faqs.org - Internet FAQ Archives

RFC 1102 - Policy routing in Internet protocols


Or Display the document by number




Network Working Group                                           D. Clark
Request for Comments: 1102        M.I.T. Laboratory for Computer Science
                                                                May 1989

                  Policy Routing in Internet Protocols

1. Status of this Memo

   The purpose of this RFC is to focus discussion on particular problems
   in the Internet and possible methods of solution.  No proposed
   solutions in this document are intended as standards for the
   Internet.  Distribution of this memo is unlimited.

2. Introduction

   An integral component of the Internet protocols is the routing
   function, which determines the series of networks and gateways a
   packet will traverse in passing from the source to the destination.
   Although there have been a number of routing protocols used in the
   Internet, they share the idea that one route should be selected out
   of all available routes based on minimizing some measure of the
   route, such as delay.  Recently, it has become important to select
   routes in order to restrict the use of network resources to certain
   classes of customers.  These considerations, which are usually
   described as resource policies, are poorly enforced by the existing
   technology in the Internet.  This document proposes an approach to
   integrating policy controls into the Internet.

   I assume that the resources of the Internet: networks, links, and
   gateways, are partitioned into Administrative Regions or ARs.  Each
   AR is governed by a somewhat autonomous administration, with distinct
   goals as to the class of customers it intends to serve, the qualities
   of service it intends to deliver, and the means for recovering its
   cost.  To construct a route across the Internet, a sequence of ARs
   must be selected that collectively supply a path from the source to
   the destination.  This sequence of ARs will be called a Policy Route,
   or PR.  Each AR through which a Policy Route passes will be concerned
   that the PR has been properly constructed.  To this end, each AR may
   wish to insure that the user of the PR is authorized, the requested
   quality of service is supported, and that the cost of the service can
   be recovered.

   In the abstract, a Policy Route is a series of ARs, which are assumed
   to be named with globally distinct identifiers.  (The requirement for
   global names for ARs suggests that the name space of ARs is flat.
   That simplifying assumption is made in this RFC, but it should be
   possible to extend the scheme described here to permit nesting of ARs

   to reduce the amount of global information.  The problem of adding
   structure to the space of ARs is an exercise for later study.)
   Before a PR can be used, however, it must be reduced to more concrete
   terms; a series of gateways which connect the sequence of ARs.  These
   gateways will be called Policy Gateways.

   Presently, the closest mechanism to policy routing in the Internet is
   EGP, the Exterior Gateway Protocol.  EGP was constructed to permit
   regions of the Internet to communicate reachability information, even
   though they did not totally share trust.  In this respect, the
   regions hooked together by EGP could each be viewed as Administrative
   Regions.  However, the mechanisms of EGP imposed a topological
   restriction on the interconnection of the Administration Regions.  In
   practice, this has proved unsatisfactory.  Policy matters are driven
   by human concerns, and these have not turned out to be amenable to
   topological constraints, or indeed to constraints of almost any sort.

   The proposals in this memo are designed to permit as wide a latitude
   as possible in the construction and enforcement of policies.  In
   particular, no topological restrictions are assumed.  In general, the
   approach taken in this memo is driven by the belief that since
   policies reflect human concerns, the system should primarily be
   concerned with enforcement of policy, rather than synthesis of
   policy.  The proposal permits both end points and transit services to
   express and enforce local policy concerns.

3. Policy Routes

   Almost all approaches to policy control share, to some degree, the
   idea of a Policy Route.  The distinguishing component of a policy
   approach is the procedure by which the Policy Route is synthesized.
   One approach to synthesizing routes is to associate with each
   distinct policy a subset of all the gateways in the system, and then
   run a routing algorithm across the subset of the gateways.  This
   approach has several drawbacks.  It requires a distinct routing
   computation for every policy, which may be prohibitively expensive.
   It requires the global agreement on the nature and scope of each
   policy, which is at odds with the desire of Administrative Regions to
   establish their own independent policy assertions.  Finally, it
   almost inevitably implies a topological restriction on the
   interconnection of regions.

   Another synthesis approach is to have each Policy Gateway examine
   incoming packets and determine, based on local policy constraints,
   the most appropriate next AR.  This approach might possibly work, but
   again has several drawbacks.  First, it implies a substantial amount
   of computation at each Policy Gateway.  More importantly, it removes
   the route selection from the location where it would most naturally

   be executed, the end-points of the connection.

   It is useful to think of the interconnected ARs as a marketplace, in
   which various services are offered and users select among these
   services to obtain packet transport.  By this analogy, it seems
   appropriate that the actual selection of the Policy Route should be
   made by the end ARs desiring to send the packets, rather than by the
   Policy Gateways.  Looking to the phone system for comparison, it is
   the customer of the phone system who selects which of the long
   distance carriers to use, whether to purchase a fixed price service
   or pay incrementally for usage, and so on.  In this proposal,
   therefore, Policy Routes are synthesized at the end point, where the
   packet originates, and are attached to packets in order to direct
   them through the appropriate series of ARs.  In other words, Policy
   Routes are a form of source routing.  The role of synthesizing a
   Policy Route is shared between the source AR and the particular
   source host.

   In this architecture, therefore, the function of the Policy Gateway
   is not to synthesize the Policy Route, but to verify it.  In the
   following sections, we will address the two questions of how a Policy
   Route is verified, and how a Policy Route is synthesized.

