Maximum Transfer Unit(MTU):

• Each data link layer protocol has its own frame format and one of the field is maximum size of data field.
• Thus MTU is nothing but the maximum size of data which can be encapsulated in a layer 2 frame. It depends upon hardware and software used in the network
• The common data link layer protocol that is ethernet has MTU of 1500 bytes.
• If the data happens to be more than MTU, it is fragmented.

MTU for different data link layer protocols are:

Fragmentation:

• The process of braking the packets into multiple units, if the size of packet happens to be more than MTU of the underlying network is called fragmentation.
• The total length of IPv4 field is 16 bits, means it can support maximum of ((2^16) -1 ) = 65535 bytes but practically it is not possible to send this big data in single frame. It has to be fragmented into multiple segments.
• When a packet is fragmented into multiple small unit, most of the header field remains same in each fragment except for flags, fragment offset and total length.
• All the fragments will have same identification number.
• Fragmentation occurs at the routers not at the source while reassembly occurs at destination.

Flags related to Fragmentation:

Identification: This is of 16 bit and each fragment will have same identification number as that of the main packets. This will help in reassembly of these fragments at the receiver side.

Flags: This is 3 bit, where first bit is reserved.

• Dont Fragment(DF):

If this flag is set, the packets cannot be fragmented, even if, the data size is more than MTU. Such packets are dropped/discarded.

• More Fragment(MF):

If this flag is set, means there are more fragments to be followed. This is 0 if the fragments happens to be last or if the data is not fragmented.

Fragment Offset:

The fragment offset is 13 bits and indicates where a fragment belongs in the original IPv4 datagram. This value is a multiple of eight bytes. The offset of the first fragment is 0(0/8).

Example:

Datagram of size 4000 and the MTU is say 1500, so this packet has to be fragmented into three segments.

Disadvantages of Fragmentation:

• Fragmentation causes inefficient use of resources:

Poor choice of fragment sizes can greatly increase the cost of delivering a datagram. Additional bandwidth is used for the additional header information, intermediate gateways need resources to make additional routing decisions, and the receiving host must reassemble the fragments.

• Loss of fragments leads to degraded performance:

Reassembly of IP fragments is not very robust. Loss of a single fragment requires the higher level protocol to retransmit all of the data in the original datagram, even if most of the fragments were received correctly.

So its always efficient to get the MTU size beforehand and send the data accordingly.

For Transport layer : MSS = MTU – 40 bytes(L3 + L4 header)

For Ethernet:

MTU : 1500

So MSS : 1460 bytes

Advantages of Fragmentation:

Fragmentation allows higher level protocols to be unconcerned with the characteristics of the transmission channel, and to send data in conveniently sized pieces.

Categories: Networking