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.

       .                                                               .
       .   Payload                                                     .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The priority (PRI) value should be set to 0x7 (all one's) when
   sending control packets and should be queued to the highest priority
   transmit queue available.  The Time to Live is not relevant since all





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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   packets will be received and stripped by the nearest downstream
   neighbor and can be set to any value (preferably this should be set
   to 001).

4.5.1.  Control Ver

   This one octet field is the version number associated with the
   control type field.  Initially, all control types will be version 0.

4.5.2.  Control Type

   This one octet field represents the control message type. Table 4
   contains the currently defined control types.

        TABLE 4. Control Types

        Control Type    Description

        0x01            Topology Discovery

        0x02            IPS message

        0x03-
        0xFF            Reserved

4.5.3.  Control TTL

   The Control TTL is a control layer hop-count that must be decremented
   every time a node forwards a control packet.  If a node receives a
   control packet with a control TTL <= 1, then it should accept the
   packet but not forward it.

   Note that the control layer hop count is separate from the SRP L2 TTL
   which is always set to 1 for control messages.

   The originator of the control message should set the initial value of
   the control TTL to the SRP L2 TTL normally used for data packets.

4.5.4.  Control Checksum

   The checksum field is the 16 bit one's complement of the one's
   complement sum of all 16 bit words starting with the control version.
   If there are an odd number of octets to be checksummed, the last
   octet is padded on the right with zeros to form a 16 bit word for
   checksum purposes.  The pad is not transmitted as part of the
   segment.  While computing the checksum, the checksum field itself is
   replaced with zeros.  This is the same checksum algorithm as that
   used for TCP.  The checksum does not cover the 32 bit SRP FCS.



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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.5.5.  Payload

   The payload is a variable length field dependent on the control type.

4.5.6.  Addressing

   All nodes must have a globally unique IEEE 48 bit MAC address. A
   multicast bit is defined using canonical addressing conventions i.e.
   the multicast bit is the least significant bit of the most
   significant octet in the destination address.  It is acceptable but
   not advisable to change a node's MAC address to one that is known to
   be unique within the administrative layer 2 domain (that is the SRP
   ring itself along with any networks connected to the SRP ring via a
   layer 2 transparent bridge).

   FIGURE 12. Multicast bit position

                   Destination Address
        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             |M|                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      ^
                      |----Multicast bit

   Note that for SONET media, the network order is MSB of each octet
   first, so that as viewed on the line, the multicast bit will be the
   8th bit of the destination address sent. (For SRP on Ethernet media,
   the multicast bit would be sent first).

4.6.  Topology Discovery

   Each node performs topology discovery by sending out topology
   discovery packets on one or both rings.  The node originating a
   topology packet marks the packet with the egressing ring id, appends
   the node's mac binding to the packet and sets the length field in the
   packet before sending out the packet. This packet is a point-to-point
   packet which hops around the ring from node to node. Each node
   appends its mac address binding, updates the length field and sends
   it to the next hop on the ring. If there is a wrap on the ring, the
   wrapped node will indicate a wrap when appending its mac binding and
   wrap the packet. When the topology packets travel on the wrapped
   section with the ring identifier being different from that of the
   topology packet itself, the mac address bindings are not added to the
   packet.





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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   Eventually the node that generated the topology discovery packet gets
   back the packet. The node makes sure that the packet has the same
   ingress and egress ring id before excepting the packet. A topology
   map is changed only after receiving two topology packets which
   indicate the same new topology (to prevent topology changes on
   transient conditions).

   Note that the topology map only contains the reachable nodes. It does
   not correspond to the failure-free ring in case of wraps and ring
   segmentations.

   FIGURE 13. Topology Packet Format

       Topology

        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=0 | Control Type=1|    Control Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Control TTL                 |   Topology Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Originator's Globally Unique                            |
       +       MAC Address  (48 bits)  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |  MAC Type     |               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
       |                   MAC Address (48 bits)                       |
       +               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               |   Other MAC bindings                          |
       +-+-+-+-+-+-+-+-+                                               +
       |                                                               |
       +                                                               +

   Note that the Source address should be set to the source address of
   the TRANSMITTING node (which is not necessarily the ORIGINATING
   node).







