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Network Working Group                                            T. SungRequest for Comments: 1791                                  Novell, Inc.Category: Experimental                                        April 1995TCP And UDP Over IPX Networks With Fixed Path MTUStatus of this Memo   This document defines an Experimental Protocol for the Internet   community.  This does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.IESG Note:   Internet Engineering Steering Group comment from the Area Director   for Transport Services: Please note well that this memo is an   individual product of the author.  Implementation experience,   particularly on the effectiveness of the protocols in dual-stack   environments, is needed.1.  Introduction   Most of network applications run on some sort of transports.  And, if   one is to let such applications to run over a foreign network   protocol, the simplest way would be to allow the applications'   transports to run over that network protocol. For TCP/IP   applications, that transport is TCP or UDP.  Hence, to let TCP/IP   applications run over IPX, we would need to  have TCP and UDP run   over IPX.  And, once TCP and UDP are allowed to run over IPX, all TCP   and UDP based applications, such as HTTP for WWW, or NFS, can easily   be made to work over IPX networks.   DLsw is another example of such applications.  As it is a TCP   application (and TCP requires IP), the administrator is forced to run   IP on his network in order to support DLsw.  If the site was an IPX   shop, it means that he now must manage IP protocol/addresses in   addition to IPX.  If TCP could be made to run on IPX, then he would   not have to add IP to his repertoire of network protocols to manage.   TCP/IPX allows TCP/IP applications to run over IPX networks by   letting TCP and UDP run over IPX.  And this memo specifies the packet   format and operational procedures for running TCP and UDP over IPX.Sung                                                            [Page 1]

RFC 1791                  TCP And UDP Over IPX                April 19952.  Running UDP Over IPX   Since UDP datagrams can be up to 64K octets long, and the size of IPX   packet is limited to that of the path MTU, large UDP datagrams must   be fragmented.  And, since IPX does not support fragmentation, large   UDP datagrams must be fragmented before they are passed to IPX.  For   that purpose, a new protocol called IPXF (IPX Fragmentation layer),   is invented.  UDP must run on IPXF rather than directly on IPX.  IPXF   layer is described insection 4.   To IPXF service users, IPXF behaves just like IPX except that IPXF   accepts datagram larger than the IPX path MTU.  As such, we describe   UDP in this section as if it is running on IPX.   UDP must send and receive the packets on IPX/IPXF socket 0x9092.   Though it may be possible to send a packet from sockets other than   0x9092, such sockets cannot receive UDP datagram destined to a well   known socket 0x9092.  Hence, the bidirectional communcation may not   be established if a socket other than 0x9092 is used to send UDP   datagram.  For that reason.  UDP/IPX does not allow source sockets   other than 0x9092.  If a datagram with source socket number other   than 0x9092 is received, UDP/IPX should discard the packet silently.   (And increment udpInDatagramErr MIB counter if it is instrumented.)   UDP over IPX uses the IPX packet type 4, a normal IPX packet type.   The IPX packet type has no meaning to TCP/IPX protocol.  It simply is   a number required by IPX for general IPX packets.   SeeAppendix B.1 and B.2 for UDP over IPX packet format.   The UDP/IPX checksum uses a pseudo header similar to UDP/IP pseudo   header.  The only difference is that IP addresses and protocol ID are   replaced by IPX addresses and socket numbers.   SeeAppendix B.3 for the UDP/IPX pseudo header format.3.  Running TCP Over IPX   Unlike UDP, TCP runs directly over IPX. Since IPX does not support   fragmentation, no TCP segment sent over IPX can be larger than the   path MTU for the connection.  The discovery of the path MTU is   outside of scope of this paper.  If the  implementation does not have   a way to dynamically determine the path MTU for each connection, it   should at least allow a way to statically configure a reasonable   value for all connections.  For example, if the internetwork made of   ethernets only, the user may configure the segment size to be 1470   including the TCP header.  If the configuration of the segment size   is not possible, the implementation should assume that the IPX pathSung                                                            [Page 2]