   In determining that Policy Routes should be synthesized at the end
   point, it is important to distinguish between those aspects of
   routing that reflect legitimate policy concerns, and those aspects of
   routing which, in reality, relate to the detailed operation of the
   ARs.  For example, if one were to represent Policy Routes using the
   existing Internet source route mechanism, which allows the end point
   to specify a series of gateways through which the packet should pass,
   the result would be that too much function has been transferred from
   the internals of the Internet to the end points.  The end point would
   have to have knowledge of exactly which gateways are up and
   operational at a particular moment, and this degree of knowledge
   cannot be justified by policy concerns.  Further, it would be
   necessary to run a systemwide gateway reachability protocol.

   This proposal attempts to strike a balance between end point
   specification of those concerns legitimately related to policy, and
   local determination in the Policy Gateways of the more specific
   details necessary for reliable operation.  This leads to a two-level
   routing model, in which the abstract Policy Route, a series of
   administrative regions, is specified by the end point as a form of
   source route, and each Policy Gateway selects the next actual Policy
   Gateway that is to be used to forward this packet.  In other words,
   the abstract Policy Route is made concrete incrementally.  This
   division of function does require that the source AR know if there
   are faults that have partitioned pairs of ARs that are normally

   connected together.  This implies a global reachability protocol to
   be run for the purpose of providing information to the source AR, but
   it need only concern itself at the level of ARs, not at the level of
   gateways.  In a later section on cost-recovery, the topic of gateway
   selection will be discussed in more detail.

   An objection to a scheme such as source routing is that the
   potentially bulky source route must be in every packet, and must be
   evaluated for each packet.  One solution to this performance problem
   is to employ a limited form of route setup, in which the actual
   Policy Route is carried only in the first packet of a sequence, and a
   short identifier or "handle" is included in subsequent packets of the
   sequence.  Each Policy Gateway evaluates the PR on first encounter,
   and caches the result, which is then retrieved for later packets
   using the handle in the packet.  The idea of a handle and caching,
   and the need for a form of route setup, is discussed later.

4. Verification of Policy Routes

   As a packet arrives at a Policy Gateway, attempting to enter an AR,
   the Policy Gateway must decide whether it is legitimate to forward
   this packet, and if so, at what next Policy Gateway the packet should
   exit the AR (assuming that the final destination is not within the
   AR).  The information available to the Policy Gateway to support its
   decision determines the range of policies that can be enforced.
   Determining what information is to be available is therefore a
   central feature of our proposal.

4.1. Identifying the User

   Classic routing decisions, those minimizing some cost, are typically
   driven only by the destination of the packet.  At a minimum, policy
   decisions must be based both on the source and the destination of the
   packet.  In fact, source and destination addresses may not be
   sufficient to determine policy, for an AR may support different users
   with different rights, moreover a single user may wish to exercise
   different rights at different times.  I suggest that to identify the
   user who is proposing to use this particular Policy Route, it is
   sufficient that the packets contain the source host and AR, the
   destination host and AR, and, optionally, a User Class Identifier, or
   UCI.  In a later section, I discuss how to prevent misuse of the user
   class identifier.

   In fact, the source and destination host address may not be needed to
   support the practical range of policy decisions required at
   intermediate ARs.  Only the source and destination AR information may
   be necessary.  If individual host addresses are to be used, that
   implies that intermediate ARs will want to keep track of the rights

   of individual hosts.  It would be much simpler if the source AR could
   be trusted to permit only the proper hosts to use certain PRs.  I
   will consider this further in a later section when I discuss the role
   of the Policy Controller.

4.2. Verifying the Route

   The packet contains an abstract Policy Route: a series of AR
   identifiers.  To validate this route, each Policy Gateway could store
   the complete selection of acceptable policy routes, and require that
   an incoming packet have a Policy Route that exactly matched one of
   the stored entries.  This degree of constraint probably overspecifies
   the situation, and causes an information explosion.  At the other end
   of the scale, Policy Gateways could simply be sensitive to the source
   AR and the destination AR.  In some cases, particularly as regards to
   billing, this does not provide sufficient constraints.  This proposal
   suggests that in deciding whether a given Policy Route is valid, a
   Policy Gateway should look at the source and destination ARs, and
   also the ARs immediately abutting the AR in question, called the
   entry and exit ARs.

   One can think of the verification information in the Policy Gateway
   as a number of templates.  Each template is associated with a valid
   set of users, as described by the source and destination host address
   and the optional User Class, and contains the four ARs described
   above, Source, Destination, Exit, and Entry.  An incoming packet
   should be forwarded if, and only if, there is a template matching the
   information in the packet.  These templates will be called Policy
   Terms.

4.3. Conditions

   The Policy Terms, as described so far, do not permit the expression
   of a realistic range of policies.  What is needed is the ability to
   attach to a Policy Term a number of conditions, which describe
   circumstances under which the term is valid.  These might include
   what type of service (TOS) is available, what times of day the term
   is valid, what accounting options are valid, and so on.  A time-of-
   day condition, for example, would permit networks, like time-sharing
   systems, to offer their off-peak capacity to a wider community.

   In general, these conditions could be quite arbitrary.  The important
   constraint on these conditions is that any condition imposed by the
   Policy Gateway must be understood by the end point, so that it can
   generate Policy Routes which will conform to the condition.  If this
   is not so, and the Policy Gateway attaches capricious conditions to
   its policy terms, then the end points will construct Policy Routes in
   good faith which are rejected, leading to a failure to obtain service

   and serious dissatisfaction among users.  For this reason, it is
   necessary that the nature of policy conditions be negotiated in
   advance.

