| Network Working Group | F. L. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Standards Track | October 25, 2011 |
| Expires: April 27, 2012 |
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-34.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 it soon became apparent that 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 a new Internetworking architecture based on Routing and Addressing in Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139] and the Internet Routing Overlay Network (IRON) [I-D.templin-ironbis].
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 to the previous hop on the path toward the original source even if the network path between the ITE and ETE filters ICMP messages.
SEAL further ensures packet header integrity, data origin authentication and anti-replay [I-D.ietf-savi-framework][RFC4301][RFC4302]. The SEAL encapsulation in many respects is simply a lightweight version of the IP Security (IPsec) Authentication Payload (AUTH), however its purpose is to provide minimal authenticating services along multiple hops of a bridged segment within a path while leaving data integrity services as an end-to-end consideration.
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 for SEAL are discussed in [I-D.templin-intarea-vet].
SEAL-enabled ITEs encapsulate each inner packet in a SEAL header and any outer encapsulations as shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
~ outer trailers ~
+--------------------+
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.)
The following sections specify the SEAL header format and SEAL-related operations of the ITE and ETE.
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|A|R| RSV | NEXTHDR | PREFLEN | LINK_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the header fields are defined as:
Setting of the various bits and fields of the SEAL header is specified in the following sections.
The ITE maintains a per-ETE checksum calculation algorithm and secret key to verify the Checksum in the SEAL header.
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.
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.
The ITE maintains HLEN as the sum of the lengths of the SEAL header and any outer headers and trailers. The ITE must include the length of the uncompressed outer headers and trailers when calculating HLEN even if the tunnel is using header compression. The ITE then prepares each inner packet/fragment admitted into the tunnel interface for encapsulation according to its length.
For IPv4 inner packets with DF=0 in the IPv4 header, the ITE fragments the packet into IPv4 fragments of a length that (when added to HLEN) is unlikely to incur additional fragmentation on the path to the ETE. (It is crucial that the ITE be conservative in it's selection of an inner fragment size, since the ETE will discard any packet that arrives as multiple IPv4 fragments after reassembly.)The ITE then submits each fragment for SEAL encapsulation as specified in Section 4.4.4.
For all other inner packets, the ITE checks whether the length of the packet plus HLEN is larger than the MTU of the outgoing interface. If the packet is not too large, the ITE submits it for SEAL encapsulation as specified in Section 4.4.4. Otherwise, the ITE sends a PTB message toward the source address of the inner packet.
To send the PTB 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 (see: Section 4.6.1). Otherwise, the ITE sends a PTB message appropriate to the inner protocol version back to the source. (In both cases, the ITE sets the MTU field in the (S)PTB message to the MTU of the underlying interface minus HLEN.) The ITE then discards the 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 Internet Protocol number corresponding to the encapsulated inner packet. For example, the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets [RFC2003], the value '41' for encapsulated IPv6 packets [RFC2473][RFC4213], the value '80' for encapsulated OSI packets [RFC1070], etc.
The ITE then 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 not advertise a length that it is not authorized to use. Next, the ITE sets R=1 if redirects are permitted (see: [I-D.templin-intarea-vet]). (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 chain of ITE/ETEs that must all be authorized to represent the prefix.)
The ITE next sets C=0 and sets A=1 if an explicit acknowledgement is required from the ETE (see: Section 4.4.6). The ITE 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 vale 0 in the first packet transmitted..
The ITE finally sets the Checksum field to 0, calculates the Checksum over the first 128 bytes of the packet beginning with the SEAL header and leading portion of the inner packet, then writes the value in the Checksum field. (If there are fewer than 128 bytes, the Checksum is calculated up to the end of the inner packet.) The Checksum is calculated using an algorithm agreed on by the ITE and ETE. The algorithm uses a shared secret key so that the ETE can verify that the Checksum was generated by the ITE.
Following SEAL encapsulation, the ITE next encapsulates the packet in the requisite outer headers and trailers 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 finally sets the DF flag in the IPv4 header of each segment. If the path to the ETE correctly implements IP fragmentation (see: Section 4.4.6), the ITE sets DF=0; otherwise, it sets DF=1.
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.
Following outer encapsulation, the ITE sends each outer packet via the underlying link corresponding to LINK_ID.
When IPv4 is used as the outer encapsulation layer, the ITE can perform a qualification exchange over an underlying link to determine whether the subnetwork path to the ETE correctly implements IP fragmentation. This procedure could be employed, e.g., to determine whether there are any middleboxes on the path that violate the [RFC1812], Section 5.2.6 requirement that: "A router MUST NOT reassemble any datagram before forwarding it".
