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					   A Suite of Schemes for User-level
Network Diagnosis without Infrastructure


             Yao Zhao, Yan Chen
   Lab for Internet and Security Technology,
           Northwestern University

                                               1
            Motivation
• How do end users, with no special
  privileges, identify packet loss
  inside the network with one or two
  computers?




                                       2
             Motivation
• How do end users, with no special
  privileges, identify packet loss
  inside the network with one or two
  computers?
• Take-home
  – We propose three user-level loss rate
    diagnosis approaches
  – The combo of our approaches and
    Tulip [SOSP03] is much better than
    any single approach
                                        3
                Outline
• Motivation
• Related Works
• Lossy Link Diagnosis
  – Fragmentation Aided Diagnosis (FAD)
    • Algebraic FAD
    • Opportunistic FAD
  – Striped Probe Analysis (SPA)
• Evaluations
• Conclusions
                                      4
                 Related Work I
• Internet Tomography
    – Multicast based (not practical)
    – Unicast based
        • Mimic multicast             S




      The more
  cooperating end
hosts, the shorter the
     virtual links




                            L1   L2       L3       L4
                                               5
               Related Work II
• Tulip [SOSP03]
    – Leverage on consecutive IPID
    – Tend to underestimate forward loss
      rates
         • Suffer from the packet loss correlation
S         D          S        D        S        D


     x                                      x

                          x                 x




Forward Loss        Reverse Loss           ?
                                                     6
                Outline
• Motivation
• Related Works
• Lossy Link Diagnosis
  – Fragmentation Aided Diagnosis (FAD)
    • Algebraic FAD
    • Opportunistic FAD
  – Striped Probe Analysis (SPA)
• Evaluations
• Conclusions
                                      7
Link Diagnosis=> Forward Path Diagnosis

• If we can infer the loss rates of
  forward path F1 and F2, we can
  infer the link loss rate of l3
             F1
        l1        l2         l3   D
    S        R1         R2


                   F2


 • The more diagnosable forward path
   segments, the better the diagnosis
   granularity
                                      8
       Basic Idea of FAD


                       P
S P          R N
                       R




                        P1
                       P2
  P2
S P1         R   N
                       R




                             9
               Algebraic FAD
• Let pf and pr be the loss rate of the
  forward and reverse path respectively
                                      P
 (1 - pf)×(1 - pr)=1 – p   (1)
                                      R



                                       P1
                                      P2
  (1 - pf)2×(1 - pr)=1 – p’ (2)
                                      R




  p and p’ are measurable. Solve pf and pr
  using (1) and (2)
                                             10
           How to Achieve FAD
IP Fragmentation
  – Fragment a packet longer than MTU
  – Required to be supported in IPv4
  – Some routers disable it for security reason
• Support of IP Fragmentation
  – 64,320 router IP addresses probed by using
    Traceroute
  – About 80% of routers support IP fragmentation
• Degree of Rate Limiting on Responses
  – 99% of routers allow a rate of 100 probes/s for
    ICMP Echo, ICMP Timestamp and TCP probes
  – Response to UDP probe is severely rate-
    limited
                                                  11
           Opportunistic FAD




   F1      +      F2           P
aaaaaaaa       bbbbbbbb    aaaaaaaabbbbbb




   F1      +      F’2           P’
aaaaaaaa       ccccccccc   aaaaaaaacccccccc




                                              12
              Opportunistic FAD

    No Loss                     Forward Loss


S             N                 S            N



                                      x
                  F1+F2

                                                 F1+F’2




                          Similar to Tulip, but OFAD allows
                          large gap between fragments
                                                          13
        Striped Probe Analysis (SPA)

                                    S
        p1                              p1
                 p2
    S        R        D             R
                                   p2        p3
        p3                     D                  S




• S sends a probe to D and we get the path p1->p2
• S sends UDP packet with a certain TTL so that R
  returns an ICMP TTL-Exceeded response. Hence
  we get path p1->p3
                                                      14
Striped Probe Analysis (SPA)
            S
           P1 P2
             p1

            R

      p2           p3


  D                     S

(1) Loss on shared link




                               15
          Striped Probe Analysis (SPA)
                       S                                 S
                                                        P1 P2
                       p1                                 p1

                       R                                 R

                  p2        p3                     p2           p3


              D                  S             D                     S

            (1) Loss on shared link         (2) Loss on non-shared link
• Success rate of p1≈n1×n2 / (n×n12)
    –   n: number of striped probes sent,
    –   n1: number of P1 received by D,
    –   n2: number of P2 received by S,
    –   n12: number of cases that both P1 and P2 are received
• Unbiased if packet loss has perfect correlation and loss rates of
  different links are independent                               16
                              Summary


             Requirement                 Accuracy


Tulip                                    Inaccurate w/ strong loss
             Consecutive IPID (70%)
[SOSP03]                                 correlation

