Subject: [Isis-wg] OSI ??

From: "Swati Rastogi" <[email-masked]>
Date: Tue, 7 Aug 2001 20:36:26 +0530

Hi All,
This is slightly off-track but any help in this regard will be really very
much appreciated.

Here goes my long list of doubts

(1) Is CLNP (OSI) analogous to IP (TCP/IP) .. If so then why have i never
come across a CLNP packet format (analogy .. we have IP packet formats)
(2) Routing is making sure that a packet reaches the destination safe 'n
sound .. while a routing protocol ensures that the out of the available
paths choosing the best one. So if ISIS runs along IP then who is taking
care of the "routing" aspect i.e taking the packet from the source to the
destination. ISIS is a routing protocol and its job shud be to just choose
the optimal path .. once its done that it can then hand over the packet to
CLNP for forwarding .. So where did i loose the track ??
(3) In my transport layer i could be running tp0, tp1, tp2 etc. How does the
routing module know which transport layer to forward the packet to. There is
only 1 byte field "SEL" aka selector in the NSAP addressing .. is this used
to determine this ??
To summarize .. how when a module recvs a packet from the data link layer
determines whether to use CONP or CLNP in the networking layer and then
later which transport layer protocol is to be used in the transport layer!

Which transport layer protocol does ISIS use ??

Please Help .. i am very confused with all the OSI terminology !

Thanks and Regards,
Swati

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---

From: "Manav Bhatia" <[email-masked]>
Date: Wed, 8 Aug 2001 13:59:05 +0530

> come across a CLNP packet format (analogy .. we have IP packet formats)

Refer to ISO/IEC 8473, Section 7

> CLNP for forwarding .. So where did i loose the track ??

ISIS protocol is designed to work in close conjunction with ISO 8473 and ISO
9542. The latter is used to establish connectivity and reachability between
end systems and Intermediate systems. Data is carried by ISO 8473. It
provides connectionless mode network service.

> only 1 byte field "SEL" aka selector in the NSAP addressing .. is this
used

For ISIS you dont access the upper transport layer .. and hence you dont
need to know that ..
I hope i am right ..
SEL byte is always zero for Intermediate systems

> later which transport layer protocol is to be used in the transport layer!

The OSI pdu has a "network layer protocol identifier" parameter in its fixed
part .. which probably tells about whether its CLNP or CONP.

> Which transport layer protocol does ISIS use ??

It wont be running on top of any transport layer.

> Please Help .. i am very confused with all the OSI terminology !

Regards,
Manav Bhatia

---

From: "Swati Rastogi" <[email-masked]>
Date: Wed, 8 Aug 2001 14:23:17 +0530

> provides connectionless mode network service.
Does it imply that all ISIS PDUs like L1 lan/p2p IIS Hello, L1/2 LSPs, L1/2
CSNPs, L1/2 PSNPs form the data part in the ISO 8473 data PDU ?
Quoting Jeff . .
" Be careful when you read "encapsulated in CLNS packets".  It is absolutely
correct, but does NOT mean that they are encapsulated in CLNP packets.  It
does mean that they are OSI connectionless layer three packets and therefore
begin with a Network Layer Protocol ID (variously called NPID, NLPI, or
NLPID).  After that first octet, IS-IS and CLNP headers have little in
common."

He says that ISIS packets are not encapsulated in CLNP packets .. Moreover
he also mentions that ISIS and CLNP headers have a little in common except
for NLPID ?
Does it imply that there will be different values in NLPID for CLNP,CONP and
ISIS packets ??

Pl. clarify

Thanks and Regards,
Swati

---

From: Jeff Learman <[email-masked]>
Date: Wed, 08 Aug 2001 13:40:33 -0400

Swati,

>come across a CLNP packet format (analogy .. we have IP packet formats)

Yes, CLNP provides very similar services to IP.  You don't see
packet formats everywhere because it never became popular -- mostly
because of the incredible success of IP.

>destination. 

