Delayed Binding Syn -> LB -> ack (sent back) TCP packets (sIP, sP, dIP dP, seq, ack, ....) Seq # is the # of the packet The ack the LB has to choose a "right to left 42 -> LB LB ->ACK 43 , seq -101(made up by LB) send me more of the header Back and forth Syn to info server 3 seq-42 Ack back to LB s seq 17 ack =43 Every packet going left to right : seq fiel is OK but the seq is wrong (it needs to have the seq # of the initial server seq # ) (change both direction) ack # ok but now the delta has be LB has to keep a table that maps the ID of the TCP stream and keep track of the deltas for the ack / seq to maintain the correct connections .. Load Balancing: - add-a-box weaknesses: -- singel point of failure -- session maintenance Failover: - keep a hot/warm standby : state ready machine example Dynamic DNS - reliable, change, a name to IP binding in order of a minute TTL to 1min or 5 min .( includes waiting for client cache timeout ) 100 min MTTR 10000 min MTTF ~ .99 up time LB - IP 1 AS AS LB - IP 2 every 10000 minutes my LB fails, and in one minute it returns for 100min I only have one LB and no fail over. failure every 10K minutes 1K recovery . 9999 of up time the driver is the response of the failover.. Amazon feature: IP remapping - the place you want to put dynamic DNS DNS " mysite = sIP goes to sIP --> LB LB fails client cache timeout and DNS timeout before you can remap we can us an "interface IP" to use a last hop change (sub second response) Can use * round robin DNS gives me several equal bandwidth, * IP remapping just in front of LB * standby hardware for LB (customers hate this as the HW is not being use in a good state)