Request for Comments: 1195 Digital Equipment Corporation
December 1990
Use of OSI IS-IS for Routing in TCP/IP and Dual Environments
Status of this Memo
This RFCspecifies a protocol on the IAB Standards Track for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "IAB
Official Protocol Standards" for the standardization state and status
of this protocol. Distribution of this memo is unlimited.
This RFCis available in both postscript and text versions. Where
possible, use of the postscript version is recommended. For example,
this text version may have figures which are less informative or
missing.
Abstract
This RFCspecifies an integrated routing protocol, based on the OSI
Intra-Domain IS-IS Routing Protocol, which may be used as an interior
gateway protocol (IGP) to support TCP/IP as well as OSI. This allows
a single routing protocol to be used to support pure IP environments,
pure OSI environments, and dual environments. This specification was
developed by the IS-IS working group of the Internet Engineering Task
Force.
The OSI IS-IS protocol has reached a mature state, and is ready for
implementation and operational use. The most recent version of the
OSI IS-IS protocol is contained in ISO DP 10589 [1]. The proposed
standard for using IS-IS for support of TCP/IP will therefore make
use of this version (with a minor bug correction, as discussed in
Annex B). We expect that future versions of this proposed standard
will upgrade to the final International Standard version of IS-IS
when available.
Comments should be sent to "isis@merit.edu".
Contents
1 Introduction: Overview of the Protocol
1.1 What the Integrated IS-IS offers
1.2 Overview of the ISO IS-IS Protocol
1.3 Overview of the Integrated IS-IS
1.4 Support of Mixed Routing Domains
1.5 Advantages of Using Integrated IS-IS
2 Symbols and Abbreviations
3 Subnetwork Independent Functions
3.1 Exchange of Routing Information
3.2 Hierarchical Abbreviation of IP Reachability Information
3.3 Addressing Routers in IS-IS Packets
3.4 External Links
3.5 Type of Service Routing
3.6 Multiple LSPs and SNPs
3.7 IP-Only Operation
3.8 Encapsulation
3.9 Authentication
3.10 Order of Preference of Routes / Dijkstra Computation
4 Subnetwork Dependent Functions
4.1 Link Demultiplexing
4.2 Multiple IP Addresses per Interface
4.3 LANs, Designated Routers, and Pseudonodes
4.4 Maintaining Router Adjacencies
4.5 Forwarding to Incompatible Routers
5 Structure and Encoding of PDUs
5.1 Overview of IS-IS PDUs
5.2 Overview of IP-Specific Information for IS-IS
5.3 Encoding of IP-Specific Fields in IS-IS PDUs
6 Security Considerations
7 Author's Address
8 References
A Inter-Domain Routing Protocol Information
A.1 Inter-Domain Information Type
A.2 Encoding
B Encoding of Sequence Number Packets
B.1 Level 1 Complete Sequence Numbers PDU
B.2 Level 2 Complete Sequence Numbers PDU
B.3 Level 1 Partial Sequence Numbers PDU
B.4 Level 2 Partial Sequence Numbers PDU
C Dijkstra Calculation and Forwarding
C.1 SPF Algorithm for IP and Dual Use
C.2 Forwarding of IP packets
D Use of the Authentication Field
D.1 Authentication Field in IS-IS packets
D.2 Authentication Type 1 - Simple Password
E Interaction of the Integrated IS-IS with Brouters
E.1 The Problem
E.2 Possible Solutions
Figures
1 ISO Hierarchical Address Structure
2 An Example
3 Encoding of Variable Length Fields
1 Introduction: Overview of the Protocol
The TCP/IP protocol suite has been growing in importance as a multi-
vendor communications architecture. With the anticipated emergence of
OSI, we expect coexistence of TCP/IP and OSI to continue for an
extended period of time. There is a critical need for routers to
support both IP traffic and OSI traffic in parallel.
There are two main methods that are available for routing protocols
to support dual OSI and IP routers. One method, known as "Ships in
the Night", makes use of completely independent routing protocols for
each of the two protocol suites. This specification presents an
alternate approach, which makes use of a single integrated protocol
for interior routing (i.e., for calculating routes within a routing
domain) for both protocol suites.
This integrated protocol design is based on the OSI Intra-domain IS-
IS routing protocol [1], with IP-specific functions added. This RFC
is considered a companion to the OSI IS-IS Routing spec, and will
only describe the required additional features.
By supporting both IP and OSI traffic, this integrated protocol
design supports traffic to IP hosts, OSI end systems, and dual end
systems. This approach is "integrated" in the sense that the IS-IS
protocol can be used to support pure-IP environments, pure-OSI
environments, and dual environments. In addition, this approach
allows interconnection of dual (IP and OSI) routing domains with
other dual domains, with IP-only domains, and with OSI-only domains.
The protocol specified here is based on the work of the IETF IS-IS
working group.
