Routing over Large Clouds Working Group Dave Katz
INTERNET-DRAFT (cisco Systems)
<draft-ietf-rolc-nhrp-06.txt> David Piscitello
(Core Competence, Inc.)
Bruce Cole
(cisco Systems)
James V. Luciani
(Ascom Nexion)
November 1995
NBMA Next Hop Resolution Protocol (NHRP)
Status of this Memo
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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 IP 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. Although
this document focuses on NHRP in the context of IP, the technique is
applicable to other internetwork layer protocols (e.g., IPX, CLNP,
Appletalk) as well.
This document is intended to be a functional superset of the NBMA
Address Resolution Protocol (NARP) documented in [1].
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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.
1. Introduction
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 IP and NBMA
addresses of the "NBMA next hop" 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.
An NBMA subnetwork may, in general, consist of multiple logically
independent IP subnets (LISs), defined in [3] and [4] as having the
following properties:
1) All members of a LIS have the same IP network/subnet number
and address mask.
2) All members within a LIS are directly connected to the same
NBMA subnetwork.
3) All members outside of the LIS are accessed via a router.
IP routing 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 IP routing may not be sufficient to
resolve the "NBMA next hop" toward the destination station, and IP
packets may traverse the NBMA subnetwork more than once.
NHRP describes a routing method that relaxes the forwarding
restrictions of the LIS model. With NHRP, 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 and QOS characteristics (in a connection-oriented NBMA
subnetwork such as ATM).
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NHRP in its most basic form provides a simple IP-to-NBMA-address
binding service. This may be sufficient for hosts which 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
alongside existing ARP services.
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.
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.)
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
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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 Next Hop 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 participate in protocols
used to disseminate routing information across (and beyond the
boundaries of) the NBMA subnetwork, and may support "classical" ARP
service as well.
An NHS maintains a "next-hop resolution" cache, which is a table of
address mappings (IP-to-NBMA address). This table can be constructed
from information gleaned from NHRP Register packets (see Section
5.2.3 and 5.2.4), extracted from Next Hop Resolution Requests or
replies that traverse the NHS as they are forwarded, or through
mechanisms outside the scope of this document (examples of such
mechanisms include ARP [2, 3, 4] and pre-configured tables). Section
6.2 further describes cache management issues.
A host or router that is not an NHRP server must be configured with
the identity of the NHS which serves it (see Configuration, Section
4).
[Note: for NBMA subnetworks that offer group or multicast addressing
features, it may be desirable to configure stations with a group
identity for NHSs, i.e., addressing information that would solicit a
response from "all NHSs". The means whereby a group of NHSs divide
responsibilities for next hop resolution are not described here.]
Whether or not a particular station within the NBMA subnetwork which
is making use of the NHRP protocol needs to be able to act as an NHS
is a local matter. For a station to avoid providing NHS
functionality, there must be one or more NHSs within the NBMA
subnetwork which are providing authoritative NBMA information on its
behalf. If NHRP is to be able to resolve the NBMA address for
stations that lack NHS functionality, these serving NHSs must exist
along all routed paths between Next Hop Resolution Requesters and the
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station which cannot answer Next Hop Resolution Requests.
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 IP address). If the next hop is reachable through its
NBMA interface, S constructs an Next Hop Resolution Request packet
(see Section 5.2.1) containing station D's IP address as the (target)
destination address, S's own IP 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 reply (i.e., station S only wishes to receive a reply
from the NHS-speaker that maintains the NBMA-to-IP address mapping
for this destination). Station S emits the Next Hop Resolution
Request packet towards the destination, using the NBMA address of the
next routed hop.
If the Next Hop Resolution Request is triggered by a data packet,
station S may choose to dispose of the data packet while awaiting an
NHRP reply in one of the following ways:
(a) Drop the packet
(b) Retain the packet until the reply arrives and a more optimal
path is available
(c) Forward the packet along the routed path toward D
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 Next Hop Resolution Request for a given destination MUST
NOT be triggered on every packet, though periodically retrying a Next
Hop Resolution Request is permitted.
When the NHS receives an Next Hop Resolution Request, a check is made
to see if it "serves" station D, i.e., the NHS checks to see if there
is a "next hop" entry for D in its next-hop resolution cache. If the
NHS does not serve D, the NHS forwards the Next Hop Resolution
Request to another NHS. (Mechanisms for determining how to forward
the Next Hop Resolution Request are discussed in Section 3,
Deployment.) Note that because NHRP packets are encapsulated with the
NBMA address of neighboring stations (see encapsulation discussion,
section 5), NHSs must be next hops to one another in order for
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forwarding of these packets to be possible.
If this NHS serves D, the NHS resolves station D's NBMA address, and
generates a positive NHRP reply on D's behalf. (NHRP replies in this
scenario are always marked as "authoritative".) The NHRP reply
packet contains the next hop IP and NBMA address for station D and is
sent back to S. (Note that if station D is not on the NBMA
subnetwork, the next hop IP address will be that of the egress router
through which packets for station D are forwarded.)
An NHS receiving an NHRP reply may cache the NBMA next hop
information contained therein. To a subsequent Next Hop Resolution
Request, this NHS may respond with the cached, non-authoritative,
NBMA next hop information or with cached negative information, if the
NHS is allowed to do so, see section 6.2. Non-authoritative NHRP
replies are distinguished from authoritative replies so that if a
communication attempt based on non-authoritative information fails, a
source station can choose to send an authoritative Next Hop
Resolution Request. NHSs MUST NOT respond to authoritative Next Hop
Resolution Requests with cached information.
[Note: An NHRP reply can be returned directly to the Next Hop
Resolution Request initiator, i.e., without traversing the list of
NHSs that forwarded the request, if all of the following criteria
are satisfied:
(a) Direct communication is available via datagram transfer
(e.g., SMDS) or the NHS has an existing virtual circuit
connection to the Next Hop Resolution Request initiator or is permitted
to open one.
(b) The Next Hop Resolution Request initiator has not included the NHRP
Reverse NHS record Extension (see Section 5.3.5).
(c) The authentication policy in force permits direct
communication between the NHS and the Next Hop Resolution Request
initiator.
The purpose of allowing an NHS to reply directly is to reduce
response time. A consequence of allowing a direct reply is that
NHSs that would under normal circumstances be traversed by the
reply would not cache next hop information contained therein.]
