RFC2332 - NBMA Next Hop Resolution Protocol (NHRP)

Network Working Group J. LUCiani

Request for Comments: 2332 Bay Networks

Category: Standards Track D. Katz

cisco Systems

D. Piscitello

Core Competence, Inc.

B. Cole

Juniper Networks

N. Doraswamy

Bay Networks

April 1998

NBMA Next Hop Resolution Protocol (NHRP)

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document describes the NBMA Next Hop Resolution Protocol (NHRP).

NHRP can be used by a source station (host or router) connected to a

Non-Broadcast, Multi-Access (NBMA) subnetwork to determine the

internetworking layer address and NBMA subnetwork addresses of the

"NBMA next hop" towards a destination station. If the destination is

connected to the NBMA subnetwork, then the NBMA next hop is the

destination station itself. Otherwise, the NBMA next hop is the

egress router from the NBMA subnetwork that is "nearest" to the

destination station. NHRP is intended for use in a multiprotocol

internetworking layer environment over NBMA subnetworks.

Note that while this protocol was developed for use with NBMA

subnetworks, it is possible, if not likely, that it will be applied

to BMA subnetworks as well. However, this usage of NHRP is for

further study.

This document is intended to be a functional superset of the NBMA

Address Resolution Protocol (NARP) documented in [1].

Operation of NHRP as a means of establishing a transit path across an

NBMA subnetwork between two routers will be addressed in a separate

document (see [13]).

1. Introduction

The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,

SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this

document, are to be interpreted as described in [15].

The NBMA Next Hop Resolution Protocol (NHRP) allows a source station

(a host or router), wishing to communicate over a Non-Broadcast,

Multi-Access (NBMA) subnetwork, to determine the internetworking

layer addresses and NBMA addresses of suitable "NBMA next hops"

toward a destination station. A subnetwork can be non-broadcast

either because it technically doesn't support broadcasting (e.g., an

X.25 subnetwork) or because broadcasting is not feasible for one

reason or another (e.g., an SMDS multicast group or an extended

Ethernet would be too large). If the destination is connected to the

NBMA subnetwork, then the NBMA next hop is the destination station

itself. Otherwise, the NBMA next hop is the egress router from the

NBMA subnetwork that is "nearest" to the destination station.

One way to model an NBMA network is by using the notion of logically

independent IP subnets (LISs). LISs, as defined in [3] and [4], have

the following properties:

1) All members of a LIS have the same IP network/subnet number

and address mask.

2) All members of a LIS are directly connected to the same

NBMA subnetwork.

3) All hosts and routers outside of the LIS are accessed via

a router.

4) All members of a LIS access each other directly (without

routers).

Address resolution as described in [3] and [4] only resolves the next

hop address if the destination station is a member of the same LIS as

the source station; otherwise, the source station must forward

packets to a router that is a member of multiple LIS's. In multi-LIS

configurations, hop-by-hop address resolution may not be sufficient

to resolve the "NBMA next hop" toward the destination station, and IP

packets may have multiple IP hops through the NBMA subnetwork.

Another way to model NBMA is by using the notion of Local Address

Groups (LAGs) [10]. The essential difference between the LIS and the

LAG models is that while with the LIS model the outcome of the

"local/remote" forwarding decision is driven purely by addressing

information, with the LAG model the outcome of this decision is

decoupled from the addressing information and is coupled with the

Quality of Service and/or traffic characteristics. With the LAG

model any two entities on a common NBMA network could establish a

direct communication with each other, irrespective of the entities'

addresses.

Support for the LAG model assumes the existence of a mechanism that

allows any entity (i.e., host or router) connected to an NBMA network

to resolve an internetworking layer address to an NBMA address for

any other entity connected to the same NBMA network. This resolution

would take place regardless of the address assignments to these

entities. Within the parameters described in this document, NHRP

describes such a mechanism. For example, when the internetworking

layer address is of type IP, once the NBMA next hop has been

resolved, the source may either start sending IP packets to the

destination (in a connectionless NBMA subnetwork such as SMDS) or may

first establish a connection to the destination with the desired

bandwidth (in a connection-oriented NBMA subnetwork such as ATM).

Use of NHRP may be sufficient for hosts doing address resolution when

those hosts are directly connected to an NBMA subnetwork, allowing

for straightforward implementations in NBMA stations. NHRP also has

the capability of determining the egress point from an NBMA

subnetwork when the destination is not directly connected to the NBMA

subnetwork and the identity of the egress router is not learned by

other methods (such as routing protocols). Optional extensions to

NHRP provide additional robustness and diagnosability.

Address resolution techniques such as those described in [3] and [4]

may be in use when NHRP is deployed. ARP servers and services over

NBMA subnetworks may be required to support hosts that are not

capable of dealing with any model for communication other than the

LIS model, and deployed hosts may not implement NHRP but may continue

to support ARP variants such as those described in [3] and [4]. NHRP

is intended to reduce or eliminate the extra router hops required by

the LIS model, and can be deployed in a non-interfering manner with

existing ARP services [14].

The operation of NHRP to establish transit paths across NBMA

subnetworks between two routers requires additional mechanisms to

avoid stable routing loops, and will be described in a separate

document (see [13]).

2. Overview

2.1 Terminology

The term "network" is highly overloaded, and is especially confusing

in the context of NHRP. We use the following terms:

Internetwork layer--the media-independent layer (IP in the case of

TCP/IP networks).

Subnetwork layer--the media-dependent layer underlying the

internetwork layer, including the NBMA technology (ATM, X.25, SMDS,

etc.)

The term "server", unless eXPlicitly stated to the contrary, refers

to a Next Hop Server (NHS). An NHS is an entity performing the

Next Hop Resolution Protocol service within the NBMA cloud. An NHS

is always tightly coupled with a routing entity (router, route

server or edge device) although the converse is not yet guaranteed

until ubiquitous deployment of this functionality occurs. Note

that the presence of intermediate routers that are not coupled with

an NHS entity may preclude the use of NHRP when source and

destination stations on different sides of such routers and thus

such routers may partition NHRP reachability within an NBMA

network.

The term "client", unless explicitly stated to the contrary, refers

to a Next Hop Resolution Protocol client (NHC). An NHC is an

entity which initiates NHRP requests of various types in order to

oBTain access to the NHRP service.

The term "station" generally refers to a host or router which

contains an NHRP entity. Occasionally, the term station will

describe a "user" of the NHRP client or service functionality; the

difference in usage is largely semantic.

2.2 Protocol Overview

In this section, we briefly describe how a source S (which

potentially can be either a router or a host) uses NHRP to determine

the "NBMA next hop" to destination D.

For administrative and policy reasons, a physical NBMA subnetwork may

be partitioned into several, disjoint "Logical NBMA subnetworks". A

Logical NBMA subnetwork is defined as a collection of hosts and

routers that share unfiltered subnetwork connectivity over an NBMA

subnetwork. "Unfiltered subnetwork connectivity" refers to the

absence of closed user groups, address screening or similar features

that may be used to prevent direct communication between stations

connected to the same NBMA subnetwork. (Hereafter, unless otherwise

specified, we use the term "NBMA subnetwork" to mean *logical* NBMA

subnetwork.)

Placed within the NBMA subnetwork are one or more entities that

implement the NHRP protocol. Such stations which are capable of

answering NHRP Resolution Requests are known as "Next Hop Servers"

(NHSs). Each NHS serves a set of destination hosts, which may or may

not be directly connected to the NBMA subnetwork. NHSs cooperatively

resolve the NBMA next hop within their logical NBMA subnetwork. In

addition to NHRP, NHSs may support "classical" ARP service; however,

this will be the subject of a separate document [14].

An NHS maintains a cache which contains protocol layer address to

NBMA subnetwork layer address resolution information. This cache can

be constructed from information obtained from NHRP Register packets

(see Section 5.2.3 and 5.2.4), from NHRP Resolution Request/Reply

packets, or through mechanisms outside the scope of this document

(examples of such mechanisms might include ARP[3] and pre-configured

tables). Section 6.2 further describes cache management issues.

For a station within a given LIS to avoid providing NHS

functionality, there must be one or more NHSs within the NBMA

subnetwork which are providing authoritative address resolution

information on its behalf. Such an NHS is said to be "serving" the

station. A station on a LIS that lacks NHS functionality and is a

client of the NHRP service is known as NHRP Client or just NHCs. If

a serving NHS is to be able to supply the address resolution

information for an NHC then NHSs must exist at each hop along all

routed paths between the NHC making the resolution request and the

destination NHC. The last NHRP entity along the routed path is the

serving NHS; that is, NHRP Resolution Requests are not forwarded to

destination NHCs but rather are processed by the serving NHS.

An NHC also maintains a cache of protocol address to NBMA address

resolution information. This cache is populated through information

obtained from NHRP Resolution Reply packets, from manual

configuration, or through mechanisms outside the scope of this

document.

The protocol proceeds as follows. An event occurs triggering station

S to want to resolve the NBMA address of a path to D. This is most

likely to be when a data packet addressed to station D is to be

emitted from station S (either because station S is a host, or

station S is a transit router), but the address resolution could also

be triggered by other means (a routing protocol update packet, for

example). Station S first determines the next hop to station D

through normal routing processes (for a host, the next hop may simply

be the default router; for routers, this is the "next hop" to the

destination internetwork layer address). If the destination's

address resolution information is already available in S's cache then

that information is used to forward the packet. Otherwise, if the

next hop is reachable through one of its NBMA interfaces, S

constructs an NHRP Resolution Request packet (see Section 5.2.1)

containing station D's internetwork layer address as the (target)

destination address, S's own internetwork layer address as the source

address (Next Hop Resolution Request initiator), and station S's NBMA

addressing information. Station S may also indicate that it prefers

an authoritative NHRP Resolution Reply (i.e., station S only wishes

to receive an NHRP Resolution Reply from an NHS serving the

destination NHC). Station S emits the NHRP Resolution Request packet

towards the destination.

If the NHRP Resolution Request is triggered by a data packet then S

may, while awaiting an NHRP Resolution Reply, choose to dispose of

the data packet in one of the following ways:

(a) Drop the packet

(b) Retain the packet until the NHRP Resolution Reply arrives

and a more optimal path is available

The choice of which of the above to perform is a local policy matter,

though option (c) is the recommended default, since it may allow data

to flow to the destination while the NBMA address is being resolved.

Note that an NHRP Resolution Request for a given destination MUST NOT

be triggered on every packet.

When the NHS receives an NHRP Resolution Request, a check is made to

see if it serves station D. If the NHS does not serve D, the NHS

forwards the NHRP Resolution Request to another NHS. Mechanisms for

determining how to forward the NHRP Resolution Request are discussed

in Section 3.

If this NHS serves D, the NHS resolves station D's NBMA address

information, and generates a positive NHRP Resolution Reply on D's

behalf. NHRP Resolution Replies in this scenario are always marked

as "authoritative". The NHRP Resolution Reply packet contains the

address resolution information for station D which is to be sent back

to S. Note that if station D is not on the NBMA subnetwork, the next

hop internetwork layer address will be that of the egress router

through which packets for station D are forwarded.

