Hi Ben, Just trying to position our understanding of the position between the ICMP PTB and the IKE PTB.
If an incoming Encrypted packet is larger than the Link MTU, an ICMP PTB is sent, otherwise the packet is accepted. If fragments are received, a reassembly operation happens and the packet may be too large to be built or decrypted. I am unaware of any ICMP signaling between the gateway that could carry this information. As far as I understand, ICMP PTB is not issued for a reassembled packet as long as each of the fragments are below the MTU. This is the reason we send the EMTU_R which designates the maximum size the egress gateway can handle. EMTU_R could be a configuration parameter that would not change, but that value also considers the MTU of the router part which can be changed. As soon as it passes the interface, as I understand it, an ICMP PTB will be sent to the Source and not the gateway. As I understand it, EMTU_R defines the maximum payload the Link network is able to handle. In our case, the payload is the encrypted ESP packet that may have been reassembled. The packet is passed to the decryption. Once decrypted, the clear text packet is passed to the router of the node. The router may send an ICMP PTB, which will be sent to the Source or treat that packet. I see EMTU_R as reflecting the node of the router with Tunnel MTU = EMTU_R - ESP overhead Considering a ping encapsulated in esp - we may discover the MTU as well as the EMTU_R by fragmenting unless we meet EMTU_R. Note also that since we want to avoid fragmentation having a discovery mechanism that relies on fragmentation may not be the best idea. Yours, Daniel On Mon, Jul 31, 2023 at 1:22 PM Daniel Migault <[email protected]> wrote: > An encapsulated ICMP ECHO would get a response from the router (not the > interface) of the security gateway. I have not read carefully > draft-colitti-ipsecme-esp-ping but this is potentially what we could use > to discover that path upon which we could set DF=1. That said, if MTU > changes, we need to receive a notification. > I tend to think that extending colitti-ipsecme-esp-ping to other ICMP > plus defining PLMTU could replace our IKE PTB. > > > On Mon, Jul 31, 2023 at 12:57 PM Ben Schwartz <[email protected]> wrote: > >> It seems to me that the statement "This packet is too big for me to >> decrypt" is quite different from "This packet arrived fragmented". The >> former can generally be negotiated in the handshake, whereas the latter is >> a dynamic behavior of the underlying path. >> >> Monitoring the Path MTU is important, even when the path traverses an >> ICMP blackhole. So while I don't see the value of the PTB extension, I can >> understand the rationale for the LMAP extension. However, I would like to >> see a bit more description of the whole system. How do I send path probes >> to elicit these responses? Can I use ICMP ECHO inside the tunnel, or do we >> need draft-colitti-ipsecme-esp-ping? If we have path probes, why not just >> set DF=1 on the outer header for PMTUD? >> >> --Ben Schwartz >> ------------------------------ >> *From:* Daniel Migault <[email protected]> >> *Sent:* Monday, July 31, 2023 12:10 PM >> *To:* Ben Schwartz <[email protected]> >> *Cc:* Harold Liu <[email protected]>; >> [email protected] <[email protected]> >> *Subject:* Re: [IPsec] -ikev2-mtu-dect: IKEv2 PTB Notification >> >> Hi Ben, Please see my comments. On Mon, Jul 31, 2023 at 10: 47 AM Ben >> Schwartz <bemasc@ meta. com> wrote: Hi Harold, It sounds like you're >> describing a different problem. Daniel mentioned a concern about cases in >> which "the encrypted >> Hi Ben, >> Please see my comments. >> On Mon, Jul 31, 2023 at 10:47 AM Ben Schwartz <[email protected]> wrote: >> >> Hi Harold, >> >> It sounds like you're describing a different problem. Daniel mentioned a >> concern about cases in which "the encrypted packet is too big and so cannot >> be decrypted". >> >> We see the MTU indicating the size the packet the egress interface is >> able to handle which includes the ability to reassemble and decrypt the >> packet. In that sense, I see sending the EMTU_R as very similar to an ICMP >> PTB except. I am wondering if you see any reasons for these issues to be >> considered differently and how you think such distinction could help. >> >> That's quite different from an MTU limit on the forwarding path, which >> can be dealt with using ordinary IP fragmentation and PMTUD. >> >> Fragmentation works, but costs too much resources and this draft is >> aiming at reducing such operations. Our concern is with IPv4, where DF=1 >> leads to a blackholing situation. PMTUD is extremely difficult as ICMP >> messages are not received by the ingress gateway. >> PLMTUD I-D.spiriyath-ipsecme-dynamic-ipsec-pmtu for ESP is another path, >> but it would take a lot of effort. >> >> Yours, >> Daniel >> >> >> --Ben SchwartzI-D.spiriyath-ipsecme-dynamic-ipsec-pmtu >> ------------------------------ >> *From:* Harold Liu <[email protected]> >> *Sent:* Sunday, July 30, 2023 9:28 PM >> *To:* Ben Schwartz <[email protected]>; Daniel Migault <[email protected] >> > >> *Cc:* [email protected] <[email protected]> >> *Subject:* RE: [IPsec] -ikev2-mtu-dect: IKEv2 PTB Notification >> >> Ben, thanks for your comment. Yes at