| Network Working Group | F. L. Templin, Ed. |
| Internet-Draft | Boeing Research & Technology |
| Intended status: Informational | June 15, 2012 |
| Expires: December 15, 2012 |
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-43.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, packet reordering, source address spoofing and traversal of links with diverse Maximum Transmission Units (MTUs). This document specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that addresses these issues.
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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 encapsulation headers often include insufficiently provisioned per-packet identification values. This can present issues for duplicate packet detection and detection of packet reordering within the subnetwork. IP encapsulation also allows an attacker to produce encapsulated packets with spoofed source addresses even if the source address in the encapsulating header cannot be spoofed. A denial-of-service vector that is not possible in non-tunneled subnetworks is therefore presented.
Additionally, the insertion of an outer IP header reduces the effective path MTU visible to the inner network layer. When IPv6 is used as the encapsulation protocol, original sources expect to be informed of the MTU limitation through IPv6 Path MTU discovery (PMTUD) [RFC1981]. 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 IPv4 PMTUD [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, IP is ubiquitously deployed as the Layer 3 protocol. The primary functions of IP are to provide for routing, addressing, and a fragmentation and reassembly capability used to accommodate links with diverse MTUs. While it is well known that the IP address space is rapidly becoming depleted, there is a lesser-known but growing consensus that other IP protocol limitations have already or may soon become problematic.
First, the Internet historically provided no means for discerning whether the source addresses of IP packets are authentic. This shortcoming is being addressed more and more through the deployment of site border router ingress filters [RFC2827], however the use of encapsulation provides a vector for an attacker to circumvent filtering for the encapsulated packet even if filtering is correctly applied to the encapsulation header. Secondly, the IP header does not include a well-behaved identification value unless the source has included a fragment header for IPv6 or unless the source permits fragmentation for IPv4. These limitations preclude an efficient means for routers to detect duplicate packets and packets that have been re-ordered within the subnetwork.
For IPv4 encapsulation, when fragmentation is permitted the header includes a 16-bit Identification field, meaning that at most 2^16 unique packets with the same (source, destination, protocol)-tuple can be active in the Internet at the same time [I-D.ietf-intarea-ipv4-id-update]. (When middleboxes such as Network Address Translators (NATs) re-write the Identification field to random values, the number of unique packets is even further reduced.) Due to the escalating deployment of high-speed links, however, these numbers have 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 have been known 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 PMTUD failures for both IPv4 and IPv6 in the Internet today [TBIT][WAND][SIGCOMM].
The issues with both IP fragmentation and this “classical” PMTUD method 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. If the ITE allows IP fragmentation on the encapsulated packets, persistent fragmentation could lead to undetected data corruption due to Identification field wrapping and/or reassembly congestion at the ETE. If the ITE instead uses classical IP PMTUD it must rely on ICMP error messages coming from the subnetwork that may be suspect, subject to loss due to filtering middleboxes, or insufficiently provisioned 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 PMTUD 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 inner network layer protocol packets 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 mid-layer encapsulation that accommodates links with diverse MTUs, and allows routers in the subnetwork to perform efficient duplicate packet and packet reordering detection. The encapsulation further ensures data origin authentication, packet header integrity and anti-replay in environments in which these functions are necessary.
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 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 PMTUD feedback even if the network path between the ITE and ETE filters ICMP messages.
SEAL further provides mechanisms to ensure data origin authentication, packet header integrity, and anti-replay. The SEAL framework is therefore similar to the IP Security (IPsec) Authentication Header (AH) [RFC4301][RFC4302], however it provides only minimal hop-by-hop authenticating services while leaving full data integrity, authentication and confidentiality services as an end-to-end consideration. While SEAL performs data origin authentication, the origin site must also perform the necessary ingress filtering in order to provide full source address verification [I-D.ietf-savi-framework].
In many aspects, SEAL also very closely resembles the Generic Routing Encapsulation (GRE) framework [RFC1701]. SEAL can therefore be applied in the same use cases that are traditionally addressed by GRE, and can also provide additional capabilities as described in this document.
