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There are two types of packet formats: data packets and control packets. Data packets contain the user data, the LCN, the send and receive sequence numbers, and the M-, D-, and Q-bits. Control packets contain the LCN, a packet type identifier, and additional information, depending on the control information.
The following basic packet attributes are supported by NetWare Link/X.25:
After a call or virtual circuit is established between two DTEs, the logical channel is in the data transfer phase. Data is sent through the network in packets. The sending DTE breaks up data into units of maximum length. X.25 specifies that a network must support a maximum user data field length of at least 128 bytes. The network can also allow a different maximum field length of 16 to 4,096 bytes. The length requirement can be different at either end of a virtual circuit.
NetWare Link/X.25 supports the standard maximum user data size of 128 bytes. Other, nonstandard default maximum user data fields between 16 and 4,096 bytes are also available.
When the Q-bit is set to 0, the user data field contains data for the receiving user. When the Q-bit is set to 1, the user data field contains high-level control information that is used in packet assembler/disassembler (PAD) operations or other protocols.
When the D-bit is set to 0, delivery is acknowledged locally. When the D-bit is set to 1, the remote DTE acknowledges delivery (end-to-end acknowledgment). This is an acknowledgment of the entire complete packet sequence. The DTE can set the D-bit to 1 for individual packets or all packets. The D-bit set on all packets gives the maximum security, but also adds delay.
When the M-bit is set to 1, additional complete packet sequences follow, and the next packet is a logical continuation of the data in the current packet. This is useful for long information streams that do not fit in one data packet. Smaller sequences can form a part of a large sequence, and end-to-end acknowledgment can occur before the end of a larger sequence. The M-bit can also be set by the network if the network truncates a large packet into smaller ones.
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