IP Datagram Fragmentation

Transcription

1 IP Datagram Fragmentation Each network interface on a host or router has a unique Maximum Transfer Unit (MTU) property, which is the maximum data payload that the link layer can carry. Ethernet interfaces typically supports MTU of 1500, whereas PPP interfaces using regular phone lines support 576. MTU is a property of the Link/Physical layer, but IP has to be aware of it in order to prepare IP datagrams for transmission. Suppose a datagram arrives at one router interface with 1500 of data, and that the outgoing interface has MTU of 576. Obviously, the datagram needs to be chopped up into smaller fragments. This process is called IP Fragmentation. Incoming MTU=1500 Bytes Outgoing MTU=576 Bytes Figure 1. Once a datagram gets fragmented, it is re-assembled only at the destination host. Each fragment becomes an IP datagram by itself and travels independently of the others. Each of these fragments may get fragmented further. The destination host's IP layer has to re-assemble the whole datagram from fragments before delivering to the transport layer. This process is called Re-assembly of the fragments. Each fragment should carry enough information to enable the re-assembly. IP datagram fragmentation and re-assembly is facilitated by the ID, Flags and Fragment Offset fields in the IP header. The figure below shows bits 32 through 63 of the IP header that carry these information: bit 32 bit 48 bit 49 bit 50 bit 63 Identification 0 D M Fragment offset

2 Using the example in Figure 1, suppose the original incoming 1500-byte datagram has a 20-byte IP header and 1480 of data. Further assume that the ID field is (in decimal). Then the fragmentation process chops up the data into three parts, each with 556, 556, and 368 (total = 1480 ) respectively, attaches a 20-byte IP header to each fragment and sends them off to the destination as separate and completely independent datagrams. The offset values and the flag bits are set to mark the packets. The datagram's unique ID field is copied to all the fragments. The process is shown in the figure below:

3 bit 48 bit 49 bit 50 ORIGINAL Datagram 1480 data (ID) (Frag. offset) IP Header First data byte is byte 0 of the original datagram Fragment (ID) More fragments flag is set byte 0 byte 551 Fragment First data byte is byte 552 of the original datagram so frag. offset is (ID) byte 552 byte 1103 First data byte is byte 1104 of the original datagram Fragment (ID) byte 1104 byte 1479 No more fragments

4 Note the use of the third flag bit (50 th bit of the header, called More Fragments flag, or M flag). Each fragment, except the last one, has this bit set (to 1). This informs the destination IP that this datagram is a fragment of a complete datagram. The offset value informs the destination IP where the fragment belongs in the datagram this is needed for the re-assembly process. A requirement of fragmentation is that all the fragments, except possibly the last, must have multiples of 8 of data in them. That is why the first two fragments in the above example had 552, instead of 556 allowed by the MTU of 576. The ID field in the original datagram is preserved in the fragments this uniquely identifies the fragments as belonging to the same datagram. Re-assembly of fragments When an IP receives a datagram and if the M flag is set, then it knows that it has received a fragment of a datagram. IP puts the fragment in a re-assembly buffer and starts a timer. If it receives all the fragments within a prescribed time, the datagram is re-assembled and sent to the transport layer. If the timer expires before all the fragments arrive, a reassembly time exceeded message is sent to the source (this is an ICMP message). If a datagram arrives with M flag not set, does that mean that it is not a fragment? Then IP has to look at the offset value if offset is 0, then this is the entire datagram; if offset is not zero, then this is the last fragment, and IP needs to wait for the other fragments. If a datagram arrives with M flag set and offset field not zero, what does that mean? Note that fragments of a datagram are themselves full-fledged datagrams and that they need not arrive at a destination in order, nor do they need to take the same route. The Don't Fragment bit, (bit 49 of the header, or DF bit) is set if a datagram is not to be fragmented. If a router encounters a datagram with DF flag set, and if fragmentation is required because of MTU considerations, then the datagram is dropped and ICMP error message is sent to the sender saying fragmentation required but DF bit set.

5 Fragmentation is complicated and takes time and resources - therefore it is desirable to avoid it. IPv6 has no provision for fragmentation.