NAME

  PMTU_UseSmallest.seq - Association PMTU is the smallest Path MTU of all destination addresses if its peer is multi-homed

PURPOSE

  To check whether the endpoint used the smallest PMTU discovered of all
  destination addresses if its peer is multi-homed.

SYNOPSIS

  ./PMTU_UseSmallest.seq [-tooloption ...] -pkt ./PMTU_UseSmallest.def
    -tooloption : v6eval tool option
  See Also: ../common/STD_PKT_COMMON.def
            ../common/SCTP_COMMON.def

PRE-TEST CONDITION

  Association not established between endpoint A and B. Also arrange the
  data in endpoint B such that upper layers send associate primitive to
  startup an association with endpoint A.

TEST PROCEDURE

  Endpoint A                           Endpoint B                ULP
  (CLOSED)                             (CLOSED)

  ICMP (IP=y)     ----------------->
  (Fragmentation Needed
  or Packet Too Big)
                                                   <-----    Association
                  ......
                                                   <-----    Send
  
                  <----------------       DATA (Fragment 1)
                                          (IP=x)  
                  <----------------       DATA (Fragment 2)
                                          (IP=x)  
  SACK            ----------------->

  TEST DESCRIPTION:

  1. Attempt to change PMTU between endpoint A (IP=y) and endpoint B by
     sending ICMP message.
  2. Associate primitive to startup an association with endpoint A.
  4. Check B: Data be retransmission be fragmented used the PMTU of y
     on IP=x.

NOTE

  None

REFERENCE

  RFC 4960

  7.3.  Path MTU Discovery

    [RFC4821], [RFC1981], and [RFC1191] specify "Packetization Layer Path
    MTU Discovery", whereby an endpoint maintains an estimate of the
    maximum transmission unit (MTU) along a given Internet path and
    refrains from sending packets along that path that exceed the MTU,
    other than occasional attempts to probe for a change in the Path MTU
    (PMTU).  [RFC4821] is thorough in its discussion of the MTU discovery
    mechanism and strategies for determining the current end-to-end MTU
    setting as well as detecting changes in this value.

    An endpoint SHOULD apply these techniques, and SHOULD do so on a
    per-destination-address basis.

    There are two important SCTP-specific points regarding Path MTU
    discovery:

    1)  SCTP associations can span multiple addresses.  An endpoint MUST
        maintain separate MTU estimates for each destination address of
        its peer.

    2)  The sender should track an association PMTU that will be the
        smallest PMTU discovered for all of the peer's destination
        addresses.  When fragmenting messages into multiple parts this
        association PMTU should be used to calculate the size of each
        fragment.  This will allow retransmissions to be seamlessly sent
        to an alternate address without encountering IP fragmentation.