MBONED Working Group H. Asaeda
Internet-Draft NICT
Intended status: Standards Track W. Lee, Ed.
Expires: April 25, 2013 Juniper Networks, Inc.
October 22, 2012
Mtrace Version 2: Traceroute Facility for IP Multicast
draft-ietf-mboned-mtrace-v2-09
Abstract
This document describes the IP multicast traceroute facility, named
Mtrace version 2 (Mtrace2). Unlike unicast traceroute, Mtrace2
requires special implementations on the part of routers. This
specification describes the required functionality in multicast
routers, as well as how an Mtrace2 client invokes a query and
receives a reply.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 25, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Definitions . . . . . . . . . . . . . . . . . . . . . . . 6
3. Packet Formats . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Mtrace2 TLV format . . . . . . . . . . . . . . . . . . . . 8
3.2. Defined TLVs . . . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Mtrace2 Query . . . . . . . . . . . . . . . . . . . . 9
3.2.2. Mtrace2 Extended Query Block . . . . . . . . . . . . . 10
3.2.3. Mtrace2 Request . . . . . . . . . . . . . . . . . . . 11
3.2.4. Mtrace2 Reply . . . . . . . . . . . . . . . . . . . . 11
3.2.5. IPv4 Mtrace2 Standard Response Block . . . . . . . . . 12
3.2.6. IPv6 Mtrace2 Standard Response Block . . . . . . . . . 16
3.2.7. Mtrace2 Augmented Response Block . . . . . . . . . . . 19
4. Router Behavior . . . . . . . . . . . . . . . . . . . . . . . 20
4.1. Receiving Mtrace2 Query . . . . . . . . . . . . . . . . . 20
4.1.1. Query Packet Verification . . . . . . . . . . . . . . 20
4.1.2. Query Normal Processing . . . . . . . . . . . . . . . 21
4.2. Receiving Mtrace2 Request . . . . . . . . . . . . . . . . 21
4.2.1. Request Packet Verification . . . . . . . . . . . . . 21
4.2.2. Request Normal Processing . . . . . . . . . . . . . . 22
4.3. Forwarding Mtrace2 Request . . . . . . . . . . . . . . . . 23
4.3.1. Destination Address . . . . . . . . . . . . . . . . . 24
4.3.2. Source Address . . . . . . . . . . . . . . . . . . . . 24
4.3.3. Appending Standard Response Block . . . . . . . . . . 24
4.4. Sending Mtrace2 Reply . . . . . . . . . . . . . . . . . . 24
4.4.1. Destination Address . . . . . . . . . . . . . . . . . 25
4.4.2. Source Address . . . . . . . . . . . . . . . . . . . . 25
4.4.3. Appending Standard Response Block . . . . . . . . . . 25
4.5. Proxying Mtrace2 Query . . . . . . . . . . . . . . . . . . 25
4.6. Hiding Information . . . . . . . . . . . . . . . . . . . . 26
5. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 26
5.1. Sending Mtrace2 Query . . . . . . . . . . . . . . . . . . 26
5.1.1. Destination Address . . . . . . . . . . . . . . . . . 26
5.1.2. Source Address . . . . . . . . . . . . . . . . . . . . 26
5.2. Determining the Path . . . . . . . . . . . . . . . . . . . 26
5.3. Collecting Statistics . . . . . . . . . . . . . . . . . . 27
5.4. Last Hop Router (LHR) . . . . . . . . . . . . . . . . . . 27
5.5. First Hop Router (FHR) . . . . . . . . . . . . . . . . . . 27
5.6. Broken Intermediate Router . . . . . . . . . . . . . . . . 27
5.7. Non-Supported Router . . . . . . . . . . . . . . . . . . . 28
5.8. Mtrace2 Termination . . . . . . . . . . . . . . . . . . . 28
5.8.1. Arriving at Source . . . . . . . . . . . . . . . . . . 28
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5.8.2. Fatal Error . . . . . . . . . . . . . . . . . . . . . 28
5.8.3. No Upstream Router . . . . . . . . . . . . . . . . . . 28
5.8.4. Reply Timeout . . . . . . . . . . . . . . . . . . . . 28
5.9. Continuing after an Error . . . . . . . . . . . . . . . . 28
6. Protocol-Specific Considerations . . . . . . . . . . . . . . . 29
6.1. PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . . . 29
6.2. Bi-Directional PIM . . . . . . . . . . . . . . . . . . . . 29
6.3. PIM-DM . . . . . . . . . . . . . . . . . . . . . . . . . . 30
6.4. IGMP/MLD Proxy . . . . . . . . . . . . . . . . . . . . . . 30
7. Problem Diagnosis . . . . . . . . . . . . . . . . . . . . . . 30
7.1. Forwarding Inconsistencies . . . . . . . . . . . . . . . . 30
7.2. TTL or Hop Limit Problems . . . . . . . . . . . . . . . . 30
7.3. Packet Loss . . . . . . . . . . . . . . . . . . . . . . . 30
7.4. Link Utilization . . . . . . . . . . . . . . . . . . . . . 31
7.5. Time Delay . . . . . . . . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
8.1. Forwarding Codes . . . . . . . . . . . . . . . . . . . . . 32
8.2. UDP Destination Port . . . . . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
9.1. Addresses in Mtrace2 Header . . . . . . . . . . . . . . . 32
9.2. Topology Discovery . . . . . . . . . . . . . . . . . . . . 32
9.3. Characteristics of Multicast Channel . . . . . . . . . . . 32
9.4. Limiting Query/Request Rates . . . . . . . . . . . . . . . 33
9.5. Limiting Reply Rates . . . . . . . . . . . . . . . . . . . 33
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
Given a multicast distribution tree, tracing from a multicast source
to a receiver is difficult, since we do not know which branch of the
multicast tree the receiver lies. This means that we have to flood
the whole tree to find the path from a source to a receiver. On the
other hand, walking up the tree from a receiver to a source is easy,
as most existing multicast routing protocols know the upstream router
for each source. Tracing from a receiver to a source can involve
only the routers on the direct path.
This document specifies the multicast traceroute facility named
Mtrace version 2 or Mtrace2 which allows the tracing of an IP
multicast routing path. Mtrace2 is usually initiated from a Mtrace2
client towards a specified source, or a Rendezvous Point (RP) if no
source address is specified. RP is a special router where the source
and receiver meet in PIM-SM [1]. Moreover, Mtrace2 provides
additional information such as the packet rates and losses, as well
as other diagnosis information. Especially, Mtrace2 can be used for
the following purposes:
o To trace the path that a packet would take from a source to a
receiver.
o To isolate packet loss problems (e.g., congestion).
o To isolate configuration problems (e.g., TTL threshold).
