PCP Server Selection
draft-ietf-pcp-server-selection-01

Abstract

This document specifies the behavior to be followed by the PCP client to contact its PCP server(s) when one or several PCP server names are configured. Multiple names may be configured to a PCP client in some deployment contexts such as multi-homing.

This document updates RFC6887.

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.

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/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on November 23, 2013.

Copyright Notice

Copyright (c) 2013 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 (http://trustee.ietf.org/license-info) 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

• 1. Introduction
• 2. Terminology
• 3. Name Resolution
• 4. IP Address Selection
• 4.1. Serial Queries
• 5. Examples
• 5.1. Example 1
• 5.2. Example 2
• 5.3. Example 3
• 6. Security Considerations
• 7. IANA Considerations
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Multi-homing
• A.1. IPv6 Multi-homing
• A.2. IPv4 Multi-homing
• A.3. Multiple interfaces and Servers
• Authors' Addresses

1. Introduction

This document specifies the behavior to be followed by the PCP client [RFC6887] to contact its PCP server(s) [RFC6887] when receiving one or several PCP server names (e.g., using DHCP [I-D.ietf-pcp-dhcp]). This document is not specific to DHCP; any other mechanism can be used to configure PCP server names.

Multiple names may be configured to a PCP client in some deployment contexts such as multi-homing (see Appendix A). It is out of scope of this document to enumerate all deployment scenarios which require multiple names to be configured.

This document assumes appropriate name resolution means (e.g., Section 6.1.1 of [RFC1123]) are available on the host client.

2. Terminology

This document makes use of the following terms:

• PCP server denotes a functional element which receives and processes PCP requests from a PCP client. A PCP server can be co-located with or be separated from the function (e.g., Network Address Translation (NAT), firewall) it controls. Refer to [RFC6887].
• PCP client denotes a PCP software instance responsible for issuing PCP requests to a PCP server. Refer to [RFC6887].
• Name is a string that can be passed to getaddrinfo (Section 6.1 of [RFC3493]), such as a DNS name, address literals, etc. A name may be a fully qualified domain name (e.g., "myservice.example.com."), IPv4 address in dotted-decimal form (e.g., 192.0.2.33) or textual representation of an IPv6 address (e.g., 2001:db8::1).

3. Name Resolution

Each configured name is passed to the name resolution library (e.g., Section 6.1.1 of [RFC1123] or [RFC6055]) to retrieve the corresponding IP address(es) (IPv4 or IPv6). Then, the PCP client follows the procedure specified in Section 4 to contact its PCP server(s).

A host may have multiple network interfaces (e.g, 3G, IEEE 802.11, etc.); each configured differently. Each PCP server learned MUST be associated with the interface via which it was learned.

4. IP Address Selection

This section specifies the behavior to be followed by the PCP client to contact its PCP server(s) when receiving one or several PCP server names:

1. If only one PCP server name is configured: if a list of IP addresses is returned as a result of resolving the PCP server name, the PCP client follows the procedure specified in Section 4.1.
2. If several PCP server names are configured: each name is treated as a separate PCP server. Moreover, each name may be resolved into one IP address or a list of IP addresses. The PCP client contacts in parallel the first IP address of each name and follows the procedure specified in Section 4.1 for the list of IP addresses returned for each name. Section 5 provides some examples to illustrate this procedure.

This procedure does not require any knowledge of the capabilities of the PCP-controlled device(s). Instead, the PCP client assumes each configured name refer to a separate PCP server.

This procedure may result in a PCP client instantiating multiple mappings maintained by distinct PCP servers. The decision to use all these mappings or delete some of them is deployment-specific. Only the PCP client can decide whether all the mappings are needed or only a subset of them.

4.1. Serial Queries

The PCP client initializes its Maximum Retransmission Count (MRC) to 4.

The PCP client sends its PCP message to the PCP server following the retransmission procedure specified in Section 8.1.1 of [RFC6887]. If no response is received after MRC attempts, the PCP client tries with the next IP address in its list of PCP server addresses. If it has exhausted its list, the procedure is repeated every fifteen minutes until the PCP request is successfully answered. If, when sending PCP requests the PCP client receives an ICMP error (e.g., port unreachable, network unreachable) it SHOULD immediately try the next IP address in the list. Once the PCP client has successfully received a response from a PCP server address on that interface, it sends subsequent PCP requests to that same server address until that PCP server becomes non-responsive, which causes the PCP client to attempt to re-iterate the procedure starting with the first PCP server address on its list.

5. Examples

The following sub-sections provide three examples to illustrate the procedure.

For all these examples, let's suppose pcpserver-x, pcpserver-y and pcpserver-z are configured as PCP server names.

