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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
INTERNET DRAFT Anant Kumar
Expiration Date: April 12, 1994 Steve Hotz
<draft-ietf-dns-ixfr-00.txt> Jon Postel
USC/ISI
Oct. 1993
Incremental Transfer and Fast Convergence in DNS
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet-Drafts. Internet-Drafts are draft
documents valid for a maximum of six months. Internet-Drafts may be
updated, replaced, or obsoleted by other documents at any time. It
is not appropriate to use Internet-Drafts as reference material or to
cite them other than as a ``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or
munnari.oz.au.
This Internet Draft expires April 12, 1994.
Abstract
This memo proposes extensions to the DNS protocols to provide for an
incremental zone transfer (IXFR) procedure. A companion mechanism,
the NOTIFY procedure, is also proposed to allow secondaries to learn
of changes to the primary database in a timely manner.
Introduction
The last few years have witnessed an exponential growth in the number
of machines in the internet, and a corresponding dependence on DNS.
As a result, zone files have grown to near HOSTS.TXT proportions.
Each zone file is maintained at a primary server. All modifications
to this file are made at the single site and propagated to secondary
servers using the Zone Transfer protocol [RFC 1035].
Whenever any change is made to the zone file, the zone administrator
increments the SOA serial number. Secondary servers poll the primary
every REFRESH interval, and if the serial number has changed, the
entire zone file is transferred. More often than not, the change
Kumar, Hotz, Postel [Page 1]
INTERNET DRAFT October, 1993
made to the zone file is a very small percentage of the zone file.
Thus, an incremental transfer protocol that will propagate only the
changes to the zone file, may allow substantial savings of bandwidth
overhead.
In addition, secondaries only check to see if they are consistent
with the primary every REFRESH period. While setting REFRESH to be a
relatively large value reduces bandwidth overhead, there can be large
time intervals during which at least one secondary has data that is
inconsistent with the primary. The proposed NOTIFY mechanism (where
the primary sends a message to known secondaries) facilitates fast
convergence of servers vis-a-vis consistency of data in the zones
(without requiring the overhead implied by a short REFRESH period).
These two mechanisms can be used to reduce the bandwidth overhead of
DNS while maintaining server-to-server consistency for any particular
zone. These mechanisms could prove particularly useful if a DNS of
the future were required to support dynamic updates (e.g. frequent
changes to a zone, possibly from multiple entities making changes by
sending "update packets"). Dynamic updates imply small database
changes, and a need for fast convergence among authoritative servers.
This memo does not specifically address a Dynamic Update scheme, but
the IXFR and NOTIFY mechanisms were designed in light of possible
requirements for dynamic update schemes.
The following changes to the DNS protocols are required to support
the two new mechanisms: a new query type called IXFR is defined, an
opcode called NOTIFY is defined, a new RR type called ISOA is
defined, and two fields (a serial number and a flag) are added to
each resource record.
1. Resource Record support for Incremental Transfers
To support incremental zone transfers, an RR will now have 7 fields
as follows:
<owner> <ttl> <class> <type> <data> <serial number> <Zflag>
The current DNS mechanism does not provide a way to identify the
chronological history of the data in the zone files. To support
incremental transfers, it is necessary to know when a resource record
was added to the database (relative to other updates). Thus, a
serial number is associated with each resource record in a zone file.
The "Zflag" (zombie flag) is necessary to convey incremental state
information with respect to a resource record (i.e. should the RR be
added or deleted from a secondaries zone information). Section 1.2
details the use of "Zflag".
Kumar, Hotz, Postel [Page 2]
INTERNET DRAFT October, 1993
For backward compatibility reasons, the serial number and "Zflag"
will only be propagated along with IXFR transfers (further, a DNS
client has no use for serial number information at this point).
Future DNS clients might want to make use of this information, and
new query types could be defined that would return serial numbers.
1.1 IXFR Use of RR Serial Numbers
The RR serial numbers must be a strictly monotonically increasing
function. This will allow servers to differentiate between two sets
of RRs: those added before a certain serial number, and those added
after a certain serial number.