1.1 What the Integrated IS-IS offers
The integrated IS-IS provides a single routing protocol which will
simultaneously provide an efficient routing protocol for TCP/IP, and
for OSI. This design makes use of the OSI IS-IS routing protocol,
augmented with IP-specific information. This design provides explicit
support for IP subnetting, variable subnet masks, TOS-based routing,
and external routing. There is provision for authentication
information, including the use of passwords or other mechanisms. The
precise form of authentication mechanisms (other than passwords) is
outside of the scope of this document.
Both OSI and IP packets are forwarded "as is" -- i.e., they are
transmitted directly over the underlying link layer services without
the need for mutual encapsulation. The integrated IS-IS is a dynamic
routing protocol, based on the SPF (Dijkstra) routing algorithm.
The protocol described in this specification allows for mixing of
IP-only, OSI-only, and dual (IP and OSI) routers, as defined below.
An IP-only IS-IS router (or "IP-only" router) is defined to be a
router which: (i) Uses IS-IS as the routing protocol for IP, as
specified in this report; and (ii) Does not otherwise support OSI
protocols. For example, such routers would not be able to forward OSI
CLNP packets.
An OSI-only router is defined to be a router which uses IS-IS as the
routing protocol for OSI, as specified in [1]. Generally, OSI-only
routers may be expected to conform to OSI standards, and may be
implemented independent of this specification.
A dual IS-IS router (or "dual" router) is defined to be a router
which uses IS-IS as a single integrated routing protocol for both IP
and OSI, as specified in this report.
This approach does not change the way that IP packets are handled.
IP-only and dual routers are required to conform to the requirements
of Internet Gateways [4]. The integrated IS-IS protocol described in
this report outlines an Interior Gateway Protocol (IGP) which will
provide routing within a TCP/IP routing domain (i.e., autonomous
system). Other aspects of router functionality (e.g., operation of
ICMP, ARP, EGP, etc.) are not affected by this proposal.
Similarly, this approach does not change the way that OSI packets are
handled. There will be no change at all to the contents nor to the
handling of ISO 8473 Data packets and Error Reports, nor to ISO 9542
Redirects and ES Hellos. ISO 9542 IS Hellos transmitted on LANs are
similarly unchanged. ISO 9542 IS Hellos transmitted on point-to-point
links are unchanged except for the addition of IP-related
information. Similarly, other OSI packets (specifically those
involved in the IS-IS intra-domain routing protocol) remain unchanged
except for the addition of IP-related information.
This approach makes use of the existing IS-IS packets, with IP-
specific fields added. Specifically: (i) authentication information
may be added to all IS-IS packets; (ii) the protocols supported by
each router, as well as each router's IP addresses, are specified in
ISO 9542 IS Hello, IS-IS Hello and Link State Packets; (iii)
internally reachable IP addresses are specified in all Link State
Packets; and (iv) externally reachable IP addresses, and external
routing protocol information, may be specified in level 2 Link State
Packets. The detailed encoding and interpretation of this in
formation is specified in sections 3, 4, and 5 of this RFC.
The protocol described in this report may be used to provide routing
in an IP-only routing domain, in which all routers are IP-only.
Similarly, this protocol may be used to provide routing in a pure
dual domain, in which all routers are dual. Finally, this protocol
may be used to provide routing in a mixed domain, in which some
routers are IP-only, some routers are OSI-only, and some routers are
dual. The specific topological restrictions which apply in this
latter case are described in detail in section 1.4 ("Support of Mixed
Routing Domains"). The use of IS-IS for support of pure OSI domains
is specified in [1].
This protocol specification does not constrain which network
management protocol(s) may be used to manage IS-IS-based routers.
Management information bases (MIBs) for managing IP-only, OSI-only,
and dual routers, compatible with CMIP, CMOT, and/or SNMP, are the
subject of a separate, companion document [8].
1.2 Overview of the ISO IS-IS Protocol
The IS-IS Routing Protocol has been developed in ISO to provide
routing for pure OSI environments. In particular, IS-IS is designed
to work in conjunction with ISO 8473 (The ISO Connectionless Network
Layer Protocol [2]), and ISO 9542 (The ISO End System to Intermediate
System Protocol [3]). This section briefly describes the manner in
which IS-IS is used to support pure OSI environments. Enhancements
for support of IP and dual environments are specified elsewhere in
this report.
In IS-IS, the network is partitioned into "routing domains". The
boundaries of routing domains are defined by network management, by
setting some links to be "exterior links". If a link is marked as
"exterior", no IS-IS routing messages are sent on that link.
Currently, ISO does not have a standard for inter-domain routing
(i.e., for routing between separate autonomous routing domains).
Instead, manual configuration is used. The link is statically
configured with the set of address prefixes reachable via that link,
and with the method by which they can be reached (such as the DTE
address to be dialed to reach that address, or the fact that the DTE
address should be extracted from the IDP portion of the ISO address).
OSI IS-IS routing makes use of two-level hierarchical routing. A
routing domain is partitioned into areas. Level 1 routers know the
topology in their area, including all routers and end systems in
their area. However, level 1 routers do not know the identity of
routers or destinations outside of their area. Level 1 routers
forward all traffic for destinations outside of their area to a level
2 router in their area. Similarly, level 2 routers know the level 2
topology, and know which addresses are reachable via each level 2
router. However, level 2 routers do not need to know the topology
within any level 1 area, except to the extent that a level 2 router
may also be a level 1 router within a single area. Only level 2
routers can exchange data packets or routing information directly
with external routers located outside of the routing domains.
