Host-to-IMP Protocol - Revision history

Jnc: /* See also */ Bitsavers has reorganized a bit

2026-10-02T15:29:27Z

See also: Bitsavers has reorganized a bit

← Older revision		Revision as of 15:29, 2 October 2026
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	==See also==		==See also==

	* [[Bolt, Beranek, and Newman\|BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/~~BBN1822_Jan1976~~.pdf Specification for the Interconnection of a Host and an IMP] - the 1976 revision of this key document, it includes the earliest specification of the new long leader format		* [[Bolt, Beranek, and Newman\|BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN-1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - the 1976 revision of this key document, it includes the earliest specification of the new long leader format

	==External links==		==External links==

Jnc: Tweaks, typo

2024-12-30T10:20:01Z

Tweaks, typo

← Older revision		Revision as of 10:20, 30 December 2024
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	The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another~~, as the lowest layer of [[Network Control Protocol\|NCP]]~~.		The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[Network Control Protocol\|NCP]] [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another.

	Communication ~~and~~ between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].		Communication between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].

	==Host addresses==		==Host addresses==
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	:4 - NOP		:4 - NOP
	:5 - RFNM		:5 - RFNM
	:: 0 - Original Message ~~Nno~~-Refusable		:: 0 - Original Message Non-Refusable
	:: 1 - Original Message Refusable		:: 1 - Original Message Refusable
	:6 - Dead Host Status		:6 - Dead Host Status
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	:: 1 - Currently unused		:: 1 - Currently unused
	:: 2 - Connection not available		:: 2 - Connection not available
	:: 3 - ~~Reassemly~~ space not available at destination		:: 3 - Reassembly space not available at destination
	:: 4 - Message number not available		:: 4 - Message number not available
	:: 5 - Transaction block for message not available		:: 5 - Transaction block for message not available
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	==See also==		==See also==

	* [[Bolt, Beranek, and Newman\|BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - ~~The~~ 1976 revision of this key document, it includes the earliest specification of the new long leader format		* [[Bolt, Beranek, and Newman\|BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - the 1976 revision of this key document, it includes the earliest specification of the new long leader format

	==External links==		==External links==

Jnc: More NCP fixups

2022-06-29T12:56:32Z

More NCP fixups

← Older revision		Revision as of 12:56, 29 June 2022
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	The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control ~~Program (ARPANET)~~\|NCP]].		The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Protocol\|NCP]].

	Communication and between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].		Communication and between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].
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	==See also==		==See also==

	* [[BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - The 1976 revision of this key document, it includes the earliest specification of the new long leader format		* [[Bolt, Beranek, and Newman\|BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - The 1976 revision of this key document, it includes the earliest specification of the new long leader format

	==External links==		==External links==

Jnc: dab NCP

2022-06-04T21:36:40Z

dab NCP

← Older revision		Revision as of 21:36, 4 June 2022
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	The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program\|NCP]].		The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program (ARPANET)\|NCP]].

	Communication and between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].		Communication and between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].

Jnc: better cat

2021-11-06T00:24:47Z

better cat

← Older revision		Revision as of 00:24, 6 November 2021
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	** [http://www.chiappa.net/~jnc/tech/arpaprot.html ARPANET Technical Information: ARPANET Protocol Handbook]		** [http://www.chiappa.net/~jnc/tech/arpaprot.html ARPANET Technical Information: ARPANET Protocol Handbook]

	[[Category: ~~Networking~~]]		[[Category: ARPANET]]

Jnc: Split of 1822 stuff, for use in PRNET, etc

2021-11-02T17:39:30Z

Split of 1822 stuff, for use in PRNET, etc

← Older revision		Revision as of 17:39, 2 November 2021
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	The '''Host-to-IMP Protocol''' ~~includes~~ the lowest ~~several layers (Physical, etc) of the communication channel in the~~ [[~~ARPANET~~]] ~~between a~~ [[~~host~~]] ~~and an~~ [[~~Interface Message Processor\|IMP~~]]. It used both for the host to communicate with its IMP, and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program\|NCP]].		The '''Host-to-IMP Protocol''' is the lowest [[protocol]] layer of the [[protocol suite\|protocol stack]] in the [[ARPANET]]. It was used both for the [[host]] to communicate with its [[Interface Message Processor\|IMP]], and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program\|NCP]].

	~~The~~ service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].		Communication and between IMPs and their connected hosts was all done with packets (for which the formal ARPANET term was "messages"); the host to host data carriage service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].

	~~==Physical layer==~~		==Host addresses==

	~~Connection of a host to an IMP required a special [[network interface]], an [[IMP interface]].~~

	~~At the very lowest layer, there are three different choices for the [[physical layer]]; 'Local Host' (LH), 'Distant Host' (DH), and 'Very Distant Host' (VDH).~~

	The first two are logically the same, and differ only in the physical details - DH uses [[transistor-transistor logic\|TTL]] signals over cable using [[twisted pair]]s, with a [[ground]] being the other [[conductor]] in the pair, whereas LH uses [[differential pair]]s.

