IMPLEMENTING THE ISO-OSI REFERENCE MODEL by R. Popescu ...

IMPLEMENTING

THE

ISO-OSI

REFERENCE

MODEL

by R. P o p e s c u - Z e l e t i n

Bereich Hahn-Meitner-Institut

Datenverarbeitungund Elektronik fur K e r n f o r s c h u n g B e r l i n GmbH, Berlin,

Abstract

c e r t a i n layer in o r d e r to p r o v i d e indep e n d e n c e b e t w e e n s t a n d a r d s and c o r r e s p o n d i n g p r o d u c t s at d i f f e r e n t layers.

T h e ISO-OSI r e f e r e n c e m o d e l was the d e s i g n f r a m e w o r k for the i m p l e m e n t a t i o n of a h e t e r o g e n e o u s n e t w o r k in a local e n v i r o n m e n t . The H M I N E T - 2 is a n e t w o r k b a s e d on s t a n d a r d s in d a t a c o m m u n i c a t i o n . The p a p e r focus on the OSI m o d e l feasab i l i t y and o u t l i n e s the e n c o u n t e r e d prob l e m s and the a d o p t e d s o l u t i o n s in the n e t w o r k d e s i g n and i m p l e m e n t a t i o n .

T h e r e f e r e n c e c h a r a c t e r of the ISOOSI m o d e l has and w i l l h a v e a d o m i n a n t role in the f u t u r e of s t a n d a r d s and prod u c t s in d a t a c o m m u n i c a t i o n since it imposes a r i g o r o u s d i s c i p l i n e for the d e v e l o p m e n t of p r o d u c t s and systems to a c h i e v e compatibility. Many organizations (CCITT, IEEE) and p r o d u c t s s u p p l i e r s (DIX etc.) h a v e a d o p t e d the m o d e l and are r e f e r e n cing t h e i r systems and p r o d u c t s to the model.

1. I n t r o d u c t i o n In the last five y e a r s t r e m e n d o u s r e s e a r c h and s t a n d a r d i z a t i o n e f f o r t s can be w i t n e s s e d in the d a t a c o m m u n i c a t i o n field in o r d e r to s u p p o r t the r e q u i r e m e n t s of s t a n d a r d c o m m u n i c a t i o n f e a t u r e s in h e t e r o g e n e o u s e n v i r o n m e n t s .

T h e p a p e r o u t l i n e s the e x p e r i e n c e and p r o b l e m s c o m i n g up f r o m a p i l o t p r o j e c t in n e t w o r k i n g w h i c h has b e e n s p o n s o r e d by the G e r m a n M i n i s t r y of R e s e a r c h and T e c h n o l o g ~ T h e scope of the p r o j e c t was the d e v e l o p m e n t of a h e t e r o g e n e o u s n e t w Q r k b a s e d on ISO-OSI r e f e r e n c e m o d e l and the r e l a t e d s t a n d a r d i z a t i o n output.

The fast t e c h n o l o g i c a l d e v e l o p m e n t s in the field of d a t a c o m m u n i c a t i o n h a v e b e e n m a t e r i a l i z e d in a v a r i e t y of products w h i c h c l a i m to solve the u s e r ' s req u i r e m e n t s for n e t w o r k i n g . On the o t h e r hand, the users h a v e p r e c i p i t a t e d this i n v a s i o n b y c a l l i n g for i n c r e a s e d c o n n e c tivity, h i g h e r r e l i a b i l i t y , lower costs in n e t w o r k i n g and s u p p o r t for i n t e r c o n n e c t i n g h e t e r g e n e o u s equipment. T h e result of this s i t u a t i o n is that the u s e r has a d i f f i c u l t t a s k in d e v e l o p i n g strategies for its e n v i r o n m e n t in o r d e r to g e t u n s c a t h e d ~ t h r o u g h the p r o d u c t jungle of the m a r k e t v.

F i r s t we o u t l i n e the m a i n c h a r a c t e r i s tics of the h a r d w a r e and s o f t w a r e c h a r a c t e r i s t i c s and the n e t w o r k e m b e d m e n t in the n a t i o n a l and i n t e r n a t i o n a l p u b l i c networks. C h a p t e r 3 f o c u s s e s on some g e n e r a l c o n c e p t u a l p r o b l e m s of the ISo-OSI m o d e l e n c o u n t e r e d in the d e s i g n of the network. T h e n we d i s c u s s the p r a c t i c a l and c o n c e p tual p r o b l e m s in e a c h layer and we desc r i b e the a d o p t e d solutions.

T h e r e s e a r c h and s t a n d a r d i z a t i o n efforts h a v e b e e n m a t e r i a l i z e d in the ISOO p e n S y s t e m I n t e r c o n n e c t i o n R e f e r e n c e Model. The l a y e r e d jectives:

model

Germany

S o m e m a i n p o i n t s of c o n c e r n neral c o n c l u s i o n s are s u m m a r i z e d last c h a p t e r of this paper.

and gein the

We a s s u m e that the r e a d e r of this p a p e r is f a m i l i a r w i t h the c o n c e p t s and the t e r m i n o l o g y u s e d in the I S O / T C 9 7 / S C 1 6 documents.

has two m a i n ob-

- to p r o v i d e a l o g i c a l f r a m e w o r k to b r i n g o r d e r into a v e r y c o m p l e x domain, to s e p a r a t e f u n c t i o n s c h a r a c t e r i z i n g a -

2. H M I N E T 2

based

on s t a n d a r d s

HMI is a n u c l e a r r e s e a r c h i n s t i t u t e in W e s t - B e r l i n . T h e r e q u i r e m e n t s for netw o r k i n g and d i s t r i b u t e d p r o c e s s i n g h a v e b e e n i d e n t i f i e d at the b e g i n n i n g of 7O's. T h e r e s u l t was the d e v e l o p m e n t of a local area n e t w o r k (HMINET-1) w h i c h w a s b a s e d

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56 © 1983 ACM 0-89791-113-X/83/0010/0056500.75

- a network

on a d e s i g n c l o s e l y i n f l u e n c e d by the local e n v i r o n m e n t at t h a t time. The n e t w o r k was a r e s u l t of a c o o p e r a t i o n b e t w e e n HMI and S I E M E N S AG. and was i n ~ t a l l e d at different p l a c e s in the w o r l d . The e x p e r i e n ce g a i n e d in the o p e r a t i o n of HMINET-I has i n f l u e n c e d the d e c i s i o n to start the d e s i g n of a n e w n e t w o r k c a l l e d HMINET-2. The m a i n aspects w h i c h have i n f l u e n c e d this d e c i s i o n are:

O u t s i d e HMI, in the B e r l i n area and in o t h e r i n s t i t u t i o n s of i n t e r e s t for HMI users, a large v a r i e t y of c o m p u t e r types are d i r e c t l y c o n n e c t e d to the G e r m a n public n e t w o r k (DATEX-P). Some of t h e m are: IBM, CD, UNIVAC, $77OO etc. The h i g h d e g r e e of h e t e r o g e n i t y explains also w h y the d e s i g n had to rely on a r e f e r e n c e m o d e l (ISO/OSI) and the corr e s p o n d i n g standards.

