# Mobility-Multihoming Duality
## 2 Combined Mobility and Multihoming
### Host Mobility  
_Host mobility_ is a feature where a host with multiple connectivity moves from one point of attachment (PoA) to another PoA in a different network. This could make use of different sub-network technologies, i.e., across different layer 3 networks, or _vertical handover_. This is not the same as handovers in layer 2/layer 1 connectivity, which are typically handled within a specific network technology, e.g., radio-level handover in 5G networks [4]. Due to the way naming and addressing is used in the current IP architecture, moving across different IP networks and PoAs requires special handling. One or more of the following mechanisms must be used, which we describe with their relative advantages and disadvantages:  
* _Transport protocols._ Specific transport protocol mechanisms could enable mobility and multipath communication through multiple paths, e.g., using Multipath-TCP (MP-TCP) [5] or multipath QUIC [6]. Strictly speaking, transport protocols do not provide multihoming, but they can use multiple paths--_multipath transport_--where multihoming is available at the site or directly to a host. Transport protocols offer a direct interface to an application, so they are important to consider. However, such an approach will, of course, require that applications use only those transport protocols that directly support multipath. This in turn might require the redesign and/or re-engineering of the application to make use of mobility and multipath connectivity, which is true in the case of QUIC, for example. * _Middleboxes and proxies._ A middlebox or proxy entity can act as a relay or coordination point to aid a host that moves from one PoA to another. Such an entity typically will use additional control plane protocols with the host to provide such a capability. The use of middleboxes and proxies gives rise to a number of general problems, even though they are widely used. A middlebox or proxy approach entails the following: 1. It could become a performance bottleneck; 2. It is potentially a single point of failure (SPoF); 3. It increases the attack surface for potential security and privacy threats, for example, a proxy is the ideal point from which to launch a Man-in-the-Middle (MITM) attack. 4. It introduces additional operations, administration, and maintenance (OAM) overhead; 5. It can become more complex in proportion to the scale of deployment, especially with respect to OAM. * _Application-specific mechanism._ Application-specific mechanisms might use special control plane and user plane protocols for a host, or they might be enabled through the use of a proxy or middlebox. Such an approach has the advantage that it could provide tailored, efficient capability for a particular application's needs. Application-specific mechanisms might pose challenges in terms of deployment and uptake, however, as legacy applications could be left with limited connectivity. Application deployments might even be left as non-interoperable, especially if the particular approach prevents an older version from functioning with a newer version. So, overall, there is a deployment and OAM challenge. * _Special in-network (vendor and provider) support._ Support from a specially designed or implemented mechanism from a vendor or network provider could provide a highly integrated, tailored, and efficient solution for a particular use case. Such an approach might rely on a proprietary protocol and network configuration. However, such a solution might be constrained to a single sub-network technology, could be more difficult to deploy at scale, or could tie a user into a specific vendor and/or network provider, limiting networks that the host can potentially'move' to and from. This approach will narrow the scope of deployment when considering global connectivity. This could be especially so when there is a need to provide connectivity _across_ different sub-network technologies, i.e., not any one of 4G, 5G, or IEEE 802.11 by itself offers complete mobile coverage globally.