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Keywords: Reverse, Address, Resolution, Protocol
Network Working Group D. Brownell
Request For Comments: 1931 Sun Microsystems, Inc.
Category: Informational April 1996
Dynamic RARP Extensions for
Automatic Network Address Acquisition
Status of this Memo
This memo provides information for the Internet community. This memo
does not define an Internet standard of any kind. Distribution of
this memo is unlimited.
1. Introduction
This memo describes extensions to the Reverse Address Resolution
Protocol (RARP [2]) and called Dynamic RARP (DRARP, pronounced D-
RARP). The role of DRARP, and to some extent the configuration
protocol used in conjunction with it, has subsequently been addressed
by the DHCP protocol [9]. This memo is being published now to
document this protocol for the record.
DRARP is used to acquire (or allocate) a protocol level address given
the fixed hardware address for a host. Its clients are systems being
installed or reconfigured, and its servers are integrated with other
network administration services. The protocol, along with adjunct
protocols as briefly described here, supports several common styles
of "Intranet" administration including networks which choose not to
support the simplified installation and reconfiguration features
enabled by DRARP.
The rest of this introductory section summarizes the system design of
which the DRARP protocol was a key part. The second section presents
the DRARP protocol, and the third section discusses requirements
noted for an "Address Authority" managing addresses in conjunction
with one or more cooperating DRARP servers.
1.1 Automatic System Installation
Dynamic RARP was used by certain Sun Microsystems platforms beginning
in 1988. (These platforms are no longer sold by Sun.) In conjunction
with other administrative protocols, as summarized in the next
subsection, it was part of a simplified network and domain
administration framework for SunOS 4.0. Accordingly, there was a
product requirement to extend (rather than replace) the RARP/TFTP two
phase booting model [3], in order to leverage the existing system
infrastructure. This is in contrast to the subsequent DHCP [9] work,
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which extended BOOTP.
The "hands-off" installation of all kinds of systems (including
diskless workstations, and servers) was required, as supported by
LocalTalk networks [8]. However, Internet administrative models are
not set up to allow that: there is no way to set up a completely
functional IP network by just plugging machines into a cable and
powering them up. That procedure doesn't have a way to input the
network number (and class) that must be used, or to bootstrap the
host naming system. An approach based on administered servers was
needed for IP-based "Intranet" systems, even though that
unfortunately called for networks to be initially set up by
knowledgeable staff before any "hands-off" installations could be
performed.
1.2 System Overview
DRARP was used by systems in the first phase of joining a network, to
acquire a network address without personal intervention by a network
administrator. Once a system was given a network address, it would
perform whatever network operations it desired, subject to a site's
access control policies. During system installation, those network
operations involved a (re)configuration protocol ("Plug'n'Play", or
PNP). Diskless sytems used TFTP to download code which could speak
the PNP protocol.
The PNP protocol would register the names of newly installed hosts in
the naming service, using the address which was acquired using DRARP.
These names could be chosen by users installing the system, but could
also be assigned automatically. Diskless systems used the PNP
protocol to assign booting resources (e.g. filesystem space) on
servers. All systems were assigned public and private keys, also
initial (quasi-secret) "root" passwords, so that they could use what
was then the strongest available ONC RPC authentication system.
Servers for DRARP and for the configuration protocol (as well as
other administrative tools) needed to consult an authoritative
database of which Internet addresses which were allocated to which
hosts (as identified by hardware addresses). This "address
authority" role was implemented using a name service (NIS) and an
RPC-based centralized IP address allocation protocol ("IPalloc").
Address allocation could be performed only by authorized users,
including network administrators and DRARP servers.
Most systems used DRARP and PNP each time they started, to
automatically reconfigure applicable system and network policies.
For example, network addresses and numbers were changed using these
protocols; host names changed less often. The naming service (NIS)
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held most information, such as the locations of printers and users'
home directories.
