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.
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
solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s).
Casner, Fenner Expires April 1998 [Page 1]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 2]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 3]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 4]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 5]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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).
Casner, Fenner Expires April 1998 [Page 6]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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:
Casner, Fenner Expires April 1998 [Page 7]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 8]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 9]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 10]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 11]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 12]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 13]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 14]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 15]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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.
Casner, Fenner Expires April 1998 [Page 16]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 17]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
Requirement Levels", RFC 2119/BCP 14, Harvard
University, March 1997.
Katz97 Katz, D., "IP Router Alert Option," RFC 2113, Cisco
Systems, February 1997.
Casner, Fenner Expires April 1998 [Page 18]
Internet Draft draft-ietf-idmr-traceroute-ipm-02.txt November 21, 1997
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
Casner, Fenner Expires April 1998 [Page 19]