1 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP over Satellite Networks Mohammed Atiquzzaman School of Computer Science University of Oklahoma. Web: Co-authors: Shaojian Fu and William Ivancic

2 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Introduction A number of enhancements to TCP have been proposed to enhance its performance over satellite networks. No extensive study to investigate the suitability of SCTP, a new transport protocol being standardized by IETF, over satellite networks. Main contributions of this study:  Provide insights into the suitability of SCTP over satellite networks;  Highlight the different features of SCTP which may help SCTP to achieve the performance of “TCP with enhancements” in satellite environments;  Determine the effects of the unique features of SCTP in improving its performance over satellite links;  Provide recommendations on using SCTP over satellite networks.

3 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Satellite Networking Characteristics Long propagation delay:  Propagation delay between an earth station and a GEO satellite is around ms.  Requires sender long time to probe the network capacity and detect the possible loss of segments.  Expensive satellite bandwidth is wasted. Large delay-bandwidth product:  GEO satellite link is a typical case of the Long Fat Pipe (LFP), which features a large delay bandwidth product. Corruption loss during transmission:  Large transmission distance of satellite links results in a low SNR and consequently a high Bit Error Rate (BER).  Cause TCP and SCTP senders to reduce their transmission rates unnecessarily.

4 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Stream Control Transmission Protocol SCTP (RFC 2960) is being developed by IETF as the next generation transport protocol.  Reliable: retransmission of lost packets, ack of packets.  In-order delivery: re-sequencing at the destination. Transport layer protocol which operates on top of an unreliable connectionless network layer such as IP.  Transparent to IPv4 or IPv6 Key Unique features:  Support for multiple logical streams to improve data transmission throughput;  Support for multiple network interfaces to achieve high availability;  More secure mechanisms to prevent threats such as Denial of Service (DoS) attack.

5 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP multi-streaming and multihoming

6 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Illustration of SCTP multihoming

7 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Illustration of Multi-streaming

8 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP and TCP: Common features Congestion control mechanism  Slow Start and Congestion Avoidance; Transmission error Recovery  Fast Retransmit;  SCTP doesn't have an explicit Fast Recovery phase, but achieves this automatically with the use of SACK. Path MTU discovery  SCTP has a slightly different support for path MTU discovery - separate path MTU estimates must be maintained for each destination IP address. Selective acknowledgement (SACK)  Use of SACK is mandatory in SCTP, whereas it is optional in TCP.

9 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP Selective Acknowledgment | Type = 3 |Chunk Flags | Chunk Length | | Cumulative TSN Ack | | Advertised Receiver Window Credit (a_rwnd) | | Number of Gap Ack Blocks = N | Number of Duplicate TSNs = X | | Gap Ack Block #1 Start | Gap Ack Block #1 End | / /... / / | Gap Ack Block #N Start | Gap Ack Block #N End | | Duplicate TSN 1 | / /... / / | Duplicate TSN X | Total available chunk space: 2 16 Bytes Maximum number of Gap Ack Blocks: ( ×4)/4=16380 Space used by other fields: 4×4 Bytes Space required for each Gap Ack Block: 4 Bytes

10 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, TCP Selective Acknowledgment | Kind=5 | Length | | Left Edge of 1st Block | | Right Edge of 1st Block | / /... / / | Left Edge of nth Block | | Right Edge of nth Block | Space required for one SACK block: 8 bytes Maximum number of TCP SACK Blocks: Integer[(40-2)/8]=4 4-bit “Header Length” field in TCP limits the maximum space available for TCP options to 40bytes

11 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Advantage of large number of SCTP SACK blocks Satellite link features high BER and large TCP window. There is larger possibility of multiple losses in a single window. 3 or 4 blocks in TCP may not be sufficient for reporting segment losses. Larger number of SACK blocks make SCTP more robust in the case of multiple losses.

12 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Large Window Support in SCTP 32-bit SCTP Advertised Receiver Window Credit 16-bit TCP window 14 bits by window scaling { Maximum TCP window size: 65535×2 14 bytes Maximum SCTP window size: bytes

13 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP Large window support TCP supports windows up to 2 16 bytes; TCP requires window scaling option (RFC 1323) to support large windows; SCTP has a natural support for large windows up to 2 32 bytes to fill out the pipe. ACK Data DestinationSource ACK Data SourceDestination

14 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Delayed Acknowledgment & Byte Counting Byte counting: the cwnd is increased based on the number of bytes acknowledged by the SACK instead of by the number of ACKs as in TCP. Byte counting decouples the cwnd increase from the arrival frequency of the SACKs, which is important in satellite environments by speeding up the slow start stage. SCTP limits the cwnd increase to one PMTU per SACK. When the total number of bytes acknowledged by a single SACK exceeds PMTU, the benefit of byte counting is impaired.

15 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Delayed Acknowledgment & Byte Counting (cont.) cwnd=10 segments S1=536bytes S2=536bytes cwnd=11 segments TCP delayed Ack cwnd=5360 bytes S1=536bytes S2=536bytes cwnd=6432 bytes (12 segments) SCTP delayed SACK (PMTU=1500 bytes) cwnd=15000 bytes S1=1500 bytes cwnd=16500 bytes (11 segments) SCTP delayed SACK (PMTU=1500 bytes) S2=1500 bytes AckSACK We recommend increasing the byte counting limit to 2 PMTU by considering the delayed SACK. Benefits of byte counting is lost

16 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, SCTP support for ECN Explicit Congestion Notification (ECN) helps determining the exact reason (congestion vs. corruption losses) of segment losses, preventing the sender from unnecessarily entering congestion control. SCTP has explicit support for ECN:  Endpoints can negotiate about ECN capabilities during association setup;  When the SCTP receiver detects the “CE” bit in the IP header of a received segment, it will use an Explicit Congestion Notification Echo (ECNE) to notify sender about the congestion;  Sender will respond with Congestion Window Reduce (CWR) indicating that the cwnd has been reduced.

17 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Recommended Use of SCTP over Satellite Networks MechanismTCP Use (RFC 2488) SCTP UseWhere Path MTU DiscoveryRecommended S Slow StartRequired S Congestion AvoidanceRequired S Fast RetransmitRecommended S Fast RecoveryRecommendedImplicitly UsedS SACKRecommendedImplicitly UsedS, R Delayed SACKRecommended R Large Receiver WindowRecommendedImplicitly UsedS, R SCTP Multi-HomingN/ARecommendedS, R SCTP Multi-StreamingN/ARecommendedS, R Byte CountingN/AImplicitly UsedS, R Larger Byte Counting LimitN/ARecommendedS Larger Initial cwndN/ARecommendedS ECNN/ARecommendedS, R Common to TCP/SCTP Unique to SCTP

18 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Conclusion New SCTP features make this new transport protocol suitable for long-delay, high BER satellite links. Some issues for using SCTP over satellite links remains unresolved:  SCTP/IP Header Compression in high BER environment  Bias against long-RTT associations during congestion avoidance  SCTP over asymmetrical forward and backward satellite links Some TCP enhancements, such as Protecting Against Wrapped Sequence (PAWS) numbers and Round Trip Time Measurement (RTTM) require timestamp option which is not available in SCTP. New chunk type needs to be defined.

19 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, Acknowledgements  National Aeronautics and Space Administration (NASA) for supporting this research through grant no. NAG Further Information Dr. Mohammed Atiquzzaman (405) 325 These slides are available at