Segment Routing Prefix Segment Identifier Extensions for BGP

Segment Routing Prefix Segment Identifier Extensions for BGP Huawei Technologies

Italy


        
        stefano@previdi.net


    
      Cisco Systems
      
        
          
          Brussels
          Belgium
          
        
        
        cfilsfil@cisco.com
      

    
    
      Cisco Systems
      
        
          301 Midenhall Way
          Cary, NC
          United States of America
          27513
        
        
        acee@cisco.com
      
    
    
      
        arjunhrs@gmail.com
      
    
    
      RtBrick Inc.
      
        hannes@rtbrick.com
      
    
    
    Routing
    IDR
    SR
    MPLS
    BGP
    Prefix-SID
    Label-Index
    SRGB
    
      Segment Routing (SR) leverages the source-routing paradigm. A node
      steers a packet through an ordered list of instructions called
      "segments". A segment can represent any instruction, topological or
      service based. The ingress node prepends an SR header to a packet
      containing a set of segment identifiers (SIDs). Each SID represents a
      topological or service-based instruction. Per-flow state is maintained
      only on the ingress node of the SR domain. An "SR domain" is defined as a
      single administrative domain for global SID assignment.
      This document defines an optional, transitive BGP attribute for
      announcing information about BGP Prefix Segment Identifiers (BGP Prefix-SIDs)
      and the specification for SR-MPLS SIDs.
    
    
      
        Status of This Memo
        
            This is an Internet Standards Track document.
        
        
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.
        
        
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            .
        
      
      
        Copyright Notice
        
            Copyright (c) 2019 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
            () 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.
        
      
    
    
      
        Table of Contents
        
          
            .  Introduction
          
          
            .  MPLS BGP Prefix-SID
          
          
            .  BGP Prefix-SID Attribute
            
              
                .  Label-Index TLV
              
              
                .  Originator SRGB TLV
              
            
          
          
            .  Receiving BGP Prefix-SID Attribute
            
              
                .  MPLS Data Plane: Labeled Unicast
              
            
          
          
            .  Advertising BGP Prefix-SID Attribute
            
              
                .  MPLS Data Plane: Labeled Unicast
              
            
          
          
            .  Error Handling of BGP Prefix-SID Attribute
          
          
            .  IANA Considerations
          
          
            .  Manageability Considerations
          
          
            .  Security Considerations
          
          
            . References
            
              
                .  Normative References
              
              
                .  Informative References
              
            
          
          
            Acknowledgements
          
          
            Contributors
          
          
            Authors' Addresses


  
    
      Introduction
      The Segment Routing (SR) architecture leverages the source-routing
      paradigm. A segment represents either a topological instruction, such as
      "go to prefix P following shortest path", or a service instruction.
      Other types of segments may be defined in the future.
      A segment is identified through a Segment Identifier (SID). 
      An "SR domain" is defined as a single administrative domain for
      global SID assignment. It may be comprised of a single Autonomous System (AS)
      or multiple ASes under consolidated global SID administration. Typically, the ingress
      node of the SR domain prepends an SR header containing SIDs to an incoming packet.
      As described in ,
      when SR is applied to the MPLS data plane (), the SID consists of a
      label.
       also 
      describes how Segment Routing can be applied to an IPv6 data plane (SRv6) using
      an IPv6 routing header containing a stack of SR SIDs encoded as 
      IPv6 addresses .
      The applicability and support for Segment Routing over IPv6 is beyond the
      scope of this document.
      A BGP Prefix Segment is a BGP prefix with a Prefix-SID attached.
      A BGP Prefix-SID is always a global SID () within the SR domain
      and identifies an instruction to forward
      the packet over the Equal-Cost Multipath (ECMP) best path 
      computed by BGP to the related
      prefix. The BGP Prefix-SID is the identifier of the BGP Prefix Segment.
      In this document, we always refer to the BGP Prefix Segment by the BGP
      Prefix-SID.
      This document describes the BGP extensions to signal the BGP
      Prefix-SID. Specifically, this document defines a BGP attribute
      known as the "BGP Prefix-SID attribute" and specifies the rules to
      originate, receive, and handle error conditions for the attribute.
      The BGP Prefix-SID attribute defined in this document can be attached
       to prefixes from Multiprotocol BGP IPv4/IPv6 Labeled 
       Unicast ( ).
       Usage of the BGP Prefix-SID attribute for other 
       Address Family Identifier (AFI) / Subsequent Address 
       Family Identifier (SAFI) combinations
       is not defined herein but may be specified in
       future specifications.
       describes 
      example use cases where the BGP Prefix-SID is used for the above 
      AFI/SAFI combinations.
      It should be noted that:
      