   The most interesting and difficult conditions are those that relate
   to the dynamic state of the network.  An excellent example is a
   bilateral mutual aid agreement between two transit ARs in which each
   agrees to carry the load of the other if the other should go down.
   To capture this agreement, each might wish to put in Policy Terms
   with the condition that they are valid only if some other AR is non-
   functional.  In the earlier discussion of Policy Route synthesis, it
   was necessary for the ARs to run a global up-down protocol to
   describe the connectivity of ARs.  This protocol is sufficient to
   allow the Policy Gateway to know that some other AR is non-
   functional, but care is required in the dynamics of this system to
   ensure that the end point in the PR have a consistent view of the
   up-down status of the world.  Otherwise, there would be transient
   service outages, which again would lead to user dissatisfaction.

   In general, this proposal asserts that policies should not be based
   on highly dynamic phenomenon.  Administrative Regions should be
   thought of as stable entities which do not change state rapidly.
   Highly dynamic characteristics like queue length should be dealt with
   by proper engineering internal to the AR.  Precisely because
   conditions must be propagated globally, attempting to base a
   condition on a highly dynamic parameter is liable to lead to system
   instability.

4.4. Ownership of Policy Gateways

   In Section 1, all the resources of the network were described as
   being partitioned among the ARs.  This statement does not extend to
   the Policy Gateways, which sit on the boundary between ARs.  Either
   the Policy Gateway must be composed of two physical halves, connected
   by a wire, or there must be a joint agreement for the ownership and
   operation of the gateway.  This is a matter for further study.

5. Examples of Policy Terms

   This section presents examples of how policy terms would be used to
   express a range of practical policies.  In order to give examples, it
   is necessary to define a notation for policy terms.  The following is
   not necessarily the most compact form, but will be sufficient for
   some simple examples.

        A Policy Term will be expressed as follows:

        ((Hs,ARs,ARent),(Hd,ARd,ARexit),UCI,Cg)

   where:

        Hs is the source host address,
        ARs is the source AR,
        ARent is the entry AR,

   and these three values comprise the first "element" of the term,
   describing the permitted access looking toward the source.
   Similarly, for the destination, there is an element describing the
   host address, the adjacent AR, and the ultimate AR.

   In addition to the two directional elements of the term, there is
   global information:

        UCI is the User Class Id, and
        Cg are any global conditions.

   In many cases, an element will not want to constrain one of the
   values, and we will use the "*" symbol to indicate a "wild-card"
   match.

   To construct some simple examples, here is a topology, where H
   elements are hosts, G elements are Policy Gateways, and Numbered
   elements are ARs.

      H1 ---  1 --- G1 -----  2 ------ G2 ----- 3 ----- H2
              |                                 |
              |                                 |
              |                                 |
              |---- G3 -----  4 ------ G4 ------|------ G5 --- 5
                              |                                |
                              |                                |
                              |                               H4
                              H3

   In this picture, there are four hosts, five gateways, and five
   Administrative Regions.

   First, consider AR two.  It has no hosts attached, and models a
   transit service, such as the NSF network.  It may have a very simple
   policy: it will carry any traffic between universities, without
   further constraint.  If we let AR1 and AR3 be the regions of two

   particular universities, then its policy term could be written as:

      AR2: ((*,1,*),(*,3,*),*,*).

   This says that AR 2 agrees to carry traffic from AR 1 to AR 3,
   without concern as to the entry and exit AR, and for any hosts in
   these ARs.

   This notation works, but is very bulky, as a new term is required for
   every pair of universities.  There are several ways to compact the
   notation.  First, we can use the * and a new symbol, "-", to broaden
   the terms a bit.  For example:

      AR2: ((*,1,*),(*,*,-),*,*)

   would assert that AR 1 can use AR 2 to talk to any directly attached
   AR, where we use the "-" to mean that the exit AR must be the
   destination AR.  In other words, the destination AR must be directly
   attached to AR2.  If AR 2 only attaches to universities, then this
   would provide the proper constraint.

   Another approach is to use the User Class ID:

      AR2:((*,*,*),(*,*,*),University,*)

   says that any traffic of any sort that has the User Class of
   University is acceptable.

   Another, and perhaps most suitable notation, is to observe that the
   distinction between source and destination is actually artificial.
   While it helps in this memo to have names for the two ends, either
   end can be a source, depending on who sends the first packet. (A
   later section explores the bi-directional nature of PRs).  A more
   general form of a PR is thus to permit any number of elements.  That
   is, a Policy Term can have more than two elements, and the meaning of
   this is that a PR is valid if it uses any two of these.

   For example, if university 5 wanted to use the AR2 service, AR2 might
   write a Policy term as follows:

      AR2:((*,1,*),(*,3,*),(*,5,*),*,*)

   which would permit a policy route between hosts in any two of the ARs
   1, 3 and 5.

   All the terms so far relate to the policies of AR2.  If university 1
   wanted to subscribe to this service, and use it to reach any other
   site, it would specify terms of its own.  For example:

      AR1: ((*,1, -),(*,*,2),*,*).

   This term says that any host in AR 1 can use AR 2 as a path to any
   host in any AR.  Again we use the "-" notation to indicate that the
   entry AR is the same as the source AR, in this case the AR writing
   the term.

   The ARs numbered 3 and 5 are more interesting.  While 3 is directly
   attached to 2, 5 is not.  Instead, 5 has attached to 3.  If 3 wants
   to use 2 for general transit service, it must provide a term similar
   to the one provided by 1:

      AR3: ((*,3,-),(*,*,2),*,*).

