BROCADE IP PRIMER

Co ns ol ePwrCC U1 PU 2 PU 3TxActRxActActi ve1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 6J -F16GCJ -F16GCServerIron 850J-F16GCJ -F16GCJ -F16GCJ-F16GCJ -F16GCThe Health CheckLayer 7Now, we're getting sophisticated. Layer 7 uses Layer 3 and Layer 4, but adds acustomized element to verify that all seven layers are functioning as expected.The load balancer can be configured to perform a rudimentary Layer 7 task toverify that the server will respond in a desirable way. This will be differentdepending on the Layer 7 protocol you're load balancing (i.e, HTTP, DNS, SMTP,etc.).If we go back to our HTTP example, we've done the Layer 3 check, so theserver's responded to a ping. We've done the Layer 4 check, so we know theserver is listening on TCP port 80 (HTTP). Now, we'll do the Layer 7 check. Thisstarts out similarly to the Layer 4 check. The load balancer sends a TCP SYN.The server replies with a TCP SYN/ACK. This time, the load balancer finishesthe handshake, and sends a TCP ACK. Now the load balancer will make anHTTP request. Let's say the web application is called “webapp.cgi.” The loadbalancer will make an “HTTP GET /webapp.cgi” request of the server. Theserver will send a TCP ACK (acknowledging the request). The server will thensend an “HTTP Status 200 OK” message. This tells the requester that it understoodthe request, and it is ready and able to reply. Usually before the actualrequested data is sent, the load balancer will send a TCP RST (reset) messageto the server. It does this because it already knows that the server will comply.It knows that it's operating at Layer 7.2 M M S W1 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 61 2 3 4 5 6 7 8 9 1 0 1 1 12 1 3 14 1 5 1 6TCP SYNTCP SYN/ACKTCP ACKHTTP GET/webapp.cgiTCP ACKHTTP Status 200 OKTCP RSTWhat if the “webapp.cgi” isn't there (for whatever reason)? The load balancerwill establish a three-way handshake. It will send an “HTTP GET /webapp.cgi”request to the server. The server will send a TCP ACK, but this time, the serverwill reply with an “HTTP Status 404 File Not Found” message. The load balancerrecognizes the HTTP status codes. It knows that if it does not receive anHTTP 200 status code, something is wrong, and it will deem that serverinactive.But what if you're not load balancing HTTP? No problem. The load balancercan handle all kinds of different Layer 7 protocols, and it will perform its Layer7 check accordingly. If you're load balancing DNS, it will attempt a namelookup. If you're load balancing FTP, it will make certain it receives an initialgreeting with a proper 220 status code. What if you're using a Layer 7 protocolthat the load balancer doesn't know? No problem. You can customize yourLayer 7 health check to look for a specific text in its response. In other words,you can teach the load balancer what it should expect to see when trying tocommunicate with the protocol.Brocade IP Primer 107