Figure 1 shows a typical case on how Mtrace2 is used. FHR represents
the first-hop router, LHR represents the last-hop router, and the
arrow lines represent the Mtrace2 messages that are sent from one
node to another. The numbers before the Mtrace2 messages represent
the sequence of the messages that would happen. Source, Receiver and
Mtrace2 client are typically a host on the Internet.
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2. Request 2. Request
+----+ +----+
| | | |
v | v |
+--------+ +-----+ +-----+ +----------+
| Source |----| FHR |----- The Internet -----| LHR |----| Receiver |
+--------+ +-----+ | +-----+ +----------+
\ | ^
\ | /
\ | /
\ | /
3. Reply \ | / 1. Query
\ | /
\ | /
\ +---------+ /
v | Mtrace2 |/
| client |
+---------+
Figure 1
When an Mtrace2 client initiates a multicast trace anywhere on the
Internet, it sends an Mtrace2 Query packet to the LHR for a multicast
group and a source address. The LHR turns the Query packet into a
Request, appends a standard response block containing its interface
addresses and packet statistics to the Request packet, then forwards
the packet towards the source. The Request packet is either
unicasted to its upstream router towards the source, or multicasted
to the group if the upsteam router's IP address is not known. In a
similar fashion, each router along the path to the source appends a
standard response block to the end of the Request packet before
forwarding it to its upstream router. When the FHR receives the
Request packet, it appends its own standard response block, turns the
Request packet into a Reply, and unicasts the Reply back to the
Mtrace2 client.
The Mtrace2 Reply may be returned before reaching the FHR if it
reaches the RP first, or a fatal error condition such as "no route"
is encountered along the path, or the hop count is exceeded.
The Mtrace2 client waits for the Mtrace2 Reply message and displays
the results. When not receiving an Mtrace2 Reply message due to
network congestion, a broken router (see Section 5.6), or a non-
responding router (see Section 5.7), the Mtrace2 client may resend
another Mtrace2 Query with a lower hop count (see Section 3.2.1), and
repeat the process until it receives an Mtrace2 Reply message. The
details are Mtrace2 client specific, and it is outside the scope of
this document.
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Note that when a router's control plane and forwarding plane are out
of sync, the Mtrace2 Requests might be forwarded based on the control
states instead. In which case, the traced path might not represent
the real path the data packets would follow.
Mtrace2 supports both IPv4 and IPv6. Unlike the previous version of
Mtrace, which implements its query and response as IGMP messages [8],
all Mtrace2 messages are UDP-based. Although the packet formats of
IPv4 and IPv6 Mtrace2 are different because of the address families,
the syntax between them is similar.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2],
and indicate requirement levels for compliant Mtrace2
implementations.
2.1. Definitions
Since Mtrace2 Queries and Requests flow in the opposite direction to
the data flow, we refer to "upstream" and "downstream" with respect
to data, unless explicitly specified.
Incoming interface
The interface on which data is expected to arrive from the
specified source and group.
Outgoing interface
The interface to which data from the source or RP is expected to
transmit for the specified source and group. It is also the
interface on which the Mtrace2 Request will be received.
Upstream router
The router, connecting to the Incoming interface of the current
router, which is responsible for forwarding data for the specified
source and group to the current router.
First-hop router (FHR)
The router that is directly connected to the source the Mtrace2
Query specifies.
Last-hop router (LHR)
The router that is directly connected to the receivers. It is
also the router that receives the Mtrace2 Query from an Mtrace2
client.
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Group state
It is the state a shared-tree protocol, such as PIM-SM [1], uses
to choose the upstream router towards the RP for the specified
group. In this state, source-specific state is not available for
the corresponding group address on the router.
Source-specific state
It is the state that is used to choose the path towards the source
for the specified source and group.
ALL-[protocol]-ROUTERS.MCAST.NET
It is a link-local multicast address for multicast routers to
communicate with their adjacent routers that are running the same
routing protocol. For instance, the address of ALL-PIM-
ROUTERS.MCAST.NET [1] is '224.0.0.13' for IPv4 and 'ff02::d' for
IPv6.
3. Packet Formats
This section describes the details of the packet formats for Mtrace2
messages.
All Mtrace2 messages are encoded in TLV format (see Section 3.1). If
an implementation receives an unknown TLV, it SHOULD ignored and
silently discarded the unknown TLV. If the length of a TLV exceeds
the length specified in the TLV, the TLV SHOULD be accepted; however,
any additional data after the TLV SHOULD be ignored.
All Mtrace2 messages are UDP packets. For IPv4, Mtrace2 Query and
Request messages MUST NOT be fragmented. For IPv6, the packet size
for the Mtrace2 messages MUST NOT exceed 1280 bytes, which is the
smallest MTU for an IPv6 interface [3]. The source port is uniquely
selected by the local host operating system. The destination port is
the IANA reserved Mtrace2 port number (see Section 8). All Mtrace2
messages MUST have a valid UDP checksum.
Additionally, Mtrace2 supports both IPv4 and IPv6, but not mixed.
For example, if an Mtrace2 Query or Reply message arrives in as an
IPv4 packet, all addresses specified in the Mtrace2 messages MUST be
IPv4 as well. Same rule applies to IPv6 Mtrace2 messages.
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3.1. Mtrace2 TLV format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 8 bits
Describes the format of the Value field. For all the available
types, please see Section 3.2
Length: 16 bits
Length of Type, Length, and Value fields in octets. Minimum
length required is 6 octets. The maximum TLV length is not
defined; however the entired Mtrace2 packet length should not
exceeed the available MTU.
Value: variable length
The format is based on the Type value. The length of the value
field is Length field minus 3. All reserved fields in the Value
field MUST be transmitted as zeros and ignored on receipt.
3.2. Defined TLVs
The following TLV Types are defined:
Code Type
==== ================================
0x01 Mtrace2 Query
0x02 Mtrace2 Request
0x03 Mtrace2 Reply
0x04 Mtrace2 Standard Response Block
0x05 Mtrace2 Augmented Response Block
0x06 Mtrace2 Extended Query Block
Each Mtrace2 message MUST begin with either a Query, Request or Reply
TLV. The first TLV determines the type of each Mtrace2 message.
Following this TLV, there can be a sequence of optional Extended
Query Blocks. In the case of the Request and Reply message, it is
then followed by a sequence of Standard Response Blocks, each from a
multicast router on the path towards the source or the RP. In the
case more information is needed, a Standard Response Block can be
followed by one or multiple Augmented Response Blocks.
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We will describe each message type in details in the next few
sections.
3.2.1. Mtrace2 Query
An Mtrace2 query is usually originated by an Mtrace2 client which
sends an Mtrace2 Query message to the LHR. When tracing towards the
source or the RP, the intermediate routers MUST NOT modify the Query
message except the Type field.
An Mtrace2 Query message is shown as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | # Hops |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Multicast Address |
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| |
| Source Address |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Mtrace2 Client Address |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query ID | Client Port # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
# Hops: 8 bits
This field specifies the maximum number of hops that the Mtrace2
client wants to trace. If there are some error conditions in the
middle of the path that prevent an Mtrace2 Reply from being
received by the client, the client MAY issues another Mtrace2
Query with the lower number of hops until it receives a Reply from
the FHR.
Multicast Address: 32 bits or 128 bits
This field specifies an IPv4 or IPv6 address, which can be either:
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m-1: a multicast group address to be traced; or,
m-2: all 1's in case of IPv4 or the unspecified address (::) in
case of IPv6 if no group-specific information is desired.
Source Address: 32 bits or 128 bits
This field specifies an IPv4 or IPv6 address, which can be either:
s-1: an unicast address of the source to be traced; or,
s-2: all 1's in case of IPv4 or the unspecified address (::) in
case of IPv6 if no source-specific information is desired.
For example, the client is tracing a (*,g) group state.
Note that it is invalid to have a source-group combination of
(s-2, m-2). If a router receives such combination in an Mtrace2
Query, it MUST silently discard the Query.
Mtrace2 Client Address: 32 bits or 128 bits
This field specifies the Mtrace2 client's IPv4 address or IPv6
global address. This address MUST be a valid unicast address, and
therefore, MUST NOT be all 1's or an unspecified address. The
Mtrace2 Reply will be sent to this address.
Query ID: 16 bits
This field is used as a unique identifier for this Mtrace2 Query
so that duplicate or delayed Reply messages may be detected.
Client Port #: 16 bits
This field specifies the destination UDP port number for receiving
the Mtrace2 Reply packet.
3.2.2. Mtrace2 Extended Query Block
There may be a sequence of optional Extended Query Blocks that follow
an Mtrace2 Query to further specify any information needed for the
Query. For example, an Mtrace2 client might be interested in tracing
the path the specified source and group would take based on a certain
topology. In which case, the client can pass in the multi-topology
ID as the Value for an Extended Query Type (see below). The Extended
Query Type is extensible and the behavior of the new types will be
addressed by seperate documents.
The Mtrace2 Extended Query Block is formatted as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MBZ |T|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Query Type | Value .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MBZ: 7 bits
This field must be zeroed on transmission and ignored on
reception.
T-bit (Transitive Attribute): 1 bit
If the TLV type is unrecognized by the receiving router, then this
TLV is either discarded or forwarded along with the Query,
depending on the value of this bit. If this bit is set, then the
router MUST forward this TLV. If this bit is clear, the router
MUST send an mtrace2 Reply with an UNKNOWN_QUERY error.
Extended Query Type: 16 bits
This field specifies the type of the Extended Query Block.
Value: 16 bits
This field specifies the value of this Extended Query.
3.2.3. Mtrace2 Request
The format of an Mtrace2 Request message is similar to an Mtrace2
Query except the Type field is 0x02.
When a LHR receives an Mtrace2 Query message, it would turn the Query
into a Request by changing the Type field of the Query from 0x01 to
0x02. The LHR would then append an Mtrace2 Standard Response Block
(see Section 3.2.5) of its own to the Request message before sending
it upstream. The upstream routers would do the same without changing
the Type field until one of them is ready to send a Reply.
3.2.4. Mtrace2 Reply
The format of an Mtrace2 Reply message is similar to an Mtrace2 Query
except the Type field is 0x03.
When a FHR or a RP receives an Mtrace2 Request message which is
destined to itself, it would append an Mtrace2 Standard Response
Block (see Section 3.2.5) of its own to the Request message. Next,
it would turn the Request message into a Reply by changing the Type
field of the Request from 0x02 to 0x03. The Reply message would then
be unicated to the Mtrace2 client specified in the Mtrace2 Client
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Address field.
There are a number of cases an intermediate router might return a
Reply before a Request reaches the FHR or the RP. See Section 4.1.1,
Section 4.2.2, Section 4.3.3, and Section 4.5 for more details.
3.2.5. IPv4 Mtrace2 Standard Response Block
This section describes the message format of an IPv4 Mtrace2 Standard
Response Block. The Type field is 0x04.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query Arrival Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Upstream Router Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Input packet count on incoming interface .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Output packet count on outgoing interface .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Total number of packets for this source-group pair .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol | Multicast Rtg Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fwd TTL | MBZ |S| Src Mask |Forwarding Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MBZ: 8 bits
This field must be zeroed on transmission and ignored on
reception.
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Query Arrival Time: 32 bits
The Query Arrival Time is a 32-bit NTP timestamp specifying the
arrival time of the Mtrace2 Query or Request packet at this
router. The 32-bit form of an NTP timestamp consists of the
middle 32 bits of the full 64-bit form; that is, the low 16 bits
of the integer part and the high 16 bits of the fractional part.
The following formula converts from a UNIX timeval to a 32-bit NTP
timestamp:
query_arrival_time
= (tv.tv_sec + 32384) << 16 + ((tv.tv_usec << 10) / 15625)
The constant 32384 is the number of seconds from Jan 1, 1900 to
Jan 1, 1970 truncated to 16 bits. ((tv.tv_usec << 10) / 15625) is
a reduction of ((tv.tv_usec / 100000000) << 16).
Note that Mtrace2 does not require all the routers on the path to
have synchronized clocks in order to measure one-way latency.
Additionally, Query Arrival Time is useful for measuring the
packet rate. For example, suppose that a client issues two
queries, and the corresponding requests R1 and R2 arrive at router
X at time T1 and T2, then the client would be able to compute the
packet rate on router X by using the packet count information
stored in the R1 and R2, and the time T1 and T2.
Incoming Interface Address: 32 bits
This field specifies the address of the interface on which packets
from the source or the RP are expected to arrive, or 0 if unknown
or unnumbered.
Outgoing Interface Address: 32 bits
This field specifies the address of the interface on which packets
from the source or the RP are expected to transmit towards the
receiver, or 0 if unknown or unnumbered. This is also the address
of the interface on which the Mtrace2 Query or Request arrives.
