Network Working Group
Request for Comments: 1992
Category: Informational
I. Castineyra
N. Chiappa
M. Steenstrup
August 1996

The Nimrod Routing 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.


We present a scalable internetwork routing architecture, called Nimrod. The Nimrod architecture is designed to accommodate a dynamic internetwork of arbitrary size with heterogeneous service requirements and restrictions and to admit incremental deployment throughout an internetwork. The key to Nimrod's scalability is its ability to represent and manipulate routing-related information at multiple levels of abstraction.

Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2. Overview of Nimrod . . . . . . . . . . . . . . . . . . . . . . . 3
     2.1 Constraints of the Internetworking Environment  . . . . . . . 3
     2.2 The Basic Routing Functions . . . . . . . . . . . . . . . . . 5
     2.3 Scalability Features  . . . . . . . . . . . . . . . . . . . . 6
       2.3.1 Clustering and Abstraction  . . . . . . . . . . . . . . . 6
       2.3.2 Restricting Information Distribution  . . . . . . . . . . 7
       2.3.3 Local Selection of Feasible Routes  . . . . . . . . . . . 8
       2.3.4 Caching . . . . . . . . . . . . . . . . . . . . . . . . . 8
       2.3.5 Limiting Forwarding Information . . . . . . . . . . . . . 8
   3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     3.1 Endpoints   . . . . . . . . . . . . . . . . . . . . . . . . . 9
     3.2 Nodes and Adjacencies . . . . . . . . . . . . . . . . . . . . 9
     3.3 Maps  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
       3.3.1 Connectivity Specifications  . . . . . . . . . . . . . . 10
     3.4  Locators  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.5 Node Attributes  . . . . . . . . . . . . . . . . . . . . . . 11
       3.5.1 Adjacencies  . . . . . . . . . . . . . . . . . . . . . . 11
       3.5.2 Internal Maps  . . . . . . . . . . . . . . . . . . . . . 11
       3.5.3 Transit Connectivity . . . . . . . . . . . . . . . . . . 12
       3.5.4 Inbound Connectivity . . . . . . . . . . . . . . . . . . 12
       3.5.5 Outbound Connectivity  . . . . . . . . . . . . . . . . . 12
   4. Physical Realization  . . . . . . . . . . . . . . . . . . . . . 13
     4.1 Contiguity   . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.2 An Example . . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.3 Multiple Locator Assignment  . . . . . . . . . . . . . . . . 15
   5. Forwarding  . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     5.1 Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     5.2 Trust  . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     5.3 Connectivity Specification (CSC) Mode  . . . . . . . . . . . 24
     5.4 Flow Mode  . . . . . . . . . . . . . . . . . . . . . . . . . 25
     5.5 Datagram Mode  . . . . . . . . . . . . . . . . . . . . . . . 25
     5.6 Connectivity Specification Sequence Mode . . . . . . . . . . 26
   6. Security Considerations . . . . . . . . . . . . . . . . . . . . 26
   7. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 26
   7. Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . 27

1. Introduction

Nimrod is a scalable routing architecture designed to accommodate a continually expanding and diversifying internetwork. First suggested by Noel Chiappa, the Nimrod architecture has undergone revision and refinement through the efforts of the Nimrod working group of the IETF. In this document, we present a detailed description of this architecture.

The goals of Nimrod are as follows:

  1. To support a dynamic internetwork of arbitrary size by providing mechanisms to control the amount of routing information that must be known throughout an internetwork.
  1. To provide service-specific routing in the presence of multiple constraints imposed by service providers and users.
  1. To admit incremental deployment throughout an internetwork.

We have designed the Nimrod architecture to meet these goals. The key features of this architecture include:

  1. Representation of internetwork connectivity and services in the form of maps at multiple levels of abstraction.
  1. User-controlled route generation and selection based on maps and traffic service requirements.
  1. User-directed packet forwarding along established paths.

Nimrod is a general routing architecture that can be applied to routing both within a single routing domain and among multiple routing domains. As a general internetwork routing architecture designed to deal with increased internetwork size and diversity, Nimrod is equally applicable to both the TCP/IP and OSI environments.

2. Overview of Nimrod

Before describing the Nimrod architecture in detail, we provide an overview. We begin with the internetworking requirements, followed by the routing functions, and concluding with Nimrod's scaling characteristics.

2.1 Constraints of the Internetworking Environment

Internetworks are growing and evolving systems, in terms of number, diversity, and interconnectivity of service providers and users, and therefore require a routing architecture that can accommodate internetwork growth and evolution. A complicated mix of factors such as technological advances, political alliances, and service supply and demand economics will determine how an internetwork will change over time. However, correctly predicting all of these factors and all of their effects on an internetwork may not be possible. Thus, the flexibility of an internetwork routing architecture is its key to handling unanticipated requirements.

