rfc2892.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.  Packet Formats

   This section describes the packet formats used by SRP. Packets can be
   sent over any point to point link layer (e.g. SONET/SDH, ATM, point
   to point ETHERNET connections). The maximum transfer unit (MTU) is
   9216 octets.  The minimum transfer unit for data packets is 55
   octets.  The maximum limit was designed to accommodate the large IP
   MTUs of IP over AAL5.  SRP also supports ATM cells.  ATM cells over
   SRP are 55 octets.  The minimum limit corresponds to ATM cells
   transported over SRP.  The minimum limit does not apply to control
   packets which may be smaller.

   These limits include everything listed in Figure 7: but are exclusive
   of the frame delineation (e.g. for SRP over SONET/SDH, the flags used
   for frame delineation are not included in the size limits).

   The following packet and cell formats do not include any layer 1
   frame delineation.  For SRP over POS, there will be an additional
   flag that delineates start and end of frame.

4.1.  Overall Packet Format

   The overall packet format is show below in Figure 7:

   FIGURE 7. Overall Packet Format

                     ---------------------------------
                     |       SRP Header              |
                     ---------------------------------
                     |       Dest. Addr.             |
                     ---------------------------------
                     |       Source Addr.            |
                     ---------------------------------
                     |       Protocol Type           |
                     ---------------------------------
                     |       Payload                 |
                     |                               |
                     |                               |
                     |                               |
                     ---------------------------------
                     |       FCS                     |
                     ---------------------------------

   The frame check sequence (FCS) is a 32-bit cyclic redundancy check
   (CRC) as specified in RFC-1662 and is the same CRC as used in Packet
   Over SONET (POS - specified in RFC-2615).  The generator polynomial
   is:




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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   CRC-32:

   x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 +
   x2 + x + 1

   The FCS is computed over the destination address, source address,
   protocol type and payload.  It does not include the SRP header.

   Note that the packet format after the SRP header is identical to
   Ethernet Version 2.

4.2.  Generic Packet Header Format

   Each packet has a fixed-sized header. The packet header format is
   shown in Figure 8.

   FIGURE 8. Detailed Packet Header Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Time to Live |R| MOD | PRI |P|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Destination Address       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +    Source Address             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |     Protocol Type             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                         Payload                               |
       .                                                               .
       .                                                               .
       .                                                               .
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields are described below.

4.2.1.  Time To Live (TTL)

   This 8 bit field is a hop-count that must be decremented every time a
   node forwards a packet. If the TTL reaches zero it is stripped off
   the ring. This allows for a total node space of 256 nodes on a ring.
   However, due to certain failure conditions (e.g. when the ring is





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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   wrapped) the total number of nodes that are supported by SRP is 128.
   When a packet is first sent onto the ring the TTL should be set to at
   least twice the total number of nodes on the ring.

4.2.2.  Ring Identifier (R)

   This single bit field is used to identify which ring this packet is
   designated for. The designation is as follows:

        TABLE 1. Ring Indicator Values

        Outer Ring      0
        Inner Ring      1

4.2.3.  Priority Field (PRI)

   This three bit field indicates the priority level of the SRP packet
   (0 through 7). The higher the value the higher the priority. Since
   there are only two queues in the transit buffer (HPTB and LPTB) a
   packet is treated as either low or high priority once it is on the
   ring.  Each node determines the threshold value for determining what
   is considered a high priority packet and what is considered a low
   priority packet.  However, the full 8 levels of priority in the SRP
   header can be used prior to transmission onto the ring (transmit
   queues) as well as after reception from the ring (receive queues).

4.2.4.  MODE

   This three bit field is used to identify the mode of the packet. The
   following modes are defined in Table 2 below.

        TABLE 2. MODE Values

        Value   Description

        000     Reserved
        001     Reserved
        010     Reserved
        011     ATM cell
        100     Control Message (Pass to host)
        101     Control Message (Locally Buffered for host)
        110     Usage Message
        111     Packet Data

   These modes will be further explained in later sections.






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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.2.5.  Parity Bit (P-bit)

   The parity bit is used to indicate the parity value over the 15 bits
   of the SRP header to provide additional data integrity over the
   header. Odd parity is used (i.e. the number of ones including the
   parity bit shall be an odd number).

4.2.6.  Destination Address

   The destination address is a globally unique 48 bit address assigned
   by the IEEE.

4.2.7.  Source Address

   The source address is a globally unique 48 bit address assigned by
   the IEEE.

4.2.8.  Protocol Type

   The protocol type is a two octet field like that used in EtherType
   representation. Current defined values relevant to SRP are defined in
   Table 3 below.

        TABLE 3. Defined Protocol Types

        Value   Protocol Type

        0x2007  SRP Control
        0x0800  IP version 4
        0x0806  ARP

4.3.  SRP Cell Format

   SRP also supports the sending of ATM cells.  The detailed cell format
   is shown below:
















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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   FIGURE 9. SRP Cell Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Time to Live |R| MOD | PRI |P|         VPI/VCI               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        VCI            | PTI |C|     HEC       |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                                                               |
       .                                                               .
       .                    ATM   Payload                              .
       .                    ( 48 Bytes )               +-+-+-+-+-+-+-+-+
       |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Packet nodes would typically ignore (never receive or strip) and
   always forward ATM-cells.  The idea is that ATM switches and routers
   could coexist in a ring.  Note that SRP cells do not contain an FCS.
   Data integrity is handled at the AAL layer.

4.4.  SRP Usage Packet Format

   SRP usage packets are sent out periodically to propagate allowed
   usage information to upstream nodes.  SRP usage packets also perform
   a keepalive function.  SRP usage packets should be sent approximately
   every 106 usec.

   If a receive interface has not seen a usage packet within the
   keepalive timeout interval it will trigger an L2 keepalive timeout
   interrupt/event. The IPS software will subsequently mark that
   interface as faulty and initiate a protection switch around that
   interface.  The keepalive timeout interval should be set to 16 times
   the SRP usage packet transmission interval.

   FIGURE 10. Usage Packet Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Time to Live |R| MOD | PRI |P|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  Originator MAC Address       +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Reserved                     |    Usage                      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A USAGE of all ones indicates a value of NULL.



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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.5.  SRP Control Packet Format

   If the MODE bits are set to 10X (SRP control) then this indicates a
   control message. Control messages are always received and stripped by
   the adjacent node.  They are by definition unicast, and do not need
   any addressing information.  The destination address field for
   control packets should be set to 0's.  The source address field for a
   control packet should be set to the source address of the
   transmitting node.

   Two types of controls messages are defined : Pass to host and Locally
   buffered. Pass to host messages can be passed to the host software by
   whatever means is convenient. This is most often the same path used
   to transfer data packets to the host. Locally buffered control
   messages are usually reserved for protection messages.  These are
   normally buffered locally in order to not contend for resources with
   data packets. The actual method of handling these messages is up to
   the implementor.

   The control packet format is shown in Figure 11.

   FIGURE 11. Control Packet Format

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Time to Live |R| MOD | PRI |P|                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     Destination Address       |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +    Source Address             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |     Protocol Type = 0x2007    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Control Ver   | Control Type  |    Control Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Control TTL                 |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +