diff --git a/ompi/mca/pml/ob1/pml_ob1.c b/ompi/mca/pml/ob1/pml_ob1.c
index 0369a19b92..5adf19028a 100644
--- a/ompi/mca/pml/ob1/pml_ob1.c
+++ b/ompi/mca/pml/ob1/pml_ob1.c
@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2010 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2012 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
@@ -250,9 +250,9 @@ int mca_pml_ob1_add_comm(ompi_communicator_t* comm)
             continue;
         }
 
-      add_fragment_to_unexpected:
-
         if (((uint16_t)hdr->hdr_seq) == ((uint16_t)pml_proc->expected_sequence) ) {
+
+        add_fragment_to_unexpected:
             /* We're now expecting the next sequence number. */
             pml_proc->expected_sequence++;
             opal_list_append( &pml_proc->unexpected_frags, (opal_list_item_t*)frag );
@@ -264,17 +264,16 @@ int mca_pml_ob1_add_comm(ompi_communicator_t* comm)
              * situation as the cant_match is only checked when a new fragment is received from
              * the network.
              */
-            OPAL_LIST_FOREACH(frag, &pml_proc->frags_cant_match, mca_pml_ob1_recv_frag_t) {
-               hdr = &frag->hdr.hdr_match;
-               /* If the message has the next expected seq from that proc...  */
-               if(hdr->hdr_seq != pml_proc->expected_sequence)
-                   continue;
-
-               opal_list_remove_item(&pml_proc->frags_cant_match, (opal_list_item_t*)frag);
-               goto add_fragment_to_unexpected;
-           }
+            if( NULL != pml_proc->frags_cant_match ) {
+                frag = check_cantmatch_for_match(pml_proc);
+                if( NULL != frag ) {
+                    hdr = &frag->hdr.hdr_match;
+                    goto add_fragment_to_unexpected;
+                }
+            }
         } else {
-            opal_list_append( &pml_proc->frags_cant_match, (opal_list_item_t*)frag );
+            append_frag_to_ordered_list(&pml_proc->frags_cant_match, frag,
+                                        pml_proc->expected_sequence);
         }
     }
     return OMPI_SUCCESS;
@@ -561,6 +560,23 @@ static void mca_pml_ob1_dump_frag_list(opal_list_t* queue, bool is_req)
     }
 }
 
+void mca_pml_ob1_dump_cant_match(mca_pml_ob1_recv_frag_t* queue)
+{
+    mca_pml_ob1_recv_frag_t* item = queue;
+
+    do {
+        mca_pml_ob1_dump_hdr( &item->hdr );
+        if( NULL != item->range ) {
+            mca_pml_ob1_recv_frag_t* frag = item->range;
+            do {
+                mca_pml_ob1_dump_hdr( &frag->hdr );
+                frag = (mca_pml_ob1_recv_frag_t*)frag->super.super.opal_list_next;
+            } while( frag != item->range );
+        }
+        item = (mca_pml_ob1_recv_frag_t*)item->super.super.opal_list_next;
+    } while( item != queue );
+}
+
 int mca_pml_ob1_dump(struct ompi_communicator_t* comm, int verbose)
 {
     struct mca_pml_comm_t* pml_comm = comm->c_pml_comm;
@@ -596,9 +612,9 @@ int mca_pml_ob1_dump(struct ompi_communicator_t* comm, int verbose)
             opal_output(0, "expected specific receives\n");
             mca_pml_ob1_dump_frag_list(&proc->specific_receives, true);
         }
-        if( opal_list_get_size(&proc->frags_cant_match) ) {
+        if( NULL != proc->frags_cant_match ) {
             opal_output(0, "out of sequence\n");
-            mca_pml_ob1_dump_frag_list(&proc->frags_cant_match, false);
+            mca_pml_ob1_dump_cant_match(proc->frags_cant_match);
         }
         if( opal_list_get_size(&proc->unexpected_frags) ) {
             opal_output(0, "unexpected frag\n");
diff --git a/ompi/mca/pml/ob1/pml_ob1.h b/ompi/mca/pml/ob1/pml_ob1.h
index 4826587564..1f4bfbb589 100644
--- a/ompi/mca/pml/ob1/pml_ob1.h
+++ b/ompi/mca/pml/ob1/pml_ob1.h
@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2016 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
diff --git a/ompi/mca/pml/ob1/pml_ob1_comm.c b/ompi/mca/pml/ob1/pml_ob1_comm.c
index 40c54771a8..510704849d 100644
--- a/ompi/mca/pml/ob1/pml_ob1_comm.c
+++ b/ompi/mca/pml/ob1/pml_ob1_comm.c
@@ -2,7 +2,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2006 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
@@ -29,7 +29,7 @@ static void mca_pml_ob1_comm_proc_construct(mca_pml_ob1_comm_proc_t* proc)
     proc->ompi_proc = NULL;
     proc->expected_sequence = 1;
     proc->send_sequence = 0;
-    OBJ_CONSTRUCT(&proc->frags_cant_match, opal_list_t);
+    proc->frags_cant_match = NULL;
     OBJ_CONSTRUCT(&proc->specific_receives, opal_list_t);
     OBJ_CONSTRUCT(&proc->unexpected_frags, opal_list_t);
 }
@@ -37,7 +37,7 @@ static void mca_pml_ob1_comm_proc_construct(mca_pml_ob1_comm_proc_t* proc)
 
