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2 Internet Area WG R. Bonica
3 Internet-Draft Juniper Networks
4 Intended status: Best Current Practice F. Baker
5 Expires: December 16, 2019 Unaffiliated
6 G. Huston
7 APNIC
8 R. Hinden
9 Check Point Software
10 O. Troan
11 Cisco
12 F. Gont
13 SI6 Networks
14 June 14, 2019
16 IP Fragmentation Considered Fragile
17 draft-ietf-intarea-frag-fragile-11
19 Abstract
21 This document describes IP fragmentation and explains how it
22 introduces fragility to Internet communication.
24 This document also proposes alternatives to IP fragmentation and
25 provides recommendations for developers and network operators.
27 Status of This Memo
29 This Internet-Draft is submitted in full conformance with the
30 provisions of BCP 78 and BCP 79.
32 Internet-Drafts are working documents of the Internet Engineering
33 Task Force (IETF). Note that other groups may also distribute
34 working documents as Internet-Drafts. The list of current Internet-
35 Drafts is at https://datatracker.ietf.org/drafts/current/.
37 Internet-Drafts are draft documents valid for a maximum of six months
38 and may be updated, replaced, or obsoleted by other documents at any
39 time. It is inappropriate to use Internet-Drafts as reference
40 material or to cite them other than as "work in progress."
42 This Internet-Draft will expire on December 16, 2019.
44 Copyright Notice
46 Copyright (c) 2019 IETF Trust and the persons identified as the
47 document authors. All rights reserved.
49 This document is subject to BCP 78 and the IETF Trust's Legal
50 Provisions Relating to IETF Documents
51 (https://trustee.ietf.org/license-info) in effect on the date of
52 publication of this document. Please review these documents
53 carefully, as they describe your rights and restrictions with respect
54 to this document. Code Components extracted from this document must
55 include Simplified BSD License text as described in Section 4.e of
56 the Trust Legal Provisions and are provided without warranty as
57 described in the Simplified BSD License.
59 Table of Contents
61 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
62 1.1. IP-in-IP Tunnels . . . . . . . . . . . . . . . . . . . . 3
63 2. IP Fragmentation . . . . . . . . . . . . . . . . . . . . . . 3
64 2.1. Links, Paths, MTU and PMTU . . . . . . . . . . . . . . . 3
65 2.2. Fragmentation Procedures . . . . . . . . . . . . . . . . 5
66 2.3. Upper-Layer Reliance on IP Fragmentation . . . . . . . . 6
67 3. Requirements Language . . . . . . . . . . . . . . . . . . . . 7
68 4. Increased Fragility . . . . . . . . . . . . . . . . . . . . . 7
69 4.1. Policy-Based Routing . . . . . . . . . . . . . . . . . . 7
70 4.2. Network Address Translation (NAT) . . . . . . . . . . . . 8
71 4.3. Stateless Firewalls . . . . . . . . . . . . . . . . . . . 9
72 4.4. Equal Cost Multipath, Link Aggregate Groups and Stateless
73 Load-Balancers . . . . . . . . . . . . . . . . . . . . . 9
74 4.5. IPv4 Reassembly Errors at High Data Rates . . . . . . . . 10
75 4.6. Security Vulnerabilities . . . . . . . . . . . . . . . . 11
76 4.7. PMTU Blackholing Due to ICMP Loss . . . . . . . . . . . . 12
77 4.7.1. Transient Loss . . . . . . . . . . . . . . . . . . . 12
78 4.7.2. Incorrect Implementation of Security Policy . . . . . 13
79 4.7.3. Persistent Loss Caused By Anycast . . . . . . . . . . 13
80 4.7.4. Persistent Loss Caused By Unidirectional Routing . . 14
81 4.8. Blackholing Due To Filtering or Loss . . . . . . . . . . 14
82 5. Alternatives to IP Fragmentation . . . . . . . . . . . . . . 15
83 5.1. Transport Layer Solutions . . . . . . . . . . . . . . . . 15
84 5.2. Application Layer Solutions . . . . . . . . . . . . . . . 16
85 6. Applications That Rely on IPv6 Fragmentation . . . . . . . . 17
86 6.1. Domain Name Service (DNS) . . . . . . . . . . . . . . . . 17
87 6.2. Open Shortest Path First (OSPF) . . . . . . . . . . . . . 18
88 6.3. Packet-in-Packet Encapsulations . . . . . . . . . . . . . 18
89 6.4. UDP Applications Enhancing Performance . . . . . . . . . 18
90 7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 19
91 7.1. For Application and Protocol Developers . . . . . . . . . 19
92 7.2. For System Developers . . . . . . . . . . . . . . . . . . 19
93 7.3. For Middle Box Developers . . . . . . . . . . . . . . . . 19
94 7.4. For ECMP, LAG and Load-Balancer Developers And Operators 20
95 7.5. For Network Operators . . . . . . . . . . . . . . . . . . 20
96 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
97 9. Security Considerations . . . . . . . . . . . . . . . . . . . 21
98 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
99 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
100 11.1. Normative References . . . . . . . . . . . . . . . . . . 21
101 11.2. Informative References . . . . . . . . . . . . . . . . . 23
102 Appendix A. Contributors' Address . . . . . . . . . . . . . . . 26
103 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
105 1. Introduction
107 Operational experience [Kent] [Huston] [RFC7872] reveals that IP
108 fragmentation introduces fragility to Internet communication. This
109 document describes IP fragmentation and explains the fragility it
110 introduces. It also proposes alternatives to IP fragmentation and
111 provides recommendations for developers and network operators.
113 While this document identifies issues associated with IP
114 fragmentation, it does not recommend deprecation. Legacy protocols
115 that depend upon IP fragmentation SHOULD be updated to break that
116 dependency. However, some applications and environments (see
117 Section 6) require IP fragmentation. In these cases, the protocol
118 will continue to rely on IP fragmentation, but the designer should to
119 be aware that fragmented packets may result in blackholes; a design
120 should include appropriate safeguards (e.g. PLPMTU).
122 Rather than deprecating IP Fragmentation, this document recommends
123 that upper-layer protocols address the problem of fragmentation at
124 their layer, reducing their reliance on IP fragmentation to the
125 greatest degree possible.
