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1 | ============ |
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2 | SNMP counter |
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3 | ============ |
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4 | |
5 | This document explains the meaning of SNMP counters. |
6 | |
7 | General IPv4 counters |
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8 | ===================== |
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9 | All layer 4 packets and ICMP packets will change these counters, but |
10 | these counters won't be changed by layer 2 packets (such as STP) or |
11 | ARP packets. |
12 | |
13 | * IpInReceives |
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14 | |
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15 | Defined in `RFC1213 ipInReceives`_ |
16 | |
17 | .. _RFC1213 ipInReceives: https://tools.ietf.org/html/rfc1213#page-26 |
18 | |
19 | The number of packets received by the IP layer. It gets increasing at the |
20 | beginning of ip_rcv function, always be updated together with |
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21 | IpExtInOctets. It will be increased even if the packet is dropped |
22 | later (e.g. due to the IP header is invalid or the checksum is wrong |
23 | and so on). It indicates the number of aggregated segments after |
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24 | GRO/LRO. |
25 | |
26 | * IpInDelivers |
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27 | |
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28 | Defined in `RFC1213 ipInDelivers`_ |
29 | |
30 | .. _RFC1213 ipInDelivers: https://tools.ietf.org/html/rfc1213#page-28 |
31 | |
32 | The number of packets delivers to the upper layer protocols. E.g. TCP, UDP, |
33 | ICMP and so on. If no one listens on a raw socket, only kernel |
34 | supported protocols will be delivered, if someone listens on the raw |
35 | socket, all valid IP packets will be delivered. |
36 | |
37 | * IpOutRequests |
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38 | |
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39 | Defined in `RFC1213 ipOutRequests`_ |
40 | |
41 | .. _RFC1213 ipOutRequests: https://tools.ietf.org/html/rfc1213#page-28 |
42 | |
43 | The number of packets sent via IP layer, for both single cast and |
44 | multicast packets, and would always be updated together with |
45 | IpExtOutOctets. |
46 | |
47 | * IpExtInOctets and IpExtOutOctets |
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48 | |
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49 | They are Linux kernel extensions, no RFC definitions. Please note, |
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50 | RFC1213 indeed defines ifInOctets and ifOutOctets, but they |
51 | are different things. The ifInOctets and ifOutOctets include the MAC |
52 | layer header size but IpExtInOctets and IpExtOutOctets don't, they |
53 | only include the IP layer header and the IP layer data. |
54 | |
55 | * IpExtInNoECTPkts, IpExtInECT1Pkts, IpExtInECT0Pkts, IpExtInCEPkts |
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56 | |
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57 | They indicate the number of four kinds of ECN IP packets, please refer |
58 | `Explicit Congestion Notification`_ for more details. |
59 | |
60 | .. _Explicit Congestion Notification: https://tools.ietf.org/html/rfc3168#page-6 |
61 | |
62 | These 4 counters calculate how many packets received per ECN |
63 | status. They count the real frame number regardless the LRO/GRO. So |
64 | for the same packet, you might find that IpInReceives count 1, but |
65 | IpExtInNoECTPkts counts 2 or more. |
66 | |
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67 | * IpInHdrErrors |
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68 | |
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69 | Defined in `RFC1213 ipInHdrErrors`_. It indicates the packet is |
70 | dropped due to the IP header error. It might happen in both IP input |
71 | and IP forward paths. |
72 | |
73 | .. _RFC1213 ipInHdrErrors: https://tools.ietf.org/html/rfc1213#page-27 |
74 | |
75 | * IpInAddrErrors |
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76 | |
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77 | Defined in `RFC1213 ipInAddrErrors`_. It will be increased in two |
78 | scenarios: (1) The IP address is invalid. (2) The destination IP |
79 | address is not a local address and IP forwarding is not enabled |
80 | |
81 | .. _RFC1213 ipInAddrErrors: https://tools.ietf.org/html/rfc1213#page-27 |
82 | |
83 | * IpExtInNoRoutes |
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84 | |
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85 | This counter means the packet is dropped when the IP stack receives a |
86 | packet and can't find a route for it from the route table. It might |
87 | happen when IP forwarding is enabled and the destination IP address is |
88 | not a local address and there is no route for the destination IP |
89 | address. |
90 | |
91 | * IpInUnknownProtos |
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92 | |
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93 | Defined in `RFC1213 ipInUnknownProtos`_. It will be increased if the |
94 | layer 4 protocol is unsupported by kernel. If an application is using |
95 | raw socket, kernel will always deliver the packet to the raw socket |
96 | and this counter won't be increased. |
97 | |
98 | .. _RFC1213 ipInUnknownProtos: https://tools.ietf.org/html/rfc1213#page-27 |
99 | |
100 | * IpExtInTruncatedPkts |
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101 | |
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102 | For IPv4 packet, it means the actual data size is smaller than the |
103 | "Total Length" field in the IPv4 header. |
104 | |
105 | * IpInDiscards |
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106 | |
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107 | Defined in `RFC1213 ipInDiscards`_. It indicates the packet is dropped |
108 | in the IP receiving path and due to kernel internal reasons (e.g. no |
109 | enough memory). |
110 | |
111 | .. _RFC1213 ipInDiscards: https://tools.ietf.org/html/rfc1213#page-28 |
112 | |
113 | * IpOutDiscards |
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114 | |
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115 | Defined in `RFC1213 ipOutDiscards`_. It indicates the packet is |
116 | dropped in the IP sending path and due to kernel internal reasons. |
117 | |
118 | .. _RFC1213 ipOutDiscards: https://tools.ietf.org/html/rfc1213#page-28 |
119 | |
120 | * IpOutNoRoutes |
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121 | |
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122 | Defined in `RFC1213 ipOutNoRoutes`_. It indicates the packet is |
123 | dropped in the IP sending path and no route is found for it. |
124 | |
125 | .. _RFC1213 ipOutNoRoutes: https://tools.ietf.org/html/rfc1213#page-29 |
126 | |
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127 | ICMP counters |
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128 | ============= |
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129 | * IcmpInMsgs and IcmpOutMsgs |
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130 | |
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131 | Defined by `RFC1213 icmpInMsgs`_ and `RFC1213 icmpOutMsgs`_ |
132 | |
133 | .. _RFC1213 icmpInMsgs: https://tools.ietf.org/html/rfc1213#page-41 |
134 | .. _RFC1213 icmpOutMsgs: https://tools.ietf.org/html/rfc1213#page-43 |
135 | |
136 | As mentioned in the RFC1213, these two counters include errors, they |
137 | would be increased even if the ICMP packet has an invalid type. The |
138 | ICMP output path will check the header of a raw socket, so the |
139 | IcmpOutMsgs would still be updated if the IP header is constructed by |
140 | a userspace program. |
141 | |
142 | * ICMP named types |
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143 | |
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144 | | These counters include most of common ICMP types, they are: |
145 | | IcmpInDestUnreachs: `RFC1213 icmpInDestUnreachs`_ |
146 | | IcmpInTimeExcds: `RFC1213 icmpInTimeExcds`_ |
147 | | IcmpInParmProbs: `RFC1213 icmpInParmProbs`_ |
148 | | IcmpInSrcQuenchs: `RFC1213 icmpInSrcQuenchs`_ |
149 | | IcmpInRedirects: `RFC1213 icmpInRedirects`_ |
150 | | IcmpInEchos: `RFC1213 icmpInEchos`_ |
151 | | IcmpInEchoReps: `RFC1213 icmpInEchoReps`_ |
152 | | IcmpInTimestamps: `RFC1213 icmpInTimestamps`_ |
153 | | IcmpInTimestampReps: `RFC1213 icmpInTimestampReps`_ |
154 | | IcmpInAddrMasks: `RFC1213 icmpInAddrMasks`_ |
155 | | IcmpInAddrMaskReps: `RFC1213 icmpInAddrMaskReps`_ |
156 | | IcmpOutDestUnreachs: `RFC1213 icmpOutDestUnreachs`_ |
157 | | IcmpOutTimeExcds: `RFC1213 icmpOutTimeExcds`_ |
158 | | IcmpOutParmProbs: `RFC1213 icmpOutParmProbs`_ |
159 | | IcmpOutSrcQuenchs: `RFC1213 icmpOutSrcQuenchs`_ |
160 | | IcmpOutRedirects: `RFC1213 icmpOutRedirects`_ |
161 | | IcmpOutEchos: `RFC1213 icmpOutEchos`_ |
162 | | IcmpOutEchoReps: `RFC1213 icmpOutEchoReps`_ |
163 | | IcmpOutTimestamps: `RFC1213 icmpOutTimestamps`_ |
164 | | IcmpOutTimestampReps: `RFC1213 icmpOutTimestampReps`_ |
165 | | IcmpOutAddrMasks: `RFC1213 icmpOutAddrMasks`_ |
166 | | IcmpOutAddrMaskReps: `RFC1213 icmpOutAddrMaskReps`_ |
167 | |
168 | .. _RFC1213 icmpInDestUnreachs: https://tools.ietf.org/html/rfc1213#page-41 |
169 | .. _RFC1213 icmpInTimeExcds: https://tools.ietf.org/html/rfc1213#page-41 |
170 | .. _RFC1213 icmpInParmProbs: https://tools.ietf.org/html/rfc1213#page-42 |
171 | .. _RFC1213 icmpInSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-42 |
172 | .. _RFC1213 icmpInRedirects: https://tools.ietf.org/html/rfc1213#page-42 |
173 | .. _RFC1213 icmpInEchos: https://tools.ietf.org/html/rfc1213#page-42 |
174 | .. _RFC1213 icmpInEchoReps: https://tools.ietf.org/html/rfc1213#page-42 |
175 | .. _RFC1213 icmpInTimestamps: https://tools.ietf.org/html/rfc1213#page-42 |
176 | .. _RFC1213 icmpInTimestampReps: https://tools.ietf.org/html/rfc1213#page-43 |
177 | .. _RFC1213 icmpInAddrMasks: https://tools.ietf.org/html/rfc1213#page-43 |
178 | .. _RFC1213 icmpInAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-43 |
179 | |
180 | .. _RFC1213 icmpOutDestUnreachs: https://tools.ietf.org/html/rfc1213#page-44 |
