Transcript Chapter8 (Internet Protocol)

Chapter 8
Internet Protocol
(IP)
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Position of IP in TCP/IP protocol suite
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Introduction
IP is the transmission mechanism used by the
TCP/IP protocol
It is unreliable and connectionless datagram
protocol
Providing Best-effort delivery service (best-effort :
no error checking and tracking)
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8.1 Datagram
Packets in the IP layer : called datagrams
IP datagram format
Variable-length packet consisting of header and data

Header
–
20 ~ 60 bytes
–
Containing information that is essential for routing and delivery
IP header
Version (VER) : Version 4 or 6 (IPng)
Header length (HLEN) : represented by in 4 byte words

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Ex) if HLEN = 5, the real header length is 20 bytes
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Datagram (cont’d)
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Datagram (cont’d)
Service Type

Defining how the datagram should be handled by the
routers

Precedence : 3 bits

–
Defining the priority of the datagram in issues such as
congestion
–
Ex) a datagram for network management vs. optional
information to a group of people
–
At present, not used in version 4
service type : 4 bits (TOS bits)
–

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With only one bit set at a time
Remaining bit : not used
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Datagram (cont’d)
 Service type or Differentiated Services
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Datagram (cont’d)
Types of service
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TOS bits
Description
0000
Normal
0001
Minimize cost
0010
Maximize reliability
0100
Maximize throughput
1000
Minimize delay
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Datagram (cont’d)
Default types for some applications in use of TOS
Protocol
TOS bits
Description
ICMP
0000
Normal
BOOTP
0000
Normal
NNTP
0001
Minimize cost
IGP
0010
Maximize reliability
SNMP
0010
Maximize reliability
TELNET
1000
Minimize delay
FTP (data)
0100
Maximize throughput
FTP (control)
1000
Minimize delay
TFTP
1000
Minimize delay
SMTP (command)
1000
Minimize delay
SMTP (data)
0100
Maximize throughput
DNS (UDP query)
1000
Minimize delay
DNS (TCP query)
0000
Normal
0100
Maximize throughput
DNS (zone)
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Datagram (cont’d)
 Differentiated Services
The first 6 bits : codepoint subfield
Values for codepoints
Category
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Codepoint
Assigning Authority
1
XXXXX0
Internet
2
XXXX11
Local
3
XXXX01
Temporary or
experiment
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Datagram (cont’d)
Total Length : head + data

Defining the total length of the datagram including the
header

Length of data = total length – header length

Limited to 65,535 (216 – 1) bytes

Encapsulation of a small datagram in an Ethernet Frame
Ethernet Frame size : 46 ~ 1500 bytes
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Datagram (cont’d)
Flags : used in fragmentation
Fragmentation offset : used in fragmentation
Time to live
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
Used to control the maximum number of hops (routers)
visited by the datagram

If the value is Zero, the routers discarded

If the source wants to confine the packet to the local
network, it can store 1 in this field
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Datagram (cont’d)
Protocol

Defining the higher level protocol that uses the services of
the IP layer
–
–
TCP, UDP, ICMP, and IGMP
Multiplexing data from different higher level protocols
Value
1
2
6
8
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Protocol
ICMP
IGMP
TCP
EGP
UDP
OSPF
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Datagram (cont’d)
 Example 1
- An arriving IP packet :  01000010
The receiver discards the packet, Why ?
- 2 x 4 bytes = 8bytes : Minimum number of bytes
in the header must be 20
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Datagram (cont’d)
 Example 2
The value of HLEN is 1000 in binary
How many bytes of options are being carried by this
packet ?

8 x 4 bytes = 32 bytes : 20 bytes + 12 bytes (option)
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Datagram (cont’d)
 Example 3
- In an IP packet, the value of HLEN is 5 16 and the
value of the total length field is 002816. How many
bytes of data are being carried by this packet?
Answer
The HLEN value is 5, which means the total number
of bytes in the header is 5  4 or 20 bytes (no
options). The total length is 40 bytes, which
means the packet is carrying 20 bytes of data (40 20).
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Datagram (cont’d)
 Example 4
An IP packet has arrived with the first few
hexadecimal digits as shown below:
 45000028000100000102...................
How many hops can this packet travel before
being dropped? The data belong to what upper
layer protocol?
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Datagram (cont’d)
 Answer
To find the time-to-live field, we should skip 8
bytes (16 hexadecimal digits). The time-to-live field
is the ninth byte, which is 01. This means the
packet can travel only one hop. The protocol field
is the next byte (02), which means that the upper
layer protocol is IGMP.
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Datagram (cont’d)
Checksum : header checksum- 16 bits
Source IP address : 32 bit-field
Destination IP address : 32 bit-field
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8.2 Fragmentation
The format and size of the received frame depend on
the protocol used by the physical network
Ex) A router connecting Ethernet to token ring
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Fragmentation (cont’d)
MTU (Maximum Transfer Unit)
When a datagram is encapsulated in a frame, the
total size of the datagram must be less than this
maximum size
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Fragmentation (cont’d)
MTUs for different networks
Protocol
MTU
Hyperchannel
65,535
Token ring (16Mbps)
17,914
Token ring (4Mbps)
4,464
FDDI
4,352
Ethernet
1,500
X.25
576
PPP
296
Hyperchannel : Network Systems Corporation, 1988 (RFC 1044)
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Fragmentation (cont’d)
The maximum length of the IP datagram equals to the
largest MTU defined so far (65,535 bytes)
Therefore, for the other physical networks we must
divide the datagram : fragmentation
datagram that can be fragmented by the source host
or any router in the path, but the reassembly of
datagram is done by the destination
When a datagram is fragmented, required parts of
the header must be copied by all fragments.