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 is configured over one or more underlying interfaces attached to subnetwork links. The VET interface connects an ITE to one or more ETE "neighbors" via tunneling across an underlying subnetwork, where the tunnel neighbor relationship 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 and any outer header encapsulations as shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +----------+---------+
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, the ITE inserts the SEAL header following the outer IP header and before the inner packet as: IP/SEAL/{inner packet}.
For encapsulations over transports such as UDP, the ITE inserts the SEAL header following the outer transport layer header and before the inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the UDP header is seen as an "other outer header" as depicted in Figure 1.
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 additional tunnels. 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 arrives at 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 SEAL ITE considers each underlying interface as the ingress attachment point to a subnetwork link path to the ETE. The ITE therefore may experience different path MTUs on different ETE link paths.
Finally, the SEAL ITE ensures that the inner network layer protocol will see a minimum MTU of 1500 bytes over each ETE link path regardless of the outer network layer protocol version, i.e., even if a small amount of fragmentation and reassembly are necessary. This is necessary to avoid path MTU "black holes" for the minimum MTU configured by the vast majority of links in the Internet.
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|L|I|V|F|M| Reserved | NEXTHDR | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Vector (ICV) (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 ETE link paths 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.
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., 1280 bytes for IPv6, 576 bytes for IPv4, etc.).
The ITE can alternatively set an indefinite MTU on the tunnel interface such that all inner packets are admitted into the interface regardless of their 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").
In light of the above considerations, the ITE should configure an indefinite MTU on tunnel *router* interfaces so that subnetwork adaptation is handled from within the interface. The ITE can instead set a smaller MTU on tunnel *host* interfaces (e.g., the smallest MTU among all of the underlying links minus the size of the encapsulation headers) but should not set an MTU smaller than 1500 bytes.
The tunnel virtual interface maintains a number of soft state variables for each ETE and for each ETE link path.
When per-packet identification is required, the ITE maintains a per ETE window of Identification values for the packets it has recently sent to this ETE. The ITE then sets a variable "USE_ID" to TRUE, and includes an Identification in each packet it sends to this neighbor; otherwise, it sets USE_ID to FALSE.
When data origin authentication and integrity checking is required, the ITE also maintains a per ETE integrity check vector (ICV) calculation algorithm and a symmetric secret key to calculate the ICV in each packet it will send to this ETE. The ITE then sets a variable "USE_ICV" to TRUE, and includes an ICV in each packet it sends to this ETE; otherwise, it sets USE_ICV to FALSE.
For IPv4 ETE link paths, the ITE further maintains a variable "RATE_LIMIT" initialized to FALSE. If the link path subsequently exhibits unavoidable IPv4 fragmentation the ETE sets RATE_LIMIT to TRUE.
For each ETE link path, the ITE must also account for encapsulation header lengths. The ITE therefore maintains the per ETE link path constant values "SHLEN" set to length of the SEAL header, "THLEN" set to the length of the outer encapsulating transport layer headers (or 0 if outer transport layer encapsulation is not used), "IHLEN" set to the length of the outer IP layer header, and "HLEN" set to (SHLEN+THLEN+IHLEN). (The ITE must include the length of the uncompressed headers even if header compression is enabled when calculating these lengths.) In addition, the ITE maintains a per ETE link path variable "PATH_MTU" initialized to the maximum of 1500 bytes and the MTU of the underlying link minus HLEN. (Thereafter, the ITE must not reduce PATH_MTU to a value smaller than 1500 bytes.)
The ITE may instead maintain the packet sizing variables and constants as per ETE (rather than per ETE link path) values. In that case, the values reflect the lowest-common-denominator MTU across all of the ETE's link paths.
For each inner packet admitted into the tunnel interface, if the packet is itself a SEAL packet (i.e., one with the port number for SEAL in the transport layer header or one with the protocol number for SEAL in the IP layer header) and the LEVEL field of the SEAL header contains the value 0, the ITE silently discards the packet.
Otherwise, for non-atomic inner packets (i.e., a non-SEAL packet with (DF==0 || MF!=0 || Offset !=0) in the IP header), if the packet is larger than (1280-HLEN) bytes the ITE fragments the packet into random length inner fragments between 500 - 1000 bytes each. The ITE then submits each inner fragment for SEAL encapsulation as specified in Section 4.4.4.
