draft-ietf-idmr-traceroute-ipm-01.txt   draft-ietf-idmr-traceroute-ipm-02.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-01.txt Xerox PARC draft-ietf-idmr-traceroute-ipm-02.txt Xerox PARC
S. Casner S. Casner
Precept Software Precept Software
November 26, 1996 November 21, 1997
Expires: 3/31/97 Expires April 1998
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
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Abstract Abstract
This draft describes the IGMP multicast traceroute facility. As This draft describes the IGMP multicast traceroute facility. As
the deployment of IP multicast has spread, it has become clear that the deployment of IP multicast has spread, it has become clear that
a method for tracing the route that a multicast IP packet takes a method for tracing the route that a multicast IP packet takes
from a source to a particular receiver is absolutely required. from a source to a particular receiver is absolutely required.
Unlike unicast traceroute, multicast traceroute requires a special Unlike unicast traceroute, multicast traceroute requires a special
packet type and implementation on the part of routers. This packet type and implementation on the part of routers. This
specification describes the required functionality. specification describes the required functionality.
This document is a product of the Inter-Domain Multicast Routing working This document is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force. Comments are soli- group within the Internet Engineering Task Force. Comments are
cited and should be addressed to the working group's mailing list at solicited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s). idmr@cs.ucl.ac.uk and/or the author(s).
1. Introduction 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 The unicast "traceroute" program allows the tracing of a path from one
machine to another, using mechanisms that already existed in IP. Unfor- machine to another, using a mechanism that already existed in IP.
tunately, no such existing mechanisms can be applied to IP multicast Unfortunately, no such existing mechanism can be applied to IP multicast
paths. The key mechanism for unicast traceroute is the ICMP TTL exceeded paths. The key mechanism for unicast traceroute is the ICMP TTL exceeded
message, which is specifically precluded as a response to multicast message, which is specifically precluded as a response to multicast
packets. Thus, we specify the multicast "traceroute" facility to be packets. Thus, we specify the multicast "traceroute" facility to be
implemented in multicast routers and accessed by diagnostic programs. implemented in multicast routers and accessed by diagnostic programs.
While it is a disadvantage that a new mechanism is required, the multi- While it is a disadvantage that a new mechanism is required, the
cast traceroute facility can provide additional information about packet multicast traceroute facility can provide additional information about
rates and losses that the unicast traceroute cannot, and generally packet rates and losses that the unicast traceroute cannot, and
requires fewer packets to be sent. generally requires fewer packets to be sent.
Goals: Goals:
+ To be able to trace the path that a packet would take from some + To be able to trace the path that a packet would take from some
source to some destination. source to some destination.
+ To be able to isolate packet loss problems (e.g., congestion). + To be able to isolate packet loss problems (e.g., congestion).
+ To be able to isolate configuration problems (e.g., TTL threshold). + To be able to isolate configuration problems (e.g., TTL threshold).
+ To minimize packets sent (e.g. no flooding, no implosion). + To minimize packets sent (e.g. no flooding, no implosion).
2. Overview 3. Overview
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
nation to source is easy, as all existing multicast routing protocols destination to source is easy, as all existing multicast routing
know the previous hop for each source. Tracing from destination to protocols know the previous hop for each source. Tracing from
source can involve only routers on the direct path. destination to 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 Query 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 turns multicast router for the given destination. The last-hop router turns
the Query into a Request packet by adding a response data block contain- the Query into a Request packet by adding a response data block
ing its interface addresses and packet statistics, and then forwards the containing its interface addresses and packet statistics, and then
Request packet via unicast to the router that it believes is the proper forwards the Request packet via unicast to the router that it believes
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
such as "no route" is encountered along the path. 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.
3. Multicast Traceroute header 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: 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 4.1. IGMP Type: 8 bits
The IGMP type field is defined to be 0x1F for traceroute queries The IGMP type field is defined to be 0x1F for traceroute queries
and requests. The IGMP type field is changed to 0x1E when the and requests. The IGMP type field is changed to 0x1E when the
packet is completed and sent as a response from the first hop packet is completed and sent as a response from the first hop
router to the querier. Two codes are required so that multicast router to the querier. Two codes are required so that multicast
routers won't attempt to process a completed response in those routers won't attempt to process a completed response in those
cases where the initial query was issued from a router or the cases where the initial query was issued from a router or the
response is sent via multicast. response is sent via multicast.
3.2. # hops: 8 bits 4.2. # 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 4.3. Checksum: 16 bits
This is the standard IGMP checksum. 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.
