SNMP - The Simple Network Management Protocol
From Web (09.06.1997)
Table of Contents
When we were young, we wanted to become a pilot,
astronaut or a fire engine driver.
- Our mothers would laugh at us.
When we were in our pre-teens, we wanted to become programmers.
- Our fathers would laugh at us.
When we became programmers, we wanted to become network administrators.
- The world (our girlfriends included) would laugh at us.
The above is not a figment of our imagination but facts relating to our daily
we started off as programmers we proved our fathers wrong. Sorry, we could not
do the same
regarding our mothers. This left us with the world and our girlfriends to deal
It took us some time to devise ways to prove them wrong. But before we let you
secret of how to do it, lets try and justify their laughs.
Network administrators of yore were perfect nerds, living in ivory towers
(actually dingy low-lit rooms , disconnected from the world). In short, they were
people to be avoided, basically belonging to class called social rejects. The
rationale behind this was very simple. Network administrators had no control over
their networks. The designation 'Network administrator' sounded glamorous but
that was where the glamour ended. All he was supposed to do was to add/remove
users to the network , add/remove network paraphernalia ( or was it peripherals?).
If he got bored of doing all this, he would set up the network all over again and
life went on. (By the way, the world and his girlfriend had left him)
When he coined the title, George Lucas did not even dream that it may become a
mission statement for many a network administrators on this planet. The network
administrator it was high time to take control of the life (life had become
synonymous with routers, bridges and other peripherals.) To a layman, getting
control over his life meant knowing his wife, family, his pay-check, his dues
(however unpleasant they may seem.) But to a network administrator, it meant
knowing the traffic on his routers, the load on the server etc. But the big
question was 'How does he do it? Simple SNMP ???
Sorry, we did not mean to shock you. We thought that you would have got impatient
reading this stuff and so we decided to get down to business. SNMP is an acronym
for Simple Network Management Protocol. It belongs to the rare breed of computer
jargon which justify their name, i.e it is actually simple. As seeing is believing
let us check out SNMP.
Imagine a situation where a company has 50 routers , 20 servers and all sorts of
network hardware that runs the TCP/IP suite (A network administrators utopian dream!!).
In such a situation questions like 'What is the load on the server?', 'What time
does this loading take place ?' or 'When to expand the lease line?' etc formed
part of the network administrators nightmare ( almost lead to his downfall).
To add to his sorrow, he had to work with diverse piece of equipment from diverse
companies running equally diverse protocols. Variety is the spice of life you may
say but this is the case of food ( for thought) getting too spicy. The only thing
that was common between the network elements was that they ran the TCP/IP suite.
Therefore TCP/IP formed the lowest common denominator as it was the only thing we
could standardize on. Beyond this standardization was almost zero, in some cases
it approached the negative index. The Network Administrator had to figure out a
way to manage his network which was running the TCP/IP suite. Therefore this gained
the generic term network management under TCP/IP. Here is how the Network
went about their work.
The first thing he did was to built himself another ivory tower ( a cleaner
one this time). This he named it as the 'Network Management Station'. He then
went about stating the guidelines for the protocol to be called 'Simple Network
Management Protocol (SNMP). The guidelines were as follows [ in no order of
The protocol should be cross-platform compatible ( No we DON'T use JAVA). Put
simply, it meant that the rules for talking to a router from CISCO should not differ
from those used to talk to a router from 3COM. ( All trademarks acknowledge)
The language used ( if any) and the output generated should not increase the
network traffic. Rationale:- Choking of network by the traffic should not lead the
network administrator being fired.
It should be fast. Faster network always gave the network administrator a
Well that is one of the reasons as to why a network administrator did not buy a
The system was to work as if it were operating in a client-server
the software running at the network management station acting as the client and
software running at the other network elements ( jargonsss.... but basically
like routers, bridge , etc. ) acting as the server. Perfect case of 'cart
the horse". (Taming of the shrew ( or the network)). The factors or the
affecting the server were to be stored in a database.
But before he did all this, he implemented the most important item on his
He thought life was getting too simple due to all the 'simple' stuff floating
He decided to rename everything that he set his eyes on. So the client software
a manager ( to reflect his new status), the server software became the agent.
( it would have been a better idea to call them spies because all they do was
to pass on information about the network element). The database seemed to
people of likes of ORACLE, SYBASE etc. So he decided to rename it as the
Management Information Base or the MIB. The variables were formatted using a
definition language called the ASN.1, but more on this later. The variable
was renamed as an object. The final objective of the entire exercise was to
a case of name changed to protect identity. We recommended you take a break
(coffee or otherwise) because it is going to be a hell of a day today.
