MBONED Working Group H. Asaeda
Internet-Draft Keio University
Expires: May 15, 2008 T. Jinmei
Toshiba Corporation
W. Fenner
AT&T Research
S. Casner
Packet Design, Inc.
November 12, 2007
Mtrace Version 2: Traceroute Facility for IP Multicast
draft-ietf-mboned-mtrace-v2-00
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Copyright Notice
Copyright (C) The IETF Trust (2007).
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Abstract
This document describes the IP multicast traceroute facility. Unlike
unicast traceroute, multicast traceroute requires special
implementations on the part of routers. This specification describes
the required functionality in multicast routers, as well as how
management applications can use the new router functionality.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. IPv4 Multicast Traceroute Header . . . . . . . . . . . . . . . 8
4.1. Type: 8 bits . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. # hops: 8 bits . . . . . . . . . . . . . . . . . . . . . . 8
4.3. Checksum: 16 bits . . . . . . . . . . . . . . . . . . . . 9
4.4. Multicast Address . . . . . . . . . . . . . . . . . . . . 9
4.5. Source Address . . . . . . . . . . . . . . . . . . . . . . 9
4.6. Destination Address . . . . . . . . . . . . . . . . . . . 9
4.7. Response Address . . . . . . . . . . . . . . . . . . . . . 9
4.8. Resp TTL: 8 bits . . . . . . . . . . . . . . . . . . . . . 9
4.9. Query ID: 24 bits . . . . . . . . . . . . . . . . . . . . 9
5. IPv4 Multicast Traceroute Response Data . . . . . . . . . . . 10
5.1. Query Arrival Time: 32 bits . . . . . . . . . . . . . . . 10
5.2. Incoming Interface Address . . . . . . . . . . . . . . . . 11
5.3. Outgoing Interface Address . . . . . . . . . . . . . . . . 11
5.4. Previous-Hop Router Address . . . . . . . . . . . . . . . 11
5.5. Packet counts . . . . . . . . . . . . . . . . . . . . . . 11
5.6. Input packet count on incoming interface . . . . . . . . . 11
5.7. Output packet count on incoming interface . . . . . . . . 11
5.8. Total number of packets for this source-group pair . . . . 11
5.9. Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 12
5.10. Fwd TTL: 8 bits . . . . . . . . . . . . . . . . . . . . . 12
5.11. MBZ: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . 12
5.12. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.13. Src Mask: 6 bits . . . . . . . . . . . . . . . . . . . . . 12
5.14. Forwarding Code: 8 bits . . . . . . . . . . . . . . . . . 13
6. IPv6 Multicast Traceroute Header . . . . . . . . . . . . . . . 15
6.1. Type: 8 bits . . . . . . . . . . . . . . . . . . . . . . . 16
6.2. # hops: 8 bits . . . . . . . . . . . . . . . . . . . . . . 16
6.3. Checksum: 16 bits . . . . . . . . . . . . . . . . . . . . 16
6.4. Reserved: 32 bits . . . . . . . . . . . . . . . . . . . . 16
6.5. Multicast Address . . . . . . . . . . . . . . . . . . . . 16
6.6. Source Address . . . . . . . . . . . . . . . . . . . . . . 16
6.7. Destination Address . . . . . . . . . . . . . . . . . . . 16
6.8. Response Address . . . . . . . . . . . . . . . . . . . . . 16
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6.9. Resp Hop Limit: 8 bits . . . . . . . . . . . . . . . . . . 17
6.10. Query ID: 24 bits . . . . . . . . . . . . . . . . . . . . 17
7. IPv6 Multicast Traceroute Response Data . . . . . . . . . . . 18
7.1. Query Arrival Time: 32 bits . . . . . . . . . . . . . . . 19
7.2. Incoming Interface ID: 32 bits . . . . . . . . . . . . . . 19
7.3. Outgoing Interface ID: 32 bits . . . . . . . . . . . . . . 19
7.4. Local Address . . . . . . . . . . . . . . . . . . . . . . 19
7.5. Remote Address . . . . . . . . . . . . . . . . . . . . . . 19
7.6. Input packet count on incoming interface . . . . . . . . . 20
7.7. Output packet count on incoming interface . . . . . . . . 20
7.8. Total number of packets for this source-group pair . . . . 20
7.9. Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 20
7.10. Fwd Hop Limit: 8 bits . . . . . . . . . . . . . . . . . . 20
7.11. MBZ: 7 bits . . . . . . . . . . . . . . . . . . . . . . . 20
7.12. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.13. Src Prefix Len: 8 bits . . . . . . . . . . . . . . . . . . 20
7.14. Forwarding Code: 8 bits . . . . . . . . . . . . . . . . . 21
7.15. Reserved: 24 bit . . . . . . . . . . . . . . . . . . . . . 21
8. Router Behavior . . . . . . . . . . . . . . . . . . . . . . . 22
8.1. Traceroute Query . . . . . . . . . . . . . . . . . . . . . 22
