Internet Engineering Task Force (IETF) J. Luo, Ed. Request for Comments: 7743 ZTE Updates: 4379 L. Jin, Ed. Category: Standards Track ISSN: 2070-1721 T. Nadeau, Ed. Brocade G. Swallow, Ed. Cisco January 2016
Relayed Echo Reply Mechanism for Label Switched Path (LSP) Ping
Abstract
In some inter-AS (Autonomous System) and inter-area deployment scenarios for RFC 4379 ("Label Switched Path (LSP) Ping and Traceroute"), a replying Label Switching Router (LSR) may not have the available route to an initiator, and the Echo Reply message sent to the initiator would be discarded, resulting in false negatives or a complete failure of operation of the LSP Ping and Traceroute. This document describes extensions to the LSP Ping mechanism to enable the replying LSR to have the capability to relay the Echo Response by a set of routable intermediate nodes to the initiator. This document updates RFC 4379.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7743.
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Copyright Notice
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This document describes extensions to the Label Switched Path (LSP) Ping specified in [RFC4379] by adding a Relayed Echo Reply mechanism that could be used to report data-plane failures for inter-AS (Autonomous System) and inter-area LSPs. Without these extensions, the ping functionality provided by [RFC4379] would fail in many deployed inter-AS scenarios, since the replying Label Switching Router (LSR) in one AS may not have an available route to the initiator in the other AS. The mechanism in this document defines a new Message Type referred to as the "Relayed Echo Reply message" and a new TLV referred to as the "Relay Node Address Stack TLV".
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
LSP Ping [RFC4379] defines a mechanism to detect data-plane failures and localize faults. The mechanism specifies that the Echo Reply should be sent back to the initiator using a UDP packet with the IPv4/IPv6 destination address set to an address of the LSR that originated the Echo Request. This works in administrative domains where IP-address reachability is allowed among LSRs and every LSR is able to route back to the originating LSR. However, in practice, this is often not the case due to intra-provider routing policy, route hiding, and network address translation at Autonomous System Border Routers (ASBRs). In fact, it is almost always the case that in inter-AS scenarios, the only node in one AS to which direct routing is allowed from the other AS is the ASBR, and routing information from within one AS is not distributed into another AS.
Figure 1 demonstrates a case where an LSP is set up between PE1 and PE2. If PE1's IP address is not distributed to AS2, a traceroute from PE1 directed towards PE2 can result in a failure because an LSR in AS2 may not be able to send the Echo Reply message. For example, P2 cannot forward packets back to PE1 given that it is a routable IP address in AS1 but not routable in AS2. In this case, PE1 would
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detect a path break, as the Echo Reply messages would not be received. Then, localization of the actual fault would not be possible.
Note that throughout the document, "routable address" means that it is possible to route an IP packet to this address using the normal information exchanged by the IGP and BGP (or EGP) operating in the AS.
A second example that illustrates how [RFC4379] would be insufficient would be the inter-area situation in a seamless MPLS architecture [MPLSARCH] as shown below in Figure 2. In this example, LSRs in the core network would not have an IP-reachable route to any of the access nodes (ANs). When tracing an LSP from one AN to the remote AN, the LSR1/LSR2 node cannot send the Echo Reply either, like the P2 node in the inter-AS scenario in Figure 1.
RFC 7743 MPLS LSP Ping Relayed Echo Reply January 2016
This document describes extensions to the LSP Ping mechanism to facilitate a response from the replying LSR by defining a mechanism that uses a relay node (e.g., ASBR) to relay the message back to the initiator. Every designated or learned relay node must be reachable to the next relay node or to the initiator. Using a recursive approach, a relay node could relay the message to the next relay node until the initiator is reached.
The LSP Ping relay mechanism in this document is defined for unicast. How to apply the LSP Ping relay mechanism in multicast is out of scope.
[RFC4379] defines two Message Types: Echo Request and Echo Reply. This document defines a new Message Type: Relayed Echo Reply. The Relayed Echo Reply message is used in place of the Echo Reply message when an LSR is replying LSR to a relay node.
A new TLV named the "Relay Node Address Stack TLV" is defined in this document to carry the IP addresses of the relay nodes for the replying LSR.
In addition, the Maximum Transmission Unit (MTU) Exceeded Return Code is defined to indicate to the initiator that one or more TLVs will not be returned due to the MTU size.