In developing the Nimrod architecture, we first assembled a list of internetwork environmental constraints that have implications for routing. This list, enumerated below, includes observations about the present Internet; it also includes predictions about internetworks five to ten years in the future.

  1. The Internet will grow to include O(10^9) networks.
  1. The number of internetwork users may be unbounded.
  1. The capacity of internetwork resources is steadily increasing but so is the demand for these resources.
  1. Routers and hosts have finite processing capacity and finite memory, and networks have finite transmission capacity.
  1. Internetworks comprise different types of communications media -- including wireline, optical and wireless, terrestrial and satellite, shared multiaccess and point-to-point -- with different service characteristics in terms of throughput, delay, error and loss distributions, and privacy.
   6. Internetwork elements -- networks, routers, hosts, and processes --
      may be mobile.
  1. Service providers will specify offered services and restrictions on access to those services. Restrictions may be in terms of when a service is available, how much the service costs, which users may subscribe to the service and for what purposes, and how the user must shape its traffic in order to receive a service guarantee.
  1. Users will specify traffic service requirements which may vary widely among sessions. These specifications may be in terms of requested qualities of service, the amounts they are willing to pay for these services, the times at which they want these services, and the providers they wish to use.
  1. A user traffic session may include m sources and n destinations, where m, n > or = 1.
  1. Service providers and users have a synergistic relationship. That is, as users develop more applications with special service requirements, service providers will respond with the services to meet these demands. Moreover, as service providers deliver more services, users will develop more applications that take advantage of these services.
  1. Support for varied and special services will require more processing, memory, and transmission bandwidth on the part of both the service providers offering these services and the users requesting these services. Hence, many routing-related activities will likely be performed not by routers and hosts but rather by independent devices acting on their behalf to process, store, and distribute routing information.
   12. Users requiring specialized services (e.g., high guaranteed
       throughput) will usually be willing to pay more for these services
       and to incur some delay in obtaining them.
  1. Service providers are reluctant to introduce complicated protocols into their networks, because they are more difficult to manage.
  1. Vendors are reluctant to implement complicated protocols in their products, because they take longer to develop.

Collectively, these constraints imply that a successful internetwork routing architecture must support special features, such as service- specific routing and component mobility in a large and changing internetwork, using simple procedures that consume a minimal amount of internetwork resources. We believe that the Nimrod architecture meets these goals, and we justify this claim in the remainder of this document.

2.2 The Basic Routing Functions

The basic routing functions provided by Nimrod are those provided by any routing system, namely:

  1. Collecting, assembling, and distributing the information necessary for route generation and selection.
  1. Generating and selecting routes based on this information.
  1. Establishing in routers information necessary for forwarding packets along the selected routes.
  1. Forwarding packets along the selected routes.

The Nimrod approach to providing this routing functionality includes map distribution according to the "link-state" paradigm, localization of route generation and selection at traffic sources and destinations, and specification of packet forwarding through path establishment by the sources and destinations.

Link-state map distribution permits each service provider to have control over the services it offers, through both distributing restrictions in and restricting distribution of its routing information. Restricting distribution of routing information serves to reduce the amount of routing information maintained throughout an internetwork and to keep certain routing information private. However, it also leads to inconsistent routing information databases throughout an internetwork, as not all such databases will be complete or identical. We expect routing information database inconsistencies to occur often in a large internetwork, regardless of whether privacy is an issue. The reason is that we expect some devices to be incapable of maintaining the complete set of routing information for the internetwork. These devices will select only some of the distributed routing information for storage in their databases.

Route generation and selection, based on maps and traffic service requirements, may be completely controlled by the users or, more likely, by devices acting on their behalf and does not require global coordination among routers. Thus these devices may generate routes specific to the users' needs, and only those users pay the cost of generating those routes. Locally-controlled route generation allows incremental deployment of and experimentation with new route generation algorithms, as these algorithms need not be the same at each location in an internetwork.

Packet forwarding according to paths may be completely controlled by the users or the devices acting on their behalf. These paths may be specified in as much detail as the maps permit. Such packet forwarding provides freedom from forwarding loops, even when routers in a path have inconsistent routing information. The reason is that the forwarding path is a route computed by a single device and based on routing information maintained at a single device.

We note that the Nimrod architecture and Inter-Domain Policy Routing (IDPR) [1] share in common link-state routing information distribution, localized route generation and path-oriented message forwarding. In developing the Nimrod architecture, we have drawn upon experience gained in developing and experimenting with IDPR.

