1.1. Prior Work and Motivation

Encapsulating IKE connections within TCP streams is a common approach to solve the problem of UDP packets being blocked by network middleboxes. The specific goals of this document are as follows:¶

• To promote interoperability by defining a standard method of framing IKE and ESP messages within TCP streams.¶
• To be compatible with the current IKEv2 standard without requiring modifications or extensions.¶
• To use IKE over UDP by default to avoid the overhead of other alternatives that always rely on TCP or Transport Layer Security (TLS) [RFC5246] [RFC8446].¶

Some previous alternatives include:¶

Cellular Network Access:

Interworking Wireless LAN (IWLAN) uses IKEv2 to create secure connections to cellular carrier networks for making voice calls and accessing other network services over Wi-Fi networks. 3GPP has recommended that IKEv2 and ESP packets be sent within a TLS connection to be able to establish connections on restrictive networks.¶

ISAKMP over TCP:

Various non-standard extensions to the Internet Security Association and Key Management Protocol (ISAKMP) have been deployed that send IPsec traffic over TCP or TCP-like packets.¶

Secure Sockets Layer (SSL) VPNs:

Many proprietary VPN solutions use a combination of TLS and IPsec in order to provide reliability. These often run on TCP port 443.¶

IKEv2 over TCP:

IKEv2 over TCP as described in [IPSECME-IKE-TCP] is used to avoid UDP fragmentation.¶

TCP encapsulation for IKE and IPsec was defined in [RFC8229]. This document updates the specification for TCP encapsulation by including additional clarifications obtained during implementation and deployment of this method.¶

In particular:¶

• The interpretation of the Length field preceding every message is clarified (Section 3).¶
• The use of the NAT_DETECTION_*_IP notifications is clarified (Sections 5.1, 6.5, and 7.1).¶
• Retransmission behavior is clarified (Section 6.2).¶
• The use of cookies and puzzles is described in more detail (Section 6.3).¶
• Error handling is clarified (Section 6.4).¶
• Implications of TCP encapsulation on IPsec SA processing are expanded (Section 6.8).¶
• Section 7 describing interactions with other IKEv2 extensions is added.¶
• The interaction of TCP encapsulation with IKEv2 Mobility and Multihoming (MOBIKE) is clarified (Section 7.1).¶
• The recommendation for TLS encapsulation (Appendix A) now includes TLS 1.3.¶
• Examples of TLS encapsulation are provided using TLS 1.3 (Appendix B).¶
• More security considerations are added.¶

1.2. Terminology and Notation

This document distinguishes between the IKE peer that initiates TCP connections to be used for TCP encapsulation and the roles of Initiator and Responder for particular IKE messages. During the course of IKE exchanges, the role of IKE Initiator and Responder may swap for a given SA (as with IKE SA rekeys), while the Initiator of the TCP connection is still responsible for tearing down the TCP connection and re-establishing it if necessary. For this reason, this document will use the term "TCP Originator" to indicate the IKE peer that initiates TCP connections. The peer that receives TCP connections will be referred to as the "TCP Responder". If an IKE SA is rekeyed one or more times, the TCP Originator MUST remain the peer that originally initiated the first IKE SA.¶

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.1. TCP-Encapsulated IKE Message Format

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Non-ESP Marker                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                     IKE Message (RFC 7296)                    ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The IKE message is preceded by a 16-bit Length field in network byte order that specifies the length of the IKE message (including the non-ESP marker) within the TCP stream. As with IKE over UDP port 4500, a zeroed 32-bit non-ESP marker is inserted before the start of the IKE header in order to differentiate the traffic from ESP traffic between the same addresses and ports.¶

Length (2 octets, unsigned integer):

Length of the IKE message, including the Length field and non-ESP marker. The value in the Length field MUST NOT be 0 or 1. The receiver MUST treat these values as fatal errors and MUST close the TCP connection.¶

Non-ESP Marker (4 octets):

Four zero-valued bytes.¶

3.2. TCP-Encapsulated ESP Packet Format

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                                +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                |            Length             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
~                     ESP Packet (RFC 4303)                     ~
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The ESP packet is preceded by a 16-bit Length field in network byte order that specifies the length of the ESP packet within the TCP stream.¶

The Security Parameter Index (SPI) field [RFC7296] in the ESP header MUST NOT be a zero value.¶

Length (2 octets, unsigned integer):

Length of the ESP packet, including the Length field. The value in the Length field MUST NOT be 0 or 1. The receiver MUST treat these values as fatal errors and MUST close TCP connection.¶