IS-IS sends its packets directly over the link layer -- IP is not
involved.  There are two exceptions to this, (a) partition repair
tunneling and (b) Integrated IS-IS over IP tunnels.  (a) didn't
work until a rather recent correction, and (b) was never popular.
Ignore both of these until you have everything else VERY well
understood.

>ISIS is a routing protocol and its job shud be to just choose

You are correct and that's how it works.

>later which transport layer protocol is to be used in the transport layer!

These are not IS-IS questions, so I will reply separately.

>Which transport layer protocol does ISIS use ??

None, as mentioned above.

>Please Help .. i am very confused with all the OSI terminology !

You're not the first!

---

From: Jeff Learman <[email-masked]>
Date: Wed, 08 Aug 2001 13:42:05 -0400

>CSNPs, L1/2 PSNPs form the data part in the ISO 8473 data PDU ?

No, none of these are carried in the data part of the 8473 data PDU.

>Does it imply that there will be different values in NLPID for CLNP,CONP and

Yes, except for CONS, which is more complicated (remember, the network
access protocol for CONS is X.25, and there is no standard protocol
to implement X.25 in the service provider's network).

CLNP is 0x81, ESIS is 0x82, ISIS is 0x83, IDRP has 0x84 (but doesn't
use it), and Network Layer Security Protocol (ISO NLSP) uses 0x85.

Jeff

---

From: "Ken Larmer" <[email-masked]>
Date: Wed, 8 Aug 2001 13:42:00 -0400

Hi,

	Let me try to explain the difference between CLNS (Connection-Less Network
Service) and CONS (Connection-Oriented Network Service). Furthermore, I will
attempt to explain their use with IS-IS (OSI only) and I will then attempt
to tie their use with Integrated IS-IS (Dual OSI and IP).

	One must remember that OSI was conceived in the 1980s by Digital with its
primary reason being to increase DECnet Phase IV address space. Thus,
Digital designed DECnet Phase V, which in turn spawned the better part of
OSI. In the 1980s the predominate LAN technology was Ethernet and the
predominate WAN technology was X.25. Ethernet circuits are considered to be
Connection-Less, while X.25 networks are considered to be
Connection-Oriented.

The difference being that when an ES wants to transmit a data packet across
an Ethernet, it simply sends it to the destination or to the default router.
There is no need to establish a prior connection because all MTU sizes
should be the same and there is no need to negotiate flow-control below the
transport layer. However, on an X.25 network, an ES or IS must setup the
connection to the remote DTE prior to transmission of data packets. X.25 has
many negotiable options for data transfer, there are really no defaults, per
say!

	So, CONS was employed as a mechanism for communicating with ISO-8208 X.25
packet switch networks to facilitate call setup, parameter negotiation,
circuit maintenance and call teardown. SNDCF (Sub-Network Dependent
Convergence Functions) are utilized by the OSI network layer to communicate
with the X.25 PLP (Packet Level Protocol) for execution of these DTE
functions. Through this CONS mechanism, we can setup, maintain and teardown
SVCs between X.25 DTEs.

Furthermore, since X.25 already possessed a method for guaranteed delivery
(LAPB – Link Access Procedure Balanced) at the datalink DTE<>DCE and packet
sequencing at the X.25 network layer DTE<cloud>DTE, there is no need to
sequence packets at the transport layer when using a CONS network. So, most
applications (both of them :^) use OSI TP0 or TP2 for an OSI Transport
mechanism across CONS networks. TP0 and TP2 are much like UDP.

	Ethernet on the other hand does not require call establishment prior to
transport of data. So, CLNS is used to communicate across this type of
network. Since there is no guaranteed delivery mechanism with Ethernet
(Broadcast circuits for that matter), we need to make sure we use a
higher-layer to guarantee delivery of packets. This is where OSI TP4 comes
into play. OSI TP4 is much like TCP in that it sequences packets and
basically guarantees their delivery.

