Document Number TWG-1993 / 042
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SOURCE: Novell, Inc.
        Jay E. Israel
        M/S F6-91-2
        2275 Trade Zone Blvd.
        San Jose, CA  95131
        Telephone: 408-473-8478
        Fax: 408-435-1706
        Email: jay@novell.com
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TYPE OF CONTRIBUTION:   INFORMATIONAL SPECIFICATION
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ISSUE ADDRESSED: Transmission of Novell IPX Datagrams over the 
                 SMDS Service, Version 1.0
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DATE: July 15, 1993
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DISTRIBUTION:   UNLIMITED
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Abstract

This informational specification defines operation of Novell IPX-
based protocols over Switched Multi-megabit Data Service (SMDS).
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SMDS Interest Group's Disclaimer

"This is an Informational Specification which has been submitted 
by the originating organization for review and comment by members 
of the SIG TWG.  However, the SIG TWG does not approve or disap-
prove of any information contained in this document, nor can it 
provide assurance that its contents will not be revised in the 
future.  Distribution of this Informational Specification is 
unlimited.  Readers are advised to check with the originating 
organization for any possible updates to this document." 
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1. Scope

This document defines implementation of Novell IPX-based 
Protocols over Switched Multi-megabit Data Service (SMDS).  This 
information should be considered subject to change.  

This specification applies to the situations delimited in the 
following subsections.  Future versions of this specification 
will expand the situations covered, as circumstances warrant.

1.1  The SMDS service provider supplies a separate virtual 
private network (VPN) to each separate administrative entity sub-
scribing to an SMDS service.  (Sometimes, this is called a 
"closed user group"; sometimes, a "logical subnetwork".)  A VPN 
can carry IPX traffic, or other protocols, or multiple protocols 
concurrently, at the discretion of the SMDS subscriber.  Stations 
within the same VPN access each other directly over SMDS.  
Stations of different VPNs access each other through a network-
layer router.

1.2  Currently, two alternative routing protocols are specified 
for IPX:

  - RIP distance-vector routing [1], together with SAP (Service 
    Advertising Protocol).  SAP provides internetwork resource 
    location.  

  - NLSP link-state routing [2].

Since all packets are conveyed with IPX headers for both of these 
routing protocols, this specification supports them both.

1.3  An IPX router connected to SMDS may route between two (or 
more) VPNs.


2. Configurable values

2.1 Each point of attachment to SMDS has an eight-byte SMDS in-
dividual station address (station_ia).  This address is obtained 
from the SMDS service provider.

2.2 For each VPN, there is an eight-byte SMDS "global" group 
address (global_ga) signifying "all nodes belonging to the VPN, 
whether they use IPX, a different protocol, or multiple proto-
cols."  This address is obtained from the SMDS service provider.

2.3 For each VPN, there is an eight-byte SMDS "IPX" group address 
(ipx_ga) signifying "all nodes belonging to the VPN that use 
IPX."  This address is obtained from the SMDS service provider.  
Implementations must allow that ipx_ga = global_ga.  IPX broad-
casts are sent to ipx_ga.

2.4 For each VPN, there is an eight-byte SMDS "IPX ARP Request" 
address (ipx_arp_ga).  This address is obtained from the SMDS 
service provider.  "ARP" refers to the Address Resolution 
Protocol specified in Reference [3].  Implementations must allow 
that the value of ipx_arp_ga be the same as the ARP request 
address used by other network-layer protocols.  They must also 
allow that it be distinct.  Typically, ipx_arp_ga = ipx_ga, and 
ipx_arp_ga is a group address.  The value of ipx_arp_ga is con-
figured separately so that in the future ARP replies may be pro-
vided by a central service (rather than by the node owning the 
requested address), in which case ipx_arp_ga could become an 
individual (rather than a group) address.  

2.5 For each VPN conveying NLSP, there is an eight-byte SMDS 
"NLSP group address" (ipx_nlsp_ga) used to convey the NLSP 
"multicast" packets specified in Reference [2].  This address is 
obtained from the SMDS service provider.  Typically, ipx_nlsp_ga 
= ipx_ga, but implementations must allow the two to be different.

