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Versions: 01 02 03 04 RFC 2498

Network Working Group        J. Mahdavi, Pittsburgh Supercomputer Center
Internet Draft          V. Paxson, Lawrence Berkeley National Laboratory
Expiration Date: May 1999                                  November 1998


                IPPM Metrics for Measuring Connectivity
                 <draft-ietf-ippm-connectivity-04.txt>


1. Status of this Memo

   This document is an Internet  Draft.   Internet  Drafts  are  working
   documents  of  the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may  also  distribute
   working documents as Internet Drafts.

   Internet Drafts are draft  documents  valid  for  a  maximum  of  six
   months, and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet Drafts as reference
   material or to cite them other than as ``work in progress''.

   To view the entire list of current Internet-Drafts, please check  the
   "1id-abstracts.txt"  listing  contained in the Internet-Drafts Shadow
   Directories  on  ftp.is.co.za   (Africa),   ftp.nordu.net   (Northern
   Europe),  ftp.nis.garr.it  (Southern  Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

   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.


2. Introduction

   Connectivity is the basic stuff from  which  the  Internet  is  made.
   Therefore,  metrics determining whether pairs of hosts (IP addresses)
   can reach each other must form the base of a measurement  suite.   We
   define  several  such metrics, some of which serve mainly as building
   blocks for the others.

   This memo defines a series of metrics for connectivity between a pair
   of  Internet hosts.  It builds on notions introduced and discussed in
   RFC 2330, the IPPM framework document.  The reader is assumed  to  be
   familiar with that document.

   The structure of the memo is as follows:





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ID              IPPM Metrics for Measuring Connectivity    November 1998


 +    An analytic  metric,  called  Type-P-Instantaneous-Unidirectional-
      Connectivity, will be introduced to define one-way connectivity at
      one moment in time.
 +    Using  this  metric,  another  analytic  metric,  called   Type-P-
      Instantaneous-Bidirectional-Connectivity,  will  be  introduced to
      define two-way connectivity at one moment in time.
 +    Using these  metrics,  corresponding  one-  and  two-way  analytic
      metrics are defined for connectivity over an interval of time.
 +    Using  these  metrics,  an  analytic  metric,  called  Type-P1-P2-
      Interval-Temporal-Connectivity,  will  be  introduced  to define a
      useful notion of two-way connectivity between two  hosts  over  an
      interval of time.
 +    Methodologies are then  presented  and  discussed  for  estimating
      Type-P1-P2-Interval-Temporal-Connectivity    in   a   variety   of
      settings.
   Careful definition of  Type-P1-P2-Interval-Temporal-Connectivity  and
   the  discussion of the metric and the methodologies for estimating it
   are the two chief contributions of the memo.


3. Instantaneous One-way Connectivity


3.1. Metric Name:

   Type-P-Instantaneous-Unidirectional-Connectivity


3.2. Metric Parameters:
 +    Src, the IP address of a host
 +    Dst, the IP address of a host
 +    T, a time


3.3. Metric Units:

   Boolean.


3.4. Definition:

   Src has *Type-P-Instantaneous-Unidirectional-Connectivity* to Dst  at
   time  T if a type-P packet transmitted from Src to Dst at time T will
   arrive at Dst.







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3.5. Discussion:

   For  most  applications  (e.g.,  any  TCP  connection)  bidirectional
   connectivity   is   considerably  more  germane  than  unidirectional
   connectivity, although unidirectional connectivity can be of interest
   for  some  security  applications  (e.g.,  testing whether a firewall
   correctly filters out a "ping of  death").   Most  applications  also
   require   connectivity   over  an  interval,  while  this  metric  is
   instantaneous,  though,  again,  for   some   security   applications
   instantaneous  connectivity  remains of interest.  Finally, one might
   not have instantaneous connectivity due to a transient event such  as
   a full queue at a router, even if at nearby instants in time one does
   have connectivity.  These  points  are  addressed  below,  with  this
   metric serving as a building block.

