User`s Manual for the SCPS Transport Protocol MITRE
Contents
1. int cc bytes_to_send tp_prev_bytes int push 0 int errno Default buffer size for initiator static int tp_write_cnt 0 scps_init ifdef SCPS_RATECONTROL int rate_control SCPS_RATECONTROL scps_setsockopt route_sock SCPS_ROUTE SCPS_RATE amp rate_control sizeof rate_control endif SCPS_RATECONTROL if tp_sock scps_socket AF_INET SOCK_STREAM 0 lt 0 printf Error Failed to create a tp_socket n 43 exit 1 Bind to a local port hisaddress sin_port htons config_tp_init_port bzero amp hisaddress sin_addr sizeof u_long scps_bind tp_sock struct sockaddr amp hisaddress sizeof hisaddress ifdef SCPS_SOCKBUFSIZI GI int sockbufsize SCPS_SOCKBUFSIZI Gl scps_setsockopt tp_sock SCPS_SOCKET SO_RCVBUF amp sockbufsize sizeof sockbufsize scps_setsockopt tp_sock SCPS_SOCKET SO_SNDBUF amp sockbufsize sizeof sockbufsize GI endif SCPS_SOCKBUFSIZ ifdef SCPS_COMPRESS scps_setsockopt tp_sock PROTO_SCPSTP SCPSTP_COMPRESS amp one sizeof one endif SCPS_COMPRESS E ifdef ACKDELAY int ackdelay SCPS_ACK_DELAY scps_setsockopt tp_sock PROTO_SCPSTP SCPSTP_ACKDELAY amp ackdelay sizeof ackdelay scps_setsockopt tp_sock PROTO_SCPSTP SCPSTP_ACKFLOOR amp ackdelay sizeof ackdelay2. RACY E PRODUCT Di ANY GEN F RAL D F NTAL EXEMPLA SPECIAL DAMAGES EV EN IF MITR POSSIBILITY OF SUCH You accept this sof fy and hold Trustees officers all liability or da attorneys indemni fees co and expenses arisi irrespective of the for liability arisi infringements control restriction DAMAGES tware on t harmless M agents an mages to t ULE COSES ng our of cause of ng from cl Ss F Di ITR de hird F ur and your said aims The export from the United States or reexport of this software is subject with United States export control and You agree that seek to export this software you assu licenses and approvals and for assuri HAS BE mployees F N ADVI he condition tha its Board parties RY OR SED OF TH E t you of from any and including other related costs use of the liability of US paten the sub to comp in the me full with applicable reexport restrictions Product except sequent liance munitions event you responsibility for obtaining all necessary export ng compliance HR HR 22 2 2 2 2 2 2 2 2 2 22 2 2 2 2 2 2 2 2 2 2 2 22 RK KK RK KK SCPSTP initiator 41 Opens a SCPSTP socket and source data 7 include lt stdio h gt
3. include lt ctype h gt include lt sys types h gt include lt sys socket h gt include lt netinet in h gt include scps h void scps_init void void initiator_application void To turn on any of these SCPS options remove the comment blocks from that particular SCPS define statement Ay define SCPS_ACKDELAY 20 Sets the acknowledgment timer for SCPS TP to 20 milliseconds define SCPS_SOCKBUFSIZI GI 65536 Sets the maximum send and receive buffer to 65536 bytes define SCPS_COMPRESS 1 Enables SCPS Compression define SCPS_RATECONTROL 1000000 Sets SCPS TP rate control to 1 million packets per second extern int errno extern int optind extern char optarg int config_tp_pkt_count 10 int config_tp_pkt_size 1000 unsigned short config_tp_init_port 0x8000 unsigned short config_tp_resp_port Ox8fff unsigned int mtu_val 42 struct sockaddr_in t tp_sock hisaddress in Di T define MAX_BUF_SIZE 65536 byte outbuf MAX_BUF_SIZE byte inbuf MAX_BUF_SIZE un F Di F Di unsigned long buffsize 32768 int optlen char peer_host int one 1 truct timeval un select_time tatic int total_in_data tatic tp_total_bytes nt sched_counter 0 0 nt options ongword host s s i i 1 i nt select_val 0 void initiator_application
4. sets scps_errno to indicate the error and the descriptor EINVAL accept called with no socket descriptors identified When selecting on a socket using SCPS TP with BETS enabled the application there is no need to call 31 the creation of a BETS hole will cause select to an EBETS error message Since the creation of a BETS hole also indicates that there is new data available for consumption by select return with again prior to attempting to perform a read recv operation finclude lt scps h gt int scps_getsockopt int sockid int level int optname void optval int optlen scps_setsockopt int sockid int level int optname void optval int optlen PARAMETERS sockfd The sockfd of a currently valid socket level SOL_SOCKET SCPSPPROTO_TP SCPS_ROUTE optname dependent on level RETURN VALUES O on success 1 on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor EFAULT address length is invalid ENOPROTOOPT The option is unknown at the level indicated EPROTONOSUPPORT Attempt made to perform SCPSTP operations on another type of socket EOPNOTSUPP Operation not supported ESOCKOUTSTATE Connection in incorrect state for this operation ENOBETS BETS not supported on this connection ENOMEM Unable to build a list of current BETS holes 32
5. SCPSTP_TIMESTAMP Enables or disables use of TIMESTAMP on a TP connection SCPSTP_COMPRESS Enables or disables use of header compression on a TP connection SCPSTP_SNACK Enables or disables use of Selective Negative Acks ona TP connection SCPSTP_BETS Enables or disables use of BETS on a TP connection SCPSTP_CONGEST Enables or disables use of congestion control on a TP connection Disabling of congestion control on a specific transport connection should only be done in managed network environments SCPSTP_VJ_CONGEST Enables or disables the use of the TCP standard congestion control algorithm Van Jacobson Congestion Control on a TP connection SCPSTP_CONGEST must be enabled for this to have an effect SCPSTP_VEGAS_CONGEST Enables or disables the use of TCP Vegas congestion control on a TP connection SCPSTP_CONGEST must be enabled for this to have an effect 35 Route specific options LEVEL SCPS_ROUTE SCPS allows management agents or applications running with sufficient privilege to change the certain default values on a route specific basis SCPS_RATE Sets reads the global routing SCPS_MTU Sets reads the global routing SCPS_RTT Sets reads the connections rate control setting in bps for the socket maximum MTU size in bytes for the socket initial RTT estimate used by TP 36 Section 4 Suggested Reading This section presents some suggested re
6. The value of result provides a boolean indication as to whether header compression is enabled on this socket Similarly the setsockopt call can be used to enable or disable the header compression capability on a particular socket scps_setsockopt socket SCPSPROTO_TP OPT_COMP amp result sizeof result 2 3 4 Rate Control SCPS TP provides a capability for imposing a maximum transmission rate on a connection The rate is configured on per route basis all connections sharing a route also share the aggregate allocated rate The rate available on a routing socket can be queried using the scps_getsockopt call scps_getsockopt route_socket SCPS_ROUTE SCPS_RATE amp rate sizeof rate returns the current rate allocated in bits per second Applications with sufficient runtime authority may change the maximum transmission rate for a routing socket by using the scps_setsockopt call rate 1000000 scps_setsockopt route_socket SCPS_ROUTE SCPS_RATE amp rate sizeof rate 16 changes the maximum transmission rate to 1Mbps 2 3 5 Acknowledgment Frequency Reduction An application may query and change the acknowledgment frequency on a socket basis To query the current acknowledgment frequency the application must make use of scps_getsockopt The following call will return the current delay between acknowledgments in milliseconds in the variable del scps_getsockopt socket SCPSSOCK_TP SCPSTP_ACKDELAY amp del si