3.4. Group address 4.4. Group address
This field specifies the group address to be traced, or zero if no This field specifies the group address to be traced, or zero if no
group-specific information is desired. Note that non-group- group-specific information is desired. Note that non-group-
specific traceroutes may not be possible with certain multicast specific traceroutes may not be possible with certain multicast
routing protocols. routing protocols.
3.5. Source address 4.5. 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, or 0xFFFFFFFF if no source-specific information
the intended multicast receiver towards this source. is desired. Note that non-source-specific traceroutes may not be
possible with certain multicast routing protocols.
3.6. Destination address 4.6. 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 traffic source.
3.7. Response Address 4.7. 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 6.2. explained in section 6.2.
3.8. resp ttl: 8 bits 4.8. 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 4.9. 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
to minimize collisions when a multicast response address is used. to minimize collisions when a multicast response address is used.
4. Response data 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.
6. Response data
Each router adds a "response data" segment to the traceroute packet be- Each router adds a "response data" segment to the traceroute packet be-
fore it forwards it on. The response data looks like this: fore it forwards it on. The response data looks like this:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query Arrival Time | | Query Arrival Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming Interface Address | | Incoming Interface Address |
skipping to change at page 5, line 27 skipping to change at page 6, line 27
| Outgoing Interface Address | | Outgoing Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous-Hop Router Address | | Previous-Hop Router Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Input packet count on incoming interface | | Input packet count on incoming interface |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Output packet count on outgoing interface | | Output packet count on outgoing interface |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Total number of packets for this source-group pair | | Total number of packets for this source-group pair |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol | FwdTTL |MBZ| Src Mask | ForwardingErr | | | |M| | | |
| Rtg Protocol | FwdTTL |B|S| Src Mask |Forwarding Code|
| | |Z| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1. Query Arrival Time 6.1. Query Arrival Time
The Query Arrival Time is a 32-bit NTP timestamp specifying the The Query Arrival Time is a 32-bit NTP timestamp specifying
arrival time of the traceroute request packet at this router. The the arrival time of the traceroute request packet at this
32-bit form of an NTP timestamp consists of the middle 32 bits of router. The 32-bit form of an NTP timestamp consists of the
the full 64-bit form; that is, the low 16 bits of the integer part middle 32 bits of the full 64-bit form; that is, the low 16
and the high 16 bits of the fractional part. bits of the integer part and the high 16 bits of the
fractional part.
4.2. Incoming Interface Address The following formula converts from a UNIX timeval to a 32-bit
NTP timestamp:
This field specifies the address of the interface on which packets query_arrival_time = (tv.tv_sec + 32384) << 16 + ((tv.tv_usec
from this source are expected to arrive, or 0 if unknown. << 10) / 15625)
4.3. Outgoing Interface Address 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).
This field specifies the address of the interface on which packets 6.2. Incoming Interface Address
from this source flow to the specified destination, or 0 if unk-
nown.
4.4. Previous-Hop Router 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 This field specifies the router from which this router expects
packets from this source, or 0 if unknown. 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.
4.5. Input packet count on incoming interface 6.5. Input packet count on incoming interface
This field contains the number of multicast packets received for This field contains the number of multicast packets received
all groups and sources on the incoming interface, or 0xffffffff if for all groups and sources on the incoming interface, or
no count can be reported. 0xffffffff if no count can be reported.
4.6. Output packet count on outgoing interface 6.6. Output packet count on outgoing interface
This field contains the number of multicast packets that have been This field contains the number of multicast packets that have
transmitted for all groups and sources on the outgoing interface, been transmitted for all groups and sources on the outgoing
or 0xffffffff if no count can be reported. interface, or 0xffffffff if no count can be reported.
4.7. Total number of packets for this source-group pair 6.7. Total number of packets for this source-group pair
This field counts the number of packets from the specified source This field counts the number of packets from the specified
forwarded by this router to the specified group, or 0xffffffff if source forwarded by this router to the specified group, or
no count can be reported. 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.
4.8. Rtg Protocol: 8 bits 6.8. 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 5 PIM using special routing table
6 - PIM using a static route 6 PIM using a static route
7 - DVMRP using a static route 7 DVMRP using a static route
4.9. FwdTTL: 8 bits 6.9. 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 6.10. MBZ: 1 bit
This field contains the number of 1's in the netmask this router Must be zeroed on transmission and ignored on reception.
has for the source (i.e. a value of 24 means the netmask is
0xffffff00)
4.11. ForwardingErr: 8 bits 6.11. S: 1 bit
This field contains a forwarding error code. Specified values If this bit is set, it indicates that the packet count for the
include: 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 0x00 No error
0x01 Traceroute request arrived on an interface 0x01
to which this router would not forward Traceroute request arrived on an interface to
for this source,group,destination. which this router would not forward for this
0x02 This router has sent a prune upstream for the group. source,group,destination.
