work based on the .
The management of the MSD features within an ietf segment-routing
stack is also an ongoing work based on the .
Management of the segment routing in IGPs is ongoing work for ISIS ,
and OSPF .The 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 document does not introduce additional security issues beyond discussed in ,
and .
However, is being revised in to provide additional clarification in several portions of the specification after
receiving feedback from implementers. One of the places that is being clarified is the error handling and security.
It is expected that after is released that implementers will update all BGP-LS base implementations
improving the error handling for protocol work (including this document) that depend on this function.
We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene
for their reviews and valuable comments.