TSIN02 - Internetworking

Transcription

1 Lecture 5: SCTP Litterature: RFC3257 SCTP Applicability Statement RFC3286 Introduction to SCTP Forouzan 3 rd ed, Chapter 13 (optional) RFC2960 (optional extra material) RFC3309 (optional extra material) 2004 Image Coding Group, Linköpings Universitet

2 Lecture 5: SCTP Outline: What is SCTP? Why SCTP? SCTP Architecture SCTP Header Establishing connections 2

3 What is SCTP? SCPT is a messageoriented, reliable protocol that combines features from TCP and UDP. But we already have TCP, UDP (and RTP) at the transport layer! Figure from Forouzan 3

4 Why SCTP? There are some limitations in TCP: TCP use a strict order-of-transmission delivery of data TCP is stream-oriented on a byte level TCP have no support for multihoming TCP does not scale well TCP does not support timer control TCP is relatively vulnerable to denial-of-service attacks UDP limitations: Unreliable Transmission No error or congestion control No check for out-of-order or duplicated messages PSTN signaling has been the main motivation for SCTP 4

5 PSTN Signaling Requirements When (1998) the SIGTRAN working group started to design a transport protocol for transport of signaling protocols they identified the following points as important (RFC2719): Transport a variety of SCN (Switched Circuit Network) protocol types, eg MTP3, ISUP, SCCP, TCAP etc., with the ability of providing a way to identify the specific SCN protocol being transported. Provide a common base protocol defining header formats, security extensions and procedures for signaling transport, and support extensions to add individual SCN protocols if needed. Together with IP, provide the relevant functionality as defined by the SCN lower layer. Support the ability to multiplex several higher layer SCN sessions on one underlying signaling transport session. This allows, for example, several DSS1 D-channel session sto be carried in one signaling transport session Be able to transport complete messages of greater length than the underlying SCN segmentation/reassembly limitations. Allow for a range of suitable robust security schemes to protect signaling information being carried across networks. Provide for congestion avoidance on the internet by supporting appropriate controls on signaling traffic generation and reaction to network congestion. 5

6 PSTN Signaling Requirements cont. Relevant lower layer functionality may include: flow control in sequence delivery of signalling messages within a control stream logical identification of the entities on which the signaling messages originate or terminate logical identification of the physical interface controlled by the signalling message error detection recovery from failure of components in teh transit path retransmission and other error correcting methods detection of unavailability of peer entities 6

7 More desirable features Ability to discover the Maximum Tranfer Unit (MTU) of the path used and possiblity to fragment user data to conform to this MTU Possibility of sending user messages within multiple streams inside the same association. Sequenced delivery of the user messages sent through the same stream, and possibility of order-of-arrival delivery of individual user messages. Possibility of bundling multiple user messages into a single packet. 7

8 SCTP - early proposals Many protocols were suggested, eg Reliable UDP UDP for TCAP (T/UDP) Simple SCCP Tunneling Protocol PURDET Multi-Network Datagram Transmission Protocol (MDTP) All of the above were designed to run on top of UDP! Finally SCTP were developed from MDTP. 8

9 SCTP History In 1998 The SIGTRAN working group was formed by the IETF. The motivation was to move existing telephone signaling protocols to run over IP. At that time several different protocols were suggested, all running over UDP. Finally SCTP was designed based on a protocol called MDTP (Multi-network Datagram Transmission Protocol). RFC2960 was published in October After that, the Transport Area Working Group (TSVWG) took over the responsibility for the continued work on SCTP. Some updates and extentions have been designed, eg an updated checksum algorithm. 9

10 SCTP Services SCTP offers the following services: acknowledged error-free non-duplicated transfer of user data data fragmentation to conform to discovered path MTU size sequenced delivery of user messages within multiple streams, with an option for order-of-arrival delivery of individual user messages optional bundling of multiple user messages into a single SCTP packet network-level fault tolerance through supporting of multihoming at either or both ends of an association The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks. 10

11 SCTP Features Connection state at endpoints Reliable data transfer Congestion control and avoidance Message boundary conservation Path MTU discovery and message fragmentation Message bundling Multi-homed hosts support Multi-stream support Unordered data delivery possible State cookie against SYN flood attack Built in heartbeat (reachability check) 11

12 Multiple Streams In SCTP the data reliability mechanism has been separated from the message ordering mechanism A Stream in SCTP means a sequence of messages Independent messages can be transferred in parallel without any interdependency (ie in different streams) Unordered message delivery is possible Figure from Forouzan 12

13 Multi-homed Support Several IP addresses can be listed during handshake Both IPv4 and IPv6 addresses are valid. One path is chosen as primary Other paths are checked for availability through Heartbeat Retransmissions are send on different path Switch path if congested. Figure from Forouzan 13

14 Security Considerations SCTP have been designed with these security objectives: availability of reliable and timely data transport services integrity of the user-to-user information carried by SCTP SCTP includes mechanisms that protects against blind denial-of -service attacks (A blind attack is one where the attacker is unable to intercept or othervise see the content of data flows passing to and from the target SCTP node) These include the 4-way handshake with a cookie and delayed commitment to resources. 14

