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Internet Engineering Task Force   Inter-Domain Multicast Routing Working Group
INTERNET-DRAFT                                                       W. Fenner
draft-ietf-idmr-traceroute-ipm-02.txt                               Xerox PARC
                                                                     S. Casner
                                                              Precept Software
                                                             November 21, 1997
                                                            Expires April 1998


               A ''traceroute'' facility for IP Multicast.

Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups.  Note that other groups may also distribute working
documents as Internet Drafts.

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Distribution of this document is unlimited.

                                Abstract

     This draft describes the IGMP multicast traceroute facility.  As
     the deployment of IP multicast has spread, it has become clear that
     a method for tracing the route that a multicast IP packet takes
     from a source to a particular receiver is absolutely required.
     Unlike unicast traceroute, multicast traceroute requires a special
     packet type and implementation on the part of routers.  This
     specification describes the required functionality.

This document is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force.  Comments are
solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s).





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1.  Key Words

     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
     [Bradner97].

2.  Introduction

The unicast "traceroute" program allows the tracing of a path from one
machine to another, using a mechanism that already existed in IP.
Unfortunately, no such existing mechanism can be applied to IP multicast
paths. The key mechanism for unicast traceroute is the ICMP TTL exceeded
message, which is specifically precluded as a response to multicast
packets.  Thus, we specify the multicast "traceroute" facility to be
implemented in multicast routers and accessed by diagnostic programs.
While it is a disadvantage that a new mechanism is required, the
multicast traceroute facility can provide additional information about
packet rates and losses that the unicast traceroute cannot, and
generally requires fewer packets to be sent.

Goals:

+    To be able to trace the path that a packet would take from some
     source to some destination.

+    To be able to isolate packet loss problems (e.g., congestion).

+    To be able to isolate configuration problems (e.g., TTL threshold).

+    To minimize packets sent (e.g. no flooding, no implosion).

3.  Overview

Tracing from a source to a multicast destination is hard, since you
don't know down which branch of the multicast tree the destination lies.
This means that you have to flood the whole tree to find the path from
one source to one destination.  However, walking up the tree from
destination to source is easy, as all existing multicast routing
protocols know the previous hop for each source.  Tracing from
destination to source can involve only routers on the direct path.

The party requesting the traceroute (which need be neither the source
nor the destination) sends a traceroute Query packet to the last-hop
multicast router for the given destination.  The last-hop router turns
the Query into a Request packet by adding a response data block
containing its interface addresses and packet statistics, and then
forwards the Request packet via unicast to the router that it believes



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is the proper previous hop for the given source and group.  Each hop
adds its response data to the end of the Request packet, then unicast
forwards it to the previous hop.  The first hop router (the router that
believes that packets from the source originate on one of its directly
connected networks) changes the packet type to indicate a Response
packet and sends the completed response to the response destination
address.  The response may be returned before reaching the first hop
router if a fatal error condition such as "no route" is encountered
along the path.

Multicast traceroute uses any information available to it in the router
to attempt to determine a previous hop to forward the trace towards.
Multicast routing protocols vary in the type and amount of state they
keep; multicast traceroute endeavors to work with all of them by using
whatever is available.  For example, if a DVMRP router has no active
state for a particular source but does have a DVMRP route, it chooses
the parent of the DVMRP route as the previous hop.  If a PIM-SM router
is on the (*,G) tree, it chooses the parent towards the RP as the
previous hop.  In these cases, no source/group-specific state is
available, but the path may still be traced.

4.  Multicast Traceroute header

The header for all multicast traceroute packets is as follows:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    IGMP Type  |    # hops     |           checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Multicast Group Address                      |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                     Source Address                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Destination Address                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Response Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    resp ttl   |               Query ID                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.  IGMP Type: 8 bits

     The IGMP type field is defined to be 0x1F for traceroute queries
     and requests.  The IGMP type field is changed to 0x1E when the
     packet is completed and sent as a response from the first hop
     router to the querier.  Two codes are required so that multicast



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     routers won't attempt to process a completed response in those
     cases where the initial query was issued from a router or the
     response is sent via multicast.

4.2.  # hops: 8 bits

     This field specifies the maximum number of hops that the requester
     wants to trace.  If there is some error condition in the middle of
     the path that keeps the traceroute request from reaching the
     first-hop router, this field can be used to perform an expanding-
     length search to trace the path to just before the problem.

