Evaluation of Header Field Entropy for Hash-Based Packet Selection Evaluation of Header Field Entropy for Hash-Based Packet Selection Christian Henke, Carsten Schmoll, Tanja Zseby Fraunhofer Institute FOKUS, Berlin, Germany

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Outline 2 1.Introduction Multipoint Sampling 2.Problem Statement 3.Approach 4.Measurement Setup 5.Measurement Results 6.Conclusion

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Introduction Multipoint Sampling 3 Passive Multipoint Measurements –at observation points a packet ID and timestamp exported for each packet –trace observable based on occurrence of packet ID –delay = timestamp A – timestamp B of packets with equal ID Multipoint Collector Point A Point B Point C

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Introduction Multipoint Sampling 4 CChallenge in Passive Multipoint Measurements  immense amounts of measurement data  High infrastructure costs: processing, storing, exporting Random Packet Selection and Estimation Random Sampling (n-out-of-N, probabilistic) unsuitable -> inconsistent sample at observation points Duffield and Grossglauser in “Trajectory Sampling for Direct Traffic Observation” propose hash-based packet selection.

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Introduction Multipoint Sampling 5 IP HeaderTransport HeaderPayload hash input hash function packet selectedpacket not selected consistent selected subset if x, h and S are equal at all observation points Hash-Based Paket Selection

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Problem Statement Which packet content to use as hash input? Requirements for header fields 1.static between network nodes ( IP TTL and checksum) 2.variable among packets Challenge:  HBS is deterministic; but goal is to emulate random selection  choice of hash input can introduce bias to the selection 6

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Problem Statement 7 How bias is introduced -packets in a hash input collision have same hash input -selection decision is not independent -the more packets in collision the more grievous the bias -unsuitable to use whole packet because hash value calculation time increases with hash input length

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Approach Approach –packets differ more often in high variable bytes –entropy per byte used to measure variability Entropy Information Efficiency p i probability that hash value i occurs H(B) entropy dependent on discrete Variant of Byte Values 8

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Evaluation dependent on analyzed traces -6 IPv4 trace groups – 1 IPv6 -geographical locations (NZ, AUT, FR, NED – 2 LEO) -network location (university, peering point, large ISP) -application mix Measurement Setup 9

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Measurement Results Entropy IPv4 10

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Measurement Results High Entropy Header Fields  IPv4: Identification, Length LSB, Src/Dst Address 2 LSB  TCP: Chksum, SeqNo, AckNo, Src/Dst Port 2 LSB  UDP: Chksum, Length LSB, Src/Dst Port 2 LSB  ICMP: Chksum, Bytes 12,13,18,19  IPv6: Length LSB –more IPv6 traces required for further evaluation –Addresses anonymized and no transport header - only 8 bytes could be evaluated Recommended 8 byte Configuration IP ID field + 6 Transport Header Bytes:  TCP (Checksum, 2 LSB of Seq and AckNo)  UDP (Checksum, Source Port, LSB Destination Port, LSB Length)  ICMP (Checksum, Bytes 12,13,18,19) 11

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Measurement Results 12 Empirical Hash Input Collisions Evaluation  4 configurations used 1.whole IP and transport header (minimum reachable collisions) 2.only IP header (bad configuration) 3.8 high entropy bytes 4.Molina‘s 16 bytes  sum of packets on 20 largest collisions of each trace –Large collision: all or none decision of all packets that have same attributes –Small collisions: packets equal in one collision but different between

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Measurement Results Hash Input Collision Comparison  recommended 8 bytes better than Molina’s 16 bytes  LEO2 traces include a large VPN traffic flow with UDP Checksum==0 – more high entropy bytes should be used 13

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Conclusion Outcome  give a recommendation of 8 bytes for use as hash input for HBS  8 recommended bytes sufficient to gain unique hash inputs Henke, Schmoll, Zseby “Empirical Evaluation of Hash Functions for Multipoint Measurements”  hash calculation time linear increase with input length  hash functions are able to select representative subset based on 8 bytes 14

Evaluation of Header Field Entropy for Hash-Based Packet Selection PAM 2008, Cleveland Future Work Correlation between Bytes  Correlation between address bytes  entropy of combined bytes expected to be average of entropy IPv6  entropy evaluation of IPv6 addresses  transport headers