The Network Administrator had by now renamed most ( if not
all the) factors ( name unchanged) involved in making up a network . Rule of nature state
that every simple answer gives rise to complex questions. So the question was how does the
Network Administrator know which element he was to communication with? He, therefore went
around drawing a 'tree diagram' of the network. He arranged the factors involved
as part of the tree. The tree diagram was used to
- define administrative relationships
- organize network management data
- assign a unique identifier to every network management variable.
As he was still in the mood to rename things, so he decided to call the tree as the
Structure of Management Information (SMI) . A sample of the tree diagram (or the SMI)
is shown below.
The elements of the SIM were called as Groups or modules. These groups were
a collection of objects ( the good old variable) relating to that group. For
the Ethernet module contained further groups of Ethernet related stuff like
and so on. These were further subdivided to obtain groups or objects as the
case may be.
As each module/group and objects were numbered, accessing them became a
play ( not necessarily the network administrator's child) The modules at the
end of any node was called the leaf node ( network administrators are the most
eco-friendly people on the planet)
The elements in the SMI have been assigned numbers. This makes traversing through the
SMI easier. You always travel from the roots up ( or down). For example to access the
interfaces 'leaf', we traverse through root - iso - org - dod - internet - mgmt -
mib - interfaces. As numbering has been enabled to access the same interface all
we say is 220.127.116.11.2.1.2 (eco-friendly again - less waste paper).
By now, or so we hope you will be able to visualize the setup of the entire
If you don't then please hit the Page Up and go through it all over again. If
understand then it is time for some action, Let's put all the pieces together.
The manager can seek any data from the agent at any point of time. To do this
manager sends a UDP packet to the agent. Actually any other protocol can be
instead of UDP. Despite being unreliable UDP won the battle for the post of
messenger because of it's ability to have a very small packet size. This
fulfill the second guideline of the specifications. The manager speaks to the
on port 161 whereas the agent responds by communicating with the manager on
The SNMP version 1 has only 5 (unbelievable but true)
commands in it's instruction set. They are listed as below
- Get_Request - used to request the value of 1 or more MIB variable
- Get_Next_Request - used to read the next value , works in a sequential way
- Set_Request - used to update one or more MIB value
- Get_Response - Returns an answer to the above 3 commands
Trap - used to report significant events on the network. Such
events include 'cold or warm restart' or a failed link.
By the way, it reminds us to tell you that the instruction set is no longer called
an instruction set but an SNMP Protocol Data Unit or a PDU. In the above PDU,
the first 3 are generated by the manager. They are self explanatory . Hence lets
not waste disk space talking about them . The other two are generated by the client.
The differentiation comes in the fact that 'Get_Response' works in the polling #
mode i.e it forms the reply to a query from the manager, whereas 'trap' is generated
by the agent only under certain condition independent of the presence or absence
of a query i.e it is interrupt driven. For the uninitiated, lets put it in a
simpler format. Polling (Get_Response) is akin to you asking your date for kiss.
Interrupt driven (Trap) is your date giving you a kiss without you asking for it.
Well we must admit, keeping your trap shut all the time does not do you any good !!!
SNMP version 2 is the latest version of SNMP commercially available. For once
developers have retained the earthly flavor of the previous version. They struck
to their task and have kept the SNMP as simple as possible. SNMP v2 is just a
superset of SNMP v1. The 5 basic instruction of the SNMP v1 . PDU have been
retained though some names have been changed (moods at play again). Get_Response
whereas trap has been renamed as SNMPv2 - trap. This however does not affect them.
Functionality remain the same. The 'Get' function have also been strengthened.
In SNMP v1 the get_request and get_next request would fail in case one of the
read processes failed. Generation of error signals was virtually non-existant.
However in SNMPv2 request failure leads to appropriately error messages generation.
Besides these 5 instructions, SNMPv2 includes 2 new instruction
- inform request
Inform request is used by a network administrator to talk to his brethren i.e.
administrator at other network management stations. Birds of a feather flock
This enables a large complex network to be further subdivided into smaller
each with his own network management station. Get_Bulk_Request is used to
large block of data from the MIB to the Manager in a single transfer operation.
That's all that consists the PDU of SNMPv2.
Lets turn our attention to the objects that constitute the MIB of a
device . An object is actually a variable which holds information about a particular
device. An object is known by the following
- A unique name, called object identifier ( remember the mood)
- Attributes :- This include
- - data type
- details required for the correct implementation.