8.1.1. Packet Verification . . . . . . . . . . . . . . . . . 22
8.1.2. Normal Processing . . . . . . . . . . . . . . . . . . 22
8.2. Traceroute Request . . . . . . . . . . . . . . . . . . . . 22
8.2.1. Packet Verification . . . . . . . . . . . . . . . . . 23
8.2.2. Normal Processing . . . . . . . . . . . . . . . . . . 23
8.3. Traceroute Response . . . . . . . . . . . . . . . . . . . 24
8.4. Forwarding Traceroute Requests . . . . . . . . . . . . . . 24
8.5. Sending Traceroute Responses . . . . . . . . . . . . . . . 25
8.5.1. Destination Address . . . . . . . . . . . . . . . . . 25
8.5.2. TTL and Hop Limit . . . . . . . . . . . . . . . . . . 25
8.5.3. Source Address . . . . . . . . . . . . . . . . . . . . 25
8.5.4. Sourcing multicast responses . . . . . . . . . . . . . 25
8.6. Hiding information . . . . . . . . . . . . . . . . . . . . 26
9. Using multicast traceroute . . . . . . . . . . . . . . . . . . 27
9.1. Sample client . . . . . . . . . . . . . . . . . . . . . . 27
9.1.1. Sending initial query . . . . . . . . . . . . . . . . 27
9.1.2. Determining the Path . . . . . . . . . . . . . . . . . 27
9.1.3. Collecting statistics . . . . . . . . . . . . . . . . 27
9.2. Last hop router . . . . . . . . . . . . . . . . . . . . . 28
9.3. First hop router . . . . . . . . . . . . . . . . . . . . . 28
9.4. Broken intermediate router . . . . . . . . . . . . . . . . 28
9.5. Mtrace2 termination . . . . . . . . . . . . . . . . . . . 29
9.5.1. Arriving at source . . . . . . . . . . . . . . . . . . 29
9.5.2. Fatal error . . . . . . . . . . . . . . . . . . . . . 29
9.5.3. No previous hop . . . . . . . . . . . . . . . . . . . 29
9.5.4. Traceroute shorter than requested . . . . . . . . . . 29
9.6. Continuing after an error . . . . . . . . . . . . . . . . 29
9.7. Multicast Traceroute and shared tree routing protocols . . 30
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9.7.1. PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . 30
9.7.2. Bi-directional PIM . . . . . . . . . . . . . . . . . . 30
9.7.3. CBT . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.8. Protocol-specific considerations . . . . . . . . . . . . . 31
9.8.1. DVMRP . . . . . . . . . . . . . . . . . . . . . . . . 31
9.8.2. PIM-DM . . . . . . . . . . . . . . . . . . . . . . . . 31
10. Problem Diagnosis . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Forwarding Inconsistencies . . . . . . . . . . . . . . . . 32
10.2. TTL or hop limit problems . . . . . . . . . . . . . . . . 32
10.3. Packet loss . . . . . . . . . . . . . . . . . . . . . . . 32
10.4. Link Utilization . . . . . . . . . . . . . . . . . . . . . 33
10.5. Time delay . . . . . . . . . . . . . . . . . . . . . . . . 33
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
11.1. Routing protocols . . . . . . . . . . . . . . . . . . . . 34
11.2. Forwarding codes . . . . . . . . . . . . . . . . . . . . . 34
11.3. UDP destination port and IPv6 address . . . . . . . . . . 34
12. Security Considerations . . . . . . . . . . . . . . . . . . . 35
12.1. Topology Discovery . . . . . . . . . . . . . . . . . . . . 35
12.2. Traffic Rates . . . . . . . . . . . . . . . . . . . . . . 35
12.3. Unicast Replies . . . . . . . . . . . . . . . . . . . . . 35
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
14.1. Normative References . . . . . . . . . . . . . . . . . . . 37
14.2. Informative References . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . . . . 39
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1. Introduction
The unicast "traceroute" program allows the tracing of a path from
one machine to another. The key mechanism for unicast traceroute is
the ICMP TTL exceeded message, which is specifically precluded as a
response to multicast packets. On the other hand, the multicast
traceroute facility allows the tracing of an IP multicast routing
paths. In this document, we specify the new multicast "traceroute"
facility to be implemented in multicast routers and accessed by
diagnostic programs. The new multicast traceroute, mtrace version 2
or mtrace2, can provide additional information about packet rates and
losses that the unicast traceroute cannot, and generally requires
fewer packets to be sent.
o. To be able to trace the path that a packet would take from some
source to some destination.
o. To be able to isolate packet loss problems (e.g., congestion).
o. To be able to isolate configuration problems (e.g., TTL
threshold).
o. To minimize packets sent (e.g. no flooding, no implosion).
