GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1337

Network Working Group R. Braden Request for Comments: 1337 ISI

                                                              May 1992
               TIME-WAIT Assassination Hazards in TCP

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard.  Distribution of this memo is
 unlimited.

Abstract

 This note describes some theoretically-possible failure modes for TCP
 connections and discusses possible remedies.  In particular, one very
 simple fix is identified.

1. INTRODUCTION

 Experiments to validate the recently-proposed TCP extensions [RFC-
 1323] have led to the discovery of a new class of TCP failures, which
 have been dubbed the "TIME-WAIT Assassination hazards".  This note
 describes these hazards, gives examples, and discusses possible
 prevention measures.
 The failures in question all result from old duplicate segments.  In
 brief, the TCP mechanisms to protect against old duplicate segments
 are [RFC-793]:
 (1)  The 3-way handshake rejects old duplicate initial <SYN>
      segments, avoiding the hazard of replaying a connection.
 (2)  Sequence numbers are used to reject old duplicate data and ACK
      segments from the current incarnation of a given connection
      (defined by a particular host and port pair).  Sequence numbers
      are also used to reject old duplicate <SYN,ACK> segments.
      For very high-speed connections, Jacobson's PAWS ("Protect
      Against Wrapped Sequences") mechanism [RFC-1323] effectively
      extends the sequence numbers so wrap-around will not introduce a
      hazard within the same incarnation.
 (3)  There are two mechanisms to avoid hazards due to old duplicate
      segments from an earlier instance of the same connection; see
      the Appendix to [RFC-1185] for details.

Braden [Page 1] RFC 1337 TCP TIME-WAIT Hazards May 1992

      For "short and slow" connections [RFC-1185], the clock-driven
      ISN (initial sequence number) selection prevents the overlap of
      the sequence spaces of the old and new incarnations [RFC-793].
      (The algorithm used by Berkeley BSD TCP for stepping ISN
      complicates the analysis slightly but does not change the
      conclusions.)
 (4)  TIME-WAIT state removes the hazard of old duplicates for "fast"
      or "long" connections, in which clock-driven ISN selection is
      unable to prevent overlap of the old and new sequence spaces.
      The TIME-WAIT delay allows all old duplicate segments time
      enough to die in the Internet before the connection is reopened.
 (5)  After a system crash, the Quiet Time at system startup allows
      old duplicates to disappear before any connections are opened.
 Our new observation is that (4) is unreliable: TIME-WAIT state can be
 prematurely terminated ("assassinated") by an old duplicate data or
 ACK segment from the current or an earlier incarnation of the same
 connection.  We refer to this as "TIME-WAIT Assassination" (TWA).
 Figure 1 shows an example of TIME-WAIT assassination.  Segments 1-5
 are copied exactly from Figure 13 of RFC-793, showing a normal close
 handshake.  Packets 5.1, 5.2, and 5.3 are an extension to this
 sequence, illustrating TWA.   Here 5.1 is *any* old segment that is
 unacceptable to TCP A.  It might be unacceptable because of its
 sequence number or because of an old PAWS timestamp.  In either case,
 TCP A sends an ACK segment 5.2 for its current SND.NXT and RCV.NXT.
 Since it has no state for this connection, TCP B reflects this as RST
 segment 5.3, which assassinates the TIME-WAIT state at A!

