Avoiding Traffic Black-Holes for Route Aggregation in IS-IS
draft-chen-isis-black-hole-avoid-01

Network Working Group                                            Z. Chen
Internet-Draft                                                     X. Xu
Intended status: Standards Track                     Huawei Technologies
Expires: March 17, 2018                               September 13, 2017

      Avoiding Traffic Black-Holes for Route Aggregation in IS-IS
                  draft-chen-isis-black-hole-avoid-01

Abstract

   When the Intermediate System to Intermediate System (IS-IS) routing
   protocol is adopted by a highly symmetric network such as the Leaf-
   Spine or Fat-Tree network, the Leaf nodes (e.g., Top of Rack switches
   in datacenters) are recommended to be prevented from receiving other
   nodes' explicit routes in order to achieve scalability.  However,
   such a setup would cause traffic black-holes or suboptimal routing if
   link failure happens in the network.  This document extends IS-IS to
   solve this problem.

Requirements Language

   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 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 17, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Description . . . . . . . . . . . . . . . . . . . . .   3
   3.  IS-IS Extensions  . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  TLV Encoding  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Unreachable Prefixes Advertisement  . . . . . . . . . . .   5
   4.  Alternative Solution  . . . . . . . . . . . . . . . . . . . .   6
   5.  IPv6 Support  . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   When running the Intermediate System to Intermediate System (IS-IS)
   routing protocol in a highly symmetric network such as the Leaf-Spine
   or Fat-Tree network, the Leaf nodes (e.g., Top of Rack switches in
   datacenters) are recommended to be prevented from receiving other
   nodes' explicit routes in order to achieve scalability, as proposed
   in [IS-IS-SL-Extension], [IS-IS-Overhead-Reduction], [RIFT], and
   [OpenFabric].  In particular, each Leaf node SHOULD simply maintain a
   default (or aggregated) route (e.g., 0.0.0.0/0) in its routing table,
   of which the next hop SHOULD be an Equal Cost Multi Path (ECMP) group
   including all Spines nodes that the Leaf node connects to.  However,
   such a setup would cause traffic black-holes or suboptimal routing if
   link failure happens in the network, since the Leaf nodes are not
   aware of any topology information.

   To solve this problem, this document extends IS-IS to advertise
   unreachable prefixes, which are defined as the prefixes that a
   default (or aggregated) route's next hop can no longer reach.  When
   link failure happens between a Spine node and a Leaf node, the Spine
   node SHOULD advertise all prefixes attached to the Leaf node (i.e.,
   the unreachable prefixes) to every other Leaf node it connects to.
   On receiving the unreachable prefixes, each Leaf node SHOULD add the

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   unreachable prefixes to its routing table, thus avoiding traffic
   black-holes and suboptimal routing.

2.  Problem Description

   This section illustrates why link failure would cause traffic black-
   hole or suboptimal routing when Leaf nodes only maintain default (or
   aggregated) routes.

           +--------+          +--------+          +--------+
           | Spine1 |          | Spine2 |          | Spine3 |
           +-+-+-+-++          +-+-+-+-++          +-+-+-+-++
      +------+ | | |             | | | |             | | | |
      | +------|-|-|-------------+ | | |             | | | X
      | | +----|-|-|---------------|-|-|-------------+ | | X
      | | |    | | |        +------+ | |               | | X
      | | |    | | |        | +------|-|---------------+ | |
      | | |    | | |        | |      | |                 | |
      | | |    | | |        | |      | |                 | |
      | | |    | | |        | |      | |         +-------+ +-----+
      | | |    | | |        | |      | +---------|-------------+ |
      | | |    | | |        | |      +---------+ |             | |
      | | |    | | +--------|-|----------------|-|-----------+ | |
      | | |    | +----------|-|--------------+ | |           | | |
      | | |    +----------+ | |              | | |           | | |
    +-+-+-+-+           +-+-+-+-+          +-+-+-+-+       +-+-+-+-+
    | Leaf1 |           | Leaf2 |          | Leaf3 |       | Leaf4 |
    +-------+           +-------+          +-------+       +-------+
                                                            |     |
                                                           ---   ---
                                                       prefixA   prefixB

                      Figure 1: Topology Example

   Figure 1 shows a Spine-Leaf topology example where Leaf1 to Leaf4 are
   connected to Spine1 to Spine3, and prefixA and prefixB are attached
   to Leaf4.  To achieve scalability, as proposed in [IS-IS-SL-
   Extension], [IS-IS-Overhead-Reduction], [RIFT], and [OpenFabric],
   Leaf1 to Leaf4 SHOULD NOT receive explict routes from each other nor
   the Spine nodes.  Instead, each of them maintains a default (or
   aggregated) route (e.g., 0.0.0.0/0) in the routing table, of which
   the next hop is an ECMP group including Spine1, Spine2, and Spine3.
   Flows from one Leaf node to another are shared among Spine1, Spine2,
   and Spine3 based on the well known 5-tuple hashing.

