Network Working Group D. Piscitello
Request for Comments: 1209 J. Lawrence
Bell Communications Research
March 1991 The Transmission of IP Datagrams over the SMDS Service
Status of this Memo
This memo defines a protocol for the transmission of IP and ARP
packets over a Switched Multi-megabit Data Service Network configured
as a logical IP subnetwork. This RFC specifies an IAB standards
track protocol for the Internet community, and requests discussion
and suggestions for improvements. Please refer to the current
edition of the "IAB Official Protocol Standards" for the
standardization state and status of this protocol. Distribution of
this memo is unlimited.
This memo describes an initial use of IP and ARP in an SMDS service
environment configured as a logical IP subnetwork, LIS (described
below). The encapsulation method used is described, as well as
various service-specific issues. This memo does not preclude
subsequent treatment of the SMDS Service in configurations other than
LIS; specifically, public or inter-company, inter-enterprise
configurations may be treated differently and will be described in
future documents. This document considers only directly connected IP
end-stations or routers; issues raised by MAC level bridging are
beyond the scope of this paper.
This memo draws heavily in both concept and text from , written by
Jon Postel and Joyce K. Reynolds of ISI and , written by David
Katz of Merit, Inc. The authors would also like to acknowledge the
contributions of the IP Over SMDS Service working group of the
Internet Engineering Task Force.
The following language conventions are used in the items of
specification in this document:
o MUST, SHALL, or MANDATORY -- the item is an absolute
requirement of the specification.
o SHOULD or RECOMMENDED -- the item should generally be followed
for all but exceptional circumstances.
o MAY or OPTIONAL -- the item is truly optional and may be
followed or ignored according to the needs of the implementor.
The goal of this specification is to allow compatible and
interoperable implementations for transmitting IP datagrams and ARP
requests and replies.
The characteristics of the SMDS Service and the SMDS Interface
Protocol (SIP) are presented in , , and in . Briefly, the
SMDS Service is a connectionless, public, packet-switched data
service. The operation and features of the SMDS Service are similar
to those found in high-speed data networks such as LANs:
o The SMDS Service provides a datagram packet transfer, where each
data unit is handled and switched separately without the prior
establishment of a network connection.
o The SMDS Service exhibits high throughput and low delay, and
provides the transparent transport and delivery of up to 9188
octets of user information in a single transmission.
o No explicit flow control mechanisms are provided; instead, the
rate of information transfer on the access paths is controlled
both in the subscriber-to-network direction and in the network-
to-subscriber direction through the use of an access class
o Both individually and group-addressed (multicast) packets can
o In addition to these LAN-like features, a set of addressing-
related service features (source address validation, source and
destination address screening) are provided to enable a
subscriber or set of subscribers to create a logical private
network, or closed user group, over the SMDS Service. The
access control provided by the closed user group mechanism is
supplied by the SMDS provider according to the specifications
stated in .
o SMDS addresses are 60 bits plus a 4 bit Address Type. The
Address Type subfield occupies the 4 most significant bits of
the destination and source address fields of the SIP Level 3
Protocol Data Unit (PDU). It contains the value 1100 to
indicate an individual address and the value 1110 for a 60-bit
The SMDS Interface Protocol is based on the IEEE Standard 802.6,
Distributed Queue Dual Bus (DQDB) Connectionless MAC protocol .
The SMDS service layer corresponds to the IEEE 802 MAC sublayer. The
remainder of the Data Link Service is provided by the IEEE 802.2
Logical Link Control (LLC) service . The resulting stack of
services is illustrated in Figure 1:
| IP/ARP |
|IEEE 802.2 LLC/SNAP |
| SIP LEVEL 3 (MAC) |
| SIP LEVELS 1 & 2 |
Figure 1. Protocol stack for IP over SMDS Service
This memo describes an initial use of IP and ARP in an SMDS Service
environment configured as a logical IP subnetwork (described below).
It does not preclude subsequent treatment of SMDS Service in
configurations other than logical IP subnetworks; specifically,
public or inter-company, inter-enterprise configurations may be
treated differently and will be described in future documents. This
document does not address issues related to transparent data link
Logical IP Subnetwork Configuration
This section describes the scenario for an SMDS Service that is
configured with multiple logical IP subnetworks, LIS (described
below). The scenario considers only directly connected IP end-
stations or routers; issues raised by MAC level bridging are beyond
the scope of this paper.