127 1.1. IP-in-IP Tunnels
129 This document acknowledges that in some cases, packets must be
130 fragmented within IP-in-IP tunnels [I-D.ietf-intarea-tunnels].
131 Therefore, this document makes no additional recommendations
132 regarding IP-in-IP tunnels.
134 2. IP Fragmentation
136 2.1. Links, Paths, MTU and PMTU
138 An Internet path connects a source node to a destination node. A
139 path can contain links and routers. If a path contains more than one
140 link, the links are connected in series and a router connects each
141 link to the next.
143 Internet paths are dynamic. Assume that the path from one node to
144 another contains a set of links and routers. If a link fails, the
145 path can also change so that it includes a different set of links and
146 routers.
148 Each link is constrained by the number of bytes that it can convey in
149 a single IP packet. This constraint is called the link Maximum
150 Transmission Unit (MTU). IPv4 [RFC0791] requires every link to
151 support a specified MTU (see NOTE 1). IPv6 [RFC8200] requires every
152 link to support an MTU of 1280 bytes or greater. These are called
153 the IPv4 and IPv6 minimum link MTU's.
155 Likewise, each Internet path is constrained by the number of bytes
156 that it can convey in a single IP packet. This constraint is called
157 the Path MTU (PMTU). For any given path, the PMTU is equal to the
158 smallest of its link MTU's. Because Internet paths are dynamic, PMTU
159 is also dynamic.
161 For reasons described below, source nodes estimate the PMTU between
162 themselves and destination nodes. A source node can produce
163 extremely conservative PMTU estimates in which:
165 o The estimate for each IPv4 path is equal to the IPv4 minimum link
166 MTU.
168 o The estimate for each IPv6 path is equal to the IPv6 minimum link
169 MTU.
171 While these conservative estimates are guaranteed to be less than or
172 equal to the actual PMTU, they are likely to be much less than the
173 actual PMTU. This may adversely affect upper-layer protocol
174 performance.
176 By executing Path MTU Discovery (PMTUD) [RFC1191] [RFC8201]
177 procedures, a source node can maintain a less conservative estimate
178 of the PMTU between itself and a destination node. In PMTUD, the
179 source node produces an initial PMTU estimate. This initial estimate
180 is equal to the MTU of the first link along the path to the
181 destination node. It can be greater than the actual PMTU.
183 Having produced an initial PMTU estimate, the source node sends non-
184 fragmentable IP packets to the destination node (see NOTE 2). If one
185 of these packets is larger than the actual PMTU, a downstream router
186 will not be able to forward the packet through the next link along
187 the path. Therefore, the downstream router drops the packet and
188 sends an Internet Control Message Protocol (ICMP) [RFC0792] [RFC4443]
189 Packet Too Big (PTB) message to the source node (see NOTE 3). The
190 ICMP PTB message indicates the MTU of the link through which the
191 packet could not be forwarded. The source node uses this information
192 to refine its PMTU estimate.
194 PMTUD produces a running estimate of the PMTU between a source node
195 and a destination node. Because PMTU is dynamic, the PMTU estimate
196 can be larger than the actual PMTU. In order to detect PMTU
197 increases, PMTUD occasionally resets the PMTU estimate to its initial
198 value and repeats the procedure described above.
200 Ideally, PMTUD operates as described above. However, in some
201 scenarios, PMTUD fails. For example:
203 o PMTUD relies on the network's ability to deliver ICMP PTB messages
204 to the source node. If the network cannot deliver ICMP PTB
205 messages to the source node, PMTUD fails.
207 o PMTUD is susceptible to attack because ICMP messages are easily
208 forged [RFC5927] and not authenticated by the receiver. Such
209 attacks can cause PMTUD to produce unnecessarily conservative PMTU
210 estimates.
212 NOTE 1: In IPv4, every host must be capable of receiving a packet
213 whose length is equal to 576 bytes. However, the IPv4 minimum link
214 MTU is not 576. Section 3.2 of RFC 791 explicitly states that the
215 IPv4 minimum link MTU is 68 bytes. But for practical purposes, many
216 network operators consider the IPv4 minimum link MTU to be 576 bytes,
217 to minimize the requirement for fragmentation en route. So, for the
218 purposes of this document, we assume that the IPv4 minimum path MTU
219 is 576 bytes.
221 NOTE 2: A non-fragmentable packet can be fragmented at its source.
222 However, it cannot be fragmented by a downstream node. An IPv4
223 packet whose DF-bit is set to zero is fragmentable. An IPv4 packet
224 whose DF-bit is set to one is non-fragmentable. All IPv6 packets are
225 also non-fragmentable.
227 NOTE 3:: The ICMP PTB message has two instantiations. In ICMPv4
228 [RFC0792], the ICMP PTB message is a Destination Unreachable message
229 with Code equal to (4) fragmentation needed and DF set. This message
230 was augmented by [RFC1191] to indicate the MTU of the link through
231 which the packet could not be forwarded. In ICMPv6 [RFC4443], the
232 ICMP PTB message is a Packet Too Big Message with Code equal to (0).
233 This message also indicates the MTU of the link through which the
234 packet could not be forwarded.
236 2.2. Fragmentation Procedures
238 When an upper-layer protocol submits data to the underlying IP
239 module, and the resulting IP packet's length is greater than the
240 PMTU, the packet is divided into fragments. Each fragment includes
241 an IP header and a portion of the original packet.
243 [RFC0791] describes IPv4 fragmentation procedures. An IPv4 packet
244 whose DF-bit is set to one can be fragmented by the source node, but
245 cannot be fragmented by a downstream router. An IPv4 packet whose
246 DF-bit is set to zero can be fragmented by the source node or by a
247 downstream router. When an IPv4 packet is fragmented, all IP options
248 appear in the first fragment, but only options whose "copy" bit is
249 set to one appear in subsequent fragments.
251 [RFC8200] describes IPv6 fragmentation procedures. An IPv6 packet
252 can be fragmented at the source node only. When an IPv6 packet is
253 fragmented, all extension headers appear in the first fragment, but
254 only per-fragment headers appear in subsequent fragments. Per-
255 fragment headers include the following:
257 o The IPv6 header.
