1. Datagram Fragmentation, ICMP & IPv6
Lecture 10
Lecture 10
Datagram Fragmentation, ICMP & IPv6
IP Datagram Encapsulation
Network Maximum Transmission Unit (MTU)
IP Datagram Fragmentation
ICMP (Internet Control Message Protocol)
- Error Report Mechanism
- Information Query Mechanism
- ICMP Message format and Transmission
- ping and traceroute Utilities
IPv6
- IPv6 Features
- IPv6 Header and Format
- IPv6 Address

2. Internet Transmission Paradigm
Lecture 10
Internet Transmission Paradigm
IP D
IP D
IP D
IP D
IP D
Source
host
router
router
router
Destination
host
network
Routing
Table
## ***
…………
network
Routing
Table
## ***
…………
network
Routing
Table
## ***
…………
network
Routing
Table
## ***
…………
Routing
Table
## ***
…………
Source host
- Forms datagram with destination address
- Sends to nearest router
Intermediate routers
- Forward datagram to next router
Final router
- Delivers to destination host
Note: Datagram must be passed to network interface & sent across physical network.
Network hardware does not recognize IP datagram format and IP address !!
How is datagram transmitted across physical network ??
 Address Resolution (ARP) and Encapsulation !!

3. IP Datagram Encapsulation
Lecture 10
IP Datagram Encapsulation
IP Datagram/Packet
IP Header
IP Data Area
Encapsulated into a frame/packet in lower layer
Frame Header
Frame Data
Hardware Network Frame/Packet
Entire datagram treated like data encapsulated in a frame for transmission
Frame type (0800 for Ethernet)
identifies contents as IP datagram
Frame destination address gives next hop
Next hop Frame/Hardware Address is obtained by address resolution protocol (ARP)
IP address will not be changed while frame address is different in different network
Ethernet Frame

4. Encapsulation Across Multiple Hops
Lecture 10
Encapsulation Across Multiple Hops
Animation
Each router extracts datagram, discard frame,
determines next hop via ARP, encapsulates datagram in outgoing frame
Frame headers may differ depended upon network types
Datagram survives in entire trip, but frame only survives one hop

5. Maximum Transmission Unit (MTU)
Lecture 10
Maximum Transmission Unit (MTU)
Every hardware technology specification includes the definition of the
maximum size of the frame data area - called maximum transmission unit (MTU)
IP datagrams can be larger than most hardware MTUs
- IP: (216 – 1) bytes = 64K bytes
- Ethernet: bytes
- Token ring: 4464 bytes
- FDDI: 4352 bytes
- X.25: 576 bytes
- PPP: 296 bytes (Point-to-Point Protocol)
Any datagram encapsulated in a hardware frame must be smaller than the
MTU for that hardware
An internet may have networks with different MTUs
Ethernet Frame

6. Datagram Fragmentation
Lecture 10
Datagram Fragmentation
Fragmentation: a technique to limit datagram size to smallest MTU of any network
IP uses fragmentation – split datagrams into pieces to fit in network with small MTU
Router detects datagram larger than network MTU
- Splits into pieces called fragments
- Each piece smaller than output network MTU
Each fragment has datagram header and is sent separately
Ultimate destination reassembles fragments
> MTU
Each <= MTU
Fragment 1
Fragment 2
Fragment 3
Fragmentation
Fragmentation
No-fragmentation
Assemble fragments
No-assemble
No-assemble

7. Datagram Fragmentation & Reassembly
Lecture 10
Datagram Fragmentation & Reassembly
Network links have MTU
- Different link types
with Different MTUs
* 1500 bytes for Ethernet
* 296 bytes for PPP
large IP datagram divided (“fragmented”) within net
one datagram becomes several datagrams
“reassembled” only at the final destination
IP header bits used to identify, order related fragments
Fragmentation:
in: one large datagram
out: 3 smaller datagrams
Reassembly

8. Fragment Related Fields in IP Header
Lecture 10
Fragment Related Fields in IP Header
Identification
- Datagram ID
- 16 bits counter
Flag
- Signal fragment.
- 3 bits, ABC
A: reserved
B: 1 – no fragment
0 - fragmented
C: 1 - not last fragment
0 - last fragment
Fragment offset
- Payload data location
- Numbers of 8 bytes
- 13 bits

9. An Example of Datagram Fragmentation
Lecture 10
An Example of Datagram Fragmentation
Example
MTU = 1500 bytes
4020 byte IP datagram
20 byte IP header
4000 byte payload
3 fragments: F1, F2, F3
4000= ID =x
offset =0
fragflag
length
=4020 =1
=1500
=185
=370
=1040
One large datagram becomes
several smaller datagrams
If one fragment is lost, IP discards all fragments F1
1480 bytes in data field F2
offset = multiple of 8 bytes so
1480/8 = 185
= 370 F3
ID: set by sending host IP layer; typically increments ID num for each datagram it sends. Last fragment sent has flag field set to 0 to indicate it’s the last fragment; all other fragments have flag set to 1