 static void mca_pml_ob1_comm_proc_destruct(mca_pml_ob1_comm_proc_t* proc)
 {
-    OBJ_DESTRUCT(&proc->frags_cant_match);
+    assert(NULL == proc->frags_cant_match);
     OBJ_DESTRUCT(&proc->specific_receives);
     OBJ_DESTRUCT(&proc->unexpected_frags);
     if (proc->ompi_proc) {
diff --git a/ompi/mca/pml/ob1/pml_ob1_comm.h b/ompi/mca/pml/ob1/pml_ob1_comm.h
index 33f1695519..a6f3215325 100644
--- a/ompi/mca/pml/ob1/pml_ob1_comm.h
+++ b/ompi/mca/pml/ob1/pml_ob1_comm.h
@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2016 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
@@ -40,7 +40,7 @@ struct mca_pml_ob1_comm_proc_t {
 #else
     int32_t send_sequence; /**< send side sequence number */
 #endif
-    opal_list_t frags_cant_match;  /**< out-of-order fragment queues */
+    struct mca_pml_ob1_recv_frag_t* frags_cant_match;  /**< out-of-order fragment queues */
     opal_list_t specific_receives; /**< queues of unmatched specific receives */
     opal_list_t unexpected_frags;  /**< unexpected fragment queues */
 };
diff --git a/ompi/mca/pml/ob1/pml_ob1_recvfrag.c b/ompi/mca/pml/ob1/pml_ob1_recvfrag.c
index 83b7a44902..eb261029ff 100644
--- a/ompi/mca/pml/ob1/pml_ob1_recvfrag.c
+++ b/ompi/mca/pml/ob1/pml_ob1_recvfrag.c
@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2017 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2007 High Performance Computing Center Stuttgart,
@@ -70,6 +70,7 @@ OBJ_CLASS_INSTANCE( mca_pml_ob1_recv_frag_t,
  * initialize the fragment (if necessary) and then will add it to the specified
  * queue. The allocated fragment is not returned to the caller.
  */
+
 static void
 append_frag_to_list(opal_list_t *queue, mca_btl_base_module_t *btl,
                     mca_pml_ob1_match_hdr_t *hdr, mca_btl_base_segment_t* segments,
@@ -83,51 +84,183 @@ append_frag_to_list(opal_list_t *queue, mca_btl_base_module_t *btl,
 }
 