+----------------------+-------------------------------+
| IDP | DSP |
+----------------------+-------------------------------+
. . .
. . .
. . .
+-----+----------------+----------+--------------+-----+
| AFI | IDI | HO-DSP | ID | SEL |
+-----+----------------+----------+--------------+-----+
Figure 1 - ISO Hierarchical Address Structure
As illustrated in figure 1, ISO addresses are subdivided into the
Initial Domain Part (IDP), and the Domain Specific Part (DSP). The
IDP is the part which is standardized by ISO, and specifies the
format and authority responsible for assigning the rest of the
address. The DSP is assigned by whatever addressing authority is
specified by the IDP. The DSP is further subdivided into a "High
Order Part of DSP" (HO-DSP), a system identifier (ID), and an NSAP
selector (SEL). The HO-DSP may use any format desired by the
authority which is identified by the IDP. Together, the combination
of [IDP, HO-DSP] identify both the routing domain and the area within
the routing domain. The combination of [IDP,HO-DSP] may therefore be
referred to as the "Area Address".
Usually, all nodes in an area have the same area address. However,
sometimes an area might have multiple addresses. Motivations for
allowing this are:
- It might be desirable to change the address of an area. The most
graceful way of changing an area from having address A to having
address B is to first allow it to have both addresses A and B, and
then after all nodes in the area have been modified to recognize
both addresses, then one by one the nodes can be modified to
"forget" address A.
- It might be desirable to merge areas A and B into one area. The
method for accomplishing this is to, one by one, add knowledge of
address B into the A partition, and similarly add knowledge of
address A into the B partition.
- It might be desirable to partition an area C into two areas, A
and B (where "A" might equal "C", in which case this example
becomes one of removing a portion of an area). This would be
accomplished by first introducing knowledge of address A into
the appropriate nodes (those destined to become area A), and
knowledge of address B into the appropriate nodes, and then one
by one removing knowledge of address C.
Since OSI addressing explicitly identifies the area, it is very easy
for level 1 routers to identify packets going to destinations outside
of their area, which need to be forwarded to level 2 routers.
In IS-IS, there are two types of routers:
- Level 1 intermediate systems -- these nodes route based on the ID
portion of the ISO address. They route within an area. They
recognize, based on the destination address in a packet, whether
the destination is within the area. If so, they route towards
the destination. If not, they route to the nearest level 2 router.
- Level 2 intermediate systems -- these nodes route based on the area
address (i.e., on the combination of [IDP, HO-DSP]). They route
towards areas, without regard to the internal structure of an area.
A level 2 IS may also be a level 1 IS in one area.
A level 1 router will have the area portion of its address manually
configured. It will refuse to become a neighbor with a node whose
area addresses do not overlap its area addresses. However, if level 1
router has area addresses A, B, and C, and a neighbor has area
addresses B and D, then the level 1 router will accept the other node
as a neighbor.
A level 2 router will accept another level 2 router as a neighbor,
regardless of area address. However, if the area addresses do not
overlap, the link would be considered by both routers to be "level 2
only", and only level 2 LSPs would flow on the link. External links
(to other routing domains) must be from level 2 routers.
IS-IS provides an optional partition repair function. In the unlikely
case that a level 1 area become partitioned, this function, if
implemented, allows the partition to be repaired via use of level 2
routes.
IS-IS requires that the set of level 2 routers be connected. Should
the level 2 backbone become partitioned, there is no provision for
use of level 1 links to repair a level 2 partition.
In unusual cases, a single level 2 router may lose connectivity to
the level 2 backbone. In this case the level 2 router will indicate
in its level 1 LSPs that it is not "attached", thereby allowing level
1 routers in the area to route traffic for outside of the domain to a
different level 2 router. Level 1 routers therefore route traffic to
destinations outside of their area only to level 2 routers which
indicate in their level 1 LSPs that they are "attached".
An end system may autoconfigure the area portion of its address by
extracting the area portion of a neighboring router's address. If
this is the case, then an endnode will always accept a router as a
neighbor. Since the standard does not specify that the end system
MUST autoconfigure its area address, an end system may be configured
with an area address. In this case the end system would ignore router
neighbors with non-matching area addresses.
Special treatment is necessary for broadcast subnetworks, such as
LANs. This solves two sets of issues: (i) In the absence of special
treatment, each router on the subnetwork would announce a link to
every other router on the subnetwork, resulting in n-squared links
reported; (ii) Again, in the absence of special treatment, each
router on the LAN would report the same identical list of end systems
on the LAN, resulting in substantial duplication.
These problems are avoided by use of a "pseudonode", which represents
the LAN. Each router on the LAN reports that it has a link to the
pseudonode (rather than reporting a link to every other router on the
LAN). One of the routers on the LAN is elected "designated router".
