| Network Working Group | F. L. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Standards Track | October 31, 2011 |
| Expires: May 03, 2012 |
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-36.txt
For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected IP network routing region and bounded by encapsulating border nodes. These virtual topologies are manifested by tunnels that may span multiple IP and/or sub-IP layer forwarding hops, and can introduce failure modes due to packet duplication and/or links with diverse Maximum Transmission Units (MTUs). This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that accommodates such virtual topologies over diverse underlying link technologies.
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As Internet technology and communication has grown and matured, many techniques have developed that use virtual topologies (including tunnels of one form or another) over an actual network that supports the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual topologies have elements that appear as one hop in the virtual topology, but are actually multiple IP or sub-IP layer hops. These multiple hops often have quite diverse properties that are often not even visible to the endpoints of the virtual hop. This introduces failure modes that are not dealt with well in current approaches.
The use of IP encapsulation (also known as "tunneling") has long been considered as the means for creating such virtual topologies. However, the insertion of an outer IP header reduces the effective path MTU visible to the inner network layer. When IPv4 is used, this reduced MTU can be accommodated through the use of IPv4 fragmentation, but unmitigated in-the-network fragmentation has been found to be harmful through operational experience and studies conducted over the course of many years [FRAG][FOLK][RFC4963]. Additionally, classical path MTU discovery [RFC1191] has known operational issues that are exacerbated by in-the-network tunnels [RFC2923][RFC4459]. The following subsections present further details on the motivation and approach for addressing these issues.
Before discussing the approach, it is necessary to first understand the problems. In both the Internet and private-use networks today, IPv4 is ubiquitously deployed as the Layer 3 protocol. The two primary functions of IPv4 are to provide for 1) addressing, and 2) a fragmentation and reassembly capability used to accommodate links with diverse MTUs. While it is well known that the IPv4 address space is rapidly becoming depleted, there is a lesser-known but growing consensus that other IPv4 protocol limitations have already or may soon become problematic.
First, the IPv4 header Identification field is only 16 bits in length, meaning that at most 2^16 unique packets with the same (source, destination, protocol)-tuple may be active in the Internet at a given time [I-D.ietf-intarea-ipv4-id-update]. Due to the escalating deployment of high-speed links, however, this number may soon become too small by several orders of magnitude for high data rate packet sources such as tunnel endpoints [RFC4963]. Furthermore, there are many well-known limitations pertaining to IPv4 fragmentation and reassembly – even to the point that it has been deemed “harmful” in both classic and modern-day studies (see above). In particular, IPv4 fragmentation raises issues ranging from minor annoyances (e.g., in-the-network router fragmentation [RFC1981]) to the potential for major integrity issues (e.g., mis-association of the fragments of multiple IP packets during reassembly [RFC4963]).
As a result of these perceived limitations, a fragmentation-avoiding technique for discovering the MTU of the forward path from a source to a destination node was devised through the deliberations of the Path MTU Discovery Working Group (PMTUDWG) during the late 1980’s through early 1990’s (see Appendix D). In this method, the source node provides explicit instructions to routers in the path to discard the packet and return an ICMP error message if an MTU restriction is encountered. However, this approach has several serious shortcomings that lead to an overall “brittleness” [RFC2923].
In particular, site border routers in the Internet are being configured more and more to discard ICMP error messages coming from the outside world. This is due in large part to the fact that malicious spoofing of error messages in the Internet is trivial since there is no way to authenticate the source of the messages [RFC5927]. Furthermore, when a source node that requires ICMP error message feedback when a packet is dropped due to an MTU restriction does not receive the messages, a path MTU-related black hole occurs. This means that the source will continue to send packets that are too large and never receive an indication from the network that they are being discarded. This behavior has been confirmed through documented studies showing clear evidence of path MTU discovery failures in the Internet today [TBIT][WAND][SIGCOMM].
The issues with both IPv4 fragmentation and this “classical” method of path MTU discovery are exacerbated further when IP tunneling is used [RFC4459]. For example, an ingress tunnel endpoint (ITE) may be required to forward encapsulated packets into the subnetwork on behalf of hundreds, thousands, or even more original sources within the end site that it serves. If the ITE allows IPv4 fragmentation on the encapsulated packets, persistent fragmentation could lead to undetected data corruption due to Identification field wrapping. If the ITE instead uses classical IPv4 path MTU discovery, it may be inconvenienced by excessive ICMP error messages coming from the subnetwork that may be either suspect or contain insufficient information for translation into error messages to be returned to the original sources.
Although recent works have led to the development of a robust end-to-end MTU determination scheme [RFC4821], they do not excuse tunnels from delivering path MTU discovery feedback when packets are lost due to size restrictions. Moreover, in current practice existing tunneling protocols mask the MTU issues by selecting a "lowest common denominator" MTU that may be much smaller than necessary for most paths and difficult to change at a later date. Therefore, a new approach to accommodate tunnels over links with diverse MTUs is necessary.
For the purpose of this document, a subnetwork is defined as a virtual topology configured over a connected network routing region and bounded by encapsulating border nodes. Example connected network routing regions include Mobile Ad hoc Networks (MANETs), enterprise networks and the global public Internet itself. Subnetwork border nodes forward unicast and multicast packets over the virtual topology across multiple IP and/or sub-IP layer forwarding hops that may introduce packet duplication and/or traverse links with diverse Maximum Transmission Units (MTUs).
This document introduces a Subnetwork Encapsulation and Adaptation Layer (SEAL) for tunneling network layer protocols (e.g., IP, OSI, etc.) over IP subnetworks that connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border nodes. It provides a modular specification designed to be tailored to specific associated tunneling protocols. A transport-mode of operation is also possible, and described in Appendix C.
