Subject: [rohc] Content of TCP packet formats

From: Kristofer Sandlund <[email-masked]>
Date: Fri, 08 Oct 2004 11:29:37 +0200

In parallell with discussing how to notate these, I figured I'd try to get some 
input of their actual content even though this has proven to be a 
near-impossible task in the past.

So, reviewing Mark's latest packet formats I've summarized them as follows:

Common (1 packet) - (swiss army knife, can transmit basically any dynamic field)
Seq-IPID (17 packets) (sequential and sequential-byteswapped)
Random-IPID (12 packets) - Random, none or zero IPID

List of the number of times each field appears in the CO packets (variable 
number of bits for some fields):
Note that all packets WILL contain discriminator, crc, push bit and msn

Sequential packets:
----
IP-ID: ALL
Sequence number - ALL
ECN behavior - 6 (too many, I sugggest only the common packet can change the 
behavior. All can still transmit ecn if on or off. Keeping 2 is ok I guess if 
crc is 7 bit, but a 3-bit CRC for something that changes the irregular chain 
seems weak)
ACK number - 8 packets
Hoplimit/TTL - 6 packets (too much, I suggest a reduction to max 2 packets, and 
maybe these should have a 7-bit CRC)
TOS/TC (not including ECN bits)- 5 packets (a bit too much, I suggest reduction 
to 2 or 3 (w/ crc-7) or even down to 0 and let common handle this)
window size: 4 (ok? actually I don't know)

Random packets:
----
Sequence number - ALL
ecn-behavior - 2 (as above, I think this should only be in common, but 2 is ok I 
guess if crc is 7 bit)
ACK: 9 packets (high percentage compared to above, but ok I guess)
rsf flags (syn,reset,fin) - 1 (why can't these be changed in seq-packets? Maybe 
add these to one seq packet as well)
ttl/hopl - 4 (suggtgest down to max 2 again)
tc/tos - 0 (5 for seqeuntial and none here? this seems more reasonable)
window - 3
urgent pointer - 1 (common should be enough since this is supposed to be rare)

So, generally, I'm for removing fields from packets, and since the WG earlier 
has received feedback that we have too many packet formats, I suggest we try to 
remove some of them when we remove fields.
(note that I don't have access to the scripts Mark has used to generate these 
packet formats, so I'm trying to base packet formats on the TCP-BEH draft instead).

Now is a REALLY good time for people to comment, 'cause I'll try to change these 
during the weekend and I don't want to do this over more than once before the 
next revision of the draft

/Kristofer Sandlund, Effnet AB

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