7. SCPS introduces a number of new options that are not present in the standard Berkeley socket interface These are grouped according to operations that are e socket specific level SOL_SOCKET e transport connection specific level SCPSPROTO_TP e route specific level SCPS_ROUTE Socket specific option handling level SOL_SOCKET SCPS enhancements to the SOL_SOCKET level socket option processing are for the querying of BETS holes in the transmission stream both on the sending and receiving sides of a connection SO_BETS_RHOLE_SIZE Retrieves the size of a signaled BETS hole at the receiver SO_BETS_RHOLE_START Retrieves the relative sequence number of the start of the signaled BETS hole SO_BETS_NUM_SEND_HOLES Retrieves the number of BETS holes currently detected at the sender sender has experienced one or more BETS timeouts on its data stream SO_BETS_SEND_HOLES Retrieves the first 50 BETS holes currently detected at the sender and then resets the counter SEND_HOLE counter to zero Connection specific socket option handling LEVEL SCPSPROTO_TP SCPS TP provides the application using the transport protocol much more flexibility in the configuration of connection specific parameters These values should be changed from their system wide default values with great care and planning 33 SCPSTP_MAXSEG Sets reads the maximum se
8. This user s guide continues in Section 2 with an overview of the major SCPS TP capabilities The material in Section 2 explains and motivates the options available in the SCPS TP programming interface Section 3 documents this interface by means of a series of Unix manual pages for SCPS TP Finally Section 4 presents suggested reading for those readers not familiar with TCP UDP or the Berkeley Socket Interface Section 2 Overview of SCPS Transport Protocol Capabilities The SCPS Transport Protocol is designed to support current and upcoming tactical and space communication environments The modifications to the base protocols are intended to address the communication environments and resource constraints that systems fielded into these environments typically face As a result of the study phase several technical requirements were allocated to the transport layer of the Open Systems Interconnect model These requirements include the following support for communication with full reliability partial reliability and minimal reliability efficient operation in a wide range of delay bandwidth and error conditions efficient operation in space based processing environments support for precedence priority support for connectionless multicasting support for packet oriented applications The SCPS Transport Protocol SCPS TP refers collectively to the protocols that provide the full reliability best effo
9. main argv argv Initialize the thread scheduler data structures init_scheduler create a thread for the SCPS protocols scheduler run_queue 0 create_thread tp create a thread for the source application scheduler run_queue 1 create_thread source_application Initialize the SCPS protocols data structures void scps_InitQ Pass control to the thread scheduler start_threads Exit when done exit 0 Writing programs using SCPS TP sockets is not very different than writing programs that make use of TCP sockets The SCPS socket calls are prefaced with scps_ but otherwise their use is basically identical to their TCP counterparts For example the SCPS equivalent to the TCP socket call accept is scps_accept 39 To set the various SCPS options the scps_setsockopt function is used The corresponding scps_getsockopt function is used to get the various SCPS options In this example the following constants are used define SCPS_ACKDELAY 20 Sets the acknowledgment timer for SCPS TP to 20 milliseconds define SCPS_SOCKBUFSIZE 65536 Sets the maximum send and receive buffer to 65536 bytes define SCPS_COMPRESS l Enable SCPS Compression define SCPS_RATECONTROL 1000000 Sets SCPS TP rate control to 1 million packets per second To set a particular option remove the comment block from the corresponding define statement in the
10. 2 2 5 3 Memory buffer strategies The reference implementation of SCPS TP provides memory management that is optimized for efficient use of memory This is the intrinsic behavior and requires no user action 2 2 6 Communication Goals SCPS TP addresses the communication goals of the space and tactical communication environments with five enhancements to TCP To address the goals of maximizing throughput and link utilization during a contact period TCP s congestion control mechanisms are made optional Header precomputation reduces the protocol processing required at the time that the link is available And SCPS TP s corruption response and SNACK capabilities maintain high link utilization when experiencing bit errors SCPS TP defines a partial reliability service to address the goal of selectable reliability 2 2 6 1 Optional congestion control SCPS TP makes optional the standard congestion control capabilities within TCP However if TCP congestion control is not enabled system designers must ensure that congestion is either controlled by other means or is not possible in the network due to resource reservation 2 2 6 2 Header precomputation Header precomputation is described in section 2 2 1 3 Header precomputation can improve link utilization by reducing the amount of protocol processing required during the time that the link is available This benefit accrues if the processor is the performance bottleneck in the system 2 2
11. 2 3 Configuring SCPS TP to Enable Specific Capabilities SCPS TP provides a large degree of flexibility in the enabling and disabling of capabilities as well as queries and modification of operational parameters This section describes the mechanisms that allow an application to perform such operations 2 3 1 Congestion Control Applications when using congestion control have the ability to select the standard TCP congestion control algorithm Van Jacobson Congestion Control VJCC or the newer and experimental TCP Vegas congestion control algorithm The standard algorithm is robust but interprets all loss as congestion and will reduce TCP s transmission rate accordingly TCP Vegas s algorithm uses changes increases in round trip time to infer congestion rather than using loss As a result TCP Vegas is better at avoiding congestion and doesn t suffer from a misinterpretation of loss as congestion However TCP Vegas may be susceptible to misinterpretation of changing round trip times as congestion Note that enabling one congestion control algorithm or another must be done before a connection is established An application may enable standard TCP congestion control VJCC on a socket in the following manner one 1 scps_setsockopt socket SCPSTP_VJ_CONGEST one sizeof one Similarly an application may enable TCP Vegas congestion control on a socket in the following manner one 1 scps_setsockopt socket SC
12. 6 3 Separate corruption response The SCPS TP corruption response described in section 2 2 1 1 improves link utilization by not interpreting data loss due to bit errors as data loss due to congestion When responding to corruption the transmission rate and therefore link utilization is not reduced 13 2 2 6 4 Selective Negative Acknowledgment The Selective Negative Acknowledgment SNACK capability improves link utilization by providing a means to unambiguously identify and request immediate retransmission of missing data The SNACK capability is described in section 2 2 1 2 2 2 6 5 Partial reliability service SCPS TP provides a partial reliability service called BETS to ensure correct in sequence but possibly incomplete data delivery When the BETS capability is enabled SCPS TP on the sending side attempts to retransmit packets a user specified number of times then continues on as if the packets had been acknowledged rather than aborting the connection as standard TCP does If no retransmissions are desired the sender discards the packet after its initial transmission At the receive side the receiving SCPS TP entity waits for retransmissions until its receive buffers fill to a user specified level then the missing data is signaled to the user After receiving the signal that a block of data is missing the receiver can continue reading data beyond that block 2 2 7 Primary Sources of Data Loss SCPS TP addresses mixed lo
