Path Computation Element (PCE) Discovery using Domain Name System(DNS)

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Document Type Active Internet-Draft (individual)
Authors Qin Wu  , Dhruv Dhody 
Last updated 2013-07-15
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PCE Working Group                                                  Q. Wu
Internet-Draft                                                  D. Dhody
Intended status: Standards Track                                  Huawei
Expires: January 16, 2014                                  July 15, 2013

 Path Computation Element (PCE) Discovery using Domain Name System(DNS)


   Discovery of the Path Computation Element (PCE) within an IGP area or
   domain is possible using OSPF [RFC5088] and IS-IS [RFC5089].
   However, in some deployment scenarios PCEs may not wish, or be able,
   to participate within the IGP process,therefore it would be
   beneficial for the Path Computation Client (PCC) (or other PCEs) to
   discover PCEs via an alternative mechanism to those proposed in
   [RFC5088] and [RFC5089].

   This document specifies the requirements, use cases, procedures and
   extensions to support discovery via DNS for PCE.

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 January 16, 2014.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   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

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions used in this document  . . . . . . . . . . . . . .  4
   3.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Load Sharing of Path Computation Requests  . . . . . . . .  5
     3.2.  Network Address Translation Gateway  . . . . . . . . . . .  5
     3.3.  Multiple-Provider Domains  . . . . . . . . . . . . . . . .  5
     3.4.  Multiple PCE Servers . . . . . . . . . . . . . . . . . . .  6
     3.5.  End to End Path Computation  . . . . . . . . . . . . . . .  6
   4.  Discovering a Path Computation Element . . . . . . . . . . . .  7
     4.1.  Determining the PCE Service and transport protocol . . . .  8
     4.2.  Determining the IP Address of the PCE  . . . . . . . . . .  8
     4.3.  Determining path computation scope,the PCE domains and
           Neighbor PCE domains . . . . . . . . . . . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  Acknowledges . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   The Path Computation Element Communication Protocol (PCEP) is a
   transaction-based protocol carried over TCP[RFC4655].  In order to be
   able to direct path computation requests to the Path Computation
   Element (PCE), a Path Computation Client (PCC) (or other PCEs) needs
   to know the location and capability of a PCE.

   In a network where an IGP is used and where the PCE participates in
   the IGP, discovery mechanisms exist for PCC (or PCE) to learn the
   identity and capability of each PCE.  [RFC5088] defines a PCE
   Discovery (PCED) TLV carried in an OSPF Router LSA.  Similarly,
   [RFC5089] defines the PCED sub-TLV for use in PCE Discovery using
   IS-IS.  Scope of the advertisement is limited to IGP area/level or
   Autonomous System (AS).

   However in certain scenarios not all PCEs will participate in the IGP
   instance, section 3 (Motivation) outlines a number of use cases.  In
   these cases, current PCE Discovery mechanisms are therefore not
   appropriate and another PCE discovery function would be required.

1.1.  Requirements

   As described in [RFC4674], the PCE Discovery information should at
   least be composed of:

   o  The PCE location: an IPv4 and/or IPv6 address that is used to
      reach the PCE.  It is RECOMMENDED to use an address that is always
      reachable if there is any connectivity to the PCE;

   o  The PCE path computation scope (i.e., intra-layer, inter-area,
      inter-AS, or inter-layer);

   o  The set of one or more PCE-Domain(s) into which the PCE has
      visibility and for which the PCE can compute paths;

   o  The set of zero, one, or more neighbor PCE-Domain(s) toward which
      the PCE can compute paths;

   that allows PCCs to select appropriate PCEs:

   This document specifies an extension to DNS for the above PCE
   information discovery, which is complements the existing discovery

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2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC2119 [RFC2119].

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3.  Motivation

   This section discusses in more detail the motivation and use cases
   for an alternative DNS based PCE discovery mechanism.

3.1.  Load Sharing of Path Computation Requests

   Multiple PCE servers can be present in a single network domain for
   redundancy.  However load balance decision is made by PCC,it doesn't
   enable real load balance across the PCE servers if PCC still tries
   PCE one by one and PCE doesn't indicate the load status to the PCC.

   Inherent DNS based load balancing may be used for inbound load
   balancing and implemented at the application level in both servers
   and clients.  Multiple host IP addresses are configured in DNS for a
   single host server name.  Also DNS is query-response based mechanism
   and capable of automatically detecting and reacting to errors.  These
   allow you to provide load balancing across two separate Systems and
   facilitate PCE system failover and recovery.

