| PCP Working Group | M. Boucadair |
| Internet-Draft | France Telecom |
| Intended status: Standards Track | July 30, 2013 |
| Expires: January 31, 2014 |
Learning NAT64 PREFIX64s using PCP
draft-ietf-pcp-nat64-prefix64-04
This document defines a new PCP extension to learn the IPv6 prefix(es) used by a PCP-controlled NAT64 device to build IPv4-converted IPv6 addresses. This extension is needed for successful communications when IPv4 addresses are used in referrals.
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This document defines a new PCP extension [RFC6887] to inform PCP clients about the Pref64::/n and suffix [RFC6052] used by a PCP-controlled NAT64 device [RFC6146]. It does so by defining a new PREFIX64 option.
This extension is a deterministic solution to help establish communications between IPv6-only hosts and remote IPv4-only hosts. Unlike [I-D.ietf-behave-nat64-discovery-heuristic], this extension solves all the issues identified in [I-D.ietf-behave-nat64-learn-analysis].
Some illustration examples are provided in Section 5. Detailed experiment results are available at [I-D.boucadair-pcp-nat64-experiments].
The use of this PCP extension for NAT64 load balancing purposes ([I-D.zhang-behave-nat64-load-balancing]) is 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].
This document proposes a deterministic solution to solve the following issues:
A more elaborated discussion can be found at [I-D.ietf-behave-nat64-learn-analysis].
This section provides some use cases to illustrate the problem space. More details can be found at Section 4 of [I-D.ietf-behave-nat64-learn-analysis].
The extension defined in this document can be used for hosts with DNS64 capability [RFC6147] added to the host's stub-resolver.
The stub resolver on the host will try to obtain (native) AAAA records and if they are not found, the DNS64 function on the host will query for A records and then synthesizes AAAA records. Using the PREFIX64 PCP extension, the host's stub-resolver can learn the prefix used for IPv6/IPv4 translation and synthesize AAAA records accordingly.
Learning the Pref64::/n used to construct IPv4-converted IPv6 addresses allows the use of DNSSEC.
As discussed in [I-D.carpenter-behave-referral-object], a frequently occurring situation is that one entity A connected to a network needs to inform another entity B how to reach either A itself or some third-party entity C. This is known as address referral.
In the particular context of NAT64 [RFC6146], applications relying on address referral will fail because an IPv6-only client won't be able to make use of an IPv4 address received in a referral. A non-exhaustive list of such applications is provided below:
Learning the Pref64::/n solves the issues listed above.
The format of the PREFIX64 option is depicted in Figure 1. This option follows the guidelines specified in Section 7.3 of [RFC6887].
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pref64 Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
: Pref64 (Variable) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
: Suffix (Variable) :
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (optional) |
: IPv4 Prefix List (Variable) :
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Prefix64 PCP Option
The description of the fields is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix Count | IPv4 Prefix Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Address (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IPv4 Prefix Length | IPv4 Address (32 bits)... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... IPv4 Address (continued) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of IPv4 Prefix List field
Option Name: PREFIX64
Number: <to be assigned in the optional-to-process range>
Purpose: Learn the prefix used by the NAT64 to build
IPv4-converted IPv6 addresses. This is used by a host
is for local address synthesis (e.g., when an IPv4 address
is present in referrals).
Valid for Opcodes: MAP, ANNOUNCE
Length: Variable
May appear in: request, response.
Maximum occurrences: 1 for a request. As many as fit within
the maximum PCP message size for a response.
The PCP client includes a PREFIX64 option in a MAP or ANNOUNCE request to learn the IPv6 prefix and suffix used by an upstream PCP-controlled NAT64 device. When enclosed in a PCP request, the Prefix64 MUST be set to ::/96. The PREFIX64 option can be inserted in a MAP request used to learn the external IP address as detailed in Section 11.6 of [RFC6887].
