HIGHER LAYER PROTOCOLS (TCP/IP) OVER SATELLITES1

TCP/IP

Transport control protocol/Internet protocol (TCP/IP) is the protocol suite on which the Internet is based. TCP/IP is now very widely deployed. However, it was developed without taking into consideration its performance over very high speed (fiber optic) links or long-delay (satellite) links, with the result that efforts are now underway to remedy some of the shortcomings that are evident on links that have large bandwidth-delay product. At the present time, considerable low bit rate TCP/IP commercial traffic is being carried over GEO satellites. With suitable TCP/IP performance enhancements, data rates in excess of 500 Mbps have been demonstrated over GEO located satellites."

Internet Protocol

IP is a network layer protocol whose function is to permit data traffic to flow seamlessly between different types of transport mechanisms (Ethernet, ATM, Frame Relay, etc). IP resides in the terminal devices and in routers which function as switches in the network, routing datagrams (packets) towards their destination based on an address field contained in the datagram. Figure 4.21 shows the protocol stack for a network operating on TCP/IP.

The routers in the network are required to translate between different addressing schemes. For example, local area networks operating on the IEEE 802 LAN standard address attached devices with 16 or 48-bit binary addresses. An X.25 public packet-switching network, on the other hand, uses 12-digit decimal addresses. IP provides a global addressing scheme and a directory service. The current version (Ipv4) of IP has address space limitations that threaten to inhibit the growth of the Internet, with the result that a new version (Ipv6) is under development (Stallings 1997).



Fig. 4.21. The protocol stack for a network running on TCP/IP (Stallings 1997).

Routers are also required to handle differences in the size of packets that can be carried on different networks. X.25 networks commonly operate with packets having a maximum size of 1,000 bytes-in contrast to Ethernet, which permits packets of 1,500 bytes. To overcome these differences datagrams may have to be broken into smaller packets (this is known as fragmentation) and reassembled when they reach their destination.

The IP protocol does not guarantee delivery, or that packets will arrive in the proper sequence. (Packets can get out of order since they may follow different paths through the network, thereby encountering different amounts of delay.) Packets can fail to be delivered for several reasons. If the network becomes congested one or more routers may become overloaded and their buffers may begin to overflow. Rather than simply discarding all newly arriving packets, the routers are programmed discard packets in a random fashion to prevent buffer overflow. This is best implemented in a "fair" way so that the data stream having the largest volume suffers the largest number of dropped packets. The links in the network are not error free so that it is possible for a packet's address to become corrupted making the packet undeliverable. Again it must be discarded lest the network become clogged with undeliverable traffic. In sum, IP is engineered to make a best effort to deliver a message but does not guarantee to do so.

Transport Control Protocol (TCP)

It is the function of the TCP protocol residing in the end devices (computers)-see Figure 4.21-to ensure the proper delivery of a complete message. TCP achieves this by assigning each byte of information a unique sequence number. The receiver keeps track of these sequence numbers and sends acknowledgements (ACKS) to indicate that it has received each datagram up to a particular byte number.

Window Size

A problem for links via geostationary satellites that involve a response time of almost 0.5 seconds is that TCP will not allow for more data to be sent beyond a certain "window" size before receiving an acknowledgement. This is currently set at 64 kB and is limited by the fact that only 16 bits are available in the header to describe the packet size. This limits the throughput to 216 bytes divided by the response time (round trip delay) of the circuit. For a GEO path with a round trip delay of 600 msec this corresponds to approximately 840 kbps (Partridge and Shepard 1997).

Figure 4.22 shows the effect of the round trip time (RTT) on throughput as a function of window size. It can be seen that a long terrestrial fiber link with a 200 msec round trip time would be limited to 2.6 Mbps by the maximum window size of 64 kbytes. Because TCP resides in the users' computers the only way to "spoof" it is to place at the forwarding earth station a terminal device that acknowledges receipt of data segments as if it were the distant receiver. However this has its own drawbacks as discussed below.



Fig. 4.22. Maximum throughput for a single TCP connection as
a function of window size and round trip time (RTT).

