| TRAM | M. Petit-Huguenin |
| Internet-Draft | Impedance Mismatch |
| Intended status: Standards Track | January 05, 2015 |
| Expires: July 9, 2015 |
Path MTU Discovery Using Session Traversal Utilities for NAT (STUN)
draft-petithuguenin-tram-stun-pmtud-00
This document describes a Session Traversal Utilities for NAT (STUN) usage for discovering the path MTU between a client and a server.
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The Packetization Layer Path MTU Discovery specification [RFC4821] describes a method to discover the path MTU but does not describe a practical protocol to do so with UDP.
This document only describe how probing mechanisms are implemented with STUN. The algorithm to find the path MTU is described in [RFC4821].
Two probing mechanisms are described, a simple probing mechanism and a more complete mechanism that can converge quicker.
The simple probing mechanism is implemented by sending a Probe Request with a PADDING [RFC5780] attribute and the DF bit set over UDP. A router on the path to the server can reject this request with an ICMP message or drop it. The client SHOULD cease retransmissions after 3 missing responses.
The complete probing mechanism is implemented by sending one or more Probe Indication with a PADDING attribute and the DF bit set over UDP then a Report Request to the same server. A router on the path to the server can reject this indication with an ICMP message or drop it. The server keeps a time ordered list of identifiers of all packets received (including retransmitted packets) and sends this list back to the client in the Report Response. The client analyzes this list to find which packets were not received. Because UDP packets does not contain an identifier, the complete probing mechanism needs a way to identify each packet received. As example, this document describes two different ways to identify a specific packet.
In the first packet identifier mechanism, the server computes a checksum over each packet received and sends back to the sender the ordered list of checksums. The client compares this list to its own list of checksums.
In the second packet identifier mechanism, the client adds a sequential number in front of each UDP packet sent. The server sends back the ordered list of sequential numbers received that the client compares to its own list
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]. When these words are not in ALL CAPS (such as "must" or "Must"), they have their usual English meanings, and are not to be interpreted as RFC 2119 key words.
A client MUST NOT send a probe if it does not have knowledge that the server supports this specification. This is done by an external mechanism specific to each UDP protocol. Section 6 describes some of this mechanisms.
The probe mechanism is used to discover the path MTU in one direction only, from the client to the server.
A client forms a Probe Request by following the rules in Section 7.1 of [RFC5389]. No authentication method is used. The client adds a PADDING [RFC5780] attribute with a length that, when added to the IP and UDP headers and the other STUN components, is equal to the Selected Probe Size, as defined in [RFC4821] section 7.3. The client MUST add the FINGERPRINT attribute.
Then the client sends the Probe Request to the server over UDP with the DF bit set. The client SHOULD stop retransmitting after 3 missing responses.
A server receiving a Probe Request MUST process it as specified in [RFC5389]. The server MUST NOT challenge the client.
The server then creates a Probe Response. The server MUST add the FINGERPRINT attribute. The server then sends the response to the client.
A client receiving a Probe Response MUST process it as specified in [RFC5389]. If a response is received this is interpreted as a Probe Success as defined in [RFC4821] section 7.6.1. If an ICMP packet "Fragmentation needed" is received then this is interpreted as a Probe Failure as defined in [RFC4821] section 7.6.2. If the Probe transactions fails in timeout, then this is interpreted as a Probe Inconclusive as defined in [RFC4821] section 7.6.4.
A client forms a Probe Indication by following the rules in [RFC5389] section 7.1. The client adds to the Probe Indication a PADDING attribute with a size that, when added to the IP and UDP headers and the other STUN components, is equal to the Selected Probe Size, as defined in [RFC4821] section 7.3. The client MUST add the FINGERPRINT attribute.
Then the client sends the Probe Indication to the server over UDP with the DF bit set.
Then the client forms a Report Request by following the rules in [RFC5389] section 7.1. No authentication method is used. The client MUST add the FINGERPRINT attribute.
Then the client waits half the RTO if it is known or 50 milliseconds after sending the Probe Indication and sends the Report Request to the server over UDP.
