many ARQ links may exist in series along an arbitrary Internet path
between endhosts, especially as the path taken and its links may
change over time.
In summary, when links cannot classify traffic flows and treat them
separately, low persistence is generally desirable; preserving packet
ordering is generally desirable. Extremely high persistence and
perfect persistence are generally undesirable; highly-persistent ARQ
is a bad idea unless flow classification and detailed and accurate
knowledge of flow requirements make it possible to deploy high
persistency where it will be beneficial.
There is currently insufficient experience to recommend a specific
ARQ scheme for any class of link. It is also important to realize
that link ARQ is just one method of error recovery, and that other
complementary physical-layer techniques may be used instead of, or
together with, ARQ to improve overall link throughput for IP traffic.
The choice of potential schemes includes adapting the data rate,
adapting the signal bandwidth, adapting the transmission power,
adaptive modulation, adaptive information redundancy / forward error
control, and interleaving. All of these schemes can be used to
improve the received signal energy per bit, and hence reduce error,
frame loss and resulting packet loss rates given specific channel
conditions.
There is a need for more research to more clearly identify the
importance of and trade-offs between the above issues over various
types of link and over various types of channels. It would be useful
if researchers and implementers clearly indicated the loss model,
link capacity and characteristics, link and end-to-end path delays,
details of TCP, and the number (and details) of flows sharing a link
when describing their experiences. In each case, it is recommended
that specific details of the link characteristics and mechanisms also
be considered; solutions vary with conditions.
5. Security Considerations
No security implications have been identified as directly impacting
IP traffic. However, an unreliable link service may adversely impact
some existing link-layer key management distribution protocols if
link encryption is also used over the link.
Denial-of-service attacks exploiting the behaviour of the link
protocol, e.g., using knowledge of its retransmission behaviour and
propagation delay to cause a particular form of jamming, may be
specific to an individual link scenario.
6. IANA Considerations
No assignments from the IANA are required.
7. Acknowledgements
Much of what is described here has been developed from a summary of a
subset of the discussions on the archived IETF PILC mailing list. We
thank the contributors to PILC for vigorous debate.
In particular, the authors would like to thank Spencer Dawkins, Aaron
Falk, Dan Grossman, Merkourios Karaliopoulos, Gary Kenward, Reiner
Ludwig and Jean Tourrilhes for their detailed comments.
8. References
References of the form RFCnnnn are Internet Request for Comments
(RFC) documents available online at http://www.rfc-editor.org/.
8.1 Normative References
[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC768,
August 1980.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC791,
September 1981.
[RFC793] Postel, J., "Transmission Control Protocol", RFC793,
September 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC1122, October 1989.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC2406, November 1998.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Services", RFC2475, December 1998.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC2581, April 1999.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's
Retransmission Timer", RFC2988, November 2000.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
Shelby, "Performance Enhancing Proxies Intended to
Mitigate Link-Related Degradations", RFC3135, June
2001.
[RFC3260] Grossman, D., "New Terminology and Clarifications for
Diffserv", RFC3260, April 2002.
8.2 Informative References
[BAL95] Balakrishnan, H., Seshan, S. and R. H. Katz,
"Improving Reliable Transport and Handoff Performance
in Cellular Wireless Networks", ACM MOBICOM, Berkeley,
1995.
[BAL97] Balakrishnan, H., Padmanabhan, V. N., Seshan, S. and
R. H. Katz, "A Comparison of Mechanisms for Improving
TCP Performance over Wireless Links", IEEE/ACM
Transactions on Networking, 5(6), pp. 756-759, 1997.
[BEN00] Bennett, J. C., Partridge, C. and N. Schectman, "Packet
Reordering is Not Pathological Network Behaviour",
IEEE/ACM Transactions on Networking, 7(6), pp. 789-798,
2000.
[BHA97] Bhagwat, P., Bhattacharya, P., Krishna A. and S. K.
Tripathi, "Using channel state dependent packet
scheduling to improve TCP throughput over wireless
LANs", ACM/Baltzer Wireless Networks Journal, (3)1,
1997.
[CHE00] Cheng, H. S., G. Fairhurst et al., "An Efficient
Partial Retransmission ARQ Strategy with Error Codes
by Feedback Channel", IEE Proceedings - Communications,
(147)5, pp. 263-268, 2000.
[DRAFTKARN02] Karn, P., Ed., "Advice for Internet Subnetwork
Designers", Work in Progress.
[DRAFTHAN01] Handley, M., Floyd, S. and J. Widmer, "TCP Friendly
Rate Control (TFRC): Protocol Specification", Work in
Progress.
[ECK98] Eckhardt, D. A. and P. Steenkiste, "Improving Wireless
LAN Performance via Adaptive Local Error Control",
IEEE ICNP, 1998.