The Internet Engineering Task Force (IETF) has been at work recommending changes to TCP/IP to overcome this and other limitations inherent in the current design. An increase in the size of the window to 230 bytes is proposed (RFC 1323) which would raise the throughput over a GEO satellite link to about 15 Gbps. Since even at Q/V-band, satellite frequency assignments are likely to have no more than 3 GHz bandwidth, this will probably not impose any limitation for the foreseeable future.

Selective Acknowledgement

TCP ensures the complete delivery of data over a link by retransmitting anything for which it does not receive an acknowledgement. That is, it retransmits everything that was sent since the last acknowledged datagram. This "ARG" scheme is clearly inefficient in a situation where many bytes in a packet were received correctly and only one or two arrived corrupted. In these situations it is preferable to retransmit only the corrupted information i.e., perform a selective acknowledgement. TCP has yet to be modified and widely deployed with this capability. It is, however, possible to implement this capability on satellite links by introducing suitable interface units at the earth stations at each end of the link.

A relatively straightforward modification to TCP that goes some way towards remedying its current shortcomings has been approved by the IETF. This permits the acknowledgement of datagrams received correctly, but out of order. This new feature has been termed selective acknowledgement (SACK).

Congestion Control

TCP employs two strategies for avoiding or mitigating congestion in the network. The first scheme is a "slow start" mechanism. Under this protocol a transmission commences with the sending of a single segment (datagram) of information. The size of a segment is negotiated between sender and receiver at the start of transmission, and may be limited by features of the network, but typically might be 1,000 bytes. Once this is acknowledged two segments are sent, then four, eight, etc. This exponential growth continues until limited by the maximum window size. This slow start algorithm will cause the throughput on long delay links to rarely reach its maximum. It is particularly troublesome when transmitting Web pages formatted by HTTP, since TCP treats each item in the image as requiring a separate transmission sequence. An IETF proposal to reduce the impact of the slow start protocol is to commence by sending four segments.

In the event the network becomes congested and a packet is dropped (or lost due to error) the sender will fail to receive an acknowledgement. Regardless of the cause, the sender is obliged to assume that the problem is congestion and institute a congestion control algorithm. This requires that the sending rate be immediately reduced to about half, and then increased only gradually (by one segment at a time). This introduces a linear increase as opposed to the initial exponential increase. On long delay circuits the consequences of this congestion algorithm are particularly severe since it now takes an inordinately long time to reach maximum throughput. This is also potentially very severe for satellite circuits with their higher error rates, since any loss is interpreted as being caused by congestion. The best means of avoiding this error loss problem appears to be to operate the link with sufficient (concatenated) coding to ensure very low BER. Commercial devices such as COMSAT Link Accelerator for IP, CLA-2000/IP, address this problem in a similar manner as described before for ASIU. CLA-2000/IP invokes link error dependent amount of Reed Solomon outer coding, resulting in a very low TCP packet error ratio. This dynamic adaptive coding method, coupled with the data compression, improves considerably the throughput of applications such as file transfer protocol (FTP) running over TCP, as illustrated in Figure 4.23.



Fig. 4.23. Improvement in the throughput of data over a satellite link as a Function of bit error rate with and without a link conditioning unit (ASIU). The example shown here is for a commercial unit (COMSAT Link Accelerator).

Current Research

Current research involving TCP over satellite channels is generally focused in two areas. The first area is the slow start algorithm. By beginning slow start by sending more than 1 segment, transfer time can be reduced by several round trip times. This change has been shown to be effective in the satellite environment. This change is being proposed in the IETF by researchers from NASA Lewis Research Center, Lawrence Berkeley Laboratory and BBN. In addition, researchers at NASA Lewis Research Center and Ohio University are further investigating the impact of this proposed change.

In addition to starting with a larger number of segments, NASA Lewis Research Center is investigating alternate methods for generating and utilizing acknowledgments that will provide more rapid speedup during slow start. This will be especially useful in the long-delay satellite environment, but should benefit all networks including terrestrial networks.

The second broad area for study is loss recovery. Traditionally TCP has used the lack of an acknowledgment from the receiver to indicate a segment was dropped. However, with the recent introduction of a selective acknowledgment option for TCP, the TCP sender is able to better manage which segments are retransmitted, as it has more information. With SACK, instead of the receiver returning the highest in-order segment received, it informs the sender about all the segments it has received (and therefore, all the segments that have not been received). This allows the sender to implement network-friendly retransmission. In addition, since the sender knows much more about the state of the network it can safely determine when it is appropriate to inject new segments into the network during recovery. This allows better utilization of the network and therefore better performance.

