Network Working Group Dave Katz
INTERNET DRAFT Juniper Networks, Inc.
Expiration Date: July 2002 Rajesh Saluja
Nortel Networks, Inc.
January 2002
Three-Way Handshake for IS-IS Point-to-Point Adjacencies
draft-ietf-isis-3way-04.txt
Status of this Memo
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Abstract
The IS-IS routing protocol (ISO 10589) requires reliable protocols at
the link layer for point-to-point links. As a result, it does not use
a three-way handshake when establishing adjacencies on point-to-point
media. This paper defines a backward-compatible extension to the
protocol that provides for a three-way handshake. It is fully
interoperable with systems that do not support the extension.
Additionally, the extension allows the robust operation of more than
256 point-to-point links on a single router.
This extension has been implemented by multiple router vendors; this
paper is provided as information to the Internet community in order
to allow interoperable implementations to be built by other vendors.
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1.0 Introduction
The IS-IS protocol [1] assumes certain requirements stated in ISO
10589 (section 6.7.2) for the operation of IS-IS over point-to-point
links and hence provides only a two-way handshake when establishing
adjacencies on point-to-point links. The protocol does not operate
correctly if these subnetwork requirements for point-to-point links
are not met. The basic mechanism defined in the standard is that each
side declares the other side to be reachable if a Hello packet is
heard from it. Once this occurs, each side then sends a Complete
Sequence Number PDU (CSNP) to trigger database synchronization.
Three failure modes are known. First, if the link goes down and then
comes back up, or one of the systems restarts, and the CSNP packet is
lost, and the network has a cut set of one through the link, the link
state databases on either side of the link will not synchronize for a
full LSP refresh period (up to eighteen hours).
A second, more serious failure, is that if the link fails in only one
direction, the failure will only be detected by one of the systems.
Normally, this is not a serious issue; only one of the two systems
will announce the adjacency in its link state packets, and the SPF
algorithm will thus ignore the link. However, if there are two
parallel links between systems and one of them fails in one
direction, SPF will still calculate paths between the two systems,
and the system that does not notice the failure will attempt to pass
traffic down the failed link (in the direction that does not work).
The third issue is that on some physical layers, the
interconnectivity between endpoints can change without causing a
link-layer-down condition. In this case, a system may receive packets
that are actually destined for a different system (or a different
link on the same system). The receiving system may end up thinking
that it has an adjacency with the remote system when in fact the
remote system is adjacent with a third system.
The solution proposed here ensures correct operation of the protocol
over unreliable point-to-point links. As part of the solution to the
three-way handshaking issue, a method is defined for supporting more
than 256 point-to-point interfaces which is more robust than the ad
hoc methods currently in use.
2.0 Overview of Extensions
2.1 Handshaking
The intent is to provide a three-way handshake for point-to-point
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adjacency establishment in a backward compatible fashion. This is
done by providing an optional mechanism that allows each system to
report its adjacency three-way state; this allows a system to only
declare an adjacency to be up if it knows that the other system is
receiving its IS-IS Hello (IIH) packets.
The adjacency three-way state that is reported by this mechanism is
not equal or equivalent to the adjacency state that is described in
ISO 10589 [1]. If this mechanism is supported then an adjacency may
have two states, its state as defined in ISO 10589[1], and its
three-way state. For example according to ISO 10589 [1] receipt of an
ISH will cause an adjacency to go to Initializing state; however
receipt of an ISH will have no effect on the three-way state of an
adjacency, which remains firmly Down until it receives an IIH from a
neighbor that contains the three-way handshaking option.
In addition, the neighbor's system ID and (newly-defined) extended
circuit ID are reported in order to detect the case where the same
stream is being received by multiple systems (only one of which can
talk back).
The mechanism is quite similar to the one defined in the Netware Link
Services Protocol (NLSP) [2], a variant of IS-IS used for routing IPX
traffic. The difference between this mechanism and the one used in
NLSP is the location where the information is carried (NLSP uses two
of the reserved bits in the IIH header, whereas this solution adds a
separate option to the IIH), and the presence of the neighbor's
system ID and circuit ID. In theory, using the reserved header bits
should be backward compatible, since systems are supposed to ignore
them. However, it was felt that this was risky, as the use of
untested mechanisms such as this have led to problems in the past in
other protocols. New option codes, on the other hand, have been
demonstrated to work properly, as the deployment of Integrated IS-IS
for IP [3] has done exactly this. It is also worth noting that the
encoding used in the NLSP solution does not lend itself to backward
compatibility.
The new mechanism only comes into play when the remote system
includes the new option in its IIH packet; if the option is not
present, it is assumed that the system does not support the new
mechanism, and so the old procedures are used.
2.2 More Than 256 Interfaces
The IS-IS specification has an implicit limit of 256 interfaces, as
constrained by the eight bit Circuit ID field carried in various
packets. Moderately clever implementors have realized that the only
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true constraint is that of 256 LAN interfaces, and for that matter
only 256 LAN interfaces for which a system is the Designated IS. This
is because the only place that the circuit ID is advertised in LSPs
is in the pseudonode LSP ID.
Implementors have treated the point-to-point Circuit ID number space
as being independent from that of the LAN interfaces, since these
Circuit IDs appear only in IIH PDUs and are only used for detection
of a change in identity at the other end of a link. More than 256
point-to-point interfaces have been supported by sending the same
circuit ID on multiple interfaces. This reduces the robustness of the
ID change detection algorithm, since it would then be possible to
switch links between interfaces on a system without detecting the
change.