4.3.  Checksum: 16 bits

     The checksum is the 16-bit one's complement of the one's complement
     sum of the whole IGMP message (the entire IP payload).  For
     computing the checksum, the checksum field is set to zero.  When
     transmitting packets, the checksum MUST be computed and inserted
     into this field.  When receiving packets, the checksum MUST be
     verified before processing a packet.

4.4.  Group address

     This field specifies the group address to be traced, or zero if no
     group-specific information is desired.  Note that non-group-
     specific traceroutes may not be possible with certain multicast
     routing protocols.

4.5.  Source address

     This field specifies the IP address of the multicast source for the
     path being traced, or 0xFFFFFFFF if no source-specific information
     is desired.  Note that non-source-specific traceroutes may not be
     possible with certain multicast routing protocols.

4.6.  Destination address

     This field specifies the IP address of the multicast receiver for
     the path being traced.  The trace starts at this destination and
     proceeds toward the traffic source.

4.7.  Response Address

     This field specifies where the completed traceroute response packet
     gets sent.  It can be a unicast address or a multicast address, as
     explained in section 6.2.





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4.8.  resp ttl: 8 bits

     This field specifies the TTL at which to multicast the response, if
     the response address is a multicast address.

4.9.  Query ID: 24 bits

     This field is used as a unique identifier for this traceroute
     request so that duplicate or delayed responses may be detected and
     to minimize collisions when a multicast response address is used.

5.  Definitions

Since multicast traceroutes flow in the opposite direction to the data
flow, we always refer to "upstream" and "downstream" with respect to
data, unless explicitly specified.

Incoming Interface
     The interface on which traffic is expected from the specified
     source and group.

Outgoing Interface
     The interface on which traffic is forwarded from the specified
     source and group towards the destination.  Also called the
     "Reception Interface", since it is the interface on which the
     multicast traceroute Request was received.

Previous-Hop Router
     The router, on the Incoming Interface, which is responsible for
     forwarding traffic for the specified source and group.





















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6.  Response data

Each router adds a "response data" segment to the traceroute packet  be-
fore it forwards it on.  The response data looks like this:

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Query Arrival Time                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Incoming Interface Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Outgoing Interface Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Previous-Hop Router Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Input packet count on incoming interface            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Output packet count on outgoing interface           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Total number of packets for this source-group pair       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |               |M| |           |               |
| Rtg Protocol  |    FwdTTL     |B|S| Src Mask  |Forwarding Code|
|               |               |Z| |           |               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     6.1.  Query Arrival Time

          The Query Arrival Time is a 32-bit NTP timestamp specifying
          the arrival time of the traceroute request packet at this
          router.  The 32-bit form of an NTP timestamp consists of the
          middle 32 bits of the full 64-bit form; that is, the low 16
          bits of the integer part and the high 16 bits of the
          fractional part.

          The following formula converts from a UNIX timeval to a 32-bit
          NTP timestamp:

          query_arrival_time = (tv.tv_sec + 32384) << 16 + ((tv.tv_usec
          << 10) / 15625)

          The constant 32384 is the number of seconds from Jan 1, 1900
          to Jan 1, 1970 truncated to 16 bits.  ((tv.tv_usec << 10) /
          15625) is a reduction of ((tv.tv_usec / 100000000) << 16).




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     6.2.  Incoming Interface Address

          This field specifies the address of the interface on which
          packets from this source and group are expected to arrive, or
          0 if unknown.

     6.3.  Outgoing Interface Address

          This field specifies the address of the interface on which
          packets from this source and group flow to the specified
          destination, or 0 if unknown.

     6.4.  Previous-Hop Router Address

          This field specifies the router from which this router expects
          packets from this source.  This may be a multicast group if
          the previous hop is not known because of the workings of the
          multicast routing protocol.  However, it should be 0 if the
          incoming interface address is unknown.

     6.5.  Input packet count on incoming interface

          This field contains the number of multicast packets received
          for all groups and sources on the incoming interface, or
          0xffffffff if no count can be reported.

     6.6.  Output packet count on outgoing interface

          This field contains the number of multicast packets that have
          been transmitted for all groups and sources on the outgoing
          interface, or 0xffffffff if no count can be reported.

