rfc1234.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                          D. Provan
Request for Comments: 1234                                  Novell, Inc.
                                                               June 1991


               Tunneling IPX Traffic through IP Networks

Status of this Memo

   This memo describes a method of encapsulating IPX datagrams within
   UDP packets so that IPX traffic can travel across an IP internet.
   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.

Introduction

   Internet Packet eXchange protocol (IPX) is the internetwork protocol
   used by Novell's NetWare protocol suite.  For the purposes of this
   paper, IPX is functionally equivalent to the Internet Datagram
   Protocol (IDP) from the Xerox Network Systems (XNS) protocol suite
   [1].  This memo describes a method of encapsulating IPX datagrams
   within UDP packets [2] so that IPX traffic can travel across an IP
   internet [3].

   This RFC allows an IPX implementation to view an IP internet as a
   single IPX network.  An implementation of this memo will encapsulate
   IPX datagrams in UDP packets in the same way any hardware
   implementation might encapsulate IPX datagrams in that hardware's
   frames.  IPX networks can be connected thusly across internets that
   carry only IP traffic.

Packet Format

   Each IPX datagram is carried in the data portion of a UDP packet.
   All IP and UDP fields are set normally.  Both the source and the
   destination ports in the UDP packet should be set to the UDP port
   value allocated by the Internet Assigned Numbers Authority for the
   implementation of this encapsulation method.

   As with any UDP application, the transmitting party has the option of
   avoiding the overhead of the checksum by setting the UDP checksum to
   zero.  Since IPX implementations never use the IPX checksum to guard
   IPX packets from damage, UDP checksumming is highly recommended for
   IPX encapsulation.




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   +---------------------+------------+-------------------------------+
   |                     |            |             |                 |
   |     IP Header       | UDP Header | IPX Header  | IPX packet data |
   | (20 or more octets) | (8 octets) | (30 octets) |                 |
   |                     |            |             |                 |
   +---------------------+------------+-------------------------------+

         Figure 1: An IPX packet carried as data in a UDP packet.

Reserved Packets

   The first two octets of the IPX header contain the IPX checksum.  IPX
   packets are never sent with a checksum, so every IPX header begins
   with two octets of FF hex.  Implementations of this encapsulation
   scheme should ignore packets with any other value in the first two
   octets immediately following the UDP header.  Other values are
   reserved for possible future enhancements to this encapsulation
   protocol.

Unicast Address Mappings

   IPX addresses consist of a four octet network number and a six octet
   host number.  IPX uses the network number to route each packet
   through the IPX internet to the destination network.  Once the packet
   arrives at the destination network, IPX uses the six octet host
   number as the hardware address on that network.

   Host numbers are also exchanged in the IPX headers of packets of
   IPX's Routing Information Protocol (RIP).  This supplies end nodes
   and routers alike with the hardware address information required for
   forwarding packets across intermediate networks on the way towards
   the destination networks.

   For implementations of this memo, the first two octets of the host
   number will always be zero and the last four octets will be the
   node's four octet IP address.  This makes address mapping trivial for
   unicast transmissions: the first two octets of the host number are
   discarded, leaving the normal four octet IP address.  The
   encapsulation code should use this IP address as the destination
   address of the UDP/IP tunnel packet.

Broadcasts between Peer Servers

   IPX requires broadcast facilities so that NetWare servers and IPX
   routers sharing a network can find one another.  Since internet-wide
   IP broadcast is neither appropriate nor available, some other
   mechanism is required.  For this memo, each server and router should
   maintain a list of the IP addresses of the other IPX servers and



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   routers on the IP internet.  I will refer to this list as the "peer
   list", to individual members as "peers", and to all the peers taken
   together, including the local node, as the "peer group".  When IPX
   requests a broadcast, the encapsulation implementation simulates the
   broadcast by transmitting a separate unicast packet to each peer in
   the peer list.

   Because each peer list is constructed by hand, several groups of
   peers can share the same IP internet without knowing about one
   another.  This differs from a normal IPX network in which all peers
   would find each other automatically by using the hardware's broadcast
   facility.

   The list of peers at each node should contain all other peers in the
   peer group.  In most cases, connectivity will suffer if broadcasts
   from one peer consistently fail to reach some other peer in the
   group.

   The peer list could be implemented using IP multicast [4], but since
   multicast facilities are not widely available at this time, no well-
   known multicast address has been assigned and no implementations
   using multicast exist.  As IP multicast is deployed in IP
   implementations, it can be used by simply including in the peer list
   an IP multicast address for IPX servers and routers.  The IP
   multicast address would replace the IP addresses of all peers which
   will receive IP multicast packets sent from this peer.