 /**
- * Append an unexpected descriptor to an ordered queue. This function will allocate and
- * initialize the fragment (if necessary) and then will add it to the specified
- * queue respecting the sequence number. The allocated fragment is not returned to the caller.
+ * Append an unexpected descriptor to an ordered queue.
+ *
+ * use the opal_list_item_t to maintain themselves on an ordered list
+ * according to their hdr_seq. Special care has been taken to cope with
+ * overflowing the uint16_t we use for the hdr_seq. The current algorithm
+ * works as long as there are no two elements with the same hdr_seq in the
+ * list in same time (aka. no more than 2^16-1 left out-of-sequence
+ * messages. On the vertical layer, messages with contiguous sequence
+ * number organize themselves in a way to minimize the search space.
  */
-static void
-append_frag_to_ordered_list(opal_list_t* queue, mca_btl_base_module_t *btl,
-                            mca_pml_ob1_match_hdr_t *hdr, mca_btl_base_segment_t* segments,
-                            size_t num_segments, mca_pml_ob1_recv_frag_t* frag)
+void
+append_frag_to_ordered_list(mca_pml_ob1_recv_frag_t** queue,
+                            mca_pml_ob1_recv_frag_t *frag,
+                            uint16_t seq)
 {
-    mca_pml_ob1_recv_frag_t* tmpfrag;
-    mca_pml_ob1_match_hdr_t* tmphdr;
+    mca_pml_ob1_recv_frag_t  *prior, *next;
+    mca_pml_ob1_match_hdr_t *hdr;
 
-    if(NULL == frag) {
-        MCA_PML_OB1_RECV_FRAG_ALLOC(frag);
-        MCA_PML_OB1_RECV_FRAG_INIT(frag, hdr, segments, num_segments, btl);
+    frag->super.super.opal_list_next = (opal_list_item_t*)frag;
+    frag->super.super.opal_list_prev = (opal_list_item_t*)frag;
+    frag->range = NULL;
+    hdr = &frag->hdr.hdr_match;
+
+    if( NULL == *queue ) {  /* no pending fragments yet */
+        *queue = frag;
+        return;
     }
 