A transit NHS receiving an NHRP Resolution Reply may cache the

address resolution information contained therein. To a subsequent

NHRP Resolution Request, this NHS may respond with the cached, "non-

authoritative" address resolution information if the NHS is permitted

to do so (see Sections 5.2.2 and 6.2 for more information on non-

authoritative versus authoritative NHRP Resolution Replies). Non-

authoritative NHRP Resolution Replies are distinguished from

authoritative NHRP Resolution Replies so that if a communication

attempt based on non-authoritative information fails, a source

station can choose to send an authoritative NHRP Resolution Request.

NHSs MUST NOT respond to authoritative NHRP Resolution Requests with

cached information.

If the determination is made that no NHS in the NBMA subnetwork can

reply to the NHRP Resolution Request for D then a negative NHRP

Resolution Reply (NAK) is returned. This occurs when (a) no next-hop

resolution information is available for station D from any NHS, or

(b) an NHS is unable to forward the NHRP Resolution Request (e.g.,

connectivity is lost).

NHRP Registration Requests, NHRP Purge Requests, NHRP Purge Replies,

and NHRP Error Indications follow a routed path in the same fashion

that NHRP Resolution Requests and NHRP Resolution Replies do.

Specifically, "requests" and "indications" follow the routed path

from Source Protocol Address (which is the address of the station

initiating the communication) to the Destination Protocol Address.

"Replies", on the other hand, follow the routed path from the

Destination Protocol Address back to the Source Protocol Address with

the following exceptions: in the case of a NHRP Registration Reply

and in the case of an NHC initiated NHRP Purge Request, the packet is

always returned via a direct VC (see Sections 5.2.4 and 5.2.5); if

one does not exists then one MUST be created.

NHRP Requests and NHRP Replies do NOT cross the borders of a NBMA

subnetwork however further study is being done in this area (see

Section 7). Thus, the internetwork layer data traffic out of and

into an NBMA subnetwork always traverses an internetwork layer router

at its border.

NHRP optionally provides a mechanism to send a NHRP Resolution Reply

which contains aggregated address resolution information. For

example, suppose that router X is the next hop from station S to

station D and that X is an egress router for all stations sharing an

internetwork layer address prefix with station D. When an NHRP

Resolution Reply is generated in response to a NHRP Resolution

Request, the responder may augment the internetwork layer address of

station D with a prefix length (see Section 5.2.0.1). A subsequent

(non-authoritative) NHRP Resolution Request for some destination that

shares an internetwork layer address prefix (for the number of bits

specified in the prefix length) with D may be satisfied with this

cached information. See section 6.2 regarding caching issues.

To dynamically detect subnetwork-layer filtering in NBMA subnetworks

(e.g., X.25 closed user group facility, or SMDS address screens), to

trace the routed path that an NHRP packet takes, or to provide loop

detection and diagnostic capabilities, a "Route Record" may be

included in NHRP packets (see Sections 5.3.2 and 5.3.3). The Route

Record extensions are the NHRP Forward Transit NHS Record Extension

and the NHRP Reverse Transit NHS Record Extension. They contain the

internetwork (and subnetwork layer) addresses of all intermediate

NHSs between source and destination and between destination and

source respectively. When a source station is unable to communicate

with the responder (e.g., an attempt to open an SVC fails), it may

attempt to do so successively with other subnetwork layer addresses

in the NHRP Forward Transit NHS Record Extension until it succeeds

(if authentication policy permits such action). This approach can

find a suitable egress point in the presence of subnetwork-layer

filtering (which may be source/destination sensitive, for instance,

without necessarily creating separate logical NBMA subnetworks) or

subnetwork-layer congestion (especially in connection-oriented

media).

3. Deployment

NHRP Resolution Requests traverse one or more hops within an NBMA

subnetwork before reaching the station that is expected to generate a

response. Each station, including the source station, chooses a

neighboring NHS to which it will forward the NHRP Resolution Request.

The NHS selection procedure typically involves applying a destination

protocol layer address to the protocol layer routing table which

causes a routing decision to be returned. This routing decision is

then used to forward the NHRP Resolution Request to the downstream

NHS. The destination protocol layer address previously mentioned is

carried within the NHRP Resolution Request packet. Note that even

though a protocol layer address was used to acquire a routing

decision, NHRP packets are not encapsulated within a protocol layer

header but rather are carried at the NBMA layer using the

encapsulation described in Section 5.

Each NHS/router examines the NHRP Resolution Request packet on its

way toward the destination. Each NHS which the NHRP packet traverses

on the way to the packet's destination might modify the packet (e.g.,

updating the Forward Record extension). Ignoring error situations,

the NHRP Resolution Request eventually arrives at a station that is

to generate an NHRP Resolution Reply. This responding station

"serves" the destination. The responding station generates an NHRP

Resolution Reply using the source protocol address from within the

NHRP packet to determine where the NHRP Resolution Reply should be

sent.

Rather than use routing to determine the next hop for an NHRP packet,

an NHS may use other applicable means (such as static configuration

information ) in order to determine to which neighboring NHSs to

forward the NHRP Resolution Request packet as long as such other

means would not cause the NHRP packet to arrive at an NHS which is

not along the routed path. The use of static configuration

information for this purpose is beyond the scope of this document.

The NHS serving a particular destination must lie along the routed

path to that destination. In practice, this means that all egress

routers must double as NHSs serving the destinations beyond them, and

that hosts on the NBMA subnetwork are served by routers that double

as NHSs. Also, this implies that forwarding of NHRP packets within

an NBMA subnetwork requires a contiguous deployment of NHRP capable

routers. It is important that, in a given LIS/LAG which is using

NHRP, all NHSs within the LIS/LAG have at least some portion of their

resolution databases synchronized so that a packet arriving at one

router/NHS in a given LIS/LAG will be forwarded in the same fashion

as a packet arriving at a different router/NHS for the given LIS/LAG.

One method, among others, is to use the Server Cache Synchronization

Protocol (SCSP) [12]. It is RECOMMENDED that SCSP be the method used

when a LIS/LAG contains two or more router/NHSs.

During migration to NHRP, it cannot be expected that all routers

within the NBMA subnetwork are NHRP capable. Thus, NHRP traffic

which would otherwise need to be forwarded through such routers can

be expected to be dropped due to the NHRP packet not being

recognized. In this case, NHRP will be unable to establish any

transit paths whose discovery requires the traversal of the non-NHRP

speaking routers. If the client has tried and failed to acquire a

cut through path then the client should use the network layer routed

path as a default.

If an NBMA technology offers a group, an anycast, or a multicast

addressing feature then the NHC may be configured with such an

address (appropriate to the routing realm it participates in) which

would be assigned to all NHS serving that routing realm. This

address can then be used for establishing an initial connection to an

NHS to transmit a registration request. This address may not be used

for sending NHRP requests. The resulting VC may be used for NHRP

requests if and only if the registration response is received over

that VC, thereby indicating that one happens to have anycast

connected to an NHS serving the LIS/LAG. In the case of non-

connection oriented networks, or of multicast (rather than anycast)

addresses, the addres MUST NOT be used for sending NHRP resolution

requests.

When an NHS "serves" an NHC, the NHS MUST send NHRP messages destined

for the NHC directly to the NHC. That is, the NHRP message MUST NOT

transit through any NHS which is not serving the NHC when the NHRP

message is currently at an NHS which does serve the NHC (this, of

course, assumes the NHRP message is destined for the NHC). Further,

an NHS which serves an NHC SHOULD have a direct NBMA level connection

to that NHC (see Section 5.2.3 and 5.2.4 for examples).

With the exception of NHRP Registration Requests (see Section 5.2.3

and 5.2.4 for details of the NHRP Registration Request case), an NHC

MUST send NHRP messages over a direct NBMA level connection between

the serving NHS and the served NHC.

It may not be desirable to maintain semi-permanent NBMA level

connectivity between the NHC and the NHS. In this case, when NBMA

level connectivity is initially setup between the NHS and the NHC (as

described in Section 5.2.4), the NBMA address of the NHS should be

obtained through the NBMA level signaling technology. This address

should be stored for future use in setting up subsequent NBMA level

connections. A somewhat more information rich technique to obtain

the address information (and more) of the serving NHS would be for

the NHC to include the Responder Address extension (see Section

5.3.1) in the NHRP Registration Request and to store the information

returned to the NHC in the Responder Address extension which is

subsequently included in the NHRP Registration Reply. Note also

that, in practice, a client's default router should also be its NHS;

thus a client may be able to know the NBMA address of its NHS from

the configuration which was already required for the client to be

able to communicate. Further, as mentioned in Section 4, NHCs may be

configured with the addressing information of one or more NHSs.

4. Configuration

Next Hop Clients

An NHC connected to an NBMA subnetwork MAY be configured with the

Protocol address(es) and NBMA address(es) of its NHS(s). The

NHS(s) will likely also represent the NHC's default or peer

routers, so their NBMA addresses may be obtained from the NHC's

existing configuration. If the NHC is attached to several

subnetworks (including logical NBMA subnetworks), the NHC should

also be configured to receive routing information from its NHS(s)

and peer routers so that it can determine which internetwork layer

networks are reachable through which subnetworks.

Next Hop Servers

An NHS is configured with knowledge of its own internetwork layer

and NBMA addresses. An NHS MAY also be configured with a set of

internetwork layer address prefixes that correspond to the

internetwork layer addresses of the stations it serves. The NBMA

addresses of the stations served by the NHS may be learned via NHRP

Registration packets.

If a served NHC is attached to several subnetworks, the

router/route-server coresident with the serving NHS may also need

to be configured to advertise routing information to such NHCs.

If an NHS acts as an egress router for stations connected to other

subnetworks than the NBMA subnetwork, the NHS must, in addition to

the above, be configured to exchange routing information between

the NBMA subnetwork and these other subnetworks.

In all cases, routing information is exchanged using conventional

intra-domain and/or inter-domain routing protocols.

5. NHRP Packet Formats

This section describes the format of NHRP packets. In the following,

unless otherwise stated explicitly, the unqualified term "request"

refers generically to any of the NHRP packet types which are

"requests". Further, unless otherwise stated explicitly, the

unqualified term "reply" refers generically to any of the NHRP packet

types which are "replies".

An NHRP packet consists of a Fixed Part, a Mandatory Part, and an

Extensions Part. The Fixed Part is common to all NHRP packet types.

The Mandatory Part MUST be present, but varies depending on packet

type. The Extensions Part also varies depending on packet type, and

need not be present.

The length of the Fixed Part is fixed at 20 octets. The length of

the Mandatory Part is determined by the contents of the extensions

offset field (ar$extoff). If ar$extoff=0x0 then the mandatory part

length is equal to total packet length (ar$pktsz) minus 20 otherwise

the mandatory part length is equal to ar$extoff minus 20. The length

of the Extensions Part is implied by ar$pktsz minus ar$extoff. NHSs

may increase the size of an NHRP packet as a result of extension

processing, but not beyond the offered maximum packet size of the

NBMA network.