5.1. Recommended Fallback from UDP

Since UDP is the preferred method of transport for IKE messages, implementations that use TCP encapsulation should have an algorithm for deciding when to use TCP after determining that UDP is unusable. If an Initiator implementation has no prior knowledge about the network it is on and the status of UDP on that network, it SHOULD always attempt to negotiate IKE over UDP first. IKEv2 defines how to use retransmission timers with IKE messages and, specifically, IKE_SA_INIT messages [RFC7296]. Generally, this means that the implementation will define a frequency of retransmission and the maximum number of retransmissions allowed before marking the IKE SA as failed. An implementation can attempt negotiation over TCP once it has hit the maximum retransmissions over UDP, or slightly before to reduce connection setup delays. It is recommended that the initial message over UDP be retransmitted at least once before falling back to TCP, unless the Initiator knows beforehand that the network is likely to block UDP.¶

When switching from UDP to TCP, a new IKE_SA_INIT exchange MUST be initiated with the Initiator's new SPI and with recalculated content of NAT_DETECTION_*_IP notifications.¶

6.1. Connection Establishment and Teardown

When the IKE Initiator uses TCP encapsulation, it will initiate a TCP connection to the Responder using the Responder's preconfigured TCP port. The first bytes sent on the TCP stream MUST be the stream prefix value (Section 4). After this prefix, encapsulated IKE messages will negotiate the IKE SA and initial Child SA [RFC7296]. After this point, both encapsulated IKE (Figure 1) and ESP (Figure 2) messages will be sent over the TCP connection. The TCP Responder MUST wait for the entire stream prefix to be received on the stream before trying to parse out any IKE or ESP messages. The stream prefix is sent only once, and only by the TCP Originator.¶

In order to close an IKE session, either the Initiator or Responder SHOULD gracefully tear down IKE SAs with DELETE payloads. Once the SA has been deleted, the TCP Originator SHOULD close the TCP connection if it does not intend to use the connection for another IKE session to the TCP Responder. If the TCP connection is no longer associated with any active IKE SA, the TCP Responder MAY close the connection to clean up IKE resources if the TCP Originator didn't close it within some reasonable period of time (e.g., a few seconds).¶

An unexpected FIN or a TCP Reset on the TCP connection may indicate a loss of connectivity, an attack, or some other error. If a DELETE payload has not been sent, both sides SHOULD maintain the state for their SAs for the standard lifetime or timeout period. The TCP Originator is responsible for re-establishing the TCP connection if it is torn down for any unexpected reason. Since new TCP connections may use different IP addresses and/or ports due to NAT mappings or local address or port allocations changing, the TCP Responder MUST allow packets for existing SAs to be received from new source IP addresses and ports. Note that the IPv6 Flow-ID header MUST remain constant when a new TCP connection is created to avoid ECMP load balancing.¶

A peer MUST discard a partially received message due to a broken connection.¶

Whenever the TCP Originator opens a new TCP connection to be used for an existing IKE SA, it MUST send the stream prefix first, before any IKE or ESP messages. This follows the same behavior as the initial TCP connection.¶

Multiple IKE SAs MUST NOT share a single TCP connection, unless one is a rekey of an existing IKE SA, in which case there will temporarily be two IKE SAs on the same TCP connection.¶

If a TCP connection is being used to continue an existing IKE/ESP session, the TCP Responder can recognize the session using either the IKE SPI from an encapsulated IKE message or the ESP SPI from an encapsulated ESP packet. If the session had been fully established previously, it is suggested that the TCP Originator send an UPDATE_SA_ADDRESSES message if MOBIKE is supported and an empty informational message if it is not.¶

The TCP Responder MUST NOT accept any messages for the existing IKE session on a new incoming connection, unless that connection begins with the stream prefix. If either the TCP Originator or TCP Responder detects corruption on a connection that was started with a valid stream prefix, it SHOULD close the TCP connection. The connection can be corrupted if there are too many subsequent messages that cannot be parsed as valid IKE messages or ESP messages with known SPIs, or if the authentication check for an IKE message or ESP message with a known SPI fails. Implementations SHOULD NOT tear down a connection if only a few consecutive ESP packets have unknown SPIs since the SPI databases may be momentarily out of sync. If there is instead a syntax issue within an IKE message, an implementation MUST send the INVALID_SYNTAX notify payload and tear down the IKE SA as usual, rather than tearing down the TCP connection directly.¶

A TCP Originator SHOULD only open one TCP connection per IKE SA, over which it sends all of the corresponding IKE and ESP messages. This helps ensure that any firewall or NAT mappings allocated for the TCP connection apply to all of the traffic associated with the IKE SA equally.¶

As with TCP Originators, a TCP Responder SHOULD send packets for an IKE SA and its Child SAs over only one TCP connection at any given time. It SHOULD choose the TCP connection on which it last received a valid and decryptable IKE or ESP message. In order to be considered valid for choosing a TCP connection, an IKE message must be successfully decrypted and authenticated, not be a retransmission of a previously received message, and be within the expected window for IKE message IDs. Similarly, an ESP message must be successfully decrypted and authenticated, and must not be a replay of a previous message.¶

