rfc908.txt
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If EACK set
Send <SEQ=SEG.ACK + 1><RST>
Discard segment
Return
Endif
If ACK set
If SEG.ACK = SND.ISS
Set State = OPEN
else
Send <SEQ=SEG.ACK + 1><RST>
Discard segment
Return
Endif
else
Discard segment
Return
Endif
If Data in segment or NUL set
If the received segment is in sequence
Copy the data (if any) to user buffers
Set RCV.CUR=SEG.SEQ
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK>
else
If out-of-sequence delivery permitted
Copy the data (if any) to user buffers
Endif
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK><EACK><RCVDSEQNO1>
...<RCVDSEQNOn>
Endif
Endif
Endif
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RFC-908 July 1984
If State = OPEN
If RCV.CUR < SEG.SEQ =< RCV.CUR + (RCV.MAX * 2)
Segment sequence number acceptable
else
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK>
Discard segment and return
Endif
If RST set
Set State = CLOSE-WAIT
Signal "Connection Reset"
Return
Endif
If NUL set
Set RCV.CUR=SEG.SEQ
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK>
Discard segment
Return
Endif
If SYN set
Send <SEQ=SEG.ACK + 1><RST>
Set State = CLOSED
Signal "Connection Reset"
Discard segment
Deallocate connection record
Return
Endif
If ACK set
If SND.UNA =< SEG.ACK < SND.NXT
Set SND.UNA = SEG.ACK
Flush acknowledged segments
Endif
Endif
If EACK set
Flush acknowledged segments
Endif
Page 28
RDP Specification Protocol Operation
If Data in segment
If the received segment is in sequence
Copy the data to user buffers
Set RCV.CUR=SEG.SEQ
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK>
else
If out-of-sequence delivery permitted
Copy the data to user buffers
Endif
Send <SEQ=SND.NXT><ACK=RCV.CUR><ACK><EACK><RCVDSEQNO1>
...<RCVDSEQNOn>
Endif
Endif
Endif
3.7.3 Timeout Events
Timeout events occur when a timer expires and signals the
RDP. Two types of timeout events can occur, as described below:
RETRANSMISSION TIMEOUTS
If timeout on segment at head of retransmission queue
Resend the segment at head of queue
Restart the retransmission timer for the segment
Requeue the segment on retransmission queue
Return
Endif
CLOSE-WAIT TIMEOUTS
Set State = CLOSED
Deallocate connection record
Return
Page 29
RFC-908 July 1984
Page 30
RDP Specification RDP Segments and Formats
CHAPTER 4
RDP Segments and Formats
The segments sent by the application layer are encapsulated
in headers by the transport, internet and network layers, as
follows:
+----------------+
| Network Access |
| Header |
+----------------+
| IP Header |
+----------------+
| RDP Header |
+----------------+
| D |
| A |
| T |
| A |
+----------------+
Segment Format
Figure 4
4.1 IP Header Format
When used in the internet environment, RDP segments are sent
using the version 4 IP header as described in RFC791, "Internet
Protocol." The RDP protocol number is ??? (decimal). The time-
to-live field should be set to a reasonable value for the
network.
All other fields should be set as specified in RFC-791.
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RFC-908 July 1984
4.2 RDP Header Format
Every RDP segment is prefaced with an RDP header. The
format of the header is shown in Figure 5 below. The RDP header
is variable in length and its size is indicated by a field in a
fixed location within the header.
0 0 0 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+---+---------------+
|S|A|E|R|N| |Ver| Header |
0 |Y|C|A|S|U|0|No.| Length |
|N|K|K|T|L| | | |
+-+-+-+-+-+-+---+---------------+
1 | Source Port | Dest. Port |
+---------------+---------------+
2 | Data Length |
+---------------+---------------+
3 | |
+--- Sequence Number ---+
4 | |
+---------------+---------------+
5 | |
+--- Acknowledgement Number ---+
6 | |
+---------------+---------------+
7 | |
+--- Checksum ---+
8 | |
+---------------+---------------+
9 | Variable Header Area |
. .
. .
| |
+---------------+---------------+
RDP Header Format
Figure 5
Page 32
RDP Specification RDP Segments and Formats
4.2.1 RDP Header Fields
Control Flags
This 8-bit field occupies the first octet of word one in the
header. It is bit encoded with the following bits currently
defined:
Bit # Bit Name Description
0 SYN Establish connection and
synchronize sequence numbers.
1 ACK Acknowledge field significant.
2 EACK Non-cumulative (Extended) acknowledgement.
3 RST Reset the connection.
4 NUL This is a null (zero data length) segment.
5 Unused.
Note that the SYN and RST are sent as separate segments and
may not contain any data. The ACK may accompany any
message. The NUL segment must have a zero data length, but
may be accompanied by ACK and EACK information. The other
control bit is currently unused and is defined to be zero.
Version Number
This field occupies bits 6-7 of the first octet. It
contains the version number of the protocol described by
this document. Current value is one (1).
Header Length
The length of the RDP header in units of two (2) octets,
including this field. This field allows RDP to find the
start of the Data field, given a pointer to the head of the
segment. This field is 8 bits in length. For a segment
with no variable header section, the header length field
will have the value 9.
Source and Destination Ports
The Source and Destination Ports are used to identify the
processes in the two hosts that are communicating with each
other. The combination of the port identifiers with the
source and destination addresses in the network access
Page 33
RFC-908 July 1984
protocol header serves to fully qualify the connection and
constitutes the connection identifier. This permits RDP to
distinguish multiple connections between two hosts. Each
field is 8 bits in length, allowing port numbers from 0 to
255 (decimal).
Data Length
The length in octets of the data in this segment. The data
length does not include the RDP header. This field is 16
bits in length.
Sequence Number
The sequence number of this segment. This field is 32 bits
in length.
Acknowledgement Number
If the ACK bit is set in the header, this is the sequence
number of the segment that the sender of this segment last
received correctly and in sequence. Once a connection is
established this should always be sent. This field is 32
bits in length.
Checksum
This field is a 32-bit checksum of the segment header and
data. The algorithm description below includes two
variables, the checksum accumulator and the checksum
pointer. The checksum accumulator is an actual 32-bit
register in which the checksum is formed. The checksum
pointer is included for purposes of description, to
represent the operation of advancing through the data four
octets (32-bits) at a time. It need not be maintained in a
register by an implementation.
1) The checksum pointer is set to zero, to correspond to the
beginning of the area to be checksummed. The checksum
accumulator is al⌨️ 快捷键说明
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