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Network Working Group                                          T. DixonRequest for Comments: 1454                                         RARE                                                               May 1993Comparison of Proposals for Next Version of IPStatus 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.Dixon                                                           [Page 1]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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.Dixon                                                           [Page 2]

RFC 1454        Comparison of Next Version IP Proposals         May 19932. WHAT THE PROPOSALS HAVE IN COMMON2.1 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.Dixon                                                           [Page 3]

RFC 1454        Comparison of Next Version IP Proposals         May 19932.3 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.Dixon                                                           [Page 4]

RFC 1454        Comparison of Next Version IP Proposals         May 1993      - 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.Dixon                                                           [Page 5]

RFC 1454        Comparison of Next Version IP Proposals         May 19932.7 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 AT3.1 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:Dixon                                                           [Page 6]

RFC 1454        Comparison of Next Version IP Proposals         May 1993      - 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.Dixon                                                           [Page 7]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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,Dixon                                                           [Page 8]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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.Dixon                                                           [Page 9]

RFC 1454        Comparison of Next Version IP Proposals         May 19934. WHAT THE PROPOSALS DON'T REALLY MENTION4.1 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.Dixon                                                          [Page 10]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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 addressDixon                                                          [Page 11]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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:Dixon                                                          [Page 12]

RFC 1454        Comparison of Next Version IP Proposals         May 1993      - 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:Dixon                                                          [Page 13]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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.Dixon                                                          [Page 14]

RFC 1454        Comparison of Next Version IP Proposals         May 1993   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.ukDixon                                                          [Page 15]