| MBONED Working Group | H. Asaeda |
| Internet-Draft | NICT |
| Intended status: Standards Track | K. Meyer |
| Expires: October 4, 2018 | |
| W. Lee, Ed. | |
| April 2, 2018 |
Mtrace Version 2: Traceroute Facility for IP Multicast
draft-ietf-mboned-mtrace-v2-23
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.
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This Internet-Draft will expire on October 4, 2018.
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Given a multicast distribution tree, tracing hop-by-hop downstream from a multicast source to a given multicast receiver is difficult because there is no efficient and deterministic way to determine the branch of the multicast routing tree on which that receiver lies. 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 an Mtrace2 client by sending an Mtrace2 Query to a Last Hop Router (LHR) or to a Rendezvous Point (RP). The RP is a special router where sources and receivers meet in Protocol Independent Multicast - Sparse Mode (PIM-SM). From the LHR/RP receiving the query, the tracing is directed towards a specified source if a source address is specified and source specific state exists on the receiving router. If no source address is specified or if no source specific state exists on a receiving LHR, the tracing is directed toward the RP for the specified group address. Moreover, Mtrace2 provides additional information such as the packet rates and losses, as well as other diagnostic information. Mtrace2 is primarily intended for the following purposes:
Figure 1 shows a typical case on how Mtrace2 is used. First-hop router (FHR) represents the first-hop router, LHR represents the last-hop router (LHR), 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 hosts.
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, it sends an Mtrace2 Query packet to an LHR or RP for a multicast group and, optionally, a source address. The LHR/RP turns the Query packet into a Request. The Request message type enables each of the upstream routers processing the message to apply different packet and message validation rules than those required for handling of a Query message. The LHR/RP then appends a standard response block containing its interface addresses and packet statistics to the Request packet, then forwards the packet towards the source/RP. The Request packet is either unicasted to its upstream router towards the source/RP, or multicasted to the group if the upstream router's IP address is not known. In a similar fashion, each router along the path to the source/RP 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 under some circumstances. This can happen if a Request packet is received at an RP or gateway, or when any of several types of error or exception conditions occur which prevent sending of a request to the next upstream router.
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 outside the scope of this document.
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 this 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 Internet Group Management Protocol (IGMP) messages, 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.
This document describes the base specification of Mtrace2 that can serve as a basis for future proposals such as Mtrace2 for Automatic Multicast Tunneling (AMT) [9] and Mtrace2 for Multicast in MPLS/BGP IP VPNs (MVPN) [10]. They are therefore out of the scope of this document.
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, and indicate requirement levels for compliant Mtrace2 implementations.
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.
This section describes the details of the packet formats for Mtrace2 messages.
All Mtrace2 messages are encoded in the Type/Length/Value (TLV) format (see Section 3.1). The first TLV of a message is a message header TLV specifying the type of message and additional context information required for processing of the message and for parsing of subsequent TLVs in the message. Subsequent TLVs in a message, referred to as Blocks, are appended after the header TLV to provide additional information associated with the message. If an implementation receives an unknown TLV type for any TLV in a message, it SHOULD ignore and silently discard the entire packet. If the length of a TLV exceeds the available space in the containing packet, the implementation MUST ignore and silently discard the TLV and any remaining portion of the containing packet.
All Mtrace2 messages are UDP packets. For IPv4, Mtrace2 Query/Request/Reply messages MUST NOT be fragmented. Therefore, Mtrace2 clients and LHRs/RPs MUST set the IP header do-not-fragment (DF) bit for all Mtrace2 messages. For IPv6, the packet size for the Mtrace2 messages MUST NOT exceed 1280 bytes, which is the smallest Maximum Transmission Unit (MTU) for an IPv6 interface [2]. 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 Request 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.
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
Length: 16 bits
Value: variable length
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 a Query TLV, there can be a sequence of optional Extended Query Blocks. In the case of a Request or a Reply TLV, 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.
We will describe each message type in detail in the next few sections.
An Mtrace2 Query is originated by an Mtrace2 client which sends an Mtrace2 Query message to the LHR. The LHR modifies only the Type field of the Query TLV (to turn it into a "Request") before appending a Standard Response Block and forwarding it upstream. The LHR and intermediate routers handling the Mtrace2 message when tracing upstream MUST NOT modify any other fields within the Query/Request TLV. Additionally, intermediate routers handling the message after the LHR has converted the Query into a Request MUST NOT modify the type field of the Request TLV. If the actual number of hops is not known, an Mtrace2 client could send an initial Query message with a large # Hops (e.g., 0xff), in order to try to trace the full path.
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
The Mtrace2 Request TLV is exactly the same as an Mtrace2 Query except for identifying the Type field of 0x02.
When a LHR receives an Mtrace2 Query message, it turns the Query into a Request by changing the Type field of the Query from 0x01 to 0x02. The LHR then appends an Mtrace2 Standard Response Block (see Section 3.2.4) of its own to the Request message before sending it upstream. The upstream routers do the same without changing the Type field until one of them is ready to send a Reply.