   Comparing with advertisement based PCE discovery
   [RFC5088][RFC5089],it can mitigate flooding issue (see section 3.2 of
   [RFC5088])and avoid unwanted traffic and reduce a large amount of
   unnecessary advertisement, especially when PCE information needs
   frequent changes.

3.2.  Network Address Translation Gateway

   PCEP uses TCP as the transport [RFC5440].  To secure TCP connection
   that underly PCEP sessions, TLS can be used besides using TCP-MD5.
   When NAT gateway is in place, a TCP or TCP/TLS connection can be
   opened by ICE for the purpose of connectivity checks.  However the
   TCP connection cannot be established in cases where one of the agents
   is behind a NAT with connection-dependent filtering properties
   [RFC5382].  Therefore IGP discovery is limited within an IGP domain
   and cannot be used in this case.

3.3.  Multiple-Provider Domains

   Backward recursive path computation (BRPC) [RFC5441] MAY be used by
   cooperating PCEs to compute inter-domain path.  In which case these
   cooperating PCEs should known to other PCEs.  In case of inter-AS
   where the PCE do not participate in a common IGP, the existing IGP
   discovery mechanism cannot be used to discover inter-AS PCE.

   Also in the case of multiple ASes within different service provider
   networks, the H-PCE [RFC6805] architecture does not require
   disclosure of internals of a child domain to the parent PCE.  It may

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   be necessary for a third party to manage the parent PCEs according to
   commercial and policy agreements from each of the participating
   service providers.

   [RFC6805] specifies that a child PCE must be configured with the
   address of its parent PCE in order for it to interact with its parent
   PCE.  However handling changes in parent PCE identities and coping
   with failure events would be an issue for a configured system.

   There is no scope for parent PCEs to advertise their presence,
   however there is potential for directory systems (such as DNS
   [RFC4848] as used in the ALTO discovery function [I-D.ietf-alto-

3.4.  Multiple PCE Servers

   In some cases, each network domain may have multiple PCE server, only
   one main PCE sever is responsible for Establish topology database by
   participating in OSPF/ISIS routing protocol, the other PCE server
   gains knowledge of Topology information either from TED maintained by
   the main PCE server or some management system(e.g.,NMS/OSS).  In such
   cases, it is desirable to use DNS based mechanism to discover PCE.

3.5.  End to End Path Computation

   To compute end to end paths across domains, PCE has the following

   o  Within a single area, the PCE can not offers enhanced
      computational power for end to end path computation,e.g.,
      coordination of computation across the whole area.

   o  A single router participating in IGP area lacks visibility of
      complete topology with its own TED.

   Per domain path computation mechanism[RFC5152]can be used to compute
   end to end path, however it may lead to sub-optimal paths or result
   in no end to end path to be found when the PCE only has visibility
   into the IGP area it serves.  This issue can be resolved when one
   powerful PCE is responsible for multiple areas,i.e., PCE sits in one
   area it serves and also can get access to topology information
   provided by PCE server in other IGP area using BGP.  In such case, it
   will be desirable to use DNS based mechanism to discover those PCE
   that has visibility to multiple areas.

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4.  Discovering a Path Computation Element

   The Dynamic Delegation Discovery System (DDDS) [RFC3401] is used to
   implement lazy binding of strings to data, in order to support
   dynamically configured delegation systems.  The DDDS functions by
   mapping some unique string to data stored within a DDDS database by
   iteratively applying string transformation rules until a terminal
   condition is reached.  When DDDS uses DNS as a distributed database
   of rules, these rules are encoded using the Naming Authority Pointer
   (NAPTR) Resource Record (RR).  One of these rules is the First Well
   Known Rule, which says where the process starts.

   In current specifications, the First Well Known Rule in a DDDS
   application [RFC3403] is assumed to be fixed, i.e., the domain in the
   tree where the lookups are to be routed to, is known.  This document
   proposes the input to the First Well Known Rule to be dynamic, based
   on the search path the resolver discovers or is configured to use.

   The search path of the resolver can either be pre-configured, or
   discovered using DHCP.

   When the PCC or other PCEs needs to discover PCEs in the domain into
   which the PCEP speaker has visibility (e.g.,local domain), the input
   to the First Well Known Rule MUST be the domain the PCC knows, which
   is assumed to be pre- configured in the PCC or discovered using DHCP.

