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Network Working Group                                         N. ChiappaRequest for Comments: 1753                                 December 1994Category: InformationalIPng Technical RequirementsOf the Nimrod Routing and Addressing ArchitectureStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   This document was submitted to the IETF IPng area in response toRFC1550.  Publication of this document does not imply acceptance by the   IPng area of any ideas expressed within.  Comments should be   submitted to the big-internet@munnari.oz.au mailing list.   This document presents the requirements that the Nimrod routing and   addressing architecture has upon the internetwork layer protocol. To   be most useful to Nimrod, any protocol selected as the IPng should   satisfy these requirements. Also presented is some background   information, consisting of i) information about architectural and   design principles which might apply to the design of a new   internetworking layer, and ii) some details of the logic and   reasoning behind particular requirements.1. Introduction   It is important to note that this document is not "IPng Requirements   for Routing", as other proposed routing and addressing designs may   need different support; this document is specific to Nimrod, and   doesn't claim to speak for other efforts.   However, although I don't wish to assume that the particular designs   being worked on by the Nimrod WG will be widely adopted by the   Internet (if for no other reason, they have not yet been deployed and   tried and tested in practise, to see if they really work, an   absolutely necessary hurdle for any protocol), there are reasons to   believe that any routing architecture for a large, ubiquitous global   Internet will have many of the same basic fundamental principles as   the Nimrod architecture, and the requirements that these generate.Chiappa                                                         [Page 1]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   While current day routing technologies do not yet have the   characteristics and capabilities that generate these requirements,   they also do not seem to be completely suited to routing in the   next-generation Internet. As routing technology moves towards what is   needed for the next generation Internet, the underlying fundamental   laws and principles of routing will almost inevitably drive the   design, and hence the requirements, toward things which look like the   material presented here.   Therefore, even if Nimrod is not the routing architecture of the   next-generation Internet, the basic routing architecture of that   Internet will have requirements that, while differing in detail, will   almost inevitably be similar to these.   In a similar, but more general, context, note that, by and large, the   general analysis of sections3.1 ("Interaction Architectural Issues")   and 3.2 ("State and Flows in the Internetwork Layer") will apply to   other areas of a new internetwork layer, not just routing.   I will tackle the internetwork packet format first (which is   simpler), and then the whole issue of the interaction with the rest   of the internetwork layer (which is a much more subtle topic).2. Packet Format2.1 Packet Format Issues   As a general rule, the design philosophy of Nimrod is "maximize the   lifetime (and flexibility) of the architecture". Design tradeoffs   (i.e., optimizations) that will adversely affect the flexibility,   adaptability and lifetime of the design are not not necessarily wise   choices; they may cost more than they save. Such optimizations might   be the correct choices in a stand-alone system, where the replacement   costs are relatively small; in the global communication network, the   replacement costs are very much higher.   Providing the Nimrod functionality requires the carrying of certain   information in the packets. The design principle noted above has a   number of corollaries in specifying the fields to contain that   information.   First, the design should be "simple and straightforward", which means   that various functions should be handled by completely separate   mechanisms, and fields in the packets. It may seem that an   opportunity exists to save space by overloading two functions onto   one mechanism or field, but general experience is that, over time,   this attempt at optimization costs more, by restricting flexibility   and adaptability.Chiappa                                                         [Page 2]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   Second, field lengths should be specified to be somewhat larger than   can conceivably be used; the history of system architecture is   replete with examples (processor address size being the most   notorious) where fields became too short over the lifetime of the   system. The document indicates what the smallest reasonable   "adequate" lengths are, but this is more of a "critical floor" than a   recommendation. A "recommended" length is also given, which is the   length which corresponds to the application of this principle. The   wise designer would pick this length.   