        A BGP Prefix-SID will be global across ASes when the
          interconnected ASes are part of the same SR domain.
          Alternatively, when interconnecting ASes, the ASBRs of each
          domain will have to handle the advertisement of unique SIDs. The
          mechanisms for such interconnection are outside the scope of the
          protocol extensions defined in this document.
        A BGP Prefix-SID MAY be attached to a BGP prefix.
          This implies that each prefix is advertised individually, reducing the
          ability to pack BGP advertisements (when sharing common
          attributes).
      
      
    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.
      
    
    
      MPLS BGP Prefix-SID
      The BGP Prefix-SID is realized on the MPLS data plane () in the following
      way:
      
        The operator
            assigns a globally unique label index, L_I, to a locally originated
            prefix of a BGP speaker N, which is advertised to all other BGP
            speakers in the SR domain.
        According to ,
            each BGP speaker is configured with a label block called the
            Segment Routing Global Block (SRGB). While  recommends using the
            same SRGB across all the nodes within the SR domain, the SRGB of a
            node is a local property and could be different on different
            speakers. The drawbacks of the use case where BGP speakers have
            different SRGBs are documented in  and .
        If traffic engineering within the SR domain is required, each
            node may also be required to advertise topological information and
            Peer SIDs for each of its links and peers. This information is
            required to perform the explicit path computation and to
            express an explicit path as a list of SIDs. The advertisement
            of topological information and peer segments (Peer SIDs) is done 
            through .
        If a prefix segment is to be included in an MPLS label stack,
        e.g., for traffic-engineering purposes, knowledge of the prefix
        originator's SRGB is required in order to compute the local label used
        by the originator.
        This document assumes that Border Gateway Protocol - Link State
	    (BGP-LS) is the preferred method for a
            collecting both peer segments (Peer SIDs) and SRGB
            information through , , and . However, as an
            optional alternative for the advertisement of the local SRGB
            without the topology or the peer SIDs and, therefore, without
            applicability for TE, the Originator SRGB TLV of the BGP Prefix-SID
            attribute is specified in  of this
            document.
        A BGP speaker will derive its local MPLS label L from the 
               label index L_I and its local SRGB as 
               described in . The 
               BGP speaker then programs the MPLS label L in its MPLS data plane as
               its incoming/local label for the prefix.
               See  for more details.
        The outgoing label for the prefix is found in the 
            Network Layer Reachability Information (NLRI) of the
            Multiprotocol BGP IPv4/IPv6 Labeled Unicast prefix advertisement as
            defined in .
            The label index L_I is only used as a hint to derive the local/incoming
            label.
        
           of this document specifies the
            Label-Index TLV of the BGP Prefix-SID attribute; this TLV can be
            used to advertise the label index for a given prefix.
      
    
    
      BGP Prefix-SID Attribute
      The BGP Prefix-SID attribute is an optional, transitive BGP path
      attribute. The attribute type code 40 has been assigned by IANA (see
      ).
      The BGP Prefix-SID attribute is defined here to be a set of elements
      encoded as "Type/Length/Value" tuples (i.e., a set of TLVs). All BGP 
      Prefix-SID attribute TLVs will start with a 1-octet type and a 2-octet
      length. The following TLVs are defined in this 
      document:
      
        Label-Index TLV
        Originator SRGB TLV
      
      The Label-Index and Originator SRGB TLVs are used only when SR is applied
      to the MPLS data plane.
      For future extensibility, unknown TLVs MUST be ignored and propagated 
         unmodified.
      
