An Experiment in DNS Based IP Routing

Network Working Group                                         C. Huitema
Request for Comments: 1383                                         INRIA
                                                           December 1992


                 An Experiment in DNS Based IP Routing

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  Discussion and suggestions for improvement are requested.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Table of Contents

   1. Routing, scaling and hierarchies ......................    1
   2. Routing based on MX records ...........................    2
   3. Evaluation of DNS routing .............................    3
   3.1 Loops and relays .....................................    4
   3.2 Performances and scaling .............................    5
   3.3 Tunneling or source routing ..........................    6
   3.4 Choosing a gateway ...................................    6
   3.5 Routing dynamics .....................................    6
   3.6 DNS connectivity .....................................    7
   3.7 On the way back ......................................    8
   3.8 Flirting with policy routing .........................    8
   4. Rationales for deployment .............................    9
   4.1 The good citizens ....................................   10
   4.2 The commercial approach ..............................   10
   5. The experimental development ..........................   11
   5.1 DNS record ...........................................   11
   5.2 Interface with the standard IP router ................   12
   5.3 The DNS query manager ................................   12
   5.4 The real time forwarder ..............................   12
   5.5 Interaction with routing protocols ...................   13
   6. Acknowledgments .......................................   13
   7. Conclusion ............................................   13
   8. References ............................................   14
   9. Security Considerations ...............................   14
   10. Author's Address .....................................   14

1.  Routing, scaling and hierarchies

   Several recent studies have outlined the risk of "routing explosion"
   in the current Internet: there are already more than 5000 networks
   announced in the NSFNET routing tables, more than 7000 in the EBONE
   routing tables.  As these numbers are growing, several problems
   occur:

      *    The size of the routing tables grows linearly with the
           number of connected networks; handling this larger tables
           requires more resources in all "intelligent" routers, in
           particular in all "transit" and "external" routers that
           cannot rely on default routes.

      *    The volume of information carried by the route exchange
           protocols such as BGP grows with the number of networks,
           using more network resources and making the reaction to
           routing events slower.

      *    Explicit administrative decisions have to be exercised by
           all transit networks administrators which want to
           implement "routing policies" for each and every
           additional "multi-homed" network.

   The current "textbook" solution to the routing explosion problem is
   to use "hierarchical routing" based on hierarchical addresses. This
   is largely documented in routing protocols such as IDRP, and is one
   of the rationales for deploying the CIDR [3] addressing structure in
   the Internet. This textbook solution, while often perfectly adequate,
   as a number of inconveniences, particularly in the presence of
   "multihomed stubs", e.g., customer networks that are connected to
   more than one service providers.

   The current proposal presents a scheme that allows for simple
   routing. It is complementary with the classic "hierarchical routing"
   approach, but provides an easy to implement and low cost solution for
   "multi-homed" domains. The solution is a generalization of the "MX
   record" scheme currently used for mail routing.

2.  Routing based on MX records

   The "MX records" are currently used by the mail routing application
   to introduce a level of decoupling between the "domain names" used
   for user registration and the mailbox addresses. They are
   particularly useful for sending mail to "non connected" domains: in
   that case, the MX record points to one or several Internet hosts that
   accept to relay mail towards the target domain.

   We propose to generalize this scheme for packet routing.  Suppose a
   routing domain D, containing several networks, subnetwork and hosts,
   and connected to the Internet through a couple of IP gateways. These
   gateways are dual homed: they each have an address within the domain
   D -- say D1 and D2 -- and an address within the Internet -- say I1
   and I2 --. These gateways also have a particularity: they retain
   information, and don't try to announce to the Internet any
   reachibility information on the networks contained within "D". These
   networks however have been properly registered; a name server
   accessible from the Internet contains the "in-addr.arpa" records that
   enable reverse "address to name" lookup, and also contains the
   network level equivalent of "MX records", say "RX records". Given any
   host address Dx within D, one can get "RX records" pointing to the
   Internet addresses of the gateways, I1 and I2.

   A standard Internet router Ix cannot in principle send a packet to
   the address Dx: it does not have any corresponding routing
   information. However, if the said Internet router has been modified
   to exploit our scheme, it will query the DNS with the name build up
   from "Dx" in the "in-addr.arpa" domain, obtain the RX records, and
   forward the packet towards I1 (or I2), using some form of "source
   routing". The gateway I1 (or I2) will receive the packet; its routing
   tables contain information on the domain D and it can relay the
   packet to the host Dx.

