📄 rfc1553.txt
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Network Working Group S. Mathur
Request for Comments: 1553 M. Lewis
Category: Standards Track Telebit Corporation
December 1993
Compressing IPX Headers Over WAN Media (CIPX)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state and
status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes a method for compressing the headers of IPX
datagrams (CIPX). With this method, it is possible to
significantly improve performance over lower speed wide area
network (WAN) media. For normal IPX packet traffic, CIPX can
provide a compression ratio of approximately 2:1 including both IPX
header and data. This method can be used on various type of WAN
media, including those supporting PPP and X.25.
This memo ia a product of the Point-to-Point Protocol Extensions
(PPPEXT) Working Group of the IETF. Comments should be sent to
the authors and the ietf-ppp@ucdavis.edu mailing list.
Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized.
MUST
This word, or the adjective "required", means that the
definition is an absolute requirement of the specification.
MUST NOT
This phrase means that the definition is an absolute
prohibition of the specification.
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RFC 1553 CIPX December 1993
SHOULD
This word, or the adjective "recommended", means that there
may exist valid reasons in particular circumstances to
ignore this item, but the full implications should be
understood and carefully weighed before choosing a
different course.
MAY
This word, or the adjective "optional", means that this
item is one of an allowed set of alternatives. An
implementation which does not include this option MUST be
prepared to interoperate with another implementation which
does include the option.
Introduction
Internetwork Packet Exchange (IPX) is a protocol defined by the
Novell Corporation [1]. It is derived from the Internet Datagram
Protocol (IDP) protocol of the Xerox Network Systems (XNS) family
of protocols. IPX is a datagram, connectionless protocol that does
not require an acknowledgment for each packet sent. The IPX
protocol corresponds to the network layer of the ISO model.
Usually, there is a transport layer protocol above IPX. The most
common transport protocol is the Netware Core Protocol (NCP), which
is used for file server access. The Sequenced Packet Exchange
(SPX) is the reliable connection-based transport protocol commonly
used by applications.
The IPX packet consists of a 30 octet IPX header, usually followed
by the transport layer protocol header. The NCP header is 6 octets
in length. The SPX header is 12 octets in length.
Two strategies are described below for compressing IPX headers.
This specification requires that implementations of CIPX support
both IPX header compression strategies. These header compression
algorithms are based on those Van Jacobson described [2] for TCP/IP
packets.
The first strategy is to compress only the IPX header. This
compression algorithm can be used to compress any IPX packet,
without affecting the transport protocol. This algorithm
compresses a 30 octet IPX header into a one to seven octet header.
The second strategy is to compress the combined IPX and NCP
headers. This algorithm compresses only NCP packets with NCP type
of 0x2222 and 0x3333. This algorithm compresses a 36 octet NCP/IPX
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RFC 1553 CIPX December 1993
header into a one to eight octet header.
Lastly, it is possible and many times desirable, to use this type
of header compression in conjunction with some type of data
compression.
Data compression technology takes many forms. Link bit stream
compression is a common approach over very low speed asynchronous
links, normally performed by modems transparently. Transparent bit
stream compression is also offered in some DSUs, routers and
bridges. Data compression can be provided using protocols such as
CCITT V.42bis[3], MNP 5, Lempel-Ziv, or LAPB[4].
When using both header and data compression, the sequence of
compression is important. When sending packets, data compression
MUST be done after header compression. Conversely when receiving
packets, data decompression MUST be done before header
decompression.
IPX Compression Algorithm
The normal IPX header consists of the following fields: checksum,
packet length, transport control (hop count), packet type,
destination and source address fields.
+-----------------------+
| Checksum |
+-----------------------+
| Packet Length |
+-----------+-----------+
| Hops |Packet Type|
+-----------+-----------+
| Destination |
| Address |
| (12 Octets) |
+-----------------------+
| Source |
| Address |
| (12 Octets) |
+-----------------------+
IPX PACKET HEADER
The IPX header diagram above is shown without the field alignment
details. Consider each field of the IPX header separately, and how
it typically changes.
Historically, Novell has not used the Checksum field in the IPX
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RFC 1553 CIPX December 1993
header, and has required that this field be set to 0xFFFF. Since the
Checksum field remains constant, it is clear that the value can be
compressed.
