Shim6 Working Group I. van Beijnum Internet-Draft June 19, 2006 Expires: December 21, 2006 Shim6 Reachability Detection draft-van-beijnum-shim6-reach-detect-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 21, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract The shim6 working group is developing a mechanism that allows multihoming by using multiple addresses. When communication between the initially chosen addresses for a transport session is no longer possible, a "shim" layer makes it possible to switch to a different set of addresses without breaking current transport protocol assumptions. This draft discusses the issues of detecting failures in a currently used address pair between two hosts ("current pair reachability") and and picking a new address pair to be used when a failure occurs ("full path exploration"). Note: the previous version of this document was draft-ietf-shim6-reach-detect-01.txt. Van Beijnum Expires December 21, 2006 [Page 1] Internet-Draft Shim6 Reachability Detection June 2006 1 Introduction Exploring the full set of communication options between two hosts that both have two or more addresses is an expensive operation as the number of combinations to be explored increases very quickly with the number of addresses. For instance, with two addresses on both sides, there are four possible address pairs. Since we can't assume that reachability in one direction automatically means reachability for the complement pair in the other direction, the total number of two-way combinations is eight. (Combinations = nA * nB * 2.) For three addresses on each end that increases to 18, and with four it's 32. The input for the full path exploration process is ordered lists of local and remote addresses that are reasonably likely to work. (I.e., not include addresses that are known to be unreachable for local reasons.) These lists must be available at both ends of the communication, although the ordering may differ. Building these address lists from locally available information and synchronizing them with the remote end are outside the scope of this document. In hop-by-hop forwarding the source address doesn't have any effect on reachability, but in the presence of filters or source address based routing, it may. And although links almost always work in two directions, routing protocols and filters only work in one direction so unidirectional reachability can happen. Without additional mechanisms, the practice of ingress filtering by ISPs makes unidirectional connectivity likely. Being able to use the working leg in a unidirectional path is useful, it's not an essential requirement. It is essential, however, to avoid assuming bidirectional connectivity when there is in fact a unidirectional failure. An important observation in multihoming is that failures are relatively infrequent, so that a path that worked a few seconds ago is very likely to work now as well. So it makes sense to have a light-weight protocol that confirms existing reachability, and only invoke the much heavier protocol that can determine full reachability when a there is a suspected failure. This text is for the most part based on discussions on the multi6 list, several multi6 design team lists and the shim6 list, with notable contributions from Erik Nordmark, Marcelo Bagnulo and Jari Arkko. Suggestions and additions are more than welcome. 2 Determining reachability for the current pair Reachability for the currently used address pair in a shim context is determined by making sure that whenever there is data traffic in one direction, there is also traffic in the other direction. This can be data traffic as well, but also transport layer acknowledgments or a shim Van Beijnum Expires December 21, 2006 [Page 2] Internet-Draft Shim6 Reachability Detection June 2006 reachability keepalive if there is no other traffic. This way, it is no longer possible to have traffic in only one direction, so whenever there is data traffic going out, but there are no return packets, there must be a failure, so the full path exploration mechanism is started. The behavior of the mechanism depends on the kind of packet that was received or sent last: a data packet or a non-data packet. The difference between the two is that data packets are packets that should generate return traffic in most transport protocols, while non-data packets are the house keeping packets, such as TCP ACKs, which don't necessarily require any return traffic. The host should use the information available to it to determine whether a packet is a data or a non-data packet. Examples of non-data packets are TCP ACKs and shim keepalive packets. If there is any doubt, a packet should be considered a data packet. A more detailed description of the current pair reachability evaluation mechanism: 1. The base timing unit for this mechanism is named ShimKeepT. Until a negotiation mechanism to negotiate different values for ShimKeepT becomes available, a value of 10 seconds MUST be used. 2. Whenever outgoing packets are generated that are part of a shim context, one of two timestamps belonging to the shim context is updated to record the point in time that a packet of this type was sent for the last time: the timestamp for outgoing data packets, or the timestamp for outgoing non-data packets. 3. Whenever incoming packets are received that are part of a shim context, one of two timestamps belonging to the shim context is updated: the timestamp for incoming data packets, or the timestamp of incoming non-data packets. For incoming packets, it's less critical that packets are labeled as data or non-data correctly. In the absence of better information, hosts may assume that any IPv6 packet with a total length field with a value of 20 or lower is a non-data packet. 