2. Dynamic RARP Extensions
Dynamic RARP (DRARP) service is provided by any of a small active set
of cooperating server systems on a network segment (network or
subnetwork). Those servers are contacted through link level
procedures, normally a packet broadcast. One or more servers may
respond to a given request. It was intended that network segments
will be administered together in domains [5] consisting of one or
more network segments. Domains sharing a network segment need to
share information about network addresses, both hardware level and
protocol level, so an address authority (see section 3) can avoid
reallocating protocol addresses which are already allocated or in
use.
Dynamic RARP benefits from link layer addresses which are scoped more
widely than just the local network segment. It takes advantage of
such scoping to detect hosts which move between network segments.
Such scoping is provided by IEEE 802 48-bit addresses [7], but not by
all other kinds of network address. Without such a widely scoped ID,
the case of systems roaming between networks can't be detected by
Dynamic RARP.
2.1 Mixing RARP and DRARP Servers
DRARP is an extension to RARP, so that all Dynamic RARP servers are
also RARP servers. However, DRARP provides a more manageable service
model than RARP does: while RARP allows multiple servers to respond
to RARP requests, it does not expect all those servers to be able to
respond, or to respond identically. A given RARP server can not be
relied upon to know whether a given link level address can be mapped
into a protocol address, and some other RARP server may have a
different answer.
Dynamic RARP addresses this problem by requiring that all Dynamic
RARP servers on a network segment must communicate with the same
address authority. That address authority controls name and address
bindings, records bindings between host identifiers and addresses,
makes decisions about how to allocate addresses, and keeps records
about addresses in use.
This means that in effect there may be a number of independent RARP
services offered along with a single DRARP service. DRARP service
may well be offered through multiple servers, and the persistent
address bindings it serves will be accessible as from a set of
coordinated RARP servers.
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Not all networks want to support dynamic address allocation services.
Even those that do support it will need control over implementation
of the address authority. So DRARP servers need policy controls such
as "restricting" them from assigning addresses (applied to an entire
network segment) as well as disabling use of DRARP entirely. (One
may need to disable servers that would otherwise allocate new
addresses, in order to enable ones which can speak to the "correct"
address authority. Standards do not exist for protocols and security
options used to talk to address authorities.)
2.2 Packet Format
The packet format is identical to RARP and is encapsulated using RARP
frames, with the same Ethernet/SNAP type field. [1, 2, 6]. That is,
a DRARP packet looks like a RARP packet, but it uses opcodes which
are ignored by RARP servers; DRARP servers must also support RARP
requests, and hence ARP requests [1].
2.2.1 RARP Packets
The two RARP opcodes are described here, in order to clarify the
overall presentation. The name "REVARP", used in the opcode
descriptions, is a synonym for "RARP".
REVARP_REQUEST (3)
REVARP_REQUEST packets are sent to RARP servers as a request to
map the target hardware address (tha) into the corresponding
target protocol address (tpa), sending the response to the
source hardware address (sha) as encoded in the packet. The
source hardware address will usually be the same as the target
hardware address, that of the system sending the packet. RARP
servers will consult their name and address databases, and
return a REVARP_REPLY packet if they can perform the reverse
address resolution as requested.
REVARP_REPLY (4)
This packet is sent by RARP servers in response to
REVARP_REQUEST packets. The target protocol address (tpa) is
filled in as requested, and the source hardware and protocol
addresses (sha, spa) correspond to the RARP server. The target
hardware address (tha) is from the corresponding REVARP_REQUEST
packet, and the packet is sent to the source hardware address
(sha) from that packet.
This packet is also sent by Dynamic RARP servers in response to
DRARP_REQUEST packets, if the protocol address returned was not
a temporary one, but was instead what it would have returned
given an otherwise identical REVARP_REQUEST packet.
