| Network Working Group | J. McCann |
| Internet-Draft | Digital Equipment Corporation |
| Obsoletes: 1981 (if approved) | S. Deering |
| Intended status: Standards Track | Retired |
| Expires: October 9, 2017 | J. Mogul |
| Digital Equipment Corporation | |
| R. Hinden, Ed. | |
| Check Point Software | |
| April 7, 2017 |
Path MTU Discovery for IP version 6
draft-ietf-6man-rfc1981bis-06
This document describes Path MTU Discovery for IP version 6. It is largely derived from RFC 1191, which describes Path MTU Discovery for IP version 4. It obsoletes RFC1981.
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When one IPv6 node has a large amount of data to send to another node, the data is transmitted in a series of IPv6 packets. These packets can have a size less than or equal to the Path MTU (PMTU). Alternatively, they can be larger packets that are fragmented into a series of fragments each with a size less than or equal to the PMTU.
It is usually preferable that these packets be of the largest size that can successfully traverse the path from the source node to the destination node without the need for IPv6 fragmentation. This packet size is referred to as the Path MTU, and it is equal to the minimum link MTU of all the links in a path. This document defines a standard mechanism for a node to discover the PMTU of an arbitrary path.
IPv6 nodes SHOULD implement Path MTU Discovery in order to discover and take advantage of paths with PMTU greater than the IPv6 minimum link MTU [I-D.ietf-6man-rfc2460bis]. A minimal IPv6 implementation (e.g., in a boot ROM) may choose to omit implementation of Path MTU Discovery.
Nodes not implementing Path MTU Discovery MUST use the IPv6 minimum link MTU defined in [I-D.ietf-6man-rfc2460bis] as the maximum packet size. In most cases, this will result in the use of smaller packets than necessary, because most paths have a PMTU greater than the IPv6 minimum link MTU. A node sending packets much smaller than the Path MTU allows is wasting network resources and probably getting suboptimal throughput.
Nodes implementing Path MTU Discovery and sending packets larger than the IPv6 minimum link MTU are susceptible to problematic connectivity if ICMPv6 [ICMPv6] messages are blocked or not transmitted. For example, this will result in connections that complete the TCP three-way handshake correctly but then hang when data is transferred. This state is referred to as a black hole connection. Path MTU Discovery relies on such messages to determine the MTU of the path.
An extension to Path MTU Discovery defined in this document can be found in [RFC4821]. RFC4821 defines a method for Packetization Layer Path MTU Discovery (PLPMTUD) designed for use over paths where delivery of ICMPv6 messages to a host is not assured.
This memo describes a technique to dynamically discover the PMTU of a path. The basic idea is that a source node initially assumes that the PMTU of a path is the (known) MTU of the first hop in the path. If any of the packets sent on that path are too large to be forwarded by some node along the path, that node will discard them and return ICMPv6 Packet Too Big messages. Upon receipt of such a message, the source node reduces its assumed PMTU for the path based on the MTU of the constricting hop as reported in the Packet Too Big message. The decreased PMTU causes the source to send smaller fragments or change EMTU_S to cause upper layer to reduce the size of IP packets it sends.
The Path MTU Discovery process ends when the node's estimate of the PMTU is less than or equal to the actual PMTU. Note that several iterations of the packet-sent/Packet-Too-Big-message-received cycle may occur before the Path MTU Discovery process ends, as there may be links with smaller MTUs further along the path.
Alternatively, the node may elect to end the discovery process by ceasing to send packets larger than the IPv6 minimum link MTU.
The PMTU of a path may change over time, due to changes in the routing topology. Reductions of the PMTU are detected by Packet Too Big messages. To detect increases in a path's PMTU, a node periodically increases its assumed PMTU. This will almost always result in packets being discarded and Packet Too Big messages being generated, because in most cases the PMTU of the path will not have changed. Therefore, attempts to detect increases in a path's PMTU should be done infrequently.
Path MTU Discovery supports multicast as well as unicast destinations. In the case of a multicast destination, copies of a packet may traverse many different paths to many different nodes. Each path may have a different PMTU, and a single multicast packet may result in multiple Packet Too Big messages, each reporting a different next-hop MTU. The minimum PMTU value across the set of paths in use determines the size of subsequent packets sent to the multicast destination.
Note that Path MTU Discovery must be performed even in cases where a node "thinks" a destination is attached to the same link as itself. In a situation such as when a neighboring router acts as proxy [ND] for some destination, the destination can to appear to be directly connected but is in fact more than one hop away.
As discussed in Section 1, IPv6 nodes are not required to implement Path MTU Discovery. The requirements in this section apply only to those implementations that include Path MTU Discovery.