The process of forwarding the Next Hop Resolution Request is repeated
until the request is satisfied, or an error occurs (e.g., no NHS in
the NBMA subnetwork can resolve the request.) If the determination is
made that station D's next hop cannot be resolved, a negative reply
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 Next Hop Resolution Request (e.g., connectivity is lost).
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Next Hop Resolution Requests and replies MUST NOT cross the borders
of a logical NBMA subnetwork (an explicit NBMA subnetwork identifier
may be included as an extension in the Next Hop Resolution Request,
see section 5.3.2). Thus, IP traffic out of and into a logical NBMA
subnetwork always traverses an IP router at its border. Internetwork
layer filtering can then be implemented at these border routers.
NHRP optionally provides a mechanism to reply with aggregated NBMA
next hop information. Suppose that router X is the NBMA next hop
from station S to station D. Suppose further that X is an egress
router for all stations sharing an IP address prefix with station D.
When an NHRP reply is generated in response to a request, the
responder may augment the IP address of station D with a mask
defining this prefix (see Section 5.3.1). A subsequent (non-
authoritative) Next Hop Resolution Request for some destination that
shares an IP address prefix 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), as
well as to provide loop detection and diagnostic capabilities, NHRP
optionally incorporates a "Route Record" in requests and replies (see
Sections 5.3.4 and 5.3.5). The Route Record extensions contain the
internetwork (and subnetwork layer) addresses of all intermediate
NHSs between source and destination (in the forward direction) and
between destination and source (in the reverse direction). 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 Route Record 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).
NHRP messages, with the exception of Purge packets, are sent using a
best effort delivery service. Next Hop Resolution Requests should be
retransmitted periodically until either a Reply or an Error packet is
received.
3. Deployment
Next Hop 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
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neighboring NHS to forward the request on to. The NHS selection
procedure typically involves performing a routing decision based upon
the network layer destination address of the Next Hop Resolution
Request. Ignoring error situations, the Next Hop Resolution Request
eventually arrives at a station that is to generate an NHRP reply.
This responding station either serves the destination, or is the
destination itself if both NHRP client and server functionality are
co-resident in the same station. The responding station generates a
reply using the source address from within the NHRP packet to
determine where the reply should be sent.
The Next Hop Resolution Request packet is carried at the NBMA layer,
with a destination NBMA address set to that of the locally determined
NHS. If the addressed NHS does not serve the destination address
specified in the Next Hop Resolution Request, the request packet is
routed at the network layer based upon the request's destination
address, and forwarded to the neighboring NHS determined by the
routing decision. Alternately, the NHS may use static configuration
information in order to determine which neighboring NHSs to forward
the request packet to. Each NHS/router examines the NHRP request
packet on its way toward the destination, optionally modifying it on
the way (such as updating the Forward Record extension), and
continues to forward it until it reaches the NHS that serves the
destination network layer address.
In order to forward NHRP packets to a neighboring NHS, NHRP clients
must nominally be configured with the NBMA address of at least one
NHS. In practice, a client's default router should also be its NHS.
A client may be able to derive the NBMA address of its NHS from the
configuration that was already required for the client to be able to
communicate with its next hop router.
Forwarding of NHRP packets within an NBMA subnetwork requires a
contiguous deployment of NHRP capable stations. During migration to
NHRP, it cannot be expected that all stations within the NBMA
subnetwork are NHRP capable. NHRP traffic which would otherwise need
to be forwarded through such stations can be expected to be dropped
due to the NHRP packet being unrecognized. In this case, NHRP will
be unable to establish any transit paths whose discovery requires the
traversal of the non-NHRP speaking stations. In such a scenario,
NHRP is still able to provide basic address resolution functionality
between stations which do support NHRP.
The path taken by Next Hop Resolution Requests will normally be the
same as the path taken by data packets which are routed at the
network layer to the desired destination. (The paths may be
different in situations where NHSs have been statically configured to
forward traffic by other means. For example, an Next Hop Resolution
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Request may be forwarded to a group multicast address.)
NHSs should acquire knowledge about destinations other NHSs serve as
a direct consequence of participating in intradomain and interdomain
routing protocol exchange. In this case, 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.
NHSs (and end stations) may alternately be statically configured with
the NBMA addresses of their neighbors, the identities of the
destinations that each of them serves, and optionally a logical NBMA
subnetwork identifier. Such static configurations may be necessary
in cases where NHSs do not contain network layer routing protocol
implementations.
If the NBMA subnetwork offers a group addressing or multicast
feature, the client (station) may be configured with a group address
assigned to the group of next-hop servers. The client might then
submit Next Hop Resolution Requests to the group address, eliciting a
response from one or more NHSs, depending on the response strategy
selected. Note that the constraints described in Section 2 regarding
direct replies may apply.
NHSs may also be deployed with the group or multicast address of
their peers, and an NHS might use this as a means of forwarding Next
Hop Resolution Requests it cannot satisfy to its peers. This might
elicit a response (to the NHS) from one or more NHSs, depending on
the response strategy. The NHS would then forward the NHRP reply to
the Next Hop Resolution Request originator. The purpose of using
group addressing or a similar multicast mechanism in this scenario
would be to eliminate the need to preconfigure each NHS in a logical
NBMA subnetwork with both the individual identities of other NHSs as
well as the destinations they serve. It reduces the number of NHSs
that might be traversed to process an Next Hop Resolution Request (in
those configurations where NHSs either respond or forward via the
multicast, only two NHSs would be traversed), and allows the NHS that
serves the Next Hop Resolution Request originator to cache next hop
information associated with the reply (again, within the constraints
described in Section 2).
4. Configuration
Stations
To participate in NHRP, a station connected to an NBMA subnetwork
should be configured with the NBMA address(es) of its NHS(s)
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(alternatively, it should be configured with a means of acquiring
them, i.e., the group address that members of a NHS group use for
the purpose of address or next-hop resolution.) The NHS(s) will
likely also represent the stations's default or peer routers, so
their NBMA addresses may be obtained from the station's existing
configuration. If the station is attached to several subnetworks
(including logical NBMA subnetworks), the station should also be
configured to receive routing information from its NHS(s) and peer
routers so that it can determine which IP networks are reachable
through which subnetworks.
