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How Layer 4 Switching Works: SLB
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SLB works by creating virtual server farms and displaying them to the outside world as a single IP address SLB then handles which real server is contacted regarding the client's request using its load-balancing algorithm Thus, SLB controls the loads on the real servers dynamically based on traffic loads Unlike other methods of load balancing (such as roundrobin DNS), SLB enables dynamic response to varying server loads SLB works based on a few fundamental rules:
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Virtual servers are defined on the SLB device These virtual servers are configured with the IP addresses that remote clients use to connect to the resource The IP address for the virtual server is translated into an IP address of the real server deemed best able to respond to the request by the SLB device The real server address is chosen from a pool of real server addresses defined for the specific application that the client is requesting In this manner, multiple servers are displayed as a single server to the client Because the pools of real servers are definable by application, a single virtual server IP address can be used to redirect traffic to several pools of real servers, with different real servers serving different requests SLB is transparent to the client and can be transparent to the internal server pools SLB can also increase security for the server pools by enforcing protocol restrictions, as well as performing more complex packet analysis (like SYN flood protection) SLB can operate as a NAT server for the clients, the servers, both, or neither
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To examine how SLB works in more detail, let's take the example network shown in Figure 21-1, and go through the process as a remote client requests a web page Note that in this example, NAT is enabled on the 4840 for the servers
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Figure 21-1: Example SLB network First, the client (Client1) attempts to connect to the target web site, wwwcorpcom, and the DNS server resolves this name to the IP address of the virtual server configured on the SLB device (69411) The SLB device (4840G) then looks at its listing of real servers for this application (Port 80, HTTP) and forwards the packet to the server it believes is the least busy in this case, Server1 (10111) by modifying the destination IP to 10111 (You will learn more about how the least busy server is determined later in this section) Server1 responds, and the 4840 forwards the packet back to Client1, this time modifying the source IP from 10111 to 69411 so that Client1 can't tell which server is actually responding While this process is occurring, let's assume that Client2 is also attempting to reach wwwcorpcom Client2 visits the exact same IP address as Client1 (69411), but this time, the 4840 decides that Server2 is best able to handle the request The 4840 modifies the destination IP to 10112 and forwards the packet Server2 responds, and the 4840 again modifies the IP address in the packet to 10111 to keep the client unaware of this process Now that you have seen basic SLB functioning, you're ready to learn about some of the more advanced functionality First, let's examine the load-balancing mechanisms available with SLB It offers two different load-balancing mechanisms to suit your particular needs: weighted round-robin and weighted least connections Weighted round-robin is the simplest With a nonweighted round-robin algorithm, the pool of servers is simply accessed in a circular fashion, with no particular priority given to one server over another Therefore, if you had three servers, it sends the first request to Server1, the second to Server2, and the third to Server3, and then restarts with Server1 With weighted round-robin, however, you could apply a weight of four to Server1, five to Server2, and one to Server3 As a result, the first four connections go to Server1, the next five go to Server2, and the last connection goes to Server3
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This strategy is a very simple method of weighing access to your servers, however, and its primary downfall is the same as any other simple round-robin technique: if a server fails, that server has to be accessed x number of times before the algorithm switches the connection to a new server A second problem with this technique is that time and session length are not taken into account Sure, maybe Server1 should be able to respond to four times as many connections as Server3, but what if they all exist at the same time Or what if they are all activated during a short time (like within milliseconds of each other) Weighted round-robin cannot take these factors into account, and, therefore, slow response times can still be a problem in certain situations It is simple to design and implement, however, and works well for applications with fairly static data access and connection establishment requirements Weighted least connections, on the other hand, is a more complicated, and consequently more robust, algorithm Weighted least connections examines current connections established to all of the servers in the pool and assigns new connections based on the current traffic load of the server, as well as the server's configured weight Although