N e w c o m p u t e r types in the HMINET-1 en ~ v i r o n m e n t impose a large i m p l e m e n t a tion effort. - New operating-system versions require c h a n g e s in the n e t w o r k software. - R e q u i r e m e n t for o n - l i n e c o m m u n i c a t i o n w i t h o t h e r s i m i l a r i n s t i t u t i o n s in G e r m a n y or elsewhere. - The r e q u i r e m e n t of a global c o n c e p t w h i c h covers n e t w o r k and r e a l - t i m e a p p l i c a t i o n s in order to m i n i m i z e the i m p l e m e n t a t i o n effort. -

The n e t w o r k t o p o l o g y and its embedm e n t in the n a t i o n a l and i n t e r n a t i o n a l public n e t w o r k s is o u t l i n e d in fig. 1. The a d o p t e d c o m m u n i c a t i o n ture and the r e l a t e d s t a n d a r d s ted in fig. 2.

3. C o n c e p t u a l

architecare d e p i c -

problems

T h e r e are a n u m b e r of issues w h i c h are not clear in th~ ISO-OSI r e f e r e n c e m o d e l s p e c i f i c a t i o n - for a n e t w o r k d e s i g ner.

These c o n s i d e r a t i o n s and the n e w impacts and k n o w - h o w in n e t w o r k technology h a v e d e t e r m i n e d the b a s i c aims of the H M I N E T - 2 design:

Some of t h e m are: the m a n u f a c t u r e r i n d e p e n d e n c e by a d o p t i n g a c c e p t e d standards, 2) - d e s i g n of an o p e n - e n d e d n e t w o r k architecture, 3) - h i g h i n t e r c o n n e c t i v i t y and f l e x i b i lity by a l l o w i n g c o m p u t e r to participate at d i f f e r e n t n e t w o r k technologies u s i n g the same s t a n d a r d application 4) - c o m p a t i b i l i t y w i t h the n a t i o n a l public n e t w o r k to a l l o w c o m m u n i c a tion to the o u t s i d e world. I)

-

1. L a y e r i n d e p e n d e n c e . The a d o p t e d l a y e r i n g t e c h n i q u e is char a c t e r i z e d by two m a j o r concepts: -The layer service w h i c h is the set of c a p a b i l i t i e s o f f e r e d at the b o u n d a r y of a layer to a u s e r in the n e x t h i g h e r layer. N o t e that the service is an abs t r a c t i o n by w h i c h the c a p a b i l i t i e s offered by a layer (in u s i n g all lower layers) are s p e c i f i e d and that the serv i c e d e f i n i t i o n is i n d e p e n d e n t of any particular implementation. -The p r o t o c o l w h i c h d e f i n e s the rules of i n t e r a c t i o n s b e t w e e n e n t i t i t i e s w h i c h are s i t u a t e d in the same layer but p e r t a i n to d i f f e r e n t systems. It is o b v i o u s that by the d e f i n i t i o n of the service o f f e r e d by a layer and the d e f i n i t i o n of the service o f f e r e d by the lower layer the f u n c t i o n a l i t y of a layer is c o m p l e t e l y specified. N o t e that d i f f e r e n t p r o t o c o l s m a y c a r r y out this f u n c t i o n a l i t y . A l t h o u g h these a r c h i t e c t u r a l c o n c e p t s aim to layer i n d e p e d e n c e the services d e f i n e d in the r e l a t e d s t a n d a r d s for each layer do not p r o v i d e c o m p l e t e l y this goal. This is m a i n l y due to the fact that the s e r v i c e s are d e f i n e d as a three p a r t y c o m m u n i c a t i o n . The three c o m m u n i c a t i n g e n t i t i e s are the two users of the service and the layer below as service provider. The i m p l i c a t i o n of this fact is that the s p e c i f i c a t i o n s of the s e r v i c e s prov i d e d by each layer d e f i n e the capabilities o f f e r e d by the layer and the beh a v i o u r of the two users of the s e r v i c e in the layer above.

T h e d e v e l o p m e n t s in the s t a n d a r d i z a tion o r g a n i z a t i o n s , the e v o l u t i o n of the ISO-OSI r e f e r e n c e m o d e l and its i m p l i c a tion in the d a t a c o m m u n i c a t i o n market, the CCITT r e c o m m e n d a t i o n s and their imp a c t on the d e v e l o p m e n t of E u r o p e a n networks are some of the facts w h i c h h a v e i n f l u e n c e d the d e c i s i o n to d e v e l o p a netw o r k b a s e d on standards. The c o m p u t e r types corresponding operating picted i n t h e f o l l o w i n g Host type I

~.

of ~ s t s

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in HMI and the systems are detable:

I ORG

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57

A n e u t r a l service d e f i n i t i o n w i t h o u t the i n v o l v e m e n t of the b e h a v i o u r of the e n t i t i e s in the layer above is p r o b a b l y the o n l y w a y to a c h i e v e the aimed layer i n d e p e n d e n c e . 2.