NHRP packets are actually members of a wider class of address mapping

and management protocols being developed by the IETF. A specific

encapsulation, based on the native formats used on the particular

NBMA network over which NHRP is carried, indicates the generic IETF

mapping and management protocol. For example, SMDS networks always

use LLC/SNAP encapsulation at the NBMA layer [4], and an NHRP packet

is preceded by the following LLC/SNAP encapsulation:

[0xAA-AA-03] [0x00-00-5E] [0x00-03]

The first three octets are LLC, indicating that SNAP follows. The

SNAP OUI portion is the IANA's OUI, and the SNAP PID portion

identifies the mapping and management protocol. A field in the Fixed

Header following the encapsulation indicates that it is NHRP.

ATM uses either LLC/SNAP encapsulation of each packet (including

NHRP), or uses no encapsulation on VCs dedicated to a single protocol

(see [7]). Frame Relay and X.25 both use NLPID/SNAP encapsulation or

identification of NHRP, using a NLPID of 0x0080 and the same SNAP

contents as above (see [8], [9]).

Fields marked "unused" MUST be set to zero on transmission, and

ignored on receipt.

Most packet types (ar$op.type) have both internetwork layer

protocol-independent fields and protocol-specific fields. The

protocol type/snap fields (ar$pro.type/snap) qualify the format of

the protocol-specific fields.

5.1 NHRP Fixed Header

The Fixed Part of the NHRP packet contains those elements of the NHRP

packet which are always present and do not vary in size with the type

of packet.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$afn ar$pro.type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$pro.snap

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$pro.snap ar$hopcnt ar$pktsz

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$chksum ar$extoff

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$op.version ar$op.type ar$shtl ar$sstl

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

ar$afn

Defines the type of "link layer" addresses being carried. This

number is taken from the 'address family number' list specified in

[6]. This field has implications to the coding of ar$shtl and

ar$sstl as described below.

ar$pro.type

field is a 16 bit unsigned integer representing the following

number space:

0x0000 to 0x00FF Protocols defined by the equivalent NLPIDs.

0x0100 to 0x03FF Reserved for future use by the IETF.

0x0400 to 0x04FF Allocated for use by the ATM Forum.

0x0500 to 0x05FF Experimental/Local use.

0x0600 to 0xFFFF Protocols defined by the equivalent Ethertypes.

(based on the observations that valid Ethertypes are never smaller

than 0x600, and NLPIDs never larger than 0xFF.)

ar$pro.snap

When ar$pro.type has a value of 0x0080, a SNAP encoded extension is

being used to encode the protocol type. This snap extension is

placed in the ar$pro.snap field. This is termed the 'long form'

protocol ID. If ar$pro != 0x0080 then the ar$pro.snap field MUST be

zero on transmit and ignored on receive. The ar$pro.type field

itself identifies the protocol being referred to. This is termed

the 'short form' protocol ID.

In all cases, where a protocol has an assigned number in the

ar$pro.type space (excluding 0x0080) the short form MUST be used

when transmitting NHRP messages; i.e., if Ethertype or NLPID

codings exist then they are used on transmit rather than the

ethertype. If both Ethertype and NLPID codings exist then when

transmitting NHRP messages, the Ethertype coding MUST be used (this

is consistent with RFC1483 coding). So, for example, the

following codings exist for IP:

SNAP: ar$pro.type = 0x00-80, ar$pro.snap = 0x00-00-00-08-00

NLPID: ar$pro.type = 0x00-CC, ar$pro.snap = 0x00-00-00-00-00

Ethertype: ar$pro.type = 0x08-00, ar$pro.snap = 0x00-00-00-00-00

and thus, since the Ethertype coding exists, it is used in

preference.

ar$hopcnt

The Hop count indicates the maximum number of NHSs that an NHRP

packet is allowed to traverse before being discarded. This field

is used in a similar fashion to the way that a TTL is used in an IP

packet and should be set accordingly. Each NHS decrements the TTL

as the NHRP packet transits the NHS on the way to the next hop

along the routed path to the destination. If an NHS receives an

NHRP packet which it would normally forward to a next hop and that

packet contains an ar$hopcnt set to zero then the NHS sends an

error indication message back to the source protocol address

stating that the hop count has been exceeded (see Section 5.2.7)

and the NHS drops the packet in error; however, an error

indication is never sent as a result of receiving an error

indication. When a responding NHS replies to an NHRP request, that

NHS places a value in ar$hopcnt as if it were sending a request of

its own.

ar$pktsz

The total length of the NHRP packet, in octets (excluding link

layer encapsulation).

ar$chksum

The standard IP checksum over the entire NHRP packet starting at

the fixed header. If the packet is an odd number of bytes in

length then this calculation is performed as if a byte set to 0x00

is appended to the end of the packet.

ar$extoff

This field identifies the existence and location of NHRP

extensions. If this field is 0 then no extensions exist otherwise

this field represents the offset from the beginning of the NHRP

packet (i.e., starting from the ar$afn field) of the first

extension.

ar$op.version

This field indicates what version of generic address mapping and

management protocol is represented by this message.

0 MARS protocol [11].

1 NHRP as defined in this document.

0x02 - 0xEF Reserved for future use by the IETF.

0xF0 - 0xFE Allocated for use by the ATM Forum.

0xFF Experimental/Local use.

ar$op.type

When ar$op.version == 1, this is the NHRP packet type: NHRP

Resolution Request(1), NHRP Resolution Reply(2), NHRP Registration

Request(3), NHRP Registration Reply(4), NHRP Purge Request(5), NHRP

Purge Reply(6), or NHRP Error Indication(7). Use of NHRP packet

Types in the range 128 to 255 are reserved for research or use in

other protocol development and will be administered by IANA as

described in Section 9.

ar$shtl

Type & length of source NBMA address interpreted in the context of

the 'address family number'[6] indicated by ar$afn. See below for

more details.

ar$sstl

Type & length of source NBMA subaddress interpreted in the context

of the 'address family number'[6] indicated by ar$afn. When an

NBMA technology has no concept of a subaddress, the subaddress

length is always coded ar$sstl = 0 and no storage is allocated for

the subaddress in the appropriate mandatory part. See below for

more details.

Subnetwork layer address type/length fields (e.g., ar$shtl, Cli Addr

T/L) and subnetwork layer subaddresses type/length fields (e.g.,

ar$sstl, Cli SAddr T/L) are coded as follows:

7 6 5 4 3 2 1 0

+-+-+-+-+-+-+-+-+

0x length

+-+-+-+-+-+-+-+-+

The most significant bit is reserved and MUST be set to zero. The

second most significant bit (x) is a flag indicating whether the

address being referred to is in:

- NSAP format (x = 0).

- Native E.164 format (x = 1).

For NBMA technologies that use neither NSAP nor E.164 format

addresses, x = 0 SHALL be used to indicate the native form for the

particular NBMA technology.

If the NBMA network is ATM and a subaddress (e.g., Source NBMA

SubAddress, Client NBMA SubAddress) is to be included in any part of

the NHRP packet then ar$afn MUST be set to 0x000F; further, the

subnetwork layer address type/length fields (e.g., ar$shtl, Cli Addr

T/L) and subnetwork layer subaddress type/length fields (e.g.,

ar$sstl, Cli SAddr T/L) MUST be coded as in [11]. If the NBMA

network is ATM and no subaddress field is to be included in any part

of the NHRP packet then ar$afn MAY be set to 0x0003 (NSAP) or 0x0008

(E.164) accordingly.

The bottom 6 bits is an unsigned integer value indicating the length

of the associated NBMA address in octets. If this value is zero the

flag x is ignored.

5.2.0 Mandatory Part

The Mandatory Part of the NHRP packet contains the operation specific

information (e.g., NHRP Resolution Request/Reply, etc.) and variable

length data which is pertinent to the packet type.

5.2.0.1 Mandatory Part Format

Sections 5.2.1 through 5.2.6 have a very similar mandatory part.

This mandatory part includes a common header and zero or more Client

Information Entries (CIEs). Section 5.2.7 has a different format

which is specified in that section.

The common header looks like the following:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Src Proto Len Dst Proto Len Flags

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Request ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source NBMA Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source NBMA Subaddress (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

And the CIEs have the following format:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code Prefix Length unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Maximum Transmission Unit Holding Time

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Cli Addr T/L Cli SAddr T/L Cli Proto Len Preference

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client NBMA Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client NBMA Subaddress (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

.....................

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Code Prefix Length unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Maximum Transmission Unit Holding Time

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Cli Addr T/L Cli SAddr T/L Cli Proto Len Preference

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client NBMA Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client NBMA Subaddress (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Client Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The meanings of the fields are as follows:

Src Proto Len

This field holds the length in octets of the Source Protocol

Address.

Dst Proto Len

This field holds the length in octets of the Destination Protocol

Address.

Flags

These flags are specific to the given message type and they are

explained in each section.

Request ID

A value which, when coupled with the address of the source,

provides a unique identifier for the information contained in a

"request" packet. This value is copied directly from an "request"

packet into the associated "reply". When a sender of a "request"

receives "reply", it will compare the Request ID and source address

information in the received "reply" against that found in its

outstanding "request" list. When a match is found then the

"request" is considered to be acknowledged.

The value is taken from a 32 bit counter that is incremented each

time a new "request" is transmitted. The same value MUST be used

when resending a "request", i.e., when a "reply" has not been

received for a "request" and a retry is sent after an appropriate

interval.

It is RECOMMENDED that the initial value for this number be 0. A

node MAY reuse a sequence number if and only if the reuse of the

sequence number is not precluded by use of a particular method of

synchronization (e.g., as described in Appendix A).

The NBMA address/subaddress form specified below allows combined

E.164/NSAPA form of NBMA addressing. For NBMA technologies without a

subaddress concept, the subaddress field is always ZERO length and

ar$sstl = 0.

Source NBMA Address

The Source NBMA address field is the address of the source station

which is sending the "request". If the field's length as specified

in ar$shtl is 0 then no storage is allocated for this address at

all.

Source NBMA SubAddress

The Source NBMA subaddress field is the address of the source

station which is sending the "request". If the field's length as

specified in ar$sstl is 0 then no storage is allocated for this

address at all.

For those NBMA technologies which have a notion of "Calling Party

Addresses", the Source NBMA Addresses above are the addresses used

when signaling for an SVC.

"Requests" and "indications" follow the routed path from Source

Protocol Address to the Destination Protocol Address. "Replies", on

the other hand, follow the routed path from the Destination Protocol

Address back to the Source Protocol Address with the following

exceptions: in the case of a NHRP Registration Reply and in the case

of an NHC initiated NHRP Purge Request, the packet is always returned

via a direct VC (see Sections 5.2.4 and 5.2.5).

