Internet Protocol: Connectionless Datagram Delivery

1
Internet Protocol Connectionless Datagram
Delivery

• D. E. Comer,Internetworking with TCP/IP
  Principles, Protocols and Architectures, Ch. 7,
  Prentice Hall, 2000

2
Agenda

• Introduction
• Internet Datagram
• Internet Datagram Options
• IPv4 vs. IPv6 Datagram
• Summary

3
Introduction

• Internet A single virtual network
• Connectionless Delivery System
• Unreliable Delivery is not guaranteed
• Best-effort Makes earnest attempt for delivery
• Connectionless Each packet is treated
  independently
• Internet Protocol
• Data Transfer Unit
• Routing
• Unreliable Packet Delivery

3 Conceptual Layers of Internet Services
4
Internet Datagram

• IP Datagram vs. Network Frame
• Datagram Format

VERS
HLEN
TOTAL LENGTH
SERVICE TYPE
5
Internet Datagram Type of Service

• Specify how the datagram should be handled
• Original 5 subfields of SERVICE TYPE
• Precedence(0-7) Specifies the importance of the
  data
• D, T, R Desired type of transport
• D Low Delay
• T High Throughput
• R High Reliability
• Transport request is considered as a hint to the
  routing algorithm

5
7
3
4
0
R
UNUSED
T
PRECEDENCE
D
6
Internet Datagram Type of Service(2)

• In 1990s, IETF redefined the meaning of service
  type
• Differentiated Services (DS) interpretation
• In new standard, a router is required to
  implement at least two priority schemes
• One for Normal traffic
• One for High-priority traffic

0
6
7
UNUSED
PRECEDENCE
D
T
R
7
Internet Datagram Encapsulation

• The idea of carrying one datagram in one network
  frame
• Which frame size should be chosen?
• The smallest MTU?
• How can it be more than Min MTU?

MTU Maximum Transfer Unit Max. amount of data in
one physical frame
8
Internet Datagram Fragmentation

• Important Design Point
• Hide Underlying Design Technology!
• Arrange a way to divide large datagrams into
  smaller pieces Fragmentation
• Each fragment has the original datagram format

Original Datagram with 1400 byte of Data
3 Fragments for network MTU 620
9
Internet Datagram Fragmentation(2)

• Reassembly is done in the destination
• 2 disadvantages
• Carrying small fragments all through the
  destination
• If any fragment is lost, the datagram cannot be
  reassembled
• Fragmentation Control
• Identification Unique integer datagram
  identifier
• Flags low-order two bits are for controlling
  fragmentation
• 1st control bit do not fragment
• 2nd control bit more fragment
• Fragment Offset The offset of the data in
  original datagram (in units of 8 bytes)

IDENTIFICATION
FLAGS
FRAGMENT OFFSET
10
Internet Datagram Other Header Fields
31
19
24
16
8
4
0
HLEN
SERVICE TYPE
TOTAL LENGTH
VERS
FRAGMENT OFFSET
FLAGS
IDENTIFICATION
PROTOCOL
HEADER CHECKSUM
TIME TO LIVE
SOURCE IP ADDRESS
DESTINATION IP ADDRESS

• Time to Live Specify how long, in second, the
  datagram is allowed to remain in the internet
  system
• Time to Live vs. Hop Limit
• Guarantees that datagrams cannot travel forever!
• Header Checksum
• Reduce processing time at routers
• Higher level protocols are forced to use their
  own checksum scheme for the data

11
Internet Datagram Options

• Options are variable in length and appear
  contiguously
• Each option has a one-byte Option Code
• Option Class The general class of the option
• Class 0 Datagram or Network Control
• Example

Description
Length
Option Number
Option Class
Record Route. Used to trace a route
Variable
7
0
Strict Source Route. Used to specify an exact
path trough the internet
Variable
9
0
12
Internet Datagram Options Record Route

• Allow the source to create an empty list of IP
  addresses and arrange for each router to add its
  IP address to the list.
• Each router adds its address to the record route
  list (if any space is still available). Then
  increments Pointer by 4.
• What about copy bit in the CODE field?

13
Internet Datagram Options Source Route

• Allow the sender to dictate a path through the
  internet.
• IP supports two forms of source routing
• Strict Source Routing
• The datagram MUST follow the specified addresses
  exactly
• Loose Source Routing
• Allows multiple network hops between successive
  addresses
• What about copy bit in the CODE field?

Strict Route Option
14
IPv4 vs. IPv6 Datagram

• In addition to Larger Address Space, IPv6 has new
  Flexible Header Format and Improved Options.
• Each datagram includes extension headers for only
  those services that it uses.
• For example, when fragmentation is needed, a
  Fragment Extension Header is inserted after the
  base header.

15
IPv4 vs. IPv6 Datagram
IPv4 Header
IPv6 Base Header
31
19
24
16
8
4
0
12
31
24
16
4
0
HLEN
SERVICE TYPE
TOTAL LENGTH
VERS
TRAFFIC CLASS
FLOW LABEL
VERS
FRAGMENT OFFSET
FLAGS
IDENTIFICATION
HOP LIMIT
NEXT HEADER
PAYLOAD LENGTH
PROTOCOL
HEADER CHECKSUM
TIME TO LIVE
SOURCE IP ADDRESS (64 bits)
SOURCE IP ADDRESS
DESTINATION IP ADDRESS
DESTINATION IP ADDRESS (64 bits)
PADDING
IP OPTIONS (IF ANY)

• Alignment has been changed from 32-bit to 64-bit
  multiples
• Total Length Payload Length
• Time to Live Hop Limit
• Service Type Traffic Class Flow Label
• Fragment information has been moved out of fixed
  fields

16
IPv6 Fragmentation Source Routing

• Fragmentation is end-to-end
• Reduce overhead in the routers
• An internet protocol that uses end-to-end
  fragmentation requires a sender to discover the
  path MTU to each destination, and to fragment any
  outgoing datagram that is larger than the path
  MTU. End-to-end fragmentation does not
  accommodate route changes.
• Source Routing is possible
• But IPv6 uses a separate extension header

17
Summary

• Fundamental service provided by TCP/IP
• A connectionless, unreliable, best-effort packet
  delivery system
• Internet protocol formally specifies the format
  of internet packets, called datagrams
• IP datagram is divided into header and data
• Header contains source and destination IP
  addresses, fragmentation control, precedence, and
  checksum
• Header also conations variable length option
  fields
• Options are normally used for monitoring and
  controlling and internet

18
Internet Protocol Connectionless Datagram
Delivery

• D. E. Comer,Internetworking with TCP/IP
  Principles, Protocols and Architectures, Ch. 7,
  Prentice Hall, 2000