DNS
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... Routing mechanisms. At a high-level, these may be classified
under: DNS Request-Routing, transport-layer Request-Routing ...
...
The memo is organized as follows: Section 2 provides a summary of
known DNS based Request-Routing techniques. Section 3 discusses
transport-layer ...
... DNS based Request-Routing techniques are common due to the ubiquity
of the DNS system [10][12][13 ...
... DNS based Request-Routing
techniques, a specialized DNS server is inserted in the DNS
resolution process. The server is capable of returning a different
...
... Routing
techniques, a specialized DNS server is inserted in the DNS
resolution process. The server is capable of returning a different
set of A, NS ...
... metrics, or a combination of both. In [11] RFC 2782prop (DNS SRV)
provides guidance on the use of DNS for load balancing ...
... 2782prop (DNS SRV)
provides guidance on the use of DNS for load balancing. The RFC
describes some of the limitations and suggests appropriate useage of
...
... load balancing. The RFC
describes some of the limitations and suggests appropriate useage of
DNS based techniques. The next sections provides a summary of some
of the used techniques.
...
... domain or a sub domain. The DNS server returns the IP address of the
best surrogate in an A record ...
... IP address of the
best surrogate in an A record to the requesting DNS server. The IP
address of the surrogate could also be a virtual IP(VIP) address ...
...
In this approach, the Request-Routing DNS server returns multiple
replies such as several A records for various surrogates. Common
...
... A records for various surrogates. Common
implementations of client site DNS server's cycles through the
multiple replies in a Round-Robin fashion. The order in which the
...
...
In this approach multiple Request-Routing DNS servers can be involved
in a single DNS resolution. The rationale of utilizing multiple
...
... Routing DNS servers can be involved
in a single DNS resolution. The rationale of utilizing multiple
Request-Routing DNS servers ...
... DNS resolution. The rationale of utilizing multiple
Request-Routing DNS servers in a single DNS resolution is to allow
one to distribute more complex decisions from a single server ...
... Request-Routing DNS servers in a single DNS resolution is to allow
one to distribute more complex decisions from a single server to
...
... single server to
multiple, more specialized, Request-Routing DNS servers. The most
common mechanisms used to insert multiple Request-Routing DNS servers ...
... DNS servers. The most
common mechanisms used to insert multiple Request-Routing DNS servers
in a single DNS resolution is the use of NS ...
... NS and CNAME records. An
example would be the case where a higher level DNS server operates
within a territory, directing the DNS lookup to a more specific DNS
server ...
... example would be the case where a higher level DNS server operates
within a territory, directing the DNS lookup to a more specific DNS
server within that territory to provide a more accurate resolution.
...
... DNS server operates
within a territory, directing the DNS lookup to a more specific DNS
server within that territory to provide a more accurate resolution.
...
... level domain to another Request-Routing DNS server. The, technique
allows multiple DNS server to be involved in the name resolution
...
... Routing DNS server. The, technique
allows multiple DNS server to be involved in the name resolution
process. For example, a client site DNS server ...
... DNS server to be involved in the name resolution
process. For example, a client site DNS server resolving
a.b.example.com [10] would eventually request a resolution of
...
... Routing NS server. In this case the Request-Routing DNS
server can either return a set of A records or can redirect the
resolution of the request a.b.example.com to the DNS server ...
... DNS
server can either return a set of A records or can redirect the
resolution of the request a.b.example.com to the DNS server that is
authoritative for example.com using NS records.
...
... NS records is that the number of
Request-Routing DNS servers are limited by the number of parts in the
DNS name. This problem results from DNS ...
... Routing DNS servers are limited by the number of parts in the
DNS name. This problem results from DNS policy that causes a client
...
... DNS servers are limited by the number of parts in the
DNS name. This problem results from DNS policy that causes a client
site DNS server ...
... DNS policy that causes a client
site DNS server to abandon a request if no additional parts of the
DNS name are resolved in an exchange with an authoritative DNS
server ...
... site DNS server to abandon a request if no additional parts of the
DNS name are resolved in an exchange with an authoritative DNS
server.
...
... DNS server to abandon a request if no additional parts of the
DNS name are resolved in an exchange with an authoritative DNS
server.
A second drawback is that the last DNS server ...
... DNS
server.
A second drawback is that the last DNS server can determine the TTL
of the entire resolution process. Basically, the last DNS server ...
... DNS server can determine the TTL
of the entire resolution process. Basically, the last DNS server can
return in the authoritative section of its response its own NS
...
... domain currently resolved, since
in this case the A records, which might be passed in the DNS
response, are discarded for security reasons. Another drawback is
the added delay in resolving the request due to the use of multiple
...
... security reasons. Another drawback is
the added delay in resolving the request due to the use of multiple
DNS servers.
...
...
In this scenario, the Request-Routing DNS server returns a CNAME
record to direct resolution to an entirely new domain ...
... the new domain might employ a new set of Request-Routing DNS servers.
One disadvantage of this approach is the additional overhead ...
... domain name. The main advantage of this approach
is that the number of Request-Routing DNS servers is independent of
the format of the domain name.
