CODEWITHSUNDEEP
c#snmpsharp-snmp
Aereal
Aereal

Reputation: 103

SharpSNMP max-repetitions increase causes buffer size exception through GPRS

I am trying to send SNMP requests to a remote location.

I am using the SharpSNMP 8.5.0 library and the Snmp.BulkWalk example from a code project post ( here ).

In the example, they use 10 as max-repetitions and using sniffing software I noticed that is creating multiple datagram packets to make the walk within in the subtree. Actually I am getting 120 packets results back every time. So I decided to try a higher max-repetitions number and I noticed that the packets number is going down, actually I can get all the data in one packet. Now I have another problem: the remote device is using GPRS when I snmpwalk on the device from the server using GPRS, I get a timeout or a buffer out of size error. When I run the same solution on my local PC and I access the remote device from my router(no GPRS involved) I don't get any errors and get all the data!

Can someone explain this behavior? Does it have to do with a GPRS limitation? GPRS is unreliable? Or is it a network limitation on the server?

(The MTU in the server is 1500). Does anyone have an experience on the best practices and the optimal packet size that can send through SNMP-UDP datagram packets?

Upvotes: 2

Views: 1038

Answers (2)

Jolta
Jolta

Reputation: 2725

Regarding best practices for SNMP packet size, I've always been told that you should avoid exceeding the network MTU. In other words, set the max-repetitions so that the Ethernet frames don't regularly exceed 1500 bytes. (Of course, this assumes that the size of your table cells is predicable.)

While using larger packets should work on most well-configured networks, it's advisable to avoid having fragmented packets on the network. Perhaps packet re-assembly might create larger overhead in the networking equipment. And if you're going to fragment the PDUs over several packets anyway, the drawback of having to do a few more back-and-forth requests is not that bad.

For example, Cisco equipment seems to follow this best practice, and it's recommended in a Microsoft article.

(BTW, next time you have two separate questions, consider posting them as two questions!)

Upvotes: 1

Lex Li
Lex Li

Reputation: 63264

Though I am the author of that library, I could not answer the GPRS part, as I am not a mobile network expert.

What I could answer is the packet number part, which is relatively simple if you check out the definition of "max-repititions",

https://www.webnms.com/snmp/help/snmpapi/snmpv3/v2c/maxrepetition.html

By setting a larger value to this parameter, a single packet can contain more results, and obviously less packets are needed.

I used 10 in that Code Project article, because it was just an example. You might see from the link above that other libraries might use 50 as the default.

Upvotes: 1

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