   Note also that we have  not  explicitly  defined  *when*  the  packet
   arrives  at  Dst.   The  TTL field in IP packets is meant to limit IP
   packet lifetimes to 255 seconds (RFC 791).  In practice the TTL field
   can be strictly a hop count (RFC 1812), with most Internet hops being
   much shorter than one second.  This means that most packets will have
   nowhere  near  the 255 second lifetime.  In principle, however, it is
   also possible that packets might survive  longer  than  255  seconds.
   Consideration  of  packet  lifetimes  must  be  taken into account in
   attempts to measure the value of this metric.

   Finally,  one  might  assume  that  unidirectional  connectivity   is
   difficult  to  measure  in the absence of connectivity in the reverse
   direction.  Consider, however, the  possibility  that  a  process  on
   Dst's  host  notes when it receives packets from Src and reports this
   fact either using an external channel, or later in time when Dst does
   have  connectivity to Src.  Such a methodology could reliably measure
   the unidirectional connectivity defined in this metric.


4. Instantaneous Two-way Connectivity


4.1. Metric Name:

   Type-P-Instantaneous-Bidirectional-Connectivity


4.2. Metric Parameters:








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 +    A1, the IP address of a host
 +    A2, the IP address of a host
 +    T, a time


4.3. Metric Units:

   Boolean.


4.4. Definition:

   Addresses  A1  and   A2   have   *Type-P-Instantaneous-Bidirectional-
   Connectivity*  at  time  T  if  address  A1 has Type-P-Instantaneous-
   Unidirectional-Connectivity to address A2 and address A2 has  Type-P-
   Instantaneous-Unidirectional-Connectivity to address A1.


4.5. Discussion:

   An alternative definition would be that A1 and A2 are fully connected
   if at time T address A1 has instantaneous connectivity to address A2,
   and at time T+dT address A2 has  instantaneous  connectivity  to  A1,
   where  T+dT  is  when  the  packet  sent from A1 arrives at A2.  This
   definition is more useful for measurement,  because  the  measurement
   can  use  a reply from A2 to A1 in order to assess full connectivity.
   It is a more complex  definition,  however,  because  it  breaks  the
   symmetry  between A1 and A2, and requires a notion of quantifying how
   long a particular packet from A1 takes  to  reach  A2.   We  postpone
   discussion  of  this  distinction  until the development of interval-
   connectivity metrics below.


5. One-way Connectivity


5.1. Metric Name:

   Type-P-Interval-Unidirectional-Connectivity


5.2. Metric Parameters:
 +    Src, the IP address of a host








Mahdavi and Paxson                                              [Page 4]


ID              IPPM Metrics for Measuring Connectivity    November 1998


 +    Dst, the IP address of a host
 +    T, a time
 +    dT, a duration
   {Comment:  Thus,  the  closed  interval  [T,  T+dT]  denotes  a  time
   interval.}


5.3. Metric Units:

   Boolean.


5.4. Definition:

   Address  Src  has  *Type-P-Interval-Unidirectional-Connectivity*   to
   address  Dst  during the interval [T, T+dT] if for some T' within [T,
   T+dT] it has Type-P-instantaneous-connectivity to Dst.


6. Two-way Connectivity


6.1. Metric Name:

   Type-P-Interval-Bidirectional-Connectivity


6.2. Metric Parameters:
 +    A1, the IP address of a host
 +    A2, the IP address of a host
 +    T, a time
 +    dT, a duration
   {Comment:  Thus,  the  closed  interval  [T,  T+dT]  denotes  a  time
   interval.}


6.3. Metric Units:

   Boolean.


6.4. Definition:

   Addresses A1 and A2 have *Type-P-Interval-Bidirectional-Connectivity*
   between  them during the interval [T, T+dT] if address A1 has Type-P-
   Interval-Unidirectional-Connectivity  to  address   A2   during   the
   interval   and   address   A2   has   Type-P-Interval-Unidirectional-
   Connectivity to address A1 during the interval.



