Internet Engineering Task Force Rohit Goyal INTERNET DRAFT Raj Jain File: draft-goyal-tcpsat-tcpatm-00.txt November, 1998 Expires: May, 1999 Optimizing TCP over Satellite ATM Networks Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet- Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). NOTE This document is not to be taken as a finished product. Some of the sections are rough and are included in order to obtain comments from the community that will benefit future iterations of this document. This is simply a step in the ongoing conversation about this document. Finally, all the authors of this draft do not necessarily agree with and/or advocate all the mechanisms outlined in this document. Abstract This document discusses techniques to improve the performance of TCP over satellite-ATM networks. ATM provides the ABR, UBR and GFR service categories for data traffic. TCP experiences poor performance over UBR due to bursty packet losses during congestion. Buffer management and guaranteed rate techniques to improve TCP performance over UBR are presented. The performance of TCP over ABR and the interaction of ABR congestion control mechanisms with TCP congestion control mechanisms are also described. This document is intended to be in informational document for the efficient transport of TCP over satellite-ATM networks. Goyal & Jain [Page 1] draft-goyal-tcpsat-tcpatm-00.txt - 2 - November, 1998 1. Introduction Several issues arise in optimizing the performance of TCP when ATM is deployed over satellite links. This document emphasizes that both TCP mechanisms as well as ATM mechanisms should be used to improve TCP performance over long-delay ATM networks. ATM technology provides at least 3 service categories for data: UBR, ABR, and GFR [ATMF]. Each of these categories can be improved by a number of mechanisms. Some examples of these categories and improvements are: - Unspecified Bit Rate (UBR) with tail drop, - UBR with intelligent buffer management, - UBR with guaranteed rate, - Available Bit Rate (ABR) with network feedback, and - ABR with virtual source/virtual destination (VS/VD). - Guaranteed Frame Rate (GFR), In addition, TCP provides several congestion control mechanisms including: - Vanilla TCP with slow start and congestion avoidance, - TCP Reno with fast retransmit and recovery, - TCP New Reno - TCP with selective acknowledgements (SACK) Satellite-ATM network designers as well as service providers must choose the optimal ATM services for efficient TCP transport. In the presence of TCP performance enhancing proxies at the edges of the satellite-ATM network, the service providers also have the ability to control TCP mechanisms so as to provide good performance. This document describes the design choices and performance analysis results of various options available to TCP over satellite-ATM networks. 2. Techniques for Improving TCP Performance over ATM It has been shown [LI96] that vanilla TCP over the UBR service category achieves low throughput and high unfairness over satellite networks. This is because during packet loss, TCP loses time waiting for its coarse granularity retransmission timeout. In the presence of bursty packet losses, fast retransmit and recovery (FRR) (without SACK) further hurts TCP performance over UBR for long delay-bandwidth product networks. This is because after two fast retransmissions, the congestion window is too small to send out new Goyal & Jain [Page 2] draft-goyal-tcpsat-tcpatm-00.txt - 3 - November, 1998 packets that trigger duplicate acks. In the absence of duplicate acks, the third lost packet is not retransmitted, and a timeout occurs at a small window. This results in congestion avoidance with a small window, which is very slow for long delay networks. There are four ways to significantly improve the TCP throughput over UBR: - frame-level discard policies, - intelligent buffer management policies, - TCP New Reno, - TCP SACK, and - guaranteed rates. Frame level discard policies such as early packet discard (EPD) improve the throughput significantly over cell-level discard policies. However, the fairness is not guaranteed unless intelligent buffer management using per-VC accounting is used [GOYAL97b]. Throughput increases further with more aggressive New Reno and SACK [SACK]. SACK gives the best performance in terms of throughput. It has been found that for long delay paths, the throughput improvement due to SACK is more than that from discard policies and buffer