RFC 3449 (rfc3449) - Page 2 of 41

TCP Performance Implications of Network Path Asymmetry

RFC 3449                PILC - Asymmetric Links            December 2002


Table of Contents

   1. Conventions used in this Document ...............................3
     2. Motivation ....................................................4
     2.1 Asymmetry due to Differences in Transmit
         and Receive Capacity .........................................4
     2.2 Asymmetry due to Shared Media in the Reverse Direction .......5
     2.3 The General Problem ..........................................5
   3. How does Asymmetry Degrade TCP Performance? .....................5
     3.1 Asymmetric Capacity ..........................................5
     3.2 MAC Protocol Interactions ....................................7
     3.3 Bidirectional Traffic ........................................8
     3.4 Loss in Asymmetric Network Paths ............................10
   4. Improving TCP Performance using Host Mitigations ...............10
     4.1 Modified Delayed ACKs .......................................11
     4.2 Use of Large MSS ............................................12
     4.3 ACK Congestion Control ......................................13
     4.4 Window Prediction Mechanism .................................14
     4.5 Acknowledgement based on Cwnd Estimation. ...................14
     4.6 TCP Sender Pacing ...........................................14
     4.7 TCP Byte Counting ...........................................15
     4.8 Backpressure ................................................16
   5. Improving TCP performance using Transparent Modifications ......17
     5.1 TYPE 0: Header Compression ..................................18
       5.1.1 TCP Header Compression ..................................18
       5.1.2 Alternate Robust Header Compression Algorithms ..........19
     5.2 TYPE 1: Reverse Link Bandwidth Management ...................19
       5.2.1 ACK Filtering ...........................................20
       5.2.2 ACK Decimation ..........................................21
     5.3 TYPE 2: Handling Infrequent ACKs ............................22
       5.3.1 ACK Reconstruction ......................................23
       5.3.2 ACK Compaction and Companding ...........................25
       5.3.3 Mitigating TCP packet bursts generated by
             Infrequent ACKs .........................................26
     5.4 TYPE 3: Upstream Link Scheduling ............................27
       5.4.1 Per-Flow queuing at the Upstream Bottleneck Link ........27
       5.4.2 ACKs-first Scheduling ...................................28
   6. Security Considerations ........................................29
   7. Summary ........................................................30
   8. Acknowledgments ................................................32
   9. References .....................................................32
   10. IANA Considerations ...........................................37
   Appendix: Examples of Subnetworks Exhibiting Network Path
             Asymmetry ...............................................38
   Authors' Addresses ................................................40
   Full Copyright Statement ..........................................41





Balakrishnan et. al.     Best Current Practice