Internet Engineering Task Force (IETF) J. Touch Request for Comments: 6864 USC/ISI Updates: 791, 1122, 2003 February 2013 Category: Standards Track ISSN: 2070-1721 Updated Specification of the IPv4 ID Field
AbstractThe IPv4 Identification (ID) field enables fragmentation and reassembly and, as currently specified, is required to be unique within the maximum lifetime for all datagrams with a given source address/destination address/protocol tuple. If enforced, this uniqueness requirement would limit all connections to 6.4 Mbps for typical datagram sizes. Because individual connections commonly exceed this speed, it is clear that existing systems violate the current specification. This document updates the specification of the IPv4 ID field in RFCs 791, 1122, and 2003 to more closely reflect current practice and to more closely match IPv6 so that the field's value is defined only when a datagram is actually fragmented. It also discusses the impact of these changes on how datagrams are used. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6864.
Copyright Notice Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction ....................................................3 2. Conventions Used in This Document ...............................3 3. The IPv4 ID Field ...............................................4 3.1. Uses of the IPv4 ID Field ..................................4 3.2. Background on IPv4 ID Reassembly Issues ....................5 4. Updates to the IPv4 ID Specification ............................6 4.1. IPv4 ID Used Only for Fragmentation ........................7 4.2. Encouraging Safe IPv4 ID Use ...............................8 4.3. IPv4 ID Requirements That Persist ..........................8 5. Impact of Proposed Changes ......................................9 5.1. Impact on Legacy Internet Devices ..........................9 5.2. Impact on Datagram Generation .............................10 5.3. Impact on Middleboxes .....................................11 5.3.1. Rewriting Middleboxes ..............................11 5.3.2. Filtering Middleboxes ..............................12 5.4. Impact on Header Compression ..............................12 5.5. Impact of Network Reordering and Loss .....................13 5.5.1. Atomic Datagrams Experiencing Reordering or Loss ...13 5.5.2. Non-atomic Datagrams Experiencing Reordering or Loss .................................14 6. Updates to Existing Standards ..................................14 6.1. Updates to RFC 791 ........................................14 6.2. Updates to RFC 1122 .......................................15 6.3. Updates to RFC 2003 .......................................16 7. Security Considerations ........................................16 8. References .....................................................17 8.1. Normative References ......................................17 8.2. Informative References ....................................17 9. Acknowledgments ................................................19
RFC791] [RFC1122]. As currently specified, all datagrams between a source and destination of a given protocol must have unique IPv4 ID values over a period of this MDL, which is typically interpreted as two minutes and is related to the recommended reassembly timeout [RFC1122]. This uniqueness is currently specified as for all datagrams, regardless of fragmentation settings. Uniqueness of the IPv4 ID is commonly violated by high-speed devices; if strictly enforced, it would limit the speed of a single protocol between two IP endpoints to 6.4 Mbps for typical MTUs of 1500 bytes (assuming a 2-minute MDL, using the analysis presented in [RFC4963]). It is common for a single connection to operate far in excess of these rates, which strongly indicates that the uniqueness of the IPv4 ID as specified is already moot. Further, some sources have been generating non-varying IPv4 IDs for many years (e.g., cellphones), which resulted in support for such in RObust Header Compression (ROHC) [RFC5225]. This document updates the specification of the IPv4 ID field to more closely reflect current practice and to include considerations taken into account during the specification of the similar field in IPv6. RFC2119]. In this document, the characters ">>" preceding one or more indented lines indicate a requirement using the key words listed above. This convention aids reviewers in quickly identifying or finding this document's explicit requirements.
RFC791]. o In IPv6, fragments are indicated in an extension header that includes an ID, Fragment Offset, and an M (more fragments) flag similar to their counterparts in IPv4 [RFC2460]. IPv6 fragmentation differs from IPv4 fragmentation in a few important ways. IPv6 fragmentation occurs only at the source, so a DF bit is not needed to prevent downstream devices from initiating fragmentation (i.e., IPv6 always acts as if DF=1). The IPv6 fragment header is present only when a datagram has been fragmented, or when the source has received a "packet too big" ICMPv6 error message indicating that the path cannot support the required minimum 1280-byte IPv6 MTU and is thus subject to translation [RFC2460] [RFC4443]. The latter case is relevant only for IPv6 datagrams sent to IPv4 destinations to support subsequent fragmentation after translation to IPv4. With the exception of these two cases, the ID field is not present for non-fragmented datagrams; thus, it is meaningful only for datagrams that are already fragmented or datagrams intended to be fragmented as part of IPv4 translation. Finally, the IPv6 ID field is 32 bits and required unique per source/destination address pair for IPv6, whereas for IPv4 it is only 16 bits and required unique per source address/destination address/protocol tuple. This document focuses on the IPv4 ID field issues, because in IPv6 the field is larger and present only in fragments. RFC791]. Within a given source address, destination address, and protocol, fragments of an original datagram are matched based on their IPv4 ID. This requires that IDs be unique within the source address/destination address/protocol tuple when fragmentation is possible (e.g., DF=0) or when it has already occurred (e.g., frag_offset>0 or MF=1).