Upstream Router Address: 32 bits
This field specifies the address of the upstream router from which
this router expects packets from this source. This may be a
multicast group (e.g. ALL-[protocol]-ROUTERS.MCAST.NET) if the
upstream router is not known because of the workings of the
multicast routing protocol. However, it should be 0 if the
incoming interface address is unknown or unnumbered.
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Input packet count on incoming interface: 64 bits
This field contains the number of multicast packets received for
all groups and sources on the incoming interface, or all 1's if no
count can be reported. This counter may have the same value as
ifHCInMulticastPkts from the IF-MIB [9] for this interface.
Output packet count on outgoing interface: 64 bit
This field contains the number of multicast packets that have been
transmitted or queued for transmission for all groups and sources
on the outgoing interface, or all 1's if no count can be reported.
This counter may have the same value as ifHCOutMulticastPkts from
the IF-MIB [9] for this interface.
Total number of packets for this source-group pair: 64 bits
This field counts the number of packets from the specified source
forwarded by the router to the specified group, or all 1's if no
count can be reported. If the S bit is set (see below), the count
is for the source network, as specified by the Src Mask field (see
below). 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. This counter should have the same value as
ipMcastRoutePkts from the IPMROUTE-STD-MIB [10] for this
forwarding entry.
Rtg Protocol: 16 bits
This field describes the unicast routing protocol running between
this router and the upstream router, and it is used to determine
the RPF interface for the specified source or RP. This value
should have the same value as ipMcastRouteRtProtocol from the
IPMROUTE-STD-MIB [10] for this entry. If the router is not able
to obtain this value, all 0's must be specified.
Multicast Rtg Protocol: 16 bits
This field describes the multicast routing protocol in use between
the router and the upstream router. This value should have the
same value as ipMcastRouteProtocol from the IPMROUTE-STD-MIB [10]
for this entry. If the router cannot obtain this value, all 0's
must be specified.
Fwd TTL: 8 bits
This field contains the TTL in which an Mtrace2 Request packet can
be forwarded towards the source or the RP.
S: 1 bit
If this bit is set, it indicates that the packet count for the
source-group pair is for the source network, as determined by
masking the source address with the Src Mask field.
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Src Mask: 7 bits
This field contains the number of 1's in the netmask the 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 127 (0x7f).
Forwarding Code: 8 bits
This field contains a forwarding information/error code.
Section 4.1 and Section 4.2 will explain how and when the
Forwarding Code is filled. Defined values are as follows:
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Value Name Description
----- -------------- ----------------------------------------------
0x00 NO_ERROR No error
0x01 WRONG_IF Mtrace2 Request arrived on an interface
to which this router would not forward for
the specified group towards the source or RP.
0x02 PRUNE_SENT This router has sent a prune upstream which
applies to the source and group in the
Mtrace2 Request.
0x03 PRUNE_RCVD This router has stopped forwarding for this
source and group in response to a request
from the downstream router.
0x04 SCOPED The group is subject to administrative
scoping at this router.
0x05 NO_ROUTE This router has no route for the source or
group and no way to determine a potential
route.
0x06 WRONG_LAST_HOP This router is not the proper LHR.
0x07 NOT_FORWARDING This router is not forwarding this source and
group out the outgoing interface for an
unspecified reason.
0x08 REACHED_RP Reached the Rendezvous Point.
0x09 RPF_IF Mtrace2 Request arrived on the expected
RPF interface for this source and group.
0x0A NO_MULTICAST Mtrace2 Request arrived on an interface
which is not enabled for multicast.
0x0B INFO_HIDDEN One or more hops have been hidden from this
trace.
0x0C REACHED_GW Mtrace2 Request arrived on a gateway (e.g.,
a NAT or firewall) that hides the
information between this router and the
Mtrace2 client.
0x0D UNKNOWN_QUERY A non-transitive Extended Query Type was
received by a router which does not support
the type.
0x80 FATAL_ERROR A fatal error is one where the router may
know the upstream router but cannot forward
the message to it.
0x81 NO_SPACE There was not enough room to insert another
Standard Response Block in the packet.
0x83 ADMIN_PROHIB Mtrace2 is administratively prohibited.
3.2.6. IPv6 Mtrace2 Standard Response Block
This section describes the message format of an IPv6 Mtrace2 Standard
Response Block. The Type field is also 0x04.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Query Arrival Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Incoming Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outgoing Interface ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* Local Address *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* Remote Address *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Input packet count on incoming interface .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Output packet count on outgoing interface .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. Total number of packets for this source-group pair .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol | Multicast Rtg Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MBZ 2 |S|Src Prefix Len |Forwarding Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MBZ: 8 bits
This field must be zeroed on transmission and ignored on
reception.
Query Arrival Time: 32 bits
Same definition as in IPv4.
Incoming Interface ID: 32 bits
This field specifies the interface ID on which packets from the
source or RP are expected to arrive, or 0 if unknown. This ID
should be the value taken from InterfaceIndex of the IF-MIB [9]
for this interface.
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Outgoing Interface ID: 32 bits
This field specifies the interface ID to which packets from the
source or RP are expected to transmit, or 0 if unknown. This ID
should be the value taken from InterfaceIndex of the IF-MIB [9]
for this interface
Local Address: 128 bits
This field specifies a global IPv6 address that uniquely
identifies the router. An unique local unicast address [11]
SHOULD NOT be used unless the router is only assigned link-local
and unique local addresses. If the router is only assigned link-
local addresses, its link-local address can be specified in this
field.
Remote Address: 128 bits
This field specifies the address of the upstream router, which, in
most cases, is a link-local unicast address for the upstream
router.
Although a link-local address does not have enough information to
identify a node, it is possible to detect the upstream router with
the assistance of Incoming Interface ID and the current router
address (i.e., Local Address).
Note that this may be a multicast group (e.g., ALL-[protocol]-
ROUTERS.MCAST.NET) if the upstream router is not known because of
the workings of a multicast routing protocol. However, it should
be the unspecified address (::) if the incoming interface address
is unknown.
Input packet count on incoming interface: 64 bits
Same definition as in IPv4.
Output packet count on outgoing interface: 64 bits
Same definition as in IPv4.
Total number of packets for this source-group pair: 64 bits
Same definition as in IPv4, except if the S bit is set (see
below), the count is for the source network, as specified by the
Src Prefix Len field. If the S bit is set and the Src Prefix Len
field is 255, indicating no source-specific state, the count is
for all sources sending to this group. This counter should have
the same value as ipMcastRoutePkts from the IPMROUTE-STD-MIB [10]
for this forwarding entry.
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Rtg Protocol: 16 bits
Same definition as in IPv4.
Multicast Rtg Protocol: 16 bits
Same definition as in IPv4.