The designated router then sends out an LSP on behalf of the
pseudonode, reporting links to all of the routers on the LAN. This
reduces the potential n-squared links to n links. In addition, only
the pseudonode LSP includes the list of end systems on the LAN,
thereby eliminating the potential duplication (for further
information on designated routers and pseudonodes, see [1]).
The IS-IS provides for optional Quality of Service (QOS) routing,
based on throughput (the default metric), delay, expense, or residual
error probability. This is described in greater detail in section
3.5, and in [1].
1.3 Overview of the Integrated IS-IS
The integrated IS-IS allows a single routing protocol to be used to
route both IP and OSI packets. This implies that the same two-level
hierarchy will be used for both IP and OSI routing. Each area will be
specified to be either IP-only (only IP traffic can be routed in that
particular area), OSI-only (only OSI traffic can be routed in that
area), or dual (both IP and OSI traffic can be routed in the area).
This proposal does not allow for partial overlap of OSI and IP areas.
For example, if one area is OSI-only, and an other area is IP-only,
then it is not permissible to have some routers be in both areas.
Similarly, a single backbone is used for the routing domain. There is
no provision for independent OSI and IP backbones.
Similarly, within an IP-only or dual area, the amount of knowledge
maintained by routers about specific IP destinations will be as
similar as possible as for OSI. For example, IP-capable level 1
routers will maintain the topology within the area, and will be able
to route directly to IP destinations within the area. However, IP-
capable level 1 routers will not maintain information about
destinations outside of the area. Just as in normal OSI routing,
traffic to destinations outside of the area will be forwarded to the
nearest level 2 router. Since IP routes to subnets, rather than to
specific end systems, IP routers will not need to keep nor distribute
lists of IP host identifiers (note that routes to hosts can be
announced by using a subnet mask of all ones).
The IP address structure allows networks to be partitioned into
subnets, and allows subnets to be recursively subdivided into smaller
subnets. However, it is undesireable to require any specific
relationship between IP subnet addresses and IS-IS areas. For
example, in many cases, the dual routers may be installed into
existing environments, which already have assigned IP and/or OSI
addresses. In addition, even if IP addresses are not already pre-
assigned, the address limitations of IP constrain what addresses may
be assigned. We therefore will not require any specific relationship
between IP addresses and the area structure. The IP addresses can be
assigned completely independently of the OSI addresses and IS-IS area
structure. As will be described in section 3.2 ("Hierarchical
Abbreviation of IP Reachability Information"), greater efficiency and
scaling of the routing algorithm can be achieved if there is some
correspondence between the IP address assignment structure and the
area structure.
Within an area, level 1 routers exchange link state packets which
identify the IP addresses reachable by each router. Specifically,
zero or more [IP address, subnet mask, metric] combinations may be
included in each Link State Packet. Each level 1 router is manually
configured with the [IP address, subnet mask, metric] combinations
which are reachable on each interface. A level 1 router routes as
follows:
- If a specified destination address matches an [IP address, subnet
mask, metric] reachable within the area, the packet is routed via
level 1 routing.
- If a specified destination address does not match any [IP address,
subnet mask, metric] combination listed as reachable within the
area, the packet is routed towards the nearest level 2 router.
Flexible use of the limited IP address space is important in order to
cope with the anticipated growth of IP environments. Thus an area
(and by implication a routing domain) may simultaneously make use of
a variety of different address masks for different subnets in the
area (or domain). Generally, if a specified destination address
matches more than one [IP address, subnet mask] pair, the more
specific address is the one routed towards (the one with more "1"
bits in the mask -- this is known as "best match" routing).
Level 2 routers include in their level 2 LSPs a complete list of [IP
address, subnet mask, metric] specifying all IP addresses reachable
in their area. As described in section 3, this information may be
obtained from a combination of the level 1 LSPs (obtained from level
1 routers in the same area), and/or by manual configuration. In
addition, Level 2 routers may report external reachability
information, corresponding to addresses which can be reached via
routers in other routing domains (autonomous systems)
Default routes may be announced by use of a subnet mask containing
all zeroes. Default routes should be used with great care, since they
can result in "black holes". Default routes are permitted only at
level 2 as external routes (i.e., included in the "IP External
Reachability Information" field, as explained in sections 3 and 5).
Default routes are not permitted at level 1.
The integrated IS-IS provides optional Type of Service (TOS) routing,
through use of the QOS feature from IS-IS.
1.4 Support of Mixed Routing Domains
The integrated IS-IS proposal specifically allows for three types of
routing domains:
- Pure IP
- Pure OSI
- Dual
In a pure IP routing domain, all routers must be IP-capable. IP-only
routers may be freely mixed with dual routers. Some fields
specifically related to OSI operation may be included by dual
routers, and will be ignored by IP-only routers. Only IP traffic will
be routed in an pure IP domain. Any OSI traffic may be discarded
(except for the IS-IS packets necessary for operation of the routing
protocol).