It is important to now that this does *not* mean that implementations   must support the maximum value possible in a field of that size. I   imagine that system-wide administrative limits will be placed on the   maximum values which must be supported. Then, as the need arises, we   can increase the administrative limit. This allows an easy, and   completely interoperable (with no special mechanisms) path to upgrade   the capability of the network. If the maximum supported value of a   field needs to be increased from M to N, an announcement is made that   this is coming; during the interim period, the system continues to   operate with M, but new implementations are deployed; while this is   happening, interoperation is automatic, with no transition mechanisms   of any kind needed. When things are "ready" (i.e., the proportion of   old equipment is small enough), use of the larger value commences.   Also, in speaking of the packet format, you first need to distinguish   between the host-router part of the path, and the router-router part;   a format that works OK for one may not do for another.   The issue is complicated by the fact that Nimrod can be made to work,   albeit not in optimal form, with information/fields missing from the   packet in the host to "first hop router" section of the packet's   path. The missing fields and information can then be added by the   first hop router. (This capability will be used to allow deployment   and operation with unmodified IPv4 hosts, although similar techniques   could be used with other internetworking protocols.) Access to the   full range of Nimrod capabilities will require upgrading of hosts to   include the necessary information in the packets they exchange with   the routers.   Second, Nimrod currently has three planned forwarding modes (flows,   datagram, and source-routed packets), and a format that works for one   may not work for another; some modes use fields that are not used by   other modes.  The presence or absence of these fields will make a   difference.Chiappa                                                         [Page 3]

RFC 1753         Nimrod Technical Requirements for IPng    December 19942.2 Packet Format Fields   What Nimrod would like to see in the internetworking packet is:   - Source and destination endpoint identification. There are several     possibilities here.     One is "UID"s, which are "shortish", fixed length fields which     appear in each packet, in the internetwork header, which contain     globally unique, topologically insensitive identifiers for either     i) endpoints (if you aren't familiar with endpoints, think of them     as hosts), or ii) multicast groups. (In the former instance, the     UID is an EID; in the latter, a "set ID", or SID. An SID is an     identifier which looks just like an EID, but it refers to a group     of endpoints. The semantics of SID's are not completely defined.)     For each of these 48 bits is adequate, but we would recommend 64     bits. (IPv4 will be able to operate with smaller ones for a while,     but eventually either need a new packet format, or the difficult     and not wholly satisfactory technique known as Network Address     Translators, which allows the contents of these fields to be only     locally unique.)     Another possibility is some shorter field, named an "endpoint     selector", or ESEL, which contains a value which is not globally     unique, but only unique in mapping tables on each end, tables which     map from the small value to a globally unique value, such as a DNS     name.     Finally, it is possible to conceive of overall networking designs     which do not include any endpoint identification in the packet at     all, but transfer it at the start of a communication, and from then     on infer it.  This alternative would have to have some other means     of telling which endpoint a given packet is for, if there are     several endpoints at a given destination. Some coordination on     allocation of flow-ids, or higher level port numbers, etc., might     do this.   - Flow identification. There are two basic approaches here, depending     on whether flows are aggregated (in intermediate switches) or not.     It should be emphasized at this point that it is not yet known     whether flow aggregation will be needed. The only reason to do it     is to control the growth of state in intermediate routers, but     there is no hard case made that either this growth will be     unmanageable, or that aggregating flows will be feasible     practically.Chiappa                                                         [Page 4]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994     For the non-aggregated case, a single "flow-id" field will suffice.     This *must not* use one of the two previous UID fields, as in     datagram mode (and probably source-routed mode as well) the flow-id     will be over-written during transit of the network. It could most     easily be constructed by adding a UID to a locally unique flow-id,     which will provide a globally unique flow-id. It is possible to use     non-globally unique flow-ids, (which would allow a shorter length     to this field), although this would mean that collisions would     result, and have to be dealt with. An adequate length for the local     part of a globally unique flow-id would be 12 bits (which would be     my "out of thin air" guess), but we recommend 32. For a non-     globally unique flow-id, 24 bits would be adequate, but I recommend     32.     For the aggregated case, three broad classes of mechanism are     possible.      - Option 1: The packet contains a sequence (sort of like a source        route) of flow-ids. Whenever you aggregate or deaggregate, you        move along the list to the next one. This takes the most space,        but is otherwise the least work for the routers.      - Option 2: The packet contains a stack of flow-ids, with the        current one on the top. When you aggregate, you push a new one        on; when you de-aggregate, you take one off. This takes more        work, but less space in the packet than the complete "source-        route". Encapsulating packets to do aggregation does basically        this, but you're stacking entire headers, not just flow-ids. The        clever way to do this flow-id stacking, without doing        encapsulation, is to find out from flow-setup how deep the stack        will get, and allocate the space in the packet when it's        created. That way, all you ever have to do is insert a new        flow-id, or "remove" one; you never have to make room for more        flow-ids.      - Option 3: The packet contains only the "base" flow-id (i.e., the        one with the finest granularity), and the current flow-id. When        you aggregate, you just bash the current flow-id. The tricky        part comes when you de-aggregate; you have to put the right        value back. To do this, you have to have state in the router at        the end of the aggregated flow, which tells you what the de-        aggregated flow for each base flow is. The downside here is        obvious: we get away without individual flow state for each of        the constituent flows in all the routers along the path of that        aggregated, flow, *except* for the last one.Chiappa                                                         [Page 5]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994        Other than encapsulation, which has significant inefficiency in        space overhead fairly quickly, after just a few layers of        aggregation, there appears to be no way to do it with just one        flow-id in the packet header.  Even if you don't touch the        packets, but do the aggregation by mapping some number of "base"        flow-id's to a single aggregated flow in the routers along the        path of the aggregated flow, the table that does the mapping is        still going to have to have a number of entries directly        proportional to the number of base flows going through the        switch.   - A looping packet detector. This is any mechanism that will detect a     packet which is "stuck" in the network; a timeout value in packets,     together with a check in routers, is an example. If this is a hop-     count, it has to be more than 8 bits; 12 bits would be adequate,     and I recommend 16 (which also makes it easy to update). This is     not to say that I think networks with diameters larger than 256 are     good, or that we should design such nets, but I think limiting the     maximum path through the network to 256 hops is likely to bite us     down the road the same way making "infinity" 16 in RIP did (as it     did, eventually). When we hit that ceiling, it's going to hurt, and     there won't be an easy fix. I will note in passing that we are     already seeing paths lengths of over 30 hops.   - Optional source and destination locators. These are structured,     variable length items which are topologically sensitive identifiers     for the place in the network from which the traffic originates or     to which the traffic is destined. The locator will probably contain     internal separators which divide up the fields, so that a     particular field can be enlarged without creating a great deal of     upheaval. An adequate value for maximum length supported would be     up to 32 bytes per locator, and longer would be even better; I     would recommend up to 256 bytes per locator.   - Perhaps (paired with the above), an optional pointer into the     locators.  This is optional "forwarding state" (i.e., state in the     packet which records something about its progress across the     network) which is used in the datagram forwarding mode to help     ensure that the packet does not loop. It can also improve the     forwarding processing efficiency. It is thus not absolutely     essential, but is very desirable from a real-world engineering view     point. It needs to be large enough to identify locations in either     locator; e.g., if locators can be up to 256 bytes, it would need to     be 9 bits.   - An optional source route. This is used to support the "source     routed packet" forwarding mode. Although not designed in detail     yet, we can discuss two possible approaches.Chiappa                                                         [Page 6]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994     In one, used with "semi-strict" source routing (in which a     contiguous series of entities is named, albeit perhaps at a high     layer of abstraction), the syntax will likely look much like source     routes in PIP; in Nimrod they will be a sequence of Nimrod entity     identifiers (i.e., locator elements, not complete locators), along     with clues as to the context in which each identifier is to be     interpreted (e.g., up, down, across, etc.). Since those identifiers     themselves are variable length (although probably most will be two     bytes or less, otherwise the routing overhead inside the named     object would be excessive), and the hop count above contemplates     the possibility of paths of over 256 hops, it would seem that these     might possibly some day exceed 512 bytes, if a lengthy path was     specified in terms of the actual physical assets used. An adequate     length would be 512 bytes; the recommended length would be 2^16     bytes (although this length would probably not be supported in     practise; rather, the field length would allow it).     