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Versions: 00 01                                                         
opsawg                                                       P. Lapukhov
Internet-Draft                                                  Facebook
Intended status: Standards Track                          March 18, 2016
Expires: September 19, 2016


           Data-plane probe for in-band telemetry collection
                   draft-lapukhov-dataplane-probe-00

Abstract

   Detecting and isolating network faults in IP networks has
   traditionally been done using tools like ping and traceroute (see
   [RFC7276]) or more complex systems built on similar concepts of
   active probing and path tracing.  While using active synthetic probes
   is proven to be helpful in detecting data-plane faults, isolating
   fault location has proven to be a much harder problem, especially in
   diverse networks with multiple active forwarding planes (e.g.  IP and
   MPLS).  Moreover, existing end-to-end tools do not generally support
   functionality beyond dealing with packet loss - for example, they are
   hardly useful for detecting and reporting transient (i.e. milli- or
   even micro-second) network congestion.

   Modern network forwarding hardware can enable more sophisticated
   data-plane functionality that provides substantial improvement to the
   isolation and identification capabilities of network elements.  For
   example, it has become possible to encode a snapshot of a network
   elements forwarding state within the packet payload as it transits
   the device.  One example of such device/network state would be queue
   depth on the egress port taken by that specific packet.  When
   combined with a unique device identifier embedded in the same packet,
   this could allow for precise time and topological identification of
   the the congested location within the network.

   This document proposes a standard format for embedding telemetry
   information in UDP-based probing packets, i.e. packets designated for
   testing the network while not carrying application traffic.  These
   active probes could be conveyed over multiple protocols (ICMP, UDP,
   TCP, etc.) but this document specifically focuses on UDP, given its
   simple semantics.  In addition this document provides recommendations
   on handling the active probes by devices that do not support the
   required data-plane functionality.

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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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 19, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Data plane probe  . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Probe transport . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Probe structure . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Header Format . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Telemetry Record Template . . . . . . . . . . . . . . . .   7
     2.5.  Telemetry Record  . . . . . . . . . . . . . . . . . . . .   8
   3.  Telemetry Record Types  . . . . . . . . . . . . . . . . . . .   9
     3.1.  Device Identifier . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Timestamp . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Queueing Delay  . . . . . . . . . . . . . . . . . . . . .  10
     3.4.  Ingress/Egress Port IDs . . . . . . . . . . . . . . . . .  11
     3.5.  Forwarding Information  . . . . . . . . . . . . . . . . .  11
       3.5.1.  IPv6 Route  . . . . . . . . . . . . . . . . . . . . .  12
       3.5.2.  IPv4 Route  . . . . . . . . . . . . . . . . . . . . .  12
       3.5.3.  MPLS Route  . . . . . . . . . . . . . . . . . . . . .  12
   4.  Operating in loopback mode  . . . . . . . . . . . . . . . . .  13
   5.  Processing Probe Packet . . . . . . . . . . . . . . . . . . .  14
     5.1.  Detecting a probe . . . . . . . . . . . . . . . . . . . .  14



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   6.  Non-Capable Devices . . . . . . . . . . . . . . . . . . . . .  14
   7.  Handling data-plane probes in the MPLS domain . . . . . . . .  14
   8.  Multi-chip device considerations  . . . . . . . . . . . . . .  15
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Detecting and isolating faults in IP networks may involve multiple
   tools and approaches, but by far the two most popular utilities used
   by operators are ping and traceroute.  The ping utility provides the
   basic end-to-end connectivity check by sending a special ICMP packet.
   There are other variants of ping that work using TCP or UDP probes,
   but may require a special responder application (for UDP) on the
   other end of the probed connection.

   This type of active probing approach has its limitations.  First, it
   operates end-to-end and thus it is impossible to tell where in the
   path the fault has happened from simply observing the packet loss
   ratios.  Secondly, in multipath (ECMP) scenarios it can be quite
   difficult to fully and/or deterministically exercise all the possible
   paths connecting two end-points.

   The traceroute utility has multiple variants as well - UDP, ICMP and
   TCP based, for instance, and special variant for MPLS LSP testing.
   Practically all variants follow the same model of operations: varying
   TTL field setting in outgoing probes and analyzing the returned ICMP
   unreachable messages.  This does allow isolating the fault down to
   the IP hop that is losing packets, but has its own limitations.  As
   with the ping utility, it becomes complicated to explore all possible
   ECMP paths in the network.  This is especially problematic in large
   Clos fabric topologies that are very common in large data-center
   networks.  Next, many network devices limit the rate of outgoing ICMP
   messages as well as the rate of "exception" packets "punted" to the
   control plane processor.  This puts a functional limit on the packet
   rate that the traceroute can probe a given hop with, and hence
   impacts the resolution and time to isolate a fault.  Lastly, the
   treatment for these control packets is often different from the
   packets that take regular forwarding path: the latter are normally
   not redirected to the control plane processor and handled purely in
   the data-plane hardware.

