CMPT 371
Data Communications and Networking
Network Layer
ICMP and fragmentation
0
© Janice Regan, CMPT 128, 2007-2012
IPv4 Protocol Header
0
4
Version
#
8
Header
Length
Type of
service
Identifier
.
TTL
32 bits
16
Datagram length (bytes)
.
Flags
.
Protocol
transport layer
13 bit
fragmentation offset
Header
checksum
Source IP address
Destination IP address
Options
(if any)
© Janice Regan, 2007-2012
Padding
.
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Header Fields (1)
 Version(4 bits): Now 4 (IP v6 being phased in)
 Internet header length or IHL( 4 bits): length
of IP header in 32 bit words. Minimum is 20
octets, so header length would be at least 5.
Used to locate the start of the payload
 Type of service (8 bits): Contains bits to set
priority (0 lowest to 7 highest) and to select
routing based on optimization of reliability,
precedence, delay or throughput parameters
(TOS replaced by Differential Services)
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© Janice Regan, 2007-2012
Header Fields (2)
 Total length (16 bits): This includes the header and the
data payload. Packet length is measured in octets.
Maximum length of a packet is 216 -1 = 65,535 octets
 Identification (16 bits): Identifies a particular datagram
or packet. The same Identification is used for each
fragment of a fragmented datagram. The final receiver
will use the Identification for reassembly
 Flags(3 bits): More bit, Don’t fragment bit, third bit is
undefined
© Janice Regan, 2007-2012
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Header Fields (3)
 Fragmentation offset (13 bits): Position of the
fragment in the present packet within the unfragmented
payload. (Must be a multiple of 64 bits from start of the
unfragmented payload)
 Time to live (8 bits): Measured in seconds, but must
decrement by at least 1 at each intermediate station.
Since transmission time in modern system are very
rarely in excess of one second this is essentially a hop
counter (Default 64)
 Protocol (8 bits): protocol of next higher layer (transport
layer) to receive data field at destination
© Janice Regan, 2007-2012
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Header Fields (4)
 Header checksum (16 bits): ones complement of ones
complement sum of all 16 bit words in header (assuming
header checksum is zero). Reverified and recomputed at
each intermediate station (IS) because TTL, some options
and some other fields may change at each intermediate
station. IP packet is discarded if checksum does not
match. Note this is a checksum of the header not the
entire datagram.
 Source address (32 bits): IP address of originating
station
 Destination address (32 bits): IP address of final
destination
© Janice Regan, 2007-2012
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Header Fields (5)
 Options (variable):

Security, Strict source routing (specify all ISs), Loose source
routing (Specify some ISs), record route (records address at
each hop), timestamp (records address and timestamp at each
hop)
 Padding (variable)

To fill to multiple of 32 bits long after options added
 Data field



Carries user data from next layer up
Integer multiple of 8 bits long (octet)
Max length of datagram (header plus data) 65,535 octets
© Janice Regan, 2007-2012
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IP Fragmentation (1)
 Uses fields in header

Data Unit Identifier (ID): uniquely identifies end system
originated datagram and contains






Source and destination address
Protocol layer generating data (e.g. TCP)
Identification supplied by that layer
Data length: Length of user data in octets (datagram length –
header length)
Fragmentation Offset: Position of fragment of user data in
original datagram, (offset from start of original datagram) in
multiples of 64 bits (8 octets)
More flag: Indicates that this is not the last fragment
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© Janice Regan, 2007-2012
IP Fragmentation (2)
 Copy the header frame of the incoming datagram
into each fragment
 Divide the incoming user data field into equal parts
along 64bit boundaries (last fragment may be
shorter). Length of each equal part is MTU
 For of each datagram except the last, set Data
Length to the length of each data fragment and set
more flag to 1. Add the length of the previous data
segment in octets to the Offset.
 For the last datagram set Data Length to the
length of the remaining data, Add the length of the
previous data segment in octets to the Offset.
© Janice Regan, 2007-2012
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Fragmentation: example 1
IP header UDP header
IP data 612 octets
IP header UDP header
offset=0
more=1
208 octets
offset=208/8=26 more=1
IP header
208 octets
IP header
offset=52 more=0
196 octets
© Janice Regan, 2007-2012
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Fragmentation: example 1
IP header TCP header
IP data 1400 octets
IP header TCP header
offset=0
more=1
600 octets
offset=600/8=75 more=1
IP header
600 octets
IP header
offset=150 more=0
200 octets
© Janice Regan, 2007-2012
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Dealing with Failure
 Re-assembly may fail if some fragments get lost
 Need to detect failure
 Re-assembly time out
 Assigned to first fragment to arrive
 If timeout expires before all fragments arrive, discard
partial data
 Use packet lifetime (time to live in IP)
 If time to live runs out, kill partial data
 Fragmentation is needed but Don’t fragment bit
is set: packet is dropped
© Janice Regan, 2007-2012
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Dealing with IP errors
 IP datagram delivery (network level) has a header




