Transport Layer of TCP/IP. Transmission Control Protocol (TCP) basics and network sockets explained. How TCP connection get established, error recovered and terminated. User Datagram Protocol and its comparison to TCP. Quality of Service (QoS).

1. Andriy Berestovskyy
2017
Networking Fundamentals
Transport Protocols
( ц ) А н д р
і й Б е р е с
т о в с ь к и
й
networking hour
TCP
UDP
NAT
IPsec
IPv4
IPv6
internet
protocolsAH
ESP
authentication
authorization
accounting
encapsulation
security
BGP
OSPF
ICMP
ACLSNAT
tunnel
PPPoE
GRE
ARP
discovery
NDP
OSI
broadcast
multicast
IGMP
PIM
MAC
DHCP
DNS
fragmentation
semihalf
berestovskyy

2. Networking Fundamentals
● Computer Networks
● Networking Models
● Local Networks
● Internet
● IPv4 Routing
● > Transport Protocols
2

3. IPv4 Transport Layer
3
Application
Presentation
Session
Transport
Network
Data Link
Physical
Frame
Bits
MAC Data FCS
IP Data
TCP Data
Packet
Segment
Data
7
6
5
4
3
2
1
Hi!
Salut!

4. Transmission Control Protocol
● Connection Oriented
● Ordered Delivery
● Error Recovery
● Flow Control
● Multiplexing
4
How?
Other L4
protocols?

5. Source/Destination Port — for multiplexing
Seq — seq number of the first data byte
Ack — next seq number sender is expecting
Data offset — size of header in 32-bit words, usually 5
Flags — Flags (ACK, PSH, RST, SYN, FIN, etc…)
Window Size —size of the receive window
Checksum — TCP header, data and pseudo-header
Urgent Pointer — offset to last urgent byte
Options — usually empty
TCP Header
5
Source Port Destination Port
Sequence Number
Acknowledgement Number
Off
TCP Checksum
Data
...
[Options …]
0 32
Protected by
TCP Checksum
DataOffset
Flags Window Size
Urgent Pointer
Pseudo
header?
Examples?
Ver IHL DSCP Total LengthE
ID Fl Fragment Offset
TTL Proto = TCP IP Header Checksum
Source Address
Destination Address
[Options …]
0 32
IHL
Protected by IP
Header Checksum

6. Transport Layer Multiplexing
google.com
Client
100.3.2.5
Server
1.1.1.1
DNS QUERYTo: 1.1.1.11
Src/dst IPs are
the same
How to
distinct?
HTTP GET /To: 1.1.1.12
HTTP GET logo.jpgTo: 1.1.1.13

7. Multiplexing using Sockets
google.com
Client
100.3.2.5
DNS QUERYTo: 1.1.1.1
Src: 1025
Dst: 53
1
HTTP GET /To: 1.1.1.1
Src: 1026
Dst: 80
2
HTTP GET logo.jpgTo: 1.1.1.1
Src: 1027
Dst: 8080
3
SOCKET 1:
IP: 1.1.1.1
Protocol: UDP
Port: 53
Why 1025?
SOCKET 2:
IP: 1.1.1.1
Protocol: TCP
Port: 80
SOCKET 3:
IP: 1.1.1.1
Protocol: TCP
Port: 8080
Socket?
Why 53?

8. Network socket — internal endpoint for sending or receiving data
at a single node in a computer network
— Wikipedia
8

9. Well-Known* Port Numbers
9
20
Port Proto Service
* See /etc/services for more
TCP File Transfer Protocol data connectionftp-data
Unix Name
21 TCP File Transfer protocol control connectionftp
22 TCP Secure shellssh
25 TCP Simple Mail Transfer Protocolsmtp
53 UDP Domain Name System requestdomain
69 UDP Dynamic Host Configuration Protocolbootps
69 UDP Trivial File Transfer Protocoltftp
80 TCP World Wide Webhttp

10. Socket Connections
10
Client
DNS
Resolver
1025
Web
Browser
Web
Browser
1026 1027
TCP
(100.3.2.5, TCP, 1027)
(100.3.2.5, TCP, 1026)
(100.3.2.5, UDP, 1025)
IP 100.3.2.5
UDP
Server
DNS
Server
53
Web
Server
Web
Server
80 8080
TCP
IP 100.3.2.5
UDP
(1.1.1.1, TCP, 8080)
(1.1.1.1, TCP, 80)
(1.1.1.1, UDP, 53)

