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INFORMATIONAL
Network Working Group T. Dixon
Request for Comments: 1454 RARE
May 1993
Comparison of Proposals for Next Version of IP
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This is a slightly edited reprint of RARE Technical Report
(RTC(93)004).
The following is a brief summary of the characteristics of the three
main proposals for replacing the current Internet Protocol. It is not
intended to be exhaustive or definitive (a brief bibliography at the
end points to sources of more information), but to serve as input to
the European discussions on these proposals, to be co-ordinated by
RARE and RIPE. It should be recognised that the proposals are
themselves "moving targets", and in so far as this paper is accurate
at all, it reflects the position at the 25th IETF meeting in
Washington, DC. Comments from Ross Callon and Paul Tsuchiya on the
original draft have been incorporated. Note that for a time the term
"IPv7" was use to mean the eventual next version of IP, but that the
same term was closely associated with a particilar proposal, so the
term "IPng" is now used to identify the eventual next generation of
IP.
The paper begins with a "generic" discussion of the mechanisms for
solving problems and achieving particular goals, before discussing
the proposals invidually.
1. WHY IS THE CURRENT IP INADEQUATE?
The problem has been investigated and formulated by the ROAD group,
but briefly reduces to the following:
- Exhaustion of IP Class B Address Space.
- Exhaustion of IP Address Space in General.
- Non-hierarchical nature of address allocation leading to flat
routing space.
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Although the IESG requirements for a new Internet Protocol go further
than simply routing and addressing issues, it is these issues that
make extension of the current protocol an impractical option.
Consequently, most of the discussion and development of the various
proposed protocols has concentrated on these specific problems.
Near term remedies for these problems include the CIDR proposals
(which permit the aggregation of Class C networks for routing
purposes) and assignment policies which will allocate Class C network
numbers in a fashion which CIDR can take advantage of. Routing
protocols supporting CIDR are OSPF and BGP4. None of these are pre-
requisites for the new IP (IPng), but are necessary to prolong the
life of the current Internet long enough to work on longer-term
solutions. Ross Callon points out that there are other options for
prolonging the life of IP and that some ideas have been distributed
on the TUBA list.
Longer term proposals are being sought which ultimately allow for
further growth of the Internet. The timescale for considering these
proposals is as follows:
- Dec 15 Issue selection criteria as RFC.
- Feb 12 Two interoperable implementations available.
- Feb 26 Second draft of proposal documents available.
The (ambitious) target is for a decision to be made at the 26th IETF
(Columbus, Ohio in March 1993) on which proposals to pursue.
The current likely candidates for selection are:
- PIP ('P' Internet Protocol - an entirely new protocol).
- TUBA (TCP/UDP with Big Addresses - uses ISO CLNP).
- SIP (Simple IP - IP with larger addresses and fewer options).
There is a further proposal from Robert Ullman of which I don't claim
to have much knowledge. Associated with each of the candidates are
transition plans, but these are largely independent of the protocol
itself and contain elements which could be adopted separately, even
with IP v4, to further extend the life of current implementations and
systems.
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2. WHAT THE PROPOSALS HAVE IN COMMON
2.1 Larger Addresses
All the proposals (of course) make provision for larger address
fields which not only increase the number of addressable systems, but
also permit the hierarchical allocation of addresses to facilitate
route aggregation.
2.2 Philosophy
The proposals also originate from a "routing implementation" view of
the world - that is to say they focus on the internals of routing
within the network and do not primarily look at the network service
seen by the end-user, or by applications. This is perhaps inevitable,
especially given the tight time constraints for producing
interoperable implementations. However, the (few) representatives of
real users at the 25th IETF, the people whose support is ultimately
necessary to deploy new host implementations, were distinctly
unhappy.
There is an inbuilt assumption in the proposals that IPng is
intended to be a universal protocol: that is, that the same network-
layer protocol will be used between hosts on the same LAN, between
hosts and routers, between routers in the same domain, and between
routers in different domains. There are some advantages in defining
separate "access" and "long-haul" protocols, and this is not
precluded by the requirements. However, despite the few opportunities
for major change of this sort within the Internet, the need for speed
of development and low risk have led to the proposals being
incremental, rather than radical, changes to well-proven existing
technology.