SEAL provides a minimal mid-layer encapsulation that accommodates links with diverse MTUs and allows routers in the subnetwork to perform efficient duplicate packet detection. The encapsulation further ensures packet header integrity, data origin authentication and anti-replay [I-D.ietf-savi-framework][RFC4302].
SEAL treats tunnels that traverse the subnetwork as ordinary links that must support network layer services. Moreover, SEAL provides dynamic mechanisms to ensure a maximal per-destination path MTU over the tunnel. This is in contrast to static approaches which avoid MTU issues by selecting a lowest common denominator MTU value that may be overly conservative for the vast majority of tunnel paths and difficult to change even when larger MTUs become available.
The following sections provide the SEAL normative specifications, while the appendices present non-normative additional considerations.
The following terms are defined within the scope of this document:
The following abbreviations correspond to terms used within this document and/or elsewhere in common Internetworking nomenclature:
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 used in lower case (e.g., must, must not, etc.), these words MUST NOT be interpreted as described in [RFC2119], but are rather interpreted as they would be in common English.
SEAL was originally motivated by the specific case of subnetwork abstraction for Mobile Ad hoc Networks (MANETs), however the domain of applicability also extends to subnetwork abstractions over enterprise networks, ISP networks, SOHO networks, the global public Internet itself, and any other connected network routing region. SEAL along with the Virtual Enterprise Traversal (VET) [I-D.templin-intarea-vet] tunnel virtual interface abstraction are the functional building blocks for the Internet Routing Overlay Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139] architectures.
SEAL provides a network sublayer for encapsulation of an inner network layer packet within outer encapsulating headers. SEAL can also be used as a sublayer within a transport layer protocol data payload, where transport layer encapsulation is typically used for Network Address Translator (NAT) traversal as well as operation over subnetworks that give preferential treatment to certain "core" Internet protocols (e.g., TCP, UDP, etc.). The SEAL header is processed the same as for IPv6 extension headers, i.e., it is not part of the outer IP header but rather allows for the creation of an arbitrarily extensible chain of headers in the same way that IPv6 does.
To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts as a passive observer that simply informs the Ingress Tunnel Endpoint (ITE) of any packet size limitations. This allows the ITE to return appropriate path MTU discovery feedback even if the network path between the ITE and ETE filters ICMP messages.
SEAL further ensures data origin authentication [I-D.ietf-savi-framework], packet header integrity, and anti-replay. The SEAL framework resembles a lightweight version of the IP Security (IPsec) [RFC4301] Authentication Header (AH) [RFC4302], however its purpose is to provide minimal hop-by-hop authenticating services along a path while leaving full data integrity, authentication and confidentiality services as an end-to-end consideration.
SEAL supports both "nested" tunneling and "re-encapsulating" tunneling. Nested tunneling occurs when a first tunnel is encapsulated within a second tunnel, which may then further be encapsulated within a third tunnel, etc. Nested tunneling can be useful, and stands in contrast to "recursive" tunneling which is an anomalous condition incurred due to misconfiguration or a routing loop. Considerations for nested tunneling are discussed in Section 4 of [RFC2473].
Re-encapsulating tunneling occurs when a packet emerges from a first ETE, which then acts as an ITE to re-encapsulate and forward the packet to a second ETE connected to the same subnetwork. In that case each ITE/ETE transition represents a segment of a bridged path between the ITE nearest the source and the ETE nearest the destination. Combinations of nested and re-encapsulating tunneling are also naturally supported by SEAL.
The following sections specify the operation of SEAL:
SEAL is an encapsulation sublayer used within VET non-broadcast, multiple access (NBMA) tunnel virtual interfaces. Each VET interface connects an ITE to one or more ETE "neighbors" via tunneling across an underlying subnetwork. The tunnel neighbor relationship between the ITE and each ETE may be either unidirectional or bidirectional.
A unidirectional tunnel neighbor relationship allows the near end ITE to send data packets forward to the far end ETE, while the ETE only returns control messages when necessary. A bidirectional tunnel neighbor relationship is one over which both TEs can exchange both data and control messages.
Implications of the VET unidirectional and bidirectional models are discussed in [I-D.templin-intarea-vet].
SEAL-enabled ITEs encapsulate each inner packet in a SEAL header, any outer header encapsulations, and in some instances a SEAL trailer as shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
| SEAL Trailer |
+--------------------+
The ITE inserts the SEAL header according to the specific tunneling protocol. For simple encapsulation of an inner network layer packet within an outer IP header (e.g., [RFC1070][RFC2003][RFC2473][RFC4213], etc.), the ITE inserts the SEAL header between the inner packet and outer IP headers as: IP/SEAL/{inner packet}.
For encapsulations over transports such as UDP (e.g., in the same manner as for [RFC4380]), the ITE inserts the SEAL header between the outer transport layer header and the inner packet, e.g., as IP/UDP/SEAL/{inner packet}. (Here, the UDP header is seen as an "other outer header" as depicted in Figure 1.)
Finally, in some instances the ITE appends a SEAL trailer at the end of the SEAL packet. In that case, the trailer is added after the final byte of the encapsulated packet.
The SEAL header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|P|R|T|U|Z| NEXTHDR | PREFLEN | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PREFIX (when present) ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
where the header fields are defined as:
When present, The SEAL trailer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The ITE maintains a per-ETE integrity check vector (ICV) calculation algorithm and a symmetric secret key to calculate the ICV(s) in packets it will send to this ETE. The ITE also maintains a window of PKT_ID values for the packets it has recently sent to this ETE.