To illustrate the basic scheme, for the moment consider only the case
of adding new RRs to a zone (the more subtle cases of deletion and
modification are considered in detail below). When an RR is added to
a zone, a new (higher) serial number is associated with the newly
added RR. Because RR serial numbers are monotonically increasing,
servers can distinguish when an RR was added (relative to other RRs).
A scheme to conserve serial number space is described in section
1.4.1.
The current status of zone information held at a particular server is
reflected by the highest serial number associated with the RRs of the
zone. When a secondary requests an incremental zone transfer (IXFR),
it must send its current status (highest RR serial number) as part of
this request. The primary server can then transfer all RR records
that have a higher sequence number; consequently, the status of the
zone information held at the primary and secondary will be the same.
1.2 Deleting/Modifying an existing resource Record
A modification will be treated as a deletion followed by an addition,
thus only the deletion process is described here. [NOTE: If there is
a requirement for modification atomicity, this would require a
distinct operation; this could be supported by extending the "zombie"
mechanism described below.]
When receiving IXFR updates, a secondary must receive an EXPLICIT
notification of deleted RRs (unlike a full AXFR scheme where all RRs
are considered deleted unless refreshed). Hence an RR cannot be
removed from the primary zone when it is deleted; instead it is
modified and used as the explicit notification (to the secondaries)
of removal. The RR is marked as a "ZOMBIE" (using the Zflag) and the
serial number is updated as described above. The zombie RR is kept
in memory until the primary is sure all secondaries have updated
zones to reflect the deleted RR. When the primary is certain all
secondaries have received the notification of the deleted RR; during
Kumar, Hotz, Postel [Page 3]
INTERNET DRAFT October, 1993
the interim the primary, of course, does not return the deleted RR in
response to client queries. No server should return a zombie RR in
response to a client query.
In a sense, an IXFR contains commands of two types: one that
specifies a new RR should be added to the zone information, and one
to delete an RR from the zone. To converge correctly, a server
receiving an IXFR must apply/process these commands (RRs and zombie
RRs) in order per the RR sequence numbers. Note that once a
secondary applies a ZOMBIE RR to the zone information it holds, it
does not need to maintain this ZOMBIE (unless it also serves to
update other secondaries via IXFR).
ZOMBIE RRs cannot be maintained indefinitely, because this would
cause the amount of information maintained for the zone (at the
primary) to be unnecessarily large (i.e. one does not want to
maintain some number of ADD/DELETE pairs for a particular RR that
could theoretically occur over time). Fortunately, there is no need
to maintain ZOMBIE RRs indefinitely; they can be deleted when all
servers for a zone have been notified of the deleted RR.
1.2.1 Mechanisms for Deleting "ZOMBIE" Resource Records
There are multiple mechanisms that could be used to keep track of
ZOMBIE RRs and when they can be deleted. Either or both of the
following two schemes could be used, depending on the desired
performance and overhead trade-offs.
a. The primary can maintain information (state) about all secondaries
that normally transfer zone from it. This will be "soft state",
implying that it can be rebuilt from scratch should the primary
server crash (the only impact being that ZOMBIE RRs may not be
deleted as soon as they might otherwise).
Hence, for each secondary server, the primary records the last
serial number it transferred (on recovering from a crash, this
number will be set to 0). When the minimum of these serial numbers
(for all servers of a particular zone) is greater than the serial
number on a ZOMBIE RR entry, that ZOMBIE RR can be removed.
b. This scheme requires that secondary servers may sometime be
"forced" to take an AXFR rather than an IXFR. A primary maintains
all RRs within "N" serial numbers of the zone's current serial
number (highest valued RR serial number); ZOMBIE RRs with serial
numbers lower than (current_serial_number - N) are deleted.
Any secondary server that requests a serial number smaller than the
primary's (current_serial_number - N), must AXFR instead of IXFR.