For atomic inner packets (i.e., a SEAL packet and/or a packet with (DF==1 && MF==0 && Offset==0)), if the packet is between ((1281-HLEN), 1500) bytes in length the ITE must ensure that it will traverse the tunnel. The ITE must therefore decide whether to submit the packet for SEAL encapsulation in one piece or fragment the packet using inner IP fragmentation. The ITE submits the packet in one piece if there is assurance that the packet will traverse the tunnel without fragmentation, and/or if there is assurance that the ETE will be able to reassemble it if fragmentation occurs within the tunnel.
For other atomic inner packets, if the packet is no larger than PATH_MTU for the corresponding ETE link path, the ITE submits it for SEAL encapsulation in one piece. Otherwise, the ITE sends a PTB error 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 4.6.1.1) with the MTU field set to PATH_MTU. Otherwise, the ITE sends an ordinary PTB message appropriate to the inner protocol version with the MTU field set to PATH_MTU. (For IPv4 SEAL packets with DF==0, the ITE should set DF=1 and re-calculate the IPv4 header checksum before generating the PTB message in order to avoid bogon filters.)
After sending the (S)PTB message, the ITE discards the inner packet.
For each inner packet/fragment submitted for SEAL encapsulation, the ITE next encapsulates the packet in a SEAL header formatted as specified in Section 4.3. The SEAL header includes an Identification field when USE_ID is TRUE, followed by an ICV field when USE_ICV is TRUE.
The ITE next sets C=0 in the SEAL header. The ITE also sets A=1 if necessary for ETE reachability determination (see: Section 4.4.6) or for stateful MTU determination (see Section 4.4.9). Otherwise, the ITE sets A=0. Next, when RATE_LIMIT is TRUE the ITE sets L=1; otherwise, it sets L=0.
The ITE then 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), R is instead copied from the SEAL header of the re-encapsulated packet. This implies that the R value is propagated across a re-encapsulating chain of ITE/ETEs.)
The ITE then sets LINK_ID to the value assigned to the underlying ETE link path, and 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.
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. If the inner packet is a SEAL packet that is undergoing nested encapsulation, the ITE instead sets LEVEL to the value that appears in the inner packet's SEAL header minus 1. If the inner packet is undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL value from the SEAL header of the packet to be re-encapsulated.
Next, if the inner packet/fragment is between ((1281-HLEN), 1500) bytes in length the ITE breaks it into two equal-length pieces and appends a clone of the SEAL header from the first piece onto the head of the second piece. The ITE then sets (F=1; M=1) in the first piece and sets (F=0; M=0) in the second piece. Note that for single-piece inner packets the ITE instead sets (F=1; M=0) (the code (F=0; M=1) is undefined).
When USE_ID is FALSE, the ITE next sets I=0. Otherwise, the ITE sets I=1 and writes a monotonically-increasing integer value for this ETE in the Identification field beginning with 0 in the first packet transmitted. (For SEAL packets that have been split into two pieces, each piece writes the same Identification value.)
When USE_ICV is FALSE, the ITE next sets V=0. Otherwise, the ITE sets V=1 and calculates the packet header ICV value using an algorithm agreed on by the ITE and ETE. When data origin authentication is required, the algorithm uses a symmetric secret key so that the ETE can verify that the ICV was generated by the ITE. Beginning with the SEAL header, the ITE calculates the ICV over the leading 128 bytes of the packet (or up to the end of the packet if there are fewer than 128 bytes) and places result in the ICV field. (For SEAL packets that have been split into two pieces, each piece caclulates its own ICV value.)
The ITE then adds the outer encapsulating headers and performs any necessary outer fragmentation as specified in Section 4.4.5.
Following SEAL encapsulation, the ITE next encapsulates each packet in the requisite outer transport (when necessary) and IP layer headers. When a transport layer header is included, the ITE writes the port number for SEAL in the transport destination service port field and writes the protocol number of the transport protocol in the outer IP header protocol field. Otherwise, the ITE writes the protocol number for SEAL in the outer IP header protocol field.