Multicast communication T h e ISO-OSI m o d e l p r o v i d e s p r o v i s i o n s for p o i n t - t o - p o i n t c o m m u n i c a t i o n only. A large class of a p p l i c a t i o n s r e q u i r e s m u l t i c a s t c o m m u n i c a t i o n r e g i m e s (dist r i b u t e d d a t a b a s e s , m a i l i n g and telec o n f e r e n c i n g systems etc.). At the p r e s e n t time the u s e r of the m o d e l has to solve the c o m m u n i c a t i o n aspects for m u l t i c a s t a p p l i c a t i o n s o u t s i d e the model. Since one of the aims of the m o d e l is to r e l i e v e the users of the m o d e l of c o m m u n i c a t i o n o r i e n t e d aspects, for this class of a p p l i c a t i o n s the m o d e l failed its aim.

3.

Layer entities A n o t h e r p r a c t i c a l and a r c h i t e c t u r a l p r o b l e m is the fact that in the OSI R e f e r e n c e M o d e l the e x i s t e n c e of entities in the n e x t h i g h e r layer is a s s u m e d at c o n n e c t i o n set up time (e.g. S e s s i o n E n t i t i e s for the T r a n s p o r t L a y e r in the e s t a b l i s h m e n t phase). This a s s u m p t i o n is not true for m o s t i m p l e m e n t a t i o n s if the implem e n t a t o r cares for i m p l e m e n t a t i o n eff i e n c y and so does not i m p l e m e n t a m u l t i t u d e of d u m m y p r o c e s s e s w h i c h have to w a i t to be activated. T h a t m e a n s that b e f o r e a C O N N E C T indication m a y o c c u r the c r e a t i o n of an entity in the S e s s i o n L a y e r has to be performed. It is not clear f r o m the s e r v i c e s p e c i f i c a t i o n w h i c h e n t i t y in w h i c h layer has to e n t e r the t e r m i n a tion p h a s e if the e n t i t y c a n n o t be c r e a t e d or w h e n d e a d l o c k s i t u a t i o n s occur.

4.

3.1

r e c o m m e n d a t i o n was a d o p t e d and p r o v i d e d by the h o s t m a n u f a c t u r e r for the first three layers. T h e p r a c t i c e has shown that e v e n if a s t a n d a r d is a v a i l a b l e the m a n u f a c t u r e r are r e l u c t a n t in p r o v i d i n g it. The reluctance is m o t i v a t e d by the o p t i o n s available in a s t a n d a r d and also in the ambig u i t i e s c o n t a i n e d in it. T h e s e a m b i g u i t i e s a p p e a r t h r o u g h the i n f o r m a l d e s c r i p t i o n t e c h n i q u e of the standard. A l t h o u g h the i n f o r m a l d e s c r i p t i o n eases the r e a d a b i l i t y of standard, a formal m e t h o d o l o g y a c c o m p a n i n g the i n f o r m a l one w i l l m i n i m i z e false i n t e r p r e t a t i o n s and i m p l e m e n t a t i o n effort. Since the first three layers are p r o v i d e d by d i f f e r e n t m a n u f a c t u r e r s in the H M I N E T - 2 a lot of e f f o r t was put in testing, c e r t i f i c a t i o n and a l l e v i a t i o n of i n c o m p a t i b i l i t i e s of the p r o d u c t s s u p p l i e d by the m a n u f a c t u r e r s . This task was eased by the e x i s t e n c e of the r e q u i r e m e n t t h a t all X.25 p r o d u c t s h a v e to be fully c o m p a tible w i t h the a d o p t e d v e r s i o n of the X.25 in the G e r m a n p u b l i c n e t w o r k (DATEX-P). We l e a r n e d t h a t a c e r t i f i c a t i o n a u t h o r i t y and a p p r o p r i a t e t e s t i n g m e c h a n i s m s and p r o c e dures are v i t a l in the d e v e l o p m e n t of the network. T h e c e r t i f i c a t i o n task has a c c o m p a n i e d the w h o l e p r o j e c t and even n o w for n e w c o m m u n i c a t i o n links to hosts c o n n e c ted to o t h e r n a t i o n a l p u b l i c n e t w o r k s diff e r e n c e s a p p e a r b e t w e e n n a t i o n a l X25 implementations (e.g. f a c i l i t y field). A m a j o r p r a c t i c a l and a r c h i t e c t u r a l p r o b l e m is the v i s i b i l i t y of the X.25 services. The q u e s t i o n is "where the u p p e r layers access the X.25 layer 3 services, in h o s t or in the f r o n t - e n d ? " (fig. 3) U s u a l l y b e t w e e n the h o s t o p e r a t i n g s y s t e m and f r o n t - e n d a m a n u f a c t u r e r specific protocol is ~ l ~ n e n t e d (fig.3a). Adoptinq the ISO-OSI m o d e l i ~ the n e t w o r k d e s i g n m e a n s that the u p p e r layers h a v e to use the s e r v i c e s and the c h a r a c t e r i s t i c s of the layers below. The second solution(fig3b) has the a d v a n t a g e that all layer 3 c h a r a c teristics: flow control, error d e t e c t i o n and recovery, a d d r e s s i n g are a v a i l a b l e in h o s t for the u p p e r layers. S i n c e the standards in t r a n s p o r t layer are b a s e d on b a c k pressure, error r e p o r t i n g etc. the s e c o n d s o l u t i o n eases the i m p l e m e n t a t i o n and design of the network.

D y n a m i c c h a n g e of q u a l i t y of s e r v i c e The p r e s e n t d o c u m e n t s s p e c i f y the neg o t i a t i o n of the s e r v i c e q u a l i t y o n l y at c o n n e c t i o n set UD time. There are m a n y cases w h e r e a d y n a m i c c h a n g e of the q u a l i t y of s e r v i c e is required. In the e x i s t i n g d o c u m e n t s a q u a l i t y of s e r v i c e m a y be c h a n g e d o n l y by involv i n g the t e r m i n a t i o n of the c o n n e c t i o n and then by r e - e s t a b l i s h i n g a n e w conn e c t i o n w i t h the n e w s e r v i c e quality. This s c h e m a is too r i g i d and too costly. A p r a c t i c a l p r o b l e m in the n e t w o r k design is also the c r i t e r i a c h o i c e on h o w d i f f e r e n t q u a l i t y of s e r v i c e s at d i f f e r e n t layers can be maped.