MBZ 2: 15 bits
This field must be zeroed on transmission and ignored on
reception.
S: 1 bit
Same definition as in IPv4, except the Src Prefix Len field is
used to mask the source address.
Src Prefix Len: 8 bits
This field contains the prefix length this router has for the
source. If the router is forwarding solely on group state, this
field is set to 255 (0xff).
Forwarding Code: 8 bits
Same definition as in IPv4.
3.2.7. Mtrace2 Augmented Response Block
In addition to the Standard Response Block, a multicast router on the
traced path can optionally add one or multiple Augmented Response
Blocks before sending the Request to its upstream router.
The Augmented Response Block is flexible for various purposes such as
providing diagnosis information (see Section 7) and protocol
verification. It's Type field is 0x05, and its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Augmented Response Type | Value .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MBZ: 8 bits
This field must be zeroed on transmission and ignored on
reception.
Augmented Response Type: 16 bits
This field specifies the type of various responses from a
multicast router that might need to communicate back to the
Mtrace2 client as well as the multicast routers on the traced
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path.
The Augmented Response Type is defined as follows:
Code Type
==== ===============================================
0x01 # of the returned Standard Response Blocks
When the NO_SPACE error occurs on a router, the router should send
the original Mtrace2 Request received from the downstream router
as a Reply back to the Mtrace2 client, and continue with a new
Mtrace2 Request. In the new Request, the router would add a
Standard Response Block followed by an Augmented Response Block
with 0x01 as the Augmented Response Type, and the number of the
returned Mtrace2 Standard Response Blocks as the Value.
Each upstream router would recognize the total number of hops the
Request has been traced so far by adding this number and the
number of the Standard Response Block in the current Request
message.
This document only defines one Augmented Response Type in the
Augmented Response Block. The description on how to provide
diagnosis information using the Augmented Response Block is out of
the scope of this document, and will be addressed in separate
documents.
Value: variable length
The format is based on the Augmented Response Type value. The
length of the value field is Length field minus 6.
4. Router Behavior
This section describes the router behavior in the context of Mtrace2
in details.
4.1. Receiving Mtrace2 Query
An Mtrace2 Query message is an Mtrace2 message with no response
blocks filled in, and uses TLV type of 0x01.
4.1.1. Query Packet Verification
Upon receiving an Mtrace2 Query message, a router MUST examine
whether the Multicast Address and the Source Address are a valid
combination as specified in Section 3.2.1, and whether the Mtrace2
Client Address is a valid IP unicast address. If either one is
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invalid, the Query MUST be silently ignored.
Mtrace2 supports non-local client to the LHR. It is up to the
implementation to filter out such queries.
In the case when it is a local client, the router must then examine
the Query to see if it is the proper LHR for the destination address
in the packet. It is the proper LHR if it has a multicast-capable
interface on the same subnet as the Mtrace2 Client Address and is the
router that would forward traffic from the given (S,G) or (*,G) onto
that subnet.
If the router determines that it is not the proper LHR, or it cannot
make that determination, it does one of two things depending on
whether the Query was received via multicast or unicast. If the
Query was received via multicast, then it MUST be silently discarded.
If it was received via unicast, the router turns the Query into a
Reply message by changing the TLV type to 0x03 and appending a
Standard Response Block with a Forwarding Code of WRONG_LAST_HOP.
The rest of the fields in the Standard Response Block MUST be zeroed.
The router then sends the Reply message to the Mtrace2 Client Address
on the Client Port # as specified in the Mtrace2 Query.
Duplicate Query messages as identified by the tuple (Mtrace2 Client
Address, Query ID) SHOULD be ignored. This MAY be implemented using
a cache of previously processed queries keyed by the Mtrace2 Client
Address and Query ID pair. The duration of the cached entries is
implementation specific. Duplicate Request messages MUST NOT be
ignored in this manner.
4.1.2. Query Normal Processing
When a router receives an Mtrace2 Query and it determines that it is
the proper LHR, it turns the Query to a Request by changing the TLV
type from 0x01 to 0x02, and performs the steps listed in Section 4.2.
4.2. Receiving Mtrace2 Request
An Mtrace2 Request is an Mtrace2 message that uses TLV type of 0x02.
With the exception of the LHR, whose Request was just converted from
a Query, each Request received by a router should have at least one
Standard Response Block filled in.
4.2.1. Request Packet Verification
If the Mtrace2 Request does not come from an adjacent router, or if
the Request is not addressed to this router, or if the Request is
addressed to a multicast group which is not a link-scoped group (i.e.
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224/24 for IPv4, FFx2::/16 [4] for IPv6), it MUST be silently
ignored. GTSM [12] SHOULD be used by the router to determine whether
the router is adjacent or not.
If the sum of the number of the Standard Response Blocks in the
received Mtrace2 Request and the value of the Augmented Response Type
of 0x01, if any, is equal or more than the # Hops in the Mtrace2
Request, it MUST be silently ignored.
4.2.2. Request Normal Processing
When a router receives an Mtrace2 Request message, it performs the
following steps. Note that it is possible to have multiple
situations covered by the Forwarding Codes. The first one
encountered is the one that is reported, i.e. all "note Forwarding
Code N" should be interpreted as "if Forwarding Code is not already
set, set Forwarding Code to N".
1. Prepare a Standard Response Block to be appended to the packet
and fill in the Query Arrival Time, Outgoing Interface Address
(for IPv4) or Outgoing Interface ID (for IPv6), Output Packet
Count, and Fwd TTL (for IPv4). Note that the Outgoing Interface
is the one on which the Mtrace2 Request message arrives.
2. Attempt to determine the forwarding information for the
specified source and group, using the same mechanisms as would
be used when a packet is received from the source destined for
the group. A state need not be instantiated, it can be a
"phantom" state created only for the purpose of the trace, such
as "dry-run."
If using a shared-tree protocol and there is no source-specific
state, or if no source-specific information is desired (i.e.,
all 1's for IPv4 or unspecified address (::) for IPv6), group
state should be used. If there is no group state or no group-
specific information is desired, potential source state (i.e.,
the path that would be followed for a source-specific Join)
should be used.
3. If no forwarding information can be determined, the router notes
a Forwarding Code of NO_ROUTE, sets the remaining fields that
have not yet been filled in to zero, and then sends an Mtrace2
Reply back to the Mtrace2 client.
4. Fill in the Incoming Interface Address, Upstream Router Address,
Input Packet Count, Total Number of Packets, Routing Protocol,
S, and Src Mask (or Src Prefix Len for IPv6) using the
forwarding information determined by the step 2.
