QUIC Bandwidth Delay Product Tokens

Introduction This document describes a method for a QUIC server to send calculated Congestion Control parameters to a client for storage and later use on a future connection with Careful Resume . It also allows the client to inspect CC parameters, and allows the client not use them with new connections if its aware of a change in the BDP of the path.

Requirements Language 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 when, and only when, they appear in all capitals, as shown here.

BDP Tokens The address validation token is overloaded to also store Congestion Control information. This has the neat advantage that a QUIC client unaware of this document will still be able to make use of this specification without modification. QUIC defines an address validation token as an opaque blob that the client should not inspect. This document extends this by providing some structure to the token, whilst still providing fields for server specific information that is opaque to the client. The format of a token containing BDP information is defined as follows:

BDP Token BDP Token { Address Validation Length (i), Address Validation (..), Congestion Control Data Length (i), Congestion Control Data Value (..), Capacity (i), RTT (i), Requested Capacity (i) } The fields are as follows:

Address Validation Length: A variable-length integer specifying the length of the Address Validation field, in bytes. A value of 0 indicates that the server is not using address validation and is using the token purely for CC information.
Address Validation: Opaque information specific to the server that it will use for address validation. The construction of this field MUST comply with the requirements of QUIC section 8.1.
Congestion Control Data Length: A variable-length integer specifying the length of the Congestion Control Data field, in bytes. A value of 0 indicates that the server does not intend to store Congestion Control data on the client, and that the Saved Capacity and Saved RTT fields are merely informative.
Congestion Control Data: Opaque information specific to the server that it will use to prime its congestion controller state. This field SHOULD contain expiration/lifetime information, and any additional information that the server may need to validate that the same path is being used, such as an endpoint token as defined in Careful Resume . The data MUST be signed, or otherwise protected against modification by the client.
Capacity: The estimated capacity of the path in bytes, encoded as a variable-length integer. The exact Congestion Control state value used to fill this field is left to server preference.
RTT: The RTT of the path in microseconds, encoded as a variable-length integer. The exact Congestion Control state value used to fill this field is left to server preference.
Requested Capacity: In a token sent by the server, this field is set to the same value as the Capacity field. If the client becomes aware of a change in the available bandwidth of the path, it can adjust this field to request a lower capacity be used by the server when priming its congestion controller state.

The Capacity and RTT fields are merely hints to the client and the server MUST not use these fields when priming its congestion controller state. If it wishes to use these parameters it will have to also include them in its CC Data structure, as data in this field is protected against modification by the client.

Client interaction If the client becomes aware of a change in the available bandwidth of the path, it can use the Requested Capacity field to signal to the server a change it its available bandwidth. The server MUST not accept a value higher than that of the Capacity field, as this could cause an overload of the network path. If the client sets the Requested Bandwidth field to 0 then it is signalling that the server should not attempt to prime its congestion controller from previous state and should instead treat this connection as an entirely new congestion control context.

Invalid BDP tokens If the server is unable to decode the token received from the client as a valid BDP token then this MUST be treated as a connection error BDP_TOKEN_ERROR (0x1312). A token which is merely expired MUST NOT trigger a connection error, instead it should be silently discarded.

Extension signalling The server can send the above extended BDP token to all clients without further negotiation. However a client needs some way to know that there is meaningful structure to a token its received from the server. To this end a new transport parameter is defined.

bdp_token (0x1312): The bdp_token transport parameter is a boolean value that indicates that the server is using BDP tokens. It can have the following values:
- 0, default value: BDP Tokens are not in use.
- 1: BDP Tokens are in use.

IANA Considerations

QUIC Transport Parameters Per this document, one new entry has been added to the "QUIC Transport Parameters" registry defined in section 22.3. This entry is defined below: New entries

Value	Status	Specification	Parameter name
0x1312	Provisional	This document	bdp_frame

QUIC Transport Error Codes Per this document, one new entry has been added to the "QUIC Transport Error Codes" registry defined in section 22.5. This entry is defined below: New entries

Value	Status	Code	Description	Specification
0x1312	Provisional	BDP_TOKEN_ERROR	The BDP token received from the client is invalid.	This document

Security Considerations The Congestion Control Data field MUST be protected against manipulation by malicious or mis-behaving clients. A client that can modify Congestion Control calculations could cause an overload of the network path. The Capacity and RTT fields are hints to the client and not protected from modification by a client. The server MUST ignore these when processing a received BDP token.