Inter-Domain Routing H. Gredler, Ed.
Internet-Draft Private Contributor
Intended status: Standards Track J. Medved
Expires: April 6, 2016 S. Previdi
Cisco Systems, Inc.
A. Farrel
Juniper Networks, Inc.
S. Ray
October 4, 2015

North-Bound Distribution of Link-State and 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 information. This is information typically distributed by IGP routing protocols within the network.

This document describes a mechanism by which links state and traffic engineering 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).

Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on April 6, 2016.

Copyright Notice

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Table of Contents

1. Introduction

The contents of a Link State Database (LSDB) or a Traffic Engineering Database (TED) has the scope of an IGP area. Some applications, such as end-to-end Traffic Engineering (TE), would benefit from visibility outside one area or Autonomous System (AS) in order to make better decisions.

The IETF has defined the Path Computation Element (PCE) [RFC4655] as a mechanism for achieving the computation of end-to-end TE paths that cross the visibility of more than one TED or which require CPU-intensive or coordinated computations. The IETF has also defined the ALTO Server [RFC5693] as an entity that generates an abstracted network topology and provides it to network-aware applications.

Both a PCE and an ALTO Server need to gather information about the topologies and capabilities of the network in order to be able to fulfill their function.

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 [RFC4271]. This is achieved using a new BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism is applicable to physical and virtual links. The mechanism described is subject to policy control.

A router maintains one or more databases for storing link-state information about nodes and links in any given area. Link attributes stored in these databases include: local/remote IP addresses, local/remote interface identifiers, link metric and TE metric, link bandwidth, reservable bandwidth, per CoS class reservation state, preemption and Shared Risk Link Groups (SRLG). The router's BGP process can retrieve topology from these LSDBs and distribute it to a consumer, either directly or via a peer BGP Speaker (typically a dedicated Route Reflector), using the encoding specified in this document.

The collection of Link State and TE link state information and its distribution to consumers is shown in the following figure.

                        | Consumer  |
                        |    BGP    |               +-----------+
                        |  Speaker  |               | Consumer  |
                        +-----------+               +-----------+
                          ^   ^   ^                       ^
                          |   |   |                       |
          +---------------+   |   +-------------------+   |
          |                   |                       |   |
    +-----------+       +-----------+             +-----------+
    |    BGP    |       |    BGP    |             |    BGP    |
    |  Speaker  |       |  Speaker  |    . . .    |  Speaker  |
    +-----------+       +-----------+             +-----------+
          ^                   ^                         ^
          |                   |                         |
         IGP                 IGP                       IGP

Figure 1: TE Link State info collection

A BGP Speaker may apply configurable policy to the information that it distributes. Thus, it may distribute the real physical topology from the LSDB or the TED. Alternatively, it may create an abstracted topology, where virtual, aggregated nodes are connected by virtual paths. Aggregated nodes can be created, for example, out of multiple routers in a POP. Abstracted topology can also be a mix of physical and virtual nodes and physical and virtual links. Furthermore, the BGP Speaker can apply policy to determine when information is updated to the consumer so that there is reduction of information flow from the network to the consumers. Mechanisms through which topologies can be aggregated or virtualized are outside the scope of this document

2. Motivation and Applicability

This section describes use cases from which the requirements can be derived.

2.1. MPLS-TE with PCE

As described in [RFC4655] a PCE can be used to compute MPLS-TE paths within a "domain" (such as an IGP area) or across multiple domains (such as a multi-area AS, or multiple ASes).

Previous solutions used per-domain path computation [RFC5152]. The source router could only compute the path for the first area because the router only has full topological visibility for the first area along the path, but not for subsequent areas. Per-domain path computation uses a technique called "loose-hop-expansion" [RFC3209], and selects the exit ABR and other ABRs or AS Border Routers (ASBRs) using the IGP computed shortest path topology for the remainder of the path. This may lead to sub-optimal paths, makes alternate/back-up path computation hard, and might result in no TE path being found when one really does exist.

The PCE presents a computation server that may have visibility into more than one IGP area or AS, or may cooperate with other PCEs to perform distributed path computation. The PCE obviously needs access to the TED for the area(s) it serves, but [RFC4655] does not describe how this is achieved. Many implementations make the PCE a passive participant in the IGP so that it can learn the latest state of the network, but this may be sub-optimal when the network is subject to a high degree of churn, or when the PCE is responsible for multiple areas.

The following figure shows how a PCE can get its TED information using the mechanism described in this document.

             +----------+                           +---------+
             |  -----   |                           |   BGP   |
             | | TED |<-+-------------------------->| Speaker |
             |  -----   |   TED synchronization     |         |
             |    |     |        mechanism:         +---------+
             |    |     | BGP with Link-State NLRI
             |    v     |
             |  -----   |
             | | PCE |  |
             |  -----   |
                  | Request/
                  | Response
    Service  +----------+   Signaling  +----------+
    Request  | Head-End |   Protocol   | Adjacent |
    -------->|  Node    |<------------>|   Node   |
             +----------+              +----------+

Figure 2: External PCE node using a TED synchronization mechanism

The mechanism in this document allows the necessary TED information to be collected from the IGP within the network, filtered according to configurable policy, and distributed to the PCE as necessary.

