draft-ietf-idmr-traceroute-ipm-02.txt   draft-ietf-idmr-traceroute-ipm-03.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-02.txt Xerox PARC draft-ietf-idmr-traceroute-ipm-03.txt Xerox PARC
S. Casner S. Casner
Precept Software Precept Software
November 21, 1997 August 5, 1998
Expires April 1998 Expires December 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 This document is an Internet-Draft. Internet-Drafts are working docu-
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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 speci-
specification describes the required functionality. fication 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 group within the Internet Engineering Task Force. Comments are
solicited 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. Key Words Key Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
this document are to be interpreted as described in RFC 2119 document are to be interpreted as described in RFC 2119 [Bradner97].
[Bradner97].
2. Introduction 1. 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 a mechanism that already existed in IP. machine to another, using a mechanism that already existed in IP.
Unfortunately, no such existing mechanism 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
message, which is specifically precluded as a response to multicast exceeded message, which is specifically precluded as a response to mul-
packets. Thus, we specify the multicast "traceroute" facility to be ticast packets. Thus, we specify the multicast "traceroute" facility to
implemented in multicast routers and accessed by diagnostic programs. be implemented in multicast routers and accessed by diagnostic programs.
While it is a disadvantage that a new mechanism is required, the While it is a disadvantage that a new mechanism is required, the multi-
multicast traceroute facility can provide additional information about cast traceroute facility can provide additional information about packet
packet rates and losses that the unicast traceroute cannot, and rates and losses that the unicast traceroute cannot, and generally
generally requires fewer packets to be sent. requires fewer packets to be sent.
Goals: Goals:
+ To be able to trace the path that a packet would take from some o 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). o To be able to isolate packet loss problems (e.g., congestion).
+ To be able to isolate configuration problems (e.g., TTL threshold). o To be able to isolate configuration problems (e.g., TTL threshold).
+ To minimize packets sent (e.g. no flooding, no implosion). o To minimize packets sent (e.g. no flooding, no implosion).
3. Overview 2. Overview
Tracing from a source to a multicast destination is hard, since you Given a multicast distribution tree, tracing from a source to a multi-
don't know down which branch of the multicast tree the destination lies. cast destination is hard, since you don't know down which branch of the
This means that you have to flood the whole tree to find the path from multicast tree the destination lies. This means that you have to flood
one source to one destination. However, walking up the tree from the whole tree to find the path from one source to one destination.
destination to source is easy, as all existing multicast routing However, walking up the tree from destination to source is easy, as most
protocols know the previous hop for each source. Tracing from existing multicast routing protocols know the previous hop for each
destination to source can involve only routers on the direct path. 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 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 the Query into a Request packet by adding a response data block contain-
containing its interface addresses and packet statistics, and then ing its interface addresses and packet statistics, and then forwards the
forwards the Request packet via unicast to the router that it believes Request packet via unicast to the router that it believes is the proper
is the proper previous hop for the given source and group. Each hop previous hop for the given source and group. Each hop adds its response
adds its response data to the end of the Request packet, then unicast data to the end of the Request packet, then unicast forwards it to the
forwards it to the previous hop. The first hop router (the router that previous hop. The first hop router (the router that believes that pack-
believes that packets from the source originate on one of its directly ets from the source originate on one of its directly connected networks)
connected networks) changes the packet type to indicate a Response changes the packet type to indicate a Response packet and sends the com-
packet and sends the completed response to the response destination pleted response to the response destination address. The response may
address. The response may be returned before reaching the first hop be returned before reaching the first hop router if a fatal error condi-
router if a fatal error condition such as "no route" is encountered tion such as "no route" is encountered along the path.
along the path.
Multicast traceroute uses any information available to it in the router Multicast traceroute uses any information available to it in the router
to attempt to determine a previous hop to forward the trace towards. to attempt to determine a previous hop to forward the trace towards.
Multicast routing protocols vary in the type and amount of state they Multicast routing protocols vary in the type and amount of state they
keep; multicast traceroute endeavors to work with all of them by using keep; multicast traceroute endeavors to work with all of them by using
whatever is available. For example, if a DVMRP router has no active 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 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 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 is on the (*,G) tree, it chooses the parent towards the RP as the previ-
previous hop. In these cases, no source/group-specific state is ous hop. In these cases, no source/group-specific state is available,
available, but the path may still be traced. but the path may still be traced.
4. Multicast Traceroute header 3. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1. IGMP Type: 8 bits 3.1. 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.
4.2. # 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-
first-hop router, this field can be used to perform an expanding- hop router, this field can be used to perform an expanding-length
length search to trace the path to just before the problem. search to trace the path to just before the problem.
4.3. Checksum: 16 bits 3.3. Checksum: 16 bits
The checksum is the 16-bit one's complement of the one's complement 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 sum of the whole IGMP message (the entire IP payload)[Brad88].
computing the checksum, the checksum field is set to zero. When When computing the checksum, the checksum field is set to zero.
transmitting packets, the checksum MUST be computed and inserted When transmitting packets, the checksum MUST be computed and
into this field. When receiving packets, the checksum MUST be inserted into this field. When receiving packets, the checksum
verified before processing a packet. MUST be verified before processing a packet.
