RFC 9 UNKNOWN ARPANET · HOST SOFTWARE · LINKS
Host Software
The host-to-host dialogue of the early ARPANET: logical links, a link-opening exchange, service calls and tables.
RFC EDITOR
Deloche, G., "Host Software", RFC 9, DOI 10.17487/RFC0009, May 1969, <https://www.rfc-editor.org/info/rfc9>.
BIBTEX
@misc{rfc9,
series = {Request for Comments},
number = 9,
howpublished = {RFC 9},
publisher = {RFC Editor},
doi = {10.17487/RFC0009},
url = {https://www.rfc-editor.org/info/rfc9},
author = {G. Deloche},
title = {{Host Software}},
pagetotal = 15,
year = 1969,
month = may,
} RFC text
Contents
This text was converted from the RFC Editor's PDF edition and may contain conversion errors; the layout is approximate. The PDF is the reference copy.
G. Dc1oche , U.C.L.p ,
1 i':1ay 1969
.
,..
1. Introduction
2. HOST-Hosrr Prot ocol
2.1 Logical Li.nks
2.1.1 Pr-Imary
2.1.2 Auxiliary
2.2 Link Es t ab Ll sbment
2.2.1 G~neral Proc edur es
2.2.2 Exanp'le
3. Net,. lorf~ Service Calls
3.1 List of Service Calls
4. Data Struct ure
4.1 II HOST" Tab I.e
4.2 "LWK" Tab Le
4•3 "USE ~" Tab Le
':. "
..
.1 . Introduc tio~
'fius paper conc entr-ates upon t he HOST- I-L)'ST d.l.a'logue procedure,
Chapter 2 descr-Ibe s t ile lOGical Links ccnnec t L. l g t he HOSTJ and the r..:2.'/ da t a
are exchanged over t h ese Li.nks ,
The emphasis of Chapter-a 3 J 11 J and 5 is on sof't:. rare or-gan.i zat l on c;.cl data
structure.
Figure 1 hiSllights t he different p~c-sr21:lS Lnvol ved in a HOS'l'.
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2. HOSl'-HJST Pr'c t c coL
= '-- .;..-=-= -"= -=:::.:= =
2.1 ' Lo~i c cl Lir~s (~ibJre 2)
~-- -- - --
Any n·I? C:L'1 be v.Levre d as 2.l1 Lnt erf'ace be t we en a local ce nt er and the
t.runk networ k, Locally J an r.::P may ser ve up to four HOSTs; for each
of them :!.t pr ovi de s 256 l ogical links to any r-emot e EOST .
However' J between an ]].1P and a l l t he ot her niPs no mor-e t han 64 links
may be in us e s.imu'It anecus Iy , In other words J a HOSrr die.loSruinS
with a remot e EOST c all c on sider its l ocal I:·IP a s a svd t chi.nx cent e r
offering 256 1:L'18 S t o t he r-emot e HOSrr, but only 64 can be 3.C'ti vat ed
at a t~e. (If 2. local cen ter inc luj e s n HOSTs , 64 should be shared
amount t he n EOST's ) .
The 256 Logf.c a.l Link s c onnect ing t';
, IO EOS'r s C211 be distinGUi s hed a s
fo110',.:s:
Link 0 has a s pe cial s t atus. It is' t he cen t rol lLllk (conYlec t i on
' r eque s t s , s t a tus report of 2.l1y kind ••• ).
The 255 ot hers c an be u s ed e i ther as 2£.!rr,:::.ry links, i.e., "teletype
Llke" co nn e ct i ons, or as aU:<ili2-E1~ 11r.k;.; for file transmi s s i on.
2.1.1 -
PrWE.r V Li
. nks Fe at ur es.
-----'--_
A pr-Imary lin.%
* is t he f irs t l i nk est2.b lished f or a HOST-EOST trans::1i ssion.
* is a "T'l'Y-like" connecti on that i s:
ASCI I ChCl"2.ct 2r 3 are tran3mitted .
Echos are ~):~nere.ted by t he r-ernot e HOS'1' .
The r'etnot e HOS'l' s cans for break char-ac ber-,
The t.r-ansr.tl as l on is s l.ow (les s t h an 20 ch aract er-s per second ) .
~ is mainly us e d for' t.r-ansmi.t t.Lng cont r ol CO:T:Tl.::""1d 3, L, e. j for
Log-d.n to the r er;-.ote EOST oper-at lng sys t em••
* provld~ s s pc c tal bufferinG t e chn l ques for' 5 1 0':;, shor-t
t.ransmi s al on.