	Above that, the interface is completely identical; it is [[asynchronous]], bit-serial, with a handshake on every bit. There is no fixed [[word]] length, the IMP was prepared to talk to machines with different [[word]] lengths. There are also two pairs of 'ready' lines (one from the IMP, and one from the Host), which are connected to ground via a [[relay]] when that entity wants to signal that it is ready.

	~~The VDH uses an entirely different protocol, essentially the [[modem]] to modem [[protocol]] which the IMPs use to talk to each other over a long-distance [[synchronous serial line]].~~

	~~==Network layer==~~

	~~Communication and between IMPs and their connected hosts was all done with [[packet]]s (for which the formal ARPANET term was "messages").~~

	===Host addresses===

	Host [[address]]es in the ARPANET consisted of the IMP number, and the host number on the IMP. In the original 'short leader' format (below), IMP numbers were 6 bits, and host numbers were 2 bits. In the later 'long leader' format, these were increased to 16 bits and 8 bits respectively.		Host [[address]]es in the ARPANET consisted of the IMP number, and the host number on the IMP. In the original 'short leader' format (below), IMP numbers were 6 bits, and host numbers were 2 bits. In the later 'long leader' format, these were increased to 16 bits and 8 bits respectively.

	===Packet formats===		==Packet formats==

	There were two versions of the Host-IMP headers:		There were two versions of the Host-IMP headers:

	* The so-called "short leader" format, used from 1970 until roughly the end of 1975 (the exact date is not yet ascertained), although it continued to be supported after that date, but hosts using it could not reach all of the other hosts in the network at some point shortly thereafter. It was 32 bits long.		* The so-called "short leader" format, used from 1970 until roughly the end of 1975 (the exact date is not yet ascertained); although it continued to be supported after that date, but hosts using it could not reach all of the other hosts in the network at some point shortly thereafter. It was 32 bits long.
	* The so-called "long leader" format, introduced around the end of 1975, which allowed i) more IMPs in the network, ii) more hosts per IMP, and iii) various other improvements. It was 96 bits long.		* The so-called "long leader" format, introduced around the end of 1975, which allowed i) more IMPs in the network, ii) more hosts per IMP, and iii) various other improvements. It was 96 bits long.

	For details, see the external link below.		For details, see the external link below.

	===Links===		==Links==

	In both the original 32-bit leader and the later 96-bit leader, there was a 12-bit field called the 'message ID'; the high 8 bits of this field were designated as the '''link'''.		In both the original 32-bit leader and the later 96-bit leader, there was a 12-bit field called the 'message ID'; the high 8 bits of this field were designated as the '''link'''.

	The ~~[[Network Control Program\|~~NCP~~]] [[~~protocol]] used links 0, and 2-0107; the low 4 bits of the message ID were always set to zero. The [[Internet Protocol]] used links 0233-0236. Several other [[internetworking layer]] protocols had links assigned, to allow ~~them~~ to be carried over the ARPANET: 0230 for [[PARC Universal Packet\|PUP]], and 0227 for [[~~CHAOSnet~~\|CHAOS]].		The NCP protocol used links 0, and 2-0107; the low 4 bits of the message ID were always set to zero. The [[Internet Protocol]] used links 0233-0236. Several other [[internetworking layer]] protocols had links assigned, to allow their packets to be carried over the ARPANET: 0230 for [[PARC Universal Packet\|PUP]], and 0227 for [[Chaosnet\|CHAOS]].

	===Packet Types (Host->IMP) ===		==Packet Types (Host->IMP) ==

	The network layer uses a number of different packet types; some are used by the IMP and Host to communicate with each other, others are used to send data from one Host to another.		The network layer uses a number of different packet types; some are used by the IMP and Host to communicate with each other, others are used to send data from one Host to another.

	One important type is the 'Ready For Next Message' ('''RFNM'''), used between the Host and the IMP (with cooperation from the destination Host's IMP) for flow control; it indicated when a message had been successfully delivered to the destination Host, and that Host's IMP was ready for another packet to that host.		One important type is the 'Ready For Next Message' ('''RFNM'''), used between the Host and the IMP (with cooperation from the destination Host's IMP) for flow control; it indicated when a message had been successfully delivered to the destination Host, and that Host's IMP was ready for another packet to that destination host.

	(Depending on the exact version of the code running in the IMP, there was generally a maximum of 8 outstanding packets to any given destination allowed at a time.)		(Depending on the exact version of the code running in the IMP, there was generally a maximum of 8 outstanding packets to any given destination allowed at a time from a source host.)