   At this stage, the readers should be convinced that we have presented
   a scheme that:

      *    avoid changes in host IP addresses as topology changes,
           without requiring extra overhead on routing (provided
           that the routing employs some form of hierarchical
           information aggregation/abstraction),

      *    allow to support multihomed domains without requiring
           additional overhead on routing and without requiring
           hosts to have explicit knowledge of multiple addresses.

   They should also forcingly scratch their head, and mumble that things
   can't be so simple, and that one should perhaps carefully look at the
   details before assuming that the solution really works.

3.  Evaluation of DNS routing

   Several questions come to mind immediately when confronted to such
   schemes:

       -    Should all relays access the DNS? What about possible
            loops?

       -    Will the performances be adequate?

       -    How does one choose the best gateway when several are
            announced? What happens if the gateway is overloaded, or
            unreachable?

       -    What if the directory cannot be accessed?

       -    How does it work in the reverse direction?

       -    Should we use tunnelling or loose source routing?

       -    Can we be more general?

   There may indeed be more questions, but these ones, at least, have
   been taken into account in the setting of our experiment.

3.1.  Loops and relays

   In the introduction to DNS-IP routing, we mentioned that the packets
   would be directed towards the access gateway I1 or I2 by means of
   "source routing" or "tunnelling". This is not, stricto sensu,
   necessary. One could imagine that the packet would simply be routed
   "as if it was directed towards I1 or I2". The next relay would, in
   turn, also access the DNS to get routing information and forward the
   packet.

   Such a strategy would have the advantage of leaving the header
   untouched and of letting the transit nodes choose the best routing
   towards the destination, based on their knowledge of the reachability
   status. It would however have two important disadvantages:

          -    It would oblige all intermediate relays to access the
               DNS,

          -    It would oblige all these relays to exploit consistently
               the DNS information.

   Obliging all intermediate gateways to access the DNS is impractical
   in the short term: it would mean that we would have to update each
   and every transit relay before deploying the scheme. It could also
   have an important performance impact: the "working set" of transit
   relays is typical much wider than that of stub gateways, and the
   argument presented previously on the efficiency of caches may not
   apply. This would perhaps remain impractical even in the long term,
   as it the volume of DNS traffic could well become excessive.

   The second argument would apply even if the performance problem had
   been solved. Suppose that several RX records are registered for a
   given destination, such as I1 and I2 for Dx in our example, and that
   a "hop by hop routing" strategy is used. There would be a fair risk
   that some relays would choose to route the packet towards I1 and some
   others towards I2, resulting in inefficient routing and the
   possibility of loops.

   In order to ensure coherency, we propose that all routing decisions
   be made at the source, or by one of the first relays near the source.

3.2.  Performances and scaling

   The performance impact of using the DNS for acquiring routing
   information is twofold:

      *    The initial DNS exchanges required for loading the
           information may induce a response time penalty for the
           users,

      *    The extra DNS traffic may contribute to overloading the
           network.

   We already have some experience of DNS routing in the Internet for
   the "mail" application. After the introduction of the "MX record",
   the mail routing slowly evolved from a hardwired hierarchy, e.g.,
   send all mail to the addresses in the ".FR" domain to the french
   gateway, towards a decoupling between a name hierarchy used for
   registration and the physical hierarchy used for delivery.

   If we consider that the mail application represent about 1/4th of the
   Internet traffic, and that a mail message seldom include more than
   half a dozen packets, we come to the point that DNS access is already
   needed at least once for every 24 packets. The performances are not
   apocalyptic -- or someone would have complained! In fact, if we
   generalize this, we may suppose that a given host has a "working set"
   of IP destinations, and that some caching strategy should be
   sufficient to alleviate the performance effect.

   In the scheme that we propose, the DNS is only accessed once, either
   by the source host or by an intelligent router located near the
   source host. The routing decision is only made once, and consistent
   routing is pursued in the Internet until reaching an access router to
   the remote domain.

   The volume of DNS traffic through the NSFNET, as collected by MERIT,
   is currently about 9%. When a host wants to establish communication
   with a remote host it usually need to obtain the name - IP address
   mapping. Getting extra information (I1 or I2 in our example) should
   incur in most cases one more DNS lookup at the source. That lookup
   would at most double the volume of DNS traffic.

3.3.  Tunneling or source routing

   Source directed routing, as described above, can be implemented
   through one of two techniques: source routing, or a form of
   encapsulation protocol. For the sake of simplicity, we will use
   source routing, as defined in [1]: we don't have to define a
   particular tunnelling protocol, and we don't have to require hosts to
   implement a particular encapsulation protocol.