Kumar, Hotz, Postel [Page 4]
INTERNET DRAFT October, 1993
The primary will send an AXFR reply (in response to the IXFR) and
the secondary is expected to be able to parse either IXFR or AXFR
responses to its original IXFR query. It should be built to parse it
either way.
This scheme is designed to be more efficient (and simpler) than the
alternative where the primary refuses the IXFR and the following events
happen:
client: request(IXFR)
server: refuse(IXFR)
client: request(AXFR)
server: send(AXFR)
Either or both of these schemes, (a) and (b), could be used, and this
can be an implementation-specific decision so long as the servers can
interoperate. Scheme (a) requires no additional protocol
interaction, but all [secondary] servers must accommodate a "you
asked for an IXFR *but* here is an AXFR" if different implementations
are to interoperate.
While a primary could implement either approach, one should see an
advantage by implementing both. The "soft state" method (a) will
allow ZOMBIE RRs to be deleted as early as is possible, and the
"refused IXFR" method (b) will place a bound on the amount of memory
required by the primary (useful in the case where a secondary is out
of touch for very long periods of time).
1.3 The Role of the SOA Serial Number
Since IXFR-capable servers are likely to be used together with older
servers during a transition period (of unfortunately indefinite
length), the SOA serial number functionality must be preserved for
backward compatibility. This implies that the SOA serial number must
be changed each time the zone is updated. The simplest solution is
for the SOA serial number to reflect the highest RR serial number.
This issue is not difficult in the context of simply accommodating
incremental zone transfers, since a zone file (and SOA serial number)
will necessarily be updated when RRs are added, deleted, or modified.
However, there is other ongoing work that is addressing mechanisms
for dynamically updating zone information; it would be an advantage
if the IXFR scheme considered such mechanisms, and was designed to
accomodate the additional complexities introduced by dynamically
updated zone information.
If the use of dynamic updates are ignored, the SOA serial number
could be updated in the same manner as it is today. In fact, the
Kumar, Hotz, Postel [Page 5]
INTERNET DRAFT October, 1993
manually-updated SOA serial number could be assigned to each new,
modified, or deleted/zombie RR. Other implementation-specific
schemes could be used to derive SOA serial numbers and maintain the
relationship between SOA serial number and RR serial numbers.
1.3.1 SOA Serial Numbers in light of Dynamic Updates
The IXFR scheme essentially obsoletes the function of the SOA serial
number, replacing it with finer-granularity serial numbers applied to
RRs (this is especially true if dynamically updates to the zone are
made). The highest-valued RR serial number now reflects the status
of the zone information. At this point, the question is unresolved
as to how the SOA serial number should be treated.
Without going into specifics, the crucial point is that zone
information will not always change due to a zone file update.
Further, RR serial numbers will be assigned dynamically which must be
reflected in SOA serial number (for interoperability).
An implication (which may be disturbing to the old guard of DNS
administrators) is that SOA serial number will not be updated
exclusively in the zone file. To preserve zone integrity, the
changes made dynamically must take precedence over manually-updated
SOA serial numbers; consider the case of a manually-updated SOA
serial number being less than that required by the number of dynamic
updates.
Summary: The specifics of this issue can be resolved in separately
from the dynamic update effort. However, the IXFR scheme should be
aware that the use of the SOA serial number is likely to change as
follows:
(1) The function of the SOA serial number is replaced by the highest
RR serial number. SOA serial number must also reflect changes to
the zone for backward compatability; some mapping from RR serial
numbers to SOA serial is required (the simplest being that they
are the same).
(2) A manual update of a zone file cannot specify an SOA serial number
which conflicts with (is smaller than) SOA serial number that
reflects dynamic changes to the zone.
1.4 New RR Serial Number Generation
This is an implementation specific issue, as long as the the function
is monotonically increasing, and the constraints imposed by the
relationship between RR serial numbers and the SOA serial number are
met (see Section 1.3). One obvious function is to maintain a
Kumar, Hotz, Postel [Page 6]
INTERNET DRAFT October, 1993
sequence number counter, which is updated each time a new RR (or
ZOMBIE RR) is added (see section 1.4.1 for a variation of counter-
based sequence number generation).