There is a further unstated assumption that the architecture is
targeted at the singly-connected host. It is currently difficult to
design IPv4 networks which permit hosts with more than one interface
to benefit from increased bandwidth and reliability compared with
singly-connected hosts (a consequence of the address belonging to the
interface and not the host). It would be preferable if topological
constraints such as these were documented. It has been asserted that
this is not necessarily a constraint of either the PIP or TUBA
proposals, but I believe it is an issue that has not emerged so far
amongst the comparative criteria.
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2.3 Source Routing
The existing IPv4 has provision for source-specified routes, though
this is little used [would someone like to contradict me here?],
partly because it requires knowledge of the internal structure of the
network down to the router level. Source routes are usually required
by users when there are policy requirements which make it preferable
or imperative that traffic between a source and destination should
pass through particular administrative domains. Source routes can
also be used by routers within administrative domains to route via
particular logical topologies. Source-specified routing requires a
number of distinct components:
a. The specification by the source of the policy by which the
route should be selected.
b. The selection of a route appropriate to the policy.
c. Marking traffic with the identified route.
d. Routing marked traffic accordingly.
These steps are not wholly independent. The way in which routes are
identified in step (c) may constrain the kinds of route which can be
selected in previous steps. The destination, inevitably, participates
in the specification of source routes either by advertising the
policies it is prepared to accept or, conceivably, by a negotiation
process.
All of the proposals mark source routes by adding a chain of (perhaps
partially-specified) intermediate addresses to each packet. None
specifies the process by which a host might acquire the information
needed to specify these intermediate addresses [not entirely
unreasonably at this stage, but further information is expected]. The
negative consequences of these decisions are:
- Packet headers can become quite long, depending on the number of
intermediate addresses that must be specified (although there are
mechanisms which are currently specified or which can be imagined
to specify only the significant portions of intermediate addresses).
- The source route may have to be re-specified periodically if
particular intermediate addresses are no longer reachable.
The positive consequences are:
- Inter-domain routers do not have to understand policies, they
simply have to mechanically follow the source route.
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- Routers do not have to store context identifying routes, since
the information is specified in each packet header.
- Route servers can be located anywhere in the network, provided
the hosts know how to find them.
2.4 Encapsulation
Encapsulation is the ability to enclose a network-layer packet within
another one so that the actual packet can be directed via a path it
would not otherwise take to a router that can remove the outermost
packet and direct the resultant packet to its destination.
Encapsulation requires:
a. An indication in the packet that it contains another packet.
b. A function in routers which, on receiving such a packet,
removes the encapsulation and re-enters the forwarding process.
All the proposals support encapsulation. Note that it is possible to
achieve the effect of source routing by suitable encapsulation by the
source.
2.5 Multicast
The specification of addresses to permit multicast with various
scopes can be accomodated by all the proposals. Internet-wide
multicast is, of course, for further study!
2.6 Fragmentation
All the proposals support the fragmentation of packets by
intermediate routers, though there has been some recent discussion of
removing this mechanism from some of the proposals and requiring the
use of an MTU-discovery process to avoid the need for fragmentation.
Such a decision would effectively preclude the use of transport
protocols which use message-count sequence numbering (such as OSI
Transport) over the network, as only protocols with byte-count
acknowledgement (such as TCP) can deal with MTU reductions during the
lifetime of a connection. OSI Transport may not be particularly
relevant to the IP community (though it may be of relevance to
commercial suppliers providing multiprotocol services), however the
consequences for the types of services which may be supported over
IPng should be noted.
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2.7 The End of Lifetime as We Know It
The old IPv4 "Time to Live" field has been recast in every case as a
simple hop count, largely on grounds of implementation convenience.
Although the old TTL was largely implemented in this fashion anyway,
it did serve an architectural purpose in putting an upper bound on
the lifetime of a packet in the network. If this field is recast as a
hop-count, there must be some other specification of the maximum
lifetime of a packet in the network so that a source host can ensure
that network-layer fragment ids and transport-layer sequence numbers
are never in danger of re-use whilst there is a danger of confusion.