        Label-Index TLV
        The Label-Index TLV MUST be present in the BGP Prefix-SID attribute
        attached to IPv4/IPv6 Labeled Unicast prefixes (). It MUST be ignored when received for other 
        BGP AFI/SAFI combinations. The Label-Index 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            |   RESERVED    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Flags              |       Label Index             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Label Index          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        where: 
        
          
            
              Type:
              1
              Length:
              7, the total length in octets of the value portion
               of the TLV.
              RESERVED:
              8-bit field. It MUST be clear on transmission and MUST be
            ignored on reception.
              Flags:
              16 bits of flags. None are defined by this document. The
            Flags field MUST be clear on transmission and MUST be ignored on
            reception.
              Label Index:
              32-bit value representing the index value in the
            SRGB space.
            
          
        
      
      
        Originator SRGB TLV
        The Originator SRGB TLV is an optional TLV 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      |      
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
 |     Flags     |     
 +-+-+-+-+-+-+-+-+
 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         SRGB 1 (6 octets)                                     |      
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |         SRGB n (6 octets)                                     |      
 |                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        where:
        
          
            
              Type:
              3
              Length:
              The total length in octets of the value portion of
            the TLV: 2 + (non-zero multiple of 6).
              Flags:
              16 bits of flags. None are defined in this document.
            The Flags field MUST be clear on transmission and MUST be ignored on
            reception.
              SRGB:
              3 octets specifying the first label in the range followed
            by 3 octets specifying the number of labels in the range. Note that
            the SRGB field MAY appear multiple times. If the SRGB field
            appears multiple times, the SRGB consists of multiple ranges
            that are concatenated.
            
          
        
        The Originator SRGB TLV contains the SRGB of the node originating
        the prefix to which the BGP Prefix-SID is attached. The Originator
        SRGB TLV MUST NOT be changed during the propagation of the BGP
        update. It is used to build SR policies
        when different SRGBs are used in the fabric, for example, .
        Examples of how the receiving routers concatenate the
          ranges and build their neighbor's Segment Routing Global Block (SRGB)
          are included in .
        The Originator SRGB TLV may only appear in a BGP Prefix-SID attribute
        attached to IPv4/IPv6 Labeled Unicast prefixes (). It MUST be ignored when received for other 
        BGP AFI/SAFI combinations. Since the Label-Index TLV is required 
        for IPv4/IPv6 prefix applicability, the Originator SRGB TLV will be 
        ignored if it is not specified in a manner consistent with .
        If a BGP speaker receives a node's SRGB as an attribute of the BGP-LS
         Node NLRI and the BGP speaker also receives the same node's SRGB
         in a BGP Prefix-SID attribute, then the received values should be the
         same. If the values are different, the values advertised in the BGP-LS
         NLRI SHOULD be preferred, and an error should be logged.
      
    
    
      Receiving BGP Prefix-SID Attribute
      A BGP speaker receiving a BGP Prefix-SID attribute from an External BGP (EBGP)
      neighbor residing outside the boundaries of the SR domain MUST
      discard the attribute unless it is configured to accept the attribute
      from the EBGP neighbor. A BGP speaker SHOULD log an error for further
      analysis when discarding an attribute.
      