In a pure OSI routing domain, all routers must be OSI-capable. OSI-
only routers may be freely mixed with dual routers. Some fields
specifically related to IP operation may be included by dual routers,
and will be ignored by OSI-only routers. Only OSI traffic will be
routed in a pure OSI domain. Any IP traffic may be discarded.
In a dual routing domain, IP-only, OSI-only, and dual routers may be
mixed on a per-area basis. Specifically, each area may itself be
defined to be pure IP, pure OSI, or dual.
In a pure IP area within a dual domain, IP-only and dual routers may
be freely mixed. Only IP traffic can be routed by level 1 routing
within a pure-IP area.
In a pure-OSI area within a dual domain, OSI-only and dual routers
may be freely mixed. Only OSI traffic can be routed by level 1
routing within a pure OSI area.
In a dual area within a dual routing domain only dual routers may be
used. Both IP and OSI traffic can be routed within a dual area.
Within a dual domain, if both IP and OSI traffic are to be routed
between areas then all level 2 routers must be dual.
1.5 Advantages of Using Integrated IS-IS
Use of the integrated IS-IS protocol, as a single protocol for
routing both IP and OSI packets in a dual environment, has
significant advantages over using separate protocols for
independently routing IP and OSI traffic.
An alternative approach is known as "Ships In the Night" (S.I.N.).
With the S.I.N. approach, completely separate routing protocols are
used for IP and for OSI. For example, OSPF [5] may be used for
routing IP traffic, and IS-IS [1] may be used for routing OSI
traffic. With S.I.N., the two routing protocols operate more or less
independently. However, dual routers will need to implement both
routing protocols, and therefore there will be some degree of
competition for resources.
Note that S.I.N. and the integrated IS-IS approach are not really
completely separate options. In particular, if the integrated IS-IS
is used within a routing domain for routing of IP and OSI traffic, it
is still possible to use other independent routing protocols for
routing other protocol suites.
In the future, optional extensions to IS-IS may be defined for
routing other common protocol suites. However, such future options
are outside of the scope of this document. This section will compare
integrated IS-IS and S.I.N. for routing of IP and OSI only.
A primary advantage of the integrated IS-IS relates to the network
management effort required. Since the integrated IS-IS provides a
single routing protocol, within a single coordinated routing domain
using a single backbone, this implies that there is less information
to configure. This combined with a single coordinated MIB simplifies
network management.
Note that the operation of two routing protocols with the S.I.N.
approach are not really independent, since they must share common
resources. However, with the integrated IS-IS, the interactions are
explicit, whereas with S.I.N., the interactions are implicit. Since
the interactions are explicit, again it may be easier to manage and
debug dual routers.
Another advantage of the integrated IS-IS is that, since it requires
only one routing protocol, it uses fewer resources. In particular,
less implementation resources are needed (since only one protocol
needs to be implemented), less CPU and memory resources are used in
the router (since only one protocol needs to be run), and less
network resources are used (since only one set of routing packets
need to be transmitted). Primarily this translates into a financial
savings, since each of these three types of resources cost money.
This implies that dual routers based on the integrated IS-IS should
be less expensive to purchase and operate than dual routers based on
S.I.N.
Note that the operation of two routing protocols with the S.I.N.
approach are not really independent, since they must share common
resources. For example, if one routing protocol becomes unstable and
starts to use excessive resources, the other protocol is likely to
suffer. A bug in one protocol could crash the other. However, with
the integrated IS-IS, the interactions are explicit and are defined
into the protocol and software interactions. With S.I.N., the
interactions are implicit.
The use of a single integrated routing protocol similarly reduces the
likely frequency of software upgrades. Specifically, if you have two
different routing protocols in your router, then you have to upgrade
the software any time EITHER of the protocols change. If you make use
of a single integrated routing protocol, then software changes are
still likely to be needed, but less frequently.
Finally, routing protocols have significant real time requirements.
In IS-IS, these real time requirements have been explicitly
specified. In other routing protocols, these requirements are
implicit. However, in all routing protocols, there are real time
guarantees which must be met in order to ensure correct operation. In
general, it is difficult enough to ensure compliance with real time
requirements in the implementation of a single real time system. With
S.I.N., implementation of two semi-independent real-time protocols in
a single device makes this more difficult.
Note that both integrated IS-IS and S.I.N. allow for independence of
external routes (for traffic from/to outside of the routing domain),
and allow for independent assignment of OSI and TCP/IP addresses.
2 Symbols and Abbreviations
AA Administrative Authority
(a three octet field in the GOSIP version 2.0 NSAP
address format)
AFI Authority and Format Identifier
(the first octet of all OSI NSAP addresses -- identifies
format of the rest of the address)
CLNP Connection-Less Network Protocol
(ISO 8473, the OSI connectionless network layer protocol
-- very similar to IP)
DFI DSP Format Identifier
(a one octet field in the GOSIP version 2.0 NSAP address
format)
ES End System
(The OSI term for a host)
ES-IS End System to Intermediate System Routeing Exchange
Protocol (ISO 9542 -- OSI protocol between routers
and end systems)
ICD International Code Designator
(ISO standard for identifying organizations)
IP Internetwork Protocol
(an Internet Standard Network Layer Protocol)
IS Intermediate System
(The OSI term for a router)
IS-IS Intermediate System to Intermediate System Routeing
Exchange Protocol
(the ISO protocol for routing within a single
routing domain)
IS-IS Hello An Hello packet defined by the IS-IS protocol
(a type of packet used by the IS-IS protocol)
ISH An Hello packet defined by ISO 9542 (ES-IS protocol).