Braden [Page 2] RFC 1337 TCP TIME-WAIT Hazards May 1992

     TCP A                                                TCP B
 1.  ESTABLISHED                                          ESTABLISHED
     (Close)
 2.  FIN-WAIT-1  --> <SEQ=100><ACK=300><CTL=FIN,ACK>  --> CLOSE-WAIT
 3.  FIN-WAIT-2  <-- <SEQ=300><ACK=101><CTL=ACK>      <-- CLOSE-WAIT
                                                          (Close)
 4.  TIME-WAIT   <-- <SEQ=300><ACK=101><CTL=FIN,ACK>  <-- LAST-ACK
 5.  TIME-WAIT   --> <SEQ=101><ACK=301><CTL=ACK>      --> CLOSED
  1. - - - - - - - - - - - - - - - - - - - - - - - - - - -
 5.1. TIME-WAIT   <--  <SEQ=255><ACK=33> ... old duplicate
 5.2  TIME-WAIT   --> <SEQ=101><ACK=301><CTL=ACK>    -->  ????
 5.3  CLOSED      <-- <SEQ=301><CTL=RST>             <--  ????
    (prematurely)
                       Figure 1.  TWA Example
 Note that TWA is not at all an unlikely event if there are any
 duplicate segments that may be delayed in the network.  Furthermore,
 TWA cannot be prevented by PAWS timestamps; the event may happen
 within the same tick of the timestamp clock.  TWA is a consequence of
 TCP's half-open connection discovery mechanism (see pp 33-34 of
 [RFC-793]), which is designed to clean up after a system crash.

2. The TWA Hazards

 2.1 Introduction
    If the connection is immediately reopened after a TWA event, the
    new incarnation will be exposed to old duplicate segments (except
    for the initial <SYN> segment, which is handled by the 3-way
    handshake).  There are three possible hazards that result:
    H1.  Old duplicate data may be accepted erroneously.
    H2.  The new connection may be de-synchronized, with the two ends
         in permanent disagreement on the state.  Following the spec
         of RFC-793, this desynchronization results in an infinite ACK

Braden [Page 3] RFC 1337 TCP TIME-WAIT Hazards May 1992

         loop.  (It might be reasonable to change this aspect of RFC-
         793 and kill the connection instead.)
         This hazard results from acknowledging something that was not
         sent.  This may result from an old duplicate ACK or as a
         side-effect of hazard H1.
    H3.  The new connection may die.
         A duplicate segment (data or ACK) arriving in SYN-SENT state
         may kill the new connection after it has apparently opened
         successfully.
    Each of these hazards requires that the seqence space of the new
    connection overlap to some extent with the sequence space of the
    previous incarnation.  As noted above, this is only possible for
    "fast" or "long" connections.  Since these hazards all require the
    coincidence of an old duplicate falling into a particular range of
    new sequence numbers, they are much less probable than TWA itself.
    TWA and the three hazards H1, H2, and H3 have been demonstrated on
    a stock Sun OS 4.1.1 TCP running in an simulated environment that
    massively duplicates segments.  This environment is far more
    hazardous than most real TCP's must cope with, and the conditions
    were carefully tuned to create the necessary conditions for the
    failures.  However, these demonstrations are in effect an
    existence proof for the hazards.
    We now present example scenarios for each of these hazards.  Each
    scenario is assumed to follow immediately after a TWA event
    terminated the previous incarnation of the same connection.
 2.2  HAZARD H1: Acceptance of erroneous old duplicate data.
    Without the protection of the TIME-WAIT delay, it is possible for
    erroneous old duplicate data from the earlier incarnation to be
    accepted.  Figure 2 shows precisely how this might happen.

Braden [Page 4] RFC 1337 TCP TIME-WAIT Hazards May 1992

         TCP A                                                 TCP B
    1. ESTABL.  --> <SEQ=400><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
    2. ESTABL.  <--     <SEQ=101><ACK=500><CTL=ACK>     <--   ESTABL.
    3.  (old dupl)...<SEQ=560><ACK=101><DATA=80><CTL=ACK> --> ESTABL.
    4. ESTABL.  <--     <SEQ=101><ACK=500><CTL=ACK>     <--   ESTABL.
    5. ESTABL.  --> <SEQ=500><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
    6.             ...  <SEQ=101><ACK=640><CTL=ACK>     <--   ESTABL.
  1. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    7a. ESTABL. --> <SEQ=600><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
    8a. ESTABL.  <--    <SEQ=101><ACK=640><CTL=ACK> ...
    9a. ESTABL. --> <SEQ=700><ACK=101><DATA=100><CTL=ACK> --> ESTABL.
                  Figure 2: Accepting Erroneous Data
    The connection has already been successfully reopened after the
    assumed TWA event.  Segment 1 is a normal data segment and segment
    2 is the corresponding ACK segment.  Old duplicate data segment 3
    from the earlier incarnation happens to fall within the current
    receive window, resulting in a duplicate ACK segment #4.  The
    erroneous data is queued and "lurks" in the TCP reassembly queue
    until data segment 5 overlaps it.  At that point, either 80 or 40
    bytes of erroneous data is delivered to the user B; the choice
    depends upon the particulars of the reassembly algorithm, which
    may accept the first or the last duplicate data.
    As a result, B sends segment 6, an ACK for sequence = 640, which
    is 40 beyond any data sent by A.  Assume for the present that this
    ACK arrives at A *after* A has sent segment 7a, the next full data
    segment.  In that case, the ACK segment 8a acknowledges data that
    has been sent, and the error goes undetected.  Another possible
    continuation after segment 6 leads to hazard H3, shown below.
 2.3  HAZARD H2: De-synchronized Connection
    This hazard may result either as a side effect of H1 or directly
    from an old duplicate ACK that happens to be acceptable but
    acknowledges something that has not been sent.

