| Softwire Working Group | Y. Cui |
| Internet-Draft | Tsinghua University |
| Intended status: Standards Track | Q.S. Sun |
| Expires: August 10, 2014 | China Telecom |
| M.B. Boucadair | |
| France Telecom | |
| T.T. Tsou | |
| Huawei Technologies | |
| Y. Lee | |
| Comcast | |
| I.F. Farrer | |
| Deutsche Telekom AG | |
| February 06, 2014 |
Lightweight 4over6: An Extension to the DS-Lite Architecture
draft-ietf-softwire-lw4over6-05
Dual-Stack Lite (RFC 6333) describes an architecture for transporting IPv4 packets over an IPv6 network. This document specifies an extension to DS-Lite called Lightweight 4over6 which moves the Network Address and Port Translation (NAPT) function from the centralized DS-Lite tunnel concentrator to the tunnel client located in the Customer Premises Equipment (CPE). This removes the requirement for a Carrier Grade NAT function in the tunnel concentrator and reduces the amount of centralized state that must be held to a per-subscriber level. In order to delegate the NAPT function and make IPv4 Address sharing possible, port-restricted IPv4 addresses are allocated to the CPEs.
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 August 10, 2014.
Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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Dual-Stack Lite (DS-Lite, [RFC6333]) defines a model for providing IPv4 access over an IPv6 network using two well-known technologies: IP in IP [RFC2473] and Network Address Translation (NAT). The DS-Lite architecture defines two major functional elements as follows:
[RFC3022]). To achieve this, the AFTR must dynamically maintain per-flow state in the form of active NAPT sessions. For service providers with a large number of B4 clients, the size and associated costs for scaling the AFTR can quickly become prohibitive. It can also place a large NAPT logging overhead upon the service provider in countries where legal requirements mandate this.
As the AFTR performs the centralized NAT44 function, it dynamically assigns public IPv4 addresses and ports to requesting host's traffic (as described in
This document describes a mechanism called Lightweight 4 over 6 (lw4o6), which provides a solution for these problems. By relocating the NAPT functionality from the centralized AFTR to the distributed B4s, a number of benefits can be realised:
Operator's IPv6 and IPv4 addressing architectures remain independent of each other. Therefore, flexible IPv4/IPv6 addressing schemes can be deployed.
Lightweight 4over6 provides a solution for a hub-and-spoke softwire architecture only. It does not offer direct, meshed IPv4 connectivity between subscribers without packets traversing the AFTR. If this type of meshed interconnectivity is required, [I-D.ietf-softwire-map] provides a suitable solution.
The tunneling mechanism remains the same for DS-Lite and Lightweight 4over6. This document describes the changes to DS-Lite that are necessary to implement Lightweight 4over6. These changes mainly concern the configuration parameters and provisioning method necessary for the functional elements.
Lightweight 4over6 features keeping per-subscriber state in the service provider's network. It is categorized as Binding approach in [I-D.ietf-softwire-unified-cpe] which defines a unified IPv4-in-IPv6 Softwire CPE.
This document is an extended case, which covers address sharing for [RFC7040]. It is also a variant of A+P called Binding Table Mode (see Section 4.4 of [RFC6346]).
This document focuses on architectural considerations and particularly on the expected behavior of the involved functional elements and their interfaces. Deployment-specific issues are discussed in a companion document. As such, discussions about redundancy and provisioning policy are out of scope.
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].
The document defines the following terms:
Throughout the remainder of this document, the terms B4/AFTR should be understood to refer specifically to a DS-Lite implementation. The terms lwB4/lwAFTR refer to a Lightweight 4over6 implementation.
