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Transcript of 1 M. Atiquzzaman, SCTP over satellite networks IEEE Computer Communications Workshop, Oct 20, 2003....
1M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
SCTP over Satellite Networks
Mohammed AtiquzzamanSchool of Computer Science
University of Oklahoma.Email: [email protected]
Web: www.cs.ou.edu/~atiq
Co-authors: Shaojian Fu and William Ivancic
2M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
3M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Satellite Networking Characteristics
Long propagation delay: Propagation delay between an earth station and a GEO satellite is around
120-140ms. 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.
4M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
5M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
SCTP multi-streaming and multihoming
6M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Illustration of SCTP multihoming
7M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Illustration of Multi-streaming
8M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
9M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
SCTP Selective Acknowledgment
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 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: 216 Bytes
Maximum number of Gap Ack Blocks: (216 -4×4)/4=16380
Space used by other fields: 4×4 Bytes
Space required for each Gap Ack Block: 4 Bytes
10M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
TCP Selective Acknowledgment
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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
11M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
12M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Large Window Support in SCTP
32-bit SCTP Advertised Receiver Window Credit
16-bit TCP window
16-bit TCP window 14 bits by window scaling
{Maximum TCP window size: 65535×214 bytes
Maximum SCTP window size: 232-1 bytes
13M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
SCTP Large window support
TCP supports windows up to 216 bytes; TCP requires window scaling option
(RFC 1323) to support large windows;
SCTP has a natural support for large windows up to 232 bytes to fill out the pipe.
ACK
Data
DestinationSource
ACK
DataSource Destination
14M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
15M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Delayed Acknowledgment & Byte Counting (cont.)
cwnd=10 segmentsS1=536bytesS2=536bytes
cwnd=11 segments
TCP delayed Ack
cwnd=5360 bytesS1=536bytesS2=536bytes
cwnd=6432 bytes(12 segments)
SCTP delayed SACK(PMTU=1500 bytes)
cwnd=15000 bytesS1=1500 bytes
cwnd=16500 bytes(11 segments)
SCTP delayed SACK(PMTU=1500 bytes)
S2=1500 bytes
Ack SACKSACK
We recommend increasing the byte counting limit to 2 PMTU by considering the delayed SACK.
Benefits of byte counting is lost
16M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
17M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Recommended Use of SCTP over Satellite Networks
Mechanism TCP Use
(RFC 2488)
SCTP Use Where
Path MTU Discovery Recommended Recommended S
Slow Start Required Required S
Congestion Avoidance Required Required S
Fast Retransmit Recommended Recommended S
Fast Recovery Recommended Implicitly Used S
SACK Recommended Implicitly Used S, R
Delayed SACK Recommended Recommended R
Large Receiver Window Recommended Implicitly Used S, R
SCTP Multi-Homing N/A Recommended S, R
SCTP Multi-Streaming N/A Recommended S, R
Byte Counting N/A Implicitly Used S, R
Larger Byte Counting Limit N/A Recommended S
Larger Initial cwnd N/A Recommended S
ECN N/A Recommended S, R
Com
mon to T
CP
/SC
TP
Unique to
SC
TP
18M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
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.
19M. Atiquzzaman, SCTP over satellite networks
IEEE Computer Communications Workshop, Oct 20, 2003.
Acknowledgements National Aeronautics and Space Administration (NASA) for
supporting this research through grant no. NAG3-2528.
Further Information
Dr. Mohammed Atiquzzaman
[email protected], (405) 325 8077
These slides are available at
www.cs.ou.edu/~atiq