4.4. Group address 3.4. 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-spe-
specific traceroutes may not be possible with certain multicast cific traceroutes may not be possible with certain multicast rout-
routing protocols. ing protocols.
4.5. 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, or 0xFFFFFFFF if no source-specific information path being traced, or 0xFFFFFFFF if no source-specific information
is desired. Note that non-source-specific traceroutes may not be is desired. Note that non-source-specific traceroutes may not be
possible with certain multicast routing protocols. possible with certain multicast routing protocols.
4.6. 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 traffic source. proceeds toward the traffic source.
4.7. 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 6.2. explained in section 6.2.
4.8. 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.
4.9. 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
to minimize collisions when a multicast response address is used. to minimize collisions when a multicast response address is used.
5. Definitions 4. Definitions
Since multicast traceroutes flow in the opposite direction to the data Since multicast traceroutes flow in the opposite direction to the data
flow, we always refer to "upstream" and "downstream" with respect to flow, we always refer to "upstream" and "downstream" with respect to
data, unless explicitly specified. data, unless explicitly specified.
Incoming Interface Incoming Interface
The interface on which traffic is expected from the specified The interface on which traffic is expected from the specified
source and group. source and group.
Outgoing Interface Outgoing Interface
The interface on which traffic is forwarded from the specified The interface on which traffic is forwarded from the specified
source and group towards the destination. Also called the source and group towards the destination. Also called the "Recep-
"Reception Interface", since it is the interface on which the tion Interface", since it is the interface on which the multicast
multicast traceroute Request was received. traceroute Request was received.
Previous-Hop Router Previous-Hop Router
The router, on the Incoming Interface, which is responsible for The router, on the Incoming Interface, which is responsible for
forwarding traffic for the specified source and group. forwarding traffic for the specified source and group.
6. Response data 5. 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
fore it forwards it on. The response data looks like this: before 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Interface Address | | Outgoing Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 6, line 32 skipping to change at page 6, line 32
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |M| | | | | | |M| | | |
| Rtg Protocol | FwdTTL |B|S| Src Mask |Forwarding Code| | Rtg Protocol | FwdTTL |B|S| Src Mask |Forwarding Code|
| | |Z| | | | | | |Z| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6.1. Query Arrival Time 5.1. Query Arrival Time
The Query Arrival Time is a 32-bit NTP timestamp specifying The Query Arrival Time is a 32-bit NTP timestamp specifying the
the arrival time of the traceroute request packet at this arrival time of the traceroute request packet at this router. The
router. The 32-bit form of an NTP timestamp consists of the 32-bit form of an NTP timestamp consists of the middle 32 bits of
middle 32 bits of the full 64-bit form; that is, the low 16 the full 64-bit form; that is, the low 16 bits of the integer part
bits of the integer part and the high 16 bits of the and the high 16 bits of the fractional part.
fractional part.
The following formula converts from a UNIX timeval to a 32-bit The following formula converts from a UNIX timeval to a 32-bit NTP
NTP timestamp: timestamp:
query_arrival_time = (tv.tv_sec + 32384) << 16 + ((tv.tv_usec query_arrival_time = (tv.tv_sec + 32384) << 16 + ((tv.tv_usec <<
<< 10) / 15625) 10) / 15625)
The constant 32384 is the number of seconds from Jan 1, 1900 The constant 32384 is the number of seconds from Jan 1, 1900 to Jan
to Jan 1, 1970 truncated to 16 bits. ((tv.tv_usec << 10) / 1, 1970 truncated to 16 bits. ((tv.tv_usec << 10) / 15625) is a
15625) is a reduction of ((tv.tv_usec / 100000000) << 16). reduction of ((tv.tv_usec / 100000000) << 16).
6.2. Incoming Interface Address 5.2. Incoming Interface Address
This field specifies the address of the interface on which This field specifies the address of the interface on which packets
packets from this source and group are expected to arrive, or from this source and group are expected to arrive, or 0 if unknown.
0 if unknown.
6.3. Outgoing Interface Address 5.3. Outgoing Interface Address
This field specifies the address of the interface on which This field specifies the address of the interface on which packets
packets from this source and group flow to the specified from this source and group flow to the specified destination, or 0
destination, or 0 if unknown. if unknown.
6.4. Previous-Hop Router Address 5.4. 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. This may be a multicast group if packets from this source. This may be a multicast group if the
the previous hop is not known because of the workings of the previous hop is not known because of the workings of the multicast
multicast routing protocol. However, it should be 0 if the routing protocol. However, it should be 0 if the incoming inter-
incoming interface address is unknown. face address is unknown.
6.5. Input packet count on incoming interface 5.5. Input packet count on incoming interface
This field contains the number of multicast packets received This field contains the number of multicast packets received for
for all groups and sources on the incoming interface, or all groups and sources on the incoming interface, or 0xffffffff if
0xffffffff if no count can be reported. no count can be reported.
6.6. Output packet count on outgoing interface 5.6. Output packet count on outgoing interface
This field contains the number of multicast packets that have This field contains the number of multicast packets that have been
been transmitted for all groups and sources on the outgoing transmitted for all groups and sources on the outgoing interface,
interface, or 0xffffffff if no count can be reported. or 0xffffffff if no count can be reported.