     6.7.  Total number of packets for this source-group pair

          This field counts the number of packets from the specified
          source forwarded by this router to the specified group, or
          0xffffffff if no count can be reported.  If the S bit is set,
          the count is for the source network, as specified by the Src
          Mask field.  If the S bit is set and the Src Mask field is 63,
          indicating no source-specific state, the count is for all
          sources sending to this group.

     6.8.  Rtg Protocol: 8 bits

          This field describes the routing protocol in use between this
          router and the previous-hop router.  Specified values include:





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          1   DVMRP
          2   MOSPF
          3   PIM
          4   CBT
          5   PIM using special routing table
          6   PIM using a static route
          7   DVMRP using a static route


     6.9.  FwdTTL: 8 bits

          This field contains the TTL that a packet is required to have
          before it will be forwarded over the outgoing interface.

     6.10.  MBZ: 1 bit

          Must be zeroed on transmission and ignored on reception.

     6.11.  S: 1 bit

          If this bit is set, it indicates that the packet count for the
          source-group pair is for the source network, as determined by
          masking the source address with the Src Mask field.

     6.12.  Src Mask: 6 bits

          This field contains the number of 1's in the netmask this
          router has for the source (i.e. a value of 24 means the
          netmask is 0xffffff00).  If the router is forwarding solely on
          group state, this field is set to 63 (0x2f).

     6.13.  Forwarding Code: 8 bits

          This field contains a forwarding information/error code.
          Defined values include:

          0x00        No error
          0x01
                      Traceroute request arrived on an interface to
                      which this router would not forward for this
                      source,group,destination.
          0x02
                      This router has sent a prune upstream which
                      applies to the source and group in the traceroute
                      request.
          0x03
                      This router has stopped forwarding for this source
                      and group in response to a request from the next
                      hop router.




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          0x04
                      The group is subject to administrative scoping at
                      this hop.
          0x05        This router has no route for the source.
          0x06        This router is not the proper last-hop router.
          0x07
                      This router is not forwarding this source,group
                      for an unspecified reason.
          0x08        Reached Rendez-vous Point or Core
          0x09
                      Traceroute request arrived on the expected RPF
                      interface for this source,group.
          0x0A
                      Traceroute request arrived on an interface which
                      is not enabled for multicast.
          0x81
                      There was not enough room to insert another
                      response data block in the packet.
          0x82
                      The previous hop router does not understand
                      traceroute requests.
          0x83        Traceroute is administratively prohibited.


          Note that if a router discovers there is not enough room in a
          packet to insert its response, it puts the 0x81 error code in
          the previous router's Forwarding Code field, overwriting any
          error the previous router placed there.  It is expected that a
          multicast traceroute client, upon receiving this error, will
          restart the trace at the last hop listed in the packet.

          The 0x80 bit of the Forwarding Code is used to indicate a
          fatal error.  A fatal error is one where the router may know
          the previous hop but cannot forward the message to it.

     7.  Router Behavior

     All of these actions are performed in addition to (NOT instead of)
     forwarding the packet, if applicable.  E.g. a multicast packet that
     has TTL remaining MUST be forwarded normally, as should a unicast
     packet that has TTL remaining and is not addressed to this router.

     7.1.  Traceroute Query

          A traceroute Query message is a traceroute message with no
          response blocks filled in, and uses IGMP type 0x1F.

     7.1.1.  Packet Verification

          Upon receiving a traceroute Query message, a router must
          examine the Query to see if it is the proper last-hop router
          for the destination address in the packet.  It is the proper
          last-hop router if it has a multicast-capable interface on the
          same subnet as the Destination Address and is the router that



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          would forward traffic from the given source onto that subnet.

          A router may receive a traceroute Query message via either
          unicast or multicast.  If received via multicast and it
          determines that it is not the proper last-hop router, the
          packet MUST be silently dropped.  If received via unicast and
          the packet was addressed to this router, an error code of 0x06
          should be noted and normal processing should occur.

          Duplicate Query messages as identified by the tuple (IP
          Source, Query ID) SHOULD be ignored.

     7.1.2.  Normal Processing

          When a router receives a traceroute Query and it determines
          that it is the proper last-hop router, it treats it like a
          traceroute Request and performs the steps listed under Normal
          Processing of a Traceroute Request, below.

     7.2.  Traceroute Request

          A traceroute Request is a traceroute message with some number
          of response blocks filled in, and also uses IGMP type 0x1F.
          Routers can tell the difference between Queries and Requests
          by checking the length of the packet.

