rfc3063.txt

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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


   Fig.9 shows an example of thread withdrawing.  In Fig.9(a), the red
   thread on incoming link i2 is withdrawn.  Then Hmax decreases from 3
   to  1, and node B  creates a new  green thread and extends it
   downstream, as shown in Fig.9(b).

          (bl,1)      (re,4)           (bl,1)       (gr,2)+
       ----[i1]--->B---[o1]--->     ----[i1]--->B----[o1]--->
                   ^
                   |
       ----[i2]----+
          (re,3)-

                Fig.9(a)                     Fig.9(b)

               Fig.9  Thread withdrawing (1)

   Fig.10 shows another example of thread withdrawing.  In Fig.10(a),
   the red thread on incoming link i3 is withdrawn.  In this case, Hmax
   decreases from unknown to 1, however, no thread is extended as shown
   in Fig.10(b), since the outgoing link has a colored thread and the
   hop count is unknown.

           (bl,1)      (re,U)          (bl,1)       (re,U)
       ----[i2]--->B----[o1]--->    ----[i2]--->B----[o1]--->
                   ^
                   |
       ----[i3]----+
           (re,U)-

               Fig.10(a)                     Fig.10(b)

               Fig.10    Thread withdrawing (2)

   Fig.11 shows another example of thread withdrawing.  In Fig.11(a),
   the transparent thread on incoming link i3 is withdrawn.  In this
   case, a transparent thread is extended (Fig.11(b)), since Hmax
   decreases and the outgoing link is transparent.

           (tr,1)      (tr,U)          (tr,1)       (tr,2)+
       ----[i2]--->B----[o1]--->    ----[i2]--->B----[o1]--->
                   ^
                   |
       ----[i3]----+
           (tr,U)-

               Fig.11(a)                     Fig.11(b)

               Fig.11    Thread withdrawing (3)



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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


4. Thread algorithm

   The ordered downstream-on-demand allocation is assumed here, however,
   the algorithm can be adapted to the ordered downstream allocation, as
   shown in section 5.

   In the algorithm, a next hop change event will be separated into two
   events: a next hop loss event on the old next hop and a next hop
   acquisition event on the new next hop, in this order.

   The following notations are defined:

         Hmax: the largest incoming link hop count
         Ni:   the number of unstalled incoming links

   The thread algorithm is described as follows.

   When a node acquires a new next hop, it creates a colored thread and
   extends it downstream.

   When a node loses a next hop to which it has extended a thread, it
   may withdraw that thread.  As described in section 3, this may or may
   not cause the next hop to take some action.  Among the actions the
   next hop may take are withdrawing the thread from its own next hop,
   or extending a new thread to its own next hop.

   A received colored thread is either stalled, merged, rewound, or
   extended.  A thread with TTL zero is never extended.

   When a received thread is stalled at a node, if Ni=0 and the node is
   not an eligible leaf node, initiate a thread withdrawing.  Otherwise,
   if Ni>0 and the received thread hop count is not unknown, a colored
   thread of hop count unknown is created and extended.  If the received
   thread hop count is unknown, no thread is extended and no further
   action is taken.

   When a thread being extended is rewound, if the thread hop count is
   greater than one more than Hmax, a transparent thread of hop count
   (Hmax+1) is extended downstream.

   When a node that has an transparent outgoing link receives a
   transparent thread, if Hmax decreases the node extends it downstream
   without changing color.

5. Applicability of the algorithm

   The thread algorithm described in section 4 can be applied to various
   LSP management policies.



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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


5.1.  LSP Loop prevention/detection

   The same thread algorithm is applicable to both LSP loop prevention
   and detection.

   In loop prevention mode, a node transmits a label mapping (including
   a thread object) for a particular LSP only when it rewinds the thread
   for that LSP.  No mapping message is sent until the thread rewinds.

   On the other hand, if a node operates in loop detection mode, it
   returns a label mapping message without a thread object immediately
   after receiving a colored thread.  A node which receives a label
   mapping message that does not have a thread object will not rewind
   the thread.

5.2.  Using old path while looping on new path

   When a route changes, one might want to continue to use the old path
   if the new route is looping.  This is achieved simply by holding the
   label assigned to the downstream link on the old path until the
   thread being extended on the new route gets rewound.  This is an
   implementation choice.

5.3.  How to deal with ordered downstream allocation

   The thread mechanism can be also adapted to ordered downstream
   allocation mode (or the egress-initiated ordered control) by
   regarding the event of newly receiving of a label mapping message [4]
   from the next hop as a next hop acquisition event.

   Note that a node which doesn't yet have an incoming link behaves as a
   leaf.  In the case where the tree is being initially built up (e.g.,
   the egress node has just come up), each node in turn will behave as a
   leaf for a short period of time.

5.4.  How to realize load splitting

   A leaf node can easily perform load splitting by setting up two
   different LSPs for the same FEC.  The downstream links for the two
   LSPs are simply assigned different colors.  The thread algorithm now
   prevents a loop in either path, but also allows the two paths to have
   a common downstream node.

   If some intermediate node wants to do load splitting, the following
   modification is made.  Assume that there are multiple next hops for
   the same FEC.  If there are n next hops for a particular FEC, the set
   of incoming links for that FEC's LSP can be partitioned into n
   subsets, where each subset can be mapped to a distinct outgoing link.