Braden [Page 5] RFC 1337 TCP TIME-WAIT Hazards May 1992

    Referring to Figure 2 above, suppose that the ACK generated by the
    old duplicate data segment arrived before the next data segment
    had been sent.  The result is an infinite ACK loop, as shown by
    the following alternate continuation of Figure 2.
  1. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

7b. ESTABL. ←- <SEQ=101><ACK=640><CTL=ACK> …

   (ACK something not yet
    sent => send ACK)
    8b. ESTABL.  -->    <SEQ=600><ACK101><CTL=ACK>       -->   ESTABL.
                                                     (Below window =>
                                                          send ACK)
    9b. ESTABL.  <--    <SEQ=101><ACK=640><CTL=ACK>     <--    ESTABL.
                             (etc.!)
                   Figure 3: Infinite ACK loop
 2.4  HAZARD H3:  Connection Failure
    An old duplicate ACK segment may lead to an apparent refusal of
    TCP A's next connection attempt, as illustrated in Figure 4.  Here
    <W=...> indicates the TCP window field SEG.WIND.*
      TCP A                                                     TCP B
  1.  CLOSED                                                   LISTEN
  2.  SYN-SENT    --> <SEQ=100><CTL=SYN>                 --> SYN-RCVD
  3.         ... <SEQ=400><ACK=101><CTL=SYN,ACK><W=800>  <-- SYN-RCVD
  4.  SYN-SENT    <-- <SEQ=300><ACK=123><CTL=ACK> ... (old duplicate)
  5.  SYN-SENT    --> <SEQ=123><CTL=RST>                   --> LISTEN
  6.  ESTABLISHED <-- <SEQ=400><ACK=101><CTL=SYN,ACK><W=900> ...
  7.  ESTABLISHED --> <SEQ=101><ACK=401><CTL=ACK>          --> LISTEN
  8.  CLOSED      <--  <SEQ=401><CTL=RST>                  <-- LISTEN
         Figure 4: Connection Failure from Old Duplicate

Braden [Page 6] RFC 1337 TCP TIME-WAIT Hazards May 1992

    The key to the failure in Figure 4 is that the RST segment 5 is
    acceptable to TCP B in SYN-RECEIVED state, because the sequence
    space of the earlier connection that produced this old duplicate
    overlaps the new connection space.  Thus, <SEQ=123> in segment #5
    falls within TCP B's receive window [101,900).  In experiments,
    this failure mode was very easy to demonstrate.  (Kurt Matthys has
    pointed out that this scenario is time-dependent:  if TCP A should
    timeout and retransmit the initial SYN after segment 5 arrives and
    before segment 6, then the open will complete successfully.)

