This document provides an overview of transport layer protocols TCP, UDP, and SCTP. It discusses the history and evolution of TCP, including key developments like congestion control algorithms. UDP is described as a connectionless and unreliable protocol. SCTP is introduced as a protocol developed to transport telephony signaling over IP networks. It addresses limitations of TCP like head-of-line blocking and provides features like multi-homing and message orientation. The document defines SCTP terminology and describes its chunks, states, congestion control approach, and similarities to TCP. In summary, it serves as a high-level introduction to transport protocols with a focus on motivations and capabilities of SCTP.

1. Transport Layer
TCP/UDP/SCTP FUNDAMENTAL CONCEPTS AND OVERVIEW
Hamid Reza Bolhasani
DATA SCIENTIST
PHD, COMPUTER ENGINEERING
H.BOLHASANI@GMAIL.COM
JAN 2020.

2. 2
Table of Contents
 History
 Introduction
 TCP
 UDP
 MP-TCP
 SCTP
 Motivation of Developing SCTP
 Comparison of Transport Protocols
 Features and Advantages of SCTP
 Role of SCTP in Fixed/Mobile Networks
 Question & Answer
 Conclusion

3. 3
 History

4. 4
 Introduction

5. 5
 Transport Layer Role and Services

6. 6
 Transport Layer Role and Services

7.  Transport Layer Role and Services
 End-to-End Delivery, Addressing, Reliable Delivery, Flow Control, Multiplexing
 Serve Application Layer
 Track individual communications
 Segment data and manage each peace of data
 Re-assemble segments back into streams of application data at the receiving host
 Identify the different applications using port numbers
 Communication between processes

8.  Transport Layer
 TCP
 UDP
 SCTP
 UDP-Lite
 RUDP
 MPTCP
 DCCP

9.  Evolution of TCP (1/2)
1975 1980 1985 1990
1982
TCP & IP
RFC 793 & 791
1974
TCP described by
Vint Cerf and Bob Kahn
In IEEE Trans Comm
1983
BSD Unix 4.2
supports TCP/IP
1984
Nagel’s algorithm
to reduce overhead
of small packets;
predicts congestion
collapse
1987
Karn’s algorithm
to better estimate
round-trip time
1986
Congestion collapse
observed
1988
Van Jacobson’s
algorithms
congestion avoidance and
congestion control
(most implemented in
4.3BSD Tahoe)
1990
4.3BSD Reno
fast retransmit
delayed ACK’s
1975
Three-way handshake
Raymond Tomlinson
In SIGCOMM 75

10.  Evolution of TCP (2/2)
1993 1994 1996
1994
ECN
(Floyd)
Explicit
Congestion
Notification
1993
TCP Vegas
(Brakmo et al)
real congestion
avoidance
1994
T/TCP
(Braden)
Transaction
TCP
1996
SACK TCP
(Floyd et al)
Selective
Acknowledgement
1996
Hoe
Improving TCP
startup
1996
FACK TCP
(Mathis et al)
extension to SACK

11.  TCP Introduction
 Communication Abstraction
 Reliable
 Ordered
 Point-to-Point
 Byte-Stream
 Full-Duplex
 Flow and Congestion Controlled
 Protocol Implemented entirely at the ends
 Fate Sharing
 Sliding Window with Cumulative Acks
 Ack field contains last in-order packet received.
 Duplicate Ack sent when out-of-order packet received

12.  TCP Segment Header

13.  TCP Services
Port Protocol Use
21 FTP File Transfer
23 Telnet Remote login
25 SMTP E-mail
69 TFTP Trivial File Transfer Protocol
79 Finger Lookup info about a user
80 HTTP World Wide Web
110 POP-3 Remote e-mail access
119 NNTP USENET news

14.  TCP / 3 Way Handshaking
Host A Host B

15.  TCP / 4 Way Termination
Host A Host B

16.  TCP Signaling
Host A (Client) Host B (Server)
socket
bind
listen
accept (blocks)
socket
connect (blocks)
connect returns
accept returns
read (blocks)
write
read (blocks)
write
read (blocks)
read returns

17.  TCP Window Flow Control
Host A Host B
t1
t2
t3
t4
t0

18.  TCP / Sliding Window

19.  TCP Congestion Control (1/3)
(a) A fast network feeding a low capacity receiver.
(b) A slow network feeding a high-capacity receiver.