Next Hop Servers
An NHS is configured with knowledge of its own IP and NBMA
addresses, a set of IP address prefixes that correspond to the IP
addresses of the stations it serves, and a logical NBMA subnetwork
identifier (see Section 5.3.2). If a served station is attached to
several subnetworks, the NHS may also need to be configured to
advertise routing information to such stations.
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.
The NBMA addresses of the stations served by the NHS may be learned
via NHRP Register packets or manual configuration.
5. NHRP Packet Formats
This section describes the format of NHRP packets.
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$pktsz) minus 20 otherwise
the mandatory part lenth is equal to ar$extoff minus 20. The length
of the Extensions Part is implied by ar$pktsz minus ar$extoff minus
20. NHSs may increase the size of an NHRP packet as a result of
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extension processing, but not beyond the offered maximum SDU size of
the NBMA network.
NHRP packets are encapsulated using the native formats used on the
particular NBMA network over which NHRP is carried. For example,
SMDS networks always use LLC/SNAP encapsulation at the NBMA layer,
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 NHRP (see [4]).
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$op.type) have both internetwork layer
protocol-independent fields and protocol-specific fields. The
protocol-independent fields always come first in the packet, and the
protocol type/snap fields (ar$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.
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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$hrd | ar$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$hrd
Defines the type of "link layer" addresses being carried. This
number is taken from the number hardware type from the list in [6].
***The use of the number hardware type or of the address family number
is still an open issue.
ar$pro.type
field is a 16 bit unsigned integer representing the following
number space:
0x0000 to 0x00FF Protocols defined by the equivalent NPLIDs.
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 NPLIDs 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$pro.type space (excluding 0x0080) the short form MUST be used
when transmitting NHRP messages. Additionally, where a protocol has
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valid short and long forms of identification, receivers MAY choose
to recognise the long form.
ar$hopcnt
The Hop count indicates the maximum number of NHSs that an NHRP
packet is allowed to traverse before being discarded.
ar$pktsz
The total length of the NHRP packet, in octets (exluding link layer
encapsulation).
ar$chksum
The standard IP checksum over the entire NHRP packet (starting with
the fixed header). If only the hop count field is changed, the
checksum is adjusted without full recomputation. The checksum is
completely recomputed when other header fields are changed.
ar$extoff
This field identifies the existence and location NHRP extensions.
If this field is 0 then no extensions exist otherwise this field
represents the offset from the beginning of the NHRP packet (i.e.,
starting from the ar$afn field) of the first extension.
ar$op.version
This field is set to 0x0001 for NHRP version 1.
ar$op.type
This is the NHRP packet type (request, reply, purge, or error).
ar$shtl
Type & length of source NBMA address interpreted in the context of
the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM).
ar$sstl
Type & length of source NBMA subaddress interpreted in the context
of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). When an NBMA technology has no concept of
a subaddress the subaddress is always null and ar$sstl = 0 and no
storage is allocated for the address in the appropriate mandatory
part.
The ar$shtl and ar$sstl fields are coded as follows:
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7 6 5 4 3 2 1 0
+-+-+-+-+-+-+-+-+
|0|x| 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 NBMA
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.
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 Mandatory Part
The Mandatory Part of the NHRP packet contains the operation specific
information (e.g., Next Hop Resolution request/reply, etc.) and
variable length data which is pertinent to the packet type.
5.2.1 Next Hop Resolution Request
The Next Hop Resolution Request packet has a Type code of 1. The
Mandatory Part has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q|A|P|B| Unused| Proto Length | Source Address Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Request ID
A value which, when coupled with the address of the source,
provides a unique identifier for the information contained in a
Request and its associated Next Hop Resolution Reply, and any
subsequent Purge. This value can be used by the source to aid in
matching requests with replies. This value could also be sent
across a virtual circuit (in SVC environments) to aid in matching
NHRP transactions with virtual circuits (this use is for further
study).
The value is taken from a 32 bit counter that is incremented each
time a new NHRP request is transmitted. The same value MUST be
used when sending another request for the same destination when a
previous request is still active or pending, i.e., when
retransmitting a Next Hop Resolution Request because a Next Hop
Resolution Reply was not received, or when refreshing an existing
entry to avoid holding timer expiration. A new value MUST be used
when sending a request when no cache entry is present, or a
previous cache entry was deleted for any reason.
Next Hop Resolution Request Flags
Q
Set if the requester is a router; clear if the requester is a
host.
A
A response to an NHRP request may contain cached information. If
an authoritative answer is desired, then this bit
("Authoritative") should be set. If non-authoritative (cached)
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information is acceptable, this bit should be clear.
P
Unused (clear on transmit)
B
Unused (clear on transmit)
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Source Address Holding Time
The Source Address Holding Time field specifies the number of
seconds for which the source NBMA information is considered to be
valid. Cached information SHALL be discarded when the holding time
expires.
Source NBMA Address
The Source NBMA address field is the address of the source station
which initiated the request. It is zero-filled to the nearest 32-
bit boundary. However, if its 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 initiated the request. It is zero-filled to the
nearest 32- bit boundary. However, if its length as specified in
ar$sstl is 0 then no storage is allocated for this address at all.
Source Protocol Addresses
This is the protocol address of the station which initially issued
an NHRP Request packet.
Destination Protocol Address
This is the protocol address of the station for which the NBMA next
hop is desired.
(The NBMA address/subaddress form allows combined E.164/NSAPA form of
NBMA addressing. For NBMA technologies without a subaddress concept,
the subaddress field is always ZERO length and ar$sstl = 0.)
5.2.2 NHRP Next Hop Resolution Reply
The NHRP Next Hop Resolution Reply packet has a type code of 2. The
Mandatory Part has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Q|A|P|B| Unused| Proto Length | Next Hop Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NH Addr T/L | NH SAddr T/L | Preference | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Request ID
A value which, when coupled with the address of the source,
provides a unique identifier for the information contained in a
Request and its associated Next Hop Resolution Reply, and any
subsequent Purge. This value can be used by the source to aid in
matching requests with replies. This value could also be sent
across a virtual circuit (in SVC environments) to aid in matching
NHRP transactions with virtual circuits (this use is for further
study).
The value is taken from a 32 bit counter that is incremented each
time a new NHRP request is transmitted. The same value MUST be
used when sending another request for the same destination when a
previous request is still active or pending, i.e., when
retransmitting a Next Hop Resolution Request because a Next Hop
Resolution Reply was not received, or when refreshing an existing
entry to avoid holding timer expiration. A new value MUST be used
when sending a request when no cache entry is present, or a
previous cache entry was deleted for any reason.