259 o The Hop-by-hop Options header (if present)
261 o The Destination Options header (if present and if it precedes a
262 Routing header)
264 o The Routing Header (if present)
266 o The Fragment Header
268 In both IPv4 and IPv6, the upper-layer header appears in the first
269 fragment only. It does not appear in subsequent fragments.
271 2.3. Upper-Layer Reliance on IP Fragmentation
273 Upper-layer protocols can operate in the following modes:
275 o Do not rely on IP fragmentation.
277 o Rely on IP fragmentation by the source node only.
279 o Rely on IP fragmentation by any node.
281 Upper-layer protocols running over IPv4 can operate in all of the
282 above-mentioned modes. Upper-layer protocols running over IPv6 can
283 operate in the first and second modes only.
285 Upper-layer protocols that operate in the first two modes (above)
286 require access to the PMTU estimate. In order to fulfil this
287 requirement, they can:
289 o Estimate the PMTU to be equal to the IPv4 or IPv6 minimum link
290 MTU.
292 o Access the estimate that PMTUD produced.
294 o Execute PMTUD procedures themselves.
296 o Execute Packetization Layer PMTUD (PLPMTUD) [RFC4821]
297 [I-D.ietf-tsvwg-datagram-plpmtud] procedures.
299 According to PLPMTUD procedures, the upper-layer protocol maintains a
300 running PMTU estimate. It does so by sending probe packets of
301 various sizes to its upper-layer peer and receiving acknowledgements.
302 This strategy differs from PMTUD in that it relies on acknowledgement
303 of received messages, as opposed to ICMP PTB messages concerning
304 dropped messages. Therefore, PLPMTUD does not rely on the network's
305 ability to deliver ICMP PTB messages to the source.
307 3. Requirements Language
309 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
310 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
311 "OPTIONAL" in this document are to be interpreted as described in BCP
312 14 [RFC2119] [RFC8174] when, and only when, they appear in all
313 capitals, as shown here.
315 4. Increased Fragility
317 This section explains how IP fragmentation introduces fragility to
318 Internet communication.
320 4.1. Policy-Based Routing
322 IP Fragmentation causes problems for routers that implement policy-
323 based routing.
325 When a router receives a packet, it identifies the next-hop on route
326 to the packet's destination and forwards the packet to that next-hop.
327 In order to identify the next-hop, the router interrogates a local
328 data structure called the Forwarding Information Base (FIB).
330 Normally, the FIB contains destination-based entries that map a
331 destination prefix to a next-hop. Policy-based routing allows
332 destination-based and policy-based entries to coexist in the same
333 FIB. A policy-based FIB entry maps multiple fields, drawn from
334 either the IP or transport-layer header, to a next-hop.
336 +-------+--------------+-----------------+------------+-------------+
337 | Entry | Type | Dest. Prefix | Next Hdr / | Next-Hop |
338 | | | | Dest. Port | |
339 +-------+--------------+-----------------+------------+-------------+
340 | | | | | |
341 | 1 | Destination- | 2001:db8::1/128 | Any / Any | 2001:db8::2 |
342 | | based | | | |
343 | | | | | |
344 | 2 | Policy- | 2001:db8::1/128 | TCP / 80 | 2001:db8::3 |
345 | | based | | | |
346 +-------+--------------+-----------------+------------+-------------+
348 Table 1: Policy-Based Routing FIB
350 Assume that a router maintains the FIB in Table 1. The first FIB
351 entry is destination-based. It maps the a destination prefix
352 (2001:db8::1/128) to a next-hop (2001:db8::2). The second FIB entry
353 is policy-based. It maps the same destination prefix
354 (2001:db8::1/128) and a destination port ( TCP / 80 ) to a different
355 next-hop (2001:db8::3). The second entry is more specific than the
356 first.
358 When the router receives the first fragment of a packet that is
359 destined for TCP port 80 on 2001:db8::1, it interrogates the FIB.
360 Both FIB entries satisfy the query. The router selects the second
361 FIB entry because it is more specific and forwards the packet to
362 2001:db8::3.
364 When the router receives the second fragment of the packet, it
365 interrogates the FIB again. This time, only the first FIB entry
366 satisfies the query, because the second fragment contains no
367 indication that the packet is destined for TCP port 80. Therefore,
368 the router selects the first FIB entry and forwards the packet to
369 2001:db8::2.
371 Policy-based routing is also known as filter-based-forwarding.
373 4.2. Network Address Translation (NAT)
375 IP fragmentation causes problems for Network Address Translation
376 (NAT) devices. When a NAT device detects a new, outbound flow, it
377 maps that flow's source port and IP address to another source port
378 and IP address. Having created that mapping, the NAT device
379 translates:
381 o The Source IP Address and Source Port on each outbound packet.
383 o The Destination IP Address and Destination Port on each inbound
384 packet.
386 A+P [RFC6346] and Carrier Grade NAT (CGN) [RFC6888] are two common
387 NAT strategies. In both approaches the NAT device must virtually
388 reassemble fragmented packets in order to translate and forward each
389 fragment. (See NOTE 1.)
391 Virtual reassembly in the network is problematic, because it is
392 computationally expensive and because it is prone to attacks
393 (Section 4.6).
395 NOTE 1: Virtual reassembly is a procedure in which a device
396 reassembles a packet, forwards its fragments, and discards the
397 reassembled copy. In A+P and CGN, virtual reassembly is required in
398 order to correctly translate fragment addresses.
400 4.3. Stateless Firewalls
402 As discussed in more detail in Section 4.6, IP fragmentation causes
403 problems for stateless firewalls whose rules include TCP and UDP
404 ports. Because port information is not available in the trailing
405 fragments the firewall is limited to the following options:
407 o Accept all trailing fragments, possibly admitting certain classes
408 of attack.
410 o Block all trailing fragments, possibly blocking legitimate
411 traffic.
413 Neither option is attractive.
415 4.4. Equal Cost Multipath, Link Aggregate Groups and Stateless Load-
416 Balancers
418 IP fragmentation causes problems for Equal Cost Multipath (ECMP),
419 Link Aggregate Groups (LAG) and other stateless load-balancing
420 technologies. In order to assign a packet or packet fragment to a
421 link, an intermediate node executes a hash (i.e., load-balancing)
422 algorithm. The following paragraphs describe a commonly deployed
423 hash algorithm.