11. TCP Connection Establishment
Server
Port: 80
Src: 1026, Dst: 80
SYN 132
1
Client
Port: 1026
Source Port Destination Port
Sequence Number
Acknowledgement Number
Off
TCP Checksum
Data
...
[Options …]
0 32
Flags: SYN ACK Window Size
Urgent Pointer
Src: 80, Dst: 1026
SYN 69, ACK 132+1
2
Src: 1026, Dst: 80
ACK 69+1
3
11

12. TCP Error Recovery
12
TCP Client TCP
ACK 701
Data
[ ...
HTTP
GET /
HTTP
OK
TCP
SEQ 70
...SEQ 80
… ]SEQ 90
ACK 80
...SEQ 80
2
3
4
6
TCP Server
5

13. TCP Connection Termination
Server
Port: 80
Src: 1026, Dst: 80
FIN 99
1
Client
Port: 1026
Source Port Destination Port
Sequence Number
Acknowledgement Number
Off
TCP Checksum
Data
...
[Options …]
0 32
Flags: FIN ACK Window Size
Urgent Pointer
Src: 1026, Dst: 80
ACK 142+1
4
Src: 80, Dst: 1026
ACK 99+1
2
Src: 80, Dst: 1026
ACK 99+1, FIN 142
3
13

14. User Datagram Protocol
● Connection Oriented
● Connectionless
● Ordered Delivery
● Error Recovery
● Flow Control
● Multiplexing
14
Usage?

15. Source/Destination Port — for multiplexing
Length —length of UDP header and data
Checksum — header, data and pseudo-header
UDP Header
15
Source Port Destination Port
Length
Data
...
0 32
Protected by
UDP Checksum
8B
UDP Checksum
Pseudo
header?
Ver IHL DSCP Total LengthE
ID Fl Fragment Offset
TTL Proto = UDP IP Header Checksum
Source Address
Destination Address
[Options …]
0 32
IHL
Protected by IP
Header Checksum

16. TCP vs UDP
16
UDP TCP
Header size
Feature
Data unit
Connection oriented
Reliable transport
Preserve message boundary
Ordered delivery
Data checksum
Flow control

17. TCP vs UDP
17
8
UDP TCP
20-60Header size
Feature
Datagram SegmentData unit
No YesConnection oriented
No YesReliable transport
Yes NoPreserve message boundary
No YesOrdered delivery
Optional YesData checksum
No YesFlow control Flow
control?

18. Quality of Service (QoS) — measurement of the overall
performance of a service seen by the users of the network
— Wikipedia
18

19. Bandwidth
— number of bits available per time in a communication channel
End-to-end delay
— time for a packet to be transmitted across a network
Jitter
— deviation in end-to-end delays
Packet loss
— rate of packets lost / packets sent
Quality of Service Characteristics
19
End-to-end?

20. End-to-End QoS
LAN
End-to-End QoS

21. Example Services Requirements
21
Bandwidth Delay
Web browsing
Service
Voice calling
Videoconference
Gaming
Jitter Loss
Peer-to-peer downloading

22. Example Services Requirements
22
Medium
Bandwidth Delay
MediumWeb browsing
Service
Low LowVoice calling
Medium LowVideoconference
Low LowGaming
Jitter Loss
Medium
Low
Low
Low
Medium
Low
Low
Low
High HighPeer-to-peer downloading High High
How?

23. 23
QoS Overview
Classification Policer
Drop
Scheduler
How?

24. 24
QoS Classification
Payload
...
MAC Destination
MAC Destination MAC Source
MAC Source
Ethertype
0 32
Frame Check Sequence
0x8100 VIDPCP D
Ver IHL DSCP Total LengthE
ID Fl Fragment Offset
TTL Protocol Header Checksum
Source Address
Destination Address
Payload
...
[Options …]
0 32
Ethernet Header IPv4 Header
DSCP — Differentiated Services Code Point (6 bits)
ECN — Explicit Congestion Notification (2 bits)
PCP — Priority Code Point (Class of service) (3 bits)
DEI — Drop Eligible Indicator (1 bit)

25. Transport Protocols Checklist
1. TCP services?
2. Socket?
3. TCP connection establishment?
4. UDP services?
5. TCP vs UDP?
6. QoS?
25

26. References
1. Kevin Wallace. CCNP Routing and Switching ROUTE 300-101 Official Cert Guide. Cisco Press, 2014
2. David Hucaby. CCNP Routing and Switching SWITCH 300-115 Official Cert Guide. Cisco Press, 2014
3. RFC 1122. Requirements for Internet Hosts — Communication Layers.
4. IANA Protocol Numbers: http://www.iana.org/assignments/protocol-numbers
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