Other uses have been envisioned for the IPv4 ID field. The field has been proposed as a way to detect and remove duplicate datagrams, e.g., at congested routers (noted in Section 18.104.22.168 of [RFC1122]) or in network accelerators. It has similarly been proposed for use at end hosts to reduce the impact of duplication on higher-layer protocols (e.g., additional processing in TCP or the need for application-layer duplicate suppression in UDP). This is discussed further in Section 5.1. The IPv4 ID field is used in some diagnostic tools to correlate datagrams measured at various locations along a network path. This is already insufficient in IPv6 because unfragmented datagrams lack an ID, so these tools are already being updated to avoid such reliance on the ID field. This is also discussed further in Section 5.1. The ID clearly needs to be unique (within the MDL, within the source address/destination address/protocol tuple) to support fragmentation and reassembly, but not all datagrams are fragmented or allow fragmentation. This document deprecates non-fragmentation uses, allowing the ID to be repeated (within the MDL, within the source address/destination address/protocol tuple) in those cases. RFC4963]. Readers are encouraged to consult RFC 4963 for a more detailed discussion of these issues. With the maximum IPv4 datagram size of 64 KB, a 16-bit ID field that does not repeat within 120 seconds means that the aggregate of all TCP connections of a given protocol between two IP endpoints is limited to roughly 286 Mbps; at a more typical MTU of 1500 bytes, this speed drops to 6.4 Mbps [RFC791] [RFC1122] [RFC4963]. This limit currently applies for all IPv4 datagrams within a single protocol (i.e., the IPv4 protocol field) between two IP addresses, regardless of whether fragmentation is enabled or inhibited and whether or not a datagram is fragmented. IPv6, even at typical MTUs, is capable of 18.7 Tbps with fragmentation between two IP endpoints as an aggregate across all protocols, due to the larger 32-bit ID field (and the fact that the IPv6 next-header field, the equivalent of the IPv4 protocol field, is not considered in differentiating fragments). When fragmentation is not used, the field is absent, and in that case IPv6 speeds are not limited by the ID field uniqueness.
Note also that 120 seconds is only an estimate on the MDL. It is related to the reassembly timeout as a lower bound and the TCP Maximum Segment Lifetime as an upper bound (both as noted in [RFC1122]). Network delays are incurred in other ways, e.g., satellite links, which can add seconds of delay even though the Time to Live (TTL) is not decremented by a corresponding amount. There is thus no enforcement mechanism to ensure that datagrams older than 120 seconds are discarded. Wireless Internet devices are frequently connected at speeds over 54 Mbps, and wired links of 1 Gbps have been the default for several years. Although many end-to-end transport paths are congestion limited, these devices easily achieve 100+ Mbps application-layer throughput over LANs (e.g., disk-to-disk file transfer rates), and numerous throughput demonstrations with Commercial-Off-The-Shelf (COTS) systems over wide-area paths have exhibited these speeds for over a decade. This strongly suggests that IPv4 ID uniqueness has been moot for a long time.