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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.6.1.  Topology Length

   This two octet field represents the length of the topology message in
   octets starting with the first MAC Type/MAC Address binding.

4.6.2.  Topology Originator

   A topology discovery packet is determined to have been originated by
   a node if the originator's globally unique MAC address of the packet
   is that node's globally unique MAC address (assigned by the IEEE).

   Because the mac addresses could be changed at a node, the IEEE MAC
   address ensures that a unique identifier is used to determine that
   the topology packet has gone around the ring and is to be consumed.

4.6.3.  MAC bindings

   Each MAC binding shall consist of a MAC Type field followed by the
   node's 48 bit MAC address.  The first MAC binding shall be the MAC
   binding of the originator.  Usually the originator's MAC address will
   be it's globally unique MAC Address but some implementations may
   allow this value to be overridden by the network administrator.

4.6.4.  MAC Type Format

   This 8 bit field is encoded as follows:

        TABLE 5. MAC Type Format

        Bit     Value

        0       Reserved
        1       Ring ID (1 or 0)
        2       Wrapped Node (1) / Unwrapped Node (0)
        3-7     Reserved

   Determination of whether a packet's egress and ingress ring ID's are
   a match should be done by using the Ring ID found in the MAC Type
   field of the last MAC binding as the ingress ring ID rather than the
   R bit found in the SRP header.  Although they should be the same, it
   is better to separate the two functions as some implementations may
   not provide the SRP header to upper layer protocols.

   The topology information is not required for the IPS protection
   mechanism. This information can be used to calculate the number of
   nodes in the ring as well as to calculate hop distances to nodes to
   determine the shortest path to a node (since there are two counter-
   rotating rings).



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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


   The implementation of the topology discovery mechanism could be a
   periodic activity or on "a need to discover" basis. In the periodic
   implementation, each node generates the topology packet periodically
   and uses the cached topology map until it gets a new one. In the need
   to discover implementation, each node generates a topology discovery
   packet whenever they need one e.g., on first entering a ring or
   detecting a wrap.

4.7.  Intelligent Protection Switching (IPS)

   IPS is a method for automatically recovering from various ring
   failures and line degradation scenarios. The IPS packet format is
   outlined in Figure 14 below.

   FIGURE 14. IPS 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=0 | Control Type=2|    Control Checksum           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Control TTL                 |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |             Originator MAC Address                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Ips Octet   |  Rsvd Octet   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The IPS specific fields are detailed below.

4.7.1.  Originator MAC Address

   This is the MAC address of the originator of the IPS message.  It is
   not necessarily the same as the SRP Header Source Address as a node
   may be simply propagating an IPS message (see the section "SRP IPS
   Protocol Rules" Rule P.8 as an example).







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RFC 2892            The Cisco SRP MAC Layer Protocol         August 2000


4.7.2.  IPS Octet

   The IPS octet contains specific protection information. The format of
   the IPS octet is as follows:

   FIGURE 15. IPS Octet Format:

   Bits    Values (values not listed are reserved)

   0-3     IPS Request Type

           1101 - Forced Switch (FS)
           1011 - Signal Fail (SF)
           1000 - Signal Degrade (SD)
           0110 - Manual Switch (MS)
           0101 - Wait to Restore (WTR)
           0000 - No Request (IDLE)

   4       Path indicator

           0 - short (S)
           1 - long (L)

   5-7     Status Code

           010 - Protection Switch Completed - traffic Wrapped (W)
           000 - Idle (I)

   The currently defined request types with values, hierarchy and
   interpretation are as used in SONET BLSR [3], [4], except as noted.

4.8.  Circulating packet detection (stripping)

   Packets continue to circulate when transmitted packets fail to get