IETF TCP Over Satellite Working Group

The TCPSAT Working Group of IETF is chartered to produce two documents (Glover, Allman) for those working with and studying satellite networks. The first document outlines the current standard mechanisms that can improve the bandwidth utilization of TCP over satellite channels. The second document outlines areas currently being researched and areas for future study. The mechanisms in the second document may be useful for private satellite networks, but have not yet been judged to be safe for use in a shared network such as the Internet.

Interoperability with Terrestrial Networks

It has become increasingly evident that the full potential of the emerging national and global information infrastructure (NII/GII) depends on the inclusion of satellite networks in telecommunication networks. Full inclusion requires achievement of seamless interoperability of satellite networks with terrestrial networks as both evolve to provide end-to-end services.

This panel observed that the interoperability issues are being addressed worldwide on several fronts. Standards, of course, are key to interoperability. The development of applications that are based on integrated networks also plays a role. This section highlights the research activity that is being conducted to address interoperability issues worldwide.

In Europe, the activity tends to fall into the service-driven category. Major North American activity tends to be technology-driven, while Asian activities are more evenly divided, although there are exceptions and changes.

In order to integrate satellites into the emerging global information infrastructure, both technology-driven and service-driven programs are necessary. Neither one is necessarily superior to the other; in fact, in either case, success is dependent upon implementation.

Europe's Experimental Programs

The European ACTS Program

Advanced Communications Technology and Services, known simply as ACTS, is one of the specific programs of the Fourth Framework Program of European Union (EU) activities in the field of research and technological development and demonstration (1994 to 1998). In fact, it is the focus of the EU's research effort to accelerate deployment of advanced communications infrastructures and services, and is complemented by extensive European research in the related fields of information technology and telematics. ACTS research strongly complements a broad range of EU policy initiatives, examples of which include:

ACTS builds on the work of the earlier RACE programs (Research and Development in Advanced Communications Technologies for Europe, 1985-1995), which were established to help introduce Integrated Broadband Communications (IBC), taking into account the evolving ISDN and national introduction strategies, and to bring about EU-wide services by 1995. Independent assessments have confirmed that RACE broadly achieved this objective, and that such technologies are beginning to be deployed in European countries either in specialized scientific networks or, in a few cases, as limited public services.

European Space Agency (ESA)

Like NASA, ESA is heavily involved in satellite-terrestrial interoperability R&D. ESA provided the following material from a presentation given at an ATM workshop in Paris in July of 1996.

ESA Research and Development Activities on ATM

Interconnection of ATM LANS:

System studies (CSEM, Spar) :

Technology studies (Spar) :

Results of ESA developments:

Status of the Project:

VANTAGE - VSAT ATM Network Trials for Applications Groups Across Europe

VANTAGE will unite the service flexibility of ATM and the access flexibility of satellites to provide a pan-European interconnection service. This project will include trials with a variety of earth station technologies and access techniques to interconnect isolated users, to integrate National Host networks out to remote users, and to show that ATM's flexibility in the management of signaling allows satellites to be seamlessly embedded in the terrestrial network.

VANTAGE will implement a novel architecture using a conventional transparent satellite, with its earth stations, as a distributed ATM switch. This project will be the first time such an innovative architecture has been used.

A series of three major trials, each of several weeks duration, will be mounted, with at least four simultaneous sites provided from project resources. User access will be by standard interfaces (from a few bits to many megabits per second), and total capacity will be increased through the project to over 20Mbps.

Key issues. By using this approach VANTAGE will allow "switch-in-the-sky" capabilities to be provided and operationally tested while avoiding the need for high cost, high risk, specialist satellite development or major terrestrial infrastructure.

Relationship to previous work. The VANTAGE project will elaborate on concepts developed and techniques explored and evaluated in the former RACE program.