2.3 Scalability Features

Nimrod must provide service-specific routing in arbitrarily large internetworks and hence must employ mechanisms that help to contain the amount of internetwork resources consumed by the routing functions. We provide a brief synopsis of such mechanisms below, noting that arbitrary use of these mechanisms does not guarantee a scalable routing architecture. Instead, these mechanisms must be used wisely, in order enable a routing architecture to scale with internetwork growth.

2.3.1 Clustering and Abstraction

The Nimrod architecture is capable of representing an internetwork as clusters of entities at multiple levels of abstraction. Clustering reduces the number of entities visible to routing. Abstraction reduces the amount of information required to characterize an entity visible to routing.

Clustering begins by aggregating internetwork elements such as hosts, routers, and networks according to some predetermined criteria. These elements may be clustered according to relationships among them, such as "managed by the same authority", or so as to satisfy some objective function, such as "minimize the expected amount of forwarding information stored at each router". Nimrod does not mandate a particular cluster formation algorithm.

New clusters may be formed by clustering together existing clusters. Repeated clustering of entities produces a hierarchy of clusters with a unique universal cluster that contains all others. The same clustering algorithm need not be applied at each level in the hierarchy.

All elements within a cluster must satisfy at least one relation, namely connectivity. That is, if all elements within a cluster are operational, then any two of them must be connected by at least one route that lies entirely within that cluster. This condition prohibits the formation of certain types of separated clusters, such as the following. Suppose that a company has two branches located at opposite ends of a country and that these two branches must communicate over a public network not owned by the company. Then the two branches cannot be members of the same cluster, unless that cluster also includes the public network connecting them.

Once the clusters are formed, their connectivity and service information is abstracted to reduce the representation of cluster characteristics. Example abstraction procedures include elimination of services provided by a small fraction of the elements in the cluster or expression of services in terms of average values. Nimrod does not mandate a particular abstraction algorithm. The same abstraction algorithm need not be applied to each cluster, and multiple abstraction algorithms may be applied to a single cluster.

A particular combination of clustering and abstraction algorithms applied to an internetwork results in an organization related to but distinct from the physical organization of the component hosts, routers, and networks. When a clustering is superimposed over the physical internetwork elements, the cluster boundaries may not necessarily coincide with host, router, or network boundaries. Nimrod performs its routing functions with respect to the hierarchy of entities resulting from clustering and abstraction, not with respect to the physical realization of the internetwork. In fact, Nimrod need not even be aware of the physical elements of an internetwork.

2.3.2 Restricting Information Distribution

The Nimrod architecture supports restricted distribution of routing information, both to reduce resource consumption associated with such distribution and to permit information hiding. Each cluster determines the portions of its routing information to distribute and the set of entities to which to distribute this information. Moreover, recipients of routing information are selective in which information they retain. Some examples are as follows. Each cluster might automatically advertise its routing information to its siblings (i.e., those clusters with a common parent cluster). In response to requests, a cluster might advertise information about specific portions of the cluster or information that applies only to specific users. A cluster might only retain routing information from clusters that provide universal access to their services.

2.3.3 Local Selection of Feasible Routes

Generating routes that satisfy multiple constraints is usually an NP-complete problem and hence a computationally intensive procedure. With Nimrod, only those entities that require routes with special constraints need assume the computational load associated with generation and selection of such routes. Moreover, the Nimrod architecture allows individual entities to choose their own route generation and selection algorithms and hence the amount of resources to devote to these functions.

2.3.4 Caching

The Nimrod architecture encourages caching of acquired routing information in order to reduce the amount of resources consumed and delay incurred in obtaining the information in the future. The set of routes generated as a by-product of generating a particular route is an example of routing information that is amenable to caching; future requests for any of these routes may be satisfied directly from the route cache. However, as with any caching scheme, the cached information may become stale and its use may result in poor quality routes. Hence, the routing information's expected duration of usefulness must be considered when determining whether to cache the information and for how long.

2.3.5 Limiting Forwarding Information

The Nimrod architecture supports two separate approaches for containing the amount of forwarding information that must be maintained per router. The first approach is to multiplex, over a single path (or tree, for multicast), multiple traffic flows with similar service requirements. The second approach is to install and retain forwarding information only for active traffic flows.

With Nimrod, the service providers and users share responsibility for the amount of forwarding information in an internetwork. Users have control over the establishment of paths, and service providers have control over the maintenance of paths. This approach is different from that of the current Internet, where forwarding information is established in routers independent of demand for this information.

3. Architecture

Nimrod is a hierarchical, map-based routing architecture that has been designed to support a wide range of user requirements and to scale to very large dynamic internets. Given a traffic stream's description and requirements (both quality of service requirements and usage-restriction requirements), Nimrod's main function is to manage in a scalable fashion how much information about the internetwork is required to choose a route for that stream, in other words, to manage the trade-off between amount of information about the internetwork and the quality of the computed route. Nimrod is implemented as a set of protocols and distributed databases. The following sections describe the basic architectural concepts used in Nimrod. The protocols and databases are specified in other documents.