FAD (AFAD                                Accurate w/ weak or short loss
          IP fragmentation (80%)
& OFAD)                                  correlation

             ICMP TTL-Exceeded.
                                         Accurate w/ strong loss
SPA          Access from both end
                                         correlation
             hosts
The current Internet usually has strong but short loss correlation.
                                                                      17
                Outline
• Motivation
• Related Works
• Lossy Link Diagnosis
  – FAD
  – SPA
• Evaluations
• Conclusions


                          18
          Evaluation Metrics
• Diagnosis Granularity
  – Weighted average of the lengths of the
    path’s diagnosable segments
  – For example, an 8-hop path has two
    diagnosable segments of length 3 and 5,
    and then the granularity of the path is
    (32 + 52)/8 = 4.25
• Accuracy
  – Estimation error:
  – Relative error:

                                        19
         Diagnosis Granularity
SPA is
 best



                       FAD ≈Tulip


                 Combo of
               FAD and Tulip
                  is better




                                    20
Path-Level Accuracy Evaluation




            FAD > Tulip >
               SPA

                            OFAD, Tulip and
                              SPA tends to
                             underestimate
                               loss rates

                                         21
            More Evaluations

•   Consistency Check
•   Packet Probe Size Selection
•   Lossy Link Distribution
•   More in the technical report
    http://www.cs.northwestern.edu/~yzh734/




                                         22
Conclusions and Recommendations
• We propose AFAD, OFAD and SPA which
  can conduct loss rate diagnosis without
  infrastructure
• Tulip, FAD and SPA have different working
  scenarios
  – The combination of them can achieve low
    diagnosis granularity and high accuracy
• Recommendations
  – OFAD+SPA, if we can control the two ends of
    an end-to-end path
  – OFAD+Tulip, if we can only control the source

                                              23
24
  Thanks!




Questions?


             25
Path-Level Accuracy of Combined Schemes




                                    26
27
Path-Level Accuracy Evaluation




                             28
Path-Level Accuracy of Combined Schemes




                                    29
 IP Fragmentation Is Widely Supported
• Router Collection
   – 64,320 router IP addresses probed by using traceroute from a
     machine
• Support of Different Probes

                 Echo    Timestamp UDP           TCP        Any

   1 source      85.3% 69.2%           64.5% 71.7% 88.2%

   10 sources 87.3% 72.3%              70.7% 73.3% 90.1%

• Support of IP Fragmentation
   – 90.3% of responsive routers support IP fragmentation
   – Altogether about 80% of routers support FAD.
• Degree of Rate Limiting on Responses
   – 99% of routers allow a rate of 100 probes/s for ICMP Echo,
     ICMP Timestamp and TCP probes
   – UDP probe is severely rate-limited
                                                              30
Packet Transmission Correlation
• Choose 100 PlanetLab hosts and
  randomly measure 5000 paths
• Little loss correlation with enough gap




                                            31
Forward Path Diagnosis => Link Diagnosis

• If we can infer the loss rates of
  forwarding path l1 and P1, we can
  infer the link loss rate of l2 too.




                                   D




                                        32
                 Opportunistic FAD

       P1                     P1

       P2                     P2
                                    x
     R12
           P’2                      P’2
                             R’12


       (1)                     (2)

• n: number of R12 received, n’: number of
  R’12 received
• Xi = 0 when forward packet i is lost and
  Xi =1 otherwise
• P(X2=1)≈P(X2=1|X1=1)≈n/(n+n’)
                                             33
        Striped Probe Analysis (SPA)

                                         S
        l1       l2
                                             l1
    S        R        D
                                         R
        l3                          l2            l3
                                D                      S



• No fragmented packets needed !
• S sends a probe to D and we get the path l1->l2
• S sends UDP packet with a certain TTL so that R
  returns an ICMP TTL-Exceeded response. Hence
  we get path l1->l3
                                                           34
          Striped Probe Analysis (SPA)
          S
         P1 P2
              l1

          R

    l2             l3


D                       S




                                         35
           Striped Probe Analysis (SPA)
           S                               S
                                          P1 P2
               l1                              l1


           R                               R

      l2            l3               l2             l3
P1                       P2
 D                       S       D                       S

     (1) No loss              (2) Loss on shared link




                                                             36
               Striped Probe Analysis (SPA)
               S                              S                              S
                                                                            P1 P2
                   l1                             l1                             l1


               R                              R                              R

          l2            l3               l2            l3              l2             l3
    P1                       P2
    D                        S       D                      S      D                       S

         (1) No loss              (2) Loss on shared link       (3) Loss on non-shared link

• Success rate of l1≈n1×n2 / (n×n12)
    –    n: number of striped probes sent,
    –    n1: number of P1 received by D,
    –    n2: number of P2 received by S,
    –    n12: number of cases that both P1 and P2 are received
• Unbiased if packet loss has perfect correlation and loss rates of different
  links are independent                                            37

				
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