3.4.  Choosing a gateway

   A simplification to the previous problem would be to allow only one
   RX record per destination, thus guaranteeing consistent decisions in
   the network. This would however have a number of draw-backs. A single
   access point would be a single point of failure, and would be
   connected to only one transit network thus keeping the "customer
   locking" effect of hierarchical routing.

   We propose that the RX records have a structure parallel to that of
   MX records, i.e., that they carry associated with each gateway
   address a preference identifier. The source host, when making the
   routing decision based on RX records, should do the following:

          -    List all possible gateways,

          -    Prune all gateways in the list which are known as
               "unreachable" from the local site,

          -    If the local host is present in the list with a
               preference index "x", prune all gateways whose preference
               index are larger than "x" or equal to "x".

          -    Choose one of the gateway in the list. If the list is
               empty, consider the destination as unreachable.

   Indeed, these evaluations should not be repeated for each and every
   packet. The routers should maintain a cache of the most frequently
   used destinations, in order to speed up the processing.

3.5.  Routing dynamics

   In theory, one could hope to extract "distance" information from the
   local routing table and combine it with the preference index for
   choosing the "best" gateway. In practice, as shown in the mail
   context, it is extremely difficult to perform this kind of test, and
   one has to rely on more heuristical approaches. The easiest one is to
   always choose a "preferred gateway", i.e., the gateway which has the
   minimal preference index. One could also, alternatively, choose one
   gateway at random within the list: this would spread the traffic on
   several routes, which is known to introduce better load sharing and
   more redundancy in the network.

   As this decision is done only once, the particular algorithm to use
   can be left as a purely local matter. One domain may make this
   decision based purely on the RX record, another based purely on the
   routing information to the gateways listed in the RX record, and yet
   the third one may employ some weighted combinations of both.

   Perhaps the most important feature is the ability to cope rapidly
   with network errors, i.e., to detect that one of the route has become
   "unreachable". This is clearly an area where we lack experience, and
   where the experiment will help. One can think of several possible
   solutions, e.g.,:

      *    Let intermediate gateways rewrite the loose source route
           in order to replace an unreachable access point by a
           better alternative,

      *    Monitor the LSR options in the incoming packets, and use
           the reverse LSR,

      *    Monitor the "ICMP Unreachable" messages received from
           intermediate gateways, and react accordingly,

      *    Regularly probe the LSR, in order to check that it is
           still useful.

   A particularly interesting line would be to combine these
   connectivity checks with the transport control protocol
   acknowledgments; this would however require an important modification
   of the TCP codes, and is not practical in the short term. We will not
   try any such interaction in the early experiments.

   The management of these reachability informations should be taken
   into account when caching the results of the DNS queries.

3.6.  DNS connectivity

   It should be obvious that a scheme relying on RX records is only
   valid if these records can be accessed. By definition, this is not
   the case of the target domain itself, which is located at the outer
   fringes of the Internet.

   A domain that want to obtain connectivity using the RX scheme will
   have to replicate its domain name service info, and in particular the
   RX records, so has to provide them through servers accessible from
   the core of the Internet. A very obvious way to do so is to locate
   replicated name servers for the target domain in the access gateways
   "I1" and "I2".

3.7.  On the way back

   A source located in the fringe domain, when accessing a core Internet
   host, will have to choose an access relay, I1 or I2 in our example.

   A first approach to the problem is to let the access gateway relay
   the general routing information provided by the routing domains
   through the fringe network. The fringe hosts would thus have the same
   connectivity as the core hosts, and would not have to use source
   directed routing.  This approach has the advantage of leaving the
   packets untouched, but may pose problems should the transit network
   need to send back a ICMP packet: it will have to specify a source
   route through the access gateway for the ICMP packet. This would be
   guaranteed if the IP packets are source routed, as the reverse source
   route would be automatically used for the ICMP packet. We are thus
   led to recommend that all IP packets leaving a fringe domain be
   explicitly source routed.

   The source route could be inserted by the access gateway when the
   packet exits the fringe domain, if the gateway has been made aware of
   our scheme. It can also be set by the source host, which would then
   have to explicitly choose the transit gateway, or by the first router
   in the path, usually the default router of the host sending the
   packets. As we expect that hosts will be easier to modify than
   routers, we will develop here suitable algorithms.

   The fringe hosts will have to know the set of available gateways, of
   which all temporarily unreachable gateways shall indeed be pruned. In
   the absence of more information, the gateway will be chosen according
   to some preference order, or possibly at random.

   It is very clear that if a "fringe" host wants to communicate with
   another "fringe" host, it will have to insert two relays in the LSR,
   one for the domain that sources the packet, and one for the domain
   where the destination resides.

