Subject: [MMUSIC] Use of TTL in SDP c line

From: "mo altaraireh" <[email-masked]>
Date: Fri, 12 May 2006 11:03:19 -0400

Hi,

According to RFC 2327  "IP multicast connection address must also have
     a time to live (TTL) value present in addition to the multicast
     address.  The TTL and the address together define the scope with
     which multicast packets sent in this conference will be sent. TTL
     values must be in the range 0-255.
"
How is the ttl value is used? and what does a ttl value of 0 means? Also if
the ttl value is set to 0 does the format still needs to be mcsatip/0?

Thanks,
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---

From: Kylin Wei <[email-masked]>
Date: Sat, 13 May 2006 10:37:52 +0800

TTL (Time to live): the hop number which is contained within IP data
packet before being destroyed.  
To those data packets which can't be send out, TTL can be used to avoid
them to always be transferred on the internet.
Once the router receives the IP data packet, it would first subtract 1
from original TTL value of packet. 
If TTL value is equal to 0, anyone router will discard this data packet
if this router isn't the destination of data packet.
As a rule, the TTL value can't be set to 0.
 
Kylin Wei
 
-----Original Message-----
From: mo altaraireh [mailto:[email-masked]] 
Sent: Friday, May 12, 2006 11:03 PM
To: [email-masked]
Subject: [MMUSIC] Use of TTL in SDP c line
 
Hi,
 
According to RFC 2327  "IP multicast connection address must also have
     a time to live (TTL) value present in addition to the multicast
     address.  The TTL and the address together define the scope with 
     which multicast packets sent in this conference will be sent. TTL
     values must be in the range 0-255.
"
How is the ttl value is used? and what does a ttl value of 0 means? Also
if the ttl value is set to 0 does the format still needs to be
mcsatip/0?
 
Thanks,

---

From: Colin Perkins <[email-masked]>
Date: Sat, 13 May 2006 18:01:53 +0100

On 13 May 2006, at 03:37, Kylin Wei wrote:
> TTL (Time to live): the hop number which is contained within IP  
> data packet before being destroyed.
>
> To those data packets which can’t be send out, TTL can be used to  
> avoid them to always be transferred on the internet.
>
> Once the router receives the IP data packet, it would first  
> subtract 1 from original TTL value of packet.
>
> If TTL value is equal to 0, anyone router will discard this data  
> packet if this router isn’t the destination of data packet.
>
> As a rule, the TTL value can’t be set to 0.

Sure it can, and the resulting packets won't leave the host. That's  
sometimes a useful feature.

Colin

---

From: "mo altaraireh" <[email-masked]>
Date: Mon, 15 May 2006 18:39:54 -0400

Hi Colin,

Sure this is the definition of the ttl in the IP header. My question is what
does all that mean in the context of the SDP body? I am questioning the
reasoning in the RFC to make the ttl value a "MUST" for multicast ip
addresses in the SDP. I would think that the ttl value should be optional at
best given that most GWs look at the IP header ttl value not
the SDP.

Thanks,

On 5/13/06, Colin Perkins <[email-masked]> wrote:
> > To those data packets which can't be send out, TTL can be used to
> > packet if this router isn't the destination of data packet.
> >

---

From: Colin Perkins <[email-masked]>
Date: Tue, 16 May 2006 09:27:29 +0100

On 15 May 2006, at 23:39, mo altaraireh wrote:
> Sure this is the definition of the ttl in the IP header. My  
> question is what does all that mean in the context of the SDP body?  
> I am questioning the reasoning in the RFC to make the ttl value a  
> "MUST" for multicast ip addresses in the SDP. I would think that  
> the ttl value should be optional at best given that most GWs look  
> at the IP header ttl value not
> the SDP.

Historical reasons, to do with previous use of TTL-limited scopes in  
the Mbone. Making the TTL optional when describing multicast sessions  
in SDP would not be backwards compatible, and saving four bytes isn't  
important.

Colin