	Now, how does this tie into CLNP (Connection-Less Network Protocol) and the
format of OSI packets? Well, both CONS and CLNS networks transfer OSI data
in CLNP packet formats.

On a CLNS Ethernet network, OSI packets are exchanged using IEEE 802.3
formatted ethernet frames. The data portion of an IEEE 802.3 frame, which is
carrying OSI data, must have as its first octet a value of 128 (81 hex) =
OSI data packet, 129 (82 hex) ES-IS control packet or 130 (83 hex) IS-IS
control packet. This first octet is referred to as the NLPID (Network Layer
Protocol Identifier) for ES-IS and OSI data and error packets and as IRPD
(Intradomain Routing Protocol Discriminator) for IS-IS control packets. This
is how the packet is passed up the stack to the proper network layer entity.

	On a CONS X.25 network, OSI packets are exchanged as USER DATA contained
within the X.25- PLP. Again, the first octet in the user data portion of an
X.25 packet, must have as its first octet a value of 128 (81 hex) = OSI data
packet. ES-IS and IS-IS are not normally run over an X.25 circuit. Instead
most implementations of OSI/X.25 only allow for SVCs or DED (Dynamically
Established Datalinks). In most cases, the OSI network layer uses X.121
addresses (X.25 DTE addresses), extracted from X.121 formatted NSAPs to
signal the correct DTE for the call setup to the remote DTE. Another reason
for not running ES-IS or IS-IS over an X.25 network is because some PSNs
charge by the packet. Static routes are much cheaper.

	An IS-IS OSI-only router will exchange IS-IS and ES-IS control data
encapsulated in a CLNP packet. An IS-IS OSI-only router will also forward
OSI data in a CLNP packet. An Integrated IS-IS IP-only router will exchange
IS-IS control data encapsulated in a CLNP packet. An IS-IS IP-only router
will forward IP data in an IP encapsulated packet. It will NOT exchange any
ES-IS control data nor will it forward any OSI data, just IP data in its
native format. An Integrated IS-IS Dual router will act as both an OSI-only
router and IP-only router.

	Finally, the SEL (Selector) portion of an NSAP, though not defined by any
ISO docs in this manner, was used by Digital to signal the Network layer of
a DECnet Phase V system to send this packet to either the NSP (DECnet Phase
IV transport) or OSI Transport.

Hope this helps?

Cheers,
Ken Larmer

---

From: John Harper <[email-masked]>
Date: Wed, 08 Aug 2001 13:09:25 -0700

This seems to be a rather inaccurate portrayal of the way it really was. 
I'm sure
nobody cares, but I thought I'd correct it a little anyway. (I was chair of the
OSI Network Layer group during the time it was doing all this, as well as
running the lower layers of the DECnet/OSI architecture, so I really was
There).

First, OSI was conceived long before DEC latched onto it - in the late 70s,
whilst our decision to adopt it was made in 1984. Our rationale was that if 
not,
SNA would take over the world. If OSI achieved anything at all, it was to
delay SNA for long enough that TCP/IP did in fact take over the world -
meeting all the objectives that we at DEC had for OSI, even if all the
actual protocols were different.

Incidentally our original ideas for DECnet were to have 64-bit addresses.
The OSI (NSAP) address format was a horrible compromise to accomodate
the world views of both the phone companies and the data networking people
(us and the tiny TCP/IP community at the time).

CONS and CLNS were completely different religious views of the world. CONS
came from the phone companies and from the view that X.25 was the centre
of the universe, and everything else should look like it. CLNS of course 
came from
the data networking community. Either of them had standards to allow it to
operate over all kinds of subnetwork (OSI jargon), including both LANs and
WANs such as X.25 and ISDN. (ISO/IEC 8881 tells you how to make a LAN
look like an X.25 network, should you ever want to). In practice CONS
collapsed under the combined weight of its own complexity and the general
incompetence of its protagonists, and has never seen even a serious
attempt to implement it, never mind deploy it.

