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Network Working Group                                                 Tony Li
INTERNET DRAFT                                               Procket Networks
                                                                    Henk Smit
                                                             Procket Networks
                                                               September 2000


                IS-IS extensions for Traffic Engineering

                    <draft-ietf-isis-traffic-02.txt>


Status

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 except that the right to
   produce derivative works is not granted.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   The list of Internet-Draft Shadow Directories can be accessed at
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1.0 Abstract

   This document describes extensions to the IS-IS protocol to support
   Traffic Engineering [1].  The IS-IS protocol is specified in [2],
   with extensions for supporting IPv4 specified in [3].

   This document extends the IS-IS protocol by specifying new
   information that a Intermediate System (IS) [router] can place in
   Link State Protocol Data Units (LSPs).  This information describes
   additional information about the state of the network that is useful
   for traffic engineering computations.


2.0 Introduction

   An IS-IS LSP is composed of a fixed header and a number of tuples,
   each consisting of a Type, a Length, and a Value.  Such tuples are
   commonly known as TLVs, and are a good way of encoding information in
   a flexible and extensible format.

   The changes in this document include the design of new TLVs to
   replace the existing IS Neighbor TLV, IP Reachability TLV and add
   additional information.  Mechanisms and procedures to migrate to the
   new TLVs are not discussed in this document.

   The primary goal of these extensions is to add more information about
   the characteristics of a particular link to an IS-IS's LSP.
   Secondary goals include increasing the dynamic range of the IS-IS
   metric and improving the encoding of IP prefixes.  The router id is
   useful for traffic engineering purposes because it describes a single
   address that can always be used to reference a particular router.

   This document is a publication of the IS-IS Working Group within the
   IETF, and is a contribution to ISO IEC JTC1/SC6, for eventual
   inclusion with ISO 10589.


3.0 The Traffic Engineering router ID TLV


   The Traffic Engineering router ID TLV is TLV type 134.

   The router ID TLV contains the 4-octet router ID of the router
   originating the LSP.  This is useful in several regards:

   For traffic engineering, it guarantees that we have a single stable
   address that can always be referenced in a path that will be
   reachable from multiple hops away, regardless of the state of the
   node's interfaces.

   If OSPF is also active in the domain, traffic engineering can compute
   the mapping between the OSPF and IS-IS topologies.

   If a router advertises the Traffic Engineering router ID TLV in its
   LSP, and if it advertises BGP routes with the BGP next hop attribute
   set to the BGP router ID, in that case the Traffic Engineering router
   ID should be the same as the BGP router ID.

   Implementations MUST NOT inject a /32 prefix for the router ID into
   their forwarding table, because this can lead to forwarding loops
   when interacting with systems that do not support this TLV.


4.0 The extended IP reachability TLV


   The extended IP reachability TLV is TLV type 135.

   The existing IP reachability TLV is a single TLV that carries IP
   prefixes in a format that is analogous to the IS neighbor TLV.  It
   carries four metrics, of which only the default metric is commonly
   used.  Of this, the default metric has a possible range of 0-63.
   This limitation is one of the restrictions that we would like to
   lift.

   In addition, route redistribution (a.k.a. route leaking) is a key
   problem that is not addressed by the existing IP reachability TLV.
   This problem occurs when an IP prefix is injected into a level one
   area, redistributed into level 2, subsequently redistributed into a
   second level one area, and then redistributed from the second level
   one area back into level two.  This problem occurs because the path
   that the information can take forms a loop.  The likely result is a
   forwarding loop.

   To address these issues, the proposed extended IP reachability TLV
   provides for a 32 bit metric and adds one bit to indicate that a
   prefix has been redistributed 'down' in the hierarchy.

   The proposed extended IP reachability TLV contains a new data
   structure, consisting of:
        4 bytes of metric information
        1 byte of control information, consisting of
             1 bit of up/down information
             1 bit indicating the existence of sub-TLVs
             6 bits of prefix length
        0-4 bytes of IPv4 prefix
        0-250 optional octets of sub-TVLs, if present consisting of
             1 octet of length of sub-TLVs
             0-249 octets of sub-TLVs

   This data structure can be replicated within the TLV, not to exceed
   the maximum length of the TLV.

   The up/down bit shall be 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.
   Prefixes that have the up/down bit set to 1 must not be
   redistributed.  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.

   These semantics apply even if IS-IS is extended in the future to have
   additional levels.  By insuring that prefixes follow only the IS-IS
   hierarchy, we have insured that the information does not loop,
   thereby insuring that there are no persistent forwarding loops.

   If there are no sub-TLVs associated with this IP prefix, the bit
   indicating the presence of sub-TVLs shall be set to 0.  If this bit
   is set to 1, the first octet after the prefix will be interpreted as
   the length of sub-TLVs. Please note that while the encoding allows
   for 255 octets of sub-TLVs, the maximum value cannot fit in the
   overall extended IP reachability TLV. The practical maximum is 255
   octets minus the 5-9 octets described above, or 250 octets.  No sub-
   TLVs for the extended IP reachability TLV have been defined yet.