181 | .. _RFC1213 icmpOutTimeExcds: https://tools.ietf.org/html/rfc1213#page-44 |
182 | .. _RFC1213 icmpOutParmProbs: https://tools.ietf.org/html/rfc1213#page-44 |
183 | .. _RFC1213 icmpOutSrcQuenchs: https://tools.ietf.org/html/rfc1213#page-44 |
184 | .. _RFC1213 icmpOutRedirects: https://tools.ietf.org/html/rfc1213#page-44 |
185 | .. _RFC1213 icmpOutEchos: https://tools.ietf.org/html/rfc1213#page-45 |
186 | .. _RFC1213 icmpOutEchoReps: https://tools.ietf.org/html/rfc1213#page-45 |
187 | .. _RFC1213 icmpOutTimestamps: https://tools.ietf.org/html/rfc1213#page-45 |
188 | .. _RFC1213 icmpOutTimestampReps: https://tools.ietf.org/html/rfc1213#page-45 |
189 | .. _RFC1213 icmpOutAddrMasks: https://tools.ietf.org/html/rfc1213#page-45 |
190 | .. _RFC1213 icmpOutAddrMaskReps: https://tools.ietf.org/html/rfc1213#page-46 |
191 | |
192 | Every ICMP type has two counters: 'In' and 'Out'. E.g., for the ICMP |
193 | Echo packet, they are IcmpInEchos and IcmpOutEchos. Their meanings are |
194 | straightforward. The 'In' counter means kernel receives such a packet |
195 | and the 'Out' counter means kernel sends such a packet. |
196 | |
197 | * ICMP numeric types |
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198 | |
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199 | They are IcmpMsgInType[N] and IcmpMsgOutType[N], the [N] indicates the |
200 | ICMP type number. These counters track all kinds of ICMP packets. The |
201 | ICMP type number definition could be found in the `ICMP parameters`_ |
202 | document. |
203 | |
204 | .. _ICMP parameters: https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml |
205 | |
206 | For example, if the Linux kernel sends an ICMP Echo packet, the |
207 | IcmpMsgOutType8 would increase 1. And if kernel gets an ICMP Echo Reply |
208 | packet, IcmpMsgInType0 would increase 1. |
209 | |
210 | * IcmpInCsumErrors |
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211 | |
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212 | This counter indicates the checksum of the ICMP packet is |
213 | wrong. Kernel verifies the checksum after updating the IcmpInMsgs and |
214 | before updating IcmpMsgInType[N]. If a packet has bad checksum, the |
215 | IcmpInMsgs would be updated but none of IcmpMsgInType[N] would be updated. |
216 | |
217 | * IcmpInErrors and IcmpOutErrors |
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218 | |
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219 | Defined by `RFC1213 icmpInErrors`_ and `RFC1213 icmpOutErrors`_ |
220 | |
221 | .. _RFC1213 icmpInErrors: https://tools.ietf.org/html/rfc1213#page-41 |
222 | .. _RFC1213 icmpOutErrors: https://tools.ietf.org/html/rfc1213#page-43 |
223 | |
224 | When an error occurs in the ICMP packet handler path, these two |
225 | counters would be updated. The receiving packet path use IcmpInErrors |
226 | and the sending packet path use IcmpOutErrors. When IcmpInCsumErrors |
227 | is increased, IcmpInErrors would always be increased too. |
228 | |
229 | relationship of the ICMP counters |
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230 | --------------------------------- |
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231 | The sum of IcmpMsgOutType[N] is always equal to IcmpOutMsgs, as they |
232 | are updated at the same time. The sum of IcmpMsgInType[N] plus |
233 | IcmpInErrors should be equal or larger than IcmpInMsgs. When kernel |
234 | receives an ICMP packet, kernel follows below logic: |
235 | |
236 | 1. increase IcmpInMsgs |
237 | 2. if has any error, update IcmpInErrors and finish the process |
238 | 3. update IcmpMsgOutType[N] |
239 | 4. handle the packet depending on the type, if has any error, update |
240 | IcmpInErrors and finish the process |
241 | |
242 | So if all errors occur in step (2), IcmpInMsgs should be equal to the |
243 | sum of IcmpMsgOutType[N] plus IcmpInErrors. If all errors occur in |
244 | step (4), IcmpInMsgs should be equal to the sum of |
245 | IcmpMsgOutType[N]. If the errors occur in both step (2) and step (4), |
246 | IcmpInMsgs should be less than the sum of IcmpMsgOutType[N] plus |
247 | IcmpInErrors. |
248 | |
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249 | General TCP counters |
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250 | ==================== |
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251 | * TcpInSegs |
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252 | |
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253 | Defined in `RFC1213 tcpInSegs`_ |
254 | |
255 | .. _RFC1213 tcpInSegs: https://tools.ietf.org/html/rfc1213#page-48 |
256 | |
257 | The number of packets received by the TCP layer. As mentioned in |
258 | RFC1213, it includes the packets received in error, such as checksum |
259 | error, invalid TCP header and so on. Only one error won't be included: |
260 | if the layer 2 destination address is not the NIC's layer 2 |
261 | address. It might happen if the packet is a multicast or broadcast |
262 | packet, or the NIC is in promiscuous mode. In these situations, the |
263 | packets would be delivered to the TCP layer, but the TCP layer will discard |
264 | these packets before increasing TcpInSegs. The TcpInSegs counter |
265 | isn't aware of GRO. So if two packets are merged by GRO, the TcpInSegs |
266 | counter would only increase 1. |
267 | |
268 | * TcpOutSegs |
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269 | |
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270 | Defined in `RFC1213 tcpOutSegs`_ |
271 | |
272 | .. _RFC1213 tcpOutSegs: https://tools.ietf.org/html/rfc1213#page-48 |
273 | |
274 | The number of packets sent by the TCP layer. As mentioned in RFC1213, |
275 | it excludes the retransmitted packets. But it includes the SYN, ACK |
276 | and RST packets. Doesn't like TcpInSegs, the TcpOutSegs is aware of |
277 | GSO, so if a packet would be split to 2 by GSO, TcpOutSegs will |
278 | increase 2. |
279 | |
280 | * TcpActiveOpens |
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281 | |
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282 | Defined in `RFC1213 tcpActiveOpens`_ |
283 | |
284 | .. _RFC1213 tcpActiveOpens: https://tools.ietf.org/html/rfc1213#page-47 |
285 | |
286 | It means the TCP layer sends a SYN, and come into the SYN-SENT |
287 | state. Every time TcpActiveOpens increases 1, TcpOutSegs should always |
288 | increase 1. |
289 | |
290 | * TcpPassiveOpens |
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291 | |
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292 | Defined in `RFC1213 tcpPassiveOpens`_ |
293 | |
294 | .. _RFC1213 tcpPassiveOpens: https://tools.ietf.org/html/rfc1213#page-47 |
295 | |
296 | It means the TCP layer receives a SYN, replies a SYN+ACK, come into |
297 | the SYN-RCVD state. |
298 | |
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299 | * TcpExtTCPRcvCoalesce |
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300 | |
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301 | When packets are received by the TCP layer and are not be read by the |
302 | application, the TCP layer will try to merge them. This counter |
303 | indicate how many packets are merged in such situation. If GRO is |
304 | enabled, lots of packets would be merged by GRO, these packets |
305 | wouldn't be counted to TcpExtTCPRcvCoalesce. |
306 | |
307 | * TcpExtTCPAutoCorking |
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308 | |
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309 | When sending packets, the TCP layer will try to merge small packets to |
310 | a bigger one. This counter increase 1 for every packet merged in such |
311 | situation. Please refer to the LWN article for more details: |
312 | https://lwn.net/Articles/576263/ |
313 | |
314 | * TcpExtTCPOrigDataSent |
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315 | |
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316 | This counter is explained by `kernel commit f19c29e3e391`_, I pasted the |
317 | explaination below:: |
318 | |
319 | TCPOrigDataSent: number of outgoing packets with original data (excluding |
320 | retransmission but including data-in-SYN). This counter is different from |
321 | TcpOutSegs because TcpOutSegs also tracks pure ACKs. TCPOrigDataSent is |
322 | more useful to track the TCP retransmission rate. |
323 | |
324 | * TCPSynRetrans |
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325 | |
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326 | This counter is explained by `kernel commit f19c29e3e391`_, I pasted the |
327 | explaination below:: |
328 | |
329 | TCPSynRetrans: number of SYN and SYN/ACK retransmits to break down |
330 | retransmissions into SYN, fast-retransmits, timeout retransmits, etc. |
331 | |
332 | * TCPFastOpenActiveFail |
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333 | |
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334 | This counter is explained by `kernel commit f19c29e3e391`_, I pasted the |
335 | explaination below:: |
336 | |
337 | TCPFastOpenActiveFail: Fast Open attempts (SYN/data) failed because |
338 | the remote does not accept it or the attempts timed out. |
339 | |
340 | .. _kernel commit f19c29e3e391: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f19c29e3e391a66a273e9afebaf01917245148cd |
341 | |
342 | * TcpExtListenOverflows and TcpExtListenDrops |
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343 | |
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344 | When kernel receives a SYN from a client, and if the TCP accept queue |
345 | is full, kernel will drop the SYN and add 1 to TcpExtListenOverflows. |
346 | At the same time kernel will also add 1 to TcpExtListenDrops. When a |
347 | TCP socket is in LISTEN state, and kernel need to drop a packet, |
348 | kernel would always add 1 to TcpExtListenDrops. So increase |
349 | TcpExtListenOverflows would let TcpExtListenDrops increasing at the |
350 | same time, but TcpExtListenDrops would also increase without |
351 | TcpExtListenOverflows increasing, e.g. a memory allocation fail would |
352 | also let TcpExtListenDrops increase. |
353 | |
354 | Note: The above explanation is based on kernel 4.10 or above version, on |
355 | an old kernel, the TCP stack has different behavior when TCP accept |
356 | queue is full. On the old kernel, TCP stack won't drop the SYN, it |
357 | would complete the 3-way handshake. As the accept queue is full, TCP |
