Internet Area Christopher Ranch Internet Draft Novell, Inc. July 1992 Transmitting IP Traffic over LocalTalk Networks Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' Please check the 1id- abstracts.txt listing contained in the internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. The expiration date of this draft is [six month + 5 days after submission]. 1. Introduction This draft document specifies a method of encapsulating Internet Protocol (IP) [1] and Address Resolution Protocol (ARP) [2] datagrams for transmission across LocalTalk [3]. It is intended to supplement existing technology for enabling Apple Macintoshes with built-in LocalTalk network media to communicate with other IP hosts in a TCP/IP internet. 2. LocalTalk Packet Format In 1984, Apple Computers released a network media that came to be known as LocalTalk. It is defined in "Inside AppleTalk, Second Edition" [3]. We'll begin with a brief description of that media Ranch [Page 1] Internet draft IP on LocalTalk July 1992 and associated protocols. 2.1. LocalTalk Framing LocalTalk is a CSMA/CD 230.4 Kbps. SDLC, FM-0 encoded, passive media with 8 bit addresses. Please refer to [3] for a discussion of its electrical characteristics and its bit encoding, flags, and its method for address acquisition and exchanging data frames. LocalTalk supports a total frame length of 603 bytes; 3 bytes header, and 600 bytes of data. No datalink fragmentation is supported. Thus the maximum supportable Maximum Transmission Unit (MTU) is 600 octets. LocalTalk has an extensible LLAP type space. $00 is invalid, and $01-$0F are reserved by Apple Computer. IP, ARP, and RARP use LLAP types $10, $11, and $12, respectively. All numbers are in hexadecimal. The frame format for LocalTalk is shown here. Except as noted, each block represents one octet. LocalTalk Frame +---------------+ | source | +---------------+ | destination | +---------------+ | LLAP type | +---------------+ . data field . . . . (0 to 600 . . octets) . +---------------+ 3. Transmitting IP, ARP, and RARP Datagrams IP, ARP, and RRP datagrams are carried in the client data area of LocalTalk packets. Datalink support places each datagram in an appropriate size LocalTalk frame. IP datagrams larger than 576 octets, the MTU for LocalTalk, MUST be fragmented. [6] Ranch [Page 2] Internet draft IP on LocalTalk July 1992 The frame formats for IP, ARP, and RARP in LocalTalk are shown here. Except as noted, each block represents one octet. [Shold we align these puppies to even boundries?] IP over LocalTalk ARP over LocalTalk RARP over LocalTalk +---------------+ +---------------+ +---------------+ | source | | source | | source | +---------------+ +---------------+ +---------------+ | destination | | destination | | destination | +---------------+ +---------------+ +---------------+ | type = $10 | | type = $11 | | type = $12 | +---------------+ +---------------+ +---------------+ . IP header . . ARP packet . . RARP packet . . and data . . . . . . (MTU = 576 . . ( 18 octets ) . . ( 18 octets ) . . octets) . . . . . +---------------+ +---------------+ +---------------+ 4. IP Address Mappings This section explains how each of the three basic 32-bit internet address types are mapped to 8-bit LocalTalk addresses. 4.1. Unicast Addresses A unicast IP address is mapped to an 8-bit LocalTalk address using ARP as specified in [2]. A later section covers the specific values which should be used in ARP packets sent on LocalTalk networks. 4.2. Broadcast Addresses All IP broadcast addresses must be mapped to the LocalTalk broadcast address of $FF. 4.3. Multicast Addresses Ranch [Page 3] Internet draft IP on LocalTalk July 1992 Since LocalTalk provides no support for multicasts, all IP multicast addresses must be mapped to the LocalTalk broadcast address of $FF. 5. ARP The hardware address length is 1 octet for ARP packets sent over LocalTalk networks. The ARP hardware type for LocalTalk is [z]. ARP request packets are broadcast by directing them to LocalTalk broadcast address, which is $FF. 6. RARP Reverse Address Resolution Protocol [4] packets can also be transmitted over LocalTalk. 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 LocalTalk, 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 LocalTalk, while there's an very large number of possible Ethernet addresses. Second, LocalTalk hardware addresses are more likely to be duplicated on different LocalTalk networks; Ethernet hardware addresses will normally be globally unique. Third, a LocalTalk hardware address is not as constant as an Ethernet address: LocalTalk hardware addresses are acquired dynamically, not fixed in ROM as they are on Ethernet. 7. Maximum Transmission Unit The maximum IP packet length possible is 600 bytes. Since 576 is the minimum MTU for IP, we will select it to the MTU value, as this is commonly implelemted. [6] Data that the IP process receives from one of its clients for transmission MUST fragment the IP datagram. Ranch [Page 4] Internet draft IP on LocalTalk July 1992 8. Assigned Numbers Apple Computer assigns LocalTalk protocol IDs to identify different protocols running on the same LocalTalk medium. For implementations of this draft, you MUST use $10, $11, and $12 hexadecimal for IP, ARP, and RARP, respectively. [What do we need to do to make this a reality?] The Internet Assigned Numbers Authority assigns ARP hardware type values. [I hope] It has assigned LocalTalk the ARP hardware type of [z]. [7] 9. Acknowledgements This document was mostly lifted from Don Provan's RFC 1201, "Transmitting IP Traffic over ARCNET Networks". 10. References [1] Postel, J., "Internet Protocol", RFC-791, Network Information Center, SRI, September 1981. [2] Plummer, D., "An Ethernet Address Resolution Protocol", RFC-826, Network Interformation Center, SRI, November 1982. [3] Sidhu, Andrews, and Oppenheimer, "Inside AppleTalk, Second Edition", Addison Wesley, May 1990. [4] Finlayson, R., et al, "A Reverse Address Resolution Protocol", RFC-903, Network Interformation Center, SRI, June 1984. [5] Postel, J., "Transmission Control Protocol", RFC-793, Network Information Center, SRI, September 1981. [6] Mogul, J., et al, "IP MTU Discovery Options", RFC-1063, Network Interformation Center, SRI, July 1988. [7] Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1060, Network Information Center, SRI, March 1990. Ranch [Page 5] Internet draft IP on LocalTalk July 1992 11. Security Considerations Security issues are not discussed in this memo. 12. Expiration Date This document will expire on [6 months + 5 days after submission]. 13. Author's Address Christopher Ranch Novell, Inc. 2180 Fortune Drive San Jose, California, 95131 Phone: (408)473-8667 EMail: cranch@novell.com Ranch [Page 6]