Network Sweeping

By | August 15, 2016

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.