| Internet Engineering Task Force | K. Fleischhauer, Ed. |
| Internet-Draft | O. Bonneß |
| Intended status: Informational | Deutsche Telekom AG |
| Expires: August 29, 2012 | February 28, 2012 |
draft-fleischhauer-ipv4-addr-saving-02
On demand IPv4 address provisioning in Dual-Stack PPP deployment scenarios
Today the Dual-Stack approach is the most straightforward and the most common way for introducing IPv6 into existing systems and networks. However a typical drawback of implementing Dual-Stack is that each node will still require at least one IPv4 address. Hence, solely deploying Dual-Stack does not provide a sufficient solution to the IPv4 address exhaustion problem. Assuming a situation where most of the IP communication (e.g. always-on, VoIP etc.) can be provided via IPv6, the usage of public IPv4 addresses can significantly be reduced and the unused public IPv4 addresses can under certain circumstances be returned to the public IPv4 address pool of the service provider. New Dual-Stack enabled services can be introduced without increasing the public IPv4 address demand, when IPv6 will be the preferred network layer protocol. This document describes such a solution in a Dual-Stack PPP session network scenario and explains the protocol mechanisms which are used.
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The Dual-Stack approach as defined in [RFC4213] provides the most straightforward and most common way for introducing IPv6 [RFC2460] into existing systems and networks. However a typical drawback of the Dual-Stack approach is that each network node will still require at least one IPv4 [RFC0791] address. Assuming a situation where most of the IP communication (e.g. always-on, VoIP etc.) can be provided via IPv6, the usage of public IPv4 addresses can be reduced significantly and the unused public IPv4 addresses may be returned to the public IPv4 address pool of the provider. This document describes how such a solution can be realised in a Dual-Stack PPP session scenario and details the protocol mechanisms of the solution which are also thought as contribution to [BBF-WT-242].
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]
The Broadband Forum describes in [BBF-TR-187] a target IPv4/IPv6 Dual-Stack Architecture (assuming native implementation of IPv6). TR-187 builds on the capabilities of existing protocols such as Point-to-Point Protocol (PPP) [RFC1332] and Layer 2 Tunnelling Protocol (L2TP) [RFC2661] to provide IPv6 service in addition to today's IPv4 service. These protocols allow the parallel usage of IPv4 and IPv6 within a single PPP respectively L2TP session. Usually in such a scenario the service provider offers to the customer the usage of public IPv4 and IPv6 address resources during the duration of the PPP session. Because of the potential parallel usage of IPv4 and IPv6 within such a Dual-Stack PPP scenario a public IPv4 address is always provisioned, also in the case where most of the communication is running on top of IPv6. This document extends the sketched Dual-Stack deployment scenario for PPP and L2TPv2 with a mechanism that allows a temporary assignment and a release of an unused IPv4 address within such a Dual-Stack capable PPP session scenario. This approach covers also situations where the IPv4 address may only be provided on-demand later on, after initiating the Dual-Stack PPP session with an IPv6 context only.
Basically the here described mechanism is also applicable in cable and mobile networks. The corresponding DOCSIS and 3GPP standards may be adapted as a follow-on work to this draft later on.
In order to illustrate the applicability and usefulness of the proposed “On demand IPv4 address provisioning” mechanism an illustrative network provider use case should be sketched here. Lets assume a network access and service provider which is offering Dual-Stack services via a single PPP connection to its customers. Independently of the concrete subscribed customer service (Single, Play, Double Play etc.) all customers should be produced and provisioned in the same way in order to keep the network operation costs and the network complexity as low as possible. Lets assume furthermore that the above mentioned network access and service provider has already migrated its VoIP service to IPv6, so that all Single play (VoIP) customers do only need IPv6 connectivity and have no need for an IPv4 context within their Dual-Stack PPP session. However, the standard Dual-Stack PPP connection set-up today initiates an IPCP and as well as an IPCPv6 negotiation independently of the concrete need for IPv4 and/or IPv6 connectivity, so that after a successful Dual-Stack PPP connection establishment the PPP client site is provisioned with a complete set of IPv6 and IPv4 connection parameters. As a consequence in our example, the whole Single Play customer base of the network access and service provider has also been provisioned with public IPv4 addresses, although these customers will never need IPv4 Internet connectivity during the whole lifetime of their PPP session. Hence a huge amount of unneeded and potentially unused IPv4 addresses has been wasted, that should be better kept in the provider address pool and delegated to other customers that really need IPv4 connectivity. In order to allow a more dynamic and on-demand provisioning of IPv4 parameters within Dual-Stack PPP sessions, a new mechanism is needed, that requests and also releases IPv4 addresses on-demand when they are really needed during the PPP session lifetime. Such a mechanism is proposed and described within this document.