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2.2.2 Dynamic RARP Packets
There are three opcodes defined for DRARP, in addition to the
two already defined for RARP:
DRARP_REQUEST (5)
DRARP_REQUEST packets have the same format as REVARP_REQUEST
packets, except for the operation code. The semantics are simi-
lar, except that in cases where a REVARP_REQUEST would produce
no REVARP_REPLY (no persistent address mapping is stored in an
addressing database) a DRARP_REQUEST will normally return a tem-
porary address allocation in a DRARP_REPLY packet. A
DRARP_ERROR packet may also be returned; a Dynamic RARP server
will always provide a response, unlike a REVARP server.
DRARP_REPLY (6)
DRARP_REPLY packets have the same format, opcode excepted, as
REVARP_REPLY packets. The interpretation of the fields is the
same.
There are semantic differences between the two packet types.
First, the protocol address bindings returned in DRARP_REPLY
packets are temporary ones, which will be recycled after some
period (e.g. an hour). Those bindings returned in REVARP_REPLY
packets are "persistent" addresses which typically change much
more slowly. Second, it is explicitly a protocol error for
DRARP_REPLY packets to be sent which differ except in the sender
address fields. Also, DRARP_REPLY packets are generated only in
response to DRARP_REQUEST packets.
These temporary addresses may be reallocated to another system
after some time period. A configuration protocol is normally
used to ensure that reallocation does not occur.
DRARP_ERROR (7)
DRARP_ERROR packets may also be sent in response to
DRARP_REQUESTs. The format is identical to REVARP_REPLY, except
for the opcode and that the target protocol address (tpa) field
is replaced by an error code field. The error code field must
be at least one byte long, and the first byte is used to encode
an error status describing why no target protocol address (tpa)
is being returned. The status values are:
DRARPERR_RESTRICTED (1)
This network does not support dynamic address allocation.
The response is definitive; the network is controlled so
that no other DRARP_REPLY (for this hardware address) is
legal until the network policy on dynamic address
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RFC 1931 Dynamic RARP April 1996
allocation is changed, or until the client is otherwise
assigned a persistent address binding. A REVARP_REQUEST
might yield a REVARP_REPLY, however; non-cooperating RARP
servers could be the very reason that dynamic address allo-
cation was disabled.
DRARPERR_NOADDRESSES (2)
This network supports dynamic address allocation, but all
available protocol addresses in the local segment are in
use, so none can be allocated now.
DRARPERR_SERVERDOWN (3)
The service providing access to the address authority is
temporarily unavailable. May also be returned if an
address allocation was required and the required response
took a "long time" to generate; this distinguishes the case
of a network that didn't support DRARP from the case of one
that does, but is slow.
DRARPERR_MOVED (4)
Analogous to the DRARPERR_RESTRICTED status in that no
address was dynamically allocated. This provides the addi-
tional status that this client was recognized by the
administration software for the domain as being on a dif-
ferent network segment than expected; users will be able to
remedy the problem by connecting the system to the correct
network segment.
DRARPERR_FAILURE (5)
For some reason, no address could be returned. No defined
status code known to the server explained the reason.
More opcodes for the Address Resolution Protocol (ARP) family could
be defined in the future, so unrecognized opcodes (and error codes)
should be ignored rather than treated as errors.
2.3 Protocol Exchanges
This section describes typical protocol exchanges using RARP and
Dynamic RARP, and common fault modes of each exchange.
2.3.1. RARP Address Lookup
To determine a previously published ("persistent") protocol address
for itself or another system, a system may issue a REVARP_REQUEST
packet. If a REVARP_REPLY packet arrives in response, then the
target protocol address listed there should be used.
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If no REVARP_REPLY response packet arrives within some time interval,
a number of errors may have occurred. The simplest one is that the
request or reply packet may never have arrived: most RARP client
implementations retransmit requests to partially account for this
error. There is no clear point at which to stop retransmitting a
request, so many implementations apply an exponential backoff to the
retransmit interval, to reduce what is typically broadcast traffic.
Otherwise there are many different errors which all have the same
failure mode, including: the system might not have a published
protocol address; it might be on the wrong network segment, so its
published address is invalid; the RARP servers which can supply the
published address may be unavailable; it might even be on a network
without any RARP servers at all.