Next Hop Resolution Request Flags
Q
Copied from the Next Hop Resolution Request. Set if the
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Requester is a router; clear if the requester is a host.
A
Set if the next hop in the Next Hop Resolution Reply is
authoritative; clear if the Next Hop Resolution Reply is non-
authoritative.
P
Set if the Next Hop Resolution Reply is positive; clear if the
Next Hop Resolution Reply is negative.
B
Set if the association between the destination and the next hop
information is guaranteed to be stable for the lifetime of the
information (the holding time). This is the case if the Next Hop
protocol address 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 information affects cacheing
strategies (see section 6.2).
An NHS is not allowed to reply to an Next Hop Resolution Request
for authoritative information with cached information, but may do
so for an NHRP Next Hop Resolution Request which indicates a
request for non-authoritative information. An NHS may reply to an
Next Hop Resolution Request for non-authoritative information
with authoritative information.
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Next Hop Address Holding Time
The Next Hop Address Holding Time field specifies the number of
seconds for which the next hop NBMA information is considered to be
valid. Cached information SHALL be discarded when the holding time
expires. Must be set to 0 on a NAK.
NH Addr T/L
Type & length of next hop NBMA address interpreted in the context
of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). When the address length is specified as 0
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no storage is allocated for the address. Set to 0 on a NAK.
NH SAddr T/L
Type & length of next hop NBMA subaddress interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). 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. Set to 0 on a NAK.
Preference
This field specifies the preference of the Next Hop entry, relative
to other Next Hop entries in this NHRP Next Hop Resolution Reply
packet which may be in the Additional Next Hop Entries Extension
for the given internetworking protocol. Higher values indicate
more preferable Next Hop entries. Action taken when multiple next
hop entries have the highest preference value is a local matter.
Set to 0 on a NAK.
Source NBMA Address
The Source NBMA address field is the address of the source station
which initiated the request. It is zero-filled to the nearest 32-
bit boundary. However, if its length as specified in ar$shtl is 0
then no storage is allocated for this address at all.
Source NBMA SubAddress
The Source NBMA subaddress field is the address of the source
station which initiated the request. It is zero-filled to the
nearest 32- bit boundary. However, if its length as specified in
ar$sstl is 0 then no storage is allocated for this address at all.
Source Protocol Addresses
This is the protocol address of the station which initially issued
an NHRP Request packet.
Destination Protocol Address
This is the protocol address of the station for which the NBMA next
hop is desired.
(The NBMA address/subaddress form 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.)
Next Hop entry
Next Hop Protocol Address
This internetworkin layer address specifies the next hop. This
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will be the address of the destination host if it is directly
attached to the NBMA subnetwork, or the egress router if it is
not directly attached.
Next Hop NBMA Address
This is the NBMA address of the station that is the next hop for
packets bound for the internetworking layer address specified.
The NBMA address field itself is zero-filled to the nearest 32-
bit boundary.
Next Hop NBMA SubAddress
This is the NBMA sub address of the station that is the next hop
for packets bound for the internetworking layer address
specified. The NBMA subaddress field itself is zero-filled to
the nearest 32-bit boundary.
There may be multiple Next Hop entries returned in the Next Hop
Resolution Reply by including the Additional Next Hop Entries
Extension. See Section 5.3.9 for use of these entries. The most
preferable Next Hop must be specified in the mandatory part of the
Next Hop Resolution Reply.
Any extensions present in the Next Hop Resolution Request packet MUST
be present in the NHRP Next Hop Resolution Reply packet, except for
the case of unrecognized non-Compulsory extensions.
If an unsolicited NHRP Next Hop Resolution Reply packet is received,
an Error Indication of type Invalid Next Hop Resolution Reply
Received SHOULD be sent in response.
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. The Mandatory Part has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | Register Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Request ID
A value which, when coupled with the address of the source,
provides a unique identifier for the information contained in a
Registration Request packet. This value is copied directly from a
Registration Request packet into the associated Registration Reply.
This value could also be sent across a virtual circuit (in SVC
environments) to aid in matching NHRP transactions with virtual
circuits (this use is for further study).
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Source Protocol Address
The Protocol Address of the station wishing to register its NBMA
address with an NHS.
Register Holding Time
The Register Holding Time field specifies the number of seconds for
which the registered NBMA information is considered to be valid.
Cached information SHALL be discarded when the holding time
expires.
Source NBMA Address
The Source NBMA address field is the address of the source station
which initiated the request. It is zero-filled to the nearest 32-
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bit boundary. However, if its 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 initiated the request. It is zero-filled to the
nearest 32- bit boundary. However, if its length as specified in
ar$sstl is 0 then no storage is allocated for this address at all.
Source Protocol Addresses
This is the protocol address of the station which initially issued
an NHRP Request packet.
Destination Protocol Address
This is the protocol address of the NHS for which the source NBMA
next hop information is being reistered
This packet is used to register a station's Protocol and NBMA
addresses with its NHSs, as configured or known through conventional
routing means. This allows static configuration information to be
reduced; the NHSs need not be configured with the identities of all
of the stations that they serve.
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 resend the NHRP Registration Request packet often enough
to refresh the registration, even in the face of occasional packet
loss. It is recommended that the 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 Registration Request. If the NAK Code field has
anything other than 0 zero in it then the Registration Reply is a NAK
otherwise the reply is an ACK. The Registration Reply has a Type
code of 4. Its mandatory part has the following format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NAK Code | Proto Length | Register Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Request ID
A value which, when coupled with the address of the source,
provides a unique identifier for the information contained in a
Registration Request packet. This value is copied directly from a
Registration Request packet into the associated Registration Reply.
This value could also be sent across a virtual circuit (in SVC
environments) to aid in matching NHRP transactions with virtual
circuits (this use is for further study).
NAK Code
If this field is set to zero then this packet contains a
Registration Request Acknowledgement. If this field contains any
other value then this contains a Registration Request NAK.
Currently defined NAK Codes are as follows:
4 - Can't Serve This Address
An NHS may refuse an NHRP registration attempt for
administrative reasons. If so, the NHS MUST reply with this
NAK in the Registration Reply which contains a NAK code of 4.
5 - Registration Overflow
If an NHS cannot serve a station due to a lack of resources,
the NHS MUST reply with a NAKed Registration Reply which
contains a NAK code of 5.
