4. The "record_size_limit" Extension
4. The "record_size_limit" Extension
The ExtensionData of the record_size_limit extension is RecordSizeLimit:
uint16 RecordSizeLimit;
The value of RecordSizeLimit is the maximum size of record in octets that the endpoint is willing to receive. This value is used to limit the size of records that are created when encoding application data and the protected handshake message into records.
When the record_size_limit extension is negotiated, an endpoint MUST NOT generate a protected record with plaintext that is larger than the RecordSizeLimit value it receives from its peer. Unprotected messages are not subject to this limit.
This value is the length of the plaintext of a protected record. The value includes the content type and padding added in TLS 1.3 (that is, the complete length of TLSInnerPlaintext). In TLS 1.2 and earlier, the limit covers all input to compression and encryption (that is, the data that ultimately produces TLSCiphertext.fragment). Padding added as part of encryption, such as that added by a block cipher, is not included in this count (see Section 4.1).
An endpoint that supports all record sizes can include any limit up to the protocol-defined limit for maximum record size. For TLS 1.2 and earlier, that limit is 2^14 octets. TLS 1.3 uses a limit of 2^14+1 octets. Higher values are currently reserved for future versions of the protocol that may allow larger records; an endpoint MUST NOT send a value higher than the protocol-defined maximum record size unless explicitly allowed by such a future version or extension. A server MUST NOT enforce this restriction; a client might advertise a higher limit that is enabled by an extension or version the server does not understand. A client MAY abort the handshake with an illegal_parameter alert if the record_size_limit extension includes a value greater than the maximum record size permitted by the negotiated protocol version and extensions.
Even if a larger record size limit is provided by a peer, an endpoint MUST NOT send records larger than the protocol-defined limit, unless explicitly allowed by a future TLS version or extension.
The record size limit only applies to records sent toward the endpoint that advertises the limit. An endpoint can send records that are larger than the limit it advertises as its own limit. A TLS endpoint that receives a record larger than its advertised limit MUST generate a fatal record_overflow alert; a DTLS endpoint that receives a record larger than its advertised limit MAY either generate a fatal record_overflow alert or discard the record.
Endpoints SHOULD advertise the record_size_limit extension, even if they have no need to limit the size of records. For clients, this allows servers to advertise a limit at their discretion. For servers, this allows clients to know that their limit will be respected. If this extension is not negotiated, endpoints can send records of any size permitted by the protocol or other negotiated extensions.
Endpoints MUST NOT send a record_size_limit extension with a value smaller than 64. An endpoint MUST treat receipt of a smaller value as a fatal error and generate an illegal_parameter alert.
In TLS 1.3, the server sends the record_size_limit extension in the EncryptedExtensions message.
During renegotiation or resumption, the record size limit is renegotiated. Records are subject to the limits that were set in the handshake that produces the keys that are used to protect those records. This admits the possibility that the extension might not be negotiated when a connection is renegotiated or resumed.
The Path Maximum Transmission Unit (PMTU) in DTLS also limits the size of records. The record size limit does not affect PMTU discovery and SHOULD be set independently. The record size limit is fixed during the handshake and so should be set based on constraints at the endpoint and not based on the current network environment. In comparison, the PMTU is determined by the network path and can change dynamically over time. See [PMTU] and Section 4.1.1.1 of [DTLS] for more detail on PMTU discovery.
PMTU governs the size of UDP datagrams, which limits the size of records, but does not prevent records from being smaller. An endpoint that sends small records is still able to send multiple records in a single UDP datagram.