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RFC4717 [复制链接]

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亚星游戏官网-yaxin222  新兵

注册:2010-1-28
发表于 2010-1-31 22:19:41 |显示全部楼层
5.1.  The Control Word

   The Control Words defined in this section are based on the Generic PW
   MPLS Control Word as defined in [RFC4385].  They provide the ability
   to sequence individual frames on the PW, avoidance of equal-cost
   multiple-path load-balancing (ECMP) [RFC2992], and OAM mechanisms
   including VCCV [VCCV].

   [RFC4385] states, "If a PW is sensitive to packet misordering and is
   being carried over an MPLS PSN that uses the contents of the MPLS
   payload to select the ECMP path, it MUST employ a mechanism which
   prevents packet misordering."  This is necessary because ECMP
   implementations may examine the first nibble after the MPLS label
   stack to determine whether or not the labelled packet is IP.  Thus,
   if the VPI of an ATM connection carried over the PW using N-to-one
   cell mode encapsulation, without a control word present, begins with
   0x4 or 0x6, it could be mistaken for an IPv4 or IPv6 packet.  This



Martini, et al.             Standards Track                     [Page 6]

RFC 4717            Encapsulation for ATM over MPLS        December 2006


   could, depending on the configuration and topology of the MPLS
   network, lead to a situation where all packets for a given PW do not
   follow the same path.  This may increase out-of-order frames on a
   given PW, or cause OAM packets to follow a different path than actual
   traffic (see section 4.4.3 on Frame. Ordering).

   The features that the control word provides may not be needed for a
   given ATM PW.  For example, ECMP may not be present or active on a
   given MPLS network, strict frame. sequencing may not be required, etc.
   If this is the case, and the control word is not REQUIRED by the
   encapsulation mode for other functions (such as length or the
   transport of ATM protocol specific information), the control word
   provides little value and is therefore OPTIONAL.  Early ATM PW
   implementations have been deployed that do not include a control word
   or the ability to process one if present.  To aid in backwards
   compatibility, future implementations MUST be able to send and
   receive frames without a control word present.

   In all cases, the egress PE MUST be aware of whether the ingress PE
   will send a control word over a specific PW.  This may be achieved by
   configuration of the PEs, or by signaling, as defined in [RFC4447].

   If the pseudowire traverses a network link that requires a minimum
   frame. size (Ethernet is a practical example), with a minimum frame
   size of 64 octets, then such links will apply padding to the
   pseudowire PDU to reach its minimum frame. size.  In this case, the
   control word must include a length field set to the PDU length.  A
   mechanism is required for the egress PE to detect and remove such
   padding.

5.1.1.  The Generic Control Word

   This control word is used in the following encapsulation modes:

     - ATM One-to-one Cell Mode
     - AAL5 PDU Frame. Mode

   The PWE3 control word document [RFC4385] provides the following
   structure for the generic control word:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0|          Specified by PW Encapsulation                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






Martini, et al.             Standards Track                     [Page 7]

RFC 4717            Encapsulation for ATM over MPLS        December 2006


   The detailed structure for the ATM One-to-one Cell Mode and for the
   AAL5 PDU Frame. Mode is as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0| Resvd |        Sequence Number        | ATM Specific  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the above diagram, the first 4 bits MUST be set to 0 to indicate
   PW data.  They MUST be ignored by the receiving PE.

   The next four bits are reserved and MUST be set to 0 upon
   transmission and ignored upon reception.

   The next 16 bits provide a sequence number that can be used to
   guarantee ordered packet delivery.  The processing of the sequence
   number field is OPTIONAL.

   The sequence number space is a 16-bit, unsigned circular space.  The
   sequence number value 0 is used to indicate that the sequence number
   check algorithm is not used.

   The last 8 bits provide space for carrying ATM-specific flags.  These
   are defined in the protocol-specific details below.

   There is no requirement for a length field for the One-to-one Cell
   and PDU Frame. modes because the PSN PDU is always greater than 64
   bytes; therefore, no padding is applied in Ethernet links in the PSN.

5.1.2.  The Preferred Control Word

   This control word is used in the following encapsulation modes:

     - ATM N-to-one Cell Mode
     - AAL5 SDU Frame. Mode

   It is defined as follows:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0| Flags |Res|   Length  |     Sequence Number           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In the above diagram, the first 4 bits MUST be set to 0 to indicate
   PW data.  They MUST be ignored by the receiving PE.




Martini, et al.             Standards Track                     [Page 8]

RFC 4717            Encapsulation for ATM over MPLS        December 2006


   The next 4 bits provide space for carrying protocol-specific flags.
   These are defined in the protocol-specific details below.

   The next 6 bits provide a length field, which is used as follows:  If
   the packet's length (defined as the length of the layer 2 payload
   plus the length of the control word) is less than 64 bytes, the
   length field MUST be set to the packet's length.  Otherwise, the
   length field MUST be set to zero.  The value of the length field, if
   non-zero, can be used to remove any padding.  When the packet reaches
   the service provider's egress router, it may be desirable to remove
   the padding before forwarding the packet.  Note that the length field
   is not used in the N-to-one mode and MUST be set to 0.

   The last 16 bits provide a sequence number that can be used to
   guarantee ordered packet delivery.  The processing of the sequence
   number field is OPTIONAL.

   The sequence number space is a 16-bit, unsigned circular space.  The
   sequence number value 0 is used to indicate that the sequence number
   check algorithm is not used.

5.1.3.  Setting the Sequence Number Field in the Control Word

   This section applies to the sequence number field of both the Generic
   and Preferred Control Words.

   For a given emulated VC and a pair of routers PE1 and PE2, if PE1
   supports packet sequencing, then the sequencing procedures defined in
   [RFC4385] MUST be used.

   Packets that are received out of order MAY be dropped or reordered at
   the discretion of the receiver.

   A simple extension of the processing algorithm in [RFC4385] MAY be
   used to detect lost packets.

   If a PE router negotiated not to use receive sequence number
   processing, and it received a non-zero sequence number, then it
   SHOULD send a PW status message indicating a receive fault and
   disable the PW.

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