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, RFC791, 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", RFC1338, 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