[FER99] Ferrero, A., "The Eternal Ethernet", Addison-Wesley,
1999.
[ISO4335a] HDLC Procedures: Specification for Consolidation of
Elements of Procedures, ISO 4335 and AD/1,
International Standardization Organization, 1985.
[ISO4335b] HDLC Procedures: Elements of Procedures, Amendment 4:
Multi-Selective Reject Option, ISO 4335/4,
International Standards Organization, 1991.
[ISO7776] Specification for X.25 LAPB-Compatible DTE Data Link
Procedures, ISO 4335/4, International Standards
Organization, 1985.
[KEN87] Kent, C. A. and J. C. Mogul, "Fragmentation
Considered Harmful", Proceedings of ACM SIGCOMM 1987,
ACM Computer Communications Review, 17(5), pp. 390-401,
1987.
[LIN93] Lin, S. and D. Costello, "Error Control Coding:
Fundamentals and Applications", Prentice Hall, 1993.
[LUD99a] Ludwig, R., Rathonyi, B., Konrad, A., Oden, K., and A.
Joseph, "Multi-Layer Tracing of TCP over a Reliable
Wireless Link", ACM SIGMETRICS, pp. 144-154, 1999.
[LUD99b] Ludwig, R., Konrad, A., Joseph, A. and R. H. Katz,
"Optimizing the End-to-End Performance of Reliable
Flows over Wireless Links", ACM MobiCOM, 1999.
[MEY99] Meyer, M., "TCP Performance over GPRS", IEEE Wireless
Communications and Networking Conference, 1999.
[PAR00] Parsa, C. and J. J. Garcia-Luna-Aceves, "Improving TCP
Performance over Wireless Networks at the Link Layer",
ACM Mobile Networks and Applications Journal, (5)1,
pp. 57-71, 2000.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC
1191, November 1990.
[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions
for High Performance", RFC1323, May 1992.
[RFC1350] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,
RFC1350, July 1992.
[RFC1435] Knowles, S., "IESG Advice from Experience with Path MTU
Discovery", RFC1435, March 1993.
[RFC1981] McCann, J., Deering, S. and J. Mogul, "Path MTU
Discovery for IP version 6", RFC1981, August 1996.
[RFC2488] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP
Over Satellite Channels using Standard Mechanisms",
BCP 28, RFC2488, January 1999.
[RFC2757] Montenegro, G., Dawkins, S., Kojo, M., Magret V. and
N. Vaidya, "Long Thin Networks", RFC2757, January
2000.
[RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J.,
Tran, D., Henderson, T., Heidemann, J., Touch, J.,
Kruse, H., Ostermann, S., Scott K. and J. Semke
"Ongoing TCP Research Related to Satellites",
RFC2760, February 2000.
[RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L. and V. Paxson, "Stream Control Transmission
Protocol", RFC2960, October 2000.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC3022,
January 2001.
[RFC3155] Dawkins, S., Montenegro, G., Kojo, M., Magret, V. and
N. Vaidya, "End-to-end Performance Implications of
Links with Errors", BCP 50, RFC3155, August 2001.
[SALT81] Saltzer, J. H., Reed, D. P. and D. Clark, "End-to-End
Arguments in System Design", Second International
Conference on Distributed Computing Systems, pp.
509-512, 1981. Published with minor changes in ACM
Transactions in Computer Systems (2)4, pp. 277-288,
1984.
[SAM96] Samaraweera, N. and G. Fairhurst, "Robust Data Link
Protocols for Connection-less Service over Satellite
Links", International Journal of Satellite
Communications, 14(5), pp. 427-437, 1996.
[SAM98] Samaraweera, N. and G. Fairhurst, "Reinforcement of
TCP/IP Error Recovery for Wireless Communications",
ACM Computer Communications Review, 28(2), pp. 30-38,
1998.
[STE94] Stevens, W. R., "TCP/IP Illustrated, Volume 1",
Addison-Wesley, 1994.
[STONE00] Stone, J. and C. Partridge, "When the CRC and TCP
Checksum Disagree", Proceedings of SIGCOMM 2000, ACM
Computer Communications Review 30(4), pp. 309-321,
September 2000.
[WARD95] Ward, C., et al., "A Data Link Control Protocol for LEO
Satellite Networks Providing a Reliable Datagram
Service", IEEE/ACM Transactions on Networking, 3(1),
1995.
Authors' Addresses
Godred Fairhurst
Department of Engineering
University of Aberdeen
Aberdeen AB24 3UE
United Kingdom
EMail: gorry@erg.abdn.ac.uk
http://www.erg.abdn.ac.uk/users/gorry/
Lloyd Wood
Cisco Systems Ltd
4 The Square
Stockley Park
Uxbridge UB11 1BY
United Kingdom
EMail: lwood@cisco.com
http://www.ee.surrey.ac.uk/Personal/L.Wood/
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