-    if( opal_list_is_empty(queue) ) {  /* no pending fragments yet */
-        opal_list_append(queue, (opal_list_item_t*)frag);
-        return;
-    }
-    /* Shortcut for sequence number earlier than the first fragment in the list */
-    tmpfrag = (mca_pml_ob1_recv_frag_t*)opal_list_get_first(queue);
-    tmphdr = &tmpfrag->hdr.hdr_match;
-    assert(hdr->hdr_seq != tmphdr->hdr_seq);
-    if( hdr->hdr_seq < tmphdr->hdr_seq ) {
-        opal_list_prepend(queue, (opal_list_item_t*)frag);
-        return;
-    }
-    /* Shortcut for sequence number later than the last fragment in the list */
-    tmpfrag = (mca_pml_ob1_recv_frag_t*)opal_list_get_last(queue);
-    tmphdr = &tmpfrag->hdr.hdr_match;
-    if( hdr->hdr_seq > tmphdr->hdr_seq ) {
-        opal_list_append(queue, (opal_list_item_t*)frag);
-        return;
-    }
-    /* For all other cases (sequence number missing in the list) */
-    OPAL_LIST_FOREACH(tmpfrag, queue, mca_pml_ob1_recv_frag_t) {
-        tmphdr = &tmpfrag->hdr.hdr_match;
-        if( hdr->hdr_seq < tmphdr->hdr_seq ) {
-            opal_list_insert_pos(queue, (opal_list_item_t*)tmpfrag,
-                                 (opal_list_item_t*) frag);
-            return;
+    prior = *queue;
+    assert(hdr->hdr_seq != prior->hdr.hdr_match.hdr_seq);
+
+    /* The hdr_seq being 16 bits long it can rollover rather quickly. We need to
+     * account for this rollover or the matching will fail.
+     * Extract the items from the list to order them safely */
+    if( hdr->hdr_seq < prior->hdr.hdr_match.hdr_seq ) {
+        uint16_t d1, d2 = prior->hdr.hdr_match.hdr_seq - hdr->hdr_seq;
+        do {
+            d1 = d2;
+            prior = (mca_pml_ob1_recv_frag_t*)(prior->super.super.opal_list_prev);
+            d2 = prior->hdr.hdr_match.hdr_seq - hdr->hdr_seq;
+        } while( (hdr->hdr_seq < prior->hdr.hdr_match.hdr_seq) &&
+                 (d1 > d2) && (prior != *queue) );
+    } else {
+        uint16_t prior_seq = prior->hdr.hdr_match.hdr_seq,
+        next_seq = ((mca_pml_ob1_recv_frag_t*)(prior->super.super.opal_list_next))->hdr.hdr_match.hdr_seq;
+        /* prevent rollover */
+        while( (hdr->hdr_seq > prior_seq) && (hdr->hdr_seq > next_seq) && (prior_seq < next_seq) ) {
+            prior_seq = next_seq;
+            prior = (mca_pml_ob1_recv_frag_t*)(prior->super.super.opal_list_next);
+            next_seq = ((mca_pml_ob1_recv_frag_t*)(prior->super.super.opal_list_next))->hdr.hdr_match.hdr_seq;
         }
     }
+
+    /* prior is the fragment with a closest hdr_seq lesser than the current hdr_seq */
+    mca_pml_ob1_recv_frag_t* parent = prior;
+
+    /* Is this fragment the next in range ? */
+    if( NULL == parent->range ) {
+        if( (parent->hdr.hdr_match.hdr_seq + 1) == hdr->hdr_seq ) {
+            parent->range = (mca_pml_ob1_recv_frag_t*)frag;
+            goto merge_ranges;
+        }
+        /* all other cases fallback and add the frag after the parent */
+    } else {
+        /* can we add the frag to the range of the previous fragment ? */
+        mca_pml_ob1_recv_frag_t* largest = (mca_pml_ob1_recv_frag_t*)parent->range->super.super.opal_list_prev;
+        if( (largest->hdr.hdr_match.hdr_seq + 1) == hdr->hdr_seq ) {
+            /* the frag belongs to this range */
+            frag->super.super.opal_list_prev = (opal_list_item_t*)largest;
+            frag->super.super.opal_list_next = largest->super.super.opal_list_next;
+            frag->super.super.opal_list_prev->opal_list_next = (opal_list_item_t*)frag;
+            frag->super.super.opal_list_next->opal_list_prev = (opal_list_item_t*)frag;
+            goto merge_ranges;
+        }
+        /* all other cases fallback and add the frag after the parent */
+    }
+
+    frag->super.super.opal_list_prev = (opal_list_item_t*)prior;
+    frag->super.super.opal_list_next = (opal_list_item_t*)prior->super.super.opal_list_next;
+    frag->super.super.opal_list_prev->opal_list_next = (opal_list_item_t*)frag;
+    frag->super.super.opal_list_next->opal_list_prev = (opal_list_item_t*)frag;
+    parent = frag;  /* the frag is not part of a range yet */
+
+    /* if the newly added element is closer to the next expected sequence mark it so */
+    if( parent->hdr.hdr_match.hdr_seq >= seq )
+        if( abs(parent->hdr.hdr_match.hdr_seq - seq) < abs((*queue)->hdr.hdr_match.hdr_seq - seq))
+            *queue = parent;
+
+ merge_ranges:
+    /* is the next hdr_seq the increasing next one ? */