The Lightweight 4over6 architecture is functionally similar to DS-Lite. lwB4s and an lwAFTR are connected through an IPv6-enabled network. Both approaches use an IPv4-in-IPv6 encapsulation scheme to deliver IPv4 connectivity services. The following figure shows the data plane with the main functional change between DS-Lite and lw4o6:
+--------+ +---------+ IPv4-in-IPv6 +------+ +-------------+
|IPv4 LAN|---|lwB4/NAPT|==================|lwAFTR|----|IPv4 Internet|
+--------+ +---------+ +------+ +-------------+
^ |
+-------------------------+
NAPT function relocated
to lwB4 in lw4o6
Figure 1 Lightweight 4over6 Data Plane Overview
There are three main components in the Lightweight 4over6 architecture:
The lwB4 differs from a regular B4 in that it now performs the NAPT functionality. This means that it needs to be provisioned with the public IPv4 address and port set it is allowed to use. This information is provided though a provisioning mechanism such as DHCP, PCP or TR-69.
The lwAFTR needs to know the binding between the IPv6 address of each subscriber and the IPv4 address and port set allocated to that subscriber. This information is used to perform ingress filtering upstream and encapsulation downstream. Note that this is per-subscriber state as opposed to per-flow state in the regular AFTR case.
The consequence of this architecture is that the information maintained by the provisioning mechanism and the one maintained by the lwAFTR MUST be synchronized (See figure 2). The details of this synchronization depend on the exact provisioning mechanism and will be discussed in a companion document.
The solution specified in this document allows the assignment of either a full or a shared IPv4 address requesting CPEs. [RFC7040] provides a mechanism for assigning a full IPv4 address only.
+------------+
/-------|Provisioning|<-----\
| +------------+ |
| |
V V
+--------+ +---------+ IPv4/IPv6 +------+ +-------------+
|IPv4 LAN|---|lwB4/NAPT|==================|lwAFTR|----|IPv4 Internet|
+--------+ +---------+ +------+ +-------------+
Figure 2 Lightweight 4over6 Provisioning Synchronization
With DS-Lite, the B4 element only needs to be configured with a single DS-Lite specific parameter so that it can set up the softwire (the IPv6 address of the AFTR). Its IPv4 address can be taken from the well-known range 192.0.0.0/29.
In lw4o6, due to the distributed nature of the NAPT function, a number of lw4o6 specific configuration parameters must be provisioned to the lwB4. These are:
For DHCPv6 based configuration of these parameters, the lwB4 SHOULD implement OPTION_SW46_LW as described in section 6.3 of [I-D.ietf-softwire-map-dhcp]. This means that the lifetime of the softwire and the derived configuration information (e.g. IPv4 shared address, IPv4 address) is bound to the lifetime of the DHCPv6 lease. If stateful IPv4 configuration or additional IPv4 configuration information is required, DHCPv4 [RFC2131] must be used.
Some other mechanisms which may be adapted for the provisioning of IPv4 addresses and port-sets are described in section 7 below.
An IPv6 address from an assigned prefix is also required for the lwB4 to use as the encapsulation source address for the softwire. In order to enable end-to-end provisioning, the IPv6 address is constructed by taking a /64 prefix assigned to the WAN interface and suffixing 64-bits for the interface identifier. As there may be multiple WAN prefixes, of which only one can be used for lw4o6, the CPE is provisioned with the logic to select the correct one. The /128 prefix is then constructed as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Operator assigned (64-bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Zero Padding | IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Addr cont. | PSID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 Construction of the lw4o6 /128 Prefix
In the event that the lwB4's encapsulation source address is changed for any reason (such as the DHCPv6 lease expiring), the lwB4's dynamic provisioning process must be re-initiated.
An lwB4 MUST support dynamic port-restricted IPv4 address provisioning. The port set algorithm for provisioning this is described in Section 5.1 of [I-D.ietf-softwire-map]. For lw4o6, the number of a-bits SHOULD be 0.
In the event that the lwB4 receives and ICMPv6 error message (type 1, code 5) originating from the lwAFTR, the lwB4 SHOULD interpret this to mean that no matching entry in the lwAFTR's binding table has been found. The lwB4 MAY then re-initiate the dynamic port-restricted provisioning process. The lwB4's re-initiation policy SHOULD be configurable.
The DNS considerations described in Section 5.5 and Section 6.4 of [RFC6333] SHOULD be followed.