3.1 Endpoints

The basic entity in Nimrod is the endpoint. An endpoint represents a user of the internetwork layer: for example, a transport connection. Each endpoint has at least one endpoint identifier (EID). Any given EID corresponds to a single endpoint. EIDs are globally unique, relatively short "computer-friendly" bit strings---for example, small multiples of 64 bits. EIDs have no topological significance whatsoever. For ease of management, EIDs might be organized hierarchically, but this is not required.


In practice, EIDs will probably have a second form, which we can call the endpoint label (EL). ELs are ASCII strings of unlimited length, structured to be used as keys in a distributed database (much like DNS names). Information about an endpoint---for example, how to reach it---can be obtained by querying this distributed database using the endpoint's label as key.


3.2 Nodes and Adjacencies

A node represents a region of the physical network. The region of the network represented by a node can be as large or as small as desired: a node can represent a continent or a process running inside a host. Moreover, as explained in section 4, a region of the network can simultaneously be represented by more than one node.

An adjacency consists of an ordered pair of nodes. An adjacency indicates that traffic can flow from the first node to the second.

3.3 Maps

The basic data structure used for routing is the map. A map expresses the available connectivity between different points of an internetwork. Different maps can represent the same region of a physical network at different levels of detail.

A map is a graph composed of nodes and adjacencies. Properties of nodes are contained in attributes associated with them. Adjacencies have no attributes. Nimrod defines languages to specify attributes and to describe maps.

Maps are used by routers to generate routes. In general, it is not required that different routers have consistent maps.


Nimrod has been designed so that there will be no routing loops even when the routing databases of different routers are not consistent. A consistency requirement would not permit representing the same region of the internetwork at different levels of detail. Also, a routing-database consistency requirement would be hard to guarantee in the very large internets Nimrod is designed to support.


In this document we speak only of routers. By "router" we mean a physical device that implements functions related to routing: for example, forwarding, route calculation, path set-up. A given device need not be capable of doing all of these to be called a router. The protocol specification document, see [2], splits these functionalities into specific agents.

3.3.1 Connectivity Specifications

By connectivity between two points we mean the available services and the restrictions on their use. Connectivity specifications are among the attributes associated with nodes. The following are informal examples of connectivity specifications:

  • "Between these two points, there exists best-effort service with no restrictions."
  • "Between these two points, guaranteed 10 ms delay can be arranged for traffic streams whose data rate is below 1 Mbyte/sec and that have low (specified) burstiness."
  • "Between these two points, best-effort service is offered, as long as the traffic originates in and is destined for research organizations."

3.4 Locators

A locator is a string of binary digits that identifies a location in an internetwork. Nodes and endpoint are assigned locators.

Different nodes have necessarily different locators. A node is assigned only one locator. Locators identify nodes and specify *where* a node is in the network. Locators do *not* specify a path to the node. An endpoint can be assigned more than one locator. In this sense, a locator might appear in more than one location of an internetwork.

In this document locators are written as ASCII strings that include colons to underline node structure: for example, a:b:c. This does not mean that the representation of locators in packets or in databases will necessarily have something equivalent to the colons.

A given physical element of the network might help implement more than one node---for example, a router might be part of two different nodes. Though this physical element might therefore be associated with more than one locator, the nodes that this physical element implements have each only one locator.

The connectivity specifications of a node are identified by a tuple consisting of the node's locator and an ID number.

All map information is expressed in terms of locators, and routing selections are based on locators. EIDs are *not* used in making routing decisions---see section 5.

3.5 Node Attributes

The following are node attributes defined by Nimrod.

3.5.1 Adjacencies

Adjacencies appear in maps as attributes of both the nodes in the adjacency. A node has two types of adjacencies associated with it: those that identify a neighboring node to which the original node can send data to; and those that identivy a neighboring node that can send data to the original node.

3.5.2 Internal Maps

As part of its attributes, a node can have internal maps. A router can obtain a node's internal maps---or any other of the node's attributes, for that matter---by requesting that information from a representative of that node. (A router associated with that node can be such a representative.) A node's representative can in principle reply with different internal maps to different requests---for example, because of security concerns. This implies that different routers in the network might have different internal maps for the same node.

A node is said to own those locators that have as a prefix the locator of the node. In a node that has an internal map, the locators of all nodes in this internal map are prefixed by the locator of the original node.

Given a map, a more detailed map can be obtained by substituting one of the map's nodes by one of that node's internal maps. This process can be continued recursively. Nimrod defines standard internal maps that are intended to be used for specific purposes. A node's "detailed map" gives more information about the region of the network represented by the original node. Typically, it is closer to the physical realization of the network than the original node. The nodes of this map can themselves have detailed maps.