Broadcasts by Clients

   Typically, NetWare client nodes do not need to receive broadcasts, so
   normally NetWare client nodes on the IP internet would not need to be
   included in the peer lists at the servers.

   On the other hand, clients on an IPX network need to send broadcasts
   in order to locate servers and to discover routes.  A client
   implementation of UDP encapsulation can handle this by having a
   configured list of the IP addresses of all servers and routers in the
   peer group running on the IP internetwork.  As with the peer list on
   a server, the client implementation would simulate the broadcast by
   sending a copy of the packet to each IP address in its list of IPX
   servers and routers.  One of the IP addresses in the list, perhaps
   the only one, could be a broadcast address or, when available, a
   multicast address.  This allows the client to communicate with
   members of the peer group without knowing their specific IP
   addresses.

   It's important to realize that broadcast packets sent from an IPX
   client must be able to reach all servers and routers in the server



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   peer group.  Unlike IP, which has a unicast redirect mechanism, IPX
   end systems are responsible for discovering routing information by
   broadcasting a packet requesting a router that can forward packets to
   the desired destination.  If such packets do not tend to reach the
   entire server peer group, resources in the IPX internet may be
   visible to an end system, yet unreachable by it.

Maximum Transmission Unit

   Although larger IPX packets are possible, the standard maximum
   transmission unit for IPX is 576 octets.  Consequently, 576 octets is
   the recommended default maximum transmission unit for IPX packets
   being sent with this encapsulation technique.  With the eight octet
   UDP header and the 20 octet IP header, the resulting IP packets will
   be 604 octets long.  Note that this is larger than the 576 octet
   maximum size IP implementations are required to accept [3].  Any IP
   implementation supporting this encapsulation technique must be
   capable of receiving 604 octet IP packets.

   As improvements in protocols and hardware allow for larger,
   unfragmented IP transmission units, the 576 octet maximum IPX packet
   size may become a liability.  For this reason, it is recommended that
   the IPX maximum transmission unit size be configurable in
   implementations of this memo.

Security Issues

   Using a wide-area, general purpose network such as an IP internet in
   a position normally occupied by physical cabling introduces some
   security problems not normally encountered in IPX internetworks.
   Normal media are typically protected physically from outside access;
   IP internets typically invite outside access.

   The general effect is that the security of the entire IPX
   internetwork is only as good as the security of the entire IP
   internet through which it tunnels.  The following broad classes of
   attacks are possible:

   1)  Unauthorized IPX clients can gain access to resources through
       normal access control attacks such as password cracking.

   2)  Unauthorized IPX gateways can divert IPX traffic to unintended
       routes.

   3)  Unauthorized agents can monitor and manipulate IPX traffic
       flowing over physical media used by the IP internet and under
       control of the agent.




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   To a large extent, these security risks are typical of the risks
   facing any other application using an IP internet.  They are
   mentioned here only because IPX is not normally suspicious of its
   media.  IPX network administrators will need to be aware of these
   additional security risks.

Assigned Numbers

   The Internet Assigned Numbers Authority assigns well-known UDP port
   numbers.  It has assigned port number 213 decimal to the IPX
   encapsulation technique described in this memo [5].

Acknowledgements

   This encapsulation technique was developed independently by Schneider
   & Koch and by Novell.  I'd like to thank Thomas Ruf of Schneider &
   Koch for reviewing this memo to confirm its agreement with the
   Schneider & Koch implementation and also for his other valuable
   suggestions.

References

   [1] Xerox, Corp., "Internet Transport Protocols", XSIS 028112, Xerox
       Corporation, December 1981.

   [2] Postel, J., "User Datagram Protocol", RFC 768, USC/Information
       Sciences Institute, August 1980.

   [3] Postel, J., "Internet Protocol", RFC 791, DARPA, September 1981.

   [4] Deering, S., "Host Extensions for IP Multicasting", RFC 1112,
       Stanford University, August 1989.

   [5] Reynolds, J., and J. Postel, "Assigned Numbers", RFC-1060,
       USC/Information Sciences Institute, March 1990.

Security Considerations

   See the "Security Issues" section above.












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Author's Address

   Don Provan
   Novell, Inc.
   2180 Fortune Drive
   San Jose, California, 95131

   Phone: [phone]

   EMail: [e-mail]









































Provan                                                          [Page 6]