Aug 302016
 

In OSS, we use the polling concept often to pull statistics and configuration data from the devices. If the devices we are dealing with are implementing the pull based protocols such as SNMP or FTP, we cannot get rid of this.

All types of polling processes come with a polling period. If I have 100 routers and a polling period of 5 minutes, each and every 5 minutes I will have to connect each device and pull the necessary KPIs to be injected into my DataMart.

If you look at the CPU and Memory utilization of a performance management server (poller) during the process, you will see high peaks at the start of the polling periods. If we follow the 5 minutes polling example above, we will see the peaks at the minutes, for example, 0,5,10,15,20,25,30,35,40,45,50,55. If your polling period is 5 minutes, you have 5 minutes to finish your job. If it exceeds that period, you will fall into data consistency issues. As the node and KPI count increase, you have to throw more hardware to finish soon. (For each device connection, we will most probably want to open up a separate thread until we hit the point of diminishing returns)

Considering the whole collection process does not occupy the whole 5 minutes’ period, the remaining period will be wasted in the waiting state for the server. Since the hardware configuration was designed for the peak times, our server will remain to be “expensive”.

Assigning a polling time to a specific node is the key to this problem. In this approach, we divide the polling period to sub-periods. So, if the polling period is 5 minutes, we can divide it like:

10 nodes Zeroth second of First Minute, 10 nodes Thirtieth second of First Minute, 10 nodes Zeroth second of Second Minute, 10 nodes Thirtieth second of Second Minute…

Here we put 10 nodes into each 30 seconds timeframe, to finalize polling of 100 nodes in 5 minutes.

We also need to consider the speed of these nodes. Some nodes will suffer performance problems due to weak hardware configuration or high load. The response time of those may exceed the 30 seconds timeframe.

In order to cope with this problem, we should also consider putting the slowest responding nodes to the earliest sub-frames. This way, a node’s polling can “extend” to the next subframe and can still be finalized in the given 5 minutes. This, of course, requires you to maintain a continuous baseline of node response times at the server side.

Splitting the polling period and distributing the nodes wisely to the sub-periods will help you to reduce your hardware costs.

Aug 152016
 

Today’s topic is about the Network Sweeping and how it can be optimized. As you may know from the previous topics, sweeping means searching a subnet by attempting to connect to each and every possible IP addresses it has.  Usually, the initial protocol is ICMP due to its’ low overhead. (In that case, the sweep is called Ping Sweep). SNMP and even HTTP interfaces are also used as sweep protocols.

Sweeping is used in different domains, such as;

  • Security
  • Inventory Management
  • Performance Management
  • Configuration Management

Sweeping could be time and resource consuming (both for sender and receiver side). That’s why, for most enterprise customers, it is normally done daily.

For large networks, it may take hours to complete a sweeping process. Consider the scenario of sweeping a class C IP subnet. (It will have at least 254 IP addresses.). Also, suppose that only 10 devices exist in that subnet. I am supposing I will be using ICMP for discovery. That is the simple ping request and at least I need to send 2 ICMP packets to be sure that there is a device there. (50% packet loss still means the remote side is up)

For the reachable devices, the round-trip ping time should not exceed 5ms. Considering we have 2 ICMP packets, it would be 10ms per check. We have 10 devices and it would take around 100ms which is well below 1 sec. That’s a great performance if you just consider pinging the “up” devices. But what about the remaining 244 down ones?

ICMP timeout kicks in when dealing with the dead devices or vacant IP addresses. ICMP timeout is the duration in milliseconds for the ping software will wait until an ICMP echo reply package arrives. If the packet does not arrive within that period, it will report it as “down”. The default timeout for ICMP in Cisco routers is 2 seconds. So, using the defaults, if you use 2 seconds as the timeout, for 2 packets in the test, you will have to wait 4 seconds per test. If we do the math, the total wait time for the class C subnet on hand would be 976 seconds, roughly 16 minutes. Organizations that rely on sweeping normally have much bigger subnets with thousands of possible IP addresses. The sweeping process would take hours in such kind of networks.

Luckily, we can tweak this process so it will take less time.

1: Use of Parallel Measurements:

This is the first thing we need to do. Opening multiple threads of ICMP operation at the same time. How about opening up 1000 threads? It will be finished in 4 seconds. Isn’t it great? Not really, it has some consequences.

  • Increased LAN traffic: Sending 1000 ICMP packets at the same second will generate lots of traffic in your LAN/WAN. (around 70 bytes per packet * 1000 threads = 70000 bytes/sec =560000 bits/sec = 560Kbps one-way traffic. Considering there would be replies to these requests, the total bandwidth consumption can easily reach 1Mbps.
  • CPU Cycles: Each thread will consume CPU and Memory resources. Source machine should be able to cope with this. 

This is just the sweeping part of it. In the real world scenarios, no inventory or security tool will stop there after it discovered a live IP address. It will go ahead and try to fetch more information. So these two parameters can boost if you open up too many threads.

2: Optimize your ICMP Packet Timeout

I told that the default ICMP timeout is 2 seconds. Luckily this is configurable. Go ahead and send some pings to those destination IP addresses. For the “live” ones, capture the round trip time. This is the network delay (plus the processing delay of the remote NIC). That delay will not change much on LAN links, may slightly change on WAN links. Baseline this. So if it is 100msec you can easily put a timeout of 300 msec. This is 3 times more than the baseline but still well below 2 seconds default.

Keep in mind that ICMP is one of the protocols which has the lowest overhead. Layer 7 protocols like SNMP and HTTP will have much more overhead, so above suggestions may bring greater value.

Long sweep times can also result in inconsistencies between the sweep periods. Suppose you started with 10.1.1.1 /24 and found out that 10.1.1.1 is vacant. You continue your sweeping and 10 seconds later 10.1.1.1 became up. If you sweep every day, your inventory (and other dependent OSS systems) will not know this device until the next day. (If you don’t have a change process in place for this device) That’s why there should be a mechanism to listen for new IP address activity during the sweep time. DHCP logs could be a good alternative for the networks that utilize DHCP for IP addressing. A costlier solution could be listening for Syslog events or switch span ports.