3.5.3 Transit Connectivity

For a given node, this attribute specifies the services available between nodes adjacent to the given node. This attribute is requested and used when a router intends to route traffic *through* a node. Conceptually, the traffic connectivity attribute is a matrix that is indexed by a pair of locators: the locators of adjacent nodes. The entry indexed by such a pair contains the connectivity specifications of the services available across the given node for traffic entering from the first node and exiting to the second node.

The actual format of this attribute need not be a matrix. This document does not specify the format for this attribute.

3.5.4 Inbound Connectivity

For a given node, this attribute represents connectivity from adjacent nodes to points within the given node. This attribute is requested and used when a router intends to route traffic to a point within the node but does not have, and either cannot or does not want to obtain, a detailed map of the node. The inbound connectivity attribute identifies what connectivity specifications are available between pairs of locators. The first element of the pair is the locator of an adjacent node; the second is a locator owned by the given node.

3.5.5 Outbound Connectivity

For a given node, this attribute represents connectivity from points within the given node to adjacent nodes. This attribute identifies what connectivity specifications are available between pairs of locators. The first element of the pair is a locator owned by the given node, the second is the locator of an adjacent node.

The Transit, Inbound and Outbound connectivity attributes together wiht a list of adjacencies form the "abstract map."

4. Physical Realization

   A network is modeled as being composed of physical elements: routers,
   hosts, and communication links.  The links can be either point-to-
   point---e.g., T1 links---or multi-point---e.g., ethernets, X.25
   networks, IP-only networks, etc.

The physical representation of a network can have associated with it one or more Nimrod maps. A Nimrod map is a function not only of the physical network, but also of the configured clustering of elements (locator assignment) and of the configured connectivity.

Nimrod has no pre-defined "lowest level": for example, it is possible to define and advertise a map that is physically realized inside a CPU. In this map, a node could represent, for example, a process or a group of processes. The user of this map need not necessarily know or care. ("It is turtles all the way down!", in [3] page 63.)

4.1 Contiguity

Locators sharing a prefix must be assigned to a contiguous region of a map. That is, two nodes in a map that have been assigned locators sharing a prefix should be connected to each other via nodes that themselves have been assigned locators with that prefix. The main consequence of this requirement is that "you cannot take your locator with you."

As an example of this, see figure 1, consider two providers and (these designations are *not* locators but DNS names) which appear in a Nimrod map as two nodes with locators A and B. Assume that corporation (also a DNS name) was originally connected to Locators corresponding to elements in are, in this example, A-prefixed. Corporation decides to change providers- --severing its physical connection to The connectivity requirement described in this section implies that, after the provider change has taken place, elements in will have been, in this example, assigned B-prefixed locators and that it is not possible for them to receive data destined to A-prefixed locators through

                  A                 B
               +------+          +------+
               ||          ||
               +------+         /+------+

Figure 1: Connectivity after switching providers

The contiguity requirement simplifies routing information exchange: if it were permitted for to receive A-prefixed locators through, it would be necessary that a map that contains node B include information about the existence of a group of A-prefixed locators inside node B. Similarly, a map including node A would have to include information that the set of A-prefixed locators asigned to is not to be found within A. The more situations like this happen, the more the hierarchical nature of Nimrod is subverted to "flat routing." The contiguity requirement can also be expressed as "EIDs are stable; locators are ephemeral."

4.2 An Example

Figure 2 shows a physical network. Hosts are drawn as squares, routers as diamonds, and communication links as lines. The network shown has the following components: five ethernets ---EA through EE; five routers---RA through RE; and four hosts---HA through HD. Routers RA, RB, and RC interconnect the backbone ethernets---EB, EC and ED. Router RD connects backbone EC to a network consisting of ethernet EA and hosts HA and HB. Router RE interconnects backbone ED to a network consisting of ethernet EE and hosts HC and HD. The assigned locators appear in lower case beside the corresponding physical entity.

Figure 3 shows a Nimrod map for that network. The nodes of the map are represented as squares. Lines connecting nodes represent two adjacencies in opposite directions. Different regions of the network are represented at different detail. Backbone b1 is represented as a single node. The region of the network with locators prefixed by "a" is represented as a single node. The region of the network with locators prefixed by "c" is represented in full detail.

4.3 Multiple Locator Assignment

Physical elements can form part of, or implement, more than one node. In this sense it can be said that they can be assigned more than one locator. Consider figure 4, which shows a physical network. This network is composed of routers (RA, RB, RC, and RD), hosts (HA, HB, and HC), and communication links. Routers RA, RB, and RC are connected with point-to-point links. The two horizontal lines in the bottom of the figure represent ethernets. The figure also shows the locators assigned to hosts and routers.