The PCP server controlling a NAT64 SHOULD be configured to return to requesting PCP clients the value of the Pref64::/n and suffix used to build IPv4-converted IPv6 addresses. When enabled, the PREFIX64 option conveys the value of Pref64::/n and configured suffix. If no suffix is explicitly configured to the PCP server, the null suffix is used as the default value (see Section 2.2 of [RFC6052]).
If the PCP server is configured to honor the PREFIX64 option but no Pref64::/n is explicitly configured, the PCP server MUST NOT include any PREFIX64 option in its PCP messages.
The PCP server controlling a NAT64 MAY be configured to include a PREFIX64 option in all MAP responses even if the PREFIX64 option is not listed in the associated request. The PCP server controlling a NAT64 MAY be configured to include a PREFIX64 option in its ANNOUNCE messages.
The PCP server MAY be configured with a list of destination IPv4 prefixes associated with each Pref64::/n. This list is then included by the PCP server in a PREFIX64 option sent to PCP clients.
If the PCP client receives a PREFIX64 option that includes an invalid IPv4 prefix, the PCP client ignores that IPv4 prefix. Any other valid IPv4 prefix, IPv6 prefix and suffix are not ignored by the PCP client.
When multiple prefixes are configured in a network, the PCP server MAY be configured to return multiple PREFIX64 options in the same message to the PCP client. The client then does the following:
Upon receipt of the message from the PCP server, the PCP client replaces any old prefix(es)/suffix(es) received from the same PCP server with the new one(s) included in the PREFIX64 option(s). If no PREFIX64 option includes a destination IPv4 prefix list, the host embedding the PCP client uses the prefix/suffix included in the first PREFIX64 option for local address synthesis. Other prefixes learned can be used by the host to avoid any NAT64 deployed in the network. If one or multiple received PREFIX64 options contain a destination IPv4 prefix list, the PCP client MUST associate the included IPv4 prefixes with the Pref64::/n and the suffix indicated in the same PREFIX64 option. In such case, the host embedding the PCP client MUST enforce a destination-based prefix Pref64::/n selection for local address synthesis purposes. How the content of the PREFIX64 option(s) is passed to the OS is implementation-specific.
The PCP client MUST be prepared to receive multiple prefix(es) (e.g., if several PCP servers are deployed and each of them is configured with a distinct Pref64::/n). The PCP client MUST associate each received Pref64::/n and suffix with the PCP server from which the Pref64::/n and suffix information was retrieved. If the PCP client fails to contact a given PCP server, the PCP client SHOULD clear the prefix(es) and suffix(es) it learned from that PCP server.
If a distinct Pref64::/n or suffix is configured to the PCP-controlled NAT64 device, the PCP server SHOULD issue an unsolicited PCP ANNOUNCE message to inform the PCP client about the new Pref64::/n and/or suffix. Upon receipt of this message, the PCP client replaces the old prefix/suffix received from the same PCP server with the new Pref64::/n and suffix included in the PREFIX64 option.
This section provides a non-normative description of use cases relying on the PREFIX64 option.
The usage shown in Figure 3 depicts a typical usage of the PREFIX64 option when a DNS64 capability is embedded in the host.
In the example shown in Figure 3, once the IPv6-only client discovers the IPv4 address of the remote IPv4-only server, it retrieves the Pref64::/n (i.e., 2001:db8:122:300::/56) to be used to build an IPv4-converted IPv6 address for that server. This retrieval is achieved using the PREFIX64 option (Steps (a) and (b)). The client then 2001:db8:122:300::/56 to construct an IPv6 address and then initiates a TCP connection (Steps (1) to (4)).