425 If the packet or packet fragment contains a transport-layer header,
426 the algorithm accepts the following 5-tuple as input:
428 o IP Source Address.
430 o IP Destination Address.
432 o IPv4 Protocol or IPv6 Next Header.
434 o transport-layer source port.
436 o transport-layer destination port.
438 If the packet or packet fragment does not contain a transport-layer
439 header, the algorithm accepts only the following 3-tuple as input:
441 o IP Source Address.
443 o IP Destination Address.
445 o IPv4 Protocol or IPv6 Next Header.
447 Therefore, non-fragmented packets belonging to a flow can be assigned
448 to one link while fragmented packets belonging to the same flow can
449 be divided between that link and another. This can cause suboptimal
450 load-balancing.
452 [RFC6438] offers a partial solution to this problem for IPv6 devices
453 only. According to [RFC6438]:
455 "At intermediate routers that perform load distribution, the hash
456 algorithm used to determine the outgoing component-link in an ECMP
457 and/or LAG toward the next hop MUST minimally include the 3-tuple
458 {dest addr, source addr, flow label} and MAY also include the
459 remaining components of the 5-tuple."
461 If the algorithm includes only the 3-tuple {dest addr, source addr,
462 flow label}, it will assign all fragments belonging to a packet to
463 the same link. (See [RFC6437] and [RFC7098]).
465 In order to avoid the problem described above, implementations SHOULD
466 implement the recommendations provided in Section 7.4 of this
467 document.
469 4.5. IPv4 Reassembly Errors at High Data Rates
471 IPv4 fragmentation is not sufficiently robust for use under some
472 conditions in today's Internet. At high data rates, the 16-bit IP
473 identification field is not large enough to prevent frequent
474 incorrectly assembled IP fragments, and the TCP and UDP checksums are
475 insufficient to prevent the resulting corrupted datagrams from being
476 delivered to higher protocol layers. [RFC4963] describes some easily
477 reproduced experiments demonstrating the problem, and discusses some
478 of the operational implications of these observations.
480 These reassembly issues are not easily reproducible in IPv6 because
481 the IPv6 identification field is 32 bits long.
483 4.6. Security Vulnerabilities
485 Security researchers have documented several attacks that exploit IP
486 fragmentation. The following are examples:
488 o Overlapping fragment attacks [RFC1858][RFC3128][RFC5722]
490 o Resource exhaustion attacks (such as the Rose Attack,
491 http://www.digital.net/~gandalf/Rose_Frag_Attack_Explained.htm)
493 o Attacks based on predictable fragment identification values
494 [RFC7739]
496 o Evasion of Network Intrusion Detection Systems (NIDS) [Ptacek1998]
498 In the overlapping fragment attack, an attacker constructs a series
499 of packet fragments. The first fragment contains an IP header, a
500 transport-layer header, and some transport-layer payload. This
501 fragment complies with local security policy and is allowed to pass
502 through a stateless firewall. A second fragment, having a non-zero
503 offset, overlaps with the first fragment. The second fragment also
504 passes through the stateless firewall. When the packet is
505 reassembled, the transport layer header from the first fragment is
506 overwritten by data from the second fragment. The reassembled packet
507 does not comply with local security policy. Had it traversed the
508 firewall in one piece, the firewall would have rejected it.
510 A stateless firewall cannot protect against the overlapping fragment
511 attack. However, destination nodes can protect against the
512 overlapping fragment attack by implementing the procedures described
513 in RFC 1858, RFC 3128 and RFC 8200. These reassembly procedures
514 detect the overlap and discard the packet.
516 The fragment reassembly algorithm is a stateful procedure in an
517 otherwise stateless protocol. Therefore, it can be exploited by
518 resource exhaustion attacks. An attacker can construct a series of
519 fragmented packets, with one fragment missing from each packet so
520 that the reassembly is impossible. Thus, this attack causes resource
521 exhaustion on the destination node, possibly denying reassembly
522 services to other flows. This type of attack can be mitigated by
523 flushing fragment reassembly buffers when necessary, at the expense
524 of possibly dropping legitimate fragments.
526 Each IP fragment contains an "Identification" field that destination
527 nodes use to reassemble fragmented packets. Many implementations set
528 the Identification field to a predictable value, thus making it easy
529 for an attacker to forge malicious IP fragments that would cause the
530 reassembly procedure for legitimate packets to fail.
532 NIDS aims at identifying malicious activity by analyzing network
533 traffic. Ambiguity in the possible result of the fragment reassembly
534 process may allow an attacker to evade these systems. Many of these
535 systems try to mitigate some of these evasion techniques (e.g. By
536 computing all possible outcomes of the fragment reassembly process,
537 at the expense of increased processing requirements).
539 4.7. PMTU Blackholing Due to ICMP Loss
541 As mentioned in Section 2.3, upper-layer protocols can be configured
542 to rely on PMTUD. Because PMTUD relies upon the network to deliver
543 ICMP PTB messages, those protocols also rely on the networks to
544 deliver ICMP PTB messages.
546 According to [RFC4890], ICMP PTB messages must not be filtered.
547 However, ICMP PTB delivery is not reliable. It is subject to both
548 transient and persistent loss.
550 Transient loss of ICMP PTB messages can cause transient PMTU black
551 holes. When the conditions contributing to transient loss abate, the
552 network regains its ability to deliver ICMP PTB messages and
553 connectivity between the source and destination nodes is restored.
554 Section 4.7.1 of this document describes conditions that lead to
555 transient loss of ICMP PTB messages.
557 Persistent loss of ICMP PTB messages can cause persistent black
558 holes. Section 4.7.2, Section 4.7.3, and Section 4.7.4 of this
559 document describe conditions that lead to persistent loss of ICMP PTB
560 messages.
562 The problem described in this section is specific to PMTUD. It does
563 not occur when the upper-layer protocol obtains its PMTU estimate
564 from PLPMTUD or from any other source.
566 4.7.1. Transient Loss
568 The following factors can contribute to transient loss of ICMP PTB
569 messages:
571 o Network congestion.
573 o Packet corruption.
575 o Transient routing loops.
577 o ICMP rate limiting.
579 The effect of rate limiting may be severe, as RFC 4443 recommends
580 strict rate limiting of IPv6 traffic.