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ID              IPPM Metrics for Measuring Connectivity    November 1998


6.5. Discussion:

   This metric is  not  quite  what's  needed  for  defining  "generally
   useful"  connectivity  -  that requires the notion that a packet sent
   from A1 to A2 can elicit a response from A2 that will reach A1.  With
   this  definition,  it  could be that A1 and A2 have full-connectivity
   but only, for example, at time T1 early enough in  the  interval  [T,
   T+dT] that A1 and A2 cannot reply to packets sent by the other.  This
   deficiency motivates the next metric.


7. Two-way Temporal Connectivity


7.1. Metric Name:

   Type-P1-P2-Interval-Temporal-Connectivity


7.2. Metric Parameters:
 +    Src, the IP address of a host
 +    Dst, the IP address of a host
 +    T, a time
 +    dT, a duration
   {Comment:  Thus,  the  closed  interval  [T,  T+dT]  denotes  a  time
   interval.}


7.3. Metric Units:

   Boolean.


7.4. Definition:

   Address  Src   has   *Type-P1-P2-Interval-Temporal-Connectivity*   to
   address Dst during the interval [T, T+dT] if there exist times T1 and
   T2, and time intervals dT1 and dT2, such that:
 +    T1, T1+dT1, T2, T2+dT2 are all in [T, T+dT].
 +    T1+dT1 <= T2.
 +    At time T1, Src has Type-P1 instantanous connectivity to Dst.
 +    At time T2, Dst has Type-P2 instantanous connectivity to Src.
 +    dT1 is the time taken for a Type-P1 packet sent by Src at time  T1
      to arrive at Dst.







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ID              IPPM Metrics for Measuring Connectivity    November 1998


 +    dT2 is the time taken for a Type-P2 packet sent by Dst at time  T2
      to arrive at Src.


7.5. Discussion:

   This metric defines "generally useful" connectivity -- Src can send a
   packet  to  Dst  that  elicits a response.  Because many applications
   utilize different types of packets for forward and  reverse  traffic,
   it  is  possible (and likely) that the desired responses to a Type-P1
   packet will be of a  different  type  Type-P2.   Therefore,  in  this
   metric  we  allow  for  different types of packets in the forward and
   reverse directions.


7.6. Methodologies:

   Here we sketch a class of methodologies  for  estimating  Type-P1-P2-
   Interval-Temporal-Connectivity.   It  is a class rather than a single
   methodology because the particulars will depend on the types  P1  and
   P2.


7.6.1. Inputs:
 +    Types P1 and P2, addresses A1 and A2, interval [T, T+dT].
 +    N, the number  of  packets  to  send  as  probes  for  determining
      connectivity.
 +    W, the "waiting time", which bounds for how long it is  useful  to
      wait for a reply to a packet.
   Required: W <= 255, dT > W.


7.6.2. Recommended values:

   dT = 60 seconds.
   W = 10 seconds.
   N = 20 packets.


7.6.3. Algorithm:

 +    Compute N *sending-times* that are randomly, uniformly distributed
      over [T, T+dT-W].








Mahdavi and Paxson                                              [Page 7]


ID              IPPM Metrics for Measuring Connectivity    November 1998


 +    At each sending time, transmit from A1  a  well-formed  packet  of
      type P1 to A2.
 +    Inspect  incoming  network  traffic  to  A1  to  determine  if   a
      successful  reply  is  received.   The particulars of doing so are
      dependent on types P1 & P2, discussed below.   If  any  successful
      reply  is  received,  the  value of the measurement is "true".  At
      this point, the measurement can terminate.
 +    If no successful replies are received by time T+dT, the  value  of
      the measurement is "false".


7.6.4. Discussion:

   The algorithm is inexact because  it  does  not  (and  cannot)  probe
   temporal  connectivity  at  every  instant in time between [T, T+dT].
   The value of N  trades  off  measurement  precision  against  network
   measurement load.  The state-of-the-art in Internet research does not
   yet offer solid guidance for picking N.  The values given  above  are
   just guidelines.