   If 5 wants to use 2, more terms are required.  Since 2 is not
   directly attached, it cannot be named as the exit AR in a term
   written by 5.  The directly attached AR, 3, is all that can be named:

      AR5: ((*,5,-),(*,*,3),*,*).

   Then AR3 must agree to carry the transit traffic for 5.

      AR3: ((*,5,-),(*,*,2),*,*)

   AR3 might not want to carry all forms of transit traffic for 5, but
   only of certain sorts or to certain locations.  This could be
   expressed by restricting the previous term.  For example,

      AR3: ((*,5,-),(*,2,-),*,*)

   would permit traffic from 5 to cross 3 to reach 2, but only to hosts
   directly in those ARs.

   For some further examples, consider AR 4, which might represent the
   AR of a commercial user.  It connects together the hosts of that
   user, for example, H3, and is connected to the other environment to
   permit cross-communication.  Given the terms so far, no traffic will
   flow into this AR.

   If AR 1 wants to permit communication with AR 4, it could add:

      AR1: ((*,1,-),(*,4,-),*,*)

   This would permit communication between hosts directly in each AR,
   but no transit traffic.  In particular, H3 and H2 cannot talk.  There
   are several different terms that would permit them to talk.

   The direct path would be the following:

      AR4: ((*,4,-),(H2,3,-),*,*)
      AR3: ((*,3,-),(H3,4,-),*,*).

   This would permit direct connection through G4.  Note, for variety,
   that each term has been set up so that any host in the local AR can
   match, but only one host in the other AR.  The combination happens to
   permit only H3 and H2 to communicate.

   If G4 were not there, another path would be via AR 2, which could be
   permitted by suitable terms in ARs 1,2,3 and 4.

   Even if G3 and G4 exist, no transit traffic will flow across AR 4
   from 1 to 3.  Even if 1 and 3 want it to:

      AR1: ((*,1,-),(*,3,4),*,*) and
      AR3: ((*,3,-),(*,1,4),*,*),

   the lack of a term for AR4 will prevent a valid PR via that path.
   Only if AR 4 added:

      AR4:((*,1,-),(*,3,-),*,*)

   would AR 4 start serving AR a transit path from 1 to 3.

   If AR4 added:

   AR4: ((*,4,-),(*,*,*),*,*), any host in AR 4 could talk to any host
   anywhere else, but AR 4 would still not become a transit service.

   These various examples demonstrate how individual ARs can offer
   Policy Terms that can be combined to form a route.  The notation
   proposed here is probably not adequate to express the needed range of
   policies.  For example, it may be desirable to have lists of ARs as
   part of a term, as well as single values and "*".  Other notation
   might be proposed to permit exclusion of a limited set of ARs.  It
   may also be appropriate to write elements that are directional, so
   that connections can be "opened" in one direction but not in others.
   This idea is vague in a connectionless architecture, but seems to
   relate to some real policy requirements.

   In general, the problem of expressing policy terms in compact form is
   the same as the problem of constructing compact access control lists.
   There is still an ongoing argument whether access control lists
   should be ordered, and should permit exclusion, and so on.  It would
   seem that the exact same issues arise here. Some experience
   attempting to express real policies may give guidance as to the

   expressive power needed.

6. Cost Recovery

   Almost all of the existing Internet has been paid for as a capital
   purchase and provided to the users as a free good.  There are limited
   examples of cost recovery, but these are based on an annual
   subscription fee rather than a charge related to the utilization.
   There is a growing body of opinion which says that accounting for
   usage, if not billing for it, is an important component of effective
   resource management.  For this reason, tools for accounting and
   billing must be a central part of any policy mechanism.  However,
   precisely because the administrative regions are autonomous, we
   cannot impose a uniform form of billing policy on all of the regions.
   Some of them may continue to provide service freely, or on the basis
   of an annual fee.  Others may charge on the basis of resources
   consumed, but even here there may be variations in detail, as some
   may charge by the packet and others may charge by the byte.  Again,
   in the telephone analogy, we see a variety of billing policies, with
   both local and long distance carriers selling service either on the
   basis of a monthly fee or on a fee-per-minute of usage, with time of
   day conditions attached.  The billing problem is thus a very
   complicated one, for the user would presumably desire to minimize the
   cost, in the context of the various outstanding conditions.

   If we are actually to pay for use of services, there is also the
   problem of collection.  Using the current telephone system as an
   example, there are two strategies for collecting revenues.  One is
   the pre-divestiture mode, in which the source AR (or the destination
   AR in the case of a collect call) serves as a single collection point
   for all of the ARs involved in the call.  After divestiture, we see
   another paradigm, in which the transit AR separately bills the
   customer.

   There are many reasons to support both collection formats.  The
   primary reason for separate billing is that not all regions may wish
   to charge the user in the same units of currency.  Some regions may
   wish to charge actual dollars, while others may wish to charge using
   some form of private allocation units.  On the other hand, having a
   single point of collection is very convenient, because it eliminates
   a lot of duplicate effort in collection.  It does, however, require a
   greater degree of trust and coordination among ARs.

   Single point collection also simplifies another sticky problem, lost
   packets.  For most types of service, the user would presumably be
   offended if asked to pay for a significant number of packets
   undelivered because they have been lost before reaching the
   destination.  If each region separately bills for its traffic, then

   to avoid billing for packets that are lost between that AR and the
   destination, it is necessary to have some form of lost packet
   reporting, which travels backward through system decrementing the
   counters of all the intervening ARs.  If single point collection is
   performed, then the usage meters can be put in the destination AR,
   and periodically propagated to the billing AR, if that is a different
   region.