	Here are the packet types, listed by type and (indented) sub-type:		Here are the packet types, listed by type and (indented) sub-type:
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	<nowiki>*</nowiki> = Non-blocking interface, not implemented		<nowiki>*</nowiki> = Non-blocking interface, not implemented

	===Packet Types (IMP->Host) ===		==Packet Types (IMP->Host)==

	Listed by type and (indented) sub-type:		Listed by type and (indented) sub-type:

Jnc: /* See also */ typo

2021-10-25T18:41:57Z

Jnc: more precise phrasing

2021-10-19T23:13:47Z

more precise phrasing

← Older revision		Revision as of 23:13, 19 October 2021
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	The '''Host-to-IMP Protocol''' includes the lowest several layers (Physical, etc) of the communication channel between a [[host]] and an [[Interface Message Processor\|IMP]]. It used both for the host to communicate with its IMP, and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program\|NCP]].		The '''Host-to-IMP Protocol''' includes the lowest several layers (Physical, etc) of the communication channel in the [[ARPANET]] between a [[host]] and an [[Interface Message Processor\|IMP]]. It used both for the host to communicate with its IMP, and for one host to send a [[packet]] to another, as the lowest layer of [[Network Control Program\|NCP]].

	The service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].		The service provided was reliable (although if a packet was irretrievably lost, the IMP would indicate that to the host with an error message), with [[flow control]].
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	==Network layer==		==Network layer==

	Communication and between IMPs and their connected hosts was all done with [[packet]]s (~~technically,~~ "messages").		Communication and between IMPs and their connected hosts was all done with [[packet]]s (for which the formal ARPANET term was "messages").

	===Host addresses===		===Host addresses===

	Host ~~addresses~~ in the ARPANET consisted of the IMP number, and the host number on the IMP. In the original 'short leader' format (below), IMP numbers were 6 bits, and host numbers were 2 bits. In the later 'long leader' format, these were increased to 16 bits and 8 bits respectively.		Host [[address]]es in the ARPANET consisted of the IMP number, and the host number on the IMP. In the original 'short leader' format (below), IMP numbers were 6 bits, and host numbers were 2 bits. In the later 'long leader' format, these were increased to 16 bits and 8 bits respectively.

	===Packet formats===		===Packet formats===

Jnc: /* Physical layer */ clarify

2021-10-18T12:45:21Z

Physical layer: clarify

← Older revision		Revision as of 12:45, 18 October 2021
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	At the very lowest layer, there are three different choices for the [[physical layer]]; 'Local Host' (LH), 'Distant Host' (DH), and 'Very Distant Host' (VDH).		At the very lowest layer, there are three different choices for the [[physical layer]]; 'Local Host' (LH), 'Distant Host' (DH), and 'Very Distant Host' (VDH).

	The first two are logically the same, and differ only in the physical details - DH uses [[transistor-transistor logic\|TTL]] signals over cable using [[twisted pair]]s, with a [[ground]] being the other ~~line~~ in the pair, whereas LH uses [[differential pair]]s.		The first two are logically the same, and differ only in the physical details - DH uses [[transistor-transistor logic\|TTL]] signals over cable using [[twisted pair]]s, with a [[ground]] being the other [[conductor]] in the pair, whereas LH uses [[differential pair]]s.

	Above that, the interface is completely identical; it is [[asynchronous]], bit-serial, with a handshake on every bit. There is no fixed [[word]] length, the IMP was prepared to talk to machines with different [[word]] lengths. There are also two pairs of 'ready' lines (one from the IMP, and one from the Host), which are connected to ground via a [[relay]] when that entity wants to signal that it is ready.		Above that, the interface is completely identical; it is [[asynchronous]], bit-serial, with a handshake on every bit. There is no fixed [[word]] length, the IMP was prepared to talk to machines with different [[word]] lengths. There are also two pairs of 'ready' lines (one from the IMP, and one from the Host), which are connected to ground via a [[relay]] when that entity wants to signal that it is ready.

Larsbrinkhoff: Link to BBN.

2021-08-20T10:58:17Z

Link to BBN.

← Older revision		Revision as of 10:58, 20 August 2021
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	==See also==		==See also==

	* BBN Report #1822, "[http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Inteconnection of a Host and an IMP]" - The 1976 revision of this key document, it includes the earliest specification of the new long leader format		* [[BBN]] Report #1822, "[http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Inteconnection of a Host and an IMP]" - The 1976 revision of this key document, it includes the earliest specification of the new long leader format

	==External links==		==External links==

← Older revision		Revision as of 18:41, 25 October 2021
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	==See also==		==See also==

	* [[BBN]] Report #1822, "[http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the ~~Inteconnection~~ of a Host and an IMP]" - The 1976 revision of this key document, it includes the earliest specification of the new long leader format		* [[BBN]] Report #1822, [http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf Specification for the Interconnection of a Host and an IMP] - The 1976 revision of this key document, it includes the earliest specification of the new long leader format

	==External links==		==External links==