   However, such a setup would cause traffic black-hole or suboptimal
   routing when link failure happens in the network.  For example, if

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   the link between Spine3 and Leaf4 is broken, Leaf1, Leaf2, and Leaf3
   could not get aware of the failure.  As a result, these Leaf nodes
   will still send a portion of traffic destined for prefixA or prefixB
   toward Spine3, which makes the traffic be discarded at Spine3,
   causing traffic black-hole.  On the other hand, if there is a higher
   tier of switches interconnecting Spine1, Spine2, and Spine3, the
   traffic will be steered up to the higher-tier switches by Spine3,
   causing suboptimal routing.

   Therefore, this document extends IS-IS to advertise unreachable
   prefixes thus solving this problem.

3.  IS-IS Extensions

3.1.  TLV Encoding

   This document introduces one IS-IS TLV to advertise unreachable
   prefixes, called the IP Unreachability TLV, which SHOULD be carried
   in the IS-IS Link State Packet (LSP).  The format of the IP
   Unreachability TLV is shown as follow:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type (1 octet)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Length (1 octet)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved (1 octet)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Prefix Length (1 octet)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Prefix (1 or 2 or 3 or 4 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Sub-TLV Length (1 octet)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Optional Sub-TLVs (variable)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                ......               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Prefix Length (1 octet)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Prefix (1 or 2 or 3 or 4 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Sub-TLV Length (1 octet)      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Optional Sub-TLVs (variable)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of this TLV are defined as follows:

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      Type: TBD.

      Length: Length of the Value field of the TLV.

      Reserved: Bits reserved for future usage.

      Prefix Length: The value can be 0 to 32, indicating the number of
      effective bits in the Prefix field.

      Prefix: Encoding the unreachable prefix in the minimal number of
      octets for the given number of effective bits (i.e., the Prefix
      Length field).  The remaining bits of prefix SHOULD be set zero
      and ignored upon receipt.

      Sub-TLV Length: Length of Sub-TLVs.

      Sub-TLVs: Optional Sub-TLVs for future extension.

   Note that the last four fields can appear repeatedly.

3.2.  Unreachable Prefixes Advertisement

   When link failure happens between a Spine node and a Leaf node, the
   Spine node SHOULD 1) encode all prefixes attached to the Leaf node
   (i.e., the unreachable prefixes) into the IP Unreachability TLV, 2)
   append the IP Unreachability TLV to the IS-IS LSP, and 3) send the
   LSP to every other Leaf node it connects to.

   When a Leaf node receives unreachable prefixes (contained in a LSP)
   advertised by a Spine node, it SHOULD install each of the unreachable
   prefixes into its routing table, of which the next hop SHOULD be set
   an ECMP group including all Spine nodes it connects to except the one
   who advertises the unreachable prefix.

   For example, if the link between Spine3 and Leaf4 in Figure 1 is
   broken, Spine3 SHOULD advertise prefixA and prefixB to Leaf1, Leaf2,
   and Leaf3, by sending them an IS-IS LSP containing the IP
   Unreachability TLV.  On receiving the LSP, Leaf1, Leaf2, and Leaf3
   SHOULD install prefixA and prefixB into their routing tables, and the
   next hop of prefixA or prefixB SHOULD be set an ECMP group including
   Spine1 and Spine2.  For instance, the routing table of Leaf1 before
   and after the link failure is shown in Figure 2 and Figure 3,
   respectively.

   Note that the mechanism described above could achieve minimal
   signaling latency, which helps to avoid black-hole or suboptimal
   routing rapidly when link failure happens.