15 Error Control SCTP uses checksums and retransmissions to achieve reliable transfers SCTP have built in support for selective acknowledgement 15

16 Flow and Congestion Control Flow and Congestion control in SCTP is mostly similar to that of TCP, ie sliding windows, slow start, additive increase etc Differences are due to: SCTP supports multiple streams, all are handled by the same congestion control. SCTP supports multihoming, the different paths typically have different congestion control parameters. 16

17 SCTP Association The communication relationship is called an SCTP Association There can be only one association between two endpoints. An endpoint is defined by a port number and one or more IP addresses. SCTP User App. SCTP Transport Service TCP SCTP Transport Service IP Network Service IP Network Service SCTP Node A One or more IP-address appearance SCTP Node B 17

18 Endpoint Responsibilities Stream queue management User message fragmentation Generate acknowledgements Congestion avoidance Data bundling Packet validation Path management 18

19 Connection Establishement In SCTP connection is established through a 4-way handshake: Figure from Forouzan 19

20 Closing a Connection SCTP Uses a 3-message procedure to allow for a graceful shutdown. There is also a non-graceful abort possible. SCTP does not support half-open connections Figure from Forouzan 20

21 State diagrams During connection establishement the SCTP endpoints goes through a set of states in response to various events. From any state rcv INIT (Generate cookie snd INIT ACK) CLOSED (create TCB snd INIT strt init timer) rcv valid COOKIE ECHO (create TCB snd COOKIE ACK) COOKIE WAIT COOKIE-ECHOED rcv COOKIE ECHO (snd COOKIE ECHO, stop init timer, strt cookie timer ESTABLISHED rcv COOKIE ACK (stop cookie timer) 21

22 State Example Figure from Forouzan 22

23 SCTP Building Blocks The data unit it SCTP is a chunk. Contains at most one message from the sender All chunks in an association are numbered using Transmission Sequence Numbers (TSN), in order. There are two types of chunks, data chunks and control chunks. Each stream in an association is identified with its Stream Identifier (SI). In each stream the chunks are numbered using Stream Sequence Numbers (SSN), in order. Several chunks, possibly from different streams, can be sent at the same time in SCTP packets. A packet contains a packet header and a number of chunks. 23

24 SCTP: Packet, chunks and streams Figure from Forouzan 24

25 SCTP Packet Format Figure from Forouzan 25

26 TCP SCTP Comparison Figure from Forouzan 26

27 Chunk Field Format Chunk Type (8 bits): Identifies the type of information contained in the Chunk Value field. Chunk Flags (8 bits): Usage depends on the chunk type. Chunk Length (16 bits): The size of the chunk in bytes, including type, flags, length and value fields. Chunk Value (variable length): Contains the actual information to be transmitted. Figure from Forouzan 27

28 Chunk Types Figure from Forouzan 28

29 Chunk type extensions New chunk types may be developed. Chunk types are ordered so that the highest order two bits specify the action that must be taken if the endpoint does not recognise the chunk type stop processing and discard the packet 01 - stop processing and discard the packet and send an error report 10 - skip this chunk and continue processing 11 - skip this chunk and continue processing and send an error report 29

30 DATA Chunk type U Unordered B Beginning of message E End of message Figure from Forouzan 30

31 INIT Chunk Figure from Forouzan 31

32 INIT ACK Chunk Figure from Forouzan 32

33 COOKIE ECHO and ACK Chunks Cookie Echo: Cookie Ack: Figures from Forouzan 33

34 SACK Chunk Figure from Forouzan 34

35 Applications Telephone signaling SS7, SIP, MEGACO... Web pages Ftp Mobile SCTP MPEG4 video (?) Instant messaging AAA messaging - DIAMETER... 35

36 Timers in SCTP The following timers are used within SCTP: init timer cookie timer retransmission timer shutdown timer heartbeat timer 36

37 Data Transfer Example Figure from Forouzan 37

38 Available Implementations Open source can be found at Huges Artesyn Communication Products Inc... SCTP is currently included in SUN Solaris, HP-UX, IBM AIX... Proprietary implementations have been made by Cisco Siemens Nokia Ericsson?... 38

39 References WWW: Articles: Books: Improving File Transfers Using SCTP Multistreaming by S. Lahda and P.D.Amer, IPCCC 2004 Using SCTP with Partial Reliability for MPEG-4 Multimedia Streaming by M. Molteni and M. Villari, BSDCon Europe 2002 A Modified SCTP Handover Scheme for Real Time Traffic by A. Kelly, P. Perry and J. Murphy, HETNETs '03 Stream Control Transmission Protocol (SCTP) A Reference Guide by R.R Stewart and Qiaobing Xie 39

40 SCTP Summary SCTP is a new transport protocol that like TCP provides connection-oriented and reliable tranmission of data unlike TCP supports multiple streams within one connection unlike TCP supports multihoming Have built in protection against blind denial-of-service attacks was designed for use with PSTN signaling. Can be used for transmission of SIP signals, webpages, ftp mobile SCTP... 40