this process sounds complicated, the math behind the algorithm is really fairly simple; just add up the total of all of the weights, and each server gets its corresponding percentage of the total So, in the previous example, the weight of the servers was four for Server1, five for Server2, and one for Server3 Four plus five plus one equals ten, so Server1 (weight of four) gets 40 percent of the traffic, Server2 (weight of five) gets 50 percent of the traffic, and Server3 (weight of one) gets 10 percent of the traffic If you started with these three servers and no connections were established, the first connection to come in goes to the server that was farthest away from its configured total in this case, Server2 (50 percent away from the configured traffic total) The second connection then goes to Server1 because it is 40 percent away from its configured total, Server2 is at its configured total (two connections divided by one connection to Server2 equals 50 percent of all connections allocated to Server2), and Server3 is only 10 percent away from its configured total The third connection then goes to Server2 because Server1 is only 666 percent away from its configured total (Three connections divided by one connection to Server1 equals 3333 percent; 40 percent minus 33 percent equals 666 percent) Server3 is still only 10 percent away from its configured total, but Server2 is now 1666 percent away from its configured total (Three connections divided by one connection to Server2 equals 3333 percent; 50 percent minus 33 percent equals 1666 percent) On the next connection, Server1 receives the connection because it is now 15 percent away from its configured total (Four connections divided by one connection to Server1 equals 25 percent; 40 percent minus 25 percent equals 15 percent) Server2 is at its configured total (four connections divided by two connections to Server2 equals 50 percent; 50 percent minus 50 percent equals 0 percent), and Server3 is still only 10 percent away from its configured total Somewhere around the sixth connection, Server3 finally gets a connection, but you get the idea This algorithm deals very well with the issue of having several connections open at the same time, but it is still susceptible to the problem in which several connections are established in a short time It is also still susceptible to the problem in which a server dies, and packets are still forwarded to that server instead of other servers in the pool To deal with these problems, a few additional functions need to be employed
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To eliminate the issues caused by large quantities of connections being sent to a server in a short time, two features of SLB are used: slow start and maximum connections The slow start feature limits the number of connections that can be sent to a server over a short time This restriction helps reduce errors when adding a new server to the pool because, otherwise, it will be sent a large number of connections in a short time The maximum connections feature puts limits on the maximum number of connections to a single server at any given time This restriction keeps a server from being overloaded in situations in which the hardware is reaching its capacity To help reduce errors when a server fails, an additional feature, automatic failure detection, is employed When configured, automatic failure detection keeps track of responses to client requests from each server If a server fails to respond to a client request, automatic failure detection increments a failure counter Once the failure counter reaches a certain configured threshold, SLB removes the server from the pool, assuring that no new connections are sent to that server A retry timer is then set, and after that timer expires, SLB sends the next available connection to the server If the connection succeeds, the server is again added to the pool; but if the connection fails, the retry timer is reset This functionality is called auto unfail, and it is extremely handy when a server simply times out on one or more requests (but isn't actually down) With auto unfail, the server is removed from the pool only after failing a specified number of requests, and it is automatically added back in when it begins responding normally Another feature that helps eliminate problems when a server is just overloaded or slow to respond is TCP session reassignment Remember from 5 that when a client wants to connect to a server using TCP, it sends a packet with the SYN bit set This signals the request to establish a session The SLB device tracks SYN bits sent to each server; and if a server does not respond after a configurable number of connection establishment attempts (marked by multiple SYN packets being sent from the same client), SLB automatically sends the next SYN request to a different server To allow for servers that have very specialized "tuning" for a specific application, SLB supports the use of "per application" server farms known as port-bound servers This feature allows you to configure different server pools to support different protocols while using the same external IP address For instance, Figure 21-2 shows one external IP address that all remote clients use to connect to all offered network services However, client requests destined for port 80 or 443 (HTTP and HTTPS) go to Server Farm 1, while client requests for port 20 and 21 (FTP) go to Server Farm 2