Source Protocol Address

This is the protocol address of the station which is sending the

"request". This is also the protocol address of the station toward

which a "reply" packet is sent.

Destination Protocol Address

This is the protocol address of the station toward which a

"request" packet is sent.

Code

This field is message specific. See the relevant message sections

below. In general, this field is a NAK code; i.e., when the field

is 0 in a reply then the packet is acknowledging a request and if

it contains any other value the packet contains a negative

acknowledgment.

Prefix Length

This field is message specific. See the relevant message sections

below. In general, however, this fields is used to indicate that

the information carried in an NHRP message pertains to an

equivalence class of internetwork layer addresses rather than just

a single internetwork layer address specified. All internetwork

layer addresses that match the first "Prefix Length" bit positions

for the specific internetwork layer address are included in the

equivalence class. If this field is set to 0x00 then this field

MUST be ignored and no equivalence information is assumed (note

that 0x00 is thus equivalent to 0xFF).

Maximum Transmission Unit

This field gives the maximum transmission unit for the relevant

client station. If this value is 0 then either the default MTU is

used or the MTU negotiated via signaling is used if such

negotiation is possible for the given NBMA.

Holding Time

The Holding Time field specifies the number of seconds for which

the Next Hop NBMA information specified in the CIE is considered to

be valid. Cached information SHALL be discarded when the holding

time expires. This field must be set to 0 on a NAK.

Cli Addr T/L

Type & length of next hop NBMA address specified in the CIE. This

field is interpreted in the context of the 'address family

number'[6] indicated by ar$afn (e.g., ar$afn=0x0003 for ATM).

Cli SAddr T/L

Type & length of next hop NBMA subaddress specified in the CIE.

This field is interpreted in the context of the 'address family

number'[6] indicated by ar$afn (e.g., ar$afn=0x0015 for ATM makes

the address an E.164 and the subaddress an ATM Forum NSAP address).

When an NBMA technology has no concept of a subaddress, the

subaddress is always null with a length of 0. When the address

length is specified as 0 no storage is allocated for the address.

Cli Proto Len

This field holds the length in octets of the Client Protocol

Address specified in the CIE.

Preference

This field specifies the preference for use of the specific CIE

relative to other CIEs. Higher values indicate higher preference.

Action taken when multiple CIEs have equal or highest preference

value is a local matter.

Client NBMA Address

This is the client's NBMA address.

Client NBMA SubAddress

This is the client's NBMA subaddress.

Client Protocol Address

This is the client's internetworking layer address specified.

Note that an NHS may cache source address binding information from an

NHRP Resolution Request if and only if the conditions described in

Section 6.2 are met for the NHS. In all other cases, source address

binding information appearing in an NHRP message MUST NOT be cached.

5.2.1 NHRP Resolution Request

The NHRP Resolution Request packet has a Type code of 1. Its

mandatory part is coded as described in Section 5.2.0.1 and the

message specific meanings of the fields are as follows:

Flags - The flags field is coded as follows:

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

QADUS unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Set if the station sending the NHRP Resolution Request is a

router; clear if the it is a host.

This bit is set in a NHRP Resolution Request if only

authoritative next hop information is desired and is clear

otherwise. See the NHRP Resolution Reply section below for

further details on the "A" bit and its usage.

Unused (clear on transmit)

This is the Uniqueness bit. This bit aids in duplicate address

detection. When this bit is set in an NHRP Resolution Request

and one or more entries exist in the NHS cache which meet the

requirements of the NHRP Resolution Request then only the CIE in

the NHS's cache with this bit set will be returned. Note that

even if this bit was set at registration time, there may still be

multiple CIEs that might fulfill the NHRP Resolution Request

because an entire subnet can be registered through use of the

Prefix Length in the CIE and the address of interest might be

within such a subnet. If the "uniqueness" bit is set and the

responding NHS has one or more cache entries which match the

request but no such cache entry has the "uniqueness" bit set,

then the NHRP Resolution Reply returns with a NAK code of "13 -

Binding Exists But Is Not Unique" and no CIE is included. If a

client wishes to receive non- unique Next Hop Entries, then

the client must have the "uniqueness" bit set to zero in its NHRP

Resolution Request. Note that when this bit is set in an NHRP

Registration Request, only a single CIE may be specified in the

NHRP Registration Request and that CIE must have the Prefix

Length field set to 0xFF.

Set if the binding between the Source Protocol Address and the

Source NBMA information in the NHRP Resolution Request is

guaranteed to be stable and accurate (e.g., these addresses are

those of an ingress router which is connected to an ethernet stub

network or the NHC is an NBMA attached host).

Zero or one CIEs (see Section 5.2.0.1) may be specified in an NHRP

Resolution Request. If one is specified then that entry carries the

pertinent information for the client sourcing the NHRP Resolution

Request. Usage of the CIE in the NHRP Resolution Request is

described below:

Prefix Length

If a CIE is specified in the NHRP Resolution Request then the

Prefix Length field may be used to qualify the widest acceptable

prefix which may be used to satisfy the NHRP Resolution Request.

In the case of NHRP Resolution Request/Reply, the Prefix Length

specifies the equivalence class of addresses which match the

first "Prefix Length" bit positions of the Destination Protocol

Address. If the "U" bit is set in the common header then this

field MUST be set to 0xFF.

Maximum Transmission Unit

This field gives the maximum transmission unit for the source

station. A possible use of this field in the NHRP Resolution

Request packet is for the NHRP Resolution Requester to ask for a

target MTU.

Holding Time

The Holding Time specified in the one CIE permitted to be

included in an NHRP Resolution Request is the amount of time

which the source address binding information in the NHRP

Resolution Request is permitted to cached by transit and

responding NHSs. Note that this field may only have a non-zero

value if the S bit is set.

All other fields in the CIE MUST be ignored and SHOULD be set to 0.

The Destination Protocol Address in the common header of the

Mandatory Part of this message contains the protocol address of the

station for which resolution is desired. An NHC MUST send the NHRP

Resolution Request directly to one of its serving NHSs (see Section 3

for more information).

5.2.2 NHRP Resolution Reply

The NHRP Resolution Reply packet has a Type code of 2. CIEs

correspond to Next Hop Entries in an NHS's cache which match the

criteria in the NHRP Resolution Request. Its mandatory part is coded

as described in Section 5.2.0.1. The message specific meanings of

the fields are as follows:

Flags - The flags field is coded as follows:

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

QADUS unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Copied from the NHRP Resolution Request. Set if the NHRP

Resolution Requester is a router; clear if it is a host.

Set if the next hop CIE in the NHRP Resolution Reply is

authoritative; clear if the NHRP Resolution Reply is non-

authoritative.

When an NHS receives a NHRP Resolution Request for authoritative

information for which it is the authoritative source, it MUST

respond with a NHRP Resolution Reply containing all and only

those next hop CIEs which are contained in the NHS's cache which

both match the criteria of the NHRP Resolution Request and are

authoritative cache entries. An NHS is an authoritative source

for a NHRP Resolution Request if the information in the NHS's

cache matches the NHRP Resolution Request criteria and that

information was obtained through a NHRP Registration Request or

through synchronization with an NHS which obtained this

information through a NHRP Registration Request. An

authoritative cache entry is one which is obtained through a NHRP

Registration Request or through synchronization with an NHS which

obtained this information through a NHRP Registration Request.

An NHS obtains non-authoritative CIEs through promiscuous

listening to NHRP packets other than NHRP Registrations which are

directed at it. A NHRP Resolution Request which indicates a

request for non-authoritative information should cause a NHRP

Resolution Reply which contains all entries in the replying NHS's

cache (i.e., both authoritative and non-authoritative) which

match the criteria specified in the request.

Set if the association between destination and the associate next

hop information included in all CIEs of the NHRP Resolution Reply

is guaranteed to be stable for the lifetime of the information

(the holding time). This is the case if the Next Hop protocol

address in a CIE identifies the destination (though it may be

different in value than the Destination address if the

destination system has multiple addresses) or if the destination

is not connected directly to the NBMA subnetwork but the egress

router to that destination is guaranteed to be stable (such as

when the destination is immediately adjacent to the egress router

through a non-NBMA interface).

This is the Uniqueness bit. See the NHRP Resolution Request

section above for details. When this bit is set, only one CIE is

included since only one unique binding should exist in an NHS's

cache.

Copied from NHRP Resolution Request message.

One or more CIEs are specified in the NHRP Resolution Reply. Each CIE

contains NHRP next hop information which the responding NHS has

cached and which matches the parameters specified in the NHRP

Resolution Request. If no match is found by the NHS issuing the NHRP

Resolution Reply then a single CIE is enclosed with the a CIE Code

set appropriately (see below) and all other fields MUST be ignored

and SHOULD be set to 0. In order to facilitate the use of NHRP by

minimal client implementations, the first CIE MUST contain the next

hop with the highest preference value so that such an implementation

need parse only a single CIE.

Code

If this field is set to zero then this packet contains a

positively acknowledged NHRP Resolution Reply. If this field

contains any other value then this message contains an NHRP

Resolution Reply NAK which means that an appropriate

internetworking layer to NBMA address binding was not available

in the responding NHS's cache. If NHRP Resolution Reply contains

a Client Information Entry with a NAK Code other than 0 then it

MUST NOT contain any other CIE. Currently defined NAK Codes are

as follows:

4 - Administratively Prohibited

An NHS may refuse an NHRP Resolution Request attempt for

administrative reasons (due to policy constraints or routing

state). If so, the NHS MUST send an NHRP Resolution Reply

which contains a NAK code of 4.

5 - Insufficient Resources

If an NHS cannot serve a station due to a lack of resources

(e.g., can't store sufficient information to send a purge if

routing changes), the NHS MUST reply with a NAKed NHRP

Resolution Reply which contains a NAK code of 5.

12 - No Internetworking Layer Address to NBMA Address Binding

Exists

This code states that there were absolutely no internetworking

layer address to NBMA address bindings found in the responding

NHS's cache.

13 - Binding Exists But Is Not Unique

This code states that there were one or more internetworking

layer address to NBMA address bindings found in the responding

NHS's cache, however none of them had the uniqueness bit set.

Prefix Length

In the case of NHRP Resolution Reply, the Prefix Length specifies

the equivalence class of addresses which match the first "Prefix

Length" bit positions of the Destination Protocol Address.

Holding Time

The Holding Time specified in a CIE of an NHRP Resolution Reply

is the amount of time remaining before the expiration of the

client information which is cached at the replying NHS. It is

not the value which was registered by the client.

The remainder of the fields for the CIE for each next hop are

filled out as they were defined when the next hop was registered

with the responding NHS (or one of the responding NHS's

synchronized servers) via the NHRP Registration Request.

Load-splitting may be performed when more than one Client Information

Entry is returned to a requester when equal preference values are

specified. Also, the alternative addresses may be used in case of

connectivity failure in the NBMA subnetwork (such as a failed call

attempt in connection-oriented NBMA subnetworks).