2.1.2 A~( ili 2~Y L1D~s ~e ~ tur2S
An aux iliar y l i nk
* is us ed f or tr2l1 s~:,d.s3ion of larGe '101'.1.;: :=s of d at a .
* is eatab l t shed In par'a'lLeL to the priJ-:-:':l r y link
* can be' est ab lished on l y if t he fo l lo~..linG co ndi t i ons arc
fulfille d:
user prcc r ;.:u:,-'3 , at t h e t wo ex t.rernl.tl es , mus t both r equi r e
its operrl ng,
ii is us ed f o r el t h e r 'o5.!12try or chara c t el' t.ransml.as.ion .
2.2,1 Gen er al Pr-ocedures
Eac h - HOS~L'( X) user ,,;it 1 r-escect t he follO\':i nG pr oc edure f'or
cccrcun l.cat.L ng \'iit ~l iIOSI' ( Y) A,
(a) 1..'
~,:) sh (~... O""' ;' 1 '~ V" 1 ] i nk
t-,,1-) i_ l' v
.,. v...-l. l.. . _ c.. "'
., _ L. 0
. .1...<.....U
.[(\C'P_ (Y ) •
. _
A prirnar y If[l1·: i s e :.; t aSTi s hed to H00T ( Y) t hr-ough the
control link O. The connect Lon is t hen i n a pre- loc-in
state, i.e., the re~~ te HOST expec ts its standard l OG-in
procedures.
(b) Lo~-i n Seo uen~e
Star:ct'2;:'c f.sC.:ll- char a ct e :--s are sent.Zrece lved ove r the
pr-Imary Li.nk, LI1 t hat ':ray , t he EOST(X ) us e r 5 2.Q1 3 in
to re~o te HOST(Y) by usine its s t a~dard 10g-LI1 procedures .
(c) Establish an P..UXi 1. i.9.:" .~ Li.nk t o HOST (Y )
~'his es t :~Sl ish-lle!:.s!7IU:3tEe-ex ec ut ej by both extremit i e s .
As in (a), this i s done by us i ng t he cont r ol l i nk 0,
(d) Se
__ 'ive ':'ext over
nrl/. _Pe _ce_ _ A.uxili21"!
_ J' l i n:'_::
2.2,2 Ex~p;~.
Ff.gur-c 3 f'ocus e s on t he dat a e xch an ged over t he links dur-Ing
a HOST(X)-EOST(Y) di alo ~Je.
1-100'-'
1" ('() h as t.h ..
.1"8 n etvror-x
1 .~.
lOenCll.lcn
"
. . a.on 8 •
1-' 4'
HOST(Y) has t he networ k identification 5.
r:
·* (3) and (4) const Ltut e e. trlvi.:ll exa mp'le of a 106- 1.'"1
-pr ocedure - See remark 2 beIc:..:
*(5): hOSTen , t2.l ~:i.n E; to t he ope r-at ing sys t c;n of HOST( Y),
req~ests f or U~S A . . ~5A is sUD~osed to be a us er applicatic~
pr-ogr-am in HOSrI' ('1) •
*(6) and (7) constitute tr.e a'..lxiliary Link est.ab Li.sh..rnent ,
After (5) 2...'1 aux.l Li.ary link should be established. This
is done by H03r~1(X) s ince it has the hicher identi fication
nurrber: in net:..rork, e.e't 8 agatns t 5.
'The procedure is very much like (1) and (2)
.*(8): HOS'I'(X) transmits a "file" to URSA. ':ihe t.r-ansml ss ion
is done over lLj}~ 25 ~~ich has just been established.
*(9): HOS'l'(Y) ansvrer-s back with a "file" over link 25.
And the dialozue Goes on •••
* 0): HOS'I'(X) frees t !1C links he has established
H I' : End of t.ransmi ssf.on (ASCII character).
o 0; (2): Nurro e r: of links '..ra nt.ed to be closed C3 JI~SC I I cnaracter)
{ 0'.{D8)Q>~)~): Link identificaticn nurrber (ASCII char-act.ers )
*(11) HOST(Y) acknow.ledges back as in (2) t (7).
Remar-k 1: 'I'he fiGll!'e 3 dcesn t t Sh O'.., the headf.ng of each
messa3e \~hich ~ r2 of cours e tra~s~~tted over these l~~s.
The char-acter-a repr esent.ed on eac h line should b e vl.eve d
inserted in the t2Xt zon e of a ses 3?-s e .
