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Network Working Group                                          D. ProvanRequest for Comments: 1201                                  Novell, Inc.Obsoletes:  RFC1051                                       February 1991Transmitting IP Traffic over ARCNET NetworksStatus of this Memo   This memo defines a protocol for the transmission of IP and ARP   packets over the ARCnet Local Area Network.  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.1.  Introduction   This memo specifies a method of encapsulating Internet Protocol (IP)   [1] and Address Resolution Protocol (ARP) [2] datagrams for   transmission across ARCNET [3] using the "ARCNET Packet Header   Definition Standard" [4].  This memo offers a replacement forRFC1051.RFC 1051 uses an ARCNET framing protocol which limits   unfragmented IP packets to 508 octets [5].2.  ARCNET Packet Format   In 1989, Apple Computers, Novell, ACTINET Systems, Standard   Microsystems, and Pure Data Research agreed to use the ARCNET   datalink protocol defined in "ARCNET Packet Header Definition   Standard" [4].  We'll begin with a brief description of that   protocol.2.1.  ARCNET Framing   ARCNET hardware supports two types of frames: short frames, which are   always 256 octets long, and long frames, which are always 512 octets   long.  All frames begin with a hardware header and end with the   client's data preceded by a software header.  Software places padding   in the middle of the packet between the hardware header and the   software header to make the frame the appropriate fixed length.   Unbeknown to the software, the hardware removes this padding during   transmission.   Short frames can hold from 0 to 249 octets of client data.  Long   frames can hold from 253 to 504 octets of client data.  To handle   frames with 250, 251, or 252 octets of data, the datalink protocolProvan                                                          [Page 1]

RFC 1201                      IP on ARCNET                 February 1991   introduces a third frame type: the exception frame.   These three frame formats are shown here.  Except as noted, each   block represents one octet.       Short Frame             Long Frame          Exception Frame    +---------------+      +---------------+      +---------------+    |     source    |      |     source    |      |     source    |    +---------------+      +---------------+      +---------------+    |  destination  |      |  destination  |      |  destination  |    +---------------+      +---------------+      +---------------+    |     offset    |      |       0       |      |       0       |    +---------------+      +---------------+      +---------------+    .     unused    .      |     offset    |      |     offset    |    .  (offset - 3  .      +---------------+      +---------------+    .     octets)   .      .     unused    .      .     unused    .    +---------------+      .  (offset - 4  .      .  (offset - 4  .    |  protocol ID  |      .     octets)   .      .     octets)   .    +---------------+      +---------------+      +---------------+    |  split flag   |      |  protocol ID  |      |  protocol ID  |    +---------------+      +---------------+      +---------------+    |   sequence    |      |  split flag   |      | flag: FF hex  |    +    number     +      +---------------+      +---------------+    |  (2 octets)   |      |   sequence    |      | padding: 0xFF |    +---------------+      +    number     +      +---------------+    .               .      |  (2 octets)   |      | padding: 0xFF |    .  client data  .      +---------------+      +---------------+    . (256 - offset .      .               .      | (protocol ID) |    .   - 4 octets) .      .               .      +---------------+    .               .      .               .      |  split flag   |    +---------------+      .               .      +---------------+                           .               .      |   sequence    |                           .  client data  .      +    number     +                           . (512 - offset .      |  (2 octets)   |                           .   - 4 octets) .      +---------------+                           .               .      .               .                           .               .      .  client data  .                           .               .      . (512 - offset .                           .               .      .   - 8 octets) .                           .               .      .               .                           +---------------+      +---------------+      These packet formats are presented as software would see them      through ARCNET hardware.  [3] refers to this as the "buffer      format".  The actual format of packets on the wire is a little      different: the destination ID is duplicated, the padding betweenProvan                                                          [Page 2]