For each underlying link of each ETE, the ITE maintains the boolean variable "USE_ICV2" and "USE_MIN_MTU" initialized to "FALSE".
When the ITE performs stateful MTU determination (see Section 4.4.9), it further caches a static MTU value for each ETE link path.
The tunnel interface must present a constant MTU value to the inner network layer as the size for admission of inner packets into the interface. Since VET NBMA tunnel virtual interfaces may support a large set of ETEs that accept widely varying maximum packet sizes, however, a number of factors should be taken into consideration when selecting a tunnel interface MTU.
Due to the ubiquitous deployment of standard Ethernet and similar networking gear, the nominal Internet cell size has become 1500 bytes; this is the de facto size that end systems have come to expect will either be delivered by the network without loss due to an MTU restriction on the path or a suitable ICMP Packet Too Big (PTB) message returned. When large packets sent by end systems incur additional encapsulation at an ITE, however, they may be dropped silently within the tunnel since the network may not always deliver the necessary PTBs [RFC2923].
The ITE should therefore set a tunnel interface MTU of at least 1500 bytes plus extra room to accommodate any additional encapsulations that may occur on the path from the original source. The ITE can also set smaller MTU values; however, care must be taken not to set so small a value that original sources would experience an MTU underflow. In particular, IPv6 sources must see a minimum path MTU of 1280 bytes, and IPv4 sources should see a minimum path MTU of 576 bytes.
The ITE can alternatively set an indefinite MTU on the tunnel interface such that all inner packets are admitted into the interface without regard to size. For ITEs that host applications that use the tunnel interface directly, this option must be carefully coordinated with protocol stack upper layers since some upper layer protocols (e.g., TCP) derive their packet sizing parameters from the MTU of the outgoing interface and as such may select too large an initial size. This is not a problem for upper layers that use conservative initial maximum segment size estimates and/or when the tunnel interface can reduce the upper layer's maximum segment size, e.g., by reducing the size advertised in the MSS option of outgoing TCP messages (sometimes known as "MSS clamping").
The inner network layer protocol consults the tunnel interface MTU when admitting a packet into the interface. For non-SEAL inner IPv4 packets with the IPv4 Don't Fragment (DF) bit set to 0, if the packet is larger than the tunnel interface MTU the inner IPv4 layer uses IPv4 fragmentation to break the packet into fragments no larger than the tunnel interface MTU. The ITE then admits each fragment into the interface as an independent packet.
For all other inner packets, the inner network layer admits the packet if it is no larger than the tunnel interface MTU; otherwise, it drops the packet and sends a PTB error message to the source with the MTU value set to the tunnel interface MTU. The message contains as much of the invoking packet as possible without the entire message exceeding the network layer minimum MTU (e.g., 576 bytes for IPv4, 1280 bytes for IPv6, etc.).
In light of the above considerations, the ITE SHOULD configure an indefinite MTU on tunnel *router* interfaces. The ITE MAY instead set a finite MTU on tunnel *host* interfaces.
For each inner packet, if the packet is itself a SEAL packet (i.e., one with either SEAL_PROTO in the IP protocol/next-header field, or with SEAL_PORT in the transport layer destination port field) and the LEVEL field of the SEAL header contains the value 0, the ITE silently discards the packet.
Otherwise, the ITE calculates HLEN as the sum of the lengths of the SEAL header plus outer transport and network layer headers that will be used for encapsulation of the inner packet. The ITE must include the length of the uncompressed outer headers when calculating HLEN even if the tunnel is using header compression. The ITE next sets the variable "LMTU" to the MTU of the underlying link minus HLEN. If LMTU is less than 1280, the ITE also sets the boolean variables USE_ICV2 and USE_MIN_MTU for this ETE link path to TRUE. The ITE then prepares the inner packet for encapsulation according to its length.
For IPv4 inner packets with DF=0 in the IPv4 header, if the packet is not the first fragment of a SEAL packet (i.e., not a packet that includes a SEAL header) the ITE fragments the packet into inner fragments no larger than the minimum of LMTU and 512 bytes. The ITE then submits each inner fragment for SEAL encapsulation as specified in Section 4.4.4.
For all other inner packets, if USE_MIN_MTU is TRUE the ITE resets LMTU=1280. Next, if the packet is no larger than the maximum of LMTU and 1280 bytes the ITE submits it for SEAL encapsulation. Otherwise, the ITE sends a packet-too-big error message toward the source address of the inner packet.
To send the message, the ITE first checks its forwarding tables to discover the previous hop toward the source address of the inner packet. If the previous hop is reached via the same tunnel interface, the ITE sends an SCMP PTB (SPTB) message to the previous hop with the MTU field set to the maximum of LMTU and 1280 bytes (see: Section 4.6.1). Otherwise, the ITE prepares an ordinary PTB message appropriate to the inner protocol version.
When preparing the PTB message, the ITE sets the MTU field to the maximum of LMTU and 1280 bytes. (When the inner packet is an IPv4 packet that includes the leading portion of a SEAL data packet, the ITE first sets DF=1 in the inner header then re-calculates the inner header checksum before generating the PTB. This is a precautionary step to appease network middleboxes that would discard an ICMPv4 PTB message in which the IPv4 header of the inner packet within the packet-in-error has DF=0.)
After sending the (S)PTB message, the ITE discards the inner packet.
The ITE next encapsulates the inner packet in a SEAL header formatted as specified in Section 4.3. The ITE sets NEXTHDR to the protocol number corresponding to the address family of the encapsulated inner packet. For example, the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets [RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213], '80' for encapsulated OSI/CLNP packets [RFC1070], etc.
