draft-ietf-idmr-traceroute-ipm-00.txt   draft-ietf-idmr-traceroute-ipm-01.txt 
Internet Engineering Task Force Inter-Domain Multicast Routing Working Group Internet Engineering Task Force Inter-Domain Multicast Routing Working Group
INTERNET-DRAFT W. Fenner INTERNET-DRAFT W. Fenner
draft-ietf-idmr-traceroute-ipm-00.txt Xerox PARC draft-ietf-idmr-traceroute-ipm-01.txt Xerox PARC
S. Casner S. Casner
Precept Software Precept Software
November 13, 1995 November 26, 1996
Expires: 3/31/95 Expires: 3/31/97
A "traceroute" facility for IP Multicast. A "traceroute" facility for IP Multicast.
Status of this Memo Status of this Memo
This document is an Internet Draft. Internet Drafts are working docu- This document is an Internet Draft. Internet Drafts are working docu-
ments of the Internet Engineering Task Force (IETF), its Areas, and ments of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups. Note that other groups may also distribute its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts. working documents as Internet Drafts.
skipping to change at page 2, line 41 skipping to change at page 2, line 41
Tracing from a source to a multicast destination is hard, since you 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. 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 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- one source to one destination. However, walking up the tree from desti-
nation to source is easy, as all existing multicast routing protocols nation to source is easy, as all existing multicast routing protocols
know the previous hop for each source. Tracing from destination to know the previous hop for each source. Tracing from destination to
source can involve only routers on the direct path. source can involve only routers on the direct path.
The party requesting the traceroute (which need be neither the source The party requesting the traceroute (which need be neither the source
nor the destination) sends a traceroute request packet to the last-hop nor the destination) sends a traceroute Query packet to the last-hop
multicast router for the given destination. The last-hop router adds a multicast router for the given destination. The last-hop router turns
response data block to the request packet containing its interface the Query into a Request packet by adding a response data block contain-
addresses and packet statistics, and then forwards the request packet ing its interface addresses and packet statistics, and then forwards the
via unicast to the router that it believes is the proper previous hop Request packet via unicast to the router that it believes is the proper
for the given source. Each hop adds its response data to the end of the previous hop for the given source. Each hop adds its response data to
request packet, then unicast forwards it to the previous hop. The first the end of the Request packet, then unicast forwards it to the previous
hop router (the router that believes that packets from the source ori- hop. The first hop router (the router that believes that packets from
ginate on one of its directly connected networks) changes the packet the source originate on one of its directly connected networks) changes
type to indicate a response packet and sends the completed response to the packet type to indicate a Response packet and sends the completed
the response destination address. The response may be returned before response to the response destination address. The response may be
reaching the first hop router if an error condition such as "no route" returned before reaching the first hop router if a fatal error condition
is encountered along the path. such as "no route" is encountered along the path.
3. Request / Response header 3. Multicast Traceroute header
The header for both requests and responses is as follows: The header for all multicast traceroute packets is as follows:
0 1 2 3 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 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 | | IGMP Type | # hops | checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group Address | | Multicast Group Address |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| Source Address | | Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address | | Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Response Address | | Response Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| resp ttl | Query ID | | resp ttl | Query ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1. IGMP Type: 8 bits 3.1. IGMP Type: 8 bits
The IGMP type field is defined to be 0x1F for traceroute requests The IGMP type field is defined to be 0x1F for traceroute queries
sent to the last hop router and forwarded hop by hop towards the and requests. The IGMP type field is changed to 0x1E when the
source. The IGMP type field is changed to 0x1E when the packet is packet is completed and sent as a response from the first hop
completed and sent as a response from the first hop router to the router to the querier. Two codes are required so that multicast
querier. Two codes are required so that multicast routers won't routers won't attempt to process a completed response in those
attempt to process a completed response in those cases where the cases where the initial query was issued from a router or the
initial query was issued from a router or the response is sent via response is sent via multicast.
multicast.