10. Sub-fragmentation and Fragment Loss
Lecture 10
Lecture 10
Sub-fragmentation and Fragment Loss
Fragment may encounter a subsequent network with even smaller MTU
Router fragments the fragment to fit
Resulting (sub)fragments look just like original fragments (except for size)
No need to reassemble hierarchically; (sub)fragments include position in datagram
IP may drop fragment
What happens when a fragment is lost?
 Destination drops entire original datagram
How does destination identify lost fragment?
- Sets timer with each fragment
- If timer expires before all fragments arrive, fragment assumed lost
- Datagram dropped
Source (transport/application layer protocol) assumed to retransmit
(sub)fragments
IP Hdr21
data21
IP Hdr22
data22

11. IP Datagram Errors and ICMP
Lecture 10
IP Datagram Errors and ICMP
IP provides best-effort delivery
Datagrams will be dropped if the following errors are detected
- corrupted bits  detected by header checksum
- illegal address  detected by routers (routing table) and ARP reply
- routing loop  detected by Time-To-Live (TTL) field
- fragment loss  detected by timeout
IP ignores errors, but reports some errors !!
Internet Control Message Protocol (ICMP) is a protocol to report errors and
provide some information.
- Error reporting function Report problems that a router or a destination host encounters when it
processes an packet via sending an ICMP message TO a source host
- Information query function
Help a source host or a network manager get specific information from a
router or another host

12. Error Report and Information Query Mechanism
Lecture 10
Lecture 10
Error Report and Information Query Mechanism
Error report mechanism
IP datagram
with error
Router
Dropped
Router X
with error x
Dropped
Router x
Source
Host
Destination
Host
ICMP datagram
for error report
ICMP datagram
for error report
Information query mechanism
ICMP datagram
for information query
Router
Router q q q q
Router r r
Source
Host r
ICMP datagram
for reply r
Destination
Host
ICMP datagram
for reply

13. ICMP Message Format and Transmission
Lecture 10
ICMP Message Format and Transmission
ICMP includes both error messages and information messages
ICMP message consists of ICMP header and ICMP data
ICMP encapsulates message in IP data area for transmission
ICMP datagram is processed and forwarded like conventional IP datagram
ICMP Message
ICMP Header
ICMP Data Area
ICMP Datagram
Encapsulated
IP Header
IP Data Area
IP Header: type=1 for ICMP message
ICMP Header 8 16 24 31
Type
Code
Checksum
Identifier
Sequence Num.
Encapsulated
Type: error/information type
Code: detailed error type

14. ICMP Message Types Error messages: - Source quench (type=4)
too many datagrams to buffer in a router
- Time exceeded (type=11)
TTL becomes zero in a router (code=0)
fragment reassembly timer expires
in a host (code=1)
- Destination unreachable (type=3, code=1~15)
network disconnection or
destination host is powered down or
TCP/application not run, firewall, etc
Information query messages: (a pair)
- Request/reply
(type=8: request, type=0: reply)
- Timestamp request/reply
(type=13: request, type=14: reply)
- Address mask request/reply
(type=17: request, type=18: reply)

15. ICMP, Host Reachability and Internet Route
Lecture 10
ICMP, Host Reachability and Internet Route
An internet host A is reachable from another host B if datagrams
can be delivered from A to B
ping utility tests reachability
- Sends datagram from B to A that A echoes back to B
- Uses ICMP echo request and echo reply messages
Command format: ping IP-address/Host-name
List of all routers on path from A to B is called the route from A to B
traceroute uses UDP to non-existent port and TTL field to find route
- Sends ICMP echo messages with increasing TTL
- Router that decrements TTL to 0 sends ICMP time exceeded message,
with router's address as source address
- First, with TTL=1, gets to first router, which
discards and sends time exceeded message
- Next, with TTL=2, gets through first router to second router
- Continue, with TTL=3, 4, …, until message from destination received
Command format for Unix/Linux: traceroute IP-address/Host-name
Command format for Windows: tracert IP-address/Host-name
ping & other network utilities
ICMP & TraceRT Anim1
ICMP & TraceRT Anim2

16. Motivation for Change from IPv4 to IPv6
Lecture 10
Motivation for Change from IPv4 to IPv6
Current version of IPv4 - is more than 30 years old
IPv4 has shown remarkable success !!!
Then why change?
Address space
- 32 bit address space allows for over a million networks
- But...most are Class C and too small for many organizations
- 214 = Class B network addresses already almost exhausted
Type of service
- Different applications have different requirements for delivery reliability & speed
- Current IPv4 has type of service that's not often implemented
- Effective multimedia communication
- Data encryption and authentication
Multicast
One next version is called IPv6 !