Any extensions present in the NHRP Resolution Request packet MUST be

present in the NHRP Resolution Reply even if the extension is non-

Compulsory.

If an unsolicited NHRP Resolution Reply packet is received, an Error

Indication of type Invalid NHRP Resolution Reply Received SHOULD be

sent in response.

When an NHS that serves a given NHC receives an NHRP Resolution Reply

destined for that NHC then the NHS must MUST send the NHRP Resolution

Reply directly to the NHC (see Section 3).

5.2.3 NHRP Registration Request

The NHRP Registration Request is sent from a station to an NHS to

notify the NHS of the station's NBMA information. It has a Type code

of 3. Each CIE corresponds to Next Hop information which is to be

cached at an NHS. The mandatory part of an NHRP Registration Request

is coded as described in Section 5.2.0.1. The message specific

meanings of the fields are as follows:

Flags - The flags field is coded as follows:

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

This is the Uniqueness bit. When set in an NHRP Registration

Request, this bit indicates that the registration of the protocol

address is unique within the confines of the set of synchronized

NHSs. This "uniqueness" qualifier MUST be stored in the NHS/NHC

cache. Any attempt to register a binding between the protocol

address and an NBMA address when this bit is set MUST be rejected

with a Code of "14 - Unique Internetworking Layer Address Already

Registered" if the replying NHS already has a cache entry for the

protocol address and the cache entry has the "uniqueness" bit

set. A registration of a CIE's information is rejected when the

CIE is returned with the Code field set to anything other than

0x00. See the description of the uniqueness bit in NHRP

Resolution Request section above for further details. When this

bit is set only, only one CIE MAY be included in the NHRP

Registration Request.

Request ID

The request ID has the same meaning as described in Section

5.2.0.1. However, the request ID for NHRP Registrations which is

maintained at each client MUST be kept in non-volatile memory so

that when a client crashes and reregisters there will be no

inconsistency in the NHS's database. In order to reduce the

overhead associated with updating non-volatile memory, the actual

updating need not be done with every increment of the Request ID

but could be done, for example, every 50 or 100 increments. In

this scenario, when a client crashes and reregisters it knows to

add 100 to the value of the Request ID in the non-volatile memory

before using the Request ID for subsequent registrations.

One or more CIEs are specified in the NHRP Registration Request.

Each CIE contains next hop information which a client is attempting

to register with its servers. Generally, all fields in CIEs enclosed

in NHRP Registration Requests are coded as described in Section

5.2.0.1. However, if a station is only registering itself with the

NHRP Registration Request then it MAY code the Cli Addr T/L, Cli

SAddr T/L, and Cli Proto Len as zero which signifies that the client

address information is to be taken from the source information in the

common header (see Section 5.2.0.1). Below, further clarification is

given for some fields in a CIE in the context of a NHRP Registration

Request.

Code

This field is set to 0x00 in NHRP Registration Requests.

Prefix Length

This field may be used in a NHRP Registration Request to register

equivalence information for the Client Protocol Address specified

in the CIE of an NHRP Registration Request In the case of NHRP

Registration Request, the Prefix Length specifies the equivalence

class of addresses which match the first "Prefix Length" bit

positions of the Client Protocol Address. If the "U" bit is set

in the common header then this field MUST be set to 0xFF.

The NHRP Registration Request is used to register an NHC's NHRP

information with its NHSs. If an NHC is configured with the protocol

address of a serving NHS then the NHC may place the NHS's protocol

address in the Destination Protocol Address field of the NHRP

Registration Request common header otherwise the NHC must place its

own protocol address in the Destination Protocol Address field.

When an NHS receives an NHRP Registration Request which has the

Destination Protocol Address field set to an address which belongs to

a LIS/LAG for which the NHS is serving then if the Destination

Protocol Address field is equal to the Source Protocol Address field

(which would happen if the NHC put its protocol address in the

Destination Protocol Address) or the Destination Protocol Address

field is equal to the protocol address of the NHS then the NHS

processes the NHRP Registration Request after doing appropriate error

checking (including any applicable policy checking).

When an NHS receives an NHRP Registration Request which has the

Destination Protocol Address field set to an address which does not

belong to a LIS/LAG for which the NHS is serving then the NHS

forwards the packet down the routed path toward the appropriate

LIS/LAG.

When an NHS receives an NHRP Registration Request which has the

Destination Protocol Address field set to an address which belongs to

a LIS/LAG for which the NHS is serving then if the Destination

Protocol Address field does not equal the Source Protocol Address

field and the Destination Protocol Address field does not equal the

protocol address of the NHS then the NHS forwards the message to the

appropriate NHS within the LIS/LAG as specified by Destination

Protocol Address field.

It is possible that a misconfigured station will attempt to register

with the wrong NHS (i.e., one that cannot serve it due to policy

constraints or routing state). If this is the case, the NHS MUST

reply with a NAK-ed Registration Reply of type Can't Serve This

Address.

If an NHS cannot serve a station due to a lack of resources, the NHS

MUST reply with a NAK-ed Registration Reply of type Registration

Overflow.

In order to keep the registration entry from being discarded, the

station MUST re-send the NHRP Registration Request packet often

enough to refresh the registration, even in the face of occasional

packet loss. It is recommended that the NHRP Registration Request

packet be sent at an interval equal to one-third of the Holding Time

specified therein.

5.2.4 NHRP Registration Reply

The NHRP Registration Reply is sent by an NHS to a client in response

to that client's NHRP Registration Request. If the Code field of a

CIE in the NHRP Registration Reply has anything other than zero in it

then the NHRP Registration Reply is a NAK otherwise the reply is an

ACK. The NHRP Registration Reply has a Type code of 4.

An NHRP Registration Reply is formed from an NHRP Registration

Request by changing the type code to 4, updating the CIE Code field,

and filling in the appropriate extensions if they exist. The message

specific meanings of the fields are as follows:

Attempts to register the information in the CIEs of an NHRP

Registration Request may fail for various reasons. If this is the

case then each failed attempt to register the information in a CIE of

an NHRP Registration Request is logged in the associated NHRP

Registration Reply by setting the CIE Code field to the appropriate

error code as shown below:

CIE Code

0 - Successful Registration

The information in the CIE was successfully registered with the

NHS.

4 - Administratively Prohibited

An NHS may refuse an NHRP Registration Request attempt for

administrative reasons (due to policy constraints or routing

state). If so, the NHS MUST send an NHRP Registration Reply

which contains a NAK code of 4.

5 - Insufficient Resources

If an NHS cannot serve a station due to a lack of resources,

the NHS MUST reply with a NAKed NHRP Registration Reply which

contains a NAK code of 5.

14 - Unique Internetworking Layer Address Already Registered

If a client tries to register a protocol address to NBMA

address binding with the uniqueness bit on and the protocol

address already exists in the NHS's cache then if that cache

entry also has the uniqueness bit on then this NAK Code is

returned in the CIE in the NHRP Registration Reply.

Due to the possible existence of asymmetric routing, an NHRP

Registration Reply may not be able to merely follow the routed path

back to the source protocol address specified in the common header of

the NHRP Registration Reply. As a result, there MUST exist a direct

NBMA level connection between the NHC and its NHS on which to send

the NHRP Registration Reply before NHRP Registration Reply may be

returned to the NHC. If such a connection does not exist then the

NHS must setup such a connection to the NHC by using the source NBMA

information supplied in the common header of the NHRP Registration

Request.

5.2.5 NHRP Purge Request

The NHRP Purge Request packet is sent in order to invalidate cached

information in a station. The NHRP Purge Request packet has a type

code of 5. The mandatory part of an NHRP Purge Request is coded as

described in Section 5.2.0.1. The message specific meanings of the

fields are as follows:

Flags - The flags field is coded as follows:

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

N unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

When set, this bit tells the receiver of the NHRP Purge Request

that the requester does not expect to receive an NHRP Purge

Reply. If an unsolicited NHRP Purge Reply is received by a

station where that station is identified in the Source Protocol

Address of the packet then that packet must be ignored.

One or more CIEs are specified in the NHRP Purge Request. Each CIE

contains next hop information which is to be purged from an NHS/NHC

cache. Generally, all fields in CIEs enclosed in NHRP Purge Requests

are coded as described in Section 5.2.0.1. Below, further

clarification is given for some fields in a CIE in the context of a

NHRP Purge Request.

Code

This field is set to 0x00 in NHRP Purge Requests.

Prefix Length

In the case of NHRP Purge Requests, the Prefix Length specifies

the equivalence class of addresses which match the first "Prefix

Length" bit positions of the Client Protocol Address specified in

the CIE. All next hop information which contains a protocol

address which matches an element of this equivalence class is to

be purged from the receivers cache.

The Maximum Transmission Unit and Preference fields of the CIE are

coded as zero. The Holding Time should be coded as zero but there

may be some utility in supplying a "short" holding time to be

applied to the matching next hop information before that

information would be purged; this usage is for further study. The

Client Protocol Address field and the Cli Proto Len field MUST be

filled in. The Client Protocol Address is filled in with the

protocol address to be purged from the receiving station's cache

while the Cli Proto Len is set the length of the purged client's

protocol address. All remaining fields in the CIE MAY be set to

zero although the client NBMA information (and associated length

fields) MAY be specified to narrow the scope of the NHRP Purge

Request if requester desires. However, the receiver of an NHRP

Purge Request may choose to ignore the Client NBMA information if

it is supplied.

An NHRP Purge Request packet is sent from an NHS to a station to

cause it to delete previously cached information. This is done when

the information may be no longer valid (typically when the NHS has

previously provided next hop information for a station that is not

directly connected to the NBMA subnetwork, and the egress point to

that station may have changed).

An NHRP Purge Request packet may also be sent from an NHC to an NHS

with which the NHC had previously registered. This allows for an NHC

to invalidate its registration with NHRP before it would otherwise

expire via the holding timer. If an NHC does not have knowledge of a

protocol address of a serving NHS then the NHC must place its own

protocol address in the Destination Protocol Address field and

forward the packet along the routed path. Otherwise, the NHC must

place the protocol address of a serving NHS in this field.

Serving NHSs may need to send one or more new NHRP Purge Requests as

a result of receiving a purge from one of their served NHCs since the

NHS may have previously responded to NHRP Resolution Requests for

that NHC's NBMA information. These purges are "new" in that they are

sourced by the NHS and not the NHC; that is, for each NHC that

previously sent a NHRP Resolution Request for the purged NHC NBMA

information, an NHRP Purge Request is sent which contains the Source

Protocol/NBMA Addresses of the NHS and the Destination Protocol

Address of the NHC which previously sent an NHRP Resolution Request

prior to the purge.

The station sending the NHRP Purge Request MAY periodically

retransmit the NHRP Purge Request until either NHRP Purge Request is

acknowledged or until the holding time of the information being

purged has expired. Retransmission strategies for NHRP Purge Requests

are a local matter.

When a station receives an NHRP Purge Request, it MUST discard any

previously cached information that matches the information in the

CIEs.