In the other, used with classical "loose" source routes, the source     consists of a number of locators. It is not yet clear if this mode     will be supported. If so, the header would need to be able to store     a sequence of locators (as described above). Space might be saved     by not repeating locator prefixes that match that of the previous     locator in the sequence; Nimrod will probably allow use of such     "locally useful" locators. It is hard to determine what an adequate     length would be for this case; the recommended length would be 2^16     bytes (again, with the previous caveat).   - Perhaps (paired with the above), an optional pointer into the     source route. This is also optional "forwarding state". It needs to     be large enough to identify locations anywhere in the source route;     e.g., if the source router can be up to 1024 bytes, it would need     to be 10 bits.   - An internetwork header length. I mention this since the above     fields could easily exceed 256 bytes, if they are to all be carried     in the internetwork header (see comments below as to where to carry     all this information), the header length field needs to be more     than 8 bits; 12 bits would be adequate, and I recommend 16 bits.     The approach of putting some of the data items above into an     interior header, to limit the size of the basic internetworking     header, does not really seem optimal, as this data is for use by     the intermediate routers, and it needs to be easily accessible.   - Authentication of some sort is needed. See the recent IAB document     which was produced as a result of the IAB architecture retreat on     security (draft-iab-sec-arch-workshop-00.txt),section 4, and     especiallysection 4.3. There is currently no set way of doing     "denial/theft of service" in Nimrod, but this topic is wellChiappa                                                         [Page 7]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994     explored in that document; Nimrod would use whatever mechanism(s)     seem appropriate to those knowledgeable in this area.   - A version number. Future forwarding mechanisms might need other     information (i.e., fields) in the packet header; use a version     number would allow it to be modified to contain what's needed.     (This would not necessarily be information that is visible to the     hosts, so this does not necessarily mean that the hosts would need     to know about this new format.) 4 bits is adequate; it's not clear     if a larger value needs to be recommended.2.3 Field Requirements and Addition Methods   As noted above, it's possible to use Nimrod in a limited mode where   needed information/fields are added by the first-hop router. It's   thus useful to ask "which of the fields must be present in the host-   router header, and which could be added by the router?" The only ones   which are absolutely necessary in all packets are the endpoint   identification (provided that some means is available to map them   into locators; this would obviously be most useful on UID's which are   EID's).   As to the others, if the user wishes to use flows, and wants to   guarantee that their packets are assigned to the correct flows, the   flow-id field is needed. If the user wishes efficient use of the   datagram mode, it's probably necessary to include the locators in the   packet sent to the router.  If the user wishes to specify the route   for the packets, and does not wish to set up a flow, they need to   include the source route.   How would additional information/fields be added to the packet, if   the packet is emitted from the host in incomplete form? (By this, I   mean the simple question of how, mechanically, not the more complex   issue of where any needed information comes from.)   This question is complex, since all the IPng candidates (and in fact,   any reasonable inter-networking protocol) are extensible protocols;   those extension mechanisms could be used. Also, it would possible to   carry some of the required information as user data in the   internetworking packet, with the original user's data encapsulated   further inside. Finally, a private inter-router packet format could   be defined.   It's not clear which path is best, but we can talk about which fields   the Nimrod routers need access to, and how often; less used ones   could be placed in harder-to-get-to locations (such as in an   encapsulated header). The fields to which the routers need access on   every hop are the flow-id and the looping packet detector. TheChiappa                                                         [Page 8]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   locator/pointer fields are only needed at intervals (in what datagram   forwarding mode calls "active" routers), as is the source route (the   latter at every object which is named in the source route).   Depending on how access control is done, and which forwarding mode is   used, the UID's and/or locators might be examined for access control   purposes, wherever that function is performed.   This is not a complete exploration of the topic, but should give a   rough idea of what's going on.3. Architectural Issues3.1 Interaction Architectural Issues   The topic of the interaction with the rest of the internetwork layer   is more complex. Nimrod springs in part from a design vision which   sees the entire internetwork layer, distributed across all the hosts   and routers of the internetwork, as a single system, albeit a   distributed system.   Approached from that angle, one naturally falls into a typical system   designer point of view, where you start to think of the   modularization of the system; chosing the functional boundaries which   divide the system up into functional units, and defining the   interactions between the functional units.  