Proto Length
The length in octets, of the internetwork layer protocol addresses
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appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Source Protocol Address
The Protocol Address of the client that sent a Registration
Request.
Register Holding Time
The Register Holding Time field specifies the number of seconds for
which the registered NBMA information is considered to be valid.
Cached information SHALL be discarded when the holding time
expires.
Source NBMA Address
The Source NBMA address field is the address of the source station
which initiated the request. It is zero-filled to the nearest 32-
bit boundary. However, if its 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 subaddress of the source
station which initiated the request. It is zero-filled to the
nearest 32-bit boundary. However, if its length as specified in
ar$sstl is 0 then no storage is allocated for this address at all.
Source Protocol Addresses
This is the protocol address of the station which initially issued
an NHRP request packet.
Destination Protocol Address
This is the protocol address of the station for which the NBMA next
hop is desired.
This packet is used to register a station's Protocol and NBMA
addresses with its neighboring NHSs, as configured or known through
conventional routing means. This allows static configuration
information to be reduced; the NHSs need not be configured with the
identities of all of the stations that they serve.
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
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Overflow.
In order to keep the registration entry from being discarded, the
station MUST resend the NHRP Registration Request packet often enough
to refresh the registration, even in the face of occasional packet
loss. It is recommended that the Registration Request packet be sent
at an interval equal to one-third of the Holding Time specified
therein.
5.2.5 NHRP Purge
The NHRP Purge packet has a type code of 5. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A| Unused | Proto Length | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A
Clear if this is a purge request, set if this is an acknowledgment.
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Source NBMA Address
The Source NBMA address field is the address of the source station
which initiated the Purge. It is zero-filled to the nearest 32-
bit boundary. However, if its length as specified in ar$shtl is 0
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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 initiated the Purge. It is zero-filled to the
nearest 32- bit boundary. However, if its length as specified in
ar$sstl is 0 then no storage is allocated for this address at all.
Source Protocol Address
The address of the station which is sending the purge notification.
Destination Protocol Address
The address of the station that will be receiving the purge
notification.
Target Protocol Address
The address which is to be purged from the receiver's database.
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 a client to an NHS
with which the client had previously sent a registration packet to.
This allows for a client to invalidate an NHRP registration before it
would otherwise expire.
The station sending the NHRP Purge request MUST periodically
retransmit the request until it is acknowledged, or until the holding
time of the information being purged has expired. Retransmission
strategies are for further investigation.
When a station receives an NHRP Purge request, it MUST discard any
previous cached information that matches the Target Protocol Address.
An acknowledgment MUST be returned for the Purge request even if the
station does not have a matching cache entry. The acknowledgment is
constructed by setting the Acknowledgment (A) bit and returning the
Purge request to the Source Protocol Address.
If the station wishes to reestablish communication with the
destination shortly after receiving a Purge request, it should make
an authoritative 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 Next Hop Resolution Requests be
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made for the duration of the holding time of the old information.
5.2.6 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 6. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Error Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 is set, the NHRP
request cannot be satisfied unless the extension is processed.
The responder MUST return an 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.
2 - Subnetwork ID Mismatch
This error occurs when the current station receives an NHRP
packet whose NBMA subnetwork identifier matches none of the
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locally known identifiers for the NBMA subnetwork on which the
packet is received.
3 - NHRP Loop Detected
A Loop Detected error is generated when it is determined that
an NHRP packet is being forwarded in a loop.
8 - NHRP SDU Size Exceeded
If the SDU size of the NHRP packet exceeds the maximum SDU size
of the NBMA network, 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 Next Hop Resolution Reply Received
If a client receives a Next Hop 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.
Error Offset
The offset in octets into the original NHRP packet, starting at the
NHRP Fixed Header, at which the error was detected.
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Source NBMA Address
The Source NBMA address field is the address of the source station
which observed the error. It is zero-filled to the nearest 32- bit
boundary. However, if its 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 observed the error. It is zero-filled to the nearest
32- bit boundary. However, if its length as specified in ar$sstl is
0 then no storage is allocated for this address at all.
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Source Protocol Addresses
This is the protocol address of the station which issued the Error
packet.
Destination Protocol Address
This is the protocol address of the station for which the NBMA next
hop is desired. However, if its length as specified in Proto Length
is set to 0 then no storage is allocated for this address at all.
An Error Indication packet SHALL NEVER be generated in response to
another Error Indication packet. When an Error Indication packet is
generated, the offending NHRP packet SHALL be discarded. In no case
should more than one Error Indication packet be generated for a
single NHRP packet.
5.3 Extensions Part
The Extensions Part, if present, carries one or more extensions in
{Type, Length, Value} triplets. Extensions are only present in a
Reply if they were present in the corresponding Request; therefore,
minimal NHRP station implementations that do not act as an NHS and do
not transmit extensions need not be able to receive them. An
implementation that is incapable of processing extensions SHALL
return an Error Indication of type Unrecognized Extension when it
receives an NHRP packet containing extensions.
Extensions are typically protocol-specific, as noted.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|u| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
C
"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.)
u
Unused and must be set to zero.
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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).
Each extension is padded with zero octets to a 32 bit boundary. This
padding is not included in the Length field.
When extnsions exist, 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
(other than the vendor-private extension) may occur only once in an
NHRP packet. The vendor-private extension may occur multiple times
in a packet, to allow for extensions which do not share the same
vendor ID to be represented.
The Compulsory bit provides for a means to add to the extension set.
If the bit is set, the NHRP request cannot be satisfied unless the
extension is processed, so the responder MUST return an Error
Indication of type Unrecognized Extension. If the bit is clear, the
extension can be safely ignored, though unrecognized extensions so
ignored that were received in an NHRP Request packet MUST be returned
unchanged in the corresponding NHRP Reply.
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
(in the case of a reply) and MUST NOT identify itself as an egress
router (in the Forward Record or Reverse Record extensions).
Effectively, this means that a transit NHS that encounters an
extension that it cannot process and determines that the Compulsory
bit is set MUST NOT participate in any way in the protocol exchange,
other than acting as a forwarding agent for the request.
5.3.1 Destination Prefix Extension
Compulsory = 0
Type = 1
Length = 1
This extension is used to indicate that the information carried in an
NHRP Reply pertains to an equivalence class of destinations rather
than just the destination Protocol address specified in the request.