D i s a d v a n t a g e of this s o l u t i o n is the fact that it is c o n t r a d i c t o r y w i t h the f r o n t - e n d c o n c e p t of r e l i e v i n g the m a i n frame f r o m the c o m m u n i c a t i o n aspects. We c h o o s e the standard products not a v a i l a b l e f r o m that time and have

Laye r I to 3 As a l r e a d y

mentioned

the X25 LAP B

58

s e c o n d s o l u t i o n since in the u p p e r layers w e r e the m a n u f a c t u r e r s at had to be d e s i g n e d and

implemented 3.2 L a y e r

in HMI

(fig.

4).

e a c h o p e r a t i o n a l network. T h e d e c i s i o n to implement a transport layer which offers the f u n c t i o n a l i t y of class I p r o t o c o l is m o t i v a t e d by the H M I N E T and B E R N E T e n v i r o n m e n t and scope.

4

T h e scope of the layer 4, also called t r a n s p o r t layer is to e n s u r e an endt o - e n d c o m m u n i c a t i o n b e t w e e n p r o c e s s e s sit u a t e d in d i f f e r e n t hosts. T h e p r e s e n t a v a i l a b l e ISO DP 8073 i d e n t i f i e s 5 c l a s s e s of p r o t o c o l s and m a i n l y m i r r o r s the comp r o m i z e b e t w e e n ISO and C C I T T outputs.

T h e time scale of the p r o j e c t d e v e l o p m e n t d i d n o t a l l o w to w a i t for a s t a n d a r d of the t r a n s p o r t layer. W e h a v e a d o p t e d the m e s s a g e link p r o t o c o l (MLP) a G e r m a n r e s e a r c h e n v i r o n m e n t s t a n d a r d w h i c h is e q u i v a l e n t w i t h the ISO class 1.

If c l a s s O a n d I are s u p p o s e d to supp o r t u s u a l PTT a p p l i c a t i o n s (teletex etc.) class 2 to 4 are c h a r a c t e r i z i n g the u s e r c o n c e r n s of c o s t e f f e c t i v e n e s s and e n l a r ged i n t e r c o n n e c t i v i t i y w i t h n e t w o r k s based on a c o n n e c t i o n l e s s type of service. The main practical problems encountered in the n e t w o r k d e s i g n are:

The accounting problems introduced by m u l t i p l e x i n g of t r a n s p o r t c o n n e c t i o n s o v e r a X.25 n e t w o r k c o n n e c t i o n , and the fact that H M I N E T is a c o m p u s n e t w o r k (no t r a n s m i s s i o n costs are c h a r g e d for local c o m m u n i c a t i o n ) are some o t h e r a r g u m e n t s for c h o o s i n g a p r o t o c o l f u n c t i o n a l l y equiv a l e n t w i t h class 1.

a) C h o i c e of the c l a s s and o p t i o n s to be adopted. T h i s d e c i s i o n is i n f l u e n c e d b y the e n v i r o n m e n t a l c h a r a c t e r i s t i c s and the scope of the network. T h e m u l t i t u d e of c h o i c e s and the fact t h a t a m a n u f a c t u r e r d e c i s i o n t o w a r d s a certain class and set of o p t i o n s is inp r e d i c t a b l e at the p r e s e n t time the t r a n s p o r t l a y e r d e s i g n is a d i f f i c u l t task.

A g a i n the f o r m a l d e s c r i p t i o n of the p r o t o c o l w a s an i m p o r t a n t aid t o w a r d s comp a t i b l e i m p l e m e n t a t i o n in the d i f f e r e n t hosts . T h e e m e r g e n c e of the L A N t e c h n o l o g y in d a t a c o m m u n i c a t i o n h a v e i n f l u e n c e d the d e c i s i o n to r e c o n s i d e r the i m p l e m e n t e d layer 4 p r o t o c o l . S i n c e the t e n d e n c e is a d e c e n t r a l i z a t i o n of the n e t w o r k in c l o s e d LANs for s p e c i f i c a p p l i c a t i o n s and the H M I N E T - 2 use as a b a c k - b o n e n e t w o r k (fig. I), c o n n e c t i o n and c o n n e c t i o n l e s s s e r v i c e s are r e q u i r e d at the t r a n s p o r t l a y e r b o u n dary. The m o s t s u i t a b l e class in D P 8 0 7 3 s p e c i f i c a t i o n is class 4.

b) C o s t e f f e c t i v e n e s s . If one of the scopes of the l a y e r 4 is to be cost effic i e n t then it has to p r o v i d e a d d i t i o n a l l y a s e r v i c e w h i c h w i l l a l l o w the U s e r of the T r a n s p o r t S e r v i c e to int e r r u p t the N e t w o r k C o n n e c t i o n (for w h i c h he has to pay) w i t h o u t i n t e r r u p ting the life of the T r a n s p o r t C o n n e c tion. T h a t m e a n s that the T r a n s p o r t e n t i t i e s w i l l e x i s t e v e n if the corr e s p o n d i n g N e t w o r k C o n n e c t i o n is discarded. S u c h a s i t u a t i o n m a y o c c u r w h e n an a p p l i c a t i o n t r a n s m i t s d a t a f r o m time to time and does not w a n t to pay for m a i n t a i n i n g the N e t w o r k C o n n e c t i o n for the time w h e n no c o m m u n i c a t i o n occurs.

T h e a d v a n t a g e of a r i g o r o u s l a y e r e d a r c h i t e c t u r e for the n e t w o r k i m p l e m e n t a tion is o b v i o u s s i n c e the c h a n g e s introd u c e d do n o t i n f l u e n c e the layers a b o v e and below. 3.3 S e s s i o n

layer

T h e scope of the s e s s i o n layer is to o f f e r a s y n c h r o n i z a t i o n m e c h a n i s m for entities r e s i d i n g in the layers above.

c) A u t h o r i z a t i o n . If the T r a n s p o r t L a y e r has an e n d - t o - e n d s i g n i f i c a n c e it has to p r o v i d e an a u t h o r i s a t i o n m e c h a n i s m for the a c c e s s in the end s y s t e m f r o m remote. In the p r e s e n t s p e c i f i c a t i o n no p r o t e c t i o n m e c h a n i s m s (e.g. u s e r i d e n t i f i c a t i o n , p a s s w o r d s etc.) are m e n t i Q n e d and that i m p l i e s that r e s o u r ces m a y be u s e d b y e v e r y b o d y e v e r y w h e r e . A n i n t e n d e d or u n i n t e n d e d u s e r m i s b e h a v i o u r m a y u s e all r e s o u r c e s of the rem o t e system. In the c u r r e n t s p e c i f i c a tions ~his m i s b e h a v i o u r c a n n o t be detected .