As we all know, that   modularization is the key part of the system design process.   It's rare that a group of completely independent modules form a   system; there's usually a fairly strong internal interaction. Those   interactions have to be thought about and understood as part of the   modularization process, since it effects the placement of the   functional boundaries. Poor placement leads to complex interactions,   or desired interactions which cannot be realized.   These are all more important issues with a system which is expected   to have a long lifetime; correct placement of the functional   boundaries, so as to clearly and simply break up the system into   truly fundamental units, is a necessity is the system is to endure   and serve well.3.1.1 The Internetwork Layer Service Model   To return to the view of the internetwork layer as a system, that   system provides certain services to its clients; i.e., it   instantiates a service model. To begin with, lacking a shared view of   the service model that the internetwork layer is supposed to provide,   it's reasonable to suppose that it will prove impossible to agree onChiappa                                                         [Page 9]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   mechanisms at the internetwork level to provide that service.   To answer the question of what the service model ought to be, one can   view the internetwork layer itself as a subsystem of an even large   system, the entire internetwork itself. (That system is quite likely   the largest and most complex system we will ever build, as it is the   largest system we can possibly build; it is the system which will   inevitably contain almost all other systems.)   From that point of view, the issue of the service model of the   internetwork layer becomes a little clearer. The services provided by   the internetwork layer are no longer purely abstract, but can be   thought about as the external module interface of the internetwork   layer module. If agreement can be reached on where to put the   functional boundaries of the internetwork layer, and on what overall   service the internet as a whole should provide, the service model of   the internetwork layer should be easier to agree on.   In general terms, it seems that the unreliable packet ought to remain   the fundamental building block of the internetwork layer. The design   principle that says that we can take any packet and throw it away   with no warning or other action, or take any router and turn it off   with no warning, and have the system still work, seems very powerful.   The component design simplicity (since routers don't have to stand on   their heads to retain a packet which they have the only copy of), and   overall system robustness, resulting from these two assumptions is   absolutely critical.   In detail, however, particularly in areas which are still the subject   of research and experimentation (such as resource allocation,   security, etc.), it seems difficult to provide a finished definition   of exactly what the service model of the internetwork layer ought to   be.3.1.2 The Subsystems of the Internetwork Layer   In any event, by viewing the internetwork layer as a large system,   one starts to think about what subsystems are needed, and what the   interactions among them should look like. Nimrod is simply a number   of the subsystems of this larger system, the internetwork layer. It   is *not* intended to be a purely standalone set of subsystems, but to   work together in close concert with the other subsystems of the   internetwork layer (resource allocation, security, charging, etc.) to   provide the internetwork layer service model.   One reason that Nimrod is not simply a monolithic subsystem is that   some of the interactions with the other subsystems of the   internetwork layer, for instance the resource allocation subsystem,Chiappa                                                        [Page 10]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   are much clearer and easier to manage if the routing is broken up   into several subsystems, with the interactions between them open.   It is important to realize that Nimrod was initially broken up into   separate subsystems for purely internal reasons. It so happens that,   considered as a separate problem, the fundamental boundary lines for   dividing routing up into subsystems are the same boundaries that make   interaction with other subsystems cleaner; this provides added   evidence that these boundaries are in fact the right ones.   The subsystems which comprise the functionality covered by Nimrod are   i) routing information distribution (in the case of Nimrod, topology   map distribution, along with the attributes [policy, QOS, etc.] of   the topology elements), ii) route selection (strictly speaking, not   part of the Nimrod spec per se, but functional examples will be   produced), and iii) user traffic handling.   The former can fairly well be defined without reference to other   subsystems, but the second and third are necessarily more involved.   For instance, route selection might involve finding out which links   have the resources available to handle some required level of   service. For user traffic handling, if a particular application needs   a resource reservation, getting that resource reservation to the   routers is as much a part of getting the routers ready as making sure   they have the correct routing information, so here too, routing is   tied in with other subsystems.   