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+Network Working Group                                       C. Partridge
+Request for Comments: 1809                  BBN Systems and Technologies
+Category: Informational                                        June 1995
+
+
+
+                   Using the Flow Label Field in IPv6
+
+
+Status of this Memo
+
+   This memo provides information for the Internet community.  This memo
+   does not specify an Internet standard of any kind.  Distribution of
+   this memo is unlimited.
+
+Abstract
+
+   The purpose of this memo is to distill various opinions and
+   suggestions of the End-to-End Research Group regarding the handling
+   of Flow Labels into a set of suggestions for IPv6.  This memo is for
+   information purposes only and is not one of the IPv6 specifications.
+   Distribution of this memo is unlimited.
+
+Introduction
+
+   This memo originated as the report of a discussion at an End-to-End
+   Research Group meeting in November 1994.  At that meeting the group
+   discussed several issues regarding how to manage flow identifiers in
+   IPv6.   A report of the meeting was then circulated to the IPv6
+   community.  Feedback from that community resulted in changes to this
+   memo and in changes to the IPv6 specification to fix some minor
+   problems the End-to-End Group had raised.
+
+   While many of the ideas in this memo have found their way into the
+   IPv6 specification, the explanation of why various design decisions
+   were made have not.  This memo is intended to provide some additional
+   context for interested parties.
+
+Brief Description of the Flow Label
+
+   The current draft of the IPv6 specification states that every IPv6
+   header contains a 24-bit Flow Label.  (Originally the specification
+   called for a 28-bit Flow ID field, which included the flow label and
+   a 4-bit priority field.  The priority field is now distinct, for
+   reasons discussed at the end of this memo).
+
+
+
+
+
+
+Partridge                    Informational                      [Page 1]
+
+RFC 1809                                                       June 1995
+
+
+   The Flow Label is a pseudo-random number between 1 and FFFFFF (hex)
+   that is unique when combined with the source address.  The zero Flow
+   Label is reserved to say that no Flow Label is being used.  The
+   specification requires that a source must not reuse a Flow Label
+   value until all state information for the previous use of the Flow
+   Label has been flushed from all routers in the internet.
+
+   The specification further requires that all datagrams with the same
+   (non-zero) Flow Label must have the same Destination Address, Hop-
+   by-Hop Options header, Routing Header and Source Address contents.
+   The notion is that by simply looking up the Flow Label in a table,
+   the router can decide how to route and forward the datagram without
+   examining the rest of the header.
+
+Flow Label Issues
+
+   The IPv6 specification originally left open a number of questions, of
+   which these three were among the most important:
+
+        1.   What should a router do if a datagram with a (non-zero)
+             Flow Label arrives and the router has no state for that
+             Flow Label?
+
+        2.   How does an internet flush old Flow Labels?
+
+        3.   Which datagrams should carry (non-zero) Flow Labels?
+
+   This memo summarizes the End-to-End Group's attempts to answer these
+   questions.
+
+What Does a Router Do With Flow Labels for Which It Has No State?
+
+   If a datagram with a non-zero Flow Label arrives at a router and the
+   router discovers it has no state information for that Flow Label,
+   what is the correct thing for the router to do?
+
+   The IPv6 specification allows routers to ignore Flow Labels and also
+   allows for the possibility that IPv6 datagrams may carry flow setup
+   information in their options.  Unknown Flow Labels may also occur if
+   a router crashes and loses its state.  During a recovery period, the
+   router will receive datagrams with Flow Labels it does not know, but
+   this is arguably not an error, but rather a part of the recovery
+   period.  Finally, if the controversial suggestion that each TCP
+   connection be assigned a separate Flow Label is adopted, it may be
+   necessary to manage Flow Labels using an LRU cache (to avoid Flow
+   Label cache overflow in routers), in which case an active but
+   infrequently used flow's state may have been intentionally discarded.
