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REUSE.TXT
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1988-12-10
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Reusable IP Addresses in a Dynamic Network
Robert B. Hoffman, N3CVL
_A_B_S_T_R_A_C_T
The topology of amateur packet radio networks
changes rapidly due to the frequent addition of new
stations, shutting down of old stations, and changing
location of others. This paper presents a method for
managing IP address assignments within such a network.
_1. _B_a_c_k_g_r_o_u_n_d
TCP/IP networks require
that each host have a unique
32-bit address. These
addresses are typically
assigned by the network
manager who must make sure
that no duplicate addresses
exist. In the amateur packet
radio TCP/IP network, the
assignments are done in a
hierarchal fashion. The glo-
bal coordinator (GC) assigns
blocks of addresses to Local
Area Network (LAN) coordina-
tors who, in turn, assign
individual station addresses.
The amateur packet radio
community is constantly chang-
ing due to the adding of new
stations, shutting down old
stations, changing locations,
and the like. In the AX.25
digipeater network, it becomes
difficult to maintain an accu-
rate map of reliable connec-
tion paths. In the TCP/IP
network, the job of the LAN
coordinator becomes similarly
difficult.
When a new station comes
on the air in the TCP/IP net-
work, its operator must first
contact the LAN coordinator to
get an address assignment. If
the coordinator is unavail-
able, the new user may get
frustrated and choose a random
address which may conflict
with previously assigned
addresses, causing havoc on
the network. In order to ease
the adding of new stations to
the TCP/IP network, the pro-
cess of address assignment
must be automated.
_2. _A_u_t_o_m_a_t_i_c _A_d_d_r_e_s_s _A_s_s_i_g_n_-
_m_e_n_t
It can be assumed that
the LAN coordinator operates
the router for his LAN and
that it has knowledge of all
LAN address assignments. It
therefore has enough informa-
tion to be able to assign
addresses within the block
assigned to it by the GC.
December 6, 1988
When a new station comes on
the air, it sends a broadcast
packet that contains its
callsign and a request for a
``permanent'' IP address. The
LAN router searches its tables
for the station's callsign,
and if it is found, it
responds with the previously
assigned address. If a table
entry is not found, the router
allocates a new address from
its block and assigns it to
the requesting station. It
also makes an entry in its
tables linking the station's
callsign with that address.
This is similar to the Reverse
Address Resolution Protocol
[1] that is used in booting
diskless workstations. The
router then sends a packet to
the requesting station inform-
ing it of its assignment. The
requesting station then
records the assignment in its
configuration file for subse-
quent use.
When the current block of
addresses is exhausted, a new
block would have to be
requested from the GC.
Currently, the LAN coordinator
must make a request to the GC
for another block of
addresses. As the network
develops better connectivity,
we may be able to have the LAN
router send a special packet
to the GC's system to request
another block of addresses.
The GC would take the next
available block and mark it as
being assigned to that LAN,
and send the information back
to the originating LAN router.
At the same time, the new
block-to-LAN assignment is
distributed to all other
routers so that they may
update their tables. The LAN
router may elect to send its
request when a few addresses
are still unassigned in the
old block, to allow for delays
in response from the GC.
The LAN will also have a
name server which will prob-
ably operate on the same sys-
tem as the router. Its func-
tion is to accept packets con-
taining callsigns and return
the associated IP addresses.
_3. _A_d_d_r_e_s_s _E_x_p_i_r_a_t_i_o_n
The local IP assignments
may have an expiration date
associated with them so that
seldom-seen stations don't tie
up IP addresses needlessly.
This can be an arbitrarily
long time, such as a couple of
months. As long as a station
remains active at least once
during that time period, it
retains its assignment and
stays in the name servers. If
an address expires, it is
marked as being available for
the next new station. This
will lengthen the time before
a new address block is needed.
_4. _M_o_v_i_n_g _b_e_t_w_e_e_n _L_A_N_s
When a station moves from
one LAN to another, its IP
address would be marked as
invalid in the local router,
and made to point into a for-
warding table that indicates
the station's new IP address.
This would be maintained for
some time to insure that the
new IP address has had time to
show up on the network's name
servers, and so that the old
address does not get reas-
signed locally until a reason-
able time has passed. The
rules that govern routing
decisions that are made based
on a partial IP (subnet)
address cannot allow IP
addresses to move between
December 6, 1988
LANs. This is necessary
because one cannot unplug a
computer from one
organization's network and
relocate it to another
organization's network and
expect to keep the same IP
address. With domain style
addressing, it wouldn't even
have the same hostname.
_5. _M_o_b_i_l_e _S_t_a_t_i_o_n_s
For mobile packet sta-
tions operating away from
their home territory, a tem-
porary address would be
requested from the router in
the station's current LAN.
The local router then sends a
forwarding order to his
``home'' router, cancelling
any previous forwarding order.
The home router then sends a
cancellation order to the pre-
vious router so that the pre-
vious temporary address may be
purged. The temporary address
would have a much shorter
expiration time than a regular
address. This scheme assumes
connectivity between all of
the LANs on the mobile
station's route.
_6. _C_o_n_c_l_u_s_i_o_n
As the number of stations
using TCP/IP grows, it will
become increasingly important
to respond quickly to changes
in the network. For this rea-
son, some sort of automated
network manangement is neces-
sary. The ideas presented
here represent a method for
managing IP address assign-
ments in such a network.
_R_e_f_e_r_e_n_c_e_s
1. Finlayson, R., Mann, T.,
Mogul, J., and Theimer,
M., ``Reverse Address
Resolution Protocol,''
ARPA RFC 903, June 1984.
_A_c_k_n_o_w_l_e_d_g_e_m_e_n_t_s
I wish to thank Mike
Chepponis, K3MC, and Bdale
Garbee, N3EUA, for their
assistance and encouragement
in the preparation of this
paper.
December 6, 1988