Several sections of [RFC6333] provide background information on the B4's data plane functionality and MUST be implemented by the lwB4 as they are common to both solutions. The relevant sections are:
The lwB4 element performs IPv4 address translation (NAPT44) as well as encapsulation and de-capsulation. It runs standard NAPT44 [RFC3022] using the allocated port-restricted address as its external IPv4 address and port numbers.
The working flow of the lwB4 is illustrated in figure 4.
+-------------+
| lwB4 |
+--------+ IPv4 |------+------| IPv4-in-IPv6 +----------+
|IPv4 LAN|------->| |Encap.|-------------->|Configured|
| |<-------| NAPT | or |<--------------| lwAFTR |
+--------+ | |Decap.| +----------+
+------+------+
Figure 4 Working Flow of the lwB4
Internally connected hosts source IPv4 packets with an [RFC1918] address. When the lwB4 receives such an IPv4 packet, it performs a NAPT44 function on the source address and port by using the public IPv4 address and a port number from the allocated port-set. Then, it encapsulates the packet with an IPv6 header. The destination IPv6 address is the lwAFTR's IPv6 address and the source IPv6 address is the lwB4's IPv6 tunnel endpoint address. Finally, the lwB4 forwards the encapsulated packet to the configured lwAFTR.
When the lwB4 receives an IPv4-in-IPv6 packet from the lwAFTR, it de-capsulates the IPv4 packet from the IPv6 packet. Then, it performs NAPT44 translation on the destination address and port, based on the available information in its local NAPT44 table.
If the IPv6 source address does not match the configured lwAFTR address, then the packet MUST be discarded. If the decapsulated IPv4 packet does not match the lwB4's configuration (i.e. invalid destination IPv4 address or port) then the packet MUST be dropped. An ICMPv4 error message (type 13 - Communication Administratively Prohibited) message MAY be sent back to the lwAFTR. The ICMP policy SHOULD be configurable.
The lwB4 is responsible for performing ALG functions (e.g., SIP, FTP), and other NAPT traversal mechanisms (e.g., UPnP, NAPT-PMP, manual binding configuration, PCP) for the internal hosts. This requirement is typical for NAPT44 gateways available today.
It is possible that a lwB4 is co-located in a host. In this case, the functions of NAPT44 and encapsulation/de-capsulation are implemented inside the host.
On receiving an encapsulated packet containing an IPv4 fragment, the lwB4 SHOULD wait until all other fragments have been received and de-capsulated. The original packet is then re-assembled before performing NAPT. This is necessary because layer-4 protocol information is only present in the first fragment. However, as this provides a potential security flaw (as discussed in [RFC4459] Section 5) it is RECOMMENDED that the lwB4 implements mechanisms to prevent buffer memory exhaustion.
When an lwB4 receives an IPv4 packet from a connected host that exceeds the IPv6 MTU size after encapsulation, the lwB4 SHOULD fragment the IPv4 packet before encapsulation. This lwB4 behavior will not result IPv6 fragmentation so that lwAFTR is not required to re- assemble fragmented IPv6 packets. If the the Don't Fragment (DF) bit is set in the IPv4 packet header (e.g. for PMTUD discovery), then the IPv4 packet is dropped by the lwB4 and an ICMP Fragmentation Needed (Type 3, Code 4) with the correct tunnel MTU is sent.
The lwAFTR maintains an address binding table containing the binding between the lwB4's IPv6 address, the allocated IPv4 address and restricted port-set. Unlike the DS-Lite extended binding table defined in section 6.6 of [RFC6333] which is a 5-tuple NAPT table, each entry in the Lightweight 4over6 binding table contains the following 3-tuples:
The entry has two functions: the IPv6 encapsulation of inbound IPv4 packets destined to the lwB4 and the validation of outbound IPv4-in-IPv6 packets received from the lwB4 for de-capsulation.
The lwAFTR does not perform NAPT and so does not need session entries.
The lwAFTR MUST synchronize the binding information with the port-restricted address provisioning process. If the lwAFTR does not participate in the port-restricted address provisioning process, the binding MUST be synchronized through other methods (e.g. out-of-band static update).
