As per Relevance of the word datagram, we have this rfc below:
Network Working Group J.
Request for Comments: 879
November 1983
The TCP Maximum Segment
and Related
This memo discusses the TCP Maximum Segment Size Option and
topics. The purposes is to clarify some aspects of TCP and
interaction with IP. This memo is a clarification to the
specification, and contains information that may be considered
"advice to implementers".
1.
This memo discusses the TCP Maximum Segment Size and its relation
the IP Maximum Datagram Size. TCP is specified in reference [1].
is specified in references [2,3].
This discussion is necessary because the current specification
this TCP option is ambiguous
Much of the difficulty with understanding these sizes and
relationship has been due to the variable size of the IP and
headers
There have been some assumptions made about using other than
default size for datagrams with some unfortunate results
HOSTS MUST NOT SEND DATAGRAMS LARGER THAN 576 OCTETS UNLESS
HAVE SPECIFIC KNOWLEDGE THAT THE DESTINATION HOST IS PREPARED
ACCEPT LARGER DATAGRAMS
This is a long established rule
To resolve the ambiguity in the TCP Maximum Segment Size
definition the following rule is established
THE TCP MAXIMUM SEGMENT SIZE IS THE IP MAXIMUM DATAGRAM SIZE
FORTY
The default IP Maximum Datagram Size is 576.
The default TCP Maximum Segment Size is 536.
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RFC 879 November 1983
TCP Maximum Segment Size
2. The IP Maximum Datagram
Hosts are not required to reassemble infinitely large IP datagrams
The maximum size datagram that all hosts are required to accept
reassemble from fragments is 576 octets. The maximum size
buffer every host must have is 576 octets. Hosts are allowed
accept larger datagrams and assemble fragments into larger datagrams
hosts may have buffers as large as they please
Hosts must not send datagrams larger than 576 octets unless they
specific knowledge that the destination host is prepared to
larger datagrams
3. The TCP Maximum Segment Size
TCP provides an option that may be used at the time a connection
established (only) to indicate the maximum size TCP segment that
be accepted on that connection. This Maximum Segment Size (MSS
announcement (often mistakenly called a negotiation) is sent from
data receiver to the data sender and says "I can accept TCP
up to size X". The size (X) may be larger or smaller than
default. The MSS can be used completely independently in
direction of data flow. The result may be quite different
sizes in the two directions
The MSS counts only data octets in the segment, it does not count
TCP header or the IP header
A footnote: The MSS value counts only data octets, thus it does
count the TCP SYN and FIN control bits even though SYN and FIN
consume TCP sequence numbers
4. The Relationship of TCP Segments and IP
TCP segment are transmitted as the data in IP datagrams.
correspondence between TCP segments and IP datagrams must be one
one. This is because TCP expects to find exactly one complete
segment in each block of data turned over to it by IP, and IP
turn over a block of data for each datagram received (or
reassembled).
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TCP Maximum Segment Size
5. Layering and
TCP is an end to end reliable data stream protocol with
control, flow control, etc. TCP remembers many things about
state of a connection
IP is a one shot datagram protocol. IP has no memory of
datagrams transmitted. It is not appropriate for IP to keep
information about the maximum datagram size a particular
host might be capable of accepting
TCP and IP are distinct layers in the protocol architecture, and
often implemented in distinct program modules
Some people seem to think that there must be no communication
protocol layers or program modules. There must be
between layers and modules, but it should be carefully specified
controlled. One problem in understanding the correct view
communication between protocol layers or program modules in general
or between TCP and IP in particular is that the documents
protocols are not very clear about it. This is often because
documents are about the protocol exchanges between machines, not
program architecture within a machine, and the desire to allow
program architectures with different organization of tasks
modules
6. IP Information
There is no general requirement that IP keep information on a
host basis
IP must make a decision about which directly attached network
to send each datagram to. This is simply mapping an IP address
a directly attached network address
There are two cases to consider: the destination is on the
network, and the destination is on a different network
Same
For some networks the the directly attached network address
be computed from the IP address for destination hosts on
directly attached network
For other networks the mapping must be done by table look
(however the table is initialized and maintained,
example, [4]).
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TCP Maximum Segment Size
Different
The IP address must be mapped to the directly attached
address of a gateway. For networks with one gateway to
rest of the Internet the host need only determine and
the gateway address and use it for sending all datagrams
other networks
For networks with multiple gateways to the rest of
Internet, the host must decide which gateway to use for
datagram sent. It need only check the destination network
the IP address and keep information on which gateway to use
each network
The IP does, in some cases, keep per host routing information
other hosts on the directly attached network. The IP does, in
cases, keep per network routing information
A Special
There are two ICMP messages that convey information
particular hosts. These are subtypes of the
Unreachable and the Redirect ICMP messages. These messages
expected only in very unusual circumstances. To make
use of these messages the receiving host would have to
information about the specific hosts reported on. Because
messages are quite rare it is strongly recommended that this
done through an exception mechanism rather than having the IP
per host tables for all hosts
7. The Relationship between IP Datagram and TCP Segment
The relationship between the value of the maximum IP datagram
and the maximum TCP segment size is obscure. The problem is
both the IP header and the TCP header may vary in length. The
Maximum Segment Size option (MSS) is defined to specify the
number of data octets in a TCP segment exclusive of TCP (or IP
header
To notify the data sender of the largest TCP segment it is
to receive the calculation of the MSS value to send is
MSS = MTU - sizeof(TCPHDR) - sizeof(IPHDR
On receipt of the MSS option the calculation of the size of
that can be sent is
SndMaxSegSiz = MIN((MTU - sizeof(TCPHDR) - sizeof(IPHDR)), MSS
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TCP Maximum Segment Size
where MSS is the value in the option, and MTU is the
Transmission Unit (or the maximum packet size) allowed on
directly attached network
This begs the question, though. What value should be used for
"sizeof(TCPHDR)" and for the "sizeof(IPHDR)"?
There are three reasonable positions to take: the conservative,
moderate, and the liberal
The conservative or pessimistic position assumes the worst --
both the IP header and the TCP header are maximum size, that is, 60
octets each
MSS = MTU - 60 - 60 = MTU - 120
If MTU is 576 then MSS = 456
The moderate position assumes the that the IP is maximum size (60
octets) and the TCP header is minimum size (20 octets), because
are no TCP header options currently defined that would normally
sent at the same time as data segments
MSS = MTU - 60 - 20 = MTU - 80
If MTU is 576 then MSS = 496
The liberal or optimistic position assumes the best -- that both
IP header and the TCP header are minimum size, that is, 20
each
MSS = MTU - 20 - 20 = MTU - 40
If MTU is 576 then MSS = 536
If nothing is said about MSS, the data sender may cram as much
possible into a 576 octet datagram, and if the datagram
minimum headers (which is most likely), the result will be 536
data octets in the TCP segment. The rule relating MSS to
maximum datagram size ought to be consistent with this
A practical point is raised in favor of the liberal position too
Since the use of minimum IP and TCP headers is very likely in
very large percentage of cases, it seems wasteful to limit the
segment data to so much less than could be transmitted at once
especially since it is less that 512 octets
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TCP Maximum Segment Size
For comparison: 536/576 is 93% data, 496/576 is 86% data, 456/576
is 79% data
8. Maximum Packet
Each network has some maximum packet size, or maximum
unit (MTU). Ultimately there is some limit imposed by
technology, but often the limit is an engineering choice or even
administrative choice. Different installations of the same
product do not have to use the same maximum packet size. Even
one installation not all host must use the same packet size (this
lies madness, though).
Some IP implementers have assumed that all hosts on the
attached network will be the same or at least run the
implementation. This is a dangerous assumption. It has
developed that after a small homogeneous set of host have
operational additional hosts of different types are introduced
the environment. And it has often developed that it is desired
use a copy of the implementation in a different
environment
Designers of gateways should be prepared for the fact that
gateways will be copied and used in other situation
installations. Gateways must be prepared to accept datagrams
large as can be sent in the maximum packets of the directly
networks. Gateway implementations should be easily configured
installation in different circumstances
A footnote: The MTUs of some popular networks (note that the
limit in some installations may be set lower by
policy):
ARPANET, MILNET = 1007
Ethernet (10Mb) = 1500
Proteon PRONET = 2046
9. Source
A source host would not normally create datagram fragments.
normal circumstances datagram fragments only arise when a
must send a datagram into a network with a smaller maximum
size than the datagram. In this case the gateway must fragment
datagram (unless it is marked "don't fragment" in which case it
discarded, with the option of sending an ICMP message to the
reporting the problem).
It might be desirable for the source host to send datagram
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TCP Maximum Segment Size
if the maximum segment size (default or negotiated) allowed by
data receiver were larger than the maximum packet size allowed by
directly attached network. However, such datagram fragments must
combine to a size larger than allowed by the destination host
For example, if the receiving TCP announced that it would
segments up to 5000 octets (in cooperation with the receiving IP
then the sending TCP could give such a large segment to
sending IP provided the sending IP would send it in
fragments that fit in the packets of the directly
network
There are some conditions where source host fragmentation would
necessary
If the host is attached to a network with a small packet size (
example 256 octets), and it supports an application defined
send fixed sized messages larger than that packet size (
example TFTP [5]).
If the host receives ICMP Echo messages with data it is
to send an ICMP Echo-Reply message with the same data. If
amount of data in the Echo were larger than the packet size of
directly attached network the following steps might be required
(1) receive the fragments, (2) reassemble the datagram, (3)
interpret the Echo, (4) create an Echo-Reply, (5) fragment it,
(6) send the fragments
10. Gateway
Gateways must be prepared to do fragmentation. It is not an
feature for a gateway
Gateways have no information about the size of datagrams
hosts are prepared to accept. It would be inappropriate for
to attempt to keep such information
Gateways must be prepared to accept the largest datagrams that
allowed on each of the directly attached networks, even if it
larger than 576 octets
Gateways must be prepared to fragment datagrams to fit into
packets of the next network, even if it smaller than 576 octets
If a source host thought to take advantage of the local network'
ability to carry larger datagrams but doesn't have the slightest
if the destination host can accept larger than default datagrams
expects the gateway to fragment the datagram into default
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TCP Maximum Segment Size
fragments, then the source host is misguided. If indeed,
destination host can't accept larger than default datagrams,
probably can't reassemble them either. If the gateway either
on the large datagram whole or fragments into default size
the destination will not accept it. Thus, this mode of behavior
source hosts must be outlawed
A larger than default datagram can only arrive at a gateway
the source host knows that the destination host can handle such
datagrams (probably because the destination host announced it to
source host in an TCP MSS option). Thus, the gateway should pass
this large datagram in one piece or in the largest fragments that
into the next network
An interesting footnote is that even though the gateways may
about know the 576 rule, it is irrelevant to them
11. Inter-Layer
The Network Driver (ND) or interface should know the
Transmission Unit (MTU) of the directly attached network
The IP should ask the Network Driver for the Maximum
Unit
The TCP should ask the IP for the Maximum Datagram Data Size (MDDS).
This is the MTU minus the IP header length (MDDS = MTU - IPHdrLen).
When opening a connection TCP can send an MSS option with the
equal MDDS - TCPHdrLen
TCP should determine the Maximum Segment Data Size (MSDS) from
the default or the received value of the MSS option
TCP should determine if source fragmentation is possible (by
the IP) and desirable
If so TCP may hand to IP segments (including the TCP header) up
MSDS + TCPHdrLen
If not TCP may hand to IP segments (including the TCP header)
to the lesser of (MSDS + TCPHdrLen) and MDDS
IP checks the length of data passed to it by TCP. If the length
less than or equal MDDS, IP attached the IP header and hands it
the ND. Otherwise the IP must do source fragmentation
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TCP Maximum Segment Size
12. What is the Default MSS ?
Another way of asking this question is "What transmitted value
MSS has exactly the same effect of not transmitting the option
all?".
In terms of the previous section
The default assumption is that the Maximum Transmission Unit
576 octets
MTU = 576
The Maximum Datagram Data Size (MDDS) is the MTU minus the
header length
MDDS = MTU - IPHdrLen = 576 - 20 = 556
When opening a connection TCP can send an MSS option with
value equal MDDS - TCPHdrLen
MSS = MDDS - TCPHdrLen = 556 - 20 = 536
TCP should determine the Maximum Segment Data Size (MSDS)
either the default or the received value of the MSS option
Default MSS = 536, then MSDS = 536
TCP should determine if source fragmentation is possible
desirable
If so TCP may hand to IP segments (including the TCP header)
to MSDS + TCPHdrLen (536 + 20 = 556).
If not TCP may hand to IP segments (including the TCP header
up to the lesser of (MSDS + TCPHdrLen (536 + 20 = 556))
MDDS (556).
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TCP Maximum Segment Size
13. The
The rule relating the maximum IP datagram size and the maximum
segment size is
TCP Maximum Segment Size = IP Maximum Datagram Size - 40
The rule must match the default case
If the TCP Maximum Segment Size option is not transmitted then
data sender is allowed to send IP datagrams of maximum size (576)
with a minimum IP header (20) and a minimum TCP header (20)
thereby be able to stuff 536 octets of data into each TCP segment
The definition of the MSS option can be stated
The maximum number of data octets that may be received by
sender of this TCP option in TCP segments with no TCP
options transmitted in IP datagrams with no IP header options
14. The
When TCP is used in a situation when either the IP or TCP headers
not minimum and yet the maximum IP datagram that can be
remains 576 octets then the TCP Maximum Segment Size option must
used to reduce the limit on data octets allowed in a TCP segment
For example, if the IP Security option (11 octets) were in use
the IP maximum datagram size remained at 576 octets, then the
should send the MSS with a value of 525 (536-11).
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TCP Maximum Segment Size
15.
[1] Postel, J., ed., "Transmission Control Protocol - DARPA
Program Protocol Specification", RFC 793, USC/
Sciences Institute, September 1981.
[2] Postel, J., ed., "Internet Protocol - DARPA Internet
Protocol Specification", RFC 791, USC/Information
Institute, September 1981.
[3] Postel, J., "Internet Control Message Protocol - DARPA
Program Protocol Specification", RFC 792, USC/
Sciences Institute, September 1981.
[4] Plummer, D., "An Ethernet Address Resolution Protocol
Converting Network Protocol Addresses to 48-bit
Addresses for Transmission on Ethernet Hardware", RFC 826,
MIT/LCS, November 1982.
[5] Sollins, K., "The TFTP Protocol (Revision 2)", RFC 783, MIT/LCS
June 1981.
Postel [Page 11]
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