T h e d e s i g n of this l a y e r in H M I N E T - 2 r e l i e s on the r e q u i r e m e n t s of the stand a r d a p p l i c a t i o n s offered. T h e m a i n f u n c t i o n a l i t y is a c h e c k p o i n t / r e s t a r t m e c h a n i s m s u s e d for d a t a t r a n s f e r b e t w e e n p e e r a p p l i c a t i o n entities. D e p e n d i n g on the r e l i a b i l i t y class r e q u i r e d by the a p p l i c a t i o n the f r e q u e n c y of c h e c k p o i n t s is n e g o t i a t e d at the session e s t a b l i s h m e n t time.

S o m e of the above items are i d e n t i fied in the D P 8 0 7 3 b u t c o n s i d e r e d for further study. A t l e a s t the a c c o u n t i n g and TC u s e r i d e n t i f i c a t i o n seems to be v i t a l for

T h e m a i n d i f f e r e n c e b e t w e e n the ISOOSI d o c u m e n t s on s e s s i o n and our d e s i g n and i m p l e m e n t a t i o n c o n s i s t s in the perc e p t i o n of the s e s s i o n f u n c t i o n a l i t y . If

59

3.4 A p p l i c a t i o n

the I S O / S C 1 6 d o c u m e n t s are s p e c i f y i n g a session layer which offers only a mechanism for s y n c h r o n i z a t i o n w h i c h is t r i g g e r ed b y the p r o c e s s e s in the l a y e r ~ above, the s e s s i o n l a y e r in H M I N E T - 2 p r o v i d e s the n e c e s s a r y m e c h a n i s m for r e l i a b l e comm u n i c a t i o n and s y n c h r o n i z a t i o n i n s i d e the s e s s i o n layer. By this m e a n s the a p p l i c a t i o n s p r o c e s s e s are r e l i e v e d f r o m c o m m u n i c a t i o n r e l i a b i l i t y c o n c e r n s and s y n c h r o nization problems.

1) s t a n d a r d n e t w o r k f a c i l i t i e s c o n f o r m i n g the I S O - O S I m o d e l (CBMS, v i r t u a l file), 2) f a c i l i t i e s c o n f o r m i n g o t h e r s t a n d a r d s (terminal a c c e s s C C I T T X3, X28, X29), 3) u s e r s o f t w a r e 4 or 5. We will categories terest. 3.4.1

package

relying

on layer

c o n s i d e r o n l y the f i r s t two since t h e y are of g e n e r a l in-

The Computer DINAMIT

Based Message

System

T h e D I N A M I T s y s t e m p r o v i d e s four m a i n f a c i l i t i e s for the n e t w o r k user: -

m a i l service, w h e r e l e t t e r s can be sent to one, to a g r o u p or to all n e t w o r k users,

- an on line a c c e s s to a l i b r a r y of d o c u m e n t s and h e l p files for the use of the network,

The a b o v e m e c h a n i s m has two o t h e r i m p o r t a n t s i d e - a s p e c t s w h i c h are:

-

layer

T h e f a c i l i t i e s o f f e r e d by the netw o r k c a n be c l a s s i f i e d in t h r e e c a t e g o ries:

T h e e v i d e n t i m p l i c a t i o n is t h a t d a t a is b u f f e r e d in the s e s s i o n l a y e r and t h a t the c h e c k p o i n t s r e f e r to s e s s i o n d a t a unitswhich size are s e s s i o n s p e c i f i c and n e g o t i a b l e at the s e s s i o n e s t a b l i s h m e n t time. T h e i n d e p e n d e n c e b e t w e e n t h e sess i o n d a t a u n i t and the a p p l i c a t i o n d a t a u n i t is f r o m o u r e x p e r i e n c e an i m p o r t a n t a s p e c t to a c h i e v e the l a y e r i n d e p e n d e n c e . The s e s s i o n d a t a u n i t acts like a cont a i n e r w h e r e the a p p l i c a t i o n d a t a u n i t s are p u t into for t r a n s f e r . A n a p p l i c a t i o n d a t a u n i t c a n be t r a n s f e r r e d u s i n g one or m o r e s e s s i o n d a t a u n i t s or a session d a t a u n i t m a y c o n t a i n m o r e t h a n o n e a p p l i c a t i o n d a t a units.

-

and p r e s e n t a t i o n

- a network status service where informat i o n of g e n e r a l i n t e r e s t a b o u t the h o s t s c o n n e c t e d and n e t w o r k status at the serv i c e r e q u e s t time is a v a i l a b l e ,

it o f f e r s a pflexible m e a n for r e s o u r c e a d m i n i s t r a t i o n in c o m m u n i c a t i o n b e t w e e n d i f f e r e n t hosts. T h e b u f f e r size for t h e s e s s i o n d a t a u n i t s t r a n s f e r are n e g o t i a b l e at the s e s s i o n e s t a b l i s h m e n t t i m e and are d e p e n d e n t on the c o m m u n i c a t i o n h o s t t y p e s and t h e i r load.

- network management whole network-.

capabilities

for the

T h e s y s t e m is a f u l l y d i s t r i b u t e d a p p l i c a t i o n w h e r e its u s e r s m a y send or receive messages from each terminal connected to the n e t w o r k (i.e. l o c a l i t y independent).

it o f f e r s in c e r t a i n c a s e s a p e r f o r m a n ce i m p r o v e m e n t if by one s e s s i o n d a t a u n i t m o r e t h a n o n e a p p l i c a t i o n d a t a are transferred £hrough thenehw~rk (e,g. f~le t r a n s f e ~ a p p l i c a t i o n ) .