0x03 This router has stopped forwarding in response to a 0x02
request from the next hop router. This router has sent a prune upstream which
0x04 The group is subject to administrative scoping at this hop. 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.
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 0x06 This router is not the proper last-hop router.
0x07
This router is not forwarding this source,group
for an unspecified reason. for an unspecified reason.
0x08 Reached Rendez-vous Point or Core 0x08 Reached Rendez-vous Point or Core
0x09 Traceroute request arrived on the expected 0x09
RPF interface for this source,group. Traceroute request arrived on the expected RPF
0x0A Traceroute request arrived on an interface which interface for this source,group.
0x0A
Traceroute request arrived on an interface which
is not enabled for multicast. is not enabled for multicast.
0x81 There was not enough room to insert another response data block 0x81
in the packet. There was not enough room to insert another
0x82 The next hop router does not understand traceroute requests. response data block in the packet.
0x82
The previous hop router does not understand
traceroute requests.
0x83 Traceroute is administratively prohibited. 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
previous router's ForwardingErr field, overwriting any error the the previous router's Forwarding Code field, overwriting any
previous router placed there. It is expected that a multicast tra- error the previous router placed there. It is expected that a
ceroute client, upon receiving this error, will restart the trace multicast traceroute client, upon receiving this error, will
at the last hop listed in the packet. restart the trace 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 Forwarding Code is used to indicate a
error. A fatal error is one where the router may know the previous fatal error. A fatal error is one where the router may know
hop but cannot forward the message to it. the previous hop but cannot forward the message to it.
5. Router Behavior 7. Router Behavior
All of these actions are performed in addition to (NOT instead of) for- All of these actions are performed in addition to (NOT instead of)
warding the packet, if applicable. E.g. a multicast packet that has TTL forwarding the packet, if applicable. E.g. a multicast packet that
remaining MUST still get forwarded. has TTL remaining MUST be forwarded normally, as should a unicast
packet that has TTL remaining and is not addressed to this router.
5.1. Traceroute Query 7.1. Traceroute Query
Upon receiving a traceroute Query message (a request with no A traceroute Query message is a traceroute message with no
response blocks filled in), a router must examine the traceroute response blocks filled in, and uses IGMP type 0x1F.
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 7.1.1. Packet Verification
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 Upon receiving a traceroute Query message, a router must
and it determines that it is the proper last-hop router, it inserts examine the Query to see if it is the proper last-hop router
a response block and forwards the traceroute request towards the for the destination address in the packet. It is the proper
router that it expects to be the previous hop for this source and last-hop router if it has a multicast-capable interface on the
group (or, if no group is specified, the previous hop for this same subnet as the Destination Address and is the router that
source). would forward traffic from the given source onto that subnet.
5.2. Traceroute Request 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.
When a router receives a traceroute request with some number of Duplicate Query messages as identified by the tuple (IP
response blocks filled in, it first checks the interface from which Source, Query ID) SHOULD be ignored.
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 7.1.2. Normal Processing
A router must forward all traceroute response packets normally, When a router receives a traceroute Query and it determines
with no special processing. 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.
5.4. Sending Traceroute Responses 7.2. Traceroute Request
5.4.1. Destination Address 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.
A traceroute response must be sent to the Response Address in the 7.2.1. Packet Verification
traceroute header.
5.4.2. TTL 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.
If the Response Address is unicast, the router inserts its normal 7.2.2. Normal Processing
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 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".
If the Response Address is unicast, the router may use any of its 1. Insert a new response block into the packet and fill in
interface addresses as the source address, preferring globally the Query Arrival Time, Outgoing Interface Address,
routable addresses. If the Response Address is multicast, the Output Packet Count, and FwdTTL.
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 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.
When a router sources a multicast response, the response packet 3. If no forwarding information can be determined, an error
MUST be forwarded as if it were received on the outgoing interface. 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".
6. Using multicast traceroute 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.
<<Need a section on expected client behavior (one or two attempts with 5. If traceroute is administratively prohibited or the
high hop count, then a search of some kind, then statistics later)>> previous hop router does not understand traceroute
Several problems may arise when attempting to use multicast 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".