This document supports both IPv4 and IPv6 multicast traceroute
facility. The protocol design, concept, and program behavior are
same between IPv4 and IPv6 mtrace2. Regarding the previous IPv4
multicast traceroute, mtrace, the query and response messages for
IPv4 mtrace are implemented as IGMP messages [4]. On the other hand,
mtrace2 messages are carried on UDP, whereas the packet formats of
IPv4 and IPv6 mtrace2 are different (but similar) because of the
different address family.
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2. Terminology
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 [1].
Since multicast traceroutes flow in the opposite direction to the
data flow, we 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 toward the destination. It is the interface on which the
multicast traceroute Request was received.
Previous-hop router:
The router that is on the link attached to the Incoming Interface and
is responsible for forwarding traffic for the specified source and
group.
Group state:
It is the state in which a shared-tree protocol (e.g., PIM-SM [11])
running on a router chooses the previous-hop router toward the core
router (or RP) as its parent router. In this state, source-specific
state is not available for the corresponding multicast address on the
router.
Source-specific state:
It is the state in which a routing protocol running on a router
chooses the path that would be followed for a source-specific join.
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3. Overview
Given a multicast distribution tree, 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 most 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 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.
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4. IPv4 Multicast Traceroute Header
The mtrace2 message is carried as a UDP packet. The UDP source port
is uniquely selected by the local host operating system. The UDP
destination port is the IANA reserved mtrace2 port number (see
Section 11). The UDP checksum MUST be valid in mtrace2 control
messages.
The IPv4 mtrace2 includes the common packet header as follows. The
header is only filled in by the originator of the traceroute Query;
intermediate routers MUST NOT modify any of the fields.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Address |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Resp TTL/HopLim| Query ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1. Type: 8 bits
The UDP type field is defined to be "0x1" for traceroute queries and
requests. The UDP type field is changed to "0x2" 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 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-ring
search to trace the path to just before the problem.
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4.3. Checksum: 16 bits
The checksum is the 16-bit one's complement of the one's complement
sum of the whole UDP message (the entire IP payload) [7]. When
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. Multicast Address
This field specifies the multicast 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 IP address to which the completed traceroute
response packet gets sent. It can be a unicast address or a
multicast address, as explained in Section 8.2
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.
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5. IPv4 Multicast Traceroute Response Data
Each intermediate router in a trace path appends "response data" to
the forwarded trace packet. The response data looks 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 | Fwd TTL |B|S| Src Mask |Forwarding Code|
| | |Z| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1. Query Arrival Time: 32 bits
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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5.2. 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.
5.3. 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.
5.4. Previous-Hop Router Address
This field specifies the router from which this router expects
packets from this source. This may be a multicast group (e.g. ALL-
[protocol]-ROUTERS.MCAST.NET) 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.
5.5. Packet counts
Note that these packet counts SHOULD be as up to date as possible.
If packet counts are not being maintained on the processor that
handles the traceroute request in a multi-processor router
architecture, the packet SHOULD be delayed while the counters are
gathered from the remote processor(s). If this occurs, the Query
Arrival Time should be updated to reflect the time at which the
packet counts were learned.
5.6. 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 0xffffffffffffffff
if no count can be reported. This counter should have the same value
as ifInMulticastPkts from the IF-MIB [9] for this interface.
5.7. Output packet count on incoming interface
This field contains the number of multicast packets that have been
transmitted or queued for transmission for all groups and sources on
the outgoing interface, or 0xffffffffffffffff if no count can be
reported. This counter should have the same value as
ifOutMulticastPkts from the IF-MIB for this interface.
5.8. 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
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0xffffffffffffffff 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. This counter should have the same value as
ipMcastRoutePkts from the IPMROUTE-STD-MIB [10] for this forwarding
entry.
5.9. Rtg Protocol: 8 bits
This field describes the routing protocol in use between this router
and the previous-hop router. Specified values include:
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
8 PIM using MBGP route
9 CBT using special routing table
10 CBT using a static route
11 PIM using state created by Assert processing
12 Bi-directional PIM
Note that some of the routing protocols or functions are not
supported or not used in either of IPv4 multicast nor IPv6 multicast.
5.10. Fwd TTL: 8 bits
This field contains the TTL that a packet is required to have before
it will be forwarded over the outgoing interface.
