rfc1981.txt

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RFC 1981              Path MTU Discovery for IPv6            August 1996


   Implementing Path MTU Discovery in the packetization layers
   simplifies some of the inter-layer issues, but has several drawbacks:
   the implementation may have to be redone for each packetization
   protocol, it becomes hard to share PMTU information between different
   packetization layers, and the connection-oriented state maintained by
   some packetization layers may not easily extend to save PMTU
   information for long periods.

   It is therefore suggested that the IP layer store PMTU information
   and that the ICMP layer process received Packet Too Big messages.
   The packetization layers may respond to changes in the PMTU, by
   changing the size of the messages they send.  To support this
   layering, packetization layers require a way to learn of changes in
   the value of MMS_S, the "maximum send transport-message size".  The
   MMS_S is derived from the Path MTU by subtracting the size of the
   IPv6 header plus space reserved by the IP layer for additional
   headers (if any).

   It is possible that a packetization layer, perhaps a UDP application
   outside the kernel, is unable to change the size of messages it
   sends.  This may result in a packet size that exceeds the Path MTU.
   To accommodate such situations, IPv6 defines a mechanism that allows
   large payloads to be divided into fragments, with each fragment sent
   in a separate packet (see [IPv6-SPEC] section "Fragment Header").
   However, packetization layers are encouraged to avoid sending
   messages that will require fragmentation (for the case against
   fragmentation, see [FRAG]).

5.2. Storing PMTU information

   Ideally, a PMTU value should be associated with a specific path
   traversed by packets exchanged between the source and destination
   nodes.  However, in most cases a node will not have enough
   information to completely and accurately identify such a path.
   Rather, a node must associate a PMTU value with some local
   representation of a path.  It is left to the implementation to select
   the local representation of a path.

   In the case of a multicast destination address, copies of a packet
   may traverse many different paths to reach many different nodes.  The
   local representation of the "path" to a multicast destination must in
   fact represent a potentially large set of paths.

   Minimally, an implementation could maintain a single PMTU value to be
   used for all packets originated from the node.  This PMTU value would
   be the minimum PMTU learned across the set of all paths in use by the
   node.  This approach is likely to result in the use of smaller
   packets than is necessary for many paths.



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RFC 1981              Path MTU Discovery for IPv6            August 1996


   An implementation could use the destination address as the local
   representation of a path.  The PMTU value associated with a
   destination would be the minimum PMTU learned across the set of all
   paths in use to that destination.  The set of paths in use to a
   particular destination is expected to be small, in many cases
   consisting of a single path.  This approach will result in the use of
   optimally sized packets on a per-destination basis.  This approach
   integrates nicely with the conceptual model of a host as described in
   [ND]: a PMTU value could be stored with the corresponding entry in
   the destination cache.

   If flows [IPv6-SPEC] are in use, an implementation could use the flow
   id as the local representation of a path.  Packets sent to a
   particular destination but belonging to different flows may use
   different paths, with the choice of path depending on the flow id.
   This approach will result in the use of optimally sized packets on a
   per-flow basis, providing finer granularity than PMTU values
   maintained on a per-destination basis.

   For source routed packets (i.e. packets containing an IPv6 Routing
   header [IPv6-SPEC]), the source route may further qualify the local
   representation of a path.  In particular, a packet containing a type
   0 Routing header in which all bits in the Strict/Loose Bit Map are
   equal to 1 contains a complete path specification.  An implementation
   could use source route information in the local representation of a
   path.

      Note: Some paths may be further distinguished by different
      security classifications.  The details of such classifications are
      beyond the scope of this memo.

   Initially, the PMTU value for a path is assumed to be the (known) MTU
   of the first-hop link.

   When a Packet Too Big message is received, the node determines which
   path the message applies to based on the contents of the Packet Too
   Big message.  For example, if the destination address is used as the
   local representation of a path, the destination address from the
   original packet would be used to determine which path the message
   applies to.

      Note: if the original packet contained a Routing header, the
      Routing header should be used to determine the location of the
      destination address within the original packet.  If Segments Left
      is equal to zero, the destination address is in the Destination
      Address field in the IPv6 header.  If Segments Left is greater
      than zero, the destination address is the last address
      (Address[n]) in the Routing header.



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RFC 1981              Path MTU Discovery for IPv6            August 1996


   The node then uses the value in the MTU field in the Packet Too Big
   message as a tentative PMTU value, and compares the tentative PMTU to
   the existing PMTU.  If the tentative PMTU is less than the existing
   PMTU estimate, the tentative PMTU replaces the existing PMTU as the
   PMTU value for the path.

   The packetization layers must be notified about decreases in the
   PMTU.  Any packetization layer instance (for example, a TCP
   connection) that is actively using the path must be notified if the
   PMTU estimate is decreased.

      Note: even if the Packet Too Big message contains an Original
      Packet Header that refers to a UDP packet, the TCP layer must be
      notified if any of its connections use the given path.

   Also, the instance that sent the packet that elicited the Packet Too
   Big message should be notified that its packet has been dropped, even
   if the PMTU estimate has not changed, so that it may retransmit the
   dropped data.

