If you specify FQDNs that use GSLB as your backend servers, this may result in higher latency.
- Proxy Backend
If you specify FQDNs for backend servers in a proxy, DNS resolution is periodically performed to determine the IP addresses of the backend resources. If DNS servers use GSLB they may supply a suboptimal result if the provider’s GeoIP mapping database is inaccurate for Silverline's source IP. This often manifests as additional latency for proxied connections.
Definition: Global server load balancing (GSLB): When a DNS server replies to a query for an FQDN with an answer based on the location of the client. This is determined by looking up the source IP in a GeoIP database.
An abbreviated example of how this problem can manifest is:
In the example, the GeoIP database lookup incorrectly reports that the source IP of the DNS query (198.51.100.53) is in London, UK and the IPs returned lead to higher latency as traffic to Silverline's US West POP for that proxy uses backends on a different continent.
While Silverline takes actions to avoid and correct such issues, resolving FQDNs to determine backend addresses shifts part of the responsibility for routing traffic to an external entity. We have observed some issues such as:
- In some cases, proxies have been directed to use backend IPs that are geographically distant from the region even though there are more local backend servers.
- When a GSLB provider updates their GeoIP database, they have determined that our location is now different and begun to respond with different backend servers.
Both of the above cases are outside of Silverline's and your control. Silverline doesn't have a reliable mechanism to verify whether GSLB providers return optimal DNS answers and will honor these results. This can result in high latency and can occur outside of proxy deployments.
See How To: Configure a proxy with backends in different geographies for considerations when supplying backends for managed proxies.
Silverline recognizes that you may not have an option to avoid GSLB for backend selection for some applications. We attempt to minimize this issue by:
Requesting DNS resolutions via IP addresses that are reliably known to be geographically close to our points of presence.
Working with GeoIP database vendors and popular GSLB providers to ensure that the IPs we use have accurate location data.
Warning: these techniques are reliable but do not remove the risk that a DNS server will return suboptimal results or change without notice.
If you are experiencing such issues, contact the Silverline SOC.