6.1. Last hop router 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.
The traceroute querier may not know which is the last hop router, 7. If the group is subject to administrative scoping on
or that router may be behind a firewall that blocks unicast packets either the Outgoing or Incoming interfaces, a forwarding
but passes multicast packets. In these cases, the traceroute code of 0x4 is noted.
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 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 8. If this router is the Rendez-vous Point or Core for the
destination of the request, the traceroute request may be multi- group, a forwarding code of 0x8 is noted. (NOTE: should
casted to 224.0.0.2 (ALL-ROUTERS.MCAST.NET) if the last-hop router this be earlier?)
is not known.
6.2. First hop router 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
0x7.
The traceroute querier may not be unicast reachable from the first 10. The packet is then sent on to the previous hop or the
hop router. In this case, the querier should set the traceroute requester as described in "Forwarding Traceroute
response address to a multicast address, and should set the Requests".
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 7.3. Traceroute response
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 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.
A broken intermediate router might simply not understand traceroute 7.4. Forwarding Traceroute Requests
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 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".
When performing an expanding hop-by-hop trace, it is necessary to 7.5. Sending Traceroute Responses
determine when to stop expanding.
6.4.1. Arriving at source 7.5.1. Destination Address
A trace can be determined to have arrived at the source if the last A traceroute response must be sent to the Response Address in
router in the trace has an interface on the same subnet as the the traceroute header.
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 7.5.2. TTL
A trace has encountered a fatal error if the last Forwarding Error If the Response Address is unicast, the router inserts its
in the trace has the 0x80 bit set. 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.
6.4.3. No Previous Hop 7.5.3. Source Address
A trace can not continue if the last Previous Hop in the trace is If the Response Address is unicast, the router may use any of
set to 0. 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. Problem Diagnosis 7.5.4. Sourcing Multicast Responses
7.1. Forwarding Inconsistencies 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.
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
is set to 0.
9. Problem Diagnosis
9.1. 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.
7.2. TTL problems 9.2. TTL problems
By taking the maximum of (hops from source + forwarding TTL thres- By taking the maximum of (hops from source + forwarding TTL
hold) over all hops, you can discover the TTL required for the threshold) over all hops, you can discover the TTL required
source to reach the destination. for the source to reach the destination.
7.3. Congestion 9.3. Congestion
By taking two traces, you can find packet loss information by com- By taking two traces, you can find packet loss information by
paring the difference in input packet counts to the difference in comparing the difference in input packet counts to the
output packet counts at the previous hop. On a point-to-point difference in output packet counts at the previous hop. On a
link, any difference in these numbers implies packet loss. Since point-to-point link, any difference in these numbers implies
the packet counts may be changing as the trace query is propagat- packet loss. Since the packet counts may be changing as the
ing, there may be small errors (off by 1 or 2) in these statistics. trace query is propagating, there may be small errors (off by
However, these errors will not accumulate if multiple traces are 1 or 2) in these statistics. However, these errors will not
taken to expand the measurement period. On a shared link, the accumulate if multiple traces are taken to expand the
count of input packets can be larger than the number of output measurement period. On a shared link, the count of input
packets at the previous hop, due to other routers or hosts on the packets can be larger than the number of output packets at the
link injecting packets. This appears as "negative loss" which may previous hop, due to other routers or hosts on the link
injecting packets. This appears as "negative loss" which may
mask real packet loss. mask real packet loss.