The ITE then sets R=1 if redirects are permitted (see: [I-D.templin-intarea-vet]) and sets PREFLEN to the length of the prefix to be applied to the inner source address. The ITE's claimed PREFLEN is subject to verification by the ETE; hence, the ITE MUST set PREFLEN to the exact prefix length that it is authorized to use. (Note that if this process is entered via re-encapsulation (see: Section 4.5.4), PREFLEN and R are instead copied from the SEAL header of the re-encapsulated packet. This implies that the PREFLEN and R values are propagated across a re-encapsulating chain of ITE/ETEs that must all be authorized to represent the prefix.)
The ITE next sets (C=0; P=0; Z=0), then sets LINK_ID to the value assigned to the underlying link and sets PKT_ID to a monotonically-increasing integer value, beginning with the value 0 in the first packet transmitted to this ETE. The ITE also sets U=1 if it needs to determine whether the ETE will receive it without fragmentation, e.g., to test whether a middlebox is reassembling fragmented packets before they arrive at the ETE (see: Section 4.4.8), for stateful MTU determination (see Section 4.4.9), etc. Otherwise, the ITE sets U=0.
Next, if the inner packet is not itself a SEAL packet the ITE sets LEVEL to an integer value between 0 and 7 as a specification of the number of additional layers of nested SEAL encapsulations permitted. Otherwise, the ITE sets LEVEL to the value that appears in the inner packet's SEAL header minus 1.
Next, if the outer protocol version is IPv4, the variable USE_ICV2 for this ETE link path is TRUE, and the inner packet is larger than 128 bytes minus the SEAL header length but no larger than 1280 bytes, the ITE sets T=1. Otherwise, the ITE sets T=0.
The ITE finally sets ICV1 to 0 and calculates the packet ICVs. The ICVs are calculated using an algorithm agreed on by the ITE and ETE. The algorithm uses a symmetric secret key so that the ETE can verify that the ICVs were generated by the ITE.
The ITE first calculates the ICV value over the leading 128 bytes of the packet beginning with the SEAL header (or up to the end of the packet if there are fewer than 128 bytes) then places result in the ICV1 field in the header. If T=1, the ITE next calculates the ICV over the remainder of the packet and places the result in the ICV2 field in the SEAL trailer. The ITE then submits the packet for outer encapsulation.
Following SEAL encapsulation, the ITE next encapsulates the packet in the requisite outer headers according to the specific encapsulation format (e.g., [RFC1070], [RFC2003], [RFC2473], [RFC4213], etc.), except that it writes 'SEAL_PROTO' in the protocol field of the outer IP header (when simple IP encapsulation is used) or writes 'SEAL_PORT' in the outer destination transport service port field (e.g., when IP/UDP encapsulation is used).
When UDP encapsulation is used, the ITE sets the UDP header fields as specified in Section 5.5.4 of [I-D.templin-intarea-vet]. The ITE then performs outer IP header encapsulation as specified in Section 5.5.5 of [I-D.templin-intarea-vet]. If this process is entered via re-encapsulation (see: Section 4.5.4), the ITE instead follows the outer IP/UDP re-encapsulation procedures specified in Section 5.5.6 of [I-D.templin-intarea-vet].
When IPv4 is used as the outer encapsulation layer, the ITE sets DF=0 in the IPv4 header to allow the packet to be fragmented within the subnetwork if it encounters a restricting link. The ITE MAY instead set DF=1 if it is using stateful MTU determination (see Section 4.4.9).
When IPv6 is used as the outer encapsulation layer, the "DF" flag is absent but implicitly set to 1. The packet therefore will not be fragmented within the subnetwork, since IPv6 deprecates in-the-network fragmentation.
Next, the ITE uses IP fragmentation if necessary to fragment the packet into outer IP fragments that are no larger than the MTU of the underlying link.
The ITE then sends each outer packet/fragment via the underlying link corresponding to LINK_ID.
All SEAL data packets are considered implicit probes. SEAL data packets will elicit an SCMP message from the ETE if it needs to acknowledge a probe and/or report an error condition. SEAL data packets may also be dropped by either the ETE or a router on the path, which will return an ICMP message.
The ITE can also send an SCMP Router/Neighbor Solicitation message to elicit an SCMP Router/Neighbor Advertisement response (see: [I-D.templin-intarea-vet]) as verification that the ETE is still reachable.
The ITE processes ICMP messages as specified in Section 4.4.7.
The ITE processes SCMP messages as specified in Section 4.6.2.
When the ITE sends SEAL packets, it may receive raw ICMP error messages[RFC0792][RFC4443] from either the ETE or from a router within the subnetwork. Each ICMP message includes an outer IP header, followed by an ICMP header, followed by a portion of the SEAL data packet that generated the error (also known as the "packet-in-error") beginning with the outer IP header.
The ITE should process raw ICMPv4 Protocol Unreachable messages and ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header type encountered" as a hint that the ETE does not implement the SEAL protocol. The ITE can also process other raw ICMP messages that do not include sufficient information in the packet-in-error as a hint that the path to the ETE may be failing. Specific actions that the ITE may take in these cases are out of scope.
For other ICMP messages, the ITE examines the packet-in-error beginning with the SEAL header. If the value in the PKT_ID field is not within the window of packets the ITE has recently sent to this ETE, or if the value in the SEAL header ICV1 field is incorrect, the ITE discards the message.