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All addresses that match the destination Protocol address in the bit
positions for which the mask has a one bit are part of the
equivalence class.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Mask (variable lenfth) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The size of the mask in number of octets is obtained through the
"Proto Length" in the Mandatory Part of packet.
For example, in the case of IPv4, if an initiator would like to
receive this equivalence information, it SHALL add this extension to
the NHRP Request with a value of 255.255.255.255. The responder
SHALL copy the extension to the NHRP Reply and modify the mask
appropriately.
5.3.2 NBMA Subnetwork ID Extension
Compulsory = 1
Type = 2
Length = variable
This extension is used to carry one or more identifiers for the NBMA
subnetwork. This can be used as a validity check to ensure that the
request does not leave a particular NBMA subnetwork. The extension
is placed in an NHRP Request packet by the initiator with an ID value
of zero; the first NHS fills in the field with the identifier(s) for
the NBMA subnetwork.
Multiple NBMA Subnetwork IDs may be used as a transition mechanism
while NBMA Subnetworks are being split or merged.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NBMA Subnetwork ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
Each identifier consists of a 32 bit globally unique value assigned
to the NBMA subnetwork. This value should be chosen from the IP
address space administered by the operators of the NBMA subnetwork.
This value is used for identification only, not for routing or any
other purpose.
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Each NHS processing an NHRP Request SHALL verify these values. If
none of the values matches the NHS's NBMA Subnetwork ID, the NHS
SHALL return an Error Indication of type "Subnetwork ID Mismatch" and
discard the NHRP Request.
When an NHS is building an NHRP Reply and the NBMA Subnetwork ID
extension is present in the NHRP Request, the NBMA Subnetwork ID
extension SHALL be copied from the Request to the Reply, including
all values carried therein.
Each NHS processing an NHRP Reply SHALL verify the values carried in
the NBMA Subnetwork ID extension, if present. If none of the values
matches the NHSs NBMA Subnetwork ID, the NHS SHALL return an Error
Indication of type "Subnetwork ID Mismatch" and discard the NHRP
Reply.
5.3.3 Responder Address Extension
Compulsory = 1
Type = 3
Length = 4
This extension is used to determine the Protocol address of the NHRP
Responder, that is, the entity that generates the NHRP Reply packet.
The intent is to identify the entity responding to the request, which
may be different (in the case of cached replies) than the system
identified in the Next Hop field of the reply, and to aid in
detecting loops in the NHRP forwarding path.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder's Protocol Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The size of the Responder's Protocol Address in octets is obtained
through the "Proto Length" in the Mandatory Part of packet.
If a requester desires this information, it SHALL include this
extension, with a value of zero, in the NHRP Request packet.
If an NHS is generating an NHRP Reply packet in response to a request
containing this extension, it SHALL include this extension,
containing its Protocol address, in the NHRP 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 NHS
Record, and Reverse NHS Record extensions. The choice of which of
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several Protocol addresses to include in this extension is a local
matter.
If an NHRP Next Hop Resolution Reply packet being forwarded by an NHS
contains an Protocol address of that NHS in the Responder Address
Extension, the NHS SHALL generate an Error Indication of type "NHRP
Loop Detected" and discard the Next Hop Resolution Reply.
If an NHRP Next Hop 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).
5.3.4 NHRP Forward NHS Record Extension
Compulsory = 1
Type = 4
Length = variable
The NHRP forward NHS record is a list of NHSs through which an NHRP
request traverses. Each NHS SHALL append a Next Hop element
containing its Protocol address to 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NHS Addr T/L | NHS SAddr T/L| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
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Holding Time
The Holding Time field specifies the number of seconds for which
the NBMA information is considered to be valid. Cached information
SHALL be discarded when the holding time expires. Must be set to 0
on a NAK.
NHS Addr T/L
Type & length of transit NHS's NBMA address interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). When the address length is specified as 0
no storage is allocated for the address. Set to 0 on a NAK.
NHS SAddr T/L
Type & length of transit NHS's NBMA subaddress interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). 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. Set to 0 on a NAK.
NHS Protocol Address
This internetworkin layer address specifies the transit NHS. This
will be the address of the destination host if it is directly
attached to the NBMA subnetwork, or the egress router if it is not
directly attached.
NHS NBMA Address
This is the NBMA address of the station that is the transit NHS for
packets bound for the internetworking layer address specified. The
NBMA address field itself is zero-filled to the nearest 32-bit
boundary.
NHS NBMA SubAddress
This is the NBMA sub address of the station that is the transit NHS
for packets bound for the internetworking layer address specified.
The NBMA subaddress field itself is zero-filled to the nearest 32-
bit boundary.
If a requester wishes to obtain this information, it SHALL include
this extension with a length of zero.
Each NHS SHALL append an appropriate Next Hop element to this
extension when processing an NHRP Request. The extension length
field and NHRP checksum SHALL be adjusted as necessary.
The last Hop NHS (the one that will be generating the NHRP Reply)
SHALL NOT update this extension (since this information will be in
the reply).
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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 an NHRP Request packet being forwarded by an NHS contains the
Protocol address of that NHS in the Forward NHS Record Extension, the
NHS SHALL generate an Error Indication of type "NHRP Loop Detected"
and discard the Request.
5.3.5 NHRP Reverse NHS Record Extension
Compulsory = 1
Type = 5
Length = variable
The NHRP reverse NHS record is a list of NHSs through which an NHRP
reply traverses. Each NHS SHALL append a Next Hop element containing
its Protocol address to 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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NHS Addr T/L | NHS SAddr T/L| unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Holding Time
The Holding Time field specifies the number of seconds for which
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the NBMA information is considered to be valid. Cached information
SHALL be discarded when the holding time expires. Must be set to 0
on a NAK.
NHS Addr T/L
Type & length of transit NHS's NBMA address interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). When the address length is specified as 0
no storage is allocated for the address. Set to 0 on a NAK.
NHS SAddr T/L
Type & length of transit NHS's NBMA subaddress interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). 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. Set to 0 on a NAK.
NHS Protocol Address
This internetworkin layer address specifies the transit NHS. This
will be the address of the destination host if it is directly
attached to the NBMA subnetwork, or the egress router if it is not
directly attached.
