Outline. History Introduction Packets Association/ Termination Data Transmission concepts Multihoming Streams

Transcription

1 Outline History Introduction Packets Association/ Termination Data Transmission concepts Multihoming Streams 1

2 History Developed by IETF SIGTRAN working group (Internet Engineering Task Force) (SIGnaling TRANsport) SCTP standard draft paper (RFC2960) released in October 2000 ( 2

3 Introduction Transport protocol for connectionless packet transmissions Like TCP or UDP it works on top of connectionless transport services such as IP SCTP is designed to deal with unreliability of connectionless transport 3

4 Introduction Checksums and sequence numbers acknowledgements error free transfer non- duplicated transfer congestion avoidance resistance of several attacks like blind spoofing 4

5 Introduction Difference to protocols like TCP multihoming (multible streams in one connection) Layer between SCTP user application and packet service [RFC2960] 5

6 Packets [SCTP] 6

7 Packets Consist of common header and one or more chunks Packet can be up to the maximum payload length of the transport service protocol (IP: Bytes) Chunk can contain control information or user data Some control packets have to consist of the common header and one chunk 7

8 Packets Header 2 Bytes source Port 2 Bytes destination Port Verification tag is established at association startup Checksum is calculated with Adler-32 algorithm 8

9 Packets Chunks Chunk Type Chunk Flags Chunk Length \ \ / Chunk Value / \ \ [RFC2960] ID Value Chunk Type Payload Data (DATA) 1 - Initiation (INIT) 2 - Initiation Acknowledgement (INIT ACK) 3 - Selective Acknowledgement (SACK) 4 - Heartbeat Request (HEARTBEAT) 5 - Heartbeat Acknowledgement (HEARTBEAT ACK) 6 - Abort (ABORT) 7 - Shutdown (SHUTDOWN) 8 - Shutdown Acknowledgement (SHUTDOWN ACK) 9 - Operation Error (ERROR) 10 - State Cookie (COOKIE ECHO) 11 - Cookie Acknowledgement (COOKIE ACK) 12 - Reserved for Explicit Congestion Notification Echo (ECNE) 13 - Reserved for Congestion Window Reduced (CWR) 14 - Shutdown Complete (SHUTDOWN COMPLETE) [RFC2960] 9

10 Packets Chunks Chunk flags depend on types. If there are no flags, they are set to zero and ignored. Chunk length is the byte- length of the junk including all fields. Detailed description of each chunk type can be found at [RFC2960] 10

11 Association Allocate Resources A INIT B INIT-ACK Create COOKIE Send back COOKIE COOKIE-ECHO Connection ESTABLISHED COOKIE-ACK Validate COOKIE Allocate Resources Connection ESTABLISHED 11

12 Association Association is completed after exchanging 4 messages Upper layer of A initiates association request A sends INIT- chunk and initialises all necessary data structures B receives chunk, generates secure HASH and secret key (MD5, SHA1) 12

13 Association With this values a so called COOKIE is created and sent to A in a INIT-ACK chunk A receives INIT-ACK, creates a COOKIE-ECHO chunk, puts received COOKIE into chunk and sends it back B valitates COOKIE. If valid, all necessary data for the association is allocated 13

14 Association COOKIE-ACK is sent to A and B goes to state ESTABLISHED. After receiving COOKIE-ACK, A goes to state ESTABLISHED 14

15 Termination Graceful termination All data acknowledged A SHUTDOWN B SHUTDOWN-ACK All data acknowledged SHUTDOWN-COMPLETE remove association data 15

16 Termination Graceful termination no data loss SHUTDOWN chunk is sent by A after all his data is acknowledged B responds by sending SHUTDOWN-ACK chunk after his data is acknowledged A sends SHUTDOWN-COMPLETE chunk and removes association data After receiving this chunk B removes association data. 16

17 Termination Abort the association A sends ABORT chunk, taking into account, that some data meight not be acknowledged. B validates this chunk, but does not respond Both delete association data immediately. 17

18 Data transmission concepts Multihoming only data of stream, not of connection needs to be maintained reduces waiting time by sending independend data on different streams. Data loss and duplication detection numbering each Data chunk with TSN (Transport Sequence Number) ACKs are based on these numbers 18

19 Data transmission concepts Selective ACKs (SACK) not packets, but chunks are acknowledged SACK is used to acknowledge chunks inform sender about gaps in TSNs about duplicated TSNs [RFC2960] 19

20 Data transmission concepts Congestion control Congestion control is based on TCPcongestion control Variables kept for congestion control: adertised window size (rwnd) (receiver buffer size)- kept once per association congestion control window (cwnd) (network conditions)- kept for every path in association slow start threshold (ssthresh)- kept for every path in association partial_bytes_acked (difference to TCP) because also chunks are acknowledged. 20

21 Data transmission concepts Congestion control modes of operation Slow start: for every correct received and acknowledged packet cwnd is increased (sender can send faster) Congestion avoidance: After cwnd reaches ssthresh value. cwnd is still increased, but slower. 21

22 Multihoming use of multihomed nodes reachable under several addresses possibly physically different connection paths (network failure tolerance) Addresses are exchanged in the INIT and INIT-ACK chunks Inactive paths are monitored by sending HEARTBEAT chunks (answered by HEARTBEAT-ACK chunks) 22

23 Streams Transfer and delivery are seperated Two levels of operation Data transfer level (checksum, sequence number, SACK) Data delivery level: delivering of data in correct order Enables specific adoption of protocol If strict order is not necessary, there is option of order-of-arrival delivery 23

24 Questions How many messages are exchanged during SCTP association and what is the advantage of this method? 4 messages are exchanged: INIT, INIT-ACK, COOKIE-ECHO, COOKIE-ACK The advantage is, that the receiver has to allocate all resources not until the third message was exchanged. That is a security mechanism against attacks like blind spoofing. 24

25 Questions What is the main difference in the SCTP protocol compared to TCP and what is the advantage? The concept of multihoming. More than one connection (stream) can be established during one association. If this streams use different physical paths, network failure can be compensated. 25

26 References [RFC2960] Network Working Group; Stream Control Transmission Protocol; 2000; [SCTP] A. Jungmaier; SCTP for beginners;