| 6man | J. Leddy |
| Internet-Draft | Comcast |
| Intended status: Standards Track | R. Bonica |
| Expires: November 5, 2018 | Juniper Networks |
| May 4, 2018 |
Destination Originates Internet Control Message Protocol (ICMP) Packet Too Big (PTB) Messages
draft-leddy-6man-truncate-00
This document defines procedures that enhance Path MTU Discovery (PMTUD), so that it no longer relies on the network's ability to deliver an ICMP Packet Too Big (PTB) message from a downstream router to an IPv6 source node. According to these procedures, selected packets carry a new IPv6 Destination option. When a downstream router cannot forward one of these packets because of MTU issues, it truncates the packet, marks it to indicate that it has been truncated, and forwards it towards the destination node.
When the destination node receives a packet that has been truncated, it sends an ICMP PTB message to the source node. The source node uses MTU information contained by the ICMP PTB message to update its PMTU estimate. The destination node does not deliver the truncated packet to upper layer protocols unless specifically requested to do so by the upper layer protocol.
The procedures described herein rely on the network's ability to deliver an ICMP PTB message from the packet's destination to the packet's source.
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An Internet path connects a source node to a destination node. A path can contain links and routers.
Each link is constrained by the number of bytes that it can convey in a single IP packet. This constraint is called the link Maximum Transmission Unit (MTU). IPv6 requires every link to have an MTU of 1280 bytes or greater. This value is called IPv6 minimum link MTU.
Likewise, each Internet path is constrained by the number of bytes that it can convey in a IP single packet. This constraint is called the Path MTU (PMTU). For any given path, the PMTU is equal to the smallest of its link MTUs.
IPv6 allows fragmentation at the source node only. If an IPv6 source node sends a packet whose length exceeds the destination PMTU, the packet will be discarded. In order to avoid this kind of packet loss, IPv6 nodes can either:
IPv6 nodes can execute Path MTU Discovery (PMTUD) procedures in order to maintain a running estimate of the destination PMTU. According to these procedures, the source node produces an initial PMTU estimate. This initial estimate is equal to the MTU of the first link along the path to the destination node. It can be greater than the actual PMTU.
Having produced an initial PMTU estimate, the source node sends packets to the destination node. If one of these packets is larger than the actual PMTU, a downstream router will not be able to forward the packet through the next link along the path. Therefore, the downstream router discards the packet and sends an Internet Control Message Protocol (ICMP) Packet Too Big (PTB) message to the source node. The ICMP PTB message indicates the MTU of the link through which the packet could not be forwarded. The source node uses this information to refine its PMTU estimate.
PMTUD relies on the network's ability to deliver ICMP PTB messages from the downstream router to the source node. If the network cannot deliver these messages, a persistent black hole can develop. In this scenario, the source node sends a packet whose length exceeds the destination PMTU. A downstream router discards the packet and sends an ICMP PTB message to the source. However, the network cannot deliver the ICMP PTB message to the source. Therefore, the source node does not update its PMTU estimate and it continues to send packets whose length exceeds the destination PMTU. The downstream router discards these packets and sends ICMP PTB messages to the source. These ICMP PTB messages are lost, exactly as previous ICMP PTB messages were lost.
The following are common reasons why networks cannot deliver ICMP PTB messages from downstream routers to source nodes:
This document defines procedures that enhance PMTUD, so that it no longer relies on the network's ability to deliver an ICMP PTB message from a downstream router to an IPv6 source node. According to these procedures, selected packets carry a new IPv6 Destination option. When a downstream router cannot forward one of these packets because of MTU issues, it truncates the packet, marks it to indicate that it has been truncated, and forwards it towards the destination node.
When the destination node receives a packet that has been truncated, it sends an ICMP PTB message to the source node. The source node uses MTU information contained by the ICMP PTB message to update its PMTU estimate. The destination node does not deliver the truncated packet to upper layer protocols unless specifically requested to do so by the upper layer protocol.
The procedures described herein rely on the network's ability to deliver an ICMP PTB message from the packet's destination to the packet's source.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC8174] when, and only when, they appear in all capitals, as shown here.