5.11. MBZ: 1 bit
Must be zeroed on transmission and ignored on reception.
5.12. S: 1 bit
This S bit 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.
5.13. 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).
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If the router is forwarding solely on group state, this field is set
to 63 (0x3f).
5.14. Forwarding Code: 8 bits
This field contains a forwarding information/error code. Defined
values are as follows;
Value Name Description
----- -------------- -------------------------------------------
0x00 NO_ERROR No error
0x01 WRONG_IF Traceroute request arrived on an interface
to which this router would not forward for
this source,group,destination.
0x02 PRUNE_SENT This router has sent a prune upstream which
applies to the source and group in the
traceroute request.
0x03 PRUNE_RCVD This router has stopped forwarding for this
source and group in response to a request
from the next hop router.
0x04 SCOPED The group is subject to administrative
scoping at this hop.
0x05 NO_ROUTE This router has no route for the source or
group and no way to determine a potential
route.
0x06 WRONG_LAST_HOP This router is not the proper last-hop
router.
0x07 NOT_FORWARDING This router is not forwarding this source,
group out the outgoing interface for an
unspecified reason.
0x08 REACHED_RP Reached Rendez-vous Point or Core
0x09 RPF_IF Traceroute request arrived on the expected
RPF interface for this source, group.
0x0A NO_MULTICAST Traceroute request arrived on an interface
which is not enabled for multicast.
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0x0B INFO_HIDDEN One or more hops have been hidden from this
trace.
0x81 NO_SPACE There was not enough room to insert another
response data block in the packet.
0x82 OLD_ROUTER The previous-hop router does not understand
traceroute requests.
0x83 ADMIN_PROHIB 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. A multicast traceroute client, upon receiving
this error, MAY 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.
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6. IPv6 Multicast Traceroute Header
IPv6 mtrace2 includes the common packet header as follows. Because
of the specification of the IPv6 address, all IPv6 addresses used in
each field consume 128 bits length.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # hops | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Multicast Address *
| |
* *
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
* *
| |
* Source Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Destination Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Response Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Resp Hop Limit | Query ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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6.1. Type: 8 bits
The UDP type field is defined to be "0x1" for traceroute queries and
requests. The UDP type field is changed to "0x2" 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 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.
6.2. # hops: 8 bits
Same definition described in Section 4.2
6.3. Checksum: 16 bits
As defined in [2], the checksum is the 16-bit one's complement of the
one's complement sum of the entire UDP message, starting with the UDP
message type field, and prepended with a "pseudo-header" of IPv6
header fields.
6.4. Reserved: 32 bits
Initialized to zero by the sender; ignored by receivers.
6.5. Multicast Address
Same definition described in Section 4.4
6.6. Source Address
This field specifies the IPv6 address of the multicast source for the
path being traced, or is filled with the unspecified address (::) if
no source-specific information is desired. Note that non-source-
specific traceroutes may not be possible with certain multicast
routing protocols.
6.7. Destination Address
Same definition described in Section 4.6
6.8. Response Address
Same definition described in Section 4.7
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6.9. Resp Hop Limit: 8 bits
This field specifies the hop limit at which to multicast the
response, if the response address is a multicast address.
6.10. Query ID: 24 bits
Same definition described in Section 4.9
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7. IPv6 Multicast Traceroute Response Data
Each intermediate router in a trace path appends "response data" to
the forwarded trace packet. The response data looks 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query Arrival Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Local Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Remote Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Input packet count on incoming interface |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Output packet count on outgoing interface |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Total number of packets for this source-group pair |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol | Fwd Hop Limit | MBZ |S|Src Prefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Forwarding Code| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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7.1. Query Arrival Time: 32 bits
Same definition described in Section 5.1
7.2. Incoming Interface ID: 32 bits
This field specifies the interface ID on which packets from this
source and group are expected to arrive, or 0 if unknown. This ID
should be the value taken from InterfaceIndex of the IF-MIB for this
interface. This field is carried in network byte order.
7.3. Outgoing Interface ID: 32 bits
This field specifies the interface ID on which packets from this
source and group flow to the specified destination, or 0 if unknown.
This ID should be the value taken from InterfaceIndex of the IF-MIB
for this interface. This field is carried in network byte order.
7.4. Local Address
This field specifies a global IPv6 address that uniquely identifies
the router. A unique local unicast address [6] SHOULD NOT be used
unless the node is only assigned link-local and unique local
addresses. [TBD: What if the node is only assigned link-local
addresses? It should be very unlikely case, but is possible even for
a properly working router.]
Note that since interface indices used in the Incoming and Outgoing
Interface ID fields are node-local information, a global identifier
is needed to specify the router.
