Network Working Group N. Chiappa Request for Comments: 1753 December 1994 Category: Informational IPng Technical Requirements Of the Nimrod Routing and Addressing Architecture Status 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 to RFC 1550. Publication of this document does not imply acceptance by the IPng area of any ideas expressed within. Comments should be submitted to the firstname.lastname@example.org 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.
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 sections 3.1 ("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 Format 2.1 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.
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.
2.2 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.
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.
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.
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 especially section 4.3. There is currently no set way of doing "denial/theft of service" in Nimrod, but this topic is well
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. The
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 Issues 3.1 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 on
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,
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.
3.2.1 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".
3.2.2 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.
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?
3.2.3 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 the
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.
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.