VM Live Migration's Impacts on the Running Applications

1. Will the IP address change after migration?

Both types of live migration exist, including changing and not changing IP address [5].

• Based on Google cloud [1], it can migrate clients' VM without affect the customers. That means the IP address of a VM would not be changed in this case.
• To retain the same IP address, hyper-V requires the source and destination hosts to be within the same subnet. I think Google cloud may not have this requirement.
• I think the virtual network [4] would be able to remove the restrictions on the locations of the destination hosts. "Hyper-V Network Virtualization decouples virtual networks for customer virtual machines from the physical network infrastructure." 

2. Will the migration interrupt the Internet service?

This depends on the implementation. The answer is different regarding different implementation.

• According to google cloud [1], there will be no service interruptions.
• During post-migration brownout, the VM executes on the target. The source VM is present, and may be providing supporting functionality for the target. For instance, until the network fabric has caught up the new location of the VM, and source VM provides forwarding services for packets to and from the target VM. 
• According to hyper-V [2]
• the migration is not downtime-free, the interruption is almost immeasurably brief. Usually the longest delay is the network layer while the virtual machine's MAC address is registered on the new physical switch port and its new location is propagated throughout the network. 
• According to [3], in order to use live migration the VM needs to keep the same IP address across date centers in order to achieve the goal of continuous access from clients to the virtual machine during and after the migration. 

3. How the network is migrated?

The most challenging issue in VM migration is to keep the network working.

In LAN, different hypervisors using different strategies.

• Xen
• It uses ARP to bind the IP address to the new host. 
• The VM sends ARP signal, broadcast that the IP address is moved to a new host.  But this may not be allowed for security reasons. 
• VMware
• VMotion uses VNIC to ensure the network connection. 
• The VNIC will be migrated with VM as well. Every VNIC has a unique MAC address in LAN and is connected to one or multiple NIC. 
• Since VNIC has a MAC address that is irrelevant to the physical network address, the network will be continued as normal using VM live migration. 
• Note due to the restrictions of Ethernet, the source and destination hosts have to be in the same subnet. 

In WAN

• The VM will be given a new IP address in the destination host. In order to ensure the network connection, we can use IP tunnel with combination of dynamic DNS, i.e., we can build a IP tunnel between the source IP and destination IP address, and use it to forward the packets from source host to destination host. Once migration is done, VM can response to the new network. It means the DNS is updated, and the network connection will refer to the new IP address. 

Reference
[1] Google cloud VM live migration
[2] Hyper-V live migration
[3] Live Migration -- Implementation considerations
[4] Hyper-V 网络虚拟化概述 
[5] 虚拟机迁移研究