Hello Christer and Gen-ART
thanks for the positive review! Please see inline for our responses
Christer Holmberg schreef op 30-6-2014 12:07:
Minor Issues:
Section 3.12 talks about keep alive signaling.
Q1: The sending of keep alives is a SHOULD, and there are no
procedures on how to act if keep alives are not received. There isn't
even a mechanism to negotiate the sending of keep alives.
If no keep alives are sent, a peer runs a risk of being declared dead
following the conditions of Section 8.15. Note that if it does send
normal messages, this is seen as a sign of liveliness. When a peer is
declared dead "the connection" should be closed. More correct phrasing
would be indeed "the channel should be closed", which implies no more
messages will be sent to the peer and the local administration about the
peer is cleared. We will change "connection" to "channel".
Does this answer your question?
Q2: As the sending of keep alives is a SHOULD, are there example
cases when keep alives would NOT be sent?
One example is that of a busy server that hosts complete copies of
static content. In that case, the server could refrain from sending keep
alives to save on processing and outgoing messages. In particular,
assume a client establishes a channel with the server, sends a REQUEST,
and receives a DATA message in reply. The client acknowledges it with an
ACK, but after that does not attempt any downloads, and just sends keep
alives.
Ideally, this client should be garbage collected. The server can achieve
that by not sending keep alives after the DATA message. After 3 minutes,
the client will conclude that the server is dead and discard
the channel. And after three minutes, the server can clean up his end.
If the client would send new REQUESTs in those three minutes, the
cleanup would be avoided.
Q3: The text saying "to each peer it wants to interact with in the
future" sounds a little strange to me. How does a peer know with whom
it wants to interact in the future? Perhaps the text instead should
talk about peers with whom one wants to maintain a signaling channel,
or something like that?
It is indeed about maintaining a signaling channel. Most P2P systems
have a policy that decides which peers are of interest now and in the
near future. Examples of interesting peers are peers that have chunks on
offer that this client needs, or peers that currently do not have
interesting chunks on offer (because they are still downloading
themselves, or in live streaming), but gave good performance in the
past. When they have new stuff to offer they can be used again,
so you want to keep a signaling channel open.
From a subset of those peers the client will request chunks. In P2P
systems that have tit-for-tat as freeriding prevention this is normally
a small subset as to not fragment the upload bandwidth needed to
reciprocate the peer.
Is it the phrasing that raises your question or should we explain more
about this policy for deciding which peers are interesting?
Regards,
Arno Bakker et al.
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