   Modern network processing elements (both hardware and software based)
   are capable of packet handling beyond basic forwarding and simple



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   header modifications.  Of special interest is the ability to capture
   and embed instantaneous state from the network element and encode
   this state directly into the transit packet.  One example would be to
   record the transit device's name, ingress and egress port
   identifiers, queue depths, timestamps and so on.  By collecting this
   state along each network device in the path, it becomes trivial to
   trace a probe's path through the network as well as record transit
   device characteristics.  Extending this model, one could build a tool
   that combines the useful properties of ping and traceroute using a
   single packet flight through the network, without the constraints of
   control plane (aka "slow path") processing.  To aid in the
   development of such tooling, this document defines a format for
   embedding telemetry information in the body of active probing
   packets.

2.  Data plane probe

   This section defines the structure of the active data-plane probe
   packets.

2.1.  Probe transport

   This document assumes the use of IP/UDP for data-plane probing
   (either IPv4 or IPv6).  A receiving application may listen on a pre-
   defined UDP port to collect and possibly echo back the information
   embedded in the probe.  One potential limitation to this methodology
   is the size of the probe packet, as some data-plane faults may only
   impact packets of a given size or range of sizes.  In this case, the
   data-plane probe may not be able to detect such issues, given the
   requirement to pre-allocate storage in the packet body.

2.2.  Probe structure

   The sender is responsible for constructing a packet large enough to
   hold all records to be added by the network elements.  Concurrently,
   the probes must not exceed the minimum MTU allowed along the path, so
   it is assumed that the sender either knows the needed MTU or relies
   on well-known mechanisms for path MTU discovery.  After adding the
   mandatory protocol (IP, UDP, etc.) headers, the packet payload is
   built according to the following layout:











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   +---------------------------------------------------------+
   | Header                                                  |
   +---------------------------------------------------------+
   | Telemetry Record template                               |
   +---------------------------------------------------------+
   | Placeholder for telemetry record 1                      |
   +---------------------------------------------------------+
   | Placeholder for telemetry record 2                      |
   +---------------------------------------------------------+
   .                                                         .
   .                                                         .
   .                                                         .
   +---------------------------------------------------------+
   | Placeholder for telemetry record N                      |
   +---------------------------------------------------------+

                          Figure 1: Probe layout

   Notice that all record placeholders are equal size, as prescribed by
   the telemetry record template, and that space for those must be pre-
   allocated by the sender of the packet.  Each record corresponds to a
   single network element on the path from sender to receiver of the
   packet.

2.3.  Header Format

   The probe payload starts with a fixed-size header.  The header
   identifies the packet as a probe packet, and encodes basic
   information shared by all telemetry records.






















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Probe Marker (1)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Probe Marker (2)                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Version Number        |       Must Be Zero        |S|O|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Message Type |   Hop Limit   |          Must Be Zero         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sender's Handle                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Write Offset                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 2: Header Format

   (1)  The "Probe Marker" fields are arbitrary 32-bit values generally
        used by the network elements to identify the packet as a probe
        packet.  These fields should be interpreted as unsigned integer
        values, stored in network byte order.  For example, a network
        element may be configured to recognize a UDP packet destined to
        port 31337 and having 0xDEAD 0xBEEF as the values in "Probe
        Marker" field as an active probe, and treat it respectively.

   (2)  "Version Number" is currently set to 1.

   (3)  The "Global Flags" field is 8 bits, and defines the following
        flags:

        (1)  "Overflow" (O-bit) (least significant bit).  This bit is
             set by the network element if there is no record
             placeholder available: i.e. the packet is already "full" of
             telemetry information.

        (2)  "Sealed" (S-bit).  This bit instructs the network element
             to forward the packet WITHOUT embedding telemetry data,
             even if it matches the probe identification rules.  This
             mechanism could be used to send "realistic" probes of
             arbitrary size after the network path associated with the
             combination of source/destination IP addresses and ports
             has been previously established.  The network element must
             not inspect the "Telemetry Record Template" field for
             "sealed" probes.




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   (4)  The "Message Type" field value could be either "1" - "Probe" or
        "2" - "Probe Reply"

   (5)  "Hop Limit" is defined only for "Message Type" of "1" ("Probe").
        For "Probe Reply" the "Hop Limit" field must be set to zero.
        This field is treated as an integer value and decremented by
        every network element in the path as "Probe" propagates.  See
        the Section 4 section on the intended use of the field.

   (6)  The "Sender's Handle" field is set by the sender to allow the
        receiver to identify a particular originator of probe packets.
        Along with "Sequence Number" it allows for tracking of packet
        order and loss within the network.

