SDBM: IPv4 Option for Sender Directed Multi-Destination Delivery
An IPv4 option that lists up to nine destination addresses so one datagram can reach just those hosts.
Graff, C., "IPv4 Option for Sender Directed Multi-Destination Delivery", RFC 1770, DOI 10.17487/RFC1770, March 1995, <https://www.rfc-editor.org/info/rfc1770>.
@misc{rfc1770,
series = {Request for Comments},
number = 1770,
howpublished = {RFC 1770},
publisher = {RFC Editor},
doi = {10.17487/RFC1770},
url = {https://www.rfc-editor.org/info/rfc1770},
author = {C. Graff},
title = {{IPv4 Option for Sender Directed Multi-Destination Delivery}},
pagetotal = 6,
year = 1995,
month = mar,
} RFC text
Contents
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, RFC 791
(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, RFC 1122 (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, RFC 791, DARPA, September 1981.
(3) Braden, R., Editor, "Requirements for Internet Hosts --
Communication Layers" STD 3, RFC 1122, IETF, October 1989.
(4) Postel, J., "User Datagram Protocol", STD 6, RFC 768,
USC/Information Sciences Institute, August 1980.
(5) Postel, J., "Transmission Control Protocol - DARPA Internet
Program Protocol Specification", STD 7, RFC 793, 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
EMail: bud@fotlan5.fotlan.army.mil
AI ELI5
On this page
§1Purpose and Background
This Informational memo defines an IPv4 option for Selective Directed Broadcast Mode (SDBM). It gives unreliable UDP delivery to a set of IP addresses carried in the option field of an IPv4 datagram. If data reliability is required, the application layer provides it, for example with acknowledgements.
The design was developed for sender-directed delivery to sparsely populated groups with no additional control traffic. SDBM is part of the U.S. Army standard for tactical data communication networks, MIL-STD-188-220, and the author expects it to suit the extremely bandwidth-constrained tactical military environment as well as some commercial uses.
Sometimes a sender wants to tell only a few friends something. Instead of shouting to everybody, it writes the friends' names on the message.
§2The SDBM Option Format
The option has a TYPE octet, a LENGTH octet, and then the destination IP addresses, 4 octets each. IP options are limited to 40 octets by the header length field, so the option can use 38 octets: 2 for the control fields and room for at most 9 addresses.
The TYPE field holds the copy flag, the class and the option number. The class is 0 and the option number is 21. The copy bit should be set to 1 so that the address list is copied into every fragment, which gives a TYPE value of 149. The length is calculated as 2 + 4*(number of IP addresses).
The note on the packet says how long the list is and then lists each friend's address, up to nine of them.
- 1COPY — Should be set so the option is copied into each fragment.
- 0CLASS — Option class.
- 21OPTION NUMBER — Identifies the SDBM option.
- 149TYPE — The resulting value of the TYPE octet.
§3Sending and Routing SDBM Datagrams
A source host groups the destination hosts by network ID. Each network gets at least one directed broadcast datagram, and a network with more than 9 destinations needs several. The destination address of each datagram is the directed broadcast address of that network, and the source address is the sender's own. Up to 9 full destination addresses go in the option. If other IP options are present, the number of addresses must shrink to fit within the 40 octets.
Transit routers handle the datagram as any ordinary IP datagram. The router attached to the destination network chooses one of three ways to deliver it. If the subnet can broadcast, it broadcasts and lets hosts filter by address. If it cannot broadcast, it sends a separate local packet to each listed host. If the subnet has reliable layer 2 multi-address capability, as MIL-STD-188-220 networks do, it uses one layer 2 multi-address frame.
The sender sorts friends by neighborhood and sends one note per neighborhood. The last post office either shouts it out, delivers it to each house, or uses a special group delivery if it has one.
§4Receiving and Limits
A receiving host looks inside the option for its own address and keeps processing the datagram only if it finds it. This gives the source explicit control over which hosts process the datagram.
Because the destination address is a broadcast address, SDBM can be used only with UDP and not with TCP, which supports only point-to-point connections. The memo says nothing about security: its Security Considerations section states that security issues are not discussed.
Everybody on the street hears the message, but only the people named on it act on it. It works for quick notes, not for long phone calls.