3. Fixes for TWA Hazards

 We discuss three possible fixes to TCP to avoid these hazards.
 (F1) Ignore RST segments in TIME-WAIT state.
      If the 2 minute MSL is enforced, this fix avoids all three
      hazards.
      This is the simplest fix.  One could also argue that it is
      formally the correct thing to do; since allowing time for old
      duplicate segments to die is one of TIME-WAIT state's functions,
      the state should not be truncated by a RST segment.
 (F2) Use PAWS to avoid the hazards.
      Suppose that the TCP ignores RST segments in TIME-WAIT state,
      but only long enough to guarantee that the timestamp clocks on
      both ends have ticked.  Then the PAWS mechanism [RFC-1323] will
      prevent old duplicate data segments from interfering with the
      new incarnation, eliminating hazard H1.  For reasons explained
      below, however, it may not eliminate all old duplicate ACK
      segments, so hazards H2 and H3 will still exist.
      In the language of the TCP Extensions RFC [RFC-1323]:
         When processing a RST bit in TIME-WAIT state:
             If (Snd.TS.OK is off) or (Time.in.TW.state() >= W)
                 then enter the CLOSED state, delete the TCB,
                 drop the RST segment, and return.
             else simply drop the RST segment and return.
      Here "Time.in.TW.state()" is a function returning the elapsed
      time since TIME-WAIT state was entered, and W is a constant that
      is at least twice the longest possible period for timestamp
      clocks, i.e., W = 2 secs [RFC-1323].

Braden [Page 7] RFC 1337 TCP TIME-WAIT Hazards May 1992

      This assumes that the timestamp clock at each end continues to
      advance at a constant rate whether or not there are any open
      connections.  We do not have to consider what happens across a
      system crash (e.g., the timestamp clock may jump randomly),
      because of the assumed Quiet Time at system startup.
      Once this change is in place, the initial timestamps that occur
      on the SYN and {SYN,ACK} segments reopening the connection will
      be larger than any timestamp on a segment from earlier
      incarnations.  As a result, the PAWS mechanism operating in the
      new connection incarnation will avoid the H1 hazard, ie.
      acceptance of old duplicate data.
      The effectiveness of fix (F2) in preventing acceptance of old
      duplicate data segments, i.e., hazard H1, has been demonstrated
      in the Sun OS TCP mentioned earlier.  Unfortunately, these tests
      revealed a somewhat surprising fact:  old duplicate ACKs from
      the earlier incarnation can still slip past PAWS, so that (F2)
      will not prevent failures H2 or H3.  What happens is that TIME-
      WAIT state effectively regenerates the timestamp of an old
      duplicate ACK.  That is, when an old duplicate arrives in TIME-
      WAIT state, an extended TCP will send out its own ACK with a
      timestamp option containing its CURRENT timestamp clock value.
      If this happens immediately before the TWA mechanism kills
      TIME-WAIT state, the result will be a "new old duplicate"
      segment with a current timestamp that may pass the PAWS test on
      the reopened connection.
      Whether H2 and H3 are critical depends upon how often they
      happen and what assumptions the applications make about TCP
      semantics.  In the case of the H3 hazard, merely trying the open
      again is likely to succeed.  Furthermore, many production TCPs
      have (despite the advice of the researchers who developed TCP)
      incorporated a "keep-alive" mechanism, which may kill
      connections unnecessarily.  The frequency of occurrence of H2
      and H3 may well be much lower than keep-alive failures or
      transient internet routing failures.
 (F3) Use 64-bit Sequence Numbers
      O'Malley and Peterson [RFC-1264] have suggested expansion of the
      TCP sequence space to 64 bits as an alternative to PAWS for
      avoiding the hazard of wrapped sequence numbers within the same
      incarnation.  It is worthwhile to inquire whether 64-bit
      sequence numbers could be used to avoid the TWA hazards as well.
      Using 64 bit sequence numbers would not prevent TWA - the early
      termination of TIME-WAIT state.  However, it appears that a

Braden [Page 8] RFC 1337 TCP TIME-WAIT Hazards May 1992

      combination of 64-bit sequence numbers with an appropriate
      modification of the TCP parameters could defeat all of the TWA
      hazards H1, H2, and H3.  The basis for this is explained in an
      appendix to this memo.  In summary, it could be arranged that
      the same sequence space would be reused only after a very long
      period of time, so every connection would be "slow" and "short".

