Network work group                                             Mach Chen 
Internet Draft                                              Renhai Zhang 
Expires: March 2008                          Huawei Technologies Co.,Ltd 
Category: Standards Track                              September 6, 2007 
                                    
                                      
    OSPF Traffic Engineering (OSPF-TE) Extensions in Support of Inter-AS 
     Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) 
                            Traffic Engineering 
             draft-ietf-ccamp-ospf-interas-te-extension-01.txt 


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   This Internet-Draft will expire on March 6, 2008. 

Abstract 

   This document describes extensions to the OSPF v2 and v3 Traffic 
   Engineering (OSPF-TE) mechanisms to support Multiprotocol Label 
   Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering(TE) 
   for multiple Autonomous Systems (ASes). It defines OSPF-TE extensions 
   for the flooding of TE information about inter-AS links which can be 
   used to perform inter-AS TE path computation. 



 
 
 
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Conventions used in this document 

   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]. 

Table of Contents 

    
   1. Introduction.................................................2 
   2. Problem Statement............................................3 
      2.1. A Note on Non-Objectives................................3 
      2.2. Per-Domain Path Determination...........................4 
      2.3. Backward Recursive Path Computation.....................6 
   3. Extensions to OSPF-TE........................................7 
      3.1. Remote AS Number Sub-TLV................................7 
      3.2. Inter-AS Link Type......................................8 
      3.3. Link ID.................................................8 
   4. Procedure for Inter-AS TE Links..............................8 
   5. Security Considerations......................................9 
   6. IANA Considerations.........................................10 
      6.1. OSPF LSA Sub-TLVs type.................................10 
      6.2. OSPF TE Link Type......................................10 
   7. Acknowledgments.............................................10 
   8. References..................................................11 
      8.1. Normative References...................................11 
      8.2. Informative References.................................11 
   Authors' Addresses.............................................12 
   Intellectual Property Statement................................12 
   Disclaimer of Validity.........................................13 
   Copyright Statement............................................13 
    
1. Introduction 

   [OSPF-TE] defines extensions to the OSPF protocol [OSPF] to support 
   intra-area Traffic Engineering (TE). The extensions provide a way of 
   encoding the TE information for TE-enabled links within the network 
   (TE links) and flooding this information within an area. Type 10 
   opaque LSAs [RFC2370] are used to carry such TE information. Two top-
   level TLVs are defined in [OSPF-TE]: Router Address TLV and Link TLV. 
   The Link TLV has several nested sub-TLVs which describe the TE 
   attributes for a TE link.  

   [OSPF-TE-V3] defines similar extensions to OSPFv3 [OSPFV3]. 

   Requirements for establishing Multiprotocol Label Switching (MPLS) TE 
   Label Switched Paths (LSPs) that cross multiple Autonomous Systems 
 
 
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   (ASes) are described in [INTER-AS-TE-REQ]. As described in [INTER-AS-
   TE-REQ], a method SHOULD provide the ability to compute a path 
   spanning multiple ASes. So a path computation entity that may be the 
   head-end Label Switching Router (LSR), an AS Border Router (ASBR), or 
   a Path Computation Element (PCE [PCE]) needs to know the TE 
   information not only of the links within an AS, but also of the links 
   that connect to other ASes. 

   In this document, some extensions to OSPF-TE are defined in support 
   of carrying inter-AS TE link information for inter-AS Traffic 
   Engineering. A new sub-TLV is added to the Link TLV and a new link 
   type is introduced. The extensions are equally applicable to OSPFv2 
   and OSPFv3 as identical extensions to [OSPF-TE] and [OSPF-TE-V3]. The 
   detailed definitions and procedures are discussed in the following 
   sections. 

2. Problem Statement 

   As described in [INTER-AS-TE-REQ], in the case of establishing an 
   inter-AS TE LSP traversing multiple ASes, the Path message [RFC3209] 
   may include the following elements in the Explicit Route Object (ERO) 
   in order to describe the path of the LSP: 

     - a set of AS numbers as loose hops; and/or 

     - a set of LSRs including ASBRs as loose hops. 

   Two methods for determining inter-AS paths are currently discussed. 
   The per-domain method [PD-PATH] determines the path one domain at a 
   time. The backward recursive method [BRPC] uses cooperation between 
   PCEs to determine an optimum inter-domain path. The sections that 
   follow examine how inter-AS TE link information could be useful in 
   both cases. 