4. ShimKeepT seconds after the last data packet has been received for a context, and if no other packet has been sent within this context since the data packet has been received, a shim keepalive packet is generated for the context in question and transmitted to the correspondent. The shim keepalive packet consists of an IPv6 header and a shim header containing the context tag, but no subsequent headers. Intermediate headers may be present between the IPv6 and shim headers. A host may send the shim keepalive after fewer than ShimKeepT seconds if implementation considerations Van Beijnum Expires December 21, 2006 [Page 3] Internet-Draft Shim6 Reachability Detection June 2006 warrant this, as long as the average time after which shim keepalives are sent is at least ShimKeepT / 2 seconds. 5. After potentially sending a single shim keepalive, no additional shim keepalives are sent as long as the session remains idle or only non-data packets are received. When a data packet is received within this shim context, the timestamps are re-initialized and more shim keepalives may be sent. In other words: when data is flowing in both directions, no keepalives are sent. When the session is idle, which is always, but not exclusively, the case when the last packet in both directions was a keepalive, no keepalives are sent. 6. When after a timeout period since the last transmission of a data packet no packets were received from the correspondent within this context, a full reachability exploration is started. The timeout period is ShimKeepT seconds plus additional time to accommodate for a round trip and regular variations in network-related functions. In the absence of better information, a timeout of at least ShimKeepT + 2 seconds but no more than ShimKeepT + 5 seconds is recommended. Note that this means that a full path exploration can be started after missing only one keepalive. The author believes the risk of starting a full path exploration because of incidental packet loss is preferable over the situation where the amount of keepalives is increased by a factor two or three to reduce this risk. 3 Full path exploration In its essence, address pair exploration is very simple: just send probes using every possible address pair, wait for something to come back and possibly consider the round trip time. In practice, testing the full combination of all source addresses and all destination addresses is very undesirable because of the large number of packets involved. This can be especially harmful when a lot of hosts on a link start doing this for many of their correspondents at the same time when there is a failure further upstream. 3.1 Host identifier / lookup key To reduce the amount of packets that need to be sent, path exploration happens on a host-to-host basis. So when there are several shim contexts between two hosts, only a single path exploration procedure is executed. In order to be able to stop exploring additional addresses when a working one is found, it would be necessary to know which addresses belong to the same host. The obvious way to do this is to assign a host Van Beijnum Expires December 21, 2006 [Page 4] Internet-Draft Shim6 Reachability Detection June 2006 identifier to each host, and make sure that all locator addresses point towards the host identifier of the host they belong to. However, this would entail significant complexity, and having the requirement that a host reveal a fairly stable identifier at the start of a session or context may not be acceptable for privacy reasons. To avoid these issues, the path exploration mechanism uses a different way to group addresses that belong to the same remote host and also to provide a basic level of protection against third parties using the path exploration mechanism to find out information about the local host and its correspondents: it requires the remote host to specify an identifier cq lookup key that is echoed back to it in path exploration packets. The identifier/key is a variable length opaque value of one to six 32-bit words (4 - 24 bytes). Since multiple correspondents may choose the same id/key, the id/key is extended with a disambiguation value inside the local host, but this value is not transmitted to the other side. The particulars of the disambiguation value are implementation specific. It is highly recommended that implementations do NOT use an address or other easily recognizable value for the id/key. Rather, they should use a private value, such as a pointer to a control block, or a hash of one or more known values, such as the remote ULID or FQDN, as the id/key. There is no requirement one way or the other as to whether a host uses the same id/key towards a correspondent for different sessions, associations or contexts. Whenever an address pair is determined to work, reachability exploration stopped for all addresses that share the same id/key (including the disambiguation value). (For addresses with the same or lower preference, that is, see below.) Note that it's possible for two hosts to have more than one active shim context as both have multiple addresses that each can be used as ULIDs and sessions can be set up between different ULID pairs before the hosts even realize the other side supports shim6. Having multiple contexts between two hosts may also be supported by the shim protocol explicitly. 