582 4.7.2. Incorrect Implementation of Security Policy
584 Incorrect implementation of security policy can cause persistent loss
585 of ICMP PTB messages.
587 Assume that a Customer Premise Equipment (CPE) router implements the
588 following zone-based security policy:
590 o Allow any traffic to flow from the inside zone to the outside
591 zone.
593 o Do not allow any traffic to flow from the outside zone to the
594 inside zone unless it is part of an existing flow (i.e., it was
595 elicited by an outbound packet).
597 When a correct implementation of the above-mentioned security policy
598 receives an ICMP PTB message, it examines the ICMP PTB payload in
599 order to determine whether the original packet (i.e., the packet that
600 elicited the ICMP PTB message) belonged to an existing flow. If the
601 original packet belonged to an existing flow, the implementation
602 allows the ICMP PTB to flow from the outside zone to the inside zone.
603 If not, the implementation discards the ICMP PTB message.
605 When a incorrect implementation of the above-mentioned security
606 policy receives an ICMP PTB message, it discards the packet because
607 its source address is not associated with an existing flow.
609 The security policy described above is implemented incorrectly on
610 many consumer CPE routers.
612 4.7.3. Persistent Loss Caused By Anycast
614 Anycast can cause persistent loss of ICMP PTB messages. Consider the
615 example below:
617 A DNS client sends a request to an anycast address. The network
618 routes that DNS request to the nearest instance of that anycast
619 address (i.e., a DNS Server). The DNS server generates a response
620 and sends it back to the DNS client. While the response does not
621 exceed the DNS server's PMTU estimate, it does exceed the actual
622 PMTU.
624 A downstream router drops the packet and sends an ICMP PTB message
625 the packet's source (i.e., the anycast address). The network routes
626 the ICMP PTB message to the anycast instance closest to the
627 downstream router. That anycast instance may not be the DNS server
628 that originated the DNS response. It may be another DNS server with
629 the same anycast address. The DNS server that originated the
630 response may never receive the ICMP PTB message and may never update
631 its PMTU estimate.
633 4.7.4. Persistent Loss Caused By Unidirectional Routing
635 Unidirectional routing can cause persistent loss of ICMP PTB
636 messages. Consider the example below:
638 A source node sends a packet to a destination node. All intermediate
639 nodes maintain a route to the destination node, but do not maintain a
640 route to the source node. In this case, when an intermediate node
641 encounters an MTU issue, it cannot send an ICMP PTB message to the
642 source node.
644 4.8. Blackholing Due To Filtering or Loss
646 In RFC 7872, researchers sampled Internet paths to determine whether
647 they would convey packets that contain IPv6 extension headers.
648 Sampled paths terminated at popular Internet sites (e.g., popular
649 web, mail and DNS servers).
651 The study revealed that at least 28% of the sampled paths did not
652 convey packets containing the IPv6 Fragment extension header. In
653 most cases, fragments were dropped in the destination autonomous
654 system. In other cases, the fragments were dropped in transit
655 autonomous systems.
657 Another recent study [Huston] confirmed this finding. It reported
658 that 37% of sampled endpoints used IPv6-capable DNS resolvers that
659 were incapable of receiving a fragmented IPv6 response.
661 It is difficult to determine why network operators drop fragments.
662 Possible causes follow:
664 o Hardware inability to process fragmented packets.
666 o Failure to change vendor defaults.
668 o Unintentional misconfiguration.
670 o Intentional configuration (e.g., network operators consciously
671 chooses to drop IPv6 fragments in order to address the issues
672 raised in Section 4.1 through Section 4.7, above.)
674 5. Alternatives to IP Fragmentation
676 5.1. Transport Layer Solutions
678 The Transport Control Protocol (TCP) [RFC0793]) can be operated in a
679 mode that does not require IP fragmentation.
681 Applications submit a stream of data to TCP. TCP divides that stream
682 of data into segments, with no segment exceeding the TCP Maximum
683 Segment Size (MSS). Each segment is encapsulated in a TCP header and
684 submitted to the underlying IP module. The underlying IP module
685 prepends an IP header and forwards the resulting packet.
687 If the TCP MSS is sufficiently small, the underlying IP module never
688 produces a packet whose length is greater than the actual PMTU.
689 Therefore, IP fragmentation is not required.
691 TCP offers the following mechanisms for MSS management:
693 o Manual configuration
695 o PMTUD
697 o PLPMTUD
699 Manual configuration is always applicable. If the MSS is configured
700 to a sufficiently low value, the IP layer will never produce a packet
701 whose length is greater than the protocol minimum link MTU. However,
702 manual configuration prevents TCP from taking advantage of larger
703 link MTU's.
705 Upper-layer protocols can implement PMTUD in order to discover and
706 take advantage of larger path MTUs. However, as mentioned in
707 Section 2.1, PMTUD relies upon the network to deliver ICMP PTB
708 messages. Therefore, PMTUD can only provide an estimate of the PMTU
709 in environments where the risk of ICMP PTB loss is acceptable (e.g.,
710 known to not be filtered).
712 By contrast, PLPMTUD does not rely upon the network's ability to
713 deliver ICMP PTB messages. It utilises probe messages sent as TCP
714 segments to determine whether the probed PMTU can be successfully
715 used across the network path. In PLPMTUD, probing is separated from
716 congestion control, so that loss of a TCP probe segment does not
717 cause a reduction of the congestion control window. [RFC4821]
718 defines PLPMTUD procedures for TCP.
720 While TCP will never knowingly cause the underlying IP module to emit
721 a packet that is larger than the PMTU estimate, it can cause the
722 underlying IP module to emit a packet that is larger than the actual
723 PMTU. For example, if routing changes and as a result the PMTU
724 becomes smaller, TCP will not know until the ICMP PTB message
725 arrives. If this occurs, the packet is dropped, the PMTU estimate is
726 updated, the segment is divided into smaller segments and each
727 smaller segment is submitted to the underlying IP module.
