Протокол IP: фрагментация | Курс "Компьютерные сети"

Introduction to IP Fragmentation

Overview of IP Protocol

• The lecture is presented by Andrey Sazykhin, focusing on the topic of IP fragmentation within the context of network and telecommunications systems.

• The IP protocol operates at the network layer of the OSI model, facilitating interconnection between networks using different technologies.

Maximum Transmission Unit (MTU)

• Different networks have varying maximum transmission units (MTUs); for Ethernet, it is 1500 bytes, while for other technologies like Token Ring and FDDI, it can be larger.

• When a router receives a packet larger than what the network can transmit, fragmentation occurs to divide the packet into smaller fragments.

Mechanism of Fragmentation

Fields Used in Fragmentation

• Three header fields are essential for implementing fragmentation: Packet Identifier, Flags, and Fragment Offset.

• The Packet Identifier ensures all fragments belong to the same original packet; this field must remain consistent across all fragments.

Flags in Fragmentation

• The Flags field consists of three bits; however, only two are commonly used:

• DF (Don't Fragment): Indicates that fragmentation is not allowed.

• MF (More Fragments): Signals that more fragments will follow.

Order and Assembly of Fragments

Handling Out-of-Order Fragments

• Since IP does not guarantee message delivery order, fragments may arrive out of sequence.

• The Fragment Offset field helps determine how to reassemble these fragments correctly; it measures offsets in 8-byte blocks rather than bytes.

Example of Fragmentation Process

• A practical example illustrates a large packet size of 4000 bytes with an IP header size of 20 bytes. This results in 3980 bytes of data needing transmission over an Ethernet network with a MTU limit.

Detailed Example Breakdown

Creating Fragments from Large Packets

• For a total payload size exceeding MTU limits:

• First fragment: Size = 1480 bytes (Offset = 0).

• Second fragment: Size = 1480 bytes (Offset = 185).

• Third fragment: Size = remaining data with appropriate offset adjustments.

Reassembly Process at Destination

Receiving and Assembling Fragments

• Upon receiving fragments, routers identify incomplete packets through flags and offsets.

• Once all fragments are received, they are combined based on their offsets to reconstruct the original packet accurately.

Implications of DF Flag

Consequences when DF Flag is Set

• If a packet exceeds MTU limits and has its DF flag set, routers cannot fragment it. Instead:

• They discard the packet and send an ICMP Type 3 Code 4 message back to indicate that fragmentation was required but not permitted due to the DF flag being active.

Conclusion on IPv6 Changes

Transition from IPv4 to IPv6