   The 6 bits of prefix length can have the values 0-32 and indicate the
   number of significant bits in the prefix.  The prefix is encoded in
   the minimal number of bytes for the given number of significant bits.
   This implies:

           Significant bits                Bytes
           0                               0
           1-8                             1
           9-16                            2
           17-24                           3
           25-32                           4

   The remaining bits of prefix are transmitted as zero and ignored upon
   receipt.

   If an IP prefix is advertised with a metric larger then
   MAX_PATH_METRIC (0xFE000000, see below), this IP prefix should not be
   considered during the normal SPF computation. This will allow
   advertisment of an IP prefix for other purposes than building the
   normal IP routing table.


5.0 The extended IS reachability TLV


   The extended IS reachability TLV is TLV type 22.

   The existing IS reachability TLV is a single TLV that contains
   information about a series of IS neighbors.  For each neighbor, there
   is a structure that contains the default metric, the delay, the
   monetary cost, the reliability, and the 7-octet ID of the adjacent
   neighbor.  Of this information, the default metric is commonly used.
   The default metric is currently one octet, with one bit used to
   indicate that the metric is present and one bit used to indicate
   whether the metric is internal or external.  The remaining 6 bits are
   used to store the actual metric, resulting a possible metric range of
   0-63.  This limitation is one of the restrictions that we would like
   to lift.

   The remaining three metrics (delay, monetary cost, and reliability)
   are not commonly implemented and reflect unused overhead in the TLV.
   The neighbor is identified by its system Id (typically 6-octets),
   plus one octet to indicate the pseudonode number if the neighbor is
   on a LAN interface.  Thus, the existing TLV consumes 11 octets per
   neighbor, with 4 octets for metric and 7 octets for neighbor
   identification.  To indicate multiple adjacencies, this structure is
   repeated within the IS reachability TLV.  Because the TLV is limited
   to 255 octets of content, a single TLV can describe up to 23
   neighbors.  The IS reachability TLV can be repeated within the LSP
   fragments to describe further neighbors.

   The proposed extended IS reachability TLV contains a new data
   structure, consisting of
        7 octets of system Id and pseudonode number
        3 octets of default metric
        1 octet of length of sub-TLVs
        0-244 octets of sub-TLVs

   Thus, if no sub-TLVs are used, the new encoding requires 11 octets
   and can contain up to 23 neighbors.  Please note that while the
   encoding allows for 255 octets of sub-TLVs, the maximum value cannot
   fit in the overall IS reachability TLV.  The practical maximum is 255
   octets minus the 11 octets described above, or 244 octets.  Further,
   there is no defined mechanism for extending the sub-TLV space for a
   particular neighbor.  Thus, wasting sub-TLV space is discouraged.

   The metric octets are encoded as a 24-bit unsigned integer. Note that
   the metric field in the new extended IP reachability TLV is encoded
   as a 32-bit unsigned integer. These different sizes were chosen so
   that it is very unlikely that the cost of an intra-area route has to
   be chopped off to fit in the metric field of an inter-area route.

   To preclude overflow within an SPF implementation, all metrics
   greater than or equal to MAX_PATH_METRIC shall be considered to have
   a metric of MAX_PATH_METRIC.  It is easiest to select MAX_PATH_METRIC
   such that MAX_PATH_METRIC plus a single link metric does not overflow
   the number of bits for internal metric calculation.  We assume that
   this is 32 bits.  Thus, MAX_PATH_METRIC is 4,261,412,864 (0xFE000000,
   2^32 - 2^25).

   If a link is advertised with the maximum link metric (2^24 - 1), this
   link should not be considered during the normal SPF computation.
   This will allow advertisment of a link for other purposes than
   building the normal Shortest Path Tree. An example is a link that is
   available for traffic engineering, but not for hop-by-hop routing.

   Certain sub-TLVs are proposed here:
       Sub-TLV type   Length (octets)  Name
           3               4           Administrative group (color)
           6               4           IPv4 interface address
           8               4           IPv4 neighbor address
           9               4           Maximum link bandwidth
           10              4           Reservable link bandwidth
           11              32          Unreserved bandwidth
           18              3           TE Default metric
           250-254                     Reserved for cisco specific extensions
           255                         Reserved for future expansion

   Each of these sub-TLVs is described below.  Unless stated otherwise,
   multiple occurrences of the information are supported by multiple
   inclusions of the sub-TLV.


5.1 Sub-TLV 3: Administrative group (color, resource class)


   The administrative group sub-TLV contains a 4-octet bit mask assigned
   by the network administrator.  Each set bit corresponds to one
   administrative group assigned to the interface.

   By convention the least significant bit is referred to as 'group 0',
   and the most significant bit is referred to as 'group 31'.


5.2 Sub-TLV 6: IPv4 interface address


   This sub-TLV contains a 4-octet IPv4 address for the interface
   described by the (main) TLV.  This sub-TLV can occur multiple times.