   (7)  The "Write Offset" field specifies the offset for the next
        telemetry record to be written in the probe packet body.  It
        counts from the start of the packet body and must be initially
        set to the first octet after the "Record Template" field.  It
        must be incremented by every network element that adds a
        telemetry record, without overflowing the storage.  This
        simplifies the work for the subsequent network element - it just
        needs to parse the template and then add the data at the "Write
        Offset".

2.4.  Telemetry Record Template

   The following figure defines the "Record Template".  This template
   uses type-length fields to describe the telemetry data records as
   added by network elements.  The most significant bit in the "Type"
   field must be set to zero.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      TL record count (N)      |          Must Be Zero         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|           Type 1            |            Length 1           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|           Type 2            |            Length 2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|           Type N            |           Length N            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: Record Template



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2.5.  Telemetry Record

   This section defines the structure of a telemetry record.  Every
   network element capable of reporting inband telemetry data must add a
   record as defined in the "Record Template" to the probe packet.  The
   new record must be inserted at the "Write Offset" position in the
   packet payload, with the "Write Offset" subsequenly incremented by
   the size of the new record.  The order of TLV elements must follow
   the order prescribed by the Figure 3 portion of the probe packet.
   The most significant bit in the type field ("S-bit") must be set to
   "1" if the network element was able to understand and record the
   requested telemetry type.  That bit must be set to zero otherwise,
   along with the contents of the "Value" field.  The length field is
   the TLV field length including the "Type" and "Length" fields.

   If writing a new telemetry record to the packet body would cause it
   to exceed the packet size, no record is added and the overflow
   "O-bit" must be set to "1" in the probe header.

































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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|           Type 1            |           Length 1            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                             Value 1                           .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|           Type 2            |            Length 2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                             Value 2                           .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                                               .
   .                                                               .
   .                                                               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|           Type N            |             Length N          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                             Value N                           .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 4: Telemetry Record Format

3.  Telemetry Record Types

   This section defines some of the telemetry record types that could be
   supported by the network elements.

3.1.  Device Identifier

   This is used to identify the device reporting telemetry information.
   This document does not prescribe any specific identifier format.








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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 1 (Device ID)      |           Length =  12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Device ID (1)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Device ID (2)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 5: Device Identifier

3.2.  Timestamp

   This telemetry record encodes the time that the packet enters and
   leaves the device, in UTC.  The "entering" time is recorded when the
   L2 header enters the processing pipeline.  The "exit" time is
   recorded when the network elements starts serializing L2 header on
   egress port.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 10 (Timestamp)     |           Length =  28        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Receive Seconds                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Receive Microseconds                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Receive Nanoseconds                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Send Seconds                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Send Microseconds                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Send Nanoseconds                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 6: Timestamp

3.3.  Queueing Delay

   Encodes the amount of time that the frame has spent queued in the
   network element.  This is only recorded if packet has been queued,
   and defines the time spent in memory buffers.  This could be helpful
   to detect queueing-related delays in the network.  In case of the
   cut-through switching operation this must be set to zero.




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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|  Type = 11 (Queueing Delay) |           Length =  16        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Seconds                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Microseconds                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Nanoseconds                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 7: Queueing Delay

3.4.  Ingress/Egress Port IDs

   This record stores the ingress and egress physical ports used to
   receive and send packet respectively.  Here, "physical port" means a
   unit with actual MAC and PHY devices associated - not any logical
   subdivision based, for example, on protocol level tags (e.g.  VLAN).
   The port identifiers are opaque, and defined as 32-bit entries.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 12 (Port IDs)      |           Length =  12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Ingress Port ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Egress Port ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 8: Ingress/Egress Port IDs

3.5.  Forwarding Information

   Records defined in this section require the network element to store
   forwarding information that was used to direct the packet to the
   next-hop.  In the network that uses multiple forwarding plane
   implementations (e.g.  IP and MPLS) the originator of the probe is
   required to populate the record template with all kinds of forwarding
   information it expects in the path.  The network elements then
   populate the entries they know about, e.g. in IPv4-only network the
   "IPv6 Route" record will be left unfilled, and so will be "MPLS
   Route".






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3.5.1.  IPv6 Route

   This record stores the IPv6 route that has been used for packet
   forwarding.  If not used, then S-bit is set to zero, along with the
   value field.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 20 (IPv6 Route)    |           Length =  24        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      ECMP group size    |  ECMP group index   | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv6 Address (1)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv6 Address (2)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv6 Address (3)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv6 Address (4)                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 9: IPv6 Route

3.5.2.  IPv4 Route

   This record stores the IPv4 route that has been used for packet
   forwarding.  If not used, then S-bit is set to zero, along with the
   value field.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 21 (IPv4 Route)    |           Length =  12        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     ECMP group size   |  ECMP group index     | Prefix Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       IPv4 Address                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 10: IPv4 Route