------------- ------------- -------------
| Upper | | | | Upper |
| Layer | | | | Layer |
| | | | | |
| IP |<-------->| IP |<-------->| IP |
| Layer | MTU | Layer | MTU | Layer |
------------- 9000 ------------- 1500 -------------
Source Router Destination
Node Node
Figure 1
Figure 1 depicts a reference topology that is used in this document. The reference topology includes a source node, a router, and a destination node. The link that connects the source node to the router has an MTU of 9000 bytes. The link that connects the router to the destination node has an MTU of 1500 bytes. Therefore, the PMTU between the source node and the destination node is 1500 bytes.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Opt Data Len |T|D|R| Reserved|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2
Figure 2 depicts the Truncation option. The IPv6 Destination Options header MAY include the Truncation option. The IPv6 Hop-by-hop header MUST NOT include the Truncation option.
NOTE 1: The highest-order two bits of the Option Type (i.e., the "act" bits) are 10. These bits specify the action taken by a destination node that does not recognize Truncation option. The required action is to discard the packet and, regardless of whether or not the packet's Destination Address was a multicast address, send an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type.
NOTE 2: The third highest-order bit of the Option Type (i.e., the "chg" bit) is 1. This indicates that Option Data can be modified along the path between the packet's source and its destination.
In the Reference Topology, the upper-layer protocol that resides on the source node submits a packet to its local IP layer. The packet includes a Truncation option. Within the Truncation option:
The packet length is 500 bytes. Because the packet length is less than the destination PMTU, the packet can be delivered without encountering MTU issues.
The IP layer on the source node forwards the packet to the downstream router and the downstream router forwards the packet to the destination node. The IP layer on the destination node examines the Destination Option header. Because it recognizes the Truncation option, and because the packet has not been truncated, it delivers the packet to an upper-layer protocol.
Now, the upper-layer protocol that resides on the source node submits another packet to its local IP layer. This packet is identical to the first, except that the packet length is 2000 bytes. Because the packet length is greater than the destination PMTU, the packet cannot be delivered without encountering MTU issues.
The IP layer on the source node forwards the packet to the downstream router but the downstream router cannot forward the packet because its length exceeds the MTU of the next-hop link. Because an MTU issue has been encountered, the IP Layer on the downstream router examines the Destination Options header, searching for a Truncation option. (Normally, the downstream router would ignore the Destination Options header).
Because the downstream router finds and recognizes the Truncation option, it:
The IP layer on the destination node examines the Destination Option header. Because it recognizes the Truncation option, and because the packet has been truncated, it sends an ICMP PTB message to the source node. The MTU field in the ICMP PTB message is set to the packet's length.
The IP layer on the destination node then discards the packet because the packet has been truncated and the D-bit is set to 0. It does not deliver the packet to an upper-layer protocol.
As per [RFC8201], the source node updates its PMTU estimate using information contained by the ICMP PTB message.
A packet can be truncated multiple times.
------------- ------------- ------------- -------------
| IP |<-------->| IP |<-------->| IP |<-------->| IP |
| Layer | MTU | Layer | MTU | Layer | MTU | Layer |
------------- 9000 ------------- 4000 ------------- 1500 -------------
Source Router1 Router 2 Destination
Node Node
Figure 3: Double Truncation
Figure 3 depicts a network that contains a Source Node, Router1, Router2 and a Destination Node. The link that connects the Source Node to Router1 has an MTU of 9000 bytes. The link that connects Router1 to Router2 has an MTU of 4000 bytes, and the link that connects Router2 to the Destination Node has an MTU of 1500 bytes.
Assume that the Source Node sends a packet to the Destination Node. The packet is 8000 bytes long. When Router1 receives this packet, it identifies the next-hop towards the destination. This is the link that connects Router1 to Router2. Router 1 encounters an MTU issue, because the packet length (8000 bytes) is greater than the MTU associated with the next-hop link (4000 bytes). Therefore, Router 1 truncates the packet to 4000 bytes, sets the T-bit in the Truncation Option, and forwards the packet towards Router2. When Router2 receives this packet, it identifies the next-hop towards the destination. This is the link that connects Router2 to the Destination Node. Router 2 encounters an MTU issue, because the packet length (4000 bytes) is greater than the MTU associated with the next-hop link (1500 bytes). Therefore, Router 2 truncates the packet to 1500 bytes, sets the T-bit in the Truncation Option, and forwards the packet towards the Destination Node. The Destination Node sends an ICMP PTB packet to the source node. The MTU field in the ICMP PTB field is set to 1500.