+
+
+
+
+Partridge                    Informational                      [Page 2]
+
+RFC 1809                                                       June 1995
+
+
+   In any case, it is clear that treating this situation as an error
+   and, say dropping the datagram and sending an ICMP message, is
+   inappropriate.  Indeed, it seems likely that in most cases, simply
+   forwarding the datagram as one would a datagram with a zero Flow
+   Label would give better service to the flow than dropping the
+   datagram.
+
+   Of course, there will be situations in which routing the datagram as
+   if its Flow Label were zero will cause the wrong result.  An example
+   is a router which has two paths to the datagram's destination, one
+   via a high-bandwidth satellite link and the other via a low-bandwidth
+   terrestrial link.  A high bandwidth flow obviously should be routed
+   via the high-bandwidth link, but if the router loses the flow state,
+   the router may route the traffic via the low-bandwidth link, with the
+   potential for the flow's traffic to swamp the low-bandwidth link.  It
+   seems likely, however, these situations will be exceptions rather
+   than the rule.   So it seems reasonable to handle these situations
+   using options that indicate that if the flow state is absent, the
+   datagram needs special handling.  (The options may be Hop-by-Hop or
+   only handled at some routers, depending on the flow's needs).
+
+   It would clearly be desirable to have some method for signalling to
+   end systems that the flow state has been lost and needs to be
+   refreshed.  One possibility is to add a state-lost bit to the Flow
+   Label field, however there is sensitivity to eating into the precious
+   24-bits of the field.  Other possibilities include adding options to
+   the datagram to indicate its Flow Label was unknown or sending an
+   ICMP message back to the flow source.
+
+   In summary, the view is that the default rule should be that if a
+   router receives a datagram with an unknown Flow Label, it treats the
+   datagram as if the Flow Label is zero.  As part of forwarding, the
+   router will examine any hop-by-hop options and learn if the the
+   datagram requires special handling.  The options could include simply
+   the information that the datagram is to be dropped if the Flow Label
+   is unknown or could contain the flow state the router should have.
+   There is clearly room here for experimentation with option design.
+
+Flushing Old Flow Labels
+
+   The flow mechanism assumes that state associated with a given Flow
+   Label is somehow deposited in routers, so they know how to handle
+   datagrams that carry the Flow Label.  A serious problem is how to
+   flush Flow Labels that are no longer being used (stale Flow Labels)
+   from the routers.
+
+   Stale Flow Labels can happen a number of ways, even if we assume that
+   the source always sends a message deleting a Flow Label when the
+
+
+
+Partridge                    Informational                      [Page 3]
+
+RFC 1809                                                       June 1995
+
+
+   source finishes using a Flow.  An internet may have partioned since
+   the flow was created.  Or the deletion message may be lost before
+   reaching all routers.  Furthermore, the source may crash before it
+   can send out a Flow Label deletion message.  The point here is that
+   we cannot expect the source (or, for the same reasons, a third party)
+   always to clear out stale Flow Labels.  Rather, routers will have to
+   find some mechanism to flush Flow Labels themselves.
+
+   The obvious mechanism is to use a timer.  Routers should discard Flow
+   Labels whose state has not been refreshed within some period of time.
+   At the same time, a source that crashes must observe a quiet time,
+   during which it creates no flows, until it knows that all Flow Labels
+   from its previous life must have expired.  (Sources can avoid quiet
+   time restrictions by keeping information about active Flow Labels in
+   stable storage that survives crashes).  This is precisely how TCP
+   initial sequence numbers are managed and it seems the same mechanism
+   should work well for Flow Labels.
+
+   Exactly how the Flow Label and its state should be refreshed needs
+   some study.  There are two obvious options.  The source could
+   periodically send out a special refresh message (such as an RSVP Path
+   message) to explicitly refresh the Flow Label and its state.  Or, the
+   router could treat every datagram that carries the Flow Label as an
+   implicit refresh or sources could send explicit refresh options.  The
+   choice is between periodically handling a special update message and
+   doing an extra computation on each datagram (namely noting in the
+   Flow Label's entry that the Flow Label has been refreshed).