Remark 2: These c.iarac t er-s - see (3) and (4)- Ca..T1 either
be t.r-ansmlt.t.cd one at a t irre over the line (each charac t er
consti. _ t.ut
v~ v_e sv t .he
. text of" ,., r- n ", ,'''''''''::» 0"'" be oacked bef'or'e
.l.__ l... J. <.,;;,. 1 1~ ~ ;') .:..J ~ ""2) _ ... ~. :\, ~ _.£.l::;,.;
trans:nission by t he user co 'rrnurrl.cat.L on prcgrani ,
In either case , the r-ernot e EOST can c cns lder t he link as
a noroat teletype (Sear-cas br-e ak lng cher-act er-s t provide s
eellos ••• ).
Remark 3: In (2), (7), or (11), HOST(Y) can ans wer' qz:.cl ~ a
negat
.... ~""r>10·
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ad t ~r =s '1 "- ,;,...,'l.l..::> ; lJ.. \.. d , ~1.rO oJ
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and
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meS S2.Ge could b e ~. l@ ':.'he re ~IQ..\ is a char act er'
Indf.cat ing '.i"t::y tI1'2 p! 'e '/i oiJ :~ block has b een r e f us ed. Upon
rece ...:.. ~· back
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repeat its me:3S 2.:e unt Ll, EC3T( Y)" accept s it. An emer-gen cy
procedure '... ill t ake place if teo r:Dn~~ succes sive W\ K occur.
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A us er pr cgrem a c cesa ec t~le t~ ,::;t ~/;cr i ( f 2.c "i. l i -:i e s (lir11-: c c t a. b l i s ~'L~: _ rJ.t J dat a
ani s rrts s i on . . . . . ) t· ·, r r"" c.,-;) , v,,, c·--' I-;e.... _" ceI.Ls
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traps to a mon i t or S (~ I' V:!. C0 r-c ut ine t ha t int erprets and ex ecutes t h e s er
vice. Con tro l. Ll t c en :,out c·.:l b::=.:.;l;: to the user proS-::'-'<:'1 ;" .
3..1 List of s e l".;i.ce C2.11 'j at u ~; f:r ' s di scosa l.
(a) Ooen PriiT.:'.I'.'! Link
----. . ~- -
PRII-lID: User ijent i fic 2.~ion o f' t.he p:d.r.!.'J..ry Ll.nk ,
HOS'i'I D: Re mo t e EOS·.i.' ide!l'cific2.t i on .
BUFl~ ADLJ F\: Buf'f'e; : addr ess for t ho irlCCl.u.rlG measages ,
nm{Pl'-CO D~: Cede t hat t h e nc'c\·!·.)r: : P:'OCI'c.Sl shou.ld gi ve t o t he
user P!'OG!'2.:":, , : lr:en he i s Lnt.er-rupt.ed because a r::ess 3.se
has coree b2.C;( .
on: Opt Lono S UC:l a s "full e cho" ( f or test in.::; purpose }, message
requi r ed aft er succeasf'ul, link e stab lisr-:ri;cnt , et c ••• •
. Remar k: []: not r equi r ed .
(b) Ooen aux.i l i arv l i nk
-'----- - -----
....N"U
h -X(" .. .·( I lJ , ~n " ~'": 'u...t O" ;\ 'c:;' l'' - (vv ..}....Dz.•. , r~ o:;ti
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P'u i:
Tt':'r c
OPl:!. Au ~w - · i,..l.l
Tr1}, ~ r1
r\ T' ''''''''')
,l:·H
AlfAIO': Us er Lderrt iI'Lcat Lcn of the c.1..:.xiliar y l i nk.
PRE·TID: US0~ i de n tifica t ion o f a priffi~~Y li ;~. Eefers t o a1
already esta'81i s::.ed 9 r::' r: :3. r:~: Lf.r.k ,
BUFFA..lJD:i., Ii rl'?.F'l'- CODE , a PI' S21;;e mean ing as above .
(c) 'l'ran S~:li. 3 3 i cn e ve r
'lTRANSLI NK ( I D, BU?Ff\DDI1, ''1 , [ O? T ])
10: Us er link i de nti fi ca t i on. Depend Lng on ,..ihich t ype o f 1 i nke 3
~ · :we want to t runsml t J thi s Lde nt .L f'Lca t Lon nu rb er- 'dill be e qua l
to a pre vi ously de fine d AUXID/PRE-UD .
BUFFA-DDR: Data location addr-ess f or trall.s r.1 ission .