Nodes SHOULD appropriately validate the payload of ICMPv6 PTB messages to ensure these are received in response to transmitted traffic (i.e., a reported error condition that corresponds to an IPv6 packet actually sent by the application) per [ICMPv6].
If a node receives a Packet Too Big message reporting a next-hop MTU that is less than the IPv6 minimum link MTU, it MUST discard it. A node MUST NOT reduce its estimate of the Path MTU below the IPv6 minimum link MTU.
When a node receives a Packet Too Big message, it MUST reduce its estimate of the PMTU for the relevant path, based on the value of the MTU field in the message. The precise behavior of a node in this circumstance is not specified, since different applications may have different requirements, and since different implementation architectures may favor different strategies.
After receiving a Packet Too Big message, a node MUST attempt to avoid eliciting more such messages in the near future. The node MUST reduce the size of the packets it is sending along the path. Using a PMTU estimate larger than the IPv6 minimum link MTU may continue to elicit Packet Too Big messages. Since each of these messages (and the dropped packets they respond to) consume network resources, the node MUST force the Path MTU Discovery process to end.
Nodes using Path MTU Discovery MUST detect decreases in PMTU as fast as possible. Nodes MAY detect increases in PMTU, but because doing so requires sending packets larger than the current estimated PMTU, and because the likelihood is that the PMTU will not have increased, this MUST be done at infrequent intervals. An attempt to detect an increase (by sending a packet larger than the current estimate) MUST NOT be done less than 5 minutes after a Packet Too Big message has been received for the given path. The recommended setting for this timer is twice its minimum value (10 minutes).
A node MUST NOT increase its estimate of the Path MTU in response to the contents of a Packet Too Big message. A message purporting to announce an increase in the Path MTU might be a stale packet that has been floating around in the network, a false packet injected as part of a denial-of-service attack, or the result of having multiple paths to the destination, each with a different PMTU.
This section discusses a number of issues related to the implementation of Path MTU Discovery. This is not a specification, but rather a set of notes provided as an aid for implementers.
The issues include:
In the IP architecture, the choice of what size packet to send is made by a protocol at a layer above IP. This memo refers to such a protocol as a "packetization protocol". Packetization protocols are usually transport protocols (for example, TCP) but can also be higher-layer protocols (for example, protocols built on top of UDP).
Implementing Path MTU Discovery in the packetization layers simplifies some of the inter-layer issues, but has several drawbacks: the implementation may have to be redone for each packetization protocol, it becomes hard to share PMTU information between different packetization layers, and the connection-oriented state maintained by some packetization layers may not easily extend to save PMTU information for long periods.
It is therefore suggested that the IP layer store PMTU information and that the ICMPv6 layer process received Packet Too Big messages. The packetization layers may respond to changes in the PMTU by changing the size of the messages they send. To support this layering, packetization layers require a way to learn of changes in the value of MMS_S, the "maximum send transport-message size".
MMS_S is a transport message size calculated by subtracting the size of the IPv6 header (including IPv6 extension headers) from the largest IP packet that can be sent, EMTU_S. MMS_S is limited by a combination of factors, including the PMTU, support for packet fragmentation and reassembly, and the packet reassembly limit (see [I-D.ietf-6man-rfc2460bis] section "Fragment Header"). When source fragmentation is available, EMTU_S is set to EMTU_R, as indicated by the receiver using an upper layer protocol or based on protocol requirements (1500 octets for IPv6). When a message larger than PMTU is to be transmitted, the source creates fragments, each limited by PMTU. When source fragmentation is not desired, EMTU_S is set to PMTU, and the upper layer protocol is expected to either perform its own fragmentation and reassembly or otherwise limit the size of its messages accordingly.
However, packetization layers are encouraged to avoid sending messages that will require source fragmentation (for the case against fragmentation, see [FRAG]).
Ideally, a PMTU value should be associated with a specific path traversed by packets exchanged between the source and destination nodes. However, in most cases a node will not have enough information to completely and accurately identify such a path. Rather, a node must associate a PMTU value with some local representation of a path. It is left to the implementation to select the local representation of a path.
In the case of a multicast destination address, copies of a packet may traverse many different paths to reach many different nodes. The local representation of the "path" to a multicast destination must represent a potentially large set of paths.
Minimally, an implementation could maintain a single PMTU value to be used for all packets originated from the node. This PMTU value would be the minimum PMTU learned across the set of all paths in use by the node. This approach is likely to result in the use of smaller packets than is necessary for many paths. In the case of multipath routing (e.g., Equal Cost Multipath Routing, ECMP), a set of paths can exist even for a single source and destination pair.