7.5. Remote Address
This field specifies the address of the previous-hop router, which,
in most cases, is a link-local unicast address for the queried source
and destination addresses.
Although a link-local address does not have enough information to
identify a node, it is possible to detect the previous-hop router
with the assistance of Incoming Interface ID and the current router
address (i.e., Local Address).
This may be a multicast group (e.g., ALL-[protocol]-
ROUTERS.MCAST.NET) if the previous hop is not known because of the
workings of the multicast routing protocol. However, it should be
the unspecified address (::) if the incoming interface address is
unknown.
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7.6. Input packet count on incoming interface
Same definition described in Section 5.6
7.7. Output packet count on incoming interface
Same definition described in Section 5.7
7.8. 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
0xffffffffffffffff if no count can be reported. If the S bit is set,
the count is for the source network, as specified by the Src Prefix
Len field. If the S bit is set and the Src Prefix Len field is 255,
indicating no source-specific state, the count is for all sources
sending to this group. This counter should have the same value as
ipMcastRoutePkts from the IPMROUTE-STD-MIB for this forwarding entry.
7.9. Rtg Protocol: 8 bits
Same definition described in Section 5.9
Note that some of the routing protocols or functions are not
supported or not used in IPv6 multicast.
7.10. Fwd Hop Limit: 8 bits
This field contains the hop limit that a packet is required to have
before it will be forwarded over the outgoing interface.
7.11. MBZ: 7 bits
Must be zeroed on transmission and ignored on reception.
7.12. S: 1 bit
This S bit 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 Prefix Len field.
7.13. Src Prefix Len: 8 bits
This field contains the decimal number of the prefix length this
router has for the source. If the router is forwarding solely on
group state, this field is set to 255 (0xff)
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7.14. Forwarding Code: 8 bits
Same definition described in Section 5.14
7.15. Reserved: 24 bit
Initialized to zero by the sender; ignored by receivers.
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8. 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 or the hop limit remaining MUST be forwarded normally, as
MUST a unicast packet that has TTL or the hop limit remaining and is
not addressed to this router.
8.1. Traceroute Query
A traceroute Query message is a traceroute message with no response
blocks filled in, and uses UDP type 0x1 for IPv4 and IPv6 mtrace2.
8.1.1. 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 would forward traffic from the given
source onto that subnet.
If the router determines that it is not the proper last-hop router,
or it cannot make that determination, it does one of two things
depending if the Query was received via multicast or unicast. If the
Query was received via multicast, then it MUST be silently dropped.
If it was received via unicast, a forwarding code of WRONG_LAST_HOP
is noted and processing continues as in Section 8.2
Duplicate Query messages as identified by the tuple (IP Source, Query
ID) SHOULD be ignored. This MAY be implemented using a simple 1-back
cache (i.e. remembering the IP source and Query ID of the previous
Query message that was processed, and ignoring future messages with
the same IP Source and Query ID). Duplicate Request messages MUST
NOT be ignored in this manner.
8.1.2. 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 in Section 8.2
8.2. Traceroute Request
A traceroute Request is a traceroute message with some number of
response blocks filled in, and uses UDP type 0x1 for IPv4 and IPv6
mtrace2. Routers can tell the difference between Queries and
Requests by checking the length of the packet.
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8.2.1. 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 (i.e. 224/24 for IPv4, FFx2::/16 [3] for IPv6), it MUST be
silently ignored.
8.2.2. 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. If there is room in the current buffer (or the router can
efficiently allocate more space to use), insert a new response
block into the packet and fill in the Query Arrival Time,
Outgoing Interface Address (for IPv4 mtrace2) or Outgoing
Interface ID (for IPv6 mtrace2), Output Packet Count, and Fwd
TTL or Fwd Hop Limit. If there was no room, fill in the
response code "NO_SPACE" in the *previous* hop's response block,
and forward the packet to the requester as described in
"Forwarding Traceroute Requests".
2. Attempt to determine the forwarding information for the source
and group specified, using the same mechanisms as 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.
If using a shared-tree protocol and there is no source-specific
state, or if the source is specified as 0xFFFFFFFF, group state
should be used. If there is no group state or the group is
specified as 0, potential source state (i.e. the path that would
be followed for a source-specific Join) should be used. If this
router is the Core or RP and no source-specific information is
available, note an error code of REACHED_RP.
3. If no forwarding information can be determined, the router notes
an error code of NO_ROUTE, sets the remaining fields that have
not yet been filled in to zero, and then forwards the packet 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
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was determined.