Since a connection may be broken and a new connection re-established by the TCP Originator without the TCP Responder being aware, a TCP Responder SHOULD accept receiving IKE and ESP messages on both old and new connections until the old connection is closed by the TCP Originator. A TCP Responder MAY close a TCP connection that it perceives as idle and extraneous (one previously used for IKE and ESP messages that has been replaced by a new connection).¶

6.2. Retransmissions

Section 2.1 of [RFC7296] describes how IKEv2 deals with the unreliability of the UDP protocol. In brief, the exchange Initiator is responsible for retransmissions and must retransmit request messages until a response message is received. If no reply is received after several retransmissions, the SA is deleted. The Responder never initiates retransmission, but it must send a response message again in case it receives a retransmitted request.¶

When IKEv2 uses a reliable transport protocol, like TCP, the retransmission rules are as follows:¶

• The exchange Initiator SHOULD NOT retransmit request message (*); if no response is received within some reasonable period of time, the IKE SA is deleted.¶
• If a new TCP connection for the IKE SA is established while the exchange Initiator is waiting for a response, the Initiator MUST retransmit its request over this connection and continue to wait for a response.¶
• The exchange Responder does not change its behavior, but acts as described in Section 2.1 of [RFC7296].¶

(*) This is an optimization; implementations may continue to use the retransmission logic from Section 2.1 of [RFC7296] for simplicity.¶

6.3. Cookies and Puzzles

IKEv2 provides a DoS attack protection mechanism through Cookies, which is described in Section 2.6 of [RFC7296]. [RFC8019] extends this mechanism for protection against DDoS attacks by means of Client Puzzles. Both mechanisms allow the Responder to avoid keeping state until the Initiator proves its IP address is legitimate (and after solving a puzzle if required).¶

The connection-oriented nature of TCP transport brings additional considerations for using these mechanisms. In general, Cookies provide less value in the case of TCP encapsulation; by the time a Responder receives the IKE_SA_INIT request, the TCP session has already been established and the Initiator's IP address has been verified. Moreover, a TCP/IP stack creates state once a TCP SYN packet is received (unless SYN Cookies described in [RFC4987] are employed), which contradicts the statelessness of IKEv2 Cookies. In particular, with TCP, an attacker is able to mount a SYN flooding DoS attack that an IKEv2 Responder cannot prevent using stateless IKEv2 Cookies. Thus, when using TCP encapsulation, it makes little sense to send Cookie requests without Puzzles unless the Responder is concerned with a possibility of TCP sequence number attacks (see [RFC6528] and [RFC9293] for details). Puzzles, on the other hand, still remain useful (and their use requires using Cookies).¶

The following considerations are applicable for using Cookie and Puzzle mechanisms in the case of TCP encapsulation:¶

• The exchange Responder SHOULD NOT send an IKEv2 Cookie request without an accompanied Puzzle; implementations might choose to have exceptions to this for cases like mitigating TCP sequence number attacks.¶
• If the Responder chooses to send a Cookie request (possibly along with Puzzle request), then the TCP connection that the IKE_SA_INIT request message was received over SHOULD be closed after the Responder sends its reply and no repeated requests are received within some short period of time to keep the Responder stateless (see Section 6.3.1). Note that the Responder MUST NOT include the Initiator's TCP port into the Cookie calculation (*) since the Cookie can be returned over a new TCP connection with a different port.¶
• The exchange Initiator acts as described in Section 2.6 of [RFC7296] and Section 7 of [RFC8019], i.e., using TCP encapsulation doesn't change the Initiator's behavior.¶

(*) Examples of Cookie calculation methods are given in Section 2.6 of [RFC7296] and in Section 7.1.1.3 of [RFC8019], and they don't include transport protocol ports. However, these examples are given for illustrative purposes since the Cookie generation algorithm is a local matter and some implementations might include port numbers that won't work with TCP encapsulation. Note also that these examples include the Initiator's IP address in Cookie calculation. In general, this address may change between two initial requests (with and without Cookies). This may happen due to NATs, which have more freedom to change source IP addresses for new TCP connections than for UDP. In such cases, cookie verification might fail.¶

6.4. Error Handling in IKE_SA_INIT

Section 2.21.1 of [RFC7296] describes how error notifications are handled in the IKE_SA_INIT exchange. In particular, it is advised that the Initiator should not act immediately after receiving an error notification; instead, it should wait some time for a valid response since the IKE_SA_INIT messages are completely unauthenticated. This advice does not apply equally in the case of TCP encapsulation. If the Initiator receives a response message over TCP, then either this message is genuine and was sent by the peer or the TCP session was hijacked and the message is forged. In the latter case, no genuine messages from the Responder will be received.¶