A simple alternative (especially in light of dynamic updates) is to
use a timestamp (seconds since the epoch) as determined at the
primary server. The primary advantage (other than simplicity) is
speculative; if the actual time of update is available to clients,
the DNS system, or network administrators, it might serve some useful
function in the future (e.g. perhaps network administrators can
identify irregularities in a zone by examining RR serial number and
applying heuristic that takes into account expected turn-over rate
for the zone).
Section 1.3 implies that manually-updated SOA serial numbers may not
be possible in the future, hence the semantics attached to them by
many administrators today (i.e. YYMMDDHH) may also be less clear.
The use of timestamp-based serial number might be an appropriate
replacement.
The disadvantage of timestamps is that they might not be compatible
with the existing serial number space. A future timestamp might be
much smaller (numerically) than a serial number associated with a
given zone today. This could, of course, be resolved by the rather
drastic measure of shutting down all servers for a domain, deleting
the zone backup files from all secondaries and starting afresh, with
a new serial number space based on timestamps.
1.4.1 Reducing Sequence Number Rollover
If a simple counter-based sequence number is used (rather than
timestamp-based), the sequence number does not necessarily have to be
updated each time a new RR is added. A scheme to conserve the use of
serial number space could be based on whether other servers have
received updates recently:
If (zone transferred by anybody)
set(NewNumNeeded);
and when a new RR is added:
If (NewNumNeeded) {
currentserial++;
reset(NewNumNeeded);
}
RR->serialNum = currentserial;
Thus, all RRs added between any two transfers get the same serial
Kumar, Hotz, Postel [Page 7]
INTERNET DRAFT October, 1993
number, thereby saving some amount of serial number space. Note that
when an RR is modified, this is actually a "delete" and an "add"
which must be done in the correct order. If the ordering is based on
sequence number, then the simple scheme above must accommodate this
additional constraint.
2. ISOA, the new RR type.
System administrators (for whatever reason) might not be comfortable
depending exclusively on incremental transfer to maintain zone
consistency. If this is the case, it can be resolved by configuring
periodic "checkpoints" where full zone transfers are done.
Thus, after REFRESH seconds, secondaries will use IXFR to transfer
incremental data. In addition, secondaries will do a complete zone
transfer every XXFRTIME seconds (typically at least an order of
magnitude greater than REFRESH) using the method described later in
this document (section 4).
To specify the XXFRTIME, an additional RR type is defined (so we are
backward compatible with existing DNS implementations). It has
symbolic name "ISOA" and numeric value xxx. The data section of the
ISOA record will look as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| XXFRTIME |
| |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
This RR might, in the future, be expanded to contain other fields and
we are considering giving it a more flexible structure to accommodate
for those changes.
3. The Actual Mechanism
An IXFR query packet will, necessarily, contain the highest RR serial
number the secondary last saw. Thus, the DNS IXFR query packet will
look like this.
Kumar, Hotz, Postel [Page 8]
INTERNET DRAFT October, 1993
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
/ QNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QTYPE = IXFR |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QCLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SECONDARY SERIAL NUMBER |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Note that QNAME, QTYPE and QCLASS are exactly as for any other query
type. SECONDARY SERIAL NUMBER is the serial number the secondary
must convey to the primary so the primary can send it all entries
updated since that serial number was seen.
3.1 The Client Side
The client (secondary) sends an SOA query to the server (primary) and
compares the serial numbers.
If (current > latest) {
signal a possible error;
QUIT;
}
If (current == latest) {
exit gracefully;
}
If (current < latest) {
transmit_IXFR_query(current_serial_number);
destroy_zone_file();
receive_IXFR_response(&buf);
update_zone_data(hashtab);
recreate_zone_file(backup_zone_file);
}
Note that we destroy the zone file before we actually attempt to
receive any data from the server. We do this since otherwise, if we
receive data and crash before we are able to update our zone file,
the primary will believe that we have the latest data while when we
actually recover, we will not. The "recreate_zone_file()" operations
needs to be atomic or at least, if the server crashes midway through
the operation, it should be able to detect this when restarting.