3.8.  Flirting with policy routing

   The current memo assumes that all gateways to a fringe domain are
   equivalent: the objective of the experiment is to test and evaluate a
   simple form of directory base routing, not to provide a particular
   "policy routing" solution. It should be pointed out, however, that
   some form of policy routing could be implemented as a simple
   extension to our RX scheme.

   In the proposed scheme, RX records are only qualified by an "order of
   preference".  It would not be very difficult to also qualify them
   with a "supported policy" indication, e.g., the numeric identifier of
   a particular "policy". The impact on the choice of gateways will be
   obvious:

      -    When going towards a fringe network, one should prune
           from the usable list all the gateways that do not support
           at least one of the local policies,

      -    When exiting a fringe network, one should try to assess
           the policies supported by the target, and pick a
           corresponding exit gateway,

      -    When going from a fringe network towards another fringe
           network, one should pick a pair of exit and access
           gateway that have matching policies.

   In fact, a similar but more general approach has been proposed by
   Dave Clark under the title of "route fragments". The only problem
   here are that we don't know how to identify policies, that we don't
   know whether a simple numeric identifier is good enough and that we
   probably need to provide a way for end users to assess the policy on
   a packet per packet or flow per flow basis. In short, we should try
   to keep the initial experiment simple. If it is shown to be
   successful, we will have to let it evolve towards some standard
   service; it will be reasonable to provide policy hooks at this stage.

4.  Rationales for deployment

   Readers should be convinced, after the previous section, that the
   DNS-IP routing scheme is sleek and safe. However, they also are
   probably convinced that a network which is only connected through our
   scheme will probably enjoy somewhat less services than if they add
   have full traditional connectivity.  We can see two major reasons for
   inducing users into this kind of scheme:

      -    Because they are good network citizen and want to suffer
           their share in order to ease the general burden of the
           Internet,

      -    Because they are financially induced to do so.

   We will examine these two rationales separately.

4.1.  The good citizens

   A strong tradition of the Internet is the display of cooperative
   spirit: individual users are ready to suffer a bit and "do the right
   thing" if this conduct can be demonstrated to improve the global
   state of the network -- and also is not overly painful.

   Restraining to record your internal networks in the international
   connectivity tables is mainly an advantage for your Internet
   partners, and in particular for the backbone managers. The normal way
   to relieve this burden is to follow a hierarchical addressing plan,
   as suggested by CIDR. However, when for some reason the plan cannot
   be followed, e.g., when the topology just changed while the target
   hosts have not yet been renumbered, our scheme provides an
   alternative to "just announcing one more network number in the
   tables". Thus, it can help reducing the routing explosion problem.

4.2.  The commercial approach

   Announcing network numbers in connectivity tables does have a
   significant cost for network operators:

      -    larger routing tables means more memory hence more
           expensive routres,

      -    more networks means larger and more frequent routing
           messages, hence consume more network resources,

      -    more remote networks means more frequent administrative
           decisions if policies have to be implemented.

   These costs are very significant not only for regionals, but also for
   backbone networks. It would thus be very reasonable, from an
   economical point of view, for a backbone to charge regionals
   according to the number of networks that they announce. A similar
   line of reasoning can be applied by the regionals, which could thus
   give the choice to their customers between:

      -    being charged for announcing an address of their choice,

      -    or being allocated at a lower cost a set of addresses in
           an addressing space belonging to the regional.

   Our scheme may prove an interesting tool if the charge for individual
   addresses, which are necessary for "multi homed" clients, becomes too
   high.

5.  The experimental development

   The experimental software, implemented under BSD Unix in a "socket"
   environment, contains two tasks:

          -    a real time forwarder, which is implemented inside the
               kernel and handles the source demanded routes,

          -    a DNS query manager, which transmit to the real time
               forwarder the source routing information.

   In this section, we will describe the real time forwarder, the query
   manager, the format of the DNS record, and the interface with the
   standard IP routers.

5.1.  DNS record

   In a definitive scheme, it would be necessary to define a DNS record
   type and the corresponding "RX" format. In order to deploy this
   scheme, we would then have to teach this new format to the domain
   name service software. While not very difficult to do, this would
   probably take a couple of month, and will not be used in the early
   experimentations, which will use the general purpose "TXT" record.

   This record is designed for easy general purpose extensions in the
   DNS, and its content is a text string. The RX record will contain
   three fields:

          -    A record identifier composed of the two characters "RX".
               This is used to disambiguate from other experimental uses
               of the "TXT" record.

          -    A cost indicator, encoded on up to 3 numerical digits.
               The corresponding positive integer value should be less
               that 256, in order to preserve future evolutions.

          -    An IP address, encoded as a text string following the
               "dot" notation.