13. RETURN VALUES 0 ERRORS EBADF EINVAL ENOTSOCK A valid sockfd A pointer to a sockaddr_in address structure The size of struct sockaddr_in upon success on failure and sets scps_errno to indicate the error sockfd is not a valid descriptor addrlen is invalid socket is wrong type routing socket There is no difference in use of the bind socket call for use with the SCPS protocol suite 21 finclude lt scps h gt int scps_connect int sockfd void servaddr int addrlen PARAMETERS sockfd servaddr addrlen RETURN VALUES A valid sockfd A pointer to a sockaddr_in address structure The sizeof struct sockaddr_in O upon success l on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor EFAULT invalid address length ENOMEM Could not get the buffer space required to build a SYN segment There is no difference in use of the connect socket call for use with the SCPS protocol suite 22 finclude lt scps h gt int scps_listen int sockfd int backlog PARAMETERS sockfd A valid sockfd backlog Maximum number of sockets that can be queued while waiting for an accept RETURN VALUES O upon success si on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor There is no difference in use of the listen socket call for use with the SCPS protocol suite 23 f
14. followingsource code and recompile The corresponding scps_setsockopt function will be called For example if the socket buffer size is to be set then remove the comment block from the following statement define SCPS_SOCKBUFSIZE 65536 The will result in the following block of code to be executed ifdef SCPS_SOCKBUFSIZE int sockbufsize SCPS_ SOCKBUFSIZE scps_setsockopt tp_sock SCPS_SOCKET SO_RCVBUF amp sockbufsize sizeof sockbufsize scps_setsockopt tp_sock SCPS_SOCKET SO_SNDBUF amp sockbufsize sizeof sockbufsize endif SCPS_SOCKBUFSIZE 40 ROR RR KR KK KK KKK IK RR IR RK I ORK KK KK k DAABO796 C E601 is subject This software was developed by The MITRE and was produced for the US Government F19628 94 C 0001 to Department of Defense Federal under and NAS5 Acquisition Regulation Clause 252 227 7013 Cc C lause lause 52 227 14 NOTIC MI WA RE PROVIDES THIS RRANTY EXPRESS O CAPABILITY EFFICI IN NO EVENT WILL MI CONSEQUENTIAL INDI Di T NCY SOFTWA R IMPLI OR BE F RI F Di A F TRE IA 252 227 7013 and Federal Acquisi AS IS AS TO T FUNCTIONING FOR E et B RECT INCI Rights in Data General AND MAK HE ACCU OF TH F Di Alt tion Regulation F Corporation Contracts 32607 and 2 ES NO
15. onboard software loads and to confirm correct spacecraft performance via a flow of telemetry The team adopted a two pronged approach in its study phase part of the team conducted a top down survey of representative civil and military space data communications requirements while the remainder of the team conducted a bottom up analysis of available standard data communications protocols The team compared the results to see how capabilities matched requirements and formulated recommendations for future work In evaluating existing capabilities first priority was given to commercially supported off the shelf standards However recognizing unique requirements of the space mission environment long propagation delays noise induced errors and limited spacecraft data processing resources and communications capacity the team also considered other options By the end of 1993 the team concluded that wide segments of the U S civil and military space communities have common needs for e An efficient file handling protocol capable of supporting file transfers initiated either from ground based systems or space based systems e A data transport protocol that provides the user with selectable levels of reliability based on operational need between computers that are communicating over a network containing one or more space data transmission paths e Optional data protection mechanisms to assure the end to end security and integrity of such mess
16. using SOCK_SEOPACKET sockets all write send operations are treated as atomic transfers that generate a new record and therefore a record boundary into the transmitted octet strean When operating over a transport unacknowledged holes in the ou connection with BETS enabled tbound data stream are not 27 automatically signaled to the sending application Applications that need to be aware of BETS holes on the transmitted data stream need to periodically poll for their existence location and size This polling is achieved through the getsockopt system call An application can query both the number of previously unreported BETS holes SO_BETS_NUM_SEND_HOLES and also retrieve th position and size of the first 50 unreported holes If there are more than 50 unreported holes to be retrieved then multiple getsockopt calls should be made 28 finclude lt scps h gt int scps_close int sockfd PARAMETERS sockfd A valid sockfd RETURN VALUES 0 on success on failure and sets errno to indicate the error ERRORS EBADF invalid socket descriptor ESOCKOUTSTATE TP connection is not in ESTAB state There is no difference in use of the close socket call for use with the SCPS protocol suite 29 finclude lt scps h gt int scps_shutdown int sockid int how PARAMETERS sockid a valid socket how method for closing connection The shutdow
17. CONNRESET connection was reset by peer EWOULDBLOCK operation would block EBETS BETS hole detected at receiver Read receive operations for applications utilizing the SCPS transport protocol are slightly more complicated than their TCP UDP counterparts The behavioral differences stem from the SCPS Record Boundary and Best Effort Transport Service BETS When reading data from a socket of type SOCK_SEQPACKET responses to read requests wil Attempts to read boundaries ll not cross emb dded record lengths greater next pending record will return only the next however until consumed legal to read portions of records boundaries are remain persistent in the socke a 25 than that of the record It is s the record receive buffer The length of bytes read will be the minimum of the requested length and the amount Record Boundary operations under SCPS are provided only for sockets of type SOCK_SEOPACKET When using SOCK_SEOPACKET sockets all write send operations are treated as atomic transfers that generate a new record and therefore a record boundary into the transmitted octet stream When operating over a transport connection with BETS enabled the delivery of a BETS hole in the incoming data stream is signaled via an EBETS error message Upon receiving such an error an application should immediately getsockopt using the SO_BETS_RHOLE_SIZE qualifier in order to dete
18. PSTP_VEGAS_CONGEST one sizeof one Applications have the ability to disable congestion control on a per socket basis This can be achieved by making the following scps_setsockopt call null 0 scps_setsockopt socket SCPSPROTO_TP SCPSTP_CONGEST null sizeof null Congestion control should only be disabled when operating in highly managed network environments in which other traffic control policies exist to prevent congestion collapse of the network 15 2 3 2 Selective Negative Acknowledgment The default behavior for SCPS TP should be to use SNACK capability on all connections An application can query whether or not a socket is configured to support the SNACK capability by executing the following getsockopt call scps_getsockopt socket SCPSPROTO_TP OPT_SNACK result sizeof result The value of result provides a boolean indication as to whether SNACK is enabled on this socket Similarly the setsockopt call can be used to enable or disable the SNACK capability on a particular socket scps_setsockopt socket SCPSPROTO_TP OPT_SNACK amp result sizeof result 2 3 3 SCPS TP Header Compression The default behavior for SCPS TP should be to use header compression capability on all connections An application can query whether or not a socket is configured to support the SNACK capability by executing the following getsockopt call scps_getsockopt socket SCPSPROTO_TP OPT_COMP amp result sizeof result