The test for non-atomic datagrams is the logical negative of the test for atomic datagrams; thus, all possibilities are considered. RFC 1122 suggests that the IPv4 ID field has other uses, including datagram de-duplication, such uses are already not interoperable with known implementations of sources that do not vary their ID. This document thus defines this field's value only for fragmentation and reassembly: >> The IPv4 ID field MUST NOT be used for purposes other than fragmentation and reassembly. Datagram de-duplication can still be accomplished using hash-based duplicate detection for cases where the ID field is absent (IPv6 unfragmented datagrams), which can also be applied to IPv4 atomic datagrams without utilizing the ID field [RFC6621]. In atomic datagrams, the IPv4 ID field has no meaning; thus, it can be set to an arbitrary value, i.e., the requirement for non-repeating IDs within the source address/destination address/protocol tuple is no longer required for atomic datagrams: >> Originating sources MAY set the IPv4 ID field of atomic datagrams to any value. Second, all network nodes, whether at intermediate routers, destination hosts, or other devices (e.g., NATs and other address- sharing mechanisms, firewalls, tunnel egresses), cannot rely on the field of atomic datagrams: >> All devices that examine IPv4 headers MUST ignore the IPv4 ID field of atomic datagrams. The IPv4 ID field is thus meaningful only for non-atomic datagrams -- either those datagrams that have already been fragmented or those for which fragmentation remains permitted. Atomic datagrams are detected by their DF, MF, and fragmentation offset fields as explained in Section 4, because such a test is completely backward compatible; thus, this document does not reserve any IPv4 ID values, including 0, as distinguished. Deprecating the use of the IPv4 ID field for non-reassembly uses should have little -- if any -- impact. IPv4 IDs are already frequently repeated, e.g., over even moderately fast connections and from some sources that do not vary the ID at all, and no adverse impact has been observed. Duplicate suppression was suggested
[RFC1122] and has been implemented in some protocol accelerators, but no impacts of IPv4 ID reuse have been noted to date. Routers are not required to issue ICMPs on any particular timescale, and so IPv4 ID repetition should not have been used for validation purposes; this scenario has not been observed. Besides, repetition already occurs and would have been noticed [RFC1812]. ICMP relaying at tunnel ingresses is specified to use soft state rather than a datagram cache; for similar reasons, if the latter is used, this should have been noticed [RFC2003]. These and other legacy issues are discussed further in Section 5.1. RFC 1122, if TCP retransmits a segment, it may be possible to reuse the IPv4 ID (see Section 6.2). This can make it difficult for a source to avoid IPv4 ID repetition for received fragments. RFC 1122 concludes that this behavior "is not useful"; this document formalizes that conclusion as follows: >> The IPv4 ID of non-atomic datagrams MUST NOT be reused when sending a copy of an earlier non-atomic datagram. RFC 1122 also suggests that fragments can overlap. Such overlap can occur if successive retransmissions are fragmented in different ways but with the same reassembly IPv4 ID. This overlap is noted as the result of reusing IPv4 IDs when retransmitting datagrams, which this document deprecates. However, it is also the result of in-network datagram duplication, which can still occur. As a result, this document does not change the need for receivers to support overlapping fragments. RFC791] for non-atomic datagrams, that is: >> Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID values within one MDL for a given source address/destination address/protocol tuple. Such sources include originating hosts, tunnel ingresses, and NATs (including other address-sharing mechanisms) (see Section 5.3).
This document does not relax the requirement that all network devices honor the DF bit, that is: >> IPv4 datagrams whose DF=1 MUST NOT be fragmented. >> IPv4 datagram transit devices MUST NOT clear the DF bit. Specifically, DF=1 prevents fragmenting atomic datagrams. DF=1 also prevents further fragmenting received fragments. In-network fragmentation is permitted only when DF=0; this document does not change that requirement. RFC5225]. This is discussed further in Section 5.4. Generation of IPv4 datagrams with constant (zero) IDs is also described as part of the IP/ICMP translation standard [RFC6145]. Many current devices support fragmentation that ignores the IPv4 Don't Fragment (DF) bit. Such devices already transit traffic from sources that reuse the ID. If fragments of different datagrams reusing the same ID (within the source address/destination address/protocol tuple) arrive at the destination interleaved, fragmentation would fail and traffic would be dropped. Either such interleaving is uncommon or traffic from such devices is not widely traversing these DF-ignoring devices, because significant occurrence of reassembly errors has not been reported. DF-ignoring devices do not comply with existing standards, and it is not feasible to update the standards to allow them as compliant.