It should be noted that this document focuses only on detecting the LSP that is set up using a uniform IP address family type. That is, all hops between the source and destination node use the same address family type for their LSP Ping control planes. This does not preclude nodes that support both IPv6 and IPv4 addresses simultaneously, but the entire path must be addressable using only one address family type. Support for mixed IPv4-only and IPv6-only is beyond the scope of this document.
The Relayed Echo Reply message is a UDP packet, and the UDP payload has the same format with Echo Request/Reply message. A new Message Type is requested from IANA.
New Message Type: Value Meaning ----- ------- 5 MPLS Relayed Echo Reply
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The use of TCP and UDP port number 3503 is described in [RFC4379] and has been allocated by IANA for LSP Ping messages. The Relayed Echo Reply message will use the same port number.
The Relay Node Address Stack TLV is an optional TLV. It MUST be carried in the Echo Request, Echo Reply, and Relayed Echo Reply messages if the Echo Reply relayed mechanism described in this document is required. Figure 3 illustrates the TLV format.
- Type: Value is 32768. The value has been assigned by IANA from the 32768-49161 range as suggested by [RFC4379], Section 3.
- Length: The length of the value field in octets.
- Initiator Source Port: The source UDP port that the initiator uses in the Echo Request message, and the port that is expected to receive the Echo Reply message.
- Reply Address Type: Address type of replying router. This value also implies the length of the address field as shown below.
RFC 7743 MPLS LSP Ping Relayed Echo Reply January 2016
- Reserved: This field is reserved and MUST be set to zero.
- Source Address of Replying Router: Source IP address of the originator of Echo Reply or Relay Echo Reply message.
- Destination Address Offset: The offset in octets from the top-of- stack to the destination address entry. Each entry size has been listed in this section. Please also refer to Section 4.2 for more detail of the operation.
- Number of Relayed Addresses: An integer indicating the number of relayed addresses in the stack.
- Stack of Relayed Addresses: A list of relay node address entries. This stack grows downward, with relay node addresses that are further along the LSP appearing lower down in the stack. Please refer to Section 4.2 for the relay node discovery mechanism.
The format of each relay node address entry is as below:
Type# Address Type Address Length Size of the Entry ---- ------------ ------------ ----------------- 0 Null 0 4 1 IPv4 4 8 2 IPv6 16 20
Reserved: The two fields are reserved and MUST be set to zero.
K bit: If the K bit is set to 1, then this address stack entry MUST NOT be stripped from the Relay Node Address Stack during the processing described in Section 4.2. If the K bit is clear, the entry might be stripped according to the processing described in Section 4.2.
Having the K bit set in the relay node address entry causes that entry to be preserved in the Relay Node Address Stack TLV for the entire traceroute operation. A responder node MAY set the K bit to ensure its relay node address entry remains as one of the relay nodes
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in the Relay Node Address Stack TLV. The address with the K bit set will always be a relay node address for the Relayed Echo Reply, see Section 4.3.
Relayed Address: This field specifies the node address: either IPv4 or IPv6.
This Return Code is defined to indicate that one or more TLVs were omitted from the Echo Reply or Relayed Echo Reply message to avoid exceeding the message's effective MTU size. These TLVs MAY be included in an Errored TLV's Object with their lengths set to 0 and no value. The return sub-code MUST be set to the value that otherwise would have been sent. For more detail, please refer to Section 4.2.
MTU Exceeded Return Code: Value Meaning ----- ------- 20 One or more TLVs not returned due to MTU size
This document updates step 7 in Section 4.4 of [RFC4379] to integrate the processing of the MTU Exceeded Return Code by adding the following text:
Before sending Echo Reply, the new packet size should be checked. If Best-return-code is 3 ("Replying router is an egress for the FEC at stack depth"), or 8 ("Label switched at stack-depth"), and if the packet size exceeds MTU size, then Best-return-code is 20 ("One or more TLVs not returned due to MTU size").
To perform a ping operation, the initiator first discovers the relay nodes. Once those nodes have been discovered, the initiator includes the Relay Node Address Stack TLV into any Echo Request message. The node can then ping as normal. Note that, in some cases, the repeated lack of replies to Echo Request messages may be due to a route change that has impacted the necessary stack of relay nodes. In this case, the initiator may need to rediscover the relay nodes. The following sections describe the procedures for sending and receiving Echo Request messages with the Relay Node Address Stack TLV. These procedures can be used in traceroute mode to discover the relay nodes.
