Samples of Descriptive ProblemsSamples of Descriptive Problems
CSC/ECE 573, Sections 001
Fall, 2012
IP, FragmentationIP, Fragmentation A 3674 byte IP packet was fragmented by an IP
stack for transmission over a link whose MTU and DLC protocol limit the size of the IP packet to 1100 bytes. The original IP packet did not have any IP header options. State what the total length and fragment offset fields of each fragment produced by this stack will be.
IP HeaderIP Header
Total Length in bytes (16)
Time to Live (8)
Options (if any)
Version(4)
Hdr Len(4) TOS (8)
Identification (16 bits) Flags (3) Fragment Offset (13)
Source IP Address
Destination IP Address
Header Checksum (16)Protocol (8)
PAD
Data Field
Copyright Rudra Dutta, NCSU, Spring, 2005 4
One-hop deliveryOne-hop delivery
It is assumed that physical net methods can be used– But physical net capabilities might differ
Maximum Transmission Unit (MTU)– Largest IP packet a network will accept– Arriving IP packet may be larger– Solution: break into several packets
That is, “fragment”
IP Packet
MTU
Copyright Rudra Dutta, NCSU, Spring, 2005 5
IP FragmentationIP Fragmentation If IP packet is longer than the MTU, the router
breaks packet into smaller packets– Called IP fragments– Fragments are still IP packets
Only data, header is replicated
– Router performs fragmentation
IP Packet 2 1
IP PacketsFragmentation
MTU
3
Copyright Rudra Dutta, NCSU, Spring, 2005 6
Multiple FragmentationsMultiple Fragmentations Original packet may be fragmented multiple
times along its route
DestinationHost
InternetProcess
SourceHost
InternetProcess
Fragmentation
Copyright Rudra Dutta, NCSU, Spring, 2005 7
DefragmentationDefragmentation Internet layer process on destination host
defragments, restoring the original packet IP Defragmentation only occurs once
DestinationHost
InternetProcess
Defragmentation
SourceHost
InternetProcess
Copyright Rudra Dutta, NCSU, Spring, 2005 8
Fragmentation and IP FieldsFragmentation and IP Fields More Fragments field (1 bit)
– 1 if more fragments– 0 if not– Source host internet process sets to 0– If router fragments, sets More Fragments field
in last fragment to 0– In all other fragments, sets to 1
0 0 1 1
Original IP Packet Fragments
Copyright Rudra Dutta, NCSU, Spring, 2005 9
Identification FieldIdentification Field IP packet has a 16-bit Identification field
– If router fragments, places the original Identification field value in the Identification field of each fragment
47 47 47 47
Original IP Packet Fragments
Total Length in bytes (16)
Time to Live (8)
Version(4)
Hdr Len(4) TOS (8)
Identification (16 bits) Flags (3) Fragment Offset (13)
Header Checksum (16)Protocol (8)
Copyright Rudra Dutta, NCSU, Spring, 2005 10
Identification FieldIdentification Field Purpose
– Allows receiving host’s internet layer process know what fragments belong to each original packet
– Works even if an IP packet is fragmented several times
47 47 47 47
Original IP Packet Fragments
Copyright Rudra Dutta, NCSU, Spring, 2005 11
Fragment Offset FieldFragment Offset Field Fragment offset field (13 bits) is used to
reorder fragments with the same Identification field
Contains the data field’s starting point (in octets) from the start of the data field in the original IP packet
Total Length in bytes (16)Version
(4)Hdr Len
(4) TOS (8)
Identification (16 bits) Flags (3) Fragment Offset (13)
Copyright Rudra Dutta, NCSU, Spring, 2005 12
Fragment Offset FieldFragment Offset Field Receiving host’s internet layer process
assembles fragments in order of increasing fragment offset field value
This works even if fragments arrive out of order!
Works even if fragmentation occurs multiple times
0212730
Fragment Offset Field
IP, FragmentationIP, Fragmentation Original header is replicated, other than flags
– Original has no header, so neither will fragments– 20 byte header on each packet
3654 bytes of data– Must fragment into whole number of 8-bytes– Trick to accommodate flags yet specify fragment
length– 1080, 1080, 1080 and 414 bytes of data– Lengths are 20 bytes more in each case– Offset value for first one is zero– Next one? And next? And next?
TCPTCP A TCP packet is seen to have the following bit
values in the header:– Bits 106 – 111: 000010
Which of the following fields are valid, and which invalid (i.e. the values have no significance) ?– Bytes 0 – 1– Bytes 2 – 3– Bytes 4 – 7– Bytes 8 – 11– Bytes 16 – 17– Bytes 18 – 19
TCP HeaderTCP Header
TCP FlagsTCP Flags
16
Flag Description
URG The value of the urgent pointer is valid
ACK The value of the acknowledgment number is valid
PSH Push the data (pass data to receiver as quickly as possible, later)
RST The connection must be reset
SYN Synchronize the sequence numbers during connection establishment
FIN The sender has no more data to transmit
Copyright Rudra Dutta, NCSU, Spring, 2005 17
Three-Way HandshakeThree-Way Handshake
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