Fragmentation of an internet datagram is necessary when it originates in a local net that allows a large packet size and must traverse a local net that limits packets to a smaller size to reach its destination.
An internet datagram can be marked "don't fragment." Any internet datagram so marked is not to be internet fragmented under any circumstances. If internet datagram marked don't fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead.
Fragmentation, transmission and reassembly across a local network which is invisible to the internet protocol module is called intranet fragmentation and may be used [6].
The internet fragmentation and reassembly procedure needs to be able to break a datagram into an almost arbitrary number of pieces that can be later reassembled. The receiver of the fragments uses the identification field to ensure that fragments of different datagrams are not mixed. The fragment offset field tells the receiver the position of a fragment in the original datagram. The fragment offset and length determine the portion of the original datagram covered by this fragment. The more-fragments flag indicates (by being reset) the last fragment. These fields provide sufficient information to reassemble datagrams.
The identification field is used to distinguish the fragments of one datagram from those of another. The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. The originating protocol module of a complete datagram sets the more-fragments flag to zero and the fragment offset to zero.
To fragment a long internet datagram, an internet protocol module (for example, in a gateway), creates two new internet datagrams and copies the contents of the internet header fields from the long datagram into both new internet headers. The data of the long datagram is divided into two portions on a 8 octet (64 bit) boundary (the second portion might not be an integral multiple of 8 octets, but the first must be). Call the number of 8 octet blocks in the first portion NFB (for Number of Fragment Blocks). The first portion of the data is placed in the first new internet datagram, and the total length field is set to the length of the first datagram. The more-fragments flag is set to one. The second portion of the data is placed in the second new internet datagram, and the total length field is set to the length of the second datagram. The more-fragments flag carries the same value as the long datagram. The fragment offset field of the second new internet datagram is set to the value of that field in the long datagram plus NFB.
This procedure can be generalized for an n-way split, rather than the two-way split described.
To assemble the fragments of an internet datagram, an internet protocol module (for example at a destination host) combines internet datagrams that all have the same value for the four fields: identification, source, destination, and protocol. The combination is done by placing the data portion of each fragment in the relative position indicated by the fragment offset in that fragment's internet header. The first fragment will have the fragment offset zero, and the last fragment will have the more-fragments flag reset to zero.
The internet identification field (ID) is used together with the source and destination address, and the protocol fields, to identify datagram fragments for reassembly.
The More Fragments flag bit (MF) is set if the datagram is not the last fragment. The Fragment Offset field identifies the fragment location, relative to the beginning of the original unfragmented datagram. Fragments are counted in units of 8 octets. The fragmentation strategy is designed so than an unfragmented datagram has all zero fragmentation information (MF = 0, fragment offset = 0). If an internet datagram is fragmented, its data portion must be broken on 8 octet boundaries.
This format allows 2**13 = 8192 fragments of 8 octets each for a total of 65,536 octets. Note that this is consistent with the the datagram total length field (of course, the header is counted in the total length and not in the fragments).
When fragmentation occurs, some options are copied, but others remain with the first fragment only.
Every internet module must be able to forward a datagram of 68 octets without further fragmentation. This is because an internet header may be up to 60 octets, and the minimum fragment is 8 octets.
Every internet destination must be able to receive a datagram of 576 octets either in one piece or in fragments to be reassembled.
The fields which may be affected by fragmentation include:
(1) options field
(2) more fragments flag
(3) fragment offset
(4) internet header length field
(5) total length field
(6) header checksum
If the Don't Fragment flag (DF) bit is set, then internet
fragmentation of this datagram is NOT permitted, although it may be
discarded. This can be used to prohibit fragmentation in cases
where the receiving host does not have sufficient resources to
reassemble internet fragments.One example of use of the Don't Fragment feature is to down line load a small host. A small host could have a boot strap program that accepts a datagram stores it in memory and then executes it.
The fragmentation and reassembly procedures are most easily described by examples. The following procedures are example implementations.
General notation in the following pseudo programs: "=<" means "less than or equal", "#" means "not equal", "=" means "equal", "<-" means "is set to". Also, "x to y" includes x and excludes y; for example, "4 to 7" would include 4, 5, and 6 (but not 7).
The maximum sized datagram that can be transmitted through the next network is called the maximum transmission unit (MTU).
If the total length is less than or equal the maximum transmission unit then submit this datagram to the next step in datagram processing; otherwise cut the datagram into two fragments, the first fragment being the maximum size, and the second fragment being the rest of the datagram. The first fragment is submitted to the next step in datagram processing, while the second fragment is submitted to this procedure in case it is still too large.
Notation:
FO - Fragment Offset
IHL - Internet Header Length
DF - Don't Fragment flag
MF - More Fragments flag
TL - Total Length
OFO - Old Fragment Offset
OIHL - Old Internet Header Length
OMF - Old More Fragments flag
OTL - Old Total Length
NFB - Number of Fragment Blocks
MTU - Maximum Transmission Unit
Procedure:
IF TL =< MTU THEN Submit this datagram to the next step
in datagram processing ELSE IF DF = 1 THEN discard the
datagram ELSE
To produce the first fragment:
(1) Copy the original internet header;
(2) OIHL <- IHL; OTL <- TL; OFO <- FO; OMF <- MF;
(3) NFB <- (MTU-IHL*4)/8;
(4) Attach the first NFB*8 data octets;
(5) Correct the header:
MF <- 1; TL <- (IHL*4)+(NFB*8);
Recompute Checksum;
(6) Submit this fragment to the next step in
datagram processing;
To produce the second fragment:
(7) Selectively copy the internet header (some options
are not copied, see option definitions);
(8) Append the remaining data;
(9) Correct the header:
IHL <- (((OIHL*4)-(length of options not copied))+3)/4;
TL <- OTL - NFB*8 - (OIHL-IHL)*4);
FO <- OFO + NFB; MF <- OMF; Recompute Checksum;
(10) Submit this fragment to the fragmentation test; DONE.