6.7. Total number of packets for this source-group pair 5.7. Total number of packets for this source-group pair
This field counts the number of packets from the specified This field counts the number of packets from the specified source
source forwarded by this router to the specified group, or forwarded by this router to the specified group, or 0xffffffff if
0xffffffff if no count can be reported. If the S bit is set, no count can be reported. If the S bit is set, the count is for
the count is for the source network, as specified by the Src the source network, as specified by the Src Mask field. If the S
Mask field. If the S bit is set and the Src Mask field is 63, bit is set and the Src Mask field is 63, indicating no source-spe-
indicating no source-specific state, the count is for all cific state, the count is for all sources sending to this group.
sources sending to this group.
6.8. Rtg Protocol: 8 bits 5.8. 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
6.9. FwdTTL: 8 bits 5.9. 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.
6.10. MBZ: 1 bit 5.10. MBZ: 1 bit
Must be zeroed on transmission and ignored on reception. Must be zeroed on transmission and ignored on reception.
6.11. S: 1 bit 5.11. S: 1 bit
If this bit is set, it indicates that the packet count for the If this bit is set, it indicates that the packet count for the
source-group pair is for the source network, as determined by source-group pair is for the source network, as determined by mask-
masking the source address with the Src Mask field. ing the source address with the Src Mask field.
6.12. Src Mask: 6 bits 5.12. Src Mask: 6 bits
This field contains the number of 1's in the netmask this This field contains the number of 1's in the netmask this router
router has for the source (i.e. a value of 24 means the has for the source (i.e. a value of 24 means the netmask is
netmask is 0xffffff00). If the router is forwarding solely on 0xffffff00). If the router is forwarding solely on group state,
group state, this field is set to 63 (0x2f). this field is set to 63 (0x3f).
6.13. Forwarding Code: 8 bits 5.13. Forwarding Code: 8 bits
This field contains a forwarding information/error code. This field contains a forwarding information/error code. Defined
Defined values include: values include:
0x00 No error Value Name Description
0x01 --------------------------------------------------------------------
Traceroute request arrived on an interface to 0x00 NO_ERROR No error
0x01 WRONG_IF Traceroute request arrived on an interface to
which this router would not forward for this which this router would not forward for this
source,group,destination. source,group,destination.
0x02 0x02 PRUNE_SENT This router has sent a prune upstream which
This router has sent a prune upstream which applies to the source and group in the tracer-
applies to the source and group in the traceroute oute request.
request.
0x03
This router has stopped forwarding for this source
and group in response to a request from the next
hop router.
0x04 0x03 PRUNE_RCVD This router has stopped forwarding for this
The group is subject to administrative scoping at source and group in response to a request from
this hop. the next hop router.
0x05 This router has no route for the source. 0x04 SCOPED The group is subject to administrative scoping
0x06 This router is not the proper last-hop router. at this hop.
0x07 0x05 NO_ROUTE This router has no route for the source.
This router is not forwarding this source,group 0x06 WRONG_LAST_HOP This router is not the proper last-hop router.
for an unspecified reason. 0x07 NOT_FORWARDING This router is not forwarding this
0x08 Reached Rendez-vous Point or Core source,group for an unspecified reason.
0x09 0x08 REACHED_RP Reached Rendez-vous Point or Core
Traceroute request arrived on the expected RPF 0x09 RPF_IF Traceroute request arrived on the expected RPF
interface for this source,group. interface for this source,group.
0x0A 0x0A NO_MULTICAST Traceroute request arrived on an interface
Traceroute request arrived on an interface which which is not enabled for multicast.
is not enabled for multicast. 0x81 NO_SPACE There was not enough room to insert another
0x81
There was not enough room to insert another
response data block in the packet. response data block in the packet.
0x82 0x82 OLD_ROUTER The previous hop router does not understand
The previous hop router does not understand
traceroute requests. traceroute requests.
0x83 Traceroute is administratively prohibited. 0x83 ADMIN_PROHIB 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 packet to insert its response, it puts the 0x81 error code in the
the previous router's Forwarding Code field, overwriting any previous router's Forwarding Code field, overwriting any error the
error the previous router placed there. It is expected that a previous router placed there. It is expected that a multicast
multicast traceroute client, upon receiving this error, will traceroute client, upon receiving this error, will restart the
restart the trace at the last hop listed in the packet. trace at the last hop listed in the packet.
The 0x80 bit of the Forwarding Code is used to indicate a The 0x80 bit of the Forwarding Code is used to indicate a fatal
fatal error. A fatal error is one where the router may know error. A fatal error is one where the router may know the previous
the previous hop but cannot forward the message to it. hop but cannot forward the message to it.
7. Router Behavior 6. Router Behavior
All of these actions are performed in addition to (NOT instead of) All of these actions are performed in addition to (NOT instead of) for-
forwarding the packet, if applicable. E.g. a multicast packet that warding the packet, if applicable. E.g. a multicast packet that has TTL
has TTL remaining MUST be forwarded normally, as should a unicast remaining MUST be forwarded normally, as MUST a unicast packet that has
packet that has TTL remaining and is not addressed to this router. TTL remaining and is not addressed to this router.
7.1. Traceroute Query 6.1. Traceroute Query
A traceroute Query message is a traceroute message with no A traceroute Query message is a traceroute message with no response
response blocks filled in, and uses IGMP type 0x1F. blocks filled in, and uses IGMP type 0x1F.