Expected Achievements. VANTAGE will provide a service platform allowing a variety of other projects to develop and evaluate their applications and technologies. For some projects this platform may provide the only interconnection possibility. VANTAGE will provide a unique opportunity for empirical exploration of the capabilities, strengths, weakness, and impact of the IBC network in advance of its wide terrestrial deployment. VANTAGE will demonstrate the capability of European industry to develop, and of European government to coordinate, world-leading equipment, systems and services.

To the extent that the system is ultimately exploited, VANTAGE provides the opportunity for Europe to establish world leadership with a "first" operational system setting de facto international standards.

Expected Impact. VANTAGE will provide the enabling technology to extend a variety of (many novel) applications and services beyond their current urban limits.

Relationship to other Projects. In addition to testing applications from within the project, VANTAGE will also offer a connection service to applications from other ACTS projects and to national hosts.

Japan's Experimental Programs

Communications Research Laboratory (CRL)

Although not specifically targeted at SDH, WTEC panelists were informed by the CRL project managers that one of the key issues to be addressed in CRL's program is the ability of current and next-generation satellites to interoperate with international, SDH-based, undersea fiber optic cable systems. Note that this project includes both technology-driven and service-driven research thrusts. Because of its importance to SDH-satellite interoperability, however, we include it only in this section.

The laboratory has two principal goals: first, the international interconnection of ultra high-speed telecommunication networks. This is expected to be achieved by the installation of an experimental facility on the premises of CRL, which would have state-of-the art equipment. The second goal is research and development to enhance international multimedia applications. Again, cutting edge equipment is being installed so that the research can lead to the development of international standards for applications, such as international transactions.

Project Objectives. One objective is to improve the telecommunications network infrastructure for Japanese corporations for communication between these corporations and also for communicating with overseas corporations. This is expected to promote the transformation of the Japanese economic structure by developing new markets, growth in sophistication of existing operations, and an increase in import volumes. This will relieve the negative impact of the appreciation of the yen through expansion of domestic demand for goods and services.

Another objective is to provide leadership for G7 international joint projects in order to contribute to the international standardization in terms of interconnectivity and interoperability of broadband network applications. Additional objectives are as follows:

  1. Promotion of standardization
  2. Promotion of efforts to construct global broadband network and relevant applications development
  3. Promotion of various new businesses utilizing broadband network technology.

Asia-Pacific Information Infrastructure Testbed

Kansai Advanced Research Center was established in May 1989 as a major facility to perform basic research under CRL. The center performs basic research in the areas of information science and technology, material science and laser technology and biological information science with the objective of developing advanced technology of the future in the areas of information processing and communications. Also near its facility, the Asia Pacific Information Infrastructure (APII) Technology Center has been established. Details are provided below.

Asia Pacific Information Infrastructure (APII) Technology Center. With the aim of achieving a leading role in the establishment of the information infrastructure in the Asia-Pacific region, the APII Technology Center has been constructed in Kobe City.

To respond appropriately to the globalization of information communications, there is a need to establish an information infrastructure that crosses national borders while giving full consideration to the social and cultural diversity as well as different levels of economic and technological development in the Asia-Pacific region.

International joint research and experiments on a multimedia information network are being planned. The final goal is to conduct remote joint research and distance learning activities and to establish capabilities in telemedicine, teleshopping, and an electronic museum. A related goal is to promote the use of this network, and to create an environment in which as many countries in the Asia-Pacific region as possible can participate.

The APII Technology Center offers group technical training courses on multimedia information and communications technology to the engineers from the Asia-Pacific region. The technical training program consists of trends in multimedia information technology, internet utilization technology, video on demand (VOD), application techniques and cyber space application techniques.

The APII Technology Center has been built by the Ministry of Posts and Telecommunications. This facility uses various systems, including Internet, "cyberspace" and VOD, which are offered through an ATM-LAN. The facility carries out joint development of applications and network interconnection technologies, and trains technicians.

At the APEC informal meeting of heads of government held in Manila in 1996, Prime Minister Hashimoto called for, among other things, various kinds of experiments and training to be undertaken that would use this facility as a test-bed, with the goal of becoming a nucleus in the Asian-Pacific information and communications infrastructure (APII).