The Mtrace2 Reply TLV is exactly the same as an Mtrace2 Query except for identifying the Type field of 0x03.
When a FHR or an RP receives an Mtrace2 Request message which is destined to itself, it appends an Mtrace2 Standard Response Block (see Section 3.2.4) of its own to the Request message. Next, it turns the Request message into a Reply by changing the Type field of the Request from 0x02 to 0x03 and by changing the UDP destination port to the port number specified in the Client Port number field in the Request. It then unicasts the Reply message to the Mtrace2 client specified in the Mtrace2 Client Address field.
There are a number of cases in which 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.
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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
This section describes the message format of an IPv6 Mtrace2 Standard Response Block. The Type field is also 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 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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. Its 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 .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Code Type ====== ============================================== 0x0001 # of the returned Standard Response Blocks
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 this 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 separate documents.
The Mtrace2 Extended Query Block's Type field is 0x06, and is formatted 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 |T| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Extended Query Type | Value .... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This section describes the router behavior in the context of Mtrace2 in detail.
An Mtrace2 Query message is an Mtrace2 message with no response blocks filled in, and uses TLV type of 0x01.
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 invalid, the Query MUST be silently ignored.
Mtrace2 supports a non-local client to the LHR/RP. A router SHOULD, however, support a mechanism to filter out queries from clients beyond a specified administrative boundary. The potential approaches are described in Section 9.2.
In the case where a local LHR client is required, the router must then examine the Query to see if it is the proper LHR/RP for the destination address in the packet. It is the proper local 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. It is the proper RP if the multicast group address specified in the query is 0 and if the IP header destination address is a valid RP address on this router.
If the router determines that it is not the proper LHR/RP, 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.
When a router receives an Mtrace2 Query and it determines that it is the proper LHR/RP, 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.
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.
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., 224.0.0.0/24 for IPv4, FFx2::/16 for IPv6), it MUST be silently ignored. The Generalized TTL Security Mechanism (GTSM) [14] 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.
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". Note that in the steps described below the "Outgoing Interface" is the one on which the Mtrace2 Request message arrives.
This section describes how an Mtrace2 Request should be forwarded.
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 group. It MUST be a link-scoped group (i.e., 224.0.0.0/24 for IPv4, FF02::/16 for IPv6), and MUST NOT be the all-systems multicast group (224.0.0.1) for IPv4 and All Nodes Address (FF02::1) for IPv6. It MAY also be the all-routers multicast group (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.
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 addresses as the source address.
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 3.2.4.
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.
An Mtrace2 Reply MUST be returned to the client by a router if any of the following conditions occur:
An Mtrace2 Reply MUST be sent to the address specified in the Mtrace2 Client Address field in the Mtrace2 Request.
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 addresses as the source address.
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.
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. It turns the Query or Request into 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:
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 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.
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 3.2.4 and Section 3.2.5) within the domain may be all 1's if it is hidden.
This section describes the behavior of an Mtrace2 client in detail.
An Mtrace2 client initiates an Mtrace2 Query by sending the Query to the LHR of interest.
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 multicast group (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.
An Mtrace2 Query MUST be sent with the client's interface address, which is the Mtrace2 Client Address.
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 to get a Reply, then 2, and so on. If no Reply is received at a certain hop, this hop is identified as the probable cause of forwarding failures on the path. Nevertheless, the sender may attempt to continue tracing past the non-responding hop by further increasing the hop count in the hopes that further hops may respond. Each of these attempts should be initiated only after the previous attempt has terminated either because of successful reception of a Reply or because the [Mtrace Reply Timeout] timeout has occurred.
See also Section 5.6 on receiving the results of a trace.
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.
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 the all-routers multicast group (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.
The IANA assigned 224.0.1.32 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.
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.
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 returns an Internet Control Message Protocol (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 returns 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.
When performing an expanding hop-by-hop trace, it is necessary to determine when to stop expanding.
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.
A trace has encountered a fatal error if the last Forwarding Error in the trace has the 0x80 bit set.
A trace cannot continue if the last Upstream Router in the trace is set to 0.
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.
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 with a new Request (see Section 4.3.3). In this 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 by collating the augmented response blocks of subsequent Replies sharing the same query ID, sequencing each cluster of augmented response blocks based on the order in which they are received.
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 the Simple Network Management Protocol (SNMP) [11][13]) a list of neighbors of the non-responding router. The neighbors obtained in this way could then be probed (via the multicast MIB [13]) to determine which one is the upstream neighbor (i.e., Reverse Path Forwarding (RPF) neighbor) of the non-responding router. This algorithm can identify the upstream neighbor because, even though there may be multiple neighbors, the non-responding router should only have sent a "join" to the one neighbor corresponding to its selected RPF path. Because of this, only the RPF neighbor should contain the non-responding router as a multicast next hop in its MIB output list for the affected multicast route.
This section describes the Mtrace2 behavior with the presence of different multicast protocols.