An implementation could use the destination address as the local representation of a path. The PMTU value associated with a destination would be the minimum PMTU learned across the set of all paths in use to that destination. This approach will result in the use of optimally sized packets on a per-destination basis. This approach integrates nicely with the conceptual model of a host as described in [ND]: a PMTU value could be stored with the corresponding entry in the destination cache.
If flows [I-D.ietf-6man-rfc2460bis] are in use, an implementation could use the flow id as the local representation of a path. Packets sent to a particular destination but belonging to different flows may use different paths, as with ECMP, in which the choice of path might depending on the flow id. This approach might result in the use of optimally sized packets on a per-flow basis, providing finer granularity than PMTU values maintained on a per-destination basis.
For source routed packets (i.e. packets containing an IPv6 Routing header [I-D.ietf-6man-rfc2460bis]), the source route may further qualify the local representation of a path.
Initially, the PMTU value for a path is assumed to be the (known) MTU of the first-hop link.
When a Packet Too Big message is received, the node determines which path the message applies to based on the contents of the Packet Too Big message. For example, if the destination address is used as the local representation of a path, the destination address from the original packet would be used to determine which path the message applies to.
The node then uses the value in the MTU field in the Packet Too Big message as a tentative PMTU value or the IPv6 minimum link MTU if that is larger, and compares the tentative PMTU to the existing PMTU. If the tentative PMTU is less than the existing PMTU estimate, the tentative PMTU replaces the existing PMTU as the PMTU value for the path.
The packetization layers must be notified about decreases in the PMTU. Any packetization layer instance (for example, a TCP connection) that is actively using the path must be notified if the PMTU estimate is decreased.
Also, the instance that sent the packet that elicited the Packet Too Big message should be notified that its packet has been dropped, even if the PMTU estimate has not changed, so that it may retransmit the dropped data.
It is important to understand that the notification of the packetization layer instances using the path about the change in the PMTU is distinct from the notification of a specific instance that a packet has been dropped. The latter should be done as soon as practical (i.e., asynchronously from the point of view of the packetization layer instance), while the former may be delayed until a packetization layer instance wants to create a packet. Retransmission should be done for only for those packets that are known to be dropped, as indicated by a Packet Too Big message.
Internetwork topology is dynamic; routes change over time. While the local representation of a path may remain constant, the actual path(s) in use may change. Thus, PMTU information cached by a node can become stale.
If the stale PMTU value is too large, this will be discovered almost immediately once a large enough packet is sent on the path. No such mechanism exists for realizing that a stale PMTU value is too small, so an implementation SHOULD "age" cached values. When a PMTU value has not been decreased for a while (on the order of 10 minutes), the PMTU estimate should be set to the MTU of the first-hop link, and the packetization layers should be notified of the change. This will cause the complete Path MTU Discovery process to take place again.
An upper layer must not retransmit data in response to an increase in the PMTU estimate, since this increase never comes in response to an indication of a dropped packet.
One approach to implementing PMTU aging is to associate a timestamp field with a PMTU value. This field is initialized to a "reserved" value, indicating that the PMTU is equal to the MTU of the first hop link. Whenever the PMTU is decreased in response to a Packet Too Big message, the timestamp is set to the current time.
Once a minute, a timer-driven procedure runs through all cached PMTU values, and for each PMTU whose timestamp is not "reserved" and is older than the timeout interval:
A packetization layer (e.g., TCP) must track the PMTU for the path(s) in use by a connection; it should not send segments that would result in packets larger than the PMTU, except to probe during PMTU discovery (this probe packet must not be fragmented to the PMTU). A simple implementation could ask the IP layer for this value each time it created a new segment, but this could be inefficient. An implementation typically caches other values derived from the PMTU. It may be simpler to receive asynchronous notification when the PMTU changes, so that these variables may be also updated.
A TCP implementation must also store the Maximum Segment Size (MSS) value received from its peer, which represents the EMTU_R, the largest packet that can be reassembled by the receiver, and must not send any segment larger than this MSS, regardless of the PMTU.
The value sent in the TCP MSS option is independent of the PMTU; it is determined by the receiver reassembly limit EMTU_R. This MSS option value is used by the other end of the connection, which may be using an unrelated PMTU value. See [I-D.ietf-6man-rfc2460bis] sections "Packet Size Issues" and "Maximum Upper-Layer Payload Size" for information on selecting a value for the TCP MSS option.
When a Packet Too Big message is received, it implies that a packet was dropped by the node that sent the ICMPv6 message. It is sufficient to treat this in the same way as any other dropped segment, and will be recovered by normal retransmission methods. If the Path MTU Discovery process requires several steps to find the PMTU of the full path, this could delay the connection by many round-trip times.