      Note: An implementation can avoid the use of an asynchronous
      notification mechanism for PMTU decreases by postponing
      notification until the next attempt to send a packet larger than
      the PMTU estimate.  In this approach, when an attempt is made to
      SEND a packet that is larger than the PMTU estimate, the SEND
      function should fail and return a suitable error indication.  This
      approach may be more suitable to a connectionless packetization
      layer (such as one using UDP), which (in some implementations) may
      be hard to "notify" from the ICMP layer.  In this case, the normal
      timeout-based retransmission mechanisms would be used to recover
      from the dropped packets.

   It is important to understand that the notification of the
   packetization layer instances using the path about the change in the
   PMTU is distinct from the notification of a specific instance that a
   packet has been dropped.  The latter should be done as soon as
   practical (i.e., asynchronously from the point of view of the
   packetization layer instance), while the former may be delayed until
   a packetization layer instance wants to create a packet.
   Retransmission should be done for only for those packets that are
   known to be dropped, as indicated by a Packet Too Big message.

5.3. Purging stale PMTU information

   Internetwork topology is dynamic; routes change over time.  While the
   local representation of a path may remain constant, the actual
   path(s) in use may change.  Thus, PMTU information cached by a node
   can become stale.



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RFC 1981              Path MTU Discovery for IPv6            August 1996


   If the stale PMTU value is too large, this will be discovered almost
   immediately once a large enough packet is sent on the path.  No such
   mechanism exists for realizing that a stale PMTU value is too small,
   so an implementation should "age" cached values.  When a PMTU value
   has not been decreased for a while (on the order of 10 minutes), the
   PMTU estimate should be set to the MTU of the first-hop link, and the
   packetization layers should be notified of the change.  This will
   cause the complete Path MTU Discovery process to take place again.

      Note: an implementation should provide a means for changing the
      timeout duration, including setting it to "infinity".  For
      example, nodes attached to an FDDI link which is then attached to
      the rest of the Internet via a small MTU serial line are never
      going to discover a new non-local PMTU, so they should not have to
      put up with dropped packets every 10 minutes.

   An upper layer must not retransmit data in response to an increase in
   the PMTU estimate, since this increase never comes in response to an
   indication of a dropped packet.

   One approach to implementing PMTU aging is to associate a timestamp
   field with a PMTU value.  This field is initialized to a "reserved"
   value, indicating that the PMTU is equal to the MTU of the first hop
   link.  Whenever the PMTU is decreased in response to a Packet Too Big
   message, the timestamp is set to the current time.

   Once a minute, a timer-driven procedure runs through all cached PMTU
   values, and for each PMTU whose timestamp is not "reserved" and is
   older than the timeout interval:

   - The PMTU estimate is set to the MTU of the first hop link.

   - The timestamp is set to the "reserved" value.

   - Packetization layers using this path are notified of the increase.

5.4. TCP layer actions

   The TCP layer must track the PMTU for the path(s) in use by a
   connection; it should not send segments that would result in packets
   larger than the PMTU.  A simple implementation could ask the IP layer
   for this value each time it created a new segment, but this could be
   inefficient.  Moreover, TCP implementations that follow the "slow-
   start" congestion-avoidance algorithm [CONG] typically calculate and
   cache several other values derived from the PMTU.  It may be simpler
   to receive asynchronous notification when the PMTU changes, so that
   these variables may be updated.




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RFC 1981              Path MTU Discovery for IPv6            August 1996


   A TCP implementation must also store the MSS value received from its
   peer, and must not send any segment larger than this MSS, regardless
   of the PMTU.  In 4.xBSD-derived implementations, this may require
   adding an additional field to the TCP state record.

   The value sent in the TCP MSS option is independent of the PMTU.
   This MSS option value is used by the other end of the connection,
   which may be using an unrelated PMTU value.  See [IPv6-SPEC] sections
   "Packet Size Issues" and "Maximum Upper-Layer Payload Size" for
   information on selecting a value for the TCP MSS option.

   When a Packet Too Big message is received, it implies that a packet
   was dropped by the node that sent the ICMP message.  It is sufficient
   to treat this as any other dropped segment, and wait until the
   retransmission timer expires to cause retransmission of the segment.
   If the Path MTU Discovery process requires several steps to find the
   PMTU of the full path, this could delay the connection by many
   round-trip times.

   Alternatively, the retransmission could be done in immediate response
   to a notification that the Path MTU has changed, but only for the
   specific connection specified by the Packet Too Big message.  The
   packet size used in the retransmission should be no larger than the
   new PMTU.

      Note: A packetization layer must not retransmit in response to
      every Packet Too Big message, since a burst of several oversized
      segments will give rise to several such messages and hence several
      retransmissions of the same data.  If the new estimated PMTU is
      still wrong, the process repeats, and there is an exponential
      growth in the number of superfluous segments sent.

      This means that the TCP layer must be able to recognize when a
      Packet Too Big notification actually decreases the PMTU that it
      has already used to send a packet on the given connection, and
      should ignore any other notifications.

   Many TCP implementations incorporate "congestion avoidance" and
   "slow-start" algorithms to improve performance [CONG].  Unlike a
   retransmission caused by a TCP retransmission timeout, a
   retransmission caused by a Packet Too Big message should not change
   the congestion window.  It should, however, trigger the slow-start
   mechanism (i.e., only one segment should be retransmitted until
   acknowledgements begin to arrive again).

   TCP performance can be reduced if the sender's maximum window size is
   not an exact multiple of the segment size in use (this is not the
   congestion window size, which is always a multiple of the segment



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RFC 1981              Path MTU Discovery for IPv6            August 1996