Internet Engineering Task Force Christian E. Hopps
INTERNET-DRAFT NextHop Technologies, Inc.
Expires January 2001 10 July 2000
Routing IPv6 with IS-IS
<draft-ietf-isis-ipv6-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Abstract
This draft specifies a method for exchanging IPv6 routing information
using the IS-IS routing protocol [0]. The method utilizes the same
mechanisms described in RFC 1195 [1]. This is accomplished by adding
2 new TLVs and defining their use. These new TLVs are patterned from
the ones described in "IS-IS extensions for Traffic Engineering" [2].
Just as in RFC 1195 [1] with IPv4 and OSI, this method allows one to
route both IPv4 and IPv6 using a single intra-domain routing
protocol.
1. Terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. Overview
IS-IS is an extendible intra-domain routing protocol. Each router in
the routing domain issues an LSP that contains information pertaining
to that router. The LSP contains typed variable length data often
referred to as TLVs (type-length-values). We extend the protocol
with 2 new TLVs to carry information required to perform IPv6
routing.
In [1] a method is described to route both OSI and IPv4. We utilize
this same method with some minor changes to allow for IPv6. To do so
we must define 2 new TLVs, namely "IPv6 Reachability" and "IPv6
Interface Address" and a new IPv6 protocol identifier. In our new
TLVs we utilize the extended metrics and up/down semantics of [2].
3. IPv6 Reachability TLV
The "IPv6 Reachability" TLV is TLV type 236 (0xEC).
[1] defines 2 Reachability TLVs, "IP Internal Reachability
Information" and "IP External Reachability Information". We provide
the equivalent IPv6 data with the "IPv6 Reachability" TLV and an
"external" bit.
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The "IPv6 Reachability" TLV describes network reachability through
the specification of a routing prefix, metric information, a bit to
indicate if the prefix is being advertised down from a higher level,
a bit to indicate if the prefix is being distributed from another
routing protocol and optionally the existence of sub-TLVs to allow
for later extension. This data is represented by the following
structure:
4 octets of metric information
1 octet of control information
1 bit of up/down information
1 bit indicating external origination
(e.g., from another routing protocol)
1 bit indicating the existence of sub-TLVs
5 reserved bits which must be zero and ignored
1 octet of prefix length
0-16 octets of prefix
0-249 optional octets of sub-TLVs, if present consisting of
1 octet of length of sub-TLVs
0-248 octets of sub-TLVs
This structure may appear any number of times (including none) within
the TLV.
As is described in [2], "the up/down bit is set to 0 when a prefix is
first injected into IS-IS. If a prefix is redistributed from a
higher level to a lower level (e.g., level two to level one), the bit
shall be set to 1 to indicate that the prefix has travelled down the
hierarchy. If a prefix is redistributed from an area to another area
at the same level then the up/down bit shall be set to 1."
If the prefix was distributed into IS-IS from another routing
protocol the external bit shall be set to 1. This information is
useful when distributing prefixes from IS-IS to other protocols.
If the sub-TLV bit is set to 0 then the optional octets of sub-TLVs
are not present. Otherwise the bit is 1 and the octet following the
prefix will contain the length of the sub-TLV portion of the
structure.
The prefix is "packed" in the data structure. That is, only the
required number of octets of prefix are present. This number can be
computed from the prefix length octet as follows:
prefix octets = integer of ((prefix length + 7) / 8)
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Just as in [2], if a prefix is advertised with a metric larger than
MAX_V6_PATH_METRIC (0xFE000000), this prefix must not be considered
during the normal SPF computation. This will allow advertisement of
a prefix for purposes other than building the normal IPv6 routing
table.
4. IPv6 Interface Address TLV
The "IPv6 Interface Address" TLV is TLV type 232 (0xE8).
This TLV maps directly to [1]'s "IP Interface Address" TLV. We
necessarily modify the contents to be 0-15 16 octet IPv6 interface
addresses instead of 0-63 4 octet IPv4 interface address.
We further restrict the semantics of this TLV depending on where it
is advertised. For Hello PDUs the "Interfaces Address" TLV must
contain only the link-local IPv6 addresses assigned to the interface
which is sending the Hello. For LSPs the "Interfaces Address" TLVs
must contain only the non-link-local IPv6 addresses assigned to the
IS.
5. IPv6 NLPID
The value of the IPv6 NLPID is 142 (0x8E).
As with [1] and IPv4, if the IS supports IPv6 routing using IS-IS, it
must advertise this in the "NLPID" TLV by adding the IPv6 NLPID.
6. Operation
We utilize the same changes to [1] as made in [2] for the processing
of prefix information. These changes are both related to the SPF
calculation.
Since the metric space has been extended we need to redefine the
MAX_PATH_METRIC (1023) from the original specification in [1]. This
new value MAX_V6_PATH_METRIC is the same as in [2] (0xFE000000). If
during the SPF a path metric would exceed MAX_V6_PATH_METRIC it shall
be considered to be MAX_V6_PATH_METRIC.
The order of preference between paths for a given prefix must be
modified to consider the up/down bit. The new order of preference is
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as follows (from best to worst).
1. Level 1 up prefix
2. Level 2 up prefix
3. Level 2 down prefix
4. Level 1 down prefix
If multiple paths have the same best preference then selection occurs
based on metric. Any remaining multiple paths should be considered
for equal-cost multi-path routing if the router supports this, otherwise
the router can select any one of the multiple paths.
7. Encapsulation of IS-IS PDUs
There is no method currently defined for encapsulation of IS-IS PDUs
in IPv6 packets. Based on further study, future versions of this
draft may specify an optional method for encapsulating IS-IS PDUs in
IPv6 packets.
8. Implementations
An implementation of this draft has been completed in GateD.
9. Security Considerations
This document raises no new security considerations.
10. References
[0] "Intermediate System to Intermediate System Intra-Domain Routeing
Exchange Protocol for use in Conjunction with the Protocol for
Providing the Connectionless-mode Network Service (ISO 8473)", ISO
10589, 1992.
[1] Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
Environments", RFC 1195, December 1990.
[2] Smit, H., and T. Li, "IS-IS extensions for Traffic Engineering",
Work in Progress, May 1999.
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11. Author's Address
Christian E. Hopps
NextHop Technologies, Inc.
4251 Plymouth Road,
Building 1, Suite 2000.
Ann Arbor, MI 48105
Phone: +1 734 936 0291
Email: chopps@djinesys.com
12. Full Copyright Statement
Copyright (C) The Internet Society 2000. All Rights Reserved.
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