+---------+ +-----+ +---------+
|IPv6-only| |NAT64| |IPv4-only|
| Client | | | | Server |
+---------+ +-----+ +---------+
| | |
| (a) PCP MAP Request | |
| PREFIX64 | |
|======================>| |
| (b) PCP MAP Response | |
| PREFIX64 = | |
| 2001:db8:122:300::/56 | |
|<======================| |
| (1) TCP SYN | (2) TCP SYN |
|======================>|====================>|
| (4) TCP SYN/ACK | (3) TCP SYN/ACK |
|<======================|<====================|
| (5) TCP ACK | (6) TCP ACK |
|======================>|====================>|
| | |
Figure 3: Example of a TCP session initiated from an IPv6-only host
Figure 4 shows an example of the use of the option defined in Section 4 in a SIP context. In order for RTP/RTCP flows to be exchanged between an IPv6-only SIP UA and an IPv4-only UA without requiring any ALG (Application Level Gateway) at the NAT64 nor any particular function at the IPv4-only SIP Proxy Server (e.g., Hosted NAT traversal [I-D.ietf-mmusic-latching]), the PORT_SET option [I-D.ietf-pcp-port-set] is used in addition to the PREFIX64 option.
In steps (a) and (b), the IPv6-only SIP UA retrieves a pair of ports to be used for RTP/RTCP sessions, the external IPv4 address and the Pref64::/n to build IPv4-embedded IPv6 addresses. This is achieved by issuing a MAP request that includes a PREFIX64 option and a PORT_SET option. A pair of ports (i.e., port_X/port_X+1) and an external IPv4 address are then returned by the PCP server to the requesting PCP client together with a Pref64::/n (i.e., 2001:db8:122::/48).
The returned external IPv4 address and external port numbers are used by the IPv6-only SIP UA to build its SDP offer which contains exclusively IPv4 addresses (especially in the "c=" line, the port indicated for media port is the external port assigned by the PCP server). The INVITE request including the SDP offer is then forwarded by the NAT64 to the Proxy Server which will relay it to the called party (i.e., the IPv4-only SIP UA) (Steps (1) to (3)).
The remote IPv4-only SIP UA accepts the offer and sends back its SDP answer in a "200 OK" message which is relayed by the SIP Proxy Server and NAT64 until being delivered to the IPv6-only SIP UA (Steps (4) to (6)).
The Pref64::/n (2001:db8:122::/48) is used by the IPv6-only SIP UA to construct a corresponding IPv6 address of the IPv4 address enclosed in the SDP answer made by the IPv4-only SIP UA (Step 6).
The IPv6-only SIP UA and IPv4-only SIP UA are then able to exchange RTP/RTCP flows without requiring any ALG at the NAT64 nor any special function at the IPv4-only SIP Proxy Server.
+---------+ +-----+ +------------+ +---------+
|IPv6-only| |NAT64| | IPv4 SIP | |IPv4-only|
| SIP UA | | | |Proxy Server| | SIP UA |
+---------+ +-----+ +------------+ +---------+
| (a) PCP MAP Request | | |
| PORT_SET | | |
| PREFIX64 | | |
|======================>| | |
| (b) PCP MAP Response | | |
| PORT_SET | | |
| PREFIX64: | | |
| 2001:db8:122::/48 | | |
|<======================| | |
| (1) SIP INVITE | (2) SIP INVITE | (3) SIP INVITE |
|======================>|===============>|================>|
| (6) SIP 200 OK | (5) SIP 200 OK | (4) SIP 200 OK |
|<======================|<===============|<================|
| (7) SIP ACK | (8) SIP ACK | (9) SIP ACK |
|======================>|===============>|================>|
| | | |
|src port: dst port:|src port: dst port:|
|port_A port_B|port_X port_B|
|<======IPv6 RTP=======>|<============IPv4 RTP============>|
|<===== IPv6 RTCP======>|<============IPv4 RTCP===========>|
|src port: dst port:|src port: dst port:|
|port_A+1 port_B+1|port_X+1 port_B+1|
| | |
Figure 4: Example of IPv6 to IPv4 SIP initiated Session
When the session is initiated from the IPv4-only SIP UA (see Figure 5), the IPv6-only SIP UA retrieves a pair of ports to be used for the RTP /RTCP session, the external IPv4 address and the Pref64::/n to build IPv4-converted IPv6 addresses (Steps (a) and (b)). These two steps could instead be delayed until the INVITE message is received (Step 3).
