Letters

Seamless Access to Multiple Networks
Using Domain Name Resolution
Akihiro Otaka†, Morimichi Hirahashi,and Masahide Migita
Abstract
We have developed a new method of achieving seamless access to multiple networks, such as the Internet and a content delivery network (CDN), based on domain name resolution with a novel concept ofnetwork priority. It lets users access to a CDN without manually switching network connections.

1. Seamless access to multiple networks
Communication technologies related to the Internet
protocol (IP) have been developed mainly for communication between computers. Today, however,many other types of IP-based services, such as video
distribution and IP telephony, are under development.
In these situations, IP networks will be constructedaccording to the specifications for each service. For
example, a content delivery network (CDN) could be
developed to facilitate a high-quality video distribution service, providing higher quality than could beachieved with the Internet. It would be constructed asa separate network, independent of the Internet. However, it would be inconvenient for users to have toreconfigure their computers whenever they want tochange access networks. Therefore, seamless multiple-network access will be important.
2. DNS-based seamless access method
In this article, we propose a new method of seamless access to multiple networks, based on domainname resolution with a novel concept of network priority. Our method provides seamless access by utilizing the domain name system (DNS) in an environment consisting of multiple network connections.
DNS is a technology used to translate a domain name,† NTT Access Network Service Systems Laboratories
Chiba-shi, 261-0023 Japan
E-mail: ootaka@ansl.ntt.co.jp

Vol. 2 No. 1 Jan. 2004

such as “ansl.com” that one might input into a webbrowser, into an IP address. In our method, the terminal of a user who wants to access a server while connected to multiple networks sends DNS query packets to each network. If the terminal receives morethan one DNS response packet, an optimum network
is selected according to network priority. This newconcept of network priority with DNS facilitates anew domain name space that is independent of theInternet. Figure 1 shows an example of a new mirror
service*1 using this method. In this case, two different
IP addresses in the CDN and the Internet are used forthe same domain name. The DNS server in the Internet and that in the CDN respond to different IPaddresses as a result of queries for the same domainname. Thus, the Internet access user accesses theInternet-based server by using the conventional DNS
sequence, while the CDN user accesses the CDNbased server with high-quality contents automatically by using this new access method.
We have implemented our new method in an environment with simultaneous connections to a PPPbased access network (PPP: point-to-point protocol)
and a native IP-based CDN. In this article, we illustrate two types of access based on this method: the*1 Mirror service: One of the network services for contents holders.
Using this service, one can deliver contents smoothly using multiple servers. Content in an original server is copied to some “mirror” servers and users can access the most convenient server automatically. In this article, the word “mirror service” includes thecase where a copy of higher quality than the original one is put on
mirror servers.

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LettersServerServerACDN

B
Ordinary-quality content

High-quality contentInternetDNS

DNS

ansl.com =server A
ansl.com =server BViewing
ansl.com
Internet access user

CDN userFig. 1. Example of a mirror service using the new access method.

user-terminal-type method and the network-type
method. First, we examine the user-terminal type,which is designed for user PCs connected to two networks, with the seamless access functions implemented as software on the PCs. Then, we discuss
some limitations of the user-terminal-type method
and show how the network-type method can provide
greater convenience.

through which the PC accesses the Internet by using
PPPoE (PPP over Ethernet). The other is a CDN,
through which the PC accesses a content server storing high-quality contents by using native IP. The PC
is assigned two IP addresses, one by each network.
Both the Internet and the CDN include DNS servers,
whose IP addresses are configured on the PC inadvance. The Internet and the CDN also both include
content servers, with low-bit-rate sample contents on
the Internet server and high-quality original contentson the CDN server. In this situation, the PC is a splitting point with respect to the two networks, so it musthave a function for selecting the more suitable network and content server in a given situation.

3. User-terminal-type access method
Figure 2 illustrates the user-terminal-type seamless
access method. In this figure, a user PC is connected
to two networks. One is a PPP-based access network,

DNSInternet
CDN
PPP-based
access network

BRAS

RouterDNS

To a DNS
server in theInternet

To a DNS
server inthe CDN

(1)

(1)

PPPoE
(2)

Switch(2)

Proxy resolver(3)

Access line
PPPoE
IP addresses

User terminal (PC)DNS resolver

DNS query
DNS response

Application
BRAS: broadband remote access server
PPP: point to point protocol
PPPoE: PPP over Ethernet

Fig. 2. DNS-based seamless access method: user-terminal type.

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We have implemented the DNS-based accessmethod as software, called a proxy resolver, runningon the Windows operating system. This enables us toadd this new function without changing the existing
DNS resolver in Windows.
Figure 2 shows a schematic view of this approach.
First, an application on the PC asks the proxy resolver
for the IP address of the content server. The proxy
resolver sends DNS query packets to the Internet
DNS server and the CDN DNS server simultaneously (step (1) in the figure). Each DNS server replies
with a DNS response packet (step (2)). The proxy
resolver receives these packets and chooses the optimum DNS response as the result of name resolution
according to the network priority. It then sends theresult to the application through the DNS resolver(step (3)). As a result of this sequence, if the domainname is resolved in the CDN, the application canaccess the CDN server, while if it is not resolved inthe CDN, the application accesses the Internet server.
Because the PC cannot know in advance which
response it will receive first, the proxy resolver waits
for the DNS response packet from the CDN for a certain time. If this DNS response packet does not arrive
within this time, the proxy resolver sends the resultsfrom the Internet to the application. This waiting step
enables the DNS response from the CDN to be handled with the appropriate priority.
In this method, the DNS server and content servers
in the Internet do not have to make any changes.
Therefore, the user can continue to access the Internet
server as normal through the PPP-based access network. On the other hand, the DNS server in the CDN

