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Internet Engineering Task Force Inter-Domain Multicast Routing Working Group
INTERNET-DRAFT W. Fenner
draft-ietf-idmr-traceroute-ipm-01.txt Xerox PARC
S. Casner
Precept Software
November 26, 1996
Expires: 3/31/97
A "traceroute" facility for IP Multicast.
Status of this Memo
This document is an Internet Draft. Internet Drafts are working docu-
ments 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.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
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 soli-
cited 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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Internet Draft draft-ietf-idmr-traceroute-ipm-01.txt November 1996
1. Introduction
The unicast "traceroute" program allows the tracing of a path from one
machine to another, using mechanisms that already existed in IP. Unfor-
tunately, no such existing mechanisms 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 multi-
cast 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).
2. 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 desti-
nation 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 contain-
ing 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. 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
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Internet Draft draft-ietf-idmr-traceroute-ipm-01.txt November 1996
such as "no route" is encountered along the path.
3. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1. 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
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.
3.2. # 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.
3.3. Checksum: 16 bits
This is the standard IGMP checksum.
3.4. Group address
This field specifies the group address to be traced, or zero if no
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group-specific information is desired. Note that non-group-
specific traceroutes may not be possible with certain multicast
routing protocols.
3.5. Source address
This field specifies the IP address of the multicast source for the
path being traced. The traceroute request proceeds hop-by-hop from
the intended multicast receiver towards this source.
3.6. 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 source.
3.7. 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.
3.8. resp ttl: 8 bits
This field specifies the TTL at which to multicast the response, if
the response address is a multicast address.
3.9. 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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4. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol | FwdTTL |MBZ| Src Mask | ForwardingErr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1. 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.
4.2. Incoming Interface Address
This field specifies the address of the interface on which packets
from this source are expected to arrive, or 0 if unknown.
4.3. Outgoing Interface Address
This field specifies the address of the interface on which packets
from this source flow to the specified destination, or 0 if unk-
nown.
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4.4. Previous-Hop Router Address
This field specifies the router from which this router expects
packets from this source, or 0 if unknown.
4.5. 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.
4.6. 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.
4.7. 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.
4.8. 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
4.9. FwdTTL: 8 bits
This field contains the TTL that a packet is required to have
before it will be forwarded over the outgoing interface.
4.10. 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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4.11. ForwardingErr: 8 bits
This field contains a forwarding error code. Specified 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 for the group.
0x03 This router has stopped forwarding in response to a
request from the next hop router.
0x04 The group is subject to administrative scoping at this hop.
0x05 This router has no route for the source.
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 next 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 ForwardingErr field, overwriting any error the
previous router placed there. It is expected that a multicast tra-
ceroute client, upon receiving this error, will restart the trace
at the last hop listed in the packet.
The 0x80 bit of the ForwardingErr 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.
5. Router Behavior
All of these actions are performed in addition to (NOT instead of) for-
warding the packet, if applicable. E.g. a multicast packet that has TTL
remaining MUST still get forwarded.
5.1. Traceroute Query
Upon receiving a traceroute Query message (a request with no
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response blocks filled in), a router must examine the traceroute
request to see if it is the proper last-hop router for the destina-
tion address in the packet. It is the proper last-hop router if it
has a multicast-capable interface on the same subnet as the Desti-
nation Address and is the router that would forward traffic from
the given source onto that subnet. It is also the proper last-hop
router if the Destination Address is the address of one of its
interfaces and either it is the router that would forward traffic
from the given source onto that subnet or there is no other router
on 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 should be silently
dropped. If received via unicast and it determines that it is not
the proper last-hop router, a response block with an error code of
0x1 must be inserted and the response forwarded to the response
address as described below. If the router knows which router is
the correct last-hop router, it puts that router's address in the
"Previous Hop" field of the response.
When a router receives a traceroute request with no response blocks
and it determines that it is the proper last-hop router, it inserts
a response block and forwards the traceroute request towards the
router that it expects to be the previous hop for this source and
group (or, if no group is specified, the previous hop for this
source).