   If the interface being advertised for Traffic Engineering purposes is
   unnumbered, the IPv4 interface address sub-TLV is set to the router
   ID of the advertising router. In combination with the IPv4 neighbor
   address sub-TLV this identifies the unnumbered link over which the
   advertised adjacency has been established.

   Implementations MUST NOT inject a /32 prefix for the interface
   address into their routing or forwarding table, because this can lead
   to forwarding loops when interacting with systems that do not support
   this sub-TLV.

   If a router implements the basic TLV extensions in this document, it
   is free to add or omit this sub-TLV to the description of an
   adjacency.  If a router implements traffic engineering, it must
   include this sub-TLV.





5.3 Sub-TLV 8: IPv4 neighbor address


   This sub-TLV contains a single IPv4 address for a neighboring router
   on this link.  This sub-TLV can occur multiple times.

   If the interface being advertised for Traffic Engineering purposes is
   unnumbered, the first two octets of the IPv4 neighbor address sub-TLV
   are set to zero and the next two octets are set to the interface ID
   of the unnumbered interface. In combination with the IPv4 interface
   address sub-TLV this identifies the unnumbered link over which the
   advertised adjacency has been established.

   Implementations MUST NOT inject a /32 prefix for the neighbor address
   into their routing or forwarding table, because this can lead to
   forwarding loops when interacting with systems that do not support
   this sub-TLV.

   If a router implements the basic TLV extensions in this document, it
   is free to add or omit this sub-TLV to the description of an
   adjacency.  If a router implements traffic engineering, it must
   include this sub-TLV on point-to-point adjacencies.


5.4 Sub-TLV 9: Maximum link bandwidth


   This sub-TLV contains the maximum bandwidth that can be used on this
   link in this direction (from the system originating the LSP to its
   neighbors). This is useful for traffic engineering.

   The maximum link bandwidth is encoded in 32 bits in IEEE floating
   point format. The units are bytes (not bits!) per second.


5.5 Sub-TLV 10: Maximum reservable link bandwidth


   This sub-TLV contains the maximum amount of bandwidth that can be
   reserved in this direction on this link.  Note that for
   oversubscription purposes, this can be greater than the bandwidth of
   the link.

   The maximum reservable link bandwidth is encoded in 32 bits in IEEE
   floating point format. The units are bytes (not bits!) per second.


5.6 Sub-TLV 11: Unreserved bandwidth


   This sub-TLV contains the amount of bandwidth reservable on this
   direction on this link.  Note that for oversubscription purposes,
   this can be greater than the bandwidth of the link.

   Because of the need for priority and preemption, each head end needs
   to know the amount of reserved bandwidth at each priority level.
   Thus, this sub-TLV contains eight 32 bit IEEE floating point numbers.
   The units are bytes (not bits!) per second.  The values correspond to
   the bandwidth that can be reserved with a holding of priority 0
   through 7, arranged in increasing order with priority 0 occurring at
   the start of the sub-TLV, and priority 7 at the end of the sub-TLV.

   For stability reasons, rapid changes in the values in this sub-TLV
   should not cause rapid generation of LSPs.


5.7 Sub-TLV 18: Traffic Engineering Default metric


   This sub-TLV contains a 24-bit unsigned integer.  This metric is
   administratively assigned and can be used to present a differently
   weighted topology to traffic engineering SPF calculations.

   To preclude overflow within an SPF implementation, all metrics
   greater than or equal to MAX_PATH_METRIC shall be considered to have
   a metric of MAX_PATH_METRIC.  It is easiest to select MAX_PATH_METRIC
   such that MAX_PATH_METRIC plus a single link metric does not overflow
   the number of bits for internal metric calculation.  We assume that
   this is 32 bits.  Thus, MAX_PATH_METRIC is 4,261,412,864 (0xFE000000,
   2^32 - 2^25).

   If a link is advertised without this sub-TLV, traffic engineering SPF
   calculations must use the normal default metric of this link, which
   is advertised in the fixed part of the extended IS reachability TLV.


6.0 Security Considerations

   This document raises no new security issues for IS-IS.


7.0 Acknowledgments

   The authors would like to thank Yakov Rekhter and Dave Katz for their
   comments on this work.


8.0 References

   [1] RFC 2702, "Requirements for Traffic Engineering Over MPLS," D.
   Awduche, J. Malcolm, J. Agogbua, M. O'Dell, and J. McManus, September
   1999.

   [2] ISO 10589, "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)" [Also republished as RFC 1142]

   [3] RFC 1195, "Use of OSI IS-IS for routing in TCP/IP and dual
   environments", R.W. Callon, Dec. 1990


9.0 Authors' Addresses

   Tony Li
   Procket Networks, Inc.
   3850 North First Street
   San Jose, CA 95134
   Email: tli@procket.com
   Voice: +1 408 9547900
   Fax: +1 408 9876166

   Henk Smit
   Procket Networks, Inc.
   3850 North First Street
   San Jose, CA 95134
   Email: henk@procket.com
   Voice: +1 408 9547900
   Fax: +1 408 9876166


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