-sfatables is a tool for defining access and admission control policies in an SFA network, in much the same way as iptables is for ip networks. This file gives an overview of the tool and then describes its design and implementation.
+sfatables is a tool for defining access and admission control policies
+in an SFA network, in much the same way as iptables is for ip
+networks. This file gives an overview of the tool and then describes
+its design and implementation.
Example command
---------------
-An sfatables configuration consists of lists of rules that are applied to incoming and outgoing rspecs. Each rule consists of a 'match', to evaluate a given request against a certain set of criteria and a 'target', to perform a corresponding action. Rules are manipulated by using the 'sfatables' command.
+An sfatables configuration consists of lists of rules that are applied
+to incoming and outgoing rspecs. Each rule consists of a 'match', to
+evaluate a given request against a certain set of criteria and a
+'target', to perform a corresponding action. Rules are manipulated by
+using the 'sfatables' command.
Consider the following example:
sfatables -A INCOMING -- -m hrn --user-hrn plc.princeton -- -j RESTRICT_TO_NODES --blacklist plc.mit
-The statement in this example has three parts: the command, the match and the target, separated by the token '--'.
+The statement in this example has three parts: the command, the match
+and the target, separated by the token '--'.
-- The command is '-A', which means 'Add rule.'
-- The match is defined in the segment '-m hrn --user-hrn plc.princeton.' Here, '-m hrn' specifies the 'match type', and '--user-hrn' provides an argument specific to the match type.
-- The target is defined by the segment '-j RESTRICT_TO_NODES --blacklist plc.princeton.' '-j RESTRICT_TO_NODES' defines the target type (RESTRICT_TO_NODES) and '--blacklist' defines a parameter specific to this type.
+* The command is '-A', which means 'Add rule.'
-sfatables comes with a default set of matches and targets, which can be extended using a simple interface.
+* The match is defined in the segment '-m hrn --user-hrn
+ plc.princeton.' Here, '-m hrn' specifies the 'match type', and
+ '--user-hrn' provides an argument specific to the match type.
-When you execute this command, you should see it in your current configuration by running 'sfatables -L INCOMING'
+* The target is defined by the segment '-j RESTRICT_TO_NODES
+ --blacklist plc.princeton.' '-j RESTRICT_TO_NODES' defines the
+ target type (RESTRICT_TO_NODES) and '--blacklist' defines a
+ parameter specific to this type.
+
+sfatables comes with a default set of matches and targets, which can
+be extended using a simple interface.
+
+When you execute this command, you should see it in your current
+configuration by running 'sfatables -L INCOMING'
# ./sfatables -L INCOMING
# Rule Match Arguments Target Arguments
# 1 hrn user-hrn=plc.princeton.* RESTRICT_TO_NODES blacklist=plc.mit
-With this configuration, every time a request is received from plc.princeton.*, nodes matching the blacklist prefix (plc.mit) are dropped from the rspec.
+With this configuration, every time a request is received from
+plc.princeton.*, nodes matching the blacklist prefix (plc.mit) are
+dropped from the rspec.
-The basis for deploying rules using sfatables is the library of matches and
-targets. Furthermore, a set of rules constitutes a 'policy', which as we will
-see is a portable piece of information that can be exchanged with users, peers, and policy makers to make resource allocation and peering a more effective process.
+The basis for deploying rules using sfatables is the library of
+matches and targets. A set of such rules constitutes a 'policy', which
+as we will see is a portable piece of information that can be
+exchanged with users, peers, and policy makers to make resource
+allocation and peering a more effective process.
-XPath crash course -- read this now, or deal with frustration in the remainder of the document
-----------------------------------------------------------------------------------------------
+XPath crash course -- read this now, or deal with frustration in the
+remainder of the document
+-----------------------------------------------------
-XPath is used to select sets of nodes in an XML file. It is like the 'SELECT' command in SQL, but has the advantage of applying to tree structures, which are more general than relations. I.e., while a relation (a table) has a depth = 2, a tree can have an arbitrary depth. This property allows us to consicely refer to 'the nodes in the site corresponding to a user named Alice.' This particular command might look like: '/user[name='Alice']/site/node.'
+XPath is used to select sets of nodes in an XML file. It is like the
+'SELECT' command in SQL, but has the advantage of applying to tree
+structures, which are more general than relations. That is, while a
+relation (a table) has a depth = 2, a tree can have an arbitrary
+depth. This property allows us to consicely refer to 'the nodes in the
+site corresponding to a user named Alice.' This particular command
+might look like: '/user[name='Alice']/site/node.'
-An XPath expression is like a directory path, with the following key differences.
+An XPath expression is like a directory path, with the following key
+differences.
+
+* In a directory path the relationship between X/Y is a parent-child
+ relationship. In XPath, this can be one of a large number of
+ relationships, including 'sibling', 'parent', 'ancestor',
+ 'descendant' etc. The most frequently used relationships are:
-* In a directory path the relationship between X/Y is a parent-child relationship. In XPath, this can be one of a large number of relationships, including 'sibling', 'parent', 'ancestor','descendant' etc. The most frequently used relationships are:
child - e.g. site/node
-and
+
+ and
+
descendant - e.g. user//node
-* Each level can be filtered with a predicate. E.g. 'site[startswith(@hrn,'plc')]/nodes' means all nodes in sites that have the prefix 'plc'.