To perform this qualification, the ITE prepares a SEAL Neighbor Solicitation (SNS) message as specified in [I-D.templin-intarea-vet] then splits the packet into two outer IP fragments and sends both fragments to the ETE over the same underlying link. If the ETE returns an SPTB message with non-zero MTU (see Section 4.6.1.1), then the subnetwork path correctly implements IP fragmentation. If the ETE instead returns a SEAL Neighbor Solicitation (SNA) message, however, then a middlebox in the subnetwork is reassembling the IP fragments before they are delivered to the ETE (i.e., in violation of [RFC1812]).
In addition to any control plane probing, all SEAL encapsulated data packets sent by the ITE are considered implicit probes. SEAL data packets that use IPv4 as the outer layer of encapsulation with DF=0 will elicit SPTB messages from the ETE if any IPv4 fragmentation occurs in the path. SEAL data packets that use either IPv6 or IPv4 with DF=1 as the outer layer of encapsulation may be dropped by a router on the path to the ETE which will return a PTB message of the appropriate outer IP protocol to the ITE.
If the PTB message includes enough information (see Section 4.4.7), the ITE can then use the identifying information in the SEAL header along with the addresses within the packet-in-error to determine whether the message corresponds to one of its recent SEAL data packet transmissions. If the previous hop toward the inner source address within the packet-in-error is reached via the same tunnel interface the SEAL data packet was sent on, the ITE translates the PTB into an SPTB message and forwards it to the previous hop. Otherwise, the ITE translates the message into a PTB appropriate for the inner header and forwards it to the inner source address.
The ITE should also send explicit probes, periodically, to verify that the ETE is still reachable. The ITE sets A=1 in the SEAL header of a packet to be used as an explicit probe. The probe will elicit an SPTB message from the ETE as an acknowledgement (see Section 4.6.1.1). The ITE can also send an SNS message to elicit an SNA response from the ETE when there are no convenient data packets to use as explicit probes.
When the ITE sends SEAL data packets, it may receive raw ICMP error messages [RFC0792][RFC4443] from either the ETE or from routers within the subnetwork. The ICMP messages include 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 can use the identifying information in the SEAL header along with the source and destination addresses within the packet-in-error to confirm that the ICMP message came from either the ETE or an on-path router, and can use any additional information to determine whether to accept or discard the message.
The ITE should specifically 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 ICMPv4 messages 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.
The ETE maintains a per-ITE checksum calculation algorithm and secret key to verify the Checksum in the SEAL header.
The ETE observes the minimum reassembly buffer sizes specified for IPv4 [RFC0791] and IPv6 [RFC2460].
If the SEAL data packet did not undergo outer IP fragmentation, the ETE submits it for decapsulation as specified in Section 4.5.4. Otherwise, the ETE submits each IP fragment for reassembly.
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 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 gathers the outer IP fragments of a fragmented SEAL packet until it has received enough initial fragments to include the first 128 bytes of the SEAL packet beyond the outer headers beginning with the SEAL header (or up to the end of the packet if the packet itself includes less than 128 bytes). Using this leading portion of the (partially) reassembled SEAL packet, the ETE then verifies the SEAL header Checksum. If the Checksum is correct, the ETE sends an SPTB message back to the ITE (see Section 4.6.1.1).
Whether or not the Checksum was correct, the ETE then discards all IP fragments of the fragmented SEAL packet (i.e., it does not submit the reassembled packet for decapsulation).
The ETE next checks the SEAL header of the (unfragmented) SEAL packet. If the PKT_ID is not within the window of acceptable next PKT_ID values from this ITE, or if the SEAL header includes an incorrect Checksum value, the ETE silently drops the packet. Otherwise, if the packet has an incorrect value in other SEAL header fields the ETE discards the packet and returns an SCMP "Parameter Problem" (SPP) message (see Section 4.6.1.2). Finally, if the SEAL header has A=1 the ETE sends an SPTB message with MTU=0 back to the ITE (see Section 4.6.1.1).