7.1.1. Packet Verification 6.1.1. Packet Verification
Upon receiving a traceroute Query message, a router must Upon receiving a traceroute Query message, a router must examine
examine the Query to see if it is the proper last-hop router the Query to see if it is the proper last-hop router for the
for the destination address in the packet. It is the proper destination address in the packet. It is the proper last-hop
last-hop router if it has a multicast-capable interface on the router if it has a multicast-capable interface on the same subnet
same subnet as the Destination Address and is the router that as the Destination Address and is the router that would forward
would forward traffic from the given source onto that subnet. traffic from the given source onto that subnet.
A router may receive a traceroute Query message via either If the router determines that it is not the proper last-hop router,
unicast or multicast. If received via multicast and it or it cannot make that determination, it does one of two things
determines that it is not the proper last-hop router, the depending if the Query was received via multicast or unicast. If
packet MUST be silently dropped. If received via unicast and the Query was received via multicast, then it MUST be silently
the packet was addressed to this router, an error code of 0x06 dropped. If it was received via unicast, a forwarding code of
should be noted and normal processing should occur. NOT_LAST_HOP is noted and processing continues as in section 7.2.
Duplicate Query messages as identified by the tuple (IP Duplicate Query messages as identified by the tuple (IP Source,
Source, Query ID) SHOULD be ignored. Query ID) SHOULD be ignored.
7.1.2. Normal Processing 6.1.2. Normal Processing
When a router receives a traceroute Query and it determines When a router receives a traceroute Query and it determines that it
that it is the proper last-hop router, it treats it like a is the proper last-hop router, it treats it like a traceroute
traceroute Request and performs the steps listed under Normal Request and performs the steps listed in section 7.2.
Processing of a Traceroute Request, below.
7.2. Traceroute Request 6.2. Traceroute Request
A traceroute Request is a traceroute message with some number A traceroute Request is a traceroute message with some number of
of response blocks filled in, and also uses IGMP type 0x1F. response blocks filled in, and also uses IGMP type 0x1F. Routers
Routers can tell the difference between Queries and Requests can tell the difference between Queries and Requests by checking
by checking the length of the packet. the length of the packet.
7.2.1. Packet Verification 6.2.1. Packet Verification
If the traceroute Request is not addressed to this router, or If the traceroute Request is not addressed to this router, or if
if the Request is addressed to a multicast group which is not the Request is addressed to a multicast group which is not a link-
a link-scoped group (e.g. 224.0.0.x), it MUST be silently scoped group (e.g. 224.0.0.x), it MUST be silently ignored.
ignored.
7.2.2. Normal Processing 6.2.2. Normal Processing
When a router receives a traceroute Request, it performs the When a router receives a traceroute Request, it performs the fol-
following steps. Note that it is possible to have multiple lowing steps. Note that it is possible to have multiple situations
situations covered by the Forwarding Codes. The first one covered by the Forwarding Codes. The first one encountered is the
encountered is the one that is reported, i.e. all "note one that is reported, i.e. all "note forwarding code N" should be
forwarding code N" should be interpreted as "if forwarding interpreted as "if forwarding code is not already set, set forward-
code is not already set, set forwarding code to N". ing code to N".
1. Insert a new response block into the packet and fill in 1. Insert a new response block into the packet and fill in the
the Query Arrival Time, Outgoing Interface Address, Query Arrival Time, Outgoing Interface Address, Output Packet
Output Packet Count, and FwdTTL. Count, and FwdTTL.
2. Attempt to determine the forwarding information for the 2. Attempt to determine the forwarding information for the source
source and group specified, using the same mechanisms as and group specified, using the same mechanisms as would be used
would be used when a packet is received from the source when a packet is received from the source destined for the
destined for the group. State need not be instantiated, group. State need not be instantiated, it can be "phantom"
it can be "phantom" state created only for the purpose of state created only for the purpose of the trace.
the trace.
3. If no forwarding information can be determined, an error 3. If no forwarding information can be determined, an error code
code of 0x05 is inserted in the Forwarding Code field, of NO_ROUTE is inserted in the Forwarding Code field, the
the remaining fields that have not yet been filled in are remaining fields that have not yet been filled in are set to
set to zero, and the packet is forwarded to the requester 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 (ADMIN_PROHIB or OLD_ROUTER). If
traceroute is administratively prohibited and any of the fields
as filled in step 4 are considered private information, zero
out the applicable fields. Then the packet is forwarded to the
requester as described in "Forwarding Traceroute Requests".
6. If the reception interface is not enabled for multicast, note
forwarding code NO_MULTICAST. If the reception interface is
the interface from which the router would expect data to arrive
from the source, a forwarding code of RPF_IF is noted. Other-
wise, if the reception interface is not one to which the router
would forward data from the source, a forwarding code of
WRONG_IF is noted.
7. If the group is subject to administrative scoping on either the
Outgoing or Incoming interfaces, a forwarding code of SCOPED is
noted.
8. If this router is the Rendez-vous Point or Core for the group,
a forwarding code of REACHED_RP is noted.