358 | stack will keep the socket in the TCP half-open queue. As it is in the |
359 | half open queue, TCP stack will send SYN+ACK on an exponential backoff |
360 | timer, after client replies ACK, TCP stack checks whether the accept |
361 | queue is still full, if it is not full, moves the socket to the accept |
362 | queue, if it is full, keeps the socket in the half-open queue, at next |
363 | time client replies ACK, this socket will get another chance to move |
364 | to the accept queue. |
365 | |
366 | |
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367 | TCP Fast Open |
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368 | ============= |
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369 | * TcpEstabResets |
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370 | |
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371 | Defined in `RFC1213 tcpEstabResets`_. |
372 | |
373 | .. _RFC1213 tcpEstabResets: https://tools.ietf.org/html/rfc1213#page-48 |
374 | |
375 | * TcpAttemptFails |
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376 | |
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377 | Defined in `RFC1213 tcpAttemptFails`_. |
378 | |
379 | .. _RFC1213 tcpAttemptFails: https://tools.ietf.org/html/rfc1213#page-48 |
380 | |
381 | * TcpOutRsts |
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382 | |
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383 | Defined in `RFC1213 tcpOutRsts`_. The RFC says this counter indicates |
384 | the 'segments sent containing the RST flag', but in linux kernel, this |
385 | couner indicates the segments kerenl tried to send. The sending |
386 | process might be failed due to some errors (e.g. memory alloc failed). |
387 | |
388 | .. _RFC1213 tcpOutRsts: https://tools.ietf.org/html/rfc1213#page-52 |
389 | |
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390 | * TcpExtTCPSpuriousRtxHostQueues |
391 | |
392 | When the TCP stack wants to retransmit a packet, and finds that packet |
393 | is not lost in the network, but the packet is not sent yet, the TCP |
394 | stack would give up the retransmission and update this counter. It |
395 | might happen if a packet stays too long time in a qdisc or driver |
396 | queue. |
397 | |
398 | * TcpEstabResets |
399 | |
400 | The socket receives a RST packet in Establish or CloseWait state. |
401 | |
402 | * TcpExtTCPKeepAlive |
403 | |
404 | This counter indicates many keepalive packets were sent. The keepalive |
405 | won't be enabled by default. A userspace program could enable it by |
406 | setting the SO_KEEPALIVE socket option. |
407 | |
408 | * TcpExtTCPSpuriousRTOs |
409 | |
410 | The spurious retransmission timeout detected by the `F-RTO`_ |
411 | algorithm. |
412 | |
413 | .. _F-RTO: https://tools.ietf.org/html/rfc5682 |
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414 | |
415 | TCP Fast Path |
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416 | ============= |
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417 | When kernel receives a TCP packet, it has two paths to handler the |
418 | packet, one is fast path, another is slow path. The comment in kernel |
419 | code provides a good explanation of them, I pasted them below:: |
420 | |
421 | It is split into a fast path and a slow path. The fast path is |
422 | disabled when: |
423 | |
424 | - A zero window was announced from us |
425 | - zero window probing |
426 | is only handled properly on the slow path. |
427 | - Out of order segments arrived. |
428 | - Urgent data is expected. |
429 | - There is no buffer space left |
430 | - Unexpected TCP flags/window values/header lengths are received |
431 | (detected by checking the TCP header against pred_flags) |
432 | - Data is sent in both directions. The fast path only supports pure senders |
433 | or pure receivers (this means either the sequence number or the ack |
434 | value must stay constant) |
435 | - Unexpected TCP option. |
436 | |
437 | Kernel will try to use fast path unless any of the above conditions |
438 | are satisfied. If the packets are out of order, kernel will handle |
439 | them in slow path, which means the performance might be not very |
440 | good. Kernel would also come into slow path if the "Delayed ack" is |
441 | used, because when using "Delayed ack", the data is sent in both |
442 | directions. When the TCP window scale option is not used, kernel will |
443 | try to enable fast path immediately when the connection comes into the |
444 | established state, but if the TCP window scale option is used, kernel |
445 | will disable the fast path at first, and try to enable it after kernel |
446 | receives packets. |
447 | |
448 | * TcpExtTCPPureAcks and TcpExtTCPHPAcks |
ae5220c6 |
449 | |
80cc4950 |
450 | If a packet set ACK flag and has no data, it is a pure ACK packet, if |
451 | kernel handles it in the fast path, TcpExtTCPHPAcks will increase 1, |
452 | if kernel handles it in the slow path, TcpExtTCPPureAcks will |
453 | increase 1. |
454 | |
455 | * TcpExtTCPHPHits |
ae5220c6 |
456 | |
80cc4950 |
457 | If a TCP packet has data (which means it is not a pure ACK packet), |
458 | and this packet is handled in the fast path, TcpExtTCPHPHits will |
459 | increase 1. |
460 | |
461 | |
462 | TCP abort |
ae5220c6 |
463 | ========= |
80cc4950 |
464 | * TcpExtTCPAbortOnData |
ae5220c6 |
465 | |
80cc4950 |
466 | It means TCP layer has data in flight, but need to close the |
467 | connection. So TCP layer sends a RST to the other side, indicate the |
468 | connection is not closed very graceful. An easy way to increase this |
469 | counter is using the SO_LINGER option. Please refer to the SO_LINGER |
470 | section of the `socket man page`_: |
471 | |
472 | .. _socket man page: http://man7.org/linux/man-pages/man7/socket.7.html |
473 | |
474 | By default, when an application closes a connection, the close function |
475 | will return immediately and kernel will try to send the in-flight data |
476 | async. If you use the SO_LINGER option, set l_onoff to 1, and l_linger |
477 | to a positive number, the close function won't return immediately, but |
478 | wait for the in-flight data are acked by the other side, the max wait |
479 | time is l_linger seconds. If set l_onoff to 1 and set l_linger to 0, |
480 | when the application closes a connection, kernel will send a RST |
481 | immediately and increase the TcpExtTCPAbortOnData counter. |
482 | |
483 | * TcpExtTCPAbortOnClose |
ae5220c6 |
484 | |
80cc4950 |
485 | This counter means the application has unread data in the TCP layer when |
486 | the application wants to close the TCP connection. In such a situation, |
487 | kernel will send a RST to the other side of the TCP connection. |
488 | |
489 | * TcpExtTCPAbortOnMemory |
ae5220c6 |
490 | |
80cc4950 |
491 | When an application closes a TCP connection, kernel still need to track |
492 | the connection, let it complete the TCP disconnect process. E.g. an |
493 | app calls the close method of a socket, kernel sends fin to the other |
494 | side of the connection, then the app has no relationship with the |
495 | socket any more, but kernel need to keep the socket, this socket |
496 | becomes an orphan socket, kernel waits for the reply of the other side, |
497 | and would come to the TIME_WAIT state finally. When kernel has no |
498 | enough memory to keep the orphan socket, kernel would send an RST to |
499 | the other side, and delete the socket, in such situation, kernel will |
500 | increase 1 to the TcpExtTCPAbortOnMemory. Two conditions would trigger |
501 | TcpExtTCPAbortOnMemory: |
502 | |
503 | 1. the memory used by the TCP protocol is higher than the third value of |
504 | the tcp_mem. Please refer the tcp_mem section in the `TCP man page`_: |
505 | |
506 | .. _TCP man page: http://man7.org/linux/man-pages/man7/tcp.7.html |
507 | |
508 | 2. the orphan socket count is higher than net.ipv4.tcp_max_orphans |
509 | |
510 | |
511 | * TcpExtTCPAbortOnTimeout |
ae5220c6 |
512 | |
80cc4950 |
513 | This counter will increase when any of the TCP timers expire. In such |
514 | situation, kernel won't send RST, just give up the connection. |
515 | |
516 | * TcpExtTCPAbortOnLinger |
ae5220c6 |
517 | |
80cc4950 |
518 | When a TCP connection comes into FIN_WAIT_2 state, instead of waiting |
519 | for the fin packet from the other side, kernel could send a RST and |
520 | delete the socket immediately. This is not the default behavior of |
521 | Linux kernel TCP stack. By configuring the TCP_LINGER2 socket option, |
522 | you could let kernel follow this behavior. |
523 | |
524 | * TcpExtTCPAbortFailed |
ae5220c6 |
525 | |
80cc4950 |
526 | The kernel TCP layer will send RST if the `RFC2525 2.17 section`_ is |
527 | satisfied. If an internal error occurs during this process, |
528 | TcpExtTCPAbortFailed will be increased. |
529 | |
530 | .. _RFC2525 2.17 section: https://tools.ietf.org/html/rfc2525#page-50 |
531 | |
712ee16c |
532 | TCP Hybrid Slow Start |
ae5220c6 |
533 | ===================== |
712ee16c |
534 | The Hybrid Slow Start algorithm is an enhancement of the traditional |
535 | TCP congestion window Slow Start algorithm. It uses two pieces of |
536 | information to detect whether the max bandwidth of the TCP path is |
537 | approached. The two pieces of information are ACK train length and |
538 | increase in packet delay. For detail information, please refer the |
539 | `Hybrid Slow Start paper`_. Either ACK train length or packet delay |
540 | hits a specific threshold, the congestion control algorithm will come |
541 | into the Congestion Avoidance state. Until v4.20, two congestion |
542 | control algorithms are using Hybrid Slow Start, they are cubic (the |
543 | default congestion control algorithm) and cdg. Four snmp counters |
544 | relate with the Hybrid Slow Start algorithm. |
545 | |
546 | .. _Hybrid Slow Start paper: https://pdfs.semanticscholar.org/25e9/ef3f03315782c7f1cbcd31b587857adae7d1.pdf |
547 | |
548 | * TcpExtTCPHystartTrainDetect |
ae5220c6 |
549 | |
712ee16c |
550 | How many times the ACK train length threshold is detected |
551 | |
552 | * TcpExtTCPHystartTrainCwnd |
ae5220c6 |
553 | |
712ee16c |
554 | The sum of CWND detected by ACK train length. Dividing this value by |
555 | TcpExtTCPHystartTrainDetect is the average CWND which detected by the |
556 | ACK train length. |
557 | |
558 | * TcpExtTCPHystartDelayDetect |
ae5220c6 |
559 | |