In the LIS scenario, each separate administrative entity configures
its hosts within a closed logical IP subnetwork. Each LIS operates
and communicates independently of other LISs over the same network
providing SMDS. Hosts connected to SMDS communicate directly to
other hosts within the same LIS. Communication to hosts outside of
an individual LIS is provided via an IP router. This router would
simply be a station attached to the SMDS Service that has been
configured to be a member of both logical IP subnetworks. This
configuration results in a number of disjoint LISs operating over the
same network supporting the SMDS Service. It is recognized that with
this configuration, hosts of differing IP networks would communicate
via an intermediate router even though a direct path over the SMDS
Service may be possible.
It is envisioned that the service will evolve to provide a more
public interconnection, allowing machines directly connected to the
SMDS Service to communicate without an intermediate router. However,
the issues raised by such a large public interconnection, such as
scalability of address resolution or propagation of routing updates,
are beyond the scope of this paper. We anticipate that future RFCs
will address these issues.
The following is a list of the requirements for a LIS configuration:
o All members have the same IP network/subnet number.
o All stations within a LIS are accessed directly over SMDS.
o All stations outside of the LIS are accessed via a router.
o For each LIS a single SMDS group address has been configured
that identifies all members of the LIS. Any packet transmitted
with this address is delivered by SMDS Service to all members
of the LIS.
The following list identifies a set of SMDS Service specific
parameters that MUST be implemented in each IP station which would
connect to the SMDS Service. The parameter values will be determined
at SMDS subscription time and will be different for each LIS. Thus
these parameters MUST be user configurable.
o SMDS Hardware Address (smds$ha). The SMDS Individual address
of the IP station as determined at subscription time. Each
host MUST be configured to accept datagrams destined for this
o SMDS LIS Group Address(smds$lis-ga). The SMDS Group address
that has been configured at subscription time to identify the
SMDS Subscriber Network Interfaces (SNI) of all members of the
LIS connected to the SMDS Service. All members of the LIS MUST
be prepared to accept datagrams addressed to smds$lis-ga.
o SMDS Arp Request Address (smds$arp-req). The SMDS address
(individual or group) to which arp requests are to be sent. In
the initial LIS configuration this value is set to smds$lis-ga.
It is conceivable that in other configurations this value would
be set to some address other than that of smds$lis-ga (see
section on Address Resolution).
It is RECOMMENDED that routers providing LIS functionality over the
SMDS service also support the ability to interconnect differing LISs.
Routers that wish to provide interconnection of differing LISs MUST
be able to support multiple sets of these parameters (one set for
each connected LIS) and be able to associate each set of parameters
with a specific IP network/subnet number. In addition, it is
RECOMMENDED that a router be able to provide this multiple LIS
support with a single physical SMDS interface that may have one or
more individual SMDS addresses.
The following list identifies LIS specific parameters that MUST be
configured in the network supporting the SMDS Service. For each LIS,
the IP network administrator MUST request the configuration of these
parameters at subscription time. The administrator of each LIS MUST
update these parameters as each new station is added to the LIS.
o SMDS LIS Group Address(smds$lis-ga). An SMDS Group address MUST
be configured at subscription time to identify the SMDS
Subscriber Network Interfaces (SNI) of all members of the LIS
connected to the SMDS Service.
o SMDS Address Screening Tables (Source and Destination). The use
of SMDS screening tables is not necessary for the operation of
IP over SMDS Service. If the SMDS screening tables are to be
used, both source and destination tables for each SNI MUST be
configured to allow, at minimum, both the direct communication
between all hosts in the same LIS and the use of the SMDS LIS
Service SHALL be encapsulated within the IEEE 802.2 LLC and IEEE
802.1A Sub-Network Access Protocol (SNAP)  Data Link layers
and the 3-level SIP. The SNAP MUST be used with an
Organizationally Unique Identifier Code indicating that the SNAP
header contains the EtherType code as listed in Assigned Numbers
 (see Figure 2). Note that values specified in this document
follow Internet conventions: multi-byte fields are described in
big-endian order and bits within bytes are described as most
significant first .
|IP/ARP | IP/ARP
| Org Code |Ethertype| | SNAP
|DSAP|SSAP|Ctrl| | LLC
|SIP..|HLPI|...| | SIP L3
Figure 2. Data Link Encapsulation
o The value of HLPI in the SIP L3 Header is 1.