An NHRP Purge Reply MUST be returned for the NHRP Purge Request even

if the station does not have a matching cache entry assuming that the

"N" bit is off in the NHRP Purge Request.

If the station wishes to reestablish communication with the

destination shortly after receiving an NHRP Purge Request, it should

make an authoritative NHRP Resolution Request in order to avoid any

stale cache entries that might be present in intermediate NHSs (See

section 6.2.2.). It is recommended that authoritative NHRP

Resolution Requests be made for the duration of the holding time of

the old information.

5.2.6 NHRP Purge Reply

The NHRP Purge Reply packet is sent in order to assure the sender of

an NHRP Purge Request that all cached information of the specified

type has been purged from the station sending the reply. The NHRP

Purge Reply has a type code of 6.

An NHRP Purge Reply is formed from an NHRP Purge Request by merely

changing the type code in the request to 6. The packet is then

returned to the requester after filling in the appropriate extensions

if they exist.

5.2.7 NHRP Error Indication

The NHRP Error Indication is used to convey error indications to the

sender of an NHRP packet. It has a type code of 7. The Mandatory

Part has the following format:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Src Proto Len Dst Proto Len unused

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Error Code Error Offset

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source NBMA Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source NBMA Subaddress (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination Protocol Address (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Contents of NHRP Packet in error (variable length)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Src Proto Len

This field holds the length in octets of the Source Protocol

Address.

Dst Proto Len

This field holds the length in octets of the Destination Protocol

Address.

Error Code

An error code indicating the type of error detected, chosen from

the following list:

1 - Unrecognized Extension

When the Compulsory bit of an extension in NHRP packet is set,

the NHRP packet cannot be processed unless the extension has

been processed. The responder MUST return an NHRP Error

Indication of type Unrecognized Extension if it is incapable of

processing the extension. However, if a transit NHS (one which

is not going to generate a reply) detects an unrecognized

extension, it SHALL ignore the extension.

3 - NHRP Loop Detected

A Loop Detected error is generated when it is determined that

an NHRP packet is being forwarded in a loop.

6 - Protocol Address Unreachable

This error occurs when a packet it moving along the routed path

and it reaches a point such that the protocol address of

interest is not reachable.

7 - Protocol Error

A generic packet processing error has occurred (e.g., invalid

version number, invalid protocol type, failed checksum, etc.)

8 - NHRP SDU Size Exceeded

If the SDU size of the NHRP packet exceeds the MTU size of the

NBMA network then this error is returned.

9 - Invalid Extension

If an NHS finds an extension in a packet which is inappropriate

for the packet type, an error is sent back to the sender with

Invalid Extension as the code.

10 - Invalid NHRP Resolution Reply Received

If a client receives a NHRP Resolution Reply for a Next Hop

Resolution Request which it believes it did not make then an

error packet is sent to the station making the reply with an

error code of Invalid Reply Received.

11 - Authentication Failure

If a received packet fails an authentication test then this

error is returned.

15 - Hop Count Exceeded

The hop count which was specified in the Fixed Header of an

NHRP message has been exceeded.

Error Offset

The offset in octets into the original NHRP packet in which an

error was detected. This offset is calculated starting from the

NHRP Fixed Header.

Source NBMA Address

The Source NBMA address field is the address of the station which

observed the error.

Source NBMA SubAddress

The Source NBMA subaddress field is the address of the station

which observed the error. If the field's length as specified in

ar$sstl is 0 then no storage is allocated for this address at all.

Source Protocol Address

This is the protocol address of the station which issued the Error

packet.

Destination Protocol Address

This is the protocol address of the station which sent the packet

which was found to be in error.

An NHRP Error Indication packet SHALL NEVER be generated in response

to another NHRP Error Indication packet. When an NHRP Error

Indication packet is generated, the offending NHRP packet SHALL be

discarded. In no case should more than one NHRP Error Indication

packet be generated for a single NHRP packet.

If an NHS sees its own Protocol and NBMA Addresses in the Source NBMA

and Source Protocol address fields of a transiting NHRP Error

Indication packet then the NHS will quietly drop the packet and do

nothing (this scenario would occur when the NHRP Error Indication

packet was itself in a loop).

Note that no extensions may be added to an NHRP Error Indication.

5.3 Extensions Part

The Extensions Part, if present, carries one or more extensions in

{Type, Length, Value} triplets.

Extensions have the following format:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Cu Type Length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Value...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

"Compulsory." If clear, and the NHS does not recognize the type

code, the extension may safely be ignored. If set, and the NHS

does not recognize the type code, the NHRP "request" is considered

to be in error. (See below for details.)

Unused and must be set to zero.

Type

The extension type code (see below). The extension type is not

qualified by the Compulsory bit, but is orthogonal to it.

Length

The length in octets of the value (not including the Type and

Length fields; a null extension will have only an extension header

and a length of zero).

When extensions exist, the extensions list is terminated by the Null

TLV, having Type = 0 and Length = 0.

Extensions may occur in any order, but any particular extension type

may occur only once in an NHRP packet unless explicitly stated to the

contrary in the extensions definition. For example, the vendor-

private extension may occur multiple times in a packet in order to

allow for extensions which do not share the same vendor ID to be

represented. It is RECOMMENDED that a given vendor include no more

than one Vendor Private Extension.

An NHS MUST NOT change the order of extensions. That is, the order

of extensions placed in an NHRP packet by an NHC (or by an NHS when

an NHS sources a packet) MUST be preserved as the packet moves

between NHSs. Minimal NHC implementations MUST only recognize, but

not necessarily parse, the Vendor Private extension and the End Of

Extensions extension. Extensions are only present in a "reply" if

they were present in the corresponding "request" with the exception

of Vendor Private extensions. The previous statement is not intended

to preclude the creation of NHS-only extensions which might be added

to and removed from NHRP packets by the same NHS; such extensions

MUST not be propagated to NHCs.

The Compulsory bit provides for a means to add to the extension set.

If the bit is set in an extension then the station responding to the

NHRP message which contains that extension MUST be able to understand

the extension (in this case, the station responding to the message is

the station that would issue an NHRP reply in response to a NHRP

request). As a result, the responder MUST return an NHRP Error

Indication of type Unrecognized Extension. If the Compulsory bit is

clear then the extension can be safely ignored; however, if an

ignored extension is in a "request" then it MUST be returned,

unchanged, in the corresponding "reply" packet type.

If a transit NHS (one which is not going to generate a "reply")

detects an unrecognized extension, it SHALL ignore the extension. If

the Compulsory bit is set, the transit NHS MUST NOT cache the

information contained in the packet and MUST NOT identify itself as

an egress router (in the Forward Record or Reverse Record

extensions). Effectively, this means, if a transit NHS encounters an

extension which it cannot process and which has the Compulsory bit

set then that NHS MUST NOT participate in any way in the protocol

exchange other than acting as a forwarding agent.

The NHRP extension Type space is subdivided to encourage use outside

the IETF.

0x0000 - 0x0FFF Reserved for NHRP.

0x1000 - 0x11FF Allocated to the ATM Forum.

0x1200 - 0x37FF Reserved for the IETF.

0x3800 - 0x3FFF Experimental use.

IANA will administer the ranges reserved for the IETF as described in

Section 9. Values in the 'Experimental use' range have only local

significance.

5.3.0 The End Of Extensions

Compulsory = 1

Type = 0

Length = 0

When extensions exist, the extensions list is terminated by the End

Of Extensions/Null TLV.

5.3.1 Responder Address Extension

Compulsory = 1

Type = 3

Length = variable

This extension is used to determine the address of the NHRP

responder; i.e., the entity that generates the appropriate "reply"

packet for a given "request" packet. In the case of an NHRP

Resolution Request, the station responding may be different (in the

case of cached replies) than the system identified in the Next Hop

field of the NHRP Resolution Reply. Further, this extension may aid

in detecting loops in the NHRP forwarding path.

This extension uses a single CIE with the extension specific meanings

of the fields set as follows:

The Prefix Length fields MUST be set to 0 and ignored.

CIE Code

5 - Insufficient Resources

If the responder to an NHRP Resolution Request is an egress point

for the target of the address resolution request (i.e., it is one

of the stations identified in the list of CIEs in an NHRP

Resolution Reply) and the Responder Address extension is included

in the NHRP Resolution Request and insufficient resources to

setup a cut-through VC exist at the responder then the Code field

of the Responder Address Extension is set to 5 in order to tell

the client that a VC setup attempt would in all likelihood be

rejected; otherwise this field MUST be coded as a zero. NHCs MAY

use this field to influence whether they attempt to setup a cut-

through to the egress router.

Maximum Transmission Unit

This field gives the maximum transmission unit preferred by the

responder. If this value is 0 then either the default MTU is used

or the MTU negotiated via signaling is used if such negotiation is

possible for the given NBMA.

Holding Time

The Holding Time field specifies the number of seconds for which

the NBMA information of the responser is considered to be valid.

Cached information SHALL be discarded when the holding time

expires.

"Client Address" information is actually "Responder Address"

information for this extension. Thus, for example, Cli Addr T/L is

the responder NBMA address type and length field.

If a "requester" desires this information, the "requester" SHALL

include this extension with a value of zero. Note that this implies

that no storage is allocated for the Holding Time and Type/Length

fields until the "Value" portion of the extension is filled out.

If an NHS is generating a "reply" packet in response to a "request"

containing this extension, the NHS SHALL include this extension,

containing its protocol address in the "reply". If an NHS has more

than one protocol address, it SHALL use the same protocol address

consistently in all of the Responder Address, Forward Transit NHS

Record, and Reverse Transit NHS Record extensions. The choice of

which of several protocol address to include in this extension is a

local matter.

If an NHRP Resolution Reply packet being forwarded by an NHS contains

a protocol address of that NHS in the Responder Address Extension

then that NHS SHALL generate an NHRP Error Indication of type "NHRP

Loop Detected" and discard the NHRP Resolution Reply.

If an NHRP Resolution Reply packet is being returned by an

intermediate NHS based on cached data, it SHALL place its own address

in this extension (differentiating it from the address in the Next

Hop field).

5.3.2 NHRP Forward Transit NHS Record Extension

Compulsory = 1

Type = 4

Length = variable

The NHRP Forward Transit NHS record contains a list of transit NHSs

through which a "request" has traversed. Each NHS SHALL append to

the extension a Forward Transit NHS element (as specified below)

containing its Protocol address. The extension length field and the

ar$chksum fields SHALL be adjusted appropriately.

The responding NHS, as described in Section 5.3.1, SHALL NOT update

this extension.

In addition, NHSs that are willing to act as egress routers for

packets from the source to the destination SHALL include information

about their NBMA Address.

This extension uses a single CIE per NHS Record element with the

extension specific meanings of the fields set as follows:

The Prefix Length fields MUST be set to 0 and ignored.