Incidentally CONS most definitely does (did) NOT transport data using CLNP
packets. It used all kinds of complicated interworking arrangements at 
subnetwork
boundaries precisely to avoid this approach, which was anethema to the CONS
religion.

As several people have already said, IS-IS does NOT use CLNP to carry its
packets, nor does ES-IS. I happen to think this was an excellent decision,
although John Moy certainly disagrees with me.

         John

At 13:42 08/08/2001 -0400, Ken Larmer wrote:
>(LAPB  Link Access Procedure Balanced) at the datalink DTE<>DCE and packet

---

From: Jeff Learman <[email-masked]>
Date: Wed, 08 Aug 2001 18:08:23 -0400

>functions. Through this CONS mechanism, we can setup, maintain and teardown

Warning: SNDCF is usually used to mean the way CLNS operates over
a particular kind of data-link, and most commonly for the specific
case of CLNP over X.25 (where CLNP is using X.25 as a link level
protocol -- sort of like tunneling).

>mechanism across CONS networks. TP0 and TP2 are much like UDP.

Not quite.  CLTP is like UDP; they both provide a connectionless service.
TP0 and TP2 are simpler protocols than TP4 or IP, but provide
connection-oriented services over a network that is already
connection-oriented, and therefore are simpler than TP4 or
TCP, as Ken says.

>basically guarantees their delivery.

True, except that there is a provision for X.25-CONS over LLC3,
which is on Ethernet.  Fortunately not very popular.

>format of OSI packets? Well, both CONS and CLNS networks transfer OSI data

False.  The protocol used to provide CONS is X.25, and involves
no CLNP packets.  CLNP can operate over X.25, but CONS is not involved.

>(Intradomain Routing Protocol Discriminator) for IS-IS control packets.

IDRP is the inTER-domain routing protocol, not inTRA-domain, which is
IS-IS.  IDRP serves the same role as BGP does for IP, and like BGP is
a distance-vector protocol, not a link-state protocol.

>This
>packet.

This is true only when running CLNS over X.25, and the NLPID is
really considered part of the CLNS PDU, according to the specs.
(When I say CLNS in this context, I mean CLNP, ES-IS, or IS-IS.)

>ES-IS and IS-IS are not normally run over an X.25 circuit.

>Instead most implementations of OSI/X.25 only allow for SVCs or DED
>(Dynamically Established Datalinks).

Really?

>In most cases, the OSI network layer uses X.121
>encapsulated in a CLNP packet.

ES-IS and IS-IS packets are NOT exchanged in CLNP packets.

>An IS-IS OSI-only router will also forward
>IS-IS control data encapsulated in a CLNP packet.

IS-IS packets are NOT exchanged in CLNP packets.

>An IS-IS IP-only router

True, the NSEL was not identified in ISO network layer addressing,
CLNP, or ES-IS specs.  It was discussed in NIST OIW agreements.
Finally, though, IS-IS mandated that the final octet of an NSAP
would not be used in routing decisions, except at the destination
end system.

Jeff

---

From: "Ken Larmer" <[email-masked]>
Date: Thu, 9 Aug 2001 10:47:46 -0400

Thanks for all the corrections :^)

I have a very bad habit of calling all OSI packets CLNP, when they are
clearly not! Furthermore, I apologize if I miss-represented the evolution of
OSI!

Just a couple of comments and I'll fade away!

Jeff, I believe you meant to say X.25 over ethernet is LLC2 not LLC3. Please
correct me if I'm wrong. BTW, Digital had a lot of customers running this
hack.

I also realize you can run IS-IS over a X.25 PVC on one IS-IS implementation
I know of. My comment was simply stating that this is not a common practice.

>no CLNP packets.  CLNP can operate over X.25, but CONS is not involved.

What is the format? CONS is used establish the connection and IS-IS does the
forwarding. I know I didn't state that, but is that true?

>a distance-vector protocol, not a link-state protocol.

