Forum Discussion
NimbostratusSNAT POOL Lists
This is talking about SNAT POOL LISTS on the LTM
In this procedure, we configure a SNAT pool. A secure network address translation (SNAT) translates the source IP address within a connection to a
BIG-IP system IP address that you define. A SNAT pool is a group of these IP addresses.
Popular websites with heavy traffic (such as Google and Facebook) may require more SNAT addresses than typical websites. To estimate number of
SNAT address you need for these high-traffic sites, you must have a number of SNAT addresses larger than the maximum number of concurrent
connections per destination IP address divided by 64,000 (Number of SNAT address > (maximum concurrent connections per destination IP address /
64,000)).
For example, if the destination IP address has 250,000 maximum concurrent connections, you would need 4 SNAT addresses in the SNAT pool
(250,000/64,000 = 3.906). You want to make sure you have enough SNAT addresses to cover the site with the highest expected traffic.
If the Destination IP is the interface of the physical server (Node) in the pool, how are the SNATs going to know the connection limit of the server (node)
If we are saying facebook needs 1,000,000 conncurrent connections, so we need 16 SNATs address, is this not back towards the F5 ?
What connection limit are we trying to fix with SNATS ??
Graham
3 Replies
- Kevin_Stewart
Employee
A SNAT or SNAT pool is really irrespective of the server node's capabilities. TCP allows for 65,535 port numbers per IP address. Subtract the 1024 "well known" ports, and you have ~64k ports. The BIG-IP server side connection (in a full proxy configuration) is the client side connection to the server node. A stateful client will establish a connection to a server using a random source port, going to a known listening destination port (ie. port 80). If the client runs out of random ports to a given destination IP address, it can no longer make new connections to that destination address. The SNAT pool, therefore, allows the BIG-IP to spread the connections across multiple source addresses. 
pete_71470Cirrostratus
The number of pool members matters, too. The F5 can reuse source ports to a degree (sol7820). The most concurrent connections we've seen with a single SNAT (AutoMap) and a pool of 4 servers is 187,000 (at which point ephemeral port exhaustion was logged). I'm sure it would have gone higher - but the servers began slowing down, leaving connections open longer and longer.
ApproxeeHi Kevin / Pete, Thanks very much for the reply. It all makes sence now - Thanks again
Graham