9. If this router has sent a prune upstream which applies to the
source and group in the traceroute Request, it notes forwarding
code PRUNE_SENT. If the router has stopped forwarding down-
stream in response to a prune sent by the next hop router, it
notes forwarding code PRUNE_RCVD. If the router should nor-
mally forward traffic for this source and group downstream but
is not, it notes forwarding code NOT_FORWARDING.
10. The packet is then sent on to the previous hop or the requester
as described in "Forwarding Traceroute Requests". as described in "Forwarding Traceroute Requests".
4. Fill in the Incoming Interface Address, Previous-Hop 6.3. Traceroute response
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 A router must forward all traceroute response packets normally,
previous hop router does not understand traceroute with no special processing. If a router has initiated a traceroute
requests, note the appropriate forwarding code. If with a Query or Request message, it may listen for Responses to
traceroute is administratively prohibited and any of the that traceroute but MUST still forward them as well.
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, 6.4. Forwarding Traceroute Requests
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 If the Previous-hop router is known for the source and group (or,
either the Outgoing or Incoming interfaces, a forwarding if no group is specified, the previous-hop router for the source,
code of 0x4 is noted. 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".
8. If this router is the Rendez-vous Point or Core for the 6.5. Sending Traceroute Responses
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 6.5.1. Destination Address
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.
10. The packet is then sent on to the previous hop or the A traceroute response must be sent to the Response Address in the
requester as described in "Forwarding Traceroute traceroute header.
Requests".
7.3. Traceroute response 6.5.2. TTL
A router must forward all traceroute response packets If the Response Address is unicast, the router inserts its normal
normally, with no special processing. If a router has unicast TTL in the IP header. If the Response Address is multi-
initiated a traceroute with a Query or Request message, it may cast, the router copies the Response TTL from the traceroute header
listen for Responses to that traceroute but MUST still forward into the IP header.
them as well.
7.4. Forwarding Traceroute Requests 6.5.3. Source Address
If the Previous-hop router is known for the source and group If the Response Address is unicast, the router may use any of its
(or, if no group is specified, the previous-hop router for the interface addresses as the source address. Since some multicast
source, or if no source is specified, the previous-hop router routing protocols forward based on source address, if the Response
for the group) and the number of response blocks is less than Address is multicast, the router MUST use an address that is known
the number requested, the packet is sent to that router. If in the multicast routing table if it can make that determination.
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 6.5.4. Sourcing Multicast Responses
7.5.1. Destination Address 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 pack-
ets.
A traceroute response must be sent to the Response Address in 7. Using multicast traceroute
the traceroute header.
7.5.2. TTL 7.1. Sample Client
If the Response Address is unicast, the router inserts its This section describes the behavior of an example multicast traceroute
normal unicast TTL in the IP header. If the Response Address client.
is multicast, the router copies the Response TTL from the
traceroute header into the IP header.
7.5.3. Source Address 7.1.1. Sending Initial Query
If the Response Address is unicast, the router may use any of When the destination of the trace is the machine running the
its interface addresses as the source address. Since some client, the traceroute Query packet can be sent to the ALL-ROUTERS
multicast routing protocols forward based on source address, multicast group (224.0.0.2). This will ensure that the packet is
if the Response Address is multicast, the router MUST use an received by the last-hop router on the subnet. Otherwise, if the
address that is known in the multicast routing table if it can proper last-hop router is known for the trace destination, the
make that determination. Query could be unicasted to that router. Otherwise, the Query
packet should be multicasted to the group being queried; if the
destination of the trace is a member of the group this will get the
Query to the proper last-hop router. In this final case, the
packet should contain the Router Alert option, to make sure that
routers that are not members of the multicast group notice the
packet. See also section 8.2 on determining the last-hop router.
7.5.4. Sourcing Multicast Responses 7.1.2. Determining the Path
When a router sources a multicast response, the response The client could send a small number of Initial Query messages with
packet MUST be sent on a single interface, then forwarded as a large "# hops" field, in order to try to trace the full path. If
if it were received on that interface. It MUST NOT source the this attempt fails, one strategy is to perform a linear search (as
response packet individually on each interface, since that the traditional unicast traceroute program does); set the "#hops"
causes duplicate packets. field to 1 and try to get a response, then 2, and so on. If no
response is received at a certain hop, the hop count can continue
past the non-responding hop, in the hopes that further hops may
respond. These attempts should continue until a user-defined time-
out has occurred.
8. Using multicast traceroute See also section 8.3 and 8.4 on receiving the results of a trace.
<<Need a section on expected client behavior (one or two attempts 7.1.3. Collecting Statistics
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 After a client has determined that it has traced the whole path or
as much as it can expect to (see section 8.5), it might collect
statistics by waiting a short time and performing a second trace.
If the path is the same in the two traces, statistics can be dis-
played as described in section 9.3 and 9.4.
The traceroute querier may not know which is the last hop Details of performing a multicast traceroute:
router, or that router may be behind a firewall that blocks
unicast packets but passes multicast packets. In these cases, 7.2. Last hop router
the traceroute request should be multicasted to the group
being traced (since the last hop router listens to that The traceroute querier may not know which is the last hop router,
group). All routers except the correct last hop router should or that router may be behind a firewall that blocks unicast packets
ignore any multicast traceroute request received via but passes multicast packets. In these cases, the traceroute
multicast. Traceroute requests which are multicasted to the request should be multicasted to the group being traced (since the
group being traced must include the Router Alert IP option last hop router listens to that group). All routers except the
[Katz97]. 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 [Katz97].