        MPLS Data Plane: Labeled Unicast
        A BGP session supporting the Multiprotocol BGP Labeled IPv4 or IPv6 Unicast () AFI/SAFI is required.
        When the  BGP Prefix-SID attribute is attached to a BGP Labeled IPv4 or IPv6
          Unicast  AFI/SAFI, it MUST contain the Label-Index TLV
          and MAY contain the Originator SRGB TLV.  A BGP Prefix-SID attribute received
          without a Label-Index TLV MUST be considered to be "invalid" by the
          receiving speaker.
        The label index provides guidance to the receiving BGP speaker as to 
          the incoming label that SHOULD be allocated to the prefix.
        A BGP speaker may be locally configured with an SRGB=[SRGB_Start,
        SRGB_End]. The preferred method for deriving the SRGB is a matter of
        local node configuration.
        The mechanisms through which a given label-index value is assigned
        to a given prefix are outside the scope of this document.
        Given a label index L_I, we refer to (L = L_I + SRGB_Start) as the
        derived label. A BGP Prefix-SID attribute is designated "conflicting" for
        a speaker M if the derived label value L lies outside the SRGB
        configured on M. Otherwise, the Label-Index TLV is designated
        "acceptable" to speaker M.
        If multiple different prefixes are received with the same label
           index, all of the different prefixes MUST have
           their BGP Prefix-SID attribute considered to be "conflicting".
        If multiple valid paths for the same prefix are received from
          multiple BGP speakers or, in the case of , 
          from the same BGP speaker, and the BGP Prefix-SID attributes do
          not contain the same label index, then the label index from
          the best path BGP Prefix-SID attribute SHOULD be chosen with 
          a notable exception being when  
          is being used to dampen route changes.
        When a BGP speaker receives a path from a neighbor with an
        "acceptable" BGP Prefix-SID attribute and that path is selected as 
        the best path, it SHOULD program the derived label
        as the label for the prefix in its local MPLS data plane.
        When a BGP speaker receives a path from a neighbor with an
        "invalid" or "conflicting" BGP Prefix-SID attribute, or when a 
        BGP speaker receives a path from a neighbor with a BGP Prefix-SID
        attribute but is unable to process it (e.g., local policy disables 
        the functionality), it MUST ignore the
        BGP Prefix-SID attribute. For the purposes of label allocation, a
        BGP speaker MUST assign a local (also called dynamic) label (non-SRGB)
        for such a prefix as per classic Multiprotocol BGP IPv4/IPv6 Labeled
        Unicast () operation.
        In the case of an "invalid" BGP Prefix-SID attribute, a BGP speaker MUST
        follow the error-handling rules specified in . 
        A BGP speaker SHOULD log an error for further analysis. In the case of a 
        "conflicting" BGP Prefix-SID attribute, a BGP speaker SHOULD NOT treat it 
        as an error and SHOULD propagate the attribute unchanged. A BGP speaker SHOULD 
        log a warning for further analysis, i.e., in the case the conflict is 
        not due to a label-index transition.
        When a BGP Prefix-SID attribute changes and transitions from
           "conflicting" to "acceptable", the BGP Prefix-SID attributes for other
           prefixes may also transition to "acceptable" as well. Implementations SHOULD
           ensure all impacted prefixes revert to using the label indices 
           corresponding to these newly "acceptable" BGP Prefix-SID attributes.
        The outgoing label is always programmed as per classic
        Multiprotocol BGP IPv4/IPv6 Labeled Unicast ()
        operation. Specifically, a BGP speaker receiving a prefix with a BGP Prefix-SID
        attribute and a label NLRI field of Implicit NULL 
         from a neighbor MUST
        adhere to standard behavior and program its MPLS data plane to pop the
        top label when forwarding traffic to the prefix. The label NLRI
        defines the outbound label that MUST be used by the receiving node.
      
    
    
      Advertising BGP Prefix-SID Attribute
      The BGP Prefix-SID attribute MAY be attached to BGP IPv4/IPv6 Labeled Unicast prefixes
      . In order to prevent distribution of the BGP
      Prefix-SID attribute beyond its intended scope of applicability,
      attribute filtering SHOULD be deployed to remove the BGP 
      Prefix-SID attribute at the administrative boundary of the 
      SR domain.
      A BGP speaker that advertises a path received from one of its
        neighbors SHOULD advertise the BGP Prefix-SID received with the path
        without modification as long as the BGP Prefix-SID was acceptable.
        If the path did not come with a BGP Prefix-SID attribute, the
        speaker MAY attach a BGP Prefix-SID to the path if configured to do so.
        The content of the TLVs present in the BGP Prefix-SID is determined by the
        configuration.
      