+    next = (mca_pml_ob1_recv_frag_t*)parent->super.super.opal_list_next;
+    uint16_t upper = parent->hdr.hdr_match.hdr_seq;
+    if( NULL != parent->range ) {
+        upper = ((mca_pml_ob1_recv_frag_t*)parent->range->super.super.opal_list_prev)->hdr.hdr_match.hdr_seq;
+    }
+    if( (upper + 1) == next->hdr.hdr_match.hdr_seq ) {
+        /* remove next from the horizontal chain */
+        next->super.super.opal_list_next->opal_list_prev = (opal_list_item_t*)parent;
+        parent->super.super.opal_list_next = next->super.super.opal_list_next;
+        /* merge next with it's own range */
+        if( NULL != next->range ) {
+            next->super.super.opal_list_next = (opal_list_item_t*)next->range;
+            next->super.super.opal_list_prev = next->range->super.super.opal_list_prev;
+            next->super.super.opal_list_next->opal_list_prev = (opal_list_item_t*)next;
+            next->super.super.opal_list_prev->opal_list_next = (opal_list_item_t*)next;
+            next->range = NULL;
+        } else {
+            next->super.super.opal_list_prev = (opal_list_item_t*)next;
+            next->super.super.opal_list_next = (opal_list_item_t*)next;
+        }
+        if( NULL == parent->range ) {
+            parent->range = next;
+        } else {
+            /* we have access to parent->range so make frag be it's predecessor */
+            frag = (mca_pml_ob1_recv_frag_t*)parent->range->super.super.opal_list_prev;
+            /* merge the 2 rings such that frag is right before next */
+            frag->super.super.opal_list_next = (opal_list_item_t*)next;
+            parent->range->super.super.opal_list_prev = next->super.super.opal_list_prev;
+            next->super.super.opal_list_prev->opal_list_next = (opal_list_item_t*)parent->range;
+            next->super.super.opal_list_prev = (opal_list_item_t*)frag;
+        }
+        if( next == *queue )
+            *queue = parent;
+    }
+}
+
+/*
+ * remove the head of ordered list and restructure the list.
+ */
+static mca_pml_ob1_recv_frag_t*
+remove_head_from_ordered_list(mca_pml_ob1_recv_frag_t** queue)
+{
+    mca_pml_ob1_recv_frag_t* frag = *queue;
+    /* queue is empty, nothing to see. */
+    if( NULL == *queue )
+        return NULL;
+    if( NULL == frag->range ) {
+        /* head has no range, */
+        if( frag->super.super.opal_list_next == (opal_list_item_t*)frag ) {
+            /* head points to itself means it is the only
+             * one in this queue. We set the new head to NULL */
+            *queue = NULL;
+        } else {
+            /* make the next one a new head. */
+            *queue = (mca_pml_ob1_recv_frag_t*)frag->super.super.opal_list_next;
+            frag->super.super.opal_list_next->opal_list_prev = frag->super.super.opal_list_prev;
+            frag->super.super.opal_list_prev->opal_list_next = frag->super.super.opal_list_next;
+        }
+    } else {
+        /* head has range */
+        mca_pml_ob1_recv_frag_t* range = frag->range;
+        frag->range = NULL;
+        *queue = (mca_pml_ob1_recv_frag_t*)range;
+        if( range->super.super.opal_list_next == (opal_list_item_t*)range ) {
+            /* the range has no next element */
+            assert( range->super.super.opal_list_prev == (opal_list_item_t*)range );
+            range->range = NULL;
+        } else {
+            range->range = (mca_pml_ob1_recv_frag_t*)range->super.super.opal_list_next;
+            /* remove the range from the vertical chain */
+            range->super.super.opal_list_next->opal_list_prev = range->super.super.opal_list_prev;
+            range->super.super.opal_list_prev->opal_list_next = range->super.super.opal_list_next;
+        }
+        /* replace frag with range in the horizontal range if not the only element */
+        if( frag->super.super.opal_list_next == (opal_list_item_t*)frag ) {
+            range->super.super.opal_list_next = (opal_list_item_t*)range;
+            range->super.super.opal_list_prev = (opal_list_item_t*)range;
+        } else {
+            range->super.super.opal_list_next = frag->super.super.opal_list_next;
+            range->super.super.opal_list_prev = frag->super.super.opal_list_prev;
+            range->super.super.opal_list_next->opal_list_prev = (opal_list_item_t*)range;
+            range->super.super.opal_list_prev->opal_list_next = (opal_list_item_t*)range;
+        }
+    }
+    frag->super.super.opal_list_next = NULL;
+    frag->super.super.opal_list_prev = NULL;
+    return frag;
 }
 