Kumar, Hotz, Postel [Page 9]
INTERNET DRAFT October, 1993
Thus, by this destruction, we ensure that we either have the latest
data or that we have nothing. This is of special importance here
since if we do not destroy the zone file and the client crashes after
it completes the transfer but before it can update the zone file, we
have a problem. The server believes the client has the updated data
(if it keeps soft state) while the client actually does not.
Thus, when we actually recover from the crash, we initiate a full
zone transfer from the server.
3.2 The Server
The server follows this simple algorithm when processing IXFR
queries.
current_serial_from_client -> csn;
Record (csn, soft_state_structure);
If (csn == SOA_serial) {
send blank response;
}
If (csn < SOA_serial - N) {
refuse IXFR.. or send AXFR response;
exit;
}
for (each entry in zone file)
if(csn >= entry_serial_number) continue;
else add entry to IXFR packet;
endfor;
Transmit IXFR packet to client.
Thus, a server sends incremental transfers only if the csn from the
client falls within N of the present value of the SOA serial number.
It could either refuse a response if it falls beyond N in which case
the client must be prepared to initiate an AXFR. Alternatively, the
server could send the AXFR packet instead and the secondary must
expect and be able to interpret that packet.
4. XXFR
Once every XXFRTIME (section 2), the IXFR query packet will be:
Kumar, Hotz, Postel [Page 10]
INTERNET DRAFT October, 1993
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
/ QNAME /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QTYPE = IXFR |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QCLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| SERIAL = 0 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Note that the SECONDARY SERIAL NUMBER field equals the value "0".
This prompts the server to send an IXFR packet that contains all the
zone data, i.e. it is the equivalent of the AXFR. The system could
use this to synchronize complete zone files at regular XXFRTIME
intervals.
5. NOTIFY
Currently, a secondary always waits "REFRESH" seconds before polling
the primary for any changes in the zone. If a primary makes any
changes (that may be rather important) and wants that all secondaries
reflect these changes immediately, the primary has no means of
talking to the secondaries.
A mechanism must be available to notify the secondary that it might
benefit from a zone transfer, right away if possible. We propose a
new opcode, "NOTIFY", to fulfill exactly this need.
When the database is updated, the primary sends a NOTIFY packet to
the secondaries. This packet contains the SOA record for the zone
and informs the secondary that it might benefit from a transfer. The
secondary can choose not to transfer, if it sees a heavy load at that
moment. The notification could be turned on, on a per zone basis,
and might need a new bootfile parameter (NOTIFY/NONOTIFY) with the
primary/secondary entry.
This mechanism will be particularly useful in dynamic update
situations where servers might need to converge to a common state,
fast.
The NOTIFY packet looks like this:
Kumar, Hotz, Postel [Page 11]
INTERNET DRAFT October, 1993
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| "NOTIFY" |AA|TC|RD|RA| Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT = 0 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT = 1 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT = 0 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT = 0 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ /
/ SOA RECORD /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
A mix of polling (current mechanism) and low-priority interrupts from
the primary may be considered as the mechanism for zone transfers.
5.1 Unregistered Secondaries
The primary server must be aware of the presence of all secondaries,
including those that aren't registered ("registered" servers are
those servers whose names are returned by DNS in response to an NS
query for that zone), in order to send them NOTIFY messages. Note
that this information is also needed if the primary is maintaining
"soft state" (per the mechanism in section 1.1.2).
Once again, there are two different means of doing this:
a. Automatic Registration
Any entity that initiates any kind of transfer (IXFR/AXFR) is
identified as a potential candidate for notification and for the
purpose of keeping soft state (as described in section 1.1.2). The
IP address of this entity may be recorded against the zone it
transferred. Old entries (more than some number of REFRESH periods
old, say 10*REFRESH) may be timed out).
b. "my-secondaries"
We keep a list of "my-secondaries" servers that are not advertised
via the DNS but are known servers for our domain (possibly serving
local resolvers). Since the system administrator typically knows
about unregistered secondaries, this merely formalizes their
Kumar, Hotz, Postel [Page 12]
INTERNET DRAFT October, 1993
existence. Such secondaries are usually local machines, mostly
time-sharing machines on a campus or organization premises (within
the domain) that serve as name servers for local queries, keeping the
load on the registered secondaries, small.