If the lwAFTR participates in the port-restricted provisioning process, then its binding table MUST be created as part of this process.
For all provisioning processes, the lifetime of binding table entries MUST be synchronized with the lifetime of address allocations.
Several sections of [RFC6333] provide background information on the AFTR's data plane functionality and MUST be implemented by the lwAFTR as they are common to both solutions. The relevant sections are:
When the lwAFTR receives an IPv4-in-IPv6 packet from an lwB4, it de-capsulates the IPv6 header and verifies the source addresses and port in the binding table. If both the source IPv4 and IPv6 addresses match a single entry in the binding table and the source port is in the allowed port-set for that entry, the lwAFTR forwards the packet to the IPv4 destination.
If no match is found (e.g., no matching IPv4 address entry, port out of range, etc.), the lwAFTR MUST discard or implement a policy (such as redirection) on the packet. An ICMPv6 type 1, code 5 (source address failed ingress/egress policy) error message MAY be sent back to the requesting lwB4. The ICMP policy SHOULD be configurable.
When the lwAFTR receives an inbound IPv4 packet, it uses the IPv4 destination address and port to lookup the destination lwB4's IPv6 address in its binding table. If a match is found, the lwAFTR encapsulates the IPv4 packet. The source is the lwAFTR's IPv6 address and the destination is the lwB4's IPv6 address from the matched entry. Then, the lwAFTR forwards the packet to the lwB4 natively over the IPv6 network.
If no match is found, the lwAFTR MUST discard the packet. An ICMPv4 type 3, code 1 (Destination unreachable, host unreachable) error message MAY be sent back. The ICMP policy SHOULD be configurable.
The lwAFTR MUST support hairpinning of traffic between two lwB4s, by performing de-capsulation and re-encapsulation of packets. The hairpinning policy MUST be configurable.
In addition to the DHCPv6 based mechanism described in section 5.1, several other IPv4 provisioning protocols have been suggested. These protocols MAY be implemented. These alternatives include:
In a Lightweight 4over6 domain, the binding information MUST be aligned between the lwB4s, the lwAFTRs and the provisioning server.
For both the lwAFTR and the lwB4, ICMPv6 MUST be handled as described in [RFC2473].
ICMPv4 does not work in an address sharing environment without special handling [RFC6269]. Due to the port-set style address sharing, Lightweight 4over6 requires specific ICMP message handling not required by DS-Lite.
For inbound ICMP messages The following behavior SHOULD be implemented by the lwAFTR to provide ICMP error handling and basic remote IPv4 service diagnostics for a port restricted CPE:
Additionally, the lwAFTR MAY implement:
The ICMP policy SHOULD be configurable.
The lwB4 SHOULD implement the requirements defined in [RFC5508] for ICMP forwarding. For ICMP echo request packets originating from the private IPv4 network, the lwB4 SHOULD implement the method described in [RFC6346] and use an available port from its port-set as the ICMP Identifier.
As the port space for a subscriber shrinks due to address sharing, the randomness for the port numbers of the subscriber is decreased significantly. This means it is much easier for an attacker to guess the port number used, which could result in attacks ranging from throughput reduction to broken connections or data corruption.
The port-set for a subscriber can be a set of contiguous ports or non-contiguous ports. Contiguous port-sets do not reduce this threat. However, with non-contiguous port-set (which may be generated in a pseudo-random way [RFC6431]), the randomness of the port number is improved, provided that the attacker is outside the Lightweight 4over6 domain and hence does not know the port-set generation algorithm.
More considerations about IP address sharing are discussed in Section 13 of [RFC6269], which is applicable to this solution.
This document does not include an IANA request.
The following are extended authors who contributed to the effort:
The authors would like to thank Ole Troan, Ralph Droms and Suresh Krishnan for their comments and feedback.
This document is a merge of three documents: [I-D.cui-softwire-b4-translated-ds-lite], [I-D.zhou-softwire-b4-nat] and [I-D.penno-softwire-sdnat].