2.1. A Note on Non-Objectives 

   It is important to note that this document does not make any change 
   to the confidentiality and scaling assumptions surrounding the use of 
   ASes in the Internet. In particular, this document is conformant to 
   the requirements set out in [INTER-AS-TE-REQ]. 

   The following lists of features are explicit exclusions. 

     o There is no attempt to distribute TE information from within one 
        AS to another AS. 


 
 
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     o There is no mechanism proposed to distribute any form of TE 
        reachability information for destinations outside the AS. 

     o There is no proposed change to the PCE architecture or usage. 

     o TE aggregation is not supported or recommended. 

     o There is no exchange of private information between ASes. 

     o No OSPF adjacencies are formed on the inter-AS link. 

   Note further that the extensions proposed in this document are 
   limited to use for information about inter-AS TE links. L1VPN Auto-
   Discovery [L1VPN-OSPF-AD] defines how TE information about links 
   between Customer Edge (CE) equipment and Provider Edge (PE) equipment 
   can be advertised in OSPF-TE alongside the auto-discovery information 
   for the CE-PE links. That is separate functionality and does not 
   overlap with the function defined in this document. 

2.2. Per-Domain Path Determination 

   In the per-domain method of determining an inter-AS path for an MPLS-
   TE LSP, when an LSR that is an entry-point to an AS receives a PATH 
   message from an upstream AS with an ERO containing a next hop that is 
   an AS number, it needs to find which LSRs (ASBRs) within the local AS 
   are connected to the downstream AS so that it can compute a TE LSP 
   segment across the AS to one of those LSRs and forward the PATH 
   message to the LSR and hence into the next AS. See the figure below 
   for an example: 

                R1------R3----R5-----R7------R9-----R11 
                        |     | \    |      / | 
                        |     |  \   |  ----  | 
                        |     |   \  | /      | 
                R2------R4----R6   --R8------R10----R12 
                           :              : 
                <-- AS1 -->:<---- AS2 --->:<--- AS3 ---> 
    
                  Figure 1: Inter-AS Reference Model 

   The figure shows three ASes (AS1, AS2, and AS3) and twelve LSRs (R1 
   through R12). R3 and R4 are ASBRs in AS1. R5, R6, R7, and R8 are 
   ASBRs in AS2. R9 and R10 are ASBRs in AS3. 

   If an inter-AS TE LSP is planned to be established from R1 to R12, 
   the AS sequence is limited as: AS1, AS2, AS3. 

 
 
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   Suppose that the Path message enters AS2 from R3. The next hop in the 
   ERO shows AS3, and R5 must determine a path segment across AS2 to 
   reach AS3. It has a choice of three exit points from AS2 (R6, R7, and 
   R8) and it needs to know which of these provide TE connectivity to 
   AS3, and whether the TE connectivity (for example, available 
   bandwidth) is adequate for the requested LSP. 

   Alternatively, if the next hop in the ERO is the entry ASBR for AS3 
   (say R9), R5 needs to know which of its exit ASBRs has a TE link that 
   connects to R9. Since there may be multiple exist ASBRs that are 
   connected to R9 (both R7 and R8 in this example), R5 also needs to 
   know the TE properties of the inter-AS TE links so that it can select 
   the correct exit ASBR. 

   Once the path message reaches the exit ASBR, any choice of inter-AS 
   TE link can be made by the ASBR if not already made by entry ASBR 
   that computed the segment. 

   More details can be found in the Section 4.0 of [PD-PATH], which 
   clearly points out why advertising of inter-AS links is desired. 

   To enable R5 to make the correct choice of exit ASBR the following 
   information is needed: 

     o List of all inter-AS TE links for the local AS. 

     o TE properties of each inter-AS TE link. 

     o AS number of the neighboring AS connected to by each inter-AS TE 
        link. 

     o Identity (TE Router ID) of the neighboring ASBR connected to by 
        each inter-AS TE link. 

   In GMPLS networks further information may also be required to select 
   the correct TE links as defined in [GMPLS-TE]. 

   The example above shows how this information is needed at the entry 
   point ASBRs for each AS (or the PCEs that provide computation 
   services for the ASBRs), but this information is also needed 
   throughout the local AS if path computation function is fully 
   distributed among LSRs in the local AS, for example to support LSPs 
   that have start points (ingress nodes) within the AS.  