There are, in fact, three separate issues here:
1. Terminating routing loops (solved by hop count).
2. Bounding lifetime of network-layer packets (a necessity,
unspecified so far) to support assumptions by the transport
layer.
3. Permitting the source to place further restrictions on packet
lifetime (for example so that "old" real-time traffic can be
discarded in favour of new traffic in the case of congestion
(an optional feature, unspecified so far).
3. WHAT THE PROPOSALS ONLY HINT AT
3.1 Resource Reservation
Increasingly, applications require a certain bandwidth or transit
delay if they are to be at all useful (for example, real-time video
and audio transport). Such applications need procedures to indicate
their requirements to the network and to have the required resources
reserved. This process is in some ways analogous to the selection of
a source route:
a. The specification by the source of its requirements.
b. The confirmation that the requirements can be met.
c. Marking traffic with the requirement.
d. Routing marked traffic accordingly.
Traffic which is routed according to the same set of resource
requirements is sometimes called a "flow". The identification of
flows requires a setup process, and it is tempting to suppose that
the same process might also be used to set up source routes, however,
there are a number of differences:
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- All the routers on a path must participate in resource
reservation and agree to it.
- Consequently, it is relatively straightforward to maintain
context in each router and the identification for flows can be
short.
- The network can choose to reroute on failure.
By various means, each proposal could carry flow-identification,
though this is very much "for future study" at present. No setup
mechansisms are defined. The process for actually reserving the
resources is a higher-order problem. The interaction between source-
routing and resource reservation needs further investigation:
although the two are distinct and have different implementation
constraints, the consequence of having two different mechanisms could
be that it becomes difficult to select routes which meet both policy
and performance goals.
3.2 Address-Assignment Policies
In IPv4, addresses were bound to systems on a long-term basis and in
many cases could be used interchangeably with DNS names. It is
tacitly accepted that the association of an address with a particular
system may be more volatile in IPng. Indeed, one of the proposals,
PIP, makes a distinction between the identification of a system (a
fixed quantity) and its address, and permits the binding to be
altered on the fly. None of the proposals defines bounds for the
lifetime of addresses, and the manner in which addresses are assigned
is not necessarily bound to a particular proposal. For example,
within the larger address space to be provided by IPng, there is a
choice to be made of assigning the "higher order" part of the
hierarchical address in a geographically-related fashion or by
reference to service provider. Geographically-based addresses can be
constant and easy to assign, but represent a renewed danger of
degeneration to "flat" addresses within the region of assignment,
unless certain topological restrictions are assumed. Provider-based
address assignment results in a change of address (if providers are
changed) or multiple addresses (if multiple providers are used).
Mobile hosts (depending on the underlying technology) can present
problems in both geographic and provider-based schemes.
Without firm proposals for address-assignment schemes and the
consequences for likely address lifetimes, it is impossible to assume
that the existing DNS model by which name-to-address bindings can be
discovered remains valid.
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Note that there is an interaction between the mechanism for
assignment of addresses and way in which automatic configuration may
be deployed.
3.3 Automatic Configuration
Amongst the biggest (user) bugbears of current IP services is the
administrative effort of maintaining basic configuration information,
such as assigning names and addresses to hosts, ensuring these are
refelected in the DNS, and keeping this information correct. Part of
this results from poor implementation (or the blind belief that vi
and awk are network management tools). However, a lot of the problems
could be alleviated by making this process more automatic. Some of
the possibilities (some mutually-exlusive) are:
- Assigning host addresses from some (relative) invariant, such
as a LAN address.
- Defining a protocol for dynamic assignment of addresses within a
subnetwork.
- Defining "generic addresses" by which hosts can without
preconfiguration reach necessary local servers (DNS, route
servers, etc.).
- Have hosts determine their name by DNS lookup.
- Have hosts update their name/address bindings when their
configuration changes.