- - status information
- Read/Write option available
On deciding the datatypes to be used in SNMP, the designers remembered their
of keeping SNMP simple. Therefore only elementary datatypes like integer or
strings are available under SNMP. The only other datatype was the structure
unfortunately doomed to be renamed as 'Sequence'. These were the specification
The answer was simple , use a Data Definition Language like
One of the greatest mystery of our time have been to know how long the server at
our ISP's end has been functioning. This mystery is deepened by some cryptic
from the help-desk of our ISP, saying that the server will be shutdown for
We decided to pip them at the post by finding out how much time the server was
(or down) This can be done using the object identifier called 'sysuptime' in
object called 'system'. The object identifier for sysuptime is 18.104.22.168.2.1.3.
The program given below uses a basic windows socket program. How to master
programming has been best explained in our
tutorial on sockets
so check it out. Here we will concentrate only on the bits & bytes that are
We sent a UDP packet on port 161 to the server at VSNL (our ISP). The IP
the server is 22.214.171.124, but you can change it to any IP address you like.
WindowsNT 4.0 has an in-built SNMP service, this program can also be tried out
The initial parts of the program involves a simple Windows socket program and
is not explained here. For details on such programs, have a look at our
sockets programming. The only section of the code that might intrigue you with
presence are the functions 'abc' and 'abc1'. These are our in-house helper
and are used to write the trapped bytes to the hard disk. The latter part of
program deals with the process of sending a data packet to the server. The
bytes have been explained below.
unsigned char kk,ll;
void abc(char *p)
void abc1(unsigned char p)
WNDCLASS a;HWND b;MSG c;char aa;SOCKET s;struct hostent h;
WSADATA ws;DWORD e;char bb;struct sockaddr_in sa;
long _stdcall zzz (HWND,UINT,WPARAM,LPARAM);
int _stdcall WinMain(HINSTANCE i,HINSTANCE j,char *k,int l)
while ( GetMessage(&c,0,0,0) )
long _stdcall zzz (HWND w,UINT x,WPARAM y,LPARAM z)
if ( x == WM_LBUTTONDOWN)
sprintf(aa,"WSAStartup e = %ld",e);
s = socket(PF_INET,SOCK_DGRAM,0);
sprintf(aa,"socket s = %ld",s);
sa.sin_addr.s_addr = inet_addr("126.96.36.199");
e=sendto(s,kk,39,0,(struct sockaddr *)&sa,sizeof(sa));
dw = sizeof(sa);
ii=recvfrom(s,ll,1000,0,(struct sockaddr *)&sa,&dw);
sprintf(aa,"Recv from %d",ii);
if ( x == WM_DESTROY)
The first byte we send across to the server (it can also be a router) at the other
end is 0x30. This stands for The Universal Sequence and is usually the first byte
in BER/DER compliant protocols.
If we break up the byte into it's individual bits, we get
The first two bits from the left, when off, imply that the query is universal, i.e.
it applies to all fields. The next bit from the left is on and that means that the
query is a constructed one. The value of the fourth bit is 16 and that's Sequence in
the RFC. So it's a Universal Sequence.
The next byte in the array holds the size of the total packet, excluding the
first two bytes.
The next byte is 0x02, which means that the information that follows is an Integer,
i.e. a number. The byte in kk is the length of the data and kk is the actual
value. The 0x00 there stands for the version number which is zero.
The byte that comes next is 0x04 which means that the following data is an
octet (8 bit)
string. The byte after that holds the length of the string and next six bytes
string itself. The string 'public' stands for the community we belong too;
a fancy way of saying it's our password!
The byte in kk is 0xa1. If we were to break this up into it's constituent
bytes, we'd get
When the first three bytes are 101, it means that the data to follow is Context
Specific i.e. you have to look up the documentation and look for it as it's
changes with every protocol. The last four bits hold the number, which is 1 in
case. So it's Context Specific 1. This stands for Get Next Request in the
The next byte holds the length.
The next three bytes hold the Request ID, which we've set to one. Kk is 0x02
which means that the data is an Integer. The next byte is the length and the data
itself is 0x01.
The next three bytes contain the error status of the present connection. Since we've
just begun talking, it is set to zero.
Right after that we have another three bytes which hold the error index. It too
is set to zero.
Now, at kk we have another structure (or sequence if you prefer) starting up.
The byte after that is the length of the data that follows.
Immediatly after the first structure, we have another one. This format is
so that if you want to ask two questions at the same time, you can. Simply add
0x30 after the end of this one and more data.
After the length byte we have the byte 0x06 which stands for the Object data type.
The byte after that is the length.
The next seven bytes are very important.
