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.

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 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.

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.

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 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 criteria such as '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.

* 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

    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'.

* 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>

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.

<match name="hrn">
    <context select="//sfa/current/user@hrn"/>
    <rule>
        <argument>
            <name>user-hrn</name>
            <help>HRN of the user requesting resouces</help>
            <operand>HRN</operand>
        </argument>
    </rule>
    <processor filename="hrn.xsl"/>
</match>

Now, when we run the command in the previous example:

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:

<match name="hrn">
    <!-- Empty context. We _always_ get the hrn of the current user -->
    <context select="//sfa/current/user@hrn"/>
    <rule>
        <argument>
            <name>user-hrn</name>
            <help>HRN of the user requesting resouces</help>
            <operand>HRN</operand>
        <value>plc.princeton</value>   <------------------
    </argument>
    </rule>
    <processor filename="hrn.xsl"/>
</match>

Notice the additional 'value' tag. Let's list the entries in the
configuration directory.

# ls -l /etc/sfatables/INCOMING

sapan@joyce ~/Projects/planetlab/sfa/sfatables/targets $ 
total 16
-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.

Finally, this is what the match processor looks like:

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:variable name="context-hrn" select="hrn"/>
    <xsl:template match="user">
                    <xsl:choose>
                    <xsl:when test="starts-with($context-hrn, hrn)">
                        True <!--Match -->
                    </xsl:when>
                    <xsl:otherwise>
                        False <!-- No match -->
                    </xsl:otherwise>
                </xsl:choose>
        <xsl:value-of select="$result"/>
    </xsl:template>
        
</xsl:stylesheet>

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

result = 
  if (starts_with(context-hrn,request-hrn)) then
    True
  else
    False
  return result

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:

<target name="RESTRICT_TO_NODES">
    <!-- The context is empty, since this target does not require any input from SFA -->
    <context select=""/>
    <rule>
        <argument>
            <name>whitelist</name>
            <help>Prefix of nodes to whitelist for this match.</help>
            <operand>PREFIX</operand>
        </argument>
        <argument>
            <name>blacklist</name>
            <help>Prefix of nodes to blacklist for this match.</help>
            <operand>PREFIX</operand>
        </argument>
    </rule>
    <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">
    <!-- Magic sauce copied from a manual. This fragment basically copies everything except for
    stuff that explicitly matches with the templates defined below. In the case of such a match,
    the matched node is treated differently.-->
    <xsl:template match="@* | node()">
        <xsl:copy>
            <xsl:apply-templates select="@* | node()"/>
        </xsl:copy>
    </xsl:template>

    <xsl:variable name="whitelist_prefix" select="//rspec//rule/argument[name='whitelist']/value"/>
    <xsl:variable name="blacklist_prefix" select="//rspec//rule/argument[name='blacklist']/value"/>

    <!-- Drop nodes that are not in the whitelist -->
    <xsl:template match="node">
            <xsl:choose>
                <xsl:when test="starts-with(@name,$whitelist_prefix) and not($blacklist_prefix and starts-with(@name,$blacklist_prefix))">
                    <xsl:copy-of select="."/>
                </xsl:when>
                <xsl:otherwise/>
            </xsl:choose>
    </xsl:template>

    <xsl:template match="sfatables-input"/>
</xsl:stylesheet>

[TODO: explain this target]


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 or target may refer to. For example, if a match needs access
to the nodes corresponding to a slice's site, then the context may be '/sfa/slice[@name=/context/slice/@name]/nodes'.


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).