RADIUS Extensions for Network-Assisted Multipath TCP (MPTCP)

One of the promising deployment scenarios for Multipath TCP (MPTCP, ) is to enable a Customer Premises Equipment (CPE) that is connected to multiple networks (e.g., DSL, LTE, WLAN) to optimize the usage of such resources, see for example . Network-assisted MPTCP deployment models are designed to facilitate the adoption of MPTCP for the establishment of multi-path communications without making any assumption about the support of MPTCP by the communicating peers. This deployment scenario relies on MPTCP proxies located on both the CPE and network sides (). MPTCP proxies are responsible for establishing multi-path communications on behalf of endpoints, thereby taking advantage of MPTCP capabilities to optimize resource usage to achieve different goals that include (but are not limited to) bandwidth aggregation, primary/backup communication paths, and traffic offload management.

Internet | | || (MPTCP proxy)|| | | |+--------------+| | | IP Network #2 | | | | _--------_ | | | | ( e.g., DSL ) | | | +=======+ +==+ | | | (_ _) | | +-----+------+ (_______) +----------------+ | ---- LAN ---- | end-nodes ]]> Within this document, an MPTCP concentrator (or concentrator) refers to a functional element that is responsible for aggregating the traffic originated by a group of CPEs. This element is located in the network. One or multiple concentrators can be deployed in the network to assist MPTCP-enabled CPEs to establish MPTCP connections via their available network attachments. On the uplink path, the concentrator terminates the MPTCP connections received from its customer-facing interfaces and transforms these connections into legacy TCP connections towards upstream servers. On the downlink path, the concentrator turns the legacy server's TCP connection into MPTCP connections towards its customer-facing interfaces. This document specifies two new Remote Authentication Dial-In User Service (RADIUS, ) attributes that carry the MPTCP concentrator IP address list (). In order to accommodate both IPv4 and IPv6 deployment contexts, and given the constraints in Section 3.4 of , two attributes are specified. Note that one or multiple IPv4 and/or IPv6 addresses may be returned to a requesting CPE. A sample use case is described in . This document assumes that the MPTCP concentrator(s) reachability information can be stored in Authentication, Authorization, and Accounting (AAA) servers while the CPE configuration is usually provided by means of DHCP (). This specification assumes an MPTCP concentrator is reachable through one or multiple IP addresses. As such, a list of IP addresses can be communicated via RADIUS. Also, it assumes the various network attachments provided to an MPTCP-enabled CPE are managed by the same administrative entity. This document adheres to for defining the new attributes.

Description The RADIUS MPTCP-Concentrator-IPv4 attribute contains the IPv4 address of an MPTCP concentrator that is assigned to a CPE. Because multiple MPTCP concentrator IP addresses may be provisioned to an authorised CPE (that is a CPE entitled to solicit the resources of a concentrator to establish MPTCP connections), multiple instances of the MPTCP-Concentrator-IPv4 attribute MAY be included; each instance of the attribute carries a distinct IP address. Both MPTCP-Concentrator-IPv4 and MPTCP-Concentrator-IPv6 attributes MAY be present in a RADIUS message. The MPTCP-Concentrator-IPv4 Attribute MAY appear in a RADIUS Access-Accept packet. It MAY also appear in a RADIUS Access-Request packet as a hint to the RADIUS server to indicate a preference, although the server is not required to honor such a hint. The MPTCP-Concentrator-IPv4 Attribute MAY appear in a CoA-Request packet. The MPTCP-Concentrator-IPv4 Attribute MAY appear in a RADIUS Accounting-Request packet. The MPTCP-Concentrator-IPv4 Attribute MUST NOT appear in any other RADIUS packet. Type TBA (see ). Length 6 Data Type The attribute MPTCP-Concentrator-IPv4 is of type ip4addr (Section 3.3 of ). Value This field includes an IPv4 address (32 bits) of the MPTCP concentrator. The MPTCP-Concentrator-IPv4 attribute MUST NOT include multicast and host loopback addresses . Anycast addresses are allowed to be included in an MPTCP-Concentrator-IPv4 attribute.