NHS NBMA Address
This is the NBMA address of the station that is the transit NHS for
packets bound for the internetworking layer address specified. The
NBMA address field itself is zero-filled to the nearest 32-bit
boundary.
NHS NBMA SubAddress
This is the NBMA sub address of the station that is the transit NHS
for packets bound for the internetworking layer address specified.
The NBMA subaddress field itself is zero-filled to the nearest 32-
bit boundary.
If a requester wishes to obtain this information, it SHALL include
this extension with a length of zero.
Each NHS SHALL append an appropriate Next Hop element to this
extension when processing an NHRP Reply. The extension length field
and NHRP checksum SHALL be adjusted as necessary.
The NHS generating the NHRP Reply SHALL NOT update this extension.
If an NHS has more than one Protocol address, it SHALL use the same
Protocol address consistently in all of the Responder Address,
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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 an NHRP Reply packet being forwarded by an NHS contains the
Protocol address of that NHS in the Reverse NHS Record Extension, the
NHS SHALL generate an 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. Note
that the Responder Address extension may be used to disambiguate the
set of NHSs that actually processed the reply.
5.3.6 NHRP QoS Extension
Compulsory = 0
Type = 6
Length = variable
The NHRP QoS Extension is carried in NHRP Request packets to indicate
the desired QoS of the path to the indicated destination. This
information may be used to help select the appropriate NBMA next hop.
It may also be carried in NHRP Register packets to indicate the QoS
to which the registration applies.
The syntax and semantics of this extension are TBD; alignment with
resource reservation may be useful.
5.3.7 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 packet type.
Authentication is done pairwise on an NHRP hop-by Hop basis; the
authentication extension is regenerated on each hop. If a received
packet fails the authentication test, the NHS SHALL generate an Error
Indication of type "Authentication Failure" and discard the packet.
In no case SHALL an Error Indication packet be generated on the
receipt of an Error Indication packet, however. Note that one
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possible authentication failure is the lack of an Authentication
Extension; the presence or absence of the Authentication Extension
is a local matter.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+ Authentication Data... -+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Authentication Type field identifies the authentication method in
use. Currently assigned values are:
1 - Cleartext Password
2 - Keyed MD5
All other values are reserved.
The Authentication Data field contains the type-specific
authentication information.
In the case of Cleartext Password Authentication, the Authentication
Data consists of a variable length password.
In the case of Keyed MD5 Authentication, the Authentication Data
contains the 16 byte MD5 digest of the entire NHRP packet, including
the encapsulated protocol's header, with the authentication key
appended to the end of the packet. The authentication key is not
transmitted with the packet.
Distribution of authentication keys is outside the scope of this
document.
5.3.8 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. This extension may be used at any time; if the receiver
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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. The first 24 bits of the extension's payload (following
the length field) contains the 802 vendor ID as assigned by the IEEE
[6]. The remaining octets in the payload are vendor-dependent.
5.3.9 Additional Next Hop Entries Extension
Compulsory = 0
Type = 9
Length = variable
This extension may be used to return multiple Next Hop entries in a
single NHRP Reply packet. This extension MUST only be used for
positive replies. The preference values are used to specify the
relative preference of the entries contained in the extension. The
same next Hop Protocol address may be associated with multiple NBMA
addresses. Load-splitting may be performed over the addresses, given
equal preference values, and 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).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | Next Hop Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NH Addr T/L | NH SAddr T/L | Preference | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....................
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unused | Proto Length | Next Hop Holding Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NH Addr T/L | NH SAddr T/L | Preference | unused |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop Protocol Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Address (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop NBMA Subaddress (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An NHS is not allowed to reply to an NHRP request for authoritative
information with cached information, but may do so for an NHRP
Request which indicates a request for non-authoritative information.
An NHS may reply to an NHRP request for non-authoritative information
with authoritative information.
Proto Length
The length in octets, of the internetwork layer protocol addresses
appearing in this packet. If this length is not a multiple of 4,
each internetwork layer address is zero-filled to the nearest 32-
bit boundary.
Next Hop Holding Time
The Next Hop Address Holding Time field specifies the number of
seconds for which the next hop NBMA information is considered to be
valid. Cached information SHALL be discarded when the holding time
expires. Must be set to 0 on a NAK.
NH Addr T/L
Type & length of next hop NBMA address interpreted in the context
of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). When the address length is specified as 0
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no storage is allocated for the address. Set to 0 on a NAK.
NH SAddr T/L
Type & length of next hop NBMA subaddress interpreted in the
context of the 'hardware' (NBMA media) indicated by ar$afn (e.g.,
ar$afn=0x0003 for ATM). 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. Set to 0 on a NAK.
Preference
This field specifies the preference of the Next Hop entry, relative
to other Next Hop entries in this NHRP Reply packet which may be in
the Additional Next Hop Entries Extension for the given
internetworking protocol. Higher values indicate more preferable
Next Hop entries. Action taken when multiple next hop entries have
the highest preference value is a local matter. Set to 0 on a NAK.
Next Hop Protocol Address
This internetworkin layer address specifies the next hop. This
will be the address of the destination host if it is directly
attached to the NBMA subnetwork, or the egress router if it is not
directly attached.
Next Hop NBMA Address
This is the NBMA address of the station that is the next hop for
packets bound for the internetworking layer address specified. The
NBMA address field itself is zero-filled to the nearest 32-bit
boundary.
Next Hop NBMA SubAddress
This is the NBMA sub address of the station that is the next hop
for packets bound for the internetworking layer address specified.
The NBMA subaddress field itself is zero-filled to the nearest 32-
bit boundary.
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
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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 Cacheing Requirements
Source Stations
Source stations MUST cache all received replies that they are
actively using. They also must cache "incomplete" entries, i.e.,
those for which a request has been sent but which a 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 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.
Source stations that are also NHSs may return cached information
learned in response to its own Next Hop Resolution Request packets
in reply to requests it receives, within the rules for Transit NHSs
below.
Serving NHSs
The NHS serving the destination (the one which responds
authoritatively to Next Hop Resolution Requests) SHOULD cache
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information about all requests to which it has responded if the
information in the reply has the possibility of changing during its
lifetime (so that an NHRP Purge request packet can be sent). The
NBMA information provided by the source station in the Next Hop
Resolution Request may be cached for the duration of its holding
time. This information is considered to be stable, since it
identifies a station directly attached to the NBMA subnetwork. An
example of unstable information is NBMA information derived from a
routing table, where that routing table information has not been
guaranteed to be stable through administrative means.