The ITE then sets the other fields of the outer transport and IP layer headers as specified in Sections 5.5.4 and 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 re-encapsulation procedures specified in Section 5.5.6 of [I-D.templin-intarea-vet].
For IPv4 ETE link paths, the ITE sets DF=0 in the IPv4 header to allow the packet to be fragmented if it encounters a restricting link. (For IPv6 link paths, the DF bit is implicitly set to 1.)
The ITE then sends each outer packet/fragment via the underlying link corresponding to LINK_ID. For IPv4 ETE link paths with RATE_LIMIT=TRUE, the ITE sends the packet subject to rate limiting so that the IPv4 Identification value is not repeated within the IPv4 Maximum Segment Lifetime (i.e., 120 seconds) [RFC1122].
All SEAL data packets sent by the ITE 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 via a specific link path.
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 ICMP error messages [RFC0792][RFC4443] from an ordinary router within the subnetwork or from another ITE on the path to the ETE (i.e., in case of nested encapsulations). 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 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 ICMP messages that do not include sufficient information in the packet-in-error as a hint that the ETE link path may be failing. Specific actions that the ITE may take in these cases are out of scope.
For other ICMP messages, the ITE should use any outer header information available as a first-pass authentication filter (e.g., to determine if the source of the message is within the same administrative domain as the ITE) and discards the message if first pass filtering fails.
Next, the ITE examines the packet-in-error beginning with the SEAL header. If the value in the Identification field (if present) is not within the window of packets the ITE has recently sent to this ETE, or if the value in the SEAL header ICV field (if present) is incorrect, the ITE discards the message.
Next, if the received ICMP message is a PTB the ITE sets the temporary variable "PMTU" for this ETE link path to the MTU value in the PTB message. If PMTU==0, the ITE consults a plateau table (e.g., as described in [RFC1191]) to determine PMTU based on the length field in the outer IP header of the packet-in-error. For example, if the ITE receives a PTB message with MTU==0 and length 4KB, it can set PMTU=2KB. If the ITE subsequently receives a PTB message with MTU==0 and length 2KB, it can set PMTU=1792, etc. to a minimum value of PMTU=(1500+HLEN). If the ITE is performing stateful MTU determination for this ETE link path (see Section 4.4.9), the ITE next sets PATH_MTU=MAX((PMTU-HLEN), 1500).
If the ICMP message was not discarded, the ITE then transcribes it into a message to return to the previous hop. 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 transcribes the ICMP message into an SCMP message. Otherwise, the ITE transcribes the ICMP message into a message appropriate for the inner protocol version.
To transcribe the message, the ITE extracts the inner packet from within the ICMP message packet-in-error field and uses it to generate a new message corresponding to the type of the received ICMP message. For SCMP messages, the ITE generates the message the same as described for ETE generation of SCMP messages in Section 4.6.1. For (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field.
The ITE finally forwards the transcribed message to the previous hop toward the inner source address.
The ITE can perform a qualification exchange to ensure that the subnetwork correctly delivers 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".
The ITE should use knowledge of its topological arrangement as an aid in determining when middlebox reassembly testing is necessary. For example, if the ITE is aware that the ETE is located somewhere in the public Internet, middlebox reassembly testing should not be necessary. 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 can perform a middlebox reassembly test by selecting a data packet to be used as a probe. While performing the test with real data packets, the ITE should select only inner packets that are no larger than (1500-HLEN) bytes for testing purposes. The ITE can also construct a dummy probe packet instead of using ordinary SEAL data packets.
To generate a dummy probe packet, the ITE creates a packet buffer beginning with the same outer headers, SEAL header and inner network layer header that would appear in an ordinary data packet, then pads the packet with random data to a length that is at least 128 bytes but no longer than (1500-HLEN) bytes. The ITE then writes the value '0' in the inner network layer TTL (for IPv4) or Hop Limit (for IPv6) field.
The ITE then sets (C=0; R=0) in the SEAL header of the probe packet and sets the NEXTHDR field to the inner network layer protocol type. (The ITE may also set A=1 if it requires a positive acknowledgement; otherwise, it sets A=0.) Next, the ITE sets LINK_ID and LEVEL to the appropriate values for this ETE link path, sets Identification and I=1 (when USE_ID is TRUE), then finally calculates the ICV and sets V=1(when USE_ICV is TRUE).