Description The RADIUS MPTCP-Concentrator-IPv6 attribute contains the IPv6 address of an MPTCP concentrator that is assigned to a CPE. Because multiple MPTCP concentrator IP addresses may be provisioned to an authorised CPE (that is a CPE entitled to solicit the resources of a concentrator to establish MPTCP connections), multiple instances of the MPTCP-Concentrator-IPv6 attribute MAY be included; each instance of the attribute carries a distinct IP address. Both MPTCP-Concentrator-IPv4 and MPTCP-Concentrator-IPv6 attributes MAY be present in a RADIUS message. The MPTCP-Concentrator-IPv6 Attribute MAY appear in a RADIUS Access-Accept packet. It MAY also appear in a RADIUS Access-Request packet as a hint to the RADIUS server to indicate a preference, although the server is not required to honor such a hint. The MPTCP-Concentrator-IPv6 Attribute MAY appear in a CoA-Request packet. The MPTCP-Concentrator-IPv6 Attribute MAY appear in a RADIUS Accounting-Request packet. The MPTCP-Concentrator-IPv6 Attribute MUST NOT appear in any other RADIUS packet. Type TBA (see ). Length 18 Data Type The attribute MPTCP-Concentrator-IPv6 is of type ip6addr (Section 3.9 of ). Value This field includes an IPv6 address (128 bits) of the MPTCP concentrator. The MPTCP-Concentrator-IPv6 attribute MUST NOT include multicast and host loopback addresses . Anycast addresses are allowed to be included in an MPTCP-Concentrator-IPv6 attribute.

This section does not aim to provide an exhaustive list of deployment scenarios where the use of the RADIUS MPTCP-Concentrator-IPv6 and MPTCP-Concentrator-IPv4 attributes can be helpful. Typical deployment scenarios are described, for instance, in . shows an example where a CPE is assigned an MPTCP concentrator. This example assumes that the Network Access Server (NAS) embeds both RADIUS client and DHCPv6 server capabilities.

| | | |----Access-Request ---->| | | | | |<----Access-Accept------| | | MPTCP-Concentrator-IPv6| |<-------DHCPv6 Advertisement----| | | (OPTION_V6_MPTCP) | | | | | |---------DHCPv6 Request-------->| | | | | |<---------DHCPv6 Reply----------| | | (OPTION_V6_MPTCP) | | DHCPv6 RADIUS]]> Upon receipt of the DHCPv6 Solicit message from a CPE, the NAS sends a RADIUS Access-Request message to the AAA server. Once the AAA server receives the request, it replies with an Access-Accept message (possibly after having sent a RADIUS Access-Challenge message and assuming the CPE is entitled to connect to the network) that carries a list of parameters to be used for this session, and which include MPTCP concentrator reachability information (namely a list of IP addresses). The content of the MPTCP-Concentrator-IPv6 attribute is then used by the NAS to complete the DHCPv6 procedure that the CPE initiated to retrieve information about the MPTCP concentrator it has been assigned. Upon change of the MPTCP concentrator assigned to a CPE, the RADIUS server sends a RADIUS CoA message that carries the RADIUS MPTCP-Concentrator-IPv6 attribute to the NAS. Once that message is accepted by the NAS, it replies with a RADIUS CoA ACK message. The NAS replaces the old MPTCP concentrator with the new one. shows another example where a CPE is assigned an MPTCP concentrator, but the CPE uses DHCPv6 to retrieve a list of IP addresses of an MPTCP concentrator.

| | | |----Access-Request ---->| | | | | |<----Access-Accept------| | |MPTCP-Concentrator-IPv4 | |<------------DHCPOFFER------------| | | (OPTION_V4_MPTCP) | | | | | |------------DHCPREQUEST---------->| | | (OPTION_V4_MPTCP) | | | | | |<-----------DHCPACK---------------| | | (OPTION_V4_MPTCP) | | DHCPv4 RADIUS]]> Some deployments may rely on the mechanisms defined in or , which allows a NAS to pass attributes obtained from a RADIUS server to a DHCP server.

RADIUS-related security considerations are discussed in . MPTCP-related security considerations are discussed in and . Traffic theft is a risk if an illegitimate concentrator is inserted in the path. Indeed, inserting an illegitimate concentrator in the forwarding path allows to intercept traffic and can therefore provide access to sensitive data issued by or destined to a host. To mitigate this threat, secure means to discover a concentrator should be enabled.

The following table provides a guide as what type of RADIUS packets that may contain these attributes, and in what quantity.

The following table defines the meaning of the above table entries:

IANA is requested to assign two new RADIUS attribute types from the IANA registry "Radius Attribute Types" located at http://www.iana.org/assignments/radius-types: MPTCP-Concentrator-IPv4 (TBA) MPTCP-Concentrator-IPv6 (TBA)

Thanks to Alan DeKok for the comments.