+
+Which Datagrams Should Carry (Non-Zero) Flow Labels?
+
+   Interestingly, this is the problem on which the least progress has
+   been made.
+
+   There were some points of basic agreement.  Small exchanges of data
+   should have a zero Flow Label, because it is not worth creating a
+   flow for a few datagrams.  Real-time flows must obviously always have
+   a Flow Label, since flows are a primary reason Flow Labels were
+   created.  The issue is what to do with peers sending large amounts of
+   best effort traffic (e.g., TCP connections).  Some people want all
+   long-term TCP connections to use Flow Labels, others do not.
+
+   The argument in favor of using Flow Labels on individual TCP
+   connections is that even if the source does not request special
+   service, a network provider's routers may be able to recognize a
+   large amount of traffic and use the Flow Label field to establish a
+   special route that gives the TCP connection better service (e.g.,
+   lower delay or bigger bandwidth).  Another argument is to assist in
+   efficient demux at the receiver (i.e., IP and TCP demuxing could be
+
+
+
+Partridge                    Informational                      [Page 4]
+
+RFC 1809                                                       June 1995
+
+
+   done once).
+
+   An argument against using Flow Labels in individual TCP connections
+   is that it changes how we handling route caches in routers.
+   Currently one can cache a route for a destination host, regardless of
+   how many different sources are sending to that destination host.
+   I.e., if five sources each have two TCP connections sending data to a
+   server, one cache entry containing the route to the server handles
+   all ten TCPs' traffic.  Putting Flow Labels in each datagram changes
+   the cache into a Flow Label cache, in which there is a cache entry
+   for every TCP connection.  So there's a potential for cache
+   explosion.  There are ways to alleviate this problem, such as
+   managing the Flow Label cache as an LRU cache, in which infrequently
+   used Flow Labels get discarded (and then recovered later).  It is not
+   clear, however, whether this will cause cache thrashing.
+
+   Observe that there is no easy compromise between these positions.
+   One cannot, for instance, let the application decide whether to use a
+   Flow Label.  Those who want different Flow Labels for every TCP
+   connection assume that they may optimize a route without the
+   application's knowledge.  And forcing all applications to use Flow
+   Labels will force routing vendors to deal with the cache explosion
+   issue, even if we later discover that we don't want to optimize
+   individual TCP connections.
+
+Note about the Priority Field
+
+   The original IPv6 specification combined the Priority and Flow Label
+   fields and allowed flows to redefine the means of different values of
+   the Priority field.  During its discussions, the End-to-End group
+   realized this meant that if a router forwarded a datagram with an
+   unknown Flow Label it had to ignore the Priority field, because the
+   priority values might have been redefined.  (For instance, the
+   priorities might have been inverted). The IPv6 community concluded
+   this behavior was undesirable.  Indeed, it seems likely that when the
+   Flow Label are unknown, the router will be able to give much better
+   service if it use the Priority field to make a more informed routing
+   decision.  So the Priority field is now a distinct field, unaffected
+   by the Flow Label.
+
+Acknowledgements
+
+   I would like to acknowledge the assistance of the members of the
+   End-To-End Research Group, chaired by Bob Braden, whose discussions
+   produced this memo.  I would also like to particularly thank Deborah
+   Estrin for her help in putting this memo together.  Also thanks to
+   Richard Fox, Noel Chiappa, and Tony Li for insightful comments on the
+   draft.
+
+
+
+Partridge                    Informational                      [Page 5]
+
+RFC 1809                                                       June 1995
+
+
+Security Considerations
+
+   Security issues are not discussed in this memo.
+
+Author's Address
+
+   Craig Partridge
+   BBN Systems and Technologies
+   10 Moulton St.
+   Cambridge, MA 02138
+
+   EMail: craig@aland.bbn.com
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+Partridge                    Informational                      [Page 6]
+