   When the PCC needs to discover PCE in the other domain (e.g., AS,
   Parent PCE in the parent domain)into which the PCC has no visibility,
   it SHOULD know the domain name of that domain and use DHCP to
   discover IP address of the PCE in that domain that provides path
   computation service along with some PCE location information useful
   to a PCC for PCE selection, and contact it directly.  In some
   instances, the discovery may result in a per protocol/application
   list of domain names that are then used as starting points for the
   subsequent NAPTR lookups.  If neither the IP address or PCE location
   information can be discovered with the above procedure, the PCC MAY
   request a domain search list, as described in [RFC3397] and
   [RFC3646], and use it as input to the DDDS application.

   When the PCC does not find valid domain names using the procedures
   above, it MUST stop the attempt to discover any PCE.

   The dynamic rule described above SHOULD NOT be used for discovering
   services other than Path computation services described in this
   document, unless stated otherwise by a future specification.

   The procedures defined here result in an IP address, PCE domain,
   neighboring PCE domain and PCE Computation Scope where the PCC can

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   contact the PCE that hosts the service the PCC is looking for.

4.1.  Determining the PCE Service and transport protocol

   The PCC should know the service identifier for the Path Computation
   Discovery service.  The service identifier for the Path Computation
   Discovery service is defined as "PCED", The PCE supporting "PCED"
   service MUST support only TCP as transport, as described in

   The services relevant for the task of transport protocol selection
   are those with NAPTR service fields with values "ID+M2X", where ID is
   the service identifier defined in the previous section, and X is a
   letter that corresponds to a transport protocol supported by the
   domain.  This specification only defines M2T for TCP.  This document
   also establishes an IANA registry for mappings of NAPTR service name
   to transport protocol.

   These NAPTR [RFC3403] records provide a mapping from a domain to the
   SRV [RFC2782] record for contacting a PCE with the specific transport
   protocol in the NAPTR services field.  The resource record MUST
   contain an empty regular expression and a replacement value, which
   indicates the domain name where the SRV record for that particular
   transport protocol can be found.  As per [RFC3403], the client
   discards any records whose services fields are not applicable.

   The PCC MUST discard any service fields that identify a resolution
   service whose value is not "M2T", for values of T that indicate TCP
   transport protocols supported by the client.  The NAPTR processing as
   described in RFC 3403 will result in the discovery of the most
   preferred transport protocol of the PCE that is supported by the
   client, as well as an SRV record for the PCE.

4.2.  Determining the IP Address of the PCE

   As an example, consider a client that wishes to find "PCED" service
   in the domain.  The client performs a NAPTR query for
   that domain, and the following NAPTR records are returned:

   Order Pref Flags  Service     Regexp       Replacement
    IN NAPTR  50   50   "s"  "PCED"    ""
    IN NAPTR  90   50   "s"  "PCED+M2T"    ""

   This indicates that the domain does have a PCE providing Path
   Computation services over TCP, in that order of preference.  Since
   the client only supports TCP, TCP will be used, targeted to a host

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   determined by an SRV lookup of  That lookup
   would return:

      ;;  Priority  Weight    Port        Target
   IN  SRV    0        1      XXXX
   IN  SRV    0        2      XXXX

   where XXXX represents the port number at which the service is

   Note that the regexp field in the NAPTR example above is empty.  The
   regexp field MUST NOT be used when discovering path computation
   services, as its usage can be complex and error prone.  Also, the
   discovery of the path computation service does not require the
   flexibility provided by this field over a static target present in
   the TARGET field.

   If the client is already configured with the information about which
   transport protocol is used for a path computation service in a
   particular domain, it can directly perform an SRV query for that
   specific transport using the service identifier of the path
   computation Service.  For example, if the client knows that it should
   be using TCP for path computation service, it can perform a SRV query

   Once the server providing the desired service and the transport
   protocol has been determined, the next step is to determine the IP

   According to the specification of SRV RRs in [RFC2782], the TARGET
   field is a fully qualified domain name (FQDN) that MUST have one or
   more address records; the FQDN must not be an alias, i.e., there MUST
   NOT be a CNAME or DNAME RR at this name.  Unless the SRV DNS query
   already has reported a sufficient number of these address records in
   the Additional Data section of the DNS response (as recommended by
   [RFC2782]), the PCC needs to perform A and/or AAAA record lookup(s)
   of the domain name, as appropriate.  The result will be a list of IP
   addresses, each of which can be contacted using the transport
   protocol determined previously.

