Re: [mpls] Review of draft-ietf-mpls-lsp-ping-relay-reply-07

["Nobo Akiya" <[e-mail]>]

Hi Lizhong,

 

Few more follow-up comments in-line with [NOBO].

 

From: [e-mail] [mailto:[e-mail]] 
Sent: March-23-15 10:48 AM
To: Nobo Akiya; draft-ietf-mpls-lsp-ping-relay-reply
Cc: mpls; loa
Subject: Re: Review of draft-ietf-mpls-lsp-ping-relay-reply-07

 

Hi Nobo,

Thank you for the detail review again. See the reply inline below.

 

to authors,

Please also check, thanks.

 

  _____  

Regards

Lizhong

 

From: Nobo Akiya <mailto:[e-mail]> 

Date: 2015-03-22 06:52

To: [e-mail] <mailto:[e-mail]> 

CC: [e-mail] <mailto:[e-mail]> ; 'Loa Andersson' <mailto:[e-mail]> 

Subject: Review of draft-ietf-mpls-lsp-ping-relay-reply-07

Hi Authors,

 

I was asked to provide a review of the following document:

 

http://tools.ietf.org/html/draft-ietf-mpls-lsp-ping-relay-reply-07 

 

The document is well written and addresses a real problem. I do have a

number of (mostly minor and some moderate) comments which I'd like to see

addressed before progressing the document.

 

Section 2

--------------

 

   Figure 1 demonstrates a case where one LSP is set up between PE1 and

   PE2.  If PE1's IP address is not distributed to AS2, a traceroute

   from PE1 directed to PE2 could fail if the fault exists somewhere

   between ASBR2 and PE2.

 

Above is not entirely accurate. The Traceroute from PE1 to PE2 can fail even

if there is no fault. It would be good to clarify this by saying "If PE1's

IP address is not distributed to AS2, a traceroute from PE1 directed towards

PE2 can result in a failure because an LSR in AS2 may not be able to send

the Echo Reply message".

[Lizhong] accepted.

 

   Note that throughout the document, routable address means that it is

   possible to route an IP packet to this address using the normal

   information exchanged by the IGP operating in the AS

 

Missing '.' (period) at the end of the paragraph.

[Lizhong] accepted.

 

   When tracing an LSP from one AN to the remote AN,

   the LSR1/LSR2 node could not make a response to the Echo Request

   either, like the P2 node in the inter-AS scenario in Figure 1.

 

"... could not make a response to the Echo Request either ..." can perhaps

be rephrased as "... cannot send the Echo Reply either ...", to be a bit

more clear. Yeah it's really a nitpick, but _making_ the response packet and

_sending_ the response packet are two different operations in a code.

[Lizhong] accepted.

 

Figure 3

--------------

 

Typically, the numbers at the top (i.e., bit positions 0, 1, 2, ...) are

aligned with columns with '-' character instead of '+' character. In other

words, let's shift the bit positions to the right by one space.

[Lizhong] good catch, thanks.

 

Section 3.3

--------------

 

s/the Reply/the Echo Reply/

[Lizhong] accepted.

 

Section 4.1

--------------

 

   When the Echo Request is first sent by the initiator included a Relay

   Node Address Stack TLV, the TLV MUST contain the initiator address as

   the only entry of the stack of relayed addresses,

 

I'd like to suggest rephrasing the first half of above sentence as follows.

 

When the initiator sends the first Echo Request for the Traceroute

operation, with a Relay Node Address Stack TLV, the TLV MUST contain ...

[Lizhong] I'd like to remove "Traceroute" as below. Because in "Ping" mode,

the Relayed Echo Reply mechanism could still work.

When the initiator sends the first Echo Request with a Relay Node Address 

Stack TLV, the TLV MUST contain ...

 

[NOBO] So perhaps what we want to say here is that the first address of the Relay Node Address Stack TLV in the MPLS Echo Request sent must be the same as the source IP address used in the sending packet. In other words, we should not have any text that restricts that there must only be one entry in the Relay Node Address Stack TLV. Then we can follow that up with texts like, when operating in the Traceroute mode, then the first Echo Request sent will have the Relay Node Address Stack TLV containing one local address.

 

 

Section 4.2

--------------

 

   Those address entries with K bit set to 1 MUST be kept in the stack.

   The receiver MUST check the addresses of the stack in sequence from

   bottom to top to find the last address in the stack with the K bit

   set (or the top of the stack if no K bit was found).

 

Shouldn't above "find the last address in the stack with the K bit set"

instead say "find the last non-Null address in the stack with the K bit

set"? Otherwise the procedure will cause relay LSRs to always refer to this

Null address with K bit set, preventing the first initiator address to be

ever reached when searching for the next relay address. On a related topic,

what is the purpose of allowing a Null address entry with K bit set?

[Lizhong]: please refer to the following in section 4.2.

   If a node spans two addressing domains (with respect to this message)

   where nodes on either side may not be able to nodes in the other

   domain, then the final address added MUST set the K bit.  K bit

   applies in the case of a NULL address, to serve as a warning to the

   initiator that further Echo Request messages may not result in

   receiving Echo Reply messages.

But I believe we need to update section 4.6 as below:

OLD:

Each time the TTL is increased, the initiator MUST copy the

   Relay Node Address Stack TLV received in the previous Echo Reply to

   the next Echo Request.

NEW:

Each time the TTL is increased, the initiator could copy the

   Relay Node Address Stack TLV in the previous Echo Reply to

   the next Echo Request. Some modifications could also be made to the stack TLV, e.g., delete the NIL entry.