(An additional advantage of such an on-demand IPv4 address releasing and provisioning mechanism consists in the fact that a very easy to operate and dynamic change customer profiles (e.g. upgrade of Single Play customers to Double Play services and vice versa) becomes possible with only minor changes to the customer service profile in the AAA platform of the service provider - no changes in he CPE or NAS port configuration are needed. Besides that, this dynamic on-demand IPv4 address provisioning and releasing approach allows it to share one public IPv4 address timely sequential between a bunch of customers.)
The following sections will describe the basic network architecture and the “On demand IPv4 address provisioning” mechanisms in more detail.
Assuming a Dual-Stack network access via PPP, end devices can in general communicate via IPv4 and/or IPv6 transport, depending on their own and their IP communication partners capabilities. The actual usage of IPv4 or IPv6 or both protocols depends on the capabilities of
The last two points are mainly in responsibility of the network and service provider. The approach, sketched in this document, is based on the assumption that the customer starts a Dual-Stack PPP session in "IPv6-only" mode and "adds" IPv4 later on only in the case that applications or services explicitly request IPv4 connectivity. When IPv4 connectivity is not needed during the whole time of PPP network connectivity than a continuous provisioning of a global IPv4 address to the customer device (e.g. end system, CPE etc.) is not necessary. Therefore mechanisms are needed to provision and release public IPv4 addresses for Dual-Stack PPP sessions dynamically and on-demand.
The goal of the solution sketched in this document, is to limit and decrease the public IPv4 address pool size of the PPP network access provider. Assuming that always-on services are reachable via IPv6, a Dual-Stack capable PPP connected device should per default request IPv4 address parameters only on demand, when the need for establishing IPv4 connectivity has explicitly been detected and IPv4 traffic towards the PPP WAN interface (e.g. of a CPE) is intended. As already described above it is sufficient, when initially only IPv6 address parameters are provisioned to the PPP customer endpoint (e.g. end systems, Home Gateway, CPE).
This means as a consequence that a customer device does not initially start a complete Dual-Stack PPP session but an "IPv6 only" PPP session. The IPv4 part of the complete Dual-Stack is initiated later on only in the case that IPv4 connectivity is explicitly requested.
The figure below illustrates the network architecture of a PPP Dual-Stack environment for providing Internet access to residential customers.