4.3.  Determining path computation scope,the PCE domains and Neighbor
      PCE domains

   DNS servers MAY use DNS TXT record and add new RRsets to the
   additional information section that are relevant to the answer and
   have the same authenticity as the data (the IP Address of the PCE)in
   the answer section.  RRsets include path computation scope, the PCE
   domains and Neighbor PCE domains associated with the PCE. the PCC MAY

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   inspect those Additional Information section and be capable of
   handling responses from nameservers that never fill in the Additional
   Information part of a response.

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5.  IANA Considerations

   The usage of NAPTR records described here requires well-known values
   for the service fields for the transport supported by Path
   Computation Services.  The table of mappings from service field
   values to transport protocols is to be maintained by IANA.

   The registration in the RFC MUST include the following information:

   Service Field: The service field being registered.

   Protocol: The specific transport protocol associated with that
   service field.  This MUST include the name and acronym for the
   protocol, along with reference to a document that describes the
   transport protocol.

   Name and Contact Information: The name, address, email address,
   and telephone number for the person performing the registration.

   The following values have been placed into the registry:

   Service Fields                    Protocol
      PCED+M2T                        TCP

   New Service Fields are to be added via Standards Action as defined in

   IANA is also requested to register PCED as service name in the
   Protocol and Service Names registry.

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6.  Security Considerations

   It is believed that this proposed DNS extension introduces no new
   security considerations (i.e., A list of known threats to services
   using DNS) beyond those described in [RFC3833].  For most of those
   identified threats, the DNS Security Extensions [RFC4033] does
   provide protection.  It is therefore recommended to consider the
   usage of DNSSEC [RFC4033] and the aspects of DNSSEC Operational
   Practices [RFC4641] when deploying Path Computation Services.

   In deployments where DNSSEC usage is not feasible, measures should be
   taken to protect against forged DNS responses and cache poisoning as
   much as possible.  Efforts in this direction are documented in

   Where inputs to the procedure described in this document are fed via
   DHCP, DHCP vulnerabilities can also cause issues.  For instance, the
   inability to authenticate DHCP discovery results may lead to the Path
   Computation service results also being incorrect, even if the DNS
   process was secured.

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7.  Acknowledges

   The author would like to thank Claire Bi,Ning Kong and Liang Xia for
   their review and comments that help improvement 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", March 1997.

   [RFC2782]  Gulbrandsen, A., "A DNS RR for specifying the location of
              services (DNS SRV)", RFC 2782, February 2000.

   [RFC3397]  Aboba, B., "Dynamic Host Configuration Protocol (DHCP)
              Domain Search Option", RFC 3397, November 2002.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC4033]  Arends, R., "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4641]  Kolkman, O., "DNSSEC Operational Practices", RFC 4641,
              September 2006.

   [RFC4674]  Droms, R., "Requirements for Path Computation Element
              (PCE) Discovery", RFC 4674, December 2003.

   [RFC4848]  Daigle, D., "Domain-Based Application Service Location
              Using URIs and the Dynamic Delegation Discovery Service
              (DDDS)", RFC 4848, April 2007.

   [RFC5226]  Narten, T., "Guidelines for Writing an IANA Considerations
              Section in RFCs", RFC 5226, May 2008.

   [RFC5440]  Le Roux, JL., "Path Computation Element (PCE)
              Communication Protocol (PCEP)", RFC 5440, April 2007.

   [RFC6805]  King, D. and A. Farrel, "The Application of the Path
              Computation Element Architecture to the Determination of a
              Sequence of Domains in MPLS and GMPLS", RFC 6805,
              November 2012.

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8.2.  Informative References

   [ALTO]     Kiesel, S., "ALTO Server Discovery",
              ID draft-ietf-alto-server-discovery-08, March 2013.

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401, October 2002.

   [RFC3833]  Atkins, D., "Threat Analysis of the Domain Name System
              (DNS)", RFC 3833, August 2004.

   [RFC5088]  Le Roux, JL., "OSPF Protocol Extensions for Path
              Computation Element (PCE) Discovery", RFC 5088,
              January 2008.

   [RFC5089]  Le Roux, JL., "IS-IS Protocol Extensions for Path
              Computation Element (PCE) Discovery", RFC 5089,
              January 2008.

   [RFC5382]  Guha, S., "NAT Behavioral Requirements for TCP", RFC 5382,
              October 2008.

   [RFC5452]  Hubert, A., "Measures for Making DNS More Resilient
              against Forged Answers", RFC 5452, January 2009.

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Authors' Addresses

   Qin Wu
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012


   Dhruv Dhody
   Leela Palace
   Bangalore, Karnataka  560008


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