Next, the ETE processes the inner packet according to the header type indicated in the SEAL NEXTHDR field. If the next hop toward the destination address of the inner packet will be via a different interface than the SEAL packet arrived on, the ETE discards the outer headers 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 inner 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, then calculates the SCMP message header 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 as well as the outer headers and trailers as shown in Figure 3:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
~ SCMP message header~ --> ~ SCMP message header~
+--------------------+ --> +--------------------+
~ SCMP message body ~ --> ~ SCMP message body ~
+--------------------+ --> +--------------------+
~ outer trailers ~
SCMP Message +--------------------+
before encapsulation
SCMP Packet
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 4:
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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 SEAL header Checksum is correct but an error must be returned, it prepares an SCMP error message as shown in Figure 4. The ETE sets the Type and Code fields in the SCMP header according to the appropriate error message type, fills out the Type-Specific Data field and includes the packet-in-error. The ETE then calculates the SCMP message checksum the same as specified for ICMPv6, except that the checksum begins with the SCMP message header, i.e., and not a pseudo-header of the outer header. The ETE writes the checksum value in the SCMP message Checksum field.
The ETE next encapsulates the SCMP message in the requisite SEAL and outer headers as shown in Figure 3. 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; A=0; R=0; NEXTHDR=0) in the SEAL header, then sets PREFLEN to 0 unless otherwise specified. If the neighbor relationship between the ETE and the source ITE is unidirectional, the ETE then writes random values in the LINK_ID and PKT_ID fields of the SEAL header. If the neighbor relationship is bidirectional, the ETE instead writes values appropriate to the bidirectional neighbor state in the LINK_ID and PKT_ID fields.
The ETE then calculates and sets the SEAL header Checksum field 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 headers the same as for SEAL data packets in Section 4.4.5. When IPv4 is used as the outer layer of encapsulation, the ETE sets the DF=1 in the outer header unless the SCMP message is an SNS message used for the path fragmentation qualification procedure described in Section 4.4.6. The ETE then sends the resulting SCMP packet to the ITE.
NB: A simplified implementation of this method entails creating a copy of the original data packet, inserting the SCMP message header and Type-Specific Data fields between the SEAL header and inner headers, truncating the resulting message to 576 bytes if necessary, then preparing the SEAL and outer header fields as described above.
The following sections describe additional considerations for various SCMP error messages:
An ETE generates an SCMP "Packet Too Big" (SPTB) message when it receives the leading 128 bytes of a SEAL protocol packet that arrived as multiple outer IP fragments. The ETE prepares the SPTB message the same as for the corresponding ICMPv6 PTB message, and writes the length of the outer IP first fragment (i.e., the fragment with MF=1 and Offset=0) in the MTU field of the message.
The ETE also generates an SPTB message when it receives an unfragmented SEAL protocol data packet with A=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. The message is therefore a control plane acknowledgement of a data plane probe, and does not signify a packet size restriction.
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.
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 TE first verifies that the outer addresses of the SCMP packet, the SEAL header Checksum, and the SCMP message header checksum are correct. If the identifying addresses and/or checksums are incorrect, the TE 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 is responsive and discards the message. If the SPTB message is the response to a fragmented SNS message used for path qualification (see Section 4.4.6), the ITE processes the message as a confirmation that the path supports IP fragmentation. Otherwise, the ITE processes the message as an indication of a packet size limitation.
If the MTU value is no less than 1280, the value is likely to represent the true MTU of the restricting link on the path to the ETE. If the MTU value is less than 1280, however, the ITE cannot determine the true MTU due to the possibility that a router on the path is generating runt first fragments. Instead, 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 its 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, and the inner packet is not an IPv4 packet with DF=0, the ITE transcribes the message into a format appropriate for the inner packet and sends the resulting transcribed message to the original source. If the inner packet is an IPv4 packet with DF=0, however, the ITE instead discards the SPTB message and caches the MTU value as the fragmentation size to use for fragmentation of future inner IPv4 packets destined to the inner destination address (see Section 4.4.3).
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 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 incorrect SEAL header field setting 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.
SEAL ensures that tunnels return the necessary path MTU discovery control messages. However, end systems are strongly encouraged to also implement their own end-to-end MTU assurance, e.g., using Packetization Layer Path MTU Discovery per [RFC4821].
IPv4 routers within the subnetwork are strongly encouraged to implement IPv4 fragmentation such that the first fragment is the largest and approximately the size of the underlying link MTU, i.e., they should not generate runt first fragments.
IPv6 routers within the subnetwork are required to generate the necessary PTB messages when they drop outer IPv6 packets due to an MTU restriction.
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 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. SEAL therefore considers full message integrity checking as an end-to-end consideration, and is therefore compatible with end-to-end 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 SEAL header Checksum 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/AUTH 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 a Checksum 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.
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 by unconditionally discarding any fragmented SEAL packets following reassembly.
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.