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   +-----------+-----+---+----+-----+-------+--------------+
   |Destination|Proto|Pre|Cost|Flags|NextHop|Interface     |
   +-----------+-----+---+----+-----+-------+--------------+
   |0.0.0.0/0  |ISIS |15 |20  |D    |Spine1 |Ethernet0/0/0 |
   |           |ISIS |15 |20  |D    |Spine2 |Ethernet0/0/1 |
   |           |ISIS |15 |20  |D    |Spine3 |Ethernet0/0/2 |
   +-----------+-----+---+----+-----+-------+--------------+

     Figure 2: Routing Table of Leaf1 before link failure

   +-----------+-----+---+----+-----+-------+--------------+
   |Destination|Proto|Pre|Cost|Flags|NextHop|Interface     |
   +-----------+-----+---+----+-----+-------+--------------+
   |0.0.0.0/0  |ISIS |15 |20  |D    |Spine1 |Ethernet0/0/0 |
   |           |ISIS |15 |20  |D    |Spine2 |Ethernet0/0/1 |
   |           |ISIS |15 |20  |D    |Spine3 |Ethernet0/0/2 |
   +-----------+-----+---+----+-----+-------+--------------+
   |prefixA    |ISIS |15 |20  |D    |Spine1 |Ethernet0/0/0 |
   |           |ISIS |15 |20  |D    |Spine2 |Ethernet0/0/1 |
   +-----------+-----+---+----+-----+-------+--------------+
   |prefixB    |ISIS |15 |20  |D    |Spine1 |Ethernet0/0/0 |
   |           |ISIS |15 |20  |D    |Spine2 |Ethernet0/0/1 |
   +-----------+-----+---+----+-----+-------+--------------+

     Figure 3: Routing Table of Leaf1 after link failure

4.  Alternative Solution

   The unreachable prefixes can alternatively be encoded as a new Sub-
   TLV of the Extended IP Reachability TLV defined in [RFC 5305].  The
   format of the Sub-TLV is shown as follow:

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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type (1 octet)          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Length (1 octet)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Reserved (1 octet)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Prefix Length (1 octet)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Prefix (1 or 2 or 3 or 4 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               ......                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Prefix Length (1 octet)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Prefix (1 or 2 or 3 or 4 octets)  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields of this Sub-TLV are defined as follows:

      Type: TBD.

      Length: Length of the Value field of the Sub-TLV.

      Reserved: Bits reserved for future usage.

      Prefix Length: The value can be 0 to 32, indicating the number of
      effective bits in the Prefix field.

      Prefix: Encoding the unreachable prefix in the minimal number of
      octets for the given number of effective bits (i.e., the Prefix
      Length field).  The remaining bits of prefix SHOULD be set zero
      and ignored upon receipt.

   Note that the last two fields can appear repeatedly.

   When link failure happens between a Spine node and a Leaf node, the
   Spine node SHOULD 1) encode all prefixes attached to the Leaf node
   (i.e., the unreachable prefixes) into the Sub-TLV described above, 2)
   encode the Sub-TLV into the Extended IP Reachability TLV, 3) append
   the Extended IP Reachability TLV to the IS-IS LSP, and 4) send the
   LSP to every other Leaf node it connects to.  The Prefix field of the
   Extended IP Reachability TLV SHOULD be set the default (or
   aggregated) route that each of the Leaf nodes already maintains.

   When a Leaf node receives unreachable prefixes (contained in a LSP)
   advertised by a Spine node, it SHOULD install each of the unreachable
   prefixes into its routing table, of which the next hop SHOULD be set

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   an ECMP group including all Spine nodes it connects to except the one
   who advertises the unreachable prefix.

5.  IPv6 Support

   Will be completed in the next version of the document.

6.  IANA Considerations

   TBD.

7.  Security Considerations

   TBD.

8.  Acknowledgements

   TBD.

9.  References

   [IS-IS-Overhead-Reduction]
              Chen, Z. and X. Xu, "Overheads Reduction for IS-IS Enabled
              Spine-Leaf Networks", draft-chen-isis-sl-overheads-
              reduction-01 (work in progress) , July 2017.

   [IS-IS-SL-Extension]
              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-04 (work in progress) , June 2017.

   [OpenFabric]
              White, R. and S. Zandi, "OpenFabric", draft-white-
              openfabric-02 (work in progress) , April 2017.

   [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,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305 , October 2008.

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   [RIFT]     Przygienda, T., Sharma, A., Drake, J., and A. Atlas,
              "RIFT: Routing in Fat Trees", draft-przygienda-rift-02
              (work in progress) , June 2017.

Authors' Addresses

   Zhe Chen
   Huawei Technologies
   No. 156 Beiqing Rd
   Beijing  100095
   China

   Email: chenzhe17@huawei.com

   Xiaohu Xu
   Huawei Technologies
   No. 156 Beiqing Rd
   Beijing  100095
   China

   Email: xuxiaohu@huawei.com

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