729 The Datagram Congestion Control Protocol (DCCP) [RFC4340] and the
730 Stream Control Transport Protocol (SCTP) [RFC4960] also can be
731 operated in a mode that does not require IP fragmentation. They both
732 accept data from an application and divide that data into segments,
733 with no segment exceeding a maximum size. DCCP offers manual
734 configuration, PMTUD, and PLPMTUD as mechanisms for managing that
735 maximum size. Datagram protocols can also implement PLPMTUD to
736 estimate the PMTU via[I-D.ietf-tsvwg-datagram-plpmtud]. This
737 proposes procedures for performing PLPMTUD with UDP, UDP-Options,
738 SCTP, QUIC and other datagram protocols.
740 Currently, User Data Protocol (UDP) [RFC0768] lacks a fragmentation
741 mechanism of its own and relies on IP fragmentation. However,
742 [I-D.ietf-tsvwg-udp-options] proposes a fragmentation mechanism for
743 UDP.
745 5.2. Application Layer Solutions
747 [RFC8085] recognizes that IP fragmentation reduces the reliability of
748 Internet communication. It also recognizes that UDP lacks a
749 fragmentation mechanism of its own and relies on IP fragmentation.
750 Therefore, [RFC8085] offers the following advice regarding
751 applications the run over the UDP.
753 "An application SHOULD NOT send UDP datagrams that result in IP
754 packets that exceed the Maximum Transmission Unit (MTU) along the
755 path to the destination. Consequently, an application SHOULD either
756 use the path MTU information provided by the IP layer or implement
757 Path MTU Discovery (PMTUD) itself to determine whether the path to a
758 destination will support its desired message size without
759 fragmentation."
761 RFC 8085 continues:
763 "Applications that do not follow the recommendation to do PMTU/
764 PLPMTUD discovery SHOULD still avoid sending UDP datagrams that would
765 result in IP packets that exceed the path MTU. Because the actual
766 path MTU is unknown, such applications SHOULD fall back to sending
767 messages that are shorter than the default effective MTU for sending
768 (EMTU_S in [RFC1122]). For IPv4, EMTU_S is the smaller of 576 bytes
769 and the first-hop MTU. For IPv6, EMTU_S is 1280 bytes [RFC8200].
770 The effective PMTU for a directly connected destination (with no
771 routers on the path) is the configured interface MTU, which could be
772 less than the maximum link payload size. Transmission of minimum-
773 sized UDP datagrams is inefficient over paths that support a larger
774 PMTU, which is a second reason to implement PMTU discovery."
776 RFC 8085 assumes that for IPv4, an EMTU_S of 576 is sufficiently
777 small is sufficiently small to be supported by most current Internet
778 paths, even though the IPv4 minimum link MTU is 68 bytes.
780 This advice applies equally to any application that runs directly
781 over IP.
783 6. Applications That Rely on IPv6 Fragmentation
785 The following applications rely on IPv6 fragmentation:
787 o DNS [RFC1035]
789 o OSPFv3 [RFC2328][RFC5340]
791 o Packet-in-packet encapsulations
793 Each of these applications relies on IPv6 fragmentation to a varying
794 degree. In some cases, that reliance is essential, and cannot be
795 broken without fundamentally changing the protocol. In other cases,
796 that reliance is incidental, and most implementations already take
797 appropriate steps to avoid fragmentation.
799 This list is not comprehensive, and other protocols that rely on IP
800 fragmentation may exist. They are not specifically considered in the
801 context of this document.
803 6.1. Domain Name Service (DNS)
805 DNS relies on UDP for efficiency, and the consequence is the use of
806 IP fragmentation for large responses, as permitted by the DNS EDNS(0)
807 options in the query. It is possible to mitigate the issue of
808 fragmentation-based packet loss by having queries use smaller EDNS(0)
809 UDP buffer sizes, or by having the DNS server limit the size of its
810 UDP responses to some self-imposed maximum packet size that may be
811 less than the preferred EDNS(0) UDP Buffer Size. In both cases,
812 large responses are truncated in the DNS, signalling to the client to
813 re-query using TCP to obtain the complete response. However, the
814 operational issue of the partial level of support for DNS over TCP,
815 particularly in the case where IPv6 transport is being used, becomes
816 a limiting factor of the efficacy of this approach [Damas].
818 Larger DNS responses can normally be avoided by aggressively pruning
819 the Additional section of DNS responses. One scenario where such
820 pruning is ineffective is in the use of DNSSEC, where large key sizes
821 act to increase the response size to certain DNS queries. There is
822 no effective response to this situation within the DNS other than
823 using smaller cryptographic keys and adoption of DNSSEC
824 administrative practices that attempt to keep DNS response as short
825 as possible.
827 6.2. Open Shortest Path First (OSPF)
829 OSPF implementations can emit messages large enough to cause
830 fragmentation. However, in order to optimize performance, most OSPF
831 implementations restrict their maximum message size to a value that
832 will not cause fragmentation.
834 6.3. Packet-in-Packet Encapsulations
836 In this document, packet-in-packet encapsulations include IP-in-IP
837 [RFC2003], Generic Routing Encapsulation (GRE) [RFC2784], GRE-in-UDP
838 [RFC8086] and Generic Packet Tunneling in IPv6 [RFC2473]. [RFC4459]
839 describes fragmentation issues associated with all of the above-
840 mentioned encapsulations.
842 The fragmentation strategy described for GRE in [RFC7588] has been
843 deployed for all of the above-mentioned encapsulations. This
844 strategy does not rely on IP fragmentation except in one corner case.
845 (see Section 3.3.2.2 of RFC 7588 and Section 7.1 of RFC 2473).
846 Section 3.3 of [RFC7676] further describes this corner case.
848 See [I-D.ietf-intarea-tunnels] for further discussion.
850 6.4. UDP Applications Enhancing Performance
852 Some UDP applications rely on IP fragmentation to achieve acceptable
853 levels of performance. These applications use UDP datagram sizes
854 that are larger than the path MTU so that more data can be conveyed
855 between the application and the kernel in a single system call.
857 To pick one example, the Licklider Transmission Protocol (LTP),
858 [RFC5326]which is in current use on the International Space Station
859 (ISS), uses UDP datagram sizes larger than the path MTU to achieve
860 acceptable levels of performance even though this invokes IP
861 fragmentation. More generally, SNMP and video applications may
862 transmit an application-layer quantum of data, depending on the
863 network layer to fragment and reassemble as needed.
865 7. Recommendations
867 7.1. For Application and Protocol Developers
869 Developers SHOULD NOT develop new protocols or applications that rely
870 on IP fragmentation. When a new protocol or application is deployed
871 in an environment that does not fully support IP fragmentation, it
872 SHOULD operate correctly, either in its default configuration or in a
873 specified alternative configuration.
