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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.


 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 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 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
 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

Chiappa [Page 3] RFC 1753 Nimrod Technical Requirements for IPng December 1994

2.2 Packet Format Fields

 What Nimrod would like to see in the internetworking packet is:
  1. 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
   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.
  1. 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

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
   For the aggregated case, three broad classes of mechanism are
  1. 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.
  1. 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
  1. 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
  1. 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.
  1. 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.
  1. 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.
  1. 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).
  1. 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.
  1. 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.
  1. 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

Chiappa [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.
  1. 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
 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

Chiappa [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 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

Chiappa [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

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 1994

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
 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 1994

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.

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 1994

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

Chiappa [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

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:
  1. 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.
  1. A unified mechanism (potentially distributed) for installing state

about flows (including multicast flows) in routers.

  1. 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.
  1. An interface to (potentially distributed) mechanism for maintaining

the membership in a multi-cast group.

  1. 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
  1. 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.
  1. 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).
  1. Support for distinction between a multicast group as a named

entity, and a multicast flow which may not reach all the members.

  1. 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 1994

Security Considerations

 Security issues are discussed in section 2.2.

Author's Address

 J. Noel Chiappa
 Phone: (804) 898-8183

Chiappa [Page 18]

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