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Network Working Group                                         C. HuitemaRequest for Comments: 1383                                         INRIA                                                           December 1992An Experiment in DNS Based IP RoutingStatus 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 Contents1. Routing, scaling and hierarchies ......................12. Routing based on MX records ...........................23. Evaluation of DNS routing .............................33.1 Loops and relays .....................................43.2 Performances and scaling .............................53.3 Tunneling or source routing ..........................63.4 Choosing a gateway ...................................63.5 Routing dynamics .....................................63.6 DNS connectivity .....................................73.7 On the way back ......................................83.8 Flirting with policy routing .........................84. Rationales for deployment .............................94.1 The good citizens ....................................104.2 The commercial approach ..............................105. The experimental development ..........................115.1 DNS record ...........................................115.2 Interface with the standard IP router ................125.3 The DNS query manager ................................125.4 The real time forwarder ..............................125.5 Interaction with routing protocols ...................136. Acknowledgments .......................................137. Conclusion ............................................138. References ............................................149. Security Considerations ...............................1410. Author's Address .....................................141.  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 EBONEHuitema                                                         [Page 1]

RFC 1383                  DNS based IP routing             December 1992   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 I1Huitema                                                         [Page 2]

RFC 1383                  DNS based IP routing             December 1992   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, orHuitema                                                         [Page 3]

RFC 1383                  DNS based IP routing             December 1992            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 someHuitema                                                         [Page 4]

RFC 1383                  DNS based IP routing             December 1992   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.Huitema                                                         [Page 5]

RFC 1383                  DNS based IP routing             December 19923.3.  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 oneHuitema                                                         [Page 6]

RFC 1383                  DNS based IP routing             December 1992   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 fromHuitema                                                         [Page 7]

RFC 1383                  DNS based IP routing             December 1992   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.Huitema                                                         [Page 8]

RFC 1383                  DNS based IP routing             December 1992   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.Huitema                                                         [Page 9]

RFC 1383                  DNS based IP routing             December 19924.1.  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.Huitema                                                        [Page 10]

RFC 1383                  DNS based IP routing             December 19925.  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| |_________________________|________|__________|___________________|Huitema                                                        [Page 11]

RFC 1383                  DNS based IP routing             December 1992   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 realHuitema                                                        [Page 12]

RFC 1383                  DNS based IP routing             December 1992   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.Huitema                                                        [Page 13]

RFC 1383                  DNS based IP routing             December 19928.  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.FRHuitema                                                        [Page 14]