20.  TCP Congestion Control (2/3)
Congestion
window
10
5
15
20
0
Round-trip times
Slow
start
Congestion
avoidance
Congestion occurs
Threshold
Source Destination

21.  TCP Congestion Control – Tahoe Algorithm (3/3)
 Slow Start
 Connection starts wit cwnd=1.
 For each Ack  cwnd:= cwnd+1
 Exponential Increase
 cwnd doubled every RTT.
 Congestion Avoidance (CA)
 Upon receiving Ack with cwnd > ssthresh  Increase cwnd by 1/cwnd
 Results in Additive Increase
 Multiplative Decrease
 Slow Start is over.
 cwnd > ssthresh
 Until (Loss Events) { Every w Segments ACKed: cwnd++}
 ssthresh:= cwnd/2, cwnd=1, perform slow-strat.

22.  UDP
 User Datagram Protocol
 Connectionless
 Unreliable
Source Port Destination Port
UDP Length UDP Checksum
Data
0 16 31

23.  UDP / Well Known Services

24.  TCP vs UDP

25. 25
History
 Primary motivation: Transportation of telephony signaling messages
over IP Networks.
1991
TCP Failure
Oct. 2000
SCTP - RFC2960
1998
MDTP submission
(UDP based)
1997
MDTP work
began1992-1997
UDP Reliability
Experiments
RFC 3257
Apr. 2002
RFC 3286
May 2002
RFC 3309
Sep. 2002
RFC 3436
Dec. 2002

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 What is SCTP?
 SCTP (Stream Transmission Control Protocol, RFC 2960) is
a transport protocol on OSI layer 4, like TCP or UDP.
 SCTP was specifically designed as a transport protocol for
telephony signaling message transport.
IP
Network
IPv4/IPv6
Application
Link Layer
UDP TCP SCTP
Physical Layer

27. 27
 RFCs
 RFC 2960 – Stream Control Transmission Protocol
 RFC 3257 – SCTP Applicability Statement
 RFC 3286 – An introduction to SCTP
 RFC 3309 – SCTP Checksum Change
 RFC 3436 – Transport Layer Security over SCTP
 RFC 3758 – SCTP Partial Reliability Extension
 RFC 4960 – Transport PSTN signaling message over IP networks

28. 28
 Motivations for Developing SCTP
 Developed by IETF SIGTRAN working group
 All-IP Networks.
 3GPP networks based on IPv6 possible.
 To create a new, IP based transport protocol.
 For transport of signaling data over IP-based networks.
 Signaling between SG, STP, MGW, HSS, MSC,…

29. 29
 Motivations for Developing SCTP
 Problems of TCP
 Byte-Stream Oriented
 No built-in support for multi-homed IP hosts
 Vulnerable for SYN flooding attacks
 Provide strict ordering of information which causes HOL Blocking problem.
 Problems of UDP
 Unreliable Data Transfer
 No Congestion/Flow control

30. 30
 SYN Flooding
SYN
victim Flooded!!
TCB
TCB
TCB
TCB
TCB
• There is no ACK in response to the SYN-ACK, hence connection
remains half-open
• Other genuine clients cannot open connections to the victim
• The victim is unable to provide service
attackers
128.3.4.5
221.3.5.10
Unavailable, reserved resources
SYN-ACK
ACK
SYN
SYN-ACK
ACK

31. 31
 Head-of-Line Blocking in TCP
Sender Receiver
ACK 1
1
2
3
4
5
ACK 2
ACK 2
ACK 2
Packet 3 is blocking the
head of the line.
1
2
Receiver’s App

32. 32
 Head-of-Line Blocking Problem in TCP
 TCP provides single data stream.
 When a segment is lost, subsequent segments must wait to be processed.
 Problem for some applications (Telephony)

33. 33
 SCTP Features (1/3)
 Provide an end-to-end reliable transmission service over IP networks
 Support multiple streams per path
 Support multi-homed hosts
 Multiple IP Addresses per host
 More tolerant to network failures
 Message-oriented: Conserve message boundaries
 Unordered delivery: SCTP can deliver messages as ordered or unordered.
 Congestion control: SCTP congestion control is similar to TCP
 SCTP is rate adaptive similar to TCP

34. 34
 SCTP Features (2/3)
 Fragmentation
 Detection of path MTU and fragmentation of data chunks to fit into available path MTU.
 Error Correction
 Acknowledged error-free, non-duplicated data transfer.
 Congestion Avoidance
 Similar functionality as in TCP to avoid congestion to build up in the network.

35. 35
 SCTP Features (3/3)

36. 36
 SCTP Terminology
 Chunk
 SCTP Association
 Path
 Stream
 Verification Tag
 Message Authentication Code
 Transmission Sequence Number (TSN)
 Stream Sequence Number (SSN)

37. 37
 SCTP Terminology

38. 38
 SCTP Packets
An SCTP-Protocol Data Unit with several chunks

39. 39
 SCTP Chunks

40. 40
 SCTP Payload (Data, ID=0)
 Stream Identifier: Identifies the stream to which the following user data belongs to.
 Stream Sequence Number: This value represents stream sequence number.