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5. If Mtrace2 is administratively prohibited, note the Forwarding
Code of ADMIN_PROHIB. If Mtrace2 is administratively prohibited
and any of the fields as filled in the step 4 are considered
private information, zero out the applicable fields.
6. If the Outgoing interface is not enabled for multicast, note
Forwarding Code of NO_MULTICAST. If the Outgoing interface is
the interface from which the router would expect data to arrive
from the source, note forwarding code RPF_IF. If the Outgoing
interface is not one to which the router would forward data from
the source or RP to the group, a Forwarding code of WRONG_IF is
noted. In the above three cases, the router will return an
Mtrace2 Reply and terminate the trace.
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 RP for the group, note a Forwarding Code
of REACHED_RP. The router will send an Mtrace2 Reply and
terminate the trace.
9. If this router has sent a prune upstream which applies to the
source and group in the Mtrace2 Request, it notes Forwarding
Code of PRUNE_SENT. If the router has stopped forwarding
downstream in response to a prune sent by the downstream router,
it notes Forwarding Code of PRUNE_RCVD. If the router should
normally forward traffic downstream for this source and group
but is not, it notes Forwarding Code of NOT_FORWARDING.
10. If this router is a gateway (e.g., a NAT or firewall) that hides
the information between this router and the Mtrace2 client, it
notes Forwarding Code of REACHED_GW. The router continues the
processing as described in Section 4.5.
11. If the total number of the Standard Response Blocks, including
the newly prepared one, and the value of the Augmented Response
Type of 0x01, if any, is less than the # Hops in the Request,
the packet is then forwarded to the upstream router as described
in Section 4.3; otherwise, the packet is sent as an Mtrace2
Reply to the Mtrace2 client as described in Section 4.4.
4.3. Forwarding Mtrace2 Request
This section describes how an Mtrace2 Request should be forwarded.
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4.3.1. Destination Address
If the upstream router for the Mtrace2 Request is known for this
request, the Mtrace2 Request is sent to that router. If the Incoming
interface is known but the upstream router is not, the Mtrace2
Request is sent to an appropriate multicast address on the Incoming
interface. The multicast address SHOULD depend on the multicast
routing protocol in use, such as ALL-[protocol]-ROUTERS.MCAST.NET.
It MUST be a link-scoped group (i.e. 224/24 for IPv4, FF02::/16 for
IPv6), and MUST NOT be ALL-SYSTEMS.MCAST.NET (224.0.0.1) for IPv4 and
All Nodes Address (FF02::1) for IPv6. It MAY also be ALL-
ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All Routers Address
(FF02::2) for IPv6 if the routing protocol in use does not define a
more appropriate multicast address.
4.3.2. Source Address
An Mtrace2 Request should be sent with the address of the Incoming
interface. However, if the Incoming interface is unnumbered, the
router can use one of its numbered interface address as the source
address.
4.3.3. Appending Standard Response Block
An Mtrace2 Request MUST be sent upstream towards the source or the RP
after appending a Standard Response Block to the end of the received
Mtrace2 Request. The Standard Response Block includes the multicast
states and statistics information of the router described in
Section 3.2.5.
If appending the Standard Response Block would make the Mtrace2
Request packet longer than the MTU of the Incoming Interface, or, in
the case of IPv6, longer than 1280 bytes, the router MUST change the
Forwarding Code in the last Standard Response Block of the received
Mtrace2 Request into NO_SPACE. The router then turns the Request
into a Reply, and sends the Reply as described in Section 4.4.
The router will continue with a new Request by copying from the old
Request excluding all the response blocks, followed by the previously
prepared Standard Response Block, and an Augmented Response Block
with Augmented Response Type of 0x01 and the number of the returned
Standard Response Blocks as the value. The new Request is then
forwarded upstream.
4.4. Sending Mtrace2 Reply
An Mtrace2 Reply MUST be returned to the client by a router if the
total number of the traced routers is equal to the # Hops in the
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Request. The total number of the traced routers is the sum of the
Standard Response Blocks in the Request (including the one just
added) and the number of the returned blocks, if any.
4.4.1. Destination Address
An Mtrace2 Reply MUST be sent to the address specified in the Mtrace2
Client Address field in the Mtrace2 Request.
4.4.2. Source Address
An Mtrace2 Reply SHOULD be sent with the address of the router's
Outgoing interface. However, if the Outgoing interface address is
unnumbered, the router can use one of its numbered interface address
as the source address.
4.4.3. Appending Standard Response Block
An Mtrace2 Reply MUST be sent with the prepared Standard Response
Block appended at the end of the received Mtrace2 Request except in
the case of NO_SPACE forwarding code.
4.5. Proxying Mtrace2 Query
When a gateway (e.g., a NAT or firewall), which needs to block
unicast packets to the Mtrace2 client, or hide information between
the gateway and the Mtrace2 client, receives an Mtrace2 Query from an
adjacent host or Mtrace2 Request from an adjacent router, it appends
a Standard Response Block with REACHED_GW as the Forwarding Code, and
turns the Query or Request as a Reply, and sends the Reply back to
the client.
At the same time, the gateway originates a new Mtrace2 Query message
by copying the original Mtrace2 header (the Query or Request without
any of the response blocks), and makes the changes as follows:
o sets the RPF interface's address as the Mtrace2 Client Address;
o uses its own port number as the Client Port #; and,
o decreases # Hops by the number of the Standard Response Block that
was just returned as a Reply.
The new Mtrace2 Query message is then sent to the upstream router or
to an appropriate multicast address on the RPF interface.
When the gateway receives an Mtrace2 Reply whose Query ID matches the
one in the original Mtrace2 header, it MUST relay the Mtrace2 Reply
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back to the Mtrace2 client by replacing the Reply's header with the
original Mtrace2 header. If the gateway does not receive the
corresponding Mtrace2 Reply within the [Mtrace Reply Timeout] period
(see Section 5.8.4), then it silently discards the original Mtrace2
Query or Request message, and terminates the trace.
4.6. Hiding Information
Information about a domain's topology and connectivity may be hidden
from the Mtrace2 Requests. The Forwarding Code of INFO_HIDDEN may be
used to note that. For example, the incoming interface address and
packet count on the ingress router of a domain, and the outgoing
interface address and packet count on the egress router of the domain
can be specified as all 1's. Additionally, the source-group packet
count (see Section 3.2.5 and Section 3.2.6) within the domain may be
all 1's if it is hidden.
5. Client Behavior
This section describes the behavior of an Mtrace2 client in details.
5.1. Sending Mtrace2 Query
An Mtrace2 client initiates an Mtrace2 Query by sending the Query to
the LHR of interest.
