Network Working Group D. Katz
Request for Comments: 5303 Juniper Networks
Obsoletes: 3373 R. Saluja
Category: Standards Track Tenet Technologies
D. Eastlake 3rd
October 2008 Three-Way Handshake for IS-IS Point-to-Point Adjacencies
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
The IS-IS routing protocol (Intermediate System to Intermediate
System, 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 to the Internet community in order to allow
interoperable implementations to be built by other vendors.
Table of Contents
1. Introduction ....................................................21.1. Terminology ................................................32. Overview of Extensions ..........................................32.1. Handshaking ................................................32.2. More than 256 Interfaces ...................................43. Details .........................................................53.1. Syntax .....................................................53.2. Elements of Procedure ......................................64. IANA Considerations .............................................85. Security Considerations .........................................96. Changes from RFC 3373 ...........................................97. Acknowledgements ................................................98. Normative References ............................................99. Informative References ..........................................91. Introduction
The IS-IS protocol [ISIS] 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 Link State Protocol Data Unit (LSP) refresh period (up to 18
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 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 to remove the
limitation of 255 point-to-point interfaces imposed by IS-IS [ISIS].
This method is more robust than the ad hoc methods currently in use.
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 [RFC2119].
2. Overview of Extensions
This section provides a general overview of the three-way handshaking
provided and how more than 256 interfaces are handled.
The intent is to provide a three-way handshake for point-to-point
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, thus allowing 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 can be one of the following types:
This is the initial point-to-point adjacency three-way state. The
system has not received any IIH packet containing the three-way
handshake option on this point-to-point circuit.
The system has received an IIH packet containing the three-way
handshake option from a neighbor but does not know whether the
neighbor is receiving its IIH packet.
The system knows that the neighbor is receiving its 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 [ISIS]. If this mechanism is supported, then an adjacency
may have two states, its state as defined in ISO 10589 [ISIS], and
its three-way state. For example, according to ISO 10589, receipt of
an Intermediate System Hello (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
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
The mechanism is quite similar to the one defined in the Netware Link
Services Protocol (NLSP) [NetLink], a variant of IS-IS used for
routing Internetwork Packet Exchange (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 [RFC1195] has done exactly
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 implementers have realized that the only
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 pseudo-node LSP ID.
Implementers 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
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.
The detailed syntax and procedures for this IS-IS option are given
A new IS-IS Option type, "Point-to-Point Three-Way Adjacency", is
Type - 0xF0 (decimal 240)
Length - total length of the value field (1 to 17 octets)
No. of Octets
| Adjacency Three-Way State | 1
| Extended Local Circuit ID | 4
| Neighbor System ID | ID Length
| Neighbor Extended Local Circuit ID| 4
Adjacency Three-Way State
The adjacency three-way state of the point-to-point adjacency.
The following values are defined:
0 - Up
1 - Initializing
2 - Down
Extended Local Circuit ID
Unique ID assigned to this circuit when it is created by this
Neighbor System ID
System ID of neighboring Intermediate system if known. The length
of this field is equal to "ID Length" of the IIH PDU described in
"Point-to-point IS to IS hello PDU" (section 9.7 of [ISIS]).
Neighbor Extended Local Circuit ID
Extended Local Circuit ID of the other end of the point-to-point
adjacency if known.
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 SHALL ignore it, and
(of course) SHALL NOT include it in its own IIH packets.
Any system that supports this mechanism MUST include the Adjacency
Three-Way State field in this option. The other fields in this
option SHOULD be included as explained below in section 3.2.
Any system that is able to process this option SHALL follow the
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.
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, because 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 "Receiving ISH PDUs by an intermediate
system" (section 8.2.2 of [ISIS]):
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 "Sending point-to-point IIH PDUs"
(section 8.2.3 of [ISIS]):
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 126.96.36.199.1 introduced later in the document) 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 188.8.131.52.1, "Three-Way Handshake", immediately prior to
"IIH PDU Processing" (section 184.108.40.206 of [ISIS]):
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 220.127.116.11 are followed.
If the option is present and contains invalid Adjacency Three-Way
State, the PDU SHALL be discarded and no further action is taken.
If the option with a valid Adjacency Three-Way State 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 18.104.22.168 are followed with the
a) In section 22.214.171.124 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 SHALL be Down.
b) If the action taken from section 126.96.36.199 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.)
Received Adjacency Three-Way State
Down Initializing Up
Down | Initialize Up Down
Adj. Initializing | Initialize Up Up
Way Up | Initialize Accept Accept
Adjacency Three-Way State Table
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", no event is generated and
the adjacency three-way state SHALL be set to "Initializing".
If the new action is "Up", an adjacencyStateChange(Up) event is
c) Skip section 188.8.131.52 c and d.
d) If the new action is "Initialize", "Up", or "Accept", follow
section 184.108.40.206 e.
4. IANA Considerations
This document specifies IS-IS Option 240 (0xF0), which has already
been allocated. See [RFC3359].
5. Security Considerations
This document raises no new security issues for IS-IS. IS-IS
security may be used to secure the IS-IS messages discussed here.
6. Changes from RFC 3373
This document is a minor edit of [RFC3373] with the intent of
advancing it from Informational to Standards Track. It also updates
the ISP 10589 reference to refer to the current "2002" version.
Thanks to Tony Li, Henk Smit, Naiming Shen, Dave Ward, Jeff Learman,
Les Ginsberg, and Philip Christian for their contributions to the
8. Normative References
[ISIS] ISO, "Intermediate System to Intermediate System intra-
domain routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)",
International Standard 10589:2002, Second Edition, 2002.
[NetLink] "Netware Link Services Protocol Specification, Version
1.0", Novell, Inc., February 1994.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9. Informative References
[RFC3373] Katz, D. and R. Saluja, "Three-Way Handshake for
Intermediate System to Intermediate System (IS-IS) Point-
to-Point Adjacencies", RFC 3373, September 2002.
[RFC3359] Przygienda, T., "Reserved Type, Length and Value (TLV)
Codepoints in Intermediate System to Intermediate System",
RFC 3359, August 2002.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, October 2008.
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