Subject: [rohc] Question about packet types

From: [email-masked]
Date: Thu, 3 Oct 2002 12:28:28 +0100

Hi

I wonder if anyone could explain something to me.

In RFC 3095 section 5.2.7 there is a paragraph stating that type 1 packets
can be used when the number of bits needed for the SN exceed those available
in packet type 0. However, looking at the structure of the packets in 5.7.1
and 5.7.2 the number of bits available for SN is exactly the same for type 1
and type 2 packets. Have I misunderstood the explanation?

Secondly I don't quite understand the difference between UO-1 and UO-1-TS.
Their appears to be only 1 bit difference in that UO-1 can have a TS which
is 1 bit larger than for UO-1-TS. Firstly why is there a TS in UO-1?
Secondly I don't see why this difference relates to value(RND), it states
that UO-1 cannot be used if the context contains at least one header with
value(RND) = 0 and it is a different statement for UO-1-TS. But the RND
value relates to IP-ID not TS.

Any help would be much appreciated

Thank you

Matthew 

_______________________________________________
Rohc mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/rohc

---

From: [email-masked] (Sukanta Kumar Hazra)
Date: Fri, 4 Oct 2002 01:07:03 +0800

Hi!

Replies inline

----- Original Message -----
From: "[email-masked]"
Date: Thu, 03 Oct, 2002 19:28 SGT
Subject: [rohc] Question about packet types

> and type 2 packets. Have I misunderstood the explanation?

Type 0 packets do not have an extension header, and thus the number of bits
of SN that you can send is limited, to what can be sent in the base
header. 5.7.1 and 5.7.2 do not refer to type 2 packets. 

> is 1 bit larger than for UO-1-TS. Firstly why is there a TS in UO-1?

Different packet formats exist so that you can convey the necessary
bits. Always choose the smallest packet that can get the work done. The
TS exist because sometimes you may need to send TS bits, and it was seen
as a common enough occurance to warrant a packet type. 

> value relates to IP-ID not TS.

The value(RND) is used as a demultiplexing mechanism. The decompressor
interprets the meaning of the packet, depending on the state. The
situation arises due to the need to send compressed IP-ID. This happens
when IPv4 is used and IP-ID is not random. Thus there are two cases.

1. No IP-ID compression is used (IP-ID is RND or no IPv4). In that case
we can use UO-1 packets. The additional 1 bit  gained for TS is
significant (that is what we are trying to do anyway)

2. Compressed IP-ID bits are sent, then the same UO-1 packet format is
used to send IP-ID bits or TS bits and the decompressor needs to know,
what is being sent, that is what T bit is for.

-- 
--------------------------------------------------------------------
Sukanta Kumar Hazra <[email-masked]>      
Institute for Communications Research
PGP-KEY: http://www.icr.a-star.edu.sg/~sukanta/skhkey.asc
PGP-ID: 0x8E084AD5 
Key-Fingerprint: 9C0D B00C C62A 7059 5314  99FC CE90 478A 8E08 4AD5
--------------------------------------------------------------------
_______________________________________________
Rohc mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/rohc