rfc1221.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

     4         |            DESTINATION HOST ADDRESS           |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     5         |              SOURCE HOST ADDRESS              |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     6         |                  PROTOCOL ID                  |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |                                               |
     7-N       :                      DATA                     :
               |                                               |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


                              STREAM MESSAGE
                                 Figure 2



     0[0]      Message Class = 0 (Data Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.



Edmond                                                         [Page 11]

RFC 1221                          HAP2                        April 1991


     0[4-7]    Reserved.

     0[8-15]   Message Number.  This field serves the same purpose as
               the message number field in the datagram message.
               Moreover, a single message number sequence is used for
               both datagram and stream messages (see Section 5).

     1[0-15]   Header Checksum.  (See datagram checksum for
               description.)

     2[0-15]   Piggybacked A/R.

     3[0]      Data Message Type = 1 (Stream).

     3[1]      IL flag.  Obsolete.  Must be zero.

     3[2]      Discard Flag.

     3[3]      Data Error Flag.

     3[4-5]    Stream message priority.  Note that all stream messages
               have priority over any datagram message.  Priority will
               not affect the order of stream message delivery.

                    0 = Low priority
                    1 = Medium priority
                    2 = High priority
                    3 = Reserved

     3[6-15]   Stream ID.  The WPS uses this field to identify the
               preallocated network resources (bandwidth allocations,
               queues, buffers, etc.) to use for delivery of the
               message.  Streams and their identifying numbers (stream
               IDs) are established by an explicit Create Stream
               request (see Section 6.1).

     4[0-15]   Destination Host Address.

     5[0-15]   Source Host Address.

     6[0-15]   Protocol ID.

     7-N       Data.  This field contains up to 16,384 bits (2048
               octets) of user data, and must be an even number of
               octets.






Edmond                                                         [Page 12]

RFC 1221                          HAP2                        April 1991


5. Flow Control Messages

   The WPS supports an acceptance/refusal (A/R) mechanism in each
   direction on the host access link.  The A/R mechanism is enabled for
   the link by the host by setting a bit in the Restart Complete control
   message (see Section 8).  Each datagram and stream message contains
   an 8-bit message number used to identify the message for flow control
   purposes.  When the A/R mechanism is enabled, the message number is
   incremented modulo 256 in successive messages, skipping over message
   number zero (zero indicates that A/R's are disabled for that
   message).  Up to 127 messages may be outstanding (awaiting acceptance
   or refusal) in each direction.  If the receiver of a message is
   unable to accept the message, a refusal indication containing the
   message number of the refused message and the reason for the refusal
   is returned.  The refusal indication may be piggybacked on data
   messages in the opposite direction over the link or may be sent in a
   separate control message in the absence of reverse data traffic.

   Acceptance indications are returned in a similar manner, either
   piggybacked on data messages or in a separate control message.  An
   acceptance is returned by the receiver to indicate that the
   identified message was received from the host access link and was not
   refused.  Acceptance indications returned by the WPS are not an end-
   to-end acknowledgement and do not imply any guarantee of delivery to
   the destination host(s), or even any assurance that the message will
   not be intentionally discarded by the network.  They are sent
   primarily to facilitate buffer management in the host.

   To reduce the number of A/R messages exchanged, a single A/R
   indication can be returned for multiple (lower numbered) previously
   unacknowledged messages.  Explicit acceptance of message number N
   implies implicit acceptance of outstanding messages with numbers N-1,
   N-2, etc., according to the definition of acceptance outlined above.
   Analogous interpretation of the refusal message number allows the
   receiver of a group of messages to reject them as a group when they
   all are being refused for the same reason.  As a further efficiency
   measure, HAP permits aggregation of any mix of A/R indications into a
   single A/R control message.  Such a message might be used, for
   example, to reject a group of messages where the refusal code on each
   is different.