        MPLS Data Plane: Labeled Unicast
        A BGP speaker that originates a prefix attaches the BGP Prefix-SID
        attribute when it advertises the prefix to its neighbors via
        Multiprotocol BGP IPv4/IPv6 Labeled Unicast (). The value of the label index in the Label-Index
        TLV is determined by configuration.
        A BGP speaker that originates a BGP Prefix-SID attribute MAY optionally
        announce the Originator SRGB TLV along with the mandatory Label-Index TLV.
        The content of the Originator SRGB TLV is determined by
        configuration.
        Since the label-index value must be unique within an SR domain, by
        default an implementation SHOULD NOT advertise the BGP Prefix-SID
        attribute outside an AS unless it is explicitly
        configured to do so.
        In all cases, the Label field of the advertised NLRI ( ) MUST be set to the
        local/incoming label programmed in the MPLS data plane for the given
        advertised prefix. If the prefix is associated with one of the BGP
        speaker's interfaces, this is the usual MPLS label (such as the
        Implicit or Explicit NULL label 
        ).
      
    
    
      Error Handling of BGP Prefix-SID Attribute
      When a BGP speaker receives a BGP UPDATE message containing a
      malformed or invalid BGP Prefix-SID attribute attached to an
      IPv4/IPv6 Labeled Unicast prefix (), it MUST
      ignore the received BGP Prefix-SID attribute and not advertise it to
      other BGP peers. In this context, a malformed BGP Prefix-SID attribute
      is one that cannot be parsed due to not meeting the minimum attribute 
      length requirement, containing a TLV length that doesn't conform to the 
      length constraints for the TLV, or containing a TLV length that would
      extend beyond the end of the attribute (as defined by the attribute
      length). 
      This is equivalent to the "Attribute discard" 
      action specified in . When discarding an 
      attribute, a BGP speaker SHOULD log an error for further analysis.
      As per , if the BGP
         Prefix-SID attribute appears more than once in an UPDATE
         message, all the occurrences of the attribute other than the
         first one SHALL be discarded and the UPDATE message will continue
         to be processed.
         Similarly, if a recognized TLV appears more than once in a BGP
         Prefix-SID attribute while the specification only allows for a single
         occurrence, then all the occurrences of the TLV other than the
         first one SHALL be discarded and the Prefix-SID attribute will continue
         to be processed.
      For future extensibility, unknown TLVs MUST be ignored and 
         propagated unmodified.
    
    
      IANA Considerations
      This document defines a BGP path attribute known as the BGP
      Prefix-SID attribute.  IANA has assigned 
      attribute code type 40 to the BGP Prefix-SID
      attribute from the "BGP Path Attributes" registry.
      This document defines two TLVs for the BGP Prefix-SID attribute. These
      TLVs have been registered with IANA. IANA has created a
      registry for BGP Prefix-SID Attribute TLVs as follows:
      Under the "Border Gateway Protocol (BGP) Parameters" registry, the new registry titled "BGP
      Prefix-SID TLV Types" has been created and points to this
      document as the reference.
      Registration Procedure(s):
      
        Values 1-254, Expert Review as defined in 
      
        Values
      0 and 255, Reserved
      
      BGP Prefix-SID TLV Types
        
        
          
            Value
            Type
            Reference
          
        
        
          
            0
            Reserved
            This document
          
          
            1
            Label-Index
            This document
          
          
            2
            Deprecated
            This document
          
          
            3
            Originator SRGB
            This document
          
          
            4-254
            Unassigned
            
          
          
            255
            Reserved
            This document
          
        
      
      The value 2 previously corresponded to the IPv6 SID TLV, which was specified 
         in previous versions of this document. It was removed, and use of 
         the BGP Prefix-SID for Segment Routing over the IPv6 data plane 
          has been deferred to 
         future specifications.
      IANA has also created the "BGP Prefix-SID Label-Index TLV Flags"
         registry under the  "Border Gateway Protocol (BGP) Parameters" registry,
         with a reference to this document. Initially, this 16-bit flags registry is
         empty. The registration policy for flag bits is Expert Review ,
         consistent with the "BGP Prefix-SID TLV Types" registry.
      Finally, IANA has created the "BGP Prefix-SID Originator SRGB TLV Flags"
         registry under the  "Border Gateway Protocol (BGP) Parameters" registry,
         with a reference to this document. Initially, this 16-bit flags registry is
         empty. The registration policy for flag bits is Expert Review 
         consistent with the BGP Prefix-SID TLV Types registry.
      The designated experts must be good and faithful stewards of the above registries, 
        ensuring that each request is legitimate and corresponds to a viable use case. Given 
        the limited number of bits in the flags registries and the applicability to a single TLV, 
        additional scrutiny should be afforded to requests for flag-bit allocation. In general, no 
        single use case should require more than one flag bit and, should the use case 
        require more, alternate encodings using new TLVs should be considered.
    