 
 
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2.3. Backward Recursive Path Computation 

   Another scenario using PCE techniques has the same problem. [BRPC] 
   defines a PCE-based TE LSP computation method (called Backward 
   Recursive Path Computation) to compute optimal inter-domain 
   constrained MPLS-TE or GMPLS LSPs. In this path computation method, a 
   specific set of traversed domains (ASes) are assumed to be selected 
   before computation starts. Each downstream PCE in domain(i) returns 
   to its upstream neighbor PCE in domain(i-1) a multipoint-to-point 
   tree of potential paths. Each tree consists of the set of paths from 
   all Boundary Nodes located in domain(i) to the destination where each 
   path satisfies the set of required constraints for the TE LSP 
   (bandwidth, affinities, etc.).  

   So a PCE needs to select Boundary Nodes (that is, ASBRs) that provide 
   connectivity from the upstream AS. In order that the tree of paths 
   provided by one PCE to its neighbor can be correlated, the identities 
   of the ASBRs for each path need to be referenced, so the PCE must 
   know the identities of the ASBRs in the remote AS reached by any 
   inter-AS TE link, and, in order that it provides only suitable paths 
   in the tree, the PCE must know the TE properties of the inter-AS TE 
   links. See the following figure as an example: 

                   PCE1<------>PCE2<-------->PCE3 
                   /       :             : 
                  /        :             : 
                R1------R3----R5-----R7------R9-----R11 
                        |     | \    |      / | 
                        |     |  \   |  ----  | 
                        |     |   \  | /      | 
                R2------R4----R6   --R8------R10----R12 
                           :              : 
                <-- AS1 -->:<---- AS2 --->:<--- AS3 ---> 
    
            Figure 2: BRPC for Inter-AS Reference Model 

   The figure shows three ASes (AS1, AS2, and AS3), three PCEs (PCE1, 
   PCE2, and PCE3), and twelve LSRs (R1 through R12). R3 and R4 are 
   ASBRs in AS1. R5, R6, R7, and R8 are ASBRs in AS2. R9 and R10 are 
   ASBRs in AS3. PCE1, PCE2, and PCE3 cooperate to perform inter-AS path 
   computation and are responsible for path segment computation within 
   their own domains.  

   If an inter-AS TE LSP is planned to be established from R1 to R12, 
   the traversed domains are assumed to be selected: AS1->AS2->AS3, and 
   the PCE chain is: PCE1->PCE2->PCE3. First, the path computation 
   request originated from the PCC (R1) is relayed by PCE1 and PCE2 
 
 
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   along the PCE chain to PCE3, then PCE3 begins to compute the path 
   segments from the entry boundary nodes that provide connection from 
   AS2 to the destination (R12). But, to provide suitable path segments, 
   PCE3 must determine which entry boundary nodes provide connectivity 
   to its upstream neighbor AS (identified by its AS number), and must 
   know the TE properties of the inter-AS TE links. In the same way, 
   PCE2 also needs to determine the entry boundary nodes according to 
   its upstream neighbor AS and the inter-AS TE link capabilities. 

   Thus, to support Backward Recursive Path Computation the same 
   information as listed in Section 2.2 is required. 

3. Extensions to OSPF-TE 

   Note that this document does not define mechanisms for distribution 
   of TE information from one AS to another, does not distribute any 
   form of TE reachability information for destinations outside the AS, 
   does not change the PCE architecture or usage, does not suggest or 
   recommend any form of TE aggregation, and does not feed private 
   information between ASes. See section 2.1. 

   The extensions defined in this document allow an inter-AS TE link 
   advertisement to be easily identified as such by the use of a new 
   link type. A new sub-TLV to the Link TLV is defined to carry the 
   information about the neighboring AS. The extensions are equally 
   applicable to TE distribution using OSPFv2 and OSPFv3. 

3.1. Remote AS Number Sub-TLV 

   As described in [OSPF-TE], the Link TLV describes a single link and 
   consists of a set of sub-TLVs. A new sub-TLV, the Remote AS Number 
   sub-TLV is added to the Link TLV when advertising inter-AS links. The 
   Remote AS Number sub-TLV specifies the AS number of the neighboring 
   AS to which the advertised link connects. The Remote AS number sub-
   TLV is mandatory for an inter-AS TE link.  