RFC 1791                  TCP And UDP Over IPX                April 1995   MTU is 576 octects, and not send any TCP segment larger than 546   octets including TCP header.  That will result in IPX packet of 576   octets which is the minimum path MTU for IPX.  The implementation is   then advised to comunicate the configured/default segment size to the   peer TCP by exchanging MSS option.   Note that this memo does not preclude the possibility of running TCP   over IPXF instead of IPX.  Running on IPXF can be done in the same   manner as running UDP over IPXF.  However, in general, TCP should   refrain from sending large segments that may result in fragmentation.   Hence, running TCP over IPXF is not recommended.   The IPX socket number 0x9091 is reserved for the TCP. All TCP packets   must be sent from and received on the socket 0x9091.  If the received   TCP/IPX packet has the source IPX socket number other than 0x9091,   the packet should be discarded silently. (And increment tcpInErrs MIB   counter if it is instrumented.)   TCP, like UDP, uses IPX packet type 4.  The IPX packet type has no   meaning to TCP/IPX protocol.  It is packet type required by IPX for   general IPX packets.   Seeappendix A.1 for TCP/IPX packet format.   The TCP pseudo header, used in checksuming for TCP over IPX, is   similar to TCP pseudo header for TCP over IP.  Again, the difference   is that IPX addresses and IPX socket number are substituted in place   of IP addresses and IP protocol number.   SeeAppendix A.2 for the TCP/IPX pseudo header format.4.  IPXF Layer   A large UDP datagram cannot be sent directly over IPX as IPX does not   support datagrams larger than the path MTU.  Hence, large UDP   datagrams must be fragmented before it can be sent over IPX.  To have   large UDP datagrams fragmented, UDP runs over IPXF layer instead of   running directly IPX.   IPXF users treats IPXF as if it is IPX layer.  That is, they pass   datagrams to IPXF specifying the destination IPX address/socket along   with the packet. They also must set the source socket number of the   datagram to its actual IPX socket number, as it would when sending   packets to IPX layer.  (For UDP, both source and destination sockets   are 0x9092.)   Datagrams passed to IPXF can be upto 64K octets long.Sung                                                            [Page 3]

RFC 1791                  TCP And UDP Over IPX                April 1995   IPXF fragments a datagram as necessary, prepends each fragment with   the IPXF header and send them to the IPX socket 0x9093 in the   destination IPX address.  The actual destination socket number   (0x9092 for UDP) in the orignal IPX datagram is preserved in IPXF   header. Refer toAppendix B.2 for UDP/IPXF/IPX packet format.   The largest possible IPX datagram that can be sent over the IPX path   is limited by the path MTU size.  The mechanism to discover the path   MTU is outside of the scope of the paper.  If an IPXF implementation   does not have a mean to determine the path MTU, it should assume that   the largest IPX packet size is 576. In that case, any UDP datagram   larger than 546 octects will have to be fragmented.   If the datagram does not require fragmentation, IPXF acts as a null   layer.  That is, the whole packet is directly sent to the actual IPX   destination socket without the IPXF fragmentation header.  Refer toAppendix B.1 for UDP/IPX packet format without the IPXF header.   An IPXF user receives datagrams by opening a socket with IPXF just as   it would with IPX.  For example, UDP opens the socket 0x9092 with   IPXF to receive UDP datagrams.  IPXF, in turn, opens IPX socket of   the same number with IPX, so that unfragmented packets directed to   that socket will be delivered by IPX directly to the IPXF user.   IPXF fragments are received by IPXF on the IPX socket 0x9093.  The   receiving IPXF then reassembles the fragments into a complete IPX   datagram, restores the actual detination IPX socket number from the   IPXF header and delivers the reassembled IPX datagram to its actual   recipient designated by the restored socket number.   Upon receiving a fragment, IPXF must ignore the source socket number   in the IPX header of the fragment.  The source IPX socket field in   IPX header contains the actual source of the IPX datagram.  As such,   the source IPX socket number in IPX header usually is not 0x9093, and   it is meaningful only to the actual recepient of the assembled   datagram.   The fragmentation/reassembly algorithm used by IPXF is identical to   that of IP, except for the following exceptions: 1) the offset of   fragments are measured in units of octets rather than in units of 8   octets.  2) if the receiving IPXF does not have sufficient resource   for the reassembly, it should discard fragments immediately.  The   receiving IPXF can determine if it has sufficient resources by   looking at the length of the original datagram included in every   fragment.   Note that, though it is required only for UDP in this memo, IPXF can   also be used by any protocol that requires IPX fragmentation support.Sung                                                            [Page 4]