3.2 Address lists In order to arrive at a desired outcome more quickly and with less packets, and also to accommodate traffic engineering needs, we'll assume a model where each address (source or destination) has two preference values: p1 and p2. P1 is a strong preference value: addresses with a higher p1 are always preferred over addresses with a lower p1. P2 is a weak preference value that is used to determine what percentage of traffic or probes is sent towards each of the set addresses sharing a p1 value. Van Beijnum Expires December 21, 2006 [Page 5] Internet-Draft Shim6 Reachability Detection June 2006 Addresses within the same set (source or destination) are ranked by their p1 value, where a higher p1 means that the address is more preferred. When there are multiple addresses with the same p1 value, an address is selected at random from the group with the same p1 value, where the likelihood of selecting any given address is relative to its p2 value compared to the sum of all p2 values. So if addresses A, B and C have the same p1 value and p2 values of 10, 30 and 60 for a total of 100, the chance that A is selected is 10%, the chance that B is selected is 30% and the chance that C is selected is 60%. Note that preference information may be related to type of service. So different contexts with different type of service requirements may see different p1 and p2 values for a given address. When a host suspects that there is a failure for a context, it gathers the set of possible source addresses and the set of possible destination addresses. Both sets are ordered such that each next address has an equal or lower p1 value. Addresses with the same p1 value are further ordered as per any heuristics that the host may employ, such as longest prefix matches on known working and/or known not working addresses along with the p2 value. The p2 value is considered relatively weak, and breaking p2 ordering is allowed if there is a sufficient reason for this. However, in the absence of other information, p2 ordering should be used. P1 ordering overrules any other information except a recent reachability failure for the address in question. In addition to the above, the most recently used address is put in front of the list regardless of its p1 and p2 values. This way, the last used address pair, which is now suspected of no longer working, is tried first. 3.3 Address pairs From the lists of eligible source and destination addresses, the host creates a list of source/destination address pairs, along with a combined preference value for this address pair. The calculation of the preference value is left to the implementation, with the only requirement being that when one address pair has a higher p1 for both the source and destination address than another pair, the pair with the higher p1 values also has a higher combined pair preference value. So when there are both source addresses with p1 values 1 and 5 and destination addresses with p1 values 3 and 8, the { 5, 8 } pair is considered best and the { 1, 3 } pair worst, but whether { 5, 3 } is better than { 1, 8 } is not specified. For instance, an implementation may choose to add the p1 values so that the { 1, 8 } = 9 is better than { 5, 3 } = 8, or choose to multiply the values so that { 5, 3 } = 15 is better than { 1, 8 } = 8. Van Beijnum Expires December 21, 2006 [Page 6] Internet-Draft Shim6 Reachability Detection June 2006 The list of address pairs from different contexts is combined into a host-wide list of address pairs. The preference values are updated to take into consideration the number of contexts that are interested in the pair. The specifics of calculating the resulting host-wide preference value are left upto the implementation, but implementations SHOULD try, within reason, to avoid using address pairs with lower p1 values when pairs with higher p1 values are available for a context. Context-specific address pair preferences may be normalized prior to calculating host-wide address pair preference values. So when context A has pairs P and Q with p1 values 10 and 1, while context B has pairs R and S with p1 values 7 and 4, the values for P and R are changed to 2 and the values for Q and S to 1. But this is optional. Assuming the full path exploration is executed for more than one host for which the currently used address pair is suspected of not working, the procedure outlined above results in a list of address pairs with a host-wide (as opposed to context-specific) preference value, along with a value that identifies the remote host. 3.4 Probing The local host now starts probing address pairs, in order from the pair with the highest pair preference to the pair with the lowest pair preference. When all address pairs have been tested, testing restarts from the pair with the highest preference. New pairs that become available are put in the list before pairs that have been probed already, regardless of the preference values. However, both the group of address pairs that haven't been probed and the group of address pairs that have may be reordered to reflect the preference values, as long as reordering is done such that starvation doesn't occur. When a probe is answered by the correspondent, the context that use the address pair in question are informed so they can start remapping address is outgoing packets to the pair in question. (All of this also happens when there is a working pair but an address pair with at least one address with a higher preference is determined to work.) At this point, the context updates its list of address pairs to probe by removing all pairs where either the source address has a lower p1 value than the p1 value of the now working source address, or the destination address has a lower p1 value than the p1 value of the now working destination address. Additionally, all address pairs where the p1 values for the source and destination addresses match the respective p1 values of the source and destination addresses in the now working pair are removed from the list. The host-wide list of address pair to probe is updated to reflect the removal of lower or equal priority addresses, so probing will only continue for pairs where at least one address has a higher p1 than the currently working pair. Van Beijnum Expires December 21, 2006 [Page 7] Internet-Draft Shim6 Reachability Detection June 2006 The time between probes (ShimProbeT) must be chosen such that the number of probes is limited to 60 per 300 second period. When no probes have been sent for some time, an implementation may send the initial group of probes at a fairly aggressive rate. For instance, when no probes have been sent for 60 seconds, a host may send a second probe 200 ms after the first one, and increase the ShimProbeT by a factor 1.25 after every probe, until ShimProbeT reaches 5 seconds. This results in sending 5 probes in the first 2 seconds and/or 14 probes within the first 20 seconds after a failure. After that, there is one probe every 5 seconds. When a context didn't see any outgoing data packets (see section 2) for four minutes, it removes all its address pairs from the host-wide list of address pairs. 