Packet Loss Calculation

Dear Experts,

I have a lot of Packet Loss on my remote routers. My remote routers are connected to my HO router via satellite link. I am using SNMP for monitoring devices health and manage engine software.
I need help how a device calculate its packet loss? and why it is so high ?
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Kanti PrasadCommented:

It is  calculated as a moving average  and each ICMP request has a success or failure value which is stored in a buffer of approx length 10 and the average is calculated.
9 success and 1 failure ping = 100 * 1 / 10 = 90% packet loss and if in memory only 2 pings 1 success and 1 failure ping = 100 * 1 / 2 = 50% packet loss.

You can have look at the below link
nainasipraAuthor Commented:
Dear Kanti,

Thanks for your response.
If I am using Monitoring Server on Head Office and SNMP is configured of remote router, On Server I think it will fetch the logs from router and show Packet Loss detail.
My confusion is how this packet loss is calculated in logs ?
is router automatically pinging all its connected interfaces ?
and monitoring software shows device overall packet loss not device interfaces ?
thanks in advance!
That is ping loss in example above (at least ping lost at default ping packet size). There is a big chance that packet loss is much bigger as packet size grows.

If you have better switch or router usually you can calculate packet loss from information under WAN interface.  You can see number of transmitted packets, and calculate number of lost packets from various reasons, and than you can calculate radio.
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Kanti PrasadCommented:

Routers send packets to its default gateway if it cannot find them in its own routing table. It goes into this same (gateway\router table) search cycle till it finds its destination. The overall average calculation is done from a source to a destination and not at individual interfaces. Also when packets take a certain route to their destination they do not have to take the same route back. So if there is a wrong setup at the destination even though your packet reached it may not return back as the destination default is not setup correctly.

Generally in static routing the administrator manually creates the routes, but in dynamic routing the routes are “learnt” and built automatically by the routers themselves. Dynamic routing allows routers to “talk” to each other to find where other networks are located.

Look at this link for more details
nainasipraAuthor Commented:
Dear Predrag,
my confusion is still, please help me to clear this concept.
How SNMP protocol calculate Packet Loss of any device?
as per me, may be I am wrong: SNMP protocol will fetch log records of any configured device and show the result but from that device how its calculated its packet loss?
That most likely depends o device itself (programming of device).
If you issue on Cisco #show interface fax/x (for 100Mb interface)

you can see report like
#sh interface fa4

FastEthernet4 is up, line protocol is up
  Hardware is PQII_PRO_UEC, address is 4403.a7b2.a862 (bia 4403.a7b2.a862)
  Description: $FW_OUTSIDE$
  Internet address is x.x.x.x/22
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/319/0 (size/max/drops/flushes); Total output drops: 6
  Queueing strategy: Class-based queueing
  Output queue: 0/1000/0 (size/max total/drops)
  5 minute input rate 50000 bits/sec, 22 packets/sec
  5 minute output rate 6000 bits/sec, 13 packets/sec
     39904524 packets input, 2705540460 bytes
     Received 13184349 broadcasts (0 IP multicasts)
     53 runts, 0 giants, 88 throttles
     98 input errors, 0 CRC, 0 frame, 98 overrun, 0 ignored
     0 watchdog
     0 input packets with dribble condition detected
     21498571 packets output, 2010623932 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     1 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

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But what portion is taken for packet loss depends on programming (I guess that is sum all error types)
What is your current packet loss?
nainasipraAuthor Commented:
my devices packet loss is 30% to 60%.
It's way to high. Do you have VPN?
nainasipraAuthor Commented:
yes, its VPN tunnel and satellite link as well.
nainasipraAuthor Commented:
one question: if you see as show interface x/x then you will see different input and output.
I think it possible that incoming traffic is more or less than output traffic of any interfaces?
this difference is not packet loss? need to clarify please
VPN can cause high packet loss if MTU and MSS size are not set to properly. In combination with satellite link, can cause big mess (that's why I was asking.
You can download 100MB file size, and, most likely, you will send during transfer less than 1MB of data.
So difference between output and input per se is not problem.
nainasipraAuthor Commented:
what should be MTU and MSS size as best practice ?
That depends on tunnel and encryption. Read this article.
MSS = MTU - 40

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