(not the same as IS-IS Hello)
ISO International Organization for Standardization
(an international body which is authorized to write
standards of many kinds)
LSP Link State Packet
(a type of packet used by the IS-IS protocol)
NLPID Network Layer Protocol ID
(A one-octet field identifying a network layer protocol)
NSAP Network Service Access Point
(a conceptual interface point at which the network
service is made available)
SEL NSAP Selector
(the last octet of NSAP addresses, also called NSEL)
OSI Open Systems Interconnection
(an international standard protocol architecture)
RD Routing Domain
(the set of routers and end systems using a single
instance of a routing protocol such as IS-IS)
SNPA Subnetwork Point of Attachment
(a conceptual interface at which a subnetwork service
is provided)
TCP Transmission Control Protocol
(an Internet Standard Transport Layer Protocol)
TCP/IP The protocol suite based on TCP, IP, and related
protocols (the Internet standard protocol
architecture)
3 Subnetwork Independent Functions
3.1 Exchange of Routing Information
The exchange of routing information between routers makes use of the
normal routing packet exchange as defined in the OSI IS-IS routing
spec, with additional IP-specific information added to the IS-IS
routing packets.
The IS-IS protocol provides for the inclusion of variable length
fields in all IS-IS packets. These fields are encoded using a "Code,
Length, Value" triplet, where the code and length are encoded in one
octet each, and the value has the length specified (from 0 to 254
octets). IS-IS requires that: "Any codes in a received PDU that are
not recognised are ignored and passed through unchanged". This
requirement applies to all routers implementing IS-IS, including
OSI-only, IP-only, and dual routers. This allows IP-specific
information to be encoded in a manner which OSI-only routers will
ignore, and also allows OSI-specific information to be encoded in a
manner which IP-only routers will ignore.
IP-capable (i.e., all IP-only and dual) routers need to know what
network layer protocols are supported by other routers in their area.
This information is made available by inclusion of a "protocols
supported" field in all IS-IS Hello and Link State Packets. This
field makes use of the NLPID (Network Layer Protocol Identifier),
which is a one-octet value assigned by ISO to identify network level
protocols. NLPID values have been assigned to ISO 8473 and to IP.
IP-capable routers need to know the IP address of the adjacent
interface of neighboring routers. This is required for sending ICMP
redirects (when an IP-capable router sends an ICMP redirect to a
host, it must include the IP address of the appropriate interface of
the correct next-hop router). This information is made available by
inclusion of the IP interface address in the IS-IS Hello packets.
Specifically, each IS-IS Hello packet contains the IP address(es) of
the interface over which the Hello is transmitted. The IS-IS allows
multiple IP addresses to be assigned to each physical interface.
In some cases, it will be useful for IP-capable routers to be able to
determine an IP address(es) of all other routers at their level
(i.e., for level 1 routers: all other routers in their area; for
level 2 routers: all other level 2 routers in the routing domain).
This is useful whenever an IP packet is to be sent to a router, such
as for encapsulation or for transmission of network management
packets. This information is made available by inclusion of IP
address in LSPs. Specifically, each IS-IS LSP includes one or more IP
addresses of the router which transmits the LSP. An IP-capable router
is required to include at least one of its IP addresses in its LSPs,
and may optionally include several or all of its IP addresses. Where
a single router operates as both a level 1 and a level 2 router, it
is required to include the same IP address(es) in its level 1 and
level 2 LSPs.
IP-capable routers need to know, for any given IP destination
address, the correct route to that destination. Specifically, level 1
routers need to know what IP addresses are reachable from each level
1 router in their area. In addition, level 1 routers need to find
level 2 routers (for traffic to IP addresses outside of their area).
Level 2 routers need to know what IP addresses are reachable
internally (either directly, or via level 1 routing) from other level
2 routers, and what addresses are reachable externally from other
level 2 routers. All of this information is made available by
inclusion of IP reachable address information in the Link State
Packets.
Internal (within the routing domain) and external (outside the
domain) reachability information is announced separately in level 2
LSPs. Reachable IP addresses include a default metric, and may
include multiple TOS-specific metrics. In general, for external
routes, metrics may be of type "internal" (i.e., directly comparable
with internal metrics) or of type "external" (i.e., not comparable
with the internal metric). A route using internal metrics (i.e.,
either announced as "IP internal reachability information", or
announced as "IP external reachability information" with an internal
metric) is always preferred to a route using external metrics (i.e.,
announced as "IP external reachability information", with an external
metric).
The detailed encoding of the IP-specific information included in
routing packets is provided in section 5 (Structure and Encoding of
PDUs).
