| Softwire Working Group | Y. Cui |
| Internet-Draft | Tsinghua University |
| Intended status: Standards Track | Q. Sun |
| Expires: April 18, 2015 | China Telecom |
| M. Boucadair | |
| France Telecom | |
| T. Tsou | |
| Huawei Technologies | |
| Y. Lee | |
| Comcast | |
| I. Farrer | |
| Deutsche Telekom AG | |
| October 15, 2014 |
Lightweight 4over6: An Extension to the DS-Lite Architecture
draft-ietf-softwire-lw4over6-11
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 April 18, 2015.
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 is a solution designed specifically for complete independence between IPv6 subnet prefix and IPv4 address with or without IPv4 address sharing. This is accomplished by maintaining state for each softwire (per-subscriber state) in the central lwAFTR and a hub-and-spoke forwarding architecture. [I-D.ietf-softwire-map] also offers these capabilities or, alternatively, allows for a reduction of the amount of centralized state using rules to express IPv4/IPv6 address mappings. This introduces an algorithmic relationship between the IPv6 subnet and IPv4 address. This relationship also allows the option of direct, meshed connectivity between users.
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. The following figure shows the data plane with the main functional change between DS-Lite and lw4o6:
+--------+ +---------+ IPv4-in-IPv6 +---------+ +-------------+
|IPv4 LAN|---| B4 |==================|AFTR/NAPT|----|IPv4Internet|
+--------+ +---------+ +---------+ +-------------+
DS-Lite NAPT model: all state in the AFTR
+--------+ +---------+ IPv4-in-IPv6 +------+ +-------------+
|IPv4 LAN|---|lwB4/NAPT|==================|lwAFTR|----|IPv4 Internet|
+--------+ +---------+ +------+ +-------------+
LW4over6 NAPT model:
subscriber state in the lwAFTR, NAPT state in lwB4
Figure 1 Comparison of DS-Lite and Lightweight 4over6 Data Plane
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, 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_S46_CONT_LW as described in section 5.3 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, DHCP 4o6 [RFC7341] must be used.
Although it would be possible to extend lw4o6 to have more than one active lw4o6 tunnel configured simultaneously, this document is only concerned with the use of a single tunnel.
The IPv6 binding prefix field is provisioned so that the CE can identify the correct prefix to use as the tunnel source. On receipt of the necessary configuration parameters listed in Section 5.1 of this document, the lwB4 performs a longest prefix match between the IPv6 binding prefix and its currently active IPv6 prefixes. The result forms the subnet to be used for sourcing the lw4o6 tunnel. The full /128 address is then constructed in the same manner as [I-D.ietf-softwire-map].
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 Prefix | . (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 IPv6 encapsulation source address is changed for any reason (such as the DHCPv6 lease expiring), the lwB4's dynamic provisioning process must be re-initiated. When the lwB4's public IPv4 address or port set ID is changed for any reason, the lwB4 must flush its NAPT table.
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, thus allocating a single contiguous port set to each lwB4.
Unless an lwB4 is being allocated a full IPv4 address, it is RECOMMENDED that PSIDs containing the well-known ports (0-1023) are not allocated to lwB4s.
In the event that the lwB4 receives an 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.
For TCP and UDP traffic the NAPT44 implemented in the lwB4 SHOULD conform with the behaviour and best current practices documented in [RFC4787], [RFC5508], and [RFC5382]. If the lwB4 supports DCCP, then the requirements in [RFC5597] SHOULD be implemented.
The NAPT44 in the lwB4 MUST implement ICMP message handling behaviour conforming to the best current practice documented in [RFC5508]. If the lwB4 receives an ICMP error (for errors detected inside the IPv6 tunnel), the node should relay the ICMP error message to the original source (the lwAFTR).
This behaviour SHOULD be implemented conforming to the section 8 of [RFC2473].
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].