Samples of Descriptive Problems CSC/ECE 573, Sections 001 Fall, 2012

IP, 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 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, One-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, IP 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 Packet21 IP Packets Fragmentation MTU 3

Copyright Rudra Dutta, NCSU, Spring, Multiple Fragmentations Original packet may be fragmented multiple times along its route Destination Host Internet Process Source Host Internet Process Fragmentation

Copyright Rudra Dutta, NCSU, Spring, Defragmentation Internet layer process on destination host defragments, restoring the original packet IP Defragmentation only occurs once Destination Host Internet Process Defragmentation Source Host Internet Process

Copyright Rudra Dutta, NCSU, Spring, Fragmentation 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 Original IP PacketFragments

Copyright Rudra Dutta, NCSU, Spring, Identification 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 Original IP PacketFragments 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, Identification 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 Original IP PacketFragments

Copyright Rudra Dutta, NCSU, Spring, Fragment 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, Fragment 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 Fragment Offset Field

IP, 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?

TCP A TCP packet is seen to have the following bit values in the header: – Bits 106 – 111: 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 Header

TCP Flags 16 FlagDescription URGThe value of the urgent pointer is valid ACKThe value of the acknowledgment number is valid PSHPush the data (pass data to receiver as quickly as possible, later) RSTThe connection must be reset SYNSynchronize the sequence numbers during connection establishment FINThe sender has no more data to transmit

Copyright Rudra Dutta, NCSU, Spring, Three-Way Handshake