[RFC2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC2507, February 1999.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Heavens, I. and B.
Volz, "Known TCP Implementation Problems", RFC2525, March
1999.
[RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link PPP",
RFC2686, September 1999.
[RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J., Henderson,
T., Heidemann, J., Kruse, H., Ostermann, S., Scott, K.,
Semke, J., Touch, J. and D. Tran, "Ongoing TCP Research
Related to Satellites", RFC2760, February 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC2988, November 2000.
[RFC3077] Duros, E., Dabbous, W., Izumiyama, H., Fujii, N. and Y.
Zhang, "A link Layer tunneling mechanism for unidirectional
links", RFC3077, March 2001.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke,
T., Yoshimura, T. and H. Zheng, "RObust Header Compression
(ROHC): Framework and four profiles: RTP, UDP ESP and
uncompressed", RFC3095, July 2001.
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
to-end Performance Implications of Slow Links", BCP 48, RFC
3150, July 2001.
[RFC3168] Ramakrishnan K., Floyd, S. and D. Black, "A Proposal to add
Explicit Congestion Notification (ECN) to IP", RFC3168,
September 2001.
[Sam99] Samaraweera, N.K.G, "Return Link Optimization for Internet
Service Provision Using DVB-S Networks", ACM Computer
Communications Review (CCR), Vol.29, No.3, 1999, pp.4-19.
[Seg00] Segura R., "Asymmetric Networking Techniques For Hybrid
Satellite Communications", NC3A, The Hague, Netherlands,
NATO Technical Note 810, August 2000, pp.32-37.
[SF98] Samaraweera, N.K.G., and G. Fairhurst. "High Speed Internet
Access using Satellite-based DVB Networks", Proc. IEEE
International Networks Conference (INC98), Plymouth, UK,
1998, pp.23-28.
[ZSC91] Zhang, L., Shenker, S., and D. D. Clark, "Observations and
Dynamics of a Congestion Control Algorithm: The Effects of
Two-Way Traffic", Proc. ACM SIGCOMM, ACM Computer
Communications Review (CCR), Vol 21, No 4, 1991, pp.133-
147.
10. IANA Considerations
There are no IANA considerations associated with this document.
Appendix - Examples of Subnetworks Exhibiting Network Path Asymmetry
This appendix provides a list of some subnetworks which are known to
experience network path asymmetry. The asymmetry in capacity of
these network paths can require mitigations to provide acceptable
overall performance. Examples include the following:
- IP service over some wide area and local area wireless networks.
In such networks, the predominant network path asymmetry arises
from the hub-and-spokes architecture of the network (e.g., a
single base station that communicates with multiple mobile
stations), this requires a Ready To Send / Clear To Send (RTS/CTS)
protocol and a Medium Access Control (MAC) protocol which needs to
accommodate the significant turn-around time for the radios. A
high per-packet transmission overhead may lead to significant
network path asymmetry.
- IP service over a forward satellite link utilizing Digital Video
Broadcast (DVB) transmission [EN97] (e.g., 38-45 Mbps), and a
slower upstream link using terrestrial network technology (e.g.,
dial-up modem, line of sight microwave, cellular radio) [CLC99].
Network path asymmetry arises from a difference in the upstream
and downstream link capacities.
- Certain military networks [KSG98] providing Internet access to
in-transit or isolated hosts [Seg00] using a high capacity
downstream satellite link (e.g., 2-3 Mbps) with a narrowband
upstream link (e.g., 2.4-9.6 kbps) using either Demand Assigned
Multiple Access (DAMA) or fixed rate satellite links. The main
factor contributing to network path asymmetry is the difference in
the upstream and downstream link capacities. Some differences
between forward and reverse paths may arise from the way in which
upstream link capacity is allocated.
- Most data over cable TV networks (e.g., DOCSIS [ITU01, DS00]),
where the analogue channels assigned for upstream communication
(i.e., in the reverse direction) are narrower and may be more
noisy than those assigned for the downstream link. As a
consequence, the upstream and downstream links differ in their
transmission rate. For example, in DOCSIS 1.0 [DS00], the
downstream transmission rate is either 27 or 52 Mbps. Upstream
transmission rates may be dynamically selected to be one of a
series of rates which range between 166 kbps to 9 Mbps. Operators
may assign multiple upstream channels per downstream channel.
Physical layer (PHY) overhead (which accompanies upstream
transmissions, but is not present in the downstream link) can also
increase the network path asymmetry. The Best Effort service,
which is typically used to carry TCP, uses a
contention/reservation MAC protocol. A cable modem (CM) sending
an isolated packet (such as a TCP ACK) on the upstream link must
contend with other CMs to request capacity from the central cable
modem termination system (CMTS). The CMTS then grants timeslots
to a CM for the upstream transmission. The CM may "piggyback"
subsequent requests onto upstream packets, avoiding contention
cycles; as a result, spacing of TCP ACKs can be dramatically
altered due to minor variations in load of the cable data network
and inter-arrival times of TCP DATA packets. Numerous other
complexities may add to, or mitigate, the asymmetry in rate and
access latency experienced by packets sent on the upstream link
relative to downstream packets in DOCSIS. The asymmetry
experienced by end hosts may also change dynamically (e.g., with
network load), and when best effort services share capacity with
services that have symmetric reserved capacity (e.g., IP telephony
over the Unsolicited Grant service) [ITU01].
- Asymmetric Digital Subscriber Line (ADSL), by definition, offers a
downstream link transmission rate that is higher than that of the
upstream link. The available rates depend upon channel quality
and system configuration. For example, one widely deployed ADSL
technology [ITU02, ANS01] operates at rates that are multiples of
32 kbps (up to 6.144 Mbps) in the downstream link, and up to 640
kbps for the upstream link. The network path asymmetry
experienced by end hosts may be further increased when best effort
services, e.g., Internet access over ADSL, share the available
upstream capacity with reserved services (e.g., constant bit rate
voice telephony).
Authors' Addresses
Hari Balakrishnan
Laboratory for Computer Science
200 Technology Square
Massachusetts Institute of Technology
Cambridge, MA 02139
USA
Phone: +1-617-253-8713
EMail: hari@lcs.mit.edu
Web: http://nms.lcs.mit.edu/~hari/
Venkata N. Padmanabhan
Microsoft Research
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1-425-705-2790
EMail: padmanab@microsoft.com
Web: http://www.research.microsoft.com/~padmanab/
Godred Fairhurst
Department of Engineering
Fraser Noble Building
University of Aberdeen
Aberdeen AB24 3UE
UK
EMail: gorry@erg.abdn.ac.uk
Web: http://www.erg.abdn.ac.uk/users/gorry
Mahesh Sooriyabandara
Department of Engineering
Fraser Noble Building
University of Aberdeen
Aberdeen AB24 3UE
UK
EMail: mahesh@erg.abdn.ac.uk
Web: http://www.erg.abdn.ac.uk/users/mahesh
Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFCEditor function is currently provided by the
Internet Society.