 /**
@@ -176,15 +309,13 @@ match_one(mca_btl_base_module_t *btl,
           mca_pml_ob1_comm_proc_t *proc,
           mca_pml_ob1_recv_frag_t* frag);
 
-static inline mca_pml_ob1_recv_frag_t* check_cantmatch_for_match(mca_pml_ob1_comm_proc_t *proc)
+mca_pml_ob1_recv_frag_t*
+check_cantmatch_for_match(mca_pml_ob1_comm_proc_t *proc)
 {
-    mca_pml_ob1_recv_frag_t *frag = NULL;
+    mca_pml_ob1_recv_frag_t *frag = proc->frags_cant_match;
 
-    frag = (mca_pml_ob1_recv_frag_t*)opal_list_get_first(&proc->frags_cant_match);
-    if( (opal_list_get_end(&proc->frags_cant_match) != (opal_list_item_t*)frag) &&
-        (frag->hdr.hdr_match.hdr_seq == proc->expected_sequence) ) {
-        opal_list_remove_item(&proc->frags_cant_match, (opal_list_item_t*)frag);
-        return frag;
+    if( (NULL != frag) && (frag->hdr.hdr_match.hdr_seq == proc->expected_sequence) ) {
+        return remove_head_from_ordered_list(&proc->frags_cant_match);
     }
     return NULL;
 }
@@ -253,8 +384,10 @@ void mca_pml_ob1_recv_frag_callback_match(mca_btl_base_module_t* btl,
          * now as we still have the lock.
          */
         if(OPAL_UNLIKELY(((uint16_t) hdr->hdr_seq) != ((uint16_t) proc->expected_sequence))) {
-            append_frag_to_ordered_list(&proc->frags_cant_match, btl,
-                                        hdr, segments, num_segments, NULL);
+            mca_pml_ob1_recv_frag_t* frag;
+            MCA_PML_OB1_RECV_FRAG_ALLOC(frag);
+            MCA_PML_OB1_RECV_FRAG_INIT(frag, hdr, segments, num_segments, btl);
+            append_frag_to_ordered_list(&proc->frags_cant_match, frag, proc->expected_sequence);
             OB1_MATCHING_UNLOCK(&comm->matching_lock);
             return;
         }
@@ -284,7 +417,12 @@ void mca_pml_ob1_recv_frag_callback_match(mca_btl_base_module_t* btl,
 
     if(OPAL_LIKELY(match)) {
         bytes_received = segments->seg_len - OMPI_PML_OB1_MATCH_HDR_LEN;
-        match->req_recv.req_bytes_packed = bytes_received;
+        /* We don't need to know the total amount of bytes we just received,
+         * but we need to know if there is any data in this message. The
+         * simplest way is to get the extra length from the first segment,
+         * and then add the number of remaining segments.
+         */
+        match->req_recv.req_bytes_packed = bytes_received + (num_segments-1);
 
         MCA_PML_OB1_RECV_REQUEST_MATCHED(match, hdr);
         if(match->req_bytes_expected > 0) {
@@ -339,7 +477,7 @@ void mca_pml_ob1_recv_frag_callback_match(mca_btl_base_module_t* btl,
      * MUST be called with communicator lock and will RELEASE the lock. This is
      * not ideal but it is better for the performance.
      */
-    if(0 != opal_list_get_size(&proc->frags_cant_match)) {
+    if(NULL != proc->frags_cant_match) {
         mca_pml_ob1_recv_frag_t* frag;
 