4 Address pair exploration packet format The address pair exploration packet may be encapsulated in different ways. An obvious way is inside a shim header. The address pair exploration packet contains the following information: - A type field that is at least 8 bits long - An 8 bit "number of probes sent" field - An 8 bit "number of probes received" field - An 8 bit "options length" field - One or more sent probes (see below) - Zero or more received probes (see below) - Zero or more bytes of option/variable length data There is currently one bit in the type field defined: the reply requested bit. If this bit is set, the other side should send a probe in reply to this probe. If the bit is zero, the host may or may not send a probe in accordance with its own progress. The option/variable length data contains zero or more options in the following format: - An 8 bit option type - An 8 bit option length - Zero or more bytes of data in this option Currently, there is one options defined: the id/key that the remote host uses to recognize the local host. This option must be present at least once, containing the value the remote host supplied earlier. Optionally, additional id/key options may be present if the address that is probed is a valid locator for more than one context, and these contexts use different id/keys. Van Beijnum Expires December 21, 2006 [Page 8] Internet-Draft Shim6 Reachability Detection June 2006 Sent and received probes contain data in the following format: - Source locator/address (128 bits) - Destination locator/address (128 bits) - Sent timestamp (32 bits in ms resolution relative to private epoch) - Time between reception and retransmission (32 bits in ms resolution, 0 on first transmission) - Nonce (32 bits) - Sequence number (32 bits) The first and only mandatory sent probe structure contains the addresses that are present in the current IPv6 packet along with a timestamp for the current time. Additional probe structures contain copies of earlier probes, presumably toward different addresses, with the appropriate field indicating how long ago the probe in question was sent. The received probes are copies of the last seen probes from the other side. Note that an application must be able to infer which addresses belong to the same host in order to perform this probing correctly 5 NAT and firewall considerations Since shim6 is chartered for IPv6 solutions only, and NAT compatibility is not expected, and by most people, not desired in IPv6, there is no requirement for this protocol to pass through Network Address Translation devices. However, the protocol may be applicable outside shim6, making NAT compatibility desirable. It is absolutely essential that the shim6 negotiations and the reachability detection packets are passed through filters or firewalls wherever application packets are passed through. If the shim6 negotiation and reachability detection packets are filtered out, shim6 can't be used. A more complex situation arises when the shim6 negotiation packets pass through a firewall, but the reachability detection packets are blocked. To avoid this complexity, it's highly desirable to make the shim6 negotiation and reachability detection part of the same protocol, so either both are allowed through or both are blocked. However, the same is true if this reachability detection mechanism is used in other protocols. This makes it desirable to define the reachability detection protocol such that it can be embedded in other protocols. Since firewalls are in wide use, it's important to consider whether a new protocol will be able to pass through most firewalls without requiring changes to the filter configuration. On the other hand, it may not be possible to come up with a protocol that would be allowed through a large percentage of all firewalls without changes, so extra effort in this area may produce limited results. Also, in the long run firewall Van Beijnum Expires December 21, 2006 [Page 9] Internet-Draft Shim6 Reachability Detection June 2006 configuration will presumably be changed, so any compromises would only have short term benefits but long term downsides. 6 Security considerations To avoid exposing information (even if it's just the fact that an address is reachable), hosts will probably want to limit themselves to taking part in reachability detection with known correspondents. Only replying to probes that carry a known id/key is suggested to accomplish this. 7 Document and Author Information This document expires December, 2006. The latest version will always be available at http://www.muada.com/drafts/. Please direct questions and comments to the shim6 mailinglist or directly to the author: Iljitsch van Beijnum Email: iljitsch@muada.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Van Beijnum Expires December 21, 2006 [Page 10] Internet-Draft Shim6 Reachability Detection June 2006 Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Van Beijnum Expires December 21, 2006 [Page 11]