the beginning we thought what you >> thought, we consider the solution as “Negotiate it up front (in IKEv2)”, >> however the challenge here is the MTU of the router on the forwarding path >> can be changed at any >> >> Ben, thanks for your comment. >> >> >> >> Yes at the beginning we thought what you thought, we consider the >> solution as “Negotiate it up front (in IKEv2)”, however the challenge here >> is the MTU of the router on the forwarding path can be changed at any time >> (for example, the router changes the configuration for some reason, or >> changes the forwarding path for some reason). >> >> >> >> If the MTU of any forwarding node on the forwarding path changes (even as >> to the whole forwarding path changes), a pre-negotiated MTU is probably not >> applicable. Therefore, we defined the solution is to discover MTU >> in-band via error responses. >> >> >> >> Brs >> >> >> >> *From:* IPsec <[email protected]> *On Behalf Of *Ben Schwartz >> *Sent:* Saturday, July 29, 2023 8:01 AM >> *To:* Daniel Migault <[email protected]> >> *Cc:* [email protected] >> *Subject:* Re: [IPsec] -ikev2-mtu-dect: IKEv2 PTB Notification >> >> >> >> +mailing list (oops) >> >> >> >> I think I understand the difficulty here. In IPv6, a "maximum >> reassembled ESP size" can be modeled as a next-hop MTU on the plaintext, >> but in IPv4 an enormous ESP could be decrypted and fragmented forward over >> a next hop with a reasonable MTU. >> >> >> >> If this kind of ESP size limit is allowed, I think the best architecture >> would be to negotiate it up front (in IKEv2) since it is a static property >> of the endpoints, rather than discovering it in-band via error responses. >> >> >> >> --Ben Schwartz >> ------------------------------ >> >> *From:* Daniel Migault <[email protected]> >> *Sent:* Friday, July 28, 2023 10:47 AM >> *To:* Ben Schwartz <[email protected]> >> *Subject:* Re: [IPsec] -ikev2-mtu-dect: IKEv2 PTB Notification >> >> >> >> I see the next link as being the network behind the egress security >> gateway in which case the paquet would be the clear text packet. In that >> case maybe we could expect a ICMP PTB being sent to the source. The >> scenario we have is the packet >> >> I see the next link as being the network behind the egress security >> gateway in which case the paquet would be the clear text packet. In that >> case maybe we could expect a ICMP PTB being sent to the source. >> >> The scenario we have is the packet being so big that decryption cannot be >> performed - for example once reassembled. The egress security gateway has >> an ESP packet that it cannot process. The normal way would be to send an >> ICMP PTB but that ICMP PTB does not contain sufficient information for the >> egress to address the issue. The IKE message could be seen as duplicating >> the ICMP PTB with additional guarantees. >> >> >> >> Yours, >> Daniel >> >> >> >> On Fri, Jul 28, 2023 at 1:33 AM Ben Schwartz <[email protected]> wrote: >> >> I don't understand what it would mean for an ESP packet to be "too big to >> be decrypted". Do you mean that the decrypted payload is too big to >> deliver on the next link? >> >> >> >> --Ben Schwartz >> ------------------------------ >> >> *From:* IPsec <[email protected]> on behalf of Daniel Migault < >> [email protected]> >> *Sent:* Thursday, July 27, 2023 9:32 PM >> *To:* IPsecME WG <[email protected]> >> *Subject:* [IPsec] -ikev2-mtu-dect: IKEv2 PTB Notification >> >> >> >> In yesterday's presentation of the -ikev2-mtu-dect draft, I was asked why >> do we have such a notification instead of using a standard ICMP PTB message >> encapsulated in ESP. I believe the confusion comes from me saying that >> the PTB message >> >> In yesterday's presentation of the -ikev2-mtu-dect draft, I was asked why >> do we have such a notification instead of using a standard ICMP PTB message >> encapsulated in ESP. >> >> >> >> I believe the confusion comes from me saying that the PTB message is >> sent AFTER the packet has been decrypted. This is not the case as the PTB >> is sent BECAUSE the encrypted packet is too big and so cannot be decrypted. >> In other words the packet that is too big is the ESP packet. >> >> >> >> If the packet is too big and cannot be decrypted a Packet Too Big >> Notification (PTB) that specifies the Link MTU (LMTU) of the router >> component of the egress node (on network N) as well as the effective MTU to >> receive (EMTU_R). Both are configuration parameters. An ICMP PTB message >> may also be sent by the egress node. Note that this ICMP may not contain >> even the SPI, and so is likely to not carry sufficient information to the >> ingress node, so any action be taken. Typically the ICMP message only >> carries the first 8 bytes start from IP header of the original packets. This >> is not sufficient when encapsulated as the 8 bytes will not contain the SPI >> and the egress gateway will not be able to identify the concerned SA and so >> the concerned flow. >> >> >> >> Yours, >> Daniel >> >> >> >> >> -- >> >> Daniel Migault >> >> Ericsson >> >> >> >> >> -- >> >> Daniel Migault >> >> Ericsson >> >> >> >> -- >> Daniel Migault >> Ericsson >> > > > -- > Daniel Migault > Ericsson > -- Daniel Migault Ericsson
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