41. 41
 Initiation (INIT)
 Initiate Tag: This value MUST be placed into the verification tag field of every
SCTP packets.
 Advertised Receiver Window Credit (a_rwnd): This value represents the
dedicated buffer space.

42. 42
 Initiation Acknowledgment (INIT ACK)
 The parameter part of INIT ack is formatted similarly to the INIT chunk. It uses
two extra variable parameters: The State Cookie and the Unrecognized parameter:

43. 43
 Selective Acknowledgment (1/5)

44. 44
 Selective Acknowledgment (2/5)
 SACK is sent to the peer endpoint to acknowledge received DATA chunks and to
inform the peer endpoint of gaps in the received subsequences of DATA chunks as
represented by their TSNs.
 Cumulative TSN Ack: This parameter contains the TSN of the last DATA chunk
received in sequence before a gap.
 Gap Ack Block Start: Indicates the Start offset TSN for this Gap Ack Block.
 Gap Ack Block End: Indicates the End offset TSN for this Gap Ack Block.
 Gap Ack Blocks: TSNs >= (Cumulative TSN Ack + Gap Ack Block Start) and
TSNs <= (Cumulative TSN Ack + Gap Ack Block End)
 These TSNs are assumed to have been received correctly.

45. 45
 Selective Acknowledgment (3/5)
 Duplicate TSN
 Indicates the number of times a TSN was received in duplicate since the
last SACK was sent.
 Every time a receiver gets a duplicate TSN (before sending the SACK) it
adds it to the list of duplicates. The duplicate count is re-initialized to
zero after sending each SACK.

46. 46
 Selective Acknowledgment (4/5)
TSN=17
miss
TSN=15
TSN=14
miss
TSN=12
TSN=11
TSN=10
Cumulative TSN Ack = 12
a_rwnd = 4660
Num of block=2 Num of dup=0
Block #1 start=2 Block #1 end=3
Block #2 start=5 Block #2 end=5
12+2 ~ 12+3
12+5 ~ 12+5

47. 47
 Selective Acknowledgment (5/5)
TSN=16
TSN=15
TSN=14
TSN=13
TSN=13
TSN=12
miss
TSN=10
Cumulative TSN Ack = 10
a_rwnd = 4660
Num of block=1 Num of dup=1
Block #1 start=2 Block #1 end=6
Duplicate TSN 13
10+2 ~ 10+6

48. 48
 SCTP States
 Association Establishment and Shutdown.
 SCTP uses a cookie mechanism in a four-way
handshake to establish an association.
 The shutdown process is a three-way handshake.

49. 49
 SCTP Congestion Control (1/5)
 According to RFC2960, the congestion control behavior of an SCTP implementation
may have an impact where timely delivery of messages is required.
 The congestion control mechanisms for SCTP have been derived from RFC 2581 - TCP
Congestion Control, and been adapted for multi-homing.
 For each destination address (i.e. each possible path), a discrete set of flow and
congestion control parameters is kept.
 From the point of view of the network, an SCTP association with a number of paths
may behave similarly as the same number of TCP connections.

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 SCTP Congestion Control (2/5)
 Similar to TCP, SCTP has two modes, Slow Start and Congestion Avoidance.
 The mode is determined by a set of congestion control variables, which are path specific.
 For successfully delivered and acknowledged data, the congestion window variable
(CWND) is steadily increased, and once it exceeds a certain boundary (called Slow
Start Threshold, SSTRESH), the mode changes from Slow Start to Congestion
Avoidance.
 In Slow Start, the CWND is increased faster (roughly one MTU per received SACK
chunk), and in Congestion Avoidance mode, it is only increased by one MTU per Round
Trip Time (RTT) measurement.

51. 51
 SCTP Congestion Control (3/5)
 RTO timeout or fast retransmission will trigger retransmission and cause the
SSTHRESH to be cut down drastically and reset the CWND.
 Fast Retransmission
 ssthresh = max(cwnd/2, 2*MTU)
 cwnd = ssthresh
 RTO timeout
 ssthresh = max(cwnd/2, 2*MTU)
 cwnd = MTU

52. 52
 SCTP Congestion Control – Fast Retransmit (4/5)
Sender Receiver
ACK 1
1
2
3
5
6
ACK 2
ACK 2
ACK 2
ACK 2
4
ACK 6
retransmit 3
four consecutive
SACKs

53. 53
 SCTP Congestion Control (5/5)

54. 54
Thanks ForYour Attention!
Hamid Reza Bolhasani
Bolhasani@gmail.com