    
      Manageability Considerations
      This document defines a BGP attribute to address use
      cases such as the one described in 
      .
      It is assumed that advertisement of the BGP Prefix-SID attribute is
      controlled by the operator in order to:
      
        Prevent undesired origination/advertisement of the BGP Prefix-SID
          attribute. By default, a BGP Prefix-SID attribute SHOULD NOT be
          attached to a prefix and advertised. Hence, BGP Prefix-SID 
          Advertisement SHOULD require explicit enablement.
        Prevent any undesired propagation of the BGP Prefix-SID
          attribute. By default, the BGP Prefix-SID is not advertised outside
          the boundary of a single SR/administrative domain that may include
          one or more ASes. The propagation to other ASes MUST be
          explicitly configured.
      
      The deployment model described in  assumes multiple
      ASes under a common administrative domain. For this
      use case, the BGP  Prefix-SID Advertisement is applicable to the inter-AS 
      context, i.e., EBGP,  while it is confined to a single
      administrative domain.
    
    
      Security Considerations
      This document introduces a BGP attribute (BGP Prefix-SID), which
      inherits the security considerations expressed in: , , and
      .
      When advertised using BGPsec as described in ,
     the BGP Prefix-SID attribute doesn't impose any unique
     security considerations. It should be noted that the BGP Prefix-SID
     attribute is not protected by the BGPsec signatures.
      It should be noted that,
      as described in , this document refers
      to a deployment model where all nodes are under the single administrative domain.
      In this context, we assume that the operator doesn't want to leak
      any information related to internal prefixes and topology outside of the 
      administrative domain.
      The internal information includes the BGP Prefix-SID. In order
      to prevent such leaking, the common BGP mechanisms (filters) are
      applied at the boundary of the SR/administrative domain.
      Local BGP-attribute-filtering policies 
      and mechanisms are not standardized and, consequently, are beyond the
      scope of this document.
      To prevent a Denial-of-Service (DoS) or Distributed-Denial-of-Service 
      (DDoS) attack due to excessive BGP updates with an invalid or conflicting 
      BGP Prefix-SID attribute, error log message rate limiting as well as suppression of
      duplicate error log messages SHOULD be deployed.
      Since BGP-LS is the preferred method for advertising SRGB information,
         the BGP speaker SHOULD log an error if a BGP Prefix-SID attribute
         is received with SRGB information different from that received as an attribute of
         the same node's BGP-LS Node NLRI.
    
  
  
    
    
    
    
      References
      
        Normative References
        
          
            Key words for use in RFCs to Indicate Requirement Levels
            
              
            
            
            
              In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.
            
          
          
          
          
        
        
          
            A Border Gateway Protocol 4 (BGP-4)
            
              
            
            
              
            
            
              
            
            
            
              This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol.
              The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems.  This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced.
              BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR).  These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP.  BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths.
              This document obsoletes RFC 1771.  [STANDARDS-TRACK]
            
          
          
          
        
        
          
            BGP/MPLS IP Virtual Private Networks (VPNs)
            
              
            
            
              
            
            
            
              This document describes a method by which a Service Provider may use an IP backbone to provide IP Virtual Private Networks (VPNs) for its customers.  This method uses a "peer model", in which the customers' edge routers (CE routers) send their routes to the Service Provider's edge routers (PE routers); there is no "overlay" visible to the customer's routing algorithm, and CE routers at different sites do not peer with each other.  Data packets are tunneled through the backbone, so that the core routers do not need to know the VPN routes.  [STANDARDS-TRACK]
            
          
          
          
        
        
          