4. Conclusions

 Of the three fixes described in the previous section, fix (F1),
 ignoring RST segments in TIME-WAIT state, seems like the best short-
 term solution.  It is certainly the simplest.  It would be very
 desirable to do an extended test of this change in a production
 environment, to ensure there is no unexpected bad effect of ignoring
 RSTs in TIME-WAIT state.
 Fix (F2) is more complex and is at best a partial fix.  (F3), using
 64-bit sequence numbers, would be a significant change in the
 protocol, and its implications need to be thoroughly understood.
 (F3) may turn out to be a long-term fix for the hazards discussed in
 this note.

APPENDIX: Using 64-bit Sequence Numbers

 This appendix provides a justification of our statement that 64-bit
 sequence numbers could prevent the TWA hazards.
 The theoretical ISN calculation used by TCP is:
     ISN = (R*T) mod 2**n.
 where T is the real time in seconds (from an arbitrary origin, fixed
 when the system is started), R is a constant, currently 250 KBps, and
 n = 32 is the size of the sequence number field.
 The limitations of current TCP are established by n, R, and the
 maximum segment lifetime MSL = 4 minutes.  The shortest time Twrap to
 wrap the sequence space is:
     Twrap = (2**n)/r
 where r is the maximum transfer rate.  To avoid old duplicate
 segments in the same connection, we require that Twrap > MSL (in
 practice, we need Twrap >> MSL).

Braden [Page 9] RFC 1337 TCP TIME-WAIT Hazards May 1992

 The clock-driven ISN numbers wrap in time TwrapISN:
     TwrapISN = (2**n)/R
 For current TCP, TwrapISN = 4.55 hours.
 The cases for old duplicates from previous connections can be divided
 into four regions along two dimensions:
  • Slow vs. fast connections, corresponding to r < R or r >= R.
  • Short vs. long connections, corresponding to duration E <

TwrapISN or E >= TwrapISN.

 On short slow connections, the clock-driven ISN selection rejects old
 duplicates.  For all other cases, the TIME-WAIT delay of 2*MSL is
 required so old duplicates can expire before they infect a new
 incarnation.  This is discussed in detail in the Appendix to [RFC-
 1185].
 With this background, we can consider the effect of increasing n to
 64.  We would like to increase both R and TwrapISN far enough that
 all connections will be short and slow, i.e., so that the clock-
 driven ISN selection will reject all old duplicates.  Put another
 way, we want to every connection to have a unique chunk of the
 seqence space.  For this purpose, we need R larger than the maximum
 foreseeable rate r, and TwrapISN greater than the longest foreseeable
 connection duration E.
 In fact, this appears feasible with n = 64 bits.  Suppose that we use
 R = 2**33 Bps; this is approximately 8 gigabytes per second, a
 reasonable upper limit on throughput of a single TCP connection.
 Then TwrapISN = 68 years, a reasonable upper limit on TCP connection
 duration.  Note that this particular choice of R corresponds to
 incrementing the ISN by 2**32 every 0.5 seconds, as would happen with
 the Berkeley BSD implementation of TCP.  Then the low-order 32 bits
 of a 64-bit ISN would always be exactly zero.
 REFERENCES
    [RFC-793]  Postel, J., "Transmission Control Protocol", RFC-793,
    USC/Information Sciences Institute, September 1981.
    [RFC-1185]  Jacobson, V., Braden, R., and Zhang, L., "TCP
    Extension for High-Speed Paths", RFC-1185, Lawrence Berkeley Labs,
    USC/Information Sciences Institute, and Xerox Palo Alto Research
    Center, October 1990.

Braden [Page 10] RFC 1337 TCP TIME-WAIT Hazards May 1992

    [RFC-1263]  O'Malley, S. and L. Peterson, "TCP Extensions
    Considered Harmful", RFC-1263, University of Arizona, October
    1991.
    [RFC-1323]  Jacobson, V., Braden, R. and D. Borman "TCP Extensions
    for High Performance", RFC-1323, Lawrence Berkeley Labs,
    USC/Information Sciences Institute, and Cray Research, May 1992.

Security Considerations

 Security issues are not discussed in this memo.

Author's Address:

 Bob Braden
 University of Southern California
 Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292
 Phone: (213) 822-1511
 EMail: Braden@ISI.EDU

Braden [Page 11]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1337.txt · Last modified: 1992/05/27 00:06 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki