APPENDIX B: Data Transmission Order

The order of transmission of the header and data described in this document is resolved to the octet level. Whenever a diagram shows a group of octets, the order of transmission of those octets is the normal order in which they are read in English. For example, in the following diagram the octets are transmitted in the order they are numbered.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 3 | 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 6 | 7 | 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 10 | 11 | 12 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Transmission Order of Bytes

Figure 10.

Whenever an octet represents a numeric quantity the left most bit in the diagram is the high order or most significant bit. That is, the bit labeled 0 is the most significant bit. For example, the following diagram represents the value 170 (decimal).

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1 0 1 0 1 0 1 0|
+-+-+-+-+-+-+-+-+

Significance of Bits

Figure 11.

Similarly, whenever a multi-octet field represents a numeric quantity the left most bit of the whole field is the most significant bit. When a multi-octet quantity is transmitted the most significant octet is transmitted first.

GLOSSARY

1822 BBN Report 1822, "The Specification of the Interconnection of a Host and an IMP". The specification of interface between a host and the ARPANET.

ARPANET leader The control information on an ARPANET message at the host-IMP interface.

ARPANET message The unit of transmission between a host and an IMP in the ARPANET. The maximum size is about 1012 octets (8096 bits).

ARPANET packet A unit of transmission used internally in the ARPANET between IMPs. The maximum size is about 126 octets (1008 bits).

Destination The destination address, an internet header field.

DF The Don't Fragment bit carried in the flags field.

Flags An internet header field carrying various control flags.

Fragment Offset This internet header field indicates where in the internet datagram a fragment belongs.

GGP Gateway to Gateway Protocol, the protocol used primarily between gateways to control routing and other gateway functions.

header Control information at the beginning of a message, segment, datagram, packet or block of data.

ICMP Internet Control Message Protocol, implemented in the internet module, the ICMP is used from gateways to hosts and between hosts to report errors and make routing suggestions.

Identification An internet header field carrying the identifying value assigned by the sender to aid in assembling the fragments of a datagram.

IHL The internet header field Internet Header Length is the length of the internet header measured in 32 bit words.

IMP The Interface Message Processor, the packet switch of the ARPANET.

Internet Address A four octet (32 bit) source or destination address consisting of a Network field and a Local Address field.

internet datagram The unit of data exchanged between a pair of internet modules (includes the internet header).

internet fragment A portion of the data of an internet datagram with an internet header.

Local Address The address of a host within a network. The actual mapping of an internet local address on to the host addresses in a network is quite general, allowing for many to one mappings.

MF The More-Fragments Flag carried in the internet header flags field.

module An implementation, usually in software, of a protocol or other procedure.

more-fragments flag A flag indicating whether or not this internet datagram contains the end of an internet datagram, carried in the internet header Flags field.

NFB The Number of Fragment Blocks in a the data portion of an internet fragment. That is, the length of a portion of data measured in 8 octet units.

octet An eight bit byte.

Options The internet header Options field may contain several options, and each option may be several octets in length.

Padding The internet header Padding field is used to ensure that the data begins on 32 bit word boundary. The padding is zero.

Protocol In this document, the next higher level protocol identifier, an internet header field.

Rest The local address portion of an Internet Address.

Source The source address, an internet header field.

TCP Transmission Control Protocol: A host-to-host protocol for reliable communication in internet environments.

TCP Segment The unit of data exchanged between TCP modules (including the TCP header).

TFTP Trivial File Transfer Protocol: A simple file transfer protocol built on UDP.

Time to Live An internet header field which indicates the upper bound on how long this internet datagram may exist.

TOS Type of Service

Total Length The internet header field Total Length is the length of the datagram in octets including internet header and data.

TTL Time to Live

Type of Service An internet header field which indicates the type (or quality) of service for this internet datagram.

UDP User Datagram Protocol: A user level protocol for transaction oriented applications.

User The user of the internet protocol. This may be a higher level protocol module, an application program, or a gateway program.

Version The Version field indicates the format of the internet header.

REFERENCES

[1] Cerf, V., "The Catenet Model for Internetworking," Information Processing Techniques Office, Defense Advanced Research Projects Agency, IEN 48, July 1978.

[2] Bolt Beranek and Newman, "Specification for the Interconnection of a Host and an IMP," BBN Technical Report 1822, Revised May 1978.

[3] Postel, J., "Internet Control Message Protocol - DARPA Internet Program Protocol Specification," RFC 792, USC/Information Sciences Institute, September 1981.

[4] Shoch, J., "Inter-Network Naming, Addressing, and Routing," COMPCON, IEEE Computer Society, Fall 1978.

[5] Postel, J., "Address Mappings," RFC 796, USC/Information Sciences Institute, September 1981.

[6] Shoch, J., "Packet Fragmentation in Inter-Network Protocols," Computer Networks, v. 3, n. 1, February 1979.

[7] Strazisar, V., "How to Build a Gateway", IEN 109, Bolt Beranek and Newman, August 1979.

[8] Postel, J., "Service Mappings," RFC 795, USC/Information Sciences Institute, September 1981.

[9] Postel, J., "Assigned Numbers," RFC 790, USC/Information Sciences Institute, September 1981.