o The total length of the LLC Header and the SNAP header is 8
o The value of DSAP and SSAP in the LLC header is 170 (decimal),
AA (Internet hexadecimal).
o The Ctrl (Control) value in the LLC header is 3 (Indicates Type
One Unnumbered Information).
o The Org Code in the SNAP header is zero (000000 Internet
o The EtherType for IP is 2048 (decimal), 0800 (Internet
hexadecimal). The EtherType for ARP is 2054 (decimal), 0806
IEEE 802.2 LLC Type One Unnumbered Information (UI) communication
(which must be implemented by all conforming IEEE 802.2 stations) is
used exclusively. The Higher Layer Protocol Id (HLPI) field in the
SIP L3_PDU header MUST be set to the IEEE 802.6 assigned Protocol Id
value for LLC (decimal 1) . All frames MUST be transmitted in
standard IEEE 802.2 LLC Type 1 Unnumbered Information format, with
the DSAP and the SSAP fields of the IEEE 802.2 header set to the
assigned global SAP value for SNAP (decimal 170) . The 24-bit
Org Code (Organizationally Unique Identifier Code) in the SNAP MUST
be set to a value of zero, and the remaining 16 bits are set to the
EtherType value from Assigned Numbers  (2048 for IP, 2054 for
The data link encapsulation for IP packets is shown in Figure 3 and
for ARP in Figure 4. All values shown are in Internet hexadecimal
| SIP | LLC / SNAP | IP |
| | | |
|SIP..|HLPI|...|DSAP|SSAP|Ctrl| Org Code |Ethertype| |
|SIP..| 01 |...| AA | AA | 03 | 000000 | 0800 | IP... |
Figure 3. IP Data Link Encapsulation and Values
| SIP | LLC / SNAP | ARP |
| | | |
|SIP..|HLPI|...|DSAP|SSAP|Ctrl| Org Code |Ethertype| |
|SIP..| 01 |...| AA | AA | 03 | 000000 | 0806 | ARP...|
Figure 4. ARP Data Link Encapsulation and Values
The dynamic mapping of 32-bit Internet addresses to SMDS addresses
SHALL be done via the dynamic discovery procedure of the Address
Resolution Protocol (ARP) .
Internet addresses are assigned independent of SMDS addresses. Each
host implementation MUST know its own Internet address and SMDS
address and respond to Address Resolution requests appropriately.
Hosts MUST also use ARP to map Internet addresses to SMDS addresses
The ARP protocol has several fields that parameterize its use in any
specific context . These fields are:
ar$hrd 16 - bits The Hardware Type Code
ar$pro 16 - bits The Protocol Type Code
ar$hln 8 - bits Octets in each hardware address
ar$pln 8 - bits Octets in each protocol address
ar$op 16 - bits Operation Code
o The hardware type code assigned to SMDS addresses is 14
(decimal), 0E (Internet hexadecimal) .
o The protocol type code for IP is 2048 (decimal), 0800
(Internet hexadecimal) .
o The hardware address length for SMDS is 8.
o The protocol address length for IP is 4.
o The operation code is 1 for request and 2 for reply.
The SMDS hardware addresses in ARP packets (ar$sha, ar$tha) MUST be
carried in SMDS native address format, with the most significant bit
of the Address Type sub-field as the high order bit of the first
octet. Although outside the scope of this document, it is
RECOMMENDED that SMDS addresses be represented in this format in all
higher layer Internet protocols (e.g., SNMP).
Traditionally, ARP requests are broadcast to all directly connected
stations. For the SMDS Service, the ARP request packet is
transmitted to the smds$arp-req hardware address. In the LIS
configuration, the smds$arp-req address is set to smds$lis-ga, (the
SMDS group address that identifies all members of the LIS). It is
conceivable that in a larger scale, public configuration, the
smds$arp-req address would be configured to the address of some ARP-
server(s) instead of the group address that identifies the entire
IP Broadcast Address
There is no facility for complete hardware broadcast addressing over
the SMDS Service. As discussed in the "LIS Configuration" section,
an SMDS group address (smds$lis-ga) SHALL be configured to include
all stations in the same LIS. The broadcast Internet address (the
address on that network with a host part of all binary ones) MUST be
mapped to smds$lis-ga (see also ).