19. WN 97W0000018 WORKING NOTE User s Manual for the SCPS Transport Protocol March 1997 Robert C Durst Patrick D Feighery Eric J Travis Sponsor SMC AXE Contract No Dept No W159 Project No Washington C Center McLean Virginia F 19628 94 C 0001 03970638 Abstract This document provides the information necessary to write programs that use the Space Communications Protocol Standards SCPS Transport Protocol SCPS TP The primary content of this document is Unix style manual pages that describe how the Berkeley Socket Interface has been extended to support SCPS TP This document does NOT provide information necessary to build a distribution of the SCPS TP software Such information is provided as documentation with reference implementation of the SCPS protocols or with whatever distribution of the software the user might have This document assumes that the reader is familiar with the Berkeley Socket Interface and with the basics of the Transmission Control Protocol TCP and the User Datagram Protocol UDP KEYWORDS SCPS SCPS TP TCP UDP Socket Table of Contents Section 1 Introduction 2 Overview of SCPS Transport Protocol Capabilities 2 1 Differences Between Communications Environments 2 1 1 2 1 2 21 3 2 1 4 219 2 1 6 2 1 7 Bit Error Rates Round Trip Delay Continuity of Connectivity Forward and Reverse Link Capacity CPU and Memory Capacity Communication Goals Primary Source of Dat
20. a Loss 2 2 Modifications to TCP to Address Communication Problems 221 222 2 2 3 2 24 225 2 2 6 2 2 7 Bit Error Rates Round Trip Delay Continuity of Connectivity Forward and Reverse Link Capacity CPU and Memory Capacity Communication Goals Primary Sources of Data Loss 2 3 Configuring SCPS TP to Enable Specific Capabilities 2 3 1 2 3 2 2 3 3 2 3 4 2 3 5 2 3 6 2 3 7 Congestion Control Selective Negative Acknowledgment SCPS TP Header Compression Rate Control Acknowledgment Frequency Reduction Record Boundaries Partial Reliability 3 Manual Pages for SCPS TP 4 Suggested Reading Appendix Glossary Example Code Page oO oO 0 Ww p e eS eS eS BRWNrF OC SS me meme me NN NU AADA A UO Q m nN wo N O List of Tables Table Page 1 Factors Affecting TCP Performance in Non Terrestrial Environments 5 2 SCPS TP Modifications to TCP to Address Communication Problems 9 Vi Section 1 Introduction In the fall of 1992 NASA and the DOD jointly established a technical team the SCPS Technical Working Group or SCPS TWG to explore possibilities for developing common space data communications standards with a principal focus on the activities associated with in flight monitoring and control of civil and military spacecraft In practical terms these activities involve a ground control center conducting a dialog with a remote spacecraft to transmit telecommands to up load and verify
21. ading for readers with varying levels of familiarity with TCP UDP and the Socket Interface Readers with little previous familiarity with TCP or UDP should consider reviewing an introductory text on the subject One excellent example is TCP IP Illustrated Volume 1 by W Richard Stevens Copyright 1994 Addison Wesley Professional Computing Series Chapters 1 11 and 17 24 are particularly relevant The Berkeley Socket interface is described in detail along with many various programming examples in Unix Network Programming by W Richard Stevens Copyright 1990 Prentice Hall Software Series Sections 1 through 6 are very appropriate The internals of the TCP and UDP code are explained in detail in TCP IP Ilustrated Volume 2 The Implementation by Gary R Wright and W Richard Stevens Copyright 1995 Addison Wesley Professional Computing Series The following Requests for Comments provide the specifications for the Internet Protocols on which SCPS TP is based Universal Resource Locators URLs are provided for world wide web access J Postel User Datagram Protocol TAB STD 6 RFC 768 August 28 1980 lt URL http ds internic net rfc rfc768 txt gt J Postel Transmission Control Protocol IAB STD 7 RFC 793 September 1 1981 lt URL http ds internic net rfc rfc793 txt gt D Borman R Braden and V Jacobson TCP Extensions for High Performance RFC 1323 May 13 1992 lt URL http ds internic net rfc
22. age exchange e An efficient protocol to support connectionless routing of messages through networks containing space data links Following the study phase the SCPS TWG began development of four specifications one for each of the protocols that address the above requirements the SCPS File Protocol SCPS FP the SCPS Transport Protocol SCPS TP the SCPS Security Protocol SCPS SP and the SCPS Network Protocol SCPS NP These draft specifications have been submitted for adoption as military standards as international standards At the completion of these standards activities resulting standards may then be adopted by any military civil or commercial organization for use in any space system It is the intent of NASA and DOD that commercial vendors produce the SCPS protocols as widely distributed commercial products thus helping to reduce the cost of space systems while increasing their interoperability Part of the protocol development includes development of a reference implementation of each of the protocols This reference implementation is being made available for the purposes of evaluation and experimentation with the protocols by potential users of the SCPS capabilities The SCPS TP has been implemented so as to present a familiar programming interface to users the Berkeley Socket interface This document describes the SCPS extensions to the Berkeley Socket interface that allow users to take advantage of the SCPS TP capabilities
23. bilities that provide the required solutions 2 1 Differences Between Communications Environments The Transmission Control Protocol TCP provides an excellent base of technology for extension It is a highly robust protocol widely distributed and is freely available Hundreds of individuals world wide work to ensure that TCP continues to meet the needs of the Internet community The Internet community currently employs a terrestrial communication environment and TCP is optimized to provide service to this environment The space and tactical communication environments may have many similar communication characteristics to the terrestrial environment by virtue of operating across terrestrial networks as part of the end to end network However there are significant differences between the terrestrial and space tactical environments that affect communication protocol performance It should be noted that many of the characteristics of the space and tactical environments are also characteristic of mobile and wireless communication As a result many of the SCPS enhancements may be applicable to the mobile and wireless communication community Table 1 presents a summary of the main differences that affect TCP performance when it operates in the tactical or space communications environments The following paragraphs discuss these factors Table 1 Factors Affecting TCP Performance in Non Terrestrial Environments Factor Terrestrial Communication Tact
24. ble SCPS TP continues to accept data from the user to the limits of its available memory and does all protocol processing possible When the link becomes available the timing related protocol processing is performed and the queued data is transmitted The effect of the header precomputation is to amortize the bulk of the protocol processing across the time that the link is unavailable reducing the spike in processing required when the link becomes available This capability is completely implementation dependent There is no protocol mechanism required to support it Further in some situations for example high rate data acquisition and low rate data transmission it is inappropriate for use However in the case of sustained observation and bursty transmission header precomputation smoothes the CPU utilization 12 over the two periods The reference implementation of SCPS TP provides header precomputation as its intrinsic behavior 2 2 5 2 Record boundaries TCP provides a byte stream oriented transmission capability That is it does not guarantee the preservation of record boundaries from end to end This forces applications to provide their own application layer framing mechanisms to delimit their data units SCPS TP provides a record boundary option that does this application data delimiting function This results in a memory savings when two or more applications have implemented independent application layer framing software