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In addition to the procedures described in Section 4.3 of [RFC4379], a Relay Node Address Stack TLV MUST be carried in the Echo Request message if the relay functionality is required. The relay function initiation is out of the scope of this document. One possible way to do this is that the operator can explicitly add an option to the ping command.
When the initiator sends the first Echo Request with a Relay Node Address Stack TLV, the TLV MUST contain the initiator address as the first entry of the stack of relayed addresses, the Destination Address Offset set to this entry, and the source address of the replying router set to null. The Initiator Source Port field MUST be set to the source UDP port. Note that the first relay node address in the stack will always be the initiator's address.
When sending a subsequent Echo Request message, refer to Section 4.6.
The Type of the Relay Node Address Stack TLV (32768) falls within the range defined in [RFC4379] as "optional TLVs" that can be silently dropped if not recognized. An LSR that does not recognize the TLV SHOULD ignore it.
In addition to the processes in Section 4.4 of [RFC4379], the procedures of the Relay Node Address Stack TLV are defined here.
Upon receiving a Relay Node Address Stack TLV in an Echo Request message, the receiver updates the "Source Address of Replying Router". The address MUST be the same as the source IP address of Relay Echo Reply (Section 4.3) or Echo Reply message (Section 4.5) being sent.
Those address entries with the K bit set to 1 MUST be kept in the stack. The receiver MUST check the addresses of the stack in sequence from bottom to top to find the last address in the stack with the K bit set (or the top of the stack if no K bit was found). The receiver then checks the stack beginning with this entry, proceeding towards the bottom to find the first routable address IP address. The Destination Address Offset MUST be set to this entry, which is also the resolved destination address. Address entries below the first routable IP address MUST be deleted. At least one address entry of the replying LSR MUST be added at the bottom of the stack. A second address entry (or more) MAY also be added if necessary, depending on implementation. The final address added MUST be an address that is reachable through the interface that the Echo
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Request message would have been forwarded to if its TTL had not expired at this node. The updated Relay Node Address Stack TLV MUST be carried in the response message.
If the replying LSR is configured to hide its routable address information, the address entry added in the stack MUST be a NIL entry with Address Type set to NULL.
If a node spans two addressing domains (with respect to this message) where nodes on either side may not be able to reach nodes in the other domain, then the final address added SHOULD set the K bit. One example of spanning two address domains is the ASBR node.
K bit applies in the case of a NULL address, to serve as a warning to the initiator that further Echo Request messages may not result in receiving Echo Reply messages.
If the full reply message would exceed the MTU size, the Relay Node Address Stack TLV SHOULD be included in the Echo Reply message (i.e., other optional TLVs are excluded).
The destination address determined as described in Section 4.2 is used as the next relay node address. If the resolved next relay node address is not routable, then the sending of the Relayed Echo Reply or Echo Reply will fail.
If the first IP address in the Relay Node Address Stack TLV is not the next relay node address, the replying LSR SHOULD send a Relayed Echo Reply message to the next relay node. The processing of Relayed Echo Reply is the same with the procedure of the Echo Reply described in Section 4.5 of [RFC4379], except the destination IP address and the destination UDP port. The destination IP address of the Relayed Echo Reply is set to the next relay node address from the Relay Node Address Stack TLV, and both the source and destination UDP port are set to 3503. The updated Relay Node Address Stack TLV described in Section 4.2MUST be carried in the Relayed Echo Reply message. The Source Address of the Replying Router field is kept unmodified, and the Source IP address field of the IP header is set to an address of the sending node.
When the next relay node address is the first one in the address list, please refer to Section 4.5.
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Upon receiving a Relayed Echo Reply message with its own address as the destination address in the IP header, the relay node MUST determine the next relay node address as described in Section 4.2, with the modification that the location of the received destination address is used instead of the bottom of stack in the algorithm. The Destination Address Offset in Relay Node Address Stack TLV will be set to the next relay node address. Note that unlike in Section 4.2, no changes are made to the Stack of Relayed Addresses.