Changing the values of the three fields : flags, fragmentation
offset, and total length

The rest of fields must be copied

Checksum must be recalculated
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Fragmentation (cont’d)
Fields related to fragmentation
Identification : 16 bit-field

Datagram id that is originated by the source host
–


Therefore, Source IP address + datagram id (identification)
All fragments having same identification number
Identification No. to be used for the destination in
reassembling the datagram
Flags : 3 bit-field

D : Do not fragment (1)
–

M : More fragment (0)
–
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If it can not pass the datagram through any available physical
network, it discards the datagram and send ICMP error
message to the source host
0 : last fragment or only fragment
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Fragmentation (cont’d)
 Fragmentation offset : 13-bit field
Showing relative position of this fragment with respect to the
whole datagram
Measured in units of 8 bytes : forcing hosts or routers that
fragment datagrams to choose the size of each fragment so that
the first byte number is divisible by eight
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Fragmentation (cont’d)
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8.3 Options
 Variable part of the IP datagram : the maximum of 40 bytes
 Format : Code, Length, and Data
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Options (cont’d)
 Code field
8 bits length and containing 3 subfields : copy,
class, and number
Copy



Controlling the presence of the option in fragmentation
0 : meaning that option must be copied only to the first
fragment
1 : meaning the option must be copied to all fragments
Class


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Defining the general purpose of the option
00 : datagram control, 01 : reserved, 10 : Debugging and
management, 11: reserved
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Options (cont’d)
Number

Defining the type of the option : only 6 options that are
currently being used
 Length
defining the total length of the option including the
code field and length field itself
 Data
containing the data that specific options require
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Options (cont’d)
 Option Types
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Options (cont’d)
No Operation

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one byte option used as a filler between options
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Options (cont’d)
End of Option
one-byte option used for padding at the end of the
option field
used as the last option
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Options (cont’d)
Record Route
used to record the internet routers that handle the
datagram
list up to 9 router IP addresses since the max. size
of the header is 60 bytes (Base header : 20 bytes)
pointer field
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
An offset integer field containing the byte number of the
first empty entry (available entry)

When leaving the source, the pointer field has a value of
four, pointing to the first empty field
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Options (cont’d)
Record route option
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Options (cont’d)
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Options (cont’d)
 Strict Source Route
used by the source to predetermine a route for the datagram as
it travels through the Internet
can choose a route with specific type of service : minimum
delay or maximum throughput
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Options (cont’d)
Strict source route concept
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Options (cont’d)
Loose Source Route
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
similar to the strict source route, but it is more relaxed

each router in the list must be visited, but the datagram can
visit other routers as well
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Options (cont’d)
Time Stamp
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
used to record the time of datagram processing by a router

expressed in millisecond from the midnight, Universal Time
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Options (cont’d)
overflow field : recording the number of routers that
could not add their timestamp because no more
fields were available
Use of flag in timestamp
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Options (cont’d)
Timestamp concept (when flag =1)
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8.4 Checksum
 Error detection method used by most TCP/IP
protocols
 Checksum calculation at the sender
The packet is divided into k sections, each of n bits (
n is usually 16)
All sections are added together using one’s
complement arithmetic
The final result is complemented to make the
checksum
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Checksum (cont’d)
 Checksum calculation at the receiver
The packet is divided into k sections, each of n bits.
All sections are added together using one’s
complement arithmetic
The result is complemented
If the final result is 0, the packet is accepted;
otherwise it is rejected
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Checksum (cont’d)
 Checksum concept
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Checksum (cont’d)
 Checksum in one’s complement arithmetic
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Checksum (cont’d)
 Checksum in the IP Packet
covering only the header, not the data
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
all higher level protocols that encapsulate data in the IP
datagram have a checksum field that covers the whole
packet

the header changes with each visited router, but data does
not. So the checksum includes only the part which has
changed