The ITE then encapsulates the probe packet in the appropriate outer headers, splits it into two outer IPv4 fragments, then sends both fragments over the same ETE link path.
The ITE should send a series of probe packets (e.g., 3-5 probes with 1sec intervals between tests) instead of a single isolated probe in case of packet loss. If the ETE returns an SCMP PTB message with MTU != 0, then the ETE link path correctly supports fragmentation; otherwise, the ITE enables stateful MTU determination for this ETE link path as specified in Section 4.4.9.
(Examples of middleboxes that may perform reassembly include stateful NATs and firewalls. Such devices could still allow for stateless MTU determination if they gather the fragments of a fragmented IPv4 SEAL data packet for packet analysis purposes but then forward the fragments on to the final destination rather than forwarding the reassembled packet.)
SEAL supports a stateless MTU determination capability, however the ITE may in some instances wish to impose a stateful MTU limit on a particular ETE link path. For example, when the ETE is situated behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8) it is imperative that fragmentation be avoided. In other instances (e.g., when the ETE link path includes performance-constrained links), the ITE may deem it necessary to cache a conservative static MTU in order to avoid sending large packets that would only be dropped due to an MTU restriction somewhere on the path.
To determine a static MTU value, the ITE can send a series of dummy probe packets of various sizes to the ETE with A=1 in the SEAL header and DF=1 in the outer IP header. The ITE can then cache the size 'S' of the largest packet for which it receives a probe reply from the ETE by setting PATH_MTU=MAX((S-HLEN), 1500) for this ETE link path.
For example, the ITE could send probe packets of 4KB, followed by 2KB, followed by 1792 bytes, etc. While probing, the ITE processes any ICMP PTB message it receives as a potential indication of probe failure then discards the message.
When stateful determination is used, the ITE can periodically reset PATH_MTU and/or re-probe the path to determine whether PATH_MTU has increased. If the path still has a too-small MTU, the ITE will receive a PTB message that reports a smaller size.
For IPv4 ETE link paths, when the path correctly implements fragmentation and RATE_LIMIT is TRUE, the ITE can periodically reset RATE_LIMIT=FALSE to determine whether the path still requires rate limiting. If the ITE receives an SPTB message for a SEAL packet with (F==0 || M==1) the ITE should again set RATE_LIMIT=TRUE.
When data origin authentication and integrity checking is required, the ETE maintains a per-ITE ICV calculation algorithm and a symmetric secret key to verify the ICV. When per-packet identification is required, the ETE also maintains a window of Identification values for the packets it has recently received from this ITE.
When the tunnel neighbor relationship is bidirectional, the ETE further maintains a per ETE link path mapping of outer IP and transport layer addresses to the LINK_ID that appears in packets received from the ITE.
The ETE should maintain conservative 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 discards the packet if it is larger than the reassembly buffer size or delivers the (fully-reassembled) packet to the appropriate upper layer protocol module.
For SEAL packets, the ITE performs any necessary IP reassembly then submits the packet for SEAL decapsulation as specified in Section 4.5.3. (Note that if the packet is larger than the reassembly buffer size, the ITE still returns the leading portion of the (partially) reassembled packet.)
For each SEAL packet submitted for decapsulation, when I==1 the ETE first examines the Identification field. If the Identification is not within the window of acceptable values for this ITE, the ETE silently discards the packet.
Next, if V==1 the ETE verifies the ICV value (with the ICV field itself reset to 0) and silently discards the packet if the value is incorrect.
Next, if the packet arrived as multiple IPv4 fragments and L ==0, the ETE sends an SPTB message back to the ITE with MTU set to the size of the largest fragment received minus HLEN (see: Section 4.6.1.1).
Next, if the packet arrived as multiple IP fragments and the inner packet is larger than 1500 bytes, the ETE silently discards the packet; otherwise, it continues to process the packet.
Next, if there is an incorrect value in a SEAL header field (e.g., an incorrect "VER" field value), the ETE discards the packet. If the SEAL header has C==0, the ETE also returns an SCMP "Parameter Problem" (SPP) message (see Section 4.6.1.2).