2.6 For each VPN, there is a four-byte IPX network number 
(ipx_net), assigned by the SMDS subscriber.

2.7 For each point of attachment to SMDS, there is a globally 
unique IEEE six-byte address (address_ieee).  It is assigned by 
the manufacturer of the network interface device, from a range 
allocated by IEEE.  The IEEE address should be readable program-
matically from the device.  [Note: if a station participates in 
more than one VPN through a single point of attachment, it is a 
local implementation choice to use either (a) the same 
address_ieee for all VPNs or (b) a different address_ieee for 
each VPN.]  Though it is taken from the "MAC" address space, this 
value is not used by the data link layer.  Rather, it is used as 
specified in Section 4 below.

2.8 For each VPN, there is a "maximum SMDS Interface Protocol 
Level 3 Service Data Unit (SDU) size" (L3sdu_max) assigned by the 
SMDS subscriber as a station configuration parameter.  All 
stations participating in a VPN must use the same value of 
L3sdu_max.  The value of L3sdu_max must be in the range 584 to 
9188 (inclusive).  Frames transmitted on SMDS must not have SMDS 
Level 3 SDU (that is, Level 3 user's information field [4]) 
larger than L3sdu_max bytes, including the eight bytes of 802.2 
and SNAP header.


3. Packet Format

Each IPX packet has the following format on SMDS:
 - three-level SMDS SIP header,
 - IEEE 802.2 LLC type 1 UI (Unnumbered Information) header,
 - five-byte IEEE 802.1A SNAP field,
 - IPX header as defined in [1],
 - IPX data, and
 - SMDS trailer.

The PI field in the SMDS SIP L3_PDU header is 1 to indicate an 
encapsulated LLC frame [4].  

The DSAP and SSAP of the LLC header contain AA (hexadecimal), 
indicating the presence of the SNAP field.  The SNAP field con-
tains 0000008137 (hexadecimal), indicating that the frame con-
tains an IPX packet.


4. IPX Address Assignment

An IPX network-layer address has three parts:
 - a four-byte network field, 
 - a six-byte node field, and 
 - a two-byte socket field.

A router or NetWare server uses the configured value ipx_net 
(section 2.6) as the network number of a VPN.  Other systems are 
not configured; they learn the network number by a protocol 
exchange with a router, as specified in Reference [1].  In the 
absence of a router, network number zero is used, meaning "the 
directly attached network."

A station uses address_ieee (section 2.7) as its own IPX node 
number for the VPN with network number ipx_net in which it 
participates.  

Various socket values refer to different upper-layer entities 
within a station.


5. IPX Address Resolution

The Address Resolution Protocol (ARP) specified in Reference [3] 
is used to determine the eight-byte SMDS destination address of 
an outgoing IPX packet.  (Although Reference [3] is expressed in 
terms of Ethernet, its "Ethernet packet data" includes a 
variable-length hardware address, so it can be applied to eight-
byte SMDS addresses as readily as to six-byte Ethernet 
addresses.)  A station maintains a cache of ARP information to 
support assignment of the destination SMDS address to outgoing 
frames.  The same ARP cache can be used to determine the six-byte 
IEEE source address of arriving frames.

ARP packets are encapsulated in IEEE 802.2 LLC type 1 UI frames 
with SNAP.  The SNAP field contains 0000000806 (hexadecimal), 
indicating that the frame contains an ARP packet.

Certain parameters tailor ARP to a specific context.  For IPX on 
SMDS, the following values are used:

- The hardware type code for SMDS is 0E (hexadecimal).
                                                        
- The protocol type code for IPX is 8137 (hexadecimal).

- The hardware address length for SMDS is eight.
                                           
- The protocol address length for IPX is ten.

- ARP requests are sent to the SMDS address ipx_arp_ga.

The SMDS hardware addresses in ARP packets are in SMDS native 
address format, with the most significant bit of the Address Type 
sub-field as the high order bit of the first byte.

The protocol address fields of ARP packets include the IPX net-
work number followed by the node number.  The bit order and byte 
order are the same as in the IPX header.

6. References

[1] Novell, Inc., "IPX Router Specification," June 16, 1992, part 
number 107-000029-001.

[2] Novell, Inc., "NetWare Link Services Protocol (NLSP) 
Specification," March 1993, part number 100-001708-001.

[3] Plummer, D., "An Ethernet Address Resolution Protocol 
-- or -- Converting Network Protocol Addresses to 48.bit Ethernet 
Address for Transmission on Ethernet Hardware," RFC 826, November 
1982.

[4] Bellcore, "Generic System Requirements in Support of Switched 
Multi-megabit Data Service," Technical Reference TR-TSV-000772, 
Issue 1, May 1991.

END OF SPECIFICATION
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