712ee16c |
560 | How many times the packet delay threshold is detected. |
561 | |
562 | * TcpExtTCPHystartDelayCwnd |
ae5220c6 |
563 | |
712ee16c |
564 | The sum of CWND detected by packet delay. Dividing this value by |
565 | TcpExtTCPHystartDelayDetect is the average CWND which detected by the |
566 | packet delay. |
567 | |
8e2ea53a |
568 | TCP retransmission and congestion control |
ae5220c6 |
569 | ========================================= |
8e2ea53a |
570 | The TCP protocol has two retransmission mechanisms: SACK and fast |
571 | recovery. They are exclusive with each other. When SACK is enabled, |
572 | the kernel TCP stack would use SACK, or kernel would use fast |
573 | recovery. The SACK is a TCP option, which is defined in `RFC2018`_, |
574 | the fast recovery is defined in `RFC6582`_, which is also called |
575 | 'Reno'. |
576 | |
577 | The TCP congestion control is a big and complex topic. To understand |
578 | the related snmp counter, we need to know the states of the congestion |
579 | control state machine. There are 5 states: Open, Disorder, CWR, |
580 | Recovery and Loss. For details about these states, please refer page 5 |
581 | and page 6 of this document: |
582 | https://pdfs.semanticscholar.org/0e9c/968d09ab2e53e24c4dca5b2d67c7f7140f8e.pdf |
583 | |
584 | .. _RFC2018: https://tools.ietf.org/html/rfc2018 |
585 | .. _RFC6582: https://tools.ietf.org/html/rfc6582 |
586 | |
587 | * TcpExtTCPRenoRecovery and TcpExtTCPSackRecovery |
ae5220c6 |
588 | |
8e2ea53a |
589 | When the congestion control comes into Recovery state, if sack is |
590 | used, TcpExtTCPSackRecovery increases 1, if sack is not used, |
591 | TcpExtTCPRenoRecovery increases 1. These two counters mean the TCP |
592 | stack begins to retransmit the lost packets. |
593 | |
594 | * TcpExtTCPSACKReneging |
ae5220c6 |
595 | |
8e2ea53a |
596 | A packet was acknowledged by SACK, but the receiver has dropped this |
597 | packet, so the sender needs to retransmit this packet. In this |
598 | situation, the sender adds 1 to TcpExtTCPSACKReneging. A receiver |
599 | could drop a packet which has been acknowledged by SACK, although it is |
600 | unusual, it is allowed by the TCP protocol. The sender doesn't really |
601 | know what happened on the receiver side. The sender just waits until |
602 | the RTO expires for this packet, then the sender assumes this packet |
603 | has been dropped by the receiver. |
604 | |
605 | * TcpExtTCPRenoReorder |
ae5220c6 |
606 | |
8e2ea53a |
607 | The reorder packet is detected by fast recovery. It would only be used |
608 | if SACK is disabled. The fast recovery algorithm detects recorder by |
609 | the duplicate ACK number. E.g., if retransmission is triggered, and |
610 | the original retransmitted packet is not lost, it is just out of |
611 | order, the receiver would acknowledge multiple times, one for the |
612 | retransmitted packet, another for the arriving of the original out of |
613 | order packet. Thus the sender would find more ACks than its |
614 | expectation, and the sender knows out of order occurs. |
615 | |
616 | * TcpExtTCPTSReorder |
ae5220c6 |
617 | |
8e2ea53a |
618 | The reorder packet is detected when a hole is filled. E.g., assume the |
619 | sender sends packet 1,2,3,4,5, and the receiving order is |
620 | 1,2,4,5,3. When the sender receives the ACK of packet 3 (which will |
621 | fill the hole), two conditions will let TcpExtTCPTSReorder increase |
622 | 1: (1) if the packet 3 is not re-retransmitted yet. (2) if the packet |
623 | 3 is retransmitted but the timestamp of the packet 3's ACK is earlier |
624 | than the retransmission timestamp. |
625 | |
626 | * TcpExtTCPSACKReorder |
ae5220c6 |
627 | |
8e2ea53a |
628 | The reorder packet detected by SACK. The SACK has two methods to |
629 | detect reorder: (1) DSACK is received by the sender. It means the |
630 | sender sends the same packet more than one times. And the only reason |
631 | is the sender believes an out of order packet is lost so it sends the |
632 | packet again. (2) Assume packet 1,2,3,4,5 are sent by the sender, and |
633 | the sender has received SACKs for packet 2 and 5, now the sender |
634 | receives SACK for packet 4 and the sender doesn't retransmit the |
635 | packet yet, the sender would know packet 4 is out of order. The TCP |
636 | stack of kernel will increase TcpExtTCPSACKReorder for both of the |
637 | above scenarios. |
638 | |
132c4e9e |
639 | * TcpExtTCPSlowStartRetrans |
640 | |
641 | The TCP stack wants to retransmit a packet and the congestion control |
642 | state is 'Loss'. |
643 | |
644 | * TcpExtTCPFastRetrans |
645 | |
646 | The TCP stack wants to retransmit a packet and the congestion control |
647 | state is not 'Loss'. |
648 | |
649 | * TcpExtTCPLostRetransmit |
650 | |
651 | A SACK points out that a retransmission packet is lost again. |
652 | |
653 | * TcpExtTCPRetransFail |
654 | |
655 | The TCP stack tries to deliver a retransmission packet to lower layers |
656 | but the lower layers return an error. |
657 | |
658 | * TcpExtTCPSynRetrans |
659 | |
660 | The TCP stack retransmits a SYN packet. |
661 | |
8e2ea53a |
662 | DSACK |
663 | ===== |
664 | The DSACK is defined in `RFC2883`_. The receiver uses DSACK to report |
665 | duplicate packets to the sender. There are two kinds of |
666 | duplications: (1) a packet which has been acknowledged is |
667 | duplicate. (2) an out of order packet is duplicate. The TCP stack |
668 | counts these two kinds of duplications on both receiver side and |
669 | sender side. |
670 | |
671 | .. _RFC2883 : https://tools.ietf.org/html/rfc2883 |
672 | |
673 | * TcpExtTCPDSACKOldSent |
ae5220c6 |
674 | |
8e2ea53a |
675 | The TCP stack receives a duplicate packet which has been acked, so it |
676 | sends a DSACK to the sender. |
677 | |
678 | * TcpExtTCPDSACKOfoSent |
ae5220c6 |
679 | |
8e2ea53a |
680 | The TCP stack receives an out of order duplicate packet, so it sends a |
681 | DSACK to the sender. |
682 | |
683 | * TcpExtTCPDSACKRecv |
65e9a6d2 |
684 | |
a6c7c7aa |
685 | The TCP stack receives a DSACK, which indicates an acknowledged |
8e2ea53a |
686 | duplicate packet is received. |
687 | |
688 | * TcpExtTCPDSACKOfoRecv |
ae5220c6 |
689 | |
8e2ea53a |
690 | The TCP stack receives a DSACK, which indicate an out of order |
2b965472 |
691 | duplicate packet is received. |
692 | |
a6c7c7aa |
693 | invalid SACK and DSACK |
65e9a6d2 |
694 | ====================== |
a6c7c7aa |
695 | When a SACK (or DSACK) block is invalid, a corresponding counter would |
696 | be updated. The validation method is base on the start/end sequence |
697 | number of the SACK block. For more details, please refer the comment |
698 | of the function tcp_is_sackblock_valid in the kernel source code. A |
699 | SACK option could have up to 4 blocks, they are checked |
700 | individually. E.g., if 3 blocks of a SACk is invalid, the |
701 | corresponding counter would be updated 3 times. The comment of the |
702 | `Add counters for discarded SACK blocks`_ patch has additional |
703 | explaination: |
704 | |
705 | .. _Add counters for discarded SACK blocks: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=18f02545a9a16c9a89778b91a162ad16d510bb32 |
706 | |
707 | * TcpExtTCPSACKDiscard |
65e9a6d2 |
708 | |
a6c7c7aa |
709 | This counter indicates how many SACK blocks are invalid. If the invalid |
710 | SACK block is caused by ACK recording, the TCP stack will only ignore |
711 | it and won't update this counter. |
712 | |
713 | * TcpExtTCPDSACKIgnoredOld and TcpExtTCPDSACKIgnoredNoUndo |
65e9a6d2 |
714 | |
a6c7c7aa |
715 | When a DSACK block is invalid, one of these two counters would be |
716 | updated. Which counter will be updated depends on the undo_marker flag |
717 | of the TCP socket. If the undo_marker is not set, the TCP stack isn't |
718 | likely to re-transmit any packets, and we still receive an invalid |
719 | DSACK block, the reason might be that the packet is duplicated in the |
720 | middle of the network. In such scenario, TcpExtTCPDSACKIgnoredNoUndo |
721 | will be updated. If the undo_marker is set, TcpExtTCPDSACKIgnoredOld |
722 | will be updated. As implied in its name, it might be an old packet. |
723 | |
724 | SACK shift |
65e9a6d2 |
725 | ========== |
a6c7c7aa |
726 | The linux networking stack stores data in sk_buff struct (skb for |
727 | short). If a SACK block acrosses multiple skb, the TCP stack will try |
728 | to re-arrange data in these skb. E.g. if a SACK block acknowledges seq |
729 | 10 to 15, skb1 has seq 10 to 13, skb2 has seq 14 to 20. The seq 14 and |
730 | 15 in skb2 would be moved to skb1. This operation is 'shift'. If a |
731 | SACK block acknowledges seq 10 to 20, skb1 has seq 10 to 13, skb2 has |
732 | seq 14 to 20. All data in skb2 will be moved to skb1, and skb2 will be |
733 | discard, this operation is 'merge'. |
734 | |
735 | * TcpExtTCPSackShifted |
65e9a6d2 |
736 | |
a6c7c7aa |
737 | A skb is shifted |
738 | |
739 | * TcpExtTCPSackMerged |
65e9a6d2 |
740 | |
a6c7c7aa |
741 | A skb is merged |
742 | |
743 | * TcpExtTCPSackShiftFallback |
65e9a6d2 |
744 | |
a6c7c7aa |
745 | A skb should be shifted or merged, but the TCP stack doesn't do it for |
746 | some reasons. |
747 | |
2b965472 |
748 | TCP out of order |
ae5220c6 |
749 | ================ |
2b965472 |
750 | * TcpExtTCPOFOQueue |
ae5220c6 |
751 | |
2b965472 |
752 | The TCP layer receives an out of order packet and has enough memory |
753 | to queue it. |
754 | |
755 | * TcpExtTCPOFODrop |
ae5220c6 |
756 | |
2b965472 |
757 | The TCP layer receives an out of order packet but doesn't have enough |
758 | memory, so drops it. Such packets won't be counted into |
759 | TcpExtTCPOFOQueue. |
760 | |
761 | * TcpExtTCPOFOMerge |
ae5220c6 |
762 | |
2b965472 |
763 | The received out of order packet has an overlay with the previous |
764 | packet. the overlay part will be dropped. All of TcpExtTCPOFOMerge |
765 | packets will also be counted into TcpExtTCPOFOQueue. |
766 | |
767 | TCP PAWS |
ae5220c6 |
768 | ======== |
2b965472 |
769 | PAWS (Protection Against Wrapped Sequence numbers) is an algorithm |
770 | which is used to drop old packets. It depends on the TCP |
771 | timestamps. For detail information, please refer the `timestamp wiki`_ |
772 | and the `RFC of PAWS`_. |
773 | |
774 | .. _RFC of PAWS: https://tools.ietf.org/html/rfc1323#page-17 |
775 | .. _timestamp wiki: https://en.wikipedia.org/wiki/Transmission_Control_Protocol#TCP_timestamps |