            Multiprotocol Extensions for BGP-4
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              This document defines extensions to BGP-4 to enable it to carry routing information for multiple Network Layer protocols (e.g., IPv6, IPX, L3VPN, etc.).  The extensions are backward compatible - a router that supports the extensions can interoperate with a router that doesn't support the extensions.  [STANDARDS-TRACK]
            
          
          
          
        
        
          
            Revised Error Handling for BGP UPDATE Messages
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable because a session reset would impact not only routes with the offending attribute but also other valid routes exchanged over the session.  This document partially revises the error handling for UPDATE messages and provides guidelines for the authors of documents defining new attributes.  Finally, it revises the error handling procedures for a number of existing attributes.
              This document updates error handling for RFCs 1997, 4271, 4360, 4456, 4760, 5543, 5701, and 6368.
            
          
          
          
        
        
          
            Advertisement of Multiple Paths in BGP
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              This document defines a BGP extension that allows the advertisement of multiple paths for the same address prefix without the new paths implicitly replacing any previous ones.  The essence of the extension is that each path is identified by a Path Identifier in addition to the address prefix.
            
          
          
          
        
        
          
            Guidelines for Writing an IANA Considerations Section in RFCs
            
              
            
            
              
            
            
              
            
            
            
              Many protocols make use of points of extensibility that use constants to identify various protocol parameters.  To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper.  For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).
              To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed.  This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.
              This is the third edition of this document; it obsoletes RFC 5226.
            
          
          
          
          
        
        
          
            Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
            
              
            
            
            
              RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.
            
          
          
          
          
        
        
          
            BGPsec Protocol Specification
            
              
            
            
              
            
            
            
              This document describes BGPsec, an extension to the Border Gateway Protocol (BGP) that provides security for the path of Autonomous Systems (ASes) through which a BGP UPDATE message passes.  BGPsec is implemented via an optional non-transitive BGP path attribute that carries digital signatures produced by each AS that propagates the UPDATE message.  The digital signatures provide confidence that every AS on the path of ASes listed in the UPDATE message has explicitly authorized the advertisement of the route.
            
          
          
          
        
        
          
            Using BGP to Bind MPLS Labels to Address Prefixes
            
              
            
            
            
              This document specifies a set of procedures for using BGP to advertise that a specified router has bound a specified MPLS label (or a specified sequence of MPLS labels organized as a contiguous part of a label stack) to a specified address prefix.  This can be done by sending a BGP UPDATE message whose Network Layer Reachability Information field contains both the prefix and the MPLS label(s) and whose Next Hop field identifies the node at which said prefix is bound to said label(s).  This document obsoletes RFC 3107.
            
          
          
          
        
        
          
            Segment Routing Architecture
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              Segment Routing (SR) leverages the source routing paradigm.  A node steers a packet through an ordered list of instructions, called "segments".  A segment can represent any instruction, topological or service based.  A segment can have a semantic local to an SR node or global within an SR domain.  SR provides a mechanism that allows a flow to be restricted to a specific topological path, while maintaining per-flow state only at the ingress node(s) to the SR domain.
              SR can be directly applied to the MPLS architecture with no change to the forwarding plane.  A segment is encoded as an MPLS label.  An ordered list of segments is encoded as a stack of labels.  The segment to process is on the top of the stack.  Upon completion of a segment, the related label is popped from the stack.
              SR can be applied to the IPv6 architecture, with a new type of routing header.  A segment is encoded as an IPv6 address.  An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing header.  The active segment is indicated by the Destination Address (DA) of the packet.  The next active segment is indicated by a pointer in the new routing header.
            
          
          
          
        
        
          
            Segment Routing with the MPLS Data Plane
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
          
          
          
        
      
      
        Informative References
        
          
            BGP-LS extensions for Segment Routing BGP Egress Peer Engineering
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              Segment Routing (SR) leverages source routing.  A node steers a packet through a controlled set of instructions, called segments, by prepending the packet with an SR header.  A segment can represent any instruction, topological or service-based.  SR segments allow steering a flow through any topological path and service chain while maintaining per-flow state only at the ingress node of the SR domain.  This document describes an extension to BGP Link-State (BGP-LS) for advertisement of BGP Peering Segments along with their BGP peering node information so that efficient BGP Egress Peer Engineering (EPE) policies and strategies can be computed based on Segment Routing.
            
