Lightning Network `channel_update` message Flag clarification

3

This is a lnd.log line:

2019-01-11 12:20:13.904 [DBG] PEER: Sending ChannelUpdate(chain_hash=000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f, short_chan_id=601495532567724033, flag=256, update_time=2019-01-11 12:19:08 +0100 CET) to 46.229.165.136:9735

Since I cannot find a direct correlation with the spec at https://github.com/lightningnetwork/lightning-rfc/blob/master/07-routing-gossip.md can someone clarify the "flag" value?

Where can I find the information below?

[1:message_flags]
[1:channel_flags]
[2:cltv_expiry_delta]
[8:htlc_minimum_msat]
[4:fee_base_msat]
[4:fee_proportional_millionths]
[8:htlc_maximum_msat] (option_channel_htlc_max)

Are they encoded in some way in this "flag"? How?

Stefano Angieri

Posted 2019-01-11T16:16:28.553

Reputation: 133

Answers

4

The flags field (2 bytes) has been split into two different (1 byte) fields, the message_flags (most significant bits) and the channel_flags field (least significant bits). Hence the flags value displayed in lnd has the message_flags byte set to 0x00 and the message_flags set to 0x01.

message_flags indicates that this channel_update has some optional fields appended to the end. In this case it has the 0th bit set, which corresponds to the option_channel_htlc_max option, hence the channel_update has an additional field htlc_maximum_msat appended.

cdecker

Posted 2019-01-11T16:16:28.553

Reputation: 7 878

So Basically the "Flag" is a 16 bits field [8 bit message_flag + 8 bit channel_flags].

Is it possible to find the others information [cltv_expiry_delta, htlc_minimum_msat, fee_base_msat, fee_proportional_millionths, htlc_maximum_msat] exploring the log or it is only possible by using the lncli getchainifo?

How can a Node, receiving a channel update message, understand and set the new channel policies? – Stefano Angieri – 2019-01-15T11:31:08.567

Not too familiar with the lnd RPC interface, I mostly work on c-lightning. But I guess it should list those with peers as well. And you can set all of these parameters on startup using cli options. – cdecker – 2019-01-15T12:55:41.303

Asked: 2019-01-11T16:16:28.553

Viewed: 101 times

Active: 2019-01-11T19:08:25.007