     7.2.1.  Packet Verification

          If the traceroute Request is not addressed to this router, or
          if the Request is addressed to a multicast group which is not
          a link-scoped group (e.g. 224.0.0.x), it MUST be silently
          ignored.

     7.2.2.  Normal Processing

          When a router receives a traceroute Request, it performs the
          following steps.  Note that it is possible to have multiple
          situations covered by the Forwarding Codes.  The first one
          encountered is the one that is reported, i.e. all "note
          forwarding code N" should be interpreted as "if forwarding
          code is not already set, set forwarding code to N".

          1.   Insert a new response block into the packet and fill in
               the Query Arrival Time, Outgoing Interface Address,
               Output Packet Count, and FwdTTL.

          2.   Attempt to determine the forwarding information for the
               source and group specified, using the same mechanisms as



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               would be used when a packet is received from the source
               destined for the group.  State need not be instantiated,
               it can be "phantom" state created only for the purpose of
               the trace.

          3.   If no forwarding information can be determined, an error
               code of 0x05 is inserted in the Forwarding Code field,
               the remaining fields that have not yet been filled in are
               set to zero, and the packet is forwarded to the requester
               as described in "Forwarding Traceroute Requests".

          4.   Fill in the Incoming Interface Address, Previous-Hop
               Router Address, Input Packet Count, Total Number of
               Packets, Routing Protocol, S, and Src Mask from the
               forwarding information that was determined.

          5.   If traceroute is administratively prohibited or the
               previous hop router does not understand traceroute
               requests, note the appropriate forwarding code.  If
               traceroute is administratively prohibited and any of the
               fields as filled in step 4 is considered private
               information, zero out the applicable fields.  Then the
               packet is forwarded to the requester as described in
               "Forwarding Traceroute Requests".

          6.   If the reception interface is not enabled for multicast,
               note forwarding code 0xA.  If the reception interface is
               the interface from which the router would expect data to
               arrive from the source, a forwarding code of 0x9 is
               noted.  Otherwise, if the reception interface is not one
               to which the router would forward data from the source, a
               forwarding code of 0x1 is noted.

          7.   If the group is subject to administrative scoping on
               either the Outgoing or Incoming interfaces, a forwarding
               code of 0x4 is noted.

          8.   If this router is the Rendez-vous Point or Core for the
               group, a forwarding code of 0x8 is noted.  (NOTE: should
               this be earlier?)

          9.   If this router has sent a prune upstream which applies to
               the source and group in the traceroute Request, it notes
               forwarding code 0x2.  If the router has stopped
               forwarding downstream in response to a prune sent by the
               next hop router, it notes forwarding code 0x3.  If the
               router should normally forward traffic for this source
               and group downstream but is not, it notes forwarding code



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               0x7.

          10.  The packet is then sent on to the previous hop or the
               requester as described in "Forwarding Traceroute
               Requests".

     7.3.  Traceroute response

          A router must forward all traceroute response packets
          normally, with no special processing.  If a router has
          initiated a traceroute with a Query or Request message, it may
          listen for Responses to that traceroute but MUST still forward
          them as well.

     7.4.  Forwarding Traceroute Requests

          If the Previous-hop router is known for the source and group
          (or, if no group is specified, the previous-hop router for the
          source, or if no source is specified, the previous-hop router
          for the group) and the number of response blocks is less than
          the number requested, the packet is sent to that router.  If
          the Incoming Interface is known but the Previous-hop router is
          not known, the packet is sent to an appropriate multicast
          address on the Incoming Interface.  The appropriate multicast
          address may depend on the routing protocol in use, MUST be a
          link-scoped group (i.e. 224.0.0.x), MUST NOT be ALL-
          SYSTEMS.MCAST.NET (224.0.0.1) and may be ALL-ROUTERS.MCAST.NET
          (224.0.0.2) if the routing protocol in use does not define a
          more appropriate group.  Otherwise, it is sent to the Response
          Address in the header, as described in "Sending Traceroute
          Responses".

     7.5.  Sending Traceroute Responses

     7.5.1.  Destination Address

          A traceroute response must be sent to the Response Address in
          the traceroute header.

     7.5.2.  TTL

          If the Response Address is unicast, the router inserts its
          normal unicast TTL in the IP header.  If the Response Address
          is multicast, the router copies the Response TTL from the
          traceroute header into the IP header.