F o u r types of p r o c e s s e s in the s y s t e m (fig. 5):

To s u m m a r i z e the i m p l e m e n t e d m e c h a n ism: d e p e n d i n g on the a p p l i c a t i o n type w h i c h i n i t i a l i z e s the s e s s i o n a c e r t a i n s e s s i o n d a t a u n i t size and the c h e c k p o i n t f r e q u e n c y is p r o p o s e d to the r e m o t e sys t e m (session entity).

are i n v o l v e d

- anuser agent process on each host from w h i c h the D I N A M I T s y s t e m is a c c e s s a b l e , -

D e p e n d i n g O n the local r e s o u r c e s and p a r a m e t e r s the r e m o t e s y s t e m a c c e p t s or p r o p o s e s in r e t u r n its choice. T h e initiator s e s s i o n e n t i t y m a y n o w d e c i d e if in this c o n d i t i o n s the a p p l i c a t i o n r e q u i r e m e n t s can be f u l f i l l e d or not. In the l a t e r case the s e s s i o n is d i s c a r d e d and the a p p l i c a t i o n p r o c e s s m a y d e c i d e to establish a new session with decreased q u a l i t y of s e r v i c e or to try l a t t e r in the e x p e c t a t i o n t h a t the r e s o u r c e set in the s e s s i o n l a y e r w i l l be able to s a t i s fy its r e q u i r e m e n t s .

a p o s t - s t a t i o n p r o c e s s for u s e r m e s s a g e s w i t c h i n g . T h e n u m b e r of p o s t s t a t i o n p r o c e s s e s and t h e i r a s s i g n m e n t to a cert a i n h o s t is d e p e n d e n t on t o p o l o g i c a l and e f f i c i e n c y c r i t e r i a .

- a m e s s a g e b a s e for the s t o r a g e m e s s a g e s to be f o r w a r d e d .

of the

- ~ n u s e r r e c o r d b a s e for e a c h u s e r of the system, w h e r e i n f o r m a t i o n a b o u t m e s s a g e s sent to h i m is a v a i l a b l e (fig. 5). A m e s s a g e to be sent is d e l i v e r e d by the UAP to the p o s t - s t a t i o n p r o c e s s to w h i c h the UAP is c o n n e c t e d . T h e p o s t - s t a t i o n p r o c e s s w i l l store the m e s s a g e in its m e s s a g e b a s e and w i l l i n f o r m the a d d r e s sed user(s) r e c o r d base(s) a b o u t the exi-

60

s t e n c e of a m e s s a g e for h i m in the PS m e s sage base. T h e user(s) for w h o m the m e s sage is d e s t i n a t e d w i l l be i n f o r m e d a b o u t the e~tistence of a m e s s a g e as soon as he is a c c e s s i n g the D I N A M I T s y s t e m at any of the u s e r a g e n t p r o c e s s e s .

the d i f f e r e n t s y s t e m s w i t h t r a n s l a t i o n m e c h a n i s m s b e t w e e n the n e w l o g i c a l s t r u c ture and the e x i s t i n g local s t r u c t u r e s . W i t h this concept, e q u i v a l e n t w i t h the def i n i t i o n of n e t w i d e d a t a m a n a g e m e n t system, the u s e r w h o a c c e s s e s d a t a s o m e w h e r e in a n e t w o r k has o n l y one interface; the i n t e r f a c e to the v i r t u a l file system. T h e v i r t u a l file can be d e s c r i b e d as a tree s t r u c t u r e w i t h n h i e r a r c h i c a l levels. E a c h level i ( 1 ~ i ~ n ) c o n s i s t s of a set of file a c c e s s d a t a u n i t s (FADUs) at level i-I or b e l o w (fig. 6). A file a c c e s s d a t a u n i t is an a c c e s s a b l e p i e c e of i n f o r m a tion t h a t has a s e m a n t i c m e a n i n g .

T h e p r e s e n t a t i o n layer f u n c t i o n a l i t y is i m p l e m e n t e d by a d o p t i n g s t a n d a r d d a t a f o r m a t s b e t w e e n the c o m m u n i c a t i n g p r o c e s ses and b y e n c o d i n g / d e c o d i n g the s t a n d a r d d a t a r e p r e s e n t a t i o n into the local one. N o t e t h a t D I N A M I T is a t y p i c a l e x a m p le of a d i s t r i b u t e d a p p l i c a t i o n in the ISOosI e n v i r o n m e n t , w h e r e d i f f e r e n t d i s t r i b u ted p r o c e s s e s are involved.

N o t e ~that the d e p t h of the tree is o p e n e n d e d and it is u s e r a p p l i c a t i o n d e p e n dent.

A l t h o u g h the I S O - O S I m o d e l d e s c r i bes o n l y a p o i n t - t o - p o i n t t y p e of c o m m u nication between autonomous application p r o c e s s e s of the same type, some comm e n t s a b o u t the f e a s i b i l i t y of the m o d e l for a p p l i c a t i o n s w h i c h are f u n c t i o n a l d i s t r i b u t e d are p r o b a b l y i n t e r e s t i n g : -

-

It is o b v i o u s t h a t the a d v a n t a g e of int r o d u c i n g an u n i f o r m s t r u c t u r e is that we c a n i n t r o d u c e an u n i f o r m set of F A D U access s e r v i c e s and one p r o t o c o l (i.e. the same s e r v i c e s and p r o t o c o l s u p p o r t file t r a n s f e r as w e l l as d a t a u n i t s a c c e s s w i t h i n a v i r t u a l file). The d e p t h of the v i r t u a l file is n e g o t i a b l e b e t w e e n the two a p p l i c a t i o n e n t i t i e s and u n i f o r m conv e n t i o n s on a u t h o r i z a t i o n , a c c e s s c o n t r o l etc. are i n t r o d u c e d in the network. T h e c o m m o n set of s e r v i c e s and the r e l a t e d p r o t o c o l are d e f i n e d in the n e s t e d p h a s e s (ISO t e r m i n o l o g y : r e g i m e s ) :

t h e r e is a n e e d of m u l t i c a s t and b r o a d c a s t c o m m u n i c a t i o n type to s u p p o r t such a p p l i c a t i o n s . T h i s f a c i l i t y has to be s u p p o r t e d by the s e s s i o n l a y e r and below in o r d e r to r e l i e v e the a p p l i c a tions and p r e s e n t a t i o n layer f r o m communication oriented concerns, e s t a b l i s h i n g and d i s c a r d i n g c o n n e c t i o n s for the t r a n s f e r of small a m o u n t s of d a t a is a v e r y c o s t l y and i n e f f i c i e n t m e c h a n i s m . A d a t a g r a m or fast c o n n e c t / d i s c o n n e c t s e r v i c e v i s i b l e in the u p p e r layers w i l l be u s e f u l l for a n u m b e r of applications,

T h e r e are some d i f f e r e n c e s b e t w e e n the i m p l e m e n t e d v i r t u a l file s y s t e m in H M I N E T - BERNET environme~ and the I S O - V F system specification . - the i m p l e m e n t e d file s t r u c t u r e is a subset of the ISO one and c o n s i s t s in the r e s t r i c t i o n t h a t t h e arc l e n g t h b e t w e e n the F A D U s is a l w a y s I. T h e d e p t h is neg o t i a b l e b e t w e e n the s y s t e m s i n v o l v e d in an a p p l i c a t i o n c o n n e c t i o n .