The retrieved IPv4 address and port numbers are used to build the SDP answer in Step (4) while the Pref64::/n is used to construct a IPv6 address corresponding to the IPv4 address enclosed in the SDP offer made by the IPv4-only SIP UA (Step 3). RTP/RTCP flows are then exchanged between the IPv6-only SIP UA and the IPv4-only UA without requiring any ALG at the NAT64 nor any special function at the IPv4-only SIP Proxy Server.
+---------+ +-----+ +------------+ +---------+
|IPv6-only| |NAT64| | IPv4 SIP | |IPv4-only|
| SIP UA | | | |Proxy Server| | SIP UA |
+---------+ +-----+ +------------+ +---------+
| (a) PCP MAP Request | | |
| PORT_SET | | |
| PREFIX64 | | |
|======================>| | |
| (b) PCP MAP Response | | |
| PORT_SET | | |
| PREFIX64: | | |
| 2001:db8:122::/48 | | |
|<======================| | |
| (3) SIP INVITE | (2) SIP INVITE | (1) SIP INVITE |
|<======================|<===============|<================|
| (4) SIP 200 OK | (5) SIP 200 OK | (6) SIP 200 OK |
|======================>|===============>|================>|
| (9) SIP ACK | (8) SIP ACK | (7) SIP ACK |
|<======================|<===============|<================|
| | | |
|src port: dst port:|src port: dst port:|
|port_a port_b|port_Y port_b|
|<======IPv6 RTP=======>|<============IPv4 RTP============>|
|<===== IPv6 RTCP======>|<============IPv4 RTCP===========>|
|src port: dst port:|src port: dst port:|
|port_a+1 port_b+1|port_Y+1 port_b+1|
| | |
Figure 5: Example of IPv4 to IPv6 SIP initiated Session
The following PCP Option Code is to be allocated in the optional-to-process range (the registry is maintained in http://www.iana.org/assignments/pcp-parameters/pcp-parameters.xml#option-rules):
PCP-related security considerations are discussed in [RFC6887].
As discussed in [RFC6147], if an attacker can manage to change the Pref64::/n used by the DNS64 function, the traffic generated by the host that receives the synthetic reply will be delivered to the altered Pref64. This can result in either a denial-of-service (DoS) attack, a flooding attack, or an eavesdropping attack. This attack could be achieved either by altering PCP messages issued by a legitimate PCP server or by using a fake PCP server.
Means to defend against attackers who can modify packets between the PCP server and the PCP client, or who can inject spoofed packets that appear to come from a legitimate PCP server SHOULD be enabled. For example, access control lists (ACLs) can be installed on the PCP client, PCP server, and the network between them, so those ACLs allow only communications from a trusted PCP server to the PCP client.
PCP server discovery is out of scope of this document. It is the responsibility of PCP server discovery document(s) to elaborate on the security considerations to discover a legitimate PCP server.
Learning a Pref64::/n via PCP allows using DNSSEC in the presence of NAT64. As such, NAT64 with DNSSEC and PCP is better than no DNSSEC at all, but it is less safe than DNSSEC without DNS64/NAT64 and PCP. The best mitigation action against Pref64::/n discovery attacks is thus to add IPv6 support in all endpoints and hence reduce the need to perform IPv6 address synthesis.
Many thanks to S. Perreault , R. Tirumaleswar, T. Tsou, D. Wing, J. Zhao, R. Penno, I. van Beijnum, T. Savolainen, S. Savikumar, and D. Thaler for the comments and suggestions.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC6887] | Wing, D., Cheshire, S., Boucadair, M., Penno, R. and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, April 2013. |
| [RFC6146] | Bagnulo, M., Matthews, P. and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, April 2011. |
| [RFC6147] | Bagnulo, M., Sullivan, A., Matthews, P. and I. van Beijnum, "DNS64: DNS Extensions for Network Address Translation from IPv6 Clients to IPv4 Servers", RFC 6147, April 2011. |
| [RFC6052] | Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010. |
| [RFC4632] | Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, August 2006. |