management [GOYAL97b]. When several TCP flows are multiplexed on to a few VCs, fairness among the TCP flows can be provided by the routers at the edges of the ATM network, while VC level fairness must be provided by the ATM network using either buffer management or per- VC queuing. The fourth method of improving the UBR performance is the so called "guaranteed rate" (GR) in which a small fraction of the bandwidth is reserved in the switches for the UBR service. This bandwidth is shared by all UBR VCs. Using guaranteed rates helps in the presence of a high load of higher priority traffic such as Constant Bit Rate (CBR) or Variable Bit Rate (VBR) traffic. It has been found that reserving just a small fraction, say 10%, of the bandwidth for UBR significantly improves TCP performance [GOYAL97a]. This is because the reserved bandwidth ensures that the flow of TCP packets and acknowledgements is continuous and prevents TCP timeouts due to temporary bandwidth starvation of UBR. Note that this mechanism is different from the GFR service category where each VC (rather than the entire UBR class) has a minimum rate guarantee. For TCP over ABR, in addition to the four methods discussed above, there are two more ways to improve the performance: - ABR with virtual source/virtual destination (VS/VD), and - ack regulation. Goyal & Jain [Page 3] draft-goyal-tcpsat-tcpatm-00.txt - 4 - November, 1998 Studies [GOYAL98] have indicated that VS/VD can be used to isolate long-delay segments from terrestrial segments. This helps in efficiently sizing buffers in routers and ATM switches. As a result, terrestrial switches only need to have buffers proportional to the bandwidth-delay products of the terrestrial segment of the TCP path. Switches connected to the satellite VS/VD loops must have buffers proportional to the satellite delay-bandwidth products. Ack regulation techniques require routers to control the flow of TCP acknowledgements based on ABR feedback [SHIV98]. These techniques can be applied in ATM edge devices or in Internet routers. These techniques can considerably improve fairness, throughput and end-to- end delay properties of TCP applications. 3. Summary In this document, we have briefly described TCP mechanisms and ATM policies to improve TCP performance over satellite-ATM networks. The TCP enhancements such as SACK and large windows require changes to the TCP stack. The ATM mechanisms such as buffer management, ABR feedback controls, guaranteed rates, and VS/VD do not require any changes to the TCP stack. 4. References [ATMF] The ATM Forum Technical Committee, "Traffic Management Specification Version 4.0," April 1996, ftp://ftp.atmforum.com/pub/approved-specs/af-tm-0056.000.pdf [LI96] H. Li, K.Y. Siu, H.T. Tzeng, C. Ikeda and H. Suzuki, "TCP over ABR and UBR Services in ATM,'' Proc. IPCCC'96, March 1996. [GOYAL97a] Rohit Goyal, Raj Jain, Shiv Kalyanaraman, Sonia Fahmy, Bobby Vandalore, Xiangrong Cai, Seong-Cheol Kim, Sastri Kota, "Guaranteed Rate for Improving TCP Performance on UBR+ over Terrestrial and Satellite Networks," ATM Forum/97-0424, April 1997, http://www.cse.wustl.edu/~jain/atmf/a97-0424.htm [GOYAL97b] Rohit Goyal, Raj Jain, Shivkumar Kalyanaraman, Sonia Fahmy, Bobby Vandalore, Sastri Kota, "TCP Selective Acknowledgments and UBR Drop Policies to Improve ATM-UBR Performance over Terrestrial and Satellite Networks", Proc. ICCCN97, Las Vegas, September 1997, pp17-27. http://www.cse.wustl.edu/~jain/papers/ic3n97.htm [GOYAL98] Rohit Goyal, Xiangrong Cai, Raj Jain, Sonia Fahmy, Bobby Vandalore " Per-VC Rate Allocation Techniques for ATM-ABR Virtual Source Virtual Destination Networks," Proceedings of Globecom'98, Goyal & Jain [Page 4] draft-goyal-tcpsat-tcpatm-00.txt - 5 - November, 1998 November 1998, http://www.cse.ohio- state.edu/~jain/papers/globecom98.htm [SACK] Sally Floyd, "Issues of TCP with SACK" Lawerence Berkeley Laboratory, Technical Report, December 1995. [SHIV98] Ramakrishna Satyavolu, Ketan Duvedi, Shivkumar Kalyanaraman, "Explicit rate control of TCP applications," ATM_Forum/98-0152R1, February 1998. Author's address Rohit Goyal Department of Computer and Information Science The Ohio State University 2015 Neil Ave, DL395 Columbus, OH 43210 Phone: +1 614-688-4482 Email: goyal@cse.wustl.edu Raj Jain Department of Computer and Information Science The Ohio State University 2015 Neil Ave, DL395 Columbus, OH 43210 Phone: +1 614-292-3989 Email: jain@cse.wustl.edu Expires: May, 1999 Goyal & Jain [Page 5]