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     7.5.3.  Source Address

          If the Response Address is unicast, the router may use any of
          its interface addresses as the source address.  Since some
          multicast routing protocols forward based on source address,
          if the Response Address is multicast, the router MUST use an
          address that is known in the multicast routing table if it can
          make that determination.

     7.5.4.  Sourcing Multicast Responses

          When a router sources a multicast response, the response
          packet MUST be sent on a single interface, then forwarded as
          if it were received on that interface.  It MUST NOT source the
          response packet individually on each interface, since that
          causes duplicate packets.

     8.  Using multicast traceroute

     <<Need a section on expected client behavior (one or two attempts
     with high hop count, then a search of some kind, then statistics
     later)>> Several problems may arise when attempting to use
     multicast traceroute.

     8.1.  Last hop router

          The traceroute querier may not know which is the last hop
          router, or that router may be behind a firewall that blocks
          unicast packets but passes multicast packets.  In these cases,
          the traceroute request should be multicasted to the group
          being traced (since the last hop router listens to that
          group).  All routers except the correct last hop router should
          ignore any multicast traceroute request received via
          multicast.  Traceroute requests which are multicasted to the
          group being traced must include the Router Alert IP option
          [Katz97].

          Another alternative is to unicast to the trace destination.
          Traceroute requests which are unicasted to the trace
          destination must include the Router Alert IP option [Katz97],
          in order that the last-hop router is aware of the packet.

          If the traceroute querier is attached to the same router as
          the destination of the request, the traceroute request may be
          multicasted to 224.0.0.2 (ALL-ROUTERS.MCAST.NET) if the last-
          hop router is not known.





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     8.2.  First hop router

          The traceroute querier may not be unicast reachable from the
          first hop router.  In this case, the querier should set the
          traceroute response address to a multicast address, and should
          set the response TTL to a value sufficient for the response
          from the first hop router to reach the querier.  It may be
          appropriate to start with a small TTL and increase in
          subsequent attempts until a sufficient TTL is reached, up to
          an appropriate maximum (such as 192).

          The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET, as the
          default multicast group for multicast traceroute responses.
          Other groups may be used if needed, e.g. when using mtrace to
          diagnose problems with the IANA-assigned group.

     8.3.  Broken intermediate router

          A broken intermediate router might simply not understand
          traceroute packets, and drop them.  The querier would then get
          no response at all from its traceroute requests.  It should
          then perform a hop-by-hop search by setting the number of
          responses field until it gets a response (both linear and
          binary search are options, but binary is likely to be slower
          because a failure requires waiting for a timeout).

     8.4.  Trace termination

          When performing an expanding hop-by-hop trace, it is necessary
          to determine when to stop expanding.

     8.4.1.  Arriving at source

          A trace can be determined to have arrived at the source if the
          Incoming Interface of the last router in the trace is non-
          zero, but the Previous Hop router is zero.  (XXX Need to
          actually check if this heuristic really works) <<Maybe a
          "previous hop" of 0xffffffff needs to mean "arrived at
          source">> <<or just a forwarding code>>

     8.4.2.  Fatal Error

          A trace has encountered a fatal error if the last Forwarding
          Error in the trace has the 0x80 bit set.

     8.4.3.  No Previous Hop

          A trace can not continue if the last Previous Hop in the trace



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          is set to 0.

     9.  Problem Diagnosis

     9.1.  Forwarding Inconsistencies

          The forwarding error code can tell if a group is unexpectedly
          pruned or administratively scoped.

     9.2.  TTL problems

          By taking the maximum of (hops from source + forwarding TTL
          threshold) over all hops, you can discover the TTL required
          for the source to reach the destination.

     9.3.  Congestion

          By taking two traces, you can find packet loss information by
          comparing the difference in input packet counts to the
          difference in output packet counts at the previous hop.  On a
          point-to-point link, any difference in these numbers implies
          packet loss.  Since the packet counts may be changing as the
          trace query is propagating, there may be small errors (off by
          1 or 2) in these statistics.  However, these errors will not
          accumulate if multiple traces are taken to expand the
          measurement period.  On a shared link, the count of input
          packets can be larger than the number of output packets at the
          previous hop, due to other routers or hosts on the link
          injecting packets.  This appears as "negative loss" which may
          mask real packet loss.

