Editing Sequenced Packet Exchange

{{Short description|Network protocol}}
'''Sequenced Packet Exchange''' ('''SPX''') is a protocol in the [[IPX/SPX]] [[protocol stack]] that corresponds to a [[connection-oriented]] [[transport layer]] protocol in the [[OSI model]]. Being reliable and connection-oriented, it is analogous to the [[Transmission Control Protocol]] (TCP) of [[TCP/IP]], but it is a datagram protocol, rather than a stream protocol.

== SPX packet structure==

Each SPX packet begins with a header with the following structure:

{|class=wikitable style="text-align:left;"
|-
! Octets !! Field
|-
| 1 || Connection Control
|-
| 1 || Datastream Type
|-
| 2 || Source Connection Id
|-
| 2 || Destination Connection Id (0xFFFF = unknown)
|-
| 2 || Sequence Number
|-
| 2 || Acknowledgement Number
|-
| 2 || Allocation Number (The number of outstanding receive buffers available)
|-
| 0–534 || data
|}

The Connection Control fields contains 4 single-bit flags:

{|class=wikitable style="text-align:left;"
|-
! Weight !! Meaning
|-
| 0x10 || End-of-message
|-
| 0x20 || Attention
|-
| 0x40 || Acknowledgement Required
|-
| 0x80 || System Packet
|}

The Datastream Type serves to close the SPX connection. For this purpose two values are used:

{|class=wikitable style="text-align:left;"
|-
! Value !! Meaning
|-
| 0x00–0xFD || Available for client use
|-
| 0xFE || End-of-Connection
|-
| 0xFF || End-of-Connection Acknowledgement
|}

==External links==
* [http://www.networksorcery.com/enp/protocol/spx.htm SPX, Sequenced Packet Exchange]
* [http://www.connectotel.com/netware/ipxpack.txt IPX / SPX Packet Structures]

[[Category:Novell NetWare]]
[[Category:Transport layer protocols]]

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