         OB1_MATCHING_LOCK(&comm->matching_lock);
@@ -348,7 +486,7 @@ void mca_pml_ob1_recv_frag_callback_match(mca_btl_base_module_t* btl,
             mca_pml_ob1_recv_frag_match_proc(frag->btl, comm_ptr, proc,
                                              &frag->hdr.hdr_match,
                                              frag->segments, frag->num_segments,
-                                             hdr->hdr_common.hdr_type, frag);
+                                             frag->hdr.hdr_match.hdr_common.hdr_type, frag);
         } else {
             OB1_MATCHING_UNLOCK(&comm->matching_lock);
         }
@@ -778,8 +916,10 @@ static int mca_pml_ob1_recv_frag_match( mca_btl_base_module_t *btl,
 
     /* If the sequence number is wrong, queue it up for later. */
     if(OPAL_UNLIKELY(frag_msg_seq != next_msg_seq_expected)) {
-        append_frag_to_ordered_list(&proc->frags_cant_match, btl, hdr, segments,
-                                    num_segments, NULL);
+        mca_pml_ob1_recv_frag_t* frag;
+        MCA_PML_OB1_RECV_FRAG_ALLOC(frag);
+        MCA_PML_OB1_RECV_FRAG_INIT(frag, hdr, segments, num_segments, btl);
+        append_frag_to_ordered_list(&proc->frags_cant_match, frag, next_msg_seq_expected);
         OB1_MATCHING_UNLOCK(&comm->matching_lock);
         return OMPI_SUCCESS;
     }
@@ -860,7 +1000,7 @@ mca_pml_ob1_recv_frag_match_proc( mca_btl_base_module_t *btl,
      * any fragments on the frags_cant_match list
      * may now be used to form new matchs
      */
-    if(OPAL_UNLIKELY(opal_list_get_size(&proc->frags_cant_match) > 0)) {
+    if(OPAL_UNLIKELY(NULL != proc->frags_cant_match)) {
         OB1_MATCHING_LOCK(&comm->matching_lock);
         if((frag = check_cantmatch_for_match(proc))) {
             hdr = &frag->hdr.hdr_match;
diff --git a/ompi/mca/pml/ob1/pml_ob1_recvfrag.h b/ompi/mca/pml/ob1/pml_ob1_recvfrag.h
index 80bcef1501..def120ccc6 100644
--- a/ompi/mca/pml/ob1/pml_ob1_recvfrag.h
+++ b/ompi/mca/pml/ob1/pml_ob1_recvfrag.h
@@ -3,7 +3,7 @@
  * Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
  *                         University Research and Technology
  *                         Corporation.  All rights reserved.
- * Copyright (c) 2004-2013 The University of Tennessee and The University
+ * Copyright (c) 2004-2018 The University of Tennessee and The University
  *                         of Tennessee Research Foundation.  All rights
  *                         reserved.
  * Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
@@ -42,6 +42,7 @@ struct mca_pml_ob1_recv_frag_t {
     opal_free_list_item_t super;
     mca_pml_ob1_hdr_t hdr;
     size_t num_segments;
+    struct mca_pml_ob1_recv_frag_t* range;
     mca_btl_base_module_t* btl;
     mca_btl_base_segment_t segments[MCA_BTL_DES_MAX_SEGMENTS];
     mca_pml_ob1_buffer_t buffers[MCA_BTL_DES_MAX_SEGMENTS];
@@ -167,7 +168,18 @@ extern void mca_pml_ob1_recv_frag_callback_fin( mca_btl_base_module_t *btl,
                                                 mca_btl_base_descriptor_t* descriptor,
                                                 void* cbdata );
 
+/**
+ * Extract the next fragment from the cant_match ordered list. This fragment
+ * will be the next in sequence.
+ */
+extern mca_pml_ob1_recv_frag_t*
+check_cantmatch_for_match(mca_pml_ob1_comm_proc_t *proc);
 
+void append_frag_to_ordered_list(mca_pml_ob1_recv_frag_t** queue,
+                                 mca_pml_ob1_recv_frag_t* frag,
+                                 uint16_t seq);
+
+extern void mca_pml_ob1_dump_cant_match(mca_pml_ob1_recv_frag_t* queue);
 END_C_DECLS
 
 #endif