If the next relay node address is not the first one in the address list, i.e., another intermediate relay node, the relay node MUST send a Relayed Echo Reply message to the determined upstream node with the payload unchanged other than the Relay Node Address Stack TLV. The TTL SHOULD be copied from the received Relay Echo Reply and decremented by 1. The Source Address of the Replying Router field is kept unmodified and the Source IP address field of the IP header is set to an address of the sending node.
When the next relay node address is the first one in the address list, please refer to Section 4.5.
1. When the replying LSR receives an Echo Request, and the first IP address in the Relay Node Address Stack TLV is the next relay node address (Section 4.3), the replying LSR would send an Echo Reply to the initiator. In addition to the procedure of the Echo Reply described in Section 4.5 of [RFC4379], the updated Relay Node Address Stack TLV described in Section 4.2MUST be carried in the Echo Reply.
If the receiver does not recognize the Relay Node Address Stack TLV, as per Sections 3 and 4.5 of [RFC4379], it will send an Echo Reply without including the TLV.
2. When the intermediate relay node receives a Relayed Echo Reply, and the first IP address in the Relay Node Address Stack TLV is the next relay node address (Section 4.4), the intermediate relay node will send the Echo Reply to the initiator, and update the Message Type field from type of "Relayed Echo Reply" to "Echo Reply". The updated Relay Node Address Stack TLV described in Section 4.4MUST be carried in the Echo Reply. The destination IP address of the Echo Reply is set to the first IP address in
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the stack, and the destination UDP port will be copied from the Initiator Source Port field of the Relay Node Address Stack TLV. The source UDP port should be 3503. The TTL of the Echo Reply SHOULD be copied from the received Relay Echo Reply and decremented by 1. The Source Address of the Replying Router field is kept unmodified, and the Source IP address field of the IP header is set to an address of the sending node.
During a traceroute operation, multiple Echo Request messages are sent. Each time the TTL is increased, the initiator SHOULD copy the Relay Node Address Stack TLV received in the previous Echo Reply to the next Echo Request. The Relay Node Address Stack TLV MUST NOT be modified except as follows. A NIL entry that does not have the K bit set MAY be removed. A NIL entry with the K bit serves as a warning that further Echo Request messages are likely not to result in a reply. If, however, the initiator decides to continue a traceroute operation, the NIL entry with the K bit set MUST be removed. The Source Address of the Replying Router and Destination Address Offset fields may be preserved or reset since these fields are ignored in the received MPLS Echo Request.
The Source IP address in Echo Reply and Relay Echo Reply should be the address of the node sending those packets, not the original responding node. Then the traceroute address output module will print the source IP address as below:
if (Relay Node Address Stack TLV exists) { Print the Source Address of Replying Router in Relay Node Address Stack TLV; } else { Print the source IP address of Echo Reply message; }
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Considering the inter-AS scenario in Figure 5 below, AS1 and AS2 are two independent address domains. In the example, an LSP has been created between PE1 to PE2, but PE1 in AS1 is not reachable by P2 in AS2.
When performing LSP traceroute on the LSP, the first Echo Request sent by PE1 with outermost label TTL=1 contains the Relay Node Address Stack TLV with PE1's address as the first relayed address.
After being processed by P1, P1's interface address facing ASBR1 without the K bit set will be added in the Relay Node Address Stack TLV address list following PE1's address in the Echo Reply.
PE1 copies the Relay Node Address Stack TLV into the next Echo Request when receiving the Echo Reply.
Upon receiving the Echo Request, ASBR1 checks the address list in the Relay Node Address Stack TLV and determines that PE1's address is the next relay address. Then it deletes P1's address, and adds its interface address facing ASBR2 with the K bit set. As a result, there will be PE1's address followed by ASBR1's interface address facing ASBR2 in the Relay Node Address Stack TLV of the Echo Reply sent by ASBR1.
PE1 then sends an Echo Request with outermost label TTL=3, containing the Relay Node Address Stack TLV copied from the received Echo Reply message. Upon receiving the Echo Request message, ASBR2 checks the address list in the Relay Node Address Stack TLV and determines that ASBR1's interface address is the next relay address in the stack TLV. ASBR2 adds its interface address facing P2 with the K bit set. Then ASBR2 sets the next relay address as the destination address of the Relayed Echo Reply and sends the Relayed Echo Reply to ASBR1.
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Upon receiving the Relayed Echo Reply from ASBR2, ASBR1 checks the address list in the Relay Node Address Stack TLV and determines that PE1's address is the next relay node. Then ASBR1 sends an Echo Reply to PE1.