Next, the ITE processes the ICMP message if there is operational assurance that it has not been crafted by a malicious middlebox, e.g., if the source of the ICMP message is within the same administrative domain as the ITE. If the received ICMP message is a PTB, and the MTU field encodes a non-zero value, the ITE sets the variable "PMTU" to the MTU value minus HLEN for this ETE link path. If PMTU is less than 1280, the ITE sets USE_ICV2 for the ETE link path to TRUE.
Next, the ITE transcribes the ICMP message into a message to return to the previous hop. If the previous hop toward the inner source address is reached via the same tunnel interface the SEAL data packet was sent on, the ITE transcribes the ICMP message into an SCMP message (see: Section 4.6.1) and forwards it to the previous hop. Otherwise, the ITE transcribes the ICMP message into a message appropriate for the inner protocol version and forwards it to the inner source address.
To transcribe the message, the ITE extracts the inner packet from within the ICMP message packet-in-error field and generates a new SCMP/ICMP message corresponding to the type of the received ICMP message.
For (S)PTB messages, the ITE writes the maximum of PMTU and 1280 bytes in the MTU field. This calculation allows the largest MTU size possible for packets that do not require fragmentation on the path to the ETE.
The ITE can perform a qualification exchange over each underlying ETE link path to ensure that the subnetwork correctly delivers IP fragments to the ETE. This procedure can be used, e.g., to determine whether there are middleboxes on the path that violate the [RFC1812], Section 5.2.6 requirement that: "A router MUST NOT reassemble any datagram before forwarding it".
When possible, the ITE should use knowledge of its topological arrangement as an aide in determining when middlebox reassembly testing is necessary. For example, if the ITE knows that the ITE is located somewhere in the public Internet, middlebox reassembly testing is unnecessary. If the ITE is aware that the ETE is located behind a NAT or a firewall, however, then middlebox reassembly testing is recommended.
The ITE performs the middlebox reassembly test by setting U=1 in the header of a SEAL data packet to be used as a probe. Next, the ITE encapsulates the packet in the appropriate outer headers, splits it into two outer IP fragments, then sends both fragments to the ETE over the same underlying link.
If the ETE returns an SCMP PTB message with MTU=0 (see Section 4.6.1.1), then a middlebox in the subnetwork is reassembling the fragments before they are delivered to the ETE. In that case, the ETE sets both USE_ICV2 and USE_MIN_MTU to TRUE so that future packets will be protected from reassembly misassociations.
NB: In order to account for reassembly buffer restrictions and/or packet loss, the ITE should select only relatively small SEAL data packets for testing purposes and should perform a series of tests (e.g., 3-5 tests with 1sec between tests) instead of a single isolated test. The ITE can also construct a NULL test packet instead of using ordinary SEAL data packets for testing. To create the test packet, the ITE prepares a data packet with (C=0; P=1; R=0; T=0; U=1; Z=0) in the SEAL header, writes the length of the ITE's claimed prefix in the PREFLEN field, and writes the ITE's claimed prefix in the PREFIX field. The ITE then sets NEXTHDR according to the address family of the PREFIX, i.e., it sets NEXTHDR to the value '4' for an IPv4 prefix, '41' for an IPv6 prefix , '80' for an OSI/CLNP prefix, etc. The ITE can further add padding following the SEAL header to a length that would not cause the size of the packet to exceed 512 bytes before outer encapsulation. The ITE then sets PKT_ID, LINK_ID and LEVEL to appropriate values for this ETE, calculates the ICV over the first 128 bytes of the packet beginning with the SEAL header, and writes the value in the ICV1 field.
The above sections specify a generic stateless MTU determination capability that works over all ITE->ETE paths. However, the stateless method can be overly-conservative when the ETE is situated behind a middlebox that performs IPv4 reassembly.
If the ITE discovers that the ETE is behind a middlebox that performs IPv4 reassembly (e.g., via static configuration, via testing as described in Section 4.4.8, etc.), it can instead implement a stateful MTU determination capability. To do so, it sends probe messages of various size to the ETE with U=1 in the SEAL header and DF=1 in the outer IPv4 header. The ITE can then cache the size of the largest packet for which it receives a probe reply from the ETE as the MTU to use for this ETE, and thereafter ignores USE_MIN_MTU and USE_ICV2.
For example, the ITE could send probes of 1300 bytes, followed by 1400 bytes, followed by 1460 bytes, followed by 1500 bytes, then set the MTU for this ETE link path to the size of the largest probe packet for which it receives a reply.
After determining the largest possible MTU, the ITE sends further SEAL packets to this ETE with DF=1 and without including ICV2. However, the ITE should periodically re-probe the path to determine whether routing changes have resulted in a reduced path MTU.
The ETE maintains a per-ITE ICV calculation algorithm and a symmetric secret key to verify the ICV(s) in the SEAL header and trailer. The ETE also maintains a window of PKT_ID values for the packets it has recently received from this ITE.
The ETE must maintain a minimum IP reassembly buffer size of 1500 bytes for both IPv4 [RFC0791] and IPv6 [RFC2460]. The ETE must also be capable of partially reassembling and delivering at least the leading 1280 byte portion of the inner packet even if the completely reassembled packet would exceed that size.
The ETE should maintain conservative IP-layer reassembly cache high- and low-water marks. When the size of the reassembly cache exceeds this high-water mark, the ETE should actively discard stale incomplete reassemblies (e.g., using an Active Queue Management (AQM) strategy) until the size falls below the low-water mark. The ETE should also actively discard any pending reassemblies that clearly have no opportunity for completion, e.g., when a considerable number of new fragments have arrived before a fragment that completes a pending reassembly arrives.