   The Remote AS number sub-TLV is TLV type 21 (which needs to be 
   confirmed by IANA), and is four octets in length. The format is as 
   follows: 







 
 
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    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            | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  
   |                       Remote AS Number                        |   
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
    
   The Remote AS number field has 4 octets. When only two octets are 
   used for the AS number, as in current deployments, the left (high-
   order) two octets MUST be set to zero. 

3.2. Inter-AS Link Type 

   To identify a link as an inter-AS link and allow easy identification 
   of these new advertisements, a new Link Type value is defined for use 
   in the Link Type sub-TLV. The value of the Link Type for an inter-AS 
   point-to-point link is 3 (which needs to be confirmed by IANA).  

   The use of multi-access inter-AS TE links is for future study. 

3.3. Link ID 

   For an inter-AS link, the Link ID carried in the Link ID sub-TLV is 
   the remote ASBR identifier which could be any address of the remote 
   ASBR(e.g., the TE Router ID, Router ID or interface address of the 
   remote ASBR reached through this inter-AS link). The TE Router ID is 
   RECOMMENDED. 

4. Procedure for Inter-AS TE Links 

   When TE is enabled on an inter-AS link and the link is up, the ASBR 
   SHOULD advertise this link using the normal procedures for OSPF-TE 
   [OSPF-TE]. When either the link is down or TE is disabled on the 
   link , the ASBR SHOULD withdraw the advertisement. When there are 
   changes to the TE parameters for the link (for example, when the 
   available bandwidth changes) the ASBR SHOULD re-advertise the link, 
   but the ASBR MUST take precautions against excessive re-
   advertisements as described in [OSPF-TE].  

   Hellos MUST NOT be exchanged (and consequently, an OSPF adjacency 
   MUST NOT be formed) over the inter-AS link. 

   The information advertised comes from the ASBR's knowledge of the TE 
   capabilities of the link, the ASBR's knowledge of the current status 
   and usage of the link, and configuration at the ASBR of the remote AS 
   number and remote ASBR TE Router ID. 
 
 
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   The TE link advertisement SHOULD be carried in a Type 10 Opaque LSA 
   if the flooding scope is to be limited to within the single IGP area 
   to which the ASBR belongs, or MAY be carried in a Type 11 Opaque LSA 
   if the information should reach all routers (including area border 
   routers, ASBRs, and PCEs) in the AS. The choice between the use of a 
   Type 10 or Type 11 Opaque LSA is a network-wide policy choice, and 
   configuration control SHOULD be provided in ASBR implementations that 
   support the advertisement of inter-AS TE links. 

   Legacy routers receiving an advertisement for an inter-AS TE link are 
   able to ignore it because the Link Type carries an unknown value. 
   They will continue to flood the LSA, but will not attempt to use the 
   information received as if the link were an intra-AS TE link. 

   Since there is no OSPF adjacency running on the inter-AS link, the 
   local ASBR SHOULD do a "proxy" advertisement for the backward 
   direction of an inter-AS TE link, which facilitates a path 
   computation entity to do a 2-way check before including the link in a 
   path computation. As the objective of such a "proxy" advertisement is 
   to avoid using an inter-AS TE link when the backward direction of the 
   inter-AS TE link is unavailable or unsuitable, only some mandatory or 
   essential TE information needs to be advertised, i.e. the Link ID, 
   the Link Type, and the Remote AS number of an inter-AS TE link.  

   Routers or PCEs that are capable of processing advertisements of 
   inter-AS TE links SHOULD NOT use such links to compute paths that 
   exit an AS to a remote ASBR and then immediately re-enter the AS 
   through another TE link. Such paths would constitute extremely rare 
   occurrences and SHOULD NOT be allowed except as the result of 
   specific policy configurations at the router or PCE computing the 
   path. 

5. Security Considerations 

   The protocol extensions defined in this document are relatively minor 
   and can be secured within the AS in which they are used by the 
   existing OSPF security mechanisms. 

   There is no exchange of information between ASes, and no change to 
   the OSPF security relationship between the ASes. In particular, since 
   no OSPF adjacency is formed on the inter-AS links, there is no 
   requirement for OSPF security between the ASes. 

   It should be noted, however, that some of the information included in 
   these new advertisements(the remote AS number and the remote ASBR ID) 
   are obtained from a neighboring administration and cannot be verified 
   in anyway. Since the means of delivery of this information is likely 
 
 
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   to be part of a commercial relationship, the source of the 
   information should be carefully checked before it is entered as 
   configuration information at the ASBR responsible for advertising the 
   inter-AS TE links. 