In addition to the ATM-LAN, which has been installed as a local area network facility, the center is connected to external experimental facilities via N-ISDN and high speed private lines. At the time of the WTEC visit, this facility was connected to external networks with six N-ISDN lines, and was connected to Tokyo via a 45 Mbps high speed private line.

Canada's Experimental Programs

Communications Research Centre (CRC) is the Canadian government's leading communications research facility. It is responsible for conducting leading edge R&D to develop the Canadian communications infrastructure. Its key objective is to support Canadian telecommunications firms in their efforts to remain globally competitive.

Broadband Applications and Demonstration Laboratory

The WTEC team was given a tour of the BADLAB, which is designed to demonstrate and test "Information Highway" applications using ATM fiber optic networks, with network extension via satellite and wireless. The staff has performed several experiments to investigate satellite/terrestrial interoperability based on ATM architecture.

BADLAB is Canada's ATM gateway to high speed communications networks around the world. It is a major node on the CANARIE National Test Network and an active partner in the Ottawa Carleton Research Institute Network Inc. (OCRInet). BADLAB is also connected to Europe through Teleglobe Canada's CANTAT-3 transatlantic fiber optic cable, and will use satellites to connect to Japan. The lab is actively working with its European partners on broadband interoperability and applications trials.

BADLAB is collaborating with the Government Telecommunications and Informatics Services (GTIS) to explore broadband service options for a range of government clients across Canada. BADLAB is connected to GTIS through a 155 Mbps line. The lab is currently connected to OCRInet through two 45 Mbps links, with the capacity to upgrade to 155 Mbps. The objective of this lab is to test and demonstrate various ATM networks such as OCRInet, Rnet, Wnet, LARG*net and others as they come on line, making use of BADLAB's satellite link capability for network extension and to make the lab available to industry, especially small and medium-sized high technology R&D companies across Canada, to develop applications that may be of commercial value.

The broadband applications experiments are being conducted in the following areas:

U.S. Experimental Programs

In mid-1995 the U.S. satellite industry responded to the call for a NII/GII initiative. In a White House briefing to Vice President Gore, the Satellite Industry Task Force (SITF) brought attention to five major areas of concern that would affect the ability of satellites to play an effective role in the NII/GII:

Subsequently, the U.S. satellite industry organized a group under the auspices of the Telecommunications Industry Association (TIA) to deal with these issues. Using TIA as a forum, the satellite industry has now successfully begun to address the standards, protocols and interoperability issues. At its inception, the Satellite Communications Division (SCD) of the TIA created two subgroups, TR34.1 and TR34.2, to address technical issues. It delegated to the TR34.1 subgroup those technical matters associated with protocol and interoperability issues. Spectrum and orbital utilization issues were delegated to the TR34.2 subgroup.

To date, both TR34.1 and TR34.2 have made progress in assuring that satellite systems can be seamlessly and transparently integrated into the NII/GII. They continue to be effective, working both internally and through the various standards-making bodies, in influencing the development of standards and regulatory matters.

Also, satellite interoperability experiments are being conducted. These experiments are driven by either application and service programs or by standards and technology programs. The experiments are listed in Table 4.6.

ATM Protocol

The TR34.1 subgroup thus far has focused on several ATM protocol related issues affecting satellite networks. These include ATM speech, ATM quality of service considerations, wireless ATM considerations, and ATM traffic management considerations.

In the area of ATM speech, TR34.1 has been successful in promoting satellite-friendly specifications for the ATM adaptation layer protocols through the ITU-T SG13 and through the ATM Forum. Similarly, TR34.1 has been successful in working with the ATM Forum to develop a Network Architectures and Requirements Document, which establishes the key interoperability specifications for both fixed and mobile satellite networks, including systems employing onboard ATM switches. Also included in that document are the radio access layer specifications for satellite ATM networks for a variety of network scenarios. The details on these proposed standards are available through the COMSAT website (http://ww.comsat.com). Additional work is needed to establish satellite-friendly specifications for packet error rates; packet loss rates; packet delay and delay variation specifications for AAL2; and for speech compression protocols, congestion control procedures; the multiplexing of voice, video, and data; and echo cancellation.