In addition to the counts of input and output packets for all mul- In addition to the counts of input and output packets for all
ticast traffic on the interfaces, the response data includes a multicast traffic on the interfaces, the response data
count of the packets forwarded by a node for the specified source- includes a count of the packets forwarded by a node for the
group pair. Taking the difference in this count between two traces specified source-group pair. Taking the difference in this
and then comparing those differences between two hops gives a meas- count between two traces and then comparing those differences
ure of packet loss just for traffic from the specified source to between two hops gives a measure of packet loss just for
the specified receiver via the specified group. This measure is traffic from the specified source to the specified receiver
not affected by shared links. 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 On a point-to-point link that is a multicast tunnel, packet
usually due to congestion in unicast routers along the path of that loss is usually due to congestion in unicast routers along the
tunnel. On native multicast links, loss is more likely in the out- path of that tunnel. On native multicast links, loss is more
put queue of one hop, perhaps due to priority dropping, or in the likely in the output queue of one hop, perhaps due to priority
input queue at the next hop. The counters in the response data do dropping, or in the input queue at the next hop. The counters
not allow these cases to be distinguished. Differences in packet in the response data do not allow these cases to be
counts between the incoming and outgoing interfaces on one node distinguished. Differences in packet counts between the
cannot generally be used to measure queue overflow in the node incoming and outgoing interfaces on one node cannot generally
because some packets may be routed only to or from other interfaces be used to measure queue overflow in the node because some
on that node. 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 In the multicast extensions for SunOS 4.1.x from Xerox PARC,
the output packet count and the packet forwarding count for the both the output packet count and the packet forwarding count
source-group pair are incremented before priority dropping for rate for the source-group pair are incremented before priority
limiting occurs and before the packets are put onto the interface dropping for rate limiting occurs and before the packets are
output queue which may overflow. These drops will appear as (posi- put onto the interface output queue which may overflow. These
tive) loss on the link even though they occur within the router. 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 In release 3.3/3.4 of the UNIX multicast extensions, a
packet generated on a router will be counted as having come in an multicast packet generated on a router will be counted as
interface even though it did not. This can create the appearance having come in an interface even though it did not. This can
of negative loss even on a point-to-point link. 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 In releases up through 3.5/3.6, packets were not counted as
on an interface if the reverse-path forwarding check decided that input on an interface if the reverse-path forwarding check
the packets should be dropped. That causes the packets to appear decided that the packets should be dropped. That causes the
as lost on the link if they were output by the upstream hop. This packets to appear as lost on the link if they were output by
situation can arise when two routers on the path for the group the upstream hop. This situation can arise when two routers
being traced are connected by a shared link, and the path for some on the path for the group being traced are connected by a
other group does not flow between those two routers because the shared link, and the path for some other group does not flow
downstream router receives packets for the other group on another between those two routers because the downstream router
interface, but the upstream router is the elected forwarder to receives packets for the other group on another interface, but
other routers or hosts on the shared link. the upstream router is the elected forwarder to other routers
or hosts on the shared link.
7.4. Link Utilization 9.4. 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
or output packet counts at some hop by the difference in time input or output packet counts at some hop by the difference in
stamps from the same hop to obtain the packet rate over the link. time stamps from the same hop to obtain the packet rate over
If the average packet size is known, then the link utilization can the link. If the average packet size is known, then the link
also be estimated to see whether packet loss may be due to the rate utilization can also be estimated to see whether packet loss
limit or the physical capacity on a particular link being exceeded. may be due to the rate limit or the physical capacity on a
particular link being exceeded.
7.5. Time delay 9.5. Time delay
If the routers have synchronized clocks, it is possible to estimate If the routers have synchronized clocks, it is possible to
propagation and queueing delay from the differences between the estimate propagation and queueing delay from the differences
timestamps at successive hops. between the timestamps at successive hops.
8. Acknowledgments 10. Acknowledgments
This specification started largely as a transcription of Van Jacobson's This specification started largely as a transcription of Van
slides from the 30th IETF, and the implementation in mrouted 3.3 by Ajit Jacobson's slides from the 30th IETF, and the implementation in
Thyagarajan. Van's original slides credit Steve Casner, Steve Deering, mrouted 3.3 by Ajit Thyagarajan. Van's original slides credit
Dino Farinacci and Deb Agrawal. A multicast traceroute client, mtrace, Steve Casner, Steve Deering, Dino Farinacci and Deb Agrawal. A
has been implemented by Ajit Thyagarajan, Steve Casner and Bill Fenner. multicast traceroute client, mtrace, has been implemented by Ajit
Thyagarajan, Steve Casner and Bill Fenner.
9. Security Considerations 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.
Security issues are not discussed in this memo. <<Topology discovery>> 11. IANA Considerations
<<Traffic rates>>
10. References 11.1. Routing Protocols
Katz96 Katz, D., "IP Router Alert Option," RFC XXXX, Cisco Sys- Should the IANA be responsible for allocating new Routing
tems, April 1996. Protocol codes?
11. Authors' Addresses 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
Requirement Levels", RFC 2119/BCP 14, Harvard
University, March 1997.
Katz97 Katz, D., "IP Router Alert Option," RFC 2113, Cisco
Systems, February 1997.
14. 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: +1 650 812 4816
Email: fenner@parc.xerox.com Email: fenner@parc.xerox.com
Stephen L. Casner Stephen L. Casner
Precept Software, Inc. Precept Software, Inc.
21580 Stevens Creek Blvd, Suite 207 1072 Arastradero Road
Cupertino, CA 95014 Palo Alto, CA 94304
Email: casner@precept.com Email: casner@precept.com
 End of changes. 

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