N: Data byt e s mr:0er f or t r-anscrls.s Lon .
aPr' ·• Oot
l. -l ..... Lor.s
. AU u~........
" .......
....11 a". ) ""'
_\... JJ, r1 ."t- ~ t-. . . ~ , n ,.,..
. . . . . . c... _ ( ch
..,~~ . ....... .:. ar-ac t.er
-....L ......~c
.J '- _ TJ.....
', . b Inary
.l.i ..
-.o.. )J
ackn o':ll e c:ge:;:'2:1t s Y'equi:::'c-d ( u t i l i z ~ t i on of ti. . e 2.ux i U a ry
' l in1-:s in a ha Lf dup l.ex mcde ) , t r a ce bit , etc ••.•
(d) ,1 TOOl' I~V I "7_
-: .J,._
i nk _.:.;;_
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~' ~..:.~-'::..
:1' ~ :> l- "'r
l".. ":"
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i'lDDIFLI N"K (ID, OP1')
ID: User l i nk Lden t i f'Lcat Lon (Equa l to e i t her AU:<ID!PR.l tI1I D)
(e) Cl os e 1in!-::
CLOSS LI J i< (ID, [ OPlI])
10: Sarr.~ riean lrr; 2.S ao ove ,
On ': CDJ1 be us ed t o c l os e all t he Ltnks in U3 e by t he us er-.
4. Da t e. S t :~uct
-_._ . ...ure
._-
----- .. -- .---
--- '
--
Tne allocat.ion and t he managemen t of t he lirl1-:s are ca r r i ed ou t by means
of three tabl e s:
A Table SQrted By HJ3T.
A Table Sor-t ed By LIJ'.l!(.
A Table Sor t ed b y USER .
4.1 ~ST Table (Se e Fi ~ure 4)
It is a bit-table ii'1di c2.tins , fer a gi ven r emote HOST, ',Thi ch l inks
are free. (bit-O means f ree Llnk )
This t able should provide 25G b its pe r HOST (256 logical l i M 3
possible). At a given t ime no mor e than 6Ll bits can be set to
1 in t he who l e t able.
4.2 l--L\fK Tab ~';: (See Fisures 4 and 5)
This t able co nta f ns as rna. .!1Y section s as links in use ..FiB;ure 5
describe s t he str uct lrre of a section .
Starting a'1d r e t r i e val are car~ led out dyTI ~dc ally upo~ U3i nc
a hashing t echn.lque b as ed on t he network link id e n~ i ficat i cns.
USER Table (See Fi~e 4)
--- - -- ~
'l'he t able structure is t;i ven on Fi:?;U2'e 4. These are as many
sections as act ive us er s. Each s ect i on cont a i ns the us er
identificat i on (8i ven by the oper-at Lng syst e.n) 2.-"1d , the i denti.f ica t i ons
of the links in use b y this us er- . Not ice t hat a l i nk has t vro
identifica t i ons: t hat of t he us er (z i ven as a paz-ameter- in t he
OPEN s ervice ca l l) and t hat of t he networ-k ( t h at i s a t tri but ed by
the net wor k proga';l) .
Ttd_s table is hasheJ by us ers.
- - - - --- -
T:'1e enl. ssf.on f'unc t Lons of t he networ k proV 'W,I3 are f u l fll l ed by moni.tor
service r out I ncs , I n that sense 5 thi s pr 0C-'?J] can be vi.e-red as be Long ing i
to the oper-at ing sys t.eu .
These Iunct I ons are con cerned ':li t h t he link e s t e.blish:r.ent s and da t a
!,
I
I
transrrd.ssion; they arc: 3 t~t ej by the s e r~lc e calls previously described .
Let's ex plain hovr t hese r out i nes allocate an d manage the links by
descr-Ib'ing t he oper a tion s Lnvo.Lved dur Ing t he exe cut i on of t he OPEi'[PRTI;I
routine.
I
Suppose tha t t he value of t he pararaet. er- HOS'l'ID is e qual to j. !
(a) j is us ed as an i ndex for the "HOSl,1I tab Ie to rea ch the "HOSE j" II
(b) In "HOS't,j" s ection, He s elect the first fr ee link (F'ir3t bit =Q )
e.g., i n bit •. I
(c) j and i det.errnln e r-espect Lve l y t he HOST-LIP de s tinat ion and t he
network link numo er , II
(d) 'l'hl.s j -i V2.J.u'] is us ed as a hashins code t o open a new sect ion Ln
(e)
the link t able. e. g. sect i on ~.