25. config_tp_pkt_count config_tp_pkt_size host longword get_remote_internet_addr peer_host tp_total_bytes config_tp_pkt_count config_tp_pkt_size 45 void main argc argv int argc char argv if argc 2 goto usage peer_host argv optind init_scheduler scheduler run_queue 0 create_thread tp scheduler run_queue 1 create_thread initiator_application void scps_Init start_threads exit 0 usage fprintf stderr Usage s host n argv 0 exit 1 46 Glossary Ack API BER BETS bps CPU IAB Mbps RFC SCMP SCPS SCPS FP SCPS NP SCPS SP SCPS TP SCPS TWG SIGCOMM SNACK STD TCP UDP URL VICC Acknowledgment Application Programming Interface Bit error rate Best Effort Transport Service Bits per second Central processing unit Internet Activities Board Million bits per second Request for Comments SCPS Control Message Protocol Space Communications Protocol Standards SCPS File Protocol SCPS Network Protocol SCPS Security Protocol SCPS Transport Protocol SCPS Technical Working Group Special Interest Group on Communications Selective Negative Acknowledgment Standard Transmission Control Protocol User Datagram Protocol Universal Resource Locator Van Jacobson Congestion Control 47
26. e at which acknowledgments will be sent If channel capacity permits this rate should be at least twice per round trip 2 2 4 3 SCPS TP Header compression SCPS TP header compression described in Section 2 2 1 3 reduces the size of SCPS TP headers By reducing the size of acknowledgments the load on the low data rate acknowledgment is correspondingly reduced SCPS TP header compression may be enabled or disabled on a per route basis 2 2 5 CPU and Memory Capacity While current TCP implementations tend to be efficient in their use of CPU and memory resources SCPS TP has implemented some further enhancements that take advantage of the environment These enhancements are header precomputation the provision of record boundaries and the implementation of some memory efficient buffering strategies Only the record boundary modification is strictly a protocol feature meaning that it has end to end significance The other enhancements are isolated to one endpoint and do not require the cooperation of the remote system 2 2 5 1 Header precomputation The SCPS TP implementation provides a header precomputation capability to improve CPU use Its application is in situations where data collection takes place over long periods of time compared to the time when the communication link is available and it assumes that the data will be transmitted at high rates once the link becomes available On an existing connection when the link becomes unavaila
27. ems Tactical and or ae SCPS TP Modifications ae ial Error Rate 10 to 107 Corruption response SNACK Header compression Round trip Delay Seconds to hours Window scaling Timer modifications Continuity of connectivity Intermittent Link outage support Forward and Reverse Link 10 1 to 1000 1 forward Rate control Data Rates to reverse link data rate Ack frequency reduction ratio Header compression CPU Capacity and Restricted Header precomputation Memory Availability Record boundaries Communication Goals Maximum throughput Congestion control optional during contact period rate control to support Maximum link utilization Header precomputation Selectable reliability Separate corruption response SNACK Partial reliability operation Primary Sources of Data Congestion Separate response per loss type Loss Corruption SCMP signaling Link Outage Configurable default source of loss 2 2 1 Bit Error Rates SCPS TP has developed three capabilities to address the possibility of data loss due to bit errors The first is an explicit response to corruption rather than congestion as a cause of loss The second is the Selective Negative Acknowledgment SNACK capability The third is the loss tolerant header compression mechanisms 2 2 1 1 Explicit Corruption Response When TCP responds to an isolated data loss it reduces its transmission rate by half and doubles its retransmission timer SCPS TP s response to corruption does neithe
28. ence of crypto synch or from explicit data link layer signaling The SCPS Network Protocol entity maintains simple state information about the availability of outbound links When the link s status changes for example from available to unavailable SCMP sends a signal indicating the change to recent users of that link This SCMP signal is received by the SCMP entity at the data source If in the case of a link transition from available to unavailable another route to the destination cannot be identified the link out signal is passed up to SCPS TP 2 2 3 2 Link outage support in SCPS TP When the SCPS TP receives a message from its local SCMP entity that a link is out it ceases to transmit new data Additionally it stops its normal retransmission timers and periodically probes the link to determine if it has been restored These probes are either packets with a single byte of data or they are acknowledgments if there is no data waiting to be transmitted The transmission of a probe is not counted as a retransmission of data so the connection will not be terminated as a result of exceeding the maximum retransmission count When SCPS TP receives an indication that the remote entity is again reachable either through new packets being received from the remote SCPS TP or from an SCMP message indicating that the link is restored it resumes its normal mode of operation 2 2 4 Forward and Reverse Link Capaci
29. endif ACKDELAY scps_getsockopt tp_sock SCPS_SOCKET SO_RCVBUF amp buffsize amp optlen printf TP Receive Buffer size is now u n buffsize scps_getsockopt tp_sock SCPS_SOCKET SO_SNDBUF amp buffsize amp optlen printf TP Send Buffer size is now u n buffsize printf TP sending d bytes with d byte writes d writes n tp_total_bytes config_tp_pkt_size config_tp_pkt_count scps_getsockopt route_sock SCPS_ROUTE SCPS_RATE amp buffsize amp optlen printf TP Rate control set to u bps n buffsize 44 scps_getsockopt route_sock SCPS_ROUTE SCPS_MTU amp mtu_val amp optlen printf TP MTU set to u bytes n mtu_val Open a connection with a distant host port hisaddress sin_port htons config_tp_resp_port bcopy amp host amp hisaddress sin_addr sizeof u_long scps_connect tp_sock struct sockaddr amp hisaddress sizeof hisaddress tp_prev_bytes tp_total_bytes while tp_total_bytes gt 0 data to send Meter TP data out one packet at a time bytes_to_send tp_total_bytes gt config_tp_pkt_size config_tp_pkt_size tp_total_bytes cc scps_write tp_sock outbuf bytes_to_send IE ec gt 0 tp_write_cntt tp_total_bytes cc No more data to send close the connection scps_close tp_sock threadExit void scps_init Int as tp_total_bytes
30. gment size bytes for use for this socket SCPSTP_ACKDELAY Sets reads the value in milliseconds of the delayed ACK timer to be used for TP operation for this socket SCPSTP_ACKFLOOR Sets reads the value in milliseconds for the minimum delay between ACK transmissions for TP operation for this socket Don t ACK more often than this SCPSTP_RTOMIN Sets reads the minimum retransmission time in milliseconds for TP operation for this socket SCPSTP_RETRANSMITTIME Sets reads the retransmission timer call interval in milliseconds for TP operation for this socket SCPSTP_PERSISTTIME Sets reads the delay in milliseconds until a zero window probe is sent SCPSTP_TIMEOUT Sets reads the maximum number of retransmissions allowed integer value before transmitter aborts the connection or declares a BETS_SEND_HOLE and moves on SCPSTP_LONGTIMEOUT Sets reads the maximum number of retransmissions allowed integer value for a SYN before aborting a connection SCPSTP_2MSLTIMEOUT Sets reads the 2MSLtimeout value in milliseconds for a TP socket In general this value should not be changed from the system default SCPSTP_BETS_RTIMEOUT Sets reads the BETS_Receive Timeout value in milliseconds 34 The following options must be changed prior to connection establishment either connect or listen in order to have any effect