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, and the RP takes the same actions as an LHR.
Bi-directional PIM 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.
Routers running PIM Dense Mode [15] 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.
When an IGMP or Multicast Listener Discovery (MLD) Proxy 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 3.2.4), 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.
This section describes different scenarios Mtrace2 can be used to diagnose the multicast problems.
The Forwarding Error code can tell if a group is unexpectedly pruned or administratively scoped.
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.
By taking multiple traces, it is possible to find packet loss information by tracking the difference between the output packet count for the specified source-group address pair at a given upstream router and the input packet count on the next hop downstream router. On a point-to-point link, any steadily increasing difference in these counts implies packet loss. Although the packet counts will differ due to Mtrace2 Request propagation delay, the difference should remain essentially constant (except for jitter caused by differences in propagation time among the trace iterations). However, this difference will display a steady increase if packet loss is occurring. 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.
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.
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.
The following new registries are to be created and maintained under the "Specification Required" registry policy as specified in.
This is an integer in the range 0-255. Assignment of a Forwarding Code requires specification of a value and a name for the Forwarding Code. Initial values for the forwarding codes are given in the table at the end of Section 3.2.4. Additional values (specific to IPv6) may also be specified at the end of Section 3.2.5. Any additions to this registry are required to fully describe the conditions under which the new Forwarding Code is used.
Assignment of a TLV Type requires specification of an integer value "Code" in the range 0-255 and a name ("Type"). Initial values for the TLV Types are given in the table at the beginning of Section 3.2.
IANA has assigned UDP user port 33435 (mtrace) for use by this protocol as the Mtrace2 UDP destination port.
This section addresses some of the security considerations related to Mtrace2.
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 Mtrace2 header is a multicast address, then a router that receives the Mtrace2 message MUST silently discard it.
A router MUST support a mechanism to filter out Queries from clients and Requests from peer router addresses that are unauthorized or that are beyond a specified administrative boundary. This filtering could, for example, be specified via a list of allowed/disallowed client and peer addresses or subnets for the Mtrace2 protocol port. If a Query or Request is received from an unauthorized address or one beyond the specified administrative boundary, the Query/Request MUST NOT be processed. The router MAY, however, perform rate limited logging of such events.
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.
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.
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.
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 path MAY need to rate-limit the Replies. The rate limit function is left to the router's implementation.
A malicious sender could generate invalid and undesirable Mtrace2 traffic to hosts and/or routers on a network by eliciting responses to spoofed or multicast client addresses. This could be done via forged or multicast client/source addresses in Mtrace2 Query or Request messages. The recommended protections against this type of attack are described in Section 9.1, Section 9.2, Section 9.5, and Section 9.6.
Because an Mtrace2 Query results in Mtrace2 Request and Mtrace2 Reply messages that are larger than the original message, the potential exists for an amplification attack from a malicious sender. This threat is minimized by restricting the set of addresses from which Mtrace2 messages can be received on a given router as specified in Section 9.2.
Mtrace2 Queries are a potential mechanism for obtaining confidential topology information for a targeted network. Section 9.2 and Section 9.4 describe required and optional methods for ensuring that information delivered with Mtrace2 messages is not disseminated to unauthorized hosts.
Forged Reply messages could potentially provide a host with invalid or incorrect topology information. This threat is mitigated by the following factors:
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 Kebler, John Kristoff, Mankamana Mishra, Heidi Ou, Eric Rescorla, Pekka Savola, Shinsuke Suzuki, Dave Thaler, Achmad Husni Thamrin, Stig Venaas, Cao Wei, and the Mboned working group members.
| [1] | Bradner, S., "Key words for use in RFCs to indicate requirement levels", RFC 2119, March 1997. |
| [2] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 8200, July 2017. |
| [3] | Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006. |
| [4] | Cotton, M., Leiba, B. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", RFC 8126, June 2017. |
| [5] | Fenner, B., Handley, M., Holbrook, H., Kouvelas, I., Parekh, R., Zhang, Z. and L. Zheng, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 7761, March 2016. |
| [6] | Handley, M., Kouvelas, I., Speakman, T. and L. Vicisano, "Bidirectional Protocol Independent Multicast (BIDIR-PIM)", RFC 5015, October 2007. |
| [7] | 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. |
| [8] | Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. |
| [9] | Bumgardner, G., "Automatic Multicast Tunneling", RFC 7450, February 2015. |
| [10] | Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP VPNs", RFC 6513, February 2012. |
| [11] | Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, November 2005. |
| [12] | McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB", RFC 2863, June 2000. |
| [13] | McWalter, D., Thaler, D. and A. Kessler, "IP Multicast MIB", RFC 5132, December 2007. |
| [14] | Gill, V., Heasley, J., Meyer, D., Savola, P. and C. Pignataro, "The Generalized TTL Security Mechanism (GTSM)", RFC 5082, October 2007. |
| [15] | Adams, A., Nicholas, J. and W. Siadak, "Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised)", RFC 3973, January 2005. |