776 | |
777 | * TcpExtPAWSActive |
ae5220c6 |
778 | |
2b965472 |
779 | Packets are dropped by PAWS in Syn-Sent status. |
780 | |
781 | * TcpExtPAWSEstab |
ae5220c6 |
782 | |
2b965472 |
783 | Packets are dropped by PAWS in any status other than Syn-Sent. |
784 | |
785 | TCP ACK skip |
ae5220c6 |
786 | ============ |
2b965472 |
787 | In some scenarios, kernel would avoid sending duplicate ACKs too |
788 | frequently. Please find more details in the tcp_invalid_ratelimit |
789 | section of the `sysctl document`_. When kernel decides to skip an ACK |
790 | due to tcp_invalid_ratelimit, kernel would update one of below |
791 | counters to indicate the ACK is skipped in which scenario. The ACK |
792 | would only be skipped if the received packet is either a SYN packet or |
793 | it has no data. |
794 | |
795 | .. _sysctl document: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt |
796 | |
797 | * TcpExtTCPACKSkippedSynRecv |
ae5220c6 |
798 | |
2b965472 |
799 | The ACK is skipped in Syn-Recv status. The Syn-Recv status means the |
800 | TCP stack receives a SYN and replies SYN+ACK. Now the TCP stack is |
801 | waiting for an ACK. Generally, the TCP stack doesn't need to send ACK |
802 | in the Syn-Recv status. But in several scenarios, the TCP stack need |
803 | to send an ACK. E.g., the TCP stack receives the same SYN packet |
804 | repeately, the received packet does not pass the PAWS check, or the |
805 | received packet sequence number is out of window. In these scenarios, |
806 | the TCP stack needs to send ACK. If the ACk sending frequency is higher than |
807 | tcp_invalid_ratelimit allows, the TCP stack will skip sending ACK and |
808 | increase TcpExtTCPACKSkippedSynRecv. |
809 | |
810 | |
811 | * TcpExtTCPACKSkippedPAWS |
ae5220c6 |
812 | |
2b965472 |
813 | The ACK is skipped due to PAWS (Protect Against Wrapped Sequence |
814 | numbers) check fails. If the PAWS check fails in Syn-Recv, Fin-Wait-2 |
815 | or Time-Wait statuses, the skipped ACK would be counted to |
816 | TcpExtTCPACKSkippedSynRecv, TcpExtTCPACKSkippedFinWait2 or |
817 | TcpExtTCPACKSkippedTimeWait. In all other statuses, the skipped ACK |
818 | would be counted to TcpExtTCPACKSkippedPAWS. |
819 | |
820 | * TcpExtTCPACKSkippedSeq |
ae5220c6 |
821 | |
2b965472 |
822 | The sequence number is out of window and the timestamp passes the PAWS |
823 | check and the TCP status is not Syn-Recv, Fin-Wait-2, and Time-Wait. |
824 | |
825 | * TcpExtTCPACKSkippedFinWait2 |
ae5220c6 |
826 | |
2b965472 |
827 | The ACK is skipped in Fin-Wait-2 status, the reason would be either |
828 | PAWS check fails or the received sequence number is out of window. |
829 | |
830 | * TcpExtTCPACKSkippedTimeWait |
ae5220c6 |
831 | |
2b965472 |
832 | Tha ACK is skipped in Time-Wait status, the reason would be either |
833 | PAWS check failed or the received sequence number is out of window. |
834 | |
835 | * TcpExtTCPACKSkippedChallenge |
ae5220c6 |
836 | |
2b965472 |
837 | The ACK is skipped if the ACK is a challenge ACK. The RFC 5961 defines |
838 | 3 kind of challenge ACK, please refer `RFC 5961 section 3.2`_, |
839 | `RFC 5961 section 4.2`_ and `RFC 5961 section 5.2`_. Besides these |
840 | three scenarios, In some TCP status, the linux TCP stack would also |
841 | send challenge ACKs if the ACK number is before the first |
842 | unacknowledged number (more strict than `RFC 5961 section 5.2`_). |
843 | |
844 | .. _RFC 5961 section 3.2: https://tools.ietf.org/html/rfc5961#page-7 |
845 | .. _RFC 5961 section 4.2: https://tools.ietf.org/html/rfc5961#page-9 |
846 | .. _RFC 5961 section 5.2: https://tools.ietf.org/html/rfc5961#page-11 |
847 | |
a6c7c7aa |
848 | TCP receive window |
132c4e9e |
849 | ================== |
a6c7c7aa |
850 | * TcpExtTCPWantZeroWindowAdv |
132c4e9e |
851 | |
a6c7c7aa |
852 | Depending on current memory usage, the TCP stack tries to set receive |
853 | window to zero. But the receive window might still be a no-zero |
854 | value. For example, if the previous window size is 10, and the TCP |
855 | stack receives 3 bytes, the current window size would be 7 even if the |
856 | window size calculated by the memory usage is zero. |
857 | |
858 | * TcpExtTCPToZeroWindowAdv |
132c4e9e |
859 | |
a6c7c7aa |
860 | The TCP receive window is set to zero from a no-zero value. |
861 | |
862 | * TcpExtTCPFromZeroWindowAdv |
132c4e9e |
863 | |
a6c7c7aa |
864 | The TCP receive window is set to no-zero value from zero. |
865 | |
866 | |
867 | Delayed ACK |
132c4e9e |
868 | =========== |
a6c7c7aa |
869 | The TCP Delayed ACK is a technique which is used for reducing the |
870 | packet count in the network. For more details, please refer the |
871 | `Delayed ACK wiki`_ |
872 | |
873 | .. _Delayed ACK wiki: https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment |
874 | |
875 | * TcpExtDelayedACKs |
132c4e9e |
876 | |
a6c7c7aa |
877 | A delayed ACK timer expires. The TCP stack will send a pure ACK packet |
878 | and exit the delayed ACK mode. |
879 | |
880 | * TcpExtDelayedACKLocked |
132c4e9e |
881 | |
a6c7c7aa |
882 | A delayed ACK timer expires, but the TCP stack can't send an ACK |
883 | immediately due to the socket is locked by a userspace program. The |
884 | TCP stack will send a pure ACK later (after the userspace program |
885 | unlock the socket). When the TCP stack sends the pure ACK later, the |
886 | TCP stack will also update TcpExtDelayedACKs and exit the delayed ACK |
887 | mode. |
888 | |
889 | * TcpExtDelayedACKLost |
132c4e9e |
890 | |
a6c7c7aa |
891 | It will be updated when the TCP stack receives a packet which has been |
892 | ACKed. A Delayed ACK loss might cause this issue, but it would also be |
893 | triggered by other reasons, such as a packet is duplicated in the |
894 | network. |
895 | |
896 | Tail Loss Probe (TLP) |
132c4e9e |
897 | ===================== |
a6c7c7aa |
898 | TLP is an algorithm which is used to detect TCP packet loss. For more |
899 | details, please refer the `TLP paper`_. |
900 | |
901 | .. _TLP paper: https://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01 |
902 | |
903 | * TcpExtTCPLossProbes |
132c4e9e |
904 | |
a6c7c7aa |
905 | A TLP probe packet is sent. |
906 | |
907 | * TcpExtTCPLossProbeRecovery |
132c4e9e |
908 | |
a6c7c7aa |
909 | A packet loss is detected and recovered by TLP. |
8e2ea53a |
910 | |
132c4e9e |
911 | TCP Fast Open |
912 | ============= |
913 | TCP Fast Open is a technology which allows data transfer before the |
914 | 3-way handshake complete. Please refer the `TCP Fast Open wiki`_ for a |
915 | general description. |
916 | |
917 | .. _TCP Fast Open wiki: https://en.wikipedia.org/wiki/TCP_Fast_Open |
918 | |
919 | * TcpExtTCPFastOpenActive |
920 | |
921 | When the TCP stack receives an ACK packet in the SYN-SENT status, and |
922 | the ACK packet acknowledges the data in the SYN packet, the TCP stack |
923 | understand the TFO cookie is accepted by the other side, then it |
924 | updates this counter. |
925 | |
926 | * TcpExtTCPFastOpenActiveFail |
927 | |
928 | This counter indicates that the TCP stack initiated a TCP Fast Open, |
929 | but it failed. This counter would be updated in three scenarios: (1) |
930 | the other side doesn't acknowledge the data in the SYN packet. (2) The |
931 | SYN packet which has the TFO cookie is timeout at least once. (3) |
932 | after the 3-way handshake, the retransmission timeout happens |
933 | net.ipv4.tcp_retries1 times, because some middle-boxes may black-hole |
934 | fast open after the handshake. |
935 | |
936 | * TcpExtTCPFastOpenPassive |
937 | |
938 | This counter indicates how many times the TCP stack accepts the fast |
939 | open request. |
940 | |
941 | * TcpExtTCPFastOpenPassiveFail |
942 | |
943 | This counter indicates how many times the TCP stack rejects the fast |
944 | open request. It is caused by either the TFO cookie is invalid or the |
945 | TCP stack finds an error during the socket creating process. |
946 | |
947 | * TcpExtTCPFastOpenListenOverflow |
948 | |
949 | When the pending fast open request number is larger than |
950 | fastopenq->max_qlen, the TCP stack will reject the fast open request |
951 | and update this counter. When this counter is updated, the TCP stack |
952 | won't update TcpExtTCPFastOpenPassive or |
953 | TcpExtTCPFastOpenPassiveFail. The fastopenq->max_qlen is set by the |
954 | TCP_FASTOPEN socket operation and it could not be larger than |
955 | net.core.somaxconn. For example: |
956 | |
957 | setsockopt(sfd, SOL_TCP, TCP_FASTOPEN, &qlen, sizeof(qlen)); |
958 | |
959 | * TcpExtTCPFastOpenCookieReqd |
960 | |
961 | This counter indicates how many times a client wants to request a TFO |
962 | cookie. |
963 | |
964 | SYN cookies |
965 | =========== |
966 | SYN cookies are used to mitigate SYN flood, for details, please refer |
967 | the `SYN cookies wiki`_. |
968 | |
969 | .. _SYN cookies wiki: https://en.wikipedia.org/wiki/SYN_cookies |
970 | |
971 | * TcpExtSyncookiesSent |
972 | |
973 | It indicates how many SYN cookies are sent. |
974 | |
975 | * TcpExtSyncookiesRecv |
976 | |
977 | How many reply packets of the SYN cookies the TCP stack receives. |
978 | |
979 | * TcpExtSyncookiesFailed |
980 | |
981 | The MSS decoded from the SYN cookie is invalid. When this counter is |
982 | updated, the received packet won't be treated as a SYN cookie and the |
983 | TcpExtSyncookiesRecv counter wont be updated. |
984 | |
985 | Challenge ACK |
986 | ============= |
987 | For details of challenge ACK, please refer the explaination of |
988 | TcpExtTCPACKSkippedChallenge. |
989 | |
990 | * TcpExtTCPChallengeACK |
991 | |
992 | The number of challenge acks sent. |
993 | |
994 | * TcpExtTCPSYNChallenge |
995 | |
996 | The number of challenge acks sent in response to SYN packets. After |
997 | updates this counter, the TCP stack might send a challenge ACK and |
998 | update the TcpExtTCPChallengeACK counter, or it might also skip to |
999 | send the challenge and update the TcpExtTCPACKSkippedChallenge. |
1000 | |
1001 | prune |
1002 | ===== |
1003 | When a socket is under memory pressure, the TCP stack will try to |
1004 | reclaim memory from the receiving queue and out of order queue. One of |
1005 | the reclaiming method is 'collapse', which means allocate a big sbk, |
1006 | copy the contiguous skbs to the single big skb, and free these |
1007 | contiguous skbs. |
1008 | |
1009 | * TcpExtPruneCalled |