Transit NHSs
A Transit NHS (lying along the NHRP path between the source station
and the responding NHS) may cache information contained in Next Hop
Resolution Request packets that it forwards. A Transit NHS may
cache information contained in NHRP Reply packets that it forwards
only if that reply has the Stable (B) bit set. It MUST discard any
cached information whose holding time has expired. It may return
cached information in response to non-authoritative 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 Next Hop 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 replies as being
"stable" (via the "B" bit).
This also applies equally well to source stations that are routers
as well as those which are hosts.
Note that the information carried in the Next Hop Resolution
Request packet is always considered "stable" because it represents
a station that is directly connected to the NBMA subnetwork.
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Destinations Off of the NBMA Subnetwork
If the source of a 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).
Strategies for maintaining NHRP cache information in the presence
of dynamic routing changes will be discussed in a separate
document.
6.3 Use of the Destination Prefix Extension
A certain amount of care needs to be taken when using the Destination
Prefix Extension, 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 Destination Prefix Extension 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 IP address in the
Next Hop Resolution Request), and extracts from this NLRI the
shortest address prefix such that: (a) the Destination IP address
(from the Next Hop Resolution Request) is covered by the prefix,
(b) the NHS does not have any routes with NLRI that forms a subset
of what is covered by the prefix. The prefix may then be used for
the Destination Prefix Extension.
The NHRP Destination Prefix Extension should be used with restraint,
in order to avoid NHRP stations choosing suboptimal transit paths
when overlapping prefixes are available. This extension SHOULD only
be used in an NHRP Reply when either:
(a) All destinations covered by the prefix are on the NBMA network, or
(b) All destinations covered by the prefix are directly attached to
the NHRP responding station.
For other cases, there may be no single optimal transit path for
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destinations encompassed by the address prefix, and an NHRP station
may fail to choose the optimal transit path simply because it is not
aware of all such paths. So for cases not covered by (a) and (b), an
NHRP Reply packet should not include the NHRP Destination Prefix
Extension.
6.4 Domino Effect
One could easy 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 Next Hop 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 Next Hop Resolution Request (as well as forward the
packet). In fact such a data packet may trigger Next Hop 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. It is recommended that
implementations provide one or more of the following solutions.
Possibly the most straightforward solution for suppressing the domino
effect would be to require transit routers to be preconfigured not to
originate Next Hop Resolution Requests for data traffic which is
simply being forwarded (not originated). In this case the routers
avoid the domino effect through an administrative policy.
A second possible solution would be to require that when a router
forwards an Next Hop Resolution Request, the router instantiates a
(short-lived) state. This state consists of the route that was used
to forward the request. If the router receives a data packet, and
the packet triggers an Next Hop Resolution Request generation by the
router, the router checks whether the route to forward the request
was recently used to forward some other Next Hop Resolution Request.
If so, then the router suppresses generation of the new request (but
still forwards the data packet). This solution also requires that
when a station attempts to originate an Next Hop Resolution Request
the station should send the Next Hop Resolution Request before the
data packet that triggered the origination of the request.
Otherwise, unnecessary Next Hop Resolution Requests may still be
generated.
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A third possible strategy would be to configure a router in such a
way that Next Hop 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 Next Hop Resolution Requests. Just as in the
first case, such a router avoids the NHRP domino effect through
administrative means.
Lastly, rate limiting of Next Hop Resolution Requests may help to
avoid the NHRP domino effect. Intermediate routers which would
otherwise generate unnecessary Next Hop Resolution Requests may
instead suppress such requests due to the measured request rate
exceeding a certain threshold. Of course, such rate limiting would
have to be very aggressive in order to completely avoid the domino
effect. Further work is needed to analyze this solution.
7. Security Considerations
As in any routing protocol, there are a number of potential security
attacks possible. Plausible examples include denial-of-service
attacks, and masquerade attacks using register and purge packets.
The use of authentication on all packets is recommended to avoid such
attacks.
The authentication schemes described in this document are intended to
allow the receiver of a packet to validate the identity of the
sender; they do not provide privacy or protection against replay
attacks.
Detailed security analysis of this protocol is for further study.
8. Discussion
The result of an Next Hop 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 the routed path to it lies entirely within the NBMA subnetwork,
the NHRP 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.
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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 IP 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
IP switching functionality.
References
[1] NBMA Address Resolution Protocol (NARP), Juha Heinanen and Ramesh
Govindan, draft-ietf-rolc-nbma-arp-00.txt.
[2] Address Resolution Protocol, David C. Plummer, RFC 826.
[3] Classical IP and ARP over ATM, Mark Laubach, RFC 1577.
[4] Transmission of IP datagrams over the SMDS service, J. Lawrence
and D. Piscitello, RFC 1209.
[5] Protocol Identification in the Network Layer, ISO/IEC TR
9577:1990.
[6] Assigned Numbers, J. Reynolds and J. Postel, RFC 1700.
[7] Multiprotocol Encapsulation over ATM Adaptation Layer 5, J. Heinanen,
RFC1483.
[8] Multiprotocol Interconnect on X.25 and ISDN in the Packet Mode,
A. Malis, D. Robinson, and R. Ullmann, RFC1356.
[9] Multiprotocol Interconnect over Frame Relay, T. Bradley, C. Brown, and
A. Malis, RFC1490.
Acknowledgments
We would like to thank 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. John Burnett of
Adaptive, Dennis Ferguson of ANS, Joel Halpern of Newbridge, Paul
Francis of NTT, Tony Li 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
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like to thank the members of the Routing Over Large Clouds working
group of the IETF, whose review and discussion of this document have
been invaluable.
Authors' Addresses
Dave Katz David Piscitello
cisco Systems Core Competence
170 W. Tasman Dr. 1620 Tuckerstown Road
San Jose, CA 95134 USA Dresher, PA 19025 USA
Phone: +1 408 526 8284 Phone: +1 215 830 0692
Email: dkatz@cisco.com Email: dave@corecom.com
Bruce Cole James V. Luciani
cisco Systems Ascom Nexion
170 W. Tasman Dr. 289 Great Road
San Jose, CA 95134 USA Acton, MA 01720-4379 USA
Phone: +1 408 526 4000 Phone: +1 508 266 3450
Email: bcole@cisco.com Email: luciani@nexen.com
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