CIE Code

5 - Insufficient Resources

If an NHRP Resolution Request contains an NHRP Forward Transit

NHS Record Extension and insufficient resources to setup a cut-

through VC exist at the current transit NHS then the CIE Code

field for NHRP Forward Transit NHS Record Extension is set to 5

in order to tell the client that a VC setup attempt would in all

likelihood be rejected; otherwise this field MUST be coded as a

zero. NHCs MAY use this field to influence whether they attempt

to setup a cut-through as described in Section 2.2. Note that

the NHRP Reverse Transit NHS Record Extension MUST always have

this field set to zero.

Maximum Transmission Unit

This field gives the maximum transmission unit preferred by the

transit NHS. If this value is 0 then either the default MTU is

used or the MTU negotiated via signaling is used if such

negotiation is possible for the given NBMA.

Holding Time

The Holding Time field specifies the number of seconds for which

the NBMA information of the transit NHS is considered to be valid.

Cached information SHALL be discarded when the holding time

expires.

"Client Address" information is actually "Forward Transit NHS

Address" information for this extension. Thus, for example, Cli Addr

T/L is the transit NHS NBMA address type and length field.

If a "requester" wishes to obtain this information, it SHALL include

this extension with a length of zero. Note that this implies that no

storage is allocated for the Holding Time and Type/Length fields

until the "Value" portion of the extension is filled out.

If an NHS has more than one Protocol address, it SHALL use the same

Protocol address consistently in all of the Responder Address,

Forward NHS Record, and Reverse NHS Record extensions. The choice of

which of several Protocol addresses to include in this extension is a

local matter.

If a "request" that is being forwarded by an NHS contains the

Protocol Address of that NHS in one of the Forward Transit NHS

elements then the NHS SHALL generate an NHRP Error Indication of type

"NHRP Loop Detected" and discard the "request".

5.3.3 NHRP Reverse Transit NHS Record Extension

Compulsory = 1

Type = 5

Length = variable

The NHRP Reverse Transit NHS record contains a list of transit NHSs

through which a "reply" has traversed. Each NHS SHALL append a

Reverse Transit NHS element (as specified below) containing its

Protocol address to this extension. The extension length field and

ar$chksum SHALL be adjusted appropriately.

The responding NHS, as described in Section 5.3.1, SHALL NOT update

this extension.

In addition, NHSs that are willing to act as egress routers for

packets from the source to the destination SHALL include information

about their NBMA Address.

This extension uses a single CIE per NHS Record element with the

extension specific meanings of the fields set as follows:

The CIE Code and Prefix Length fields MUST be set to 0 and ignored.

Maximum Transmission Unit

This field gives the maximum transmission unit preferred by the

transit NHS. If this value is 0 then either the default MTU is

used or the MTU negotiated via signaling is used if such

negotiation is possible for the given NBMA.

Holding Time

The Holding Time field specifies the number of seconds for which

the NBMA information of the transit NHS is considered to be valid.

Cached information SHALL be discarded when the holding time

expires.

"Client Address" information is actually "Reverse Transit NHS

Address" information for this extension. Thus, for example, Cli Addr

T/L is the transit NHS NBMA address type and length field.

If a "requester" wishes to obtain this information, it SHALL include

this extension with a length of zero. Note that this implies that no

storage is allocated for the Holding Time and Type/Length fields

until the "Value" portion of the extension is filled out.

If an NHS has more than one Protocol address, it SHALL use the same

Protocol address consistently in all of the Responder Address,

Forward NHS Record, and Reverse NHS Record extensions. The choice of

which of several Protocol addresses to include in this extension is a

local matter.

If a "reply" that is being forwarded by an NHS contains the Protocol

Address of that NHS in one of the Reverse Transit NHS elements then

the NHS SHALL generate an NHRP Error Indication of type "NHRP Loop

Detected" and discard the "reply".

Note that this information may be cached at intermediate NHSs; if

so, the cached value SHALL be used when generating a reply.

5.3.4 NHRP Authentication Extension

Compulsory = 1 Type = 7 Length = variable

The NHRP Authentication Extension is carried in NHRP packets to

convey authentication information between NHRP speakers. The

Authentication Extension may be included in any NHRP "request" or

"reply" only.

The authentication is always done pairwise on an NHRP hop-by-hop

basis; i.e., the authentication extension is regenerated at each

hop. If a received packet fails the authentication test, the station

SHALL generate an Error Indication of type "Authentication Failure"

and discard the packet. Note that one possible authentication failure

is the lack of an Authentication Extension; the presence or absence

of the Authentication Extension is a local matter.

5.3.4.1 Header Format

The authentication header has the following format:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved Security Parameter Index (SPI)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Src Addr...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+ Authentication Data... -+-+-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Security Parameter Index (SPI) can be thought of as an index into a

table that maintains the keys and other information such as hash

algorithm. Src and Dst communicate either offline using manual keying

or online using a key management protocol to populate this table. The

sending NHRP entity always allocates the SPI and the parameters

associated with it.

Src Addr a variable length field is the address assigned to the

outgoing interface. The length of the addr is obtained from the

source protocol length field in the mandatory part of the NHRP

header. The tuple <spi, src addr> uniquely identifies the key and

other parameters that are used in authentication.

The length of the authentication data field is dependent on the hash

algorithm used. The data field contains the keyed hash calculated

over the entire NHRP payload. The authentication data field is zeroed

out before the hash is calculated.

5.3.4.2 SPI and Security Parameters Negotiation

SPI's can be negotiated either manually or using an Internet Key

Management protocol. Manual keying MUST be supported. The following

parameters are associated with the tuple <SPI, src>- lifetime,

Algorithm, Key. Lifetime indicates the duration in seconds for which

the key is valid. In case of manual keying, this duration can be

infinite. Also, in order to better support manual keying, there may

be multiple tuples active at the same time (Dst being the same).

Algorithm specifies the hash algorithm agreed upon by the two

entities. HMAC-MD5-128 [16] is the default algorithm. Other

algorithms MAY be supported by defining new values. IANA will assign

the numbers to identify the algorithm being used as described in

Section 9.

Any Internet standard key management protocol MAY so be used to

negotiate the SPI and parameters.

5.3.4.3 Message Processing

At the time of adding the authentication extension header, src looks

up in a table to fetch the SPI and the security parameters based on

the outgoing interface address. If there are no entries in the table

and if there is support for key management, the src initiates the key

management protocol to fetch the necessary parameters. The src

constructs the Authentication Extension payload and calculates the

hash by zeroing authentication data field. The result replaces in the

zeroed authentication data field. The src address field in the

payload is the IP address assigned to the outgoing interface.

If key management is not supported and authentication is mandatory,

the packet is dropped and this information is logged.

On the receiving end, dst fetches the parameters based on the SPI and

the ip address in the authentication extension payload. The

authentication data field is extracted before zeroing out to

calculate the hash. It computes the hash on the entire payload and if

the hash does not match, then an "abnormal event" has occurred.

5.3.4.4 Security Considerations

It is important that the keys chosen are strong as the security of

the entire system depends on the keys being chosen properly and the

correct implementation of the algorithms.

The security is performed on a hop by hop basis. The data received

can be trusted only so much as one trusts all the entities in the

path traversed. A chain of trust is established amongst NHRP entities

in the path of the NHRP Message . If the security in an NHRP entity

is compromised, then security in the entire NHRP domain is

compromised.

Data integrity covers the entire NHRP payload. This guarantees that

the message was not modified and the source is authenticated as well.

If authentication extension is not used or if the security is

compromised, then NHRP entities are liable to both spoofing attacks,

active attacks and passive attacks.

There is no mechanism to encrypt the messages. It is assumed that a

standard layer 3 confidentiality mechanism will be used to encrypt

and decrypt messages. It is recommended to use an Internet standard

key management protocol to negotiate the keys between the neighbors.

Transmitting the keys in clear text, if other methods of negotiation

is used, compromises the security completely.

Any NHS is susceptible to Denial of Service (DOS) attacks that cause

it to become overloaded, preventing legitimate packets from being

acted upon properly. A rogue host can send request and registration

packets to the first hop NHS. If the authentication option is not

used, the registration packet is forwarded along the routed path

requiring processing along each NHS. If the authentication option is

used, then only the first hop NHS is susceptible to DOS attacks

(i.e., unauthenticated packets will be dropped rather than forwarded

on). If security of any host is compromised (i.e., the keys it is

using to communicate with an NHS become known), then a rogue host can

send NHRP packets to the first hop NHS of the host whose keys were

compromised, which will then forward them along the routed path as in

the case of unauthenticated packets. However, this attack requires

that the rogue host to have the same first hop NHS as that of the

compromised host. Finally, it should be noted that denial of service

attacks that cause routers on the routed path to expend resources

processing NHRP packets are also susceptable to attacks that flood

packets at the same destination as contained in an NHRP packet's

Destination Protocol Address field.

5.3.5 NHRP Vendor-Private Extension

Compulsory = 0

Type = 8

Length = variable

The NHRP Vendor-Private Extension is carried in NHRP packets to

convey vendor-private information or NHRP extensions between NHRP

speakers.

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Vendor ID Data....

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Vendor ID

802 Vendor ID as assigned by the IEEE [6]

Data

The remaining octets after the Vendor ID in the payload are

vendor-dependent data.

This extension may be added to any "request" or "reply" packet and it

is the only extension that may be included multiple times. If the

receiver does not handle this extension, or does not match the Vendor

ID in the extension then the extension may be completely ignored by

the receiver. If a Vendor Private Extension is included in a

"request" then it must be copied to the corresponding "reply".

6. Protocol Operation

In this section, we discuss certain operational considerations of

NHRP.

6.1 Router-to-Router Operation

In practice, the initiating and responding stations may be either

hosts or routers. However, there is a possibility under certain

conditions that a stable routing loop may occur if NHRP is used

between two routers. In particular, attempting to establish an NHRP

path across a boundary where information used in route selection is

lost may result in a routing loop. Such situations include the loss

of BGP path vector information, the interworking of multiple routing

protocols with dissimilar metrics (e.g, RIP and OSPF), etc. In such

circumstances, NHRP should not be used. This situation can be

avoided if there are no "back door" paths between the entry and

egress router outside of the NBMA subnetwork. Protocol mechanisms to

relax these restrictions are under investigation.

In general it is preferable to use mechanisms, if they exist, in

routing protocols to resolve the egress point when the destination

lies outside of the NBMA subnetwork, since such mechanisms will be

more tightly coupled to the state of the routing system and will

probably be less likely to create loops.

6.2 Cache Management Issues

The management of NHRP caches in the source station, the NHS serving

the destination, and any intermediate NHSs is dependent on a number

of factors.

6.2.1 Caching Requirements

Source Stations

Source stations MUST cache all received NHRP Resolution Replies

that they are actively using. They also must cache "incomplete"

entries, i.e., those for which a NHRP Resolution Request has been

sent but those for which an NHRP Resolution Reply has not been

received. This is necessary in order to preserve the Request ID

for retries, and provides the state necessary to avoid triggering

NHRP Resolution Requests for every data packet sent to the

destination.