The ETE processes non-SEAL IP packets as specified in the normative references, i.e., it performs any necessary IP reassembly then delivers the (reassembled) packet to the appropriate upper layer protocol module.
The ETE gathers the outer IP fragments of a SEAL packet until it has received at least the first 1280 bytes beyond the SEAL header or up to the end of the packet. The ETE then delivers this (partially) reassembled packet for decapsulation as specified in Section 4.5.4.
For each SEAL packet submitted for decapsulation, the ETE first examines the PKT_ID and ICV1 fields. If the PKT_ID is not within the window of acceptable values for this ITE, or if the ICV1 field includes an incorrect value, the ETE silently discards the packet.
Next, if the packet arrived as multiple IP fragments and the SEAL header has T=0, the ETE sends an SPTB message back to the ITE (see Section 4.6.1.1) with MTU set to the size of the largest IP fragment received. If the packet arrived as multiple IP fragments and the (partial) inner packet is larger than 1280 bytes, the ETE then silently discards the packet.
Next, if the SEAL header has T=1 the ETE silently discards the packet if the inner packet is larger than 1280 bytes. Otherwise, the ETE verifies the ICV2 value over the remainder of the packet if T=1 and silently discards the packet if the value is incorrect.
If the SEAL header has C=1, the ETE then processes the packet as an SCMP packet as specified in Section 4.6.2. Otherwise the ETE continues to process the packet as a SEAL data packet.
If there is an incorrect value in a SEAL header field, the returns an SCMP "Parameter Problem" (SPP) message (see Section 4.6.1.2) then discards the packet. Otherwise, if the SEAL header has U=1 and the packet did not require IP-layer reassembly, the ETE next sends an SPTB message with MTU=0 back to the ITE (see Section 4.6.1.1).
Next, if the SEAL header has P=1 the ETE then discards the packet. Otherwise, the ETE discards the outer headers and processes the inner packet according to the header type indicated in the SEAL NEXTHDR field.
If the next hop toward the inner destination address is via a different interface than the SEAL packet arrived on, the ETE discards the SEAL header and delivers the inner packet either to the local host or to the next hop interface if the packet is not destined to the local host.
If the next hop is on the same interface the SEAL packet arrived on, however, the ETE submits the packet for SEAL re-encapsulation beginning with the specification in Section 4.4.3 above.
SEAL provides a companion SEAL Control Message Protocol (SCMP) that uses the same message types and formats as for the Internet Control Message Protocol for IPv6 (ICMPv6) [RFC4443]. When the TE prepares an SCMP message, it sets the Type and Code fields to the same values that would appear in the corresponding ICMPv6 message, but it does not calculate the SCMP message checksum. The TE then formats the Message Body the same as for the corresponding ICMPv6 message. The TE then encapsulates the SCMP message in the SEAL header and trailer as well as the outer headers as shown in Figure 4:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
~ SCMP message header~ --> ~ SCMP message header~
+--------------------+ --> +--------------------+
~ SCMP message body ~ --> ~ SCMP message body ~
+--------------------+ --> +--------------------+
| SEAL Trailer |
+--------------------+
SCMP Message SCMP Packet
before encapsulation after encapsulation
The following sections specify the generation, processing and relaying of SCMP messages.
ETEs generate SCMP error messages in response to receiving certain SEAL data packets using the format shown in Figure 5:
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 | Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-Specific Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data packet as |
~ possible (beginning immediately after the SEAL header) ~
| without the SCMP packet exceeding 576 bytes (*) |
(*) also known as the "packet-in-error"
When the ETE processes a SEAL data packet for which the ICVs are correct but an error must be returned, it prepares an SCMP error message as shown in Figure 5. The ETE sets the Type and Code fields in the SCMP header according to the appropriate error message type, sets the Reserved field to 0, fills out the Type-Specific Data field and includes the packet-in-error.
The ETE next encapsulates the SCMP message in the requisite SEAL header, outer headers and SEAL trailer as shown in Figure 4. During encapsulation, the ETE sets the outer destination address/port numbers of the SCMP packet to the outer source address/port numbers of the original SEAL data packet and sets the outer source address/port numbers to its own outer address/port numbers.
The ETE then sets (C=1; P=0; R=0; U=0) in the SEAL header, then sets NEXTHDR, PREFLEN, LEVEL, LINK_ID and PKT_ID to the same values that appeared in the SEAL data packet header. If the SCMP message includes more than 128 bytes beginning with the SEAL header, the ETE next sets T=1; otherwise it sets T=0.
The ETE then calculates and sets the ICV1 field (and also the ICV2 field if T=1) the same as specified for SEAL data packet encapsulation in Section 4.4.4. Next, the ETE encapsulates the SCMP message in the requisite outer encapsulations and sends the resulting SCMP packet to the ITE the same as specified for SEAL data packets in Section 4.4.5.
The following sections describe additional considerations for various SCMP error messages:
An ETE generates an SCMP "Packet Too Big" (SPTB) message when it receives a SEAL data packet that arrived as multiple outer IP fragments but for which T=0 in the SEAL header. The ETE prepares the SPTB message the same as for the corresponding ICMPv6 PTB message, and writes the length of the largest outer IP fragment received in the MTU field of the message.
The ETE also generates an SPTB message when it accepts a SEAL protocol data packet which did not undergo IP reassembly and with U=1 in the SEAL header. The ETE prepares the SPTB message the same as above, except that it writes the value 0 in the MTU field.
An ETE generates an SCMP "Destination Unreachable" (SDU) message under the same circumstances that an IPv6 system would generate an ICMPv6 Destination Unreachable message.