5.1.1. Destination Address
If an Mtrace2 client knows the proper LHR, it unicasts an Mtrace2
Query packet to that router; otherwise, it MAY send the Mtrace2 Query
packet to the ALL-ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All
Routers Address (FF02::2) for IPv6. This will ensure that the packet
is received by the LHR on the subnet.
See also Section 5.4 on determining the LHR.
5.1.2. Source Address
An Mtrace2 Query MUST be sent with the client's interface address,
which would be the Mtrace2 Client Address.
5.2. Determining the Path
An Mtrace2 client could send an initial Query messages with a large #
Hops, in order to try to trace the full path. If this attempt fails,
one strategy is to perform a linear search (as the traditional
unicast traceroute program does); set the # Hops field to 1 and try
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to get a Reply, then 2, and so on. If no Reply 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 the [Mtrace Reply Timeout] timeout has occurred.
See also Section 5.6 on receiving the results of a trace.
5.3. Collecting Statistics
After a client has determined that it has traced the whole path or as
much as it can expect to (see Section 5.8), 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 displayed
as described in Section 7.3 and Section 7.4.
5.4. Last Hop Router (LHR)
The Mtrace2 client 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 Mtrace2 Request should
be multicasted to ALL-ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All
Routers Address (FF02::2) for IPv6. All routers except the correct
last-hop router SHOULD ignore any Mtrace2 Request received via
multicast.
5.5. First Hop Router (FHR)
The IANA assigned 224.0.1.32, MTRACE.MCAST.NET as the default
multicast group for old IPv4 mtrace (v1) responses, in order to
support mtrace clients that are not unicast reachable from the first-
hop router. Mtrace2, however, does not require any IPv4/IPv6
multicast addresses for the Mtrace2 Replies. Every Mtrace2 Reply is
sent to the unicast address specified in the Mtrace2 Client Address
field of the Mtrace2 Reply.
5.6. Broken Intermediate Router
A broken intermediate router might simply not understand Mtrace2
packets, and drop them. The Mtrace2 client will get no Reply at all
as a result. It should then perform a hop-by-hop search by setting
the # Hops field until it gets an Mtrace2 Reply. The client may use
linear or binary search; however, the latter is likely to be slower
because a failure requires waiting for the [Mtrace Reply Timeout]
period.
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5.7. Non-Supported Router
When a non-supported router receives an Mtrace2 Query or Request
message whose destination address is a multicast address, the router
will silently discard the message.
When the router receives an Mtrace2 Query which is destined to
itself, the router would return an ICMP port unreachable to the
Mtrace2 client. On the other hand, when the router receives an
Mtrace2 Request which is destined to itself, the router would return
an ICMP port unreachable to its adjacent router from which the
Request receives. Therefore, the Mtrace2 client needs to terminate
the trace when the [Mtrace Reply Timeout] timeout has occurred, and
may then issue another Query with a lower number of # Hops.
5.8. Mtrace2 Termination
When performing an expanding hop-by-hop trace, it is necessary to
determine when to stop expanding.
5.8.1. 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 Upstream Router is zero.
5.8.2. Fatal Error
A trace has encountered a fatal error if the last Forwarding Error in
the trace has the 0x80 bit set.
5.8.3. No Upstream Router
A trace can not continue if the last Upstream Router in the trace is
set to 0.
5.8.4. Reply Timeout
This document defines the [Mtrace Reply Timeout] value, which is used
to time out an Mtrace2 Reply as seen in Section 4.5, Section 5.2, and
Section 5.7. The default [Mtrace Reply Timeout] value is 10
(seconds), and can be manually changed on the Mtrace2 client and
routers.
5.9. Continuing after an Error
When the NO_SPACE error occurs, as described in Section 4.2, a router
will send back an Mtrace2 Reply to the Mtrace2 client, and continue
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with a new Request (see Section 4.3.3). In which case, the Mtrace2
client may receive multiple Mtrace2 Replies from different routers
along the path. When this happens, the client MUST treat them as a
single Mtrace2 Reply message.
If a trace times out, it is very likely that a router in the middle
of the path does not support Mtrace2. That router's address will be
in the Upstream Router field of the last Standard Response Block in
the last received Reply. A client may be able to determine (via
mrinfo or SNMP [11][10]) 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 algorithm for choosing an upstream 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.
6. Protocol-Specific Considerations
This section describes the Mtrace2 behavior with the present of
different multicast protocols.
6.1. PIM-SM
When an Mtrace2 reaches a PIM-SM RP, and the RP does not forward the
trace on, it means that the RP has not performed a source-specific
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 Mtrace2 Query
may be unicasted to the RP.
6.2. Bi-Directional PIM
Bi-directional PIM [5] is a variant of PIM-SM that builds bi-
directional shared trees connecting multicast sources and receivers.
Along the bi-directional shared trees, multicast data is natively
forwarded from the sources to the Rendezvous Point Link (RPL), and
from which, to receivers without requiring source-specific state. In
contrast to PIM-SM, Bi-directional PIM always has the state to trace.
A Designated Forwarder (DF) for a given Rendezvous Point Address
(RPA) is in charge of forwarding downstream traffic onto its link,
and forwarding upstream traffic from its link towards the RPL that
the RPA belongs to. Hence Mtrace2 Reply reports DF addresses or RPA
along the path.
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6.3. PIM-DM
Routers running PIM Dense Mode [13] do not know the path packets
would take unless traffic is flowing. Without some extra protocol
mechanism, this means that in an environment with multiple possible
paths with branch points on shared media, Mtrace2 can only trace
existing paths, not potential paths. When there are multiple
possible paths but the branch points are not on shared media, the
upstream router is known, but the LHR may not know that it is the
appropriate last hop.
When traffic is flowing, PIM Dense Mode routers know whether or not
they are the LHR for the link (because they won or lost an Assert
battle) and know who the upstream router is (because it won an Assert
battle). Therefore, Mtrace2 is always able to follow the proper path
when traffic is flowing.
6.4. IGMP/MLD Proxy
When an IGMP/MLD Proxy [6] receives an Mtrace2 Query packet on an
incoming interface, it notes a WRONG_IF in the Forwarding Code of the
last Standard Response Block (see Section 3.2.5), and sends the
Mtrace2 Reply back to the Mtrace2 client. On the other hand, when an
Mtrace2 Query packet reaches an outgoing interface of the IGMP/MLD
proxy, it is forwarded onto its incoming interface towards the
upstream router.
7. Problem Diagnosis
This section describes different scenarios Mtrace2 can be used to
diagnose the multicast problems.