Source stations MUST purge expired information from their caches.

Source stations MUST purge the appropriate cached information upon

receipt of an NHRP Purge Request packet.

When a station has a co-resident NHC and NHS, the co-resident NHS

may reply to NHRP Resolution Requests from the co-resident NHC with

information which the station cached as a result of the co-resident

NHC making its own NHRP Resolution Requests as long as the co-

resident NHS follows the rules for Transit NHSs as seen below.

Serving NHSs

The NHS serving the destination (the one which responds

authoritatively to NHRP Resolution Requests) SHOULD cache protocol

address information from all NHRP Resolution Requests to which it

has responded if the information in the NHRP Resolution Reply has

the possibility of changing during its lifetime (so that an NHRP

Purge Request packet can be issued). The internetworking to NBMA

binding information provided by the source station in the NHRP

Resolution Request may also be cached if and only if the "S" bit is

set, the NHRP Resolution Request has included a CIE with the

Holding Time field set greater than zero (this is the valid Holding

Time for the source binding), and only for non-authoritative use

for a period not to exceed the Holding Time.

Transit NHSs

A Transit NHS (lying along the NHRP path between the source station

and the responding NHS) may cache source binding information

contained in NHRP Resolution Request packets that it forwards if

and only if the "S" bit is set, the NHRP Resolution Request has

included a CIE with the Holding Time field set greater than zero

(this is the valid Holding Time for the source binding), and only

for non-authoritative use for a period not to exceed the Holding

Time.

A Transit NHS may cache destination information contained in NHRP

Resolution Reply CIE if only if the D bit is set and then only for

non-authoritative use for a period not to exceed the Holding Time

value contained in the CIE. A Transit NHS MUST NOT cache source

binding information contained in an NHRP Resolution Reply.

Further, a transit NHS MUST discard any cached information when the

prescribed time has expired. It may return cached information in

response to non-authoritative NHRP Resolution Requests only.

6.2.2 Dynamics of Cached Information

NBMA-Connected Destinations

NHRP's most basic function is that of simple NBMA address

resolution of stations directly attached to the NBMA subnetwork.

These mappings are typically very static, and appropriately chosen

holding times will minimize problems in the event that the NBMA

address of a station must be changed. Stale information will cause

a loss of connectivity, which may be used to trigger an

authoritative NHRP Resolution Request and bypass the old data. In

the worst case, connectivity will fail until the cache entry times

out.

This applies equally to information marked in NHRP Resolution

Replies as being "stable" (via the "D" bit).

Destinations Off of the NBMA Subnetwork

If the source of an NHRP Resolution Request is a host and the

destination is not directly attached to the NBMA subnetwork, and

the route to that destination is not considered to be "stable," the

destination mapping may be very dynamic (except in the case of a

subnetwork where each destination is only singly homed to the NBMA

subnetwork). As such the cached information may very likely become

stale. The consequence of stale information in this case will be a

suboptimal path (unless the internetwork has partitioned or some

other routing failure has occurred).

6.3 Use of the Prefix Length field of a CIE

A certain amount of care needs to be taken when using the Prefix

Length field of a CIE, in particular with regard to the prefix length

advertised (and thus the size of the equivalence class specified by

it). Assuming that the routers on the NBMA subnetwork are exchanging

routing information, it should not be possible for an NHS to create a

black hole by advertising too large of a set of destinations, but

suboptimal routing (e.g., extra internetwork layer hops through the

NBMA) can result. To avoid this situation an NHS that wants to send

the Prefix Length MUST obey the following rule:

The NHS examines the Network Layer Reachability Information (NLRI)

associated with the route that the NHS would use to forward towards

the destination (as specified by the Destination internetwork layer

address in the NHRP Resolution Request), and extracts from this

NLRI the shortest address prefix such that: (a) the Destination

internetwork layer address (from the NHRP Resolution Request) is

covered by the prefix, (b) the NHS does not have any routes with

NLRI which form a subset of what is covered by the prefix. The

prefix may then be used in the CIE.

The Prefix Length field of the CIE should be used with restraint, in

order to avoid NHRP stations choosing suboptimal transit paths when

overlapping prefixes are available. This document specifies the use

of the prefix length only when all the destinations covered by the

prefix are "stable". That is, either:

(a) All destinations covered by the prefix are on the NBMA network,

(b) All destinations covered by the prefix are directly attached to

the NHRP responding station.

Use of the Prefix Length field of the CIE in other circumstances is

outside the scope of this document.

6.4 Domino Effect

One could easily imagine a situation where a router, acting as an

ingress station to the NBMA subnetwork, receives a data packet, such

that this packet triggers an NHRP Resolution Request. If the router

forwards this data packet without waiting for an NHRP transit path to

be established, then when the next router along the path receives the

packet, the next router may do exactly the same - originate its own

NHRP Resolution Request (as well as forward the packet). In fact

such a data packet may trigger NHRP Resolution Request generation at

every router along the path through an NBMA subnetwork. We refer to

this phenomena as the NHRP "domino" effect.

The NHRP domino effect is clearly undesirable. At best it may result

in excessive NHRP traffic. At worst it may result in an excessive

number of virtual circuits being established unnecessarily.

Therefore, it is important to take certain measures to avoid or

suppress this behavior. NHRP implementations for NHSs MUST provide a

mechanism to address this problem. One possible strategy to address

this problem would be to configure a router in such a way that NHRP

Resolution Request generation by the router would be driven only by

the traffic the router receives over its non-NBMA interfaces

(interfaces that are not attached to an NBMA subnetwork). Traffic

received by the router over its NBMA-attached interfaces would not

trigger NHRP Resolution Requests. Such a router avoids the NHRP

domino effect through administrative means.

7. NHRP over Legacy BMA Networks

There would appear to be no significant impediment to running NHRP

over legacy broadcast subnetworks. There may be issues around

running NHRP across multiple subnetworks. Running NHRP on broadcast

media has some interesting possibilities; especially when setting up

a cut-through for inter-ELAN inter-LIS/LAG traffic when one or both

end stations are legacy attached. This use for NHRP requires further

research.

8. Discussion

The result of an NHRP Resolution Request depends on how routing is

configured among the NHSs of an NBMA subnetwork. If the destination

station is directly connected to the NBMA subnetwork and the routed

path to it lies entirely within the NBMA subnetwork, the NHRP

Resolution Replies always return the NBMA address of the destination

station itself rather than the NBMA address of some egress router.

On the other hand, if the routed path exits the NBMA subnetwork, NHRP

will be unable to resolve the NBMA address of the destination, but

rather will return the address of the egress router. For

destinations outside the NBMA subnetwork, egress routers and routers

in the other subnetworks should exchange routing information so that

the optimal egress router may be found.

In addition to NHSs, an NBMA station could also be associated with

one or more regular routers that could act as "connectionless

servers" for the station. The station could then choose to resolve

the NBMA next hop or just send the packets to one of its

connectionless servers. The latter option may be desirable if

communication with the destination is short-lived and/or doesn't

require much network resources. The connectionless servers could, of

course, be physically integrated in the NHSs by augmenting them with

internetwork layer switching functionality.

9. IANA Considerations

IANA will take advice from the Area Director appointed designated

subject matter expert, in order to assign numbers from the various

number spaces described herein. In the event that the Area Director

appointed designated subject matter expert is unavailable, the

relevant IESG Area Director will appoint another expert. Any and all

requests for value assignment within a given number space will be

accepted when the usage of the value assignment documented. Possible

forms of documentantion include, but is not limited to, RFCs or the

product of another cooperative standards body (e.g., the MPOA and

LANE subworking group of the ATM Forum).

References

[1] Heinanen, J., and R. Govindan, "NBMA Address Resolution Protocol

(NARP)", RFC1735, December 1994.

[2] Plummer, D., "Address Resolution Protocol", STD 37, RFC826,

November 1982.

[3] Laubach, M., and J. Halpern, "Classical IP and ARP over ATM", RFC

2225, April 1998.

[4] Piscitello,, D., and J. Lawrence, "Transmission of IP datagrams

over the SMDS service", RFC1209, March 1991.

[5] Protocol Identification in the Network Layer, ISO/IEC TR

9577:1990.

[6] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1700,

October 1994.

[7] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation

Layer 5", RFC1483, July 1993.

[8] Malis, A., Robinson, D., and R. Ullmann, "Multiprotocol

Interconnect on X.25 and ISDN in the Packet Mode", RFC1356, August

1992.

[9] Bradley, T., Brown, C., and A. Malis, "Multiprotocol Interconnect

over Frame Relay", RFC1490, July 1993.

[10] Rekhter, Y., and D. Kandlur, ""Local/Remote" Forwarding Decision

in Switched Data Link Subnetworks", RFC1937, May 1996.

[11] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM

Networks", RFC2022, November 1996.

[12] Luciani, J., Armitage, G., and J. Halpern, "Server Cache

Synchronization Protocol (SCSP) - NBMA", RFC2334, April 1998.

[13] Rekhter, Y., "NHRP for Destinations off the NBMA Subnetwork",

Work In Progress.

[14] Luciani, J., et. al., "Classical IP and ARP over ATM to NHRP

Transition", Work In Progress.

[15] Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, March 1997.

[16] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed Hashing

for Message Authentication", RFC2104, February 1997.

Acknowledgments

We would like to thank (in no particular order) Thomas Narten of IBM

for his comments in the role of Internet AD, Juha Heinenan of Telecom

Finland and Ramesh Govidan of ISI for their work on NBMA ARP and the

original NHRP draft, which served as the basis for this work.

Russell Gardo of IBM, John Burnett of Adaptive, Dennis Ferguson of

ANS, Andre Fredette of Bay Networks, Joel Halpern of Newbridge, Paul

Francis of NTT, Tony Li, Bryan Gleeson, and Yakov Rekhter of cisco,

and Grenville Armitage of Bellcore should also be acknowledged for

comments and suggestions that improved this work substantially. We

would also like to thank the members of the ION working group of the

IETF, whose review and discussion of this document have been

invaluable.

Authors' Addresses

James V. Luciani Dave Katz

Bay Networks cisco Systems

3 Federal Street 170 W. Tasman Dr.

Mail Stop: BL3-03 San Jose, CA 95134 USA

Billerica, MA 01821 Phone: +1 408 526 8284

Phone: +1 978 916 4734 EMail: dkatz@cisco.com

EMail: luciani@baynetworks.com

David Piscitello Bruce Cole

Core Competence Juniper Networks

1620 Tuckerstown Road 3260 Jay St.

Dresher, PA 19025 USA Santa Clara, CA 95054

Phone: +1 215 830 0692 Phone: +1 408 327 1900

EMail: dave@corecom.com EMail: bcole@jnx.com

Naganand Doraswamy

Bay Networks, Inc.

3 Federal Street

Mail Stop: Bl3-03

Billerica, MA 01801

Phone: +1 978 916 1323

EMail: naganand@baynetworks.com

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