Subject: Questions on draft-martini-l2circuit-encap-mpls-00.txt

From: Gopal Kailad <[email-masked]>
Date: Fri, 02 Feb 2001 22:12:39 -0500

Section 3:

It is not clear why padding is *required* ? We would need the length
in case of small packets, but I think padding can be taken care of at
the lower layer?

There is a requirement that PDUs carrying the control word MUST NOT
be delivered out of order.  This control word also carries the 
length. So, if in order delivery is not required for a Frame
Relay connection, but small packets are expected, should in order
delivery be forced?

Also, consider the case of VCC transport mode described in section
5.2. If a combination of ATM cells (OAM) and AAL5-CPCS PDUs are to be
carried over a connection, we need to use the control word as we need
the T-bit to differentiate cells from frames. But then this forces in
order delivery, which may not be required. (If cell level sequencing is
required, one has to anyways use only cell encapsulation and transport
each cell individually.) 

One solution would be to add a bit to indicate if the control word
contains a valid sequence number, and remove the restriction that PDUs
carrying control word should be delivered in order.

According to the draft, the endpoints should be configured with the
knowledge of whether a control word will be used for a specific
virtual circuit. But then, if a virtual circuit is set up to carry
a Frame Relay DLCI without the control word, and a small packet
is received to be encapsulated, should it be dropped because there
is no control word to contain the length? Of course, the control
word can always be enabled when a circuit is set up, but then it may
not be needed except in case of occasional small packets.

One solution would be to use one of the EXP bits in the encapsulating
label to indicate the presence/absence of the control word. This bit
will be set (and a control word will follow) for all packets on a 
virtual circuit if in-order delivery is required. Otherwise, it will
be set only when control word is necessary - in case of small packets,
for example.

Section 5.1:

The draft states that FCS will not be carried in case of Ethernet, cisco
HDLC and PPP. But in section 5.1 (Frame Relay) there is no mention of
FCS. Is there any reason to carry FCS in case of Frame Relay?

Section 5.2:

There seems to be a contradiction: "The ATM Cell encapsulation
consists of an MPLS label stack, a required generic sequencing control
word, a 4 byte ATM cell header, and the ATM cell payload." Later in the
same section, "VPC transport mode may only be used to transport ATM
*cells*. The sequencing control word is *optional* in VPC transport
mode."

Also, note that the sequence number is only 16 bits - the rollover
would be every 64 K cells in VPC mode. 32 bits would be better.

Cheers,

Gopal