InternetDNSCDNPPP-basedaccess network

does not reply to queries intended for the Internetbased DNS server, so it does not have to register any
Internet servers. This ensures that the CDN can beconstructed independently from the Internet domain.
There are two major issues with applying the userterminal-type access method. First, this approach performs network selection only for domain name resolution, and the IP routing for the server should bedetermined independently from the name resolution.
This means that although the PC can obtain a suitable
IP address with this method, it will not know which
network the address belongs to. Therefore, the IP
routing table for the PC has to be configured someother way.
The second issue is that the DNS response from theInternet must be stored by the proxy resolver forsome time, called the “waiting time”. This means that
the Internet domain name resolution cannot take lesstime than this waiting time. However, we think thatthe waiting time can be reduced sufficiently to prevent any inconvenience to users during typical usage,
such as Web access, because the only requirement for
the waiting time is that it should be longer than theDNS response time from the CDN.
4. Network-type access method
To resolve the problems with the user-terminal-type
approach, we developed the network-type seamless
access method (Fig. 3). The PC is connected only tothe Internet, through the PPP-based access network
by PPPoE. This means that only one IP address andthe DNS server’s address are assigned to the PC fromDNS

To a DNS
server in theInternet

To the Internet
(7)

BRAS

Router

(3)

PPPoE
(4)

(2)
PPPoE/IP translator

PPPoE/IPtranslator

Proxy DNS
device(1)

Access line
PPPoE
IP address

User terminal (PC)To a DNS
server inthe CDN(3)(4)

Proxy DNS device(5)(6)

To a content
server in the CDN

PPPoEPC

DNSresolver
Application

DNS query
DNS response
Server accessFig. 3. DNS-based seamless access method: network type.

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the Internet. Any IP addresses related to the CDN,
such as a default gateway address or a DNS address,
are not configured on the PC. Instead, the PC canaccess the CDN through a PPPoE/IP translator andproxy DNS device. These technologies facilitate
DNS-based seamless access and IP routing to communicate with the CDN content server. Thus, as withthe user-terminal-type method, DNS and content
servers are located in both the CDN and the Internet.
In the first step of the network-type method, asillustrated in Fig. 3, the PC sends a DNS query packet to the network (step (1) in the figure). The PC is
only connected to the Internet through the PPP-based
access network, so the destination IP address of theDNS query packet is the DNS server in the Internet,
and the packet is encapsulated by PPPoE. ThePPPoE/IP translator detects the DNS query packet,
removes the PPPoE header, and sends it to the proxyDNS device as an IP packet (step (2)). The proxy
DNS device receives the packet and resolves thedomain name as a proxy for the PC. It then sends
DNS query packets to the Internet DNS server andthe CDN DNS server simultaneously (step (3)), in thesame manner as for the user-terminal-type accessmethod. The DNS servers reply with DNS response
packets (step (4)). The proxy DNS device receives theDNS response packets, selects the DNS response
from the network with higher priority as the DNS
result, and sends the appropriate DNS response packet back to the PC through the PPPoE/IP translator(step (5)). The operating sequence of the proxy DNS
device is thus the same as that of the proxy resolver inthe user-terminal-type method. These network functions provide the PC with an environment in whichnothing changes with respect to name resolution inthe Internet.
The proxy DNS device caches the name resolution
data requested by PCs. As a result, it can provide a
DNS response immediately, without sending a new
request to the DNS servers. It can also refresh thisdata by periodically sending requests to the DNS
servers. Thus, this approach resolves the second issueof the waiting time in the user-terminal-type method.
The PPPoE/IP translator can resolve the first issue,
in which the IP routing table for the PC must be configured in advance. The PPPoE/IP translator examines the IP headers of the PPPoE-encapsulated IPpackets. If the destination of an IP packet is in theCDN, the translator removes the PPPoE header fromthe packet and sends it as an IP packet to a router in
the CDN (step (6)). On the other hand, if the IP packet is not destined for the CDN and it is not a DNS
88query packet, the translator transmits it without
removing the PPPoE header by applying the bridgetransmission approach (step (7)). Thus, the PPPoE/IPtranslator can correctly send packets to the CDN or to
the Internet through the PPP-based access network.
This sequence of operations in the DNS-based
seamless access method lets a PC access the optimum
server automatically, without requiring any additional functions or configuration.
5. Summary
We have proposed a new method for seamless
access to multiple networks, based on DNS with anovel concept of network priority. This method
allows convenient access to a CDN in an environment
with simultaneous connections to the Internet and theCDN.

Akihiro Otaka
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the M.S. degree in physics fromthe University of Tokyo, Tokyo in 1991 andjoined NTT’s LSI Laboratories the same year.
He is now working on broadband access systemsand networks. He is a member of the Institute ofElectronics, Information and Communication
Engineers and the Japan Society of Applied
Physics.

Morimichi Hirahashi
Senior Research Engineer, NTT Access Network Service Systems Laboratories.
He received the M.E. degree in civil engineering from Tohoku University, Sendai, Miyagi.
After working on broadband access systems andnetworks for NTT Access Network Service Systems Laboratories, he moved to NTT West in2003 .

Masahide Migita
Research Engineer, NTT Access Network Service Systems Laboratories.
He received the B.E. and M.E. degrees in civilengineering from Kyushu University, Fukuoka
in 1993 and 1995, respectively. After working in
optical access network planning, he moved toNTT West in 2003 and is now promoting broadband services.

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