- the s y n c h r o n i z a t i o n b e t w e e n c o n c u r r e n t a c c e s s e s to e n s u r e the c o n s i s t e n c y of d i s t r i b u t e d i n f o r m a t i o n is a s e r v i c e w h i c h is m a n d a t o r y for a large n u m b e r of f u n c t i o n a l d i s t r i b u t e d a p p l i c a t i o n s (mail systems, d i s t r i b u t e d d a t a b a s e s etc.). M e c h a n i s m s like t w o - p h a s e commit, time stamps b a s e d h a v e to be o f f e r e d by the s e s s i o n layer.

3.4.2 T h e v i r t u a l

file

- for e f f i c i e n c y r e a s o n s the i m p l e m e n t e d s y s t e m p r o v i d e s the c a p a b i l i t y of acc e s s i n g F A D U s in d i f f e r e n t v i r t u a l files s t o r e d at a c e r t a i n h o s t w i t h i n one appl i c a t i o n c o n n e c t i o n . T h e ISO s p e c i f i c a tion p r o v i d e s o n e a p p l i c a t i o n c o n n e c t i o n per a c c e s s e d file. T h e r e a s o n i n g is t h a t since m u l t i p l e x i n g is p r o v i d e d by the t r a n s p o r t layer (cost e f f i c i e n c y ) then maintaining one application connection per v i t u a l file w i l l not i n f l u e n c e the costs. B u t since the f i r s t two c l a s s e s of the t r a n s p o r t p r o t o c o l are n o t m u l t i p l e x i n g and since t h e r e is an i n h e r e n t s e q u e n t i a l i t y w i t h i n m o s t p r o g r a m s acc e s s i n g d a t a in d i f f e r e n t files, we dec i d e d to p r o v i d e the m e c h a n i s m d e p i c t e d in fig. 7b r a t h e r than the ISO one (fig. 7a) :

system

1~he v i r t u a l file c o n c e p t was introd u c e d v in o r d e r to a l l e v i a t e the i n c o m p a t i b i l i t i e s of the d i f f e r e n t d a t a m a n a g e m e n t s y s t e m types in n e t w o r k s w i t h a h i g h d e g r e e of h e t e r o g e n e i t y or for the d e s i g n of n e t w o r k s w i t h a n o n - p r e d i c t a b l e p o p u l a tion of h o s t s (open systems). T h e c o n c e p t w a s a d o p t e d in the n a t i o n a l and i n t e r n a tional s t a n d a r d i z a t i o n b o d i e s (ISO, D I N e t c . ) a ~ d w a s i m p l e m e n t e d in H M I N E T a n d ~BERNET-. T h e m a i n a i m of this a p p r o a c h is to d e f i n e a s t a n d a r d l o g i c a l file s t r u c t u r e in e a c h h o s t i n the n e t w o r k and to p r o v i d e

- a n o t h e r d i f f e r e n c e b e t w e e n the two app r o a c h e s is that the H M I N E T - B E R N E T implementation provides a distinct separa-

6!

t i o n for r e c o v e r y for data t r a n s f e r failures ( c h e c k - p o i n t / r e s t a r t in the s e s s i o n layer) and the check point / r e s t a r t m e c h a n i s m in the a p p l i c a t i o n layer r e l a t e d to the a c c e s s e d structure. In our a p p r o a c h the a p p l i c a t i o n e n t i t i e s are not c o n c e r n e d w i t h r e c o v e r y f r o m f a i l u r e s in the d a t a transfer; the f a i l u r e s b e i n g c a p t u r e d w i t h in the s e s s i o n layer.

mode

The D I A s y s t e m in HMI is o f f e r e d each host, in t e r m i n a l c o n c e n t r a t o r in the s w i t c h i n g node (fig. 8).

host.

on and

4. C o n c l u s i o n s

The formal s p e c i f i c a t i o n of the v i r t u a l file s y s t e m has a c c o m p a n i e d the i m p l e m e n t a t i o n w o r k and has p r o v i d e d the p r e c i s e b a s i s for the i m p l e m e n t a t i o n phase in the d i f f e r e n t c o m p u t e r s y s t e m types.

T h e r e are some a s p e c t s for g e n e r a l t e r e s t we have l e a r n e d in the p r o j e c t velopment:

inde-

A) The ISO-OSI m o d e l is an e f f i c i e n t tool for the d e v e l o p m e n t of the c o m p l e x h a r d w a r e and s o f t w a r e of a network.

T h e i m p l e m e n t e d v i r t u a l file structure is f l e x i b l e e n o u g h to a c c o m m o d a t e the r e q u i r e m e n t s of the t r a d i t i o n a l file t r a n s f e r and r e m o t e d a t a access (record access) s y s t e m s and at the same time to m a p d a t a s t r u c t u r e s u s e d in a p p l i c a t i o n s like g r a p h i c a l systems (metafiles), docum e n t access systems etc.

A d o p t i n g the same ISO-OSI r e f e r e n c e t e r m i n o l o g y and d e s i g n c o n c e p t s in a large t e a m of n e t w o r k d e s i g n e r s and imp l e m e n t a t o r s ' h a s m i n i m i z e d the u n d e r s t a n d i n g overhead. B) A s t r i c t l a y e r i n g a r c h i t e c t u r e in the n e t w o r k d e s i g n and i m p l e m e n t a t i o n is m a n d a t o r y for future c h a n g e s and integ r a t i o n of n e w s t a n d a r d s and products.