The ID field has been envisioned for use in duplicate detection, as discussed in Section 4.1. Although this document now allows IPv4 ID reuse for atomic datagrams, such reuse is already common (as noted above). Protocol accelerators are known to implement IPv4 duplicate detection, but such devices are also known to violate other Internet standards to achieve higher end-to-end performance. These devices would already exhibit erroneous drops for this current traffic, and this has not been reported. There are other potential uses of the ID field, such as for diagnostic purposes. Such uses already need to accommodate atomic datagrams with reused ID fields. There are no reports of such uses having problems with current datagrams that reuse IDs. Thus, as a result of previous requirements, this document recommends that IPv4 duplicate detection and diagnostic mechanisms apply IPv6-compatible methods, i.e., methods that do not rely on the ID field (e.g., as suggested in [RFC6621]). This is a consequence of using the ID field only for reassembly, as well as the known hazard of existing devices already reusing the ID field. RFC2671]. Because there is no strict definition of the MDL, reassembly hazards exist regardless of the IPv4 ID reuse interval or the reassembly timeout. As a result: >> Higher-layer protocols SHOULD verify the integrity of IPv4 datagrams, e.g., using a checksum or hash that can detect reassembly errors (the UDP and TCP checksums are weak in this regard, but better than nothing). Additional integrity checks can be employed using tunnels, as supported by the Subnetwork Encapsulation and Adaptation Layer (SEAL) [RFC5320], IPsec [RFC4301], or the Stream Control Transmission Protocol (SCTP) [RFC4960]. Such checks can avoid the reassembly
hazards that can occur when using UDP and TCP checksums [RFC4963] or when using partial checksums as in UDP-Lite [RFC3828]. Because such integrity checks can avoid the impact of reassembly errors: >> Sources of non-atomic IPv4 datagrams using strong integrity checks MAY reuse the ID within intervals that are smaller than typical MDL values. Note, however, that such frequent reuse can still result in corrupted reassembly and poor throughput, although it would not propagate reassembly errors to higher-layer protocols. RFC3022]. This is also true for other ASMs, including IPv4 Residual Deployment (4rd) [De11], IVI [RFC6219], and others in the "A+P" (address plus port) family [Bo11]. It is equally true for any other datagram-rewriting mechanism. As a result, they are subject to all the requirements of any datagram source, as has been noted. NATs/ASMs/rewriters present a particularly challenging situation for fragmentation. Because they overwrite portions of the reassembly tuple in both directions, they can destroy tuple uniqueness and result in a reassembly hazard. Whenever IPv4 source address, destination address, or protocol fields are modified, a NAT/ASM/rewriter needs to ensure that the ID field is generated appropriately, rather than simply copied from the incoming datagram.
Specifically: >> Address-sharing or rewriting devices MUST ensure that the IPv4 ID field of datagrams whose addresses or protocols are translated comply with these requirements as if the datagram were sourced by that device. This compliance means that the IPv4 ID field of non-atomic datagrams translated at a NAT/ASM/rewriter needs to obey the uniqueness requirements of any IPv4 datagram source. Unfortunately, translated fragments already violate that requirement, as they repeat an IPv4 ID within the MDL for a given source address/destination address/protocol tuple. Such problems with transmitting fragments through NATs/ASMs/rewriters are already known; translation is typically based on the transport port number, which is present in only the first fragment anyway [RFC3022]. This document underscores the point that not only is reassembly (and possibly subsequent fragmentation) required for translation, it can be used to avoid issues with IPv4 ID uniqueness. Note that NATs/ASMs already need to exercise special care when emitting datagrams on their public side, because merging datagrams from many sources onto a single outgoing source address can result in IPv4 ID collisions. This situation precedes this document and is not affected by it. It is exacerbated in large-scale, so-called "carrier grade" NATs [Pe11]. Tunnel ingresses act as sources for the outermost header, but tunnels act as routers for the inner headers (i.e., the datagram as arriving at the tunnel ingress). Ingresses can always fragment as originating sources of the outer header, because they control the uniqueness of that IPv4 ID field and the value of DF on the outer header independent of those values on the inner (arriving datagram) header. Section 5.1. RFC1144] [RFC2508] [RFC3545] [RFC5225]. Such algorithms currently assume that the IPv4 ID is preserved end-to-end. Some algorithms already allow
the assumption that the ID does not change (e.g., ROHC [RFC5225]), where others include non-changing IDs via zero deltas (e.g., Enhanced Compressed RTP (ECRTP) [RFC3545]). When compression assumes a changing ID as a default, having a non-changing ID can make compression less efficient. Such non-changing IDs have been described in various RFCs (e.g., footnote 21 of [RFC1144] and cRTP [RFC2508]). When compression can assume a non-changing IPv4 ID -- as with ROHC and ECRTP -- efficiency can be increased.
reported, it would be more productive to address non-compliant devices (that ignore DF=1), because it is impractical to define Internet specifications to tolerate devices that ignore those specifications. This is why this document emphasizes the need to honor DF=1, as well as that datagram transit devices need to retain the DF bit as received (i.e., rather than clear it). RFC 791 states that: The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. It later states that: Thus, the sender must choose the Identifier to be unique for this source, destination pair and protocol for the time the datagram (or any fragment of it) could be alive in the internet.