   The three strings will be separated by a single comma. An example of
   record would thus be:

 ___________________________________________________________________
 |         domain          |   type |   record |   value           |
 |            -            |        |          |                   |
 |*.27.32.192.in-addr.arpa |   IP   |    TXT   |   RX, 10, 10.0.0.7|
 |_________________________|________|__________|___________________|
   which means that for all hosts whose IP address starts by the three
   octets "192.32.27" the IP host "10.0.0.7" can be used as a gateway,
   and that the preference value is 10.

5.2.  Interface with the standard IP router

   We have implemented our real time forwarder "on the side" of a
   standard IP router, as if it were a particular subnetwork connection:
   we simply indicate to the IP router that some destinations should be
   forwarded to a particular "interface", i.e., through our real time
   forwarder.

   Of particular importance is indeed to know efficiently which
   destinations should be routed through our services. As the service
   would be useless for destinations which are directly reachable, we
   have to monitor the "unreachable" destinations.  We do so by
   monitoring the "ICMP" messages which signal the unreachable
   destination networks, and copying them to the DNS query manager.

   There are indeed situations, e.g., for fringe networks, where the
   router knows that destinations outside the local domain will have to
   be routed through the real time forwarder. In this case, it makes
   sense to declare the real time forwarder as the "default route" for
   the host.

5.3.  The DNS query manager

   Upon reception of the ICMP message, the query manager updates the
   local routing table, so that any new packet bound to the requested
   destination becomes routed through the real time forwarder.

   At the same time, the query manager will send a DNS request, in order
   to read the RX records corresponding to the destination. After
   reception of the response, it will select a gateway, and pass the
   information to the real time forwarder.

5.4.  The real time forwarder

   When the real time forwarder receives a packet, it will check whether
   a gateway is known for the corresponding destination.  If that is the
   case, it will look at the packet, and insert the necessary source
   routing information; it will then forward the packet, either by
   resending it through the general IP routing program, or by forwarding
   it directly to the network interface associated to the intermediate
   gateway.

   If the gateway is not yet known, the packet will be placed in a
   waiting queue. Each time the query manager will transmit to the real
   time forwarder new gateway information, the queue will be processed,
   and packets for which the information has become available will be
   forwarded. Packets in this waiting queue will "age"; their time to
   live counts will be decremented at regular intervals. If it become
   null, the packets will be destroyed; an ICMP message may be
   forwarded.

   The DNS query manager may be in some cases unable to find RX
   information for a particular destination. It will in that case signal
   to the real time forwarder that the destination is unreachable. The
   information will be kept in the destination table; queued packet for
   this destination will be destroyed, and new packets will not be
   forwarded.

   The information in the destination table will not be permanent. A
   time to live will be associated to each line of the table, and the
   aging lines will be periodically removed.

5.5.  Interaction with routing protocols

   The monitoring of the "destination unreachable" packets described
   above is mostly justified by a desire to leave standard IP routing,
   and the corresponding kernel code, untouched.

      If the IP routing code can be modified, and if the local host has
      full routing tables, it can take the decision to transmit the
      packets to the real time forwarder more efficiently, e.g., as a
      default action for the networks that are not announced in the
      local tables. This procedure is better practice, as it avoids the
      risk of loosing the first packet that would otherwise have
      triggered the ICMP message.

6.  Acknowledgments

   We would like to thank Yakov Rekhter, which contributed a number of
   very helpful comments.

7.  Conclusion

   This memo suggests an experiment in directory based routing.  The
   author believes that this technique can be deployed in the current
   Internet infrastructure, and may help us to "buy time" before the
   probably painful migration towards IPv7.

   The corresponding code is under development at INRIA. It will be
   placed in the public domain. Interested parties are kindly asked to
   contact us for more details.

8.  References

   [1] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
       Specification", STD 5, RFC 791, DARPA, September 1981.

   [2] Clark, D., "Building routers for the routing of tomorrow",
       Message to the "big-internet" mailing list, reference
       <9207141905.AA06992@ginger.lcs.mit.edu>, Tue, 14 Jul 92.

   [3] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:  an
       Address Assignment and Aggregation Strategy", RFC 1338, BARRNet,
       cisco, Merit, OARnet, June 1992.

9.  Security Considerations

   Security issues are not discussed in this memo.

10.  Author's Address

   Christian Huitema
   INRIA, Sophia-Antipolis
   2004 Route des Lucioles
   BP 109
   F-06561 Valbonne Cedex
   France

   Phone: +33 93 65 77 15
   EMail: Christian.Huitema@MIRSA.INRIA.FR