3.2. # hops: 8 bits 3.2. # hops: 8 bits
This field specifies the maximum number of hops that the requester This field specifies the maximum number of hops that the requester
wants to trace. If there is some error condition in the middle of wants to trace. If there is some error condition in the middle of
the path that keeps the traceroute request from reaching the the path that keeps the traceroute request from reaching the
first-hop router, this field can be used to perform an expanding- first-hop router, this field can be used to perform an expanding-
length search to trace the path to just before the problem. length search to trace the path to just before the problem.
3.3. Checksum: 16 bits 3.3. Checksum: 16 bits
This is the standard IGMP checksum. This is the standard IGMP checksum.
3.4. Group address 3.4. Group address
This field specifies the group address to be traced. 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.
3.5. Source address 3.5. Source address
This field specifies the IP address of the multicast source for the This field specifies the IP address of the multicast source for the
path being traced. The traceroute request proceeds hop-by-hop from path being traced. The traceroute request proceeds hop-by-hop from
the intended multicast receiver towards this source. the intended multicast receiver towards this source.
3.6. Destination address 3.6. Destination address
This field specifies the IP address of the multicast receiver for This field specifies the IP address of the multicast receiver for
the path being traced. The trace starts at this destination and the path being traced. The trace starts at this destination and
proceeds toward the source. proceeds toward the source.
3.7. Response Address 3.7. Response Address
This field specifies where the completed traceroute response packet This field specifies where the completed traceroute response packet
gets sent. It can be a unicast address or a multicast address, as gets sent. It can be a unicast address or a multicast address, as
explained in section 5.2. explained in section 6.2.
3.8. resp ttl: 8 bits 3.8. resp ttl: 8 bits
This field specifies the TTL at which to multicast the response, if This field specifies the TTL at which to multicast the response, if
the response address is a multicast address. the response address is a multicast address.
3.9. Query ID: 24 bits 3.9. Query ID: 24 bits
This field is used as a unique identifier for this traceroute This field is used as a unique identifier for this traceroute
request so that duplicate or delayed responses may be detected and request so that duplicate or delayed responses may be detected and
skipping to change at page 6, line 37 skipping to change at page 6, line 37
4.8. Rtg Protocol: 8 bits 4.8. Rtg Protocol: 8 bits
This field describes the routing protocol in use between this This field describes the routing protocol in use between this
router and the previous-hop router. Specified values include: router and the previous-hop router. Specified values include:
1 - DVMRP 1 - DVMRP
2 - MOSPF 2 - MOSPF
3 - PIM 3 - PIM
4 - CBT 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 4.9. FwdTTL: 8 bits
This field contains the TTL that a packet is required to have This field contains the TTL that a packet is required to have
before it will be forwarded over the outgoing interface. before it will be forwarded over the outgoing interface.
4.10. Src Mask: 6 bits 4.10. Src Mask: 6 bits
This field contains the number of 1's in the netmask this router 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 has for the source (i.e. a value of 24 means the netmask is
0xffffff00) 0xffffff00)
4.11. ForwardingErr: 8 bits 4.11. ForwardingErr: 8 bits
This field contains a forwarding error code. Specified values This field contains a forwarding error code. Specified values
include: include:
0x00 No error 0x00 No error
0x01 Traceroute request arrived on an interface 0x01 Traceroute request arrived on an interface
that is not the proper outgoing interface to which this router would not forward
for this source,group,destination. for this source,group,destination.
0x02 This router has sent a prune upstream for the group. 0x02 This router has sent a prune upstream for the group.
0x03 The next hop router has pruned 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. 0x04 The group is subject to administrative scoping at this hop.
0x05 This router has no route for the source. 0x05 This router has no route for the source.
0x07 This router is not forwarding this source,group 0x07 This router is not forwarding this source,group
for an unspecified reason. 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 0x81 There was not enough room to insert another response data block
in the packet. in the packet.