   The discussion of lost packets makes clear an important relationship
   between billing and policy.  If a Policy Route takes packets through
   a region of known unreliability, the regions preceding it on the path
   may be quite unwilling to forgive the charges for packets which have
   successfully crossed their region, only to be lost further down the
   route.  A billing policy is a way of asserting that one region wishes
   to divorce itself from the reliability behavior of another region.
   The conditions in the policy terms, and corresponding policy routes,
   must therefore be able to capture two distinct conditions.  The first
   is whether or not there exists a bilateral agreement between two ARs
   by which one agrees to be the collection agent for the other.  The
   concatenation of a number of these agreements permits a single
   collection point to be used for the entire policy route.  The other
   condition is whether or not the AR will accept packet and byte counts
   from the next AR downstream as the basis of billing, or whether the
   AR insists that the billing be based on the counts at the exit point
   of this AR.  This condition allows an AR to build a wall between it
   and a subsequent unreliable AR.  One can imagine certain regions
   agreeing to carry traffic into unreliable regions, but only
   grudgingly, knowing that the result is going to be user frustration
   which may be directed to all the ARs indiscriminately.  The use of a
   specific policy condition can make clear to the end user which ARs do
   not view themselves as interworking harmoniously.

   To enforce these mechanisms, the abstract PR which is included in the
   packet must be augmented with a number of conditions.  First, for
   each AR there is a 3-way flag which describes whether the billing
   should be separately collected for the region, propagated back to the
   source (which corresponds to the normal telephone company paradigm),
   or propagated towards the destination (which corresponds to a collect
   call).  Second, there is a flag which indicates whether the region is
   expected to accept from the next region downstream the packet and
   byte counts as the basis of billing.  Third, there must be a charge
   code, a unique number somewhat resembling a credit card number to
   which bills may be sent.  The Policy Terms in the Gateways must
   similarly be augmented to permit verification.  The management of the
   charge code, insuring its uniqueness and preventing its abuse, is
   discussed later.

   These conditions, which relate to agreements between two ARs, are

   somewhat different from the conditions previously discussed, such as
   time of day.  Conditions relating to AR agreements will be called
   "bilateral conditions," while the others are called "global
   conditions."  Note that even though bilateral conditions relate to
   the agreement between two ARs, they can have global effects.

7. Gateway Selection

   In Section Two, this memo proposed that the end point should specify
   an abstract Policy Route, as a series of ARs, and the Policy Gateway
   at the entry to each AR should convert the next hop to a concrete
   route, selecting the Policy Gateway to exit from this region into the
   next.  It turns out that this selection is not entirely devoid of
   policy concerns, and some additional conditions are required in the
   Policy Terms in order to make this operate properly.

   In order that each Policy Gateway be able to select the next Policy
   Gateway on the route, it is necessary to have a table which lists all
   of the potential Policy Gateways that connect together adjacent
   regions.  Presumably, this information is very slowly changing, and
   is not difficult to propagate.  The more dynamic information that is
   needed is whether each of these gateways is up.  It is therefore
   necessary that all of the Policy Gateways attached to a given AR must
   run a local up-down algorithm, one which hopefully can determine not
   only that each of the other gateways is up, but that its interfaces
   are up and that it is properly forwarding traffic.  It is slightly
   complicated to design such a test.  However, we do not have to design
   a strategy for propagating this information globally, because it is
   only needed by the other Policy Gateways attached to each region.

   The policy matter related to concrete routes arises if there are
   several gateways connecting two administrative regions.  As described
   so far, the exit Policy Gateway from any region (which is the entry
   Policy Gateway for the next region) is selected by the entry Policy
   Gateway for that region.  In other words, each region may select its
   exit gateway, but has no control over its entry gateway.  There are
   certain circumstances where a particular region might insist on being
   able to control the entry gateway used.  Imagine two parallel transit
   regions, one which charges incrementally for service, the other of
   which provides its service as a free good.  Obviously, from the point
   of view of the user, it is desirable to minimize the use of the
   charging AR, and maximize the use of the free AR.  But this may lead
   to gross overloads in the free AR, and apparent discrimination
   against the charging AR.  The owner of the free AR, therefore, might
   choose to impose a policy which says that it can be used only to
   reach certain points which are not directly connected to the AR which
   bills for its service, and the traffic must enter the free AR at the
   closest point to the destination.  In other words, the free AR

   requires that it be allowed to choose its entry gateway so that it
   minimizes its costs (which are not, in fact, being billed), with the
   intent of shifting as much as possible of the cost onto the other
   network.

   By adding more bilateral conditions to the Policy Terms and the
   Policy Route in the packet, it is possible to control the various
   options for Policy Gateway selection.  At each boundary between ARs,
   there are only a limited number of ways to select the Policy Gateway.
   Either it is selected by the entry side, by the exit side, or by some
   collaborative algorithm specified through a bilateral agreement.
   (There might be several such algorithms, which requires the
   possibility of more complexity in the specification.  In particular,
   if two adjacent ARs have agreed to use a common routing metric for
   some type of service, they may agree to make a common routing based
   on this metric.)