+* Each level can be filtered with a predicate; e.g.,
+ 'site[startswith(@hrn,'plc')]/nodes' means all nodes in sites that
+ have the prefix 'plc'.
+
+* Some terms have an '@' in them, meaning that they are attributes;
+ e.g., to retrieve the value of p in the following data, we would use
+ the expression "//x/y/@p"
-* Some terms have an '@' in them, meaning that they are attributes. E.g. to retrieve the value of p in the following data, we would use the expression "//x/y/@p"
-<x>
-<y p="q"/>
-</x>
+ <x>
+ <y p="q"/>
+ </x>
Example match
-------------
-A match specification consists of a 'context', a set of arguments and a 'processor.' The context defines the information associated with a request that this match operates on. Think of it as the input parameters to the match. The arguments define values specific to the rule. The processor refers to the program that actually evaluates the match.
+A match specification consists of a 'context', a set of arguments, and
+a 'processor.' The context defines the information associated with a
+request that this match operates on. Think of it as the input
+parameters to the match. The arguments define values specific to the
+rule. The processor refers to the program that actually evaluates the
+match.
<match name="hrn">
<context select="//sfa/current/user@hrn"/>
sfatables -A INCOMING -- -m hrn --user-hrn plc.princeton -- -j RESTRICT_TO_NODES --blacklist plc.mit
-... this match specification is parameterized and dropped in the sfatables configuration directory. The paramterized version of the match is given below:
+... this match specification is parameterized and dropped in the
+sfatables configuration directory. The paramterized version of the
+match is given below:
<match name="hrn">
<!-- Empty context. We _always_ get the hrn of the current user -->
<processor filename="hrn.xsl"/>
</match>
-Notice the additional 'value' tag. Let's list the entries in the configuration directory.
+Notice the additional 'value' tag. Let's list the entries in the
+configuration directory.
# ls -l /etc/sfatables/INCOMING
-rw-r--r-- 1 sapan sapan 671 Sep 11 12:13 sfatables-1-match
-rw-r--r-- 1 sapan sapan 646 Sep 11 12:13 sfatables-1-target
-As you can see, a configuration is simply a set of match-target pairs.
+As you can see, a configuration is simply a set of match-target pairs.
-Finally, this is how the match processor looks like:
+Finally, this is what the match processor looks like:
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
</xsl:stylesheet>
-
-It is written in XSLT. If the syntax of XSLT were not XML-based, then it might have looked as follows:
+It is written in XSLT. If the syntax of XSLT were not XML-based, then
+it might have looked as follows:
context-hrn = //sfa/user/hrn
request-hrn = //request/user/hrn
True
else
False
+ return result
-return result
-
-This is exactly what the previous fragment of code says, albeit in a different format.
+This is exactly what the previous fragment of code says, albeit in a
+different format.
Example target
---------------
+------------
-Targets are specified just like matches. If you haven't read the match example, then now is a good time to do that. Here's an example target:
+Targets are specified just like matches. If you haven't read the match
+example, then now is a good time to do that. Here's an example target:
<target name="RESTRICT_TO_NODES">
<!-- The context is empty, since this target does not require any input from SFA -->
<processor filename="restrict_to_nodes.xsl"/>
</target>
-
and the corresponding target processor:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
Contexts
--------
-Matches and targets are associated with specific contexts. A target may use a variety of criteria to process a request, and may need to look them up in the SFA database. The 'context' contains an xpath expression that isolates the items that a match refers to. The XML spec corresponding to this expression corresponds to an abstract database schema defined as part of the library. SFA is responsible for evaluating this expression, obtaining the data items that the match needs and providing them to the match at the time of evaluation.
-
-
-
+Matches and targets are associated with specific contexts. A target
+may use a variety of criteria to process a request, and may need to
+look them up in the SFA database. The 'context' contains an xpath
+expression that isolates the items that a match refers to. The XML
+spec corresponding to this expression corresponds to an abstract
+database schema defined as part of the library. SFA is responsible for
+evaluating this expression, obtaining the data items that the match
+needs and providing them to the match at the time of evaluation.
Here's a summary of the model:
------------------------------
+-------------------------
An AM can inherit from a set of elements (E).
Each element in E is associated with three things:
+
* A er... 'micro-rspec'
- * an abstract database schema - S, which the AM is expected to be able to generate on the fly.
- * a set of matches and targets.
-Matches and targets may use pieces of information from S by specifying them in their context (see the 'context' part of matches and targets above)
+ * an abstract database schema - S, which the AM is expected to be
+ able to generate on the fly.
+ * a set of matches and targets.
-
+Matches and targets may use pieces of information from S by specifying
+them in their context (see the 'context' part of matches and targets
+above).
git://git.onelab.eu / sfa.git / commitdiff