2.3.2 Dynamic RARP Address Lookup
Dynamic RARP may be used to determine previously published protocol
addresses by clients who issue DRARP_REQUEST packets. If the client
has a published protocol address on the network segment on which the
DRARP_REQUEST packet was issued, it is returned in a REVARP_REPLY
packet.
If the client has a published protocol address only on some other
network segment, then two basic responses are possible. In the case
where dynamic address reallocation is enabled, a temporary protocol
address may be allocated and returned in a DRARP_REPLY packet.
Otherwise if dynamic address reallocation is disabled, a DRARP_ERROR
packet is returned with the status DRARPERR_MOVED. Detection of host
movement can be provided only with link level addresses that are
unique over the catenet, such as are provided with IEEE 802 48 bit
addresses. Without such uniqueness guarantees, this case looks like
a request for a new address as described in the next section.
2.3.3 Dynamic RARP Address Allocation
Dynamic RARP clients who issue DRARP_REQUEST packets may acquire
newly allocated protocol addresses. If the client has no published
protocol address, there are three responses:
(a) When dynamic address allocation is enabled, a temporary protocol
address is allocated and returned in a DRARP_REPLY packet.
(b) Errors or delays in the allocation process (with dynamic address
allocation enabled) are reported in DRARP_ERROR packets with
error codes such as DRARPERR_SERVERDOWN, DRARPERR_NOADDRESSES,
DRARPERR_MOVED, or even DRARPERR_FAILURE.
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(c) When dynamic address allocation is disabled (or "restricted"), a
DRARP_ERROR packet with status DRARPERR_RESTRICTED is returned.
DRARP_REQUESTS are normally retransmitted until an address is
returned, using backoff-style algorithms to minimize needless
network traffic. When DRARP_ERROR responses are received, they
should be reported to the user. For example, knowing that the
server is busy could indicate it's time for a cup of Java, but
if the network is restricted then it might be time to contact a
network administrator for help instead.
2.3.4 Discovering Other DRARP Servers
The existence of a DRARP server can be discovered by the fact
that it puts its addressing information in all DRARP_REPLY
packets that it sends. DRARP servers can listen for such
packets, as well as announcing themselves by sending such a
packet to themselves.
It can be important to discover other DRARP servers. Users make
mistakes, and can inappropriately set up DRARP servers that do
not coordinate their address allocation with that done by the
other DRARP servers on their network segment. That causes
significant administrative problems, which can all but be
eliminated by DRARP servers which politely announce themselves,
and when they detect an apparently spurious server, report this
fact before entering a "restricted" mode to avoid creating any
problems themselves.
As no further server-to-server protocol is defined here, some
out-of-band mechanism, such as communication through the address
authority, must be used to help determine which servers are in
fact spurious.
2.4 Network Setup Concerns
Some internetwork environments connect multiple network segments
using link level bridges or routers. In such environments, a
given broadcast accessible "local" area network will have two
problems worth noting.
First, it will extend over several cable segments, and be
subject to partitioning faults. Assigning one DRARP server to
each segment (perhaps on systems acting as routers or bridges,
to serve multiple segments) can reduce the cost of such faults.
Assigning more than one such server can help reduce the cost of
failure to any single network segment; these cooperate in the
assignment of addresses through the address authority.
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Second, those networks are sometimes shared by organizations
which don't cooperate much on the management of protocol
addresses, or perhaps aren't even collocated. A DRARP server
might need help from link level bridges/routers in order to
ensure that local clients are tied to local servers (rather
than, for example, to servers across the country where they are
prone to availability problems). Or the server might need to
run in "restricted" mode so that a network administrator
manually assigns address and other resources to each system.