if each router must recalculates the checksum, it is needed
to have the more processing time for each router
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Checksum (cont’d)
 Example
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8.5 IP Package
 IP package : 8 components
Header-adding module
Processing module
Routing module
fragmentation module
reassembly module
routing table
MTU table
reassembly table
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IP Package (cont’d)
 IP components
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IP Package(cont’d)
 The operation of IP package
receiving an IP packet, either from the data link layer
or a higher level protocol
if the packet comes from a upper layer protocol, it
should be delivered to the data link layer
if the packet comes from the data link layer,
forwarding to data link or a upper layer ( the
destination is same as the station address)
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IP Package (cont’d)
 Header-adding Module
Receive : data, destination address
1. Encapsulate the data in an IP datagram
2. Calculate the checksum and insert it in the checksum
field
3. Send the data to the corresponding input queue
4. Return
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IP Package (cont’d)
 Processing Module
1. Remove one datagram from one of the input queues
2. if (destination address is 127.X.Y.Z or matches one of the local
addresses)
1. Send datagram to the reassembly module.
2. Return
3. if (machine is a router)
1. Decrement TTL
4. if (TTL less than or equal to zero)
1. Discard the datagram
2. Send an ICMP error message
3. Return
5. Send the datagram to the routing module
6. Return
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IP Package (cont’d)
 Queues
Input queues and output queues
 Routing table
used by the routing module to determine the nexthop address of the packet
 Routing module
receiving an IP packet from the processing module
sending the packet with the information to the
fragmentation module
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IP Package (cont’d)
 MTU Table
to find the maximum transfer unit of a particular
interface.
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IP Package (cont’d)

Fragmentation Module
Receive : an IP packet from routing module
1.
Extract the size of the datagram
2.
if (size > MTU of the corresponding network)
1.
If (D (do not fragment) bit is set
1.
2.
3.
2.
Discard the datagram
Send an ICMP error message
Return
Else
1.
2.
3.
4.
5.
6.
Calculate the maximum size
Divide the datagram into fragments
Add header to each fragment
Add required options to each fragment
Send the datagram
Return
3. Else
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Send the datagram
4. Return
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IP Package (cont’d)
 Reassembly Table
State field : FREE or IN-USE
Source IP address of datagram
Datagram ID
Time-out : a predetermined amount of time in which
all fragments must arrive
Fragment field : a pointer to a linked list of
fragments
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IP Package (cont’d)
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IP Package (cont’d)
 Reassembly Module
Receive : an IP packet from the processing module
1. If (offset value is zero and the M bit is 0)
1. Send the datagram to the appropriate queue
2. Return
2. Search the reassembly table for the corresponding entry
3. If (not found)
1. Create a new entry
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IP Package (cont’d)
4. Insert the fragment at the appropriate place in the linked
list
1.
if (all fragments have arrived)
1.
2.
3.
2.
Reassemble the fragments
Deliver the datagram to the corresponding upper layer protocol
Return
Else
1.
2.
Check the time-out
if (time-out expired)
1.
2.
Discard all fragment
Send an ICMP error message
5. Return
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Summary (1)
 IP is an unreliable connectionless protocol responsible for sourceto-destination delivery.
 Packets in the IP layer are called datagrams
 A datagram consists of a header (20 to 60 bytes) and data.
 The IP header contains the following information: version number,
header length, differentiated services, datagram length,
identification number, fragmentation flags, fragmentation offset,
time to live, protocol, checksum, source address, destination
address, and options.
 The maximum length of a datagram is 65,535 bytes.
 The MTU is the maximum number of bytes that a data link protocol
can encapsulate. MTUs vary from protocol to protocol.
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Summary (2)
Fragmentation is the division of a datagram into smaller
units to accommodate the MTU of a data link protocol.
The fields in the IP header that relate to fragmentation are
the identification number, the fragmentation flags, and the
fragmentation offset.
The IP datagram header consists of a fixed, 20-byte
section and a variable options section with a maximum of
40 bytes.
The options section of the IP header is used for network
testing and debugging.
The options header contains the following information: a
code field that identifies the option, option length, and the
specific data.
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Summary (3)
 The six IP options each have a specific function. They are as follows:
filler between options for alignment purposes, padding, recording the
route the datagram takes, selection of a mandatory route by the
sender, selection of certain routers that must be visited, and
recording of processing times at routers.
 The ping and traceroute utilities in UNIX can be used to implement
some of the IP options.
 The error detection method used by IP is the checksum.
 The checksum uses one's complement arithmetic to add equal-size
sections of the IP header. The complemented result is stored in the
checksum field. The receiver also uses one's complement arithmetic
to check the header.
 An IP package can consist of the following: a header-adding module,
a processing module, a forwarding module, a fragmentation module, a
reassembly module, a routing table, an MTU table, and a reassembly
table.
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