3.2 Hierarchical Abbreviation of IP Reachability Information
Level 2 routers include in their level 2 LSPs a list of all [IP
address, subnet mask, metric] combinations reachable in their area.
In general, this information may be determined from the level 1 LSPs
from all routers in the area. If we ignore resource constraints, then
it would be permissible for a level 2 router to simply duplicate all
[IP address, subnet mask, metric] entries from all level 1 routers in
its area (with appropriate metric adjustment), for inclusion in its
level 2 LSP. However, in order for hierarchical routing to scale to
large routing domain sizes, it is highly desired to abbreviate the
reachable address information.
This is accomplished by manual configuration of summary addresses.
Each level 2 router may be configured with one or more [IP address,
subnet mask, metric] entries for announcement in their level 2 LSPs.
The set of reachable addresses obtained from level 1 LSPs is compared
with the configured reachable addresses. Redundant information
obtained from level 1 LSPs is not included in level 2 LSPs. Generally
it is expected that the level 2 configured information will specify
more inclusive addresses (corresponding to a subnet mask with fewer
bits set to 1). This will therefore allow one configured
address/submask pair (or a small number of such pairs) to
hierarchically supercede the information corresponding to multiple
entries in level 1 LSPs.
The manually configured addresses are included in level 2 LSPs only
if they correspond to at least one address which is reachable in the
area. For manually configured level 2 addresses, the associated
metric values to announce in level 2 LSPs are also manually
configured. The configured addresses will supercede reachable address
entries from level 1 LSPs based only on the IP address and subnet
mask -- metric values are not considered when determining if a given
configured address supercedes an address obtained from a level 1 LSP.
Any address obtained from a level 1 LSP which is not superceded by
the manually configured information is included in the level 2 LSPs.
In this case, the metric value announced in the level 2 LSPs is
calculated from the sum of the metric value announced in the
corresponding level 1 LSP, plus the distance from the level 2 router
to the appropriate level 1 router. Note: If this sum results in a
metric value greater than 63 (the maximum value that can be reported
in level 2 LSPs), then the value 63 must be used. Delay, expense, and
error metrics (i.e., those TOS metrics other than the default metric)
will be included only if (i) the level 2 router supports the specific
TOS; (ii) the path from the level 2 router to the appropropriate
level 1 router is made up of links which support the specific TOS;
and (iii) the level 1 router which can reach the address directly
also supports the specific TOS for this route, as indicated in its
level 1 LSP.
In general, the same [IP address, subnet mask] pair may be announced
in level 1 LSPs sent by multiple level 1 routers in the same area. In
this case (assuming the entry is not superceded by a manually
configured entry), then only one such entry shall be included in the
level 2 LSP. The metric value(s) announced in level 2 LSPs correspond
to the minimum of the metric value(s) that would be calculated for
each of the level 1 LSP entries.
A level 2 router will have IP addresses which are directly reachable
via its own interfaces. For purposes of inclusion of IP reachable
address information in level 2 LSPs, these "directly reachable"
addresses are treated exactly the same as addresses received in level
1 LSPs.
Manually configured addresses may hierarchically supercede multiple
level 1 reachable address entries. However, there may be some IP
addresses which match the manually configured addresses, but which
are not reachable via level 1 routing. If a level 2 router receives
an IP packet whose IP address matches a manually configured address
which it is including in its level 2 LSP, but which is not reachable
via level 1 routing in the area, then the packet must be discarded.
In this case, an error report may be returned (as specified in RFC
1009), with the reason for discard specifying destination
unreachable.
Figure 2 - An Example Routing Domain (not shown)
An example is illustrated in figure 2. Suppose that the network
number for the entire routing domain is 17 (a class A network).
Suppose each area is assigned a subnet number consisting of the next
8 bits. The area may be further subdivided by assigning the next
eight bits to each LAN in the area, giving each a 24 bit subnet mask
(counting the network and subnet fields). Finally 8 bits are left for
the host field. Suppose that for a particular area (given subnet
number 17.133) there are a number of IP capable level 1 routers
announcing (in the special IP entry in their level 1 LSPs) subnets
17.133.5, 17.133.43, and 17.133.57.
Suppose that in this example, in order to save space in level 2 LSPs,
the level 2 routers in this area are configured to announce subnet
17.133. Only this one address needs to be announced in level 2 LSPs.
Thus if an IP packet comes along for an address in subnet 17.133.5,
17.133.43 or 17.133.57, then other level 2 routers, in other areas,
will know to pass the traffic to this area.
The inclusion of 17.133 in level 2 LSPs means that the three subnet
addresses starting with 17.133 do not all have to be listed
separately in level 2 LSPs.
If any traffic comes along that is for an unreachable address such as
17.133.124.7, then level 2 routers in other areas in this particular
domain will think that this area can handle this traffic, will
forward traffic to level 2 routers in this area, which will have to
discard this traffic.