For the Echo Request with outermost label TTL=4, P2 checks the address list in the Relay Node Address Stack TLV and determines that ASBR2's interface address is the next relay address. Then P2 sends a Relayed Echo Reply to ASBR2 with the Relay Node Address Stack TLV containing four addresses: PE1's, ASBR1's interface address, ASBR2's interface address, and P2's interface address facing PE2 in sequence.
Then, according to the process described in Section 4.4, ASBR2 sends the Relayed Echo Reply to ASBR1. Upon receiving the Relayed Echo Reply, ASBR1 sends an Echo Reply to PE1. And, as relayed by ASBR2 and ASBR1, the Echo Reply would finally be sent to the initiator PE1.
For the Echo Request with outermost label TTL=5, the Echo Reply would relayed to PE1 by ASBR2 and ASBR1, similar to the case of TTL=4.
The Echo Reply from the replying node that has no IP reachable route to the initiator is thus transmitted to the initiator by multiple relay nodes.
The Relayed Echo Reply mechanism for LSP Ping creates an increased risk of DoS by putting the IP address of a target router in the Relay Node Address Stack. These messages could then be used to attack the control plane of an LSR by overwhelming it with these packets. A rate limiter SHOULD be applied to the well-known UDP port on the relay node as suggested in [RFC4379]. The node that acts as a relay node SHOULD validate the relay reply against a set of valid source addresses and discard packets from untrusted border router addresses. An implementation SHOULD provide such filtering capabilities.
If an operator wants to obscure their nodes, it is RECOMMENDED that they replace the replying node address that originated the Echo Reply with the NIL address entry in the Relay Node Address Stack TLV.
A receiver of an MPLS Echo Request could verify that the first address in the Relay Node Address Stack TLV is the same address as the source IP address field of the received IP header.
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The Relay Node Address Stack TLV has the path information of the LSP, and such information may be maliciously used by any uncontrolled LSR/ LER. We have two ways to reduce the path information in the TLV:
o It is recommended to clear the K bit in the relay address entry unless it must be set (e.g., as listed in Section 4.2).
o It is recommended to use the NIL address entry to hide node information, if possible.
Other security considerations discussed in [RFC4379] are also applicable to this document.
When one of the nodes along the LSP does not support the mechanism specified in this document, the node will ignore the Relay Node Address Stack TLV as described in Section 4.2. Then the initiator may not receive the Relay Node Address Stack TLV in Echo Reply message from that node. In this case, an indication should be reported to the operator, and the Relay Node Address Stack TLV in the next Echo Request message should be copied from the previous Echo Request, and continue the ping process. If the node described above is located between the initiator and the first relay node, the ping process could continue without interruption.
One new Message Type from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry in the "Message Type" subregistry has been allocated:
Value Meaning ----- ------- 5 MPLS Relayed Echo Reply
The value has been assigned from the "Standards Action" [RFC5226] range (0-191) using the lowest free value within this range.
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One new TLV from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry in the "TLVs" subregistry has been allocated:
Type TLV Name ---- -------- 32768 Relay Node Address Stack TLV
The value has been assigned from the "Standards Action" range (32768-49161) as suggested by [RFC4379] Sections 3 and 7.2 using the first free value within this range.
The MTU Exceeded return code from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry in the "Return Codes"subregistry has been allocated:
Value Meaning ----- ------- 20 One or more TLVs not returned due to MTU size
The value has been assigned from the "Standards Action" range (0-191) using the lowest free value within this range.
[MPLSARCH] Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz, M., and D. Steinberg, "Seamless MPLS Architecture", Work in Progress, draft-ietf-mpls-seamless-mpls-07, June 2014.
The authors would like to thank Carlos Pignataro, Xinwen Jiao, Manuel Paul, Loa Andersson, Wim Henderickx, Mach Chen, Thomas Morin, Gregory Mirsky, Nobo Akiya, and Joel M. Halpern for their valuable comments and suggestions.
Contributors
Ryan Zheng JSPTPD 371, Zhongshan South Road Nanjing 210006 China
Email: ryan.zhi.zheng@gmail.com
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Authors' Addresses
Jian Luo (editor) ZTE 50, Ruanjian Avenue Nanjing 210012 China