Where Checksums are implemented (not 0xFFFF), the Checksum MUST be
included in the compressed packet. Recalculating the checksum would
destroy the end-to-end reliability of the connection. Note that
Checksums are now implemented in the Fault Tolerant Servers.
For most links, the Packet Length can be determined from the MAC
layer. There are cases in which the length cannot be determined from
the MAC layer. For example, some hardware devices pad packets to a
required minimum length. For links where it is not possible to
determine the IPX packet length from the MAC layer, packet length
needs to be included in the compressed packet.
The Transport Control (Hops) field usually does not change between
two end-points. For the purposes of compression, we will assume that
it never changes, and will not examine this field when determining a
connection.
The Packet Type field is constant for any connection.
The Destination and Source Address fields are each made up of 12
octets: Network (4 octets), Node (6 octets), and Socket (2 octets)
fields. An IPX connection is the logical association between two
endpoints known by a given source and destination address pair. For
any specific IPX connection, the Destination and Source Address
fields are constant.
Hence, the fields that may need to be included in the compressed IPX
header are the Checksum and the Packet Length.
While using this IPX header compression algorithm, packets can be
lost. The loss of an Initial packet presents a problem. In this
case, if the sender later tries to send a compressed packet, the
receiving end cannot decompress the packet correctly.
Sufficient information is not available in the IPX header to
determine when a re-transmission has occured. For this reason, it is
necessary that the sender of an Initial packet be guaranteed that the
packet has been received. Therefore, we provide a mechanism for
Confirmation of an Initial packet.
NCP/IPX Header Compression
Since most IPX packets are Netware Core Protocol packets (packet type
17), compressing the NCP header will give us added performance. A
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RFC 1553 CIPX December 1993
minimal CIPX implementation MUST also implement NCP/IPX compression.
+------------+
| NCP |
| Type |
+------------+
| Sequence |
| Number |
+------------+
| Connection |
|(low octet) |
+------------+
| Task |
| Number |
+------------+
| Connection |
|(high octet)|
+------------+
NCP HEADER
The NCP header is 6 octets in length consisting of the following
fields: NCP type, sequence number, connection number and task number.
The NCP type field values that are currently defined are:
1111 Create Connection
2222 NCP request from workstation
3333 NCP reply from file server
5555 Destroy Connection
7777 Burst Mode Packet
9999 Server Busy Packet
This NCP header compression algorithm only compresses packets that
have a type field value of 0x2222 or 0x3333. If the NCP type is
0x2222, this packet is a request from the client to the server.
Conversely if the NCP type is 0x3333, this is a response from the
server to the client. All other types of NCP packets are not
compressed at the NCP level, but are compressed at the IPX level.
The Create Connection (0x111), Destroy Connection (0x5555) and Server
Busy (0x9999) packets are not exchanged frequently enough to justify
special NCP compression. The Burst Mode (0x7777) packet is discussed
below.
The connection number is a constant for a given connection.
The sequence number is increased by one for each new request. Hence
the sequence number can be determined implicitly. The decompressor
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RFC 1553 CIPX December 1993
increments the sequence number for each compressed packet it receives
for a connection.
The task number can change unpredictably, although it might remain
constant for several packets. If the NCP task number is different
from the last one for this connection, the NCP task number must be
included.
If the NCP packet is lost, it will be retransmitted through the
normal transport layer mechanisms. The Initial NCP packet does not
require confirmation, as a re-transmitted packet can be easily
identified. This is accomplished by comparing the sequence number of
the packet to the sequence number of the previous packet. If the
sequence number is not exactly one greater than the previous packet,
a new Initial packet must be sent, although the same connection slot
may be used.
In the event of compressed packet loss, the sequence number will be
too small. When the IPX Checksum is present, the loss can be
determined at the destination system by an incorrect checksum. When
there is no checksum present, the loss is more likely to be detected
upon receiving a later retransmission.
NCP Burst Mode Packets
The burst mode protocol uses the NCP type value of 0x7777. This type
of packet does not have the normal NCP header described above.
Instead, it has a 36 octet burst header. The above NCP header
compression algorithm should not be used to compress this packet.
The IPX header in this packet is still compressible with the IPX
header compression algorithm described.
SPX Packets
SPX packets are typically used by applications which require
reliable service such as print servers. It is possible to apply a
similar NCP/IPX technique to SPX/IPX packets. At this time, we
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