6. IANA Considerations 

   IANA is requested to make the following allocations from registries 
   under its control. 

6.1. OSPF LSA Sub-TLVs type 

   IANA maintains the "Open Shortest Path First (OSPF) Traffic 
   Engineering TLVs" registry with sub-registry "Types for sub-TLVs in a 
   TE Link TLV". IANA is requested to assign a new sub-TLV as follows. 
   The number 21 is suggested as shown in Section 3.1. 

   Value     Meaning 

   21        Remote AS Number sub-TLV. 

6.2. OSPF TE Link Type 

   IANA is requested to create a new sub-registry "TE Link Types" of the 
   registry "Open Shortest Path First (OSPF) Traffic Engineering TLVs" 
   to track TE Link Types. 

   The sub-registry should read as follows: 

   [OSPF-TE] defines the Link Type sub-TLV of the Link TLV. The 
   following values are defined. 

   Value     Meaning                 Reference 

   1         Point-to-point link     [OSPF-TE] 

   2         Multi-access link       [OSPF-TE] 

   3         Inter-AS link           [this document] 

    New allocations from this registry are by IETF Standards Action. 

7. Acknowledgments 

   The authors would like to thank Adrian Farrel, Acee Lindem, JP 
   Vasseur, Dean Cheng, and Jean-Louis Le Roux for their review and 
   comments to this document. 
 
 
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8. References 

8.1. Normative References 

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate 
             Requirement Levels", BCP 14, RFC 2119, March 1997. 

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., 
             and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP 
             Tunnels", RFC 3209, December 2001. 

   [RFC2370]  R. Coltun, "The OSPF Opaque LSA Option", RFC2370, July 
             1998. 

   [OSPF]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998. 

   [OSPF-TE] Katz, D., Kompella, K., and Yeung, D., "Traffic Engineering 
             (TE) Extensions to OSPF Version 2", RFC 3630, September 
             2003. 

   [GMPLS-TE] Rekhter, Y., and Kompella, K., "OSPF Extensions in Support 
             of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 
             4203, October 2005. 

 

8.2. Informative References 

   [INTER-AS-TE-REQ] Zhang and Vasseur, "MPLS Inter-AS Traffic 
             Engineering Requirements", RFC4216, November 2005. 

   [PD-PATH] Ayyangar, A., Vasseur, JP., and Zhang, R., "A Per-domain 
             path computation method for establishing Inter-domain", 
             draft-ietf-ccamp-inter-domain-pd-path-comp, (work in 
             progress). 

   [BRPC] JP. Vasseur, Ed., R. Zhang, N. Bitar, JL. Le Roux, "A Backward 
             Recursive PCE-based Computation (BRPC) procedure to compute 
             shortest inter-domain Traffic Engineering Label Switched 
             Paths ", draft-ietf-pce-brpc, (work in progress) 

   [PCE] Farrel, A., Vasseur, JP., and Ash, J., "A Path Computation 
             Element (PCE)-Based Architecture", RFC4655, August 2006. 

   [OSPF-TE-V3] Ishiguro K., Manral V., Davey A., and Lindem A. "Traffic 
             Engineering Extensions to OSPF version 3", draft-ietf-ospf-
             ospfv3-traffic, {work in progress}. 
 
 
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   [OSPFV3]   Coltun, R., Ferguson, D., and J. Moy, "OSPF for IPv6", RFC 
             2740, April 1998.  

 
   [L1VPN-OSPF-AD] Bryskin, I., and Berger, L., "OSPF Based L1VPN Auto-
             Discovery", draft-ietf-l1vpn-ospf-auto-discovery, (work in 
             progress). 

Authors' Addresses 

   Mach Chen 
   Huawei Technologies Co.,Ltd 
   KuiKe Building, No.9 Xinxi Rd., 
   Hai-Dian District  
   Beijing, 100085 
   P.R. China 
      
   Email: mach@huawei.com 
 

   Renhai Zhang 
   Huawei Technologies Co.,Ltd 
   KuiKe Building, No.9 Xinxi Rd., 
   Hai-Dian District  
   Beijing, 100085 
   P.R. China 
      
   Email: zhangrenhai@huawei.com 
 

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   http://www.ietf.org/ipr. 
 
 
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