Thus, in the case of TCP encapsulation, an Initiator SHOULD NOT wait for additional messages in case it receives an error notification from the Responder in the IKE_SA_INIT exchange.¶

In the IKE_SA_INIT exchange, if the Responder returns an error notification that implies a recovery action from the Initiator (such as INVALID_KE_PAYLOAD or INVALID_MAJOR_VERSION, see Section 2.21.1 of [RFC7296]), then the Responder SHOULD NOT close the TCP connection immediately in anticipation of the fact that the Initiator will repeat the request with corrected parameters. See also Section 6.3.¶

6.5. NAT-Detection Payloads

When negotiating over UDP, IKE_SA_INIT packets include NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP payloads to determine if UDP encapsulation of IPsec packets should be used. These payloads contain SHA-1 digests of the SPIs, IP addresses, and ports as defined in [RFC7296]. IKE_SA_INIT packets sent on a TCP connection SHOULD include these payloads with the same content as when sending over UDP and SHOULD use the applicable TCP ports when creating and checking the SHA-1 digests.¶

If a NAT is detected due to the SHA-1 digests not matching the expected values, no change should be made for encapsulation of subsequent IKE or ESP packets since TCP encapsulation inherently supports NAT traversal. However, for the transport mode IPsec SAs, implementations need to handle TCP and UDP packet checksum fixup during decapsulation, as defined for UDP encapsulation in [RFC3948].¶

Implementations MAY use the information that a NAT is present to influence keepalive timer values.¶

6.6. NAT-Keepalive Packets

Encapsulating IKE and IPsec inside of a TCP connection can impact the strategy that implementations use to maintain middlebox port mappings.¶

In general, TCP port mappings are maintained by NATs longer than UDP port mappings, so IPsec ESP NAT-keepalive packets [RFC3948] SHOULD NOT be sent when using TCP encapsulation. Any implementation using TCP encapsulation MUST silently drop incoming NAT-keepalive packets and not treat them as errors. NAT-keepalive packets over a TCP-encapsulated IPsec connection will be sent as a 1-octet-long payload with the value 0xFF, preceded by the 2-octet Length specifying a length of 3 (since it includes the length of the Length field).¶

6.7. Dead Peer Detection and Transport Keepalives

Peer liveness should be checked using IKE informational packets [RFC7296].¶

Note that, depending on the configuration of TCP and TLS on the connection, TCP keep-alives [RFC1122] and TLS keep-alives [RFC6520] MAY be used. These MUST NOT be used as indications of IKE peer liveness, for which purpose the standard IKEv2 mechanism of exchanging (usually empty) INFORMATIONAL messages is used (see Section 1.4 of [RFC7296]).¶

6.8. Implications of TCP Encapsulation on IPsec SA Processing

Using TCP encapsulation affects some aspects of IPsec SA processing.¶

1. Section 8.1 of [RFC4301] requires all tunnel mode IPsec SAs to be able to copy the Don't Fragment (DF) bit from inner IPv4 header to the outer (tunnel) one. With TCP encapsulation, this is generally not possible because the TCP/IP stack manages the DF bit in the outer IPv4 header, and usually the stack ensures that the DF bit is set for TCP packets to avoid IP fragmentation. Note, that this behavior is compliant with generic tunneling considerations since the outer TCP header acts as a link-layer protocol and its fragmentation and reassembly have no correlation with the inner payload.¶
2. The other feature that is less applicable with TCP encapsulation is an ability to split traffic of different QoS classes into different IPsec SAs, created by a single IKE SA. In this case, the Differentiated Services Code Point (DSCP) field is usually copied from the inner IP header to the outer (tunnel) one, ensuring that IPsec traffic of each SA receives the corresponding level of service. With TCP encapsulation, all IPsec SAs created by a single IKE SA will share a single TCP connection; thus, they will receive the same level of service (see Section 9.3). If this functionality is needed, implementations should create several IKE SAs each over separate TCP connections and assign a corresponding DSCP value to each of them.¶

TCP encapsulation of IPsec packets may have implications on performance of the encapsulated traffic. Performance considerations are discussed in Section 9.¶

7.1. MOBIKE Protocol

The MOBIKE protocol, which allows SAs to migrate between IP addresses, is defined in [RFC4555]; [RFC4621] further clarifies the details of the protocol. When an IKE session that has negotiated MOBIKE is transitioning between networks, the Initiator of the transition may switch between using TCP encapsulation, UDP encapsulation, or no encapsulation. Implementations that implement both MOBIKE and TCP encapsulation within the same connection configuration MUST support dynamically enabling and disabling TCP encapsulation as interfaces change.¶

