Network Working Group C. Graff
Request for Comments: 1770 US Army CECOM
Category: Informational March 1995
IPv4 Option for Sender Directed Multi-Destination Delivery
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
This memo defines an IPv4 option to provide a sender directed multi-
destination delivery mechanism called Selective Directed Broadcast
Mode (SDBM). The SDBM provides unreliable UDP delivery to a set of
IP addresses included in the option field of an IPv4 datagram. Data
reliability if required will be provided by the application layer.
This approach was developed to support sender directed multi-
destination delivery to sparsely populated groups with no additional
control traffic. This approach will find application in the
extremely bandwidth constrained tactical military environment, as
well as in some commercial applications requiring sender control of
data delivery.
Background
The Selective Directed Broadcast Mode (SDBM) is an integral part of
the U.S. Army standard for tactical data communication networks as
defined in MIL-STD-188-220() (Reference 1). The MIL-STD-188-220()
defines a protocol architecture for the lower four layers of the
ISO-OSI Reference model. The MIL-STD-188-220() is currently
undergoing a reformatting to be consistent with other DoD standards
that deal with IP networking. These efforts will provide tactical IP
internetting of tactical Army broadcast radio networks, and will
support fully IP compliant internetworking to other types of IP
networks via commercial IP routers. It is the goal of the U.S. Army
to move toward a fully IP compliant internetwork architecture for all
tactical battlefield data communications. The Army does, however,
have a critical need for a reliable, sender directed multi-
destination data transfer capability that is not currently supported
by the existing or emerging internet standards. The SDBM IP option
was developed to meet this need. The required data reliability will
be provided by incorporating an acknowledgement strategy at the
application layer. It is hoped that this IP option, providing multi-
destination capability not currently provided by the current and
emerging internet standards, will be embraced by the internet
community and become an integal part of the IP family of protocols
and be incorporated in commercial IP software prodUCts.
SDBM Format
The SDBM provides the ability for an application to eXPlicitly list a
set of intended IP destinations. This capability will be implemented
as an option in the IP layer, as shown in Figure 1. This option field
is variable in length, up to a maximum of 40 octets due to the
limitation of the HLEN field as specified in STD 5, RFC791
(Reference 2). Under this option 38 of the 40 octets would be used to
contain the 2 octet control field and a maximum of 9 IP addresses.
1 8 16 31
***************************************************


TYPE LENGTH IP ADDRESS 1


*************************************************

IP ADDRESS 1(Cont) IP ADDRESS 2


*************************************************

IP ADDRESS 2(Cont) ..........


*************************************************


.......... IP ADDRESS N


*************************************************

IP ADDRESS N(Cont) UNUSED


***************************************************
Figure 1 IP Option Field Layout
The TYPE field specifies the copy flag, class, and option number.
The copy field indicates whether or not this option field is to be
copied into each fragment if the IP datagram is fragmented. The class
field and option number field are set to 0 and 21 respectively. The
format of the TYPE field is shown at Figure 2.
1 8
**************************************************

COPY CLASS OPTION NUMBER = 149

**************************************************
Figure 2 Type Field Layout
Since the IP multi-address list shall always be copied to all IP
headers during fragmentation, the COPY bit should be set to 1.
Returning to Figure 1, the LENGTH octet indicates how many octets are
in the option field. It is calculated as:
LENGTH = 2 + 4*(number of IP addresses)
The remaining octets contain the IP addresses of the specified
destination hosts. Each IP address occupies 4 octets.
Transmission of SDBM datagrams
The procedures for a source host, transit router, and destination
router are provided below. When a source host has a message to send
to multiple destination hosts, it shall,
a. Group the destination host internet addresses by their network
identifiers (Net IDs). If there are N distinct Net IDs, there will
be at least N distinct directed broadcast packets. If there are
more that 9 destination hosts on a single net, multiple directed
broadcast datagrams must be sent to that net.
b. For each Net ID, form the directed broadcast address as defined in
STD 3, RFC1122 (Reference 3) for that network. The directed
broadcast address is used as the destination address in the IP
datagram and the source address is the address of the host sending
the message.
c. Place the entire IP address for up to 9 destination hosts in the in
the same net in the option field defined above. The total length of
all IP options in a given datagram is limited to 40 octets as
determined by the HLEN (Header Length) field which defines the
number of 32 bit Words in the header. If other options are to be
included in addition to the SDBM option, the number of addresses in
the option field must be reduced accordingly.
d. The thusly formed datagram shall be transmitted and processed
according to normal datagram handling procedures.
When a IP SDBM datagram encounters a transit router (router not
connected to the destination network), the datagram shall be
processed in accordance with normal IP datagram handling procedures.
When encountering the destination router (the destination network is
directly attached to the router), the destination router shall
perform a, b or c below:
a. If the local subnet has a broadcast capability, broadcast to all
hosts in the network and let the hosts perform address filtering.
b. If the local subnet does not support broadcast, form a local subnet
packet for each destination host in the SDBM datagram and transmit
into the network.
c. If the local subnet supports reliable layer 2 multi-address
capability as provided by MIL-STD-188-220() networks, use a layer 2
multi-address frame to deliver the datagram to addresses found in
the IP option field.
Reception of SDBM datagrams
In processing received SDBM datagrams, receiving hosts shall look
inside the IP option field for their address. Processing shall
continue only if the host"s IP address is found inside this option
field. Thus the source host has explicit control over which hosts
will process its datagrams. Since SDBM uses a broadcast address in
its destination field, the SDBM can only be used with UDP (Reference
4) and not TCP (Reference 5) as the TCP supports only point-to-point
connections and not point-to-multi-point.
Source for MIL-STD-188-220()
The above mentioned MIL-STD-188-220() may be oBTained by contacting
US Army Communications Electronics Command
AMSEL-RD-SE-AIN-E (ATTN: Mr. Ted Dzik)
Fort Monmouth, NJ 07703
Comm: (908) 532-1780
Fax: (908) 532-3398
EMail: DZIK@ain3.monmouth.army.mil
Acknowledgements
The author wishes to acknowledge the major contributions to this work
made by Mr. Dave Macauley of ATT and Ms. Barbara Denny of SRI
International. Other contributions were made by members of the 188-
220() committee.
References
(1) "MIL-STD-188-220() For Task Force XXI, Interoperability Standard
for Digital Message Transfer Device Subsystems, 23 December 1994.
(2) Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", STD 5, RFC791, DARPA, September 1981.
(3) Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers" STD 3, RFC1122, IETF, October 1989.
(4) Postel, J., "User Datagram Protocol", STD 6, RFC768,
USC/Information Sciences Institute, August 1980.
(5) Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", STD 7, RFC793, September 1981.
Security Considerations
Security issues are not discussed in this memo.
Author"s Address
US Army Communications Electronics Command
AMSEL-RD-ST-LA-L ( ATTN: Charles Graff )
Ft. Monmouth, NJ 07703
Phone: (908) 544 3264
Fax: (908) 544 2150