Whilst a number of the proposals make mention of some of these
possibilities, the choice of appropriate solutions depends to some
extent on address-assignment policies. Also, dynamic configuration
results in some difficult philosopical and practical issues (what
exactly is the role of an address?, In what sense is a host "the same
host" when its address changes?, How do you handle dynamic changes to
DNS mappings and how do you authenticate them?).
The groups involved in the proposals would, I think, see most of
these questions outside their scope. It would seem to be a failure in
the process of defining and selecting candidates for IPng that
"systemness" issues like these will probably not be much discussed.
This is recognised by the participants, and it is likely that, even
when a decision is made, some of these ideas will be revisited by a
wider audience.
It is, however, unlikely that IP will make an impact on proprietary
networking systems for the non-technical environment (e.g., Netware,
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Appletalk), without automatic configuration being taken seriously
either in the architecture, or by suppliers. I believe that there are
ideas on people's heads of how to address these issues - they simply
have not made it onto paper yet.
3.4 Application Interface/Application Protocol Changes
A number of common application protocols (FTP, RPC, etc.) have been
identified which specifically transfer 32-bit IPv4 addresses, and
there are doubtless others, both standard and proprietary. There are
also many applications which treat IPv4 addresses as simple 32-bit
integers. Even applications which use BSD sockets and try to handle
addresses opaquely will not understand how to parse or print longer
addresses (even if the socket structure is big enough to accommodate
them).
Each proposal, therefore, needs to specify mechanisms to permit
existing applications and interfaces to operate in the new
environment whilst conversion takes place. It would be useful also,
to have (one) specification of a reference programming interface for
(TCP and) IPng (which would also operate on IPv4), to allow
developers to begin changing applications now. All the proposals
specify transition mechansisms from which existing application-
compatibility can be inferred. There is no sign yet of a new
interface specification independent of chosen protocol.
3.5 DNS Changes
It is obvious that there has to be a name to address mapping service
which supports the new, longer, addresses. All the proposals assume
that this service will be provided by DNS, with some suitably-defined
new resource record. There is some discussion ongoing about the
appropriateness of returning this information along with "A" record
information in response to certain enquiries, and which information
should be requested first. There is a potential tradeoff between the
number of queries needed to establish the correct address to use and
the potential for breaking existing implementations by returning
information that they do not expect.
There has been heat, but not light, generated by discussion of the
use of DNS for auto-configuration and the scaling (or otherwise) of
reverse translations for certain addressing schemes.
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4. WHAT THE PROPOSALS DON'T REALLY MENTION
4.1 Congestion Avoidance
IPv4 offers "Source Quench" control messages which may be used by
routers to indicate to a source that it is congested and has or may
shortly drop packets. TUBA/PIP have a "congestion encountered" bit
which provides similar information to the destination. None of these
specifications offers detailed instructions on how to use these
facilities. However, there has been a substantial body of analysis
over recent years that suggests that such facilities can be used (by
providing information to the transport protocol) not only to signal
congestion, but also to minimise delay through the network layer.
Each proposal can offer some form of congestion signalling, but none
specifies a mechanism for its use (or an analysis of whether the
mechanism is in fact useful).
As a user of a network service which currently has a discard rate of
around 30% and a round-trip-time of up to 2 seconds for a distance of
only 500 miles I would be most interested in some proposals for a
more graceful degradation of the network service under excess load.
4.2 Mobile Hosts
A characteristic of mobile hosts is that they (relatively) rapidly
move their physical location and point of attachment to the network
topology. This obviously has signficance for addressing (whether
geographical or topological) and routing. There seems to be an
understanding of the problem, but so far no detailed specification of
a solution.
4.3 Accounting
The IESG selection criteria require only that proposals do not have
the effect of preventing the collection of information that may be of
interest for audit or billing purposes. Consequently, none of the
proposals consider potential accounting mechanisms.
4.4 Security
"Network Layer Security Issues are For Further Study". Or secret.
However, it would be useful to have it demonstrated that each
candidate could be extended to provide a level of security, for
example against address-spoofing. This will be particularly
important if resource-allocation features will permit certain hosts
to claim large chunks of available bandwidth for specialised
applications.