875 Developers MAY develop new protocols or applications that rely on IP
876 fragmentation if the protocol or application is to be run only in
877 environments where IP fragmentation is known to be supported.
879 Legacy protocols that depend upon IP fragmentation SHOULD be updated
880 to break that dependency. However, in some cases, there may be no
881 viable alternative to IP fragmentation (e.g., IPSEC tunnel mode, IP-
882 in-IP encapsulation). In these cases, the protocol will continue to
883 rely on IP fragmentation but should only be used in environments
884 where IP fragmentation is known to be supported.
886 Protocols may be able to avoid IP fragmentation by using a
887 sufficiently small MTU (e.g. The protocol minimum link MTU),
888 disabling IP fragmentation, and ensuring that the transport protocol
889 in use adapts its segment size to the MTU. Other protocols may
890 deploy a sufficiently reliable PMTU discovery mechanism
891 (e.g.,PLMPTUD).
893 UDP applications SHOULD abide by the recommendations stated in
894 Section 3.2 of [RFC8085].
896 7.2. For System Developers
898 Software libraries SHOULD include provision for PLPMTUD for each
899 supported transport protocol.
901 7.3. For Middle Box Developers
903 Middle boxes should process IP fragments in a manner that is
904 consistent with [RFC0791] and [RFC8200]. In many cases, middle boxes
905 must maintain state in order to achieve this goal.
907 Price and performance considerations frequently motivate network
908 operators to deploy stateless middle boxes. These stateless middle
909 boxes may perform sub-optimally, process IP fragments in a manner
910 that is not compliant with RFC 791 or RFC 8200, or even discard IP
911 fragments completely. Such behaviors are NOT RECOMMENDED. If a
912 middleboxes implements non-standard behavior with respect to IP
913 fragmentation, then that behavior MUST be clearly documented.
915 7.4. For ECMP, LAG and Load-Balancer Developers And Operators
917 In their default configuration, when the IPv6 Flow Label is not equal
918 to zero, IPv6 devices that implement Equal-Cost Multipath (ECMP)
919 Routing as described in OSPF and other
920 routing protocols, Link Aggregation Grouping
921 (LAG), or other load-balancing technologies SHOULD accept only
922 the following fields as input to their hash algorithm:
924 o IP Source Address.
926 o IP Destination Address.
928 o Flow Label.
930 Operators SHOULD deploy these devices in their default
931 configuration.
933 These recommendations are similar to those presented in [RFC6438] and
934 [RFC7098]. They differ in that they specify a default configuration.
936 7.5. For Network Operators
938 Operators MUST ensure proper PMTUD operation in their network,
939 including making sure the network generates PTB packets when dropping
940 packets too large compared to outgoing interface MTU. However,
941 implementations MAY rate limit ICMP messages as per [RFC1812] and
942 [RFC4443].
944 As per RFC 4890, network operators MUST NOT filter ICMPv6 PTB
945 messages unless they are known to be forged or otherwise
946 illegitimate. As stated in Section 4.7, filtering ICMPv6 PTB packets
947 causes PMTUD to fail. Many upper-layer protocols rely on PMTUD.
949 As per RFC 8200, network operators MUST NOT deploy IPv6 links whose
950 MTU is less than 1280 bytes.
952 Network operators SHOULD NOT filter IP fragments if they are known to
953 have originated at a domain name server or be destined for a domain
954 name server. This is because domain name services are critical to
955 operation of the Internet.
957 8. IANA Considerations
959 This document makes no request of IANA.
961 9. Security Considerations
963 This document mitigates some of the security considerations
964 associated with IP fragmentation by discouraging its use. It does
965 not introduce any new security vulnerabilities, because it does not
966 introduce any new alternatives to IP fragmentation. Instead, it
967 recommends well-understood alternatives.
969 10. Acknowledgements
971 Thanks to Mikael Abrahamsson, Brian Carpenter, Silambu Chelvan,
972 Lorenzo Colitti, Gorry Fairhurst, Mike Heard, Tom Herbert, Tatuya
973 Jinmei, Jen Linkova, Paolo Lucente, Manoj Nayak, Eric Nygren, Fred
974 Templin and Joe Touch for their comments.
976 11. References
978 11.1. Normative References
980 [I-D.ietf-tsvwg-datagram-plpmtud]
981 Fairhurst, G., Jones, T., Tuexen, M., Ruengeler, I., and
982 T. Voelker, "Packetization Layer Path MTU Discovery for
983 Datagram Transports", draft-ietf-tsvwg-datagram-plpmtud-08
984 (work in progress), June 2019.
986 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
987 DOI 10.17487/RFC0768, August 1980,
988 .
990 [RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
991 DOI 10.17487/RFC0791, September 1981,
992 .
994 [RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
995 RFC 792, DOI 10.17487/RFC0792, September 1981,
996 .
998 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
999 RFC 793, DOI 10.17487/RFC0793, September 1981,
1000 .
1002 [RFC1035] Mockapetris, P., "Domain names - implementation and
1003 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
1004 November 1987, .
1006 [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
1007 DOI 10.17487/RFC1191, November 1990,
1008 .
1010 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
1011 Requirement Levels", BCP 14, RFC 2119,
1012 DOI 10.17487/RFC2119, March 1997,
1013 .
1015 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
1016 Control Message Protocol (ICMPv6) for the Internet
1017 Protocol Version 6 (IPv6) Specification", STD 89,
1018 RFC 4443, DOI 10.17487/RFC4443, March 2006,
1019 .
1021 [RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
1022 Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
1023 .
1025 [RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
1026 "IPv6 Flow Label Specification", RFC 6437,
1027 DOI 10.17487/RFC6437, November 2011,
1028 .
1030 [RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
1031 for Equal Cost Multipath Routing and Link Aggregation in
1032 Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
1033 .
1035 [RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
1036 Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
1037 March 2017, .
1039 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
1040 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
1041 May 2017, .
1043 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
1044 (IPv6) Specification", STD 86, RFC 8200,
1045 DOI 10.17487/RFC8200, July 2017,
1046 .
1048 [RFC8201] McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
1049 "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
1050 DOI 10.17487/RFC8201, July 2017,
1051 .