In t hi s s ection 1,
t he l ink 10 zone i s fi l l ed up \'Ji t h j - i , t he
I
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(Bernark: It is only when ':1e r ece ive back t he ackncvr'Iedgerten t
message I'rorn t {::~ r-emot e Hc) S'l' .-~ ec F iE~U!'~ 3: ( 2)-t h::.t t he link i s
consider ed c .:Jr:'pl e t ely est2.bli s fl t~d . Tnen vie se t t o 1 t he bit
c;. Jli
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_open (or f i nd) t he r i ght m s ect i on.
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and t he ne t :·;or';-:: link ID n umb er- (L) ,
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AI ELI5
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PROBLEMEach host on the early network needed a common way to open logical links to other hosts and software structures to manage them.
MECHANISMHosts use link 0 to ask for a primary, terminal-like link and then an auxiliary link for bulk data, using short ASCII messages. A user program reaches this through service calls, and the network program tracks links with host, link and user tables.
STATUS TODAYThis is a working memo, not a standard, and it describes one proposed design that the network group would go on to discuss.
§2.1Logical Links
An IMP can be viewed as an interface between a local center and the trunk network. Locally it may serve up to four hosts, and for each of them it provides 256 logical links to any remote host. Between an IMP and all the other IMPs, no more than 64 links may be in use simultaneously, so a host sees 256 lines but only 64 can be active at a time.
Link 0 is the control link, used for connection requests and status reports. The other 255 are either primary links or auxiliary links. A primary link is the first link set up for a host-to-host transmission and is TTY-like: ASCII characters, echoes generated by the remote host, and slow speed (less than 20 characters per second). It is mainly for control commands such as log-in. An auxiliary link carries large volumes of data, binary or character, in parallel to the primary link, and can be set up only if the user programs at both ends require it.
There is one line for planning, a slow line for chatting, and fast lines for sending big packages.
§2.2Link Establishment
A host establishes a primary link through control link 0, which leaves the connection in a pre-log-in state, then logs in over it with the remote host's standard log-in procedure. Setting up an auxiliary link must be done by both ends, again through link 0.
In the RFC's example, host X has network identification 8 and host Y has 5. X sends ENQ PRIM with a three-character link number and an option character over link 0. Y acknowledges with ACK followed by the same message, which is returned to the requester for security. X then logs in, asks for a program called URSA, and opens an auxiliary link on link 25, which X does because it has the higher identification number. Links are closed with EOT. Y may answer NAK instead of ACK, and X repeats its message until it is accepted; an emergency procedure applies if too many NAKs occur.
You knock to ask for a line, the other side says yes and repeats your request to prove it heard you, and when you are done you say goodbye and name which lines you are closing.
Host X (ID 8)Host Y (ID 5)
- ENQ PRIM 012 OPTLink 0: ask for a primary link.
- ACK ENQ PRIM 012 OPTLink 0: link 12 established.
- Log-in and URSA requestLink 12: terminal-like dialogue.
- ENQ AUX 025 OPTLink 0: ask for an auxiliary link.
- ACK ENQ AUX 025 OPTLink 0: link 25 established.
- File over link 25Y answers with a file on link 25.
- EOT 002 012 025Link 0: close two links.
- ACK EOT 002 012 025Link 0: acknowledged.
§3Network Service Calls
A user program reaches the network through service calls. A call traps to a monitor service routine that carries out the request and then returns control to the user program. The RFC lists five:
OPENPRIMopens a primary link.OPENAUXopens an auxiliary link tied to a primary one.TRANSLINKsends data over a link.MODIFLINKchanges link parameters.CLOSE LINKcloses a link, or all of the user's links.
Options include full echo, data type (character or binary), acknowledgements and a trace bit.
The program does not talk to the wires itself. It fills in a form (open, send, change or close) and the system does the rest.
§4Tables and the Network Program
Links are managed by three tables. The HOST table is a bit table with 256 bits per remote host, where a 0 bit means a free link and no more than 64 bits are set in the whole table. The LINK table has one section per link in use, found by hashing on the network link identification. The USER table has one section per active user, hashed by user, listing that user's links. A link thus has a user identification and a network identification.
The network program runs as monitor service routines, so it can be seen as part of the operating system. For OPENPRIM it picks the first free link bit, fills in a link table section, prepares the ENQ PRIM text, adds a heading and the host checksum, and queues the message for the handler.
The computer keeps three notebooks: which lines are free, which are busy, and who is using them.