5. If traceroute is administratively prohibited or the previous hop
router does not understand traceroute requests, note the
appropriate forwarding code (ADMIN_PROHIB or OLD_ROUTER). If
traceroute is administratively prohibited and any of the fields
as filled in step 4 are 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 NO_MULTICAST. If the reception interface is the
interface from which the router would expect data to arrive from
the source, note forwarding code RPF_IF. Otherwise, if the
reception interface is not one to which the router would forward
data from the source to the group, a forwarding code of WRONG_IF
is noted.
7. If the group is subject to administrative scoping on either the
Outgoing or Incoming interfaces, a forwarding code of SCOPED is
noted.
8. If this router is the Rendez-vous Point or Core for the group, a
forwarding code of REACHED_RP is noted.
9. If this router has sent a prune upstream which applies to the
source and group in the traceroute Request, it notes forwarding
code PRUNE_SENT. If the router has stopped forwarding
downstream in response to a prune sent by the next hop router,
it notes forwarding code PRUNE_RCVD. If the router should
normally forward traffic for this source and group downstream
but is not, it notes forwarding code NOT_FORWARDING.
10. The packet is then sent on to the previous hop or the requester
as described in Section 8.4.
8.3. 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.
8.4. Forwarding Traceroute Requests
If the Previous-hop router is known for this request 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
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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. 224/24 for IPv4, FF02::/16 for
IPv6), MUST NOT be ALL-SYSTEMS.MCAST.NET (224.0.0.1) for IPv4 and All
Nodes Address (FF02::1) for IPv6, and MAY be ALL-ROUTERS.MCAST.NET
(224.0.0.2) for IPv4 or All Routers Address (FF02::2) for IPv6 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 Section 8.5.
Note that it is not an error for the number of response blocks to be
greater than the number requested; such a packet should simply be
forwarded to the requester as described in Section 8.5.
8.5. Sending Traceroute Responses
8.5.1. Destination Address
A traceroute response must be sent to the Response Address in the
traceroute header.
8.5.2. TTL and Hop Limit
If the Response Address is unicast, the router inserts its normal
unicast TTL or hop limit in the IP header, and may use any of its
interface addresses as the source address. If the Response Address
is multicast, the router copies the Response TTL or hop limit from
the traceroute header into the IP header.
8.5.3. 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 topology if it can make that determination.
8.5.4. 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, in order to avoid duplicate packets.
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8.6. Hiding information
Information about a domain's topology and connectivity may be hidden
from multicast traceroute requests. The exact mechanism is not
specified here; however, the INFO_HIDDEN forwarding code may be used
to note that, for example, the incoming interface address and packet
count are for the entrance to the domain and the outgoing interface
address and packet count are the exit from the domain. The source-
group packet count may be from either router or not specified
(0xffffffff).
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9. Using multicast traceroute
9.1. Sample client
This section describes the behavior of an example multicast
traceroute client.
9.1.1. Sending initial query
When the destination of the mtrace2 is the machine running the
client, the mtrace2 Query packet can be sent to the ALL-
ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All Routers Address
(FF02::2) for IPv6. This will ensure that the packet is received by
the last-hop router on the subnet. Otherwise, if the proper last-hop
router is known for the mtrace2 destination, the Query could be
unicasted to that router. Otherwise, the Query packet should be
multicasted to the group being queried; if the destination of the
mtrace2 is a member of the group, this will get the Query to the
proper last-hop router. In this final case, the packet should
contain the Router Alert option [8], to make sure that routers that
are not members of the multicast group notice the packet.
See also Section 9.2 on determining the last-hop router.
9.1.2. Determining the Path
The client could send a small number of initial query messages with a
large "# hops" field, in order to try to trace the full path. If
this attempt fails, one strategy is to perform a linear search (as
the traditional unicast traceroute program does); set the "# hops"
field to 1 and try to get a response, then 2, and so on. If no
response is received at a certain hop, the hop count can continue
past the non-responding hop, in the hopes that further hops may
respond. These attempts should continue until a user-defined timeout
has occurred.
See also Section 9.3 and Section 9.4 on receiving the results of a
trace.
9.1.3. Collecting statistics
After a client has determined that it has traced the whole path or as
much as it can expect to (see Section 9.5), it might collect
statistics by waiting a short time and performing a second trace. If
the path is the same in the two traces, statistics can be displayed
as described in Section 10.3 and Section 10.4.
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9.2. Last hop router
The mtrace2 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 mtrace2 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 mtrace2 request received via multicast.
Mtrace2 requests which are multicasted to the group being traced must
include the Router Alert option [8].
Another alternative is to unicast to the trace destination.
Traceroute requests which are unicasted to the trace destination must
include the Router Alert option, 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 ALL-ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All Routers Address
(FF02::2) for IPv6 if the last-hop router is not known.