In figure 4, RA and RB have each been assigned one locator (a:t:r1 and b:t:r1, respectively). RC has been assigned locators a:y:r1 and b:d:r1; one of these two locators shares a prefix with RA's locator, the other shares a prefix with RB's locator. Hosts HA and HB have each been assigned three locators. Host HC has been assigned one locator. Depending on what communication paths have been set up between points, different Nimrod maps result. A possible Nimrod map for this network is given in figure 5.

                                             a:h1 +--+      a:h2 +--+
                                                  |HA|           |HB|
                                                  |  |           |  |
                                                  +--+           +--+
                                           a:e1    |              |
                                               --------------------- EA
                                 /\                    /\
                                /RB\ b1:r1            /RD\ b2:r1
                               /\  /\                 \  /
                              /  \/  \                 \/
    EB         b1:t:e1       /        \                 |   EC
    ------------------------          -------------------------- b2:e1
               /                             \
              /                               \
             /\                                \
            /RA\ b1:r2                          \/\
            \  /                                /RC\  b2:t:r2
             \/                                 \  /
               \                                 \/
                \                               /   ED
                  ----------------------------------- b3:t:e1
                                  /RE\ b3:t:r1
                                  \  /
                      EE           \/
                      -----------------------------   c:e1
                         |                   |
                        +--+                +--+
                        |HC|   c:h1         |HD|    c:h2
                        |  |                |  |
                        +--+                +--+

Figure 2: Example Physical Network

                             +-----+               +-----+
   +----------+              |     |               |     |
   |          |--------------| b2  | --------------| a   |
   |          |              |     |               |     |
   |    b1    |              +-----+               +-----+
   |          |                 |
   |          |                 |
   |          |                 |
   +----------+                 |
               \                |
                \               |
                 \              |
                  \             |
                   \         +--------+
                    \        |        |
                     ------- | b3:t:e1|
                             |        |
                             |       |
                             |       |
                 +-----+       +-----+     +-----+
                 |     |       |     |     |     |
                 | c:h1|-------| c:e1|-----| c:h2|
                 |     |       |     |     |     |
                 +-----+       +-----+     +-----+

Figure 3: Nimrod Map

                      a:t:r1              b:t:r1
                         +--+            +--+
                         +--+            +--+
                           \             /
                            \           /
                             \         /
                              \       /
                               \     /
                                \   /
                                 \ /
                                 |RC|  a:y:r1
                                 +--+  b:d:r1
                      |        |             |
             a:y:h1  +--+     +--+          +--+    a:y:h2
             b:d:h2  |HA|     |RD| c:r1     |HB|    b:d:h1
             c:h1    +--+     +--+          +--+    c:h2
                                 |HC| c:h3

Figure 4: Multiple Locators

           a                       b                   c
     +-------------+       +-------------+         +---------------+
     |             |       |             |         |               |
     |        a:t  |       |      b:t    |         |               |
     |   +--+      |       |  +--+       |         |               |
     |   |  |--------------|--|  |       |         |               |
     |   +--+      |       |  +--+       |         |               |
     |     |       |       |    |        |         |               |
     |   +--+      |       |  +--+       |         |               |
     |   +  +      |       |  +  +       |         |               |
     |   +--+ a:y  |       |  +--+ b:d   |         |               |
     |             |       |             |         |               |
     +-------------+       +-------------+         +---------------+

Figure 5: Nimrod Map

Nodes and adjacencies represent the *configured* clustering and connectivity of the network. Notice that even though a:y and b:d are defined on the same hardware, the map shows no connection between them: this connection has not been configured. A packet given to node `a' addressed to a locator prefixed with "b:d" would have to travel from node a to node b via the arc joining them before being directed towards its destination. Similarly, the map shows no connection between the c node and the other two top level nodes. If desired, these connections could be established, which would necessitate setting up the exchange of routing information. Figure 6 shows the map when these connections have been established.

In the strict sense, Nimrod nodes do not overlap: they are distinct entities. But, as we have seen in the previous example, a physical element can be given more than one locator, and, in that sense, participate in implementing more than one node. That is, two different nodes might be defined on the same hardware. In this sense, Nimrod nodes can be said to overlap. But to notice this overlap one would have to know the physical-to-map correspondence. It is not possible to know when two nodes share physical assets by looking only at a Nimrod map.