   Allowing the policy gateway to be selected by the AR which is on the
   far side of the gateway represents an interesting implementation
   problem.  It would be possible to send some message in advance of the
   packet, which requests the next AR to select its entry gateway.  To
   do this, it would figure out what its exit gateway would be, and then
   figure backwards to minimize its costs (for example) to select the
   potential entry gateway back into the immediate region.  This is
   complicated to describe, and would probably be complicated to
   implement.  One way to focus the problem is to observe that routes
   are bi-directional, because a packet flow is bi-directional, and it
   is very desirable that the packets from both directions follow the
   same route.  Once a packet has come back along the reverse route, the
   gateway from which it emerges is precisely the gateway which should
   be used for future traffic in the other direction.  But each gateway,
   in either the forward or reverse direction, must remember a decision
   made by another AR.

   For this to work it is necessary that gateways not be stateless.  If
   each Policy Gateway maintains a cache of recently computed Policy
   Routes, in particular remembering the result of computing the gateway
   for each abstract route, then by simply determining whether or not
   the forward direction or the reverse direction is allowed to
   constrain the gateway across this boundary, both policies can be
   enforced.  But this requires building gateways with state, which has
   not been culturally acceptable in the Internet.  I therefore consider
   as a separate topic the virtues of state in Policy Gateways. I
   believe that fairly simple algorithms exist to set up the required
   bindings in the Policy Gateways, but that problem is a matter for
   later study.

8. Flow States

   The previous section suggested that the gateway needed to maintain
   state in order to tie together the forward and reverse halves of a
   flow.  This solved the particular problem of tying together the
   routing decision which had been made in each direction, so that they
   could be used in the other.  There are, in fact, a number of reasons
   why the two halves of the flow should be tied together.

   - There is considerable overhead in accounting and collecting for the
     usage.  It is clearly desirable to have both halves of the flow
     metered jointly.

   - If the route is not bi-directional, then a failure in the node
     produces a uni-directional link.  Uni-directional links are known
     to cause anomalous behavior in protocols.

   - As part of resource management, it may be desirable for
     intermediate nodes to pass flow control information back to the
     source of the flow.  If identifiable reverse-direction packets
     are passing through the gateway, then this information can be
     piggy-backed onto those packets.

   An additional advantage of maintaining state in the gateway is that
   it will greatly reduce the overhead of dealing with incoming packets.
   There are a number of decisions which the Policy Gateway must make
   which are a part of forwarding a packet: it must validate the Policy
   Route against its terms, it must create or modify an accounting
   record, and it must select the next Policy Gateway.  It is
   unreasonable to imagine performing these tasks from scratch for each
   incoming packet.  Once these decisions have been made, the results
   should be cached, so that they can be used for subsequent packets.

   The stateless gateway was proposed as part of the Internet design in
   order to insure a robust architecture.  If the gateway has no state,
   then a crash of a gateway cannot endanger an on-going connection.  If
   there is state in a gateway, and that state information is lost
   because of a crash, then it is possible that a flow would be
   disrupted.

   In moving from a gateway with no state to a gateway which caches
   information, it is necessary to ensure that the cached information
   can be lost and reconstructed.  The idea of keeping in gateways only
   that state which can be easily reconstructed I call "soft state."

9. Synthesis and Selection of Policy Routes

   In this proposal, a packet contains a Policy Route, which is verified
   by each Policy Gateway along the way.  This section discusses how the
   Policy Route is created in the first place.

   PR creation cannot be done totally automatically by the system, but
   will in general require human judgment.  Policies, after all, are
   matters of human concern.  The approach to PR creation is thus a
   joint one, in which the system provides support to the persons
   setting policy.

   Most commonly, the desired PR will be selected from among those
   available by first finding all valid PRs, and then picking one that
   meets the requirements of the user and has the lowest real cost.
   These two stages will be called synthesis and selection.

   To synthesize a PR across a sequence of ARs, one must find a Policy
   Term in each AR that would permit such a PR.  The Policy Terms in
   each adjacent AR must be compatible in their billing conditions and
   other particulars.  One can imagine finding a sequence of Policy
   Terms that match, rather like dominoes, and reach from the source to
   the destination.

   For a Policy Term at some AR to be acceptable as a part of a PR, the
   following must be true:

   - The Source and Destination Host address and UCI must match the
     term,

   - The Source and Destination AR must match the term,

   - The Entry and Exit AR must match the adjacent AR in the route,

   - The conditions in the term relating to the adjacent AR (e.g.,
     billing) must match the conditions in the term from that region.

   These conditions, of course, are exactly what the Policy Gateway
   would test in validating the PR when it is used.

   As the route is synthesized from matching terms, the global
   conditions of each term are noted, and the combination of these
   become the condition under which the PR is valid.  As a starting
   point of the synthesis the user may have indicated constraints on the
   acceptable conditions in order to limit the candidate terms in the
   synthesis.

   The result of PR synthesis, which is somewhat similar to the

   computation in a link-state routing algorithm where each Policy Term
   represents an abstract link, is a potentially long list of possible
   PRs to each destination AR, each with attached conditions.  The
   selection process must identify one of these which is actually to be
   used.  The selection can be based on the conditions, and on the cost
   of each PR.

   To determine the cost, it must be possible to ask each AR to identify
   the cost of using that Policy Term in the context of this particular
   set of Entry and Exit ARs.  Either there must be an architected
   protocol for reporting these costs, or the task of cost determination
   must be left to humans to perform outside the system.  The problem
   with architected cost reporting is that while some ARs may bill using
   real dollars, others may bill in terms of abstract usage
   authorizations which have no meaning outside that AR.  Even so, I
   believe that we should attempt to define a representation for
   reporting the billing basis associated with each AR.  This is a
   matter for later study.