Next, if the SEAL header has C==1, the ETE 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.
Next, if the SEAL header has (F==0 || M==1) the ETE checks to see if the other segment of this already-segmented SEAL packet has arrived, i.e., by looking for a segment that has the same outer IP source address, source transport port number (if present) and SEAL Identification value. If the other segment has already arrived, the ETE discards the SEAL header and other outer headers from the second segment and appends the remaining portion onto the end of the first segment. Otherwise, the ETE caches the segment for at most 60 seconds while awaiting the arrival of its partner. To support this process, the ETE must be able to buffer segmented SEAL packets up to (1500+HLEN) bytes in length.
Next, if the SEAL header in the (reassembled) packet has A==1, the ETE sends an SPTB message back to the ITE with MTU=0 (see: Section 4.6.1.1).
Finally, the ETE discards the outer headers and processes the inner packet according to the header type indicated in the SEAL NEXTHDR field. If the inner (TTL / Hop Limit) field encodes the value 0, the ETE silently discards the packet. Otherwise, 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 and without decrementing the value in the inner (TTL / Hop Limit) field. In this process, the packet remains within the tunnel (i.e., it does not exit and then re-enter the tunnel); hence, the packet is not discarded if the LEVEL field in the SEAL header contains the value 0.
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]. As for ICMPv6, each SCMP message includes a 32-bit header and a variable-length body. The TE encapsulates the SCMP message in a SEAL header and outer headers 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 ~
| | --> | |
+--------------------+ +--------------------+
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 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 the invoking SEAL data packet as possible |
~ (beginning with 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 Identification and ICV values are 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 to the same values that would appear in the corresponding ICMPv6 message [RFC4443], but calculates the Checksum beginning with the SCMP message header using the algorithm specified for ICMPv4 in [RFC0792].
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 values associated with the ITE 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; L=0; F=1; M=0) in the SEAL header, then sets I, V, NEXTHDR and LEVEL to the same values that appeared in the SEAL header of the data packet. If the neighbor relationship between the ITE and ETE is unidirectional, the ETE next sets the LINK_ID field to the same value that appeared in the SEAL header of the data packet. Otherwise, the ETE sets the LINK_ID field to the value it would use in sending a SEAL packet to this ITE.
When I==1, the ETE next sets the Identification field to an appropriate value for the ITE. If the neighbor relationship between the ITE and ETE is unidirectional, the ETE sets the Identification field to the same value that appeared in the SEAL header of the data packet. Otherwise, the ETE sets the Identification field to the value it would use in sending the next SEAL packet to this ITE.
When V==1, the ETE then calculates and sets the ICV field the same as specified for SEAL data packet encapsulation in Section 4.4.4.
Finally, the ETE 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 IPv4 fragments and for which L==0. 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 minus HLEN in the MTU field of the message.
The ETE also generates an SPTB message when it accepts a 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.
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].
An ITE may receive SCMP messages with C==1 in the SEAL header after sending packets to an ETE. The ITE first verifies that the outer addresses of the SCMP packet are correct, and (when I==1) that the Identification field contains an acceptable value. The ITE next verifies that the SEAL header fields are set correctly as specified in Section 4.6.1. When V==1, the ITE then verifies the ICV value. The ITE next verifies the Checksum value in the SCMP message header. If any of these values are incorrect, the ITE silently 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 packet (see: Section 4.6.1.1). For IP SPTB messages with MTU==0, the ITE processes the message as confirmation that the ETE received a SEAL data packet with A==1 in the SEAL header. The ITE then discards the message.
For IPv4 SPTB messages with MTU != 0, the ITE instead processes the message as an indication of a packet size limitation as follows. If the SEAL header of the packet-in-error has (F==1 && M==1), the ITE sets RATE_LIMIT=TRUE. Otherwise, the ITE instead examines the SPTB message MTU field. If the MTU value is not substantially less than 1280 bytes, the value is likely to reflect 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 IPv4 SPTB message with MTU==256 and inner packet length 4KB, it can rewrite the MTU to 2KB. If the ITE subsequently receives an IPv4 SPTB message with MTU==256 and inner packet length 2KB, it can rewrite the MTU to 1792, etc If the ITE is performing stateful MTU determination for this ETE link path, it then writes the new MTU value minus HLEN in PATH_MTU.