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Note that providing some level of security implies manual
configuration of security information within the network and must be
considered in relationship to auto-configuration goals.
5. WHAT MAKES THE PROPOSALS DIFFERENT?
Each proposal is about as different to the others as it is to IPv4 -
that is the differences are small in principle, but may have
significant effects (extending the size of addresses is only a small
difference in principle!). The main distinct characteristics are:
PIP:
PIP has an innovative header format that facilitates hierarchical,
policy and virtual-circuit routing. It also has "opaque" fields in
the header whose semantics can be defined differently in different
administrative domains and whose use and translation can be
negotiated across domain boundaries. No control protocol is yet
specified.
SIP:
SIP offers a "minimalist" approach - removing all little-used
fields from the IPv4 header and extending the size of addresses to
(only) 64 bits. The control protocol is based on modifications to
ICMP. This proposal has the advantages of processing efficiency
and familiarity.
TUBA:
TUBA is based on CLNP (ISO 8473) and the ES-IS (ISO 9542) control
protocol. TUBA provides for the operation of TCP transport and UDP
over a CLNP network. The main arguments in favour of TUBA are that
routers already exist which can handle the network-layer protocol,
that the extensible addresses offer a wide margin of "future-
proofing" and that there is an opportunity for convergence of
standards and products.
5.1 PIP
PIP packet headers contain a set of instructions to the router's
forwarding processor to perform certain actions on the packet. In
traditional protocols, the contents of certain fields imply certain
actions; PIP gives the source the flexibility to write small
"programs" which direct the routing of packets through the network.
PIP addresses have an effectively unlimited length: each level in the
topological hierarchy of the network contributes part of the address
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and addresses change as the network topology changes. In a completely
hierarchical network topology, the amount of routing information
required at each level could be very small. However, in practice,
levels of hierarchy will be determined more by commercial and
practical factors than by the constraints of any particular routing
protocol. A greater advantage is that higher-order parts of the
address may be omitted in local exchanges and that lower-order parts
may be omitted in source routes, reducing the amount of topological
information that host systems are required to know.
There is an assumption that PIP addresses are liable to change, so a
further quantity, the PIP ID, is assigned to systems for the purposes
of identification. It isn't clear that this quantity has any purpose
which could not equally be served by a DNS name [it is more compact,
but equally it does not need to be carried in every packet and
requires an additional lookup]. However, the problem does arise of
how two potentially-communicating host systems find the correct
addresses to use.
The most complex part of PIP is that the meaning of some of the
header fields is determined by mutual agreement within a particular
domain. The semantics of specific processing facilities (for example,
queuing priority) are registered globally, but the actual use and
encoding of requests for these facilities in the packet header can be
different in different domains. Border routers between two domains
which use different encodings must map from one encoding to another.
Since routers may not only be adjacent physically to other domains,
but also via "tunnels", the number of different encoding rules a
router may need to understand is potentially quite large. Although
there is a saving in header space by using such a scheme as opposed
to the more familiar "options", the cost in the complexity of
negotiating the use and encoding of these facilities, together with
re-coding the packets at each domain border, is a subject of some
concern. Although it may be possible for hosts to "precompile" the
encoding rules for their local domain, there are many potential
implementaion difficulties.
Although PIP offers the most flexibility of the three proposals, more
work needs to be done on "likely use" scenarios which make the
potential advantages and disadvantages more concrete.
5.2 SIP
SIP is simply IP with larger addresses and fewer options. Its main
advantage is that it is even simpler that IPv4 to process. Its main
disadvantages are:
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- It is far from clear that, if 32 bits of address are
insufficient, 64 will be enough for the forseeable future;
- although there are a few "reserved" bits in the header, the
extension of SIP to support new features is not obvious.
There's really very little else to say!
