Computer Networking: Transport Layer

• Transport layer services
• Multiplexing and demultiplexing
• Connectionless transport: UDP
  • Why we need UDP?
  • Use case of UDP
  • Internet checksum
• Principles of reliable data transfer
  • rdt 1.0
  • rdt 2.0
  • rdt 2.1
  • rdt 2.2
  • rdt 3.0
  • Pipe lining: increased utilization
• Connection-oriented transport: TCP
  • rdt in TCP
    • TCP fast retransmit
  • Flow control
  • Connection management
    • Establishing TCP connection
      • Why 2-way handshake don’t work in all scenarios:
      • TCP 3-way handshake
    • Closing TCP connection
• TCP congestion control
  • Approaches towards congestion control
  • Throughput based TCP congestion control: AIMD
  • Delay based TCP congestion control
  • TCP fairness
• Evolution of transport-layer functionality

Transport layer services

• provide logical communication between application processes running on different hosts
• transport protocols actions
  • sender: breaks messages into segments, passes to network layer
  • receiver: reassembles segments, passes to application layer
• 2 protocols
  • TCP
  • UDP

Multiplexing and demultiplexing

multiplexing: 复用

Sender handle data from multiple sockets, add transport header (later used for demultiplexing).

Receiver use header info to deliver received segments to correct socket.

Host uses IP addresses and port numbers to direct segment to appropriate socket.

• UDP: demultiplexing using destination port number (only)
• TCP: demultiplexing using 4-tuple: source and destination IP addresses, and port numbers

Connectionless transport: UDP

• “best effort” service
• segment may be lost or out of order
• connectionless, no hand shaking

Why we need UDP?

• No connection establishment
• Simple: no state
• No congestion control

Use case of UDP

• streaming multimedia apps
• DNS
• SNMP
• HTTP/3

If reliable transfer needed over UDP (e.g. HTTP/3):

• add reliability at application layer
• add congestion control at application layer

Internet checksum

This is a weak protection, two flipped bits can have same checksum.

Principles of reliable data transfer

How to build reliable data transfer on top of unreliable channel?
rdt: reliable data transfer

rdt 1.0

Reliable transfer over a reliable channel: easy.

rdt 2.0

Channel with bit errors, how to recover?

• ACK: receiver explicitly tells sender that packet received OK
• NAK - negative ACK: receiver explicitly tells sender that packet had errors
  • sender then retransmits packet on receipt of NAK

“Stop and wait”:
Sender sends one packet, then waits for receiver response.

Flaw of rdt 2.0:

• ACK or NAK can be corrupted
• stop and wait is slow

rdt 2.1

sender

• seq added to each packet

rdt 2.2

No NAK, use ACK of last seq instead.

rdt 3.0

Add a timer, which is set when a packet is sent, and sender will resend when timer goes off.

Still, performance is poor due to only one packet is in transfer each moment.

Pipe lining: increased utilization

Go Back N

• sender
  • maintain a “window” of size up to N, with transmitted but unACKed packets
  • cumulative ACK: receiving ACK seq n means ACK all packets up to, including seq n
• receiver
  • ACK only
    • may generate duplicate ACKs
    • need only remember the seq of next needed packet (in order)
  • on receipt of out-of-order packet
    • can discard or buffer these packets
    • re-ACK the highest in-order seq

Selective repeat

• sender
  • data from above: if net available seq in window, send packet
  • timeout: resend packet n, restart timer
  • ACK n in [sendbase, sendbase + N-1]
    • mark packet n as received
    • if n smallest unACKed packet, advance window base to next unACKed seq
• receiver
  • packet n in [rcvbase, rcvbase + N-1]
    • send ACK n
    • out-of-order: buffer
    • in-order: deliver, advance window to next not-yet-received packet
  • packet n in [rcvbase - N, rcvbase - 1]
    • ACK n
  • otherwise: ignore

Connection-oriented transport: TCP

• point-to-point
• reliable, in-order byte steam
• full duplex data
• cumulative ACKs
• pipe lining
• connection-oriented
• flow controlled

rdt in TCP

Timer setting

TCP round trip time, timeout:

$$\begin{align} &EstimatedRTT = (1 - \alpha) \cdot EstimatedRTT + \alpha \cdot SampleRTT \\ &TimeoutInterval = EstimatedRTT + 4 \cdot DevRTT \\ &DevRTT = (1 - \beta) \cdot DevRTT + \beta \cdot |SampleRTT - EstimatedRTT| \end{align}$$

• $DevRTT$ here is a “safety margin”
• typically, $\alpha = 0.125$
• typically, $\beta = 0.25$

TCP fast retransmit

If sender receives 3 additional ACKs for same data (“triple duplicate ACKs”), resend unACKed segment with smallest seq, because it is likely that unACKed segment lost, so don’t wait for timeout.

Flow control

Receiver controls sender, so sender won’t overflow receiver’s buffer by transmitting too much, too fast.

TCP receiver put the free buffer space left in rwnd field in TCP header, sender limits amount of packets to the size of rwnd.

Connection management

Before exchanging data, sender and receiver do “handshake”.

• Agree to establish connection
• Agree on connection parameters (e.g. starting seq)

Establishing TCP connection

Why 2-way handshake don’t work in all scenarios:

and here is a never ending half open connection with no client

TCP 3-way handshake

Closing TCP connection

• client, server each close their side of connection
  • send TCP segment with FIN bit = 1
• respond to received FIN with ACK
  • on receiving FIN, ACK can be combined with own FIN
• simultaneous FIN exchanges can be handled

TCP congestion control

Approaches towards congestion control

• End-end congestion control
• Network-assisted congestion control

Throughput based TCP congestion control: AIMD

Additive Increase and Multiplicative Decrease

Sending rate is cut in half on loss detected by triple duplicate ACK, and cut to 1 MSS (maximum segment size) when loss detected by timeout.

Why AIMD?

• Optimize congested flow rates network wide
• have desirable stability properties

TCP sending behavior

$$TCP \; rate \approx \frac{cwnd}{RTT} \; bytes/sec$$

TCP sender limits transmission

$$LastByteSent - LastByteAcked \leq cwnd$$

TCP slow start: When connection begins, increase rate exponentially until first loss event. Initially, cwnd = 1 MSS, double cwnd every RTT. Done by incrementing cwnd for every ACK received.

TCP CUBIC: By contrast with TCP Reno, which increases linearly, CUBIC increase TCP sending rate gradually slower when approaching K.

K: point in time when TCP window size will reach $W_{max}$

Delay based TCP congestion control

$$measured\;throughput = \frac{bytes\; sent \;in\;last\;RTT\;interval}{RTT_{measured}}$$

Explicit congestion notification: “detect congestion before loss occurs”.

TCP fairness

The general goal: If K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of $\frac{R}{K}$.

Evolution of transport-layer functionality

Evolving transport layer functionality

QUIC: Quick UDP Internet Connections