9.3. First hop router
The mtrace2 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 (or
hop limit) 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 IPv4 mtrace2 responses, and will assign
MTRACE2_IPV6RESPADDR (TBD (see Section 11)) for IPv6 mtrace2
responses. Other groups may be used if needed, e.g. when using
mtrace2 to diagnose problems with the IANA-assigned group.
9.4. 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).
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9.5. Mtrace2 termination
When performing an expanding hop-by-hop trace, it is necessary to
determine when to stop expanding.
9.5.1. 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.
9.5.2. Fatal error
A trace has encountered a fatal error if the last Forwarding Error in
the trace has the 0x80 bit set.
9.5.3. No previous hop
A trace can not continue if the last Previous Hop in the trace is set
to 0.
9.5.4. Traceroute shorter than requested
If the trace that is returned is shorter than requested (i.e. the
number of Response blocks is smaller than the "# hops" field), the
trace encountered an error and could not continue.
9.6. Continuing after an error
When the NO_SPACE error occurs, the client might try to continue the
trace by starting it at the last hop in the trace. It can do this by
unicasting to this router's outgoing interface address, keeping all
fields the same. If this results in a single hop and a "WRONG_IF"
error, the client may try setting the trace destination to the same
outgoing interface address.
If a trace times out, it is likely to be because a router in the
middle of the path does not support multicast traceroute. That
router's address will be in the Previous Hop field of the last entry
in the last reply packet received. A client may be able to determine
(via mrinfo or SNMP [6][10]) a list of neighbors of the non-
responding router. If desired, each of those neighbors could be
probed to determine the remainder of the path. Unfortunately, this
heuristic may end up with multiple paths, since there is no way of
knowing what the non-responding router's algorithm for choosing a
previous-hop router is. However, if all paths but one flow back
towards the non-responding router, it is possible to be sure that
this is the correct path.
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9.7. Multicast Traceroute and shared tree routing protocols
When using shared-tree routing protocols like PIM-SM and CBT, a more
advanced client may use multicast traceroute to determine paths or
potential paths.
9.7.1. PIM-SM
When a multicast traceroute reaches a PIM-SM RP and the RP does not
forward the trace on, it means that the RP has not performed a
source-specific join so there is no more state to trace. However,
the path that traffic would use if the RP did perform a source-
specific join can be traced by setting the trace destination to the
RP, the trace source to the traffic source, and the trace group to 0.
This trace Query may be unicasted to the RP.
9.7.2. Bi-directional PIM
Bi-directional PIM [13] is a variant of PIM-SM that builds bi-
directional shared trees connecting multicast sources and receivers.
Along the bi-directional shared trees, multicast data is natively
forwarded from sources to the RPA (Rendezvous Point Address) and from
the RPA to receivers without requiring source-specific state. In
contrast to PIM-SM, RP always has the state to trace.
A Designated Forwarder (DF) for a given RPA is in charge of
forwarding downstream traffic onto its link, and forwarding upstream
traffic from its link towards the RPL (Rendezvous Point Link) that
the RPA belongs to. Hence mtrace2 reports DF addresses or RPA along
the path.
9.7.3. CBT
When a multicast traceroute reaches a CBT [12] Core, it must simply
stop since CBT does not have source-specific state. However, a
second trace can be performed, setting the trace destination to the
traffic source, the trace group to the group being traced, and the
trace source to the Core (or to 0, since CBT does not have source-
specific state). This trace Query may be unicasted to the Core.
There are two possibilities when combining the two traces:
9.7.3.1. No overlap
If there is no overlap between the two traces, the second trace can
be reversed and appended to the first trace. This composite trace
shows the full path from the source to the destination.
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9.7.3.2. Overlapping paths
If there is a portion of the path that is common to the ends of the
two traces, that portion is removed from both traces. Then, as in
the no overlap case, the second trace is reversed and appended to the
first trace, and the composite trace again contains the full path.
This algorithm works whether the source has joined the CBT tree or
not.
9.8. Protocol-specific considerations
9.8.1. DVMRP
DVMRP's dominant router election and route exchange guarantees that
DVMRP routers know whether or not they are the last-hop forwarder for
the link and who the previous hop is.
9.8.2. PIM-DM
Routers running PIM Dense Mode do not know the path packets would
take unless traffic is flowing. Without some extra protocol
mechanism, this means that in an environment with multiple possible
paths with branch points on shared media, multicast traceroute can
only trace existing paths, not potential paths. When there are
multiple possible paths but the branch points are not on shared
media, the previous hop router is known, but the last hop router may
not know that it is the appropriate last hop.