Alternatively, the retransmission could be done in immediate response to a notification that the Path MTU has changed, but only for the specific connection specified by the Packet Too Big message. The packet size used in the retransmission should be no larger than the new PMTU.
Many TCP implementations incorporate "congestion avoidance" and "slow-start" algorithms to improve performance [CONG]. Unlike a retransmission caused by a TCP retransmission timeout, a retransmission caused by a Packet Too Big message should not change the congestion window. It should, however, trigger the slow-start mechanism (i.e., only one segment should be retransmitted until acknowledgements begin to arrive again).
TCP performance can be reduced if the sender's maximum window size is not an exact multiple of the segment size in use (this is not the congestion window size).
Some transport protocols are not allowed to repacketize when doing a retransmission. That is, once an attempt is made to transmit a segment of a certain size, the transport cannot split the contents of the segment into smaller segments for retransmission. In such a case, the original segment can be fragmented by the IP layer during retransmission. Subsequent segments, when transmitted for the first time, should be no larger than allowed by the Path MTU.
Path MTU Discovery for IPv4 [RFC1191] used NFS as an example of a UDP-based application that benefits from PMTU discovery. Since then [RFC7530], states the supported transport layer between NFS and IP must be an IETF standardized transport protocol that is specified to avoid network congestion; such transports include TCP and the Stream Control Transmission Protocol (SCTP). In this case, the transport is itself responsible for determining and using an effective Path MTU, including implementing PMTU discovery when this is needed.
It is suggested that an implementation provide a way for a system utility program to:
The former can be accomplished by associating a flag with the path; when a packet is sent on a path with this flag set, the IP layer does not send packets larger than the IPv6 minimum link MTU.
These features might be used to work around an anomalous situation, or by a routing protocol implementation that is able to obtain Path MTU values.
The implementation should also provide a way to change the timeout period for aging stale PMTU information.
This Path MTU Discovery mechanism makes possible two denial-of- service attacks, both based on a malicious party sending false Packet Too Big messages to a node.
A malicious party could also cause problems if it could stop a victim from receiving legitimate Packet Too Big messages, but in this case there are simpler denial-of-service attacks available.
If ICMPv6 filtering prevents reception of ICMPv6 Packet Too Big messages, the source will not learn the actual path MTU. Packetization Layer Path MTU Discovery [RFC4821] does not rely upon network support for ICMPv6 messages and is therefore considered more robust than standard PMTUD. It is not susceptible to "black holing" of ICMPv6 message. See [RFC4890] for recommendations regarding filtering ICMPv6 messages.
We would like to acknowledge the authors of and contributors to [RFC1191], from which the majority of this document was derived. We would also like to acknowledge the members of the IPng working group for their careful review and constructive criticisms.
This document does not have any IANA actions
| [I-D.ietf-6man-rfc2460bis] | <>, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", Internet-Draft draft-ietf-6man-rfc2460bis-09, March 2017. |
| [ICMPv6] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, DOI 10.17487/RFC4443, March 2006. |
| [CONG] | Jacobson, V., "Congestion Avoidance and Control", Proc. SIGCOMM '88 Symposium on Communications Architectures and Protocols , August 1988. |
| [FRAG] | Kent, C. and J. Mogul, "Fragmentation Considered Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in Computer Communications Technology , August 1987. |
| [ND] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007. |
| [RFC1191] | Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, DOI 10.17487/RFC1191, November 1990. |
| [RFC4821] | Mathis, M. and J. Heffner, "Packetization Layer Path MTU Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007. |
| [RFC4890] | Davies, E. and J. Mohacsi, "Recommendations for Filtering ICMPv6 Messages in Firewalls", RFC 4890, DOI 10.17487/RFC4890, May 2007. |
| [RFC6691] | Borman, D., "TCP Options and Maximum Segment Size (MSS)", RFC 6691, DOI 10.17487/RFC6691, July 2012. |
| [RFC7530] | Haynes, T. and D. Noveck, "Network File System (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530, March 2015. |
| [RFC8085] | Eggert, L., Fairhurst, G. and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017. |
This document is based in large part on RFC 1191, which describes Path MTU Discovery for IPv4. Certain portions of RFC 1191 were not needed in this document:
This document is based on RFC1981 has the following changes from RFC1981:
NOTE TO RFC EDITOR: Please remove this subsection prior to RFC Publication
This section describes change history made in each Internet Draft that went into producing this version. The numbers identify the Internet-Draft version in which the change was made.
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