RFC 1791                  TCP And UDP Over IPX                April 19955.  TCP/IPX Checksuming   TCP/IPX is checksummed in exactly same manner as TCP/IP. It uses 16   bit 1's complement of 1's compliment sum of all 16 bit words in the   pseudo header and text.  SeeAppendix A.2 and B.3 for the pseudo   header format for TCP and UDP.6.  Multiplexing   TCP and UDP data over IPX are delivered to the application in the   same manner as in TCP/IP.  That is, they are delivered to the most   specific matching endpoint, with the match made on local port, remote   port, local IPX address and remote IPX address.   When TCP or UDP is running over both IPX and IP, the connection   endpoint also identifies the network layer on which the endpoint is.   Hence, the triplet of network address, network address family, and   the port number forms the socket.  And, the endpoint match must be   made on the the network address familty as well.   For exmple, an endpoint bound to IPX network layer would be   identified by AF_IPX, IPX address and TCP port number.  On the other   hand, endpoints bound to IP network layer would be identified by   AF_IP, IP address, and TCP port.  Finally, endpoints not bound to any   network layer would be identified by AF_UNSPEC and TCP port.   First, an attempt is made to deliver the data to the most specific   endpoint that is bound to the network layer that the packet arrived   from.  If there is no such endpoint,  then the packet is delivered to   the best matching endpoint that is not bound to any network layer at   all.  For example, if the packet arrived over IPX network, then the   packet is delivered to the most specific matching endpoint that is   bound to IPX. If there is no matching endpoint over IPX, then it is   delivered to an endpoint that did not specify any network layer.   The use of endpoints not bound to any network layer is similar to   TCP/IP endpoints with no IP address bound to it.  Such endpoints are   usually used for listening for connection requests from any of the   interfaces within the host.  Similarly, endpoints with no network   layer bound to it are used to field the connection requests from any   of the network layers.Acknowledgement   The author wishes to thank following folks, in alphabetical order,   and others for their helpful comments and contributions to the work:   Lester Bird, Doug Kogan, Greg Minshall and Don Provan.Sung                                                            [Page 5]

RFC 1791                  TCP And UDP Over IPX                April 1995Security Considerations   Security issues are not discussed in this memo.Author's Address   Tae Sung   Novell, Inc.   2180 Fortune Drive   San Jose, California, 95131   Phone: (408)577-8439   EMail: tae@novell.ComSung                                                            [Page 6]

RFC 1791                  TCP And UDP Over IPX                April 1995Appendix A.1 -  TCP/IPX Packet Format   A TCP/IPX Packet has following format:          +-------+-------+-------+-------+          | IPX Checksum  | IPX Pkt Len   |          +-------+-------+-------+-------+          | Zero  |IPX PT | IPX Dest -          +-------+-------+-------+-------+            Network | IPX Dest -          +-------+-------+-------+-------+            Node                          |          +-------+-------+-------+-------+          | IPX Dest Skt  | IPX Src -          +-------+-------+-------+-------+            Network       | IPX Src -          +-------+-------+-------+-------+            Node                          |          +-------+-------+-------+-------+          | IPX Src Skt   | TCP Header and          +---------------+-------+-------+            Data...          +----...   IPX PT field contains the IPX packet type.  It is set to 4 for   TCP/IPX packet.   Both Src Skt and Dest Skt field in IPX header must be set to 0x9091   for TCP/IPX packet.  If the Src Skt is not set to 0x9091, the   receiving TCP/IPX should discard the packet silently.  (And increment   tcpInErrs mib object if it is instrumented.)Sung                                                            [Page 7]

RFC 1791                  TCP And UDP Over IPX                April 1995Appendix A.2 -  TCP/IPX Pseudo Header Format   TCP/IPX uses following pseudo header to compute checksum:             +-------+-------+-------+-------+             | IPX Src Network               |             +-------+-------+-------+-------+             | IPX Src Node             +-------+-------+-------+-------+                             | IPX Src Skt   |             +-------+-------+-------+-------+             | IPX Dest Network              |             +-------+-------+-------+-------+             | IPX Dest Node             +-------+-------+-------+-------+                             | IPX Dest Skt  |             +-------+-------+-------+-------+             | Zero          | TCP Length    |             +---------------+---------------+   IPX Src/Dest Network/Node/Skt are the fields from the IPX header.   TCP Length is the IPX Pkt Len minus the IPX header length in octets.   Note that IPX Src Skt is expected to be 0x9091 for TCP.  As such, one   may insert 0x9091 in IPX Src Skt field rather than getting the value   from IPX header.  Then the implementation will not have to check the   IPX Src Skt field in the fast path since the checksum failure will   also cover the unexpected value.  In that case, the implementation   may want to examine if the checksum failure was due to the IPX Src   Skt value other than 0x9091, so that it can increment appropriate   counter, if proprietary counters other than tcpInErrs are used.Sung                                                            [Page 8]

