Guide to select best GSLB solution

There was a question to me
'Ok, If a XX ADC supports client proximity which is not based on DNS, is it best to go with. Do you suggest any other features to look for?'

As I see all the current ADCs do support most eye catching features, there are some things to check before selecting a GSLB equipped ADC.

Here is a check list
- Client proximity: Most LBs calculate Round Trip Time (RTT) by ICMP, TCP probe etc.,
But, think if this works in real time scenario.
a) RTT calculation should work with firewalls sitting at the client network
b) It should not add overhead in TCP communication with the client
c) Most importantly, RTT should be calculated to the real client. Assume a proxy between the client and ADC. If the ADC under consideration does ICMP or TCP probe, it may not reach the real client. So, the RTT is to the proxy but not the client.
d) VPN environment - If the client is behind a VPN gateway. The original location of the client is from private network. This private network can be geographically anywhere. But the source IP of the client NATs to a public IP of the VPN gateway that is connected to Internet. In this case, TCP probe or ICMP based RTT will go completely wrong. This is especially true with Hub and Spoke networks.
e) This solution should work for all protocols including HTTP, SIP, RTSP, SMTP etc.,
f) The client proximity solution should honor Layer 7 features like cookie persistency.
g) The client proximity information obtained must be in sync with all other sites.

- Site health checks- Health checks should be content aware and must be done to the remote server farms.

- system properties - The GSLB solution must consider system health properties like CPU, session utilization of all sites

- Persistency and Availability - GSLB persistency must be maintained. If a client connects to site B because site A is down. The next request should continue to go to Site B even when Site A comes back.
- Ask for throughput and connections per second with GSLB configured in the ADCs.
- Remote forward - When local servers are down, the client request must be proxied to remote site transparent to the client. Client should not even know that site is down.