Another alternative is to unicast to the trace destination. Another alternative is to unicast to the trace destination.
Traceroute requests which are unicasted to the trace Traceroute requests which are unicasted to the trace destination
destination must include the Router Alert IP option [Katz97], must include the Router Alert IP option [Katz97], in order that the
in order that the last-hop router is aware of the packet. last-hop router is aware of the packet.
If the traceroute querier is attached to the same router as If the traceroute querier is attached to the same router as the
the destination of the request, the traceroute request may be destination of the request, the traceroute request may be multicas-
multicasted to 224.0.0.2 (ALL-ROUTERS.MCAST.NET) if the last- ted to 224.0.0.2 (ALL-ROUTERS.MCAST.NET) if the last-hop router is
hop router is not known. not known.
8.2. First hop router 7.3. First hop router
The traceroute querier may not be unicast reachable from the The traceroute querier may not be unicast reachable from the first
first hop router. In this case, the querier should set the hop router. In this case, the querier should set the traceroute
traceroute response address to a multicast address, and should response address to a multicast address, and should set the
set the response TTL to a value sufficient for the response response TTL to a value sufficient for the response from the first
from the first hop router to reach the querier. It may be hop router to reach the querier. It may be appropriate to start
appropriate to start with a small TTL and increase in with a small TTL and increase in subsequent attempts until a suffi-
subsequent attempts until a sufficient TTL is reached, up to cient TTL is reached, up to an appropriate maximum (such as 192).
an appropriate maximum (such as 192).
The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET, as the The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET, as the default
default multicast group for multicast traceroute responses. multicast group for multicast traceroute responses. Other groups
Other groups may be used if needed, e.g. when using mtrace to may be used if needed, e.g. when using mtrace to diagnose problems
diagnose problems with the IANA-assigned group. with the IANA-assigned group.
8.3. Broken intermediate router 7.4. Broken intermediate router
A broken intermediate router might simply not understand A broken intermediate router might simply not understand traceroute
traceroute packets, and drop them. The querier would then get packets, and drop them. The querier would then get no response at
no response at all from its traceroute requests. It should all from its traceroute requests. It should then perform a hop-by-
then perform a hop-by-hop search by setting the number of hop search by setting the number of responses field until it gets a
responses field until it gets a response (both linear and response (both linear and binary search are options, but binary is
binary search are options, but binary is likely to be slower likely to be slower because a failure requires waiting for a time-
because a failure requires waiting for a timeout). out).
8.4. Trace termination 7.5. Trace termination
When performing an expanding hop-by-hop trace, it is necessary When performing an expanding hop-by-hop trace, it is necessary to
to determine when to stop expanding. determine when to stop expanding.
8.4.1. Arriving at source 7.5.1. Arriving at source
A trace can be determined to have arrived at the source if the A trace can be determined to have arrived at the source if the
Incoming Interface of the last router in the trace is non- Incoming Interface of the last router in the trace is non-zero, but
zero, but the Previous Hop router is zero. (XXX Need to the Previous Hop router is zero.
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 7.5.2. Fatal Error
A trace has encountered a fatal error if the last Forwarding A trace has encountered a fatal error if the last Forwarding Error
Error in the trace has the 0x80 bit set. in the trace has the 0x80 bit set.
8.4.3. No Previous Hop 7.5.3. No Previous Hop
A trace can not continue if the last Previous Hop in the trace A trace can not continue if the last Previous Hop in the trace is
is set to 0. set to 0.
9. Problem Diagnosis 7.5.4. Trace shorter than requested
9.1. Forwarding Inconsistencies If the trace that is returned is shorter than requested (i.e. the
number of Response blocks is smaller than the "# hops" field), the
trace encountered an error and could not continue.
7.6. Continuing after an error
When the NO_SPACE error occurs, the client might try to continue
the trace by starting it at the last hop in the trace. It can do
this by unicasting to this router's outgoing interface address,
keeping all fields the same. If this results in a single hop and a
"WRONG_IF" error, the client may try setting the trace destination
to the same outgoing interface address.
If a trace times out, it is likely to be because a router in the
middle of the path does not support multicast traceroute. That
router's address will be in the Previous Hop field of the last
entry in the last reply packet received. A client may be able to
determine (via mrinfo[Pusa98] or SNMP[Thal98a,Thal98b]) a list of
neighbors of the non-responding router. If desired, each of those
neighbors could be probed to determine the remainder of the path.
Unfortunately, this heuristic may end up with multiple paths, since
there is no way of knowing what the non-responding router's algo-
rithm for choosing a previous-hop router is. However, if all paths
but one flow back towards the non-responding router, it is possible
to be sure that this is the correct path.
7.7. Multicast Traceroute and shared-tree routing protocols
When using shared-tree routing protocols like PIM-SM and CBT, it is
still possible to use multicast traceroute to determine paths.