| +------------+ |
| | external | |
| | Address | |
| | Pool | |
| | Management | |
| +------------+ |
| service | provider |
| AAA | area |
+-------+ +--------------|--------------+
|Private|__ |
| Host |_ \IPv4 |
| 1 | \ \ +-------------+ |
+-------+ \ \ | CPE | IPv4 |
\ \ | App | over |
IPv6\ \_+------+------+ PPP +---------+ IPv4 +--------+
\__| CPE | CPE |--------| NAS |--------| Public |
__|Router| PPP | | (PPP | | Host |
/ _| | Peer |--------| Peer) |--------| n |
IPv6/ / +------+------+ IPv6 +---------+ IPv6 +--------+
/ / over
+-------+ / / PPP
|Private|_/ /
| Host |__/IPv4
| n |
+-------+
\_______________________/\__________________________________________/
Private Internet Public Internet
This abstract network topology consists of 3 major components:
[RFC1918] is the local area network on customer site wherein IPv4 and IPv6 communication are used. From this document point of view the Private Internet is an optional part of the PPP Dual-Stack architecture. Within this Private Internet (Customer LAN) exist one or more private hosts which are connected to the local area network interfaces of the Customer Premise Equipment. These hosts can have IPv4-only, IPv6-only or Dual-Stack communication capabilities. For an IPv6 communication inside the Customer LAN Link-local, Unique Local or/and public IPv6 addresses may be used. For an IPv6 communication to the public Internet public IPv6 addresses have to be used. For an IPv4 communication within the Customer LAN and to the public Internet it is assumed that private IPv4 addresses are used by the private hosts. In order to ensure an IPv4 communication between the private hosts of the Customer LAN and the public Internet the CPE must hence provide IPv4 Network Address Translation (NAT44) functionality and has to map the private IPv4 addresses of the private hosts to a public IPv4 address of the WAN interface of CPE NAT device and vice versa. The NAT functionality on the CPE is the border element between the private IPv4 Internet (aka. Customer LAN) and the public IPv4 Internet. In the case of using public IPv6 addresses for the communication between Customer LAN hosts and the Internet the CPE and its interfaces are an integral part of the IPv6 End-to-End public Internet architecture.
The Private Internet as defined in
To the public Internet belong the Access Network of the service provider and all other networks outside the customer LAN that are addressed with global IPv4 and / or IPv6 addresses and can be accessed from the private Internet as well as from other systems within the public Internet.
The focus of this draft is mainly directed to the access network of the service provider where in our scenario PPP is used between the CPE and the provider Network Access Server (NAS) in order to provide public Internet access to the customer.
The Service Provider AAA area is a network which consists of several systems to control the Network Access Server (NAS) and to provide AAA functionalities in order to reduce the load on the NAS. Such Service Provider AAA functionalities include also management of the public IPv4 and public IPv6 address pools inside the NAS and can hence also be integrated directly into the NAS.
The dynamic IPv4 address assigning approach, sketched in this document, is based on the assumption that the customer CPE initiates a PPP session based on IPv6 and adds IPv4 later on only if certain IPv4 applications or services require explicit IPv4 connectivity. A particular public IPv4 address can therefore be assigned sequentially to different customers for the lifetime of their IPv4 PPP connection and has not to be bound to a single customer for the whole lifetime of the Dual-Stack PPP session. This sequential assignment of public IPv4 addresses enables a smoother IPv4-to-IPv6 migration in comparison to other IPv4-to-IPv6 migration approaches that are based on Carrier Grade NATs in service provider network or shared IPv4 addresses. The customer will be provisioned with a public IPv4 address only in the case when global IPv4 connectivity is really needed and will not be provisioned with an IPv4 address per default when the Dual-Stack PPP session is initiated. Furthermore, a provisioned IPv4 address can be released (e.g. after a certain time interval) in the case that the CPE detects that there is no need any more for global IPv4 connectivity. In other words, when global IPv4 connectivity is not needed during the whole time of the Dual-Stack PPP session then a (continuous) provisioning of a public IPv4 address to the CPE is not necessary and the provisioning of a public IPv4 address can be done on-demand and dynamically.
Hence, one of the main achievement of this mechanism is to limit and decrease the pool size for public IPv4 addresses at the service provider site.
A similar effect in limiting and decreasing the IPv4 address demand could also be reached by using separate PPP sessions for IPv4 and IPv6. But in that case the following problems occur:
Because of these reasons the introduction of an additional PPP session for IPv6 as additional network layer protocol on a access line with an additional PPP session is not recommended.
As defined in RFC 2661 [RFC2661] PPP and L2TP provide the following main functionalities:
RFC 1332 [RFC1332] and Internet Protocol (Version 6) Control Protocol (IPV6CP) RFC 2472 [RFC2472] are used. Whereas IPCP is responsible for configuring, enabling, and disabling the IPv4 protocol modules on both ends of the point-to-point link, IPV6CP is responsible for configuring, enabling, and disabling the IPv6 protocol modules on both ends of the point-to-point link. Once one of both network-layer protocols has been configured, datagrams belonging to this network-layer protocol can be sent over the PPP link. Both NCP protocol mechanisms act independent of each other (see also requirement WLL-3 in [RFC6204]) and can be used to establish and pull-down IPv4 and IPv6 connection contexts within a Dual-Stack PPP session independently.