It seems then that a sending protocol module needs to keep a table of Identifiers, one entry for each destination it has communicated with in the last maximum datagram lifetime for the internet. However, since the Identifier field allows 65,536 different values, some host may be able to simply use unique identifiers independent of destination. It is appropriate for some higher level protocols to choose the identifier. For example, TCP protocol modules may retransmit an identical TCP segment, and the probability for correct reception would be enhanced if the retransmission carried the same identifier as the original transmission since fragments of either datagram could be used to construct a correct TCP segment. This document changes RFC 791 as follows: o IPv4 ID uniqueness applies to only non-atomic datagrams. o Retransmitted non-atomic IPv4 datagrams are no longer permitted to reuse the ID value. RFC 1122 states in Section 22.214.171.124 ("Identification: RFC 791 Section 3.2") that: When sending an identical copy of an earlier datagram, a host MAY optionally retain the same Identification field in the copy. DISCUSSION: Some Internet protocol experts have maintained that when a host sends an identical copy of an earlier datagram, the new copy should contain the same Identification value as the original. There are two suggested advantages: (1) if the datagrams are fragmented and some of the fragments are lost, the receiver may be able to reconstruct a complete datagram from fragments of the original and the copies; (2) a congested gateway might use the IP Identification field (and Fragment Offset) to discard duplicate datagrams from the queue.
This document changes RFC 1122 as follows: o The IPv4 ID field is no longer permitted to be used for duplicate detection. This applies to both atomic and non-atomic datagrams. o Retransmitted non-atomic IPv4 datagrams are no longer permitted to reuse the ID value. RFC2003], but only in the same way as for any other IPv4 datagram source. Specifically, RFC 2003 states the following, where  refers to RFC 791: Identification, Flags, Fragment Offset These three fields are set as specified in ... This document changes RFC 2003 as follows: o The IPv4 ID field is set as permitted by RFC 6864. RFC4302]. The IPv4 ID also now adds much less to the entropy of the header of a datagram. Such entropy might be used as input to cryptographic algorithms or pseudorandom generators, although IDs have never been assured sufficient entropy for such purposes. The IPv4 ID had previously been unique (for a given source/address pair, and protocol field) within one MDL, although this requirement was not enforced and clearly is typically ignored. The IPv4 ID of atomic datagrams is not required unique and so contributes no entropy to the header. The deprecation of the IPv4 ID field's uniqueness for atomic datagrams can defeat the ability to count devices behind a NAT/ASM/rewriter [Be02]. This is not intended as a security feature, however.
[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers", RFC 1812, June 1995. [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [Be02] Bellovin, S., "A Technique for Counting NATted Hosts", Internet Measurement Conference, Proceedings of the 2nd ACM SIGCOMM Workshop on Internet Measurement, November 2002. [Bo11] Boucadair, M., Touch, J., Levis, P., and R. Penno, "Analysis of Solution Candidates to Reveal a Host Identifier in Shared Address Deployments", Work in Progress, September 2011. [De11] Despres, R., Ed., Matsushima, S., Murakami, T., and O. Troan, "IPv4 Residual Deployment across IPv6-Service networks (4rd) ISP-NAT's made optional", Work in Progress, March 2011. [Pe11] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida, "Common requirements for Carrier Grade NATs (CGNs)", Work in Progress, December 2012. [RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial Links", RFC 1144, February 1990. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for Low-Speed Serial Links", RFC 2508, February 1999. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3545] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and P. Ruddy, "Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering", RFC 3545, July 2003. [RFC3828] Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., Ed., and G. Fairhurst, Ed., "The Lightweight User Datagram Protocol (UDP-Lite)", RFC 3828, July 2004. [RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005. [RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December 2005. [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 4443, March 2006. [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, September 2007. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, July 2007. [RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and UDP-Lite", RFC 5225, April 2008. [RFC5320] Templin, F., Ed., "The Subnetwork Encapsulation and Adaptation Layer (SEAL)", RFC 5320, February 2010. [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation Algorithm", RFC 6145, April 2011.