5. Forwarding

Nimrod supports four forwarding modes:

  1. Connectivity Specification Chain (CSC) mode: In this mode, packets carry a list of connectivity specifications. The packet is required to go through the nodes that own the connectivity specifications using the services specified. The nodes associated with the listed connectivity specifications should define a continuous path in the map. A more detailed description of the requirements of this mode is given in section 5.3.
   +--------+                                               +--------+
   |        |                                               |        |
   | a:t:r1 |-----------------------------------------------| b:t:r1 |
   |        |                                               |        |
   +--------+                                               +--------+
     |                                                             |
     |                                                             |
     |         /-----------------------------------------\         |
     |         |                                         |         |
     |         |                                         |         |
     |  +--------+       +--------+                    +--------+  |
     |  |        |       |        |                    |        |  |
     |  | a:y:h1 --------|  c:h1  |--------------------| b:d:h1 |  |
     |  |        |       |        |                    |        |  |
     |  +--------+       +--------+                    +--------+  |
     |    |    |           |    |                        |    |    |
   +--------+  |           |  +------+  +------+         |  +--------+
   |        |  |           |  |      |  |      |         |  |        |
   | a:y:r1 |  |           |  | c:r1 |--| c:h3 |         |  | b:d:r1 |
   |        |  |           |  |      |  |      |         |  |        |
   +--------+  |           |  +------+  +------+         |  +--------+
     |    |    |           |    |                        |    |    |
     |  +--------+       +--------+                    +--------+  |
     |  |        |       |        |                    |        |  |
     |  | a:y:h2 |--------  c:h2  |--------------------| b:d:h2 |  |
     |  |        |       |        |                    |        |  |
     |  +--------+       +--------+                    +--------+  |
     |         |                                         |         |
     |         |                                         |         |
     |         |                                         |         |
     |         \-----------------------------------------/         |

Figure 6: Nimrod Map II

  1. Connectivity Specifications Sequence (CSS) mode: In this mode, packets carry a list of connectivity specifications. The packet is supposed to go sequentially through the nodes that own each one of the listed connectivity specifications in the order they were specified. The nodes need not be adjacent. This mode can be seen as a generalization of the CSC mode. Notice that CSCs are said to be a *chains* of locators, CSSs are *sequences* of locators. This difference emphasizes the contiguity requirement in CSCs. A detailed description of this mode is in section 5.6.
  1. Flow mode: In this mode, the packet includes a path-id that indexes state that has been previously set up in routers along the path. Packet forwarding when flow state has been established is relatively simple: follow the instructions in the routers' state. Nimrod includes a mechanism for setting up this state. A more detailed description of this mode can be found in section 5.4.
  1. Datagram mode: in this mode, every packet carries source and destination locators. This mode can be seen as a special case of the CSS mode. Forwarding is done following procedures as indicated in section 5.5.

The obvious parallels are between CSC mode and IPV4's strict source route and between CSS mode and IPV4's loose source route.


In all of these modes, the packet may also carry locators and EIDs for the source and destinations. In normal operation, forwarding does not take the EIDs into account, only the receiver does. EIDs may be carried for demultiplexing at the receiver, and to detect certain error conditions. For example, if the EID is unknown at the receiver, the locator and EID of the source included in the packet could be used to generate an error message to return to the source (as usual, this error message itself should probably not be allowed to be the cause of other error messages). Forwarding can also use the source locator and EID to respond to error conditions, for example, to indicate to the source that the state for a path-id cannot be found.

Packets can be visualized as moving between nodes in a map. A packet indicates, implicitly or explicitly, a destination locator. In a packet that uses the datagram, CSC, or CSS forwarding mode, the destination locator is explicitly indicated . In a packet that uses the flow forwarding mode, the destination locator is implied by the path-id and the distributed state in the network (it might also be included explicitly). Given a map, a packet moves to the node in this map to which the associated destination locator belongs. If the destination node has a "detailed" internal map, the destination locator must belong to one of the nodes in this internal map (otherwise it is an error). The packet goes to this node (and so on, recursively).

5.1 Policy

CSC and CSS mode implement policy by specifying the connectivity specifications associated with those nodes that the packet should traverse. Strictly speaking, there is no policy information included in the packet. That is, in principle, it is not possible to determine what criteria were used to select the route by looking at the packet. The packet only contains the results of the route generation process. Similarly, in a flow mode packet, policy is implicit in the chosen route.

A datagram-mode packet can indicate a limited form of policy routing by the choice of destination and source locators. For this choice to exist, the source or destination endpoints must have several locators associated with them. This type of policy routing is capable of, for example, choosing providers.

5.2 Trust

A node that chooses not to divulge its internal map can work internally any way its administrators decide, as long as the node satisfies its external characterization as given in its Nimrod map advertisements. Therefore, the advertised Nimrod map should be consistent with a node's actual capabilities. For example, consider the network shown in figure 7 which shows a physical network and the advertised Nimrod map. The physical network consists of hosts and a router connected together by an ethernet. This node can be sub- divided into component nodes by assigning locators as shown in the figure and advertising the map shown. The map seems to imply that it is possible to send packets to node a:x without these being observable by node a:y; however, this is actually not enforceable.