For provisioning of IPv4 or IPv6 communication parameters (e.g. addresses, DNS resolver) as network-layer protocols only the NCPs Internet Protocol (Version 4) Control Protocol (IPCP)
As an example an implementation that wishes to close a dedicated NCP connection (e.g. IPCP or IPv6CP) SHOULD transmit a Terminate-Request to the peer. Upon reception of a Terminate-Request, a Terminate-Ack MUST be transmitted to the sender of the Terminate-Request. The PPP session itself and the other NCP connection inside the PPP session will remain existent. Only in the case that both NCP connections are closed, the Dual-Stack PPP session will be terminated.
This chapter identifies the network elements that are involved in the message flows to enable the on-demand IPv4 address provisioning functionality and describes their functionalities related to this mechanism.
[RFC6204]. In the case of IPv4 an additional Network Address Translation (NAT) functionality is implemented on the router part. So within the Local Area Network private IPv4 addresses can be used as defined in [RFC1918]. Therefore the demand for global IPv4 connectivity of such a Customer Edge Router will be triggered either by own applications or by receiving IPv4 packets on its customer network facing interfaces that are addressed to the public Internet.
Within the context of this document the CER/End System is any device implementing a Dual-Stack PPP stack and acting as a PPP client to the PPP server in the service provider network in order to achieve connectivity to the service provider network. In the case of a Customer Edge Router (CER) this is a node (e.g. intended for home or small office usage) which forwards IPv4 and IPv6 packets those are not explicitly addressed to itself between the Local Area Network and WAN interface. The CER itself is separated into three devices or functional blocks, one that carries the PPP session (e.g. a standalone DSL modem), one that is operating simply as a local router which includes the NAPT44 function and any IPV6 PD/ND, DHCPv6 and DHCP for both stacks and one which includes the local CPE functionalities (e.g. DNS forwarder/cache, VoIP Clients). The PPP interface of this device is also called WAN (Wide Area Network) interface
In the case of an End System, this system is a node that intends to send IPv4 and/or IPv6 packets. On an End System the IPv4 connectivity demand can only be triggered by own applications. However, in both cases (CER or End system) the IPv4_idle_timer resides on the WAN interface in order to detect IPv4 packets passing the WAN interface (incoming/ outgoing) and to measure the related IPv4 idle time when no IPv4 packet has been sent or received.
The Network Access Server (NAS) is a device providing local Dual- Stack PPP connectivity to the Service Provider network and acting as a PPP server to the PPP client on the Customer Edge Router or customer end system. Within a RFC 2661 architecture the PPP server within the service provider network is the L2TP Network Server (LNS). The address pool management can be provided locally on the NAS/LNS or remotely. In the case of a local address pool management no information exchange to an external address pool management system is needed in order to assign or release IPv4 addresses. In the case of an external address pool management an information exchange between the NAS/LNS and the address pool management system is required.
[RFC2865] or DIAMETER as specified in [RFC3588] can be used as protocol between NAS/LNS and the External Address Pool Management System.
External Address Pool Management is used in the case when no local Address Pool Management system is implemented in the NAS/LNS. In this case it is necessary that the NAS/LNS communicates with an External Address Pool Management System for signalling assignment or release of IPv4 addresses. RADIUS as specified in
A PPP client implementation wishing to open a connection MUST transmit a NCP Configure-Request to the PPP server. If every Configuration Option received in a NCP Configure-Request is recognizable and all values are acceptable, then the PPP server implementation MUST transmit a NCP Configure-Ack to the initiator of the NCP Configure-Request.
Applied to the above sketched Dual-Stack PPP session use case the configuration and enabling of the IPv6 protocol module will be done immediately after a successful LCP data link configuration (and maybe successful authentication) of the PPP session. Assuming that this IPCPv6 configuration exchange finished successfully the PPP session is now established and operational containing an IPv6-only network layer connection.