          In addition to the counts of input and output packets for all
          multicast traffic on the interfaces, the response data
          includes a count of the packets forwarded by a node for the
          specified source-group pair.  Taking the difference in this
          count between two traces and then comparing those differences
          between two hops gives a measure of packet loss just for
          traffic from the specified source to the specified receiver
          via the specified group.  This measure is not affected by
          shared links.

          On a point-to-point link that is a multicast tunnel, packet
          loss is usually due to congestion in unicast routers along the
          path of that tunnel.  On native multicast links, loss is more
          likely in the output queue of one hop, perhaps due to priority
          dropping, or in the input queue at the next hop.  The counters
          in the response data do not allow these cases to be
          distinguished.  Differences in packet counts between the



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          incoming and outgoing interfaces on one node cannot generally
          be used to measure queue overflow in the node because some
          packets may be routed only to or from other interfaces on that
          node.

          In the multicast extensions for SunOS 4.1.x from Xerox PARC,
          both the output packet count and the packet forwarding count
          for the source-group pair are incremented before priority
          dropping for rate limiting occurs and before the packets are
          put onto the interface output queue which may overflow.  These
          drops will appear as (positive) loss on the link even though
          they occur within the router.

          In release 3.3/3.4 of the UNIX multicast extensions, a
          multicast packet generated on a router will be counted as
          having come in an interface even though it did not.  This can
          create the appearance of negative loss even on a point-to-
          point link.

          In releases up through 3.5/3.6, packets were not counted as
          input on an interface if the reverse-path forwarding check
          decided that the packets should be dropped.  That causes the
          packets to appear as lost on the link if they were output by
          the upstream hop.  This situation can arise when two routers
          on the path for the group being traced are connected by a
          shared link, and the path for some other group does not flow
          between those two routers because the downstream router
          receives packets for the other group on another interface, but
          the upstream router is the elected forwarder to other routers
          or hosts on the shared link.

     9.4.  Link Utilization

          Again, with two traces, you can divide the difference in the
          input or output packet counts at some hop by the difference in
          time stamps from the same hop to obtain the packet rate over
          the link.  If the average packet size is known, then the link
          utilization can also be estimated to see whether packet loss
          may be due to the rate limit or the physical capacity on a
          particular link being exceeded.

     9.5.  Time delay

          If the routers have synchronized clocks, it is possible to
          estimate propagation and queueing delay from the differences
          between the timestamps at successive hops.





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     10.  Acknowledgments

     This specification started largely as a transcription of Van
     Jacobson's slides from the 30th IETF, and the implementation in
     mrouted 3.3 by Ajit Thyagarajan.  Van's original slides credit
     Steve Casner, Steve Deering, Dino Farinacci and Deb Agrawal.  A
     multicast traceroute client, mtrace, has been implemented by Ajit
     Thyagarajan, Steve Casner and Bill Fenner.

     The idea of unicasting a multicast traceroute Query to the
     destination of the trace with RA set is due to Tony Ballardie.  The
     idea of the "S" bit to allow statistics for a source subnet is due
     to Tom Pusateri.

     11.  IANA Considerations


     11.1.  Routing Protocols

          Should the IANA be responsible for allocating new Routing
          Protocol codes?

     11.2.  Forwarding Codes

          Should the IANA be responsible for allocating new Forwarding
          Codes?

     12.  Security Considerations


     12.1.  Topology discovery

          mtrace can be used to discover any actively-used topology.  If
          your network topology is a secret, you should restrict mtrace
          at the border of your domain.

     12.2.  Traffic rates

          mtrace can be used to discover what sources are sending to
          what groups and at what rates.  If this information is a
          secret, you should restrict mtrace at the border of your
          domain.

      ...more...

     13.  References

     Bradner97      Bradner, S., "Key words for use in RFCs to Indicate



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                    Requirement Levels", RFC 2119/BCP 14, Harvard
                    University, March 1997.

     Katz97         Katz, D., "IP Router Alert Option," RFC 2113, Cisco
                    Systems, February 1997.














































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     14.  Authors' Addresses

        William C. Fenner
        Xerox PARC
        3333 Coyote Hill Road
        Palo Alto, CA 94304
        Phone: +1 650 812 4816
        Email: fenner@parc.xerox.com

        Stephen L. Casner
        Precept Software, Inc.
        1072 Arastradero Road
        Palo Alto, CA 94304
        Email: casner@precept.com





































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