0x82 The next hop router does not understand traceroute requests. 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 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 packet to insert its response, it puts the 0x81 error code in the
previous router's ForwardingErr field, overwriting any error the previous router's ForwardingErr field, overwriting any error the
previous router placed there. It is expected that a multicast tra- previous router placed there. It is expected that a multicast tra-
ceroute client, upon receiving this error, will restart the trace ceroute client, upon receiving this error, will restart the trace
at the last hop listed in the packet. at the last hop listed in the packet.
The 0x80 bit of the ForwardingErr code is used to indicate a fatal The 0x80 bit of the ForwardingErr code is used to indicate a fatal
error. A fatal error is one that causes a router to be unable to error. A fatal error is one where the router may know the previous
forward this traceroute request on to the next hop. hop but cannot forward the message to it.
<<< Note that 0x01 and 0x05 should be fatal, but renumbering would 5. Router Behavior
be painful as backwards-compatibility is required. It's not clear
that an explicit fatal bit is required because a response issued
when the number of hops has not reached the maximum indicates that
the trace cannot go further. Feedback from implementors is
requested. >>>
5. Using multicast traceroute 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
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
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. Several problems may arise when attempting to use multicast traceroute.
5.1. Last hop router 6.1. Last hop router
The traceroute querier may not know which is the 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 or that router may be behind a firewall that blocks unicast packets
but passes multicast packets. In these cases, the traceroute but passes multicast packets. In these cases, the traceroute
request should be multicasted to the group being traced (since the request should be multicasted to the group being traced (since the
last hop router listens to that group). All routers except the last hop router listens to that group). All routers except the
correct last hop router should ignore any multicast traceroute correct last hop router should ignore any multicast traceroute
request received via multicast. request received via multicast. Traceroute requests which are mul-
ticasted to the group being traced must include the Router Alert IP
option [Katz96].
5.2. First hop router 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 The traceroute querier may not be unicast reachable from the first
hop router. In this case, the querier should set the traceroute hop router. In this case, the querier should set the traceroute
response address to a multicast address, and should set the response address to a multicast address, and should set the
response TTL to a value sufficient for the response from the first response TTL to a value sufficient for the response from the first
hop router to reach the querier. It may be appropriate to start hop router to reach the querier. It may be appropriate to start
with a small TTL and increase in subsequent attempts until a suffi- with a small TTL and increase in subsequent attempts until a suffi-
cient TTL is reached, up to an appropriate maximum (such as 192). 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 standard The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET, as the default
multicast group for multicast traceroute responses. 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.
5.3. Broken intermediate router 6.3. Broken intermediate router
A broken intermediate router might simply not understand traceroute A broken intermediate router might simply not understand traceroute
packets, and drop them. The querier would then get no response at packets, and drop them. The querier would then get no response at
all from its traceroute requests. It should then perform a search all from its traceroute requests. It should then perform a hop-
by setting the number of responses field until it gets a response by-hop search by setting the number of responses field until it
(both linear and binary search are options, but binary is likely to gets a response (both linear and binary search are options, but
be slower because a failure requires waiting for a timeout). binary is likely to be slower because a failure requires waiting
for a timeout).
6. Problem Diagnosis 6.4. Trace termination
6.1. Forwarding Inconsistencies 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
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 The forwarding error code can tell if a group is unexpectedly
pruned or administratively scoped. pruned or administratively scoped.
6.2. TTL problems 7.2. TTL problems
By taking the maximum of (hops from source + forwarding TTL thres- By taking the maximum of (hops from source + forwarding TTL thres-
hold) over all hops, you can discover the TTL required for the hold) over all hops, you can discover the TTL required for the
source to reach the destination. source to reach the destination.
6.3. Congestion 7.3. Congestion
By taking two traces, you can find packet loss information by com- By taking two traces, you can find packet loss information by com-
paring the difference in input packet counts to the difference in paring the difference in input packet counts to the difference in
output packet counts at the previous hop. On a point-to-point output packet counts at the previous hop. On a point-to-point
link, any difference in these numbers implies packet loss. Since link, any difference in these numbers implies packet loss. Since
the packet counts may be changing as the trace query is propagat- 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. ing, there may be small errors (off by 1 or 2) in these statistics.