Jeff, did you mean IDRPI (Inter-Domain Routing Protocol Information) from
RFC 1195. My statement refers to ISO-10589 control packets, the first octet
of which is the IRPD (Intradomain Routing Protocol Discriminator)field. Of
course, we could throw another 1,000 acronyms into OSI so it would be a bit
more confusing.

I believe that this open discussion is good for this working group. Much to
often I see questions go unanswered because many folks do not have all the
right answers or the ones that do are too busy to respond. I would rather
see this type of discussion, than have a bunch of folks who are to afraid to
respond because they may get some portion of their response wrong. This
stuff is extremely complex and needs many minds to get the facts 100%
correct.

Just my thoughts!

Cheers,
Ken

---

From: Jeff Learman <[email-masked]>
Date: Thu, 09 Aug 2001 12:29:14 -0400

>correct me if I'm wrong. BTW, Digital had a lot of customers running this

OOPS!  You're right, LLC2 is connection-oriented LLC.  LLC3 was from MAP
(GM's manufacturing automation protocol).

>I know of. My comment was simply stating that this is not a common practice.

You can also run it over an SVC, as long as it is statically assigned,
rather than than one that comes up and goes down

>forwarding. I know I didn't state that, but is that true?

IS-IS never does forwarding.  It's just a routing protocol, which
derives routing information for the use of the network layer data
protocol, which forwards the data.

IS-IS was designed for use with CLNP, and then extended for use with
IP.  It has never been standardized for use with CONS.

CONS itself, like CLNS itself, is an abstract service definition and
does not impose a protocol.  There are other specifications that
state how to provide CONS using X.25 as a network access protocol.
(The actual network delivery protocol and routing protocols for
X.25 are not standard, because it's the X.25 service provider's
job -- in this case the X.25 service provider is a company or
national organization.  They can do whatever they want, as
long as they provide a service that can be interfaced to using
X.25.)

>course, we could throw another 1,000 acronyms into OSI so it would be a bit

No, I wasn't referring to IDRPI.  IDRP stands for "Interdomain Routeing
Protocol", which is like BGP.  The first octet of an OSI connectionless
network layer protocol packet isthe NLPI, "Network layer protocol ID", aka
NPID or NLPID.  I don't recall an IRPD (I thought that was a transposition
on your part), so my response was non-sequitur.  Where is that IRDP defined?

---

From: "Ken Larmer" <[email-masked]>
Date: Thu, 9 Aug 2001 14:44:23 -0400

Sorry Jeff, I just have a couple more questions?

> protocol, which forwards the data.

Jeff, please help me out here, I am really confused! You stated that IS-IS
never does forwarding. I understand that it never does forwarding of IP
packets, but what about CLNP, does this still hold true?

> IP.  It has never been standardized for use with CONS.

Jeff, ISO-10589 states the following, which I thought pertained to CONS
circuits? Please help to clear this up for me???

ISO 8208 subnetworks

8.3.1 Network layer protocols

The way in which the underlying service assumed by ISO 8473 is provided for
ISO 8208 subnetworks is described in clause 8 of ISO 8473. This defines a
set of Subnetwork Dependent Convergence Functions (SNDCFs) that relate the
service provided by specific individual ISO International Standard
subnetworks to the abstract "underlying service" defined in 5.5 of ISO 8473.
In particular 8.4.3 describes the Subnetwork Dependent Convergence Functions
used with ISO 8208 Subnetworks.

Use of ISO 8473 subnetwork dependent convergence functions

SVCs shall be established according to the procedures defined in the ISO
8208 Subnetwork Dependent Convergence Functions of ISO 8473 (this may be on
system management action or on arrival of data depending on the type of
circuit). The Call Request shall contain a Protocol Discriminator specifying
ISO 8473 in the first octet of Call Userdata.

> on your part), so my response was non-sequitur.  Where is that

Jeff, are you talking about the DSAP/SSAP of an IEEE 802.3 packet? The IDRP
(83) is what identifies an IS-IS packet within an IEEE 802.3 ethernet frame.

Cheers,
Ken