RFC 1201                      IP on ARCNET                 February 1991      the offset field and the protocol ID field is not transmitted, and      there's some hardware framing information.  In addition, the      hardware transmits special packets for buffer allocation and      reception acknowledgement which are not described here [3].2.2.  Datalink Layer Fragmentation   ARCNET hardware limits individual frames to 512 octets, which allows   504 octets of client data.  This ARCNET datalink protocol allows the   datalink layer to break packets into as many as 120 fragments for   transmission.  This allows ARCNET clients to transmit up to 60,480   octets in each packet.   The "split flag" describes datalink layer packet fragments.  There   are three cases: an unfragmented packet, the first fragment of a   fragmented packet, and any other fragment of a fragmented packet.   Unfragmented packets always have a split flag of zero.   The first fragment of a fragmented packet has a split flag equal to   ((T-2)*2)+1, where T is the total number of fragments to expect for   the packet.   Subsequent fragments of a fragmented packet have a split flag equal   to ((N-1)*2), where N is the number of this fragment.  For example,   the fourth fragment of a packet will always have the split flag value   of six ( (4-1)*2 ).   The receiving station can identify the last fragment of a packet   because the value of its 8-bit split flag will be one greater than   the split flag of the first fragment of the packet.      A previous version of this ARCNET datalink protocol definition      only allowed packets which could be contained in two fragments.      In this older standard, the only legal split flags were 0, 1, and      2.  Compatibility with this older standard can be maintained by      configuring the maximum client data length to 1008 octets.   No more that 120 fragments are allowed.  The highest legal split flag   value is EE hex.  (Notice that the split flag value FF hex is used to   flag exception packets in what would otherwise be a long packet's   split flag field.)   All fragments of a single packet carry the same sequence number.2.3.  Datalink Layer Reassembly   The previous section provides enough information to implementProvan                                                          [Page 3]

RFC 1201                      IP on ARCNET                 February 1991   datalink reassembly.  To avoid buffer allocation problems during   reassembly, we recommend allocating enough space for the entire   reassembled packet when the first fragment arrives.   Since fragments are sent in order, the reassembly procedure can give   up on a packet if it receives a fragment out of order.  There is one   exception, however.  It is possible for successfully received   fragments to be retransmitted.  Reassembly software should ignore   repetitious fragments without giving up on the packet.   Since fragments will be sent briskly, the reassembly procedure can   give up on a partially reassembled packet if no additional fragments   for it arrive within a few seconds.2.4.  Datalink Layer Retransmission   For each unicast ARCNET packet, the hardware indicates to the sender   whether or not the receiver acknowledged the packet.  To improve   reliability, datalink implementations are encouraged to retransmit   unacknowledged packets or packet fragments.  Several retransmissions   may be necessary.  Broadcast packets, however, are never acknowledged   and, therefore, they should never be retransmitted.   Packets which are successfully received may not be successfully   acknowledged.  Consequently, retransmission by the datalink   implementation can cause duplicate packets or duplicate fragments.   Duplicate packets are not a problem for IP or ARP.  As mentioned in   the previous section, ARCNET reassembly support should ignore any   redundant fragments.3.  Transmitting IP and ARP Datagrams   IP and ARP datagrams are carried in the client data area of ARCNET   packets.  Datalink support places each datagram in an appropriate   size ARCNET frame, fragmenting IP datagrams larger than 504 octets   into multiple frames as described in the previous section.4.  IP Address Mappings   This section explains how each of the three basic 32-bit internet   address types are mapped to 8-bit ARCNET addresses.4.1.  Unicast Addresses   A unicast IP address is mapped to an 8-bit ARCNET address using ARP   as specified in [2].  A later section covers the specific values   which should be used in ARP packets sent on ARCNET networks.Provan                                                          [Page 4]

RFC 1201                      IP on ARCNET                 February 1991      It is possible to assign IP addresses such that the last eight      bits are the same as the 8-bit ARCNET address.  This would allow      direct mapping of IP address to ARCNET address without using a      discovery protocol.  Some implementations might provide this as an      option, but it is not recommended practice.  Although such hard-      wired mapping is initially appealing, experience shows that ARP is      a much more flexible and convenient approach which has a very      small cost.4.2.  Broadcast Addresses   All IP broadcast addresses must be mapped to the ARCNET broadcast   address of 0.      Unlike unicast packets, ARCNET does not attempt to insure delivery      of broadcast packets, so they may be lost.  This will not have a      major impact on IP since neither IP nor ARP expect all packets to      be delivered.4.3.  Multicast Addresses   Since ARCNET provides no support for multicasts, all IP multicast   addresses must be mapped to the ARCNET broadcast address of 0.5.  ARP   The hardware address length is 1 octet for ARP packets sent over   ARCNET networks.  The ARP hardware type for ARCNET is 7.  ARP request   packets are broadcast by directing them to ARCNET broadcast address,   which is 0.6.  RARP   Reverse Address Resolution Protocol [6] packets can also be   transmitted over ARCNET.  For the purposes of datalink transmission   and reception, RARP is identical to ARP and can be handled the same   way.  There are a few differences to notice, however, between RARP   when running over ARCNET, which has a one octet hardware address, and   Ethernet, which has a six octet hardware address.   First, there are only 255 different hardware addresses for any given   ARCNET while there's an very large number of possible Ethernet   addresses.  Second, ARCNET hardware addresses are more likely to be   duplicated on different ARCNET networks; Ethernet hardware addresses   will normally be globally unique.  Third, an ARCNET hardware address   is not as constant as an Ethernet address:  ARCNET hardware addresses   are set by switches, not fixed in ROM as they are on Ethernet.Provan                                                          [Page 5]

