some skbuff questions

hello,
1) Is there only one alloc_skb for each type of packet(like
TCP,ICMP,UDP) to be built?
2) Is that function is used for allocating a total space for packet so
that it will contain all layer headers as well as application data?
3) what is the significance of 15 value in alloc_skb functions in many
source files?
actually i want to know explaination about following two lines about
value 15 and 16.
tcp_output.c:1219: buff = alloc_skb(MAX_TCP_HEADER + 15,
sk->allocation);
linux/skbuff.h:1042: skb = alloc_skb(length+16, gfp_mask);
i am using kernel 2.4.24.
regards,
cranium

"cranium" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> hello,
> 1) Is there only one alloc_skb for each type of packet(like
> TCP,ICMP,UDP) to be built?
> 2) Is that function is used for allocating a total space for packet so
> that it will contain all layer headers as well as application data?
> 3) what is the significance of 15 value in alloc_skb functions in many
> source files?
> actually i want to know explaination about following two lines about
> value 15 and 16.
> tcp_output.c:1219: buff = alloc_skb(MAX_TCP_HEADER + 15,
> sk->allocation);
> linux/skbuff.h:1042: skb = alloc_skb(length+16, gfp_mask);
> i am using kernel 2.4.24.
> regards,
> cranium

This is a 100% guess, but the +15 and +16 might be to round up to a 16-byte
boundary because of some loop unrolling optimizations that make it process
data 16-bytes-per-loop.