Since the Circuit ID is an integral part of the new handshaking
mechanism, a backward compatible mechanism for expanding the circuit
ID number space is included in this specification.
3.0 Details
3.1 Syntax
A new IS-IS Option type, "Point-to-Point Three-Way Adjacency", is
defined [4]:
Type = 0xF0 (decimal 240)
Length = 1 to 17 octets
Value:
Adjacency Three-Way State (one octet):
0 = Up
1 = Initializing
2 = Down
Extended Local Circuit ID (four octets)
Neighbor System ID if known (zero to eight octets)
Neighbor Extended Local Circuit ID (four octets, if Neighbor
System ID is present)
Any system that supports this mechanism shall include this option in
its Point-to-Point IIH packets.
Any system that does not understand this option will ignore it, and
(of course) will not include it in its own IIH packets.
Any system that supports this mechanism MUST include Adjacency
Three-Way State field in this option. The other fields in this option
should be included as explained below in section 3.2.
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Any system that is able to process this option shall follow the
procedures below.
3.2 Elements of Procedure
The new handshake procedure is added to the IS-IS point-to-point IIH
state machine after the PDU acceptance tests have been performed.
The existing procedures are only executed if the neighbor is in the
proper state for the adjacency to come up.
Although the extended circuit ID is only used in the context of the
three-way handshake, it is worth noting that it effectively protects
against the unlikely event where a link is moved to another interface
on a system that has the same local circuit ID, as the received PDUs
will be ignored (via the checks defined below) and the existing
adjacency will fail.
Add a clause e) to the end of section 8.2.2 of [1]:
Set the state to be reported in the Adjacency Three-Way State field
of the Point-to-Point Three-Way Adjacency option to Down."
Add a clause e) to the end of section 8.2.3 of [1]:
The IS shall include the Point-to-Point Three-Way Adjacency option
in the transmitted Point-to-Point IIH PDU. The current three-way
state of the adjacency with its neighbor on the link (as defined in
new section 8.2.4.1.1) shall be reported in the Adjacency Three-Way
State field. If no adjacency exists, the state shall be reported as
Down.
The Extended Local Circuit ID field shall contain a value assigned
by this IS when the circuit is created. This value shall be unique
among all the circuits of this Intermediate System. The value is
not necessarily related to that carried in the Local Circuit ID
field of the IIH PDU.
If the system ID and Extended Local Circuit ID of the neighboring
system are known (in adjacency three-way state Initializing or Up),
the neighbor's system ID shall be reported in the Neighbor System
ID field, and the neighbor's Extended Local Circuit ID shall be
reported in the Neighbor Extended Local Circuit ID field.
Add a section 8.2.4.1.1, Three-Way Handshake, immediately prior to
section 8.2.4.2:
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A received Point-to-Point IIH PDU may or may not contain the
Point-to-Point Three-Way Adjacency option. If it does not, the link
is assumed to be functional in both directions, and the procedures
described in section 8.2.4.2 are followed.
If the option is present, the Neighbor System ID and Neighbor
Extended Local Circuit ID fields, if present, shall be examined. If
they are present, and the Neighbor System ID contained therein does
not match the local system's ID, or the Neighbor Extended Local
Circuit ID does not match the local system's extended circuit ID,
the PDU shall be discarded and no further action is taken.
If the Neighbor System ID and Neighbor Extended Local Circuit ID
fields match those of the local system, or are not present, the
procedures described in section 8.2.4.2 are followed with following
changes:
a) In section 8.2.4.2 a and b, the action "Up" from state tables
5, 6, 7 and 8 may create a new adjacency but the three-way
state of the adjacency will be Down.
b) If the action taken from section 8.2.4.2 a or b is "Up" or
"Accept", the IS shall perform the action indicated by the
new adjacency three-way state table below, based on the
current adjacency three-way state and the received Adjacency
Three-Way State value from the option. (Note that the
procedure works properly if neither field is ever included.
This provides backward compatibility to an earlier version of
this option.)
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Received Adjacency Three-Way State
Down Initializing Up
-------------------------------------------------
Down | Initialize Up Down
|
adj Initializing | Initialize Up Up
three |
-way Up | Initialize Accept Accept
state |
|
If the new action is "Down", an adjacencyStateChange(Down)
event is generated with the reason "Neighbor restarted" and the
adjacency shall be deleted.
If the new action is "Initialize", the adjacency three-way state
shall be set to "Initializing".
If the new action is "Up", an adjacencyStateChange(Up)
event is generated.
c) Skip section 8.2.4.2 c and d.
d) If the new action is "Initialize", "Up" or "Accept", follow section
8.2.4.2 e.
4.0 References
[1] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)", ISO/IEC 10589:1992.
[2] "Netware Link Services Protocol Specification, Version 1.0",
Novell, Inc., February 1994.
[3] Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
December 1990.
[4] Tony Przygienda, "Reserved TLV Codepoints in ISIS", Work in
Progress, July 2001
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Acknowledgements
The authors would like to thank Tony Li, Henk Smit, Naiming Shen,
Dave Ward, Jeff Learman, Les Ginsberg and Philip Christian for their
contributions to this document.
Author's Addresses:
Dave Katz
Juniper Networks
385 Ravendale Drive
Mountain View, CA 94043 USA
Phone: +1 650 526 8073
Email: dkatz@juniper.net
Rajesh Saluja
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821 USA
Phone: +1 978 288 7791
Email: rsaluja@nortelnetworks.com
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