1010 | |
1011 | The TCP stack tries to reclaim memory for a socket. After updates this |
1012 | counter, the TCP stack will try to collapse the out of order queue and |
1013 | the receiving queue. If the memory is still not enough, the TCP stack |
1014 | will try to discard packets from the out of order queue (and update the |
1015 | TcpExtOfoPruned counter) |
1016 | |
1017 | * TcpExtOfoPruned |
1018 | |
1019 | The TCP stack tries to discard packet on the out of order queue. |
1020 | |
1021 | * TcpExtRcvPruned |
1022 | |
1023 | After 'collapse' and discard packets from the out of order queue, if |
1024 | the actually used memory is still larger than the max allowed memory, |
1025 | this counter will be updated. It means the 'prune' fails. |
1026 | |
1027 | * TcpExtTCPRcvCollapsed |
1028 | |
1029 | This counter indicates how many skbs are freed during 'collapse'. |
1030 | |
b08794a9 |
1031 | examples |
ae5220c6 |
1032 | ======== |
b08794a9 |
1033 | |
1034 | ping test |
ae5220c6 |
1035 | --------- |
b08794a9 |
1036 | Run the ping command against the public dns server 8.8.8.8:: |
1037 | |
1038 | nstatuser@nstat-a:~$ ping 8.8.8.8 -c 1 |
1039 | PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data. |
1040 | 64 bytes from 8.8.8.8: icmp_seq=1 ttl=119 time=17.8 ms |
1041 | |
1042 | --- 8.8.8.8 ping statistics --- |
1043 | 1 packets transmitted, 1 received, 0% packet loss, time 0ms |
1044 | rtt min/avg/max/mdev = 17.875/17.875/17.875/0.000 ms |
1045 | |
1046 | The nstayt result:: |
1047 | |
1048 | nstatuser@nstat-a:~$ nstat |
1049 | #kernel |
1050 | IpInReceives 1 0.0 |
1051 | IpInDelivers 1 0.0 |
1052 | IpOutRequests 1 0.0 |
1053 | IcmpInMsgs 1 0.0 |
1054 | IcmpInEchoReps 1 0.0 |
1055 | IcmpOutMsgs 1 0.0 |
1056 | IcmpOutEchos 1 0.0 |
1057 | IcmpMsgInType0 1 0.0 |
1058 | IcmpMsgOutType8 1 0.0 |
1059 | IpExtInOctets 84 0.0 |
1060 | IpExtOutOctets 84 0.0 |
1061 | IpExtInNoECTPkts 1 0.0 |
1062 | |
1063 | The Linux server sent an ICMP Echo packet, so IpOutRequests, |
1064 | IcmpOutMsgs, IcmpOutEchos and IcmpMsgOutType8 were increased 1. The |
1065 | server got ICMP Echo Reply from 8.8.8.8, so IpInReceives, IcmpInMsgs, |
1066 | IcmpInEchoReps and IcmpMsgInType0 were increased 1. The ICMP Echo Reply |
1067 | was passed to the ICMP layer via IP layer, so IpInDelivers was |
1068 | increased 1. The default ping data size is 48, so an ICMP Echo packet |
1069 | and its corresponding Echo Reply packet are constructed by: |
1070 | |
1071 | * 14 bytes MAC header |
1072 | * 20 bytes IP header |
1073 | * 16 bytes ICMP header |
1074 | * 48 bytes data (default value of the ping command) |
1075 | |
1076 | So the IpExtInOctets and IpExtOutOctets are 20+16+48=84. |
80cc4950 |
1077 | |
1078 | tcp 3-way handshake |
ae5220c6 |
1079 | ------------------- |
80cc4950 |
1080 | On server side, we run:: |
1081 | |
1082 | nstatuser@nstat-b:~$ nc -lknv 0.0.0.0 9000 |
1083 | Listening on [0.0.0.0] (family 0, port 9000) |
1084 | |
1085 | On client side, we run:: |
1086 | |
1087 | nstatuser@nstat-a:~$ nc -nv 192.168.122.251 9000 |
1088 | Connection to 192.168.122.251 9000 port [tcp/*] succeeded! |
1089 | |
1090 | The server listened on tcp 9000 port, the client connected to it, they |
1091 | completed the 3-way handshake. |
1092 | |
1093 | On server side, we can find below nstat output:: |
1094 | |
1095 | nstatuser@nstat-b:~$ nstat | grep -i tcp |
1096 | TcpPassiveOpens 1 0.0 |
1097 | TcpInSegs 2 0.0 |
1098 | TcpOutSegs 1 0.0 |
1099 | TcpExtTCPPureAcks 1 0.0 |
1100 | |
1101 | On client side, we can find below nstat output:: |
1102 | |
1103 | nstatuser@nstat-a:~$ nstat | grep -i tcp |
1104 | TcpActiveOpens 1 0.0 |
1105 | TcpInSegs 1 0.0 |
1106 | TcpOutSegs 2 0.0 |
1107 | |
1108 | When the server received the first SYN, it replied a SYN+ACK, and came into |
1109 | SYN-RCVD state, so TcpPassiveOpens increased 1. The server received |
1110 | SYN, sent SYN+ACK, received ACK, so server sent 1 packet, received 2 |
1111 | packets, TcpInSegs increased 2, TcpOutSegs increased 1. The last ACK |
1112 | of the 3-way handshake is a pure ACK without data, so |
1113 | TcpExtTCPPureAcks increased 1. |
1114 | |
1115 | When the client sent SYN, the client came into the SYN-SENT state, so |
1116 | TcpActiveOpens increased 1, the client sent SYN, received SYN+ACK, sent |
1117 | ACK, so client sent 2 packets, received 1 packet, TcpInSegs increased |
1118 | 1, TcpOutSegs increased 2. |
1119 | |
1120 | TCP normal traffic |
ae5220c6 |
1121 | ------------------ |
80cc4950 |
1122 | Run nc on server:: |
1123 | |
1124 | nstatuser@nstat-b:~$ nc -lkv 0.0.0.0 9000 |
1125 | Listening on [0.0.0.0] (family 0, port 9000) |
1126 | |
1127 | Run nc on client:: |
1128 | |
1129 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1130 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1131 | |
1132 | Input a string in the nc client ('hello' in our example):: |
1133 | |
1134 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1135 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1136 | hello |
1137 | |
1138 | The client side nstat output:: |
1139 | |
1140 | nstatuser@nstat-a:~$ nstat |
1141 | #kernel |
1142 | IpInReceives 1 0.0 |
1143 | IpInDelivers 1 0.0 |
1144 | IpOutRequests 1 0.0 |
1145 | TcpInSegs 1 0.0 |
1146 | TcpOutSegs 1 0.0 |
1147 | TcpExtTCPPureAcks 1 0.0 |
1148 | TcpExtTCPOrigDataSent 1 0.0 |
1149 | IpExtInOctets 52 0.0 |
1150 | IpExtOutOctets 58 0.0 |
1151 | IpExtInNoECTPkts 1 0.0 |
1152 | |
1153 | The server side nstat output:: |
1154 | |
1155 | nstatuser@nstat-b:~$ nstat |
1156 | #kernel |
1157 | IpInReceives 1 0.0 |
1158 | IpInDelivers 1 0.0 |
1159 | IpOutRequests 1 0.0 |
1160 | TcpInSegs 1 0.0 |
1161 | TcpOutSegs 1 0.0 |
1162 | IpExtInOctets 58 0.0 |
1163 | IpExtOutOctets 52 0.0 |
1164 | IpExtInNoECTPkts 1 0.0 |
1165 | |
1166 | Input a string in nc client side again ('world' in our exmaple):: |
1167 | |
1168 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1169 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1170 | hello |
1171 | world |
1172 | |
1173 | Client side nstat output:: |
1174 | |
1175 | nstatuser@nstat-a:~$ nstat |
1176 | #kernel |
1177 | IpInReceives 1 0.0 |
1178 | IpInDelivers 1 0.0 |
1179 | IpOutRequests 1 0.0 |
1180 | TcpInSegs 1 0.0 |
1181 | TcpOutSegs 1 0.0 |
1182 | TcpExtTCPHPAcks 1 0.0 |
1183 | TcpExtTCPOrigDataSent 1 0.0 |
1184 | IpExtInOctets 52 0.0 |
1185 | IpExtOutOctets 58 0.0 |
1186 | IpExtInNoECTPkts 1 0.0 |
1187 | |
1188 | |
1189 | Server side nstat output:: |
1190 | |
1191 | nstatuser@nstat-b:~$ nstat |
1192 | #kernel |
1193 | IpInReceives 1 0.0 |
1194 | IpInDelivers 1 0.0 |
1195 | IpOutRequests 1 0.0 |
1196 | TcpInSegs 1 0.0 |
1197 | TcpOutSegs 1 0.0 |
1198 | TcpExtTCPHPHits 1 0.0 |
1199 | IpExtInOctets 58 0.0 |
1200 | IpExtOutOctets 52 0.0 |
1201 | IpExtInNoECTPkts 1 0.0 |
1202 | |
1203 | Compare the first client-side nstat and the second client-side nstat, |
1204 | we could find one difference: the first one had a 'TcpExtTCPPureAcks', |
1205 | but the second one had a 'TcpExtTCPHPAcks'. The first server-side |
1206 | nstat and the second server-side nstat had a difference too: the |
1207 | second server-side nstat had a TcpExtTCPHPHits, but the first |
1208 | server-side nstat didn't have it. The network traffic patterns were |
1209 | exactly the same: the client sent a packet to the server, the server |
1210 | replied an ACK. But kernel handled them in different ways. When the |
1211 | TCP window scale option is not used, kernel will try to enable fast |
1212 | path immediately when the connection comes into the established state, |
1213 | but if the TCP window scale option is used, kernel will disable the |
1214 | fast path at first, and try to enable it after kerenl receives |
1215 | packets. We could use the 'ss' command to verify whether the window |
1216 | scale option is used. e.g. run below command on either server or |
1217 | client:: |
1218 | |
1219 | nstatuser@nstat-a:~$ ss -o state established -i '( dport = :9000 or sport = :9000 ) |
1220 | Netid Recv-Q Send-Q Local Address:Port Peer Address:Port |
1221 | tcp 0 0 192.168.122.250:40654 192.168.122.251:9000 |
1222 | ts sack cubic wscale:7,7 rto:204 rtt:0.98/0.49 mss:1448 pmtu:1500 rcvmss:536 advmss:1448 cwnd:10 bytes_acked:1 segs_out:2 segs_in:1 send 118.2Mbps lastsnd:46572 lastrcv:46572 lastack:46572 pacing_rate 236.4Mbps rcv_space:29200 rcv_ssthresh:29200 minrtt:0.98 |
1223 | |
1224 | The 'wscale:7,7' means both server and client set the window scale |
1225 | option to 7. Now we could explain the nstat output in our test: |
1226 | |
1227 | In the first nstat output of client side, the client sent a packet, server |
1228 | reply an ACK, when kernel handled this ACK, the fast path was not |
1229 | enabled, so the ACK was counted into 'TcpExtTCPPureAcks'. |
1230 | |
1231 | In the second nstat output of client side, the client sent a packet again, |
1232 | and received another ACK from the server, in this time, the fast path is |
1233 | enabled, and the ACK was qualified for fast path, so it was handled by |
1234 | the fast path, so this ACK was counted into TcpExtTCPHPAcks. |
1235 | |
1236 | In the first nstat output of server side, fast path was not enabled, |
1237 | so there was no 'TcpExtTCPHPHits'. |
1238 | |
1239 | In the second nstat output of server side, the fast path was enabled, |
1240 | and the packet received from client qualified for fast path, so it |
1241 | was counted into 'TcpExtTCPHPHits'. |
1242 | |
1243 | TcpExtTCPAbortOnClose |
ae5220c6 |
1244 | --------------------- |
80cc4950 |
1245 | On the server side, we run below python script:: |
1246 | |
1247 | import socket |
1248 | import time |
1249 | |
1250 | port = 9000 |
1251 | |
1252 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1253 | s.bind(('0.0.0.0', port)) |
1254 | s.listen(1) |
1255 | sock, addr = s.accept() |
1256 | while True: |
1257 | time.sleep(9999999) |
1258 | |
1259 | This python script listen on 9000 port, but doesn't read anything from |
1260 | the connection. |
1261 | |
1262 | On the client side, we send the string "hello" by nc:: |
1263 | |
1264 | nstatuser@nstat-a:~$ echo "hello" | nc nstat-b 9000 |
1265 | |
1266 | Then, we come back to the server side, the server has received the "hello" |
1267 | packet, and the TCP layer has acked this packet, but the application didn't |
1268 | read it yet. We type Ctrl-C to terminate the server script. Then we |