When a MOBIKE-enabled Initiator changes networks, the INFORMATIONAL exchange with the UPDATE_SA_ADDRESSES notification SHOULD be initiated first over UDP before attempting over TCP. If there is a response to the request sent over UDP, then the ESP packets should be sent directly over IP or over UDP port 4500 (depending on if a NAT was detected), regardless of if a connection on a previous network was using TCP encapsulation. If no response is received within a certain period of time after several retransmissions, the Initiator ought to change its transport for this exchange from UDP to TCP and resend the request message. A new INFORMATIONAL exchange MUST NOT be started in this situation. If the Responder only responds to the request sent over TCP, then the ESP packets should be sent over the TCP connection, regardless of if a connection on a previous network did not use TCP encapsulation.¶

The value of the timeout and the specific number of retransmissions before switching to TCP can vary depending on the Initiator's configuration. Implementations ought to provide reasonable defaults to ensure that UDP attempts have a chance to succeed, but can shorten the timeout based on historical data or metrics.¶

If the TCP transport was used for the previous network connection, the old TCP connection SHOULD be closed by the Initiator once MOBIKE finishes migration to a new connection (either TCP or UDP).¶

Since switching from UDP to TCP can happen during a single INFORMATIONAL message exchange, the content of the NAT_DETECTION_*_IP notifications will in most cases be incorrect (since UDP and TCP ports will most likely be different), and the peer may incorrectly detect the presence of a NAT. Section 3.5 of [RFC4555] states that a new INFORMATIONAL exchange with the UPDATE_SA_ADDRESSES notify is initiated in case the address (or transport) is changed while waiting for a response.¶

Section 3.5 of [RFC4555] also states that once an IKE SA is switched to a new IP address, all outstanding requests in this SA are immediately retransmitted using this address. See also Section 6.2.¶

The MOBIKE protocol defines the NO_NATS_ALLOWED notification that can be used to detect the presence of NAT between peer and to refuse to communicate in this situation. In the case of TCP, the NO_NATS_ALLOWED notification SHOULD be ignored because TCP generally has no problems with NAT boxes.¶

Section 3.7 of [RFC4555] describes an additional optional step in the process of changing IP addresses called "Return Routability Check". It is performed by Responders in order to be sure that the new Initiator's address is, in fact, routable. In the case of TCP encapsulation, this check has little value since a TCP handshake proves the routability of the TCP Originator's address; thus, the Return Routability Check SHOULD NOT be performed.¶

7.2. IKE Redirect

A redirect mechanism for IKEv2 is defined in [RFC5685]. This mechanism allows security gateways to redirect clients to another gateway either during IKE SA establishment or after session setup. If a client is connecting to a security gateway using TCP and then is redirected to another security gateway, the client needs to reset its transport selection. In other words, with the next security gateway, the client MUST first try UDP and then fall back to TCP while establishing a new IKE SA, regardless of the transport of the SA the redirect notification was received over (unless the client's configuration instructs it to instantly use TCP for the gateway it is redirected to).¶

7.3. IKEv2 Session Resumption

Session resumption for IKEv2 is defined in [RFC5723]. Once an IKE SA is established, the server creates a resumption ticket where information about this SA is stored and transfers this ticket to the client. The ticket may be later used to resume the IKE SA after it is deleted. In the event of resumption, the client presents the ticket in a new exchange, called IKE_SESSION_RESUME. Some parameters in the new SA are retrieved from the ticket and others are renegotiated (more details are given in Section 5 of [RFC5723]).¶

Since network conditions may change while the client is inactive, the fact that TCP encapsulation was used in an old SA SHOULD NOT affect which transport is used during session resumption. In other words, the transport should be selected as if the IKE SA is being created from scratch.¶

7.4. IKEv2 Protocol Support for High Availability

[RFC6311] defines a support for High Availability in IKEv2. In case of cluster failover, a new active node must immediately initiate a special INFORMATION exchange containing the IKEV2_MESSAGE_ID_SYNC notification, which instructs the client to skip some number of Message IDs that might not be synchronized yet between nodes at the time of failover.¶

Synchronizing states when using TCP encapsulation is much harder than when using UDP; doing so requires access to TCP/IP stack internals, which is not always available from an IKE/IPsec implementation. If a cluster implementation doesn't synchronize TCP states between nodes, then after failover event the new active node will not have any TCP connection with the client, so the node cannot initiate the INFORMATIONAL exchange as required by [RFC6311]. Since the cluster usually acts as TCP Responder, the new active node cannot re- establish TCP connection because only the TCP Originator can do it. For the client, the cluster failover event may remain undetected for long time if it has no IKE or ESP traffic to send. Once the client sends an ESP or IKEv2 packet, the cluster node will reply with TCP RST and the client (as TCP Originator) will reestablish the TCP connection so that the node will be able to initiate the INFORMATIONAL exchange informing the client about the cluster failover.¶

