PCE Working Group Q. Wu
Internet-Draft D. Dhody
Intended status: Standards Track Huawei
Expires: February 13, 2014 D. King
Old Dog Consulting
D. Lopez
Telefonica I+D
August 12, 2013

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


Discovery of the Path Computation Element (PCE) within an IGP area or routing 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.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 13, 2014.

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Table of Contents

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.

This document describes PCE discovery via DNS. The mechanism with which DNS comes to know about the PCE and its capability is out of scope of this document.

1.1. Terminology

The following terminology is used in this document.

As per [RFC4655], any collection of network elements within a common sphere of address management or path computational responsibility. Examples of domains include Interior Gateway Protocol (IGP) areas and Autonomous Systems (ASs).

1.2. Requirements

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

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 mechanism.

2. 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 RFC2119 [RFC2119].

3. Motivation

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

3.1. Outside the Routing Domain

When the PCE is a router participating in the Interior Gateway Protocol (IGP), or even a server participating passively in the IGP, with all PCEP speakers in the same routing domain, a simple and efficient way to announce PCEs consists of using IGP flooding.

But the existing mechanism does not work in following situations:

Per domain path computation mechanism [RFC5152] or Backward recursive path computation (BRPC) [RFC5441] MAY be used by cooperating PCEs to compute inter-domain path. In which case these cooperating PCEs should be known to other PCEs. In case of inter-AS where the PCEs do not participate in a common IGP, the existing IGP discovery mechanism cannot be used to discover inter-AS PCE.
Hierarchy of PCE:
The H-PCE [RFC6805] architecture does not require disclosure of internals of a child domain to the parent PCE. It may be necessary for a third party to manage the parent PCEs according to commercial and policy agreements from each of the participating service providers [PCE-QUESTION]. [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 to child PCEs when they are not a part of the same routing domain.
[I.D.draft-ietf-idr-ls-distribution] 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. An external PCE MAY use this mechanism to populate its TED and not take part in the same IGP routing domain.
PCE server MAY gain the knowledge of Topology information from some management system (e.g.,NMS/OSS) and not take part in the same routing domain. Also note that in some case PCC may not be a router and instead be a management system like NMS and may not be able to discover PCE via IGP discovery.

3.2. Query-Response v/s Advertisement

Advertisement based IGP PCE discovery [RFC5088] and [RFC5089] floods the PCE information to an area, a subset of areas or to a full routing domain. By the very nature of flooding and advertisements it generates unwanted traffic and may lead to unnecessary advertisement, especially when PCE information needs frequent changes.

DNS is a query-response based mechanism, a client (say PCC) can use DNS to discover a PCE only when it needs it and does not require any other node in the network to be involved.

Incase of Intermittent PCEP session, where PCEP sessions are systematically open and closed for each PCEP request, a DNS based query-response mechanism is more suitable. One may utilize DNS based load-balancing and recovery functions.

3.3. Network Address Translation Gateway

PCEP uses TCP as the transport [RFC5440]. To secure TCP connection that underlay PCEP sessions, TLS can be used besides using TCP-MD5 [RFC2385] and TCP-AUTH [RFC5295]. When PCC and PCE support TCP-MD5 or TCP-AUTH while NAT does not, TCP connection establishment fails. When NAT gateway is in presence, a TCP or TCP/TLS connection can be opened by Interactive Connectivity Establishment (ICE) [RFC5245] for the purpose of connectivity checks. However the TCP connection cannot be established in cases where one of the peers 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.

4. Other Considerations

4.1. Load Sharing of Path Computation Requests

Multiple PCE servers can be present in a single network domain for redundancy. DNS supports inherent load balancing where multiple PCEs (with different IP addresses) are known in DNS for a single PCE server name and are hidden from the PCC.

In an IGP advertisement based PCE discovery, one learns of all the PCEs and it is the job of the PCC to do load-balancing.

A DNS based load-balancing mechanism works well in case of Intermittent PCEP sessions and request are load-balanced among PCEs similar to HTTP request without any complexity at the client.