A truncated packet MUST contain the basic IPv6 header, all extension headers and the first upper-layer header. When a router cannot forward a packet through the next-hop link due to MTU issues, and the total length of the basic IPv6 header, all extension headers, and first upper-layer header exceeds the MTU of the next-hop link, the router MUST discard the packet and send and ICMP PTB message to the source.
Source nodes MUST NOT emit packets that contain both the Fragment Header and Truncation Option.
Routers MUST NOT truncate packets that include the Fragment header. When a router cannot forward a packet through the next-hop link due to MTU issues, and the packet includes a Fragment header, the router MUST discard the packet and send and ICMP PTB message to the source.
Routers MUST NOT emit truncated packets whose length is less than 1280 bytes.
When a destination node receives a truncated packet whose length is less than 1280 bytes, the destination node MUST discard the packet. It MUST NOT send an ICMP PTB message to the packet's source and it MUST NOT deliver the packet to an upper-layer protocol.
A destination node MUST NOT deliver a truncated packet to an upper-layer protocol if the D-bit is set to zero. However, a destination node SHOULD deliver a truncated packet to an upper-layer protocol if the D-bit is set to one.
The upper-layer protocol on the source node determines the D-bit value. Some existing upper-layer protocols never set the D-bit. However, upper-layer protocols developed in the future might always set the D-bit. Alternatively, other upper-layer protocols developed in the future might set the D-bit only after receiving a Truncation option with the R-bit set.
The procedures described in Section 5 of this document assume that the source node, downstream router and destination node all recognized the Truncation option. This section explores backwards compatibility, where one or more nodes do not recognize the Truncation option.
If the destination node does not recognize the Truncation option, and it receives a packet that includes the Truncation option, it discards the packet and, regardless of whether or not the packet's Destination Address was a multicast address, sends an ICMP Parameter Problem, Code 2, message to the packet's Source Address, pointing to the unrecognized Option Type. This behavior is determined by the highest-order two bytes of the Option Type. When the source node receives the ICMP Parameter Problem message, it refrains from sending packets that contain the Truncation option.
If the downstream router does not recognize the Truncation option and it receives a packet that contains the Truncation option and that packets length does not exceed the next-hop MTU, the downstream router forwards the packet, without examining the Truncation option or any other Destination option. If the downstream router does not recognize the Truncation option and it receives a packet that contains the Truncation option and that packets length exceeds the next-hop MTU, the downstream discards the packet and sends an ICMP PTB message to the source node, as per [RFC8200].
In all cases mentioned above, PMTUD continues to function as specified in [RFC8201].
The procedures described in Section 5 of this document can be optimized by omitting the Truncation option on packets whose length is known to be less than the destination PMTU (e.g., packets whose length is less than 1280 bytes).
PMTUD is vulnerable to ICMP PTB forgery attacks. The procedures described herein do nothing to mitigate that vulnerability.
The procedures described herein are susceptible to a new variation on that attack, in which an attacker forges a truncated packet. In this case, the attackers cause the destination node to produce an ICMP PTB message on their behalf. To some degree, this vulnerability is mitigated, because the destination node will not emit an ICMP PTB message in response to a truncated packet whose length is less than 1280 bytes.
IANA is requested to allocate a codepoint from the Destination Options and Hop-by-hop Options registry (https://www.iana.org/assignments/ipv6-parameters/ipv6-parameters.xhtml#ipv6-parameters-2). This option is called "Truncation". The "act" bits are 10 and the "chg" bit is 1.
Special thanks to Andy Smith, who reviewed and commented on this document.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4291] | Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006. |
| [RFC4443] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, March 2006. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [RFC8200] | Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017. |
| [RFC8201] | McCann, J., Deering, S., Mogul, J. and R. Hinden, "Path MTU Discovery for IP version 6", STD 87, RFC 8201, DOI 10.17487/RFC8201, July 2017. |