31. ic connectivity characteristics Low Earth Orbiting satellites typically have connectivity through a single ground station 10 of the time or less Changes to the number of ground stations or the satellite s orbit can improve this but even NASA s Tracking and Data Relay Satellite System TDRSS offers only about 90 coverage Further tactical systems have unpredictable connectivity characteristics due to system mobility and potential system mortality 2 1 4 Forward and Reverse Link Capacity In the terrestrial communication environment communication links are typically duplex with the same data rate in both directions This is not the case in space environments Rather it is not unusual to have large differences in forward and reverse link capacities Ratios of 1000 1 are not unusual This degree of asymmetry causes problems for TCP which uses a stream of acknowledgments as a self clocking mechanism for transmitting data packets Thus very low capacity acknowledgment channels limit the transmission rate of data packets 2 1 5 CPU and Memory Capacity In the terrestrial communications environment the availability of computing resources is essentially unrestricted This is not the case in spacecraft and tactical systems in which power weight and volume are all precious commodities The amount of computational resource available to any subsystem in a tactical or spacecraft system must be traded off against the benefits of applying that res
32. ical and or Space Communication Bit BitError Rate Rate Typically lt 10 10 to 10 Round trip Delay Milliseconds to seconds Forward and Reverse Link i 10 1 to 1000 1 forward to Data Rates reverse link data rate ratio CPU and Memory Capacity Relatively large Relatively small Communication Goals Fair access over time Maximum throughput during High aggregate throughput contact period High reliability Maximum link utilization Selectable reliability level Primary Sources of Data Congestion Congestion Loss Corruption Link Outage 2 1 1 Bit Error Rates The error performance of typical terrestrial networks has improved to a point that it is no longer considered as a typical source of data loss With sufficient channel coding and application of radiated power some tactical and satellite links can approach the error performance of terrestrial networks However this is not the typical case especially in situations in which the power weight and volume of the communications gear is constrained The loss of data due to bit errors has a disproportionately bad effect on TCP performance because TCP interprets any loss as an indication of network congestion The appropriate response to network congestion is to reduce the offered load to the network TCP s congestion response reduces the offered load by half then builds back slowly over several subsequent round trips The effect of this in response to bit errors is to significantl
33. include lt scps h gt int scps_accept int sockfd void peer int addrlen PARAMETERS sockfd The sockfd of a currently listening socket peer A pointer to a sockaddr_in address structure to be filled in upona successful accept This is the address of the client addrlen The size in bytes of the peer address provided above RETURN VALUES positive integer representing new sockfd on success 1 on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor There is no difference in use of the accept socket call for use with the SCPS protocol suite 24 finclude lt scps h gt int scps_read int sockfd void data int size scps_recv int sockfd void data int size int flags scps_recvfrom int sockfd void data int size void ina int ina_len PARAMETERS sockfd The sockfd of a currently valid socket data A pointer to the data that is to be written read size The amount of data to be written read flags MSG_OOB MSG_PEEK MSG_DONTROUTE ina Pointer to sockaddr_in structure for remote address ina_len Integer values corresponding to length of RETURN VALUES address contained in ina The actual number of bytes read upon success l on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor ENOTCONN socket is not currently connected E
34. ion specified in the SCPS Transport Protocol This capability provides the ability for an application to select correct in sequence but possibly incomplete delivery of data Header compression specified in the SCPS Transport Protocol adapted from RFC 1144 This technique reduces the overhead of TCP headers typically by more than 50 for application in low bandwidth environments Low loss congestion control or optional non use of congestion control specified in the SCPS Transport Protocol Congestion control mechanisms prevent TCP from overdriving shared links If links are not shared users can disable congestion control and depend solely on SCPS TP s rate control mechanism to prevent overdriving the communication channel Alternatively SCPS TP provides a congestion control mechanism that does not depend on data loss to signal congestion Retransmission strategies for mixed loss environments that accommodate loss due to data corruption link outages and congestion specified in the SCPS Transport Protocol Acknowledgment strategies for highly asymmetric communication channel capacities specified in the SCPS Transport Protocol This section discusses the problems that might be encountered in the tactical and spacecraft communications environments then identifies the solutions that SCPS TP uses to address those problems Finally this section discusses how to configure SCPS TP to enable the specific capa
35. mmunity and one defined in SCPS TP 2 2 2 1 Window scaling The Window Scaling option defined in RFC 1323 permits TCP to have more than 64k bytes of data outstanding unacknowledged at one time Note that at T1 data rates 1 54 Mbps a one half second round trip delay would result in over 96k bytes of data outstanding The window scaling option simply imposes a scaling factor to the advertised window increasing the maximum data that could be outstanding by powers of two up to 2 2 2 2 2 Timer modifications SCPS TP increases the range of typical TCP timers to allow round trip delays of minutes to hours Further SCPS TP initializes its retransmission timer based on data from the routing structure This allows routes to remote systems to be configured with a reasonable initial estimate of the round trip time thus avoiding retransmission time outs at the beginning of a connection 2 2 3 Continuity of Connectivity SCPS TP depends on signaling from the network layer the SCPS Network Protocol s SCPS Control Message Protocol SCMP to identify link outages This permits SCPS TP to differentiate between link outages and other causes of packet loss 10 2 2 3 1 Signaling of link outages The SCPS Control Message Protocol entity depends on information from local link interfaces for example a satellite communications channel to determine whether the link is available or not Such information can be inferred from for example the pres
36. n call causes all or part of a full duplex connection on the socket associated with s to be shut down If how is 0 then further receives will be disallowed If how is 1 then further sends will be disallowed If how is 2 then further sends and receives will be disallowed RETURN VALUES 0 on success 1 on failure and sets scps_errno to indicate the error ERRORS EBADF s is not a valid scps socket descriptor ENOTCONN The specified socket is not connected ESOCKOUTSTATE TP connection is not in ESTAB state There is no difference in use of the shutdown socket call for use with the SCPS protocol suite 30 finclude lt scps h gt scps_select int sockid scps_fd_set read scps_fd_set writeset set scps_fd_set nullval struct timeval time PARAMETERS sockid a valid scps socket identifier readset the file descriptors to be tested for reading writeset the file descriptors to be tested for writing nullval a null value for compatibility with select time the amount of time to wait before returning if no descriptors are readable or writable RETURN VALUES select returns a non negative value on success A positive value indicat ready descriptors in tes the number of the descr iptor sets O indicates that the time limit referred to by time expired On failure select returns 1 sets are not changed ERRORS