This document makes the following recommendation: if support for High Availability in IKEv2 is negotiated and TCP transport is used, a client that is a TCP Originator SHOULD periodically send IKEv2 messages (e.g., by initiating liveness check exchange) whenever there is no IKEv2 or ESP traffic. This differs from the recommendations given in Section 2.4 of [RFC7296] in the following: the liveness check should be periodically performed even if the client has nothing to send over ESP. The frequency of sending such messages should be high enough to allow quick detection and restoration of broken TCP connections.¶

7.5. IKEv2 Fragmentation

IKE message fragmentation [RFC7383] is not required when using TCP encapsulation since a TCP stream already handles the fragmentation of its contents across packets. Since fragmentation is redundant in this case, implementations might choose to not negotiate IKE fragmentation. Even if fragmentation is negotiated, an implementation SHOULD NOT send fragments when going over a TCP connection, although it MUST support receiving fragments.¶

If an implementation supports both MOBIKE and IKE fragmentation, it SHOULD negotiate IKE fragmentation over a TCP-encapsulated session in case the session switches to UDP encapsulation on another network.¶

9.1. TCP-in-TCP

If the outer connection between IKE peers is over TCP, inner TCP connections may suffer negative effects from using TCP within TCP. Running TCP within TCP is discouraged since the TCP algorithms generally assume that they are running over an unreliable datagram layer.¶

If the outer (tunnel) TCP connection experiences packet loss, this loss will be hidden from any inner TCP connections since the outer connection will retransmit to account for the losses. Since the outer TCP connection will deliver the inner messages in order, any messages after a lost packet may have to wait until the loss is recovered. This means that loss on the outer connection will be interpreted only as delay by inner connections. The burstiness of inner traffic can increase since a large number of inner packets may be delivered across the tunnel at once. The inner TCP connection may interpret a long period of delay as a transmission problem, triggering a retransmission timeout, which will cause spurious retransmissions. The sending rate of the inner connection may be unnecessarily reduced if the retransmissions are not detected as spurious in time.¶

The inner TCP connection's round-trip-time estimation will be affected by the burstiness of the outer TCP connection if there are long delays when packets are retransmitted by the outer TCP connection. This will make the congestion control loop of the inner TCP traffic less reactive, potentially permanently leading to a lower sending rate than the outer TCP would allow for.¶

TCP-in-TCP can also lead to "TCP meltdown", where stacked instances of TCP can result in significant impacts to performance [TCP-MELTDOWN]. This can occur when losses in the lower TCP (closer to the link) increase delays seen by the higher TCP (closer to the application) that create timeouts, which, in turn, cause retransmissions that can then cause losses in the lower TCP by overrunning its buffer. The very mechanism intended to avoid loss (retransmission) interacts between the two layers to increase loss. To limit this effect, the timeouts of the two TCP layers need to be carefully managed, e.g., such that the higher layer has a much longer timeout than the lower layer.¶

Note that any negative effects will be shared among all flows going through the outer TCP connection. This is of particular concern for any latency-sensitive or real-time applications using the tunnel. If such traffic is using a TCP-encapsulated IPsec connection, it is recommended that the number of inner connections sharing the tunnel be limited as much as possible.¶

9.2. Added Reliability for Unreliable Protocols

Since ESP is an unreliable protocol, transmitting ESP packets over a TCP connection will change the fundamental behavior of the packets. Some application-level protocols that prefer packet loss to delay (such as Voice over IP or other real-time protocols) may be negatively impacted if their packets are retransmitted by the TCP connection due to packet loss.¶

9.3. Quality-of-Service Markings

Quality-of-Service (QoS) markings, such as the Differentiated Services Code Point (DSCP) and Traffic Class, should be used with care on TCP connections used for encapsulation. Individual packets SHOULD NOT use different markings than the rest of the connection since packets with different priorities may be routed differently and cause unnecessary delays in the connection.¶

9.4. Maximum Segment Size

A TCP connection used for IKE encapsulation SHOULD negotiate its MSS in order to avoid unnecessary fragmentation of packets.¶

9.5. Tunneling ECN in TCP

Since there is not a one-to-one relationship between outer IP packets and inner ESP/IP messages when using TCP encapsulation, the markings for Explicit Congestion Notification (ECN) [RFC3168] cannot easily be mapped. However, any ECN Congestion Experienced (CE) marking on inner headers should be preserved through the tunnel.¶

Implementations SHOULD follow the ECN compatibility mode for tunnel ingress as described in [RFC6040]. In compatibility mode, the outer tunnel TCP connection marks its packet headers as not ECN-capable.¶

Upon egress, if the arriving outer header is marked with CE, the implementation will drop the inner packet since there is not a distinct inner packet header onto which to translate the ECN markings.¶