7.6.5. Specific methodology for TCP:

   A TCP-port-N1-port-N2 methodology sends TCP SYN packets  with  source
   port  N1 and dest port N2 at address A2.  Network traffic incoming to
   A1 is interpreted as follows:
 +    A SYN-ack packet from A2 to A1  with  the  proper  acknowledgement
      fields and ports indicates temporal connectivity.  The measurement
      terminates immediately with a value of "true".  {Comment: if, as a
      side  effect  of  the  methodology, a full TCP connection has been
      established between A1 and A2  --  that  is,  if  A1's  TCP  stack
      acknowledges   A2's   SYN-ack  packet,  completing  the  three-way
      handshake -- then the connection now established between A1 and A2
      is  best  torn down using the usual FIN handshake, and not using a
      RST packet, because RST packets are not  reliably  delivered.   If
      the  three-way  handshake  is  not  completed, however, which will
      occur if the measurement tool on A1 synthesizes  its  own  initial
      SYN packet rather than going through A1's TCP stack, then A1's TCP
      stack will automatically terminate the connection  in  a  reliable
      fashion  as A2 continues transmitting the SYN-ack in an attempt to
      establish the connection.  Finally, we note that  using  A1's  TCP
      stack  to  conduct  the measurement complicates the methodology in
      that the stack may retransmit the initial SYN packet, altering the
      number of probe packets sent.}







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ID              IPPM Metrics for Measuring Connectivity    November 1998


 +    A RST packet from  A2  to  A1  with  the  proper  ports  indicates
      temporal  connectivity  between  the  addresses  (and  a *lack* of
      service connectivity  for  TCP-port-N1-port-N2  -  something  that
      probably should be addressed with another metric).
 +    An  ICMP  port-unreachable  from  A2  to  A1  indicates   temporal
      connectivity  between the addresses (and again a *lack* of service
      connectivity    for    TCP-port-N1-port-N2).     {Comment:     TCP
      implementations   generally   do  not  need  to  send  ICMP  port-
      unreachable messages because a  separate  mechanism  is  available
      (sending a RST).  However, RFC 1122 states that a TCP receiving an
      ICMP port-unreachable MUST treat it the  same  as  the  equivalent
      transport-level mechanism (for TCP, a RST).}
 +    An  ICMP  host-unreachable  or  network-unreachable  to  A1   (not
      necessarily from A2) with an enclosed IP header matching that sent
      from A1 to A2 *suggests* a lack of temporal connectivity.   If  by
      time  T+dT no evidence of temporal connectivity has been gathered,
      then the receipt of the ICMP can be used as additional information
      to the measurement value of "false".

   {Comment: Similar methodologies are needed for ICMP Echo, UDP, etc.}


8. Acknowledgments

   The comments of Guy Almes, Martin Horneffer, Jeff Sedayao,  and  Sean
   Shapira are appreciated.


9. Security Considerations

   As noted in RFC 2330, active measurement techniques,  such  as  those
   defined in this document, can be abused for denial-of-service attacks
   disguised as legitimate measurement activity.   Furthermore,  testing
   for  connectivity  can  be used to probe firewalls and other security
   mechnisms for weak spots.


10. References


   [RFC1812]
        F. Baker, "Requirements for IP Version 4 Routers", June 1995.

   [RFC1122]
        R. Braden, Editor, "Requirements for Internet Hosts --  Communi-
        cation Layers," Oct. 1989.

   [RFC2330]



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ID              IPPM Metrics for Measuring Connectivity    November 1998


        V. Paxson, G. Almes, J. Mahdavi, and M. Mathis,  "Framework  for
        IP Performance Metrics", May 1998.

   [RFC791]
        J. Postel, "Internet Protocol", September 1981.


11. Authors' Addresses

   Jamshid Mahdavi <mahdavi@psc.edu>
   Pittsburgh Supercomputing Center
   4400 5th Avenue
   Pittsburgh, PA  15213
   USA

   Vern Paxson <vern@ee.lbl.gov>
   MS 50A-3111
   Lawrence Berkeley National Laboratory
   University of California
   Berkeley, CA  94720
   USA
   Phone: +1 510/486-7504





























Mahdavi and Paxson                                             [Page 10]


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