2.2. ALTO Server Network API

An ALTO Server [RFC5693] is an entity that generates an abstracted network topology and provides it to network-aware applications over a web service based API. Example applications are p2p clients or trackers, or CDNs. The abstracted network topology comes in the form of two maps: a Network Map that specifies allocation of prefixes to Partition Identifiers (PIDs), and a Cost Map that specifies the cost between PIDs listed in the Network Map. For more details, see [RFC7285].

ALTO abstract network topologies can be auto-generated from the physical topology of the underlying network. The generation would typically be based on policies and rules set by the operator. Both prefix and TE data are required: prefix data is required to generate ALTO Network Maps, TE (topology) data is required to generate ALTO Cost Maps. Prefix data is carried and originated in BGP, TE data is originated and carried in an IGP. The mechanism defined in this document provides a single interface through which an ALTO Server can retrieve all the necessary prefix and network topology data from the underlying network. Note an ALTO Server can use other mechanisms to get network data, for example, peering with multiple IGP and BGP Speakers.

The following figure shows how an ALTO Server can get network topology information from the underlying network using the mechanism described in this document.

  | Client |<--+
  +--------+   |
               |    ALTO    +--------+     BGP with    +---------+
  +--------+   |  Protocol  |  ALTO  | Link-State NLRI |   BGP   |
  | Client |<--+------------| Server |<----------------| Speaker |
  +--------+   |            |        |                 |         |
               |            +--------+                 +---------+
  +--------+   |
  | Client |<--+

Figure 3: ALTO Server using network topology information

3. Carrying Link State Information in BGP

This specification contains two parts: definition of a new BGP NLRI that describes links, nodes and prefixes comprising IGP link state information, and definition of a new BGP path attribute (BGP-LS attribute) that carries link, node and prefix properties and attributes, such as the link and prefix metric or auxiliary Router-IDs of nodes, etc.

It is desired to keep the dependencies on the protocol source of this attributes to a minimum and represent any content in an IGP neutral way, such that applications which do want to learn about a Link-state topology do not need to know about any OSPF or IS-IS protocol specifics.

3.1. TLV Format

Information in the new Link-State NLRIs and attributes is encoded in Type/Length/Value triplets. The TLV format is shown in Figure 4.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  |              Type             |             Length            |
  //                        Value (variable)                     //

Figure 4: TLV format

The Length field defines the length of the value portion in octets (thus a TLV with no value portion would have a length of zero). The TLV is not padded to four-octet alignment. Unrecognized types MUST be preserved and propagated. In order to compare NLRIs with unknown TLVs all TLVs MUST be ordered in ascending order by TLV Type. If there are more TLVs of the same type, then the TLVs MUST be ordered in ascending order of the TLV value within the TLVs with the same type by treating the entire value field an opaque hexadecimal string and comparing leftmost octets first regardless of the length of the string. . All TLVs that are not specified as mandatory are considered optional.

3.2. The Link-State NLRI

The MP_REACH_NLRI and MP_UNREACH_NLRI attributes are BGP's containers for carrying opaque information. Each Link-State NLRI describes either a node, a link or a prefix.

All non-VPN link, node and prefix information SHALL be encoded using AFI 16388 / SAFI 71. VPN link, node and prefix information SHALL be encoded using AFI 16388 / SAFI TBD.

In order for two BGP speakers to exchange Link-State NLRI, they MUST use BGP Capabilities Advertisement to ensure that they both are capable of properly processing such NLRI. This is done as specified in [RFC4760], by using capability code 1 (multi-protocol BGP), with AFI 16388 / SAFI 71 for BGP-LS, and AFI 16388 / SAFI TBD for BGP-LS-VPN.

The format of the Link-State NLRI is shown in the following figure.

   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
  |            NLRI Type          |     Total NLRI Length         |
  |                                                               |
  //                  Link-State NLRI (variable)                 //
  |                                                               |

Figure 5: Link-State AFI 16388 / SAFI 71 NLRI 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
  |            NLRI Type          |     Total NLRI Length         |
  |                                                               |
  +                       Route Distinguisher                     +
  |                                                               |
  |                                                               |
  //                  Link-State NLRI (variable)                 //
  |                                                               |

Figure 6: Link-State VPN AFI 16388 / SAFI TBD NLRI Format

The 'Total NLRI Length' field contains the cumulative length, in octets, of rest of the NLRI not including the NLRI Type field or itself. For VPN applications, it also includes the length of the Route Distinguisher.

NLRI Types
Type NLRI Type
1 Node NLRI
2 Link NLRI
3 IPv4 Topology Prefix NLRI
4 IPv6 Topology Prefix NLRI

Route Distinguishers are defined and discussed in [RFC4364].

The Node NLRI (NLRI Type = 1) is shown in the following figure.

   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
  |  Protocol-ID  |
  |                           Identifier                          |
  |                            (64 bits)                          |
  //                Local Node Descriptors (variable)            //

Figure 7: The Node NLRI format

The Link NLRI (NLRI Type = 2) is shown in the following figure.