   While PR synthesis may be an automated process, selection probably is
   not.  While cost minimization will help prune the list, and some
   routes may be rejected automatically on the basis of conditions, part
   of the selection will in general require human judgment.  This
   observation, together with the observation that PR synthesis may be
   costly, suggests first that synthesis and selection cannot be done
   for each packet or indeed each time a transport connection is
   established, and second that it should not be done separately for
   each host in the AR.

   Instead, each AR should have one (or more) Policy Servers, servers
   inside the AR which support the management of PRs.  The Policy Server
   would perform a number of functions.

   - It would store the Policy Terms for the AR, and make them available
     to the Policy Gateways and the Servers of other ARs as appropriate.

   - It would synthesize potential PRs to reach other ARs, and remember
     which of these have been selected for use.

   - It will respond to requests from hosts in the AR for PRs, and
     return them so that they can be included in outgoing packets.

   - It will participate on behalf of the AR in AR up-down protocols,
     and other inter-AR routing algorithms.

   - It will remember the location of all Policy Gateways attached to
     this AR.

   - It will provide the management interface for those persons who must
     establish AR policy: setting of local Policy Terms, selection of
     Policy Routes, and so on.

   A host wishing to send packets outside the local AR must first obtain
   a PR to put into the packets.  In the normal case, it would do so by
   directing a request to the local Policy Server, supplying the desired
   destination and other negotiable conditions.  (For example, the TOS
   is negotiable, the current time is not.)  The Server, based on this
   input, must select the most appropriate PR and return it.

   At this point in the process, human intervention is not reasonable,
   as it would take much too long.  By now, sufficient selection must
   have been done so that automated PR selection is possible.  The most
   direct implementation is that the manual selection process should
   yield an ordered (or partially ordered) list of potential PRs, and
   the list is searched in order until a PR is found that matches the
   destination and conditions.  That PR is then returned.

10. Security

   There are a number of aspects of this scheme which present
   opportunities for abuse.  In essentially all cases, the possible
   abuse is theft of network resources or improper charging.  They thus
   have a somewhat different nature than problems related to corruption
   or disclosure of data.  Mechanism to insure proper use and charging
   of resources often tolerate minor abuse in exchange for ease of
   operation.  Also, control is often based on detection and recovery
   rather than prevention.  Assumptions of this sort are probably
   acceptable here as well.  An isolated packet, which is not a part of
   any sequence of packets, may be too small an item to account for or
   control.  But if a significant stream of packets goes unaccounted,
   this is less acceptable.

   There are three general options for abuse.  One is to falsify the
   user identification information in the PR, the source and destination
   host, the User Class Id and the charge code.  Another is to take a
   valid PR and misuse it intact.  And the third is to read out a valid
   charge code from a PR and then make additional charges against it.

   To protect against putting false user identification information into
   a PR, the PRs should be sealed or signed, using a crypto sealing
   technique.  Since Policy Servers are the source of PRs, the sealing
   can be done by the Server.  This would require that the seal or
   digital signature of each Server be known, but avoids the need to
   have each host known.  The Server would be trusted to seal only valid
   PRs.  It must only put User Class Ids and charge codes into PRs from
   a source permitted to use them, for example.

   Assuming a public key system, each Policy Server could have a
   separate key pair, the public half of which was advertised in some
   way.  It is a matter for further study exactly what parts of the PR
   need be sealed.

   If the Policy Server violates this trust, and uses a UCI or charge
   code with an unauthorized host, there are two sub-cases: the false
   source host is in the same AS, or is outside it.  If it is outside,
   this can be detected by inspection of the PR, since the relation
   between AR and network number is (almost) static.  One approach is to
   make an AR identifier part of the charge code, so that use of the
   code can be rejected unless that AR is the source AR for the packet.
   This works, but prevents using charge codes from a foreign location.
   Other more general techniques could probably be proposed.

   If the false source host is inside the AR, then further steps are
   required to prevent the problem.  One general solution is to note
   that a PR is valid only if sealed by a Policy Server.  Any AR
   attempting to collect for usage should be required to keep a copy of
   the PR as proof that the route was used.  If there seems to be
   unauthorized use of a charge code, the owner can ask to see the PR
   which generated the charge, which will show the Policy Server which
   constructed the route.  If this is an unauthorized use, action can be
   taken against the AR owning that Server, with the sealed PR as
   evidence. In other words, detection and redress may be more effective
   than prevention.

   If we can assume that the Policy Server for a particular region is as
   trustworthy as that AR requires, there is still the problem of a
   Server of one region trying to steal from another AR.  This could be
   done, for example, by taking a valid PR, and sending data forward
   along it from the "middle" of the route, so that what appears to be
   coming from one source is actually coming from another in a different
   AR.

   This would require that packets coming back along the route towards
   the original source be rerouted to the false source, which would
   require that the whole routing function within the AR be corrupted.
   It is unlikely that this would go long undetected, but if direct
   control of this class of fraud is needed, it could be achieved by
   requiring any AR intending to charge against a particular PR to
   obtain from time to time a confirmation, sealed by the Server of the
   source AR, that its policy gateway has in fact forwarded some number
   of packets using this PR. This sort of function is probably overkill,
   but this class of fraud needs to be considered.

   Obviously, a more detailed study will be required of the problem of
   resource theft, but I believe that a mechanism can be made to work

   based on:

   - Local trust of the Policy Server within each AR.