In general, it is reasonable to ask how much trust should be put in the maps obtained by a router. Even when a node is "trustworthy," and the information received from the node has been authenticated, there is always the possibility of an honest mistake.

                                 |RA| a:r1
                         |                       |
                        +--+                    +--+
                        |Ha| a:x:h1             |Ha| a:y:h2
                        +--+                    +--+

Physical Network

                      a             |
                   |                |                   |
                   |              +----+                |
                   |              |a:r1|                |
                   |   a:x        +----+  a:y           |
                   |   +------+  /      \ +-------+     |
                   |   |      | /        \|       |     |
                   |   |      |           |       |     |
                   |   |      |           |       |     |
                   |   +------+           +-------+     |
                   |                                    |
                   + -----------------------------------+
                               Advertised Nimrod Map

Figure 7: Example of Misleading Map

5.3 Connectivity Specification (CSC) Mode

   Routing for a CSC packet is specified by a list of connectivity
   specifications carried in the packet.  These are the connectivity
   specifications that make the specified path, in the order that they
   appear along the path.  These connectivity specifications are
   attributes of nodes.  The route indicated by a CSC packet is specifed
   in terms of connectivity specifications rather than physical
   entities:  a connectivity specification in a CSC-mode packet would

correspond to a type of service between two points of the network without specifying the physical path.

Given two connectivity specifications that appear consecutively in the a CSC-mode packet, there should exist an adjacency going from the node corresponding to the first connectivity specification to the node corresponding to the second connectivity specification. The first connectivity specification referenced in a CSC-mode packet should be an outbound connectivity specification; similarly, the last connectivity specification referenced in a CSC-mode packet should be an inbound connectivity specification; the rest should be transit connectivity specifications.

5.4 Flow Mode

A flow mode packet includes a path-id field. This field identifies state that has been established in intermediate routers. The packet might also contain locators and EIDs for the source and destination. The setup packet also includes resource requirements. Nimrod includes protocols to set up and modify flow-related state in intermediate routers. These protocols not only identify the requested route, but also describe the resources requested by the flow---e.g., bandwidth, delay, etc. The result of a set-up attempt might be either confirmation of the set-up or notification of its failure. The source-specified routes in flow mode setup are specified in terms of CSSs.

5.5 Datagram Mode

A realistic routing architecture must include an optimization for datagram traffic, by which we mean user transactions which consist of single packets, such as a lookup in a remote translation database. Either of the two previous modes contains unacceptable overhead if much of the network traffic consists of such datagram transactions. A mechanism is needed which is approximately as efficient as the existing IPv4 "hop-by-hop" mechanism. Nimrod has such a mechanism.

The scheme can be characterized by the way it divides the state in a datagram network between routers and the actual packets. In IPv4, most packets currently contain only a small amount of state associated with the forwarding process ("forwarding state")---the hop count. Nimrod proposes that enlarging the amount of forwarding state in packets can produce a system with useful properties. It was partially inspired by the efficient source routing mechanism in SIP [5], and the locator pointer mechanism in PIP [6]).

Nimrod datagram mode uses pre-set flow-mode state to support a strictly non-looping path, but without a source-route.

5.6 Connectivity Specification Sequence Mode

The connectivity specification sequence mode specifies a route by a list of connectivity specifications. There are no contiguity restrictions on consecutive connectivity specifications.


The CSS and CSC modes can be seen as combination of the datagram and flow modes. Therefore, in a sense, the basic forwarding modes of Nimrod are just these last two.


6. Security Considerations

Security issues are not addressed in this document.

7. References

   [1] Steenstrup, M., "Inter-Domain Policy Routing Protocol
       Specification: Version 1," RFC 1479, June 1993.
   [2] Steenstrup M., and R. Ramanathan, "Nimrod Functionality and
       Protocols Specification," Work in Progress, February 1996.
   [3] Wright, R., "Three Scientists and their Gods Looking for Meaning
       in an Age of Information", New York: Times Book, first ed., 1988.

[4] Deering, S., "SIP: Simple Internet Protocol," IEEE Network, vol.

7, May 1993.

   [5] Francis, P., "A Near-Term Architecture for Deploying Pip," IEEE
       Network, vol. 7, May 1993.

8. Authors' Addresses

Isidro Castineyra
BBN Systems and Technologies
10 Moulton Street
Cambridge, MA 02138

   Phone:  (617) 873-6233
   Noel Chiappa

Martha Steenstrup
BBN Systems and Technologies
10 Moulton Street
Cambridge, MA 02138

   Phone:  (617) 873-3192