   In some circumstances the overhead associated with processing A/R
   messages may prove unattractive.  For these cases, it is possible to
   disable the A/R mechanism and operate the HAP interface in a purely
   discard mode.  The ability to effect this on a link basis has already
   been noted (see Sections 2 and 8).  In addition, messages with
   sequence number zero are taken as messages for which the A/R
   mechanism is selectively disabled.  To permit critical feedback, even



Edmond                                                         [Page 13]

RFC 1221                          HAP2                        April 1991


   when operating in discard mode, HAP defines an "Unnumbered Response"
   control message.  Flow control information, and other information
   which cannot be sent as an A/R indication, is sent in an Unnumbered
   Response control message.  The format of this type of message is
   illustrated in Figure 5.

   The format shown in Figure 3 is used both for A/R indications that
   are piggybacked on data messages (word 2), and for aggregated A/R
   information in A/R control messages.  The format of A/R control
   messages is shown in Figure 4.


                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |AR|    REFUSAL CODE    |  A/R MESSAGE NUMBER   |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


                          ACCEPTANCE/REFUSAL WORD
                                 Figure 3



     [0]       Acceptance/Refusal Type.  This field identifies whether
               A/R information is an acceptance or a refusal.

                    0 = Acceptance
                    1 = Refusal

     [1-7]     Refusal Code.  When the Acceptance/Refusal Type = 1,
               this field gives the Refusal Code.

                    0 = Priority not being accepted
                    1 = Source WPS congestion
                    2 = Destination WPS congestion
                    3 = Destination host dead
                    4 = Destination WPS dead
                    5 = Illegal destination host address
                    6 = Destination host access not allowed
                    7 = Illegal source host address
                    8 = Message lost in access link
                    9 = Invalid stream ID
                   10 = Illegal source host for stream ID
                   11 = Message length too long
                   12 = Stream message too early
                   13 = Illegal control message type
                   14 = Illegal refusal code in A/R
                   15 = Can't implement loop



Edmond                                                         [Page 14]

RFC 1221                          HAP2                        April 1991


                   16 = Destination host congestion
                   17 = Delivery refused
                   18 = Odd byte length packet (not allowed)
                   19 = Invalid stream time-to-live value
                   20 = "Reliability length" exceeds message length

     [8-15]    A/R Message Number.  This field contains the number of
               the message to which this acceptance/refusal refers.
               It also applies to all outstanding messages with
               earlier numbers.  Note that this field can never be
               zero since a message number of zero implies that the
               A/R mechanism is disabled.



                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     0         | 1|LB|GOPRI|     0     |  LENGTH   |     1     |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     1         |                HEADER CHECKSUM                |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
               |                                               |
     2-N       :                     A/R's                     :
               |                                               |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


                        ACCEPTANCE/REFUSAL MESSAGE
                                 Figure 4



     0[0]      Message Class = 1 (Control Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.

     0[4-7]    Reserved.

     0[8-11]   Message Length.  This field contains the total length
               of this message in words (N+1).

     0[12-15]  Control Message Type = 1 (Acceptance/Refusal).

     1[0-15]   Header Checksum.  The checksum is the 2's-complement of
               the 2's-complement sum of words 0-N (excluding the
               checksum word itself).



Edmond                                                         [Page 15]

RFC 1221                          HAP2                        April 1991


     2[0-15]   Acceptance/Refusal Word.

     3-N       Additional Acceptance/Refusal Words (optional).


                 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     0         | 1|LB|GOPRI|     0     | RES-CODE  |     5     |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     1         |                HEADER CHECKSUM                |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     2         |                 RESPONSE INFO                 |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
     3         |                 RESPONSE INFO                 |
               +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+


                            UNNUMBERED RESPONSE
                                 Figure 5



     0[0]      Message Class = 1 (Control Message).

     0[1]      Loopback indicator.

     0[2-3]    Go-Priority.

     0[4-7]    Reserved.

     0[8-11]   Response Code.

                    3 = Destination unreachable
                    5 = Illegal destination host address
                    7 = Illegal source host address
                    9 = Nonexistent stream ID
                   10 = Illegal stream ID
                   13 = Protocol violation
                   15 = Can't implement loop

     0[12-15]  Control Message Type = 5 (Unnumbered Response).