In any event, although we can talk about the relationship between the   Nimrod subsystems, and the other functional subsystems of the   internetwork layer, until the service model of the internetwork layer   is more clearly visible, along with the functional boundaries within   that layer, such a discussion is necessarily rather nebulous.3.2 State and Flows in the Internetwork Layer   The internetwork layer as whole contains a variety of information, of   varying lifetimes. This information we can refer to as the   internetwork layer's "state". Some of this state is stored in the   routers, and some is stored in the packets.   In the packet, I distinguish between what I call "forwarding state",   which records something about the progress of this individual packet   through the network (such as the hop count, or the pointer into a   source route), and other state, which is information about what   service the user wants from the network (such as the destination of   the packet), etc.Chiappa                                                        [Page 11]

RFC 1753         Nimrod Technical Requirements for IPng    December 19943.2.1 User and Service State   I call state which reflects the desires and service requests of the   user "user state". This is information which could be sent in each   packet, or which can be stored in the router and applied to multiple   packets (depending on which makes the most engineering sense). It is   still called user state, even when a copy is stored in the routers.   User state can be divided into two classes; "critical" (such as   destination addresses), without which the packets cannot be forwarded   at all, and "non-critical" (such as a resource allocation class),   without which the packets can still be forwarded, just not quite in   the way the user would most prefer.   There are a range of possible mechanisms for getting this user state   to the routers; it may be put in every packet, or placed there by a   setup. In the latter case, you have a whole range of possibilities   for how to get it back when you lose it, such as placing a copy in   every Nth packet.   However, other state is needed which cannot be stored in each packet;   it's state about the longer-term (i.e., across the life of many   packets) situation; i.e., state which is inherently associated with a   number of packets over some time-frame (e.g., a resource allocation).   I call this state "server state".   This apparently changes the "stateless" model of routers somewhat,   but this change is more apparent than real. The routers already   contain state, such as routing table entries; state without which is   it virtually impossible to handle user traffic. All that is being   changed is the amount, granularity, and lifetime, of state in the   routers.   Some of this service state may need to be installed in a fairly   reliable fashion; e.g., if there is service state related to billing,   or allocation of resources for a critical application, one more or   less needs to be guaranteed that this service state has been   correctly installed.   To the extent that you have state in the routers (either service   state, or user state), you have to be able to associate that state   with the packets it goes with. The fields in the packets that allow   you to do this are "tags".Chiappa                                                        [Page 12]

RFC 1753         Nimrod Technical Requirements for IPng    December 19943.2.2 Flows   It is useful to step back for a bit here, and think about the traffic   in the network. Some of it will be from applications with are   basically transactions; i.e., they require only a single packet, or a   very small number.  (I tend to use the term "datagram" to refer to   such applications, and use the term "packet" to describe the unit of   transmission through the network.) However, other packets are part of   longer-lived communications, which have been termed "flows".   A flow, from the user's point of view, is a sequence of packets which   are associated, usually by being from a single application instance.   In an internetwork layer which has a more complex service model   (e.g., supports resource allocation, etc.), the flow would have   service requirements to pass on to some or all of the subsystems   which provide those services.   To the internetworking layer, a flow is a sequence of packets that   share all the attributes that the internetworking layer cares about.   This includes, but is not limited to: source/destination, path,   resource allocation, accounting/authorization,   authentication/security, etc., etc.   There isn't necessarily a one-one mapping from flows to *anything*   else, be it a TCP connection, or an application instance, or   whatever. A single flow might contain several TCP connections (e.g.,   with FTP, where you have the control connection, and a number of data   connections), or a single application might have several flows (e.g.,   multi-media conferencing, where you'd have one flow for the audio,   another for a graphic window, etc., with different resource   requirements in terms of bandwidth, delay, etc., for each.)   Flows may also be multicast constructs, i.e., multiple sources and   destinations; they are not inherently unicast. Multicast flows are   more complex than unicast (there is a large pool of state which must   be made coherent), but the concepts are similar.   There's an interesting architectural issue here. Let's assume we have   all these different internetwork level subsystems (routing, resource   allocation, security/access-control, accounting), etc. Now, we have   two choices.   