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Figure 21-2: Example of port-bound servers For even more advanced performance tuning and monitoring, SLB includes a feature known as Dynamic Feedback Protocol (DFP) DFP monitors servers using either one or more agents (software designed to track performance statistics on devices), HTTP probes, or both, and alerts SLB to any performance problems (including, with DFP, general server slowdowns due to memory leaks, high RAM use, hardware issues, and so on) The DFP functionality allows SLB to dynamically adjust the weights of the servers in a pool to deal with unexpected performance issues, including either under- or overuse of a given server HTTP probes, on the other hand, allow for a very simple mechanism to verify that HTTP servers and firewalls are up and responding to requests With an HTTP probe, a request is sent to the configured web servers at specified intervals If a configurable positive status code is returned (4xx is the default), the server is assumed to be functioning SLB, although solving many of the problems associated with consolidating high volumes of traffic, does introduce its own new set of issues Primarily, the problems with SLB are attributable to the fact that most TCP/IP applications were not designed to enable several servers to handle a given stream of requests As a result, a server may not understand a subsequent client request that relies on a previous request because a different server handled the initial client request To deal with these issues, SLB makes use of two features: "sticky" connections and delayed removal of TCP connection contexts Sticky connections allows a request from a given client to an application to always go to the same server, as long as a timer is not exceeded Thus, applications that must rely on previous session information (like some types of active web content) can function because all subsequent client requests will go to the same server as the initial request Delayed removal of connection contexts allows the SLB switch to delay the removal of terminated sessions from its tables until a delay timer has expired Therefore, packets that are received out of order can be processed correctly by the server before the session is terminated SLB also includes some anti-DOS features, most notably, the ability to perform NAT for both clients and servers, and Synguard (SYN flood protection) The NAT functionality of SLB is similar to other NAT servers, and it conforms to RFC 1631 SLB can perform translation for
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both the client (translating public client addresses into internal, private IPs) and the servers (translating the server's public or private address into the NAT server's IP address) When enabled for the client side, NAT ensures that packets back to the client are always automatically sent back through the SLB device (Otherwise, you must make sure the servers are configured to point back to the NAT server, typically as their default gateway) When enabled for the server side, SLB NAT can ensure that the clients cannot reach the server farms directly, increasing the overall security of the server farms SLB Synguard defends against a well-known DOS attack that was used to bring down several web sites (most notably, CNNcom) in early 2000 A SYN flood is when a device sends a packet with the SYN bit set and with a fake return address The end host attempts to keep track of the partially open connection, which consumes resources If enough SYN packets of this type are sent in a short time, the device may run out of resources (typically RAM) and crash Synguard in SLB attempts to eliminate this problem by rejecting any SYN packets over a configurable threshold (measured in unanswered SYN requests) within a configurable time (measured in milliseconds) Note Synguard can protect devices located only behind it in the network Routers in front of it need to be protected using access lists or firewall devices Finally, what if you are configuring one box to keep your servers clustered so that you have redundancy, but the SLB switch fails Cisco has a solution for this problem as well You can achieve SLB redundancy through two methods: Hot Standby Router Protocol (HSRP) and SLB stateful backup The basic idea of HSRP is that it clusters your routers (or, in this case, your SLB switches "clustering your clusterers," you could say) by assigning the group of devices a virtual IP and MAC address, similar to SLB The major differences between SLB and HSRP lie in the implementation and logic (HSRP is implemented on the devices in the "farm" instead of on other devices, and HSRP does not offer any type of load balancing only redundancy) Note Due to page constraints, HSRP is not covered in detail in this book However, for basic HSRP information, you can visit the following address: http://wwwciscocom/warp/ public/cc/so/cuso/epso/entdes/hsrp_wphtm SLB stateful backup works in conjunction with HSRP to make sure the backup switches in the HSRP cluster have the most recent SLB information from the primary switch This functionality allows for minimal data loss if the primary device fails and HSRP has to failover to the backup switch All things considered, SLB is a robust method of controlling access to your servers and providing for complex load balancing in high-traffic, high-availability environments
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