RFC 1201                      IP on ARCNET                 February 19917.  Maximum Transmission Unit   The maximum IP packet length possible using this encapsulation method   is 60,480 octets.  Since this length is impractical, all ARCNET   implementations on a given ARCNET network will need to agree on a   smaller value.  Therefore, the maximum packet size MUST be   configurable in implementations of this specification.   In any case, implementations must be able to send and receive IP   datagrams up to 576 octets in length, and are strongly encouraged to   handle IP datagrams up to 1500 octets in length.   Implementations may accept arriving IP datagrams which are larger   than their configured maximum transmission unit.  They are not   required to discard such datagrams.   To minimize the amount of ARCNET fragmentation, implementations may   want to aim at an optimum IP packet size of 504 bytes.  This avoids   the overhead of datalink fragmentation, but at the expense of   increasing the number of IP packets which must be handled by each   node in the path.  In addition to encouraging local applications to   generate smaller packets, an implementation might also use the TCP   maximum segment size option to indicate a desire for 464 octet TCP   segments [7], or it might  announce an IP MTU of 504 octets through   an MTU discovery mechanism such as [8].  These would inform non-   ARCNET nodes of the smaller optimum packet size.8.  Assigned Numbers   Datapoint Corporation assigns ARCNET protocol IDs to identify   different protocols running on the same ARCNET medium.  For   implementations of this specification, Datapoint has assigned 212   decimal to IP, 213 decimal to ARP, and 214 decimal to RARP.  These   are not the numbers assigned to the IP encapsulation defined byRFC1051 [5].  Implementations ofRFC 1051 can exist on the same ARCNET   as implementations of this specification, although the two would not   be able to communicate with each other.   The Internet Assigned Numbers Authority (IANA) assigns ARP hardware   type values.  It has assigned ARCNET the ARP hardware type of 7 [9].Acknowledgements   Several people have reviewed this specification and provided useful   input.  I'd like to thank Wesley Hardell at Datapoint and Troy Thomas   at Novell's Provo office for helping me figure out ARCNET.  In   addition, I particularly appreciate the effort by James VanBokkelen   at FTP Software who picked on me until all the fuzzy edges wereProvan                                                          [Page 6]

RFC 1201                      IP on ARCNET                 February 1991   smoothed out.   The pioneering work in transmitting IP traffic on ARCNET networks was   done by Philippe Prindeville.References   [1] Postel, J., "Internet Protocol",RFC 791, DARPA, September 1981.   [2] Plummer, D., "An Ethernet Address Resolution Protocol",RFC 826,       MIT, November 1982.   [3] Datapoint, Corp., "ARCNET Designer's Handbook", Document Number       61610, 2nd Edition, Datapoint Corporation, 1988.   [4] Novell, Inc., "ARCNET Packet Header Definition Standard", Novell,       Inc., November 1989.   [5] Prindeville, P., "A Standard for the Transmission of IP Datagrams       and ARP Packets over ARCNET Networks",RFC 1051, McGill       University, March 1988.   [6] Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse       Address Resolution Protocol",RFC 903, Stanford, June 1984.   [7] Postel, J., "Transmission Control Protocol",RFC 793, DARPA,       September 1981.   [8] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP MTU       Discovery Options",RFC 1063, DEC, BBN, TWG, July 1988.   [9] Reynolds, J., and J. Postel, "Assigned Numbers",RFC 1060,       USC/Information Sciences Institute, March 1990.Security Considerations   Security issues are not discussed in this memo.Author's Address   Don Provan   Novell, Inc.   2180 Fortune Drive   San Jose, California, 95131   Phone: (408) 473-8440   EMail: donp@Novell.ComProvan                                                          [Page 7]