RFC 1791                  TCP And UDP Over IPX                April 1995Appendix B.1 -  UDP/IPX Packet Format without Fragmentation   IPXF transmits UDP packets over IPX in this format if the UDP   datagram does not have to be fragmented:             +-------+-------+-------+-------+             | IPX Checksum  | IPX Pkt Len   |             +-------+-------+-------+-------+             | Zero  |IPX PT | IPX Dest -             +-------+-------+-------+-------+               Network       | IPX Dest -             +-------+-------+-------+-------+               Node                          |             +-------+-------+-------+-------+             | IPX Dest Skt  | IPX Src -             +-------+-------+-------+-------+               Network       | IPX Src -             +-------+-------+-------+-------+               Node                          |             +-------+-------+-------+-------+             | IPX Src Skt   | UDP Header and             +---------------+-------+-------+               Data...             +----...   The IPX PT field contains IPX packet type.  It should be set to 4 for   all UDP/IPX packets.   Both IPX Src Skt and IPX Dest Skt field must be set 0x9092.  The   receiving UDP/IPX should discard the packet silently if the IPX Src   Skt field is not set to 0x9092.  (And increment udpInErrors mib   object if it is instrumented.)Sung                                                            [Page 9]

RFC 1791                  TCP And UDP Over IPX                April 1995Appendix B.2 -  UDP/IPX Packet Format With Fragmentation   IPXF transmits fragmented datagrams over IPX in the following format:             +-------+-------+-------+-------+             | IPX Checksum  | IPX Pkt Len   |             +-------+-------+-------+-------+             | Zero  |IPX PT | IPX Dest -             +-------+-------+-------+-------+               Network       | IPX Dest -             +-------+-------+-------+-------+               Node                          |             +-------+-------+-------+-------+               IPX Dest Skt   | IPX Src -             +-------+-------+-------+-------+               Network       | IPX Src -             +-------+-------+-------+-------+               Node                          |             +-------+-------+-------+-------+             | IPX Src Skt   | IPXF Offset   |             +---------------+-------+-------+             | IPXF Frag Identification      |             +-------------------------------+             | IPXF Dest Skt | IPXF DG Len   |             +-------------------------------+             | UDP Header and Data ...             +--------...   The IPX PT field contains IPX packet type.  It is set to the value   set by the IPXF user in the IPX packet passed to IPXF. (UDP sets it   to 4.)   IPX Dest Skt field must be set to 0x9093 for all IPXF Packets.   The value for IPX Src Skt field is variable, and must be set to the   actual IPX socket number of the IPXF user.  (For example, it must be   set to 0x9092 for UDP.)   IPXF Offset field indicates where the fragment belongs in the   datagram.  The offset is measured is octet from the begining of the   UDP datagram.  The first fragment has the offset of 0.   IPXF Frag Identification field is assigned a same value by the sender   for all fragements belonging to the same datagram.  The receiver then   uses this field to reassemble all fragments with same ID into a   datagram.Sung                                                           [Page 10]

RFC 1791                  TCP And UDP Over IPX                April 1995   IPXF Dest Skt field contains the IPX socket number of the actual   recipient that the reassembled datagram will be delivered to.  (It is   0x9092 for UDP.)  All fragments of a datagram must have the same   value in this field.   IPXF DG Len field is the total length of the IPX datagram before the   fragmentation.  The sender should set it to the value of IPX Pkt Len   of the original IPX datagram.  All fragments of a IPX datagram must   have the same value in this field.Sung                                                           [Page 11]

RFC 1791                  TCP And UDP Over IPX                April 1995Appendix B.3 -  UDP/IPX Pseudo Header Format   UDP/IPX uses following pseudo header for computing the checksum:             +-------+-------+-------+-------+             | IPX Src Network               |             +-------+-------+-------+-------+             | IPX Src Node             +-------+-------+-------+-------+                             | IPX Src Skt   |             +-------+-------+-------+-------+             | IPX Dest Network              |             +-------+-------+-------+-------+             | IPX Dest Node             +-------+-------+-------+-------+                             | IPX Dest Skt  |             +-------+-------+-------+-------+             | Zero          | UDP Length    |             +---------------+---------------+   IPX Src/Dest Network/Node/Skt fields are from the IPX packet.  Note   that, if UDP is running over IPXF, the IPX Dest Skt field in IPX   packet header is copied over from IPXF header before the reassembled   IPX packet is delivered to UDP,  Hence, the pseudo header must be   derived from the reassembled IPX header.   UDP Length is from UDP header.   Note that IPX Src Skt is expected to be 0x9092 for UDP.  As such, one   may insert 0x9092 in IPX Src Skt field rather than getting the value   from IPX header.  Then the implementation will not have to check the   IPX Src Skt field in the fast path since the checksum failure will   also cover the unexpected value.  In that case, the implementation   may want to examine if the checksum failure was due to the IPX Src   Skt value other than 0x9092, so that it can increment appropriate   counter, if proprietary counters other than udpInDatagramErr are   DatagrSung                                                           [Page 12]