Transport Layer Protocols

TCP & UDP

Transport Services and Protocols

• provide logical communication between app’ processes running on different hosts
• transport protocols run in end systems
• network layer: data transfer between end systems
• transport layer: data transfer between processes
  • relies on, enhances, network layer services

Transport-layer protocols

Internet transport services

• reliable, in-order unicast delivery (TCP)
  • congestion
  • flow control
  • connection setup
• unreliable (“best-effort”), unordered unicast or multicast delivery: UDP
• services not available:
  • real-time
  • bandwidth guarantees
  • reliable multicast

Multiplexing / Demultiplexing: What

segment - unit of data exchanged between transport layer entities

Demultiplexing - delivering received segments to correct app layer processes

Multiplexing / Demultiplexing: How

Multiplexing - gathering data from multiple app processes, enveloping data with header (later used for demultiplexing)

• based on sender, receiver port numbers, IP addresses
  • source, destination port #s in each segment
  • well-known port numbers for specific applications

Multiplexing / Demultiplexing: Examples

Socket

• Logical address assigned to a specific process running on a host computer
• The socket’s address combines the host computer’s IP address with the port number associated with a process

Port Numbers

• 0 to 1023 : “well-known” ports
  • systems restrict to use by privileged processes only.
  • IANA controls the mapping of service names to port numbers
• 1024 to 49151 “registered” ports
  • systems permit ordinary user processes to use.
  • IANA maintains a mapping of names to port numbers but does not exert control over them
• 49152 to 65535 : “dynamic” or “private” ports
  • are not subject to IANA registration.
• http://www.iana.org/assignments/port-numbers
• /etc/services – list of ports and services

UDP: User Datagram Protocol [RFC 768]

• “no frills,” “bare bones” Internet transport protocol
• “best effort” service, UDP segments may be:
• lost
• delivered out of order to app
• connectionless:
  • no handshaking between UDP sender, receiver
  • each UDP segment handled independently of others

Why is there a UDP?

• no connection establishment (which can add delay)
• simple: no connection state at sender, receiver
• small segment header
• no congestion control: UDP can blast away as fast as desired

UDP Uses and Format

• often used for streaming multimedia apps
  • loss tolerant
  • rate sensitive
• other UDP uses
  • DNS
  • SNMP
• reliable transfer over UDP: add reliability at application layer
  • application-specific error recovery

TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581

• point-to-point:
  • one sender, one receiver
• reliable, in-order byte stream:
  • no “message boundaries”
• pipelined:
  • TCP congestion and flow control set window size
• send & receive buffers

TCP: Overview (cntd)

• full duplex data:
  • bi-directional data flow in same connection
  • MSS: maximum segment size
• **connection-oriented: **
  • handshaking (exchange of control msgs) init’s sender, receiver state before data exchange
• flow controlled:
  • sender will not overwhelm receiver

TCP segment structure

TCP Sequence #’s and Acknowledgements

Seq. #’s:

• byte stream “number” of first byte in segment’s data

ACKs:

• seq # of next byte expected from other side
  cumulative ACK

TCP spec doesn’t say how receiver handles out-of-order segments

TCP in action: States

TCP States in Action

#ss -tn
State      Recv-Q Send-Q       	Local Address:Port               Peer Address:Port 
ESTAB      0      0		142.232.107.85:43420           	216.58.193.78:443 
FIN-WAIT-1 0      1		142.232.107.85:40359           	142.232.2.1:80
ESTAB      0      0		142.232.107.85:34221          	142.232.204.52:445
ESTAB      0      0          	142.232.107.85:48256          	142.232.204.50:445
SYN-SENT   0      1           	142.232.107.85:41974           	142.232.2.1:10123
ESTAB      0      0            	172.16.100.6:47180             	10.0.255.26:22
FIN-WAIT-1 0      1           	142.232.107.85:54207           	23.58.117.226:80
ESTAB      0      0            	142.232.107.85:36948          	142.232.204.52:445 

Reading List

• Managing firewall with nftables