B.1. Establishing an IKE Session

                Client                              Server
              ----------                          ----------
  1)  --------------------  TCP Connection  -------------------
      (IP_I:Port_I  -> IP_R:Port_R)
      TcpSyn                   ------->
                               <-------              TcpSyn,Ack
      TcpAck                   ------->
  2)  ---------------------  TLS Session  ---------------------
      ClientHello              ------->
                                                    ServerHello
                                          {EncryptedExtensions}
                                                 {Certificate*}
                                           {CertificateVerify*}
                               <-------              {Finished}
      {Finished}               ------->
  3)  ---------------------- Stream Prefix --------------------
      "IKETCP"                 ------->
  4)  ----------------------- IKE Session ---------------------
      Length + Non-ESP Marker  ------->
      IKE_SA_INIT
      HDR, SAi1, KEi, Ni,
      [N(NAT_DETECTION_SOURCE_IP)],
      [N(NAT_DETECTION_DESTINATION_IP)]
                               <------- Length + Non-ESP Marker
                                                    IKE_SA_INIT
                                            HDR, SAr1, KEr, Nr,
                                  [N(NAT_DETECTION_SOURCE_IP)],
                              [N(NAT_DETECTION_DESTINATION_IP)]
      Length + Non-ESP Marker  ------->
      first IKE_AUTH
      HDR, SK {IDi, [CERTREQ]
      CP(CFG_REQUEST), IDr,
      SAi2, TSi, TSr, ...}
                               <------- Length + Non-ESP Marker
                                                 first IKE_AUTH
                                    HDR, SK {IDr, [CERT], AUTH,
                                           EAP, SAr2, TSi, TSr}
      Length + Non-ESP Marker  ------->
      IKE_AUTH (repeat 1..N times)
      HDR, SK {EAP}
                               <------- Length + Non-ESP Marker
                                   IKE_AUTH (repeat 1..N times)
                                                   HDR SK {EAP}
      Length + Non-ESP Marker  ------->
      final IKE_AUTH
      HDR, SK {AUTH}
                               <------- Length + Non-ESP Marker
                                                 final IKE_AUTH
                                  HDR, SK {AUTH, CP(CFG_REPLY),
                                             SA, TSi, TSr, ...}
      -------------- IKE and IPsec SAs Established ------------
      Length + ESP Frame       ------->

1. The client establishes a TCP connection with the server on port 4500 or on an alternate preconfigured port that the server is listening on.¶
2. If configured to use TLS, the client initiates a TLS handshake. During the TLS handshake, the server SHOULD NOT request the client's certificate since authentication is handled as part of IKE negotiation.¶
3. The client sends the stream prefix for TCP-encapsulated IKE (Section 4) traffic to signal the beginning of IKE negotiation.¶
4. The client and server establish an IKE connection. This example shows EAP-based authentication, although any authentication type may be used.¶

B.2. Deleting an IKE Session

                Client                              Server
              ----------                          ----------
  1)  ----------------------- IKE Session ---------------------
      Length + Non-ESP Marker  ------->
      INFORMATIONAL
      HDR, SK {[N,] [D,]
             [CP,] ...}
                               <------- Length + Non-ESP Marker
                                                  INFORMATIONAL
                                             HDR, SK {[N,] [D,]
                                                     [CP], ...}
  2)  ---------------------  TLS Session  ---------------------
      close_notify             ------->
                               <-------            close_notify
  3)  --------------------  TCP Connection  -------------------
      TcpFin                   ------->
                               <-------                     Ack
                               <-------                  TcpFin
      Ack                      ------->
      --------------------  IKE SA Deleted  -------------------

1. The client and server exchange informational messages to notify IKE SA deletion.¶
2. The client and server negotiate TLS session deletion using TLS CLOSE_NOTIFY.¶
3. The TCP connection is torn down.¶

The deletion of the IKE SA should lead to the disposal of the underlying TLS and TCP state.¶

B.3. Re-establishing an IKE Session

                Client                              Server
              ----------                          ----------
  1)  --------------------  TCP Connection  -------------------
      (IP_I:Port_I  -> IP_R:Port_R)
      TcpSyn                   ------->
                               <-------              TcpSyn,Ack
      TcpAck                   ------->
  2)  ---------------------  TLS Session  ---------------------
      ClientHello              ------->
                                                    ServerHello
                                          {EncryptedExtensions}
                               <-------              {Finished}
      {Finished}               ------->
  3)  ---------------------- Stream Prefix --------------------
      "IKETCP"                 ------->
  4)  <---------------------> IKE/ESP Flow <------------------>

1. If a previous TCP connection was broken (for example, due to a TCP Reset), the client is responsible for re-initiating the TCP connection. The TCP Originator's address and port (IP_I and Port_I) may be different from the previous connection's address and port.¶
2. The client SHOULD attempt TLS session resumption if it has previously established a session with the server.¶
3. After TCP and TLS are complete, the client sends the stream prefix for TCP-encapsulated IKE traffic (Section 4).¶
4. The IKE and ESP packet flow can resume. If MOBIKE is being used, the Initiator SHOULD send an UPDATE_SA_ADDRESSES message.¶