1053 11.2. Informative References
1055 [Damas] Damas, J. and G. Huston, "Measuring ATR", April 2018,
1056 .
1058 [Huston] Huston, G., "IPv6, Large UDP Packets and the DNS
1059 (http://www.potaroo.net/ispcol/2017-08/xtn-hdrs.html)",
1060 August 2017.
1062 [I-D.ietf-intarea-tunnels]
1063 Touch, J. and M. Townsley, "IP Tunnels in the Internet
1064 Architecture", draft-ietf-intarea-tunnels-09 (work in
1065 progress), July 2018.
1067 [I-D.ietf-tsvwg-udp-options]
1068 Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
1069 udp-options-07 (work in progress), March 2019.
1071 [Kent] Kent, C. and J. Mogul, ""Fragmentation Considered
1072 Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in
1073 Computer Communications Technology, DOI
1074 10.1145/55483.55524", August 1987,
1075 .
1078 [Ptacek1998]
1079 Ptacek, T. and T. Newsham, "Insertion, Evasion and Denial
1080 of Service: Eluding Network Intrusion Detection", 1998,
1081 .
1083 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
1084 Communication Layers", STD 3, RFC 1122,
1085 DOI 10.17487/RFC1122, October 1989,
1086 .
1088 [RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
1089 RFC 1812, DOI 10.17487/RFC1812, June 1995,
1090 .
1092 [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
1093 Considerations for IP Fragment Filtering", RFC 1858,
1094 DOI 10.17487/RFC1858, October 1995,
1095 .
1097 [RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
1098 DOI 10.17487/RFC2003, October 1996,
1099 .
1101 [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
1102 DOI 10.17487/RFC2328, April 1998,
1103 .
1105 [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
1106 IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
1107 December 1998, .
1109 [RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
1110 Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
1111 DOI 10.17487/RFC2784, March 2000,
1112 .
1114 [RFC3128] Miller, I., "Protection Against a Variant of the Tiny
1115 Fragment Attack (RFC 1858)", RFC 3128,
1116 DOI 10.17487/RFC3128, June 2001,
1117 .
1119 [RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
1120 Congestion Control Protocol (DCCP)", RFC 4340,
1121 DOI 10.17487/RFC4340, March 2006,
1122 .
1124 [RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
1125 Network Tunneling", RFC 4459, DOI 10.17487/RFC4459, April
1126 2006, .
1128 [RFC4890] Davies, E. and J. Mohacsi, "Recommendations for Filtering
1129 ICMPv6 Messages in Firewalls", RFC 4890,
1130 DOI 10.17487/RFC4890, May 2007,
1131 .
1133 [RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
1134 RFC 4960, DOI 10.17487/RFC4960, September 2007,
1135 .
1137 [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
1138 Errors at High Data Rates", RFC 4963,
1139 DOI 10.17487/RFC4963, July 2007,
1140 .
1142 [RFC5326] Ramadas, M., Burleigh, S., and S. Farrell, "Licklider
1143 Transmission Protocol - Specification", RFC 5326,
1144 DOI 10.17487/RFC5326, September 2008,
1145 .
1147 [RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
1148 for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
1149 .
1151 [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
1152 RFC 5722, DOI 10.17487/RFC5722, December 2009,
1153 .
1155 [RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927,
1156 DOI 10.17487/RFC5927, July 2010,
1157 .
1159 [RFC6346] Bush, R., Ed., "The Address plus Port (A+P) Approach to
1160 the IPv4 Address Shortage", RFC 6346,
1161 DOI 10.17487/RFC6346, August 2011,
1162 .
1164 [RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa,
1165 A., and H. Ashida, "Common Requirements for Carrier-Grade
1166 NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888,
1167 April 2013, .
1169 [RFC7098] Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
1170 Flow Label for Load Balancing in Server Farms", RFC 7098,
1171 DOI 10.17487/RFC7098, January 2014,
1172 .
1174 [RFC7588] Bonica, R., Pignataro, C., and J. Touch, "A Widely
1175 Deployed Solution to the Generic Routing Encapsulation
1176 (GRE) Fragmentation Problem", RFC 7588,
1177 DOI 10.17487/RFC7588, July 2015,
1178 .
1180 [RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
1181 for Generic Routing Encapsulation (GRE)", RFC 7676,
1182 DOI 10.17487/RFC7676, October 2015,
1183 .
1185 [RFC7739] Gont, F., "Security Implications of Predictable Fragment
1186 Identification Values", RFC 7739, DOI 10.17487/RFC7739,
1187 February 2016, .
1189 [RFC7872] Gont, F., Linkova, J., Chown, T., and W. Liu,
1190 "Observations on the Dropping of Packets with IPv6
1191 Extension Headers in the Real World", RFC 7872,
1192 DOI 10.17487/RFC7872, June 2016,
1193 .
1195 [RFC8086] Yong, L., Ed., Crabbe, E., Xu, X., and T. Herbert, "GRE-
1196 in-UDP Encapsulation", RFC 8086, DOI 10.17487/RFC8086,
1197 March 2017, .
1199 Appendix A. Contributors' Address
1201 Authors' Addresses
1203 Ron Bonica
1204 Juniper Networks
1205 2251 Corporate Park Drive
1206 Herndon, Virginia 20171
1207 USA
1209 Email: rbonica@juniper.net
1211 Fred Baker
1212 Unaffiliated
1213 Santa Barbara, California 93117
1214 USA
1216 Email: FredBaker.IETF@gmail.com
1218 Geoff Huston
1219 APNIC
1220 6 Cordelia St
1221 Brisbane, 4101 QLD
1222 Australia
1224 Email: gih@apnic.net
1226 Robert M. Hinden
1227 Check Point Software
1228 959 Skyway Road
1229 San Carlos, California 94070
1230 USA
1232 Email: bob.hinden@gmail.com
1233 Ole Troan
1234 Cisco
1235 Philip Pedersens vei 1
1236 N-1366 Lysaker
1237 Norway
1239 Email: ot@cisco.com
1241 Fernando Gont
1242 SI6 Networks
1243 Evaristo Carriego 2644
1244 Haedo, Provincia de Buenos Aires
1245 Argentina
1247 Email: fgont@si6networks.com