rfc2723.txt
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An ELSE Clause provides a statement which will be executed if the
IF's test fails. The statement following ELSE will often be another
IF statement, providing SRL's version of a 'select' statement. Note
that an ELSE clause always matches the immediately preceding IF.
3.2 Compound_statement
-------+-------------+----- { ---+---- Statement ----+--- } -------
| | | |
+-- label : --+ +--------<----------+
A compound statement is a sequence of statements enclosed in braces.
Each statement will terminate with a semicolon, unless it is another
compound statement (which terminates with a right brace).
A compound statement may be labelled, i.e. preceded by an identifier
followed by a semi-colon. Each statement inside the braces is
executed in sequence unless an EXIT statement is performed, as
explained below.
Labels have a well-defined scope, within which they must be unique.
Labels within a subroutine (i.e. between a SUBROUTINE and its
matching ENDSUB) are local to that subroutine and are not visible
outside it. Labels outside subroutines are part of a program's outer
block.
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3.3 Imperative_statement
------+---------------------------------------------------+------ ;
| |
+-- SAVE attrib --+--+-----------+--+---------------+
| | | | | |
| | +- / width -+ | |
| | | | | |
| | +- & mask --+ | |
| | | |
| +--- = operand ---+ |
| |
+-- COUNT ------------------------------------------+
| |
+-- EXIT label ------------------------------------+
| |
+-- IGNORE -----------------------------------------+
| |
+-- NOMATCH ----------------------------------------+
| |
+-- RETURN --+-------+------------------------------+
| | | |
| +-- n --+ |
| |
+-- STORE variable := value ------------------------+
3.3.1 SAVE Statement
The SAVE statement saves information which will (later) identify the
flow in the meter's flow table. It does not actually record anything
in the table; this is done when a subsequent COUNT statement
executes.
SAVE has two possible forms:
SAVE attrib = operand ; saves the attribute, mask and value as given
in the statement. This form of the SAVE statement is similar to
that allowed in an IF statement, except that - since imperative
statements do not perform a test - you may save an arbitrary
value.
SAVE attrib ;
SAVE attrib / width ;
SAVE attrib & mask ; saves the attribute and mask from the statement,
and the value resulting from their application to the current
packet. This is most useful when used to save a value with a
wider mask than than was used to select the packet. For
example:
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IF DestPeerAddress == 130.216/16
NOMATCH;
ELSE IF SourcePeerAddress == 130.216/16 {
SAVE SourcePeerAddress /24;
COUNT;
}
ELSE IGNORE;
3.3.2 COUNT Statement
The COUNT statement appears after all testing and saving is complete;
it instructs the PME to build the flow identifier from the attributes
which have been SAVEd, find it in the meter's flow table (creating a
new entry if this is the first packet observed for the flow), and
increment its counters. The meter then moves on to examine the next
incoming packet.
3.3.3 EXIT Statement
The EXIT statement exits a labelled compound statement. The next
statement to be executed will be the one following that compound
statement. This provides a well-defined way to jump to a clearly
identified point in a program. For example:
outer: {
...
if SourcePeerAddress == 192.168/16
exit outer; # exits the statement labelled 'outer'
...
}
# execution resumes here
In practice the language provides sufficient logical structure that
one seldom - if ever - needs to use the EXIT statement.
3.3.4 IGNORE Statement
The IGNORE statement terminates examination of the current packet
without saving any information from it. The meter then moves on to
examine the next incoming packet, beginning again at the first
statement of its program.
3.3.5 NOMATCH Statement
The NOMATCH statement indicates that matching has failed for this
execution of the program. If it is executed when a packet is being
processed with its addresses in 'on the wire' order, the PME will
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perform the program again from the beginning with source and
destination addresses interchanged. If it is executed following such
an interchange, the packet will be IGNOREd.
NOMATCH is illustrated in the SAVE example (section 3.3.1), where it
is used to ensure that flows having 130.216/16 as an end-point are
counted as though 130.216 had been those flows' source peer (IP)
address.
3.3.6 STORE Statement
The STORE statement assigns a value to an SRL variable and SAVEs it.
There are six SRL variables:
SourceClass SourceKind
DestClass DestKind
FlowClass FlowKind
Their names have no particular significance; they were arbitrarily
chosen as likely RTFM attributes but can be used to store any
single-byte integer values. Their values are set to zero each time
examination of a new packet begins. For example:
STORE SourceClass := 3;
STORE FlowKind := 'W'
3.3.7 RETURN Statement
The RETURN statement is used to return from subroutines and can be
used only within the context of a subroutine. It is described in
detail below (CALL statement).
3.4 Subroutine_declaration
-- SUBROUTINE subname ( --+-----------------------------+-- ) -->
| |
+--+-- ADDRESS --- pname --+--+
| |
+-- VARIABLE -- pname --+
| |
+------<------- , ------+
>------+-------- Statement ---------+----- ENDSUB -------- ;
| |
+-------------<--------------+
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A Subroutine declaration has three parts:
the subname is an identifier, used to name the subroutine.
the parameter list specifies the subroutine's parameters. Each
parameter is preceded with a keyword indicating its type -
VARIABLE indicates an SRL variable (see the STORE statement
above), ADDRESS indicates any other RTFM attribute. A
parameter name may be any identifier, and its scope is limited
to the subroutine's body.
the body specifies what processing the subroutine will perform.