The ISO, in it's infinite wisdom, has chopped up the Internet into a tree like
ccitt(0) ISO(1) joint-ISO-ccitt(2)
directory(1) mgmt(2) experimental(3) private(4)
system(1) interfaces(2) at(3) ip(4) icmp(5) tcp(6) udp(7)
Right at the top is the root, then comes the ISO (1), then org under ISO (1.3),
the Department of Defense or the dod under org (1.3.6). Right after that we
Internet (188.8.131.52) under the Internet we have mgmt (184.108.40.206.2) and under mgmt
MIB or the Management Information Database (220.127.116.11.2.1). Since we're using
which is used to manage a network system, we have system (18.104.22.168.2.1). So the
hierarchy we're under is 22.214.171.124.2.1. The last 3, in kk, is Sysuptime under
So the bytes from kk to kk hold the hierarchy of our query. What you
find a little confusing is that the first byte, kk, is 0x2b instead of 1.3
should be. Actually, this is the ISO being little clever (for once!). To save
transmitting an extra byte, they decided to multiply the first 1 in
with 40 and add it to the second number i.e. 3. So we get 1*40+3=43 or 0x2b!!
Confusing, but neat.
The last two bytes in the packet are 0x05 and 0x00 which collectively stand for a
NULL value. This is because we can't have an answer in a query!
The output file provides the key to understanding SNMP. The contents of the
file is reproduced below to save you the trouble of actually running the
finding the results. The column on the left are the actual contents of the
file whereas the column on the right hand side represent the analysis of the
that were 'collected' by us.
Recv from 44 - Inserted by the program and signifies that length of packet is
|1||2a||42||*|| Len - 42|
|2||2||2|| ||Version number (int)|
|3||1||1|| ||Len -1|
|4||0||0||value - 0|
|5||4||4|| ||Community (Octet String)|
|6||6||6|| ||Len -6|
|13||a2||162||¢||Context Specific 2|
|14||1d||29|| ||Len -29|
|15||2||2|| ||ID (int)|
|16||1||1|| ||Len -1|
|17||1||1|| ||value -1|
|18||2||2|| ||Error Status (int)|
|19||1||1|| ||Len -1|
|20||0||0|| ||Value -0|
|21||2||2|| ||Error Index (int)|
|22||1||1|| ||Len -1|
|23||0||0|| ||Value -1|
|25||12||18|| ||Len -18|
|27||10||16|| ||Len - 16|
|29||8||8|| ||Len -8|
|31||6||6|| ||Len -6|
|32||1||1|| || |
|33||2||2|| || |
|34||1||1|| || |
|35||1||1|| || |
|36||3||3|| || |
|39||4||4|| ||Len -4|
|40||0||0|| ||the time in milliseconds|
Lets examine the output now.
The first byte we see is 0x30 which stands, as before, for the start of a structure
(or a sequence). The next byte after that is the length of the packet to follow. The
next three bytes hold an Integer whose value is zero. This is the version number we
sent across in our query. Now come eight bytes for a string (strings always start
with a 0x04) which holds our password.
Byte number 13 is 0xa2. If broken up into it's individual bits it looks like this
Since the first three bits are 101 it means that this is 'Context Specific';
the last four bits hold the number which is two in this case. So it is Context Specific
2 which means that this is our asnwer.
The byte after that is the length of the data to follow.
The next three bytes contain an Integer which is the ID of the packet (it's the
as our ID i.e. it's one). The three bytes after that hold the Error status,
still zero and the following three bytes hold the error index, which is also
Now come two bytes for a structure and it's length. The next two bytes also
contains a structure and it's length.
Then we have the number 0x06 which as mentioned before represents an Object. It
is immediately followed by it's length.
Now comes the tree like hierarchy our query belongs to and then another 6 bytes
that hold our answer. The last zero means that the bytes following it hold the answer to
our query about SysUpTime.
Byte 38 contains the number 0x43 which stands for Time Ticks. The string that
follows holds the time, in milliseconds, that the server 126.96.36.199 has been up.
We have not taken the trouble of calculating the corresponding time in days and
hours for a simple reason that the answer obtained from the server was to large to
fit into the calculator provided with Windows95. We hope that this program infuses in
you a desire to use the various objects provided with SNMP. To do this just replace
the object identifier used in the above program with the desired identifier. Then
you can check the output file for the answer to your query. Do let us know about your
success and travails with SNMP. Also provide us with your feedback as to how you liked
The above tutorial is a joint effort of
Mr. Vijay Mukhi
Ms. Sonal Kotecha
Mr. Arsalan Zaidi
Mr. Vinesh Kurup
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Tel : 91-22-496 4335 /6/7/8/9
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