checksum to detect transmission errors in the IP header
TCP has a checksum which covers the TCP header,
pseudo header and data
Higher level protocols (for example TCP) also handle
more types of errors
Higher level protocols need to be informed that the
errors have occurred in order to handle them more
efficiently that with only timeouts and retransmissions
Within IP need an error reporting mechanism to report
such errors to TCP, one such mechanism is the ICMP
protocol.
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© Janice Regan, 2007-2012
Why fragment




The maximum size of a TCP (or UDP) segment can be configured
for each network
Therefore different networks can have different MSS (maximum
segment size)
When a segment from a network with a larger MSS must pass
through a network with a smaller MSS the segment must be broken
into parts small enough to pass through the network with the
smaller MSS
The parts will be reassembled at the destination
 any intermediate network may have multiple routers,
 not all segments will leave the intermediate network through the
same router, so they cannot be reassembled as they leave the
intermediate network
© Janice Regan, 2007-2012
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Errors in Packet Switching Networks
 Possible causes of errors include
 Hardware failure
 Network congestion
 Inability to fragment (DF set)
 Routing loops
 Unavailable host (disconnected or failed)
 Queue overrun on routers
 IP offers best effort delivery, it needs a mechanism to
inform the source of packets dropped because of errors
(except transmission errors). In the remainder of this
lecture errors will mean errors not cause by transmission
impairments
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© Janice Regan, 2007-2012
ICMP encapsulation
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ICMP message format
 Internet Control Message Protocol (RFC792)
 There are several types of ICMP messages
designed to report different types of errors
 Each ICMP message has its own format, but all
start with the same three fields

A type field (1 octet) indicating which type of ICMP
message follows
 A 1 octet code following the type that further defines
the message (see text for list)
 For example type specifies destination
unreachable, code specifies router or host
 The 3rd common field is a 2 octet checksum.
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© Janice Regan, 2007-2012
ICMP Message Formats
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ICMP Message Types
Comer
2000:
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Regan, 2007-2012
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What ICMP messages do

Time exceeded


Parameter problem


Measures transmission time (not time in queue) for strict source routing
Redirect message


IP header parameter cannot be interpreted
Timestamp/timestamp reply


Lifetime of datagram expired
Sends router a ‘better’ route to the destination that it currently has
Router discovery / solicitation



Can be used by a booting host on the network to find the routers
wait for discovery message which lists routers and is periodically
transmitted
ask for immediate discovery message by sending a solicitation message
© Janice Regan, 2007-2012
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ICMP Echo Request/Reply
 Echo request is sent by the ping command to test for





reachability
Echo reply is sent in response to a received echo reply
to confirm reachability
Type: request 8, reply 0, Code 0 : no additional
qualifying codes
Identifier and sequence number are optional, they can
be used to match replies with requests
The optional data in a echo request must be returned in
the resulting echo reply
Linux ping has a record route and a timestamp option
© Janice Regan, 2007-2012
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traceroute
 The unix traceroute command is an example of the use






of the time exceeded message
A UDP packet with a hop count of 1 is sent
The first router reached sends back a time exceeded
message, identifying itself
A packet with a hop count of 2 is sent
The second router in the path sends back a time
exceeded message identifying itself
This is repeated, incrementing the hop count by 1 until
the packet reaches its destination
Ubuntu sends 3 copies of each packet sent in the description above
© Janice Regan, 2012
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ICMP destination unreachable
 Sent when a router or host cannot deliver a
datagram due to an identified failure (not all
failures are identified)
 Can be disabled, not all hosts or routers will
send ICMP messages
 The codes indicate what destination could not
be reached and why (see table in text)
 The header and datagram information is
provided to identify the packet needing
retransmission (port numbers and sequence
number for TCP UDP)
© Janice Regan, 2007-2012
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ICMP Source Quench Message
 Used to help control congestion
 When a packet must be dropped due to
congestion a source quench packet may be
sent to the source
 When the source receives a source quench
message it may reduce the rate at which it
transmits to the network
 1 quench message per round trip travel time
should cause change
© Janice Regan, 2007-2012
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