7.7.1. PIM-SM
When a multicast traceroute reaches a PIM-SM RP and the RP does not for-
ward the trace on, it means that the RP has not performed a source-spe-
cific join so there is no more state to trace. However, the path that
traffic would use if the RP did perform a source-specific join can be
traced by setting the trace destination to the RP, the trace source to
the traffic source, and the trace group to 0. This trace Query may be
unicasted to the RP.
7.7.2. CBT
When a multicast traceroute reaches a CBT Core, it must simply stop
since CBT does not have source-specific state. However, a second trace
can be performed, setting the trace destination to the traffic source,
the trace group to the group being traced, and the trace source to the
Core (or to 0, since CBT does not have source-specific state). This
trace Query may be unicasted to the Core. There are two possibilities
when combining the two traces:
7.7.2.1. No overlap
If there is no overlap between the two traces, the second trace can
be reversed and appended to the first trace. This composite trace
shows the full path from the source to the destination.
7.7.2.2. Overlapping paths
If there is a portion of the path that is common to the ends of the
two traces, that portion is removed from both traces. Then, as in
the no overlap case, the second trace is reversed and appended to
the first trace, and the composite trace again contains the full
path.
This algorithm works whether the source has joined the CBT tree or not.
8. Problem Diagnosis
8.1. 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.
9.2. TTL problems 8.2. TTL problems
By taking the maximum of (hops from source + forwarding TTL By taking the maximum of (hops from source + forwarding TTL thresh-
threshold) over all hops, you can discover the TTL required old) over all hops, you can discover the TTL required for the
for the source to reach the destination. source to reach the destination.
9.3. Congestion 8.3. Congestion
By taking two traces, you can find packet loss information by By taking two traces, you can find packet loss information by com-
comparing the difference in input packet counts to the paring the difference in input packet counts to the difference in
difference in output packet counts at the previous hop. On a output packet counts at the previous hop. On a point-to-point
point-to-point link, any difference in these numbers implies link, any difference in these numbers implies packet loss. Since
packet loss. Since the packet counts may be changing as the the packet counts may be changing as the trace query is propagat-
trace query is propagating, there may be small errors (off by ing, there may be small errors (off by 1 or 2) in these statistics.
1 or 2) in these statistics. However, these errors will not However, these errors will not accumulate if multiple traces are
accumulate if multiple traces are taken to expand the taken to expand the measurement period. On a shared link, the
measurement period. On a shared link, the count of input count of input packets can be larger than the number of output
packets can be larger than the number of output packets at the packets at the previous hop, due to other routers or hosts on the
previous hop, due to other routers or hosts on the link link injecting packets. This appears as "negative loss" which may
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 In addition to the counts of input and output packets for all mul-
multicast traffic on the interfaces, the response data ticast traffic on the interfaces, the response data includes a
includes a count of the packets forwarded by a node for the count of the packets forwarded by a node for the specified source-
specified source-group pair. Taking the difference in this group pair. Taking the difference in this count between two traces
count between two traces and then comparing those differences and then comparing those differences between two hops gives a mea-
between two hops gives a measure of packet loss just for sure of packet loss just for traffic from the specified source to
traffic from the specified source to the specified receiver the specified receiver via the specified group. This measure is
via the specified group. This measure is not affected by not affected by shared links.
shared links.
On a point-to-point link that is a multicast tunnel, packet On a point-to-point link that is a multicast tunnel, packet loss is
loss is usually due to congestion in unicast routers along the usually due to congestion in unicast routers along the path of that
path of that tunnel. On native multicast links, loss is more tunnel. On native multicast links, loss is more likely in the out-
likely in the output queue of one hop, perhaps due to priority put queue of one hop, perhaps due to priority dropping, or in the
dropping, or in the input queue at the next hop. The counters input queue at the next hop. The counters in the response data do
in the response data do not allow these cases to be not allow these cases to be distinguished. Differences in packet
distinguished. Differences in packet counts between the counts between the incoming and outgoing interfaces on one node
incoming and outgoing interfaces on one node cannot generally cannot generally be used to measure queue overflow in the node
be used to measure queue overflow in the node because some because some packets may be routed only to or from other interfaces
packets may be routed only to or from other interfaces on that on that node.
node.
In the multicast extensions for SunOS 4.1.x from Xerox PARC, In the multicast extensions for SunOS 4.1.x from Xerox PARC, both
both the output packet count and the packet forwarding count the output packet count and the packet forwarding count for the
for the source-group pair are incremented before priority source-group pair are incremented before priority dropping for rate
dropping for rate limiting occurs and before the packets are limiting occurs and before the packets are put onto the interface
put onto the interface output queue which may overflow. These output queue which may overflow. These drops will appear as (posi-
drops will appear as (positive) loss on the link even though tive) loss on the link even though they occur within the router.
they occur within the router.
In release 3.3/3.4 of the UNIX multicast extensions, a In release 3.3/3.4 of the UNIX multicast extensions, a multicast
multicast packet generated on a router will be counted as packet generated on a router will be counted as having come in an
having come in an interface even though it did not. This can interface even though it did not. This can create the appearance
create the appearance of negative loss even on a point-to- of negative loss even on a point-to-point link.
point link.