17. New Features in IPv6 Lecture 10
Large address size – 128 bits = 16 bytes
Better header format - entirely different
Base header – 40 bytes
Extension headers - Additional information stored in optional extension headers
Support for resource allocation (QoS) - flow labels and quality of service allow
audio and video applications to establish appropriate connections
Support for more security
Extensible - new features can be added more easily
No checksum field - to reduce processing time in a router
No fragmentation - to reduce load of routers
- Potential for the Internet of Things (IoT)
40 bytes

18. IPv6 Base Header Format Lecture 10
It contains less information than IPv4 header
- VERS = 6 for IPv6
- PRIORITY (8 bits) for traffic classes, such as delay, jitter, reliability requirements
- PAYLOAD LENGTH (16 bits): Length excluding the base header
- NEXT HEADER points to first extension header
- HOP LIMIT (8 bits) same as TTL in IPv4
- FLOW LABEL (20 bits) used to associate datagrams belonging to a flow or communication
between two applications
- Specific path
- Routers use FLOW LABEL to forward datagrams along prearranged path

19. IPv6 Next Header Lecture 10
Purpose of multiple headers: economy and extensibility
Next header codes
 0 - Hop-by-hop option
 2 - ICMP
 6 - TCP
17 - UDP
43 - Source routing
44 - Fragmentation
50 - Encrypted security payload
51 - Authentication
59 - Null (no next header)
60 - Destination option

20. IPv6 Addressing Lecture 10 128-bit addresses: Type + Rest of address
Groups of 16-bit numbers in hex separated by colons - colon hexadecimal (or colon hex)
69DC:8864:FFFF:FFFF:0:1280:8C0A:FFFF
Special types of addresses: unicast, multicast, anycast - collection of computers with same prefix
Type:
        Reserved           - ISO network addresses           - IPX (Novell)         010          - Provided-based unicast addresses         100          - Geographic unicast addresses         Multicast address
Provider-based unicast addresses for normal host
| 010 | RegID(5) | ProviderID(16) | SubscriberID(24) | SubnetID(32) | HostID(48) |
Register ID: INTERNIC for North America                   RIPNIC for European countries                    APNIC for Asian and Pacific countries
Address hierarchy
Reserved addresses
- Loopback address:
- IPv4 address: IPv4 address = Ipv6 address
IPv6 Introduction Video

21. Tunneling – Transition from IPv4 toIPv6
Lecture 10
Tunneling – Transition from IPv4 toIPv6 A B E F
Tunnel
Logical view:
IPv6
IPv6
IPv6
IPv6 A B C D E F
Physical view:
IPv6
IPv6
IPv4
IPv4
IPv6
IPv6
Not all routers can be upgraded simultaneous
How will the network operate with mixed IPv4 and IPv6 routers?
Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers
Flow: X
Src: A
Dest: F
data
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
Src:B
Dest: E
Flow: X
Src: A
Dest: F
data
A-to-B:
IPv6
E-to-F:
IPv6
B-to-C:
IPv6 inside
IPv4
B-to-C:
IPv6 inside
IPv4

22. Exercise 10 Toking Ring MTU=4464 Ethernet MTU=1500 FDDI MTU=4352 H1 H2
Lecture 10
Exercise 10
byte data is sent using IP across an Ethernet. Before sent, the data will be first
formed an IP datagram and then the datagram will be encapsulated into an Ethernet
Frame. Calculate the percentage of headers in sending the 100 byte data. Assume
no optional field in IP header.
2. Suppose a file of 20 Kbytes to be sent from host H1 to host H2 across three networks as
shown in the following figure. How many IP datagrams will be sent from H1? And how many
IP datagrams will be received by H2? Assume no datagram loss, duplication and disorder
during the transmissions.
3. Host A sends a message to host B and never receive reply from B. However, host A receives
an ICMP message with a header in hexadecimal format as the follows
A C B7
Give possible reasons that A does not receive reply from B.
4. Explain how traceroute utility works. Use the utility in a Windows OS environment to probe
the Internet organization web server. The command is tracert . How many routes
have been passed when your packet travel to the web server? Which one is the slowest?
5. Summarize main features of IPv6 as compared with IPv4.
Toking Ring
MTU=4464
Ethernet
MTU=1500
FDDI
MTU=4352 H1 R1 R2 H2