Fragmentation occurs underneath the tcp udp layer

Basic Internet Architecture 57

Link layernetwork

Destination Destination

MAC.2
MAC.1 Source MAC.2

Destination Source EtherType ARP Request “Where is <broadcast> MAC.1 0 ⋅ 806 136.80.1.5?”

Time
Time

1. Interface 1 transmits a broadcast ARP Request for 136.80.1.5.

2. Interface 2 unicasts back an ARP Reply (it knows the Ethernet address of Interface 1 from the initial ARP Request)
3. Interface 1 unicasts the IP packet to Interface 2.

Transient errors in router forwarding tables can sometimes create loops in the IP network, known as routing loops. Routing loops tend to occur shortly after topology changes, while the network’s routing protocol converges on a new set of shortest-path trees. Routing loops often act like black holes in the network – packets head into the region of the routing loop, and then get stuck, consuming bandwidth as they circulate. Under extreme circumstances, the routing loop can disrupt the routing protocol itself, by saturating links carrying routing protocol update messages.

To prevent endless looping, IP packets carry an 8-bit Time to Live (TTL) field (see Figure 4.5). In practice, the TTL represents ‘hops to live’ – a limit on the maximum number of router hops a packet can traverse before it expires in transit. A packet’s TTL field is set to a nonzero value by the source, and is decremented by one every time the packet passes through a router. The packet is discarded when its TTL field is decremented to zero (whether or not it has reached its final destination).

IP fragmentation tends to occur when a packet’s path originates on a link with a large MTU, and then at some point along the route passes across a link with a smaller MTU. It is not considered a good thing, as it creates less efficient data transfer along the path [RFC1191][RFC1981].