An ETE generates an SCMP "Parameter Problem" (SPP) message when it receives a SEAL packet with an incorrect value in the SEAL header. IN THIS CASE ALONE, the ETE prepares the packet-in-error beginning with the SEAL header instead of beginning immediately after the SEAL header.
TEs generate other SCMP message types using methods and procedures specified in other documents. For example, SCMP message types used for tunnel neighbor coordinations are specified in VET [I-D.templin-intarea-vet].
For each SCMP error message it receives, the ITE first verifies that the outer addresses of the SCMP packet are correct and that the PKT_ID is within its window of values for this ETE. The ITE then verifies the ICV1 and ICV2 values. If the identifying information and/or ICVs are incorrect, the ITE discards the message; otherwise, it processes the message as follows:
After an ITE sends a SEAL data packet to an ETE, it may receive an SPTB message with a packet-in-error containing the leading portion of the inner packet (see: Section 4.6.1.1). If the SPTB message has MTU=0, the ITE processes the message as confirmation that the ETE received an unfragmented SEAL data packet with U=1 in the SEAL header. The ITE then processes the SPTB message as a path qualification probe response and discards the message. If the SPTB message has MTU!=0, the ITE instead processes the message as an indication of a packet size limitation.
If the MTU value in the SPTB message is no larger than 1280+HLEN, and the length of the inner packet within the packet-in-error is no larger than 1280, the ITE sets the boolean variable USE_ICV2 for this ETE link path to TRUE. The ITE then discards the SPTB message.
If the MTU value in the SPTB message is not substantially less that 1500, the value is likely to represent the true MTU of the restricting link on the path to the ETE; otherwise, a router on the path may be generating runt fragments. In that case, the ITE can consult a plateau table (e.g., as described in [RFC1191]) to rewrite the MTU value to a reduced size. For example, if the ITE receives an SPTB message with MTU=256 and inner header length 1500, it can rewrite the MTU to 1400. If the ITE subsequently receives an SPTB message with MTU=256 and inner header length 1400, it can rewrite the MTU to 1300, etc.
The ITE then checks it's forwarding tables to determine the previous hop on the reverse path toward the source address of the inner packet in the packet-in-error. If the previous hop is reached over a different interface than the SPTB message arrived on, the ITE transcribes the message into a format appropriate for the inner packet (i.e., the same as described for transcribing ICMP messages in Section 4.4.7) and sends the resulting transcribed message to the original source.
If the previous hop is reached over the same tunnel interface that the SPTB message arrived on, the ITE instead relays the message to the previous hop. In order to relay the message, the ITE rewrites the SEAL header fields with values corresponding to the previous hop. Next, the ITE replaces the SPTB's outer headers with headers of the appropriate protocol version and fills in the header fields as specified in Sections 5.5.4-5.5.6 of [I-D.templin-intarea-vet], where the destination address/port correspond to the previous hop and the source address/port correspond to the ITE. The ITE then sends the message to the previous hop the same as if it were issuing a new SPTB message.
An ITE may receive an SDU message with an appropriate code under the same circumstances that an IPv6 node would receive an ICMPv6 Destination Unreachable message. The ITE either transcribes or relays the message toward the source address of the inner packet within the packet-in-error the same as specified for SPTB messages in Section 4.6.2.1.
An ITE may receive an SPP message when the ETE receives a SEAL packet with an incorrect value in the SEAL header. The ITE should examine the SEAL header within the packet-in-error to determine whether a different setting should be used in subsequent packets, but does not relay the message further.
TEs process other SCMP message types using methods and procedures specified in other documents. For example, SCMP message types used for tunnel neighbor coordinations are specified in VET [I-D.templin-intarea-vet].
Subnetwork designers are expected to follow the recommendations in Section 2 of [RFC3819] when configuring link MTUs.
End systems are encouraged to implement end-to-end MTU assurance (e.g., using Packetization Layer Path MTU Discovery per [RFC4821]) even if the subnetwork is using SEAL.
Routers within the subnetwork are expected to observe the router requirements found in the normative references, including the implementation of IP fragmentation and reassembly [RFC1812][RFC2460] as well as the generation of ICMP messages [RFC0792][RFC4443].
The IANA is instructed to allocate an IP protocol number for 'SEAL_PROTO' in the 'protocol-numbers' registry.
The IANA is instructed to allocate a Well-Known Port number for 'SEAL_PORT' in the 'port-numbers' registry.
The IANA is instructed to establish a "SEAL Protocol" registry to record SEAL Version values. This registry should be initialized to include the initial SEAL Version number, i.e., Version 0.
SEAL provides a segment-by-segment data origin authentication and anti-replay service across the (potentially) multiple segments of a re-encapsulating tunnel. It further provides a segment-by-segment integrity check of the headers of encapsulated packets, but does not verify the integrity of the rest of the packet beyond the headers unless fragmentation is unavoidable. SEAL therefore considers full message integrity checking, authentication and confidentiality as end-to-end considerations in a manner that is compatible with securing mechanisms such as TLS/SSL [RFC5246].
An amplification/reflection attack is possible when an attacker sends IP first fragments with spoofed source addresses to an ETE in an attempt to generate a stream of SCMP messages returned to a victim ITE. The SCMP message ICVs, PKT_ID, as well as the inner headers of the packet-in-error, provide mitigation for the ETE to detect and discard SEAL segments with spoofed source addresses.
The SEAL header is sent in-the-clear the same as for the outer IP and other outer headers. In this respect, the threat model is no different than for IPv6 extension headers. Unlike IPv6 extension headers, however, the SEAL header is protected by an integrity check that also covers the inner packet headers.