However, these errors will not accumulate if multiple traces are However, these errors will not accumulate if multiple traces are
taken to expand the measurement period. On a shared link, the taken to expand the measurement period. On a shared link, the
count of input packets can be larger than the number of output count of input packets can be larger than the number of output
skipping to change at page 9, line 33 skipping to change at page 12, line 23
because some packets may be routed only to or from other interfaces because some packets may be routed only to or from other interfaces
on that node. on that node.
In the multicast extensions for SunOS 4.1.x from Xerox PARC, both In the multicast extensions for SunOS 4.1.x from Xerox PARC, both
the output packet count and the packet forwarding count for the the output packet count and the packet forwarding count for the
source-group pair are incremented before priority dropping for rate source-group pair are incremented before priority dropping for rate
limiting occurs and before the packets are put onto the interface limiting occurs and before the packets are put onto the interface
output queue which may overflow. These drops will appear as (posi- output queue which may overflow. These drops will appear as (posi-
tive) loss on the link even though they occur within the router. tive) loss on the link even though they occur within the router.
In release 3.3/3.4 of the multicast extensions, a multicast packet In release 3.3/3.4 of the UNIX multicast extensions, a multicast
generated on a router will be counted as having come in an inter- packet generated on a router will be counted as having come in an
face even though it did not. This can create the appearance of interface even though it did not. This can create the appearance
negative loss even on a point-to-point link. of negative loss even on a point-to-point link.
In releases up through 3.5/3.6, packets were not counted as input 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 on an interface if the reverse-path forwarding check decided that
the packets should be dropped. That causes the packets to appear 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 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 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 being traced are connected by a shared link, and the path for some
other group does not flow between those two routers because the other group does not flow between those two routers because the
downstream router receives packets for the other group on another downstream router receives packets for the other group on another
interface, but the upstream router is the elected forwarder to interface, but the upstream router is the elected forwarder to
other routers or hosts on the shared link. other routers or hosts on the shared link.
6.4. Link Utilization 7.4. Link Utilization
Again, with two traces, you can divide the difference in the input Again, with two traces, you can divide the difference in the input
or output packet counts at some hop by the difference in time 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. 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 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 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. limit or the physical capacity on a particular link being exceeded.
6.5. Time delay 7.5. Time delay
If the routers have synchronized clocks, you can estimate propaga- If the routers have synchronized clocks, it is possible to estimate
tion and queueing delay from the differences between the timestamps propagation and queueing delay from the differences between the
at successive hops. timestamps at successive hops.
7. Acknowledgments 8. Acknowledgments
This specification started largely as a transcription of Van Jacobson's 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 slides from the 30th IETF, and the implementation in mrouted 3.3 by Ajit
Thyagarajan. Van's original slides credit Steve Casner, Steve Deering, Thyagarajan. Van's original slides credit Steve Casner, Steve Deering,
Dino Farinacci and Deb Agrawal. A multicast traceroute client, mtrace, Dino Farinacci and Deb Agrawal. A multicast traceroute client, mtrace,
has been implemented by Ajit Thyagarajan and Steve Casner. has been implemented by Ajit Thyagarajan, Steve Casner and Bill Fenner.
8. Security Considerations 9. Security Considerations
Security issues are not discussed in this memo. Security issues are not discussed in this memo. <<Topology discovery>>
<<Traffic rates>>
9. Authors' Addresses 10. References
Katz96 Katz, D., "IP Router Alert Option," RFC XXXX, Cisco Sys-
tems, April 1996.
11. Authors' Addresses
William C. Fenner William C. Fenner
Xerox PARC Xerox PARC
3333 Coyote Hill Road 3333 Coyote Hill Road
Palo Alto, CA 94304 Palo Alto, CA 94304
Phone: +1 415 812 4816 Phone: +1 415 812 4816
Email: fenner@parc.xerox.com Email: fenner@parc.xerox.com
Stephen L. Casner Stephen L. Casner
Precept Software, Inc. Precept Software, Inc.
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