Separately from that, the IPv4 protocol module can (later on and dynamically on-demand) be configured and enabled using IPCP. However this SHALL only be done in the case that an IPv4 connectivity demand has been detected on the PPP customer end system or CPE (PPP client). Therefore the NAS MUST not initiate the negotiation of IPCP.
The following diagram illustrates the appropriate IPCP (and accounting) message exchange that is needed to configure the IPv4 protocol modules of an existing (Dual-Stack) PPP session on-demand.
CPE/End System NAS ext. Address
(PPP Peer) (PPP Peer) Pool management
(if necessary)
| | |
1. ->| | |
2. |-IPCP-Configure-Request->| |
3. | |----Access-Request--->|
4. | |<---Access-Accept-----|
5. |<-IPCP-Configure-Request-| |
6. |---IPCP-Configure-Ack--->| |
7. |<--IPCP-Configure-Nack---| |
8. |-IPCP-Configure-Request->| |
9. |<---IPCP-Configure-Ack---| |
10. | |--Accounting-Request->|
11. | |<---Accounting-Resp.--|
In the diagram above, the CPE/End System is triggered (1) to set up IPv4 connectivity via an already existing PPP session. The CPE/End System detects that there is no public IPv4 address for its WAN interface available and starts the negotiation of the needed IPv4 address parameter by sending an IPCP Configure-Request to the NAS (2). The NAS will request the corresponding IPv4 connectivity parameters (e.g. IPv4 address, DNS resolver address) from a local (e.g. within the NAS) or remote database representing the Address Pool Management System(e.g. via RADIUS/DIAMETER) (3, 4). After this the PPP peers use the standard IPCP procedures to finalize the IPv4 address parameter negotiation (5, 6, 7, 8, 9). After the successful provisioning of the IPv4 address parameter the CPE/End system has full global IPv4 connectivity and can proceed with the IPv4 communication (in parallel to IPv6). In case of an external Address Pool Management, the NAS will send an Accounting-Request message (10) to the external Address Pool Management System in order to signal the successful negotiation of the IPv4 address parameter. The external Address Pool Management System will answer with an Accounting-Response (11) message.
An implementation wishing to close a dedicated NCP connection (e.g. IPCP or IPv6CP) SHOULD transmit a Terminate-Request to the peer. Upon reception of a NCP Terminate-Request, a Terminate-Ack MUST be transmitted to the sender of the Terminate-Request.
In the PPP Dual-Stack session scenario discussed here, the generation of the Terminate-Request message for the IPCP part of the PPP Dual-Stack session MUST be triggered by an IPv4 traffic idle timer within the PPP client (e.g. end system, CPE). As long as there is still an ongoing IPv6 connection within the PPP session, the PPP session MUST be kept open. Equivalently, when no IPv6 connectivity is detected the IPV6CP session can be terminated again by sending an IPv6CP Terminate-Request and accepting this by a Terminate-Ack. Afterwards the link layer connectivity and hence the whole PPP connection can be terminated by exchanging the LCP Terminate-Request and Terminate-Ack messages.
CPE/End System NAS ext. Address
(PPP Peer) (PPP Peer) Pool Management
| | |
1. ->| | |
2. |--IPCP-Termin.-Request-->| |
3. |<----IPCP-Termin.-Ack.---| |
4. | |-Interim-Acc.-Requ.-->|
5. | |<---Accounting-Resp.--|
The termination of an IPCP connection within a Dual-Stack PPP session is illustrated in figure 3 above.
For this exemplary message flow it is assumed that there is still an IPV6CP connection active inside the Dual-Stack PPP session. After the expiration of the IPv4 traffic idle timer (1) the CPE/End system sends an IPCP terminate request to the peer (2). The request will be answered with an Terminate-Ack message (3). The IPv4 address can be returned to the local address pool (e.g. within the NAS) or to the remote IPv4 address pool by sending Interim-Accounting messages (4, 5) (e.g. via RADIUS/DIAMETER).