Security issues that apply to tunneling in general are discussed in [RFC6169].
Section 3.1.7 of [RFC2764] provides a high-level sketch for supporting large tunnel MTUs via a tunnel-level segmentation and reassembly capability to avoid IP level fragmentation. This capability was implemented in the first edition of SEAL, but is now deprecated.
Section 3 of [RFC4459] describes inner and outer fragmentation at the tunnel endpoints as alternatives for accommodating the tunnel MTU.
Section 4 of [RFC2460] specifies a method for inserting and processing extension headers between the base IPv6 header and transport layer protocol data. The SEAL header is inserted and processed in exactly the same manner.
IPsec/AH is [RFC4301][RFC4301] is used for full message integrity verification between tunnel endpoints, whereas SEAL only ensures integrity for the inner packet headers. The AYIYA proposal [I-D.massar-v6ops-ayiya] uses similar means for providing full message authentication and integrity.
The concepts of path MTU determination through the report of fragmentation and extending the IP Identification field were first proposed in deliberations of the TCP-IP mailing list and the Path MTU Discovery Working Group (MTUDWG) during the late 1980's and early 1990's. An historical analysis of the evolution of these concepts, as well as the development of the eventual path MTU discovery mechanism for IP, appears in Appendix D of this document.
The following individuals are acknowledged for helpful comments and suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci, Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James Woodyatt, and members of the Boeing Research & Technology NST DC&NT group.
Discussions with colleagues following the publication of RFC5320 have provided useful insights that have resulted in significant improvements to this, the Second Edition of SEAL.
Path MTU determination through the report of fragmentation was first proposed by Charles Lynn on the TCP-IP mailing list in 1987. Extending the IP identification field was first proposed by Steve Deering on the MTUDWG mailing list in 1989.
| [RFC0791] | Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. |
| [RFC0792] | Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. |
| [RFC4443] | Conta, A., Deering, S. and M. Gupta, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. |
| [RFC3971] | Arkko, J., Kempf, J., Zill, B. and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC2460] | Deering, S.E. and R.M. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. |
Although a SEAL tunnel may span an arbitrarily-large subnetwork expanse, the IP layer sees the tunnel as a simple link that supports the IP service model. Links with high bit error rates (BERs) (e.g., IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to increase packet delivery ratios, while links with much lower BERs typically omit such mechanisms. Since SEAL tunnels may traverse arbitrarily-long paths over links of various types that are already either performing or omitting ARQ as appropriate, it would therefore often be inefficient to also require the tunnel endpoints to also perform ARQ.
The SEAL header includes an ICV field that covers the SEAL header and at least the inner packet headers. This provides for header integrity verification on a segment-by-segment basis for a segmented re-encapsulating tunnel path. When fragmentation is needed, the SEAL packet also contains a trailer with a secondary ICV that covers the remainder of the packet.
Fragmentation and reassembly schemes must consider packet-splicing errors, e.g., when two fragments from the same packet are concatenated incorrectly, when a fragment from packet X is reassembled with fragments from packet Y, etc. The primary sources of such errors include implementation bugs and wrapping IP ID fields.
In terms of wrapping ID fields, when IPv4 is used as the outer IP protocol, the 16-bit IP ID field can wrap with only 64K packets with the same (src, dst, protocol)-tuple alive in the system at a given time [RFC4963] increasing the likelihood of reassembly mis-associations
SEAL avoids reassembly mis-associations through the use of extended ICVs, and also discards any reassembled packets larger than 1280 bytes.
SEAL can also be used in "transport-mode", e.g., when the inner layer comprises upper-layer protocol data rather than an encapsulated IP packet. For instance, TCP peers can negotiate the use of SEAL (e.g., by inserting a 'SEAL_OPTION' TCP option during connection establishment) for the carriage of protocol data encapsulated as IPv4/SEAL/TCP. In this sense, the "subnetwork" becomes the entire end-to-end path between the TCP peers and may potentially span the entire Internet.
If both TCPs agree on the use of SEAL, their protocol messages will be carried as IPv4/SEAL/TCP and the connection will be serviced by the SEAL protocol using TCP (instead of an encapsulating tunnel endpoint) as the transport layer protocol. The SEAL protocol for transport mode otherwise observes the same specifications as for Section 4.
The topic of Path MTU discovery (PMTUD) saw a flurry of discussion and numerous proposals in the late 1980's through early 1990. The initial problem was posed by Art Berggreen on May 22, 1987 in a message to the TCP-IP discussion group [TCP-IP]. The discussion that followed provided significant reference material for [FRAG]. An IETF Path MTU Discovery Working Group [MTUDWG] was formed in late 1989 with charter to produce an RFC. Several variations on a very few basic proposals were entertained, including:
Option 1) seemed attractive to the group at the time, since it was believed that routers would migrate more quickly than hosts. Option 2) was a strong contender, but repeated attempts to secure an "RF" bit in the IPv4 header from the IESG failed and the proponents became discouraged. 3) was abandoned because it was perceived as too complicated, and 4) never received any apparent serious consideration. Proposal 5) was a late entry into the discussion from Steve Deering on Feb. 24th, 1990. The discussion group soon thereafter seemingly lost track of all other proposals and adopted 5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a "contract" is first established between the peers, as in proposal 4) and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on Feb 19. 1990. These proposals saw little discussion or rebuttal, and were dismissed based on the following the assertions:
The first four assertions, although perhaps valid at the time, have been overcome by historical events. The final assertion is addressed by the mechanisms specified in SEAL.