1269 | could find TcpExtTCPAbortOnClose increased 1 on the server side:: |
1270 | |
1271 | nstatuser@nstat-b:~$ nstat | grep -i abort |
1272 | TcpExtTCPAbortOnClose 1 0.0 |
1273 | |
1274 | If we run tcpdump on the server side, we could find the server sent a |
1275 | RST after we type Ctrl-C. |
1276 | |
1277 | TcpExtTCPAbortOnMemory and TcpExtTCPAbortOnTimeout |
ae5220c6 |
1278 | --------------------------------------------------- |
80cc4950 |
1279 | Below is an example which let the orphan socket count be higher than |
1280 | net.ipv4.tcp_max_orphans. |
1281 | Change tcp_max_orphans to a smaller value on client:: |
1282 | |
1283 | sudo bash -c "echo 10 > /proc/sys/net/ipv4/tcp_max_orphans" |
1284 | |
1285 | Client code (create 64 connection to server):: |
1286 | |
1287 | nstatuser@nstat-a:~$ cat client_orphan.py |
1288 | import socket |
1289 | import time |
1290 | |
1291 | server = 'nstat-b' # server address |
1292 | port = 9000 |
1293 | |
1294 | count = 64 |
1295 | |
1296 | connection_list = [] |
1297 | |
1298 | for i in range(64): |
1299 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1300 | s.connect((server, port)) |
1301 | connection_list.append(s) |
1302 | print("connection_count: %d" % len(connection_list)) |
1303 | |
1304 | while True: |
1305 | time.sleep(99999) |
1306 | |
1307 | Server code (accept 64 connection from client):: |
1308 | |
1309 | nstatuser@nstat-b:~$ cat server_orphan.py |
1310 | import socket |
1311 | import time |
1312 | |
1313 | port = 9000 |
1314 | count = 64 |
1315 | |
1316 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1317 | s.bind(('0.0.0.0', port)) |
1318 | s.listen(count) |
1319 | connection_list = [] |
1320 | while True: |
1321 | sock, addr = s.accept() |
1322 | connection_list.append((sock, addr)) |
1323 | print("connection_count: %d" % len(connection_list)) |
1324 | |
1325 | Run the python scripts on server and client. |
1326 | |
1327 | On server:: |
1328 | |
1329 | python3 server_orphan.py |
1330 | |
1331 | On client:: |
1332 | |
1333 | python3 client_orphan.py |
1334 | |
1335 | Run iptables on server:: |
1336 | |
1337 | sudo iptables -A INPUT -i ens3 -p tcp --destination-port 9000 -j DROP |
1338 | |
1339 | Type Ctrl-C on client, stop client_orphan.py. |
1340 | |
1341 | Check TcpExtTCPAbortOnMemory on client:: |
1342 | |
1343 | nstatuser@nstat-a:~$ nstat | grep -i abort |
1344 | TcpExtTCPAbortOnMemory 54 0.0 |
1345 | |
1346 | Check orphane socket count on client:: |
1347 | |
1348 | nstatuser@nstat-a:~$ ss -s |
1349 | Total: 131 (kernel 0) |
1350 | TCP: 14 (estab 1, closed 0, orphaned 10, synrecv 0, timewait 0/0), ports 0 |
1351 | |
1352 | Transport Total IP IPv6 |
1353 | * 0 - - |
1354 | RAW 1 0 1 |
1355 | UDP 1 1 0 |
1356 | TCP 14 13 1 |
1357 | INET 16 14 2 |
1358 | FRAG 0 0 0 |
1359 | |
1360 | The explanation of the test: after run server_orphan.py and |
1361 | client_orphan.py, we set up 64 connections between server and |
1362 | client. Run the iptables command, the server will drop all packets from |
1363 | the client, type Ctrl-C on client_orphan.py, the system of the client |
1364 | would try to close these connections, and before they are closed |
1365 | gracefully, these connections became orphan sockets. As the iptables |
1366 | of the server blocked packets from the client, the server won't receive fin |
1367 | from the client, so all connection on clients would be stuck on FIN_WAIT_1 |
1368 | stage, so they will keep as orphan sockets until timeout. We have echo |
1369 | 10 to /proc/sys/net/ipv4/tcp_max_orphans, so the client system would |
1370 | only keep 10 orphan sockets, for all other orphan sockets, the client |
1371 | system sent RST for them and delete them. We have 64 connections, so |
1372 | the 'ss -s' command shows the system has 10 orphan sockets, and the |
1373 | value of TcpExtTCPAbortOnMemory was 54. |
1374 | |
1375 | An additional explanation about orphan socket count: You could find the |
1376 | exactly orphan socket count by the 'ss -s' command, but when kernel |
1377 | decide whither increases TcpExtTCPAbortOnMemory and sends RST, kernel |
1378 | doesn't always check the exactly orphan socket count. For increasing |
1379 | performance, kernel checks an approximate count firstly, if the |
1380 | approximate count is more than tcp_max_orphans, kernel checks the |
1381 | exact count again. So if the approximate count is less than |
1382 | tcp_max_orphans, but exactly count is more than tcp_max_orphans, you |
1383 | would find TcpExtTCPAbortOnMemory is not increased at all. If |
1384 | tcp_max_orphans is large enough, it won't occur, but if you decrease |
1385 | tcp_max_orphans to a small value like our test, you might find this |
1386 | issue. So in our test, the client set up 64 connections although the |
1387 | tcp_max_orphans is 10. If the client only set up 11 connections, we |
1388 | can't find the change of TcpExtTCPAbortOnMemory. |
1389 | |
1390 | Continue the previous test, we wait for several minutes. Because of the |
1391 | iptables on the server blocked the traffic, the server wouldn't receive |
1392 | fin, and all the client's orphan sockets would timeout on the |
1393 | FIN_WAIT_1 state finally. So we wait for a few minutes, we could find |
1394 | 10 timeout on the client:: |
1395 | |
1396 | nstatuser@nstat-a:~$ nstat | grep -i abort |
1397 | TcpExtTCPAbortOnTimeout 10 0.0 |
1398 | |
1399 | TcpExtTCPAbortOnLinger |
ae5220c6 |
1400 | ---------------------- |
80cc4950 |
1401 | The server side code:: |
1402 | |
1403 | nstatuser@nstat-b:~$ cat server_linger.py |
1404 | import socket |
1405 | import time |
1406 | |
1407 | port = 9000 |
1408 | |
1409 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1410 | s.bind(('0.0.0.0', port)) |
1411 | s.listen(1) |
1412 | sock, addr = s.accept() |
1413 | while True: |
1414 | time.sleep(9999999) |
1415 | |
1416 | The client side code:: |
1417 | |
1418 | nstatuser@nstat-a:~$ cat client_linger.py |
1419 | import socket |
1420 | import struct |
1421 | |
1422 | server = 'nstat-b' # server address |
1423 | port = 9000 |
1424 | |
1425 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1426 | s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', 1, 10)) |
1427 | s.setsockopt(socket.SOL_TCP, socket.TCP_LINGER2, struct.pack('i', -1)) |
1428 | s.connect((server, port)) |
1429 | s.close() |
1430 | |
1431 | Run server_linger.py on server:: |
1432 | |
1433 | nstatuser@nstat-b:~$ python3 server_linger.py |
1434 | |
1435 | Run client_linger.py on client:: |
1436 | |
1437 | nstatuser@nstat-a:~$ python3 client_linger.py |
1438 | |
1439 | After run client_linger.py, check the output of nstat:: |
1440 | |
1441 | nstatuser@nstat-a:~$ nstat | grep -i abort |
1442 | TcpExtTCPAbortOnLinger 1 0.0 |
712ee16c |
1443 | |
1444 | TcpExtTCPRcvCoalesce |
ae5220c6 |
1445 | -------------------- |
712ee16c |
1446 | On the server, we run a program which listen on TCP port 9000, but |
1447 | doesn't read any data:: |
1448 | |
1449 | import socket |
1450 | import time |
1451 | port = 9000 |
1452 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1453 | s.bind(('0.0.0.0', port)) |
1454 | s.listen(1) |
1455 | sock, addr = s.accept() |
1456 | while True: |
1457 | time.sleep(9999999) |
1458 | |
1459 | Save the above code as server_coalesce.py, and run:: |
1460 | |
1461 | python3 server_coalesce.py |
1462 | |
1463 | On the client, save below code as client_coalesce.py:: |
1464 | |
1465 | import socket |
1466 | server = 'nstat-b' |
1467 | port = 9000 |
1468 | s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) |
1469 | s.connect((server, port)) |
1470 | |
1471 | Run:: |
1472 | |
1473 | nstatuser@nstat-a:~$ python3 -i client_coalesce.py |
1474 | |
1475 | We use '-i' to come into the interactive mode, then a packet:: |
1476 | |
1477 | >>> s.send(b'foo') |
1478 | 3 |
1479 | |
1480 | Send a packet again:: |
1481 | |
1482 | >>> s.send(b'bar') |
1483 | 3 |
1484 | |
1485 | On the server, run nstat:: |
1486 | |
1487 | ubuntu@nstat-b:~$ nstat |
1488 | #kernel |
1489 | IpInReceives 2 0.0 |
1490 | IpInDelivers 2 0.0 |
1491 | IpOutRequests 2 0.0 |
1492 | TcpInSegs 2 0.0 |
1493 | TcpOutSegs 2 0.0 |
1494 | TcpExtTCPRcvCoalesce 1 0.0 |
1495 | IpExtInOctets 110 0.0 |
1496 | IpExtOutOctets 104 0.0 |
1497 | IpExtInNoECTPkts 2 0.0 |
1498 | |
1499 | The client sent two packets, server didn't read any data. When |
1500 | the second packet arrived at server, the first packet was still in |
1501 | the receiving queue. So the TCP layer merged the two packets, and we |
1502 | could find the TcpExtTCPRcvCoalesce increased 1. |
1503 | |
1504 | TcpExtListenOverflows and TcpExtListenDrops |
ae5220c6 |
1505 | ------------------------------------------- |
712ee16c |
1506 | On server, run the nc command, listen on port 9000:: |
1507 | |
1508 | nstatuser@nstat-b:~$ nc -lkv 0.0.0.0 9000 |
1509 | Listening on [0.0.0.0] (family 0, port 9000) |
1510 | |
1511 | On client, run 3 nc commands in different terminals:: |
1512 | |
1513 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1514 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1515 | |
1516 | The nc command only accepts 1 connection, and the accept queue length |
1517 | is 1. On current linux implementation, set queue length to n means the |
1518 | actual queue length is n+1. Now we create 3 connections, 1 is accepted |
1519 | by nc, 2 in accepted queue, so the accept queue is full. |
1520 | |
1521 | Before running the 4th nc, we clean the nstat history on the server:: |
1522 | |
1523 | nstatuser@nstat-b:~$ nstat -n |
1524 | |
1525 | Run the 4th nc on the client:: |
1526 | |
1527 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1528 | |
1529 | If the nc server is running on kernel 4.10 or higher version, you |
1530 | won't see the "Connection to ... succeeded!" string, because kernel |
1531 | will drop the SYN if the accept queue is full. If the nc client is running |
1532 | on an old kernel, you would see that the connection is succeeded, |
1533 | because kernel would complete the 3 way handshake and keep the socket |
1534 | on half open queue. I did the test on kernel 4.15. Below is the nstat |
1535 | on the server:: |
1536 | |
1537 | nstatuser@nstat-b:~$ nstat |
1538 | #kernel |
1539 | IpInReceives 4 0.0 |
1540 | IpInDelivers 4 0.0 |
1541 | TcpInSegs 4 0.0 |
1542 | TcpExtListenOverflows 4 0.0 |
1543 | TcpExtListenDrops 4 0.0 |
1544 | IpExtInOctets 240 0.0 |
1545 | IpExtInNoECTPkts 4 0.0 |
1546 | |
1547 | Both TcpExtListenOverflows and TcpExtListenDrops were 4. If the time |