In releases up through 3.5/3.6, packets were not counted as In releases up through 3.5/3.6, packets were not counted as input
input on an interface if the reverse-path forwarding check on an interface if the reverse-path forwarding check decided that
decided that the packets should be dropped. That causes the the packets should be dropped. That causes the packets to appear
packets to appear as lost on the link if they were output by as lost on the link if they were output by the upstream hop. This
the upstream hop. This situation can arise when two routers situation can arise when two routers on the path for the group
on the path for the group being traced are connected by a being traced are connected by a shared link, and the path for some
shared link, and the path for some other group does not flow other group does not flow between those two routers because the
between those two routers because the downstream router downstream router receives packets for the other group on another
receives packets for the other group on another interface, but interface, but the upstream router is the elected forwarder to
the upstream router is the elected forwarder to other routers other routers or hosts on the shared link.
or hosts on the shared link.
9.4. Link Utilization 8.4. Link Utilization
Again, with two traces, you can divide the difference in the Again, with two traces, you can divide the difference in the input
input or output packet counts at some hop by the difference in or output packet counts at some hop by the difference in time
time stamps from the same hop to obtain the packet rate over stamps from the same hop to obtain the packet rate over the link.
the link. If the average packet size is known, then the link If the average packet size is known, then the link utilization can
utilization can also be estimated to see whether packet loss also be estimated to see whether packet loss may be due to the rate
may be due to the rate limit or the physical capacity on a limit or the physical capacity on a particular link being exceeded.
particular link being exceeded.
9.5. Time delay 8.5. Time delay
If the routers have synchronized clocks, it is possible to If the routers have synchronized clocks, it is possible to estimate
estimate propagation and queueing delay from the differences propagation and queueing delay from the differences between the
between the timestamps at successive hops. timestamps at successive hops.
10. Acknowledgments 9. Acknowledgments
This specification started largely as a transcription of Van This specification started largely as a transcription of Van Jacobson's
Jacobson's slides from the 30th IETF, and the implementation in slides from the 30th IETF, and the implementation in mrouted 3.3 by Ajit
mrouted 3.3 by Ajit Thyagarajan. Van's original slides credit Thyagarajan. Van's original slides credit Steve Casner, Steve Deering,
Steve Casner, Steve Deering, Dino Farinacci and Deb Agrawal. A Dino Farinacci and Deb Agrawal. A multicast traceroute client, mtrace,
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 has been implemented by Ajit Thyagarajan, Steve Casner and Bill Fenner.
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 The idea of unicasting a multicast traceroute Query to the destination
of the trace with Router Alert set is due to Tony Ballardie. The idea
of the "S" bit to allow statistics for a source subnet is due to Tom
Pusateri.
11.1. Routing Protocols 10. IANA Considerations
Should the IANA be responsible for allocating new Routing 10.1. Routing Protocols
Protocol codes?
11.2. Forwarding Codes The IANA is responsible for allocating new Routing Protocol codes.
The Routing Protocol code is somewhat problematic, since in the
case of protocols like CBT and PIM it must encode both a unicast
routing algorithm and a multicast tree-building protocol. The
space was not divided into two fields because it was already small
and some combinations (e.g. DVMRP) would be wasted.
Should the IANA be responsible for allocating new Forwarding Routing Protocol codes should be allocated for any combination of
Codes? protocols that are in common use in the Internet.
12. Security Considerations 10.2. Forwarding Codes
12.1. Topology discovery New Forwarding codes must only be created by an RFC that modifies
this document's section 7, fully describing the conditions under
which the new forwarding code is used. The IANA may act as a cen-
tral repository so that there is a single place to look up forward-
ing codes and the document in which they are defined.
mtrace can be used to discover any actively-used topology. If 11. Security Considerations
your network topology is a secret, you should restrict mtrace
at the border of your domain.
12.2. Traffic rates 11.1. Topology discovery
mtrace can be used to discover what sources are sending to mtrace can be used to discover any actively-used topology. If your
what groups and at what rates. If this information is a network topology is a secret, mtrace may be restricted at the bor-
secret, you should restrict mtrace at the border of your der of your domain, using the ADMIN_PROHIB forwarding code.
domain.
...more... 11.2. Traffic rates
13. References mtrace can be used to discover what sources are sending to what
groups and at what rates. If this information is a secret, mtrace
may be restricted at the border of your domain, using the
ADMIN_PROHIB forwarding code.
11.3. Unicast replies
The "Response address" field may be used to send a single packet
(the traceroute Reply packet) to an arbitrary unicast address. It
is possible to use this facility as a packet amplifier, as a small
multicast traceroute Query may turn into a large Reply packet.
12. References
Brad88 Braden, B., D. Borman, C. Partridge, "Computing the
Internet Checksum", RFC 1071, ISI, September 1988.
Bradner97 Bradner, S., "Key words for use in RFCs to Indicate Bradner97 Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119/BCP 14, Harvard Requirement Levels", RFC 2119/BCP 14, Harvard University,
University, March 1997. March 1997.
Katz97 Katz, D., "IP Router Alert Option," RFC 2113, Cisco Katz97 Katz, D., "IP Router Alert Option," RFC 2113, Cisco Sys-
Systems, February 1997. tems, February 1997.
14. Authors' Addresses 13. 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: +1 650 812 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. Cisco Systems
1072 Arastradero Road 1072 Arastradero Road
Palo Alto, CA 94304 Palo Alto, CA 94304
Email: casner@precept.com Email: casner@precept.com
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