          
          
          
          Work in Progress
        
        
          
            BGP Link-State extensions for Segment Routing
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological sub-paths, called "segments".  These segments are advertised by routing protocols e.g. by the link state routing protocols (IS-IS, OSPFv2 and OSPFv3) within IGP topologies.  This document defines extensions to the BGP Link-state address-family in order to carry segment routing information via BGP.
            
          
          
          
          Work in Progress
        
        
          
            IPv6 Segment Routing Header (SRH)
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header. This document describes the Segment Routing Header and how it is used by Segment Routing capable nodes.
            
          
          
          
          Work in Progress
        
        
          
            MPLS Label Stack Encoding
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              This document specifies the encoding to be used by an LSR in order to transmit labeled packets on Point-to-Point Protocol (PPP) data links, on LAN data links, and possibly on other data links as well.  This document also specifies rules and procedures for processing the various fields of the label stack encoding.  [STANDARDS-TRACK]
            
          
          
          
        
        
          
            Avoid BGP Best Path Transitions from One External to Another
            
              
            
            
              
            
            
            
              In this document, we propose an extension to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions.  The proposed extension would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external path from one BGP speaker contributes to the churn.  [STANDARDS-TRACK]
            
          
          
          
        
        
          
            North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
            
              In a number of environments, a component external to a network is called upon to perform computations based on the network topology and current state of the connections within the network, including Traffic Engineering (TE) information.  This is information typically distributed by IGP routing protocols within the network.
              This document describes a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol.  This is achieved using a new BGP Network Layer Reachability Information (NLRI) encoding format.  The mechanism is applicable to physical and virtual IGP links.  The mechanism described is subject to policy control.
              Applications of this technique include Application-Layer Traffic Optimization (ALTO) servers and Path Computation Elements (PCEs).
            
          
          
          
        
        
          
            BGP Prefix Segment in Large-Scale Data Centers
            
              
            
            
              
            
            
              
            
            
              
            
            
              
            
            
          
          
          
        
      
    
    
      Acknowledgements
      The authors would like to thank Satya Mohanty for his contribution
      to this document.
      The authors would like to thank Alvaro Retana for substantive 
         comments as part of the Routing AD review.
      The authors would like to thank Bruno Decraene for substantive 
         comments and suggested text as part of the Routing Directorate
         review.
      The authors would like to thank Shyam Sethuram for comments and
         discussion of TLV processing and validation.
      The authors would like to thank Robert Raszuk for comments and
         suggestions regarding the MPLS data-plane behavior.
      The authors would like to thank Krishna Deevi, 
         Juan Alcaide, Howard Yang, and Jakob Heitz for discussions 
         on conflicting BGP Prefix-SID label indices and BGP add paths.
      The authors would like to thank Peter Yee, Tony Przygienda,
         Mirja Kuhlewind, Alexey Melnikov, Eric Rescorla, Suresh
         Krishnan, Warren Kumari, Ben Campbell Sue Hares, and Martin
         Vigoureux for IDR Working Group last call, IETF Last Call,
         directorate, and IESG reviews.
    
    
      Contributors
      Keyur Patel
Arrcus, Inc.
United States of America

Email: Keyur@arrcus.com
      Saikat Ray
Unaffiliated
United States of America

Email: raysaikat@gmail.com
    
    
      Authors' Addresses
      
        Huawei Technologies
        
          
            
            
            Italy
            
          
          
          stefano@previdi.net
        

      
      
        Cisco Systems
        
          
            
            Brussels
            Belgium
            
          
          
          cfilsfil@cisco.com
        

      
      
        Cisco Systems
        
          
            301 Midenhall Way
            Cary, NC
            United States of America
            27513
          
          
          acee@cisco.com
        
      
      
        
          arjunhrs@gmail.com
        
      
      
        RtBrick Inc.
        
          hannes@rtbrick.com

Value	Type	Reference
0	Reserved	This document
1	Label-Index	This document
2	Deprecated	This document
3	Originator SRGB	This document
4-254	Unassigned
255	Reserved	This document