The ITE then 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 SPTB message arrived on, the ITE relays the message to the previous hop. In order to relay the message, the first writes zero in the Identification and ICV fields of the SEAL header within the packet-in-error. The ITE next rewrites the outer SEAL header fields with values corresponding to the previous hop and recalculates the ICV using the ICV calculation parameters associated with 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. (Note that, in this process, the values within the SEAL header of the packet-in-error are meaningless to the previous hop and therefore cannot be used by the previous hop for authentication purposes.)
If the previous hop is not reached via the same tunnel interface, the ITE instead 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. (NB: if the inner packet within the SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set DF=1 and re-calculate the IPv4 header checksum while transcribing the message in order to avoid bogon filters.) The ITE then discards the 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 PMTUD 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].
SEAL supports nested tunneling for up to 8 layers of encapsulation. In this model, the SEAL ITE has a tunnel neighbor relationship only with ETEs at its own nesting level, i.e., it does not have a tunnel neighbor relationship with other ITEs, nor with ETEs at other nesting levels.
Therefore, when an ITE 'A' within an inner nesting level needs to return an error message to an ITE 'B' within an outer nesting level, it generates an ordinary ICMP error message the same as if it were an ordinary router within the subnetwork. 'B' can then perform message validation as specified in Section 4.4.7, but full message origin authentication is not possible.
Since ordinary ICMP messages are used for coordinations between ITEs at different nesting levels, nested SEAL encapsulations should only be used when the ITEs are within a common administrative domain and/or when there is no ICMP filtering middlebox such as a firewall or NAT between them. An example would be a recursive nesting of mobile networks, where the first network receives service from an ISP, the second network receives service from the first network, the third network receives service from the second network, etc.
NB: As an alternative, the SCMP protocol could be extended to allow ITE 'A' to return an SCMP message to ITE 'B' rather than return an ICMP message. This would conceptually allow the control messages to pass through firewalls and NATs, however it would give no more message origin authentication assurance than for ordinary ICMP messages. It was therefore determined that the complexity of extending the SCMP protocol was of little value within the context of the anticipated use cases for nested encapsulations.
The IANA is instructed to allocate a System Port number for "SEAL" in the 'port-numbers' registry for the TCP, UDP, DCCP and SCTP protocols.
The IANA is further instructed to allocate an IP protocol number for "SEAL" in the "protocol-numbers" registry.
Considerations for port and protocol number assignments appear in [RFC2780][RFC5226][RFC6335].
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/buffer overflow attack is possible when an attacker sends IP fragments with spoofed source addresses to an ETE in an attempt to clog the ETE's reassembly buffer and/or cause the ETE to generate a stream of SCMP messages returned to a victim ITE. The SCMP message ICV, Identification, 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 can be 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.
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 message authentication and integrity.
The concepts of path MTU determination through the report of fragmentation and extending the IPv4 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 PMTUD mechanism, appears in Appendix D of this document.
An early implementation of the first revision of SEAL [RFC5320] is available at: http://isatap.com/seal/pre-rfc5320.txt
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. |
| [RFC1122] | Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. |
| [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 be inefficient to require the tunnel endpoints to also perform ARQ.
The SEAL header includes an integrity check 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 also 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 IPv4 ID fields.
In particular, the IPv4 16-bit ID field can wrap with only 64K packets with the same (src, dst, protocol)-tuple alive in the system at a given time [RFC4963]. When the IPv4 ID field is re-written by a middlebox such as a NAT or Firewall, ID field wrapping can occur with even fewer packets alive in the system.
When outer IPv4 fragmentation is unavoidable, SEAL institutes rate limiting so that the number of packets admitted into the tunnel by the ITE does not exceed the number of unique packets that may be alive within the Internet.
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 an unspecified 'SEAL_OPTION' TCP option during connection establishment) for the carriage of protocol data encapsulated as IP/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 IP/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.