Author: tuexen
Date: Tue Feb  5 10:29:31 2019
New Revision: 343770
URL: https://svnweb.freebsd.org/changeset/base/343770

Log:
  Only reduce the PMTU after the send call. The only way to increase it, is
  via PMTUD.
  
  This fixes an MTU issue reported by Timo Voelker.
  
  MFC after:            3 days

Modified:
  head/sys/netinet/sctp_output.c

Modified: head/sys/netinet/sctp_output.c
==============================================================================
--- head/sys/netinet/sctp_output.c      Tue Feb  5 10:13:51 2019        
(r343769)
+++ head/sys/netinet/sctp_output.c      Tue Feb  5 10:29:31 2019        
(r343770)
@@ -4289,10 +4289,12 @@ sctp_lowlevel_chunk_output(struct sctp_inpcb *inp,
                                                if (net->port) {
                                                        mtu -= sizeof(struct 
udphdr);
                                                }
-                                               if ((stcb != NULL) && 
(stcb->asoc.smallest_mtu > mtu)) {
-                                                       
sctp_mtu_size_reset(inp, &stcb->asoc, mtu);
+                                               if (mtu < net->mtu) {
+                                                       if ((stcb != NULL) && 
(stcb->asoc.smallest_mtu > mtu)) {
+                                                               
sctp_mtu_size_reset(inp, &stcb->asoc, mtu);
+                                                       }
+                                                       net->mtu = mtu;
                                                }
-                                               net->mtu = mtu;
                                        }
                                } else if (ro->ro_rt == NULL) {
                                        /* route was freed */
@@ -4647,10 +4649,12 @@ sctp_lowlevel_chunk_output(struct sctp_inpcb *inp,
                                                if (net->port) {
                                                        mtu -= sizeof(struct 
udphdr);
                                                }
-                                               if ((stcb != NULL) && 
(stcb->asoc.smallest_mtu > mtu)) {
-                                                       
sctp_mtu_size_reset(inp, &stcb->asoc, mtu);
+                                               if (mtu < net->mtu) {
+                                                       if ((stcb != NULL) && 
(stcb->asoc.smallest_mtu > mtu)) {
+                                                               
sctp_mtu_size_reset(inp, &stcb->asoc, mtu);
+                                                       }
+                                                       net->mtu = mtu;
                                                }
-                                               net->mtu = mtu;
                                        }
                                } else if (ifp) {
                                        if (ND_IFINFO(ifp)->linkmtu &&
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