In the above procedure each fragment (except the last) was made
the maximum allowable size. An alternative might produce less
than the maximum size datagrams. For example, one could implement
a fragmentation procedure that repeatly divided large datagrams in
half until the resulting fragments were less than the maximum
transmission unit size.
If no other fragment with this buffer identifier is on hand then reassembly resources are allocated. The reassembly resources consist of a data buffer, a header buffer, a fragment block bit table, a total data length field, and a timer. The data from the fragment is placed in the data buffer according to its fragment offset and length, and bits are set in the fragment block bit table corresponding to the fragment blocks received.
If this is the first fragment (that is the fragment offset is zero) this header is placed in the header buffer. If this is the last fragment ( that is the more fragments field is zero) the total data length is computed. If this fragment completes the datagram (tested by checking the bits set in the fragment block table), then the datagram is sent to the next step in datagram processing; otherwise the timer is set to the maximum of the current timer value and the value of the time to live field from this fragment; and the reassembly routine gives up control.
If the timer runs out, the all reassembly resources for this buffer identifier are released. The initial setting of the timer is a lower bound on the reassembly waiting time. This is because the waiting time will be increased if the Time to Live in the arriving fragment is greater than the current timer value but will not be decreased if it is less. The maximum this timer value could reach is the maximum time to live (approximately 4.25 minutes). The current recommendation for the initial timer setting is 15 seconds. This may be changed as experience with this protocol accumulates. Note that the choice of this parameter value is related to the buffer capacity available and the data rate of the transmission medium; that is, data rate times timer value equals buffer size (e.g., 10Kb/s X 15s = 150Kb).
Notation:
FO - Fragment Offset
IHL - Internet Header Length
MF - More Fragments flag
TTL - Time To Live
NFB - Number of Fragment Blocks
TL - Total Length
TDL - Total Data Length
BUFID - Buffer Identifier
RCVBT - Fragment Received Bit Table
TLB - Timer Lower Bound
Procedure:
(1) BUFID <- source|destination|protocol|identification;
(2) IF FO = 0 AND MF = 0
(3) THEN IF buffer with BUFID is allocated
(4) THEN flush all reassembly for this BUFID;
(5) Submit datagram to next step; DONE.
(6) ELSE IF no buffer with BUFID is allocated
(7) THEN allocate reassembly resources
with BUFID;
TIMER <- TLB; TDL <- 0;
(8) put data from fragment into data buffer with
BUFID from octet FO*8 to
octet (TL-(IHL*4))+FO*8;
(9) set RCVBT bits from FO
to FO+((TL-(IHL*4)+7)/8);
(10) IF MF = 0 THEN TDL <- TL-(IHL*4)+(FO*8)
(11) IF FO = 0 THEN put header in header buffer
(12) IF TDL # 0
(13) AND all RCVBT bits from 0
to (TDL+7)/8 are set
(14) THEN TL <- TDL+(IHL*4)
(15) Submit datagram to next step;
(16) free all reassembly resources
for this BUFID; DONE.
(17) TIMER <- MAX(TIMER,TTL);
(18) give up until next fragment or timer expires;
(19) timer expires: flush all reassembly with this BUFID; DONE.
In the case that two or more fragments contain the same data
either identically or through a partial overlap, this procedure
will use the more recently arrived copy in the data buffer and
datagram delivered.
Identification
The choice of the Identifier for a datagram is based on the need to
provide a way to uniquely identify the fragments of a particular
datagram. The protocol module assembling fragments judges fragments
to belong to the same datagram if they have the same source,
destination, protocol, and Identifier. Thus, the sender must choose
the Identifier to be unique for this source, destination pair and
protocol for the time the datagram (or any fragment of it) could be
alive in the internet.It seems then that a sending protocol module needs to keep a table of Identifiers, one entry for each destination it has communicated with in the last maximum packet lifetime for the internet.
However, since the Identifier field allows 65,536 different values, some host may be able to simply use unique identifiers independent of destination.
It is appropriate for some higher level protocols to choose the identifier. For example, TCP protocol modules may retransmit an identical TCP segment, and the probability for correct reception would be enhanced if the retransmission carried the same identifier as the original transmission since fragments of either datagram could be used to construct a correct TCP segment.
Example 1: This is an example of the minimal data carrying internet datagram:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 21 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 1 | header checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+
Example Internet Datagram Figure 5.
Note that each tick mark represents one bit position.
This is a internet datagram in version 4 of internet protocol; the internet header
consists of five 32 bit words, and the total length of the datagram is 21 octets.
This datagram is a complete datagram (not a fragment).
Example 2: In this example, we show first a moderate size internet datagram
(452 data octets), then two internet fragments that might result from the
fragmentation of this datagram if the maximum sized transmission allowed were 280
octets (see Professor's notes).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 472 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 123 | Protocol = 6 | header checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Datagram Figure 6.
Now the first fragment that results from splitting the datagram after 256
data octets.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 276 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=1| Fragment Offset = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 119 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Fragment Figure 7.
And the second fragment.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver= 4 |IHL= 5 |Type of Service| Total Length = 216 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification = 111 |Flg=0| Fragment Offset = 32 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time = 119 | Protocol = 6 | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| source address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| destination address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
\ \
\ \
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Example Internet Fragment Figure 8.