We propose an entry in the bootfile of each server that expects to
see IXFR queries from other servers that reads:
my-secondaries zone server1, server2, ....., serverN
This is a list of secondaries that would normally transfer zone from
this server.
This entry may immediately follow the entry that says,
primary zone datafile
or
secondary zone primary
Thus, "my-secondaries" servers are associated with a given zone and
could transfer from either a secondary or the primary server. Thus,
a secondary that serves as source for zone data for other secondaries
needs to maintain such a list, like the primary.
In either case, please note that we do not address the issue of
cached data at other servers. TTL values could be used to ensure
that data is not cached for longer than it is likely to stay valid.
5.2 Timing and Security issues.
Notification procedure necessitates that we ensure the following:
a. There must be a minimum time between notifications. This prevents a
malicious primary from bogging the secondary down, with back-to-back
notifications. The secondary must maintain a timer (optionally) to
enforce such restrictions.
b. A secondary must transfer zone within a maximum interval (existing
REFRESH mechanism should suffice). This ensures the state is not
inconsistent for more than a fixed maximum interval.
c. Modification Notification should be accepted only if coming from
primary or the server you normally transfer from. A malicious network
entity could pose as a primary and transfer incorrect data to you.
This does not really solve the problem of impersonation since a
masquerading entity could just as well act as the primary when
sending the NOTIFY message. This just provides an additional hurdle
so somebody actually sending you a NOTIFY does need to impersonate
Kumar, Hotz, Postel [Page 13]
INTERNET DRAFT October, 1993
the primary.
d. When accepted, zone should be transferred only from primary or the
server you normally transfer from.
6. Performance Issues
DNS, like any other replicated, distributed system, has various
parameters that can be tuned to get the desired nature of
performance. For example, a shorter REFRESH cycle ensures a faster
convergence among authoritative servers, at the cost of extra network
bandwidth used in transferring zone data. Thus, system
administrators tune this figure to the desired mix of consistency and
bandwidth usage.
Similarly, the TTL associated with each RR presents a trade-off
between how often you query an authoritative server and how current
your data is. A low TTL would keep the data current at the cost of
extra network traffic while a high TTL conserves bandwidth but allows
for the data to be inconsistent. System administrators balance
between the two requirements and choose a reasonable TTL.
Similar figures in the above scheme, such as the frequency of NOTIFY
acceptance, the TTL of dynamic data and possibly the number of
secondaries allowed, present trade-offs that need to be made to tune
performance. A higher rate of NOTIFY acceptance will imply greater
network traffic but very speedy convergence. A lower figure will
conserve network bandwidth but will allow for data to be inconsistent
for longer.
A single server, serving a zone will imply instantaneous convergence
but will provide very low availability and reliability. This might
be alright for a low traffic volume zone.
We do not, by way of the IXFR and NOTIFY mechanisms, hope to provide
servers that converge instantaneously with minimal traffic. Studies
in the future will show how effective these mechanisms will be and
how best the various parameters can be used to tune the performance.
7. Acknowledgements
We express our sincere thanks to Clifford Neuman, Masataka Ohta, Don
Lewis, Philip Wood and Paul Vixie for their comments on earlier
versions of this draft.
Kumar, Hotz, Postel [Page 14]
INTERNET DRAFT October, 1993
8. Authors' Addresses:
Anant Kumar, Steve Hotz, Jon Postel
<anant, hotz, postel> @isi.edu
USC Information Sciences Institute
4676 Admiralty Way
Marina Del Rey CA 90292-6695
Phone:(310) 822-1511
FAX: (310) 823-6714
This Internet Draft expires April 12, 1994.
Kumar, Hotz, Postel [Page 15]
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