 

[NOBO] That’s a reasonable approach to address this, as long as the document states that the initiator MUST delete the NIL entry in subsequent Echo Requests. Otherwise, you will have a case where last entry with K bit set in the TLV is NIL, and described procedures will never reach the top (initiator) address.

 

 

   If a node spans two addressing domains (with respect to this message)

   where nodes on either side may not be able to nodes in the other

   domain,

 

s/may not be able to nodes/may not be able to reach nodes/

[Lizhong] accepted

 

   If a node spans two addressing domains (with respect to this message)

   where nodes on either side may not be able to nodes in the other

   domain, then the final address added MUST set the K bit.

 

Above procedures defines a strict operation (i.e., MUST) for the K bit

usage. However, how an LSR determines "If a node spans two addressing

domains where nodes on either side may not be ..." is very vague. It might

be possible that we can end up with different implementations of the K bit

setting because of this vagueness. What's your thoughts?

[Lizhong] this is the principle of the implementation. How to know the 

address information is implementation specific. If fail to get the information,

then it is possible to fail for the echo reply. Maybe "SHOULD" is better here.

 

[NOBO] Yes I would be more comfortable with SHOULD than MUST.

 

 

   If the full reply message would exceed the MTU size, the Relay Node

   Address Stack TLV MUST be returned back in the Echo Reply message.

   Some other TLV(s) MUST be dropped.

 

Well, it's possible that the Relay Node Address Stack TLV has grown so big

and that is the only optional TLV to be included in the Echo Reply. In that

case, the Relay Node Address Stack TLV cannot be included despite the

"MUST". I believe what you are trying to imply here is that the Relay Node

Address Stack TLV takes precedence over other optional TLVs, when

determining which optional TLVs to keep in the Echo Reply. In that case,

perhaps it is better to say "If the full reply message would exceed the MTU

size, the Relay Node Address Stack TLV SHOULD be included in the Echo Reply

message (i.e., other optional TLVs are excluded)."

[Lizhong] agreed.

 

Section 4.3

--------------

 

   The Destination Address determined in section 4.2 is used as the next

   relay node address.

 

Section 4.2 only describes how to update the Relay Node Address Stack TLV.

It also specifies how to update the Destination Address Pointer field.

However, nowhere in section 4.2 talk about how the Destination Address is

determined. I'm assuming "the Destination Address determined ..." is

referring to this address which the Destination Address Pointer is pointing

to. Perhaps it'll be a good idea, somewhere in section 4.2, to say "...

_this_ address is determined to be the Destination Address".

[Lizhong] good catch. See changes below:

OLD in section 4.2:

The Destination Address Pointer MUST be set to this entry. 

NEW

The Destination Address Pointer MUST be set to this entry which is also the Destination Address.

 

Section 4.4

--------------

 

   Upon receiving an Relayed Echo Reply message with its own address as

   the destination address in the IP header, the relay node MUST

   determine the next relay node address as described in section 4.3,

   with the modification that the location of the received Destination

   Address is used instead of the bottom of stack in the algorithm.

 

In above, I think you meant section 4.2 instead of section 4.3.

[Lizhong] accepted

 

   The

   destination address in Relay Node Address Stack TLV will be updated

   with the next relay node address.

 

By "destination address" above, do you mean the Destination Address Pointer

field? If so, a bit of clarification is required. Otherwise, I'm not sure

what you mean by "destination address" above.

[Lizhong] yes, changed as below:

The Destination Address Pointer in Relay Node Address Stack TLV will be set to

   the next relay node address. 

 

The document is not very clear on how these two fields are used.

- Source IP address field of the IP header

- Source Address of Replying Router field of the Relay Node Address Stack

TLV

 

I'd imagine you'd want one field to be set by the egress LSR and unmodified

by every upstream relay LSRs, and one field to be updated by the egress LSR

and every upstream relay LSR. This is sort of clarified in section 4.7, but

really should be part of the procedures.

[Lizhong] in section 4.2, it says as below:

   Upon receiving a Relay Node Address Stack TLV in an Echo Request

   message, the receiver updates the "Source Address of Replying

   Router".  The address MUST be same as the source IP address of Relay

   Echo Reply (section 4.3) or Echo Reply message (section 4.5) being

   sent.

How to set Source IP address field of the IP header is a standard behavior, and

not speicified in this document.

 

[NOBO] Yes but when receiving an Relayed Echo Reply, how are those two fields updated? I’m guessing that the Source Address of Replying Router field is unmodified, and Source IP address field of the IP header is set to an address of the relay node? Clarifying this point will  be beneficial.





 

Section 4.6

--------------

 

   During a traceroute operation, multiple Echo Request messages are

   sent.  Each time the TTL is increased, the initiator MUST copy the

   Relay Node Address Stack TLV received in the previous Echo Reply to

   the next Echo Request.

 

True but don't we want the initiator to "reset" some fields such as Reply

Add Type, Source Address of Replying Router and Destination Address Pointer?

[Lizhong] these fields will be reset by the reply node. But this section should still

be updated as described in previous comments.

 

[NOBO] I simply assumed that we want to clear:

-        the Source Address of Replying Router field (along with reply Add Type)

-        Destination Address Pointer

Since value of those fields are meaningless to the subsequent Echo Request. Is there any reason why you want to preserve those values for the subsequent Echo Request?

 

Thanks,

Nobo

 

 

Section 6

--------------

 

As an added security, a receiver of an MPLS Echo Request should verify that

the first address in the Relay Node Address Stack TLV is the same address as

the source IP address field of the received IP header.

[Lizhong] accepted, thanks.

 

Thanks!

 

-Nobo