37. ource elsewhere Therefore it is important to be aware of these constraints Note that restrictions on power may affect other factors listed here Notably power restrictions may increase error rates decrease data rates increasing delay and may affect continuity of connectivity 2 1 6 Communication Goals A major TCP goal is to provide to its users fair access to the network over time By fair access we mean that no single user can monopolize a communication channel when others need to use it TCP also attempts to provide high aggregate throughput and provides high reliability None of these communication goals are inherently bad However the tactical and space communication environments may explicitly NOT wish to provide fair access to the communication resources Rather access may need to be on a strict precedence basis with low precedence users getting starved for resources in favor of high precedence users Further TCP does not assume that maximization of link utilization is a priority It intentionally under drives the link at the beginning of a connection and after loss in an attempt to determine the sustainable capacity of the link Finally TCP offers a fully reliable service preserving completeness sequence and correctness TCP trades delay incurred as a result of retransmission and buffer space to provide these features Its companion protocol the User Datagram Protocol UDP provides an unreliable service with no prese
38. r of these things Rather both the transmission rate controlled by the congestion window and the retransmission time out value remain unchanged 2 2 1 2 Selective Negative Acknowledgment The SCPS TP Selective Negative Acknowledgment SNACK capability has been developed to identify specific data that requires retransmission and to request immediate retransmission of that data The SNACK capability is invoked when the receiver creates and transmits the SNACK option to the data sender on a regular acknowledgment The receiver does not send the SNACK option immediately upon detecting a data loss in case packets have become misordered within the network 2 2 1 3 SCPS TP Header Compression SCPS TP defines a header compression capability to reduce the size of transmitted packets This header compression capability operates at the endpoints of the SCPS TP connection As a result headers are only compressed once regardless of the number of hops that the data requires Further this header compression scheme is loss tolerant meaning that the loss of one packet does not render subsequent packets unintelligible This trait is an unfortunate characteristic of the TCP IP header compression in common use with dial up serial lines and makes it unsuitable for use in the space or tactical communication environment 2 2 2 Round Trip Delay SCPS TP addresses the problems imposed by round trip delay with two capabilities one defined by the Internet co
39. rfc1323 txt gt J Nagle Congestion Control in IP TCP Internetworks RFC 896 January 6 1984 lt URL http ds internic net rfc rfc896 txt gt K McCloghrie and M Rose Management Information Base for Network Management of TCP IP Based Internets MIB II AIB STD 17 RFC 1213 March 26 1991 lt URL http ds internic net rfc rfc1213 txt gt Several articles have been published that explain particular aspects of TCP operation The following are of particular interest 37 P Karn amp C Partridge Round Trip Time Estimation In Proceedings of SIGCOMM 87 Symposium on Communications Architectures and Protocols August 1987 V Jacobson Congestion Avoidance and Control In Proceedings of SIGCOMM 88 Symposium on Communications Architectures and Protocols August 1988 S Brakmo S W O Malley and L L Peterson TCP Vegas New Techniques for Congestion Detection and Avoidance In Proceedings of SIGCOMM 94 Symposium on Communications Architectures and Protocols August 1994 38 Appendix Example Code The following is an example of an application developed using the SCPS software reference implementation This code must be compiled and linked with the SCPS library libscps a This particular example is the source of a typical source sink networking application The programs main is minimalistic and contains the following code necessary for any application to use the SCPS protocol stack
40. rmine th size of the missing data The next read receive operation will resume the delivery of the sequenced data stream 26 finclude lt scps h gt scps_write int sockfd void data scps_send int sockfd void data scps_sendto int sockfd int size int size void data int flags int size int flags to int addrlen PARAMETERS sockfd The sockfd of a currently valid socket data A pointer to the data that is to be written size The amount of data to be written flags MSG_OOB MSG_PEEK MSG _DONTROUTE to Pointer to sockaddr_in structure containing destination address addrlen Integer value corresponding to the length of the address in to RETURN VALUES The actual number of bytes written upon success void 1 on failure and sets scps_errno to indicate the error ERRORS EBADF invalid socket descriptor ENOTCONN socket is not currently connected ECONNRESET connection was reset by peer EWOULDBLOCK operation would block EBETS BETS hole detected at receiver Write send operations for applications utilizing the SCPS transport protocol are slightly more complicated than their TCP UDP counterparts The behavioral differences stem from the SCPS Record Boundary and Best Effort Transport Service BETS Record Boundary operat tions under SCPS are provided only for sockets of type SOCK_SEOPACKET When
41. rt reliability and minimal reliability services The full reliability service is provided by the Transmission Control Protocol TCP The partial reliability service is provided by TCP with minor modifications The minimal reliability service is provided by the User Datagram Protocol UDP The SCPS TP addresses the constraints posed by the operating environment with the following extensions and modifications to TCP Window scaling specified in RFC 1323 addresses communication environments that may have more than 65k bytes of data in transit at one time Round Trip Time Measurement specified in RFC 1323 This enhancement addresses environments that have high loss changing delays or large amounts of data in transit at one time Protect Against Wrapped Sequence Numbers specified in RFC 1323 This capability which uses the Round Trip Time Measurement capability addresses very long delay or very high bandwidth environments Selective Negative Acknowledgment SNACK specified in the SCPS Transport Protocol adapted from RFC 1106 SNACK improves performance in high loss environments by providing specific information about missing data and requesting its immediate retransmission Record Boundary Indication specified in the SCPS Transport Protocol This feature provides the ability to mark and reliably carry end of record indications for packet oriented applications Partial Reliability Communicat