1548 | between the 4th nc and the nstat was longer, the value of |
1549 | TcpExtListenOverflows and TcpExtListenDrops would be larger, because |
1550 | the SYN of the 4th nc was dropped, the client was retrying. |
8e2ea53a |
1551 | |
1552 | IpInAddrErrors, IpExtInNoRoutes and IpOutNoRoutes |
ae5220c6 |
1553 | ------------------------------------------------- |
8e2ea53a |
1554 | server A IP address: 192.168.122.250 |
1555 | server B IP address: 192.168.122.251 |
1556 | Prepare on server A, add a route to server B:: |
1557 | |
1558 | $ sudo ip route add 8.8.8.8/32 via 192.168.122.251 |
1559 | |
1560 | Prepare on server B, disable send_redirects for all interfaces:: |
1561 | |
1562 | $ sudo sysctl -w net.ipv4.conf.all.send_redirects=0 |
1563 | $ sudo sysctl -w net.ipv4.conf.ens3.send_redirects=0 |
1564 | $ sudo sysctl -w net.ipv4.conf.lo.send_redirects=0 |
1565 | $ sudo sysctl -w net.ipv4.conf.default.send_redirects=0 |
1566 | |
1567 | We want to let sever A send a packet to 8.8.8.8, and route the packet |
1568 | to server B. When server B receives such packet, it might send a ICMP |
1569 | Redirect message to server A, set send_redirects to 0 will disable |
1570 | this behavior. |
1571 | |
1572 | First, generate InAddrErrors. On server B, we disable IP forwarding:: |
1573 | |
1574 | $ sudo sysctl -w net.ipv4.conf.all.forwarding=0 |
1575 | |
1576 | On server A, we send packets to 8.8.8.8:: |
1577 | |
1578 | $ nc -v 8.8.8.8 53 |
1579 | |
1580 | On server B, we check the output of nstat:: |
1581 | |
1582 | $ nstat |
1583 | #kernel |
1584 | IpInReceives 3 0.0 |
1585 | IpInAddrErrors 3 0.0 |
1586 | IpExtInOctets 180 0.0 |
1587 | IpExtInNoECTPkts 3 0.0 |
1588 | |
1589 | As we have let server A route 8.8.8.8 to server B, and we disabled IP |
1590 | forwarding on server B, Server A sent packets to server B, then server B |
1591 | dropped packets and increased IpInAddrErrors. As the nc command would |
1592 | re-send the SYN packet if it didn't receive a SYN+ACK, we could find |
1593 | multiple IpInAddrErrors. |
1594 | |
1595 | Second, generate IpExtInNoRoutes. On server B, we enable IP |
1596 | forwarding:: |
1597 | |
1598 | $ sudo sysctl -w net.ipv4.conf.all.forwarding=1 |
1599 | |
1600 | Check the route table of server B and remove the default route:: |
1601 | |
1602 | $ ip route show |
1603 | default via 192.168.122.1 dev ens3 proto static |
1604 | 192.168.122.0/24 dev ens3 proto kernel scope link src 192.168.122.251 |
1605 | $ sudo ip route delete default via 192.168.122.1 dev ens3 proto static |
1606 | |
1607 | On server A, we contact 8.8.8.8 again:: |
1608 | |
1609 | $ nc -v 8.8.8.8 53 |
1610 | nc: connect to 8.8.8.8 port 53 (tcp) failed: Network is unreachable |
1611 | |
1612 | On server B, run nstat:: |
1613 | |
1614 | $ nstat |
1615 | #kernel |
1616 | IpInReceives 1 0.0 |
1617 | IpOutRequests 1 0.0 |
1618 | IcmpOutMsgs 1 0.0 |
1619 | IcmpOutDestUnreachs 1 0.0 |
1620 | IcmpMsgOutType3 1 0.0 |
1621 | IpExtInNoRoutes 1 0.0 |
1622 | IpExtInOctets 60 0.0 |
1623 | IpExtOutOctets 88 0.0 |
1624 | IpExtInNoECTPkts 1 0.0 |
1625 | |
1626 | We enabled IP forwarding on server B, when server B received a packet |
1627 | which destination IP address is 8.8.8.8, server B will try to forward |
1628 | this packet. We have deleted the default route, there was no route for |
1629 | 8.8.8.8, so server B increase IpExtInNoRoutes and sent the "ICMP |
1630 | Destination Unreachable" message to server A. |
1631 | |
1632 | Third, generate IpOutNoRoutes. Run ping command on server B:: |
1633 | |
1634 | $ ping -c 1 8.8.8.8 |
1635 | connect: Network is unreachable |
1636 | |
1637 | Run nstat on server B:: |
1638 | |
1639 | $ nstat |
1640 | #kernel |
1641 | IpOutNoRoutes 1 0.0 |
1642 | |
1643 | We have deleted the default route on server B. Server B couldn't find |
1644 | a route for the 8.8.8.8 IP address, so server B increased |
1645 | IpOutNoRoutes. |
2b965472 |
1646 | |
1647 | TcpExtTCPACKSkippedSynRecv |
ae5220c6 |
1648 | -------------------------- |
2b965472 |
1649 | In this test, we send 3 same SYN packets from client to server. The |
1650 | first SYN will let server create a socket, set it to Syn-Recv status, |
1651 | and reply a SYN/ACK. The second SYN will let server reply the SYN/ACK |
1652 | again, and record the reply time (the duplicate ACK reply time). The |
1653 | third SYN will let server check the previous duplicate ACK reply time, |
1654 | and decide to skip the duplicate ACK, then increase the |
1655 | TcpExtTCPACKSkippedSynRecv counter. |
1656 | |
1657 | Run tcpdump to capture a SYN packet:: |
1658 | |
1659 | nstatuser@nstat-a:~$ sudo tcpdump -c 1 -w /tmp/syn.pcap port 9000 |
1660 | tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes |
1661 | |
1662 | Open another terminal, run nc command:: |
1663 | |
1664 | nstatuser@nstat-a:~$ nc nstat-b 9000 |
1665 | |
1666 | As the nstat-b didn't listen on port 9000, it should reply a RST, and |
1667 | the nc command exited immediately. It was enough for the tcpdump |
1668 | command to capture a SYN packet. A linux server might use hardware |
1669 | offload for the TCP checksum, so the checksum in the /tmp/syn.pcap |
1670 | might be not correct. We call tcprewrite to fix it:: |
1671 | |
1672 | nstatuser@nstat-a:~$ tcprewrite --infile=/tmp/syn.pcap --outfile=/tmp/syn_fixcsum.pcap --fixcsum |
1673 | |
1674 | On nstat-b, we run nc to listen on port 9000:: |
1675 | |
1676 | nstatuser@nstat-b:~$ nc -lkv 9000 |
1677 | Listening on [0.0.0.0] (family 0, port 9000) |
1678 | |
1679 | On nstat-a, we blocked the packet from port 9000, or nstat-a would send |
1680 | RST to nstat-b:: |
1681 | |
1682 | nstatuser@nstat-a:~$ sudo iptables -A INPUT -p tcp --sport 9000 -j DROP |
1683 | |
1684 | Send 3 SYN repeatly to nstat-b:: |
1685 | |
1686 | nstatuser@nstat-a:~$ for i in {1..3}; do sudo tcpreplay -i ens3 /tmp/syn_fixcsum.pcap; done |
1687 | |
1688 | Check snmp cunter on nstat-b:: |
1689 | |
1690 | nstatuser@nstat-b:~$ nstat | grep -i skip |
1691 | TcpExtTCPACKSkippedSynRecv 1 0.0 |
1692 | |
1693 | As we expected, TcpExtTCPACKSkippedSynRecv is 1. |
1694 | |
1695 | TcpExtTCPACKSkippedPAWS |
ae5220c6 |
1696 | ----------------------- |
2b965472 |
1697 | To trigger PAWS, we could send an old SYN. |
1698 | |
1699 | On nstat-b, let nc listen on port 9000:: |
1700 | |
1701 | nstatuser@nstat-b:~$ nc -lkv 9000 |
1702 | Listening on [0.0.0.0] (family 0, port 9000) |
1703 | |
1704 | On nstat-a, run tcpdump to capture a SYN:: |
1705 | |
1706 | nstatuser@nstat-a:~$ sudo tcpdump -w /tmp/paws_pre.pcap -c 1 port 9000 |
1707 | tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes |
1708 | |
1709 | On nstat-a, run nc as a client to connect nstat-b:: |
1710 | |
1711 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1712 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1713 | |
1714 | Now the tcpdump has captured the SYN and exit. We should fix the |
1715 | checksum:: |
1716 | |
1717 | nstatuser@nstat-a:~$ tcprewrite --infile /tmp/paws_pre.pcap --outfile /tmp/paws.pcap --fixcsum |
1718 | |
1719 | Send the SYN packet twice:: |
1720 | |
1721 | nstatuser@nstat-a:~$ for i in {1..2}; do sudo tcpreplay -i ens3 /tmp/paws.pcap; done |
1722 | |
1723 | On nstat-b, check the snmp counter:: |
1724 | |
1725 | nstatuser@nstat-b:~$ nstat | grep -i skip |
1726 | TcpExtTCPACKSkippedPAWS 1 0.0 |
1727 | |
1728 | We sent two SYN via tcpreplay, both of them would let PAWS check |
1729 | failed, the nstat-b replied an ACK for the first SYN, skipped the ACK |
1730 | for the second SYN, and updated TcpExtTCPACKSkippedPAWS. |
1731 | |
1732 | TcpExtTCPACKSkippedSeq |
ae5220c6 |
1733 | ---------------------- |
2b965472 |
1734 | To trigger TcpExtTCPACKSkippedSeq, we send packets which have valid |
1735 | timestamp (to pass PAWS check) but the sequence number is out of |
1736 | window. The linux TCP stack would avoid to skip if the packet has |
1737 | data, so we need a pure ACK packet. To generate such a packet, we |
1738 | could create two sockets: one on port 9000, another on port 9001. Then |
1739 | we capture an ACK on port 9001, change the source/destination port |
1740 | numbers to match the port 9000 socket. Then we could trigger |
1741 | TcpExtTCPACKSkippedSeq via this packet. |
1742 | |
1743 | On nstat-b, open two terminals, run two nc commands to listen on both |
1744 | port 9000 and port 9001:: |
1745 | |
1746 | nstatuser@nstat-b:~$ nc -lkv 9000 |
1747 | Listening on [0.0.0.0] (family 0, port 9000) |
1748 | |
1749 | nstatuser@nstat-b:~$ nc -lkv 9001 |
1750 | Listening on [0.0.0.0] (family 0, port 9001) |
1751 | |
1752 | On nstat-a, run two nc clients:: |
1753 | |
1754 | nstatuser@nstat-a:~$ nc -v nstat-b 9000 |
1755 | Connection to nstat-b 9000 port [tcp/*] succeeded! |
1756 | |
1757 | nstatuser@nstat-a:~$ nc -v nstat-b 9001 |
1758 | Connection to nstat-b 9001 port [tcp/*] succeeded! |
1759 | |
1760 | On nstat-a, run tcpdump to capture an ACK:: |
1761 | |
1762 | nstatuser@nstat-a:~$ sudo tcpdump -w /tmp/seq_pre.pcap -c 1 dst port 9001 |
1763 | tcpdump: listening on ens3, link-type EN10MB (Ethernet), capture size 262144 bytes |
1764 | |
1765 | On nstat-b, send a packet via the port 9001 socket. E.g. we sent a |
1766 | string 'foo' in our example:: |
1767 | |
1768 | nstatuser@nstat-b:~$ nc -lkv 9001 |
1769 | Listening on [0.0.0.0] (family 0, port 9001) |
1770 | Connection from nstat-a 42132 received! |
1771 | foo |
1772 | |
1773 | On nstat-a, the tcpdump should have caputred the ACK. We should check |
1774 | the source port numbers of the two nc clients:: |
1775 | |
1776 | nstatuser@nstat-a:~$ ss -ta '( dport = :9000 || dport = :9001 )' | tee |
1777 | State Recv-Q Send-Q Local Address:Port Peer Address:Port |
1778 | ESTAB 0 0 192.168.122.250:50208 192.168.122.251:9000 |
1779 | ESTAB 0 0 192.168.122.250:42132 192.168.122.251:9001 |
1780 | |
1781 | Run tcprewrite, change port 9001 to port 9000, chagne port 42132 to |
1782 | port 50208:: |
1783 | |
1784 | nstatuser@nstat-a:~$ tcprewrite --infile /tmp/seq_pre.pcap --outfile /tmp/seq.pcap -r 9001:9000 -r 42132:50208 --fixcsum |
1785 | |
1786 | Now the /tmp/seq.pcap is the packet we need. Send it to nstat-b:: |
1787 | |
1788 | nstatuser@nstat-a:~$ for i in {1..2}; do sudo tcpreplay -i ens3 /tmp/seq.pcap; done |
1789 | |
1790 | Check TcpExtTCPACKSkippedSeq on nstat-b:: |
1791 | |
1792 | nstatuser@nstat-b:~$ nstat | grep -i skip |
1793 | TcpExtTCPACKSkippedSeq 1 0.0 |