rfc2678.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.


RFC 2678        IPPM Metrics for Measuring Connectivity   September 1999


6.4. 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.
 +    dT2 is the time taken for a Type-P2 packet sent by Dst at time T2
      to arrive at Src.

6.5. 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.

6.6. 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.

6.6.1. 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.

6.6.2. Recommended values:

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





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6.6.3. Algorithm:

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

6.6.4. 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.

6.6.5. 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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RFC 2678        IPPM Metrics for Measuring Connectivity   September 1999


 +    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.}

7. Acknowledgments

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

8. 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.

9. References

   [RFC1812]  Baker, F., "Requirements for IP Version 4 Routers", RFC
              1812, June 1995.

   [RFC1122]  Braden, R., Editor, "Requirements for Internet Hosts --
              Communication Layers", STD, 3, RFC 1122,  October 1989.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J. and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330, May
              1998.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.



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RFC 2678        IPPM Metrics for Measuring Connectivity   September 1999


10. Authors' Addresses

   Jamshid Mahdavi
   Pittsburgh Supercomputing Center
   4400 5th Avenue
   Pittsburgh, PA  15213
   USA

   EMail: [e-mail]


   Vern Paxson
   MS 50A-3111
   Lawrence Berkeley National Laboratory
   University of California
   Berkeley, CA  94720
   USA

   Phone: [phone]
   EMail: [e-mail]































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RFC 2678        IPPM Metrics for Measuring Connectivity   September 1999


11.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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