First, we could allow each individual subsystem which uses the   concept of flows to define itself what it thinks a "flow" is, and   define which values in which fields in the packet define a given   "flow" for it. Now, presumably, we have to allow 2 flows for   subsystem X to map onto 1 flow for subsystem Y to map onto 3 flows   for subsystem Z; i.e., you can mix and match to your heart's content.Chiappa                                                        [Page 13]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   Second, we could define a standard "flow" mechanism for the   internetwork layer, along with a way of identifying the flow in the   packet, etc. Then, if you have two things which wish to differ in   *any* subsystem, you have to have a separate flow for each.   The former has the advantages that it's a little easier to deploy   incrementally, since you don't have to agree on a common flow   mechanism. It may save on replicated state (if I have 3 flows, and   they are the same for subsystem X, and different for Y, I only need   one set of X state). It also has a lot more flexibility. The latter   is simple and straightforward, and given the complexity of what is   being proposed, it seems that any place we can make things simpler,   we should.   The choice is not trivial; it all depends on things like "what   percentage of flows will want to share the same state in certain   subsystems with other flows". I don't know how to quantify those, but   as an architect, I prefer simple, straightforward things. This system   is pretty complex already, and I'm not sure the benefits of being   able to mix and match are worth the added complexity. So, for the   moment I'll assume a single, system-wide, definition of flows.   The packets which belong to a flow could be identified by a tag   consisting of a number of fields (such as addresses, ports, etc.), as   opposed to a specialized field. However, it may be more   straightforward, and foolproof, to simply identify the flow a packet   belongs to with by means of a specialized tag field (the "flow-id" )   in the internetwork header. Given that you can always find situations   where the existing fields alone don't do the job, and you *still*   need a separate field to do the job correctly, it seems best to take   the simple, direct approach , and say "the flow a packet belongs to   is named by a flow-id in the packet header".   The simplicity of globally-unique flow-id's (or at least a flow-id   which unique along the path of the flow) is also desirable; they take   more bits in the header, but then you don't have to worry about all   the mechanism needed to remap locally-unique flow-id's, etc., etc.   From the perspective of designing something with a long lifetime, and   which is to be deployed widely, simplicity and directness is the only   way to go. For me, that translates into flows being named solely by   globally unique flow-id's, rather than some complex semantics on   existing fields.   However, the issue of how to recognize which packets belong to flows   is somewhat orthogonal to the issue of whether the internetwork level   recognizes flows at all. Should it?Chiappa                                                        [Page 14]

RFC 1753         Nimrod Technical Requirements for IPng    December 19943.2.3 Flows and State   To the extent that you have service state in the routers you have to   be able to associate that state with the packets it goes with. This   is a fundamental reason for flows. Access to service state is one   reason to explicitly recognize flows at the internetwork layer, but   it is not the only one.   If the user has requirements in a number of areas (e.g., routing and   access control), they can theoretically communicate these to the   routers by placing a copy of all the relevant information in each   packet (in the internetwork header). If many subsystems of the   internetwork are involved, and the requirements are complex, this   could be a lot of bits.   (As a final aside, there's clearly no point in storing in the routers   any user state about packets which are providing datagram service;   the datagram service has usually come and gone in the same packet,   and this discussion is all about state retention.)   There are two schools of thought as to how to proceed. The first says   that for reasons of robustness and simplicity, all user state ought   to be repeated in each packet. For efficiency reasons, the routers   may cache such user state, probably along with precomputed data   derived from the user state.  (It makes sense to store such cached   user state along with any applicable server state, of course.)   The second school says that if something is going to generate lots of   packets, it makes engineering sense to give all this information to   the routers once, and from then on have a tag (the flow-id) in the   packet which tells the routers where to find that information. It's   simply going to be too inefficient to carry all the user state around   all the time. This is purely an engineering efficiency reason, but   it's a significant one.   There is a slightly deeper argument, which says that the routers will   inevitably come to contain more user state, and it's simply a   question of whether that state is installed by an explicit mechanism,   or whether the routers infer that state from watching the packets   which pass through them.  To the extent that it is inevitable anyway,   there are obvious benefits to be gained from recognizing that, and an   explicit design of the installation is more likely to give   satisfactory results (as opposed to an ad-hoc mechanism).   