Computer Networking Lecture Notes

27 September 2012 • IP, UDP, & TCP

---

Outline

• Internet Protocol
• Processes, addresses, and ports.
• UDP
• TCP
  • Reliability, sequence numbers, and, timeouts.

Internet Protocol

• The Internet Protocol (IP, rfc 791).
  • Basic datagram service.
  • Route determination.
  • Network management (errors, address resolution).
• Most deployed IP is version 4.
  • A slow transition to IP version 6.
    • Larger, more structured addresses.
    • IP security (IPSec).

IP Service

• IP offers unreliable (best effort) datagrams.
  • “Datagram” means “message” (and usually, unreliable).
  • “Unreliable” means loss, duplication, corruption, reordering.
  • No delay, latency, or throughput guarantees.
• There are ancillary IP services too.
  • Address resolution (ARP), control messages (ICMP), …

IP Architecture

• IP’s hourglass architecture is the secret to TCP/IP success.

  IP can be ported (relatively) easily to new networks and data-link layers.

IPv4 Packets

• Protocols communicate by exchanging protocol data units (PDUs) or packets.
• Like most packets, IP packets have two parts.

  

• The payload is a TCP, UDP or other packet.
  • IP packets encapsulate TCP packets.
  • An argument for IP being a network-level protocol.

IPv4 Packet Header

IPv4 Addresses

• IPv4 has 32-bit addresses.
  • Traditionally printed as four eight-bit values: a.b.c.d (dotted quad notation).
  • Example: 192.100.64.153 (www.monmouth.edu).
• An IPv4 address has three parts (usually).

  

Addressing Example

IPv4 Address Classes

• There are five IPv4 address classes.

  

• Address classes became a problem.
  • Solved by CIDR.

Hosts and Addresses

• IP addresses interfaces rather than hosts.
  • This allows for multi-homed hosts.

  

• But this isn’t fine-grained enough.

Processes and Addresses

• An interface address doesn’t identify Internet-connected processes on a host.

  

  • All processes have the same IP address.
• An IP address needs more bits to identify a process on a host.

Ports

• A port is a 16-bit identifier assigned to processes.

  

• This is an example of multiplexing.
  • Multiplexing many process connections over a single IP address.

UDP

• The User Datagram Protocol (UDP, rfc 768) wraps IP to allow port addressing.
  • Otherwise, UDP provides IP service.
• Candidate UDP applications might be
  • Delay intolerant and loss tolerant; streaming media (Real-Time Protocol, RTP).
  • Datagram based; Domain Name Service (DNS, RPC).
• Implementing non-trivial session protocols in UDP is hard.

The UDP Packet

• The UDP header is simple:

  

• The UDP header works with the IP header in the encapsulated UDP packet:

  

TCP

• The Transmission Control Protocol (TCP, rfc 793, rfc 1122) provides a reliable byte-stream service.
  • “Reliable” means ordered, uncorrupted, unduplicated.
  • Still no delay, latency, or throughput guarantees.
• TCP connections go from process to process, but is reliable only from host to host.

TCP Connections

• TCP is connection-oriented; a TCP connection is virtual.
  • It exists at the end-points, not in the network.
  • That’s mostly the case.
• A TCP connection is full-duplex point-to-point.
  • Full duplex: two-way, both at once.
    Half duplex: two-way, one at a time.
    Simplex: one-way.
  • There are multi-point extensions.

The TCP Header

The TCP Packet

• Like UDP, the TCP header works with the IP header in the encapsulated packet:

  

• Unlike UDP, a TCP packet represents a segment of the input data stream.

  

  • Data boundaries are not respected.

Reliability

• The TCP protocol has to manufacture a reliable service using an unreliable service.
  • The protocol has other responsibilities too.
• Sequence numbers, timeouts, and retransmissions protect against loss and reordering.
  • Sequence numbers: loss, reordering, duplication.
  • Timeouts: loss.
  • Retransmission: loss

Sequence Numbers

• A TCP sequence number is a 32-bit unsigned integer.
• Sequence numbers identify bytes in the stream.
  • Including the SYN and FIN bits.
• Initial sequence numbers are (should be) randomly generated.
  • Number wrap-around becomes a problem as pipes get fatter.

Acknowledgments

• The receiving side acknowledges the next byte it expects to get.
  • Implicitly acknowledging bytes received.
• This is a positive acknowledgment: this thing did happen.
  • As opposed to negative acknowledgment: this thing didn't happen.
• Missing sequence numbers trigger retransmissions.

Example

Timeouts

• But when does an acknowledgment go missing?
• The sender sets an retransmission timer on each segment sent.
  • When the timer triggers, the associated segment is resent.
  • And the timer is reset.
• Receiving acknowledgments tosses the segment and clears the associated timer.

Computing Timeouts

• Timer values are related to round-trip times (rtt).
• Internet rtts exhibit high variance, and do so rapidly.
  • Too short rtts kill the network.
  • Too long rtts kill connection performance.
• Compute a moving average for time-out values from connection measurements.

Timeout Computations

• The smoothed round-trip time (SRTT) is

  SRTT = αSRTT + (1 - α)R

  R = acknowledgment time - send time
  α = 7/8 or so

• The smoothed rtt variation (RTTV) is

  RTTV = βRTTV + (1 - β)|RTTV - R|

  β = 3/4 or so.

• The round-trip time is SRTT + 4RTTV

TCP Connection Establishment

• Note: “SYN: X” means “SYN, SEQ: X.”

TCP States

More end-point state means more complexity maintaining state.

Summary

• IP is kudzu in the networking world.
• UDP is IP with per-process addressing.
• TCP provides reliable, byte-stream connections.
  • And the connections are implemented at the end-points.
• Reliability comes from sequence numbers, acknowledgments, time-outs, and retransmissions.

Credits

• TCP connection state diagram by Niklas Gustavsson under a Creative Commons BY license.

---

This page last modified on 2012 September 26.