Suppose that subnet number 17.133.125 was actually reachable via some
other area, such as the lower right hand area. In this case, the
level 2 router in the left area would be announcing (in its level 2
LSPs according to manually configured information) reachability to
subnet 17.133. However, the level 2 router in the lower right area
would be announcing (in its level 2 LSPs according to information
taken from its received level 1 LSPs), reachability to subnet
17.133.125. Due to the use of best match routing, this works
correctly. All traffic from other areas destined to subnet 17.133.125
would be sent to the level 2 router in the lower right area, and all
other traffic to subnet 17.133 (i.e., traffic to any IP address
starting with 17.133, but not starting with 17.133.125) would be sent
to the level 2 router in the leftmost area.
3.3 Addressing Routers in IS-IS Packets
The IS-IS packet formats explicitly require that OSI-style addresses
of routers appear in the IS-IS packets. For example, these addresses
are used to determine area membership of routers. It is therefore
necessary for all routers making use of the IS-IS protocol to have
OSI style addresses assigned. For IP-only routers, these addresses
will be used only in the operation of the IS-IS protocol, and are not
used for any other purpose (such as the operation of EGP, ICMP, or
other TCP/IP protocols).
For OSI-only and dual routers, assignment of NSAP addresses is
straight forward, but is outside of the scope of this specification.
Address assignment mechanisms are being set up by standards bodies
which allow globally unique OSI NSAP addresses to be assigned. All
OSI-only and dual routers may therefore make use of normal OSI
addresses in the operation of the IS-IS protocol.
For IP-only routers, there are two ways in which NSAP addresses may
be obtained for use with the IS-IS protocol.
1) For those environments in which OSI is being used, or in which it
is anticipated that OSI will be used in the future, it is
permissible to obtain NSAP address assignments in the normal
manner, assign normal NSAP addresses to IP-only routers, and use
these addresses in the operation of IS-IS. This approach is
recommended even for pure IP routing domains, as it will simplify
future migration from IP-only to dual operation.
2) In some cases, routers may have only TCP/IP addresses, and it may
be undesireable to have to go through the normal mechanisms for
assignment of NSAP addresses. Instead, an alternate mechanim is
provided below for algorithmically generating a valid OSI style
address from existing IP address and autonomous system number
assignments.
Where desired, for IP-only routers, for use in IS-IS packet formats
only, OSI-style addresses (compatible with the USA GOSIP version 2.0
NSAP address format [9]) may be derived as follows:
AFI 1 octet value "47" (specifies ICD format)
ICD 2 octet value "00 05" (specifies Internet/Gosip)
DFI 1 octet value "xx"
AA 3 octets value "xx xx xx" (specifies special
IP-only use of NSAPs)
Reserved 2 octets must be "00 00"
RD 2 octets contains autonomous system number
Area 2 octets must be assigned as described below
ID 6 octets must be assigned as described below
SEL 1 octet used as described below
The AFI value of "47" and the ICD value of "00 05" specifies the
Gosip Version 2.0 addressing format. The DFI number of "xx" and the
AA of "xx xx xx" specify that this special NSAP address format is
being used, solely for IS-IS packet formats in an IP-only
environment. The reserved field must contain "00 00", as specified in
GOSIP version 2.0.
The routing domain field contains the Autonomous System number.
Strictly speaking, this is not necessary, since the IS-IS packets are
exchanged within a single AS only. However, inclusion of the AS
number in this address format will ensure correct operation in the
event that routers from separate routing domains/ASs are incorrectly
placed on the same link. The AS number in this context is used only
for definition of unique NSAP addresses, and does not imply any
coupling with exterior routing protocols.
The Area field must be assigned by the authority responsible for the
routing domain, such that each area in the routing domain must have a
unique Area value.
The ID must be assigned by the authority responsible for the routing
domain. The ID must be assigned such that every router in the routing
domain has a unique value. It is recommended that one of the
following methods is used:
1)use a unique IEEE 802 48 bit station ID
2)use the value hex "02 00" prepended to an IP address of the router.
IEEE 802 addresses, if used, must appear in IEEE canonical format.
Since the IEEE 802 station IDs are assigned to be globally unique,
use of these values clearly assures uniqueness in the area. Also, all
assigned IEEE 802 station IDs have the global/local bit set to zero.
Prepending the indicated pattern to the front of the IP address
therefore assures that format (2) illustrated above cannot produce
addresses which collide with format (1). Finally, to the extent that
IP addresses are also globally unique, format (2) will produce unique
IDs for routers.
The indicated hex value is specified in IEEE 802 canonical form [10].
In IEEE 802 addresses, the multicast bit is the least significant bit
of the first byte. The global/local bit is the next least significant
bit of the first byte. The indicated prefix therefore sets the
global/local bit to 1, and all other bits in the first two octets to
0.
Note that within an area, whether ISO addresses are configured into
the routers through ISO address assignment, or whether the ISO-style
address is generated directly from the AS number and IP address, all
routers within an area must have the same high order part of address
(AFI, ICD, DFI, AA, RD, and Area). This ISO-style address is used in
IS-IS Hello messages and is the basis by which routers recognize
whether neighbor nodes are in or out of their area.
3.4 External Links
External connectivity (i.e., communications with routers outside of