IPv4_Idle_Timer
The sending of the Terminate-Request message MUST be triggered by an IPv4 traffic idle timer within the PPP client (e.g. end system, CPE). The timer value MUST be configurable to adopt the mechanism due to the needs of the applications which are using IPv4 and with respect to an optimization of the IPv4 address saving potential.
The efficiency of the “On demand IPv4 address provisioning” mechanism can be measured in the ratio of IPCP/RADIUS/DIAMETER signalling traffic to the amount of the saved global IPv4 addresses. Hence different options to optimize the efficiency of the proposed solution are possible, by supressing unnecessary signalling load and blocking forbidden IPv4 connectivity requests.
Unnecessary signalling load between PPP peers as well as between NAS and external Address Pool Management can for instance occur when a IPv6-only customer requests IPv4 address parameter. This can be prevented by restricting the usage of a Dual-Stack CPE/CER for IPv6-only customers to IPv6 only and/or by administratively refusing the IPCP configure requests of such an IPv6-only customer inside the NAS.
The former case is more or less a business and customer relationship related issue which needs no engineering concepts.
The latter case can be solved by answering an IPCP Configure Request message from a IPv6-only customer with a LCP reject message as described in chapter 5.7 of [RFC1661]. The field Rejected-Protocol of the LCP reject message contains the value 0x8021 for IPCP and the Rejected-Information field contains a copy of the IPCP packet which is being rejected. Due to [RFC1661] upon reception of a Protocol-Reject, the implementation of the IPv4 capable CER/CPE of the IPv6-only customer MUST immediately stop sending packets of the indicated protocol at the earliest opportunity. So the transmission of unnecessary IPCP and RADIUS messages during the running PPP session should be prevented.
Another opportunity to reduce IPCP signalling load and the corresponding signalling overhead between NAS and external Address Pool Management is the definition of default IPv4 traffic idle timer values for always-on applications that are sending periodic messages (see chapter 3.3). The value of this IPv4 traffic idle timer should be chosen a bit larger than the interval between periodic messages of always-on applications. Such an approach avoids problems for these applications when IPv4 is used and optimize IPv4 address release and address assign message exchange. Very short and periodic IPv4 address renewal cycles can be avoided by such an approach.
The easiest way to reduce IPv4 traffic demand (and hence the need for public IPv4 addresses) is to shift applications from usage of IPv4 to IPv6. In using the Dual-Stack approach which is a prerequisite of the here described mechanism no differences regarding the service level of both protocols are expected. Each service can be provided with the same quality level independently of the chosen version of the Internet Protocol.
But regarding applications on end systems the Internet access provider has only very limited influence. However for applications and services running on the CER/CPE itself (e.g. VoIP User Agent) the internet access provider should be able to define and require their IPv6 readiness.
An additional point is the preferred usage of IPv6 on all external (WAN) interfaces in the case when the CPE/CER acts as a relay and caches on behalf of certain protocols (e.g. DNS). When on a LAN interface a request message for such a protocol is received via IPv4 and a relaying to the external WAN interface is needed IPv6 should be the preferred network protocol. Such a requirement has already been defined for relaying/caching devices in [BBF-TR-124-i2] (section LAN.DNSv6, item 6).
The author and contributors also wish to acknowledge the assistance and feedback of the following individuals or groups.
Tina Tsou
Alain Durand
Sven Schmidtke
Dan Wing
Vernon Schryer
Mark Townsley
Wesley George
Joel M. Halpern
This memo includes no request to IANA.
TBD.
All drafts are required to have an IANA considerations section (see Guidelines for Writing an IANA Considerations Section in RFCs [RFC5226] for a guide). If the draft does not require IANA to do anything, the section contains an explicit statement that this is the case (as above). If there are no requirements for IANA, the section will be removed during conversion into an RFC by the RFC Editor.
TBD.
All drafts are required to have a security considerations section. See RFC 3552 [RFC3552] for a guide.
| [RFC3552] | Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003. |
| [RFC5226] | Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. |
v00 2011-03-07 KF/OB initial version
v01 2011-09-06 adding figures + explanation + Feedback IETF80 &mail discussion
v02 before IETF 82 review + feedback mail discussion