5.2. Traceroute Request
When a router receives a traceroute request with some number of
response blocks filled in, it first checks the interface from which
it received the traceroute request. If the reception interface is
not one to which the router would forward data from the source, an
error code of 0x1 is noted and processing continues. If the recep-
tion interface is the interface from which the router would expect
data to arrive from the source, an error code of 0x9 is noted and
processing continues. If it receives a traceroute Request with
some number of response blocks filled in and the packet destination
is a multicast address, it must silently drop the packet. If a
router has no way to determine a route for the source, an error
code of 0x5 is noted and processing continues. The router fills in
as many fields as possible in the response packet, and then for-
wards the packet on or returns it to the requester. 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) and the
number of response blocks is less than the number requested, the
packet is forwarded to that router. Otherwise, it is sent to the
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Internet Draft draft-ietf-idmr-traceroute-ipm-01.txt November 1996
Response Address in the header, with the indicated TTL if the
Response Address is a multicast address.
5.3. Traceroute response
A router must forward all traceroute response packets normally,
with no special processing.
5.4. Sending Traceroute Responses
5.4.1. Destination Address
A traceroute response must be sent to the Response Address in the
traceroute header.
5.4.2. TTL
If the Response Address is unicast, the router inserts its normal
unicast TTL in the IP header. If the Response Address is multi-
cast, the router copies the Response TTL from the traceroute header
into the IP header.
5.4.3. Source Address
If the Response Address is unicast, the router may use any of its
interface addresses as the source address, preferring globally
routable addresses. If the Response Address is multicast, the
router MUST use a globally routable source address, if it has one.
If the router does not have a globally routable address attached to
any interface, then it SHOULD NOT try to send a multicast response.
5.4.4. Sourcing Multicast Responses
When a router sources a multicast response, the response packet
MUST be forwarded as if it were received on the outgoing interface.
6. 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.
6.1. 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
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Internet Draft draft-ietf-idmr-traceroute-ipm-01.txt November 1996
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 mul-
ticasted to the group being traced must include the Router Alert IP
option [Katz96].
If the traceroute querier is attached to the same router as the
destination of the request, the traceroute request may be multi-
casted to 224.0.0.2 (ALL-ROUTERS.MCAST.NET) if the last-hop router
is not known.
6.2. 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 suffi-
cient 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.
6.3. 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).
6.4. Trace termination
When performing an expanding hop-by-hop trace, it is necessary to
determine when to stop expanding.
6.4.1. Arriving at source
A trace can be determined to have arrived at the source if the last
router in the trace has an interface on the same subnet as the
source. (***BAD HEURISTIC***! A router might have secondary sub-
nets attached to it but not have an address on any of those sub-
nets) <<Maybe a "previous hop" of 0xffffffff needs to mean "arrived
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at source">>
6.4.2. Fatal Error
A trace has encountered a fatal error if the last Forwarding Error
in the trace has the 0x80 bit set.
6.4.3. No Previous Hop
A trace can not continue if the last Previous Hop in the trace is
set to 0.
7. Problem Diagnosis
7.1. Forwarding Inconsistencies
The forwarding error code can tell if a group is unexpectedly
pruned or administratively scoped.
7.2. TTL problems
By taking the maximum of (hops from source + forwarding TTL thres-
hold) over all hops, you can discover the TTL required for the
source to reach the destination.
7.3. Congestion
By taking two traces, you can find packet loss information by com-
paring 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 propagat-
ing, 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 mul-
ticast 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 meas-
ure 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.
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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 out-
put 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
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 (posi-
tive) 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.
7.4. 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.
7.5. 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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8. 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.
9. Security Considerations
Security issues are not discussed in this memo. <<Topology discovery>>
<<Traffic rates>>
10. References
Katz96 Katz, D., "IP Router Alert Option," RFC XXXX, Cisco Sys-
tems, April 1996.
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11. Authors' Addresses
William C. Fenner
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304
Phone: +1 415 812 4816
Email: fenner@parc.xerox.com
Stephen L. Casner
Precept Software, Inc.
21580 Stevens Creek Blvd, Suite 207
Cupertino, CA 95014
Email: casner@precept.com
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