If an ICMP packet "Fragmentation needed" is received then this is interpreted as a Probe Failure as defined in [RFC4821] section 7.5.
A server supporting this specification and knowing that the client also supports it will keep the identifiers of all packets received in a list ordered by receiving time. The same identifier can appear multiple times in the list because of retransmission. The maximum size of this list is calculated so that when the list is added to the Report Response, the total size of the packet does not exceed the unknown path MTU as defined in [RFC5389] section 7.1. Older identifiers are removed when new identifiers are added to a list already full.
A server receiving a Report Request MUST process it as specified in [RFC5389]. The server MUST NOT challenge the client.
The server creates a Report Response and adds an IDENTIFIERS attribute that contains the list of all identifiers received so far. The server MUST add the FINGERPRINT attribute. The server then sends the response to the client.
A client receiving a Report Response processes it as specified in [RFC5389]. If the response IDENTIFIERS attribute contains the identifier of the Probe Indication, then this is interpreted as a Probe Success for this probe as defined in [RFC4821] Section 7.5. If the Probe Indication identifier cannot be found in the Report Response, this is interpreted as a Probe Failure as defined in [RFC4821] Section 7.5. If the Probe Indication identifier cannot be found in the Report Response but other packets identifier sent before or after the Probe Indication cannot also be found, this is interpreted as a Probe Inconclusive as defined in [RFC4821] Section 7.5. If the Report Transaction fails in timeout, this is interpreted as a Full-Stop Timeout as defined in [RFC4821] Section 3.
When using checksum as packet identifiers, the client calculate the checksum for each packet sent over UDP and keep this checksum in an ordered list. The server does the same thing and send back this list in the Report Response.
It could have been possible to use the checksum generated in the UDP checksum for this, but this value is generally not accessible to applications. Also sometimes the checksum is not calculated or off-loaded to the network card.
When using sequential numbers, a small header similar to the TURN ChannelData header is added in front of all non-STUN packets. The sequential number is incremented for each packet sent. The server collects the sequence number of the packets sent.
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Channel Number | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Application Data / / / | | | +-------------------------------+ | | +-------------------------------+
The Channel Number is always 0xFFFF.
An endpoint acting as a client for the STUN usage described in this specification MUST also act as a server for this STUN usage. This means that a server receiving a probe can assumes that it can acts as a client to discover the path MTU to the IP address and port from which it received the probe.
A TURN client supporting this STUN usage will add a PMTUD-SUPPORTED attribute to the Allocate Request sent to the TURN server. The TURN server can immediately start to send probes to the TURN client on reception of an Allocation Request with a PMTUD-SUPPORTED attribute. The TURN client will then use the Implicit Mechanism described above to send probes.
An ICE [RFC5245] client supporting this STUN usage will add a PMTUD-SUPPORTED attribute to the Binding Request sent during a connectivity check. The ICE server can immediately start to send probes to the ICE client on reception of a Binding Request with a PMTUD-SUPPORTED attributed. Local candidates receiving Binding Request with the PMTUD-SUPPORTED flag must not start PMTUD with the remote candidate if already done so. The ICE client will then use the Implicit Mechanism described above to send probes.
This specification defines the following new STUN methods:
This specification defines the following new STUN attributes:
The IDENTIFIERS attribute is used in Report Response. It contains a list of UDP packet identifiers.
The PMTUD-SUPPORTED attribute is used in STUN usages and extensions to signal the support of this specification. This attribute has no content.
TBD
TBD
Thanks to Eilon Yardeni for his comments, suggestions and questions that helped to improve this document.
Special thanks to Dan Wing, who supported this document since its first publication back in 2008.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC4821] | Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, March 2007. |
| [RFC5389] | Rosenberg, J., Mahy, R., Matthews, P. and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008. |
| [RFC5245] | Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010. |
| [RFC5780] | MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery Using Session Traversal Utilities for NAT (STUN)", RFC 5780, May 2010. |
| [RFC6982] | Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, July 2013. |
This section must be removed before publication as an RFC.