B.4. Using MOBIKE between UDP and TCP Encapsulation

                  Client                              Server
                ----------                          ----------
  1)  --------------------- IKE_session ----------------------
      (IP_I1:UDP500 -> IP_R:UDP500)
      IKE_SA_INIT              ------->
      HDR, SAi1, KEi, Ni,
      [N(NAT_DETECTION_SOURCE_IP)],
      [N(NAT_DETECTION_DESTINATION_IP)]
                               <-------            IKE_SA_INIT
                                            HDR, SAr1, KEr, Nr,
                                  [N(NAT_DETECTION_SOURCE_IP)],
                              [N(NAT_DETECTION_DESTINATION_IP)]
      (IP_I1:UDP4500 -> IP_R:UDP4500)
      Non-ESP Marker           ------->
      IKE_AUTH
      HDR, SK { IDi, CERT, AUTH,
      SAi2, TSi, TSr,
      N(MOBIKE_SUPPORTED) }
                               <-------          Non-ESP Marker
                                                       IKE_AUTH
                                     HDR, SK { IDr, CERT, AUTH,
                                                SAr2, TSi, TSr,
                                          N(MOBIKE_SUPPORTED) }
      <---------------------> IKE/ESP Flow <------------------>
  2)  ------------ MOBIKE Attempt on New Network --------------
      (IP_I2:UDP4500 -> IP_R:UDP4500)
      Non-ESP Marker           ------->
      INFORMATIONAL
      HDR, SK { N(UPDATE_SA_ADDRESSES),
      N(NAT_DETECTION_SOURCE_IP),
      N(NAT_DETECTION_DESTINATION_IP) }
  3)  --------------------  TCP Connection  -------------------
      (IP_I2:Port_I -> IP_R:Port_R)
      TcpSyn                   ------->
                               <-------              TcpSyn,Ack
      TcpAck                   ------->
  4)  ---------------------  TLS Session  ---------------------
      ClientHello              ------->
                                                    ServerHello
                                          {EncryptedExtensions}
                                                 {Certificate*}
                                           {CertificateVerify*}
                               <-------              {Finished}
      {Finished}               ------->
  5)  ---------------------- Stream Prefix --------------------
      "IKETCP"                 ------->

  6)  ------------ Retransmit Message from step 2 -------------
      Length + Non-ESP Marker  ------->
      INFORMATIONAL
      HDR, SK { N(UPDATE_SA_ADDRESSES),
      N(NAT_DETECTION_SOURCE_IP),
      N(NAT_DETECTION_DESTINATION_IP) }
                               <------- Length + Non-ESP Marker
                                                  INFORMATIONAL
                          HDR, SK { N(NAT_DETECTION_SOURCE_IP),
                              N(NAT_DETECTION_DESTINATION_IP) }
  7)  -- New Exchange with recalculated  NAT_DETECTION_*_IP ---
      Length + Non-ESP Marker  ------->
      INFORMATIONAL
      HDR, SK { N(UPDATE_SA_ADDRESSES),
      N(NAT_DETECTION_SOURCE_IP),
      N(NAT_DETECTION_DESTINATION_IP) }
                               <------- Length + Non-ESP Marker
                                                  INFORMATIONAL
                          HDR, SK { N(NAT_DETECTION_SOURCE_IP),
                              N(NAT_DETECTION_DESTINATION_IP) }
  8)  <---------------------> IKE/ESP Flow <------------------>

1. During the IKE_AUTH exchange, the client and server exchange MOBIKE_SUPPORTED notify payloads to indicate support for MOBIKE.¶
2. The client changes its point of attachment to the network and receives a new IP address. The client attempts to re-establish the IKE session using the UPDATE_SA_ADDRESSES notify payload, but the server does not respond because the network blocks UDP traffic.¶
3. The client brings up a TCP connection to the server in order to use TCP encapsulation.¶
4. The client initiates a TLS handshake with the server.¶
5. The client sends the stream prefix for TCP-encapsulated IKE traffic (Section 4).¶
6. The client sends the UPDATE_SA_ADDRESSES notify payload in the INFORMATIONAL exchange on the TCP-encapsulated connection. Note that this IKE message is the same as the one sent over UDP in step 2; it should have the same message ID and contents.¶
7. Once the client receives a response on the TCP-encapsulated connection, it immediately starts a new INFORMATIONAL exchange with an UPDATE_SA_ADDRESSES notify payload and recalculated NAT_DETECTION_*_IP notify payloads in order to get correct information about the presence of NATs.¶
8. The IKE and ESP packet flow can resume.¶