   - Sealing of the PR by the Server.

   - Selective validation of the seal at the Policy Gateway.

   - Selective consistency checking of the PR at the Policy Gateway.

   - Use of seal on PR as evidence of the source of the PR.

11. An Experimental Program -- Migration to Policy Routing

   The proposal above calls for several Internet components not present
   today: the Policy Route IP option, Policy Gateways, Policy Servers,
   and support protocols such as the global AS up-down protocol and the
   local (to the AS) Policy Gateway up-down protocol.  Any plan for
   introduction of policy routing must provide a method to experiment
   with the concept without changing all the hosts and the gateways now
   in place.

   Since the Policy Server is a new component which can be added to the
   Internet without changing any existing components, it is easy to put
   that facility in place.  This, then, becomes the central part of an
   experimental plan. Later, it is possible to imagine adding the policy
   controls to some of the gateways.  Most difficult will be modifying
   all the hosts to use the PR IP option.  Based on our experience with
   adding minor features such as IP subnetworks, it will never be
   possible to get the PR option into all the hosts, and policy routing
   must be made to work anyway.

   Taking into account these difficulties, here is a concrete
   experimental plan, in three phases.

   In Phase I, software for a Policy Server is created, and made
   available to all potential ARs.  As a part of its function, it has
   two "temporary" feature, to mimic the function of the missing host
   and gateway support.

   To mimic the function of the policy gateway, two policy Servers are
   placed "near" a current function gateway which happens to connect the
   two ARs, one on each side of the current gateway, and representing
   their respective ARs.  These two Servers then proceed to fool the
   current gateway as follows.

   - The current gateway is given the two Servers as neighbors in its
     routing exchanges.  In this way, the Servers can control which

     network numbers are advertised.  This is similar to the way "gated"
     is used today to control routes.

   - A packet entering the AR is directed to the "near" Server inside
     the AR, which performs the functions of the Policy Gateway and
     then resends the packet.  This may require the use of a regular
     source route in some cases, but can probably just be done by
     rewriting the destination IP address in the packet.  (Note that
     the IP PR option proposed in the Appendix has fields for the
     original IP source and destination, so that these fields can be
     reused in forwarding the packet from gateway to gateway.)

   To deal with the lack of host support for the PR option, we again
   make use of the Server.  Since the Server is the recipient of all
   routing information coming into the AR (since it has been set up as
   the neighbor of the current gateway at the actual AR boundary) it
   alone knows the proper routes out.  Internally, it advertises itself
   as the default gateway to all networks outside the AR, so that it
   receives all the packets intending to leave the region.  It, rather
   than the host, adds the PR option and then sends the packet on the
   Policy Gateway (or the matching Server in the next AR playing its
   part) for relaying.

   By controlling how routes are propagated by the regular gateways, it
   is possible to prevent hosts from manually setting up routes to
   bypass the Servers.  In any event, enforcement is not the primary
   concern in Phase I of the experiment.

   In Phase II, certain of the current gateways are augmented with the
   Policy Gateway functions.  This will make enforcement easier, and
   eliminate the extra hop which the packet had make in Phase I, as it
   passed from one Server to another through the current gateway.  At
   the same time, some of the hosts are modified to insert the IP PR
   option into the packet at the source.  This will explore the problems
   of PR selection.

   In Phase III, the PR design is proposed for general implementation.

12. Policy Route Setup

   One objection to this scheme is the large size of the IP PR option.
   With all the information proposed in this memo, it is larger than the
   IP header itself.  However, this problem can easily be avoided; the
   PR option seldom need be sent.

   Since the Policy Gateways are going to cache the result of processing
   the PR, the cache holds the equivalent of the PR.  All that is
   required is a very short option in the packet which is a handle that

   permits the gateway to find the correct cache entry.  This handle
   would be included in the original IP PR option, and then repeated in
   every packet.  The Policy Server which generated the PR could select
   the handle, so it would be unique for each AR.  Perhaps the AR id and
   a 16 bit UID would be sufficient.

   The full PR option needs to be in the packet only if the cached
   Information in the gateway is lost.  If a gateway crashes or the
   route changes, the end point must reconstruct the caches in the
   series of gateways that form the route.  The end point could
   determine that this was necessary either when a gateway reports
   explicitly that it does not have an entry corresponding to a handle,
   or when the host determines that it is not getting the desired
   service.

   This sort of action can be thought of as an extension to the idea of
   retransmitting.  In transport protocols such as TCP, the host keeps
   track of the behavior of the network, and if it believes that
   something is wrong (e.g., there is a lack of an acknowledgment), it
   takes action to restore the desired service.  Other examples include
   switching to another gateway if the currently active adjacent gateway
   seems to be down.  Sending the full PR option in the packet is just
   another example of allowing the end node to restore the state of the
   connection if it seems to be broken.

   Using this model, most packets would have only a short option
   (perhaps 12 bytes).

   This idea of restoring the state in the gateway as needed achieves
   the idea of "soft state" mentioned earlier, and allows gateways with
   state to achieve the same robustness associated with datagram
   networks.

Author's Address

   David D. Clark
   Massachusetts Institute of Technology
   Laboratory for Computer Science
   545 Main Street
   Cambridge, MA 02139

   Phone: (617) 253-6003

   Email: ddc@LCS.MIT.EDU

 

User Contributions:

Comment about this RFC, ask questions, or add new information about this topic:

CAPTCHA