Table 4.6
Satellite ATM Projects

Project Name

Development Team

Transponder

Access Method

Speed

Vantage

BT Telecom

Ku-band

Point-to-Point

2 Mbps

QoS over an IDR Link

AT&T, KDD (Japan), Tetstra (Australia)

Ku-band

Point-to-Point

44.736 Mbps

ATM Satellite Experiments

AFRL (U.S.)

CRC (Canada)

NCCOSC (U.S.)

Ku-band

Point-to-Point

Multicast

44.736 Mbps

RACE

Eutelsat

Alcatal

Ku-band

Point-to-Point

34 Mbps

ACTS

NASA Lewis Research Center

Ka-band

microwave

switch matrix

TDMA

622 Mbps

Bandwidth on Demand

COMSAT

Interstate

Ku-band

MF-TDMA

2-8 Mbps

Teledesic* Broadband/Global/Net

Teledesic

Ka-band

LEO Onboard Processing

MF-TDMA

16 kbps - 2 Mbps




* 1st of many proposed systems

In the area of ATM traffic management, Raj Jain (Ohio State University), working under the sponsorship of NASA Lewis Research Center, is exploring the performance characteristics of new ATM networking concepts. These include simulations of available-bit-rate (ABR) and unspecified-bit-rate (UBR) traffic flows versus constant-bit-rate (CBR) traffic flows. Additionally, Jain is investigating the performance characteristics of guaranteed-frame-rate (GFR) networking concepts, point-to-multipoint service concepts, and multipoint-to-point service concepts for high latency satellite links. He is working closely with the ATM Forum and the Internet Engineering Task Force (IETF) in the development of these concepts. His work is available for review on the OSU website: http://www.cis.ohio-state.edu/~jain.

In the area of ATM quality of service, NASA Lewis Research Center (LeRC) has conducted a series of experiments to characterize the effects of link bit error rates and errored cell losses on video picture quality. The objective of this work is to ensure that the requirements for ATM video standards are not overly stringent, thereby needlessly burdening satellite network interoperability requirements. The results of this work have been presented to the ATM Forum.

TCP/IP Protocol for Satellites

The performance issues of TCP/IP over high-latency links are being investigated by the Internet Engineering Task Force (IETF), by TR34.1 through its Internet Protocol over Satellite (IPoS) group, and by Craig Partridge (BBN and the IETF) under a grant from NASA LeRC. The majority opinion has been that there must be a fundamental change to the protocol to make it work efficiently over high-latency satellite links. The minority opinion, promoted primarily by van Jacobson and others in the IETF, is that globally modifying the installed base of TCP applications it is not only impractical, but that it is totally unnecessary. Instead, they feel it should be possible to correct the problem by changing the slow-start and ack/nak processes that reside only in the earth stations themselves. Simulation studies need to be done to confirm or disprove that conjecture.

The Advanced Communication Technology Satellite (ACTS) program at NASA Lewis Research Center has demonstrated that it is possible to create an all-digital Ka-band system that can overcome rain fade. The ACTS is a TDMA-based system that uses spot beam (or multibeam) technology, on-board storage and processing, and all-digital transmission. ACTS can transfer TCP/IP data at OC-12 speeds (622 megabits per second). Additional information can be obtained at http://acts.lerc.nasa.gov.

Common Air Interface for Communication Satellites

One of the most significant outcomes of the organization of the satellite industry under the TIA has been the cooperative efforts that have been mounted within the industry to standardize the air interface for mobile communications. This activity, which is being managed by TR34.1, was initiated jointly by Hughes and Ericsson. The objective is to reduce the cost of satellite transceivers through economies of scale. To date, the group has agreed on a top-down approach in developing the standard, and is in the process of drafting a specification. The draft is available for review on the COMSAT website (http://www.comsat.com).

Hybrid Access Services

Currently, a number of standards bodies are in the early stages of developing hybrid service concepts that would provide IP based access to traditional telephony services, and conversely, circuit-switched access to IP based content. Work is on-going in the ITU-T under SG13 and SG16, in ETSI under project TIPHON, in the IETF under working groups MUSIC, AVT, and PINT, and in the Voice-Over-IP Forum of the Multimedia Teleconferencing Consortium (IMTC).


1Prakash Chitre of COMSAT Laboratories contributed to this section.

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Published: December 1998; WTEC Hyper-Librarian