This is simply a sequence of Statements, terminated by the
ENDSUB keyword.
Note that EXITs in a subroutine may not refer to labels outside it.
The only way to leave a subroutine is via a RETURN statement.
3.5 CALL_statement
---- CALL subname ( --+---------------------+-- ) ---->
| |
+--+-- parameter --+--+
| |
+----<--- , ----+
>---+-------------------------------------+--- ENDCALL ---- ;
| |
+---+--+-- n : --+--- Statement --+---+
| | | |
| +----<----+ |
| |
+--------------<--------------+
The CALL statement invokes an SRL subroutine. The parameters are SRL
variables or other RTFM attributes, and their types must match those
in the subroutine declaration. Following the parameters is a
sequence of statements, each preceded by an integer label. These
labels will normally be 1:, 2:, 3:, etc, but they do not have to be
contiguous, nor in any particular order. They are referred to in
RETURN statements within the subroutine body.
e.g. RETURN 2; would return to the statement labelled 2:
within in the CALL statement.
Execution of the labelled statement completes the CALL.
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If the return statement does not specify a return label, the first
statement executed after RETURN will be the statement immediately
following ENDCALL.
4 Example Programs
4.1 Classify IP Port Numbers
#
# Classify IP port numbers
#
define IPv4 = 1; # Address Family number from [ASG-NBR]
#
define ftp = (20, 21); # Well-Known Port numbers from [ASG-NBR]
define telnet = 23;
define www = 80;
#
define tcp = 6; # Protocol numbers from [ASG-NBR]
define udp = 17;
#
if SourcePeerType == IPv4 save;
else ignore; # Not an IPv4 packet
#
if (SourceTransType == tcp || SourceTransType == udp) save, {
if SourceTransAddress == (www, ftp, telnet) nomatch;
# We want the well-known port as Dest
#
if DestTransAddress == telnet
save, store FlowKind := 'T';
else if DestTransAddress == www
save, store FlowKind := 'W';
else if DestTransAddress == ftp
save, store FlowKind := 'F';
else {
save DestTransAddress;
store FlowKind := '?';
}
}
else save SourceTransType = 0;
#
save SourcePeerAddress /32;
save DestPeerAddress /32;
count;
#
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This program counts only IP packets, saving SourceTransType (tcp, udp
or 0), Source- and DestPeerAddress (32-bit IP addresses) and FlowKind
('W' for www, 'F' for ftp, 'T' for telnet, '?' for unclassified).
The program uses a NOMATCH action to specify the packet direction -
its resulting flows will have the well-known ports as their
destination.
4.2 Classify Traffic into Groups of Networks
#
# SRL program to classify traffic into network groups
#
define my_net = 130.216/16;
define k_nets = ( 130.217/16, 130.123/16, 130.195/16,
132.181/16, 138.75/16, 139.80/16 );
#
call net_kind (SourcePeerAddress, SourceKind)
endcall;
call net_kind (DestPeerAddress, DestKind)
endcall;
count;
#
subroutine net_kind (address addr, variable net)
if addr == my_net save, {
store net := 10; return 1;
}
else if addr == k_nets save, {
store net := 20; return 2;
}
save addr/24; # Not my_net or in k_nets
store net := 30; return 3;
endsub;
#
The net_kind subroutine determines whether addr is my network
(130.216), one of the Kawaihiko networks (in the k_nets list), or
some other network. It saves the network address from addr (16 bits
for my_net and the k_net networks, 24 bits for others), stores a
value of 10, 20 or 30 in net, and returns to 1:, 2: or 3:. Note
that the network numbers used are contained within the two DEFINEs,
making them easy to change.
net_kind is called twice, saving Source- and DestPeerAddress and
Source- and DestKind; the COUNT statement produces flows identified
by these four RTFM attributes, with no particular source-dest
ordering.
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In the program no use is made of return numbers and they could have
been omitted. However, we might wish to re-use the subroutine in
another program doing different things for different return numbers,
as in the version below.
call net_kind (DestPeerAddress, DestKind)
1: nomatch; # We want my_net as source
endcall;
call net_kind (SourcePeerAddress, SourceKind)
1: count; # my_net -> other networks
endcall;
save SourcePeerAddress /24;
save DestPeerAddress /24;
count;
This version uses a NOMATCH statement to ensure that its resulting
flows have my_net as their source. The NOMATCH also rejects my_net
-> my_net traffic. Traffic which doesn't have my_net as source or
destination saves 24 bits of its peer addresses (the subroutine might
only have saved 16) before counting such an unusual flow.
5 Security Considerations
SRL is a language for creating rulesets (i.e. configuration files)
for RTFM Traffic Meters - it does not present any security issues in
itself.
On the other hand, flow data gathered using such rulesets may well be
valuable. It is therefore important to take proper precautions to
ensure that access to the meter and its data is secure. Ways to
achieve this are discussed in detail in the Architecture and Meter
MIB documents [RTFM-ARC, RTFM-MIB].
6 IANA Considerations
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