3. The Address Authority
While not part of the DRARP protocol, the Address Authority used
by the DRARP servers on a network segment is critical to
providing the address allocation functionality. It manages the
data needed to implement such service, which is required not
just for dynamic address allocation tools. This section is
provided to record one set of requirements for such an
authority, ignoring implementation isssues such as whether
protocol support for replication or partitioning is needed.
3.1 Basic Requirements
For each network segment under its control, an Address Authority
maintains at least:
- persistent bindings between hardware and protocol addresses
(for at least those hosts which are DRARP clients);
- temporary bindings between such addresses;
- protocol addresses available for temporary bindings;
The Address Authority is also responsible for presenting and managing
those bindings. DRARP clients need it to support:
- creating temporary bindings initially,
- looking up bindings (the distinction between temporary and
persistent bindings is not usually significant here),
- deleting temporary or persistent bindings on request,
- purging them automatically by noticing that a binding is
now persistent or that the temporary address is available
for reuse.
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Those clients will frequently make concurrent requests, and should be
required to pass some kind of authorization check before they create
or change any bindings. They may also need to know about other
clients, in order to determine (for example) if a given DRARP server
is spurious.
3.2 Multiple Authorities and Segments
Note there is only a single address authority on a given network
segment. It may be desirable to partition that authority, though
that complicates implementation and administration of the authority
substantially.
If detection of systems moving between network segments is to be
provided, then the authorities for those two network segments must
either be the same or (equivalently) must communicate with one
another. Also, as noted earlier, hardware addresses must be scoped
widely enough that the two segments do not assign the same link level
address to different hosts.
3.3 Quality of Service
The records of temporary address bindings must be persistent for at
least long enough to install a system and propagate its records
through the site's administrative databases, even in the case of
server or network faults. A timeout mechanism could be used to
ensure that the limited address space was not used up too quickly.
The initial implementation found that an hour's worth of caching,
before deleting temporary bindings, was sufficient.
Experience has shown that many networks have addresses in use which
are not listed in their name services (or other administrative
databases). On such networks, the Address Authority should have a
way to learn when an address which it thinks is available for
allocation is instead being actively used. Probing the network for
"the truth" before handing out what turns out to be a duplicate IP
address is a worthwhile. Both ARPing for the address and ICMP echo
request have been used for this.
4. Security Considerations
Security concerns are not addressed in this memo. They are
recognized as significant, but they also interact with site-specific
network administration policies. Those policies need to be addressed
at higher levels before ramifications at this level can be
understood.
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5. References
[1] Plummer, D., "An Ethernet Address Resolution Protocol", STD 37,
RFC 826, MIT, November 1982.
[2] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse
Address Resolution Protocol", STD 38, RFC 903, Stanford, June
1984.
[3] Finlayson, R., "Bootstrap Loading using TFTP", RFC 906,
Stanford, June 1984.
[4] Postel, J., "Multi-LAN Address Resolution", RFC 925,
USC/Information Sciences Institute, October 1984.
[5] Mockapetris, P., "Domain Names -- Concepts and Facilities", STD
13, RFC 1034, USC/Information Sciences Institute, November 1987.
[6] Postel, J., and J. Reynolds, "A Standard for the Transmission of
IP Datagrams over IEEE802 Networks", STD 43, RFC 1042,
USC/Information Sciences Institute, February 1988.
[7] IEEE; "IEEE Standards for Local Area Networks: Logical Link
Control" (IEEE 802.2); IEEE, New York, NY; 1985.
[8] United States Patent No. 4,689,786; "Local Area Network with
Self Assigned Address Method"; Issued August 25, 1987;
Inventors: Sidhu, et al.; Assignee: Apple Computer, Inc.
[9] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
Bucknell University, October 1993.
[10] Srinivasan, R., "RPC: Remote Procedure Call Protocol
Specification, Version 2", RFC 1831, Sun Microsystems, August
1995.
Author's Address:
David Brownell
SunSoft, Inc
2550 Garcia Way, MS 19-215
Mountain View, CA 94043
Phone: +1-415-336-1615
EMail: dbrownell@sun.com
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