Packets 1-49 made it and thus the window is at 49. During this period, the packets may arrive at the receiver in an unintended order. If there is congestion on the link, or reliability issue of the path, then packet-loss will be observed. Here are the 6 major causes of the вЂњ %IPSEC-3-REPLAY_ERROR: IPSec SA receives anti-replay errorвЂќ log. Sources of Information for IPSEC anti-Replay Errors:.

QoS changes on active or in use policy-maps Security Association Re-key Operations (normal rekeying after lifetime expires). However this can easily be changed (increased) or disabled as weвЂ™ll see below. Thus packets must fall within this 64 sequence sliding window, or else be dropped. Now, back to the IPSEC anti-replay window-size problem and solutionsвЂ¦Д¬urrently, the default ipsec anti-replay window-size on IOS and IOS XE is 64 packets. Not to mention preventing certain other denial of services. Checking the sequence number before decrypting the packet makes sense or else we would just be wasting CPU cycles for no reason, because potentially the packet could be dropped after decryption due to sequence check failures. This also PROBABLY provides a mechanism to verify anti-replay before decryption, which is the most CPU intensive operation of the receiver. Thus no man (or woman) in the middle can tamper with the packetsвЂ™ sequence as they are authenticated. anti-replay ESP sequence is authenticated but not encrypted You might now be thinking, well if itвЂ™s not encrypted, whatвЂ™s to stop the malicious actor from just editing the packets since they are the middle?ДЄlthough the ESP header is NOT encrypted, it is authenticated via the ESP AUTH for both tunnel and transport mode. This allows for more advanced attacks on both IKEv1 and IKEv2.Д®arlier I mentioned this sequence is not encrypted. This feature is beneficial in scenarios where a malicious actor is sitting in between or inline with the traffic and is actively spoofing both sides. This behavior is further documented in RFC 2401. Note: This is specifically talking about if a packet with same ESP sequence number arrives, itвЂ™s not talking about TCP retrans which would have a new sequence number in the ESP header. If packet 40 arrived and was accepted, then it arrived again somehow, that second packet would be dropped. Both are accepted as they fall within the 64 packet window. The highest sequence number packet successfully received was 40, thus thatвЂ™s the current window size on the receiver. If packet 2 arrived after packet 12, or 63, it will be accepted as itвЂ™s within the 64 packet window. If packet 1 arrived after packet 69, it would be dropped. Packets 2-69 arrive, thus the current window size is 69. Here are some examples of what could happen: The other side then receives this and references its sliding window. Once that packet makes it to the other end (receiving end) is when the sequence is checked. HereвЂ™s what this looks like in a wireshark capture (ESP Sequence is the name in the header): ipsec anti-replay window-size as ESP sequence number Packets are literally marked in the data plane with a sequence number that is NOT encrypted.

Each new packet is encapsulated/encrypted and gets +1 added to its sequence number (in the ESP header) and is sent on.Д«asically, this numbering system provides anti-replay attacks for the receiving end. IPSEC Anti-Replay is a feature available to the ESP data plane that sequentially marks packets as they are encapsulated with a number.

Here is everything you need to know regarding the feature, the causes of the syslog, and the solutions to it. IвЂ™m sure youвЂ™ve all logged into a VPN Router once or twice and seen this syslog: %IOSXE-3-PLATFORM: R0/0: cpp_cp: QFP:0.0 Thread:000 TS: %IPSEC-3-REPLAY_ERROR: IPSec SA receives anti-replay error, DP Handle X, src_addr x.x.x.x, dest_addr y.y.y.y, SPI 0x0

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