464 Exam 1

ifconfig

Lists network interfaces

Network Reference Model

1. Application (HTTP, DNS, IMAP, SMTP)

2. Transport (TCP, UDP, MPTCP, QUIC)

3. Network (IP, DSR, AODV, DSDV)

4. Data Link (Ethernet, 802.11, Bluetooth)

5. Physical (Bits on wire)

Basic TCP Server

1. listen() - server

2. connect() - client

3. accept() - server

4. send() - server / client

5. recv() - client / server

6. close() - server / client

TCP

• Reliable ordered delivery (with retransmission if necessary)

• Congestion Avoidance and Control

• ACK sent to sender as confirmation after data received by receiver

• Issues: Often guesses wrong reason for packet loss in wireless (mobility and interference usually are the reason more than congestion)

• sudo sysctl -a | grep tcp: to see tcp configs

• Establish connection: Handshake 🙂 SYN (send open packet), SYN ACK (acknowledgement from dest), ACK (acknowledgement of dest ACK from src)

• Closing connection: FIN (Close and receive remaining bytes), FIN ACK (acknowledgement), RST (Reset to close and NOT receive remaining packets)

• ACKs is src’s sequence number + 1

Window Size

• Amount of data sent per round trip time

• Receiver’s advertised window (available buffer space)

• Congestion window (determined by sender from network feedback)

Packet Loss

• Detected by Retransmission Time Out (RTO) and DUPACKs

• DUPACK: Fast Retransmission indicates packet loss if 3 DUPACKs received. Retransmit after.

• RTO: If ACK not received before RTO timer fires, packet lost. RTO doubles for each time-out.

Congestion Avoidance & Control

• Slow Start: Exponential cwnd

• Congestion Avoidance: Linear cwnd

• Slow Start Threshold (ssthresh) = max[min(min(cwnd, receiver’s adv. window / 2), 2 * Min Segment Size)]

• Fast Recovery after Fast Retransmit (No Slow Start needed)

MPTCP

• Multipath TCP increases throughput

• Use sequence numbers to reassemble segments at the receiver

• If failure occurs, fall back on TCP

• Benefits: Higher throughput, failover from one path to another, seamless mobility

• Challenges: Out of order packets (RTT differences), hard to retransmit packet on different subflow

• Connection Established same way as TCP, except SYN ACK contains MP_CAPABLE

• ADD_ADDR and REMOVE_ADDR used to add/remove IP addresses to MPTCP connection

• MP_JOIN used to associate new subflow with existing MPTCP connection

• All subflows should add up to one TCP flow

• Picking a path depends. Least congested path has low loss, but high RTT, etc

• Uses key during connection setup to verify authenticity of created subflows

• Ex: Primary TCP Wifi connection, Backup TCP cellular connection

NAT

• Network Address Translators

• Change IP/Port numbers

TLS/SSL (OpenSSL)

• Transport Layer Security and Secure Socket Layer

• Standard for Internet Security

• Uses shared secret keys between client and sever

• Certificate Authorities (CA) verify public keys from websites

QUIC

• UDP is connectionless, so less reliable but faster than TCP

• Used for time-sensitive transmissions mostly

• QUIC combines UDP speed with TCP reliability

• Since it’s hard to make changes in TCP, its faster to implement new protocol on UDP

• Lost packets only impact the individual resource

• Encrypted

• Helps with congestion control and loss recovery

Mobile AdHoc Networks

• Mobility means route changes

• Formed without needing infrastructure

• Routes can contain multiple hops between nodes

Routing Protocols

• Next-Hop Table Based: Each node maintains a routing table

• Source Routing: Src node controls entire routing path

  • Easy to control loops

  • Src can choose between multiple routes