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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


   This provides n LSPs for the FEC.  Each such LSP uses a distinct
   color for its outgoing link.  The thread algorithm now prevents a
   loop in any of the paths, but also allows two or more of the paths to
   have a common downstream node.

   In this case, an interesting situation may happen.  Let's say that in
   Fig.12, node B has two incoming links, i1 and i2, and two outgoing
   links, o1 and o2, such that i1 is mapped to o1, while i2 is mapped to
   o2.

   If a blue thread received on i1 and extended on o1 is again received
   at node B on i2, the blue thread is not regarded as forming a loop,
   since i1 and i2 are regarded as belonging to different subsets.
   Instead, the blue thread received on i2 is extended on o2.  If the
   thread extended on o2 is rewound, a single loop-free LSP which
   traverses node B twice is established.

           +------------------...--------------------+
           .        (bl,3)          (bl,4)           |
           .     ----[i1]---+     +--[o1]---> .... --+
           .                 \   /
           .                  v /
           |                   B
           |
           +-----------[i2]--->B----[o2]--->
                     (bl,10)+      (bl,11)


            Fig.12  Load splitting at intermediate node

   There is another type of load splitting, in which packets arrived at
   single incoming link can be label switched to any one of multiple
   outgoing links.  This case does not seem to be a good load-splitting
   scheme, since the packet order in the same FEC is not preserved.
   Thus, this document does not focus on this case.

   Whether that's a good type of load splitting or not, the fact remains
   that ATM-LSRs cannot load split like this because ATM switches just
   don't have the capability to make forwarding decisions on a per-
   packet basis.

6.  Why this works

6.1.  Why a thread with unknown hop count is extended

   In the algorithm, a thread of unknown hop count is extended when a
   thread loop is detected.  This reduces the number of loop prevention




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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


   messages by merging threads (of known hop count) that are flowing
   inside or outside the loop.  See Appendix A.12.

6.2.  Why a rewound thread cannot contain a loop

6.2.1.  Case1: LSP with known link hop counts

   How can we be sure that an established path does not contain a loop
   when the outgoing link hop count is NOT "unknown"?

   Consider a sequence of LSRs <R1, ..., Rn>, such that there is a loop
   traversing the LSRs in the sequence.  (I.e., packets from R1 go to
   R2, then to R3, etc., then to Rn, and then from Rn to R1.)

   Suppose that the thread hop count of the link between R1 and R2 is k.
   Then by the above procedures, the hop counts between Rn and R1 must
   be k+n-1.  But the algorithm also ensures that if a node has an
   incoming hop count of j, its outgoing link hop count must be at least
   of j+1.  Hence, if we assume that the LSP established as a result of
   thread rewinding contains a loop, the hop counts between R1 and R2
   must be at least k+n.  From this we may derive the absurd conclusion
   that n=0, and we may therefore conclude that there is no such
   sequence of LSRs.

6.2.1.  Case2: LSP with unknown link hop counts

   An established path does not contain a loop as well, when the
   outgoing link hop count is "unknown".  This is because a colored
   thread of unknown hop count is never rewound unless it reaches
   egress.

6.3.  Why L3 loop is detected

   Regardless of whether the thread hop count is known or unknown, if
   there is a loop, then some node in the loop will be the last node to
   receive a thread over a new incoming link.  This thread will always
   arrive back at that node, without its color having changed.  Hence
   the loop will always be detected by at least one of the nodes in the
   loop.

6.4.  Why L3 loop is not mis-detected

   Since no node ever extends the same colored thread downstream twice,
   a thread loop is not detected unless there actually is an L3 routing
   loop.






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RFC 3063             MPLS Loop Prevention Mechanism        February 2001


6.5.  How a stalled thread automatically recovers from loop

   Once a thread is stalled in a loop, the thread (or the path setup
   request) effectively remains in the loop, so that a path
   reconfiguration (i.e., thread withdrawing on the old path and thread
   extending on the new path) can be issued from any node that may
   receive a route change event so as to break the loop.

6.6.  Why different colored threads do not chase each other

   In the algorithm, multiple thread color and/or hop count updates may
   happen if several leaf nodes start extending threads at the same
   time.  How can we prevent multiple threads from looping unlimitedly?

   First, when a node finds that a thread forms a loop, it creates a new
   thread of hop count "unknown".  All the looping threads of a known
   hop count which later arrive at the node would be merged into this
   thread.  Such a thread behaves like a thread absorber.

   Second, the "thread extending with changing color" prevents two
   threads from chasing each other.

   Suppose that a received thread were always extended without changing
   color.  Then we would encounter the following situation.

                                G        Y
                                |        |
                                v        v
                                R1------>R2
                                ^        |
                                |        v
                                R4<------R3

                   Fig.13   Example of thread chasing

   In Fig.13, (1) node G acquires R1 as a next hop, and starts to extend
   a green thread of hop count 1, (2) node Y acquires R2 as a next hop,
   and starts to extend a yellow thread of hop count 1, and (3) both
   node G and node Y withdraws their threads before these threads go
   round.

   In this case, the yellow and green threads would go round and get
   back to R2 and R1, respectively.  When the threads get back to R2 and
   R1, however, the incoming links that store the yellow and green