N e w e n h a n c e m e n t s of the v i r t u a l file p r o t o c o l for c o n c u r r e n c y c o n t r o l h a v e b e e n s p e c i f i e d and are in d e v e l o p m e n t . T h e e n h a n c e d p r o t o c o l is b a s e d on a time stamp m e c h a n i s m for a ~ i s t r i b u t e d database management system .

system

for a r e m o t e

A n o t h e r i m p o r t a n t a s p e c t to be o b s e r v e d is the fact that the 3X r e c o m m e n d a tions m a i n l y d e s c r i b e a p r o t o c o l and that the s e r v i c e d e f i n i t i o n is p o o r if not inexistent. The r e s u l t is that the h o s t b e h a v i o u r is not s p e c i f i e d and is implem e n t a t i o n d e p e n d e n t (e.g r e s e t behaviour, e c h o i n g etc.).

The d a t a types s u p p o r t e d by the p r e s e n t a tion layer as s t a n d a r d t r a n s f e r s y n t a x encompass: c h a r a c t e r s , integers, reals etc. The d a t a type set is d e f i n e d o p e n - e n d e d and m o r e c o m p l e x s t r u c t u r e s for n e w a p p l i c a t i o n s like: graphics, d o c u m e n t transfer, r e m o t e job e n t r y etc. are i n d e v e l o p m e n t .

3.4.3 T h e t e r m i n a l access (CCITT X3, X28, X29)

or as a t e r m i n a l

A n e t w o r k b a s e d on the s t a n d a r d i z a t i o n o u t p u t m i n i m i z e s in long terms the m a i n t e n a n c e and i n t e g r a t i o n w o r k since it can be e x p e c t e d that m o r e and m o r e c o m m u n i c a t i o n s t a n d a r d s w i l l be p r o v i d e d by the c o m p u t e r m a n u f a c t u r e r s as parts of their o p e r a t i n g systems.

DIA

T h e d e c i s i o n to s u p p o r t in H M I N E T - 2 the C C I T T 3X r e c o m m e n d a t i o n s was m o t i v a t e d b y its n a t i o n a l and i n t e r n a t i o n a l d i s s e m i n a t i o n a l t h o u g h these s t a n d a r d s do not rely on the I S O - O S I model.

C) A n a c t i v e p a r t i c i p a t i o n in the stand a r d i z a t i o n w o r k has s h o r t e d the information path standardization output implementation. -

D) T h e use of formal d e s c r i p t i o n t e c h n i ques in the s p e c i f i c a t i o n p h a s e is m a n d a t o r y in o r d e r to a c h i e v e c o m p a tible i m p l e m e n t a t i o n s b y d i f f e r e n t teams on d i f f e r e n t m a c h i n e s .

The 3X r e c o m m e n d a t i o n s d e s c r i b e the p r o t o c o l for s t a r t - s t o p t e r m i n a l s in a X25 network. T h e s p e c i f i c a t i o n was i n f l u e n c e d by the initial t h o u g h t s that p a c k e t - s w i t c h ing n e t w o r k s w i l l s u p p o r t o n l y t e r m i n a l h o s t c o m m u n i c a t i o n and that the n e t w o r k nodes w i l l o f f e r PADs (packet a s s e m b l y disassembly) as a c o m m o n c o m m u n i c a t i o n c a r r i e r service. T h e initial PAD s p e c i f i c a t i o n for DCE (data c o m m u n i c a t i o n e q u i p ~ ment) has p r e c i p i t a t e the i n v a s i o n of p r o ducts like PAD t e r m i n a l c o n c e n t r a t o r s and i m p l e m e n t a t i o n of PAD in hosts.

E) The d e v e l o p m e n t of t e s t i n g p r o c e d u r e s and r e f e r e n c e i m p l e m e n t a t i o n s as cert i f i c a t i o n c e n t e r s is a n e c e s s i t y for h e t e r o g e n e o u s n e t w o r k s and has to be c o n s i d e r e d as part of the n e t w o r k design f r o m the beginning.

Literature: I

This was m o t i v a t e d by the costs for the P A D / t e r m i n a l c o n n e c t i o n s and the flexib i l i t y in u s i n g the same t e r m i n a l in local

62

B o c h m a n n G.v., " F o r m a l i z e d s p e c i f i c a tion of the MLP" R e p o r t Univ. of M o n t r e a l J u n e 1979

2

B o c h m a n G.v. et al., " F o r m a l i z e d spec i f i c a t i o n and a n a l y s i s of the V i r t u a l F i l e System" HMI R e p o r t B367 Feb. 1982

I

B a u e r f e l d W., et al., " D I N A M I T - an adaptable, distributed computer based m e s s a g e system" to be p u b l i s h e d

4

ISO/TC97/SC16 dard

7498

Draft

International

Stan-

(NATIONAL)

1982

L e n z i n i L., P o p e s c u - Z e l e t i n R., V i s s e r s C. "State of the art s t u d y on the stand a r d i z a t i o n of level 4" R e p o r t for the C o m m i s s i o n of E u r o p e a n C o m m u n i t i e s " B r u s s e l 1981 Mier, E d w i n E. "High level p r o t o c o l s and the OSI r e f e r e n c e m o d e l " D a t a C o m m u n i c a t i o n 1982

X.,

P o p e s c u - Z e l e t i n R., W e b e r H. "The l o g i c a l a r c h i t e c t u r e for D - D B M S in the I S O - O S I f r a m e w o r k " Proc. GI C o n g r e s s B e r l i n 1983 S p r i n g e r Verlag 8

.

P o p e s c u - Z e l e t i n R., et al. "The virtual file s p e c i f i c a t i o n - final d r a f t - "HMI R e p o r t B333 1980 P o p e s c u - Z e l e t i n R., B a u m D., B u t s c h e r B. " A local c o m p u t e r netw o r k in a r e s e a r c h e n v i r o n m e n t : T h e H M I N E T Proc. C o m p u t e r n e t w o r k i n g s y m p o s i u m NBS 1979

Fig. 1

10 P o p e s c u - Z e l e t i n R., B a u e r f e l d W. "A v i r t u a l file c o n c e p t for h e t e r o g e n e o u s c o m p u t e r n e t w o r k s " Proc. A I C A c o n g r e s s BARI I t a l y 1979

USER U S E R

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63

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65

L_.-..2 s -- -

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-FCONNECT HOST A

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Fig.7a

Yig.7b

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Fig.8

66