It is worth noting that although the term "flow" is often used to   refer to this state in the routers along the path of the flow, it is   important to distinguish between i) a flow as a sequence of packets   (i.e., the definition given in 3.2.2 above), and ii) a flow, as theChiappa                                                        [Page 15]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   thing which is set up in the routers. They are different, and   although the particular meaning is usually clear from the context,   they are not the same thing at all.   I'm not sure how much use there is to any intermediate position, in   which one subsystem installs user state in the routers, and another   carries a copy of its user state in each packet.   (There are other intermediate positions. First, one flow might use a   given technique for all its subsystems, and another flow might use a   different technique for its; there is potentially some use to this,   although I'm not sure the cost in complexity of supporting both   mechanisms is worth the benefits. Second, one flow might use one   mechanism with one router along its path, and another for a different   router. A number of different reasons exist as to why one might do   this, including the fact that not all routers may support the same   mechanisms simultaneously.)   It seems to me that to have one internetwork layer subsystem (e.g.,   resource allocation) carry user state in all the packets (perhaps   with use of a "hint" in the packets to find potentially cached copies   in the router), and have a second (e.g., routing) use a direct   installation, and use a tag in the packets to find it, makes little   sense. We should do one or the other, based on a consideration of the   efficiency/robustness tradeoff.   Also, if there is a way of installing such flow-associated state, it   makes sense to have only one, which all subsystems use, instead of   building a separate one for each flow.   It's a little difficult to make the choice between installation, and   carrying a copy in each packet, without more information of exactly   how much user state the network is likely to have in the future. (For   instance, we might wind up with 500 byte headers if we include the   full source route, resource reservation, etc., in every header.)   It's also difficult without consideration of the actual mechanisms   involved. As a general principle, we wish to make recovery from a   loss of state as local as possible, to limit the number of entities   which have to become involved. (For instance, when a router crashes,   traffic is rerouted around it without needing to open a new TCP   connection.) The option of the "installation" looks a lot more   attractive if it's simple, and relatively cheap, to reinstall the   user state when a router crashes, without otherwise causing a lot of   hassle.Chiappa                                                        [Page 16]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994   However, given the likely growth in user state, the necessity for   service state, the requirement for reliable installation, and a   number of similar considerations, it seems that direct installation   of user state, and explicit recognition of flows, through a unified   definition and tag mechanism in the packets, is the way to go, and   this is the path that Nimrod has chosen.3.3 Specific Interaction Issues   Here is a very incomplete list of the things which Nimrod would like   to see from the internetwork layer as a whole:   - A unified definition of flows in the internetwork layer, and a     unified way of identifying, through a separate flow-id field, which     packets belong to a given flow.   - A unified mechanism (potentially distributed) for installing state     about flows (including multicast flows) in routers.   - A method for getting information about whether a given resource     allocation request has failed along a given path; this might be     part of the unified flow setup mechanism.   - An interface to (potentially distributed) mechanism for maintaining     the membership in a multi-cast group.   - Support for multiple interfaces; i.e., multi-homing. Nimrod does     this by decoupling transport identification (done via EID's) from     interface identification (done via locators). E.g., a packet with     any valid destination locator should be accepted by the TCP of an     endpoint, if the destination EID is the one assigned to that     endpoint.   - Support for multiple locators ("addresses") per network interface.     This is needed for a number of reasons, among them to allow for     less painful transitions in the locator abstraction hierarchy as     the topology changes.   - Support for multiple UID's ("addresses") per endpoint (roughly, per     host). This would definitely include both multiple multicast SID's,     and at least one unicast EID (the need for multiple unicast EID's     per endpoint is not obvious).   - Support for distinction between a multicast group as a named     entity, and a multicast flow which may not reach all the members.   - A distributed, replicated, user name translation system (DNS?) that     maps such user names into (EID, locator0, ... locatorN) bindings.Chiappa                                                        [Page 17]

RFC 1753         Nimrod Technical Requirements for IPng    December 1994Security Considerations   Security issues are discussed insection 2.2.Author's Address   J. Noel Chiappa   Phone: (804) 898-8183   EMail: jnc@lcs.mit.eduChiappa                                                        [Page 18]