5.1. Example 1

Let's also suppose:

* IPx1 and IPx2 are returned for pcpserver-x; IPx1 is not reachable.
* IPy1 and IPy2 are returned for pcpserver-y; IPy1 is reachable
* IPz1 and IPz2 are returned for pcpserver-z; IPz1 is reachable

The procedure to contact the PCP servers is as follows:

* Send PCP requests to all servers: IPx1, IPy1 and IPz1
* Responses are received from IPy1 and IPz1 but not from IPx1
  - The request is re-sent to IPx1
  - If no response is received after four attempts, the request 
    is sent to IPx2

5.2. Example 2

Now, if the following conditions are made:

* IPx1 and IPx2 are returned for pcpserver-x; IPx1 is not reachable.
* IPy1 and IPy2 are returned for pcpserver-y; IPy1 is reachable
* IPz1 and IPz2 are returned for pcpserver-z; IPz1 is not reachable

The procedure to contact the PCP servers lead to the following:

* Send PCP requests to all servers: IPx1, IPy1 and IPz1
* A response is received from IPy1 but not from IPx1 and IPz1
  - the requests are re-sent to IPx1 and IPz1
  - If no response is received after four attempts, the request 
    is then sent to IPx2 and IPz2

5.3. Example 3

Let's suppose now that:

* IPx1 and IPx2 are returned for pcpserver-x; IPx1 is not reachable.
* IPy1 and IPy2 are returned for pcpserver-y; IPy1 is not reachable
* IPz1 and IPz2 are returned for pcpserver-z; IPz1 is not reachable

The procedure to contact the PCP servers is as follows:

* Send PCP requests to all servers: IPx1, IPy1 and IPz1
* No answer is received for all requests
  - the requests are re-sent to IPx1, IPy1 and IPz1
  - If no response is received after four attempts, the request
    is then sent to IPx2, IPy2 and IPz2

6. Security Considerations

PCP-related security considerations are discussed in [RFC6887].

This document does not specify how PCP server names are provisioned to the PCP client. It is the responsibility of PCP server provisioning document(s) to elaborate on the security considerations to discover a legitimate PCP server.

7. IANA Considerations

This document does not request any action from IANA.

8. Acknowledgements

Many thanks to D. Thaler for the review and comments.

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Multi-homing

The main problem of a PCP multi-homing situation can be succinctly described as 'one PCP client, multiple PCP servers'. As described in Section 4, if a PCP client discovers multiple PCP server names, it should send requests to all of them in parallel with the following assumptions:

• There is no requirement that multiple PCP servers have the same capabilities.
• PCP requests to different servers are independent, the result of a PCP request to one server does not influence another.
• If PCP servers provide NAT, it is out of scope how the client manages ports across PCP servers. For example, whether PCP client requires all external ports to be the same or whether there are ports available at all.

The following sub-sections describe multi-homing examples to illustrate PCP client behavior.

A.1. IPv6 Multi-homing

In this example of an IPv6 multi-homed network, two or more routers co-located with firewalls are present on a single link shared with the host(s). Each router is in turn connected to a different service provider network and the host in this environment would be offered multiple prefixes and advertised multiple DNS/NTP servers. Consider a scenario in which firewalls within an IPv6 multi-homing environment also implement a PCP server. PCP client learns of the available PCP servers using DHCP [I-D.ietf-pcp-dhcp] or any other PCP server discovery technique defined in future specifications. The PCP client will send PCP requests in parallel to each of the PCP servers.

                       ==================
                       |    Internet    |
                       ==================
                          |          |
                          |          | 
                     +----+-+      +-+----+
                     | ISP1 |      | ISP2 |               
                     +----+-+      +-+----+      ISP Network
                          |          |
    .........................................................
                          |          | 
                          |          |        Subscriber Network
                  +-------+---+ +----+------+
                  | rtr1 with | | rtr2 with | 
                  |   FW1     | |    FW2    |
                  +-------+---+ +----+------+
                          |          |
                          |          | 
                          |          |
                   -----+-+-----+------
                          |
                        +-+-----+ 
                        | Hosts | 
                        +-------+

                     ====================
                     |    Internet       |
                     =====================
                        |              |
                        |              | 
                   +----+--------+   +-+------------+
                   | ISP1        |   | ISP2         |
                   |             |   |              |
                   +----+--------+   +-+------------+ ISP Network  
                        |              |               
                        |              |        
      ..............................................................
                        |              |
                        | Port1        | Port2    Subscriber Network
                        |              |                 
                   +----+-----------------+            
                   |   NAT & PCP servers  |         
                   |       GW Router      |             
                   +----+-----------------+             
                        |                                
                        |    
                        |
                   -----+-+-----+------
                        |
                      +-+-----+ 
                      | Hosts |  (private address space)
                      +-------+