When traffic is flowing, PIM Dense Mode routers know whether or not
they are the last-hop forwarder for the link (because they won or
lost an Assert battle) and know who the previous hop is (because it
won an Assert battle). Therefore, multicast traceroute is always
able to follow the proper path when traffic is flowing.
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10. Problem Diagnosis
10.1. Forwarding Inconsistencies
The forwarding error code can tell if a group is unexpectedly pruned
or administratively scoped.
10.2. TTL or hop limit problems
By taking the maximum of (hops from source + forwarding TTL (or hop
limit) threshold) over all hops, you can discover the TTL required
for the source to reach the destination.
10.3. Packet loss
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 incoming and outgoing interfaces on one node cannot
generally be used to measure queue overflow in the node.
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10.4. 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.
10.5. Time delay
If the routers have synchronized clocks, it is possible to estimate
propagation and queuing delay from the differences between the
timestamps at successive hops. However, this delay includes control
processing overhead, so is not necessarily indicative of the delay
that data traffic would experience.
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11. IANA Considerations
The following new assignments can only be made via a Standards Action
as specified in [5].
11.1. Routing protocols
The IANA is responsible for allocating new Routing Protocol codes.
The Routing Protocol code is somewhat problematic, since in the case
of protocols like CBT and PIM it must encode both a unicast routing
algorithm and a multicast tree-building protocol. The space was not
divided into two fields because it was already small and some
combinations (e.g. DVMRP) would be wasted.
11.2. Forwarding codes
New Forwarding codes must only be created by an RFC that modifies
this document's Section 9, fully describing the conditions under
which the new forwarding code is used. The IANA may act as a central
repository so that there is a single place to look up forwarding
codes and the document in which they are defined.
11.3. UDP destination port and IPv6 address
The IANA should allocate UDP destination port for multicast
traceroute version 2 upon publication of the first RFC.
Additionally, the well-known multicast address (MTRACE2_IPV6RESPADDR)
intended for default use by IPv6 multicast traceroute should be
registered and defined by the first RFC published.
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12. Security Considerations
12.1. Topology Discovery
Mtrace2 can be used to discover any actively-used topology. If your
network topology is a secret, mtrace2 may be restricted at the border
of your domain, using the ADMIN_PROHIB forwarding code.
12.2. Traffic Rates
Mtrace2 can be used to discover what sources are sending to what
groups and at what rates. If this information is a secret, mtrace2
may be restricted at the border of your domain, using the
ADMIN_PROHIB forwarding code.
12.3. Unicast Replies
The "Response address" field may be used to send a single packet (the
traceroute Reply packet) to an arbitrary unicast address. It is
possible to use this facility as a packet amplifier, as a small
multicast traceroute Query may turn into a large Reply packet.
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13. Acknowledgements
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. The original
multicast traceroute client, mtrace (version 1), 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 Router Alert set is due to Tony
Ballardie. The idea of the "S" bit to allow statistics for a source
subnet is due to Tom Pusateri.
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14. References
14.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[4] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, October 2002.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, October 1998.
14.2. Informative References
[6] Draves, R. and D. Thaler, "Default Router Preferences and More-
Specific Routes", RFC 4191, November 2005.
[7] Braden, B., Borman, D., and C. Partridge, "Computing the
Internet Checksum", RFC 1071, September 1988.
[8] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.
[9] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
RFC 2863, June 2000.
[10] McWalter, D., Thaler, D., and A. Kessler, "IP Multicast MIB",
draft-ietf-mboned-ip-mcast-mib-05.txt (work in progress),
March 2007.
[11] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[12] Ballardie, T., "Core Based Trees (CBT version 2) Multicast
Routing -- Protocol Specification --", RFC 2189,
September 1997.
[13] Handley, M., Kouvelas, I., and T. Speakman, "Bi-directional
Protocol Independent Multicast (BIDIR-PIM)",
draft-ietf-pim-bidir-09.txt (work in progress), February 2007.
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Authors' Addresses
Hitoshi Asaeda
Keio University
Graduate School of Media and Governance
Fujisawa, Kanagawa 252-8520
Japan
Email: asaeda@wide.ad.jp
Tatsuya Jinmei
Toshiba Corporation
Corporate Research & Development Center
Kawasaki, Kanagawa 212-8582
Japan
Email: jinmei@isl.rdc.toshiba.co.jp
William C. Fenner
AT&T Research
Menlo Park, CA 94025
US
Email: fenner@research.att.com
Stephen L. Casner
Packet Design, Inc.
Palo Alto, CA 94304
US
Email: casner@packetdesign.com
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Asaeda, et al. Expires May 15, 2008 [Page 39]