42. rvation of sequence or completeness However for some types of data such as image data a partial reliability service that preserves sequence and correctness but possibly not completeness may be appropriate In the case of image data the idea is that the possible loss of a single scan line or a part of a scan line should not significantly delay the delivery of the remainder of the image but that the order of the scan lines is important to preserve 2 1 7 Primary Source of Data Loss As previously mentioned data loss due to bit errors and to topological instability is rare in the terrestrial environment The primary source of loss in terrestrial networks is congestion and TCP is optimized to control congestion The space and tactical communication environments present mixed loss environments with losses occurring due to all three causes bit errors topology changes link outages and congestion To treat all losses in as congestion results in unnecessary reductions in offered load The increased round trip times in these environments delays the restoration of full rate transmission 2 2 Modifications to TCP to Address Communication Problems To address the problems listed above the SCPS Transport Protocol includes some already specified enhancements to TCP and some new enhancements These enhancements are summarized in Table 2 and described in the following paragraphs Table 2 SCPS TP Modifications to TCP to Address Communication Probl
43. ss environments by providing the ability to respond to different types of loss with responses that are appropriate for that type The SCPS Control Message Protocol SCMP provides signaling mechanisms to inform SCPS TP about the types of loss being experienced Finally the SCPS TP default response can be configured to invoke either the congestion response or the corruption response 2 2 7 1 Separate responses for each type of loss SCPS TP has separate responses for congestion corruption and link outages The congestion response is the same as that in TCP The corruption response is described in section 2 2 1 1 The link outage response is described in section 2 2 3 2 2 2 7 2 SCMP signaling for different loss types The SCPS Control Message Protocol SCMP provides separate signals for congestion the source quench signal corruption the corruption experienced signal and link outage the link out and link redirect signals Upon receipt of these signals SCMP informs SCPS TP and updates local state information 2 2 7 3 Configurable default source of loss If SCMP cannot determine the cause of loss or the signal does not reach SCPS TP SCPS TP must invoke some response to that loss The reference implementation of SCPS TP allows each route to be configured with a default source of loss congestion or corruption If no signals are received indicating the cause of loss SCPS TP will invoke its default response 14
44. ty Operation of TCP over highly asymmetric channels tends to result in sustained under utilization of the high capacity channel as mentioned above This is a result of TCP s use of acknowledgments as clocking mechanisms for transmitting data SCPS TP has three capabilities that work together to improve the utilization of the high capacity channel rate control acknowledgment frequency reduction and header compression 2 2 4 1 Rate control SCPS TP provides a rate control mechanism to spread the transmission of data across a time interval replacing TCP s acknowledgment clocking mechanism SCPS TP uses a token bucket rate control mechanism with the rate control parameters associated with a particular route All SCPS TP users on a single host that share that route share the capacity of that route The rate control also provides a means of limiting the rate of transmission of acknowledgments something that TCP cannot do 11 2 2 4 2 Acknowledgment frequency reduction TCP attempts to acknowledge at least every other packet that is received If TCP detects that a packet is missing it sends an acknowledgment for every packet As previously mentioned limitations on acknowledgment channel capacity result in under utilization of the data channel SCPS TP breaks the dependency on acknowledgments as clocking mechanisms and therefore allows the acknowledgment rate to be reduced SCPS TP permits the user to explicitly specify the rat
45. vailable but required 17 Section 3 Manual Pages for SCPS TP The following manual pages specify the Application Programming Interface API to the reference implementation of the SCPS Transport Protocol SCPS TP The style and content of these manual pages is consistent with the documentation provided with the Unix socket interface 19 finclude lt scps h gt int scps_socket int family int type int protocol PARAMETERS family PF_INET PF_SCPS type SOCK_STREAM SOCK_DGRAM SOCK_SEQPACKET protocol UDP TCP 0 default protocol RETURN VALUES positive integer upon success 1 on failure and sets scps_errno to indicate the error ERRORS ENOBUFS could not allocate a socket for use The SCPS transport API introduces a new protocol family PF_SCPS When a socket is created and the SCPS protocol family is specified this signals that the socket is to use SCPS TP for data transport rather than its Internet protocol equivalent Protocol Family Socket Type Protocol Resulting Protocol PF_INET CK_DGRAM IPPROT PF_INET K_STREAM IPPROTO_ PF_INET K_SEQPACKE PF_SCPS K_DGRAM IPPROTO_ UDP PF_SCPS K_STREAM IPPROTO_1 SCPS TP PF_SCPS K_SEOPACKE IPPROTO_T SCPS TP with Record Boundaries 20 finclude lt scps h gt int scps_bind int sockfd void myaddr int addrlen PARAMETERS sockfd myaddr addrlen
46. y underutilize the communication channel 2 1 2 Round Trip Delay Round trip delays in the terrestrial communication environment are typically in the tens of milliseconds to low hundreds of milliseconds Round trips across the continental United States average approximately one hundred milliseconds In the spacecraft communication environment round trip times of five hundred milliseconds are the minimum that one expects when communicating through a geostationary satellite with each hop through a satellite adding another five hundred milliseconds Deep space communications can increase round trip delays to hours Long round trip delays limit the usefulness and effectiveness of TCP s or any closed loop system s feedback from the remote communication endpoint This causes problems when the protocol needs to react to changes in the network but does not receive feedback about those changes until long after the change has occurred Note that long delays are not exclusively a result of speed of light propagation times Low data transmission rates add delay to a network as can half duplex operation Finally queuing in intermediate systems is a source of delay and the primary source in the terrestrial communication environment 2 1 3 Continuity of Connectivity The terrestrial communication environment can be characterized as a network with a very infrequently changing topology Orbiting systems have predictable but possibly highly dynam
47. zeof del Similarly the acknowledgment frequency for a given socket can be modified using the scps_setsockopt call scps_setsockopt socket SCPSSOCK_TP SCPSTP_ACKDELAY amp del sizeof del 2 3 6 Record Boundaries Record boundaries are enabled by creating a SOCK_SEQPACKET socket instead of the more typical SOCK_STREAM packet sock socket PF_SCPS SOCK_SEQPACKET 0 When using a SOCK_SEQPACKET socket all writes are treated as atomic records by the transport stream Read requests also respect record boundaries Read requests that attempt to span record boundaries are truncated at the termination of the currently pending record 2 3 7 Partial Reliability Partial reliability transport service BETS must be negotiated at the establishment of a transport connection If both parties in the connection do not offer the BETS capability partial reliability will not be supported on the resulting connection Applications must explicitly enable BETS capability on a socket prior to attempting to initiate a connection connect or listen BETS is enabled through the scps_setsockopt call scps_setsockopt socket SCPSSOCK_TP OPT_BETS amp value sizeof value Following the establishment of a connection an application can verify whether partial reliability service is indeed available using the scps_getsockopt call Depending on the outcome the application may decide to terminate the connection if partial reliability is not a
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