...
... simply type the name of the server and be connected to the nearest
one. By using anycast, DNS resolvers would no longer have to be
configured with the IP addresses of their servers, but rather could
...
... Furthermore, to combine measurement and redirection, the
Request-Routing DNS server can advertise an anycast address as its IP
address. The same address ...
... IP
address. The same address is used by multiple physical DNS servers.
In this scenario, the Request-Routing DNS server ...
... DNS servers.
In this scenario, the Request-Routing DNS server that is the closest
to the client site DNS server ...
... DNS server that is the closest
to the client site DNS server in terms of OSPF and BGP routing will
...
... OSPF and BGP routing will
receive the packet containing the DNS resolution request. The server
can use this information to make a Request-Routing decision.
...
... Drawbacks of this approach are listed below:
o The DNS server may not be the closest server in terms of routing
to the client ...
...
Since only DNS names are visible during the DNS Request-Routing, some
solutions encode the object type, object hash ...
... solutions encode the object type, object hash, or similar information
into the DNS name. This might vary from a simple division of objects
based on object type (such as images.a.b.example.com and
...
... available at resolution time. The disadvantage is that the client
site DNS server has to perform multiple resolutions to retrieve a
single Web page, which might increase rather than decrease the
overall latency ...
... Routing techniques.
o DNS only allows resolution at the domain level. However, an ideal
request resolution system should service ...
... DNS based Request-Routing systems servers may be required to
return DNS entries with a short time-to-live (TTL) values. This
...
... may be needed in order to be able to react quickly in the face of
outages. This in return may increase the volume of requests to
DNS servers.
o Some DNS ...
... DNS servers.
o Some DNS implementations do not always adhere to DNS standards.
For example, many DNS ...
...
o Some DNS implementations do not always adhere to DNS standards.
For example, many DNS implementations do not honor the DNS TTL ...
... DNS implementations do not always adhere to DNS standards.
For example, many DNS implementations do not honor the DNS TTL
field.
...
... DNS Request-Routing is based only on knowledge of the client DNS
server, as client addresses are not relayed within DNS ...
... DNS
server, as client addresses are not relayed within DNS requests.
This limits the ability of the Request-Routing system to determine
...
... client's proximity to the surrogate.
o DNS servers can request and allow recursive resolution of DNS
names. For recursive resolution of requests, the Request-Routing
...
...
o DNS servers can request and allow recursive resolution of DNS
names. For recursive resolution of requests, the Request-Routing
DNS server ...
... DNS
names. For recursive resolution of requests, the Request-Routing
DNS server will not be exposed to the IP address of the client's
...
... site DNS server. In this case, the Request-Routing DNS server
will be exposed to the address of the DNS server ...
... DNS server
will be exposed to the address of the DNS server that is
recursively requesting the information on behalf of the client's
...
... recursively requesting the information on behalf of the client's
site DNS server. For example, imgs.example.com might be resolved
by a CN ...
...
o Users that share a single client site DNS server will be
redirected to the same set of IP addresses during the TTL ...
... a flash crowd.
o Some implementations of bind can cause DNS timeouts to occur while
handling exceptional situations. For example, timeouts can occur
for NS ...
... domains.
DNS based request routing techniques can suffer from serious
limitations. For example, the use of such techniques can overburden
...
... limitations. For example, the use of such techniques can overburden
third party DNS servers, which should not be allowed [19]. In [11]
...
... 11]
RFC 2782prop provides warnings on the use of DNS for load balancing.
Readers are encouraged to read the RFC for better understanding of
...
... client to newly selected
surrogate) will flow through the surrogate originally chosen by DNS.
The reverse-flow (surrogate to client ...
... In general, transport-layer Request-Routing can be combined with DNS
based techniques. As stated earlier, DNS based methods ...
... Routing can be combined with DNS
based techniques. As stated earlier, DNS based methods resolve
clients ...
... client's DNS server IP address. Hence, the DNS based methods could
be used as a first step in deciding on an appropriate surrogate with
...
... specifics for Request-Routing. However, it can be used in
combination with DNS Request-Routing to direct related DNS queries
...
... combination with DNS Request-Routing to direct related DNS queries
into the domain name space of the service provider ...
... mechanisms can be used in combination.
A basic problem of DNS Request-Routing is the resolution granularity
that allows resolution on a per-domain ...
... domain level only. A per-object
redirection cannot easily be achieved. However, content modification
can be used together with DNS Request-Routing to overcome this
problem. With content modification, references to different objects
...
... on the same origin server can be rewritten to point into different
domain name spaces. Using DNS Request-Routing, requests for those
objects can now dynamically be directed to different surrogates.
...
... direct users to the "closest" surrogate. In this document proximity
means round-trip time. In a DNS Request-Routing system, the
measurements are made to the client ...
... Routing system, the
measurements are made to the client's local DNS server. However,
when the IP address of the client ...
... Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV ...
... A. Shaikh, "On the effectiveness of DNS-based Server Selection", INFOCOM 2001, August 2001. ...