IP Multicast Support
A method of supporting IP multicasting is specified in . It
would be desirable to fully utilize the SMDS group address
capabilities to support IP multicasting. However, the method in 
requires a Network Service Interface which provides multicast-like
ability to provide dynamic access to the local network service
o JoinLocalGroup (group-address)
o LeaveLocalGroup (group-address)
The SMDS group address ability does not currently support dynamic
subscription and removal from group address lists. Therefore, it is
RECOMMENDED that in the LIS configuration, if IP multicasting is to
be supported, the method of IP multicasting described for pure
broadcast media, such as the Experimental Ethernet, be used. For
this method, all Multicast IP addresses are mapped to the same SMDS
address which the broadcast Internet address is mapped for a given
LIS. Thus all Multicast IP addresses are mapped to smds$lis-ga.
Filtering of multicast packets MUST be performed in the destination
Some versions of Unix 4.x BSD use a different encapsulation method in
order to get better network performance with the VAX virtual memory
architecture. Trailers SHALL not be used over the SMDS Service.
As described in Appendix B of the Internet Protocol specification
, the IP datagram is transmitted over the SMDS Service as a series
of 8-bit bytes. The byte order of the IP datagram shall be mapped
directly onto the native SMDS byte order.
MAC Sublayer Details
The SMDS Service defines a maximum service data unit size of 9188
information octets. This leaves 9180 octets for user data after the
LLC/SNAP header is taken into account. Therefore, the MTU for IP
stations operating over the network supporting the SMDS Service SHALL
be 9180 octets.
There is no minimum packet size restriction defined for the SMDS
Other MAC Sublayer Issues
The SMDS Service requires that the publicly administered 60-bit
address plus 4-bit type field format SHALL be used in both source and
destination address fields of the SIP L3_PDU .
IEEE 802.2 Details
While not necessary for supporting IP and ARP, all implementations
MUST support IEEE 802.2 standard Class I service in order to be
compliant with IEEE 802.2. Some of the functions are not related
directly to the support of the SNAP SAP (e.g., responding to XID and
TEST commands directed to the null or global SAP addresses), but are
part of a general LLC implementation. Both  and  describe the
minimum functionality necessary for a conformant station.
Implementors should also consult IEEE Std. 802.2  for details.
1. Postel, J., "Internet Protocol", RFC 791, USC/Information
Sciences Institute, September 1981.
2. Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Protocol Addresses to 48.bit Ethernet Address
for Transmission on Ethernet Hardware", RFC 826, MIT, November
3. "Generic Systems Requirements in support of Switched Multi-
megabit Data Service", Technical Advisory TA-TSY-000772, Bellcore
Technical Advisory, Issue 3, October 1989.
4. Postel, J., and J. Reynolds, "A Standard for the Transmission of
IP Datagrams over IEEE 802 Networks", RFC 1042, USC/Information
Sciences Institute, February 1988.
5. Katz, D., "A Proposed Standard for the Transmission of IP
Datagrams over FDDI Networks", RFC 1188, Merit/NSFNET, October
6. Dix, F., Kelly, M., and R. Klessig, "Access to a Public Switched
Multi-Megabit Data Service Offering", ACM SIGCOMM CCR, July 1990.
7. Hemrick, C. and L. Lang, "Introduction to Switched Multi-megabit
Data Service (SMDS), an Early Broadband Service", publication
pending in the Proceedings of the XIII International Switching
Symposium (ISS 90), May 27, 1990 - June 1, 1990.
8. Institute of Electrical & Electronic Engineers, Inc. IEEE
Standard 802.6, "Distributed Queue Dual Bus (DQDB) Subnetwork of
a Metropolitan Area Network (MAN) Standard", December 1990.
9. IEEE, "IEEE Standards for Local Area Networks: Logical Link
Control", IEEE, New York, New York, 1985.
10. IEEE, "Draft Standard P802.1A--Overview and Architecture", 1989.
11. Reynolds, J., and J. Postel, "Assigned Numbers", RFC 1060,
USC/Information Sciences Institute, March 1990.
12. Braden, R., and J. Postel, "Requirements for Internet Gateways",
RFC 1009, USC/Information Sciences Institute, June 1987.
13. Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
Stanford University, August 1989.
14. IEEE,"ANSI/IEEE Std 802.2-1985, ISO Draft International Standard
8802/2", IEEE, New York, New York, 1985.
Security issues are not discussed in this memo.
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