5.3 TUBA
The characteristics of ISO CLNS are reasonably well known: the
protocol bears a strong cultural resemblance to IPv4, though with
20-byte network-layer addressing. Apart from a spurious "Not Invented
Here" prejudice, the main argument againt TUBA is that it is rather
too like IPv4, offering nothing other than larger, more flexible,
addresses. There is proof-by-example that routers are capable of
handling the (very) long addresses efficiently, rather less that the
longer headers do not adversely impact network bandwidth.
There are a number of objections to the proposed control protocol
(ISO 9542):
- My early experience is that the process by which routers
discover hosts is inefficient and resource consuming for
routers - and requires quite fine timer resolution on hosts -
if large LANs are to be accomodated reasonably. Proponents of
TUBA suggest that recent experience suggests that ARP is no
better, but I think this issue needs examination.
- The "redirect" mechanism is based on (effectively) LAN
addresses and not network addresses, meaning that local routers
can only "hand-off" complex routing decisions to other routers
on the same LAN. Equally, redirection schemes (such as that of
IPv4) which redirect to network addresses can result in
unnecessary extra hops. Analysis of which solution is better
is rather dependent on the scenarios which are constructed.
To be fair, however, the part of the protocol which provides for
router-discovery provides a mechanism, absent from other proposals,
by which hosts can locate nearby gateways and potentially
automatically configure their addresses.
6. Transition Plans
It should be obvious that a transition which permits "old" hosts to
talk to "new" hosts requires:
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Either:
(a) That IPng hosts can also use IPv4 or
(b) There is translation by an intermediate system
and either:
(c) The infrastructure between systems is capable of carrying both
IPng and IPv4 or (d) Tunneling or translation is used to carry
one protocol within another in parts of the network
The transition plans espoused by the various proposals are simply
different combinations of the above. Experience would tend to show
that all these things will in fact happen, regardless of which
protocol is chosen.
One problem of the tunneling/translation process is that there is
additional information (the extra address parts) which must be
carried across IPv4 tunnels in the network. This can either be
carried by adding an extra "header" to the data before encapsulation
in the IPv4 packet, or by encoding the information as new IPv4 option
types. In the former case, it may be difficult to map error messages
correctly, since the original packet is truncated before return; in
the latter case there is a danger of the packet being discarded (IPv4
options are not self-describing and new ones may not pass through
IPv4 routers). There is thus the possibility of having to introduce a
"new" version of IPv4 in order to support IPng tunneling.
The alternative (in which IPng hosts have two stacks and the
infrastructure may or may not support IPng or IPv4) of course
requires a mechanism for resolving which protocols to try.
7. Random Comments
This is the first fundamental change in the Internet protocols that
has occurred since the Internet was manageable as an entity and its
development was tied to US government contracts. It was perhaps
inevitable that the IETF/IESG/IAB structure would not have evolved to
manage a change of this magnitude and it is to be hoped that the new
structures that are proposed will be more successful in promoting a
(useful) consensus. It is interesting to see that many of the
perceived problems of the OSI process (slow progress, factional
infighting over trivia, convergence on the lowest-common denominator
solution, lack of consideration for the end-user) are in danger of
attaching themselves to IPng and it will be interesting to see to
what extent these difficulties are an inevitable consequence of wide
representation and participation in network design.
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It could be regarded either as a sign of success or failure of the
competitive process for the selection of IPng that the three main
proposals have few really significant differences. In this respect,
the result of the selection process is not of particular
significance, but the process itself is perhaps necessary to repair
the social and technical cohesion of the Internet Engineering
process.
8. Further Information
The main discussion lists for the proposals listed are:
TUBA: tuba@lanl.gov
PIP: pip@thumper.bellcore.com
SIP: sip@caldera.usc.edu
General: big-internet@munnari.oz.au
(Requests to: <list name>-request@<host>)
Internet-Drafts and RFCs for the various proposals can be found in
the usual places.
Security Considerations
Security issues are not discussed in this memo.
Author's Address
Tim Dixon
RARE Secretariat
Singel 466-468
NL-1017AW Amsterdam
(Netherlands)
Phone: +31 20 639 1131 or + 44 91 232 0936
EMail: dixon@rare.nl or Tim.Dixon@newcastle.ac.uk
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