5. 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 nor other 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 contact the PCE that hosts the service the PCC is looking for.

5.1. 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 [RFC5440].

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.

5.2. Determining the IP Address of the PCE

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

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

   ;;  Priority  Weight    Port        Target
IN  SRV    0        1      XXXX   server1.example.com
IN  SRV    0        2      XXXX   server2.example.com

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 determined by an SRV lookup of _PCED._tcp.example.com. That lookup would return:

where XXXX represents the port number at which the service is reachable.

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 directly for_PCED._tcp.example.com.

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

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.

5.3. Determining path computation scope,capability,the PCE domains and Neighbor PCE domains

DNS servers MAY use DNS TXT record to give additional information about PCE service and add TXT record to the additional information section that are relevant to the answer and have the same authenticity as the data (Generally this will be made up of A and SRV records)in the answer section. The additional information includes path computation scope, capability, the PCE domains and Neighbor PCE domains associated with the PCE. the PCC MAY inspect those Additional Information section and be capable of handling responses from nameservers that never fill in the Additional Information part of a response.

5.4. Relationship between PCE-Domain and DNS Domain-Name

As per [RFC4655], PCE-Domain is a collection of network elements within a common sphere of address management or path computational responsibility. Examples of PCE-domains include Interior Gateway Protocol (IGP) areas and Autonomous Systems (ASs). The DNS domain-name should have a mechanism to link the IGP area or AS to the DNS namespace.

Editors Note - To be discussed further

6. 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.

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 registration in the RFC MUST include the following information:

Service Fields                    Protocol
   PCED+M2T                        TCP

The following values have been placed into the registry:

New Service Fields are to be added via Standards Action as defined in [RFC5226].

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

7. 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 [RFC6781] 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 [RFC5452].

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.

8. Acknowledges

The author would like to thank Claire Bi,Ning Kong, Liang Xia, Stephane Bortzmeyer,Yi Yang, Ted Lemon and Adrian Farrel for their review and comments that help improvement to this document.

9. References

9.1. 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.
[RFC5226] Narten, T., "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 5226, May 2008.
[RFC4655] Farrel, A., Vasseur, J.P. and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, August 2006.
[RFC4674] Droms, R., "Requirements for Path Computation Element (PCE) Discovery", RFC 4674, December 2003.
[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.
[RFC4848] Daigle, D., "Domain-Based Application Service Location Using URIs and the Dynamic Delegation Discovery Service (DDDS)", RFC 4848, April 2007.
[RFC5440] Le Roux, JL., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, April 2007.
[RFC6781] Kolkman, O., Mekking, W. and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, December 2012.
[RFC4033] Arends, R., "DNS Security Introduction and Requirements", RFC 4033, March 2005.

9.2. Informative References

[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.
[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.
[RFC5452] Hubert, A., "Measures for Making DNS More Resilient against Forged Answers", RFC 5452, January 2009.
[RFC5382] Guha, S., "NAT Behavioral Requirements for TCP", RFC 5382, October 2008.
[RFC5295] Touch, J., "The TCP Authentication Option", RFC 5295, June 2010.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.
[ALTO] Kiesel, S., "ALTO Server Discovery", ID draft-ietf-alto-server-discovery-08, March 2013.
[BGP-LS] Gredler, H., "North-Bound Distribution of Link-State and TE Information using BGP", ID draft-ietf-idr-ls-distribution-03, May 2013.
[PCE-QUESTION] Farrel, A., "Unanswered Questions in the Path Computation Element Architecture", ID http://tools.ietf.org/html/draft-ietf-pce-questions-00, July 2013.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010.

Authors' Addresses

Qin Wu Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu 210012 China EMail: sunseawq@huawei.com
Dhruv Dhody Huawei Leela Palace Bangalore, Karnataka 560008 INDIA EMail: dhruv.dhody@huawei.com
Daniel King Old Dog Consulting UK EMail: daniel@olddog.co.uk
Diego R. Lopez Telefonica I+D EMail: diego@tid.es