3.5.3.  MPLS Route

   This record stores the MPLS label mapping that has been used for
   packet forwarding.  It is possible that inbound or outbound label set
   set to zero, if it was not used (e.g. on ingress or egress of the
   domain).  At the edge of IP2MPLS or MPLS2IP domain it is expected



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   that the device would fill in the "MPLS Route" telemetry record along
   with the corresponding "IPv6 Route" or "IPv4 Route" records.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |S|   Type = 22 (MPLS Route)    |           Length =  16        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Operation   |      ECMP group size    |  ECMP group index   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Must Be Zero     |         Incoming MPLS Label           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Must Be Zero     |         Outgoing MPLS Label           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 11: MPLS Route

   There are three MPLS operations defined

      "1" - Push

      "2" - Pop

      "3" - Swap

4.  Operating in loopback mode

   In "loopback" mode the flow of probes is "turned back" at a given
   network element.  The network element that "turns" packets around is
   identified using the "Hop Limit" field.  The network element that
   receives a "Probe" type packet having "Hop Limit" value of "1" is
   required to perform the following:

      Change the "Message Type" field to "Probe Reply" and set the "Hop
      Limit" to zero.

      Swap the destination/source addresses and port values in the IP/
      UDP headers of the probe packet.

      Add a telemetry record as required using the newly build IP/UDP
      headers to determine forwarding information.

   This way, the original probe is routed back to originator.  Notice
   that the return path may be different from the path that the original
   probe has taken.  This path will be recorded by the network elements
   as the reply is transported back to the sender.  Using this technique
   one may progressively test a path until its breaking point.  Unlike




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   the traditional traceroute utility, however, the returning packets
   are the original probes, not the ICMP messages.

5.  Processing Probe Packet

5.1.  Detecting a probe

   Since the probe looks like a regular UDP packet, the data-plane
   hardware needs a way to recognize it for special processing.  This
   document does not prescribe a specific way to do that.  For example,
   classification could be based on only the destination UDP port, or
   using more complex pattern matching techniques, e.g matching on the
   contents of "Probe Marker" field.

6.  Non-Capable Devices

   Non-capable devices are those that cannot process a probe natively in
   the fast-path data plane.  Further, there could be two types of such
   devices: those that can still process it via the control-plane
   software, and those that can not.  The control-plane processing
   should be triggered by use of the "Router-Alert" option for IPv4 of
   IPv6 packets (see [RFC2113] or [RFC2711]) added by the originator of
   the probe.  A control-plane capable device is expected to interpret
   and fill-in as much telemetry-record data as it possibly could, given
   the limited abilities.

   Network elements that are not capable of processing the data-plane
   probes are expected to perform regular packet forwarding.  If a
   network element receives a packet with the router-alert option set,
   but has no special configuration to detect such probes, it should
   process it according to [RFC6398].  Absence of the router alert
   option leaves the non dataplane-capable devices with the only option
   of processing the probe using traditional forwarding.

7.  Handling data-plane probes in the MPLS domain

   In general, the payload of an MPLS packet is opaque to the network
   element.  However, in many cases the network element still performs a
   lookup beyond the MPLS label stack, e.g. to obtain information such
   as L4 ports for load balancing.  It may be possible to perform data-
   plane probe classification in the same manner, additionally using the
   "Probe Marker" to distinguish the probe packets.

   In accordance to [RFC6178] Label Edge Routers (LERs) are required not
   to impose an MPLS router-alert label for packets carrying the router-
   alert option.  It may be beneficial to enable such translation, so
   that an end-to-end validation could be performed if a control-plane
   capable MPLS network element is present on the probe's path.



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8.  Multi-chip device considerations

   TBD

9.  IANA Considerations

   None

10.  Acknowledgements

   The author would like to thank L.J.  Wobker and Changhoom Kim for
   reviewing and providing valuable comments for the initial version of
   this document.

11.  References

11.1.  Normative References

   [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113,
              DOI 10.17487/RFC2113, February 1997,
              <http://www.rfc-editor.org/info/rfc2113>.

   [RFC2711]  Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
              RFC 2711, DOI 10.17487/RFC2711, October 1999,
              <http://www.rfc-editor.org/info/rfc2711>.

   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
              2011, <http://www.rfc-editor.org/info/rfc6398>.

   [RFC6178]  Smith, D., Mullooly, J., Jaeger, W., and T. Scholl, "Label
              Edge Router Forwarding of IPv4 Option Packets", RFC 6178,
              DOI 10.17487/RFC6178, March 2011,
              <http://www.rfc-editor.org/info/rfc6178>.

11.2.  Informative References

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,
              <http://www.rfc-editor.org/info/rfc7276>.

Author's Address







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   Petr Lapukhov
   Facebook
   1 Hacker Way
   Menlo Park, CA  94025
   US

   Email: petr@fb.com












































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