7.1. Forwarding Inconsistencies
The Forwarding Error code can tell if a group is unexpectedly pruned
or administratively scoped.
7.2. TTL or Hop Limit Problems
By taking the maximum of hops from the source and forwarding TTL
threshold over all hops, it is possible to discover the TTL or hop
limit required for the source to reach the destination.
7.3. Packet Loss
By taking two traces, it is possible to find packet loss information
by comparing the difference in input packet counts to the difference
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in output packet counts for the specified source-group address pair
at the previous hop. On a point-to-point link, any difference in
these numbers implies packet loss. Since the packet counts may be
changing as the Mtrace2 Request is propagating, there may be small
errors (off by 1 or 2 or more) in these statistics. However, these
errors will not accumulate if multiple traces are taken to expand the
measurement period. On a shared link, the count of input packets can
be larger than the number of output packets at the previous hop, due
to other routers or hosts on the link injecting packets. This
appears as "negative loss" which may mask real packet loss.
In addition to the counts of input and output packets for all
multicast traffic on the interfaces, the Standard Response Block
includes a count of the packets forwarded by a node for the specified
source-group pair. Taking the difference in this count between two
traces and then comparing those differences between two hops gives a
measure of packet loss just for traffic from the specified source to
the specified receiver via the specified group. This measure is not
affected by shared links.
On a point-to-point link that is a multicast tunnel, packet loss is
usually due to congestion in unicast routers along the path of that
tunnel. On native multicast links, loss is more likely in the output
queue of one hop, perhaps due to priority dropping, or in the input
queue at the next hop. The counters in the Standard Response Block
do not allow these cases to be distinguished. Differences in packet
counts between the incoming and outgoing interfaces on one node
cannot generally be used to measure queue overflow in the node.
7.4. Link Utilization
Again, with two traces, you can divide the difference in the input or
output packet counts at some hop by the difference in time stamps
from the same hop to obtain the packet rate over the link. If the
average packet size is known, then the link utilization can also be
estimated to see whether packet loss may be due to the rate limit or
the physical capacity on a particular link being exceeded.
7.5. Time Delay
If the routers have synchronized clocks, it is possible to estimate
propagation and queuing delay from the differences between the
timestamps at successive hops. However, this delay includes control
processing overhead, so is not necessarily indicative of the delay
that data traffic would experience.
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8. IANA Considerations
The following new assignments can only be made via a Standards Action
as specified in [7].
8.1. Forwarding Codes
New Forwarding Codes must only be created by an RFC that modifies
this document's Section 3.2.5 and Section 3.2.6, fully describing the
conditions under which the new Forwarding Code is used. The IANA may
act as a central repository so that there is a single place to look
up Forwarding Codes and the document in which they are defined.
8.2. UDP Destination Port
The IANA should allocate UDP destination port for Mtrace2 upon
publication of the first RFC.
9. Security Considerations
This section addresses some of the security considerations related to
Mtrace2.
9.1. Addresses in Mtrace2 Header
An Mtrace2 header includes three addresses, source address, multicast
address, and Mtrace2 client address. These addresses MUST be
congruent with the definition defined in Section 3.2.1 and forwarding
Mtrace2 messages having invalid addresses MUST be prohibited. For
instance, if Mtrace2 Client Address specified in an Mtrace header is
a multicast address, then a router that receives the Mtrace2 message
MUST silently discard it.
9.2. Topology Discovery
Mtrace2 can be used to discover any actively-used topology. If your
network topology is a secret, Mtrace2 may be restricted at the border
of your domain, using the ADMIN_PROHIB forwarding code.
9.3. Characteristics of Multicast Channel
Mtrace2 can be used to discover what sources are sending to what
groups and at what rates. If this information is a secret, Mtrace2
may be restricted at the border of your domain, using the
ADMIN_PROHIB forwarding code.
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9.4. Limiting Query/Request Rates
A router may limit Mtrace2 Queries and Requests by ignoring some of
the consecutive messages. The router MAY randomly ignore the
received messages to minimize the processing overhead, i.e., to keep
fairness in processing queries, or prevent traffic amplification.
The rate limit is left to the router's implementation.
9.5. Limiting Reply Rates
The proxying and NO_SPACE behaviors may result in one Query returning
multiple Reply messages. In order to prevent abuse, the routers in
the traced MAY need to rate-limit the Replies. The rate limit
function is left to the router's implementation.
10. Acknowledgements
This specification started largely as a transcription of Van
Jacobson's slides from the 30th IETF, and the implementation in
mrouted 3.3 by Ajit Thyagarajan. Van's original slides credit Steve
Casner, Steve Deering, Dino Farinacci and Deb Agrawal. The original
multicast traceroute client, mtrace (version 1), has been implemented
by Ajit Thyagarajan, Steve Casner and Bill Fenner. The idea of the
"S" bit to allow statistics for a source subnet is due to Tom
Pusateri.
For the Mtrace version 2 specification, the authors would like to
give special thanks to Tatsuya Jinmei, Bill Fenner, and Steve Casner.
Also, extensive comments were received from David L. Black, Ronald
Bonica, Yiqun Cai, Liu Hui, Bharat Joshi, Robert W. Kebler, Heidi Ou,
Pekka Savola, Shinsuke Suzuki, Dave Thaler, Achmad Husni Thamrin, and
Cao Wei.
11. References
11.1. Normative References
[1] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", RFC 2119, March 1997.
[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
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[4] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[5] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
RFC 5015, October 2007.
[6] Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet
Group Management Protocol (IGMP) / Multicast Listener Discovery
(MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")",
RFC 4605, August 2006.
[7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 5226, May 2008.
11.2. Informative References
[8] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version 3",
RFC 3376, October 2002.
[9] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
RFC 2863, June 2000.
[10] McWalter, D., Thaler, D., and A. Kessler, "IP Multicast MIB",
RFC 5132, December 2007.
[11] Draves, R. and D. Thaler, "Default Router Preferences and More-
Specific Routes", RFC 4191, November 2005.
[12] Gill, V., Heasley, J., Meyer, D., Savola, P., and C. Pignataro,
"The Generalized TTL Security Mechanism (GTSM)", RFC 5082,
October 2007.
[13] Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent
Multicast - Dense Mode (PIM-DM): Protocol Specification
(Revised)", RFC 3973, January 2005.
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Authors' Addresses
Hitoshi Asaeda
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi
Koganei, Tokyo 184-8795
Japan
Email: asaeda@nict.go.jp
WeeSan Lee (editor)
Juniper Networks, Inc.
1194 North Mathilda Avenue
Sunnyvale, CA 94089-1206
US
Email: weesan@juniper.net
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